Posts Tagged place constraints

Recent Postings from place constraints

The Chandra Planetary Nebula Survey (ChanPlaNS). II. X-ray Emission from Compact Planetary Nebulae

We present results from the most recent set of observations obtained as part of the Chandra X-ray observatory Planetary Nebula Survey (ChanPlaNS), the first comprehensive X-ray survey of planetary nebulae (PNe) in the solar neighborhood (i.e., within ~1.5 kpc of the Sun). The survey is designed to place constraints on the frequency of appearance and range of X-ray spectral characteristics of X-ray-emitting PN central stars and the evolutionary timescales of wind-shock-heated bubbles within PNe. ChanPlaNS began with a combined Cycle 12 and archive Chandra survey of 35 PNe. ChanPlaNS continued via a Chandra Cycle 14 Large Program which targeted all (24) remaining known compact (R_neb <~ 0.4 pc), young PNe that lie within ~1.5 kpc. Results from these Cycle 14 observations include first-time X-ray detections of hot bubbles within NGC 1501, 3918, 6153, and 6369, and point sources in HbDs 1, NGC 6337, and Sp 1. The addition of the Cycle 14 results brings the overall ChanPlaNS diffuse X-ray detection rate to ~27% and the point source detection rate to ~36%. It has become clearer that diffuse X-ray emission is associated with young (<~5×10^3 yr), and likewise compact (R_neb<~0.15 pc), PNe with closed structures and high central electron densities (n_e>~1000 cm^-3), and rarely associated with PNe that show H_2 emission and/or pronounced butterfly structures. Hb 5 is one such exception of a PN with a butterfly structure that hosts diffuse X-ray emission. Additionally, of the five new diffuse X-ray detections, two host [WR]-type CSPNe, NGC 1501 and NGC 6369, supporting the hypothesis that PNe with central stars of [WR]-type are likely to display diffuse X-ray emission.

KIC 10526294: a slowly rotating B star with rotationally split quasi-equally spaced gravity modes

Massive stars are important for the chemical enrichment of the universe. Since internal mixing processes influence their life, it is of high importance to place constraints on the corresponding physical parameters, such as core overshooting and the internal rotation profile, to calibrate their stellar structure and evolution models. Although asteroseismology was shown to be able to deliver the most precise constraints so far, the number of detailed seismic studies delivering quantitative results is limited. Our goal is to extend this limited sample with an in-depth case study and provide a well constrained set of asteroseismic parameters, contributing to the ongoing mapping efforts of the instability strips of the beta Cep and SPB stars. We derived fundamental parameters from high-resolution spectra using spectral synthesis techniques. We used custom masks to obtain optimal light curves from the original pixel level data from the Kepler satellite. We used standard time-series analysis tools to construct a set of significant pulsation modes which provide the basis for the seismic analysis carried out afterwards. We find that KIC 10526294 is a cool SPB star, one of the slowest rotators ever found. Despite this fact, the length of Kepler observations is sufficient to resolve narrow rotationally split multiplets for each of its nineteen quasi-equally spaced dipole modes. The number of detected consecutive (in radial order) dipole modes in this series is higher than ever before. The observed amount of splitting shows an increasing trend towards longer periods, which – largely independent of the seismically calibrated stellar models – points towards a non-rigid internal rotation profile. From the average splitting we deduce a rotation period of 188 d. From seismic modelling we find that the star is young with a central hydrogen mass fraction X_c>0.64; it has a core overshooting alpha_ov<=0.15.

The Effect of Anisotropic Viscosity on Cold Fronts in Galaxy Clusters

Cold fronts–contact discontinuities in the intracluster medium (ICM) of galaxy clusters–should be disrupted by Kelvin-Helmholtz (K-H) instabilities due to the associated shear velocity. However, many observed cold fronts appear stable. This opens the possibility to place constraints on microphysical mechanisms that stabilize them, such as the ICM viscosity and/or magnetic fields. We performed exploratory high-resolution simulations of cold fronts arising from subsonic gas sloshing in cluster cores using the grid-based Athena MHD code, comparing the effects of isotropic Spitzer and anisotropic Braginskii viscosity (expected in a magnetized plasma). Magnetized simulations with full Braginskii viscosity or isotropic Spitzer viscosity reduced by a factor f ~ 0.1 are both in qualitative agreement with observations in terms of suppressing K-H instabilities. The RMS velocity and turbulence within the sloshing region is only modestly reduced by Braginskii viscosity. We also performed unmagnetized simulations with and without viscosity and find that magnetic fields have a substantial effect on the appearance of the cold fronts, even if the initial field is weak and the viscosity is the same. This suggests that determining the dominant suppression mechanism of a given cold front from X-ray observations (e.g. viscosity or magnetic fields) by comparison with simulations is not straightforward. Finally, we performed simulations including anisotropic thermal conduction, and find that including Braginskii viscosity in these simulations does not significant affect the evolution of cold fronts; they are rapidly smeared out by thermal conduction, as in the inviscid case.

The Effect of Anisotropic Viscosity on Cold Fronts in Galaxy Clusters [Replacement]

Cold fronts–contact discontinuities in the intracluster medium (ICM) of galaxy clusters–should be disrupted by Kelvin-Helmholtz (K-H) instabilities due to the associated shear velocity. However, many observed cold fronts appear stable. This opens the possibility to place constraints on microphysical mechanisms that stabilize them, such as the ICM viscosity and/or magnetic fields. We performed exploratory high-resolution simulations of cold fronts arising from subsonic gas sloshing in cluster cores using the grid-based Athena MHD code, comparing the effects of isotropic Spitzer and anisotropic Braginskii viscosity (expected in a magnetized plasma). Magnetized simulations with full Braginskii viscosity or isotropic Spitzer viscosity reduced by a factor f ~ 0.1 are both in qualitative agreement with observations in terms of suppressing K-H instabilities. The RMS velocity of turbulence within the sloshing region is only modestly reduced by Braginskii viscosity. We also performed unmagnetized simulations with and without viscosity and find that magnetic fields have a substantial effect on the appearance of the cold fronts, even if the initial field is weak and the viscosity is the same. This suggests that determining the dominant suppression mechanism of a given cold front from X-ray observations (e.g. viscosity or magnetic fields) by comparison with simulations is not straightforward. Finally, we performed simulations including anisotropic thermal conduction, and find that including Braginskii viscosity in these simulations does not significantly affect the evolution of cold fronts; they are rapidly smeared out by thermal conduction, as in the inviscid case.

Herschel/PACS Observations of the Host Galaxy of GRB 031203

We present Herschel/PACS observations of the nearby (z=0.1055) dwarf galaxy that has hosted the long gamma ray burst (LGRB) 031203. Using the PACS data we have been able to place constraints on the dust temperature, dust mass, total infrared luminosity and infrared-derived star-formation rate (SFR) for this object. We find that the GRB host galaxy (GRBH) 031203 has a total infrared luminosity of 3×10^10 L_sun placing it in the regime of the IR-luminous galaxy population. Its dust temperature and specific SFR are comparable to that of many high-redshift (z=0.3-2.5) infrared (IR)-detected GRB hosts (T_dust>40K ; sSFR>10 Gyr^-1), however its dust-to-stellar mass ratio is lower than what is commonly seen in IR-luminous galaxies. Our results suggest that GRBH 031203 is undergoing a strong starburst episode and its dust properties are different to those of local dwarf galaxies within the same metallicity and stellar mass range. Furthermore, our measurements place it in a distinct class to the well studied nearby host of GRB 980425 (z=0.0085), confirming the notion that GRB host galaxies can span a large range in properties even at similar cosmological epochs, making LGRBs an ideal tool in selecting samples of star-forming galaxies up to high redshift.

