arXiv subjects

C. Ian Short

Publications and source records attributed to C. Ian Short.

23 records · Page 2Linked to original sources

arXiv2012

Modeling the near-UV band of GK stars, Paper II: NLTE models

We present a grid of atmospheric models and synthetic spectral energy distributions (SEDs) for late-type dwarfs and giants of solar and 1/3 solar metallicity with many opacity sources computed in self-consistent Non-Local Thermodynamic Equilibrium (NLTE), and compare them to the LTE grid of Short & Hauschildt (2010) (Paper I). We describe, for the first time, how the NLTE treatment affects the thermal equilibrium of the atmospheric structure (T(tau) relation) and the SED as a finely sampled function of Teff, log g, and [A/H] among solar metallicity and mildly metal poor red giants. We compare the computed SEDs to the library of observed spectrophotometry described in Paper I across the entire visible band, and in the blue and red regions of the spectrum separately. We find that for the giants of both metallicities, the NLTE models yield best fit Teff values that are ~30 to 90 K lower than those provided by LTE models, while providing greater consistency between \log g values, and, for Arcturus, Teff values, fitted separately to the blue and red spectral regions. There is marginal evidence that NLTE models give more consistent best fit Teff values between the red and blue bands for earlier spectral classes among the solar metallicity GK giants than they do for the later classes, but no model fits the blue band spectrum well for any class. For the two dwarf spectral classes that we are able to study, the effect of NLTE on derived parameters is less significant.

astro-ph.SR↗

arXiv2011

Spectral Type and Radial Velocity Variations in Three SRC Variables

SRC variables are M supergiants, precursors to Type II supernovae, that vary in brightness with moderately regular periods of order 100-1000 days. Although identified as pulsating stars that obey their own period-luminosity relation, few have been examined in enough detail to follow the temperature and spectral changes that they undergo during their long cycles. The present study examines such changes for several SRC variables revealed by CCD spectra obtained at the Dominion Astrophysical Observatory (DAO) during 2005-2009, as well as by archival spectra from the DAO (and elsewhere) for some stars from the 1960s to 1980s, and Cambridge radial velocity spectrometer measures for Betelgeuse. Described here is our classification procedure and information on the spectral type and radial velocity changes in three of the stars. The results provide insights into the pulsation mechanism in M supergiants.

astro-ph.SR↗

arXiv2010

Modeling the near-UV band of GK stars, Paper I: LTE models

We present a grid of LTE atmospheric models and synthetic spectra that cover the spectral class range from mid-G to mid-K, and luminosity classes from V to III, that is dense in Teff sampling (Delta Teff=62.5 K), for stars of solar metallicity and moderately metal poor scaled solar abundance ([A/H]=0.0 and -0.5). All models have been computed with two choices of atomic line list: a) the "big" line lists of Kurucz (1992) that best reproduce the broad-band solar blue and near UV flux level, and b) the "small" lists of Kurucz & Peytremann (1975) that provide the best fit to the high resolution solar blue and near-UV spectrum. We compare our model SEDs to a sample of stars carefully selected from the large catalog of uniformly re-calibrated spectrophotometry of Burnashev (1985) with the goal of determining how the quality of fit varies with stellar parameters, especially in the historically troublesome blue and near-UV bands. We confirm that our models computed with the "big" line list recover the derived Teff values of the PHOENIX NextGen grid, but find that the models computed with the "small" line list provide greater internal self-consistency among different spectral bands, and closer agreement with the empirical Teff scale of Ramirez & Melendez (2005), but not to the interferometrically derived Teff values of Baines et al. (2010). We find no evidence that the near UV band discrepancy between models and observations for Arcturus (alpha Boo) reported by Short & Hauschildt (2003 and 2009) is pervasive, and that Arcturus may be peculiar in this regard.

astro-ph.SR↗

arXiv2008

Non-LTE modeling of the near UV band of late-type stars

We investigate the ability of both LTE and Non-LTE models to fit the near UV band absolute flux distribution and individual spectral line profiles of three standard stars for which high quality spectrophotometry and high resolution spectroscopy are available: The Sun (G2 V), Arcturus (K2 III), and Procyon (F5 IV-V). We investigate 1) the effect of the choice of atomic line list on the ability of NLTE models to fit the near UV band flux level, 2) the amount of a hypothesized continuous thermal absorption extinction source required to allow NLTE models to fit the observations, and 3) the semi-empirical temperature structure required to fit the observations with NLTE models and standard continuous near UV extinction. We find that all models that are computed with high quality atomic line lists predict too much flux in the near UV band for Arcturus, but fit the warmer stars well. The variance among independent measurements of the solar irradiance in the near UV is sufficiently large that we cannot definitely conclude that models predict too much near UV flux, in contrast to other recent results. We surmise that the inadequacy of current atmospheric models of K giants in the near UV band is best addressed by hypothesizing that there is still missing continuous thermal extinction, and that the missing near UV extinction becomes more important with decreasing effective temperature for spectral classes later than early G, suggesting a molecular origin.

astro-ph↗

arXiv1999

Massive Multi-species, Multi-level NLTE Model Atmospheres for Novae in Outburst

We have used our PHOENIX multi-purpose model atmosphere code to calculate atmospheric models that represent novae in the optically thick wind phases of their outburst. We have improved the treatment of NLTE effects by expanding the number of elements that are included in the calculations from 15 to 19, and the number of ionization stages from 36 to 87. The code can now treat a total of 10713 levels and 102646 lines in NLTE. Aluminum, P, K, and Ni are included for the first time in the NLTE treatment and most elements now have at least the lowest six ionization stages included in the NLTE calculation. We have investigated the effects of expanded NLTE treatment on the chemical concentration of astrophysically significant species in the atmosphere, the equilibrium structure of the atmosphere, and the emergent flux distribution. Although we have found general qualitative agreement with previous, more limited NLTE models, the expanded NLTE treatment leads to significantly different values for the size of many of the NLTE deviations. In particular, for the hottest model presented here (effective temperature of 35000 K), for which NLTE effects are largest, we find that the expanded NLTE treatment reduces the NLTE effects for these important variables: neutral Hydrogen concentration, pressure structure, and emergent far UV flux. Moreover, we find that the addition of new NLTE species may greatly affect the concentration of species that were already treated in NLTE, so that, generally, all species that contribute significantly to the electron reservoir or to the total opacity, or whose line spectrum overlaps or interlocks with that of a species of interest, must be treated in NLTE to insure an accurate result for any particular species.

astro-ph↗