Recent Postings from Solar and Stellar

Magnetohydrodynamic Slow Mode with Drifting He$^{++}$: Implications for Coronal Seismology and the Solar Wind [Cross-Listing]

The MHD slow mode wave has application to coronal seismology, MHD turbulence, and the solar wind where it can be produced by parametric instabilities. We consider analytically how a drifting ion species (e.g. He$^{++}$) affects the linear slow mode wave in a mainly electron-proton plasma, with potential consequences for the aforementioned applications. Our main conclusions are: 1. For wavevectors highly oblique to the magnetic field, we find solutions that are characterized by very small perturbations of total pressure. Thus, our results may help to distinguish the MHD slow mode from kinetic Alfv\’en waves and non-propagating pressure-balanced structures, which can also have very small total pressure perturbations. 2. For small ion concentrations, there are solutions that are similar to the usual slow mode in an electron-proton plasma, and solutions that are dominated by the drifting ions, but for small drifts the wave modes cannot be simply characterized. 3. Even with zero ion drift, the standard dispersion relation for the highly oblique slow mode cannot be used with the Alfv\’en speed computed using the summed proton and ion densities, and with the sound speed computed from the summed pressures and densities of all species. 4. The ions can drive a non-resonant instability under certain circumstances. For low plasma beta, the threshold drift can be less than that required to destabilize electromagnetic modes, but damping from the Landau resonance can eliminate this instability altogether, unless $T_{\mathrm e}/T_{\mathrm p}\gg1$.

On the origin of stars with and without planets. Tc trends and clues to Galactic evolution

We explore a sample of 148 solar-like stars to search for a possible correlation between the slopes of the abundance trends versus condensation temperature (known as the Tc slope) with stellar parameters and Galactic orbital parameters in order to understand the nature of the peculiar chemical signatures of these stars and the possible connection with planet formation. We find that the Tc slope significantly correlates (at more than 4sigma) with the stellar age and the stellar surface gravity. We also find tentative evidence that the Tc slope correlates with the mean galactocentric distance of the stars (Rmean), suggesting that those stars that originated in the inner Galaxy have fewer refractory elements relative to the volatiles. While the average Tc slope for planet-hosting solar analogs is steeper than that of their counterparts without planets, this difference probably reflects the difference in their age and Rmean. We conclude that the age and probably the Galactic birth place are determinant to establish the star’s chemical properties. Old stars (and stars with inner disk origin) have a lower refractory-to-volatile ratio.

Next generation population synthesis of accreting white dwarfs: I. Hybrid calculations using BSE + MESA

Accreting, nuclear-burning white dwarfs have been deemed to be candidate progenitors of SNe Ia, and to account for supersoft X-ray sources, novae, etc. We have carried out a binary population synthesis (BPS) study of hydrogen-accreting WDs. First, we use the BPS code \textsf{BSE} as a baseline for the commonly used "rapid" approach. Second, we apply a "hybrid" approach: we use \textsf{BSE} to generate a population of WDs with non-degenerate companions on the verge of mass transfer. We then follow their evolution using the detailed stellar evolutionary code \textsf{MESA}. We investigate the evolution of the number of rapidly accreting white dwarfs (RAWDs), stably nuclear-burning white dwarfs (SNBWDs), and the SNe Ia rate produced by "single-degenerate" systems (SD). The two algorithms differ significantly in the predicted numbers of SNBWDs at early and late times, and also in the delay time distribution (DTD) of SD SNe Ia. The differences in the treatment of mass transfer may partially account for differences in the SNe Ia rate and DTD found by different groups. Using our hybrid algorithm, we found the number of RAWDs to be $\simeq (160 – 180)$ and $\simeq (2250 – 2500)$ in $10^{11}\,M_{\odot}$ elliptical and spiral-like galaxies, respectively. The numbers of SNBWDs found are $\simeq (750 – 1900)$ and $\simeq (4550 – 6550)$, respectively, larger than in previous studies. For 100% retention of accumulated helium, our calculations produce a SD SN Ia rate in a Milky-Way-like galaxy of $2.0\times10^{-4}\rm{yr}^{-1}$, an order of magnitude below that observed. In agreement with previous studies, our calculated SD DTD is inconsistent with observations. Subsequent articles will be devoted to the electromagnetic output from accreting WDs, and comparison of populations in varying accretion states with observations. (abridged)

Constraints on the winds of hot subdwarf stars from X-ray observations of two sdB binaries with compact companions: CD -30 11223 and PG 1232-136

Little observational data are available on the weak stellar winds of hot subdwarf stars of B spectral type (sdB). Close binary systems composed of an sdB star and a compact object (white dwarf, neutron star or black hole) could be detected as accretion-powered X-ray sources. The study of their X-ray emission can probe the properties of line-driven winds of sdB stars that can not be derived directly from spectroscopy because of the low luminosity of these stars. Here we report on the first sensitive X-ray observations of two sdB binaries with compact companions. CD -30 11223 is the sdB binary with the shortest known orbital period (1.2 h) and its companion is certainly a white dwarf. PG 1232-136 is an sdB binary considered the best candidate to host a black hole companion. We observed these stars with XMM-Newton in August 2013 for 50 ks and in July 2009 for 36 ks, respectively. None of them was detected and we derived luminosity upper limits of about 1.5×10^29 erg/s for CD -30 11223 5×10^29 erg/s for PG 1232-136. The corresponding mass loss rate for PG 1232-136 is poorly constrained, owing to the unknown efficiency for black hole accretion. On the other hand, in the case of CD -30 11223 we could derive, under reasonable assumptions, an upper limit of about 3×10^-13 solar masses/yr on the wind mass loss rate from the sdB star. This is one of the few observational constraints on the weak winds expected in this class of low mass hot stars. We also report the results on the X-ray emission from a cluster of galaxies serendipitously discovered in the field of CD -30 11223.

Perspectives of current-layer diagnostics in solar flares

A reconnecting current layer is a `heart’ of a solar flare, because it is a place of magnetic-field energy release. However there are no direct observations of these layers. The aim of our work is to understand why we actually do not directly observe current layers and what we need to do it in the future. The method is based on a simple mathematical model of a super-hot (T ~ 1E8 K) turbulent-current layer (SHTCL) and a model of plasma heating by the layer. The models allow us to study a correspondence between the main characteristics of the layer, such as temperature and dimensions, and the observational features, such as differential and integral emission measure of heated plasma, intensity of spectral lines Fe XXVI (1.78 and 1.51A) and Ni XXVII (1.59 A). This method provides a theoretical basis for determining parameters of the current layer from observations. Observations of SHTCLs are difficult, because the spectral line intensities are faint, but it is theoretically possible in the future. Observations in X-ray range 1.5–1.8 A with high spectral resolution (better than 0.01 A) and high temporal resolution (seconds) are needed. It is also very important to interpret the observations using a multi-temperature approach instead of the usual single or double temperature method.

Companions of Stars: From Other Stars to Brown Dwarfs to Planets: The Discovery of the First Methane Brown Dwarf

The discovery of the first methane brown dwarf provides a framework for describing the important advances in both fundamental physics and astrophysics that are due to the study of companions of stars. I present a few highlights of the history of this subject along with details of the discovery of the brown dwarf Gliese 229B. The nature of companions of stars is discussed with an attempt to avoid biases induced by anthropocentric nomenclature. With the newer types of remote reconnaissance of nearby stars and their systems of companions, an exciting and perhaps even more profound set of contributions to science is within reach in the near future. This includes an exploration of the diversity of planets in the universe and perhaps soon the first solid evidence for biological activity outside our Solar System.