Herschel/PACS Observations of the Host Galaxy of GRB 031203 [Replacement]

We present Herschel/PACS observations of the nearby (z=0.1055) dwarf galaxy that has hosted the long gamma ray burst (LGRB) 031203. Using the PACS data we have been able to place constraints on the dust temperature, dust mass, total infrared luminosity and infrared-derived star-formation rate (SFR) for this object. We find that the GRB host galaxy (GRBH) 031203 has a total infrared luminosity of 3×10^10 L_sun placing it in the regime of the IR-luminous galaxy population. Its dust temperature and specific SFR are comparable to that of many high-redshift (z=0.3-2.5) infrared (IR)-detected GRB hosts (T_dust>40K ; sSFR>10 Gyr^-1), however its dust-to-stellar mass ratio is lower than what is commonly seen in IR-luminous galaxies. Our results suggest that GRBH 031203 is undergoing a strong starburst episode and its dust properties are different to those of local dwarf galaxies within the same metallicity and stellar mass range. Furthermore, our measurements place it in a distinct class to the well studied nearby host of GRB 980425 (z=0.0085), confirming the notion that GRB host galaxies can span a large range in properties even at similar cosmological epochs, making LGRBs an ideal tool in selecting samples of star-forming galaxies up to high redshift.

The Carnegie Supernova Project: Intrinsic Colors of Type Ia Supernovae

We present an updated analysis of the intrinsic colors of SNe Ia using the latest data release of the Carnegie Supernova Project. We introduce a new light-curve parameter very similar to stretch that is better suited for fast-declining events, and find that these peculiar types can be seen as extensions to the population of "normal" SNe Ia. With a larger number of objects, an updated fit to the Lira relation is presented along with evidence for a dependence on the late-time slope of the B-V color-curves with stretch and color. Using the full wavelength range from u to H band, we place constraints on the reddening law for the sample as a whole and also for individual events/hosts based solely on the observed colors. The photometric data continue to favor low values of Rv, though with large variations from event to event, indicating an intrinsic distribution. We confirm the findings of other groups that there appears to be a correlation between the derived reddening law, Rv, and the color excess, E(B-V), such that larger E(B-V) tends to favor lower Rv. The intrinsic u-band colors show a relatively large scatter that cannot be explained by variations in Rv or by the Goobar (2008) power-law for circumstellar dust, but rather is correlated with spectroscopic features of the supernova and is therefore likely due to metallicity effects.

Parametric models of the periodogram

The maximum likelihood estimator is used to determine fit parameters for various parametric models of the Fourier periodogram followed by the selection of the best fit model amongst competing models using the Akaike information criteria. This analysis, when applied to light curves of active galactic nuclei can be used to infer the presence of quasi-periodicity and break or knee frequencies. The extracted information can be used to place constraints on the mass, spin and other properties of the putative central black hole and the region surrounding it through theoretical models involving disk and jet physics.

A Three-Year Multi-Wavelength Study of the Very High Energy Gamma-ray Blazar 1ES 0229+200

The high-frequency-peaked BL Lacertae object 1ES 0229+200 is a relatively distant (z = 0.1396), hard-spectrum (Gamma ~ 2.5), very-high-energy-emitting (E > 100 GeV) gamma-ray blazar. Very-high-energy measurements of this active galactic nucleus have been used to place constraints on the intensity of the extragalactic background light and the intergalactic magnetic field. A multi-wavelength study of this object centered around very-high-energy observations by VERITAS is presented. This study obtained, over a period of three years, an 11.7 standard deviation detection and an average integral flux F(E>300 GeV) = (23.3 +- 2.8_stat +- 5.8_sys) x 10^-9 photons m^-2 s^-1, or 1.7% of the Crab Nebula’s flux (assuming the Crab Nebula spectrum measured by H.E.S.S). Supporting observations from Swift and RXTE are analyzed. The Swift observations are combined with previously published Fermi observations and the very-high-energy measurements to produce an overall spectral energy distribution which is then modeled assuming one-zone synchrotron-self-Compton emission. The chi^2 probability of the TeV flux being constant is 1.6%. This, when considered in combination with measured variability in the X-ray band, and the demonstrated variability of many TeV blazars, suggests that the use of blazars such as 1ES 0229+200 for intergalactic magnetic field studies may not be straightforward and challenges models that attribute hard TeV spectra to secondary gamma-ray production along the line of sight.

Large-scale structure growth rate measurement cosmological constraints

We compile a list of $14$ independent measurements of large-scale structure growth rate between redshifts $0.067 \leq z \leq 0.8$ and use this to place constraints on model parameters of constant and time-evolving general-relativistic dark energy cosmologies. With the assumption that gravity is well-modeled by general relativity, we discover that growth-rate data provide restrictive cosmological parameter constraints. In combination with type Ia supernova apparent magnitude versus redshift data and Hubble parameter measurements, the growth rate data are consistent with the standard spatially-flat $\Lambda$CDM model, as well as with mildly evolving dark energy density cosmological models.

Cosmological constraints from large-scale structure growth rate measurements [Replacement]

We compile a list of $14$ independent measurements of large-scale structure growth rate between redshifts $0.067 \leq z \leq 0.8$ and use this to place constraints on model parameters of constant and time-evolving general-relativistic dark energy cosmologies. With the assumption that gravity is well-modeled by general relativity, we discover that growth-rate data provide restrictive cosmological parameter constraints. In combination with type Ia supernova apparent magnitude versus redshift data and Hubble parameter measurements, the growth rate data are consistent with the standard spatially-flat $\Lambda$CDM model, as well as with mildly evolving dark energy density cosmological models.

Interstellar Absorption Lines in the Direction of the Cataclysmic Variable SS Cygni

We present an analysis of interstellar absorption lines in high-resolution optical echelle spectra of SS Cyg obtained during an outburst in 2013 June and in archival Hubble Space Telescope and Far Ultraviolet Spectroscopic Explorer data. The Ca II K and Na I D lines toward SS Cyg are compared with those toward nearby B and A stars in an effort to place constraints on the distance to SS Cyg. We find that the distance constraints are not very robust from this method due to the rather slow increase in neutral gas column density with distance and the scatter in the column densities from one sight line to another. However, the optical absorption-line measurements allow us to derive a precise estimate for the line-of-sight reddening of E(B-V) = 0.020+/-0.005 mag. Furthermore, our analysis of the absorption lines of O I, Si II, P II, and Fe II seen in the UV spectra yields an estimate of the H I column density and depletion strength in this direction.

The Kappa Andromedae System: New Constraints on the Companion Mass, System Age & Further Multiplicity

Kappa Andromedae is a B9IVn star at 52 pc for which a faint substellar companion separated by 55 AU was recently announced. In this work, we present the first spectrum of the companion, "kappa And B," using the Project 1640 high-contrast imaging platform. Comparison of our low-resolution YJH-band spectra to empirical brown dwarf spectra suggests an early-L spectral type. Fitting synthetic spectra from PHOENIX model atmospheres to our observed spectrum allows us to constrain the effective temperature to ~2000K, as well as place constraints on the companion surface gravity. Further, we use previously reported log(g) and effective temperature measurements of the host star to argue that the kappa And system has an isochronal age of 220 +/- 100 Myr, older than the 30 Myr age reported previously. This interpretation of an older age is corroborated by the photometric properties of kappa And B, which appear to be marginally inconsistent with other 10-100 Myr low-gravity L-dwarfs for the spectral type range we derive. In addition, we use Keck aperture masking interferometry combined with published radial velocity measurements to rule out the existence of any tight stellar companions to kappa And A that might be responsible for the system’s overluminosity. Further, we show that luminosity enhancements due to a nearly "pole-on" viewing angle coupled with extremely rapid rotation is unlikely. Kappa And A is thus consistent with its slightly evolved luminosity class (IV) and we propose here that kappa And, with a revised age of 220 +/- 100 Myr, is an interloper to the 30 Myr Columba association with which it was previously associated. The photometric and spectroscopic evidence for kappa And B combined with our re-assesment of the system age implies a substellar companion mass of 50^{+16}_{-13} Jupiter Masses, consistent with a brown dwarf rather than a planetary mass companion.

