A&A 382, 554-555 (2002)
DOI: 10.1051/0004-6361:20011625
Research Note
K. M. V. Apparao
34, Vibha, opp. Cardinal School, R.P. Road, Bandra East, Bombay 400051, India
Received 12 October 2001 / Accepted 6 November 2001
Abstract
The recently discovered orbital period for
Cas
and the modulation period in X-rays and optical
are consistent with the orbital period-spin period relation
for white dwarfs derived from a similar relation found for Be
star - neutron star binaries. This lends support for the existence of a
white dwarf in the
Cas binary.
Key words: stars:
Cas - white dwarf - stars: emission line, Be stars - stars: binaries - stars: neutron
Cas is a Be star of spectral type B0.5IV. X-ray emission
was observed (see Apparao 1994a for references)
with a luminosity of
1033 ergs s-1. Intensity variations and flaring in X-rays were observed
(Parmar et al. 1993).
The high temperature required for the X-ray emission and
X-ray brightness (see Marlborough 1977) prompted a binary hypothesis for
the system, with the X-ray emission occuring from accretion onto a compact
object (white dwarf or neutron star).
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Figure 1: The orbital period-spin period diagram for Be star binaries. The lines for neutron stars and white dwarfs are from Apparao (1994b). The square is for the modulation period derived by Robinson & Smith (2000), while the circle is for the modulation period derived by Harmanec et al. (1999). |
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Corbet (1984, 1986) found that a correlation exists between the spin period
and the orbital period
in Be star binaries. The basis of
the relation is set out by Corbet (1984) and Waters & van Kerkwijk (1989).
The relation given by Corbet (1986) is for neutron stars and with a
radius
cm and magnetic field
G,
relation is shown
in Fig. 1. Apparao (1994b), using the expressions for the relation given by
Corbet (1986) and Waters & van Kerkwijk (1989), obtained a similar relation
for white dwarf
stars in Be star binaries. The line shown for white dwarfs in Fig. 1
is for a radius
cm and a surface magnetic field
G.
As mentioned earlier, the nature of the periodicity of 1 day
is not established. However it is interesting if the period is attributed
to the spin period of a white dwarf companion to the Be star and
we can find its
relationship with the orbital period. The points corresponding
to the two periods
(Robinson & Smith 2000) and
(Harmanec et al. 2000) and the orbital period of
are plotted in Fig. 1.
It is interesting to note that the points corresponding to the orbital period
of
Cas and the
1 day period fall on the line attributed
to a white dwarf in the spin period-orbital period relation in Be stars.
This lends support to the hypothesis that the compact object in the binary
system is a white dwarf.
Kubo et al. (1998) argue that
Cas contains a white dwarf, while
Robinson & Smith (2000)
suggest that the observed phenomena in X-rays and UV can
be explained as
being due to processes near the primary Be star. In the following,
we will discuss their arguments and give our comments:
i) The X-ray luminosity of
Cas is
1033 ergs s-1. This is higher than what is expected of an early type B-star.
Kubo et al. (1998) suggest that the X-ray emission is from a white dwarf companion
to the Be star. They assume Bondi-Hoyle type of accretion from a "wind"
emanating from the Be star. Using a wind velocity of 200 km s-1, they
obtain the requisite X-ray luminosity. Robinson & Smith (2000) argue that
Kubo et al. have not taken into account the efficiency of conversion of the
accretion energy into X-ray, which is less than one and therefore cannot
acheive the requisite luminosity.
However, as Waters et al. (2000) have suggested, Be star disks are Keplerian with very
small radial velocities (see also Apparao 1985). The orbital motion of the
compact object, while it is immersed in the gas disk, is also Keplerian.
Consequently, the relative velocity between the white dwarf and the gas in
the disk is very small, resulting in large accretion rates. This is
shown by the large
accretion rate needed to explain the super-Eddington
X-ray luminosity of the system A0538-66 and also the high luminosity of
many Be star-neutron star systems (see Apparao 1994a). Thus the accretion
rates needed to explain the X-ray luminosity of
Cas can be
acheived.
ii)
Smith et al. (1994) and Smith & Robinson (2000) observed flares and rapid
flickering in the X-ray emission of
Cas. They suggested that the
observed phenomena can be explained by processes near the Be star.
Kubo et al. (1998) have argued that similar flaring and flickering occurs in the
X-ray emission of SS Cygni, which contains an accreting white dwarf. Smith
& Robinson (2000) point out that SS Cygni is a close binary and the
accretion process is different from that in
Cas. However flares
can occur if the white dwarf passes through clumps of gas that may be
present in the gas disk.
Smith & Robinson (2000) have also observed an anti-correlation in the intensities of X-ray and UV emissions. Such anti-correlation might result if an increased X-ray emission is due to increased density of gas in the disk, which can cause a decrease in the UV luminosity.
The detection of the white dwarf in the
Cas system directly is difficult.
Cyclotron lines have been observed in magnetic white dwarfs (Wickramasinghe
1982) in the optical and UV regions. However, these lines will not be
visible due to the higher luminosity of the Be star in this region.
Accreting magnetic
white dwarfs give a luminosity in the far UV and soft X-ray region a factor
10-100 larger than the hard X-ray luminosity (Kylafis & Lamb 1982; Mason
et al. 1978). The effects due to this higher luminosity may be detectable.
This aspect will be discussed in a future publication.
It is clear that further work is needed to understand the phenomena
observed in
Cas. However, our observation that the orbital
period and the
1 day period in
Cas are consistent
with the orbital period-spin period relation expected from Be star-white
dwarf binaries, lends support for a white dwarf companion to the Be star in
Cas.
Acknowledgements
I thank the Director of Tata Institute of Fundamental Research for allowing me the use of their Library and other facilities for this research. I thank Prof. P. C. Agrawal for discussions.