Astron. Astrophys. 355, L35-L38 (2000)
1. Introduction
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
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