New theory of stellar convection without the mixing-length parameter: New stellar atmosphere model

Stellar convection is usually described by the mixing-length theory, which makes use of the mixing-length scale factor to express the convective flux, velocity, and temperature gradients of the convective elements and stellar medium. The mixing-length scale is proportional to the local pressure scale height of the star, and the proportionality factor (i.e. mixing-length parameter) is determined by comparing the stellar models to some calibrator, i.e. the Sun. No strong arguments exist to suggest that the mixing-length parameter is the same in all stars and all evolutionary phases and because of this, all stellar models in the literature are hampered by this basic uncertainty. In a recent paper [1] we presented a new theory that does not require the mixing length parameter. Our self-consistent analytical formulation of stellar convection determines all the properties of stellar convection as a function of the physical behavior of the convective elements themselves and the surrounding medium. The new theory of stellar convection is formulated starting from a conventional solution of the Navier-Stokes/Euler equations expressed in a non-inertial reference frame co-moving with the convective elements. The motion of stellar convective cells inside convective-unstable layers is fully determined by a new system of equations for convection in a non-local and time-dependent formalism. The predictions of the new theory are compared with those from the standard mixing-length paradigm with positive results for atmosphere models of the Sun and all the stars in the Hertzsprung-Russell diagram

Blazhko modulation in the infrared

We present first direct evidence of modulation in the K band of Blazhko-type RR Lyrae stars that are identified by their secular modulations in the I-band data of Optical Gravitational Lensing Experiment-IV. A method has been developed to decompose the K-band light variation into two parts originating from the temperature and the radius changes using synthetic data of atmosphere-model grids. The amplitudes of the temperature and the radius variations derived from the method for non-Blazhko RRab stars are in very good agreement with the results of the Baade-Wesselink analysis of RRab stars in the M3 globular cluster confirming the applicability and correctness of the method. It has been found that the Blazhko modulation is primarily driven by the change in the temperature variation. The radius variation plays a marginal part, moreover it has an opposite sign as if the Blazhko effect was caused by the radii variations. This result reinforces the previous finding based on the Baade-Wesselink analysis of M3 (NGC 5272) RR Lyrae, that significant modulation of the radius variations can only be detected in radial-velocity measurements, which relies on spectral lines that form in the uppermost atmospheric layers. Our result gives the first insight into the energetics and dynamics of the Blazhko phenomenon, hence it puts strong constraints on its possible physical explanations

Kron 3: a fourth intermediate age cluster in the SMC with evidence of multiple populations

We present the results of a spectroscopic study of the intermediate age (approximately 6.5 Gyr) massive cluster Kron 3 in the Small Magellanic Cloud. We measure CN and CH band strengths (at 3839 and 4300 Angstroms respectively) using VLT FORS2 spectra of 16 cluster members and find a sub-population of 5 stars enriched in nitrogen. We conclude that this is evidence for multiple populations in Kron 3, the fourth intermediate age cluster, after Lindsay 1, NGC 416 and NGC 339 (ages 6-8 Gyr), to display this phenomenon originally thought to be a unique characteristic of old globular clusters. At 6.5 Gyr this is one of the youngest clusters with multiple populations, indicating that the mechanism responsible for their onset must operate until a redshift of at least 0.75, much later than the peak of globular cluster formation at redshift ~3

Evidence for multiple populations in the intermediate age cluster Lindsay 1 in the SMC

Lindsay 1 is an intermediate age (approx 8 Gyr) massive cluster in the Small Magellanic Cloud (SMC). Using VLT FORS2 spectra of 16 probable cluster members on the lower RGB of the cluster, we measure CN and CH band strengths (at 3883 and 4300 Angstroms respectively), along with carbon and nitrogen abundances and find that a sub-population of stars has significant nitrogen enrichment. A lack of spread in carbon abundances excludes evolutionary mixing as the source of this enrichment, so we conclude that this is evidence of multiple populations. Therefore, L1 is the youngest cluster to show such variations, implying that the process triggering the onset of multiple populations must operate until at least redshift ~1

Single-lined Spectroscopic Binary Star Candidates in the RAVE Survey

Repeated spectroscopic observations of stars in the RAdial Velocity Experiment (RAVE) database are used to identify and examine single-lined binary (SB1) candidates. The RAVE latest internal database (VDR3) includes radial velocities, atmospheric parameters, and other parameters for approximately a quarter of a million different stars with slightly less than 300,000 observations. In the sample of ~20,000 stars observed more than once, 1333 stars with variable radial velocities were identified. Most of them are believed to be SB1 candidates. The fraction of SB1 candidates among stars with several observations is between 10% and 15% which is the lower limit for binarity among RAVE stars. Due to the distribution of time spans between the re-observation that is biased toward relatively short timescales (days to weeks), the periods of the identified SB1 candidates are most likely in the same range. Because of the RAVE's narrow magnitude range most of the dwarf candidates belong to the thin Galactic disk while the giants are part of the thick disk with distances extending to up to a few kpc. The comparison of the list of SB1 candidates to the VSX catalog of variable stars yielded several pulsating variables among the giant population with radial velocity variations of up to few tens of km s–1. There are 26 matches between the catalog of spectroscopic binary orbits ($S_{B^9}$) and the whole RAVE sample for which the given periastron time and the time of RAVE observation were close enough to yield a reliable comparison. RAVE measurements of radial velocities of known spectroscopic binaries are consistent with their published radial velocity curves

