Time-Series Photometry of Beta Cephi Variables in NGC 6231

September 10, 1999

NGC 6231 is one of the youngest galactic open clusters with most of the brightest stars very close to the zero-age main sequence (ZAMS). NGC 6231 has been found to contain six beta Cep variables. The discovery of beta Cep stars in open clusters is useful because it allows a more accurate determination of their physical parameters and the modes of pulsation.

Time-series UBV photometry of beta Cepehi variables in NGC 6231 was taken with 1.54m Danish telescope at La Silla in July 30/31 1999. There are 128 frames in each band and total number of CCD-frames (including Flats and Bias) are 482 1050x1026 pix. CCD procesing was done with IRAF and after that multi-aperture photometry was performed.

For analyzing the periodicities of the new data in the B bands for each star, we used both parametrical methods for searching the periods. First, the FT (Fourier Transform) was applied to identify significant patterns in time. The Deeming power spectrum was computed. After the detection of a basic period from the power spectrum, a LS (Least Square) fit of a model with two harmonics was applied to determine a more accurate value of the period. This reminement was obtained by fitting of the original data with the model:

M(t,v,c,a₁,b₁,a₂,b₂)=c+a₁cos(2PIvt) + b₁ sin(2PIvt) + a₂cos(4PIvt)+b₂sin(4PIvt):

Here, the constant parameter c is the mean magnitude in the respective band, and a₁,b₁,a₂,b₂ are the amplitudes of the harmonics of the sine and cosine functions.

The LS fit gives the estimates of the parameters c, (a_r,b_r) with their standard deviations (dc, da_r, db_r) and residuals of the data for a possible search for another period. In this way, a second period may be identified. The second period was found and reddened by analyzing the residuals using the same techniques as above after subtracting the best fitting model corresponding to the first, more significant period.

Stars	S 261	S 110	S 150	S 253
P (days)	0.0967 +/-0.0034	0.184 +/-0.022	0.136 +/-0.014	0.1452 +/-0.0050, 0.0669 +/-0.0017
Mag (mmag)	14.5	10.3	9.0	7.6, 11.2

Table 1: The results of the periods from the LS modeling process. The periods with their errors have also been listed for each star.

Figure 1: CCD image of NGC6231 open cluster in B band

Figure 2: Light curve fitting for S 261