Some suggestions for Thesis projects ("speciale"):

Thesis advisor: Uffe Gråe Jørgensen, AO (3532 5998, e-mail uffgj@nbi.dk)

GAIA spectrophotometry of the Ca triplet.

GAIA will perform its stellar radial velocity determinations based on a small piece of spectrum around the infrared Ca triplet at 8600 Å. As a by-product the mission will therefore produce the largest ever obtained sample of Ca IR triplet spectra. We see at least two studies which could be very useful to perform on the Ca triplet before the launch of GAIA as a preparation of the analysis of the data.

(1) The Ca lines are quite strong in a wide range of spectral types, and their equivalent widths (EW) are sensitive to the metallicity of the stars, but also to the effective temperature and the gravity. By modelling carefully the EW as function of the fundamental stellar parameters, one can compute the integrated spectra of populations of stars; for example globular clusters, galaxies of various types, etc. If the EW is well determined as function of the fundamental stellar parameters, one can determine the stellar population of the system. With the huge data set from GAIA one would be able to construct an empirical EW = EW(Teff,logg,Z) function. This function can be compared with the stellar atmosphere computations to gain knowledge about the stars, and it can be applied to integrated spectra of stellar systems to gain knowledge about the population of the systems.

(2) The variation in the EW of the Ca lines (mainly the visual Ca H&K lines) of the Sun has been studied systematically through several decades. It is now well established that the EW of the Ca lines is a good measure of the solar magnetic activity (i.e., the sunspot number, the flare activity, etc). Research mainly through the 1990'ies have indicated that the variations in the magnetic activity may have a major influence on the climate of the Earth. Some researchers even believe that the increase in the Earth's mean temperature through the 20th century is caused by the increased solar magnetic activity through the century rather than by the increased atmospheric content of greenhouse gases. Historically, long cold periods on Earth have been associated with periods of low solar activity, and periods of warm global weather have been associated with periods of high solar activity. However, it is not known how often the Sun is in general active periods (like in the Viking age), and how often it is in long quiet periods (like the Maunder minimum in the 17th century). By studying the Ca EW of solar type stars observed with GAIA, we will get the best ever statistical sample of activity measurements of solar type stars, and hence knowledge of how often we shall expect the Sun to be in various of its activity levels.


Litterature:
(1) U.G.Jørgensen, M.Carlsson and H.R.Johnson: The Calcium Infrared Triplet lines in Stellar Spectra, Astron. Astrophys., 254, 258-265, 1992.
(2) U.G.Jørgensen: Solen -- en livgivende stjerne, Gyldendal 1993.
(3) P.R. Wilson, Solar and stellar activity cycles, Cambridge University Press, 1994.
(4) B.A.Twarog, B.J.Anthony-Twarog, Ca H&K photometry on the ubvy system, II, AJ 109, 2828-2854, 1995.
(5) S.Baliunas, W.Soon, The Sun-Climate Connection, Sky & Telescope December 1996, p.38--41.
(6) Various papers in Cool Stars, Stellar Systems, and the Sun, ASP.Conf.Ser. vol.154, 1998.
(7) M.F.Sterzik, J.H.M.M.Schmitt, Young cool stars in the solar neighbourhood, AJ, 114, 1673--1678, 1997.
(8) Information on GAIA is given on the website: http://astro.estec.esa.nl/GAIA/