On the Weak-Wind Problem in Massive Stars: X-Ray Spectra Reveal a Massive Hot Wind in Mu Columbae
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East Tennessee State University From the SelectedWorks of Richard Ignace September 10, 2012 On the Weak-Wind Problem in Massive Stars: X- Ray Spectra Reveal a Massive Hot Wind in mu Columbae. David P. Huenemoerder, Massachusetts nI stitute of Technology Lidia M. Oskinova, University of Potsdam Richard Ignace, East Tennessee State University Wayne L. Waldron, Eureka Scientific nI c. Helge Todt, University of Potsdam, et al. Available at: https://works.bepress.com/richard_ignace/61/ The Astrophysical Journal Letters, 756:L34 (5pp), 2012 September 10 doi:10.1088/2041-8205/756/2/L34 C 2012. The American Astronomical Society. All rights reserved. Printed in the U.S.A. ON THE WEAK-WIND PROBLEM IN MASSIVE STARS: X-RAY SPECTRA REVEAL A MASSIVE HOT WIND IN μ COLUMBAE David P. Huenemoerder1, Lidia M. Oskinova2, Richard Ignace3, Wayne L. Waldron4, Helge Todt2, Kenji Hamaguchi5,6, and Shunji Kitamoto7 1 Massachusetts Institute of Technology, Kavli Institute for Astrophysics and Space Research, 70 Vassar Street, Cambridge, MA 02139, USA 2 Institute for Physics and Astronomy, University of Potsdam, D-14476 Potsdam, Germany 3 Department of Physics and Astronomy, East Tennessee State University, Johnson City, TN 37614, USA 4 Eureka Scientific Inc., 2452 Dellmer Street, Suite 100, Oakland, CA 94602, USA 5 CRESST and X-ray Astrophysics Laboratory, NASA/GSFC, Greenbelt, MD 20771, USA 6 Department of Physics, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA 7 Department of Physics, Rikkyo University, Tokyo 171-8501, Japan Received 2012 June 16; accepted 2012 August 3; published 2012 August 22 ABSTRACT μ Columbae is a prototypical weak-wind O star for which we have obtained a high-resolution 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 five 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. Key words: stars: early-type – stars: individual (mu Col) – stars: mass-loss – X-rays: stars Online-only material: color figures is a theoretical degeneracy in that different values of mass-loss 1. INTRODUCTION ˙ rate (M) and X-ray luminosity (Lx) can produce very similar The outflow of stellar winds from massive OB-type stars is an UV line profiles (Puls et al. 2008; Marcolino et al. 2009); direct important process which affects both the chemical enrichment knowledge of the X-ray spectrum is thus important for reliable and kinetics of the interstellar medium (e.g., Leitherer et al. determination of wind parameters. Another possibility is that 1992). The mass loss itself is enough to change the evolution cool and hot plasma emission originate from different volumes of the star, which ends its life in a supernova explosion, or densities; that is, clumping can affect the interpretation also profoundly changing its environment. Hence, quantitative (Hamann et al. 2008). μ Col belongs to the weak-wind domain understanding of massive star winds is important not only defined by Lucy (2010) in which a star’s rate of mechanical as a basic component of stellar astrophysics, but also for energy loss in a radiatively driven wind is less than the radiative understanding cosmic feedback on galactic scales throughout output from nuclear burning; Lucy showed that there is a huge ˙ cosmic history. disparity between the theoretically expected M and values While some basic physics of stellar winds in massive stars derived from UV and optical spectra. Lucy (2012) developed a is well established–that winds are accelerated by photoelectric phenomenological model and suggested that in low-luminosity absorption of the intense ultraviolet radiation field by a multitude O-type stars, the volumetric roles of hot and cool gas are possibly of metal lines and that an instability can lead to wind shocks reversed compared to O-type stars of high luminosity; thus in which generate X-rays–there are still puzzles to be solved. the weak-wind stars, a larger volume is occupied by the hot gas