COMMISSION 27 OF THE I. A. U.
           INFORMATION BULLETIN ON VARIABLE STARS
                        Number 2711

                                        Konkoly Observatory
                                        Budapest
                                        24 April 1985

                                        HU ISSN 0374 - 0676

           STARSPOTS AND THE Mg II EMISSION OF VW CEPHEI



   The idea that dark starspots make the more massive components of cool W
UMa binaries appear cooler is an attractive explanation of light curves (Mullan
1975; Eaton, Wu, and Rucinski 1980; Eaton 1985). Observed light curves require
the spots to be spread quite uniformly over one component of the system, a
distribution that may show up in chromospheric heating. The chromospheric
emission observed in the ultraviolet, however, seems to be practically
independent of phase in several systems (Vilhu and Rucinski 1985; Rucinski,
Vilhu, and Whelan 1985; Eaton 1985), and this might be taken to indicate that
the active regions associated with spots are spread evenly over both
components.

   If evenly distributed spots are the cause of W-type light curves, we can
predict their effects on chromospheric emission through analogy with RS CVn
binaries. In the RS CVn system II Peg, Marstad et al. (1982) observed a
particularly luminous active region that enhanced Mg II 12800 emission by ~30%
and transition-region emission by an order of magnitude. This region was
visible when the star was faintest, as might be expected from the sunspot
analogy, but Poe and Eaton (1985) find spots were visible at all phases, not
just when the active region was in view. Further, the active region producing
the enhanced emission was found by Marstad to cover only ~6% of the stellar
surface, much less than the coverage by spots (Linsky 1984). This suggests that
changes in the spot distribution act to excite strong active regions, probably
by tearing closed magnetic flux tubes as spots move past one another. Further
evidence for changing spot distributions producing the enhanced chromospheric
and transition-region emission of active stars comes from a flare observed on
UX Ari (Simon, Linsky, and Schiffer 1980). In this case the flare seemed to be
associated with interaction of active regions on different components of the
binary. Simon, Linsky, and Schiffer proposed that it is this sort of
interaction that gives the episodic giant flares observed in RS CVn systems.
   The importance of this result is that it predicts W UMa systems, which
would have many relatively small spots being dragged about by differential
rotation, should be literally covered with material producing strong
chromospheric as well as transition-region lines. The bulk of the line emission





should be confined to the spotted component. On the other hand, if most of the
chromospheric emission does not come from such active regions (perhaps it comes
from an enhanced chromospheric network), the surface flux in Mg II would not be
especially elevated over the quiet phases of RS CVn binaries. We have already
seen in SW Lac, which has the most extensive observations of any system studied
to date, that Mg II surface flux is not highly correlated with phase (Eaton
1985). VW Cep, with a later spectral type and stronger Mg II emission, is an
even better star to compare with II Peg.
   We have used 30 IUE spectra of VW Cep, obtained at three different epochs
as part of observing programs CBDJE and CBEJE, to investigate the Mg II
emission. Emission strength is measured by an index formed by taking the ratio
of average flux in a 22 angstrom band at 2799 angstrom to average flux between
2585 and 3200 angstrom (Mg II omitted), expressed as a magnitude difference.
The ultraviolet continuum flux is roughly proportional to surface area, so 
the Mg II index is a measure of surface flux. Figure 1 shows the phase
dependence of Mg II emission,
which is very slight indeed. Not only is there little phase dependence, but the
average level seems to be roughly constant from year to year. If we compare
flux in the 22 angstrom Mg II band to flux at V, we get (MgII-V)= 2.5. The


                             [FIGURE 1]

 Figure 1. VW Cep: Strength of Mg II emission relative to average
 brightness of continuum between 2580 and 3200 A. This is roughly a
 measurement of surface flux in the emission lines. Observations were
 obtained with the IUE satellite.




observations of II Peg by Marstad et al. yield (MgII-V) = 2.2 for epochs of
weak emission. The cooler II Peg [(B-V) = 1.0, vs. 0.87 for VW Cep] has a
larger bolometric correction by -0.4 mag, and (MgII-Mbol) for the two stars
must therefore be equal. Differences in calibration noted, this equality of
F(MgII)/F(bol) means that VW Cep is not covered with the very many strong
active regions predicted for extensive numbers of starspots. Note also that
Vilhu and Rucinski (1983) and Rucinski (1985) find that W UMa binaries are no
stronger emitters of Mg II on the average than other late-type rapidly rotating
stars.





                        JOEL A. EATON

                        Astronomy Department
                        Indiana University
                        Swain Hall West 319
                        Bloomington, IN 47405 U.S.A.





References:

Eaton, J. A. 1985, "SW Lacertae, W Ursae Majoris, YY Eridani, and the
      Prevalence of Starspots in Cool Contact Binaries," Ap. J.,
      in preparation. [BIBCODE 1986AcA....36...79E ]
Eaton, J. A., Wu, C.-C., and Rucinski, S. M. 1980, Ap. J., 239, 919. [BIBCODE 1980ApJ...239..919E ]
Linsky, J. L. 1984, in Third Harvard Workshop on Cool Stars, Stellar
      Systems, and the Sun, ed. S. L. Baliunas and L. Hartmann
      (New York: Springer Verlag), p. 244. [BIBCODE 1984LNP...193..244L ]
Marstad, N., et al. 1982, in Advances in Ultraviolet Astronomy: Four Years
      of IUE Research, ed. Y. Kondo, J. M. Mead, and R. D. Chapman
      (NASA CP-2238), P. 554. [BIBCODE 1982NASCP2338..554M ]
Mullan, D. J. 1975, Ap. J., 198, 563. [BIBCODE 1975ApJ...198..563M ]
Poe, C. H., and Eaton, J. A. 1985, Ap. J., 289, 644. [BIBCODE 1985ApJ...289..644P ]
Rucinski, S. 1985, "The Mg II Emission in W UMa-type Binaries," preprint. [BIBCODE 1985MNRAS.215..615R ]
Rucinski, S. M., Vilhu, O., and Whelan, J. A. J. 1985, Astron. Ap., 143, 153. [BIBCODE 1985A&A...143..153R ]
Simon, T., Linsky, J. L., and Schiffer, F. H., III 1980, Ap. J., 239, 911. [BIBCODE 1980ApJ...239..911S ]
Vilhu, O., and Rucinski, S. M. 1983, Astron. Ap., 127, 5. [BIBCODE 1983A&A...127....5V ]
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