Posts Tagged stellar regions

Recent Postings from stellar regions

A spectroscopic census in young stellar regions: the Sigma Orionis cluster [Replacement]

We present a spectroscopic survey of the stellar population of the Sigma Orionis cluster. We have obtained spectral types for 340 stars. Spectroscopic data for spectral typing come from several spectrographs with similar spectroscopic coverage and resolution. More than a half of stars of our sample are members confirmed by the presence of lithium in absorption, strong H$\alpha$ in emission or weak gravity-sensitive features. In addition, we have obtained high resolution (R~34000) spectra in the H$\alpha$ region for 169 stars in the region. Radial velocities were calculated from this data set. The radial velocity distribution for members of the cluster is in agreement with previous work. Analysis of the profile of the H$\alpha$ line and infrared observations reveals two binary systems or fast rotators that mimic the H$\alpha$ width expected in stars with accretion disks. On the other hand there are stars with optically thick disks and narrow H$\alpha$ profile not expected in stars with accretion disks. This contribution constitutes the largest homogeneous spectroscopic data set of the Sigma Orionis cluster to date.

A spectroscopic census in young stellar regions: the Sigma Orionis cluster

We present a spectroscopic survey of the stellar population of the Sigma Orionis cluster. We have obtained spectral types for 340 stars. Spectroscopic data for spectral typing come from several spectrographs with similar spectroscopic coverage and resolution. More than a half of stars of our sample are members confirmed by the presence of lithium in absorption, strong H$\alpha$ in emission or weak gravity-sensitive features. In addition, we have obtained high resolution (R~34000) spectra in the H$\alpha$ region for 169 stars in the region. Radial velocities were calculated from this data set. The radial velocity distribution for members of the cluster is in agreement with previous work. Analysis of the profile of the H$\alpha$ line and infrared observations reveals two binary systems or fast rotators that mimic the H$\alpha$ width expected in stars with accretion disks. On the other hand there are stars with optically thick disks and narrow H$\alpha$ profile not expected in stars with accretion disks. This contribution constitutes the largest homogeneous spectroscopic data set of the Sigma Orionis cluster to date.

Flavor stability analysis of dense supernova neutrinos with flavor-dependent angular distributions [Cross-Listing]

Numerical simulations of the supernova (SN) neutrino self-induced flavor conversions, associated with the neutrino-neutrino interactions in the deepest stellar regions, have been typically carried out assuming the "bulb-model". In this approximation, neutrinos are taken to be emitted half-isotropically by a common neutrinosphere. In the recent Ref. \cite{Mirizzi:2011tu} we have removed this assumption by introducing flavor-dependent angular distributions for SN neutrinos, as suggested by core-collapse simulations. We have found that in this case a novel multi-angle instability in the self-induced flavor transitions can arise. In this work we perform an extensive study of this effect, carrying out a linearized flavor stability analysis for different SN neutrino energy fluxes and angular distributions, in both normal and inverted neutrino mass hierarchy. We confirm that spectra of different nu species which cross in angular space (where F_{\nu_e}=F_{\nu_x} and F_{\bar\nu_e}=F_{\bar\nu_x}) present a significant enhancement of the flavor instability, and a shift of the onset of the flavor conversions at smaller radii with respect to the case of an isotropic neutrino emission. We also illustrate how a qualitative (and sometimes quantitative) understanding of the dynamics of these systems follows from a stability analysis.

Optically Selected Compact Stellar Regions and Tidal Dwarf Galaxies in (Ultra)-Luminous Infrared Galaxies

