2. Metallicism and pulsations: observations
CP stars are found almost everywhere in the HR diagram. Some of these stars are evolved and their observed abundance peculiarities reflect nuclear processes. Others are compact objects in which diffusion is well established (Michaud & Fontaine 1984). Many of these objects are variable and they are very favourable for asteroseismology.
We concentrate mainly on main-sequence stars in which diffusion is thought to be the principal cause of abundance anomalies. These are found from early F-type (Fm) stars to late B-type (HgMn) stars, including many varieties of A stars [Am, Ap, Bootis, Puppis]. Within these spectral classes many types of variable stars are also found [ Doradus, several classes of Scuti, and Slowly Pulsating B (SPB) stars].
In A-type stars, almost 70% of non-CP stars are Scuti variables at current levels of sensitivity. Most non-variable A stars are Am stars.
Amongst the CP stars which happen to be pulsating, the most conspicuous are the "rapidly oscillating Ap stars" (roAp) first discovered by Kurtz (1978a). These are found amongst the coolest of the magnetic Ap stars which exhibit large abundance anomalies of many heavy elements. They have generated considerable interest because of the relatively large number of observed overstable modes which makes them promising objects for asteroseismology. See, for example, Matthews (1991) for a review.
In non-magnetic stars, however, variability and anomalous abundances are found in very few stars simultaneously. Over the years some mildly metallic stars have been found to exhibit some variability. Baglin (1972) suggested that if diffusion is the cause of the Am phenomenon, Am stars should not pulsate. Some mild Am stars and evolved CP stars were then already known to be variable (Kurtz 1976) but all classical Am stars which were thought to be variable in the early 70's were subsequently found to be stable. Since then Kurtz and his collaborators have been the principal investigators in the search for variable CP stars. They have assembled a short list of metallic stars, most if not all of them fairly evolved, which are also Scuti-type pulsators (Kurtz 1978b, 1984, 1989; Kurtz et al. 1995; Kurtz 2000).
Amongst these stars, some have been thought to be problematic. In particular, Kurtz (1989) claimed to have discovered a pulsating classical Am star. The problem lies in its apparently large abundance anomalies, not typical of the other known variable non-magnetic CP stars. Also, the variable evolved Am star HD40765 has been considered by Kurtz et al. (1995) to be problematic because of the possibly large surface velocities involved. A point that has been made repeatedly is that the large velocities caused by pulsations, estimated in this star to be of the order of at the surface, might generate turbulence in the interior which would in turn hinder the formation of the required surface-abundance anomalies.
In addition to variable CP stars there are some Scuti stars that are particularly interesting from our point of view. First there are some otherwise seemingly run-of-the-mill Scuti stars 1 in which peculiar abundances (Russell 1995; Rachkovskaya 1994) superficially consistent with what is expected as a result of diffusion seem to have been found.
Second, the high-amplitude Scuti stars are interesting because they are characterized by very small (Solano & Fernley 1997). As CP stars are mostly slow rotators (), one should find different velocity distributions in CP stars and in Scuti stars. Indeed, observations show that Scuti stars are on the average fairly fast-rotating stars, with the exception of the high-amplitude stars. In other respects the high-amplitude Scuti stars do not differ from their more common low-amplitude counterparts. They are, however, known to be evolved stars and as such might not feature significant abundance anomalies.
Although observations do not completely rule out variability in Am stars, they do pose rigorous constraints on them. Either it is an extremely rare occurrence or pulsations are of extremely low amplitude. An extensive search of the Hipparcos database by many authors revealed many new variable stars (Aerts et al. 1998; Waelkens et al. 1998; Paunzen & Maitzen 1998). Significantly, all the newly discovered variable CP stars were found to be magnetic (Ap or Bp). All the other variable stars found in this way are of the known families of variable stars, i.e. , Doradus, SPB and a few Cephei stars. The expected sensitivity of these surveys ranges from as low as 3 to 55 mmag depending on the brightness of the object (Eyer & Grenon 1998).
The CP stars found most recently to be variable are Bootis stars in which nonradial pulsations have been detected (Paunzen et al. 1998).
© European Southern Observatory (ESO) 2000
Online publication: August 17, 2000