COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 2136

                                  Konkoly Observatory
                                  Budapest
                                  1982 May 3

                                  HU ISSN 0374-0676




        ENERGY DISTRIBUTION IN THE SPECTRUM OF FU Ori




  As known in 1936-1937 FU Ori increased its brightness from 16m to 10m
during half a year. Since then it has been decreasing slowly, only 0.15m in
B, for ten years. However, more rapid fluctuations of brightness with an
amplitude of about 0.1m have been also observed. The flare of FU Ori appeared
to be the first observable manifestation of a new type of variability of
young stars. The investigations and characteristics of such stars were
entirely described by Herbig (1965, 1977).
  The observations were made in the range lambdalambda 3110-8640 A by the five-channel
scanning spectrophotometer in the Cassegrain focus of the 0.7 m reflector
AZT-8 at Crimean Astrophysical Observatory (Lagutin, 1979; Bukach, 1979).
At lambda 3750 A the spectral with of exit slit was 23.4 A and the step of
scanning was 18 A in shortward spectral regions these parameters were two
times less. Time integration for a counting was 4 sec, with a full-time
record of one scan equal to 12 min.
 phi_1 Ori was used as a standard star with the absolute energy distribution
according to data from Charitonov et al. (1978) and Johnson and Mitchell.
(1975) corrected respectively for the absolute calibration of the primary
standard alpha Lyr (Charitonov et al., 1980). The standard star was observed
directly before or after scanning FU Ori so that the difference in the air
mass was negligible. In order to reduce the contribution of noise to the
final data every three serial contribution of noise to at lambda 3750 A and five 
countings in shortward regions were smoothed. The absolute energy distribution 
of FU Ori was obtained on three nights: 27.I.80, 21.II.80, 26.III.80.
The difference of the absolute energy distribution: obtained on these
nights was within the errors of observations. Figure 1 shows the dependence
of the intrinsic spectral density E_lambda (erg cm^-2 sec^-1 cm^-1) for these
three dates.

                             [FIGURE 1]

The r.m.s. error determined by the internal agreement does not exceed 12%
in. lambda < 3750 A, 3% in lambdalambda 3750 - 5700 A and 5% in more longward region. The
same figure shows the run of the energy distribution in: the spectrum of
FU Ori obtained by Kuhi (1974) in 1966. According to Charitonov et al. (1980)
the difference between our standards and the ones used by Kuhi (Hayes
standards) does not exceed 3%. is it is seen in Figure 1 the brightness of
FU Ori decreased approximately by 40% (0.35m) as compared to 1966. This
decrease of brightness agrees with the tendency that this star
faded in B light (Herbig, 1977). Considering that the star is surrounded
by a reflecting nebula, while comparing the observational data it is necessary
to notice the difference of apertures: 17" in our observations and
10" - 14" in Kuhi's observations (Kuhi, 1974). Therefore, such difference
cannot be the cause of the observed decrease of brightness.
  Following the picture, the decrease of E_lambda within the accuracy of the
observations and calibrations is the same for the whole spectral region
lambdalambda 3300 - 8000 A. Hence, the observed decrease of brightness in 1966-1980
(by 0.35m) was not accompanied by the noticeable energy distribution 
variation in the spectrum of the star.
  So, the result obtained allows to reject the hypothesis that the cooling
of the star is responsible for the decrease of its brightness. Really, a
simple calculation by Planck's formulae shows that, if a star with initial
temperature 7500d (temperature of F2 type stars; according to the spectrum

FU Ori belongs to this class), is cooling so that at lambda 5500 A E_lambda5500 has
decreased by 40%, then E_lambda8000 must decrease by 26% and E_lambda3600 by 66%,
which is noticeably different from the observed energy distribution.
  Herbig (1977) supposes that the decrease of brightness of FU Ori after
the flare is connected with the decrease of brightness of the reflecting
nebulae surrounding these stars that agrees with our observations. The same
decrease of brightness in the reflecting nebula is also observed on direct
photos of the star V 1057 Cyg (Herbig, 1977). In such a case the nebula flux
must contribute a significant part of the total flux registered.





          N.I. BONDAR, A.B. BUKACH, N.I. SHAKHOVSKAYA
          Crimean Astrophysical Observatory
          USSR





References:

Bukach, A.B., 1979, Izv. Crimean Astrophysical Observatory, 60, p. 197
Charitonov, A.B., Tereshenko, W.M., Knyazeva, L.N., 1978, Svodny 
   spectrophotometrichesky catalog zvezd" Nauka, Alma-Ata
Charitonov, A.B., Tereshenko, B.M., Knyazeva, L.N., Boyko, P.P., 1980,
   Astronomichesky Journal, 57, p. 287-295 [BIBCODE 1980AZh....57..287K ]
Herbig, G.H., 1966, Vistas in Astronomy, vol. 8, ed. A. Beer and K. Aa. Strand
   (Oxford Pergamon), p. 109 [BIBCODE 1966VA......8..109H ]
Herbig, G.H., 1977, Astrophys. J., 217, p. 693-715 [BIBCODE 1977ApJ...217..693H ]
Johnson, H.L. y Mitchell, R.I., 1975, Revista de Mexicana Astronomia y
   astrofisica, v. 1, n. 3, p. 299 [BIBCODE 1975RMxAA...1..299J ]
Kuhi, L.V., 1974, Astr. and Ap. Suppl., 15, 47 [BIBCODE 1974A&AS...15...47K ]
Lagutin, A.F., 1979, Izv. Crimean Astrophysical Observatory, 60, p. 197