On the Weak-Wind Problem in Massive Stars: X-ray Spectra Reveal a Massive Hot Wind in μ Columbae
Huenemoerder D.P., Oskinova L.M., Ignace R., Waldron W.L., Todt H., Hamaguchi K., Kitamoto S.
μ Columbae is a prototypical weak-wind O-star for which we have
obtained a highresolution X-ray spectrum with the Chandra LETG/ACIS
instrument and a low resolution spectrum with Suzaku. This allows us,
for the first time, to investigate the role of X-rays on the wind
structure in a bona fide weak-wind system and to determine whether
there actually is a massive, hot wind. The X-ray emission measure
indicates that the outflow is an order of magnitude greater than that
derived from UV lines and is commensurate with the nominal
wind-luminosity relationship for O-stars. Therefore, the "weak-wind
problem"-identified from cool wind UV/optical spectra-is largely
resolved by accounting for the hot wind seen in X-rays. From X-ray line
profiles, Doppler shifts, and relative strengths, we find that this
weak-wind star is typical of other late O dwarfs. The X-ray spectra do
not suggest a magnetically confined plasma-the spectrum is soft and
lines are broadened; Suzaku spectra confirm the lack of emission above
2 keV. Nor do the relative line shifts and widths suggest any wind
decoupling by ions. The He-like triplets indicate that the bulk of the
X-ray emission is formed rather close to the star, within 5 stellar
radii. Our results challenge the idea that some OB stars are
"weak-wind" stars that deviate from the standard wind-luminosity
relationship. The wind is not weak, but it is hot and its bulk is only
detectable in X-rays.
Fetch Pdf-File (oskinova-2012-mucol.pdf, 184kB)