Ian D. Howarth

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.

Be‐star rotation: how close to critical?

We argue that, in general, observational studies of Be-star rotation have paid insufficient attention to the effects of equatorial gravity darkening. We present new line-profile calculations that emphasize the insensitivity of line width to rotation for fast rotators. Coupled with a critical review of observational procedures, these calculations suggest that the observational parameter v sin i may systematically underestimate the true projected equatorial rotation velocity, v e sin i, by some tens of per cent for rapid rotators. A crucial implication of this work is that Be stars may be rotating much closer to their critical velocities than is generally supposed, bringing a range of new processes into contention for the elusive physical mechanism responsible for the circumstellar disc thought to be central to the Be phenomenon.

The Young Massive Stellar Objects of M17

We present a multiwavelength spectroscopic survey that reveals the ionizing cluster of M17 and uncovers a population of young stellar objects (YSOs) of high mass (M similar to 5-20 M.). The masses of the stars have been determined fairly accurately through optical or near-infrared spectral classification. We find strong circumstantial evidence for disks around the massive YSOs in the following forms: near-infrared excess, optical veiling, CO band-head emission, and/or Pa delta emission. We find a direct correlation between those YSOs that show CO band-head emission at 2.3 mu m and these stars in our survey that show Pa delta emission; in three of the four Pa delta emission stars this line is double peaked (suggestive of a bipolar wind or a rotating disk or envelope). Our data suggest that circumstellar material, possibly in the form of a disk, is prevalent among very young objects of fairly high mass. Based on considerations of disk lifetimes in other young clusters, the M17 cluster appears to be very young, perhaps less than 1 Myr.We have also identified at least nine O stars and a couple of late-O/early-B stars, most behind more than 8 mag of visible extinction using either optical or near-infrared spectral types. Several stars have inferred masses in excess of 60 M., and they look to be very close to the predicted zero-age main sequence with an estimated age of about 1 Myr, consistent with the age of the massive YSOs in the cluster. We have used the O stars to determine the distance to M17, which assumes the stars to lie on the zero-age main sequence (1300(-200)(+400) pc). While we attempt to determine an initial mass function for the cluster, it is incomplete even at high masses because of regions of extremely high extinction (A(V) > 20) in the cluster. We have also used the M17 O stars to study the dust properties in the local cloud and the behavior of the diffuse interstellar bands (DIBs) along this sight line, over the extinction range of A(V) = 3-10. The DIBs over this extinction range show little change in spectral shape nor a significant increase in strength. We suggest the features are already saturated at small A(V), or the material local to M17, where the increased extinction is being traced, does not contain the carriers of the DIB feature.

A new spectral classification system for the earliest O stars: Definition of type O2

High-quality, blue-violet spectroscopic data are collected for 24 stars that have been classified as type O3 and that display the hallmark N IV and N V lines. A new member of the class is presented; it is the second known in the Cyg OB2 association, and only the second in the northern hemisphere. New digital data are also presented for several of the other stars. Although the data are inhomogeneous, the uniform plots by subcategory reveal some interesting new relationships. Several issues concerning the classification of the hottest O-type spectra are discussed, and new digital data are presented for the five original O3 dwarfs in the Carina Nebula, in which the N IV, N V features are very weak or absent. New spectral types O2 and O3.5 are introduced here as steps toward resolving these issues. The relationship between the derived absolute visual magnitudes and the spectroscopic luminosity classes of the O2–O3 stars shows more scatter than at later O types, at least partly because some overluminous dwarfs are unresolved multiple systems, and some close binary systems of relatively low luminosity and mass emulate O3 supergiant spectra. However, it also appears that the behavior of He II λ4686, the primary luminosity criterion at later O types, responds to other phenomena in addition to luminosity at spectral types O2–O3. There is evidence that these spectral types may correspond to an immediate pre-WN phase, with a correspondingly large range of luminosities and masses. A complete census of spectra classified into the original O3 subcategories considered here (not including intermediate O3/WN types or O3 dwarfs without N IV, N V features) totals 45 stars; 34 of them belong to the Large Magellanic Cloud and 20 of the latter to 30 Doradus.

Bright OB stars in the Galaxy - III. Constraints on the radial stratification of the clumping factor in hot star winds from a combined H

Context. Recent results strongly challenge the canonical picture of massive star winds: various evidence indicates that currently accepted mass-loss rates, u M, may need to be revised downwards, by factors extending to one magnitude or even more. This is because the most commonly used mass-loss diagnostics are affected by “clumping” (small-scale density inhomogeneities), influencing our interpretation of observed spectra and fluxes. Aims. Such downward revisions would have dramatic consequences for the evolution of, and feedback from, massive stars, and thus robust determinations of the clumping properties and mass-loss rates are urgently needed. We present a first attempt concerning this objective, by means of constraining the radial stratification of the so-called clumping factor. Methods. To this end, we have analyzed a sample of 19 Galactic O-type supergiants/giants, by combining our own and archival data for Hα, IR, mm and radio fluxes, and using approximate methods, calibrated to more sophisticated models. Clumping has been included into our analysis in the “conventional” way, by assuming the inter-clump matter to be void. Because (almost) all our diagnostics depends on the square of density, we cannot derive absolute clumping factors, but only factors normalized to a certain minimum. Results. This minimum was usually found to be located in the outermost, radio-emitting region, i.e., the radio mass-loss rates are the lowest ones, compared to u M derived from Hα and the IR. The radio rates agree well with those predicted by theory, but are only upper limits, due to unknown clumping in the outer wind. Hα turned out to be a useful tool to derive the clumping properties inside r < 3 ... 5R� . Our most important result concerns a (physical) difference between denser and thinner winds: for denser winds, the innermost region is more strongly clumped than the outermost one (with a normalized clumping factor of 4.1 ± 1.4), whereas thinner winds have similar clumping properties in the inner and outer regions. Conclusions. Our findings are compared with theoretical predictions, and the implications are discussed in detail, by assuming different scenarios regarding the still unknown clumping properties of the outer wind.