One of these is the “weak-wind” problem in which UV line than by the cool gas. diagnostics clearly show a wind signature in classical P Cygni μ Col (HD 38666) is an O9.5 V runaway (and single) star line profiles, but modeled mass-loss rates can be discrepant by and is one of the weakest wind Galactic O-type stars (e.g., more than an order of magnitude from values expected based on see Figures 39 and 41 in Martins et al. 2005). These factors O star statistical trends and theoretical foundations, specifically are what motivated our spectroscopic study of the prototypical the wind momentum–luminosity relation (e.g., Puls et al. 1996). weak-wind system, μ Col, at high resolution with Chandra and Factors of a few in mass loss are enough to be significant for at low resolution but greater sensitivity at higher energies with stellar evolution and cosmic feedback (e.g., Puls et al. 2008). Suzaku. In this Letter, we concentrate on the primary empirical It has long been known that photoionization by X-rays can results from the X-ray spectral analysis of μ Col. A subsequent alter the ionization balance in the wind regions where the UV paper will investigate the influence of the X-rays on the cool lines are formed (Waldron 1984; MacFarlane et al. 1993). There wind component (H. Todt et. al., in preparation). 1 The Astrophysical Journal Letters, 756:L34 (5pp), 2012 September 10 Huenemoerder et al. Figure 1. μ Col Chandra/LETG/ACIS spectrum (left); black: observed count rate; thin-gray line (red in the online version): model; the lower panel shows the χ 2 residuals against a broken power-law APEC model modified for photoexcitation of triplets. Lines are broader than the instrumental width. The Suzaku spectrum is shown on the right (black). For conciseness, we have summed the counts from the three detectors; such is not recommended when fitting, but it provides a good summary visualization of the data. The folded Chandra-derived model is in gray (red in the online version), and residuals below. We emphasize that the Chandra-derived model was not fit to the Suzaku spectrum, only folded through the response to provide model counts and residuals; the model agrees very well without any adjustments. It is significant that there is little or no flux detected above 2 keV where Suzaku has substantial sensitivity. (A color version of this figure is available in the online journal.) Table 1 forbidden-line level, weakening it while strengthening the in- μ Col Properties tercombination lines (Gabriel & Jordan 1969; Blumenthal et al. Property Value Property Value 1972); in O stars, the UV field typically dominates the depop- ulation and hence He-like line ratios are diagnostic of radius Spectral type O9.5 V d (pc)a 408 b b,c of formation (Waldron & Cassinelli 2001). Very close to the Teff (K) 33000 R/R 4.6 b,c −1 b photosphere, density effects could also become significant. log(Lbol/L) 4.4 v∞(kms ) 1200 ˙ −1 b −9.5 −2 d 19 We have fit a global model to the X-ray spectrum, using Muv (M yr ) 10 NH (cm ) 5 × 10 e,f −13 e fx (1–40 Å) (cgs) 6 × 10 log(Lx/Lbol) −6.9 standard products produced by CIAO (version 4.3 and asso- −3 e 54.1 e EMx (cm ) 10 Tmax(MK) 4.4 ciated calibration database; Fruscione et al. 2006), using ISIS −1 e v∞,x (kms ) 1600 (±275) (Houck & Denicola 2000), and the collisional ionization equilib- f/i(O vii)g <0.01 r(O vii)g <3.3 rium emissivities in AtomDB (version 2.0; Smith & Brickhouse f/i(Ne ix)g 0.04–0.14 r(Ne ix)g 2.3–4.4 2008). For the plasma model, we used a broken power-law emis- f/i(Mg xi)g >0.2 r(Mg xi)g >2.1 sion measure distribution (EMD; which in differential form is defined as nenhdV/dT), with variable abundances for signifi- Notes. cant ions, a Doppler shift, and line profiles defined by a global a From the Hipparcos parallax, as re-evaluated by van Leeuwen (2007). b Gaussian–Doppler broadening term. Our models show that the Martins et al. (2005). absorption of X-rays in the cool stellar wind is negligible and c Adjusted for the adopted distance. d Cassinelli et al. (1994), Howk et al.