One Hundred Years of Observations of the Be Star HDE 245770
(the X-ray Binary A0535+26/V725 Tau):
The End of an Active Phase?
V.M. Lyuty
and G.V. Zaitseva
Sternberg Astronomical Institute,
Universitetskii pr. 13, Moscow, 119899 Russia
UBV observations of the X-ray binary system A0535+26/V725 Tau
at the Crimean Station of the Sternberg Astronomical Institute in
1980-1998 are presented. Based on our and published data, we
analyze the photometric history of the star from 1898. Over a period
of 100 years, the star apparently showed all three activity phases
(B, Be, Be-shell) of Be stars. We conclude that the X-ray activity
of the object is attributable to the 1970-1997 outburst of the Be star
due to envelope ejection. The star's colors during the minimum light
of 1998 and its 1953-1956 colors (before the outburst) correspond to
the spectral type B0-B1 at the color excesses
EB-V = 0.74
and EU-B = 0.48,
in agreement with the current spectral type O9.7. The minimim
light of 1998 and the color excesses are used to determine the colors
of the additional radiation, analyze their evolution during the
1973-1997 outburst, and refine the distance to the object (3 kpc).
The colors of the additional radiation at maximum light of the star
(1973-1980) match the colors of a hydrogen plasma with
Te = 1.5×104 K
which is optically thick in the Balmer continuum. The brightness
decline corresponds to a decrease in the optical depth of the plasma;
at V = 9.1 mag, it becomes optically thin in
the Balmer continuum with
Te = 104 K and
Ne = 1010-1012 cm-3.
This conclusion is consistent with the model of a circumstellar envelope
but is inconsistent with the existence of an accretion disk around the
neutron star. All the additional radiation responsible for the optical
variability is produced by a single source. The intensity of the
H emission line at maximum light
(1975-1980) is triple its intensity in 1987-1997, when quasi-periodic
light fluctuations with P = 1400d were observed.
At this time, the line intensity correlated with brightness. The
H line was in absorption at the
minimum of 1998, and, at present, the star's active phase appears to
have ended. © 2000 MAIK "Nauka/Interperiodica".
Astronomy Letters, Vol. 26, No. 1, 2000
Translated from Pis'ma v Astronomicheskii Zhurnal, Vol. 26,
No. 1, 2000
Reprints on the web at
astro-ph/0007321
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