This thesis work is devoted to the analysis of compact star forming regions (knots) in a representative sample of 32 (U)LIRGs. The project is based mainly on optical high angular resolution images taken with the ACS and WFPC2 cameras on board the HST telescope, data from a high spatial resolution simulation of a major galaxy encounter, and with the combination of optical integral field spectroscopy (IFS) taken with the INTEGRAL (WHT) and VIMOS (VLT) instruments. A few thousand knots -a factor of more than one order of magnitude higher than in previous studies- are identified and their photometric properties are characterized as a function of the infrared luminosity of the system and of the interaction phase. These properties are compared with those of compact objects identified in simulations of galaxy encounters. Finally, and with the additional use of IFS data, we search for suitable candidates to tidal dwarf galaxies, setting up constraints on the formation of these objects for the (U)LIRG class. Knots in (U)LIRGs are likely to contain sub-strucutre. Evidence is found that in ULIRGs they are intrinsically more luminous than in less luminous interacting systems due to size-of-sample effects. Furthermore, their sizes and masses are similar to stellar complexes or clumps detected in galaxies at z > 1, unlike local stellar complexes. The star formation in (U)LIRGs is charaterized by a luminosity function with a slope consistent with \alpha = 2, independent of the luminosity of the system. However, it may flatten slightly due to, as simulations suggest, higher knot formation rates at early phases of the interaction. Candidates to tidal dwarf galaxies are identified in the sample. With a production rate of 0.1 candidates per (U)LIRG system, only a few fraction (< 10%) of the general dwarf satellite population could be of tidal origin.

Eccentric black hole-neutron star mergers: effects of black hole spin and equation of state

There is a high level of interest in black hole-neutron star binaries, not only because their mergers may be detected by gravitational wave observatories in the coming years, but also because of the possibility that they could explain a class of short duration gamma-ray bursts. We study black hole-neutron star mergers that occur with high eccentricity as may arise from dynamical capture in dense stellar regions such as nuclear or globular clusters. We perform general relativistic simulations of binaries with a range of impact parameters, three different initial black hole spins (zero, aligned and anti-aligned with the orbital angular momentum), and neutron stars with three different equations of state. We find a rich diversity across these parameters in the resulting gravitational wave signals and matter dynamics, which should also be reflected in the consequent electromagnetic emission. Before tidal disruption, the gravitational wave emission is significantly larger than perturbative predictions suggest for periapsis distances close to effective innermost stable separations, exhibiting features reflecting the zoom-whirl dynamics of the orbit there. Guided by the simulations, we develop a simple model for the change in orbital parameters of the binary during close encounters. Depending upon the initial parameters of the system, we find that mass transfer during non-merging close encounters can range from essentially zero to a sizable fraction of the initial neutron star mass. The same holds for the amount of material outside the black hole post-merger, and in some cases roughly half of this material is estimated to be unbound. We also see that non-merging close encounters generically excite large oscillations in the neutron star that are qualitatively consistent with f-modes.

Eccentric black hole-neutron star mergers: effects of black hole spin and equation of state [Replacement]

There is a high level of interest in black hole-neutron star binaries, not only because their mergers may be detected by gravitational wave observatories in the coming years, but also because of the possibility that they could explain a class of short duration gamma-ray bursts. We study black hole-neutron star mergers that occur with high eccentricity as may arise from dynamical capture in dense stellar regions such as nuclear or globular clusters. We perform general relativistic simulations of binaries with a range of impact parameters, three different initial black hole spins (zero, aligned and anti-aligned with the orbital angular momentum), and neutron stars with three different equations of state. We find a rich diversity across these parameters in the resulting gravitational wave signals and matter dynamics, which should also be reflected in the consequent electromagnetic emission. Before tidal disruption, the gravitational wave emission is significantly larger than perturbative predictions suggest for periapsis distances close to effective innermost stable separations, exhibiting features reflecting the zoom-whirl dynamics of the orbit there. Guided by the simulations, we develop a simple model for the change in orbital parameters of the binary during close encounters. Depending upon the initial parameters of the system, we find that mass transfer during non-merging close encounters can range from essentially zero to a sizable fraction of the initial neutron star mass. The same holds for the amount of material outside the black hole post-merger, and in some cases roughly half of this material is estimated to be unbound. We also see that non-merging close encounters generically excite large oscillations in the neutron star that are qualitatively consistent with f-modes.

