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

Abstract

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, α 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, τ e , which we have used to investigate the relationship between the X-ray emission level and an X-ray-based Rossby number R e = P rot /τ e : we show that our empirical time scale τ e resembles the theoretical convective turnover time for 0.4 º M/M º 1.2, but it also has the same functional dependence on B − V as L −1/2 bol in the color range 0.5 º B − V º 1.5. Our results imply that -for non-saturated coronae -the L x -P rot relation is equivalent to the L x /L bol vs. R e relation.

X-ray emission regimes and rotation sequences in M 35

“…The X-ray emission from late-type stars in open clusters exhibits two kinds of dependences on stellar rotation (e.g. Patten & Simon 1996;Randich 2000;Feigelson et al 2003;Pizzolato et al 2003). Fast rotators show a relatively constant X-ray to bolometric luminosity ratio at the so-called (L X /L bol ) ≈ 10 −3 saturation level.…”

The Solar-Stellar Connection

“…Using such proxy it has been shown that magnetic activity increases with rotation rate and that above a given rotation rate (that depends on the stellar spectral type but is around Ros ∼ 0.1), stellar magnetic activity first saturates (Pizzolato et al 2003;Wright et al 2011) and then possibly "over-saturates" for extremely fast rotation rate such as the ones found in young solar-like T-Tauri stars (Feigelson and Montmerle 1999). One can thus simply seek to characterize how the saturation of the dynamo generated field can be linked to the saturation of solar-like star X-ray emissivity.…”

Long-term stellar variability

“…Moreover, the measured widths of the coronal forbidden lines are generally close to their thermal values (FWHM 40-90 km s −1 at T∼10 6.2 -10 7.0 K), except for some of the fastest rotating stars in the observed sample (AB Dor, 31 Com, and the G1 star of Capella) all showing broadening exceeding the convolution between the thermal and the rotational widths, consistently with excess turbulence or with emitting regions at ∼0.4-1.3 stellar radii from the photosphere. Right: X-ray to bolometric luminosity ratio vs. empirical Rossby number including field dwarfs (crosses) and cluster stars (squares) (from Pizzolato et al 2003).…”

“…Figure 3). Pizzolato et al (2003) have demonstrated that the critical rotation period, below which saturation occours, increases with decreasing stellar mass, suggesting that the X-ray emission in late-type main sequence stars can be considered as an effect depending on the properties of the convective region as well as an effect depending on the total energy budget available in the star.…”