TESTING MODEL ATMOSPHERES OF COOL DWARF STARS WITH ISO

PI :

David L. Lambert

The University of Texas at Austin
Dept. of Astronomy, RLM Hall, 15.308,
Austin, TX 78712,
USA.

Participants:

Ramon J. Garcia Lopez
Carlos Allende Prieto
Basilio Ruiz Cobo
Rafael Rebolo

Instituto de Astrofisica de Canarias
E-38200 La Laguna, Tenerife, Spain.

Bengt Gustafsson

Uppsala Astronomical Observatory
Box 515, S-751 20 Uppsala, Sweden.

Abstract :
Analyses of photospheric spectra of stars are vital tools for our understanding of issues from the origin of the Universe, the evolution of the Galaxy to the lives of stars. Contemporary analyses of spectra depend on model stellar atmospheres computed usually from a set of simplifying assumptions: plane-parallel homogeneous layers in radiative/convective hydrostatic and local themodynamic equilibrium. Only few attempts have been made to derive atmospheric structures as direct as possible from observations, i.e. to construct empirical atmospheres. With new observational methods, and new wavelength regions opening up, it is very important to try such methods. In this respect the infrared spectral region offers new and unique possibilities.

This proposal considers such atmospheres of cool dwarf stars whose chemical compositions are unique clues to the early evolution of the Galaxy. The study is motivated by the power of ISO photometry to measure the temperature in the upper photospheric layers: the opacity in the infrared is provided by the free-free transitions of the H- ion and scales as wavelength-squared. Hence, a large range of depths is sampled by the flux between 3 to 120 micron. Previous studies of optical line and continuous spectra suggest that theoretical model atmospheres do not yet adequately represent the real atmospheres of cool dwarf stars.

We propose to observe the stars Gmb1830 (HD103095) and Epsilon Eri (HD22049) using ISOPHOT. Six filters have been selected for the PHT03 AOT: 3.6, 4.85, 7.3, 11.5, 16 and 25 micron, with an aperture of 79 arcsec. Two additional broad-band filters will be used in the PHT22 AOT: 60 and 90 micron.

Allocated time and instruments/detectors Summary :

- 13320 sec with ISOPHOT for proposal DLL002. 3.6, 4.85, 7.3, 11, 16, and 60 micron filters used to observe the star Gmb1830 in May 15, 1996.

- 7200 sec of priority 1 time for proposal DLL004, to observe the star Epsilon Eri in 1997, using ISOPHOT with the following filters: 3.6, 4.85, 7.3, 11, 16, 25, 60, and 90 micron.

Significant Results :

The cool metal-poor star Gmb1830 (HD103095) was observed in 3.6, 4.85, 7.3, 11, 16, and 60 micron by our group with ISOPHOT (proposal DLL002), providing very good data for the shorter wavelength filters, and showing that it was possible to detect this very faint source in the longest wavelength filter.

These data will be complemented by observing the metal-rich star Epsilon Eri (HD22049, with similar stellar parameters than Gmb1830) in these six filters plus the 25 and 90 micron filters, for making a direct comparison of the IR flux distribution in both objects and having a detailed photometric information of Epsilon Eri.

Updated: January 1997