E. Baron

The Dartmouth Stellar Evolution Database

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, [α/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 M☉, 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 Web site at http://stellar.dartmouth.edu/~models/ where all tracks, isochrones, and additional files can be downloaded.

The NextGen Model Atmosphere Grid for 3000 ≤ Teff ≤ 10,000 K

We present our NextGen Model Atmosphere grid for low-mass stars for effective temperatures larger than 3000 K. These LTE models are calculated with the same basic model assumptions and input physics as the VLMS part of the NextGen grid so that the complete grid can be used, e.g., for consistent stellar evolution calculations and for internally consistent analysis of cool star spectra. This grid is also the starting point for a large grid of detailed NLTE model atmospheres for dwarfs and giants. The models were calculated from 3000 to 10,000 K (in steps of 200 K) for 3.5 ≤ log g ≤ 5.5 (in steps of 0.5) and metallicities of -4.0 ≤ [M/H] ≤ 0.0. We discuss the results of the model calculations and compare our results to the Kurucz grid. Some comparisons to standard stars like Vega and the Sun are presented and compared with detailed NLTE calculations.

The NEXTGEN Model Atmosphere Grid. II. Spherically Symmetric Model Atmospheres for Giant Stars with Effective Temperatures between 3000 and 6800 K

We present the extension of our NextGen model atmosphere grid to the regime of giant stars. The input physics of the models presented here is nearly identical to that of the NextGen dwarf atmosphere models; however, spherical geometry is used self-consistently in the model calculations (including the radiative transfer). We revisit the discussion of the effects of spherical geometry on the structure of the atmospheres and the emitted spectra and discuss the results of non-LTE calculations for a few selected models.

Evidence for a Spectroscopic Sequence among Type Ia Supernovae

In this Letter we present evidence for a spectral sequence among Type Ia supernovae (SN Ias). The sequence is based on the systematic variation of several features seen in the near-maximum light spectrum. This sequence is analogous to the recently noted photometric sequence among SN Ias which shows a relationship between the peak brightness of a SN Ia and the shape of its light curve. In addition to the observational evidence we present a partial theoretical explanation for the sequence. This has been achieved by producing a series of non-LTE synthetic spectra in which only the effective temperature is varied. The synthetic sequence nicely reproduces most of the differences seen in the observed one and presumably corresponds to the amount of 56Ni produced in the explosion.

Numerical solution of the expanding stellar atmosphere problem

Abstract In this paper we discuss numerical methods and algorithms for the solution of NLTE stellar atmosphere problems involving expanding atmospheres, e.g., found in novae, supernovae and stellar winds. We show how a scheme of nested iterations can be used to reduce the high dimension of the problem to a number of problems with smaller dimensions. As examples of these sub-problems, we discuss the numerical solution of the radiative transfer equation for relativistically expanding media with spherical symmetry, the solution of the multi-level nonLTE statistical equilibrium problem for extremely large model atoms, and our temperature correction procedure. Although modern iteration schemes are very efficient, parallel algorithms are essential in making large-scale calculations feasible, therefore we discuss some parallelization schemes that we have developed.

The ACS Survey of Galactic Globular Clusters. II. Stellar Evolution Tracks, Isochrones, Luminosity Functions, and Synthetic Horizontal-Branch Models

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 MV ~ 14 up to the tip of the red giant branch. These are complemented by a set of He-burning 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-Teff transformations, one synthetic and one semiempirical, 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 at the Dartmouth Stellar Evolution Database and the Multimission Archive at the Space Telescope Science Institute.

Parallel Implementation of the PHOENIX Generalized Stellar Atmosphere Program

We describe the parallel implementation of our generalized stellar atmosphere and non-LTE (NLTE) radiative transfer computer program PHOENIX. We discuss the parallel algorithms we have developed for radiative transfer, spectral line opacity, and NLTE opacity and rate calculations. Our implementation uses a multiple instruction-multiple data design based on a relatively small number of MPI library calls. We report the results of test calculations on a number of different parallel computers and discuss the results of scalability tests.

