We quantitatively investigate the extent of wind absorption signatures in the X-ray grating spectra of all non-magnetic, effectively single O stars in the Chandra archive via line profile fitting. Under the usual assumption of a spherically symmetric wind with embedded shocks, we confirm previous claims that some objects show little or no wind absorption. However, many other objects do show asymmetric and blue shifted line profiles, indicative of wind absorption. For these stars, we are able to derive wind mass-loss rates from the ensemble of line profiles, and find values lower by an average factor of 3 than those predicted by current theoretical models, and consistent with Hα if clumping factors of f cl ≈ 20 are assumed. The same profile fitting indicates an onset radius of X-rays typically at r ≈ 1.5 R * , and terminal velocities for the X-ray emitting wind component that are consistent with that of the bulk wind. We explore the likelihood that the stars in the sample that do not show significant wind absorption signatures in their line profiles have at least some X-ray emission that arises from colliding wind shocks with a close binary companion. The one clear exception is ζ Oph, a weak-wind star that appears to simply have a very low mass-loss rate. We also reanalyse the results from the canonical O supergiant ζ Pup, using a solarmetallicity wind opacity model and findṀ = 1.8 × 10 −6 M ⊙ yr −1 , consistent with recent multi-wavelength determinations.2 Effectively single in the sense that there is no obvious windwind interaction-related X-ray emission.
Far Ultraviolet Spectroscopic Explorer (FUSE) spectra of 22 Galactic halo stars are studied to determine the amount of O vi in the Galactic halo between $0.3 and $10 kpc from the Galactic midplane. Strong O vi 1031.93 absorption was detected toward 21 stars, and a reliable 3 upper limit was obtained toward HD 97991. The weaker member of the O vi doublet at 1037.62 Å could be studied toward only six stars because of stellar and interstellar blending problems. The measured logarithmic total column densities vary from 13.65 to 14.57 with hlog Ni ¼ 14:17 AE 0:28 (1 ). The observed columns are reasonably consistent with a patchy exponential O vi distribution with a midplane density of 1:7 Â 10 À8 cm À3 and scale height between 2.3 and 4 kpc. We do not see clear signs of strong high-velocity components in O vi absorption along the Galactic sight lines, which indicates the general absence of high-velocity O vi within 2-5 kpc of the Galactic midplane. This result is in marked contrast to the findings of Sembach et al., who reported high-velocity O vi absorption toward $60% of the complete halo sight lines observed by FUSE. The line centroid velocities of the O vi absorption do not reflect Galactic rotation well. The O vi velocity dispersions range from 33 to 78 km s À1 , with an average of hbi ¼ 45 AE 11 km s À1 (1 ). These values are much higher than the value of $18 km s À1 expected from thermal broadening for gas at T $ 3 Â 10 5 K, the temperature at which O vi is expected to reach its peak abundance in collisional ionization equilibrium. Turbulence, inflow, and outflow must have an effect on the shape of the O vi profiles. Kinematical comparisons of O vi with Ar i reveal that eight of 21 sight lines are closely aligned in LSR velocity (jDV LSR j 5 km s À1 ), while nine of 21 exhibit significant velocity differences (jDV LSR j ! 15 km s À1 ). This dual behavior may indicate the presence of two different types of O vi-bearing environments toward the Galactic sight lines. The correlation between the H i and O vi intermediate-velocity absorption is poor. We could identify the known H i intermediate-velocity components in the Ar i absorption but not in the O vi absorption in most cases. Comparison of O vi with other highly ionized species suggests that the high ions are produced primarily by cooling hot gas in the Galactic fountain flow and that turbulent mixing also has a significant contribution. The role of turbulent mixing varies from negligible to dominant. It is most important toward sight lines that sample supernova remnants like Loops I and IV. The average N(C iv)/N(O vi) ratios for the nearby halo (this work) and complete halo (Savage et al.) are similar ($0.6), but the dispersion is larger in the sample of nearby halo sight lines. We are able to show that the O vi enhancement toward the Galactic center region that was observed in the FUSE survey of complete halo sight lines (Savage et al.) is likely associated with processes occurring near the Galactic center by comparing the observations toward the ...
We have studied superionization and X-ray line formation in the spectra of z Pup using our new stellar atmosphere code (XCMFGEN) that can be used to simultaneously analyze optical, UV, and X-ray observations. Here, we present results on the formation of the O vi ll1032, 1038 doublet. Our simulations, supported by simple theoretical calculations, show that clumped wind models that assume void in the interclump space cannot reproduce the observed O vi profiles. However, enough O vi can be produced if the voids are filled by a lowdensity gas. The recombination of O vi is very efficient in the dense material, but in the tenuous interclump region an observable amount of O vi can be maintained. We also find that different UV resonance lines are sensitive to different density regimes in z Pup: C iv is almost exclusively formed within the densest regions, while the majority of O vi resides between clumps. N v is an intermediate case, with contributions from both the tenuous gas and clumps.
