Bright OB Stars Research Articles

The Young Massive Star Cluster Westerlund 2 Observed with MUSE. II. MUSEpack—A Python Package to Analyze the Kinematics of Young Star Clusters

Abstract We mapped the Galactic young massive star cluster Westerlund 2 with the integral field spectrograph MUSE (spatial resolution: 0.2 arcsec px−1, spectral resolution: Δλ = 1.25 Å, wavelength range: 4600–9350 Å) mounted on the Very Large Telescope. We present the fully reduced data set and introduce our new Python package “MUSEpack,” which we developed to measure stellar radial velocities (RVs) with an absolute precision of 1–2 km s−1 without the necessity of a spectral template library. This novel method uses the two-dimensional spectra and an atomic transition line library to create templates around strong absorption lines for each individual star. Automatic, multi-core processing makes it possible to efficiently determine stellar RVs of a large number of stars with the necessary precision to measure the velocity dispersion of young star clusters. MUSEpack also provides an enhanced method for removing telluric lines in crowded fields without sky exposures, and a Python wrapper for ESO’s data reduction pipeline. We observed Westerlund 2 with a total of 11 short and 5 long exposures (survey area: ∼11 arcmin2 or 15.8 pc2) to cover the bright nebular emission and OB stars, as well as the fainter pre-main-sequence stars (≥1 M ⊙). We extracted 1725 stellar spectra with a mean signal-to-noise ratio of S/N > 5 per pixel. Typical RV uncertainties of 4.78 km s−1, 2.92 km s−1, and 1.1 km s−1 are reached for stars with a mean S/N > 10, S/N > 20, and S/N > 50 per pixel, respectively. It is possible to reach RV accuracies of 0.9 km s−1, 1.3 km s−1, and 2.2 km s−1 with ≥5, 3–4, and 1–2 spectral lines used to measure the RVs, respectively. The combined statistical uncertainty on the RV measurements is 1.10 km s−1.

Chemical abundances of fast-rotating massive stars

Aims: Past observations of fast-rotating massive stars exhibiting normal nitrogen abundances at their surface have raised questions about the rotational mixing paradigm. We revisit this question thanks to a spectroscopic analysis of a sample of bright fast-rotating OB stars, with the goal of quantifying the efficiency of rotational mixing at high rotation rates. Method: Our sample consists of 40 fast rotators on the main sequence, with spectral types comprised between B0.5 and O4. We compare the abundances of some key element indicators of mixing (He, CNO) with the predictions of evolutionary models for single objects and for stars in interacting binary systems. Results: The properties of half of the sample stars can be reproduced by single evolutionary models, even in the case of probable or confirmed binaries that can therefore be true single stars in a pre-interaction configuration. The main problem for the rest of the sample is a mismatch for the [N/O] abundance ratio (we confirm the existence of fast rotators with a lack of nitrogen enrichment) and/or a high helium abundance that cannot be accounted for by models. Modifying the diffusion coefficient implemented in single- star models does not solve the problem as it cannot simultaneously reproduce the helium abundances and [N/O] abundance ratios of our targets. Since part of them actually are binaries, we also compared their chemical properties with predictions for post-mass transfer systems. We found that these models can explain the abundances measured for a majority of our targets, including some of the most helium-enriched, but fail to reproduce them in other cases. Our study thus reveals that some physical ingredients are still missing in current models.

Line profile variability in spectra of hot massive stars

AbstractWe report the results of our study of the fast line profile variability (LPV) (hours – few days) in the spectra of bright OB and WR stars. All spectra were obtained with 6-m and 1-m telescope of Russian Special Astrophysical Observatory (SAO) and 1.8-m telescope of Bohyunsan Optical Astronomy Observatory, Korea (BOAO). We detected both the stochastic LPV, connected with the formation of small-scale structures in the stellar wind and the regular LPV induced by the large-scale structures in the wind.

DUSTY OB STARS IN THE SMALL MAGELLANIC CLOUD. II. EXTRAGALACTIC DISKS OR EXAMPLES OF THE PLEIADES PHENOMENON?