Calibrating the updated overshoot mixing model on eclipsing binary stars: HY Vir, YZ Cas, X2 Hya & VV Crv

The detached eclipsing binary stars with convective cores provide a good tool to investigate the convective core overshoot. It has been performed on some binary stars to restrict the classical overshoot model which simply extends the boundary of fully mixed region. However, the classical overshoot model is physically unreasonable and inconsistent with the helioseismic investigation. An updated model of the overshoot mixing was established recently. There is a key parameter in the model. In this paper, we use the observations of four eclipsing binary stars, i.e., HY Vir, YZ Cas, $\rm{\chi^2}$ Hya and VV Crv, to investigate the suitable value for the parameter. It is found that the suggested value by the calibrations on eclipsing binary stars is same as the recommended value by other ways. And we have studied the effects of the updated overshoot model on the stellar structure. The diffusion coefficient of the convective/overshoot mixing is very high in the convection zone, then quickly decreases near the convective boundary, and exponentially decreases in the overshoot region. The low value of the diffusion coefficient in the overshoot region leads to weak mixing and the partially mixed overshoot region. The semi-convection, which appears in the standard stellar models of low-mass stars with convective core, is removed by the partial overshoot mixing.

The Light Curve Shapes as a Key to Resolving the Origin of Long Secondary Periods in Red Giant Stars

We present a study of OGLE light curves of red giant stars exhibiting long secondary periods (LSPs) – an enigmatic phenomenon commonly observed in stars on the upper red giant branch and asymptotic giant branch. We show that the light curves of LSP stars are essentially identical to those of the spotted variables with one dark spot on their photospheres. Such a behavior can be explained by a presence of a dusty cloud orbiting the red giant together with a low-mass companion in a close, circular orbit. We argue that the binary scenario is in agreement with most of the observational properties of LSP variables, including non-sinusoidal shapes of their radial velocity curves.

KOI-3278: A Self-Lensing Binary Star System

Over 40% of Sun-like stars are bound in binary or multistar systems. Stellar remnants in edge-on binary systems can gravitationally magnify their companions, as predicted 40 years ago. By using data from the Kepler spacecraft, we report the detection of such a "self-lensing" system, in which a 5-hour pulse of 0.1% amplitude occurs every orbital period. The white dwarf stellar remnant and its Sun-like companion orbit one another every 88.18 days, a long period for a white dwarf-eclipsing binary. By modeling the pulse as gravitational magnification (microlensing) along with Kepler’s laws and stellar models, we constrain the mass of the white dwarf to be ~63% of the mass of our Sun. Further study of this system, and any others discovered like it, will help to constrain the physics of white dwarfs and binary star evolution.

A 3D radiative transfer framework: XI. multi-level NLTE

Multi-level non-local thermodynamic equilibrium (NLTE) radiation transfer calculations have become standard throughout the stellar atmospheres community and are applied to all types of stars as well as dynamical systems such as novae and supernovae. Even today spherically symmetric 1D calculations with full physics are computationally intensive. We show that full NLTE calculations can be done with fully 3 dimensional (3D) radiative transfer. With modern computational techniques and current massive parallel computational resources, full detailed solution of the multi-level NLTE problem coupled to the solution of the radiative transfer scattering problem can be solved without sacrificing the micro physics description. We extend the use of a rate operator developed to solve the coupled NLTE problem in spherically symmetric 1D systems. In order to spread memory among processors we have implemented the NLTE/3D module with a hierarchical domain decomposition method that distributes the NLTE levels, radiative rates, and rate operator data over a group of processes so that each process only holds the data for a fraction of the voxels. Each process in a group holds all the relevant data to participate in the solution of the 3DRT problem so that the 3DRT solution is parallelized within a domain decomposition group. We solve a spherically symmetric system in 3D spherical coordinates in order to directly compare our well-tested 1D code to the 3D case. We compare three levels of tests: a) a simple H+He test calculation, b) H+He+CNO+Mg, c) H+He+Fe. The last test is computationally large and shows that realistic astrophysical problems are solvable now, but they do require significant computational resources. With presently available computational resources it is possible to solve the full 3D multi-level problem with the same detailed micro-physics as included in 1D modeling.

Looking for activity cycles in late-type Kepler stars using time-frequency analysis

We analyse light curves covering four years of 39 fast-rotating ($P_\mathrm{rot}< 1d$) late-type active stars from the Kepler database. Using time-frequency analysis (Short-Term Fourier-Transform), we find hints for activity cycles of 300-900 days at 9 targets from the changing typical latitude of the starspots, which, with the differential rotation of the stellar surface change the observed rotation period over the activity cycle. We also give a lowest estimation for the shear parameter of the differential rotation, which is ~0.001 for the cycling targets. These results populate the less studied, short period end of the rotation-cycle length relation.

Rotational and Radial Velocities of 1.3-2.2 M_Sun Red Giants in Open Clusters

This study presents the rotational distribution of red giant stars (RGs) in eleven old to intermediate age open clusters. The masses of these stars are all above the Kraft break, so that they lose negligible amounts of their birth angular momentum (AM) during the main sequence evolution. However, they do span a mass range with quite different AM distributions imparted during formation, with the stars less massive than ~1.6 M_Sun arriving on the main sequence with lower rotation rates than the more massive stars. The majority of RGs in this study are slow rotators across the entire red giant branch regardless of mass, supporting the picture that intermediate mass stars rapidly spin down when they evolve off the main sequence and develop convection zones capable of driving a magnetic dynamo. Nevertheless, a small fraction of RGs in open clusters show some level of enhanced rotation, and faster rotators are as common in these clusters as in the field red giant population. Most of these enhanced rotators appear to be red clump stars, which is also true of the underlying stellar sample, while others are clearly RGs that are above or below the clump. In addition to rotational velocities, the radial velocities and membership probabilities of individual stars are also presented. Cluster heliocentric radial velocities for NGC 6005 and Pismis 18 are reported for the first time.

A comprehensive study of Kepler phase curves and secondary eclipses -- temperatures and albedos of confirmed Kepler giant planets

We present a comprehensive study of phase curves and secondary eclipses in the Kepler data set using all available data from 15 quarters. Our original sample consists of 489 Kepler Objects of Interest (KOI) with R_p > 4 R_e , P < 10d, V_mag < 15 from the latest data release. Here we focus on 20 confirmed planets from that sample and derive their temperatures and albedos. Our results confirm and in most cases improve parameters derived by previous studies. We present new results for Kepler 1b-8b, 12b-15b, 17b, 40b, 41b, 43b, 44b, 76b, 77b, and 412b derived in a consistent manner. Furthermore we present a lightcurve analysis of Kepler 91b and Kepler 74b. Both show extra dimmings at times other than of the expected primary and secondary eclipses. Corrected for thermal emission we find the 20 planets we analyzed separate into two groups of high (>0.1) and low (<0.1) albedos, with no significant correlation to any stellar or planetary parameters. However the most massive planets from our sample are all low in albedo.

