Research Note

Interferometric radius and limb darkening of the asteroseismic red giant η Serpentis with the CHARA Array

¹ European Organisation for Astronomical Research in the Southern Hemisphere (ESO) Alonso de Córdova 3107, Casilla 19001, Santiago 19, Chile e-mail: Antoine.Merand@eso.org
² LESIA, Observatoire de Paris, CNRS UMR 8109, UPMC, Université Paris Diderot, 5 Place Jules Janssen, 92195 Meudon, France
³ Groupe de Recherche en Astronomie et Astrophysique du Languedoc (GRAAL), Université Montpellier II, CNRS UMR 5024, 34095 Montpellier Cedex 05, France
⁴ Center for High Angular Resolution Astronomy, Georgia State University, PO Box 3965, Atlanta, Georgia 30302-3965, USA
⁵ National Optical Astronomy Observatories, 950 North Cherry Avenue, Tucson, AZ 85719, USA

Received: 18 March 2009
Accepted: 4 May 2010

Abstract

Context. The radius of a star is a very important constraint to evolutionary models, particularly when combined with asteroseismology. Diameters can now be measured interferometrically with great precision (better than 1%), but the center-to-limb darkening (CLD) remains a potential source of bias. Measuring this bias is possible by completely resolving the star using long-baseline interferometry, and has only been achieved for a handful of stars.

Aims. The red giant η Ser (K0III-IV) is a particularly interesting target, as asteroseismic oscillations have recently been detected in this star by spectroscopy. We aim to measure its radius with high accuracy, debiased from limb darkening, in order to bring new constraints to its models.

Methods. We obtained interferometric observations of η Ser in the near-infrared using the CHARA/FLUOR instrument, in particular in the so-called second lobe of visibility in order to constrain the CLD and debias our diameter estimation.

Results. The limb darkened angular diameter of η Ser is 2.944 ± 0.010 mas (using spherical photosphere models PHOENIX and MARCS for the limb darkening), that converts into a radius of 5.897 ± 0.028  with the Hipparcos parallax. Thanks to a precise visibility measurement in the second lobe of the visibility function of η Ser and a one-parameter limb-darkened visibility profile, we were able to show that the photosphere models have the best agreement possible.

Conclusions. Our limb darkening measurement of η Ser is in agreement with existing atmosphere models of this star, with a slightly better agreement for models using spherical geometry. This is a strong indication that interferometric angular diameter measurements for red giants, corrected for the CLD using models, are unbiased at a very small level (a fraction of 1%). In particular, this strengthens our confidence in the existing catalogues of calibrator stars for interferometry that are based on giant stars similar to η Ser. The high accuracy of our measurement brings a new and strong constraint for the asteroseismic modeling of this star.