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Astron. Astrophys. 326, 228-236 (1997)

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The ROSAT bright source RX J0222.4+4729: an active nearby short-period binary of the BY Draconis type *

Claude Chevalier and Sergio A. Ilovaisky

Observatoire de Haute-Provence (CNRS), F-04870 St. Michel l'Observatoire, France

Received 4 December 1996 / Accepted 25 March 1997

Abstract

We report the discovery of a new BY Dra-type binary identified as the optical counterpart of the bright source RX J0222.4+4729 detected during the ROSAT All-Sky Survey (Voges et al. 1996). The star is a [FORMULA], near-by ([FORMULA] pc), close spectroscopic binary with an orbital period [FORMULA] d. The absorption-line radial velocities were obtained at the 1.93-m Haute-Provence (OHP) telescope with the Elodie echelle spectrograph by on-line numerical cross-correlation.

The M0Ve primary exhibits strong Balmer and Ca II H+K line emission, placing this system amongst the most active BY Dra stars. The width of the cross-correlation function yields a projected rotational velocity of [FORMULA] km/s. While only the primary contributes to the continuum and the absorption line spectrum, the dM5e secondary is detected through its H [FORMULA] emission. The mass ratio, estimated from the amplitudes of the emission radial velocity curves, is [FORMULA].

CCD photometry in the B and V bands, obtained with the OHP 1.2-m and 0.80-m telescopes, shows that the optical flux is modulated at the spectroscopic period with a total amplitude of 0.2 mag and little or no color change in [FORMULA]. The light curve, which can be attributed to rotational modulation of the synchronized active primary star, shows extrema near quadratures and also exhibits long-term variations in average brightness (by 0.1 mag), which are accompanied by changes around the photometric minimum. A secondary minimum appears at phase 0.5, indicating a partial eclipse of the primary star.

In contrast with many other BY Dra systems, the equivalent width of the H [FORMULA] emission from the RX J0222.4+4729 primary is directly correlated with photospheric brightness, i.e. maxima and minima occur around the same phases in both curves. However, the minimum at mid-phase in the H [FORMULA] equivalent width is broader and deeper than the V-band minimum at [FORMULA] and appears shifted towards phase 0.45, suggesting that H [FORMULA] emission comes from extended regions connecting the main starspot groups.

We find an X-ray to bolometric luminosity ratio of [FORMULA] which supports the concept of saturation of coronal X-ray emission for the most rapidly rotating late-type stars.

Key words: stars: activity – stars: starspots – binaries: spectroscopic – X-rays: stars – stars: variable: other

* Based on observations obtained at Observatoire de Haute-Provence (CNRS) with the 1.93-m, 1.20-m and 0.80-m telescopes

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© European Southern Observatory (ESO) 1997

Online publication: April 20, 1998
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