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G. Taresch 1,
R.P. Kudritzki 1, 2,
M. Hurwitz 3,
S. Bowyer 3, 4,
A.W.A. Pauldrach 1,
J. Puls 1,
K. Butler 1,
D.J. Lennon 1, 2 and
S.M. Haser 1
Received 17 June 1996 / Accepted 16 August 1996
A quantitative analysis of the extremely hot and massive galactic supergiant HD 93129A is carried out using stellar wind and pseudo photospheric lines observed in the FUV, UV and optical spectrum together with hydrodynamical NLTE model atmospheres. The analysis in the FUV is combined with spectrum synthesis of the molecular and atomic/ionic interstellar spectrum to disentangle stellar and interstellar blends. It is demonstrated that the combined stellar/interstellar spectrum synthesis technique is crucial for the determination of both interstellar column densities and stellar properties.
The fraction of hydrogen atoms in molecular form in the Carina interstellar clouds is found to be 0.1, smaller than one would expect for its E(B-V) value of 0.54. We attribute this to dissociation by the strong FUV radiation field of HD 93129A. The excitation temperature of ortho-hydrogen (J=1) is about 80K, whereas the excitation to higher levels requires temperatures up to 230 K in accordance with NLTE effects for interstellar H2 as discussed in the literature. The abundance of HD relative to H2 is of the order of 10-5. For CO we obtain an upper limit of 2.6 . Abundances for the interstellar atomic and ionic species are also derived.
The terminal velocity of the stellar wind of HD 93129A is 3200 200 km/s and the rate of mass-loss is 18 10-6 /yr. The ionization equilibrium of the optical emission and P-Cygni lines of N III, N IV and N V is used to determine the effective temperature as =52000 1000 K in reasonable agreement with previous values obtained from the helium ionization equilibrium. This high temperature is confirmed independently by an analysis of the Ar VI /Ar VII ionization equilibrium in the FUV. The luminosity of HD 93129A is log L / =6.4 0.1 corresponding to a zero age main sequence mass of slightly in excess of 120 . This very high mass is consistent with the mass determined from the stellar gravity and with the mass derived from using the theory of radiation driven winds. HD 93129A is thus the most luminous and most massive star known in our galaxy. The abundance determinations yield clear evidence of contamination with CNO-cycled matter in the atmosphere. The abundances of heavier elements are about solar. The presence of high ionization stages such as O VI can be explained by X-ray emission due to stellar wind shocks of low temperature (2.5 106 K) corresponding to the jump velocity of 500 km/s obtained from UV and FUV P-Cygni profiles. Their luminosity is 1.6 dex smaller than the luminosity of the high temperature shocks (1.1 107 K) observed directly with the ROSAT PSPC. Using effective temperature, gravity, radius and abundances as input parameters we calculate radiation driven wind models for HD 93129A. We find that the theory is able to reproduce the extreme stellar wind properties very precisely.
Key words: stars: atmospheres stars: fundamental parameter stars: individual: HD93129A ISM: abundances ISM: molecules X-rays: stars
* Based on the development and utilization of ORFEUS (Orbiting and Retrievable Far and Extreme Ultraviolet Spectrometers), a collaboration of the Astronomical Institute of the University of Tuebingen, the Space Astrophysics Group, University of California, Berkeley, and the Landessternwarte Heidelberg
Send offprint requests to: G. Taresch
Online publication: June 30, 1998