Astrophysical parameters and orbital solution of the peculiar X-ray transient IGR J00370+6122

BD+6073 is the optical counterpart of the X-ray source IGR J00370+6122, a probable accretion-powered X-ray pulsar. The X-ray light curve of this binary system shows clear periodicity at 15.7 d, which has been interpreted as repeated outbursts around the periastron of an eccentric orbit. We obtained high-resolution spectra of BD+6073 at different epochs. We used the FASTWind code to generate a stellar atmosphere model to fit the observed spectrum and obtain physical magnitudes. The synthetic spectrum was used as a template for cross-correlation with the observed spectra to measure radial velocities. The radial velocity curve provided an orbital solution for the system. We have also analysed the RXTE/ASM and Swift/BAT light curves to confirm the stability of the periodicity. BD +6073 is a BN0.7 Ib low-luminosity supergiant located at an approximate distance of 3.1 kpc, in the CasOB4 association. We derive Teff=24000 K and log gc=3.0, and chemical abundances consistent with a moderately high level of evolution. The spectroscopic and evolutionary masses are consistent at the 1 sigma level with a mass of 15 solar masses. The recurrence time of the X-ray flares is the orbital period of the system. The NS is in a high eccentricity (e=0.56) orbit, and the X-ray emission is strongly peaked around orbital phase 0.2, though the observations are consistent with some level of X-ray activity happening at all orbital phases. The X-ray behaviour of IGR J00370+6122 is reminiscent of intermediate SFXTs, though its peak luminosity is rather low. The orbit is somewhat wider than those of classical persistent supergiant X-ray binaries, which, combined with the low luminosity of the mass donor, explains the low X-ray luminosity. IGR J00370+6122 will likely evolve towards a persistent supergiant system, highlighting the evolutionary connection between different classes of wind-accreting X-ray sources.

The angular momentum transport by unstable toroidal magnetic fields

We demonstrate with a nonlinear MHD code that angular momentum can be transported due to the magnetic instability of toroidal fields under the influence of differential rotation, and that the resulting effective viscosity may be high enough to explain the almost rigid-body rotation observed in radiative stellar cores. The fields are assumed strong enough and the density stratification weak enough that the influence of the ‘negative’ buoyancy in the radiative zones can be neglected. Only permanent current-free fields and only those combinations of rotation rates and magnetic field amplitudes which provide maximal numerical values of the viscosity are considered. We find that the dimensionless ratio of the turbulent over molecular viscosity, \nu_T/\nu, linearly grows with growing magnetic Reynolds number of the rotating fluid multiplied by the square root of the magnetic Prandtl number – which is of order unity for the considered red subgiant KIC 7341231, in contrast to the smaller values of the solar radiative interior. The outward angular momentum transport is thus stronger for hot and fast rotators than for solar-type stars. For the considered interval of magnetic Reynolds numbers – which is restricted by numerical constraints of the nonlinear MHD code – there is a remarkable influence of the magnetic Prandtl number on the relative importance of the contributions of the Reynolds stress and the Maxwell stress to the total viscosity, which is magnetically dominated only for Pm > 0.5. We also find that the magnetized plasma behaves as a non-Newtonian fluid, i.e. the resulting effective viscosity depends on the shear in the rotation law. The decay time of the differential rotation thus depends on its shear and becomes longer and longer during the spin-down of a stellar core, as the viscosity is reduced when the rotation law becomes flat.

On the RZ Draconis Sub-stellar Circumbinary Companions Stability Study of the Proposed Sub-stellar Circumbinary System

In this work we revisit the proposed multi-circumbinary system RZ Dra. We find the proposed system to be highly unstable. We attempt to find a best-fit light-travel time model rendering the orbits to follow stable orbits. We found a best-fit solution, but the resulting orbits also exhibit short-term orbital instabilities. We therefore conclude that the observed timing variation must be due to other astrophysical effects causing a change in the binary orbital period. Follow-up observations of this system is encouraged.

SiO masers from AGB stars in the vibrationally excited v=1,v=2, and v=3 states

The v=1 and v=2 J=1-0 (43 GHz), and v=1 J=2-1 (86 GHz) SiO masers are intense in AGB stars and have been mapped using VLBI showing ring-like distributions. Those of the v=1, v=2 J=1-0 masers are similar, but the spots are rarely coincident, while the v=1 J=2-1 maser arises from a well separated region farther out. These relative locations can be explained by models tools that include the overlap of two IR lines of SiO and H2O. The v=3 J=1-0 line is not directly affected by any line overlap and its spot structure and position, relative to the other lines, is a good test to the standard pumping models. We present single-dish and simultaneous VLBI observations of the v=1, v=2, and v=3 J=1-0 maser transitions of 28SiO in several AGB stars. The spatial distribution of the SiO maser emission in the v=3 J=1-0 transition from AGB stars is systematically composed of a series of spots that occupy a ring-like structure. The overall ring structure is extremely similar to that found in the other 43 GHz transitions and is very different from the structure of the v=1 J=2-1 maser. The positions of the individual spots of the different 43 GHz lines are, however, very rarely coincident, which in general is separated by about 0.3 AU (between 1 and 5 mas). These results are very difficult to reconcile with standard pumping models, which predict that the masers of rotational transitions within a given vibrational state require very similar excitation conditions, while the transitions of different vibrational states should appear in different positions. However, models including line overlap tend to predict v=1, v=2, v=3 J=1-0 population inversion to occur under very similar conditions, while the requirements for v=1 J=2-1 appear clearly different, and are compatible with the observational results.

Fast Magnetic Twister and Plasma Perturbations in a 3-D Coronal Arcade

We present results of 3-D numerical simulations of a fast magnetic twister excited above a foot-point of the potential solar coronal arcade that is embedded in the solar atmosphere with the initial VAL-IIIC temperature profile, which is smoothly extended into the solar corona. With the use of the FLASH code, we solve 3-D ideal magnetohydrodynamic equations by specifying a twist in the azimuthal component of magnetic field in the solar chromosphere. The imposed perturbation generates torsional Alfv\’en waves as well as plasma swirls that reach the other foot-point of the arcade and partially reflect back from the transition region. The two vortex channels are evident in the generated twisted flux-tube with a fragmentation near its apex that results from the initial twist as well as from the morphology of the tube. The numerical results are compared to observational data of plasma motions in a solar prominence. The comparison shows that the numerical results and the data qualitatively agree even though the observed plasma motions occur over comparatively large spatio-temporal scales in the prominence.

The solar dynamo: inferences from observations and modeling

It can be shown on observational grounds that two basic effects of dynamo theory for solar activity – production of the toroidal field from the poloidal one by differential rotation and reverse conversion of the toroidal field to the poloidal configuration by helical motions – are operating in the Sun. These two effects, however, do not suffice for constructing a realistic model for the solar dynamo. Only when a non-local version of the alpha-effect is applied, is downward diamagnetic pumping included and field advection by the equatorward meridional flow near the base of the convection zone allowed for, can the observed activity cycles be closely reproduced. Fluctuations in the alpha-effect can be estimated from sunspot data. Dynamo models with fluctuating parameters reproduce irregularities of solar cycles including the grand activity minima. The physics of parametric excitation of irregularities remains, however, to be understood.

