Observational Asteroseismology of slowly pulsating B stars

Abstract

We review the status of observational asteroseismology of slowly pulsating B (SPB) stars. Their asteroseismic potential is extremely good because the excited high-order g-modes probe the deep interior of these hot stars. To enable asteroseismic modelling, a sufficient amount of well-identified modes is mandatory. To reach this goal with ground-based observations, dedicated long-term and preferably multi-site campaigns are needed to increase the number and the accuracy of detectable frequencies. The first results for SPB stars based on observations obtained with the asteroseismic space-mission MOST are very promising, guaranteeing the success of missions like CoRoT, launched in December 2006. These results also indicate that high-precision observations are needed to detect and to study low-amplitude SPB stars. Although SPB pulsations are not restricted to slow rotators, there is some observational evidence for an amplitude drop towards high values of the projected rotational velocity. For several SPB stars, close frequency multiplets are observed. In some cases, the observed frequencies might be components of a rotationally split mode, but in other cases an alternative explanation is needed. Magnetic fields of a few hundred Gauss, that recently have been detected for fourteen confirmed members, can cause such frequency shifts. SPB stars can no longer be considered as non-magnetic stars and magnetic fields should be included in the theoretical models. We argue that mode identification of g modes still remains one of the main obstacles, although progress has been made in this field recently. Asteroseismic potential After conducting a systematic study of variability amongst B type stars, Waelkens (1991) introduced the slowly pulsating B (SPB) stars as an independent class of stars pulsating in high-order, low degree gravity modes (g modes) with typical periods of the order of days. These modes are excited by the opacity mechanism acting on the metal-bump. They are trapped deep in the interior of these hot stars, making them very interesting from an asteroseismic point of view. On the other hand, they are very difficult targets for in-depth asteroseismic studies because the theoretical frequency spectra of SPB stars are very dense, the observed amplitudes are low (cf. Fig. 4), and most of the currently known SPBs are multi-periodic, giving rise to beat periods of the order of months or even years. Currently, at least 51 confirmed and 65 candidate galactic SPB stars are known, of which 15 are in open clusters. Thanks to the OGLE-II and MACHO databases, extra-galactic SPBs were recently found: 59 in the LMC and 11 in the SMC (Ko laczkowski et al. 2006). For the SPB stars observed in the Geneva photometric system, the effective temperatures and surface gravities were determined with the code CALIB in the same way as described by De Cat et al. (2007). As shown in Fig. 1, these stars cover the (young) part of the theoretical SPB instability strip. This figure also illustrates the existence of a common part of the theoretical instability strip of the β Cep and SPB stars. At least 6 β Cep/SPB hybrids are currently known: 53Psc (LeContel et al. 2001), ιHer (Chapellier et al. 2000), ν Eri (Jerzykiewicz et al. 2005), HD 886 (Chapellier et al. 2006), HD13745, and HD19374 (De Cat et al. 2007). Since they simultaneously pulsate in low-order p/g modes and high-order g modes probing both the outer layers and the deep interior of these stars, they are ideal asteroseismic targets. 168 Observational Asteroseismology of slowly pulsating B stars Figure 1: Position in the (log(Teff ),log g)-diagram of the candidate (open symbols) and confirmed (full symbols) SPB stars for which Geneva photometry is available. The triangles indicate the hybrid β Cep/SPB stars. The stars with a detected magnetic field are given in black. The lower and upper dotted lines show the ZAMS and TAMS, respectively. The dashed lines denote evolution tracks for stars with M= 15, 12, 9, 6, and 3 M . The dash-dot-dot-dotted and dash-dotted lines represent the theoretical instability strips for β Cep and SPB modes provided by De Cat et al. (2007). A typical error bar is given in the lower left corner.