The field high-amplitude SX Phoenicis variable BL Camelopardalis: results from a multisite photometric campaign

Abstract

Context. BL Cam is an extreme metal-deficient field high-amplitude SX Phe-type variable where a very complex frequency spectrum is detected, with a number of independent nonradial modes excited, unusual among the high-amplitude pulsators in the Lower Classical Instability Strip.Aims. An extensive and detailed study has been carried out to investigate the pulsational content and properties of this object. Methods. The analysis is based on 283 h of CCD observations obtained in the Johnson V filter, during a long multisite photometric campaign carried out along the Northern autumn-winter of [2005][2006]. Additionally, multicolour BI photometry was also collected to study the phase shifts and amplitude ratios, between light curves obtained in different filters, for modal discrimination of the main excited modes. Results. The detailed frequency analysis revealed a very rich and dense pulsational content consisting of 25 significant peaks, 22 of them corresponding to independent modes: one is the already known main periodicity f 0 = 25.5765 cd −1 (∆V = 153 mmag) and the other 21 are excited modes showing very small amplitudes. Some additional periodicities are probably still remaining in the residuals. This represents the most complex spectrum ever detected in a high-amplitude pulsator of this type. The majority of the secondary modes suspected from earlier works are confirmed here and, additionally, a large number of new peaks are detected. The amplitude of the main periodicity f 0 seems to be stable during decades, but the majority of the secondary modes show strong amplitude changes from one epoch to another. The suspected fundamental radial nature of the main periodicity of BL Cam is confirmed, while the secondary peak f 1 = 25.2523 cd −1 is identified as a nonradial mixed mode g 4 with = 1. The radial double-mode nature, claimed by some authors for the main two frequencies of BL Cam, is not confirmed. Nevertheless, the frequency f 6 = 32.6464 cd −1 could correspond to the first radial overtone.

SX Phoenicis period-luminosity relations and the blue straggler connection

“…The photometry of the two SX Phe in the LMC cluster NGC 2155 has been dereddened using the distance modulus and extinction law employed by Otulakowska et al (2011). For this cluster and the LMC δ Scuti, the reported amplitudes A I have been converted to A V assuming an amplitude ratio of A V / A I = 1.9 from Rodríguez et al (2007) 3…”

“…Pych et al 2001; Olech et al 2005), secure identification of pulsation modes is often difficult, especially in the case where only one pulsation mode is observed. This problem is exacerbated by the fact that the non‐radial modes have been observed to cluster near radial modes in frequency space for at least one field SX Phe (Rodríguez et al 2007) as well as some δ Scuti (Breger, Lenz & Pamyatnykh 2009). Observationally expensive solutions to this problem do exist, including the use of phase shifts and amplitude ratios from multiband time series photometry (Daszyńska‐Daszkiewicz 2008), time series spectroscopy (e.g.…”

“…3 3 This value is based on fig. 7 of Rodríguez et al (2007), and their Sections 5.2 and 5.3 imply that this value should be relatively insensitive to changes in T eff and log g . However, these amplitudes should still be regarded as somewhat uncertain, especially in the case of the high‐amplitude SX Phe in NGC 2155. …”

The Overlooked Role of Stellar Variability in the Extended Main Sequence of LMC Intermediate-Age Clusters

“…Since Fourier parameters exist only for the observations in I, light curves in V were generated as scaled up versions of the I light curves applying to A 1 the A V /A I = 1.7 empirical factor given by Rodríguez et al (2007). The period distribution for these simulated variables was taken as a normal distribution centered in 0.075 days and with dispersion of 0. light curves were "observed" at one fixed time for each filter, that is, taking for each variable a measurement from their light curve at a random phase and putting back these magnitudes into the CMDs.…”

New short-period stellar pulsators at large Galactocentric distances

“…For sources which fell within our search range, we manually inspected each light curve to verify that the light curve was consistent with that of a stellar pulsator (cf. Rodríguez et al 2007 for a recent example of a light curve for an SX Phe star) and to exclude light curves of low quality. We found 31 sources which showed a modulation in their light curve during a period between 51 and 83 min and had a mean brightness of g = 15.9-20.8.…”