5. Strategy for the future
Important progress is being made in the study of stars with masses around 2 , typical for Scuti stars. If by asteroseismology one understands the ability to derive firm results about the physics of the stellar interior using observed pulsation frequencies, then we are starting to see results on stars applying seismic techniques. Still, even for Scuti stars with more than 10 frequencies detected, mode identification is often ambiguous and therefore little information about the interior can be derived.
For single, isolated stars a breakthrough took place recently, when mode identification was announced for 8 out of 24 frequencies in FG Vir. Different methods gave the same result for 8 modes (Viskum et al. 1997b; Breger 1997). The observational efforts involved were large, and are not easily repeated for another star.
For open clusters, one cannot use quite the same techniques as for a bright star. But, analysing a group of stars simultaneously, and therefore knowing temperature and magnitude differences accurately, information about the modes can be derived. But, this only works, when the age, distance and metallicity of the cluster is well determined. Also, rotational velocities are needed and make it necessary to measure high quality spectra. The lack of good quality 'classical' data has so far prevented a completion of the analysis of the pulsators in NGC 6134.
With this in mind we have modified our strategy to reach our goal: a new, improved test of stellar evolution. The new scheme can be split into three parts.
5.1. Locating a target
From the experience gained so far, a sufficient number of variables in a cluster will only be detected either by sheer luck (NGC 6134 to some extent was such a case) or by a systematic approach (NGC 1817). One has to optimize the choice of targets, in which a search for variables is carried out, and to optimize the techniques applied.
The parameters from the literature are in many cases unreliable, and the photometry of bad quality or only done for the brightest stars. New CCD observations, covering a large dynamic range, are needed, since they will lead to more accurate values for the reddening, distance and age of the clusters. This enables us to find the apparent magnitudes of possible Scuti stars.
Open clusters, in which the isochrone turns upwards and almost follows the instability strip, seem to have the largest number of pulsators. Typically, this corresponds to an age of just below 1 Gy.
A section appears in The Book on techniques and methods with a few diagrams to help decide on the instrumental parameters and limitations. The reduction of series of CCD frames can be done efficiently using the software package MOMF (Kjeldsen & Frandsen, 1992).
5.2. Classifying the target
When a good target has been located (NGC 6134, NGC 1817), the theoretical interpretation is difficult or even impossible, if detailed information is missing for the variable stars. A complete set of data must be obtained.
A lot of nights are needed to obtain data of comparable quality to the data we have seen for FG Vir (Breger et al. 1997, 1998). This represents the largest investment in terms of observing hours and manpower.
The second step therefore consists of a determination of further parameters and more information about the target:
This second step can be quite a problem to carry out. The pulsating stars in most clusters are faint (). Some of the observations are not easily done unless one has access to a large telescope ( m).
5.3. The campaign
With all the basic data in store, the time is ripe for a large, multisite campaign of relative CCD photometry. If we take FG Vir as a typical example of a Scuti star, detection levels around 0.3 mmag should be aimed at, and time coverage to give a frequency resolution better than 0.2 µHz in order to separate multiple, close frequencies.
As mode identification is so important for the analysis of the pulsation spectra, a time series of short exposure ( min) spectra for at least one pulsating star in the cluster would be of great value.
© European Southern Observatory (ESO) 1998
Online publication: April 20, 1998