Pre-Main Sequence Stars
The study of the first stages in the formation of stars is one of the currently most active research fields in stellar astronomy. Although much progress has been made since the discovery of T Tauri stars in 1942 several questions still remain open. There are basically two classes of problems that need to be assessed: the detailed structure and evolution of single stars from the protostellar phase to the main sequence, and the nature of the processes that determine and regulate the onset of star-formation within molecular clouds in the first place. Pre-main sequence (PMS) stars are characterized by a high degree of activity like winds, jets, outflows etc. They are interacting with the circumstellar environment in which they are embedded. Mostly they have strong infrared and/or UV excesses and show photometric and spectroscopic variability on time scales from minutes to years. On time scales of weeks to hours they are generated by variable extinction due to cirumstellar dust, by clumped accretion and chromospheric activity. On time scales of half an hour to some hours the variation may be due to pulsation if the star lies in or near the instability strip. Stars over essentially the whole mass spectrum can become pulsationally unstable during various stages of their evolution. The fact that young stars during their evolution to the main sequence move across the instability region raises the possibility that at least part of the observed stellar activity could also be due to stellar pulsations. One method to separate the different contaminations (from reflection nebulae, aligned interstellar dust, gas and dust shells...) of the intrinsic star light is to study the variability (if any) in all the observable physical parameters as a function of time. The discovery of pulsation in PMS stars is extremely important, since it provides a unique means for constraining the internal structure of young stars and for testing evolutionary models. Although it is difficult to put constraints on the characteristics of young stars to define whether they are in their PMS phase of evolution or not, two different types of (possible) PMS objects can be identified:
Pulsation In Pre-Main Sequence Stars
Most of the studies of variability in PMS stars have examined the large-amplitude photometric
fluctuations and thus have investigated the behaviour of the obscuring medium which causes
the fluctuations. Only few studies have been devoted to the photometric variations on shorter
time scales and smaller amplitudes in order to detect delta Scuti like pulsation
(Breger 1972, ApJ 171, 539; Kurtz & Marang 1995, MNRAS 276, 191; etc.).
'Regular' delta Scuti stars are slightly evolved, have spectral types A to F,
belong to luminosity classes V to III and lie in the classical instability strip.
They pulsate in radial and non-radial modes with periods between 30
minutes and 8 hours. As a mature delta Scuti star differs from a young one mostly in the
inner regions (central density) and in its higher luminosity (see Fig.1), the discovery of
pulsation could potentially provide constraints on the internal structure
of an acknowledged pre-main sequence star.
Fig.1: Internal density profile (normalized to the central value) of a 3 Msun
star in different evolutionary stages.
Upper curve: A protostar on the birthline;
middle curve: PMS star with
L = 55 Lsun, Teff = 6900K, R = 5.3 Rsun;
post-MS star with the same
Teff but L = 135 Lsun and R = 8.3 Rsun.
(Figure from Marconi & Palla, 1998, ApJ 507, L141)
In 1995 Kurtz & Marang (MNRAS 276, 191) discovered delta Scuti pulsation with a period of
4.99 hours in the well-studied pre-main sequence Herbig Ae
star HR 5999 (spectral type A7 III-IVe), which is embedded in an obscured region of
Scorpius with many T Tauri stars in the associated dark cloud. The
observations clearly show periodic light variations with a visual
amplitude of 0.013 mag (peak-to-peak) in the presence of 0.35 mag of
non-periodic variability caused by variations in the obscurations by dust
in the disc surrounding HR 5999, which confirms the PMS nature of this star.
Left: Location of the instability strip of PMS stars in the HR diagram.
The detection of PMS stars as members of young clusters has been the subject of several works in the past and has some important advantages:
NGC 6383 belongs to the Sgr OB1 association and is only about 6 million years old. Van den Ancker et al. (2000, AA 362, 580) investigated the cetral part spectroscopically and confirmed that stars later than A0 lie above the ZAMS. In August 2001 the project was granted 14 nights at the Cerro Tololo Interamerican Observatory (CTIO) 0.9m telescope where timeseries photometry of this cluster was obtained in Johnson B and V colors. Currently these data are being reduced using the MOMF software written by Soeren Frandsen. (For more information see the STACC Homepage)
IC 4996 is located in the direction of Cygnus and is part of a large region with active star formation. It is slightly younger than 107 years and its PMS candidates span a range of spectral types around A to F and therefore represent counterparts to the HAEBE stars. Recent studies of the cluster (Delgado et al 1998, AJ, 116, 1801; Delgado et al 1999, AJ, 118, 1759) focussed on the reddening features in the uvby system, the relations between Strömgren and Johnson color indexes, the estimation of radial velocities and spectral types for the proposed PMS candidates and the search for possible spectral features indicative of PMS nature. No timeseries photometry of the PMS candidates has been obtained up to now in order to investigate whether they pulsate or not.
Quite similar is the situation of NGC 6910: It is also a young Cygnus cluster for which a number of PMS members is already known. The recent work of Delgado and Alfaro (2000, AJ, 119, 1848) presents UBV observations down to 18th magnitude in order to establish the photometric evidence for the membership of PMS stars in the cluster. For the 11 known PMS stars of spectral types A to G a time series analysis would allow to search for pulsation among these very young stars and probably detect some new PMS pulsators.
Last Update: 11.03.2002
In charge of this page is: Konstanze Zwintz