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Aims. We have previously analysed the spectra of 135 early B-type stars in the Large Magellanic Cloud (LMC) and found several groups of stars that have chemical compositions that conflict with the theory of rotational mixing. Here we extend this study to Galactic and Small Magellanic Cloud (SMC) metallicities. Methods. We provide chemical compositions for ~50 Galactic and ~100 SMC early B-type stars and compare these to the LMC results. These samples cover a range of projected rotational velocities up to ~300 km s-1 and hence are well suited to testing rotational mixing models. The surface nitrogen abundances are utilised as a probe of the mixing process since nitrogen is synthesized in the core of the stars and mixed to the surface. Results. In the SMC, we find a population of slowly rotating nitrogen-rich stars amongst the early B type core-hydrogen burning stars, which is comparable to that found previously in the LMC. The identification of non-enriched rapid rotators in the SMC is not possible due to the relatively high upper limits on the nitrogen abundance for the fast rotators. In the Galactic sample we find no significant enrichment amongst the core hydrogen-burning stars, which appears to be in contrast with the expectation from both rotating single-star and close binary evolution models. However, only a small number of the rapidly rotating stars have evolved enough to produce a significant nitrogen enrichment, and these may be analogous to the non-enriched rapid rotators previously found in the LMC sample. Finally, in each metallicity regime, a population of highly enriched supergiants is observed, which cannot be the immediate descendants of core-hydrogen burning stars. Their abundances are, however, compatible with them having gone through a previous red supergiant phase. Together, these observations paint a complex picture of the nitrogen enrichment in massive main sequence and supergiant stellar atmospheres, where age and binarity cause crucial effects. Whether rotational mixing is required to understand our results remains an open question at this time, but could be answered by identifying the true binary fraction in those groups of stars that do not agree with single-star evolutionary models.

, IR and radio analysis

We present a catalogue of new spectral types for hot, luminous stars in the Small Magellanic Cloud (SMC). The catalogue contains 4161 objects, giving an order-of-magnitude increase in the number of SMC stars with published spectroscopic classifications. The targets are primarily B- and A-type stars (2862 and 853 objects respectively), with one Wolf‐Rayet, 139 O-type and 306 FG stars, sampling the main sequence to ∼mid-B. The selection and classification criteria are described, and objects of particular interest are discussed, including UV-selected targets from the Ultraviolet Imaging Telescope (UIT )e xperiment, Be and B[e] stars, ‘anomalous A supergiants’ and composite-spectrum systems. We examine the incidence of Balmer-line emission, and the relationship between Hγ equivalent width and absolute magnitude for BA stars. Ke yw ords: stars: early-type ‐ stars: emission-line, Be ‐ stars: fundamental parameters ‐ Hertzsprung‐Russell (HR) diagram ‐ Magellanic Clouds.

The VLT-FLAMES survey of massive stars: constraints on stellar evolution from the chemical compositions of rapidly rotating Galactic and Magellanic Cloud B-type stars

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.

A 2dF survey of the Small Magellanic Cloud

We report the detection of a magnetic field on the Of? p star HD 108. Spectropolarimetric observations conducted in 2007, 2008 and 2009, respectively, with NARVAL@ Telescope Bernard Lyot (TBL) and Echelle SpectroPolarimetric Device for the Observation of Stars at Canada-France-Hawaii Telescope (ESPaDOnS@CFHT) reveal a clear Zeeman signature in the average Stokes V profile, stable on time-scales of days to months and slowly increasing in amplitude on time-scales of years. We speculate that this time-scale is the same as that on which Ha emission is varying and is equal to the rotation period of the star. The corresponding longitudinal magnetic field, measured during each of the three seasons, increases slowly from 100 to 150 G, implying that the polar strength of the putatively dipolar large-scale magnetic field of HD 108 is at least 0.5 kG and most likely of the order of 1-2 kG. The stellar and wind properties are derived through a quantitative spectroscopic analysis with the code CMFGEN. The effective temperature is difficult to constrain because of the unusually strong He lambda lambda 4471, 5876 lines. Values in the range of 33 000-37 000K are preferred. A mass-loss rate of about 10(-7) M(circle dot) yr(-1) (with a clumping factor f = 0.01) and a wind terminal velocity of 2000 km s(-1) are derived. The wind confinement parameter. eta(star) is larger than 100, implying that the wind of HD 108 is magnetically confined. Stochastic short-term variability is observed in the wind-sensitive lines but not in the photospheric lines, excluding the presence of pulsations. Material infall in the confined wind is the most likely origin for lines formed in the inner wind. Wind clumping also probably causes part of the Ha variability. The projected rotational velocity of HD 108 is lower than 50 km s-1, consistent with the spectroscopic and photometric variation time-scales of a few decades. Overall, HD 108 is very similar to the magnetic O star HD 191612 except for an even slower rotation.