COMMISSION 27 OF THE I. A. U.
                    INFORMATION BULLETIN ON VARIABLE STARS
                                 Number 2482

                                                       Konkoly Observatory
                                                       Budapest
                                                       24 February 1984

                                                       HU ISSN 0374-0676



            A FLARE-LIKE EVENT ON THE LIGHT CURVE OF V351 Ori



  As a part of our on-going program of UBVR photometry of antiflare stars
(Pugach, 1975) we have observed a flare-like episode of V351 Ori (BD +0d1170,
Sp A7 III) in four colours. All observations were obtained at the observing
station "Terskol" of Main Astronomical Observatory of the Academy of Sciences 
of the USSR, using the 0.5m reflecting telescope and a single-channel
automatic photon-counting photometer with a PEM-79 uncooled photomultiplier
tube.
  The comparison star was BD +0d1171 and the check star was BD-0d1067. The
magnitudes of the variable star in the UBVR-bands in normal brightness are
as follows:

      U = 9.58m,    B = 9.30m,     V = 8.98m,     R = 8.57m

  The flare-like episode occurred on September 29, 1982 when the star was
in a deep minimum - in the BVR-bands the increase of brightness appeared to
begin, while in U-filter the brightness of the variable further decreased.
From the beginning of the observations on this night and up to the beginning
of the flare, the brightness of the variable in the UBVR-bands increased by
0.095m, 0.200m, and 0.320m respectively. At the same time, the decrease of
brightness in the U-band was 0.175m. Figure 1 shows the flare-like episode in
the UBVR-bands relative to the light curve without flare. Symbol " | "
                                                                   v
indicates the moments of the comparison star observations, and symbol " || "
                                                                        vv
indicates the moments of the observations of the check+comparison stars. The
flare duration at half peak light appears to be 30 minutes.
Conventionally, the flare consists of three parts:
a. a small pre-flare brightness increase with amplitudes increasing through
  infrared,
b. the main-part of the flare - its amplitude is practically the same in the
  BVR-bands,
c. a much less intense tail appears after the main-part of the flare (this
  tail is especially well visible in the BVR-bands).


                           [FIGURE 1]


  Furthermore, the main-part of the flare in the BVR-bands is a superposition 
of a few narrow, sequent sharp mini-flares unresolved in time. However,
the thin structure of the main-part of the flare in ultraviolet is not 
observed, and the main-part and the tail are masked by much more significant
brightness increase of the variable with an amplitude up to 0m5, in addition
the moment of peak flare luminosity precedes the moment of brightness maximum
in BVR-bands by 4 min.
  No optical flares have been previously found in photometric observations
of antiflare stars with the exception of only one case: Pugach's (1975) 
observations of the variable star RZ Psc (G8-K0) showed a flare with two sharp
peaks, a small pre-flare increase of brightness, a sharp forward front and a
gradual decrease. The flare was observed, when the variable was in a deep
minimum, and the observations were obtained in a mode of continual measuring
through B filter. The flare amplitude was 0.9m.
  As for the flare activity of A and B stars, the information about it is
very scanty - the total number of such cases does not exceed ten (Bakos,
1970, Page and Page, 1970, Andrews, 1964, Kunkel, 1975).


  The energy characteristics of the flare observed were calculated by assuming 
the Planck's energy distribution in the spectrum of the variable star
with T_eff = 9000 dK and with a due accounting of the width of the instrumental
UBVR system bands. They are as follows:


Filter   The total flare   Flare   Peak flare
         energy            Base    luminosity
         x10^-35 erg               x10^-33 erg sec^-1
U        8.38              0.250   1.470
B        6.49              0.069   3.132
V        10.39             0.065   5.179
R        8.19              0.072   3.685

where the third column gives the ratio  of the flare's luminosity to the
luminosity of the quiet star over the same period of time. The ratios of energy
magnitudes radiated by the variable star in each band are as follows:

         E_U :E_B :E_V :E_R = 1.29:1.00:1.60:1.26


  At the end it should be noted that the observations of the variable 
obtained within the period from JD 2445242.426 to JD 2445242.516 in a mode of
continual measuring through a blue filter showed the presence of a periodic
component in the light variation with a period of 320s and an amplitude up
to 0.15m.




                  G.U. KOVALCHUK
              Main Astronomical Observatory
              Ukrainian Academy of Sciences,
              Kiev, USSR





References:

Andrews, A.D., 1964, Irish Astr.J. 6, 212 [BIBCODE 1964IrAJ....6..212A ]
Bakos, G.A., 1970, Sky and Telescope, 40, 214 [BIBCODE 1970S&T....40..214B ]
Kunkel, W.E., 1975, in Variable Stars and Stellar Evolution, I.A.U.
  Symposium No. 67, V. Sherwood and L. Plaut, eds. (Dordrecht:Reidel), p. 15 [BIBCODE 1975IAUS...67...15K ]
Page, A.A. and Page, B., 1970, Sky and Telescope, 40, 206
Pugach, A.F., 1975, in Variable Stars and Stellar Evolution, I.A.U. Symposium
  No. 67, V. Sherwood and L. Plaut, eds. (Dordrecht:Reidel), p. 144 [BIBCODE 1975IAUS...67..143P ]