HD 179821 (V1427 Aql, IRAS 19114+0002) – a massive post-red supergiant star?

Abstract

We have derived elemental abundances of a remarkable star, HD 179821, with unusual composition (e.g. [Na/Fe]=1.0±0.2 dex) and extra-ordinary spectral characteristics. Its metallicity at [Fe/H]=0.4 dex places it among the most metal-rich stars yet analyzed. The abundance analysis of this luminous star is based on high resolution and high quality (S/N≈120-420) optical echelle spectra from McDonald Observatory and Special Astronomy Observatory. The data includes five years of observations over twenty-one epochs. Standard 1D LTE analysis provides a fresh determination of the atmospheric parameters over all epochs: T eff = 7350±200 K, log g= +0.6±0.3, and a microturbulent velocity ξ = 6.6±1. Eu is overabundant relative to the anticipated [X/Fe] ≈ 0.0. Some peculiarities of its optical spectrum (e.g. variability in the spectral line shapes) is noticed. This includes the line profile variations for Hα line. Based on its estimated luminosity, effective temperature and surface gravity, HD 179821 is a massive star evolving to become a red supergiant and finally a Type II supernova.

LIME: Semiautomated line measurement and identification from stellar spectra

“…Thus, the manual identifications in [5] by us using the revised multiplet table as an initial guide and carrying out a systematic search that made use of LEP and gf-values were confirmed and proved to be accurate. The difference in measured line positions by LIME-IDENT was in the order of 0.01 A.…”

“…Comparison of measured EWs by LIME-EMS to [9] and [10] (see columns for 15Oct97 and 13Sep00 in table A1 of [5]) indicates significant differences in measured EWs in previous analyses (Figures 9a and 9b). For instance, the EWs measured in [9] in the McDonald spectrum (15 October 1997) appear to be systematically lower when compared to LIME-EMS measurements.…”

“…Prior to an interactive session by LIME for line measurement, the following standard steps should be followed: bias correction (step 1), cosmic-ray removal (step 2), flat fielding (step 3), scattering light subtraction (step 4), wavelength calibration (step 5), continuum normalization (step 6; blaze correction), concatenation of spectral orders to a single spectrum (for ECHELLE spectrum only; step 7), and correction for heliocentric velocity (step 8). The first five steps are essential in each spectroscopic analysis and are performed in a standard way, for instance in the same manner as in [2][3][4][5].…”

“…Similar findings were also seen for other atomic species; for instance, the line reported as an Ni I line at 6086.29Å in [9] was, in fact, a C I blend. Furthermore, our analysis of their spectra with a LIME measured line list provided 400 K hotter temperature and 0.3 dex greater metallicity ([Fe/H]) for the star (see [5] for details). Our suggestion is that the differences in model parameters likely emanate from imperfectness of the line list for Fe lines reported by [9].…”

“…Given the estimated uncertainties in abundances, the LIME-EMS measured EWs for LIME-IDENT identified lines in the Hobby-Eberly spectrum of the star led to a fair agreement in abundances by [13] for HD 179821 (see table 9 of [5]). It should equally be noted that in the process of comparing abundances to those reported in the literature, atomic data compilation for the analysis constitutes a very important and crucial stage of abundance analysis.…”