The mass fraction of halo stars contributed by the disruption of globular clusters in the E-MOSAICS simulations

Globular clusters (GCs) have been posited, alongside dwarf galaxies, as significant contributors to the field stellar population of the Galactic halo. In order to quantify their contribution, we examine the fraction of halo stars formed in stellar clusters in the suite of 25 present-day Milky Way-mass cosmological zoom simulations from the E-MOSAICS project. We find that a median of $2.3$ and $0.3$ per cent of the mass in halo field stars formed in clusters and GCs, defined as clusters more massive than $5\times 10^3$ and $10^5~M_{\odot}$, respectively, with the $25$-$75$th percentiles spanning $1.9$-$3.0$ and $0.2$-$0.5$ per cent being caused by differences in the assembly histories of the host galaxies. Under the extreme assumption that no stellar cluster survives to the present day, the mass fractions increase to a median of $5.9$ and $1.8$ per cent. These small fractions indicate that the disruption of GCs plays a sub-dominant role in the build-up of the stellar halo. We also determine the contributed halo mass fraction that would present signatures of light-element abundance variations considered to be unique to GCs, and find that clusters and GCs would contribute a median of $1.1$ and $0.2$ per cent, respectively. We estimate the contributed fraction of GC stars to the Milky Way halo, based on recent surveys, and find upper limits of $2$-$5$ per cent (significantly lower than previous estimates), suggesting that models other than those invoking strong mass loss are required to describe the formation of chemically enriched stellar populations in GCs

THE IMPRINTS OF THE GALACTIC BAR ON THE THICK DISK WITH RAVE

We study the kinematics of a local sample of stars, located within a cylinder of $500\;{\rm pc}$ radius centered on the Sun, in the RAVE data set. We find clear asymmetries in the ${{v}_{R}}$ - ${{v}_{\phi }}$ velocity distributions of thin and thick disk stars: there are more stars moving radially outward for low azimuthal velocities and more radially inward for high azimuthal velocities. Such asymmetries have been previously reported for the thin disk as being due to the Galactic bar, but this is the first time that the same type of structures are seen in the thick disk. Our findings imply that the velocities of thick-disk stars should no longer be described by Schwarzschild's, multivariate Gaussian or purely axisymmetric distributions. Furthermore, the nature of previously reported substructures in the thick disk needs to be revisited as these could be associated with dynamical resonances rather than to accretion events. It is clear that dynamical models of the Galaxy must fit the 3D velocity distributions of the disks, rather than the projected 1D, if we are to understand the Galaxy fully

The properties, origin and evolution of stellar clusters in galaxy simulations and observations

Published onlineWe investigate the properties and evolution of star particles in two simulations of isolated spiral galaxies, and two galaxies from cosmological simulations. Unlike previous numerical work, where typically each star particle represents one ‘cluster’, for the isolated galaxies we are able to model features we term ‘clusters’ with groups of particles. We compute the spatial distribution of stars with different ages, and cluster mass distributions, comparing our findings with observations including the recent LEGUS survey. We find that spiral structure tends to be present in older (100s Myr) stars and clusters in the simulations compared to the observations. This likely reflects differences in the numbers of stars or clusters, the strength of spiral arms, and whether the clusters are allowed to evolve. Where we model clusters with multiple particles, we are able to study their evolution. The evolution of simulated clusters tends to follow that of their natal gas clouds. Massive, dense, long-lived clouds host massive clusters, whilst short-lived clouds host smaller clusters which readily disperse. Most clusters appear to disperse fairly quickly, in basic agreement with observational findings. We note that embedded clusters may be less inclined to disperse in simulations in a galactic environment with continuous accretion of gas on to the clouds than isolated clouds and correspondingly, massive young clusters which are no longer associated with gas tend not to occur in the simulations. Caveats of our models include that the cluster densities are lower than realistic clusters, and the simplistic implementation of stellar feedback.We thank the referee for a useful report. The calculations for this paper were performed primarily on the DiRAC machine ‘Complexity’, as well as the supercomputer at Exeter, which is jointly funded by STFC, the Large Facilities Capital Fund of BIS, and the University of Exeter. We would like to thank Michele Fumagalli for work putting together the LEGUS cluster catalogues. CLD and CGF acknowledge funding from the European Research Council for the FP7 ERC starting grant project LOCALSTAR. CGF thanks Ben Thompson for performing data reduction. DG kindly acknowledges financial support by the German Research Foundation (DFG) through grant GO 1659/3-2. Figures in this paper were produced using splash (Price 2007)

Extragalactic Globular Clusters and Galaxy Formation

Globular cluster (GC) systems have now been studied in galaxies ranging from dwarfs to giants and spanning the full Hubble sequence of morphological types. Imaging and spectroscopy with the Hubble Space Telescope and large ground-based telescopes have together established that most galaxies have bimodal color distributions that reflect two subpopulations of old GCs: metal-poor and metal-rich. The characteristics of both subpopulations are correlated with those of their parent galaxies. We argue that metal-poor GCs formed in low-mass dark matter halos in the early universe and that their properties reflect biased galaxy assembly. The metal-rich GCs were born in the subsequent dissipational buildup of their parent galaxies and their ages and abundances indicate that most massive early-type galaxies formed the bulk of their stars at early times. Detailed studies of both subpopulations offer some of the strongest constraints on hierarchical galaxy formation that can be obtained in the near-field.Comment: 74 pages, including 14 figures. In press for Annual Reviews of Astronomy and Astrophysic