Raman K. Prinja

The stellar winds of 203 Galactic O stars - A quantitative ultraviolet survey

Measurements of the resonance lines of C IV, N V, and Si IV in a sample of 203 O stars observed using IUE in a high-resolution mode are presented. The data are discussed using a consistent set of radius, luminosity, and mass estimates. The ratio of the maximum observed velocity and the terminal (farfield) velocity is found to average at 3.0 for 131 stars with strong or saturated lines, and at 2.6 for all stars. An expression is given which is the best available predictor of O star mass loss rates, giving an accuracy better than a factor of two irrespective of luminosity class. The ionization fractions C(3+)/C and N(4+)/N are found to be independent of stellar effective temperature, but increase by a factor of about two from main sequence stars to supergiants. Two new possible ON stars are identified, and enhanced surface nitrogen abundance is confimed in ON stars. Discrete narrow absorption features are found to be virtually universal characteristic of O star P Cygni profiles. 208 references.

Terminal velocities for a large sample of O stars, B supergiants, and Wolf-Rayet stars

It is argued that easily measured, reliable estimates of terminal velocities for early-type stars are provided by the central velocity asymptotically approached by narrow absorption features and by the violet limit of zero residual intensity in saturated P Cygni profiles. These estimators are used to determine terminal velocities, v(infinity), for 181 O stars, 70 early B supergiants, and 35 Wolf-Rayet stars. For OB stars, the values are typically 15-20 percent smaller than the extreme violet edge velocities, v(edge), while for WR stars v(infinity) = 0.76 v(edge) on average. New mass-loss rates for WR stars which are thermal radio emitters are given, taking into account the new terminal velocities and recent revisions to estimates of distances and to the mean nuclear mass per electron. The relationships between v(infinity), the surface escape velocities, and effective temperatures are examined. 67 refs.

The MiMeS survey of magnetism in massive stars: introduction and overview

The Magnetism in Massive Stars (MiMeS) survey represents a highprecision systematic search for magnetic fields in hot, massive OB stars. To date, MiMeS Large Programs (ESPaDOnS@CFHT, Narval@TBL, HARPSpol@ESO3.6) and associated PI programs (FORS@VLT) have yielded nearly 1200 circular spectropolarimetric observations of over 350 OB stars. Within this sample, 20 stars are detected as magnetic. Follow-up observations of new detections reveals (i) a large diversity of magnetic properties, (ii) ubiquitous evidence for magnetic wind confinement in optical spectra of all magnetic O stars, and (iii) the presence of strong, organized magnetic fields in all known Galactic Of?p stars, and iv) a complete absence of magnetic fields in classical Be stars.

Signature of wide-spread clumping in B supergiant winds

Aims. We seek to establish additional observational signatures of the effects of clumping in OB star winds. The action of clumping on strategic wind-formed spectral lines is tested to steer the development of models for clumped winds and thus improve the reliability of mass-loss determinations for massive stars. Methods. The Siiv λλ1400 resonance line doublets of B0 to B5 supergiants are analysed using empirical line-synthesis models. The focus is on decoding information on wind clumping from measurements of ratios of the radial optical depths (τrad(w)) of the red and blue components of the Siiv doublet. We exploit in particular the fact that the two doublet components are decoupled and formed independently for targets with relatively low wind terminal velocities. Results. Line-synthesis analyses reveal that the mean ratio of τrad(w) of the blue to red Siiv components are rarely close to the canonical value of ∼2 (expected from atomic constants), and spread instead over a range of values between ∼1 and 2. These results are interpreted in terms of a photosphere that is partially obscured by optically thick structures in the outflowing gas. Conclusions. The spectroscopic signatures established in this study demonstrate the wide-spread existence of wind clumping in B supergiants. The additional information in unsaturated doublet profiles provides a means to quantify the porosity of the winds.

