Subphotospheric convection and magnetic activity dependence on metallicity and age: Models and tests

¹ Dipartimento di Scienze Fisiche ed Astronomiche, Università di Palermo, Sezione di Astronomia, Piazza del Parlamento 1, 90134 Palermo, Italy
² Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monteporzio, Italy
³ Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy

Corresponding author: N. Pizzolato, nicola@oapa23.astropa.unipa.it

Received: 9 February 2001
Accepted: 27 April 2001

Abstract

We present an extensive study on the dependence of the convective turnover time () on the stellar metallicity and age for main-sequence stars of masses 0.6-1.6 . To this aim we have used and compared predictions by stellar models based on the classical Mixing Length Theory and models incorporating a Full Spectrum of Turbulence treatment of subphotospheric convection. We show that the metallicity effect is relevant for dG stars but negligible for dK stars, while stellar age is important when computing the turnover times for red dwarfs younger than yr. A scatter by up to a factor 3 could be spuriously introduced in the activity vs. Rossby number relationships if such effects are neglected. We have tested our model predictions on the metallicity effect by comparison with the observed median X-ray luminosities for dG and dK stars in the open clusters NGC 2516, the Pleiades, and Blanco 1, having similar age but likely different metal contents. We show that our model predictions, taking into account also the dependence of the coronal plasma emissivity on the metal abundance, are in agreement with the observations in all cases except for the dK stars in the metal-rich cluster Blanco 1, where our assumption of a rotation period distribution similar to those of the other cluster need to be confirmed by future observations.