We use mid-infrared Spitzer spectroscopy and far-infrared Herschel photometry for a sample of twenty main sequence O9--B2 stars in the Small Magellanic Cloud (SMC) with strong 24 micron excesses to investigate the origin of the mid-IR emission. Either debris disks around the stars or illuminated patches of dense interstellar medium (ISM) can cause such mid-IR emission. In a companion paper, Paper I, we use optical spectroscopy to show that it is unlikely for any of these sources to be classical Be stars or Herbig Ae/Be stars. We focus our analysis on debris disks and cirrus hot spots. We find three out of twenty stars to be significantly extended in the mid-IR, establishing them as cirrus hot spots. We then fit the IR spectral energy distributions to determine dust temperatures and masses. We find the dust masses in the SMC stars to be larger than for any known debris disks, although this evidence against the debris disk hypothesis is circumstantial. Finally, we created a local comparison sample of bright mid-IR OB stars in the Milky Way (MW) by cross-matching the WISE and Hipparcos catalogs. All such local stars in the appropriate luminosity range that can be unambiguously classified are young stars with optical emission lines or are spatially resolved by WISE with sizes too large to be plausible debris disk candidates. We conclude that the very strong mid-IR flux excesses are most likely explained as cirrus hot spots, although we cannot rigorously rule out that a small fraction of the sample is made up of debris disks or transition disks. We present suggestive evidence that bow-shock heating around runaway stars may be a contributing mechanism to the interstellar emission. These sources, interpreted as cirrus hot spots, offer a new localised probe of diffuse interstellar dust in a low metallicity environment. (Abridged)

A multi-epochXMM-Newtoncampaign on the core of the massive Cygnus OB2 association

Cyg OB2 is one of the most massive associations of O-type stars in our Galaxy. Despite the large interstellar reddening towards Cyg OB2, many studies, spanning a wide range of wavelengths, have been conducted to more clearly understand this association. X-ray observations provide a powerful tool to overcome the effect of interstellar absorption and study the most energetic processes associated with the stars in Cyg OB2. We analyse XMM-Newton data to investigate the X-ray and UV properties of massive O-type stars as well as low-mass pre-main sequence stars in Cyg OB2. We obtained six XMM-Newton observations of the core of Cyg OB2. In our analysis, we pay particular attention to the variability of the X-ray bright OB stars, especially the luminous blue variable candidate Cyg OB2 #12. We find that X-ray variability is quite common among the stars in Cyg OB2. Whilst short-term variations are restricted mostly to low-mass pre-main sequence stars, one third of the OB stars display long-term variations. The X-ray flux of Cyg OB2 #12 varies by 37%, over timescales from days to years, whilst its mean log(L_X/L_bol) amounts to -6.10. These properties suggest that Cyg OB2 #12 is either an interacting-wind system or displays a magnetically confined wind. Two other X-ray bright O-type stars (MT91 516 and CPR2002 A11) display variations that suggest they are interacting wind binary systems.

THE YOUNG INTERSTELLAR BUBBLE WITHIN THE ROSETTE NEBULA

We use high-resolution International Ultraviolet Explorer (IUE) data and the interstellar (IS) features of highly ionized Si IV and C IV seen toward the young, bright OB stars of NGC 2244 in the core of the Rosette Nebula to study the physics of young IS bubbles. Two discrete velocity components in Si IV and C IV are seen toward stars in the 6.2 pc radius central cavity, while only a single velocity component is seen toward those stars in the surrounding H II region, at the perimeter and external to this cavity. The central region shows characteristics of a very young, windblown bubble. The shell around the central hot cavity is expanding at 56 km s–1 with respect to the embedded OB stars, while the surrounding H II region of the Rosette is expanding at ~13 km s–1. Even though these stars are quite young (≈2-4 Myr), both the radius and expansion velocity of the 6.2 pc inner shell point to a far younger age; t age ~ 6.4 × 104 years. These results represent a strong contradiction to theory and present modeling, where much larger bubbles are predicted around individual O stars and O associations. Specifically, the results for this small bubble and its deduced age extend the missing wind luminosity problem to young evolving bubbles. These results indicate that OB star winds mix the surrounding H II regions and the wind kinetic energy is converted to turbulence and radiated away in the dense H II regions. These winds do not form hot, adiabatically expanding cavities. True IS bubbles appear only to form at later evolutionary times, perhaps triggered by increased mass loss rates or discrete ejection events. Means for rectifying discrepancies between theory and observations are discussed.