Multi-periodic photospheric pulsations and connected wind structures in HD 64760

We report on the results of an extended optical spectroscopic monitoring campaign on the early-type B supergiant HD64760 (B0.5Ib) designed to probe the deep-seated origin of spatial wind structure in massive stars. This new study is based on high-resolution echelle spectra obtained with the Feros instrument at ESO La Silla. 279 spectra were collected over 10 nights between February 14 and 24, 2003. From the period analysis of the line-profile variability of the photospheric lines we identify three closely spaced periods around 4.810 h and a splitting of +/- 3%. The velocity-phase diagrams of the line-profile variations for the distinct periods reveal characteristic prograde non-radial pulsation patterns of high order corresponding to pulsation modes with l and m in the range 6-10. A detailed modeling of the multi-periodic non-radial pulsations with the Bruce and Kylie pulsation-model codes (Townsend 1997b, MNRAS, 284, 839) favors either three modes with l = -m and l = 8, 6, 8 or m = - 6 and l = 8, 6, 10 with the second case maintaining the closely spaced periods in the co-rotating frame. The pulsation models predict photometric variations of 0.012-0.020 mag consistent with the non-detection of any of the spectroscopic periods by photometry. The three pulsation modes have periods clearly shorter than the characteristic pulsation time scale and show small horizontal velocity fields and hence are identified as p-modes. The beating of the three pulsation modes leads to a retrograde beat pattern with two regions of constructive interference diametrically opposite on the stellar surface and a beat period of 162.8h (6.8 days). This beat pattern is directly observed in the spectroscopic time series of the photospheric lines. The wind-sensitive lines display features of enhanced emission, which appear to follow the maxima of the photospheric beat pattern. The pulsation models predict for the two regions normalized flux amplitudes of A = +0.33, -0.28, sufficiently large to raise spiral co-rotating interaction regions (Cranmer & Owocki 1996, ApJ, 462, 469). We further investigate the observed Ha wind-profile variations with a simple rotating wind model with wind-density modulations to simulate the effect of possible streak lines originating from the localized surface spots created by the NRP beat pattern. It is found that such a simple scenario can explain the time scales and some but not all characteristics of the observed Ha line-profile variations.

Wind Variability of B Supergiants. No. 1; The Rapid Rotator HD 64760 (B0.5 Ib)

We present the results of a 6 day time series of observations of the rapidly rotating B0.5 Ib star HD 64760, We point out several reasons why such intermediate luminosity B supergiants are ideal targets for wind variability studies and then present our results that show the following: (1) continuous wind activity throughout the 6 day run with the wind never in steady state for more than a few hr; (2) wind variability very near nu = 0 km s(-1) in the resonance lines from the lower ionization stages (Al III and C II); (3) a distinct correlation between variability in the Si III lambda lambda 1300 triplets, the strong C III lambda 1247 singlet, and the onset of extremely strong wind activity, suggesting a connection between photospheric and wind activity; (4) long temporal coherence in the behavior of the strong absorption events; (5) evidence for large-scale spatial coherence, implied by a whole scale, simultaneous weakening in the wind absorption over a wide range in velocities; (6) ionization variability in the wind accompanying the largest changes in the absorption strengths of the wind lines.In addition, modeling of the wind lines provides the following information about the state the wind in HD 64760. (1) The number of structures on the portion of a constant velocity surface occulting the stellar disk at a particular time must be quite small, while the number on the entire constant velocity surface throughout the wind must be large. (2) The escape probability at low velocity is overestimated by a normal beta similar to 1 velocity law, perhaps due to the presence of low-velocity shocks deep in the wind or a shallow velocity gradient at low velocity. (3) Estimates of the ionization structure in the wind indicate that the ionization ratios are not those expected from thermal equilibrium wind models or from an extrapolation of previous O star results. The large observed q(N v)/q(Si IV) ratio is almost certainly due to distributed X-rays, but the level of ionization predicted by distributed X-ray wind models is inconsistent with the predicted mass-loss rate. Thus, it is impossible to reconcile the observed ionization ratios and the predicted mass-loss rate within the framework of the available models,

Time series observations of O stars. I - IUE observations of variability in the stellar wind of Zeta Puppis