Occultation of the T Tauri Star RW Aurigae A by its Tidally Disrupted Disk

RW Aur A is a classical T Tauri star, believed to have undergone a reconfiguration of its circumstellar environment as a consequence of a recent fly-by of its stellar companion, RW Aur B. This interaction stripped away part of the circumstellar disk of RW Aur A, leaving a tidally disrupted arm and a short truncated circumstellar disk. We present photometric observations of the RW Aur system from the Kilodegree Extremely Little Telescope (KELT) survey showing a long and deep dimming that occurred from September 2010 until March 2011. The dimming has a depth of ~2 magnitudes, a duration of ~180 days and was confirmed by archival observations from American Association of Variable Star Observers (AAVSO). We suggest that this event is the result of a portion of the tidally disrupted disk occulting RW Aur A, specifically a fragment of the tidally disrupted arm. The calculated transverse linear velocity of the occulter is in excellent agreement with the measured relative radial velocity of the tidally disrupted arm. Using simple kinematic and geometric arguments, we show that the occulter cannot be a feature of the RW Aur A circumstellar disk, and we consider and discount other hypotheses. We also place constraints on the thickness and semi-major axis of the portion of the arm that occulted the star.

Placing Limits On The Transit Timing Variations Of Circumbinary Exoplanets

We present an efficient analytical method to predict the maximum transit timing variations of a circumbinary exoplanet, given some basic parameters of the host binary. We derive an analytical model giving limits on the potential location of transits for coplanar planets orbiting eclipsing binaries, then test it against numerical N-body simulations of a distribution of binaries and planets. We also show the application of the analytic model to Kepler-16b, -34b and -35b. The resulting method is fast, efficient and is accurate to approximately 1% in predicting limits on possible times of transits over a three-year observing campaign. The model can easily be used to, for example, place constraints on transit timing while performing circumbinary planet searches on large datasets. It is adaptable to use in situations where some or many of the planet and binary parameters are unknown.

Constraining Superluminal Electron and Neutrino Velocities using the 2010 Crab Nebula Flare and the IceCube PeV Neutrino Events [Replacement]

The observation of two PeV-scale neutrino events reported by Ice Cube can, in principle, allows one to place constraints on Lorentz invariance violation (LIV) in the neutrino sector. After first arguing that at least one of the IceCube events was of extragalactic origin, I derive an upper limit for {\it the difference} between putative superluminal neutrino and electron velocities of $\le \sim 5.6 \times 10^{-19}$ in units where $c = 1$, confirming that the observed PeV neutrinos could have reached Earth from extragalactic sources. I further derive a new constraint on the superluminal electron velocity, obtained from the observation of synchrotron radiation in the Crab Nebula flare of September, 2010. The inference that the $>$ 1 GeV $\gamma$-rays from synchrotron emission in the flare were produced by electrons of energy up to $\sim 5.1$ PeV indicates the non-occurrence of vacuum \’{C}erenkov radiation by these electrons. This implies a new, strong constraint on superluminal electron velocities $\delta_e \le \sim 5 \times 10^{-21}$. It immediately follows that one then obtains an upper limit on the superluminal neutrino velocity {\it alone} of $\delta_{\nu} \le \sim 5.6 \times 10^{-19}$, many orders of magnitude better than the time-of-flight constraint from the SN1987A neutrino burst. However, if the electrons are {\it subluminal} the constraint on $|\delta_e| \le \sim 8 \times 10^{-17}$, obtained from the Crab Nebula $\gamma$-ray spectrum, places a weaker constraint on superluminal neutrino velocity of $\delta_{\nu} \le \sim 8 \times 10^{-17}$.

Constraining Superluminal Electron and Neutrino Velocities using the 2010 Crab Nebula Flare and the IceCube PeV Neutrino Events [Replacement]

The observation of two PeV-scale neutrino events reported by Ice Cube can, in principle, allows one to place constraints on Lorentz invariance violation (LIV) in the neutrino sector. After first arguing that at least one of the IceCube events was of extragalactic origin, I derive an upper limit for {\it the difference} between putative superluminal neutrino and electron velocities of $\le \sim 5.6 \times 10^{-19}$ in units where $c = 1$, confirming that the observed PeV neutrinos could have reached Earth from extragalactic sources. I further derive a new constraint on the superluminal electron velocity, obtained from the observation of synchrotron radiation in the Crab Nebula flare of September, 2010. The inference that the $>$ 1 GeV $\gamma$-rays from synchrotron emission in the flare were produced by electrons of energy up to $\sim 5.1$ PeV indicates the non-occurrence of vacuum \’{C}erenkov radiation by these electrons. This implies a new, strong constraint on superluminal electron velocities $\delta_e \le \sim 5 \times 10^{-21}$. It immediately follows that one then obtains an upper limit on the superluminal neutrino velocity {\it alone} of $\delta_{\nu} \le \sim 5.6 \times 10^{-19}$, many orders of magnitude better than the time-of-flight constraint from the SN1987A neutrino burst. However, if the electrons are {\it subluminal} the constraint on $|\delta_e| \le \sim 8 \times 10^{-17}$, obtained from the Crab Nebula $\gamma$-ray spectrum, places a weaker constraint on superluminal neutrino velocity of $\delta_{\nu} \le \sim 8 \times 10^{-17}$.

Constraining Superluminal Electron and Neutrino Velocities using the 2010 Crab Nebula Flare and the IceCube PeV Neutrino Events [Replacement]

The observation of two PeV-scale neutrino events reported by Ice Cube can, in principle, allows one to place constraints on Lorentz invariance violation (LIV) in the neutrino sector. After first arguing that at least one of the IceCube events was of extragalactic origin, I derive an upper limit for {\it the difference} between putative superluminal neutrino and electron velocities of $\le \sim 5.6 \times 10^{-19}$ in units where $c = 1$, confirming that the observed PeV neutrinos could have reached Earth from extragalactic sources. I further derive a new constraint on the superluminal electron velocity, obtained from the observation of synchrotron radiation in the Crab Nebula flare of September, 2010. The inference that the $>$ 1 GeV $\gamma$-rays from synchrotron emission in the flare were produced by electrons of energy up to $\sim 5.1$ PeV indicates the non-occurrence of vacuum \’{C}erenkov radiation by these electrons. This implies a new, strong constraint on superluminal electron velocities $\delta_e \le \sim 5 \times 10^{-21}$. It immediately follows that one then obtains an upper limit on the superluminal neutrino velocity {\it alone} of $\delta_{\nu} \le \sim 5.6 \times 10^{-19}$, many orders of magnitude better than the time-of-flight constraint from the SN1987A neutrino burst. However, if the electrons are {\it subluminal} the constraint on $|\delta_e| \le \sim 8 \times 10^{-17}$, obtained from the Crab Nebula $\gamma$-ray spectrum, places a weaker constraint on superluminal neutrino velocity of $\delta_{\nu} \le \sim 8 \times 10^{-17}$.

