Proceedings of the Workshop
"The Magellanic Clouds and Other Dwarf Galaxies"
of the Bonn/Bochum-Graduiertenkolleg

Red Supergiants as a Tool for the Study of Galaxies

Th. Schmidt-Kaler

Astronomisches Institut der Ruhr-Universität Bochum, Universitätsstr. 150, D-44780 Bochum, Germany
Georg-Büchner-Str. 37, D-97276 Margetshöchheim, Germany, Fax 931-4675972

Received 13th March 1998
Early type stars, especially the O stars, and more recently also the WR stars, have been primary tools of the astronomer in
- delineating young structures of galaxies
- dating of ages, providing energy sources, studying star formation
- determining distances, radial velocities, rotation curves and masses of galaxies
It is my intention to briefly point out the importance and the advantages of the latest spectral types, the M supergiants, for the present research of galaxies.
  1. First of all note that with a typical temperature of 3000 to 4000 K the star radiates at a central wavelength around 1 mm, i.e. near the natural maximum of sensitivity of the CCD, where in addition atmospheric extinction is minimal and sky brightness still very low.

  2. M supergiants do have so many spectral features that it is easy to determine their basic spectral parameters and their radial velocities even at low resolution. To the contrary OB stars have in the terrestrial UV to IR only a few rather weak lines so that their radial velocities are difficult to determine and uncertain.

  3. The M supergiants form an evolutionary homogeneous group, contrary to the late-type AGB and normal giant stars both of which clump together with very different ages, masses and evolutionary histories.

  4. Spectra of low resolution (10 Å) are sufficient to determine accurate distances of galaxies out to the Virgo cluster and beyond (Schmidt-Kaler & Oestreicher 1998). M supergiants are in the red and infrared much brighter (MV up to -8.5, MI up to -11) than the brightest cepheids in the visual. Typical values for an M0 I star are MV = -6.5, Mbol = -7.7, (V-R)0 = 0.9, (R-I)0 = 0.8 (colours in the Cousins system). In that domain the luminosity index L of Schmidt-Kaler & Oestreicher (1998) measures MV resp. Mbol with an accuracy of better than 0.4 mag.

    Mbol = -8.45 + 73.49 exp (-12.13 L)

    The intrinsic colours (V-R)0 and (R-I)0 are well correlated to the spectral index IST as a measure of spectral type, allowing to determine accurate individual reddening values:

    (V-R)0 = 1.39 - 0.75 exp (-IST),   σ = 0.046 mag

  5. Typical ages are 107 years and can reliably be determined with high accuracy on the basis of luminosities, since the isochrones do hardly intersect each other.

  6. Young structures of and in galaxies are generally well delineated by cM stars. Imaging in the red or near infrared reveals in general more regular structures than in the blue.

  7. M supergiants occur at or near the places of actual star formation. They are connected with dust formation and stellar wind phenomena in interstellar matter. As they are very bright in the IR, they can easily be observed behind dark clouds and possibly already in those molecular clouds where the star formation is going on.

  8. The heavy element content ("metallicity") of a galaxy population can be directly determined from their spectra, even at low dispersion (Oestreicher & Schmidt-Kaler 1998), and indirectly from the ratio of red and blue supergiants (for recent references see Langer & Maeder 1995). At somewhat higher spectral resolution the various contributions of C,N,O can be separately determined. These are related to the number of cycles of stellar evolution the matter went through.

  9. M supergiants represent a laboratory for the study of rapid phases of stellar evolution (overshooting, mass loss rates, various chemical compositions, pre-supernova phases etc.)

References


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First version: 18thJune,1998
Last update: 30thAugust,1998

Jochen M. Braun   &   Tom Richtler
 (E-Mail: jbraun|richtler@astro.uni-bonn.de)