Abstract
Generally speaking, variable stars are monitored through observing campaigns which coordinate multi-site telescopes at various longitudes. A new practice is to use networks of robotic telescopes devoted to these programmes. In this issue of African Skies we review these technologies, and in the next issue we will emphasize the NORT (Network of Oriental Robotic Telescopes) project, which we are promoting in north African and middle-eastern countries.
(This paper is drawn from the Proceedings of IAU Symposium 177 on ``The Carbon Star Phenomenon'', held on May 27-31, 1996, in Antalya, Turkey; editor: Robert F. Wing; to be published by Kluwer, 1997.)
For over a decade, coordinated international campaigns have taken place from sites with sufficient longitude and latitude coverage and/or with various instruments working at complementary wavelengths. The aim is to monitor variable stars as continuously as possible to reduce the aliasing problems bound to observations at a single site. The final goal is to study and to understand their pulsational behavior, given that the pulsation on the star's surface probes its interior structure and composition in the framework of asteroseismology.
Coordinated campaigns with existing telescopes give better tools to solve scientific problems by allowing one :
Multisite campaigns also lead to a better international organization of science, since they are based on cooperative programmes among many experts in various scientific and technical fields : theoreticians, observers, engineers.
They are valuable for training of young observers, PhD students and young research associates, who follow the scientific evolution of the project, from programme definition though to the interpretation of the results, and so enabling them to choose their own speciality.
Some campaigns give access to multiwavelength techniques. They are dedicated to various wavelength intervals by using space telescopes (e.g. on board of IUE, ROSAT, HST satellites, etc.), as well as many small- and medium-sized existing ground-based telescopes around the world.
In a word, they produce best science.
The main drawbacks of multisite campaigns should be summarized as it follows.
Many nights are lost by cloudiness as some existing telescopes are not in the best sites in the world.
The costs are important due to overhead and associated expenses such as travel and accomodation costs for observers going to faraway observatories, or equipment maintenance or replacement.
There are considerable handling requirements to mount and to dismount instrumentation, to transport equipment to distant observatories, and to adapt it to various existing telescopes.
In fact, these costs and handling are so high that they seriously
restrict the possibility of numerous campaigns per year. As examples, we
cite the
WET (Whole Earth Telescope) campaigns (Sullivan, 1995) with white dwarfs
as prime targets and delta Scuti STEPHI (Stellar Photometry
International project) network campaigns (Belmonte et al., 1994). These and
other campaigns are run only once or twice by year or even once biannually for
about one week duration.
In consequence : - only one or a few stars are monitored during a campaign, - the analysed stars are short-period variables such as : white dwarfs, delta Scuti, RoAp, post-novae, cataclysmic stars, etc., having periods of some seconds to a few hours or days.
Some technical pitfalls could be added if the scientific organization is badly coordinated, as for example when data from different sites are reduced by different techniques, or obtained by not sufficiently similar instruments from different sites, or even on the same site in a long-term monitoring programmes as emphasized by Young (1994).
Problems bound to coordinated observing campaigns are reviewed for example by Sterken (1988) and by Breger (1992, 1994).
As the coordinated international campaigns run one or two times a year for about a week at a time, they are not adapted to the red giants. So, it is not surprising that this subject does not appear in the literature on cool stars. Even variables as Miras can appear `unfavourable for long-term projects' as stressed by Szabados (1994). However, it is encouraging that a first step was recently done for the solar-type stars by the presentation of a new project, the SONG (Stellar Oscillations Network Group) (see the Madison Meeting of the AAS, 9-13 June, 1996).
Roughly speaking, what do we know about the red-giant variability ?
Red giants have long periods of about one year. Superimposed on the well-known long period are also shorter periods, say of a month to a few months. Moreover, they can show rapid or slow period changes, a critical behaviour around phase 0.7-0.8 in the case of Miras, flares and very rapid variations over a day or less (for example, see a review by Querci, 1986).
This knowledge was mainly obtained by single ground-based sites dedicated to long-term monitoring programmes of observations. Let us quote some of these programmes:
Nowadays, the AAVSO includes red giants in its photoelectric photometry survey (e.g. Percy et al., 1994), as well as the other societies which give a contribution to the observations of cool stars (see GEOS Circulars, IBVS, etc.). Photographic monitoring by Latvian astronomers led to the discovery of anomalies in the light curves. In 1982, red giants were targets in the ESO-LTPV project (Sterken, 1994) : international teams collaborated for more than a decade (see Jorissen, 1994, for results on Barium and S stars).
For the past 10 years, automatic photometric telescopes (APT) have operated at Mt. Hopkins for long-term monitoring of semi-regular variables (Baliunas et al., 1987; Cristian et al., 1995).
More recently, MERLIN, a Multi telescope Radio-Linked Interferometer
Network, mapped out H2O emission in the inner CS envelope of Miras, SR,
and red supergiants, giving information about mass-loss (Yates and
Cohen, 1994), and observed OH/IR stars (Migenes et al., 1995).
A radio survey could be organized in cooperation with optical
networks.
It appears evident that we need a world-wide coordinated monitoring of
long-period variables (LPVs) and semi-regular variables which will permit to detect unknown
important features in the light curves, and, consequently, to lay the
observational foundation for red-giant asteroseismology.
(to be continued)