Narrowband Lyman-Continuum Imaging of Galaxies at z ~ 2.85

We present results from a survey for z~2.85 Lyman-Continuum (LyC) emission in the HS1549+1933 field and place constraints on the amount of ionizing radiation escaping from star-forming galaxies. Using a custom narrowband filter (NB3420) tuned to wavelengths just below the Lyman limit at z>=2.82$, we probe the LyC spectral region of 49 Lyman break galaxies (LBGs) and 70 Lya-emitters (LAEs) spectroscopically confirmed at z>=2.82, as well as 58 z~2.85 LAE photometric candidates. Four LBGs and 19 LAEs are detected in NB3420. Using V-band data probing the rest-frame non-ionizing UV, we observe that many NB3420-detected galaxies exhibit spatial offsets between their LyC and non-ionizing UV emission and are characterized by extremely blue NB3420-V colors, corresponding to low ratios of non-ionizing to ionizing radiation (F_UV/F_LyC) that are in tension with current stellar population synthesis models. We measure average values of (F_UV/F_LyC) for our spectroscopically confirmed LBG and LAE samples, correcting for foreground galaxy contamination and HI absorption in the IGM. We find (F_UV/F_LyC)_corr,LBG=82 +/- 45 and (F_UV/F_LyC)_corr,LAE=7.6 +/- 4.1. These flux-density ratios correspond respectively to LyC escape fractions of f_esc,LBG=1-2% and f_esc,LAE=5-14%, and imply a comoving LyC emissivity from star-forming galaxies of 8.7-14.7 x 10^24 ergs/s/Hz/Mpc^3. In order to study the differential properties of galaxies with and without LyC detections, we analyze narrowband Lya imaging and rest-frame near-infrared imaging, finding that while LAEs with LyC detections have lower Lya equivalent widths on average, there is no substantial difference in the rest-frame near-infrared colors of LBGs or LAEs with and without LyC detections. These observations are consistent with an orientation-dependent model where LyC emission escapes through cleared paths in a patchy ISM.

Narrowband Lyman-Continuum Imaging of Galaxies at z ~ 2.85 [Replacement]

We present results from a survey for z~2.85 Lyman-Continuum (LyC) emission in the HS1549+1933 field and place constraints on the amount of ionizing radiation escaping from star-forming galaxies. Using a custom narrowband filter (NB3420) tuned to wavelengths just below the Lyman limit at z>=2.82, we probe the LyC spectral region of 49 Lyman break galaxies (LBGs) and 91 Lya-emitters (LAEs) spectroscopically confirmed at z>=2.82. Four LBGs and seven LAEs are detected in NB3420. Using V-band data probing the rest-frame non-ionizing UV, we observe that many NB3420-detected galaxies exhibit spatial offsets between their LyC and non-ionizing UV emission and are characterized by extremely blue NB3420-V colors, corresponding to low ratios of non-ionizing to ionizing radiation (F_UV/F_LyC) that are in tension with current stellar population synthesis models. We measure average values of F_UV/F_LyC for our LBG and LAE samples, correcting for foreground galaxy contamination and HI absorption in the IGM. We find (F_UV/F_LyC)_corr^LBG=82 +/- 45 and (F_UV/F_LyC)_corr^LAE=7.4 +/- 3.6. These flux-density ratios correspond respectively to relative LyC escape fractions of f_esc,rel^LBG=5-8% and f_esc,rel^LAE=18-49%, absolute LyC escape fractions of f_esc^LBG=1-2% and f_esc^LAE=5-15%, and a comoving LyC emissivity from star-forming galaxies of 8.8-15.0 x 10^24 ergs/s/Hz/Mpc^3. In order to study the differential properties of galaxies with and without LyC detections, we analyze narrowband Lya imaging and rest-frame near-infrared imaging, finding that while LAEs with LyC detections have lower Lya equivalent widths on average, there is no substantial difference in the rest-frame near-infrared colors of LBGs or LAEs with and without LyC detections. These preliminary results are consistent with an orientation-dependent model where LyC emission escapes through cleared paths in a patchy ISM.

The parameter space of Cubic Galileon models for cosmic acceleration [Replacement]

We use recent measurements of the expansion history of the universe to place constraints on the parameter space of cubic Galileon models, in particular we concentrate on those models which contain the simplest Galileon term plus a linear potential. This gives strong constraints on the Lagrangian of these models. Most dynamical terms in the Galileon Lagrangian are constraint to be small and the acceleration is effectively provided by a constant term in the scalar potential, thus reducing, effectively, to a LCDM model for current acceleration. The effective equation of state is indistinguishable from that of a cosmological constant w = -1 and the data constraint it to have no temporal variations of more than at the few % level. The energy density of the Galileon can contribute only to about 10% of the acceleration energy density, being the other 90% a cosmological constant term. This demonstrates how useful direct measurements of the expansion history of the universe are at constraining the dynamical nature of dark energy.

Prospects for the detection of GRBs with HAWC

The observation of Gamma Ray Bursts (GRBs) with very-high-energy (VHE) gamma rays can provide understanding of the particle acceleration mechanisms in GRBs, and can also be used to probe the extra-galactic background light and place constraints on Lorentz invariance violation. We present prospects for GRB detection by the ground-based HAWC (High Altitude Water Cherenkov) gamma-ray observatory. We model the VHE spectrum of GRBs by extrapolating observations by Fermi LAT and other observatories to higher energies. Under the assumption that only e-pair production associated with extra-galactic background light is responsible for high-energy cutoffs in the spectrum, we find that HAWC will have a detection rate as high as 1.65 GRBs/year. Most of the sensitivity of HAWC to GRBs is derived from short-hard GRBs during the prompt phase. We explore the possibility of universal high-energy cutoffs in GRB spectra and find that the GRB detection rate by HAWC should be at least half of this figure as long as the typical intrinsic cutoff is above 200-300 GeV in the rest frame.

Population gradients and photometric metallicities in early- and transition-type dwarf galaxies: Clues from the Sculptor group [Replacement]

We focus on the resolved stellar populations of one early- and four transition-type dwarf galaxies in the Sculptor group, with the aim to examine the potential presence of population gradients and place constraints on their mean metallicities. We use deep HST images to construct CMDs, from which we select stellar populations that trace different evolutionary phases in order to constrain their range of ages and metallicities, as well as to examine their spatial distribution. In addition, we use the resolved stars in the RGB in order to derive photometric metallicities. All studied dwarfs contain intermediate-age stars with ages of ~1Gyr and older as traced by the luminous asymptotic giant branch and red clump stars, while the transition-type dwarfs contain also stars younger than ~1Gyr as traced by a young main sequence and vertical red clump stars. Moreover, the spatial distribution of the stars that trace different evolutionary phases shows a population gradient in all transition-type dwarfs. The derived error-weighted mean metallicities, assuming purely old stellar populations, range from -1.5dex for ESO294-G010 to -1.9dex for Scl-dE1, and should be considered as lower limits to their true metallicities. Assuming intermediate-age stellar populations to dominate the dwarfs, we derive upper limits for the metallicities that are 0.3 to 0.2 dex higher than the metallicities derived assuming purely old populations. We discuss how photometric metallicity gradients are affected by the age-metallicity degeneracy, which prevents strong conclusions regarding their actual presence. Finally, the transition-type dwarfs lie beyond the virial radius of their closest bright galaxy, as also observed for the LG transition-type dwarfs. Scl-dE1 is the only dSph in our sample and is an outlier in a potential morphology-distance relation, similar as the two isolated dSphs of the LG, Tucana and Cetus.

