Astronomy & Astrophysics

Free access article

Issue		A&A Volume 410, Number 2, November I 2003
Page(s)		671 - 684
Section		Formation and evolution of planetary systems
DOI		10.1051/0004-6361:20031203

A&A 410, 671-684 (2003)
DOI: 10.1051/0004-6361:20031203

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

S. Messina¹, N. Pizzolato², E. F. Guinan³ and M. Rodonò⁴

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

(Received 7 February 2003 / Accepted 5 August 2003 )

Abstract
The maximum amplitude ( $A_{\rm max}$ ) of spot-induced brightness variations from long-term V-band photometry and the ratio $L_{\rm X}$ / $L_{\rm bol}$ 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 , $\alpha$ 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 $A_{\rm max}$ 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 $A_{\rm max}$ - P and $A_{\rm max}$ - 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 $A_{\rm max}$ are found among K-type stars and at the ages of $\alpha$ 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 $L_{\rm X}$ / $L_{\rm bol}$ - P and $L_{\rm X}$ / $L_{\rm bol}$ - R₀ relations are fitted with linear, exponential and power laws to look for the best fitting function. Among the fastest rotating stars ( $P\le 0.3$ d) there is evidence of super-saturation. Also the highest values of $L_{\rm X}$ / $L_{\rm bol}$ 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 $L_{\rm X}$ / $L_{\rm bol}$ and $A_{\rm max}$ are found to be correlated with each other, thus confirming the dependence of coronal activity on photospheric magnetic fields. More precisely, the $L_{\rm X}$ / $L_{\rm bol}$ - $A_{\rm max}$ distribution shows the presence of an upper envelope, which is constant at the $L_{\rm X}$ / $L_{\rm bol} \simeq -3.0$ 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 $A_{\rm max}$ 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 $L_{\rm X}$ / $L_{\rm bol}$ - $A_{\rm max}$ 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.

Key words: stars: activity -- stars: late-type -- stars: magnetic fields -- stars: starspots -- X-rays: stars

Offprint request: S. Messina, sme@ct.astro.it

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