COMMISSION 27 OF THE I.A.U.
                 INFORMATION BULLETIN ON VARIABLE STARS
                             Number 3250

                                                         Konkoly Observatory
                                                         Budapest
                                                         14 October 1988

                                                         HU ISSN 0374 - 0676


               X-RAY/OPTICAL FLARES ON RS CVn STARS

Previous observers (e.g. Chambliss et al., 1978) have noted the lack of
a corresponding optical flare for quite large ultraviolet and micro-wave flares
on RS CVn stars. This has been generally interpreted as a contrast effect, 
since the non-flare continuum is more intense in these stars than, for
instance, in the dMe or flare stars where optical flares are common. An 
alternative explanation may be that the flare loop lengths and electron densities
on RS CVn stars are much larger than those on either the sun or dMe stars,
resulting in an increased column density along the loop. Assuming, as in
the sun, that we have a flare model where we have an instability at the loop
apex leading to the generation of a high energy beam of hard X-ray electrons
which move down the loop legs to the chromosphpre, theoretical calculations
from Emslie (1983) imply that all electrons with energy less than 50 keV
are stopped before they reach the chromosphere. Hence in this model, most
of the heating from this beam goes to heating the transition region/corona
with a substantially less amount going to heat the lower atmosphere than
in solar-type stars and dMe stars.
   Simultaneous optical (covering the high Balmer lines) and X-ray data should
help in distinguishing between these two suggestions, i.e. contrast effect
or increased loop length and density, since the X-ray observations will give
an estimate for the flare size and density. Simultaneous ultraviolet data
will provide. (through line ratio techniques), an estimate of the electron
density in the transition region. A good target for such a campaign is II Peg
(=HD 224085). This star has a very high X-ray luminosity, e.g. 4x10^31 erg/s
was observed by Walter et al. (1980) in the 0.2-4 keV range and 5*10^30
erg/s by Schwarz et al. (1981) its the 2-10 keV range. It is a single-
line spectroscopic binary of spectral type K2 IV-V with an orbital period
of approximately 6.7 days with photometric variations, of the type usually
interpreted as due to cool surface spots. This star has a very high rate of
flaring in the ultraviolet region, e.g. see Doyle et al. (1988). Ultraviolet
flares so-far observed have had typical decay times of 10 to 15 hours with
a total radiative power output of 10^36 erg. These long flare decay times
will mean that a high resolution (compared to the flare life-time) is
possible. Two 24 hr. shifts on the Japanese X-ray satellite GINGA has
been awarded. An application for ESA and NASA I IUE shifts have been
submitted, in addition, we have requested spectroscopy and photometric
coverage from La Palma and Mauna Kea (Hawaii). The requested dates of
observations are the 15 and 16 August 1989. Involved in this international
corroborative project is Armagh Observatory, the Institute for Astronomy at
the University of Catania, Goddard Space Flight Center, the Joint Institute
for Laboratory Astrophysics of the University of Colorado, the Rutherford-
Appleton Laboratory, the Laboratoire de Physique Stellaire et Planetaire
du CNRS and the University of Leicester.
   We welcome the participation of any observer who can contribute either 
photometric or spectroscopic observations. Interested observers are
requested to contact the undersigned and to notify us of their telephone
and telex numbers and/or their electronic mail address.


                     J.G. DOYLE

                  Armagh Observatory
                     College Hill
               Armagh. BT61 9DG, N. Ireland


Tel: (44) 861 522928
Fax: (44) 861 526187
Telex: 265871 (on the 1st line of telex,
please quote ref: MAG32510):
JANET electronic mail address: JGD%UK.AC.
QUB.ARM.STAR)



References:

Chambliss, C.R., Hall, D.S., Landis, H.J., Louth, H., Olson, E.C., Renner,
   T.R., Skillman, D.R., 1978, Astron. J. 83, 1514. [BIBCODE 1978AJ.....83.1514C ]
Doyle, J.G., Byrne, P.B., van den Oord, G.H.J., 1988, Astron. Astrophys. (in
    press). [BIBCODE 1989A&A...224..153D ]
Emslie, A.G., 1983, Solar Phys. 86, 133. [BIBCODE 1983SoPh...86..133E ]
Schwartz, D.A., Garcia, M., Ralph, E., Doxsey, R.E., Johnson, M.D., Lawrence,
   A., McHardy, I.M., Pye, J.P., 1981, MNRAS, 196, 95. [BIBCODE 1981MNRAS.196...95S ]
Walter, F.M., Cash, W., Charles, P.A., Bowyer, C.S. 1980 Ap. J. 236, 212. [BIBCODE 1980ApJ...236..212W ]