DOES STELLAR FEEDBACK CREATE H I HOLES? AHUBBLE SPACE TELESCOPE/VERY LARGE ARRAY STUDY OF HOLMBERG II

We use deep Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) F555W and F814W photometry of resolved stars in the M81 Group dwarf irregular galaxy Ho II to study the hypothesis that the holes identified in the neutral interstellar medium (H i) are created by stellar feedback. From the deep photometry, we construct color–magnitude diagrams (CMDs) and measure the star formation histories (SFHs) for stars contained in H i holes from two independent holes catalogs, as well as select control fields, i.e., similar sized regions that span a range of H i column densities. The CMDs reveal young (< 200 Myr) stellar populations inside all H i holes, which contain very few bright OB stars with ages less than 10 Myr, indicating they are not reliable tracers of H i hole locations while the recent SFHs confirm multiple episodes of star formation within most holes. Converting the recent SFHs into stellar feedback energies, we find that enough energy has been generated to have created all holes. However, the required energy is not always produced over a timescale that is less than the estimated kinematic age of the hole. A similar analysis of stars in the control fields finds that the stellar populations of the control fields and H i holes are statistically indistinguishable. However, because we are only sensitive to holes ∼100 pc in diameter, we cannot tell if there are smaller holes inside the control fields. The combination of the CMDs, recent SFHs, and locations of young stars shows that the stellar populations inside H i holes are not coherent, single-aged, stellar clusters, as previously suggested, but rather multi-age populations distributed across each hole. From a comparison of the modeled and observed integrated magnitudes, and the locations and energetics of stars inside of H i holes, we propose a potential new model: a viable mechanism for creating the observed H i holes in Ho ii is stellar feedback from multiple generations of SF spread out over tens or hundreds of Myr, and thus, the concept of an age for an H i hole is intrinsically ambiguous. For H i holes in the outer parts of Ho ii, located beyond the HST/ACS coverage, we use Monte Carlo simulations of expected stellar populations to show that low level SF could provide the energy necessary to form these holes. Applying the same method to the SMC, we find that the holes that appear to be void of stars could have formed via stellar feedback from low level SF. We further find that Hα and 24 μm emission, tracers of the most recent star formation, do not correlate well with the positions of the H i holes. However, UV emission, which traces star formation over roughly the last 100 Myr, shows a much better correlation with the locations of the H i holes.

Bright OB stars in the Galaxy

Bright OB stars in the Galaxy

Line profile variability of OB stars: Pulsation, rotation, clumps and magnetic fields

AbstractThe variability of line profiles in spectra of bright OB stars has been studied.We obtain more than 1000 high quality spectra of δ Ori A, λ Ori A, α Cam, 19 Cep, ι Her, ρ Leo and other target stars. We revealed the line profile microvariability of small amplitude (0.5–3% in the adjacent continuum units) for all observed stars. For most stars only cyclic components of the line profile variability (LPV) at the time scales from hours to days were detected. These components seem to be connected with both the non‐radial pulsations (NRP) and rotation line profile modulation. In the spectra of δ Ori A and λ Ori A we found the evidences of the stochastic LPV in spectra, probably connected with the small clumps in the stellar wind. On the basis of recent observations we discuss the origin of the magnetic field of early‐type stars. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Discovery of the Pre-Main-Sequence Population of the Stellar Association LH 95 in the Large Magellanic Cloud with Hubble Space Telescope Advanced Camera for Surveys Observations

We report the discovery of an extraordinary number of pre-main-sequence (PMS) stars in the vicinity of the stellar association LH 95 in the Large Magellanic Cloud (LMC). Using the Advanced Camera for Surveys on board the Hubble Space Telescope in wide-field mode, we obtained deep high-resolution imaging of the main body of the association and of a nearby representative LMC background field. These observations allowed us to construct the color-magnitude diagram (CMD) of the association in unprecedented detail and to decontaminate the CMD for the average LMC stellar population. The most significant result is the direct detection of a substantial population of PMS stars and their clustering properties with respect to the distribution of the higher mass members of the association. Although LH 95 represents a rather modest star-forming region, our photometry, with a detection limit of V 28 mag, reveals in its vicinity more than 2500 PMS stars with masses down to ~0.3 M☉. Thus, our observations offer a new perspective on a typical LMC association: the stellar content of LH 95 is found to extend from bright OB stars to faint red PMS stars, suggesting a fully populated initial mass function from the massive blue giants down to the subsolar mass regime.

