Peremennye Zvezdy (Variable Stars) 33, No. 7, 2013
Received 24 August; accepted 30 August.
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|The new variable star HD 305629, with a Cepheid-like light curve, is demonstrated, from our optical observations and ASAS-3 data, to actually be an ellipsoidal variable star on a highly eccentric orbit. It resembles the X-ray variable star BP Cru. HD 305629 deserves further observations in other wavelengths.|
During a systematic search of variable stars in the publicly available ASAS-3 database (Pojmanski 2002), apparent variability was found in the star HD 305629 (10513435, -604757 4, J2000.0; Fig. 1). The ASAS-3 light curve of the target is partially contaminated by HD 305630, a B1V star with , , and lying away. As a result of this, the scatter in the ASAS-3 observations is high. The light curve shape folded with a period of 5118 showed a rapid rise to maximum and a slower decline which at first sight made us think of a bright long period classical Cepheid as a possible classification. However, the spectral type in the literature was B8 (Nesterov et al. 1995). The amplitude was also barely above 01 which is too low for such a type but that could be explained by the presence of a bright companion reducing the amplitude. The B8 spectrum could belong to that companion. Tycho-2 photometry converted to the system using Bessell's tables (Bessell 2000) gives and . 2MASS color is 0.25, which indicates reddening. After closer examination, a rapid drop in brightness right at the star's maximum peak was found, but the scatter in the light curve casted some doubts about its reality.
Fig. 1. Finding chart showing the variable and the comparison and check stars. North is up. The image scale is 1 41/pixel. This ensured that appropriate apertures could be easily chosen to avoid any overlap with the comparison star.
Since a Cepheid would be redder and would display color changes, photometry (see next section) was performed to check for the presence of those changes and also to determine accurate photometry for HD 305629 without the interference of the companion.
All observations were carried out from December, 2011 to April, 2013 using a Meade 12-inch Schmidt-Cassegrain GPS telescope located at Carnes Hill Observatory. The camera used was an SBIG ST9 XE utilizing SBIG filter wheel and Custom Scientific photometric filters. Photometry was performed using AIP4WIN 2.0 with apertures carefully selected to exclude surrounding stars. The same apertures were used for all measurements to ensure consistency. Magnitudes and colors are transformed to the standard system using Southern E-field stars E4103, E4106, and E4108. Corrections for primary and secondary extinction were applied in determining the magnitudes of the comparison and check stars. Plate solutions were obtained for the field using MPO Canopus software (version 220.127.116.11) to confirm the coordinates of each of the stars of interest. The comparison stars (identified in the finding chart in Fig. 1) used were:
Comp star = HD 305630; (J2000); ; ; ; . Check star = TYC 8957-3762-1; (J2000); ; ; ; .
The light and color curves of HD 305629 from our observations are presented in Fig. 2.
Fig. 2. The light curve and color curves of HD 305629. No apparent color changes are seen which - together with the clear detection of the two different maxima and minima - rules out the Cepheid classification.
After the magnitude of our target was determined, we corrected
the ASAS-3 dataset
500,500) from the light contamination and the resulting difference in amplitude was only 005. The mean resulting colors were: ; ; ; . The relatively faint (9.34) and (9.07) magnitudes indicated that the Cepheid plus B8 companion hypothesis was not possible. Also there were no color changes revealing pulsational behavior.
Combining our dataset with the ASAS-3 observations, we determined
the light elements:
Fig. 4. A zoomed in portion of the light curve around periastron passage showing the two maxima and two minima.
Among B-type stars, we find one similar example in our Galaxy, the binary system BP Cru made up of a B2Ia supergiant and an X-ray pulsar in an eccentric 41487 orbit. This is an improved period for the system based on ASAS-3 and LTPV (Manfroid et al. 1991) data; see the light curve in Fig. 5. BP Cru undergoes X-ray outbursts just before periastron passage (Watson et al. 1982).
Fig. 5. Light curve of the ellipsoidal high mass X-ray binary BP Cru. The scatter in the phase plot is due to mean magnitude changes due to the binary interaction. The epoch of minimum is HJD 2445442.7.
Observations of HD 305629 in other wavelengths are encouraged.
Acknowledgments: This study was possible thanks to the funding and equipment provided by the Astronomical Association of Queensland, Australia. We also made use of the VizieR database operated at the Centre de Données Astronomiques (Strasbourg) in France.
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