Optical and infrared polarimetry of the transient LMXB Cen X-4 in quiescence

We present the first optical and infrared polarimetric study of the low mass transient X-ray binary Cen X-4 during its quiescent phase. This work is aimed to search for an intrinsic linear polarisation component in the system emitted radiation that might be due, e.g., to synchrotron emission from a compact jet, or to Thomson scattering with free electrons in an accretion disc. Multiband (BVRI) optical polarimetric observations were obtained during two nights in 2008 at the ESO La Silla 3.6 m telescope (EFOSC2) in polarimetric mode. These observations cover about the 30% of the 15.1 hours orbital period. J-band observations were obtained in 2007 with the NICS (TNG) instrument at La Palma, for a totality of 1 hour observation. We obtained 3-sigma upper limits to the polarisation degree in all the optical bands, with the most constraining one being in the I-band (P<0.5%). No phase-correlated variability has been noticed in all the filters. The J-band observations provided a 6% upper limit on the polarisation level. The constraining upper limits to the polarisation in the optical allowed us to evaluate the contribution of the possible emission of a relativistic particles jet to the total system radiation to be less then the 10%. This is in agreement with the observation of a spectral energy distribution typical of a single black body of a K-spectral type main sequence star irradiated from the compact object. Due to the low S/N ratio it was not possible to investigate the possible dependency of the polarisation degree from the wavelength, that could be suggestive of polarisation induced by Thomson scattering of radiation with free electrons in the outer part of the accretion disc. Observations with higher S/N ratio are required to examine in depth this hypothesis, searching for significant phase-correlated variability.

A Comprehensive Method of Estimating Electric Fields from Vector Magnetic Field and Doppler Measurements

Photospheric electric fields, estimated from sequences of vector magnetic field and Doppler measurements, can be used to estimate the flux of magnetic energy (the Poynting flux) into the corona and as time-dependent boundary conditions for dynamic models of the coronal magnetic field. We have modified and extended an existing method to estimate photospheric electric fields that combines a poloidal-toroidal (PTD) decomposition of the evolving magnetic field vector with Doppler and horizontal plasma velocities. Our current, more comprehensive method, which we dub the "{\bf P}TD-{\bf D}oppler-{\bf F}LCT {\bf I}deal" (PDFI) technique, can now incorporate Doppler velocities from non-normal viewing angles. It uses the \texttt{FISHPACK} software package to solve several two-dimensional Poisson equations, a faster and more robust approach than our previous implementations. Here, we describe systematic, quantitative tests of the accuracy and robustness of the PDFI technique using synthetic data from anelastic MHD (\texttt{ANMHD}) simulations, which have been used in similar tests in the past. We find that the PDFI method has less than $1%$ error in the total Poynting flux and a $10%$ error in the helicity flux rate at a normal viewing angle $(\theta=0$) and less than $25%$ and $10%$ errors respectively at large viewing angles ($\theta<60^\circ$). We compare our results with other inversion methods at zero viewing angle, and find that our method’s estimates of the fluxes of magnetic energy and helicity are comparable to or more accurate than other methods. We also discuss the limitations of the PDFI method and its uncertainties.

Spectroscopically confirmed brown dwarf members of Coma Berenices and the Hyades

We have obtained low and medium resolution spectra of 9 brown dwarf candidate members of Coma Berenices and the Hyades using SpEX on the NASA InfaRed Telescope Facility and LIRIS on the William Herschel Telescope. We conclude that 7 of these objects are indeed late M or early L dwarfs, and that two are likely members of Coma Berenices, and four of the Hyades. Two objects, cbd40 and Hy3 are suggested to be a field L dwarfs, although there is also a possibility that Hy3 is an unresolved binary belonging to the cluster. These objects have masses between 71 and 53 M$_{\rm Jup}$, close to the hydrogen burning boundary for these clusters, however only an optical detection of Lithium can confirm if they are truly substellar.

Additional resonant contribution to the potential model for the 12C(alpha,gamma)16O reaction [Cross-Listing]

The additional resonant contribution to the potential model is examined in $\alpha$+$^{12}$C elastic scattering and the low-energy $^{12}$C($\alpha$,$\gamma$)$^{16}$O reaction. The excitation function of elastic scattering below $E_{c.m.}= 5$ MeV seems to be reproduced by the potential model satisfactorily, and it is not profoundly disturbed by the additional resonances. The weak coupling is good enough to describe the $^{16}$O structure in the vicinity of the $\alpha$-particle threshold, especially below $E_{c.m.}= 8$ MeV, corresponding to the excitation energy $E_x \approx 15$ MeV. The additional resonances give the complement of the astrophysical $S$-factors from the simple potential model. The $S$-factor of $^{12}$C($\alpha$,$\gamma$)$^{16}$O at $E_{c.m.}=300$ keV is dominated by the $E$2 transition, which is enhanced by the subthreshold 2$^+_1$ state at $E_x= 6.92$ MeV. The contribution from the subthreshold 1$^-_1$ state at $E_x= 7.12$ MeV is predicted to be small. The additional resonances do not give the large contribution to the thermonuclear reaction rates of $^{12}$C($\alpha$,$\gamma$)$^{16}$O at helium burning temperatures.

A multiple system of high-mass YSOs surrounded by disks in NGC7538 IRS1

NGC7538 IRS1 is considered the best high-mass accretion disk candidate around an O-type young star in the northern hemisphere. We investigated the 3D kinematics and dynamics of circumstellar gas with very high linear resolution, from tens to 1500 AU, with the ultimate goal of building a comprehensive dynamical model for this YSO. We employed four different observing epochs of EVN data at 6.7 GHz, spanning almost eight years, which enabled us to measure, besides line-of-sight (l.o.s.) velocities and positions, also l.o.s. accelerations and proper motions of methanol masers. In addition, we imaged with the JVLA-B array highly-excited ammonia inversion lines, from (6,6) to (13,13), which enabled us to probe the hottest molecular gas very close to the exciting source(s). We found five 6.7 GHz maser clusters which are distributed over a region extended N-S across ~1500 AU and are associated with three peaks of the radio continuum. We proposed that these maser clusters identify three individual high-mass YSOs, named IRS1a, IRS1b, and IRS1c. We modeled the maser clusters in IRS1a and IRS1b in terms of edge-on disks in centrifugal equilibrium. In the first case, masers may trace a quasi-Keplerian thin disk, orbiting around a high-mass YSO, IRS1a, of up to 25 solar masses. This YSO dominates the bolometric luminosity of the region. The second disk is both massive (<16 Msun within ~500 AU) and thick, and the mass of the central YSO, IRS1b, is constrained to be at most a few solar masses. In summary, we present compelling evidence that NGC7538 IRS1 is not forming just one single high-mass YSO, but consists of a multiple system of high-mass YSOs, which are surrounded by accretion disks, and are probably driving individual outflows. This new model naturally explains all the different orientations and disk/outflow structures proposed for the region in previous models.

Observations of Unresolved Photospheric Magnetic Fields in Solar Flares Using Fe I and Cr I Lines

The structure of the photospheric magnetic field during solar flares is examined using echelle spectropolarimetric observations. The study is based on several Fe I and Cr I lines observed at locations corresponding to brightest H$\alpha$ emission during thermal phase of flares. The analysis is performed by comparing magnetic field values deduced from lines with different magnetic sensitivities, as well as by examining the fine structure of $I\pm V$ Stokes profiles splitting. It is shown that the field has at least two components, with stronger unresolved flux tubes embedded in weaker ambient field. Based on a two-component magnetic field model, we compare observed and synthetic line profiles and show that the field strength in small-scale flux tubes is about $2-3$ kG. Furthermore, we find that the small-scale flux tubes are associated with flare emission, which may have implications for flare phenomenology.

