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Astron. Astrophys. 352, 248-256 (1999)

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ASCA X-ray observations of the super-metal-rich stars 30 Ari and [FORMULA] Boo

A. Maggio 1, F. Favata 2, H. Negoro 3, N.S. Brickhouse 4 and A.K. Dupree 4

1 Osservatorio Astronomico di Palermo "G.S. Vaiana", Palazzo dei Normanni, 90134 Palermo, Italy
2 Astrophysics Division - Space Science Department of ESA, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands
3 Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako, Saitama, 351-0198, Japan
4 Smithsonian Astrophysical Observatory, 60 Garden Str., Cambridge, MA 02138, USA

Received 9 June 1999 / Accepted 8 September 1999

Abstract

We present X-ray observations of two late-type stars, HR 764 (30 Ari B, F4V) and HR 5235 ([FORMULA] 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 [FORMULA] = 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 FeXVII 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 [FORMULA] 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.

Key words: stars: coronae – stars: late-type – stars: abundances – stars: activity – X-rays: stars – stars: individual: 30 Ari – stars: individual: j Boo

Send offprint requests to: amaggio@oapa.astropa.unipa.it

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© European Southern Observatory (ESO) 1999

Online publication: November 23, 1999
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