Population gradients and photometric metallicities in early- and transition-type dwarf galaxies: Clues from the Sculptor group

We focus on the resolved stellar populations of one early- and four transition-type dwarf galaxies in the Sculptor group, with the aim to examine the potential presence of population gradients and place constraints on their mean metallicities. We use deep HST images to construct CMDs, from which we select stellar populations that trace different evolutionary phases in order to constrain their range of ages and metallicities, as well as to examine their spatial distribution. In addition, we use the resolved stars in the RGB in order to derive photometric metallicities. All studied dwarfs contain intermediate-age stars with ages of ~1Gyr and older as traced by the luminous asymptotic giant branch and red clump stars, while the transition-type dwarfs contain also stars younger than ~1Gyr as traced by a young main sequence and vertical red clump stars. Moreover, the spatial distribution of the stars that trace different evolutionary phases shows a population gradient in all transition-type dwarfs. The derived error-weighted mean metallicities, assuming purely old stellar populations, range from -1.5dex for ESO294-G010 to -1.9dex for Scl-dE1, and should be considered as lower limits to their true metallicities. Assuming intermediate-age stellar populations to dominate the dwarfs, we derive upper limits for the metallicities that are 0.3 to 0.2 dex higher than the metallicities derived assuming purely old populations. We discuss how photometric metallicity gradients are affected by the age-metallicity degeneracy, which prevents strong conclusions regarding their actual presence. Finally, the transition-type dwarfs lie beyond the virial radius of their closest bright galaxy, as also observed for the LG transition-type dwarfs. Scl-dE1 is the only dSph in our sample and is an outlier in a potential morphology-distance relation, similar as the two isolated dSphs of the LG, Tucana and Cetus.

M87 at metre wavelengths: the LOFAR picture

M87 is a giant elliptical galaxy located in the centre of the Virgo cluster, which harbours a supermassive black hole of mass 6.4×10^9 M_sun, whose activity is responsible for the extended (80 kpc) radio lobes that surround the galaxy. The energy generated by matter falling onto the central black hole is ejected and transferred to the intra-cluster medium via a relativistic jet and morphologically complex systems of buoyant bubbles, which rise towards the edges of the extended halo. Here we present the first observations made with the new Low-Frequency Array (LOFAR) of M87 at frequencies down to 20 MHz. Images of M87 were produced at low radio frequencies never explored before at these high spatial resolution and dynamic range. To disentangle different synchrotron models and place constraints on source magnetic field, age and energetics, we also performed a detailed spectral analysis of M87 extended radio-halo using these observations together with archival data. We do not find any sign of new extended emissions; on the contrary the source appears well confined by the high pressure of the intra-cluster medium. A continuous injection of relativistic electrons is the model that best fits our data, and provides a scenario in which the lobes are still supplied by fresh relativistic particles from the active galactic nuclei. We suggest that the discrepancy between the low-frequency radio-spectral slope in the core and in the halo implies a strong adiabatic expansion of the plasma as soon as it leaves the core area. The extended halo has an equipartition magnetic field strength of ~10 uG, which increases to ~13 uG in the zones where the particle flows are more active. The continuous injection model for synchrotron ageing provides an age for the halo of ~40 Myr, which in turn provides a jet kinetic power of 6-10×10^44 erg/s.

Broad-band timing properties of the accreting white dwarf MV Lyrae

We present a broad-band timing analysis of the accreting white dwarf system MV Lyrae based on data obtained with the Kepler satellite. The observations span 633 days at a cadence of 58.8 seconds and allow us to probe 4 orders of magnitude in temporal frequency. The modelling of the observed broad-band noise components is based on the superposition of multiple Lorentzian components, similar to the empirical modelling adopted for X-ray binary systems. We also present the detection of a frequency varying Lorentzian component in the lightcurve of MV Lyrae, where the Lorentzian characteristic frequency is inversely correlated with the mean source flux. Because in the literature similar broad-band noise components have been associated to either the viscous or dynamical timescale for different source types (accreting black holes or neutron stars), we here systematically explore both scenarios and place constraints on the accretion disk structure. In the viscous case we employ the fluctuating accretion disk model to infer parameters for the viscosity and disk scale height, and infer uncomfortably high parameters to be accommodated by the standard thin disk, whilst in the dynamical case we infer a large accretion disk truncation radius of ~10 white dwarf radii. More importantly however, the phenomenological properties between the broad-band variability observed here and in X-ray binaries and Active Galactic Nuclei are very similar, potentially suggesting a common origin for the broad-band variability.

The HST/ACS+WFC3 Survey for Lyman Limit Systems II: Science [Replacement]

We present the first science results from our Hubble Space Telescope Survey for Lyman limit absorption systems (LLS) using the low dispersion spectroscopic modes of the Advanced Camera for Surveys and the Wide Field Camera 3. Through an analysis of 71 quasars, we determine the incidence frequency of LLS per unit redshift and per unit path length, l(z) and l(x) respectively, over the redshift range 1 < z< 2.6, and find a weighted mean of l(x)=0.29 +/-0.05 for 2.0 < z < 2.5 through a joint analysis of our sample and that of Ribaudo et al. (2011). Through stacked spectrum analysis, we determine a median (mean) value of the mean free path to ionizing radiation at z=2.4 of lambda_mfp = 243(252)h^(-1) Mpc, with an error on the mean value of +/- 43h^(-1) Mpc. We also re-evaluate the estimates of lambda_mfp from Prochaska et al. (2009) and place constraints on the evolution of lambda_mfp with redshift, including an estimate of the "breakthrough" redshift of z = 1.6. Consistent with results at higher z, we find that a significant fraction of the opacity for absorption of ionizing photons comes from systems with N_HI <= 10^{17.5} cm^(-2) with a value for the total Lyman opacity of tau_lyman = 0.40 +/- 0.15. Finally, we determine that at minimum, a 5-parameter (4 power-law) model is needed to describe the column density distribution function f(N_HI, X) at z \sim 2.4, find that f(N_HI,X) undergoes no significant change in shape between z \sim 2.4 and z \sim 3.7, and provide our best fit model for f(N_HI,X).

The HST/ACS+WFC3 Survey for Lyman Limit Systems II: Science

We present the first science results from our Hubble Space Telescope Survey for Lyman limit absorption systems (LLS) using the low dispersion spectroscopic modes of the Advanced Camera for Surveys and the Wide Field Camera 3. Through an analysis of 71 quasars, we determine the incidence frequency of LLS per unit redshift and per unit path length, l(z) and l(x) respectively, over the redshift range 1 < z< 2.6, and find a weighted mean of l(x)=0.29 +/-0.05 for 2.0 < z < 2.5 through a joint analysis of our sample and that of Ribaudo et al. (2011). Through stacked spectrum analysis, we determine a median (mean) value of the mean free path to ionizing radiation at z=2.4 of lambda_mfp = 243(252)h^(-1) Mpc, with an error on the mean value of +/- 43h^(-1) Mpc. We also re-evaluate the estimates of lambda_mfp from Prochaska et al. (2009) and place constraints on the evolution of lambda_mfp with redshift, including an estimate of the "breakthrough" redshift of z = 1.6. Consistent with results at higher z, we find that a significant fraction of the opacity for absorption of ionizing photons comes from systems with N_HI <= 10^{17.5} cm^(-2) with a value for the total Lyman opacity of tau_lyman = 0.40 +/- 0.15. Finally, we determine that at minimum, a 5-parameter (4 power-law) model is needed to describe the column density distribution function f(N_HI, X) at z \sim 2.4, find that f(N_HI,X) undergoes no significant change in shape between z \sim 2.4 and z \sim 3.7, and provide our best fit model for f(N_HI,X).

