Young pre-main-sequence (PMS) stars of mass greater than 1.5 cross the region of pulsational instability during their contraction toward the main-sequence. The location of the instability strip in the H-R diagram has been recently identified by means of nonlinear models for the first three radial modes (Marconi & Palla 1998). The time spent by intermediate-mass stars within the boundaries of the strip represents a small fraction of the Kelvin-Helmoltz time scale, varying between 10% for a star of 1.5 and 5% for a 4 star. Despite the brevity of this phase, however, a number of known Herbig Ae stars have the appropriate combination of luminosity and effective temperature to become pulsationally unstable. In our previous study, we suggested to look for Scuti-type photometric variations with periods of minutes to several hours and amplitudes less than few tenths of magnitudes in a sample of Herbig Ae stars whose position in the H-R diagram coincides with the instability strip.
The identification of a few PMS objects pulsating as Scuti stars (Breger 1972), the prototype being the star HR 5999 (Kurtz & Marang 1995), has provided some support to the connection between variability and stellar pulsation. Of particular interest is the Ae star HD 104237 that shows both short- and long-term velocity changes of spectral lines (Donati et al. 1997). These variations indicate that the star is undergoing radial pulsations with a period of approximately 40 minutes and an amplitude of about 1 km s-1 (see also Böhm et al. 2000, in preparation). Interestingly, HD 104237 is the first intermediate mass PMS star with a measured magnetic field (Donati et al. 1997). A few new PMS candidate pulsators have been recently identified by Pigulski et al. (2000).
Stimulated by these initial results, we have started a photometric investigation of a sample of seven Herbig Ae stars with spectral types in the range A5 to F5, located within or near the boundaries of the instability strip. For some of them, large time scale variations have been observed during the long term monitoring program of variable stars conducted at ESO (LTPV project: Sterken et al. 1995 and references therein). However, no information is available on their variability on time scales shorter than 2 or 3 days.
The main goal of our study is to detect and characterize the pulsation properties of young stars. This way, we can improve our knowledge of their internal structure and obtain unique constraints on the theoretical predictions of the models. Ultimately, the analysis of the pulsation characteristics can yield an indirect estimate of the stellar mass. This represents a powerful method for stars that are not part of the restricted group of spectroscopic binary systems. In this Letter, we report the discovery of two additional Herbig Ae stars which show evidence for variability of the Scuti-type.
© European Southern Observatory (ESO) 2000
Online publication: March 21, 2000