COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 2432

                                       Konkoly Observatory
                                       Budapest
                                       18 November 1983

                                       HU ISSN 0374-0676



      POLARIMETRY OF Be STARS IN THE OPEN CLUSTER NGC 6611



  Be stars most probably have symmetrical, disk-shaped envelopes. The lack
of spherical symmetry produces a net amount of Linear polarization through
Thomson scattering in the envelopes. Polarimetric observations are thus a
useful tool for testing theoretical Be star models.
  In order to compare observations with theoretical predictions the interstellar
polarization in the line of sight must be determined. Statistical
methods including field stars are currently used (e.g. McLean and Clarke,
1979). These methods give better results for Be stars in open clusters, as
spatial association with field stars is easily established. Moreover, once
the cluster parameters (m-M, E(B-V)) are known, one readily obtains the 
absolute magnitude and dereddened colours of the Be star thus allowing more
complete tests of the models.
  In August, 1981 we started a program of UBVR polarimetric observations of
southern open clusters. Measurements were made at the Observatorio Astrofisico
Brasileiro 1.6 meter telescope with the Instituto Astronomico e Geofisico
(Universidade de Sao Paulo, Brasil) photopolarimeter (Magalhaes, 1979).
  Some preliminary results for the open cluster NGC 6611 are given in Table I.


                                  Table 1
Star
number   V     S_p    p^B   Theta^B  Sigma^B  p^V  Theta^V   Sigma^V   Date
------------------------------------------------------------------------------
  150   9.89  B0.5V                           2.78  93.89    0.23    7-8/8/82
  166  10.36  O9V                             3.19  97.32    0.18   12-13/8/82
  175  10.02  O6     4.24   79.37     0.30    3.24  70.63    0.19   12-13/8/82
  351  11.26  B1Vne  2.00   68.97     0.40    1.48  82.31    0.36    7-8/8/82
  367   9.43  O9.5   2.07   92.25     0.13    2.76  94.60    0.11    9-10/8/82
  401   8.96  O8V    2.37   88.50     0.14    2.79  83.68    0.08    5-6/8/82
  469  10.71  B1.5V  1.63   99.06     0.23    2.16 107.95    0.16    9-10/8/82 
------------------------------------------------------------------------------
p and sigma in %


Numbers in column 1 are from Kamp (1974), values in column 2 are from Walker
(1961), for star number 469 spectral type is from Hiltner (1956). Other
spectral. type are from Hiltner and Morgan (1969), Columns 4,5 and 6 give
the observed polarization values (in%), the polarisation angle and the error
of measurement for p (%), respectively, in the B filter, columns 7, 8 and 9
give the same for the V filter. Last column lists the dates of observations.
  Measurements will be pursued on the next observing season. More observations
are needed for determining the interstellar polarization throughout
the cluster.





                 L.A.S. REIS

                 E. JANOT-PACHECO

            Instituto Astronomico e Geofisico
            Universidade de Sao Paulo
            C. Postal 30.627
            01000 Sao Paulo
            Brasil





References:

Hiltner, W.A., 1956, Astrophys.J. (Suppl.) 2, 389 [BIBCODE 1956ApJS....2..389H ]
Hiltner, W.A., Morgan, W.W., 1969, Astron.J. 74, 1152 [BIBCODE 1969AJ.....74.1152H ]
Kamp, L.W., 1974, Astron.Astrophys.Suppl. 16, 1 [BIBCODE 1974A&AS...16....1K ]
Magalhaes, A.M., 1979, Thesis, IAG/USP, S. Paulo, Brasil
McLean, I.S., Clarke, D., 1979, Mon.Not.Roy.astr.Soc. 186, 245 [BIBCODE 1979MNRAS.186..245M ]
Walker, M.F., 1961, Astrophys.J. 133, 438 [BIBCODE 1961ApJ...133..438W ]