Astron. Astrophys. 355, 1031-1040 (2000)

1. Introduction

Rapidly oscillating Ap (roAp) stars are cool magnetic Ap SrCrEu stars which pulsate in high-overtone (), low-degree () p-modes with periods from 6 to 15 min and typical amplitudes of a few mmag. 31 such stars are currently known (Kurtz & Martinez 1993, 1994, 1995; Martinez et al. 1998; Handler & Paunzen 1999). Detailed reviews about roAp stars have been published by Kurtz (1990), Matthews (1991) and Martinez & Kurtz (1995a).

Determinations of asteroseismological luminosities (Kurtz & Martinez 1993; Martinez 1993; Matthews et al. 1999) suggest that roAp stars lie within the instability strip where the  Scuti pulsating variables are found. This leads to the suspicion that the -mechanism operating on He  II in the He  II ionization zone is driving the pulsation.

Rapidly oscillating Ap stars are ideal targets for application of the techniques of asteroseismology. By comparing the observed frequency spectrum to the asymptotic pulsation theory, it is possible to specify their rotation periods, their temperatures, luminosities, radii, masses, their atmospheric structures, their evolutionary statuses, and geometries of their magnetic fields.

In terms of the Strömgren photometric indices, the currently observed limits of the roAp phenomenon are (Martinez 1993):

In terms of the dereddened parameters, the limits are:

However, photometric indices in those ranges are not an unambiguous indicator of the roAp phenomenon. Other Ap stars co-exist in the same region of the parameter space, in which no pulsation could be detected, despite sometimes thorough searches (Martinez & Kurtz 1994).

Recent studies (e.g., Nelson & Kreidl 1993; Martinez 1993; Kupka et al. 1994; Kupka et al. 1996; Mathys 1993, 1994; Mathys et al. 1997; Mathys & Hubrig 1997) show that these Ap stars for which null results of searches for pulsations were reported (hereafter non-oscillating Ap stars, or noAp stars), are remarkably similar to the roAp stars in many respects (e.g. colour indices, abundances, magnetic fields). The first hint of a difference between the two groups was recently found through a study of their kinematical properties (Mathys et al. 1996, hereafter Paper I), based on proper motions taken from the Positions and Proper Motions Star Catalogue (PPM) (Röser & Bastian 1991; Bastian & Röser 1993) and measurements of radial velocities. It unveiled the existence of significant kinematical differences between roAp and noAp stars, suggesting that roAp stars are older than their non-pulsating counterparts. However, the conclusions drawn in that work were limited by the accuracy of the available astrometric data. In this paper we present the kinematical properties for roAp and noAp stars which we have determined by using new Hipparcos proper motions and Hipparcos parallaxes and our own radial velocity measurements.

The reader should keep in mind that we call "noAp stars" those stars in which no oscillations have been detected so far above a certain threshold (which typically differs from star to star), but that one cannot rule out that pulsations may be discovered in some of them in the future. Statistically speaking, though, noAp stars truly represent a group where pulsations have very small amplitudes, if they exist at all.

© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
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