The ever-expanding depth and quality of photometric and spectroscopic observations of stellar populations increase the need for theoretical models in regions of age-composition parameter space that are largely unexplored at present. Stellar evolution models that employ the most advanced physics and cover a wide range of compositions are needed to extract the most information from current observations of both resolved and unresolved stellar populations. The Dartmouth Stellar Evolution Database is a collection of stellar evolution tracks and isochrones that spans a range of [Fe/H] from -2.5 to +0.5, [alpha/Fe] from -0.2 to +0.8 (for [Fe/H] <=0) or +0.2 (for [Fe/H] >0), and initial He mass fractions from Y=0.245 to 0.40. Stellar evolution tracks were computed for masses between 0.1 and 4 Msun, allowing isochrones to be generated for ages as young as 250 Myr. For the range in masses where the core He flash occurs, separate He-burning tracks were computed starting from the zero age horizontal branch. The tracks and isochrones have been transformed to the observational plane in a variety of photometric systems including standard UBV(RI)c, Stromgren uvby, SDSS ugriz, 2MASS JHKs, and HST ACS-WFC and WFPC2. The Dartmouth Stellar Evolution Database is accessible through a website at http://stellar.dartmouth.edu/~models/ where all tracks, isochrones, and additional files can be downloaded.Comment: 37 pages, 11 figures, accepted by ApJ

We describe here the most ambitious survey currently planned in the optical, the Large Synoptic Survey Telescope (LSST). The LSST design is driven by four main science themes: probing dark energy and dark matter, taking an inventory of the solar system, exploring the transient optical sky, and mapping the Milky Way. LSST will be a large, wide-field ground-based system designed to obtain repeated images covering the sky visible from Cerro Pachón in northern Chile. The telescope will have an 8.4 m (6.5 m effective) primary mirror, a 9.6 deg 2 field of view, a 3.2-gigapixel camera, and six filters (ugrizy) covering the wavelength range 320-1050 nm. The project is in the construction phase and will begin regular survey operations by 2022. About 90% of the observing time will be devoted to a deep-wide-fast survey mode that will uniformly observe a 18,000 deg 2 region about 800 times (summed over all six bands) during the anticipated 10 yr of operations and will yield a co-added map to r∼27.5. These data will result in databases including about 32 trillion observations of 20 billion galaxies and a similar number of stars, and they will serve the majority of the primary science programs. The remaining 10% of the observing time will be allocated to special projects such as Very Deep and Very Fast time domain surveys, whose details are currently under discussion. We illustrate how the LSST science drivers led to these choices of system parameters, and we describe the expected data products and their characteristics.

The ACS Survey of Galactic Globular Clusters, an HST Treasury Project, will deliver high quality, homogeneous photometry of 65 globular clusters. This paper introduces a new collection of stellar evolution tracks and isochrones suitable for analyzing the ACS Survey data. Stellar evolution models were computed at [Fe/H]= -2.5, -2.0, -1.5, -1.0, -0.5, and 0; [α/Fe]= -0.2, 0, 0.2, 0.4, 0.6, and 0.8; and three initial He abundances for masses from 0.1 to 1.8 M ⊙ and ages from 2 to 15 Gyr. Each isochrone spans a wide range in luminosity from M V ∼14 -2up to the tip of the red giant branch. These are complemented by a set of Heburning tracks that extend from the zero age horizontal branch to the onset of thermal pulsations on the asymptotic giant branch. In addition, a set of computer programs are provided that make it possible to interpolate the isochrones in [Fe/H], generate luminosity functions from the isochrones, and create synthetic horizontal branch models. The tracks and isochrones have been converted to the observational plane with two different color-T eff transformations, one synthetic and one semi-empirical, in ground-based B, V, and I, and F606W and F814W for both ACS-WFC and WFPC2 systems. All models and programs presented in this paper are available from http://stellar.dartmouth.edu/∼models/ and the Multimission Archive at the Space Telescope Science Institute (MAST; http://archive.stsci.edu).

