COMMISSIONS 27 AND 42 OF THE IAU 
          INFORMATION BULLETIN ON VARIABLE STARS 
                       Number 3720
 
                                             Konkoly Observatory 
                                             Budapest 
                                             30 April 1992 
 
                                             HU ISSN 0374 - 0676 
 
 
 
 
DISCOVERY OF 5.66 DAY PERIODICITY OF HBC 379 = NTTS 041636+2743 = TAP 29 = 
 
                                 LkCa 7 
 
 
 
   While T Tauri stars (TTS) are noted for their irregular variability on 
many time-scales, there are some early detections of quasi-periodicities on 
time-scales of a few days due to the rotational modulation. A likely 
explanation of the observed rotational modulation is the presence of dark 
(or hot) spots. 
   To search for the strict periodicities, photoelectric UBVR-photometry 
of about 70 TTS was made on Mt. Maidanak in 1986-1992. The results were 
published by Berdnikov et al. (1991), Grankin et al. (1991), and Shevchenko 
et al. (1991). 
   Observations of a number of new objects, so called "naked" TTS (Walter 
et al., 1988) were began by the author in 1990. In this paper we present 
the most important results of the search for periodicity of the object HBC 
379 = NTTS 041636+2743 = TAP 29 = LkCa 7. 
   The star HBC 379 is a K7-M0 V pre-main-sequence star discovered in the 
CaII H and K emission-line survey of Herbig et al. (1986). It is a variable 
(12.3m < V < 12.6m) with weak Halpha emission (E.W. = 4 A), 
strong lithium absorption (E.W. = 0.5 A), low obscuration 
(E_(B-V) = 0.2), slow surface rotation (V sin i < 11 km/s), no infrared 
excess, bolometric luminosity of approx. 1 L_sun, and radial velocity consistent with 
membership in Tau dark cloud (Herbig et al., 1986, Hartmann et al., 1987). HBC 379 is also an 
x-ray source (Feigelson et al., 1987). 
   Our observations of HBC 379 were made in 1990-1991 on Mt. Maidanak using 
the 0.5m reflector with UBVR-pulse counting photometer. The limits of 
the light variations, average colours and number of observations are listed 
in Table I. 
   To search for a period in the light variability, the observations were 
analysed by the method of digital spectral analysis (Grankin et al., 1991). 
The analysis yields a period of 5.66+/-0.005 days. Phase diagrams for 
lightcurve in the UBVR filters for a period of 5.66 days are displayed in 
Figure 1. 


                                 [FIGURE 1]

 Figure 1. Phase diagram of HBC 379 in the UBVR filters (P=5.66 days). The
 observed data points are shown by filled dots and the solid line is a
 synthetic light-curve calculated with parameters given in Table II. Error
 bars show +-1 standard deviation.

 
                         Table I. Photometric data of HBC 379 
 
J.D.2440000+       n    Vmax Vmin     <V>      <U-B>   <B-V> <V-R> 
8182- 8230         18   12.21 12.68   12.441   1.174   1.348 1.338 
8503- 8572         36   12.20 12.81   12.443   l.063   1.390 1.350 
 
Table II.               Parameters and results of HBC 379 spot model 
Fixed parameters 
Rotation period:                                       5.66 days 
Photospheric temperature:                              3800K 
Limb Darkening Coefficient in B:                       1.0 
                        in V:                          0.89 
                        in R:                          0.74 
Results 
Spot temperature:                                      3400 +/- 25K 
Rotation axis inclination:                             36deg +/- 1deg 
Spot polar distance:                                   20deg +/- 1deg 
Spot radius (in degrees):                              65deg +/- 2deg 
 
 
 
 
 
  The amplitudes of the periodic process change considerably for each 
filter (0.37m R, 0.52m V, 0.65m B). We studied the light 
variations appearing 
as a consequence of a dark spot on the photosphere of the star. The shape 
of the light-curve may depend on the position of the spot on the stellar 
surface, the orientation of the rotational axis relative to the observer, 
the size of the spot, and the temperature difference between the spot and 
the photosphere. 
  The range of spot properties for which we find acceptable fits to the 
observed light-curves and the actual parameters used for computing the 
synthetic light-curves is given in Table II. It should be noted that the spot 
size has a lower limit due to the large changes of the colour indices 
(>0.15 mag). These changes are possible if the radius of the spot exceeds 50 
degrees. 
  Persistency of a very large cold spot or a compact cluster of spots 
(the fractional stellar surface covered by the spot approx. 30%) is 
difficult to 
explain in the frame of the hypothesis of solar-type magnetic activity. We 
mean the localization of the spot or the cluster of spots on one hemisphere 
of the stellar surface. 
 
 
                        K.N. GRANKIN 
 
                        Astronomical Institute 
                        Academy of Sciences of Uzbekistan 
                        Astronomical str. 33 
                        700052 Tashkent 
                        Uzbekistan, CIS 
 
 
 
References: 
 
Berdnikov, L.N. et al.: 1991, Pisma v Astron. Zh., 17, 1, 50.  [BIBCODE 1991PAZh...17...50B ]
Feigelson, E.D. et al.: 1987, Astron. J., 94, 1251.  [BIBCODE 1987AJ.....94.1251F ]
Grankin, K.N. et al.: 1991, I.B.V.S., No. 3658. 
Hartmann, L.W. et al.: 1987, Astron. J., 93, 907.  [BIBCODE 1987AJ.....93..907H ]
Herbig, G.H. et al.: 1986, Astron. J., 91, 575.  [BIBCODE 1986AJ.....91..575H ]
Shevchenko, V.S. et al.: 1991, I.B.V.S., No. 3652. 
Walter, F.M. et al.: 1988, Astron. J., 96, 297.  [BIBCODE 1988AJ.....96..297W ]