COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                      NUMBER 528

                              Konkoly Observatory
                              Budapest
                              1971 March 4


     UNUSUAL DISTRIBUTION OF FLARE STARS IN PLEIADES

      The distribution of stars of different classes in
stellar associations and clusters is a source of important
information on their evolution. From this point of view the
study of the distribution of flare stars in Pleiades is of
some interest.
      According to the statistics the total number of
flare stars in Pleiades exceeds 700(1). Due to the intense
observations of the Pleiades region carried out in
Tonanzintla, Byurakan, Asiago and Budapest Observatories
during the recent years 207 of them have been discovered
(1-6). The data on these flare stars have been used to 
determine the distribution of their partial density in the
Pleiades system. The main result of this study is the
following:
      The distribution of the apace partial density of
flare stars in Pleiades have been determined on the basis
of their apparent distribution on the sky. The solution of
Abel's equation has been obtained by the method given in
(7). It has been supposed that the distribution of the
flare stars in the system is a spheric-symmetrical one, the
centre of the system being the geometrical centre of the
known flare stars. It is almost coinciding with Alcyone.
      The calculated distribution of the partial density
of flare stars in Pleiades (number of flare stars per pc^3)
as a function of the distance from the centre of the system
(in pc) is presented in Fig. 1. During our calculations the
value of 125pc (8) has been taken for the distance of the
centre of the Pleiades system. The corresponding radius of
the subsystem of flare stars in Pleiades is about 8 pc.

      Fig. 1 shows that the flare stars are almost fully
absent in the central region of Pleiades. The radius of
this cavity is 1.4 pc. The density of the flare stars has a
maximum near the limit of the cavity, at the distance 1.5 pc
from the centre and is then decreasing with the distance
more rapidly than ~r^-2.
       This result concerning the absence of flare stars
in the central region of Pleiades we obtained already in
summer 1969 using the data on about 100 known flare stars.
Some indications on the similar phenomenon have been noticed
earlier for the system NGC 7023 (9). But we have been 
inclined to think that in both cases this result has been due

to the scarcity of observational data. Therefore with the
discovery of new flare stars in Pleiades this problem has
been rediscussed several times using 145,184 and 207 flares
stars, respectively. It turned  out that the conclusion of
the existence of a cavity of flare stars becomes more and
more definite with the increase of the statistical material.
      It must be noted, that in our calculations we
supposed that all observed flare stars in Pleiades belong
to the system. If one takes into account the possible
existence of background stars among them, then it must be
concluded that the real cavity is even greater.
      The probable number of flare stars inside the
sphere with a radius of 1.4 pc around the centre of the
Pleiades system can be 5-8. In order to move away the 
observed cavity it is necessary to have at least 30 additional
flare stars in this sphere. It means that in the circle of
the radius of 1.4 pc around Alcyone in projection on the
sky we must observe about 80 flare stars instead of 44 
observed.
      The absence of such a number of flare stars in the
central part of Pleiades is unlikely to be the result of
the influence of the absorbing matter, which is small and
almost uniformly distributed (10), or of serious difficulties 
in detecting the flare stars on the plates in the
nearest neighbourhood of bright stars concentrated in the
central region of Pleiades. The calculations show that
such a cavity is present in the case of comparatively
bright flare stars as well.
      The appearance of such a false cavity as a result
of fluctuations, connected with the random nature of the
detecting process of flare stars is very unprobable. The
probability of this event is less than one hundredth.
      Here is not a place to discuss all possible 
interpretations of the existence of the observed cavity of flare
stars in Pleiades. It is interesting to note only that the
explanation of this unusual phenomenon can perhaps be found
in the frames of the idea of expansion of stellar 
associations (9, 11, 12).

Byurakan Astrophysical Observatory

                          L.V. MIRZOYAN
                          M.A. MNATSAKANIAN




                      [FIGURE 1]




               REFERENCES

(1) V.A. Ambartsumian et al, Astrofizika, 6, 7, 1970. [BIBCODE 1970Afz.....6....7A ]
(2) G. Haro, Stars and Stellar Systems, Vol. 7, ed. B.M.
     Middlehurst and L.H. Aller, Univ.Press,Chicago, 1968, p, 141
(3) G. Haro, E. Chavira, Bol.Obs.Tonanzintla, 5, No. 31, 23, 1969. [BIBCODE 1970BOTT....5...23H ]
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(6) V.A. Ambartsumian et al, Astrofizika, 1971 (in press).
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(8) D.N. Limber, Ap.J., 135, 16, 1962. [BIBCODE 1962ApJ...135...16L ]
(9) L.V. Mirzoyan, E.S. Parsamian, Non-Periodic Phenomena in
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     p. 165.
(10) H.L. Johnson, R.I. Mitchell, Ap.J., 128, 31, 1958. [BIBCODE 1958ApJ...128...31J ]
(11) V.A. Ambartsumian, Astr.Zu., 26, 3, 1949.
(12) V.A. Ambartsumian et al, Problemi Sovremonnoi Kosmogonii
    Nauka. Moskwa, 1969.