Properties of minimum-flux coronae in dwarfs and giants

Journal Article · Fri Oct 01 04:00:00 EDT 1976 · Astrophys. J.; (United States)

DOI:https://doi.org/10.1086/154706· OSTI ID:7332186

Mullan, D J

Using a method due to Hearn, we examine the properties of minimum-flux coronae in dwarfs and giants. If the fraction phi of the total stellar luminosity which is used to heat the corona is equal to the solar value phi/sub s/, then red dwarfs must have coronae that are cooler than the solar corona: in UV Ceti, for example, the coronal temperature is a factor 3 less than in the Sun. This is consistent with an independent estimate of coronal temperature in a flare star. If phi=phi/sub s/, main-sequence stars hotter than the Sun have coronae which are hotter than the solar corona. Soft X-rays from Sirius suggest that the coronal temperature in Sirius is indeed hotter than the Sun by a factor of about 40 percent. Giants show an even more marked decrease in coronal temperature at later spectral type than do the dwarfs. We suggest that the reason for the presence of O V emission in ..beta.. Gem and O VI emission in ..cap alpha.. Aur, and the absence of O V emission in ..cap alpha.. Boo and ..cap alpha.. Tau, is that the coronae in the latter two stars are cooler (rather than hotter, as McClintock et al. have suggested) than in the former two. Our results explain why it is more likely that mass loss has been detected in ..cap alpha.. Aur and ..cap alpha.. Boo, but not in ..cap alpha.. Tau or ..beta.. Gem. Using a simple flare model, we show that flares in both a dwarf star (UV Ceti) and a giant (..cap alpha.. Aur) were initiated not in the corona, but in the transition region. (AIP)

• Research Organization: Bartol Research Foundation of The Franklin Institute, Swarthmore, Pennsylvania 19081
• OSTI ID: 7332186
• Journal Information: Astrophys. J.; (United States), Journal Name: Astrophys. J.; (United States) Vol. 209:1; ISSN ASJOA
• Country of Publication: United States
• Language: English

Related Subjects

640102* -- Astrophysics & Cosmology-- Stars & Quasi-Stellar
Radio & X-Ray Sources
71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
ATMOSPHERES
DWARF STARS
GIANT STARS
MAIN SEQUENCE STARS
MASS TRANSFER
MATHEMATICAL MODELS
RADIATION FLUX
SPECTRA
STAR MODELS
STARS
STELLAR ATMOSPHERES
STELLAR FLARES
STELLAR WINDS
X-RAY SPECTRA