Abstract
Near-infrared interferometers have recently imaged a number of rapidly
rotating A-type stars, finding levels of gravity darkening inconsistent
with theoretical expectations. Here, we present new imaging of both a
cooler star ß Cas (F2IV) and a hotter one ¿ Leo (B7V) using
the CHARA array and the MIRC instrument at the H band. Adopting a
solid-body rotation model with a simple gravity darkening prescription,
we modeled the stellar geometric properties and surface temperature
distributions, confirming that both stars are rapidly rotating and show
gravity darkening anomalies. We estimate the masses and ages of these
rapid rotators on L-R pol and H-R diagrams constructed for
non-rotating stars by tracking their non-rotating equivalents. The
unexpected fast rotation of the evolved sub-giant ß Cas offers a
unique test of the stellar core-envelope coupling, revealing quite
efficient coupling over the past ~0.5 Gyr. Lastly, we summarize all our
interferometric determinations of the gravity darkening coefficient for
rapid rotators, finding that none match the expectations from the widely
used von Zeipel gravity darkening laws. Since the conditions of the von
Zeipel law are known to be violated for rapidly rotating stars, we
recommend using the empirically derived ß = 0.19 for such stars
with radiation-dominated envelopes. Furthermore, we note that no
paradigm exists for self-consistently modeling heavily gravity-darkened
stars that show hot radiative poles with cool convective equators.
Original language | English |
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Number of pages | 68 |
Journal | The Astrophysical Journal |
Volume | 732 |
Issue number | 2 |
Publication status | Published - 1 May 2011 |