Constraining Variable High Velocity Winds from Broad Absorption Line Quasars with Multi-Epoch Spectroscopy

Broad absorption line (BAL) quasars probe the high velocity gas ejected by luminous accreting black holes. BAL variability timescales place constraints on the size, location, and dynamics of the emitting and absorbing gas near the supermassive black hole. We present multi-epoch spectroscopy of seventeen BAL QSOs from the Sloan Digital Sky Survey (SDSS) using the Fred Lawrence Whipple Observatory’s 1.5m telescope’s FAST Spectrograph. These objects were identified as BALs in SDSS, observed with Chandra, and then monitored with FAST at observed-frame cadences of 1, 3, 9, 27, and 81 days, as well as 1 and 2 years. We also monitor a set of non-BAL quasars with matched redshift and luminosity as controls. We identify significant variability in the BALs, particularly at the 1 and 2 year cadences, and use its magnitude and frequency to constrain the outflows impacting the broad absorption line region.

Galaxy cluster number count data constraints on cosmological parameters [Replacement]

[Abridged] We use data on massive galaxy clusters ($M_{\rm cluster} > 8 \times 10^{14} h^{-1} M_\odot$ within a comoving radius of $R_{\rm cluster} = 1.5 h^{-1}\Mpc$) in the redshift range $0.05 \lesssim z \lesssim 0.83$ to place constraints, simultaneously, on the nonrelativistic matter density parameter $\Omega_m$, on the amplitude of mass fluctuations $\sigma_8$, on the index $n$ of the power-law spectrum of the density perturbations, and on the Hubble constant $H_0$, as well as on the equation-of-state parameters $(w_0,w_a)$ of a smooth dark energy component. For the first time, we properly take into account the dependence on redshift and cosmology of the quantities related to cluster physics: the critical density contrast, the growth factor, the mass conversion factor, the virial overdensity, the virial radius and, most importantly, the cluster number count derived from the observational temperature data. We show that, contrary to previous analyses, cluster data alone prefer low values of the amplitude of mass fluctuations, $\sigma_8 \leq 0.69 (1\sigma C.L.)$, and large amounts of nonrelativistic matter, $\Omega_m \geq 0.38 (1\sigma C.L.)$, in slight tension with the $\Lambda$CDM concordance cosmological model, though the results are compatible with $\Lambda$CDM at $2\sigma$. In addition, we derive a $\sigma_8$ normalization relation, $\sigma_8 \Omega_m^{1/3} = 0.49 \pm 0.06 (2\sigma C.L.)$.

Latest Results from the DODO Survey: Imaging Planets around White Dwarfs

The aim of the Degenerate Objects around Degenerate Objects (DODO) survey is to search for very low mass brown dwarfs and extrasolar planets in wide orbits around white dwarfs via direct imaging. The direct detection of such companions would allow the spectroscopic investigation of objects with temperatures lower (< 500 K) than the coolest brown dwarfs currently observed. The discovery of planets around white dwarfs would prove that such objects can survive the final stages of stellar evolution and place constraints on the frequency of planetary systems around their progenitors (with masses between 1.5 – 8 M*, i.e., early B to mid-F). An increasing number of planetary mass companions have been directly imaged in wide orbits around young main sequence stars. For example, the planets around HR 8799 and 1RXS J160929.1 – 210524 are in wide orbits of 24 – 68 AU and 330 AU, respectively. The DODO survey has the ability to directly image planets in post-main sequence analogues of these systems. These proceedings present the latest results of our multi-epoch J band common proper motion survey of nearby white dwarfs.

The Atomic Physics Underlying the Spectroscopic Analysis of Massive Stars and Supernovae

We have developed a radiative transfer code, CMFGEN, which allows us to model the spectra of massive stars and supernovae. Using CMFGEN we can derive fundamental parameters such as effective temperatures and surface gravities, derive abundances, and place constraints on stellar wind properties. The last of these is important since all massive stars are losing mass via a stellar wind that is driven from the star by radiation pressure, and this mass loss can substantially influence the spectral appearance and evolution of the star. Recently we have extended CMFGEN to allow us to undertake time-dependent radiative transfer calculations of supernovae. Such calculations will be used to place constraints on the supernova progenitor, to place constraints on the supernova explosion and nucleosynthesis, and to derive distances using a physical approach called the “Expanding Photosphere Method”. We describe the assumptions underlying the code and the atomic processes involved. A crucial ingredient in the code is the atomic data. For the modeling we require accurate transition wavelengths, oscillator strengths, photoionization cross-sections, collision strengths, autoionization rates, and charge exchange rates for virtually all species up to, and including, cobalt. Presently, the available atomic data varies substantially in both quantity and quality.

Cosmic Lighthouses : Unveiling the nature of high-redshift galaxies

We are in the golden age for the search for high-redshift galaxies, made possible by a combination of new instruments and innovative search techniques. One of the major aims of such searches is to constrain the epoch of reionization (EoR), which marks the second major change in the ionization state of the Universe. Understanding the EoR is difficult since whilst it is galaxy evolution which drives reionization, reionization itself influences galaxy evolution through feedback effects. Unraveling the interplay of reionization and galaxy evolution is further complicated by of a lack of understanding of the metal enrichment and dust distribution in high redshift galaxies. To this end, a class of galaxies called Lyman Alpha Emitters (LAEs) have been gaining enormous popularity as probes of all these three processes. In this thesis, we couple state of the art cosmological SPH simulations (GADGET-2) with a physically motivated, self-consistent model for LAEs, so as to be able to understand the importance of the intergalactic medium (IGM) ionization state, dust and peculiar velocities in shaping their observed properties. By doing so, the aim is to gain insight on the nature of LAEs, put precious constraints on their elusive physical properties and make predictions for future instruments such as the Atacama Large Millimeter Array (ALMA). Using our LAE model in conjunction with a code that builds the MW merger tree (GAMETE), we build a bridge between the high-redshift and the local Universe. We also use SPH simulations (GADGET-2) to study the nature of the earliest galaxies that have been detected as of yet, place constraints on their contribution to reionization, and predict their detectability using the next generation of instruments, such as the James Web Space Telescope (JWST).

Do Cosmological Perturbations Have Zero Mean?

A central assumption in our analysis of cosmic structure is that cosmological perturbations have zero ensemble mean. This property is one of the consequences of statistically homogeneity, the invariance of correlation functions under spatial translations. In this article we explore whether cosmological perturbations indeed have zero mean, and thus test one aspect of statistical homogeneity. We carry out a classical test of the zero mean hypothesis against a class of alternatives in which perturbations have non-vanishing means, but homogeneous and isotropic covariances. Apart from Gaussianity, our test does not make any additional assumptions about the nature of the perturbations and is thus rather generic and model-independent. The test statistic we employ is essentially Student’s t statistic, applied to appropriately masked, foreground-cleaned cosmic microwave background anisotropy maps produced by the WMAP mission. We find evidence for a non-zero mean in a particular range of multipoles, but the evidence against the zero mean hypothesis goes away when we correct for multiple testing. We also place constraints on the mean of the temperature multipoles as a function of angular scale. On angular scales smaller than four degrees, a non-zero mean has to be at least an order of magnitude smaller than the standard deviation of the temperature anisotropies.

