COMMISSION 27 OF THE I. A. U.
            INFORMATION BULLETIN ON VARIABLE STARS
                         Number 3652

                                               Konkoly Observatory
                                               Budapest
                                               26 August 1991

                                               HU ISSN 0374 - 0676


       A SEARCH FOR ROTATIONAL MODULATION OF T TAURI STARS
                IN THE Tau AND Oph DARK CLOUDS


Many efforts were made in 1980 - 1990 by Rydgren and Vrba (1983), Vrba et
al.(1984), Bouvier et al.(1986), Herbst et al.(1987), Shevchenko (1986) and
others to search for any periodicity in T Tau stars caused by the rotational
modulation of spotted photosphere. Some results of these searches were
analysed by Bouvier (1989). The observed light and colour periodicity can be
used to determine the rotational period, the temperature of the photosphere and
spot as well as possible magnetic fields and accretion structures of T Tau
stars.

To search for the periodicity and other data, photoelectric UBVR -photometry of
T Tau stars was made on Mt. Maidanak using two 60-cm Zeiss reflectors with
pulse counting photometers. 6 stars were observed in 1978-1982. 10-18 ones
per year were observed in 1989-1986. Since 1987 the program "Rotor" including
about 70 stars has been in progress.

The periodicity of T Tau light curves is analysed by modern methods of digital
spectral analysis (Marple, 1987). The methods were adapted to real
observational sets by R.N. Grankin. Preliminary results were published by
Shevchenko (1986), Berdnikov et al.(1991), Grankin et al.(1991). In this paper
we present the most important results of the search for periodicity of 46 T
Tauri stars in the Tau and Oph dark clouds. The negative result of the
periodicity search for 12 stars seems to be caused for great part by deficiency
of observational data.

Values of the periodicity and observational data for 22 stars in TDC and ODC
are listed in Table 1. The values in columns 2-4 refer to our own
observational sets:  amplitude, number of observations and epoch. Columns 5-8
give derived data for the periods discovered:  value, statistical possibility, 
the epoch and amplitude of the periodic process. Column 9 presents the
periods referenced by Bouvier (1989).

Table 1. A search for periods in T Tau stars

Star      Amplitude         Epoch JD   Period  Pos-  Ep. of  m     Per.
name      V_max V_min   n   2440000+   days *  sibi- period  <A>   days
                                               lity   1900+         **

     SR 9 11.30-11.77  154  6607-8101   6.54*  0.99  86-90  0.35B  6.4
    SR 12 13.17-13.44  106  6964-8100   3.50   0.90   90    0.20B  3.4
HARO  1-1 13 07-13.63   98  6974-8101   3.34*  0.92  87-88  0.25B
HARO  1-8 13.88-14.24   84  6965-8101  14.58*  0.97  87-90  0.30B
HARO 1-14 13.75-14.50   88  6964-8101   8.21*  0.98  87-88  0.40B
V1121 OPH 11.13-11.62  128  6955-8123   8.50   0.90   90    0.22V
V2058 OPH 12.60-13.09  143  6607-8101   6.90*  0.87  86-88  0.15B
    T TAU  9.80-10.20  276  6665-8259   2.80   0.75  86-90  0.03B  2.8
   VY TAU 13.49-13.82  223  6378-8265   5.37*  0.97  85-90  0.10B
   AA TAU 12.55-13.90  212  6690-8265   8.17   0.90  87-90  0.43B  8.2
   BP TAU 11.66-12.37  269  6679-8230   7.60   0.70   90    0.20B  7.6
   DF TAU 11.30-13.92  515  3741-8271   7.11*  0.97  78-83  1.00B  8.5
   DG TAU 11.81-13.04  438  4465-8271  15.24*  0.95  80-83  0.35B
   DI TAU 12.80-13.10  274  6710-8271   7.57   0.80  87-90  0.05B  7.9
   DK TAU 11.57-13.85  227  7022-8271   8.20   0.95  87-90  0.90B
   DL TAU 12.60-13.89  222  7017-8270   9.40   0.95  87-90  0.40B
   DN TAU 12.14-12.50  222  6681-8265   6.30   0.85  87-90  0.14B  6.0
   GI TAU 12.48-14.91  245  6681-8270   7.02   0.7?  87-90
 V410 TAU 10.69-11.22  333  6660-8271   1.872  0.99  86-90  0.33B  1.88
 V819 TAU 12.25-12.44   61  8126-8270   5.60   0.94   90    0.19B  5.6
 V827 TAU 12.09-12.30   26  8135-8163   3.75   0.97   90    0.19B  3.75
 V830 TAU 13.10-13.30   58  8123-8270   2.75   0.97   90    0.20B  2.75

