Evidence of radius inflation in stars approaching the slow-rotator sequence^⋆

¹ Università di Catania, Dipartimento di Fisica e Astronomia, Sezione Astrofisica, via S. Sofia 78, 95123 Catania, Italy
e-mail: a.lanzafame@unict.it
² INAF–Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
³ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany

Received: 1 May 2016
Accepted: 4 September 2016

Abstract

Context. Average stellar radii in open clusters can be estimated from rotation periods and projected rotational velocities under the assumption that the spin axis has a random orientation. These estimates are independent of distance, interstellar absorption, and models, but their validity can be limited by lacking data (truncation) or data that only represent upper or lower limits (censoring).

Aims. We present a new statistical analysis method to estimate average stellar radii in the presence of censoring and truncation.

Methods. We used theoretical distribution functions of the projected stellar radius Rsini to define a likelihood function in the presence of censoring and truncation. Average stellar radii in magnitude bins were then obtained by a maximum likelihood parametric estimation procedure.

Results. This method is capable of recovering the average stellar radius within a few percent with as few as aboutten measurements. Here we apply this for the first time to the dataset available for the Pleiades. We find an agreement better than ≈10 percent between the observed R vs. M_K relationship and current standard stellar models for 1.2 ≥ M/M_⊙ ≥ 0.85 with no evident bias. Evidence of a systematic deviation at 2σ level are found for stars with 0.8 ≥ M/M_⊙ ≥ 0.6 that approach the slow-rotator sequence. Fast rotators (P < 2 d) agree with standard models within 15 percent with no systematic deviations in the whole 1.2 ≳ M/M_⊙ ≳ 0.5 range.

Conclusions. The evidence of a possible radius inflation just below the lower mass limit of the slow-rotator sequence indicates a possible connection with the transition from the fast- to the slow-rotator sequence.