Low-luminosity Type II supernovae: spectroscopic and photometric evolution

In this paper we present spectroscopic and photometric observations for four core-collapsed supernovae (SNe), namely SNe 1994N, 1999br, 1999eu and 2001dc. Together with SN 1997D, we show that they form a group of exceptionally low-luminosity events. These SNe have narrow spectral lines (indicating low expansion velocities) and low luminosities at every phase (significantly lower than those of typical core-collapsed supernovae). The very-low luminosity during the 56 Co radioactive decay tail indicates that the mass of 56 Ni ejected during the explosion is much smaller (M Ni 2-8 x 10 -3 M ○. ) than the average (M Ni 6-10 x 10 -2 M ○. ). Two supernovae of this group (SN 1999br and SN 2001dc) were discovered very close to the explosion epoch, allowing us to determine the lengths of their plateaux (100 d) as well as establishing the explosion epochs of the other, less completely observed SNe. It is likely that this group of SNe represent the extreme low-luminosity tail of a single continuous distribution of Type II plateau supernovae events. Their kinetic energy is also exceptionally low. Although an origin from low-mass progenitors has also been proposed for low-luminosity core-collapsed SNe, recent work provides evidence in favour of the high-mass progenitor scenario. The incidence of these low-luminosity SNe could be as high as 4-5 per cent of all Type II SNe.

A Comparative Study of the Absolute Magnitude Distributions of Supernovae

The Asiago Supernova Catalog is used to carry out a comparative study of supernova absolute magnitude distributions. An overview of the absolute magnitudes of the supernovae in the current observational sample is presented, and the evidence for subluminous and overluminous events is examined. The fraction of supernovae that are subluminous (MB > -15) appears to be higher (perhaps much higher) than but it remains very uncertain. The fraction that are overluminous (MB > -20) is lower (probably much lower) than 0.01. The absolute magnitude distributions for each supernova type, restricted to events within 1 Gpc, are compared. Although these distributions are affected by observational bias in favor of the more luminous events, they are useful for comparative studies. We find mean absolute blue magnitudes (for H0 = 60) of -19.46 for normal Type Ia supernovae (SNe Ia), -18.04 for SNe Ibc, -17.61 and -20.26 for normal and bright SNe Ibc considered separately, -18.03 for SNe II-L, -17.56 and -19.27 for normal and bright SNe II-L considered separately, -17.00 for SNe II-P, and -19.15 for SNe IIn.

Metallicity Effects in Non-LTE Model Atmospheres of Type Ia Supernovae

We have calculated a grid of photospheric phase atmospheres of Type Ia supernovae (SNe Ia) with metallicities from 10 times to 1/30 the solar metallicity in the C+O layer of the deflagration model, W7. We have modeled the spectra using the multipurpose non-LTE model atmosphere and spectrum synthesis code PHOENIX. We show models for the epochs 7, 10, 15, 20, and 35 days after explosion. When compared to observed spectra obtained at the approximately corresponding epochs, these synthetic spectra fit reasonably well. The spectra show variation in the overall level of the UV continuum with lower fluxes for models with higher metallicity in the unburned C+O layer. This is consistent with the classical surface cooling and line-blocking effect due to metals in the outer layers of C+O. The UV features also move consistently to the blue with higher metallicity, demonstrating that they are forming at shallower and faster layers in the atmosphere. The potentially most useful effect is the blueward movement of the Si II feature at 6150 A with increasing C+O layer metallicity. We also demonstrate the more complex effects of metallicity variations by modifying the 54Fe content of the incomplete burning zone in W7 at maximum light. We briefly address some shortcomings of the W7 model when compared to observations. Finally, we identify that the split in the Ca H+K feature produced in W7 and observed in some SNe Ia is due to a blending effect of Ca II and Si II and does not necessarily represent a complex abundance or ionization effect in Ca II.