COMMISSION 27 OF THE I.A.U.
INFORMATION BULLETIN ON VARIABLE STARS
Number 1243
Konkoly Observatory
Budapest
1977 February 18
ABSOLUTE MAGNITUDES AND LOCAL FREQUENCY OF beta CEPHEI STARS
Lesh & Aizenman (1973) have determined MK spectral types,
effective temperatures, and visual absolute magnitudes for 17
stars considered to be unambiguously certain specimens of beta
Cephei type variability. Among this small sample, there are only
two spectral sub-groups which comprise more than 2 objects, viz.,
B1 III: 4 stars, and B2 IV: 6 stars. In both these groups the
M_v as given by Lesh & Aizenman, reveal a very significant but
certainly unreal dependence on the apparent magnitudes, the
intrinsic brightness of the more distant objects being systematically
overestimated. At present, one cannot decide, whether
this effect is originating in the spectrophotometric behaviour
of the fainter stars, or may be caused by misinterpretation of
some really interstellar influence on the luminosity criteria
used by the authors of that calibration. Thus, an interpolation
formula for the absolute magnitudes of the beta Cephei stars is
to be based only on 10 objects brighter than m = 4.1. By a least
squares solution we readily get the formula:
M_v = -3.80 + 0.44 (Sp-B1,5) + 0.35 (LC-III). (1)
+-.06 +-.10 +-.08 (p.e.)
sigma(M_v) = +-0.21m (standard deviation)
This result reveals the good internal consistency of the three
parameters involved in formula (1). Indeed, if the inevitable
deviations in the practically continuous scale of magnitudes
due to the relatively coarse frame of spectral and luminosity
classes are taken into account, the formally calculated standard
deviation +-0.21 nearly equals to its theoretically possible
minimum value.
Now, this formula may be safely used to determine the
visual absolute magnitudes not only for the 17 stars considered
by Lesh & Aizenman, but for any other Beta Cephei star with
spectral and luminosity class known. Among those stars included
in the Supplement issues 1 to 3 (1971 to 1976) to the GCVS
1969, there are 11 brighter than BW Vul (m=6.52), the faintest
one in Lesh & Aizenman's list. Since beyond this limit the
completeness of information available for stars of this type is
rapidly decreasing, in the present investigation the distance
moduli E=m-M have been determined only for that relatively
complete sample of 28 beta Cephei stars, after having applied, in some
instances of spectroscopically or interferometrically known
companions, the appropriate corrections to m. The distances
themselves were derived with due regard to the galactic absorbing
layer, according to a table given previously by Ferrari & Jenkner
(1973).
Thus, it has been found that 24 out of the 28 stars
considered here are situated within a cylindrical space of 450 pc
radius, and 350 pc height perpendicular to the galactic plane.
A more realistic measure for the local frequency of beta Cephei
stars is derived from the fact that 15 of them are included in
a cylindrical space of only 250 pc radius, and 300 pc height.
from the latter data we conclude that there are, at least,
25 beta Cephei stars per 10E8 cubic parsec. (2)
The limits just given for the space under consideration are
sufficiently large so that any accidental or systematic error,
increasing the true distances of some marginal stars by about 10
percent, would not at all diminish the numerical value (2). We
arrive at the conclusion that beta Cephei stars, notwithstanding
the small number of specimens recognized with certainty, are
nearly equally frequent as, e.g., Mira type variables.
K. FERRARI D'OCCHIEPPO
Institut fur Theoretische Astronomie
A-1180 Wien/Austria
References:
Ferrari D'Occhieppo, K. & Jenkner, H., 1973, Astronomische Mitteilungen Wien
No. 12, Tab. 1
Lesh, J.R. & Aizenman, M.L., 1973, Astron. & Astrophys., 22, 229 [BIBCODE 1973A&A....22..229L ]