WATER ON THE EARLY M SUPERGIANT STARS α ORIONIS AND μ CEPHEI X-RAY EMISSION FROM COLLIDING WIND SHOCKS IN THE WOLF-RAYET BINARY WR 140

摘要

We reanalyze the spectra of α Ori (M2 Iab) and μ Cep (M2 Ia) observed with the balloon-borne tele- scope Stratoscope II more than 35 years ago, and we confirm the presence of water in these early M supergiant stars. This identification was first proposed by the Stratoscope observers themselves (Woolf, Schwarzschild, and Rose in 1964, and Danielson, Woolf, and Gaustad in l965), but this important dis- covery was overlooked for a long time without any follow-up observation. Consequently, this finding has so far had little influence on the theory of the atmosphere of red supergiant stars. A reason for this may be due to an early criticism by Wing and Spinrad, who suggested CN instead of H_2O for the spectral features observed by Stratoscope II. This alternative proposition has more easily been accepted since CN has widely been observed from the Sun to red supergiants, while H_2O has been observed only in very cool stars such as Mira variables. In fact, we confirm that the self consistent photospheric model of the early M supergiants shows CN bands but no H_2O band in the near-infrared. Nevertheless, we find that the contribution of CN is only minor and that H_2O should be the dominant absorber for the l.4 and 1.9 pm features on the Stratoscope spectra of a Ori and p Cep, a conclusion opposite to that of Wing and Spinrad. The observed spectra can best be interpreted by the water gas with the column density of the order of l0~20 cm~-2 and temperature about 1500 ± 500 K, but they cannot be originating in the photo- sphere. We suggest a possible presence of a gaseous component We analyze four ASCA X-ray observations of the Wolf Rayet binary system WR l40 obtained between 1993 and 1997 by making use of hydrodynamic colliding wind (CW) shock models. The analysis shows that the CW shock models are able to accurately reproduce the X-ray spectra at different orbital phases using mass-loss and orbital parameters that are within the ranges allowed by the uncertainties. However, some adjustment in the currently accepted values of the semimajor axis and time of periastron passage may eventually be required. Models that allow for different electron and ion temperatures provide better fits to the data. Extra absorption is inferred from CW shock models above that expected from the winds and interstellar medium, the origin of which is not yet known. We also report the seren- dipitous discovery of hot plasma at temperatures in excess of ~2 keV and X-ray emission lines in spectra extracted from the diffuse Cygnus superbubble background in the vicinity of WR l40.not as hot as the chromosphere but warmer than the cool expanding envelope. On the other hand, we notice that the mid-infrared pure- rotation lines of H_2O recently discovered on Betelgeuse (α Ori) and Antares (α Sco) by Jennings and Sada may partly be originating in the photosphere, even though the larger part should again be non- photospheric in origin. Thus, the presence of water possibly originating in the outer atmosphere of Betelgeuse is confirmed by the independent observation in the mid-infrared region. We now conclude that water should be an important new probe on the atmosphere of the early M supergiant stars, for which water has not been recognized as such until recently. Subject headings: infraredf stars -- molecular processes -- starst individual (p Cephei, a Orionis) - stars: late-type -- supergiants