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.
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.
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.
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.
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
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.
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.
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.
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.
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
Langer N., Maeder A., 1995, The Problem of the Blue-to-Red Supergiant
Ratio in Galaxies, A&A 295, 685
Oestreicher M.O., Schmidt-Kaler Th., 1998, Red Supergiants in the LMC.
II: Spectrophotometry and Model Atmospheres, MNRAS, in press
Schmidt-Kaler Th., Oestreicher M.O., 1998, Red Supergiants in the LMC.
III: The Luminosity Index for M stars and the Distance to the LMC,
MNRAS, in press