The Transient Neutral Flux in Plasma: An Explanation of Heating for the Solar Corona?

In this short note, we discuss a mechanism for the transport of energy, momentum and dipole moment via transient neutral carriers in plasma. This gives a way to rapidly convert bulk hydrodynamic flow energy into thermal energy over a distance of several mean free paths. In the transition region of the solar corona we estimate various processes and their potential to introduce the high energies needed to to reach the 2 x10^6K observed there. It implies that kinetic methods may be essential for modeling the corona and that there are more gentle but still robust means than reconnection to relax magnetic fields in plasmas.

On the Helicity of Open Magnetic Fields

We reconsider the topological interpretation of magnetic helicity for magnetic fields in open domains, and relate this to the relative helicity. Specifically, our domains stretch between two parallel planes, and each of these ends may be magnetically open. It is demonstrated that, while the magnetic helicity is gauge-dependent, its value in any gauge may be physically interpreted as the average winding number among all pairs of field lines with respect to some orthonormal frame field. In fact, the choice of gauge is equivalent to the choice of reference field in the relative helicity, meaning that the magnetic helicity is no less physically meaningful. We prove that a particular gauge always measures the winding with respect to a fixed frame, and propose that this is normally the best choice. For periodic fields, this choice is equivalent to measuring relative helicity with respect to a potential reference field. But for aperiodic fields, we show that the potential field can be twisted. We prove by construction that there always exists a possible untwisted reference field.

Astrometry of brown dwarfs with Gaia

Europe’s Gaia spacecraft will soon embark on its five-year mission to measure the absolute parallaxes of the complete sample of 1,000 million objects down to 20 mag. It is expected that thousands of nearby brown dwarfs will have their astrometry determined with sub-milli-arcsecond standard errors. Although this level of accuracy is comparable to the standard errors of the relative parallaxes that are now routinely obtained from the ground for selected, individual objects, the absolute nature of Gaia’s astrometry, combined with the sample increase from one hundred to several thousand sub-stellar objects with known distances, ensures the uniqueness of Gaia’s legacy in brown-dwarf science for the coming decade(s). We shortly explore the gain in brown-dwarf science that could be achieved by lowering Gaia’s faint-end limit from 20 to 21 mag and conclude that two spectral-type sub-classes could be gained in combination with a fourfold increase in the solar-neighbourhood-volume sampled by Gaia and hence in the number of brown dwarfs in the Gaia Catalogue.

Gravitational-wave radiation from double compact objects with eLISA in the Galaxy

The phase of in-spiral of double compact objects (DCOs: NS+WD, NS+NS, BH+NS, and BH+BH binaries) in the disk field population of the Galaxy provides a potential source in the frequency range from $10^{-4}$ to 0.1 Hz, which can be detected by the European New Gravitational Observatory (NGO: eLISA is derived from the previous LISA proposal) project. In this frequency range, much stronger gravitational wave (GW) radiation can be obtained from DCO sources because they possess more mass than other compact binaries (e.g., close double white dwarfs). In this study, we aim to calculate the gravitational wave signals from the resolvable DCO sources in the Galaxy using a binary population synthesis approach, and to carry out physical properties of these binaries using Monte Carlo simulations. Combining the sensitivity curve of the eLISA detector and a confusion-limited noise floor of close double white dwarfs, we find that only a handful of DCO sources can be detected by the eLISA detector. The detectable number of DCO sources reaches 160, in the context of low-frequency eLISA observations we find that the number of NS+WD, NS+NS, BH+NS, and BH+BH are 132, 16, 3, and 6, respectively.

Gravitational-wave radiation from double compact objects with eLISA in the Galaxy [Replacement]

The phase of in-spiral of double compact objects (DCOs: NS+WD, NS+NS, BH+NS, and BH+BH binaries) in the disk field population of the Galaxy provides a potential source in the frequency range from $10^{-4}$ to 0.1 Hz, which can be detected by the European New Gravitational Observatory (NGO: eLISA is derived from the previous LISA proposal) project. In this frequency range, much stronger gravitational wave (GW) radiation can be obtained from DCO sources because they possess more mass than other compact binaries (e.g., close double white dwarfs). In this study, we aim to calculate the gravitational wave signals from the resolvable DCO sources in the Galaxy using a binary population synthesis approach, and to carry out physical properties of these binaries using Monte Carlo simulations. Combining the sensitivity curve of the eLISA detector and a confusion-limited noise floor of close double white dwarfs, we find that only a handful of DCO sources can be detected by the eLISA detector. The detectable number of DCO sources reaches 160, in the context of low-frequency eLISA observations we find that the number of NS+WD, NS+NS, BH+NS, and BH+BH are 132, 16, 3, and 6, respectively.

AGB stars and the plate archives heritage

We report on the characterization of a number of AGB candidate stars identified with objective-prism plates of the Byurakan Observatory. Digitized photographic sky survey plates and recent CCD photometry have been used to improve the selection and distinguish variable and non-variable stars. Some comparisons among published catalog magnitudes are also made. Slit spectroscopy from the Asiago and Loiano Observatories allowed a firm spectral classification, separating C-Type, N-Type and normal M giants. Color-color plots using WISE, AKARI and 2MASS J-band data allow an efficient discrimination of spectral types, which can be used for the definition of larger statistical samples.

Chaotic flavor evolution in an interacting neutrino gas [Cross-Listing]

Neutrino-neutrino refraction can lead to non-periodic flavor oscillations in dense neutrino gases, and it has been hypothesized that some solutions are chaotic in nature. This is of particular interest in the case of neutrino emission from core-collapse supernovae where the measurement of the spectral shape for different flavors can provide crucial information about both neutrino physics and the physical conditions close to the proto-neutron star. Whether a system is chaotic or not can be assessed by the Lyapunov exponents which quantify the rate of divergence of nearby trajectories in the system. We have done a numerical case study for a simple toy model of two neutrino flavors with two momentum states traveling against each other which is known to exhibit flavor transition instabilities. We find the leading Lyapunov exponent to be positive in all cases, confirming the chaoticity of the system for both the normal and the inverted neutrino mass hierarchy. However, more Lyapunov exponents were approximately zero in the inverted hierarchy compared to the normal which has implications for the stability of the system. To investigate this, we have calculated a generalized set of normal modes, the so-called covariant Lyapunov vectors. The covariant Lyapunov vectors associated with vanishing Lyapunov exponents showed the existence of marginally stable directions in phase space for some cases. While our analysis was done for a toy model example, it should work equally well for more realistic cases of neutrinos streaming from a proto-neutron star and provide valuable insight into the nature of the flavor instability. We finally stress that our approach captures many more properties of the physical system than the linear stability analyses which have previously been performed.

