COMMISSION 27 OF THE I. A. U.
           INFORMATION BULLETIN ON VARIABLE STARS
                       Number 2557

                                       Konkoly Observatory
                                       Budapest
                                       23 July 1984

                                       HU ISSN 0374-0676


              FLARE ACTIVITY OF HD 282773 (G5)


   HD 282773 (reference star) and HD 282633 (check star) were used as 
comparison stars in observations of SU Aur over the period from 1981 to 1982.
The magnitude differences of these stars have been obtained using a 20"
reflector with UBVR counting photometer during 28 nights on the Peak Terskol.
The average standard deviations of individual UBVR differences are:

    sigma U=0.026m,    sigma B=0.018m,    sigma V=0.017m,    sigma R=0.016m.

  During the 2-year period of observations the stars seemed to be fairly
constant, but continuous photoelectric monitoring of HD 282773 on 10 October
1981 revealed a flare similar to those of UV Ceti variables. Figure 1 shows


                             [FIGURE 1]


the light curve obtained during 20-minute monitoring in B filter. The 
integration time was 1.0s. The amplitude of the flare was 0.25m, the duration 
was about 1 minute.
 On the following night additional observations were made with the same
technique but no noticeable variability was recorded.
 Since this star belongs to the G5 spectral class the energy parameter of
the observed flare is of great interest. In accordance with our measurements
the V magnitude and colours of the variable are:

               V = 9.723m +- 0.006m
                        
           B - V = 0.887m +- 0.009m
                  
           U - B = 0.557m +- 0.010m
           
           V - R = 0.724m +- 0.008m.

  The known spectral class and colour-indices make it approximately possible
to establish a luminosity class and to estimate the flare energy. The colour
excess of HD 282773 is between E(B-V) = 0.13m (if the star is a giant) and
E(B-V) = 0.25m (if the star is a dwarf). The self-consistent solution of the
equation

           m - M = 5lg r - 5 + 3.2 E(B-V)

gives:
         
        M(V) = 3.1m,  E(B-V) = 0.19m, r = 158 pc

The obtained distance r and colour-excess E(B-V) well agree with the interstellar
extinction in the direction of the star (Neckel, 1967).
  The energy distribution in the spectrum of the star has been assumed to
be a black-body with a temperature Te = 5300 K. Thus, we can estimate a part
of energy alpha B radiated by the star in the B-band of our photometric system:
    
          integral 0 to infinity F(lambda)*T(lambda)*dlambda
 alpha B= --------------------------------------------------
          integral 0 to infinity F(lambda)*dlambda

where:  F(lambda) is the Planck's function,
        T(lambda) is the relative response function of the B bandpass.
In this case, alpha B = 0.075.
As the absolute magnitude is M(V) = 3.1m, the total luminosity of the star is

          L* = 4.9L Sun = 1.9*10^34 erg 1/sec.

Hence, the energy radiated by the star in the B band is equal to

          L(B) = 1.4*10^33 erg 1/sec.

and the energy of the flare shown in Figure 1 is:

          E(B) = 7.6*10^33 ergs.

Then the total optical energy of the flare is estimated as:
         
          E(opt.) >= 10^34 ergs.

  Neither possible error in M(V) nor 10% - 20% additional energy from emission
lines (Moffet, Bopp, 1976) can essentially decrease the order of the value
E(opt.) Hence the recorded flare is one of the most powerful in the flare
stars.
  If the flare activity of the HD 282773 obeys the statistical relationship
between frequency and energy of individual flares (Gershberg, 1978), the
star must show flares with smaller amplitudes, the mean frequency of flares
increasing with the decrease of their energy. From that it follows that small
optical flares (0.01m - 0.001m) could be a cause of the potential 
microvariability of the star.


             KOVALCHUK G.U. and PUGACH A.F.
             Main Astronomical Observatory
             Ukrainian Academy of Sciences,
             Kiev, USSR

References:

Gershberg, R.E., 1978, Low-mass Flare Stars, Nauka, Moscow, 128 p [BIBCODE 1978fssm.book.....G ]
Moffet, T.J. and Bopp, B.W., 1976, Astrophys.J.Suppl.Ser., 31, 61 [BIBCODE 1976ApJS...31...61M ]
Neckel, Th., 1967, Veroff. Landessternwarte Heidelberg - Konigstuhl 19, 1 [BIBCODE 1967VeHei..19.....N ]