ASCA X-ray observations of the super-metal-rich stars 30 Ari and eta Boo

Details ASCA X-ray observations of the super-metal-rich stars 30 Ari and eta Boo ASCA X-ray observations of the super-metal-rich stars 30 Ari and eta Boo

Dec 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999a%26a...352..248m&link_type=abstract

Astronomy and Astrophysics, v.352, p.248-256 (1999)

Astronomy and Astrophysics

Astrophysics

1

Stars: Coronae, Stars: Late-Type, Stars: Abundances, Stars: Activity, X-Rays: Stars, Stars: Individual: 30 Ari, Stars: Individual: Eta Boo

Scientific paper

We present X-ray observations of two late-type stars, HR 764 (30 Ari B, F4V) and HR 5235 (eta Boo, G0IV), with photospheric metal abundances about twice the solar value. We have derived the coronal temperatures and metallicities of these stars, and compared them with those of other stellar X-ray sources with solar or sub-solar photospheric composition, already observed with ROSAT and ASCA. The X-ray spectra of the two targets can be fitted with isothermal models yielding temperatures of 6.2 MK and 3.5 MK, and X-ray luminosities log L_x = 29.6 and 28.1 erg s-1, for HR 764 and HR 5235 respectively. The best-fit model for HR 764, including a gaussian component which accounts for Fe Xvii line emission from highly excited states, yields for its corona a solar (photospheric) iron abundance, while the best-fit iron abundance for HR 5235 is about half solar; however, in both cases, the uncertainties do not allow excluding values consistent with photospheric metallicities. Instead, the data indicate significant overabundances of Mg and Si by factors 2-3 with respect to the Fe abundance, and suggest low O/Fe abundance ratios. We conclude that these two super-metal-rich stars appear to have coronal properties similar to those of other F and G main-sequence stars with ``normal'' photospheric abundances: in particular, HR 764 looks similar to the Hyades F stars, while the coronal temperature, luminosity and abundances of HR 5235 are similar to those of other field G dwarfs slightly younger than the Sun. The systematic pattern of O, Mg, Si, and Fe coronal abundances generally found for solar-type stars observed with ASCA is puzzling, because it does not appear related either to the photospheric composition or to the activity level.

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