Chaotic flavor evolution in an interacting neutrino gas

Neutrino-neutrino refraction can lead to non-periodic flavor oscillations in dense neutrino gases, and it has been hypothesized that some solutions are chaotic in nature. This is of particular interest in the case of neutrino emission from core-collapse supernovae where the measurement of the spectral shape for different flavors can provide crucial information about both neutrino physics and the physical conditions close to the proto-neutron star. Whether a system is chaotic or not can be assessed by the Lyapunov exponents which quantify the rate of divergence of nearby trajectories in the system. We have done a numerical case study for a simple toy model of two neutrino flavors with two momentum states traveling against each other which is known to exhibit flavor transition instabilities. We find the leading Lyapunov exponent to be positive in all cases, confirming the chaoticity of the system for both the normal and the inverted neutrino mass hierarchy. However, more Lyapunov exponents were approximately zero in the inverted hierarchy compared to the normal which has implications for the stability of the system. To investigate this, we have calculated a generalized set of normal modes, the so-called covariant Lyapunov vectors. The covariant Lyapunov vectors associated with vanishing Lyapunov exponents showed the existence of marginally stable directions in phase space for some cases. While our analysis was done for a toy model example, it should work equally well for more realistic cases of neutrinos streaming from a proto-neutron star and provide valuable insight into the nature of the flavor instability. We finally stress that our approach captures many more properties of the physical system than the linear stability analyses which have previously been performed.

Study of Solar Magnetic and Gravitational Energies Through the Virial Theorem

Virial theorem is important for understanding stellar structures. It produces an interesting connection between the magnetic energy and the gravitational one. Using the general form of the virial theorem including the magnetic field (toroidal magnetic field), we may explain the solar dynamo model in related to variations of the magnetic and gravitational energies. We emphasize the role of the gravitational energy in sub-surface layers which has been certainly minored up to now. We also consider two types of solar outer shape (spherical and spheroidal) to study the behavior of magnetic and gravitational energies. The magnetic energy affects the solar shape, while the gravitational energy is not changed by the considered shapes of the Sun.

Cataclysmic Variables from the Catalina Real-time Transient Survey

We present 855 cataclysmic variable candidates detected by the Catalina Real-time Transient Survey (CRTS) of which at least 137 have been spectroscopically confirmed and 705 are new discoveries. The sources were identified from the analysis of five years of data, and come from an area covering three quarters of the sky. We study the amplitude distribution of the dwarf novae CVs discovered by CRTS during outburst, and find that in quiescence they are typically two magnitudes fainter compared to the spectroscopic CV sample identified by SDSS. However, almost all CRTS CVs in the SDSS footprint have ugriz photometry. We analyse the spatial distribution of the CVs and find evidence that many of the systems lie at scale heights beyond those expected for a Galactic thin disc population. We compare the outburst rates of newly discovered CRTS CVs with the previously known CV population, and find no evidence for a difference between them. However, we find that significant evidence for a systematic difference in orbital period distribution. We discuss the CVs found below the orbital period minimum and argue that many more are yet to be identified among the full CRTS CV sample. We cross-match the CVs with archival X-ray catalogs and find that most of the systems are dwarf novae rather than magnetic CVs.

Oscillator Models of the Solar Cycle and the Waldmeier Effect

We study the behaviour of the van der Pol oscillator when either its damping parameter $\mu$ or its nonlinearity parameter $\xi$ is subject to additive or multiplicative random noise. Assuming various power law exponents for the relation between the oscillating variable and the sunspot number, for each case we map the parameter plane defined by the amplitude and the correlation time of the perturbation and mark the parameter regime where the sunspot number displays solar-like behaviour. Solar-like behaviour is defined here as a good correlation between the rise rate and cycle amplitude {\it and} the lack of a good correlation between the decay rate and amplitude, together with significant ($\ga 10$\,%) r.m.s. variation in cycle lengths and cycle amplitudes. It is found that perturbing $\mu$ alone the perturbed van der Pol oscillator does not show solar-like behaviour. When the perturbed variable is $\xi$, solar-like behaviour is displayed for perturbations with a correlation time of about 3–4 years and significant amplitude. Such studies may provide useful constraints on solar dynamo models and their parameters.

Connecting speeds, directions and arrival times of 22 coronal mass ejections from the Sun to 1 AU

Forecasting the in situ properties of coronal mass ejections (CMEs) from remote images is expected to strongly enhance predictions of space weather, and is of general interest for studying the interaction of CMEs with planetary environments. We study the feasibility of using a single heliospheric imager (HI) instrument, imaging the solar wind density from the Sun to 1 AU, for connecting remote images to in situ observations of CMEs. We compare the predictions of speed and arrival time for 22 CMEs (in 2008-2012) to the corresponding interplanetary coronal mass ejection (ICME) parameters at in situ observatories (STEREO PLASTIC/IMPACT, Wind SWE/MFI). The list consists of front- and backsided, slow and fast CMEs (up to $2700 \: km \: s^{-1}$). We track the CMEs to $34.9 \pm 7.1$ degrees elongation from the Sun with J-maps constructed using the SATPLOT tool, resulting in prediction lead times of $-26.4 \pm 15.3$ hours. The geometrical models we use assume different CME front shapes (Fixed-$\Phi$, Harmonic Mean, Self-Similar Expansion), and constant CME speed and direction. We find no significant superiority in the predictive capability of any of the three methods. The absolute difference between predicted and observed ICME arrival times is $8.1 \pm 6.3$ hours ($rms$ value of 10.9h). Speeds are consistent to within $284 \pm 288 \: km \: s^{-1}$. Empirical corrections to the predictions enhance their performance for the arrival times to $6.1 \pm 5.0$ hours ($rms$ value of 7.9h), and for the speeds to $53 \pm 50 \: km \: s^{-1}$. These results are important for Solar Orbiter and a space weather mission positioned away from the Sun-Earth line.

Interpreting the Helioseismic and Magnetic Imager (HMI) Multi-Height Velocity Measurements

The Solar Dynamics Observatory/Helioseismic and Magnetic Imager (SDO/HMI) filtergrams, taken at six wavelengths around the Fe I 6173.3 {\AA} line, contain information about the line-of-sight velocity over a range of heights in the solar atmosphere. Multi-height velocity inferences from these observations can be exploited to study wave motions and energy transport in the atmosphere. Using realistic convection simulation datasets provided by the STAGGER and MURaM codes, we generate synthetic filtergrams and explore several methods for estimating Dopplergrams. We investigate at which height each synthetic Dopplergram correlates most strongly with the vertical velocity in the model atmospheres. On the basis of the investigation, we propose two Dopplergrams other than the standard HMI-algorithm Dopplergram produced from HMI filtergrams: a line-center Dopplergram and an average-wing Dopplergram. These two Dopplergrams correlate most strongly with vertical velocities at the heights of 30 – 40 km above (line-center) and 30 – 40 km below (average-wing) the effective height of the HMI-algorithm Dopplergram. Therefore, we can obtain velocity information from two layers separated by about a half of a scale height in the atmosphere, at best. The phase shifts between these multi-height Dopplergrams from observational data as well as those from the simulated data are also consistent with the height-difference estimates in the frequency range above the photospheric acoustic cutoff frequency.

