A comprehensive study of the SX Phoenicis star BL Camelopardalis

¹ Association AstroQueyras, Le bois de Bardon, Taponnat, 16110 La Rochefoucauld, France e-mail: stephane.fauvaud@wanadoo.fr
² Instituto de Astrofísica de Andalucía, CSIC, PO Box 3004, 18080 Granada, Spain e-mail: eloy@iaa.es
³ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China
⁴ Department of Physics, Astronomy, and Materials Science, Missouri State University, 901 S. National, Springfield, MO 65897, USA
⁵ Observatoire de la Côte d'Azur, BP 4229, 06304 Nice Cedex 4, France
⁶ Institut de Ciéncies de l'Espai, CSIC, Facultat de Ciéncies, Campus UAB, Torre C5-parell, 2$^a$ planta, 08193 Bellaterra, Spain
⁷ Institut d'Estudis Espacials de Catalunya (IEEC), C/Gran Capitá 2-4, 08034 Barcelona, Spain
⁸ Norwegian Astronomical Society, Engeshaugen 15, 8616 Mo i Rana, Norway
⁹ Sulphur Flats Observatory, 449 Sulphur Creek Road, Jemez Springs, NM 87025, USA
¹⁰ Club d'Astronomie Lyon Ampère, 37 rue Paul Cazeneuve, 69008 Lyon, France
¹¹ American Association of Variable Star Observers, 25 Birch Street, Cambridge, MA 02138, USA

Received: 18 July 2005
Accepted: 5 January 2006

Abstract

Context.BL Cam is an extreme field multiperiodic short-period low-metallicity high-amplitude SX Phe-type variable where its probably complex pulsational content is subject of controversies.

Aims.A comprehensive study has been carried out to investigate the nature and pulsational properties of this object.

Methods.The analysis is based on new photometric data collected during the last few years at different sites, as well as on all available previously published data. Frequency analyses have been performed on a number of reliable data sets to analyse the pulsational content. In addition, the classical O-C method was used to study the behaviour of the main period.

Results.Our frequency analysis confirms some aspects on the multiperiodicity of BL Cam, previously found by other authors, showing a number of secondary modes close to the main frequency  cd^-1. The secondary modes present eventual amplitude variations. However, the main frequency exhibits no significant changes in its amplitude and seems to correspond to the fundamental mode of radial pulsation. Abrupt changes sometimes observed in the light curves could be due to spurious activity on or close to the star's photosphere. We confirm the existence of a secondary frequency at 31.6 or 32.6 cd^-1, with variable amplitude, and with the possibility of both frequencies being intrinsic to the star. More than one hundred of new times of light maxima have been determined in the present work. These together with those available in the literature allow us a more discriminating analysis of the O–C diagram. This shows that the observed variations of the main period can be described by two terms: (i) a secular increase of the main period at a rate of  yr^-1; and (ii) a perturbation from a companion star in a rather eccentric orbit with a period of 10.5(±0.2) yr causing a light time semi-amplitude of 148(±12) s.