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      Fundamental Stellar Astrophysics Revealed at Very High Angular Resolution

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          Abstract

          A short review of recent results in long-baseline optical interferometry pertaining to fundamental stellar parameters and the future possibilities this area over the next decade. Included are discussions of accurate stellar masses, links between asteroseismology and interferometry, precise radii and effective temperatures, limb darkening and convection, stellar rotation, and high angular resolution imaging.

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          Vega is a rapidly rotating star

          T. A. Pauls, C. Hummel, J. Armstrong … (2006)
          Vega, the second brightest star in the northern hemisphere, serves as a primary spectral type standard. While its spectrum is dominated by broad hydrogen lines, the narrower lines of the heavy elements suggested slow to moderate rotation, giving confidence that the ground-based calibration of its visibile spectrum could be safely extrapolated into the ultraviolet and near-infrared (through atmosphere models), where it also serves as the primary photometric calibrator. But there have been problems: the star is too bright compared to its peers and it has unusually shaped absorption line profiles, leading some to suggest that it is a distorted, rapidly rotating star seen pole-on. Here we report optical interferometric observations of Vega which detect the asymmetric brightness distribution of the bright, slightly offset polar axis of a star rotating at 93% of breakup speed. In addition to explaining the unusual brightness and line shape pecularities, this result leads to the prediction of an excess of near-infrared emission compared to the visible, in agreement with observations. The large temperature differences predicted across its surface call into question composition determinations, adding uncertainty to Vega's age and opening the possibility that its debris disk could be substantially older than previously thought.
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            Assessing the massive young Sun hypothesis to solve the warm young Earth puzzle

            Renu Malhotra, David A. Minton (2006)
            A moderately massive early Sun has been proposed to resolve the so-called faint early Sun paradox. We calculate the time-evolution of the solar mass that would be required by this hypothesis, using a simple parametrized energy-balance model for Earth's climate. Our calculations show that the solar mass loss rate would need to have been 2-3 orders of magnitude higher than present for a time on the order of ~2 Gy. Such a mass loss history is significantly at variance (both in timescale and in the magnitude of the mass loss rates) with that inferred from astronomical observations of mass loss in younger solar analogues. While suggestive, the astronomical data cannot completely rule out the possibility that the Sun had the required mass loss history; therefore, we also examine the effects of the hypothetical historical solar mass loss on orbital dynamics in the solar system, with a view to identifying additional tests of the hypothesis. Planetary and satellite orbits provide a few tests, but these are weak or non-unique.
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              First Results from the CHARA Array VII: Long-Baseline Interferometric Measurements of Vega Consistent with a Pole-On, Rapidly Rotating Star

              O. Absil, L. Sturmann, N. H. Turner … (2006)
              We have obtained high-precision interferometric measurements of Vega with the CHARA Array and FLUOR beam combiner in the K' band at projected baselines between 103m and 273m. The measured visibility amplitudes beyond the first lobe are significantly weaker than expected for a slowly rotating star characterized by a single effective temperature and surface gravity. Our measurements, when compared to synthetic visibilities and synthetic spectrophotometry from a Roche-von Zeipel gravity-darkened model atmosphere, provide strong evidence for the model of Vega as a rapidly rotating star viewed very nearly pole-on. Our best fitting model indicates that Vega is rotating at ~91% of its angular break-up rate with an equatorial velocity of 275 km/s. Together with the measured vsin(i), this velocity yields an inclination for the rotation axis of 5 degrees. For this model the pole-to-equator effective temperature difference is 2250 K, a value much larger than previously derived from spectral line analyses. The derived equatorial T_eff of 7900 K indicates Vega's equatorial atmosphere may be convective and provides a possible explanation for the discrepancy. The model has a luminosity of ~37 Lsun, a value 35% lower than Vega's apparent luminosity based on its bolometric flux and parallax, assuming a slowly rotating star. The model luminosity is consistent with the mean absolute magnitude of A0V stars. Our model predicts the spectral energy distribution of Vega as viewed from its equatorial plane; a model which may be employed in radiative models for the surrounding debris disk.
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                Author and article information

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                Publication date Created: 08 May 2009
                Article
                ArXiV ID: 0905.1353
                SO-VID: 76bf828b-fc26-4ced-8736-aecf16aae389
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                http://arxiv.org/licenses/nonexclusive-distrib/1.0/

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                Comments 14 pages, 5 figures, white paper submitted to the Astro2010 Decadal Survey
                Categories astro-ph.SR

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