OGLE-III Detection of the Anomalous Galactic Bulge Red Giant Branch Bump: Evidence of Enhanced Helium Enrichment

We measure the red giant branch bump (RGBB) of the Galactic bulge, the most metal-rich RGBB ever detected. The RGBB luminosity functions peaks at the expected brightness, but its number density is very low relative to Galactic globular cluster calibrations, implying the Galactic bulge has a higher helium enrichment parameter {\Delta}Y/{\Delta}Z $\ge$ 4.0 for Y~0.35 rather than the standard 2.0 with Y=0.27. The RGBB is (0.71 +/- 0.02) mag fainter than the red clump (RC) in I toward the densest stellar regions imaged by the OGLE-III Galactic bulge photometric survey, (|l| $\le$ 4, 2 <~ |b| $\le$ 4). The number density of RGBB stars is (12.7 +/- 2.0)% that of RC stars. The brightness dispersion of the RGBB is significantly lower than that of the RC, a result that is difficult to explain as the luminosity of the RGBB is known to significantly vary with metallicity. Sightlines toward the Galactic bulge that have two RCs have two RGBBs with similar properties to one another, an expected outcome if the Milky Way's bulge is X-shaped. We also find preliminary evidence of the Galactic bulge asymptotic giant branch bump, at a brightness of ~1.1 mag brighter than the RC in I and with a number density ~1.5% that of the RC. Accounting for the RGBB has a small effect on the best-fit parameters of the RC, shifting its best-fit peak brightness and reducing its brightness dispersion by ~0.015 mag each.

OGLE-III Detection of the Anomalous Galactic Bulge Red Giant Branch Bump: Evidence of Enhanced Helium Enrichment [Replacement]

We measure the red giant branch bump (RGBB) of the Galactic bulge, the most metal-rich RGBB ever detected. The RGBB luminosity functions peaks at the expected brightness, but its number density is very low relative to Galactic globular cluster calibrations, implying the Galactic bulge has a higher helium enrichment parameter {\Delta}Y/{\Delta}Z $\ge$ 4.0 for Y~0.35 rather than the standard 2.0 with Y=0.27. The RGBB is (0.71 +/- 0.02) mag fainter than the red clump (RC) in I toward the densest stellar regions imaged by the OGLE-III Galactic bulge photometric survey, (|l| $\le$ 4, 2 <~ |b| $\le$ 4). The number density of RGBB stars is (12.7 +/- 2.0)% that of RC stars. The brightness dispersion of the RGBB is significantly lower than that of the RC, a result that is difficult to explain as the luminosity of the RGBB is known to significantly vary with metallicity. Sightlines toward the Galactic bulge that have two RCs have two RGBBs with similar properties to one another, an expected outcome if the Milky Way's bulge is X-shaped. We also find preliminary evidence of the Galactic bulge asymptotic giant branch bump, at a brightness of ~1.1 mag brighter than the RC in I and with a number density ~1.5% that of the RC. Accounting for the RGBB has a small effect on the best-fit parameters of the RC, shifting its best-fit peak brightness and reducing its brightness dispersion by ~0.015 mag each.

Lithium abundance in the globular cluster M4: from the Turn-Off to the RGB Bump

We present Li and Fe abundances for 87 stars in the GC M4,obtained with GIRAFFE high-resolution spectra. The targets range from the TO up to the RGB Bump. The Li abundance in the TO stars is uniform, with an average value A(Li)=2.30+-0.02 dex,consistent with the upper envelope of Li content measured in other GCs and in the Halo stars,confirming also for M4 the discrepancy with the primordial Li abundance predicted by WMAP+BBNS. The iron content of M4 is [Fe/H]=-1.10+-0.01 dex, with no systematic offsets between dwarf and giant stars.The behaviour of the Li and Fe abundance along the entire evolutionary path is incompatible with models with atomic diffusion, pointing out that an additional turbulent mixing below the convective region needs to be taken into account,able to inhibit the atomic diffusion.The measured A(Li) and its homogeneity in the TO stars allow to put strong constraints on the shape of the Li profile inside the M4 TO stars. The global behaviour of A(Li) with T_{eff} can be reproduced with different pristine Li abundances, depending on the kind of adopted turbulent mixing.One cannot reproduce the global trend starting from the WMAP+BBNS A(Li) and adopting the turbulent mixing described by Richard et al.(2005) with the same efficiency used by Korn et al.(2006) to explain the Li content in NGC6397. Such a solution is not able to well reproduce simultaneously the Li abundance observed in TO and RGB stars.Otherwise, theWMAP+BBNS A(Li) can be reproduced assuming a more efficient turbulent mixing able to reach deeper stellar regions where the Li is burned. The cosmological Li discrepancy cannot be easily solved with the present,poor understanding of the turbulence in the stellar interiors and a future effort to well understand the true nature of this non-canonical process is needed.

