SCIENCE

The chemical history of the early Galaxy can only be understood through the study of ultra metal-poor objects. Survey of high proper-motion stars have led to the discovery of many metal-poor stars with unusual compositions. A well-known example is the metal-poor star CS 22892-052 . This star is carbon-rich but is overabundant in r-process elements. The abundances of 20 n-capture elements have been derived for it. The observed Th/Eu ratio gives an age estimate of 15.2 +/- 3.7 Gyr for CS 22892-052 which has [Fe/H] of -3.1 (Sneden et al. 1996). Similarly, detection of U~II line in CS 31082-001 by Cayrel et al. (2001) has led to an even more accurate age estimate of 12.5 +/- 3.0 Gyr for this star. The detection of stars with [Fe/H] < -3 in different parts of our Galaxy would have strong impact on our understanding of star formation in early stages of galactic evolution.

For different families of stars such as Pre-Main-Sequence stars, Post-AGB stars etc the presence of companions play very important roles in defining their evolutionary status. Some of the chemical peculiarities exhibited by post-AGB stars and RV Tau stars can be better understood in the framework of circumbinary environments. The list of known post-AGB stars with binary companions is growing steadily (Van Winckel 2003). Binary companion for Pre-Main Sequence star HD 34700 was detected by Arellano Ferro and Giridhar (2003). Hence the search of low mass companions is considered a very important area where both the long-term as well as short-term monitoring yields very interesting results.

Detection of planets of Jupiter-size around other solar-type stars demands extremely precise radial velocity measurements (of the orders of tens of ms-1). A dedicated instrument giving a complete spectral coverage from 370 to 900nm in a single CCD frame is necessary to meet the required precision. It can significantly increase the detections of such systems. It should be noted that instruments like HARPS (High Accuracy Radial Velocity Search ;cf. Queloz and Mayor 2001; Pepe, Mayor and Ruprecht 2002), SARG (Spettarografo Alta Risoluzione Galileo ;cf. Gratton et al. 2002) and FEROS (Fiber fed Extended Range Optical Spectrograph ; cf. Kaufer and Pasquini 1998) mounted on 1.5 to 2-m class telescopes have made significant contributions to this field. CORALIE and ELODIE (Baranne 1997) have made significant contributions to the detection of a large number of planets with short-period orbits (P ~ 20 days). Significant effort has gone in the detection of planets. Measurement of the 6Li/7Li isotopic ratio in planet harbouring stars is an excellent tool to test different theories of giant planet formation.

Doppler imaging technique (DPI) is an indirect elaborate computational technique to invert a series of high-resolution spectral line profile into an image of stellar surface (Donati et. al. 1997). The development and refinement of this indirect stellar imaging technique over two decades has made DPI one of the most reliable tools to estimate spatial distribution of the temperature and chemical abundances over the stellar surface. It requires high-resolution (lambda/Delta lambda >= 60,000) and high signal-to-noise ratio(S/N >=200) spectra with a good phase coverage. This method is being employed to reconstruct star-spot patterns on the stellar surface and monitor them over long periods of time. Stars like RS CVn binaries with their known surface activities could be used to explore the possibility of their surface features showing regularity similar to the sun-spot butterfly diagram (Vogt et al. 1999).

(Doppler images of HD 12545 obtained using six different spectral regions having a resolving power of 38000 with S/N∼250.)

Hide (Doppler imaging of spotted stars)

To fully understand the evolution of stars, detailed information on the processes acting in the stellar interior is needed. Since the stellar interior is not directly accessible, asteroseismologists use the indirect information contained in stellar oscillations to probe the stellar interiors. There are two families of modes carrying information on different parts of the stellar interior: pressure modes (p-modes; mainly propagating in the envelope) and gravity modes (g modes; mainly propagating in the core). For a unique asteroseismic modelling, a large number of well-identified modes are needed. Asteroseismology also provides clues to test the internal structure and chemical composition of solar type stars like alpha CMi (Procyon A) by measuring frequencies and amplitudes of p-mode oscillations in these stars. In radial velocity, p-modes have been detected but there is no agreement on the actual frequencies due to insufficient high-quality observations. Additional data with high S/N in power spectra and increased temporal coverage with multi-site campaign are needed to determine the amplitude, the rotational splitting and the damping time of p-modes in Procyon A like stars.

Theoretically predicted l = 2 oscillation spectra of a 1.8 M⊙ main sequence model, evolving from hotter to cooler effective temperature. Pulsation modes excited in this model are shown with filled circles, stable modes with open circles. The types of mode on the Zero-Age Main Sequence (ZAMS) are given. Note the g modes intruding into the p mode domain.

Hide (Asteroseismology)

PAGB stars are excellent probe for diagnostic properties in the study of dramatic morphological and chemical changes taking place at the latest stage of evolution of low and intermediate mass stars (approx mass range 0.8 to 8Mo). The extensive nucleosynthesis at AGB and mixing processes (dredge-ups) change the surface composition of the star over a short time ( a few hundred to thousand years) and synthesized elements are ejected to interstellar medium (ISM) through strong mass-loss preceding the post-AGB phase. AGB stars are therefore major contributors of C,N,F,Al, Na and s-process elements to ISM and hence play crucial role in the chemical evolution of galaxies.