Dependence of coronal X-ray emission on spot-induced brightness variations in cool main sequence stars^*

¹ Catania Astrophysical Observatory, National Institute for Astrophysics, Via S. Sofia 78, 95123 Catania, Italy
² Palermo Astronomical Observatory, National Institute for Astrophysics, Piazza del Parlamento 1, 90134 Palermo, Italy e-mail: nicola@oapa23.astropa.unipa.it
³ Department of Astronomy and Astrophysics, Villanova University, Villanova 19085, PA, USA e-mail: edward.guinan@villanova.edu
⁴ Department of Physics and Astronomy, University of Catania, Via S. Sofia 78, 95123 Catania, Italy e-mail: mrodono@ct.astro.it

Corresponding author: S. Messina, sme@ct.astro.it

Received: 7 February 2003
Accepted: 5 August 2003

Abstract

The maximum amplitude (A_max) of spot-induced brightness variations from long-term V-band photometry and the ratio L_X/L_bol between X-ray and bolometric luminosities are suitable indicators of the level of magnetic activity in the photosphere and in the corona of late-type stars, respectively. By using these activity indicators we investigate the dependence of coronal X-ray emission on the level of photospheric starspot activity in a homogeneous sample of low mass main sequence field and cluster stars of different ages (IC 2602, IC 4665, IC 2391, α Persei, Pleiades and Hyades). First, the activity-rotation connection at the photospheric level is re-analysed, as well as its dependence on spectral type and age. The upper envelope of increases monotonically with decreasing rotational period (P) and Rossby number (R₀) showing a break around 1.1 d that separates two rotation regimes where the starspot activity shows different behaviours. The –P and A_max–R₀ relations are fitted with linear, exponential and power laws to look for the function which best represents the trend of the data. The highest values of A_max are found among K-type stars and at the ages of α Persei and Pleiades. We also analyse the activity-rotation connection at the coronal level as well as its dependence on spectral type. The level of X-ray emission increases with increasing rotation rate up to a saturation level. The rotational period at which saturation occurs is colour-dependent and increases with advancing spectral type. Also the L_X/L_bol–P and L_X/L_bol–R₀ relations are fitted with linear, exponential and power laws to look for the best fitting function. Among the fastest rotating stars ( d) there is evidence of super-saturation. Also the highest values of L_X/L_bol are found among K-type stars. Finally, the photospheric-coronal activity connection is investigated by using for the first time the largest ever sample of light curve amplitudes as indicators of the magnetic filling factor. The activity parameters /L_bol and A_max are found to be correlated with each other, thus confirming the dependence of coronal activity on photospheric magnetic fields. More precisely, the L_X/L_bol–A_max distribution shows the presence of an upper envelope, which is constant at the L_X/ saturation level, and of a lower envelope. The best fit to the lower envelope is given by a power law with steepness decreasing from F-G to M spectral types. However, it is considered a tentative result, since the fit reduced chi-squares are large. Such spectral-type dependence may be related to a colour dependence of A_max on the total starspot filling factor, as well as to the coronal emission being possibly more sensitive to starspot activity variations in F- and G-type than in M-type stars. The L_X/L_bol–A_max mean values for each cluster in our sample decrease monotonically with increasing age, showing that the levels of photospheric and coronal activity evolve in time according to a single power law till the Sun's age.