2005 Young Naturalist Awards - A Search for Variable Stars in Two Northern Open Clusters: NGC 381 and NGC 637

Morgan A Search for Variable Stars in Two Northern Open Clusters: NGC 381 and NGC 637

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Figure 3: Star 4 (Click to enlarge)

Star 4 is a short-period pulsating star with a V-magnitude of 11.1. The BVRI measurements show that the star has an absolute magnitude of -2.58 magnitude and a B-V color index of -0.18 magnitude, indicating star 4 is an early B-type star (Pickles 1998). The star was found to be pulsating simultaneously in two modes. Its main frequency has an amplitude of 0.14 magnitudes and a period of 4.55 hours (0.19 days). A secondary frequency with a 3.85-day period and an amplitude of 0.04 magnitude also exists. This analysis was done with the Period 98 software. The star's light curve has a fairly sinusoidal shape. From its light curve and its colors, star 4 is most likely a Beta Cephei (BCEP) type variable. Figure 3 shows a light curve for Star 4 phased on the star's primary period.

Figure 4: NGC 637 stars 3 and 4 (Click to enlarge)

Star 3's variability can be seen in Figure 4, which shows the light curves of the stars in the I-band over five nights of study. It is 10.7 magnitude in V, with an absolute brightness of -2.91 magnitude. The color index of the star (-0.22 magnitude) indicates an early B spectral type. The star's amplitude is approximately 0.1 magnitude. The data collected suggest a 0.8-day period. While star 3 appears to be a pulsating variable, more data is needed to truly understand its variation.

Conclusions: A search was conducted for new variable stars in two northern open clusters, NGC 637 and NGC 381. Photometric measurements of more than 1,100 stars down to a limiting magnitude of about I = 15 were collected between the two target fields. Two new variables were identified in NGC 637. Because the stars are both members of NGC 637, their distance moduli, in magnitudes and reddening coefficients, are known. Both are fairly bright, approximately magnitude 11.1 and 10.7 in V. The extinction and transformation equations of the site and the equipment setup were defined, allowing absolute photometry of any star in the BVRI wavelengths. This allowed the stars' colors to be measured, showing that they are both spectral type B. Star 4 is most likely a Beta Cephei-type pulsating variable. More data is needed to fully understand the variation of star 3, but it lies within the pulsational instability regions of the upper main sequence of the HR diagram, and is very likely a pulsating variable (Pamyatnykh 1999). Star 3 offers an opportunity for further research; with more information it should be possible to identify the cause of this star's variation. Table 2 summarizes the information collected on the two new stars found in NGC 637.

Table 2:
	Period	Amplitude	Spectral type
Star 3	0.8d?	0.1m	B
Star 4	0.19d, 3.85d	0.15m, 0.04m	B

This research once again demonstrates the potential that amateur astronomy equipment has in the study of variable stars. Many amateurs own equipment similar to that which was used to conduct this research. The system described in this paper will hopefully motivate some amateurs to put their equipment to use in a similar manner.

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