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

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 , near-by ( pc), close spectroscopic binary with an orbital period 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 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 .

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 . 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 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 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

SIMBAD Objects

Contents

• 1. Introduction
• 2. Optical identification
  • 2.1. Photometric observations
  • 2.2. Low-resolution spectroscopy
  • 2.3. Average magnitude, colours, distance and X-ray luminosity
• 3. Photometric variability
  • 3.1. Time analysis of photometric data
  • 3.2. The light curve
• 4. High-resolution spectroscopy
  • 4.1. Observations
  • 4.2. Radial velocity of the absorption lines
  • 4.3. The H emission lines
    • 4.3.1. Radial velocity
    • 4.3.2. Equivalent widths
• 5. Discussion
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: April 20, 1998
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