We present a multi-wavelength (X-ray to optical) analysis, based on non-local thermodynamic equilibrium photospheric+wind models, of the B0 Ia-supergiant: ǫ Ori.The aim is to test the consistency of physical parameters, such as the mass-loss rate and CNO abundances, derived from different spectral bands. The derived mass-losswhere f ∞ is the volume filling factor. However, the S iv λλ1062,1073 profiles are too strong in the models; to fit the observed profiles it is necessary to use f ∞ <0.01. This value is a factor of 5 to 10 lower than inferred from other diagnostics, and impliesṀ 1×10 −7 M ⊙ yr −1 . The discrepancy could be related to porosity-vorosity effects or a problem with the ionization of sulfur in the wind. To fit the UV profiles of N v and O vi it was necessary to include emission from an interclump medium with a density contrast (ρ cl /ρ ICM ) of ∼100. X-ray emission in H-He like and Fe L lines was modeled using four plasma components located within the wind. We derive plasma temperatures from 1×10 6 to 7×10 6 K, with lower temperatures starting in the outer regions (R 0 ∼3-6 R * ), and a hot component starting closer to the star (R 0 2.9 R * ). From X-ray line profiles we inferṀ < 4.9×10 −7 M ⊙ yr −1 . The X-ray spectrum ( 0.1 kev) yields an X-ray luminosity L X ∼ 2.0 × 10 −7 L bol , consistent with the superion line profiles. X-ray abundances are in agreement with those derived from the UV and optical analysis: ǫ Ori is slightly enhanced in nitrogen and depleted in carbon and oxygen, evidence for CNO processed material.
Ultraviolet absorption from interstellar 12 CO and 13 CO was detected toward Oph A and Oph. The measurements were obtained at medium resolution with the Goddard High Resolution Spectrograph on the Hubble Space Telescope. Column density ratios, N( 12 CO)/N( 13 CO), of 125 AE 23 and 117 AE 35 were derived for the sight lines toward Oph A and Oph, respectively. A value of 1100 AE 600 for the ratio N( 12 C 16 O)/ N( 12 C 18 O) toward Oph A was also obtained. Absorption from vibrationally excited H 2 (v 00 ¼ 3) was clearly seen toward this star as well. The ratios are larger than the isotopic ratios for carbon and oxygen appropriate for ambient interstellar material. Since for both carbon and oxygen the more abundant isotopomer is enhanced, selective isotopic photodissociation plays the key role in the fractionation process for these directions. The enhancement arises because the more abundant isotopomer has lines that are more optically thick, resulting in more self-shielding from dissociating radiation. A simple argument involving the amount of self-shielding [from N( 12 CO)] and the strength of the ultraviolet radiation field permeating the gas (from the amount of vibrationally excited H 2 ) shows that selective isotopic photodissociation controls the fractionation seen in these two sight lines, as well as the sight line to Oph.
High-quality archival spectra of interstellar absorption from C i toward nine stars, taken with the Goddard High Resolution Spectrograph on the Hubble Space Telescope, were analyzed. Our sample was supplemented by two sight lines, 23 Ori and 1 Sco, for which the C i measurements of Federman, Welty, & Cardelli were used. Directions with known CH + absorption, but only upper limits on absorption from C 2 and CN, were considered for our study. This restriction allows us to focus on regions where CH + chemistry dominates the production of carbon-bearing molecules. Profile synthesis of several multiplets yielded column densities and Doppler parameters for the C i fine-structure levels. Equilibrium excitation analyses, using the measured column densities as well as the temperature from H 2 excitation, led to values for gas density. These densities, in conjunction with measurements of CH, CH + , C 2 , and CN column densities, provided estimates for the amount of CH associated with CH + production, which in turn set up constraints on the present theories for CH + formation in this environment. We found for our sample of interstellar clouds that on average 30%-40% of the CH originates from CH + chemistry, and in some cases it can be as high as 90%. A simple chemical model for gas containing nonequilibrium production of CH + was developed for the purpose of predicting column densities for CH, CO, HCO + , CH þ 2 , and CH þ 3 generated from large abundances of CH + . Again, our results suggest that nonthermal chemistry is necessary to account for the observed abundance of CH and probably that of CO in these clouds.
We present a method for computing the net transmission of X-rays emitted by shock-heated plasma distributed throughout a partially optically thick stellar wind from a massive star. We find the transmission by an exact integration of the formal solution, assuming that the emitting plasma and absorbing plasma are mixed at a constant mass ratio above some minimum radius, below which there is assumed to be no emission. This model is more realistic than either the slab absorption associated with a corona at the base of the wind or the exospheric approximation that assumes that all observed X-rays are emitted without attenuation from above the radius of optical depth unity. Our model is implemented in XSPEC as a pre-calculated table that can be coupled to a user-defined table of the wavelength dependent wind opacity. We provide a default wind opacity model that is more representative of real wind opacities than the commonly used neutral interstellar medium (ISM) tabulation. Preliminary modeling of Chandra grating data indicates that the X-ray hardness trend of OB stars with spectral subtype can largely be understood as a wind absorption effect.
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