Lithium abundance in the globular cluster M4: from the Turn-Off to the RGB Bump [Replacement]

We present Li and Fe abundances for 87 stars in the GC M4,obtained with GIRAFFE high-resolution spectra. The targets range from the TO up to the RGB Bump. The Li abundance in the TO stars is uniform, with an average value A(Li)=2.30+-0.02 dex,consistent with the upper envelope of Li content measured in other GCs and in the Halo stars,confirming also for M4 the discrepancy with the primordial Li abundance predicted by WMAP+BBNS. The iron content of M4 is [Fe/H]=-1.10+-0.01 dex, with no systematic offsets between dwarf and giant stars.The behaviour of the Li and Fe abundance along the entire evolutionary path is incompatible with models with atomic diffusion, pointing out that an additional turbulent mixing below the convective region needs to be taken into account,able to inhibit the atomic diffusion.The measured A(Li) and its homogeneity in the TO stars allow to put strong constraints on the shape of the Li profile inside the M4 TO stars. The global behaviour of A(Li) with T_{eff} can be reproduced with different pristine Li abundances, depending on the kind of adopted turbulent mixing.One cannot reproduce the global trend starting from the WMAP+BBNS A(Li) and adopting the turbulent mixing described by Richard et al.(2005) with the same efficiency used by Korn et al.(2006) to explain the Li content in NGC6397. Such a solution is not able to well reproduce simultaneously the Li abundance observed in TO and RGB stars.Otherwise, theWMAP+BBNS A(Li) can be reproduced assuming a more efficient turbulent mixing able to reach deeper stellar regions where the Li is burned. The cosmological Li discrepancy cannot be easily solved with the present,poor understanding of the turbulence in the stellar interiors and a future effort to well understand the true nature of this non-canonical process is needed.

On the Stability of Non Force-Free Magnetic Equilibria in Stars

The existence of stable magnetic configurations in white dwarfs, neutron stars and various non-convective stellar {regions} is now well recognized. It has recently been shown numerically that various families of equilibria, including axisymmetric mixed poloidal-toroidal configurations, are stable. Here we test the stability of an analytically-derived non force-free magnetic equilibrium, using three-dimensional magnetohydrodynamic simulations: the mixed configuration is compared with the dynamical evolution of its purely poloidal and purely toroidal components, both known to be unstable. The mixed equilibrium shows no sign of instability under white noise perturbations. {This configuration therefore provides a good description of magnetic equilibrium topology inside non-convective stellar objects and will be useful to initialize magneto-rotational transport in stellar evolution codes.

Star Formation in the Outer Filaments of NGC 1275 [Replacement]

We present photometry of the outer star clusters in NGC 1275, the brightest galaxy in the Perseus cluster. The observations were taken using the Hubble Space Telescope Advanced Camera for Surveys. We focus on two stellar regions in the south and south-east, far from the nucleus of the low velocity system (~22 kpc). These regions of extended star formation trace the H alpha filaments, drawn out by rising radio bubbles. In both regions bimodal distributions of colour (B-R)_0 against magnitude are apparent, suggesting two populations of star clusters with different ages; most of the H alpha filaments show no detectable star formation. The younger, bluer population is found to be concentrated along the filaments while the older population is dispersed evenly about the galaxy. We construct colour-magnitude diagrams and derive ages of at most 10^8 years for the younger population, a factor of 10 younger than the young population of star clusters in the inner regions of NGC 1275. We conclude that a formation mechanism or event different to that for the young inner population is needed to explain the outer star clusters and suggest that formation from the filaments, triggered by a buoyant radio bubble either rising above or below these filaments, is the most likely mechanism.

