The field high-amplitude SX Phe variable BL Cam: results from a multisite photometric campaign

WOS: 000280505000008
Context. Short-period high-amplitude pulsating stars of Population I (delta Sct stars) and II (SX Phe variables) exist in the lower part of the classical (Cepheid) instability strip. Most of them have very simple pulsational behaviours, only one or two radial modes being excited. Nevertheless, BL Cam is a unique object among them, being an extreme metal-deficient field high-amplitude SX Phe variable with a large number of frequencies. Based on a frequency analysis, a pulsational interpretation was previously given. Aims. We attempt to interpret the long-term behaviour of the residuals that were not taken into account in the previous Observed-Calculated (O-C) short-term analyses. Methods. An investigation of the O-C times has been carried out, using a data set based on the previous published times of light maxima, largely enriched by those obtained during an intensive multisite photometric campaign of BL Cam lasting several months. Results. In addition to a positive (161 +/- 3) x 10(-9) yr(-1) secular relative increase in the main pulsation period of BL Cam, we detected in the O-C data short- (144.2 d) and long-term (similar to 3400 d) variations, both incompatible with a scenario of stellar evolution. Conclusions. Interpreted as a light travel-time effect, the short- term O-C variation is indicative of a massive stellar component (0.46 to 1 M(circle dot)) with a short period orbit (144.2 d), within a distance of 0.7 AU from the primary. More observations are needed to confirm the long-term O-C variations: if they were also to be caused by a light travel-time effect, they could be interpreted in terms of a third component, in this case probably a brown dwarf star (>= 0.03 M(circle dot)), orbiting in similar to 3400 d at a distance of 4.5 AU from the primary.
UNAMUniversidad Nacional Autonoma de Mexico [PAPIIT IN114309]; Junta de AndaluciaJunta de Andalucia; Spanish Direccion General de Investigacion (DGI)Ministry of Education and Science, Spain [AYA2006-06375, AYA2009-10394]; Belgian National Lottery
E.G.H. acknowledges the use of the 1.8-m Plaskett Telescope at the Dominion Astrophysical Observatory, Herzberg Institute of Astrophysics, National Research Council of Canada. J.H.S. is very grateful to his group of students at the Physics Department of the Virginia Polytechnic Institute and State University. L.F.M. and M.A. acknowledge financial support from the UNAM under grant PAPIIT IN114309. S.F. thanks Russ Robb (University of Victoria, Canada) who provided the unpublished times of light maxima, and Jerome Berthier and Francois Colas (institut de mecanique celeste et de calcul des ephemerides, France). This investigation was partially supported by the Junta de Andalucia and the Spanish Direccion General de Investigacion (DGI) under projects AYA2006-06375 and AYA2009-10394. The observations were partially collected with the 1.5m telescope at SNO which is operated by the Instituto de Astrofisica de Andalucia. Data from Pic du Midi Observatory have been obtained with the 0.6 m telescope, a facility operated by observatoire Midi-Pyrenees and Association T60, an amateur association. Part of the data used in this work was acquired with equipment purchased thanks to a research fund financed by the Belgian National Lottery (1999). We acknowledge with thanks the variable star observations from the American Association of Variable Star Observers international database contributed by observers worldwide and used in this study. This research has made use of both the Simbad database, operated at CDS, Strasbourg, France, and the Astrophysics Data System, provided by NASA