Levels of coronal and chromospheric activity in late-type stars and various types of dynamo waves

Abstract

We analyze the X-ray emission and chromospheric activity of Sun-like stars of F, G, and K spectral classes (late-type stars) studied in the framework of the HK project. More powerful coronas are possessed by stars displaying irregular variations of their chromospheric emission, while stars with cyclic activity are characterized by comparatively modest X-ray luminosities and ratios of the X-ray to bolometric luminosity L X /L bol . This indicates that the nature of processes associated with magnetic-field amplification in the convective envelope changes appreciably in the transition from small to large dynamo numbers, directly affecting the character of the (α-Ω) dynamo. Due to the strong dependence of both the dynamo number and the Rossby number on the speed of axial rotation, earlier correlations found between various activity parameters and the Rossby number are consistent with our conclusions. Our analysis makes it possible to draw the first firm conclusions about the place of solar activity among analogous processes developing in active late-type stars.

Starspot activity in late stars: Methods and results

“…The spotting cycle is also rather poorly correlated with chromospheric emission in the Ha line (LQ Hya, VY Ari, EV Lac [42,[64][65][66]). This is probably because these stars are substantially younger than the sun and their cyclical activity has not yet been established [79]. The total radiation losses from the outer atmospheres thus turned out to be substantially less than the radiation flux deficit.…”

Magnetic cycles of Sun-like stars with different levels of coronal and chromospheric activity — comparison with the Sun

“…Bruevich et al (2001) noted the difference between stars of "Excellent", "Good", "Fair" and "Poor" classes from a position of presence and degree of development of under photospheric convective zones of stars of different color index (B − V ).…”

“…In Write et al (2011) there was done the convertation of all X-ray luminosities of the stars from the catalogue of 824 stars to the ROSAT 0.1-2.4 keV energy band. The selected data of X-ray luminosities L X /L bol of 80 HK-project stars was also taken from the ROSAT All-Sky Survey (Bruevich et al 2001). This sample also includes the Sun using the values of logL X /L bol = −6.88 (for medium level of activity) and P rot = 26.09.…”

“…The Mount Wilson HK project observational data allow us to study the Sun-like cyclic activity of stars simultaneously with their chromospheric and coronal activity. The selected data of X-ray luminosities logL X /L Bol of 80 HK-project stars were taken from the ROSAT All-Sky Survey, see Bruevich et al (2001).…”

“…This indicates that the nature of processes associated with the magnetic-field amplification in the convective envelope changes appreciably in the transition from small to large dynamo numbers, directly affecting the character of the (α − Ω) dynamo. Due to the strong dependence of both the dynamo number and the Rossby number on the speed of axial rotation, earlier correlations were found between various activity parameters and the Rossby number in Bruevich et al (2001).…”