Integrated light from distant galaxies is often compared to stellar population models via the equivalent widths of spectral features -spectral indices -whose strengths rely on the abundances of one or more elements. Such comparisons hinge not only on the overall metal abundance, but also on relative abundances. Studies have examined the influence of individual elements on synthetic spectra but little has been done to address similar issues in the stellar evolution models that underlie most stellar population models. Stellar evolution models will primarily be influenced by changes in opacities. In order to explore this issue in detail, 12 sets of stellar evolution tracks and isochrones have been created at constant heavy element mass fraction Z that self-consistently account for varying heavy element mixtures. These sets include scaled-solar, -enhanced, and individual cases where the elements C, N, O, Ne, Mg, Si, S, Ca, Ti, and Fe have been enhanced above their scaled-solar values. The variations that arise between scaled-solar and the other cases are examined with respect to the H-R diagram and main-sequence lifetimes.

We show that in order to minimize the uncertainties in the N and O abundances of low-mass, low-metallicity (O/ H 1/5 solar) emission-line galaxies, it is necessary to employ separate parameterizations for inferring T e (N + ) and T e (O + ) from T e (O +2 ). In addition, we show that for the above systems, the ionization correction factor (ICF ) for obtaining N/O from N + /O + , where the latter is derived from optical emission-line flux ratios, is ICF h i ¼ 1:08 AE 0:09. These findings are based on state-of-the-art single-star H ii region simulations, employing our own modeled stellar spectra as input. Our models offer the advantage of having matching stellar and nebular abundances. In addition, they have O/ H as low as 1/50 solar (lower than any past work), as well as log ( N/O) and log (C/O) fixed at characteristic values of À1.46 and À0.7, respectively. The above results were used to rederive N and O abundances for a sample of 68 systems with 12 þ log (O/H) 8:1, whose dereddened emission-line strengths were collected from the literature. The analysis of the log ( N/O) versus 12 þ log (O/ H ) diagram of the above systems shows that (1) the largest group of objects forms the well-known N/O plateau with a value for the mean (and its statistical error) of À1:43 þ0:0084 À0:0085 , (2) the objects are distributed within a range in log ( N/O) of À1.54 to À1.27 in Gaussian fashion around the mean with a standard deviation of ¼ þ0:071 À0:084 , and (3) a 2 analysis suggests that only a small amount of the observed scatter in log ( N /O) is intrinsic.

The first paper in this series explored the effects of altering the chemical mixture of the stellar population on an element by element basis on stellar evolutionary tracks and isochrones to the end of the red giant branch. This paper extends the discussion by incorporating the fully consistent synthetic stellar spectra with those isochrone models in predicting integrated colors, Lick indices, and synthetic spectra. Older populations display element ratio effects in their spectra at higher amplitude than younger populations. In addition, spectral effects in the photospheres of stars tend to dominate over effects from isochrone temperatures and lifetimes, but, further, the isochrone-based effects that are present tend to fall along the age-metallicity degeneracy vector, while the direct stellar spectral effects usually show considerable orthogonality.

The Virtual Atomic and Molecular Data Centre (VAMDC) Consortium is a worldwide consortium which federates atomic and molecular databases through an e-science infrastructure and an organisation to support this activity. About 90% of the inter-connected databases handle data that are used for the interpretation of astronomical spectra and for modelling in many fields of astrophysics. Recently the VAMDC Consortium has connected databases from the radiation damage and the plasma communities, as well as promoting the publication of data from Indian institutes. This paper describes how the VAMDC Consortium is organised for the optimal distribution of atomic and molecular data for scientific research. It is noted that the VAMDC Consortium strongly advocates that authors of research papers using data cite the original experimental and theoretical papers as well as the relevant databases.

Aims. We study the influence of chemi-ionization in H * (n) + H(1s) collisions and inverse chemi-recombination processes on the population of higher levels and consequently on profiles of hydrogen lines in the atmospheres of late type (M) stars. Methods. Modeling, using general stellar atmosphere code PHOENIX to reveal the importance of the inclusion of such processes. Results. We demonstrate for the first time observationally detectable effects of these chemi-processes on stellar spectra. Conclusions. It is very important to include chemi-ionization and chemi-recombination processes in modeling of atmospheres of late type stars, especially if one wants to use line profiles for diagnostics of stellar photospheres and lower chromospheres.