Magnetic diffusivity and angular momentum transport in magnetized and differentially rotating stellar radiation zones

With a linear theory the instability of a toroidal background field system with dipolar parity for inner stellar radiative zones under the presence of density stratification, differential rotation and for realistically small Prandtl numbers is analyzed. The physical parameters are the normalized latitudinal shear $a$ and the normalized field amplitude $b \simeq \Omega_A/\Omega$. Only the solutions for the wavelengths with the maximal growth rates are considered. If these scales are combined to the radial values of velocity one finds that for $b \gsim 0.1$ the (very small) radial velocity does only slightly depend on $a$ and $b$ so that it can be used as the free parameter of the eigenvalue system. The resulting instability-generated tensors of magnetic diffusivity and eddy viscosity are highly anisotropic. The eddy diffusivity in latitudinal direction exceeds the eddy diffusivity in radial direction by orders of magnitude. Its latitudinal profile shows a strong concentration to the poles and (for rigid rotation) a numerical value of $10^{12}$ cm$^2$/s. On the other hand, the instability pattern transports angular momentum equatorward even for rigid rotation producing a slightly faster rotation of the equator of the radiative zone. The resulting effective magnetic Prandtl number reaches values of $O(10^3)$ so that differential rotation decays much faster than the toroidal background field which is {\em the} necessary condition to explain the observed slow rotation of the early red-giant and subgiant cores by means of magnetic instabilities.

A Challenging Solar Eruptive Event of 18 November 2003 and the Causes of the 20 November Geomagnetic Superstorm. III. Catastrophe of the Eruptive Filament in a Magnetic Null Point and Formation of an Opposite-Handedness CME

Our analysis in Papers I and II (Grechnev et al., 2014, Solar Phys. 289, 289 and 1279) of the 18 November 2003 solar event responsible for the 20 November geomagnetic superstorm has revealed a complex chain of eruptions. In particular, the eruptive filament encountered a topological discontinuity located near the solar disk center at a height of about 100 Mm, bifurcated, and transformed into a large cloud, which did not leave the Sun. Concurrently, an additional CME presumably erupted close to the bifurcation region. The conjectures about the responsibility of this compact CME for the superstorm and its disconnection from the Sun are confirmed in Paper IV (Grechnev et al., Solar Phys., submitted), which concludes about its probable spheromak-like structure. The present paper confirms the presence of a magnetic null point near the bifurcation region and addresses the origin of the magnetic helicity of the interplanetary magnetic clouds and their connection to the Sun. We find that the orientation of a magnetic dipole constituted by dimmed regions with the opposite magnetic polarities away from the parent active region corresponded to the direction of the axial field in the magnetic cloud, while the pre-eruptive filament mismatched it. To combine all of the listed findings, we come to an intrinsically three-dimensional scheme, in which a spheromak-like eruption originates via the interaction of the initially unconnected magnetic fluxes of the eruptive filament and pre-existing ones in the corona. Through a chain of magnetic reconnections their positive mutual helicity was transformed into the self-helicity of the spheromak-like magnetic cloud.

Sensitivity of sunspot area to the tidal effect of planet Mercury during solar cycle 23

We present evidence that the allowed periods of equatorially trapped Rossby wave modes on the Sun coincide closely with the 88 day period and 176 day sub harmonic period of Mercury and evidence of Rossby waves on the Sun at the same periods. To test the hypothesis that Rossby waves trigger the emergence of sunspots we use band pass filtering to obtain the 88 day period and 176 day period components of hemispherical sunspot area and compare the variations to the tidal height variation on the surface of the Sun due to Mercury. We find that the two components of hemispherical sunspot area occur in several episodes or activations of duration 2 to 6 years during each solar cycle. When the activations are discrete the variation of the 88 day and 176 day period components are phase coherent with the tidal height variation and a 180 degree phase change is evident between successive activations. We use this result to demonstrate that Rieger type quasi-periodicities in sunspot activity are, in most reported cases, periodicities associated with sidebands of the 88 day and 176 day period variation. We find that, in variables that arise from activity in both solar hemispheres, phase coherence with the Mercury tidal effect occurs at infrequent intervals but, when it does occur, it is possible to observe phase coherence of variables such as sunspot number, solar flares, solar wind speed and cosmic ray flux with the Mercury tidal effect. Sensitivity of a range of variables to the tidal effect is calculated for such intervals in solar cycle 23.

Abundance Enhancements in Impulsive Solar Energetic-Particle Events with Associated Coronal Mass Ejections

We study the abundances of the elements He through Pb in Fe-rich impulsive solar energetic-particle (SEP) events with measurable abundances of ions with atomic number Z>2 observed on the Wind spacecraft, and their relationship with coronal mass ejections (CMEs) observed by the Large Angle and Spectrometric Coronagraph (LASCO) onboard the Solar and Heliospheric Observatory (SOHO). On average the element abundances in these events are similar to coronal abundances at low Z but, for heavier elements, enhancements rise as a power law in the mass-to-charge ratio A/Q of the ions (at coronal temperatures of 2.5-3 MK) to a factor of 3 at Ne, 9 at Fe, and 900 for 75<Z<83. Energy dependences of abundances are minimal in the 2-15 MeV/amu range. The 111 of these Fe-rich impulsive SEP events we found, between November 1994 and August 2013 using the Wind spacecraft, have a 69% association rate with CMEs. The CMEs are narrow with a median width of 75 deg, are characteristically from western longitudes on the Sun, and have a median speed of ~600 km/s. Nearly all SEP onsets occur within 1.5-5 h of the CME onset. The faster (>700 km/s), wider CMEs in our sample are related to SEPs with coronal abundances indicating hot coronal plasma with fully ionized He, C, N and O and moderate enhancements of heavier elements, relative to He, but slower (<700 km/s), narrower CMEs emerge from cooler plasma where higher SEP mass-to-charge ratios, A/Q, yield much greater abundance enhancements, even for C/He and O/He. Apparently, the open magnetic-reconnection region where the impulsive SEPs are accelerated also provides the energy to drive out CME plasma, accounting for a strong, probably universal, impulsive SEP-CME association.

Spectroscopic orbits of ten nearby solar-type dwarfs

Several nearby solar-type dwarfs with variable radial velocity were monitored to find their spectroscopic orbits. Orbital elements of HIP 179, 1989, 2981, 5276, 6439, 11218, 21443, 96434 are determined, as well as tentative orbits for HIP 28678 and 41214. We discuss each of those objects. Three of the four double-lined binaries are twins with nearly equal components. All four orbits with periods shorter than 10 days are circular, the remaining orbits are eccentric.

Evidence of Electron Acceleration around the Reconnection X-point in a Solar Flare

Particle acceleration is one of the most significant features that are ubiquitous among space and cosmic plasmas. It is most prominent during flares in the case of the Sun, with which huge amount of electromagnetic radiation and high-energy particles are expelled into the interplanetary space through acceleration of plasma particles in the corona. Though it has been well understood that energies of flares are supplied by the mechanism called magnetic reconnection based on the observations in X-rays and EUV with space telescopes, where and how in the flaring magnetic field plasmas are accelerated has remained unknown due to the low plasma density in the flaring corona. We here report the first observational identification of the energetic non-thermal electrons around the point of the ongoing magnetic reconnection (X-point); with the location of the X-point identified by soft X-ray imagery and the localized presence of non-thermal electrons identified from imaging-spectroscopic data at two microwave frequencies. Considering the existence of the reconnection outflows that carries both plasma particles and magnetic fields out from the X-point, our identified non-thermal microwave emissions around the X-point indicate that the electrons are accelerated around the reconnection X-point. Additionally, the plasma around the X-point was also thermally heated up to 10 MK. The estimated reconnection rate of this event is ~0.017.

