Astronomy & Astrophysics

Free access article

A&A 397, 147-157 (2003)
DOI: 10.1051/0004-6361:20021560

The stellar activity-rotation relationship revisited: Dependence of saturated and non-saturated X-ray emission regimes on stellar mass for late-type dwarfs

N. Pizzolato¹, A. Maggio¹, G. Micela¹, S. Sciortino¹ and P. Ventura²

¹ INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
² INAF - Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monteporzio, Italy

(Received 14 May 2002 / Accepted 1 October 2002)

Abstract
We present the results of a new study on the relationship between coronal X-ray emission and stellar rotation in late-type main-sequence stars. We have selected a sample of 259 dwarfs in the B-V range 0.5-2.0, including 110 field stars and 149 members of the Pleiades, Hyades, $\alpha$ Persei, IC 2602 and IC 2391 open clusters. All the stars have been observed with ROSAT, and most of them have photometrically-measured rotation periods available. Our results confirm that two emission regimes exist, one in which the rotation period is a good predictor of the total X-ray luminosity, and the other in which a constant saturated X-ray to bolometric luminosity ratio is attained; we present a quantitative estimate of the critical rotation periods below which stars of different masses (or spectral types) enter the saturated regime. In this work we have also empirically derived a characteristic time scale, $\tau_{\rm e}$ , which we have used to investigate the relationship between the X-ray emission level and an X-ray-based Rossby number $R_{\rm e} = P_{\rm rot}/\tau_{\rm e}$ : we show that our empirical time scale $\tau_{\rm e}$ resembles the theoretical convective turnover time for $0.4 \lesssim M/M_{\odot} \lesssim 1.2$ , but it also has the same functional dependence on B-V as $L_{\rm bol}^$ in the color range $0.5 \lesssim B-V \lesssim 1.5$ . Our results imply that - for non-saturated coronae - the $L_{\rm x}$ - $P_{\rm rot}$ relation is equivalent to the $L_{\rm x}/L_{\rm bol}$ vs. $R_{\rm e}$ relation.

Key words: stars: activity -- stars: late-type -- X-rays: stars

Offprint request: N. Pizzolato, nicola@oapa23.astropa.unipa.it

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