Fitzpatrick on “UV spectral synthesis of Vega” and the “A-star puzzle”

IUE high-resolution UV spectrum of Vega star.

An A-type main-sequence star is a main-sequence (hydrogen-burning) star of spectral type A and luminosity class V. These stars have spectra which are defined by strong hydrogen Balmer absorption lines, have masses from 1.4 to 2.1 times the mass of the Sun, and surface temperatures between 7,600 and 10,000 K. Examples include Altair, Sirius A, and Vega. These stars have since become known as the “superficially normal” stars, the poster child for such stars is Vega (HD 172167) The challenge to identify exactly what “normal” is for an A-star is the so-called “A-Star Puzzle,” the question of whether truly “normal” A-stars even exist! E.L. Fitzpatrick, “UV SPECTRAL SYNTHESIS OF VEGA,” The Astrophysical Journal 725: 2401, 2011 [ArXiv preprint 23 Nov. 2010], analyzes the UV spectrum (1280–3200 Å) of the “superficially normal” A-star Vega, as observed by the IUE satellite at nearly best possible resolution comparable. The UV spectrum is fitted remarkably well by a single-temperature synthetic spectrum based on LTE atmosphere models and a newly constructed UV line list. If Vega were a normal, equator-on, slow-rotating star, then its spectrum and our analysis would indicate a temperature of Teff ≈ 9550 K, surface gravity of log g ≈ 3.7, general surface metallicity of [m/H] ≈ −0.5, and a microturbulence velocity of vturb ≈ 2.0 km/s. These parameters must be regarded as representing averages across the observed hemisphere. Modeling the complex UV line spectrum has allowed him to determine the specific surface abundances for 17 different chemical elements, including CNO, the light metals, and the iron group elements. The main sequence A-stars continue to be a fascinating array of distinct, chemically peculiar objects. Further studies of A-stars can lead to insights into the physical processes producing the observed phenomena.

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