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Astron. Astrophys. 326, 228-236

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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


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 HFORMULA 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 B-V. 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 HFORMULA 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 HFORMULA equivalent width is broader and deeper than the V-band minimum at FORMULA and appears shifted towards phase 0.45, suggesting that HFORMULA 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

© European Southern Observatory (ESO) 1997

Online publication: September 9, 1997
Last change: April 20, 1998