Evidence for a current sheet forming in the wake of a Coronal Mass Ejection from multi-viewpoint coronagraph observations

Ray-like features observed by coronagraphs in the wake of Coronal Mass Ejections (CMEs) are sometimes interpreted as the white light counterparts of current sheets (CSs) produced by the eruption. The 3D geometry of these ray-like features is largely unknown and its knowledge should clarify their association to the CS and place constraints on CME physics and coronal conditions. With this study we test these important implications for the first time. An example of such a post-CME ray was observed by various coronagraphs, including these of the SECCHI instrument suite of the STEREO twin spacecraft and the Large Angle Spectrometric Coronagraph LASCO onboard the Solar and Heliospheric Observatory (SOHO). The ray was observed in the aftermath of a CME which occurred on 9 April 2008. The twin STEREO spacecraft were separated by about degrees on that day. This significant separation combined with a third "eye" view supplied by LASCO allow for a truly multi-viewpoint observation of the ray and of the CME. We applied 3D forward geometrical modeling to the CME and to the ray as simultaneously viewed by SECCHI-A and B and by SECCHI-A and LASCO, respectively. We found that the ray can be approximated by a rectangular slab, nearly aligned with the CME axis, and much smaller than the CME in both terms of thickness and depth (~ 0.05 and 0.15 Rsun respectively). We found that the ray and CME are significantly displaced from the associated post-CME flaring loops. The properties and location of the ray are fully consistent with the expectations of the standard CME theories for post-CME current sheets. Therefore, our multi-viewpoint observations supply strong evidence that the observed post-CME ray is indeed related to a post-CME current sheet.

NLTT 41135: a field M-dwarf + brown dwarf eclipsing binary in a triple system, discovered by the MEarth observatory

We report the discovery of an eclipsing companion to NLTT 41135, a nearby M5 dwarf that was already known to have a wider, slightly more massive common proper motion companion, NLTT 41136, at 2.4 arcsec separation. Analysis of combined-light and radial velocity curves of the system indicates that NLTT 41135B is a 31-34 +/- 3 MJup brown dwarf (where the range depends on the unknown metallicity of the host star) on a circular orbit. The visual M-dwarf pair appears to be physically bound, so the system forms a hierarchical triple, with masses approximately in the ratio 8:6:1. The eclipses are grazing, preventing an unambiguous measurement of the secondary radius, but follow-up observations of the secondary eclipse (e.g. with the James Webb Space Telescope) could permit measurements of the surface brightness ratio between the two objects, and thus place constraints on models of brown dwarfs.

Atmospheric Chemistry of Venus-like Exoplanets

We use thermodynamic calculations to model atmospheric chemistry on terrestrial exoplanets that are hot enough for chemical equilibira between the atmosphere and lithosphere, as on Venus. The results of our calculations place constraints on abundances of spectroscopically observable gases, the surface temperature and pressure, and the mineralogy of the surface. These results will be useful in planning future observations of the atmospheres of terrestrial-sized exoplanets by current and proposed space observatories such as the Hubble Space Telescope (HST), Spitzer, James Webb Space Telescope (JWST), Terrestrial Planet Finder, and Darwin.

Atmospheric Chemistry of Venus-like Exoplanets [Replacement]

We use thermodynamic calculations to model atmospheric chemistry on terrestrial exoplanets that are hot enough for chemical equilibira between the atmosphere and lithosphere, as on Venus. The results of our calculations place constraints on abundances of spectroscopically observable gases, the surface temperature and pressure, and the mineralogy of the surface. These results will be useful in planning future observations of the atmospheres of terrestrial-sized exoplanets by current and proposed space observatories such as the Hubble Space Telescope (HST), Spitzer, James Webb Space Telescope (JWST), Terrestrial Planet Finder, and Darwin.

Atmospheric Chemistry of Venus-like Exoplanets [Replacement]

We use thermodynamic calculations to model atmospheric chemistry on terrestrial exoplanets that are hot enough for chemical equilibira between the atmosphere and lithosphere, as on Venus. The results of our calculations place constraints on abundances of spectroscopically observable gases, the surface temperature and pressure, and the mineralogy of the surface. These results will be useful in planning future observations of the atmospheres of terrestrial-sized exoplanets by current and proposed space observatories such as the Hubble Space Telescope (HST), Spitzer, James Webb Space Telescope (JWST), Terrestrial Planet Finder, and Darwin.

Exploring extra dimensions through observational tests of dark energy and varying Newton's constant

We recently presented a series of dark energy theorems that place constraints on the equation of state of dark energy ($\wdark$), the ime-variation of Newton’s constant ($\dot G$), and the violation of energy conditions in theories with extra dimensions. In this paper, we explore how current and future measurements of $\wdark$ and $\dot G$ can be used to place tight limits on large classes of these theories (including some of the most well-motivated examples) independent of the size of the extra dimensions. As an example, we show that models with conformally Ricci-flat metrics obeying the null energy condition (a common ansatz for Kaluza-Klein and string constructions) are highly constrained by current ata and may be ruled out entirely by future dark energy and pulsar observations.

Flare-less long Gamma-ray Bursts and the properties of their massive star progenitors

While there is mounting evidence that long Gamma-Ray Bursts (GRBs) are associated with the collapse of massive stars, the detailed structure of their pre-supernova stage is still debatable. Particularly uncertain is the degree of mixing among shells of different composition, and hence the role of magnetic torques and convection in transporting angular momentum. Here we show that early-time afterglow observations with the Swift satellite place constraints on the allowed GRB pre-supernova models. In particular, they argue against pre-supernova models in which different elemental shells are unmixed. These types of models would produce energy injections into the GRB engine on timescales between several hundreds of seconds to a few hours. Flaring activity has {\em not} been observed in a large fraction of well-monitored long GRBs. Therefore, if the progenitors of long GRBs have common properties, then the lack of flares indicates that the massive stars which produce GRBs are mostly well mixed, as expected in low-metallicity, rapidly rotating massive stars.

Benchmark ultra-cool dwarfs in widely separated binary systems [Replacement]

Ultra-cool dwarfs as wide companions to subgiants, giants, white dwarfs and main sequence stars can be very good benchmark objects, for which we can infer physical properties with minimal reference to theoretical models, through association with the primary stars. We have searched for benchmark ultra-cool dwarfs in widely separated binary systems using SDSS, UKIDSS, and 2MASS. We then estimate spectral types using SDSS spectroscopy and multi-band colors, place constraints on distance, and perform proper motions calculations for all candidates which have sufficient epoch baseline coverage. Analysis of the proper motion and distance constraints show that eight of our ultra-cool dwarfs are members of widely separated binary systems. Another L3.5 dwarf, SDSS 0832, is shown to be a companion to the bright K3 giant Eta Cancri. Such primaries can provide age and metallicity constraints for any companion objects, yielding excellent benchmark objects. This is the first wide ultra-cool dwarf + giant binary system identified.

Benchmark ultra-cool dwarfs in widely separated binary systems

Ultra-cool dwarfs as wide companions to subgiants, giants, white dwarfs and main sequence stars can be very good benchmark objects, for which we can infer physical properties with minimal reference to theoretical models, through association with the primary stars. We have searched for benchmark ultra-cool dwarfs in widely separated binary systems using SDSS, UKIDSS, and 2MASS. We then estimate spectral types using SDSS spectroscopy and multi-band colors, place constraints on distance, and perform proper motions calculations for all candidates which have sufficient epoch baseline coverage. Analysis of the proper motion and distance constraints show that eight of our ultra-cool dwarfs are members of widely separated binary systems. Another L3.5 dwarf, SDSS 0832, is shown to be a companion to the bright K3 giant Eta Cancri. Such primaries can provide age and metallicity constraints for any companion objects, yielding excellent benchmark objects. This is the first wide ultra-cool dwarf + giant binary system identified.