Stellar wind variability in zeta-Pup (O4 I(n)f) is described based on 31 high-resolution IUE observations secured over 5 1/2 days in 1989 April. Extensive changes are evident in the absorption regions of Si IV lambda-lambda-1393.76, 1402.77 and N IV lambda-1718.55 P Cygni profiles. Both lines exhibit similar patterns of variability, which are characterized by the development (at about -1000 km s-1), and subsequent blueward migration, of discrete absorption components. The formation of four discrete features is identified over approximately 2.2 days of intensive observations, with a recurrence time of approximately 15 hr. The time scales, velocities, and accelerations of the progressive absorption enhancements are determined. These changes are accompanied by fluctuations of up to approximately 200 km s-1 in the maximum observed blue edge velocities in saturated C IV and N V P Cygni profiles.

Radio and submillimetre observations of

In common with other early-type stars, epsilon Ori (B0 Ia) shows evidence for structure in its stellar wind. Variations in optical and ultraviolet line profiles reveal the presence of large-scale structure in the inner wind. The detection of X-rays and the existence of black troughs in saturated ultraviolet lines are indicative of small-scale structure. The geometric extent of both types of structure is poorly known. In principle, large-scale structure can be detected directly from very high spatial resolution observations that resolve the stellar wind. A simpler technique is to look for the presence of additional flux compared to that expected from a smooth wind. The run of this excess flux as a function of wavelength indicates how fast structure decays in the wind. If there is variability in the excess flux, it shows us that the structure must be large-scale. Such variability is suggested by two previous 6 cm radio observations of epsilon Ori: Abbott et al. (1980) found 1.6 +/- 0.5 mJy, while Scuderi et al. (1998) measured only 0.60 +/- 0.06 mJy. This could indicate that the large-scale structure persists beyond similar to50 R-*. To further investigate this variability, we used the Very Large Array (VLA) to monitor epsilon Ori over a 5-day period in February 1999. We supplemented our data with observations from the VLA archive. In an attempt to resolve the stellar wind, we also obtained a series of high spatial resolution observations with the Multi-Element Radio Linked Interferometer Network (MERLIN) during January-March 1999. From this combined material we find no evidence for variability and we conclude that the Abbott et al. (1980) flux determination is in error. The data do show substantial excess flux at millimetre wavelengths, compared to a smooth wind. This excess is confirmed by a submillimetre observation which we obtained with the James Clerk Maxwell Telescope (JCMT). The behaviour of epsilon Ori is therefore similar to what had been found previously for alpha Cam, delta Ori A, kappa Ori and zeta Pup. While the present data do not allow very strong constraints, they show that considerable structure must persist up to at least similar to10 R-* in the wind of epsilon Ori. The combined radio fluxes are used to derive a mass-loss rate of log (M) over dot (M./yr) = 5.73 +/- 0.04. This value is in good agreement with the Halpha mass-loss rate. The good agreement between Halpha and radio mass-loss rates for hot stars in general remains puzzling, as it implies that the same amount of structure is present in very different formation regions.

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Stellar-wind variability in the archetypal nonradially pulsating O star ζ Oph (O9.5 V) is discussed on the basis of new time-series IUE and HST spectroscopy and archival results. Time-variable discrete absorption components are first observed at high velocities (≥10 3 km s −1 ≃0.8v∞) and then migrate blueward; the recurrence time scale for the phenomenon is ∼20 hr. This is the first record of this type of variability in a luminosity-class V star and provides support for the previously inferred ubiquity of such behavior across the O-star regime. The accelerations are slower than predicted by steady state wind models and are unlikely to represent the time-averaged velocity law of the outflow

Ori

A detailed case study of variability in the stellar wind of the O7.5 III star Xi Persei is presented based on 56 high-resolution IUE observations taken between 1978 and 1984. Profile-fitting techniques are described, and possible interpretations for the observed Si IV variability are discussed. The derived properties of the narrow and broad discrete absorption features are given along with other P Cygni profile characteristics, including emission-to-absorption ratios and mass-loss rates. The role of ionization conditions and episodic formation of shells in accounting for the variability is addressed together with some previously reported observations of Xi Per.