OHP Preprint No. 103 : ROSAT source RX J0222.4+4729

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

Claude Chevalier and Sergio A. Ilovaisky

Observatoire de Haute Provence (CNRS)
F-04870 Saint-Michel l'Observatoire, France
E-mail: chevalier@obs-hp.fr
Accepted in Astronomy & Astrophysics, 24 March 1997
*Based on observations obtained at Observatoire de Haute-Provence (CNRS) with the 1.93-m, 1.20-m and 0.80-m telescopes

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 V~11.1, near-by (~30 pc), close spectroscopic binary with an orbital period P=0.46543 ±0.00001 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 v sin(i)~ 85 km/s. While only the primary contributes to the continuum and the absorption line spectrum, the dM5e secondary is detected through its H emission. The mass ratio, estimated from the amplitudes of the emission radial velocity curves, is q = M2/M1~0.4.

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 H 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 equivalent width is broader and deeper than the V-band minimum at phase 0.5 and appears shifted towards phase 0.45, suggesting that H emission comes from extended regions connecting the main starspot groups.

We find an X-ray to bolometric luminosity ratio of log(Lx/Lbol) ~ -3.1 ± 0.14 which supports the concept of saturation of coronal X- ray emission for the most rapidly rotating late-type stars.

keywords : stars: activity -- stars: starspots -- binaries: spectroscopic -- X-rays: stars

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