21 cm radiation: A new probe of fundamental physics

New low frequency radio telescopes currently being built open up the possibility of observing the 21-cm radiation before the Epoch of Reionization in the future, in particular at redshifts 200 > z > 30, also known as the dark ages. At these high redshifts, Cosmic Microwave Background (CMB) radiation is absorbed by neutral hydrogen at its 21-cm hyperfine transition. This redshifted 21-cm signal thus carries information about the state of the early Universe and can be used to test fundamental physics. We study the constraints these observations can put on the variation of fundamental constants. We show that the 21-cm radiation is very sensitive to the variations in the fine structure constant and can in principle place constraints comparable to or better than the other astrophysical experiments (fractional change < 10^ {-5}). Making such observations will require radio telescopes of collecting area 10 – 10^6 sq. km compared to 1 sq. km of current telescopes. These observations will thus provide independent constraints on the fine structure constant at high redshifts, observations of quasars being the only alternative. More importantly the 21-cm absorption of CMB is the only way to probe the redshift range between recombination and reionization.

Cosmological Simulations of Normal-Branch Braneworld Gravity

We introduce a cosmological model based on the normal branch of DGP braneworld gravity with a smooth dark energy component on the brane. The expansion history in this model is identical to LambdaCDM, thus evading all geometric constraints on the DGP cross-over scale r_c. This model can serve as a first approximation to more general braneworld models whose cosmological solutions have not been obtained yet. We study the formation of large scale structure in this model in the linear and non-linear regime using N-body simulations for different values of r_c. The simulations use the code presented in (F.S., arXiv:0905.0858) and solve the full non-linear equation for the brane-bending mode in conjunction with the usual gravitational dynamics. The brane-bending mode is attractive rather than repulsive in the DGP normal branch, hence the sign of the modified gravity effects is reversed compared to those presented in arXiv:0905.0858. We compare the simulation results with those of ordinary LambdaCDM simulations run using the same code and initial conditions. We find that the matter power spectrum in this model shows a characteristic enhancement peaking at k ~ 0.7 h/Mpc. We also find that the abundance of massive halos is significantly enhanced. Other results presented here include the density profiles of dark matter halos, and signatures of the brane-bending mode self-interactions (Vainshtein mechanism) in the simulations. Independently of the expansion history, these results can be used to place constraints on the DGP model and future generalizations through their effects on the growth of cosmological structure.

Cosmological Simulations of Normal-Branch Braneworld Gravity [Replacement]

We introduce a cosmological model based on the normal branch of DGP braneworld gravity with a smooth dark energy component on the brane. The expansion history in this model is identical to LambdaCDM, thus evading all geometric constraints on the DGP cross-over scale r_c. This model can serve as a first approximation to more general braneworld models whose cosmological solutions have not been obtained yet. We study the formation of large scale structure in this model in the linear and non-linear regime using N-body simulations for different values of r_c. The simulations use the code presented in (F.S., arXiv:0905.0858) and solve the full non-linear equation for the brane-bending mode in conjunction with the usual gravitational dynamics. The brane-bending mode is attractive rather than repulsive in the DGP normal branch, hence the sign of the modified gravity effects is reversed compared to those presented in arXiv:0905.0858. We compare the simulation results with those of ordinary LambdaCDM simulations run using the same code and initial conditions. We find that the matter power spectrum in this model shows a characteristic enhancement peaking at k ~ 0.7 h/Mpc. We also find that the abundance of massive halos is significantly enhanced. Other results presented here include the density profiles of dark matter halos, and signatures of the brane-bending mode self-interactions (Vainshtein mechanism) in the simulations. Independently of the expansion history, these results can be used to place constraints on the DGP model and future generalizations through their effects on the growth of cosmological structure.

Robust Constraints on Dark Matter Annihilation into Gamma Rays and Charged Particles

Using gamma-ray data from observations of the Milky Way, Andromeda (M31), and the cosmic background, we calculate conservative upper limits on the dark matter self-annihilation cross section to a number of final states, over a wide range of dark matter masses. We first constrain annihilation to a pair of monoenergetic gamma rays, and show that in general our results are unchanged for a broader annihilation spectrum, if at least a few gamma rays are produced with energies within a factor of a few from the dark matter mass. We then place constraints on the self-annihilation cross section to an electron-positron pair, using gamma rays produced via internal bremsstrahlung radiative corrections. We also place constraints on annihilation into the other charged leptons. We make conservative assumptions about the astrophysical inputs, and demonstrate how our derived bounds would be strengthened if stronger assumptions about these inputs are adopted.

Resolving Globular Cluster Formation within a Cosmological Context [Replacement]

We place constraints on the formation redshifts for blue globular clusters (BGCs), independent of the details of hydrodynamics and population III star formation. The observed radial distribution of BGCs in the Milky Way Galaxy suggests that they formed in biased dark matter halos at high redshift. As a result, simulations of a ~1 Mpc box up to z~10 must resolve BGC formation in LCDM. We find that most halo stars could be produced from destroyed BGCs and other low-mass clusters that formed at high redshift. We present a proof-of-concept simulation that captures the formation of globular-like star clusters.

The 10 Meter South Pole Telescope

The South Pole Telescope (SPT) is a 10 m diameter, wide-field, offset Gregorian telescope with a 966-pixel, multi-color, millimeter-wave, bolometer camera. It is located at the Amundsen-Scott South Pole station in Antarctica. The design of the SPT emphasizes careful control of spillover and scattering, to minimize noise and false signals due to ground pickup. The key initial project is a large-area survey at wavelengths of 3, 2 and 1.3 mm, to detect clusters of galaxies via the Sunyaev-Zeldovich (SZ) effect and to measure the high-l angular power spectrum of the cosmic microwave background (CMB). The data will be used to characterize the primordial matter power spectrum and to place constraints on the equation of state of dark energy. 

Particle Acceleration in Relativistic Magnetized Collisionless Pair Shocks: Dependence of Shock Acceleration on Magnetic Obliquity

We investigate shock structure and particle acceleration in relativistic magnetized collisionless pair shocks by means of 2.5D and 3D particle-in-cell simulations. We explore a range of inclination angles between the pre-shock magnetic field and the shock normal. We find that only magnetic inclinations corresponding to "subluminal" shocks, where relativistic particles following the magnetic field can escape ahead of the shock, lead to particle acceleration. The downstream spectrum in such shocks consists of a relativistic Maxwellian and a high-energy power-law tail with exponential cutoff. For increasing magnetic inclination in the subluminal range, the high-energy tail accounts for an increasing fraction of particles (from ~1% to ~2%) and energy (from ~4% to ~12%). The spectral index of the power law increases with angle from -2.8+-0.1 to -2.3+-0.1. Particle energization is driven by the Diffusive Shock Acceleration process for nearly parallel shocks, and switches to Shock-Drift Acceleration for larger subluminal inclinations. For "superluminal" shocks, the downstream particle spectrum does not show any significant suprathermal tail. As seen from the upstream frame, efficient acceleration in relativistic (Lorentz factor gamma0 > 5) magnetized (sigma > 0.03) flows exists only for a very small range of magnetic inclination angles (< 34/gamma0 degrees), so relativistic astrophysical pair shocks have to be either nearly parallel or weakly magnetized to generate nonthermal particles. These findings place constraints on the models of AGN jets, Pulsar Wind Nebulae and Gamma Ray Bursts that invoke particle acceleration in relativistic magnetized shocks. (Abridged)

 

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