Spatial Nonlocality of the Small-Scale Solar Dynamo

We explore the nature of the small-scale solar dynamo by tracking magnetic features. We investigate two previously-explored categories of the small-scale solar dynamo: shallow and deep. Recent modeling work on the shallow dynamo has produced a number of scenarios for how a strong network concentration can influence the formation and polarity of nearby small-scale magnetic features. These scenarios have measurable signatures, which we test for here using magnetograms from the Narrowband Filter Imager (NFI) on Hinode. We find no statistical tendency for newly-formed magnetic features to cluster around or away from network concentrations, nor do we find any statistical relationship between their polarities. We conclude that there is no shallow or "surface" dynamo on the spatial scales observable by Hinode/NFI. In light of these results, we offer a scenario in which the sub-surface field in a deep solar dynamo is stretched and distorted via turbulence, allowing the field to emerge at random locations on the photosphere.

Magnetohydrodynamic Simulation of the X2.2 Solar Flare on 2011 February 15: I. Comparison with the Observations

We performed a magnetohydrodynamic (MHD) simulation using a nonlinear force-free field (NLFFF) in solar active region 11158 to clarify the dynamics of an X2.2-class solar flare. We found that the NLFFF never shows the drastic dynamics seen in observations, i.e., it is in stable state against the perturbations. On the other hand, the MHD simulation shows that when the strongly twisted lines are formed at close to the neutral line, which are produced via tether-cutting reconnection in the twisted lines of the NLFFF, consequently they erupt away from the solar surface via the complicated reconnection. This result supports the argument that the strongly twisted lines formed in NLFFF via tether-cutting reconnection are responsible for breaking the force balance condition of the magnetic fields in the lower solar corona. In addition to this the dynamical evolution of these field lines reveals that at the initial stage the spatial pattern of the footpoints caused by the reconnection of the twisted lines appropriately maps the distribution of the observed two-ribbon flares. Interestingly, after the flare the reconnected field lines convert into the structure like the post flare loops, which is analogous to EUV image taken by SDO. Eventually, we found that the twisted lines exceed a critical height at which the flux tube becomes unstable to the torus instability. These results illustrate the reliability of our simulation and also provide an important relationship between flare-CME dynamics.

Strong Brightness Variations Signal Cloudy-to-Clear Transition of Brown Dwarfs

We report the results of a $J$ band search for cloud-related variability in the atmospheres of 62 L4-T9 dwarfs using the Du Pont 2.5-m telescope at Las Campanas Observatory and the Canada France Hawaii Telescope on Mauna Kea. We find 9 of 57 objects included in our final analysis to be significantly variable with >99% confidence, 5 of which are new discoveries. In our study, strong variability (peak-to-peak amplitudes >2%) are confined to the L/T transition (4/16 objects with L9-T3.5 spectral types and 0/41 objects for all other spectral types). The probability that the observed occurrence rates for strong variability inside and outside the L/T transition originate from the same underlying true occurrence rate is excluded with >99.7% confidence. These observations suggest that the settling of condensate clouds below the photosphere in brown dwarf atmospheres does not occur in a spatially uniform manner. Rather, the formation and sedimentation of dust grains at the L/T transition is coupled to atmospheric dynamics, resulting in highly contrasting regions of thick and thin clouds and/or clearings. Outside the L/T transition we identify 5 weak variables (peak-to-peak amplitudes of 0.6%-1.6%). Excluding L9-T3.5 spectral types, we infer that $60^{+22}_{-18}$% of targets vary with amplitudes of 0.5%$-$1.6%, suggesting that surface heterogeneities are ubiquitous among L and T dwarfs. Our survey establishes a significant link between strong variability and L/T transition spectral types, providing evidence in support of the hypothesis that cloud holes contribute to the abrupt decline in condensate opacity and 1 micron brightening observed in this regime. More generally, fractional cloud coverage is an important model parameter for brown dwarfs and giant planets, especially those with L/T transition spectral types and colors. [ABRIDGED]

Properties of $p$- and $f$-modes in hydromagnetic turbulence

With the ultimate aim of using the fundamental or $f$-mode to study helioseismic aspects of turbulence-generated magnetic flux concentrations, we use randomly forced hydromagnetic simulations of a piecewiese isothermal layer in two dimensions with reflecting boundaries at top and bottom. We compute numerically diagnostic wavenumber-frequency diagrams of the vertical velocity at the interface between the denser gas below and the less dense gas above. For an Alfv\’en-to-sound speed ratio of about 0.1, a 5 % frequency increase of the $f$-mode can be measured when $k_x H_{\rm p}=3$-$4$, where $k_x$ is the horizontal wavenumber and $H_{\rm p}$ is the pressure scale height at the surface. Since the solar radius is about 2000 times larger than $H_{\rm p}$, the corresponding spherical harmonic degree would be 6000-8000. For weaker fields, a $k_x$-dependent frequency decrease by the turbulent motions becomes dominant. For vertical magnetic fields, the frequency is enhanced for $k_x H_{\rm p}\approx4$, but decreased relative to its nonmagnetic value for $k_x H_{\rm p}\approx9$.

A new library of theoretical stellar spectra with scaled-solar and alpha-enhanced mixtures

Theoretical stellar libraries have been increasingly used to overcome limitations of empirical libraries, e.g. by exploring atmospheric parameter spaces not well represented in the latter. This work presents a new theoretical library which covers 3000 $\leq$ Teff $\leq$ 25000\,K, -0.5 $\leq$ log g $\leq$ 5.5, and 12 chemical mixtures covering 0.0017 $\leq$ Z $\leq$ 0.049 at both scaled-solar and $\alpha$-enhanced compositions. This library complements previous ones by providing: (i) homogeneous computations of opacity distribution functions, models atmospheres, statistical surface fluxes and high resolution spectra; (ii) high resolution spectra with continua slopes corrected by the effect of predicted lines, and; (iii) two families of $\alpha$-enhanced mixtures for each scaled-solar iron abundance, to allow studies of the $\alpha$-enhancement both at `fixed iron’ and `fixed Z’ cases. Comparisons to observed spectra were performed and confirm that the synthetic spectra reproduce well the observations, although there are wavelength regions which should be still improved. The atmospheric parameter scale of the model library was compared to one derived from a widely used empirical library, and no systematic difference between the scales was found. This is particularly reassuring for methods which use synthetic spectra for deriving atmospheric parameters of stars in spectroscopic surveys.

Inbound waves in the solar corona: a direct indicator of Alfv\'en Surface location [Replacement]

The tenuous supersonic solar wind that streams from the top of the corona passes through a natural boundary — the Alfv\’en surface — that marks the causal disconnection of individual packets of plasma and magnetic flux from the Sun itself. The Alfv\’en surface is the locus where the radial motion of the accelerating solar wind passes the radial Alfv\’en speed, and therefore any displacement of material cannot carry information back down into the corona. It is thus the natural outer boundary of the solar corona, and the inner boundary of interplanetary space. Using a new and unique motion analysis to separate inbound and outbound motions in synoptic visible-light image sequences from the COR2 coronagraph on board the STEREO-A spacecraft, we have identified inbound wave motion in the outer corona beyond 6 solar radii for the first time, and used it to determine that the Alfv\’en surface is at least 12.5 solar radii from the Sun over the polar coronal holes and 17 solar radii in the streamer belt, well beyond the distance planned for NASA’s upcoming Solar Probe Plus mission. To our knowledge this is the first measurement of inbound waves in the outer solar corona, and the first direct measurement of lower bounds for the Alfv\’en surface.


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