Notes:
* the detailed information on SR-9, Haro 1-1, 1-14, and V2058
  Oph is in Berdnikov et al. (1991), VY Tau in Grankin et al.
  (1991), DF, DG Tau in Shevchenko (1986).
** periods from published data referenced by Bouvier (1989),
SR 12 : in Berdnikov et al.(1991) period is 10.2 with a low
  possibility.
DF Tau : in 1978-1983 rotational modulation due to a hot spot
  was present. The spot disappeared after 1984. In 1985-1990
  the periodicity is shown in a low possibility 7.1< P < 8.3
DG Tau : see DF Tau. A hot spot disappeared after 1982. There
  is a 455-day cycle too. In 1986-1990 the possible period is
  8.3< P <16.7 days.
GI Tau : one can see 7.0-7.5 day period against a very
  complex irregular variability.
V410 Tau : very stable period 1.872 during 1986-1990.


                               [FIGURE 1]

 Figure 1. The folded light curve for V1112 Oph (P=8.5 days).
 DK Tau (P=8.2 days) and DL Tau (P=9.4 days).


We refine and confirm periods recently discovered for 11 T Tau stars. The
period of the rotational modulation for DF Tau due to a hot spot was discovered
by Shevchenko (1986) and for the second time by Bertout et al.(1988).

Figure 1 shows summary light curves for three T Tau stars of our program.
Their periods were found by the authors.

Though the observational data for 9 T Tau stars:  RY, CI, CQ, DE, DR, DS, GK,
HN, HP Tau are representative enough, we failed in attempting to find any
periodicity in their light curves. For four more stars in TDC the periods may
be suspected with a low possibility but in this case some difficulties appear
with an irregular variability. On the other hand the periodicity may be of a
different scale.

As we noticed recently (Berdnikov et al. 1991) the assumption of spotted
rotating stars may meet with difficulties.


V.S. SHEVCHENKO, K.N. GRANKIN, M.A. IBRAGIMOV, S.Ju. MELNIKOV,
S.D. YAKUBOV, A.V. CHERNYSHEV
        Tashkent Astronomical Institute, USSR

References:

Berdnikov L.N., Grankin K.N., Chernyshev A.V. et al., 1991,
  Pisma v Astron. Zh., 17, 1, 50. [BIBCODE 1991PAZh...17...50B ]
Bertout C., Basri G., Bouvier J., 1988, Astrophys. J., 330,
  350. [BIBCODE 1988ApJ...330..350B ]
Bouvier J., 1989, Pre-publ. Inst. d'Astrophys. de Paris, N 294.
Bouvier J., Bertout C., Benz W., Mayor M., 1986, Astron.
  Astrophys., 165, 110. [BIBCODE 1986A&A...165..110B ]
Grankin K.N., Ibragimov M.A., Melnikov S. Ju., et al., 1991,
  IBVS, No. 3627.
Herbst W., Booth J.F., Koret D.L. et al., 1987, Astron. J., 94,
   137. [BIBCODE 1987AJ.....94..137H ]
Marple, Jr. S.L., 1887, Digital spectral analysis, Prentice-Hall
  New Jersey.
Rydgren A.E., Vrba F.J., 1983, Astrophys. J., 267, 191. [BIBCODE 1983ApJ...267..191R ]
Shevchenko V.S., 1986, in "Flare Stars and Related Objects", ed.
   Mirzoyan L.V., Yerevan, p.230. [BIBCODE 1986fsro.conf..230S ]
Vrba F.J., Rydgren A.E., Chugainov P.F. et al., 1984, Bull.
   Am. Astron. Soc., 16, 998. [BIBCODE 1984BAAS...16..998V ]