Star Formation in the Outer Filaments of NGC 1275

We present photometry of the outer star clusters in NGC 1275, the brightest galaxy in the Perseus cluster. The observations were taken using the Hubble Space Telescope Advanced Camera for Surveys. We focus on two stellar regions in the south and south-east, far from the nucleus of the low velocity system (~22 kpc). These regions of extended star formation trace the H alpha filaments, drawn out by rising radio bubbles. In both regions bimodal distributions of colour (B-R)_0 against magnitude are apparent, suggesting two populations of star clusters with different ages; most of the H alpha filaments show no detectable star formation. The younger, bluer population is found to be concentrated along the filaments while the older population is dispersed evenly about the galaxy. We construct colour-magnitude diagrams and derive ages of at most 10^8 years for the younger population, a factor of 10 younger than the young population of star clusters in the inner regions of NGC 1275. We conclude that a formation mechanism or event different to that for the young inner population is needed to explain the outer star clusters and suggest that formation from the filaments, triggered by a buoyant radio bubble either rising above or below these filaments, is the most likely mechanism.

Dissipational versus Dissipationless Galaxy Formation and the Dark Matter Content of Galaxies

We examine two extreme models for the build-up of the stellar component of luminous elliptical galaxies. In one case, we assume the build-up of stars is dissipational, with centrally accreted gas radiating away its orbital and thermal energy; the dark matter halo will undergo adiabatic contraction and the central dark matter density profile will steepen. For the second model, we assume the central galaxy is assembled by a series of dissipationless mergers of stellar clumps that have formed far from the nascent galaxy. In order to be accreted, these clumps lose their orbital energy to the dark matter halo via dynamical friction, thereby heating the central dark matter and smoothing the dark matter density cusp. The central dark matter density profiles differ drastically between these models. For the isolated elliptical galaxy, NGC 4494, the central dark matter densities follow the power-laws r^(-0.2) and r^(-1.7) for the dissipational and dissipationless models, respectively. By matching the dissipational and dissipationless models to observations of the stellar component of elliptical galaxies, we examine the relative contributions of dissipational and dissipationless mergers to the formation of elliptical galaxies and look for observational tests that will distinguish between these models. Comparisons to strong lensing brightest cluster galaxies yield median M*/L_B ratios of 2.1+/-0.8 and 5.2+/-1.7 at z=0.39 for the dissipational and dissipationless models, respectively. For NGC 4494, the best-fit dissipational and dissipationless models have M*/L_B=2.97 and 3.96. Comparisons to expected stellar mass-to-light ratios from passive evolution and population syntheses appear to rule out a purely dissipational formation mechanism for the central stellar regions of giant elliptical galaxies.

Theoretical values of convective turnover times and Rossby numbers for solar-like, pre-main sequence stars

Magnetic fields are at the heart of the observed stellar activity in late-type stars, and they are presumably generated by a dynamo mechanism at the interface layer between the radiative and the convective stellar regions. Since dynamo models are based on the interaction between differential rotation and convective motions, the introduction of rotation in the ATON 2.3 stellar code allows for explorations regarding a physically consistent treatment of magnetic effects in stellar structure and evolution, even though there are formidable mathematical and numerical challenges involved. As examples, we present theoretical estimates for both the local (tau_c) and global (tau_g) convective turnover times for rotating pre-main sequence solar-type stars, based on up-to-date input physics for stellar models. Our theoretical predictions are compared with the previous ones available in the literature. In addition, we investigate the dependence of the convective turnover time on convection regimes, the presence of rotation and atmospheric treatment. Those estimates, this quantities can be used to calculate the Rossby number, Ro, which is related to the magnetic activity strength in dynamo theories and, at least for main-sequence stars, shows an observational correlation with stellar activity. More important, they can also contribute for testing stellar models against observations. Our theoretical values of tau_c, tau_g and Ro qualitatively agree with those published by Kim & Demarque (1996). By increasing the convection efficiency, tau_g decreases for a given mass. FST models show still lower values. The presence of rotation shifts tau_g towards slightly higher values when compared with non-rotating models. The use of non-gray boundary conditions in the models yields values of tau_g smaller than in the gray approximation.

 

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