COMMISSIONS 27 AND 42 OF THE IAU
           INFORMATION BULLETIN ON VARIABLE STARS
                        Number 3857

                                           Konkoly Observatory
                                           Budapest
                                           12 March 1993

                                           HU ISSN 0374 - 0676


        uvby BETA PHOTOMETRY OF THE NEWLY DISCOVERED
                  PULSATING STAR HR 8851


After HR 8851 was found to be a pulsating star, possibly a delta Scuti star
(Hao and Huang, 1993), we made observation in Stromgren uvby beta system with
the 60 cm reflector located at Xinglong Station, Beijing Astronomical
Observatory on January 21, 1993. The aim of the observation is to get more
information about the physical character of this star.

The observation was performed on a very good photometric night.  We chose HD
221421 (COM1) and HD 221167 (COM2) as the comparison stars whose standard uvby
beta values are listed in Table 1.

  We use the relations by Crawford ( 1975)

               E(c_1)=0.20E(b-y)
               E(m_1)=-0.32E(b-y)
               E(b-y)=0.74E(B-V)

to correct for the interstellar extinction.  We know that HR 8851 is listed as
an F0IV type star with B-V=0.24 in Preliminary Version of the Bright Star
Catalogue, 5th Revised Edition (Hoffleit and Warren, 1991), but for an F0IV type
star, (B-V)_0=0.29 (Straizys, 1977), then E(B-V)=-0.05<0.  So we think that the
MK classification for HR 8851 adopted by BSC is uncertain.  Here we use A7V
type (see RGOB, 1968, 135, 385, as searched from Simbad) for HR 8851 to
estimate E(B-V)=B-V-(B-V)_0=0.24-0.20=0.04.  After making the correction, we
can get the results listed in the last row of Table 2.

In order to determine the absolute magnitude, we use the empirical relation
(Breger, 1990) M_v=-9(c_1)_0+15 beta -32.22

Table 1. Standard values for the two comparison stars (Olsen, 1983)

          SAO number Sp    V    b-y   m_1   C_1   Beta
HD 221421     10770  F8  8.215 0.349 0.158 0.403 2.628
HD 221167     10752  G0  7.737 0.332 0.125 0.364 2.615

            Table 2. uvby beta results observed for HR 8851

            u       v       b       y       b-y      m_1      c_1     Beta
with COM1   7.366   6.058   5.723   5.557   0.166    0.169    0.973  2.727
with COM2   7.353   6.067   5.731   5.555   0.176    0.160    0.950  2.740
average                                     0.171    0.165    0.962  2.734
                                            (b-y)_0  (m_1)_0  (c_1)_0
                                            0.141    0.175    0.956

which results in M_v=0.19+-0.3. We also use another relation (Breger, 1979)

               M_v=-3.052 log P+8.456(b-y)_0-3.121,

where P=0.272 days is the period of light variation (see Hao and Huang, 1993),
to get M_v=-0.20+-0.3.  These two independent determinations of M_v agree very
well with each other within the uncertainty.  So we can take the mean M_v=0.0
as our estimate for the absolute magnitude.  This value is too bright to fit
either F0IV or A7V type star and is due to the extremely large (c_1)_0 which is
an indicator of the luminosity.  We examined our observation carefully, but no
problems were found.

According to the uvby beta indices calibration made by Philip and Relya (1979),
we obtain T_eff=(7700+-200)K, and log g=3.65+-0.1.

  For A7 to F2 type stars, BC=0.0, M_bol=M_v. So, we use the radiation law

                log(R/R_sun)=-0.2M_bol-2logT_eff+8.472

to derive R=(5.0+-1.0)R_sun. Then from

                log g=log g_sun+log(M/M_sun)-2log(R/R_sun),

we have M=(4.0+-1.7)M_sun.

From the uvby beta photometry and the physical parameter determinations we can
find that

(a) UBV measurement and MK classification taken by BSC do not agree well with
each other, and

(b) HR 8851 has an extremely large c_1 in comparison with those stars which
have the same other indices as HR 8851.  This leads to a too bright absolute
magnitude to fit any of the calibrations for the models of A-F stars.

Further observations on this star are necessary.  High resolution spectroscopy
covering a wide range of wavelength is especially desirable for clarifying the
possible effects of the binarity which may be the cause for the problems
mentioned above.

Acknowledgement.  We are grateful to Prof. Jingyao Hu for his help in
providing us with some useful references for this work.


                          JINXIN HAO
                          LIN HUANG
                          Beijing Astronomical Observatory,
                          Chinese Academy of Sciences
                          Beijing, 100080, China

References:

Breger, M., 1979, Publ. Astron. Soc. Pac., 91, 5 [BIBCODE 1979PASP...91....5B ]
Breger, M., 1990, Delta Scuti Star Newsletter, issue 2, 14 [BIBCODE 1990DSSN....2...13B ]
Crawford, D. L., 1975, Astron. J., 80, 955 [BIBCODE 1975AJ.....80..955C ]
Hao, J., and Huang, L., 1993, IBVS, No. 3832
Hoffleit, D., and Warren, W. Jr., 1991, Preliminary Version of the Bright Star
   Catalogue, 5th Revised Edition
Olsen, E. H., 1983, A. Ap. Suppl., 54, 55 [BIBCODE 1983A&AS...54...55O ]
Philip, A. G. D., and Relya, L. J., 1979, Astron. J., 84, 1743 [BIBCODE 1979AJ.....84.1743P ]
Straizys, V., 1977, Multicolor Stellar Photometry (in Russian), p. 105 [BIBCODE 1977mspp.book.....S ]