Radial Velocities of Six OB Stars

ABSTRACTWe present new results from a radial velocity study of six bright OB stars with little or no prior measurements. One of these, HD 45314, may be a long‐period binary, but the velocity variations of this Be star may be related to changes in its circumstellar disk. Significant velocity variations were also found for HD 60848 (possibly related to nonradial pulsations) and HD 61827 (related to wind variations). The other three targets, HD 46150, HD 54879, and HD 206183, are constant‐velocity objects, but we note that HD 54879 has Hα emission that may originate from a binary companion. We illustrate the average red spectrum of each target.

Bright OB stars in the Galaxy

Bright OB stars in the Galaxy

Bright OB stars in the Galaxy

Bright OB stars in the Galaxy

High‐Resolution X‐Ray Spectra of the Brightest OB Stars in the Cygnus OB2 Association

The Cyg OB2 association contains some of the most luminous OB stars in our Galaxy, the brightest of which are also among the most luminous in X-rays. We obtained a Chandra High Energy Transmission Grating Spectrometer observation centered on Cyg OB2 No. 8A, the most luminous X-ray source in the association. Although our analysis focuses on the X-ray properties of Cyg OB2 No. 8A, we also present limited analyses of three other OB stars (Cyg OB2 Nos. 5, 9, and 12). Applying standard diagnostic techniques as used in previous studies of early-type stars, we find that the X-ray properties of Cyg OB2 No. 8A are very similar to those of other OB stars that have been observed using high-resolution X-ray spectroscopy. From analyses of the He-like ion fir emission lines (Mg XI, Si XIII, S XV, and Ar XVII), we derive radial distances of the He-like line emission sources and find that the higher energy ions have their lines form closer to the stellar surface than those of lower ion states. These fir-inferred radii are also found to be consistent with their corresponding X-ray continuum optical depth unity radii. Both of these findings are in agreement with previous O star studies and again suggest that anomalously strong shocks or high-temperature zones may be present near the base of the wind. The observed X-ray emission-line widths (HWHM ~ 1000 km s-1) are also compatible with the observations of other O star supergiants. Since Cyg OB2 No. 8A is similar in spectral type to ? Pup (the only O star that clearly shows asymmetric X-ray emission-line profiles with large blueshifts), we expected to see similar emission-line characteristics. Contrary to other O star results, the emission lines of Cyg OB2 No. 8A show a large range in line centroid shifts (~-800 to +250 km s-1). However, we argue that most of the largest shifts may be unreliable and the resulting range in shifts is much less than those observed in ? Pup. Although there is one exception, the H-like Mg XII line, which shows a blueshift of -550 km s-1, there are problems associated with trying to understand the nature of this isolated large blueshifted line. To address the degree of asymmetry in these line profiles, we present Gaussian best-fit line profile model spectra from ? Pup to illustrate the expected asymmetry signature in the ?2 residuals. Comparisons of the Cyg OB2 No. 8A best-fit line profile residuals with those of ? Pup suggest that there are no indications of any statistical significant asymmetries in these line profiles. Both the line shift characteristics and lack of line asymmetries are very puzzling results. Given the very high mass-loss rate of Cyg OB2 No. 8A (approximately 5 times larger than previous Chandra-observed O supergiants), the emission lines from this star should display a significant level of line asymmetry and blueshifts as compared to other OB stars. We also discuss the implications of our results in light of the fact that Cyg OB2 No. 8A is a member of a rather tight stellar cluster, and shocks could arise at interfaces with the winds of these other stars.

Bright OB stars in the Galaxy

We study mass-loss and wind momentum rates of 29 Galactic O-type stars with luminosity classes I, III and V by means of a pure Hα profile analysis and investigate to what extent the results compare to those originating from a state-of- the-art, complete spectral analysis. Our investigation relies on the approximate method developed by Puls et al. (1996) which we have modified to account for the effects of line-blanketing. Effective temperatures and gravities needed to obtain quanti- tative results from such a simplified approach have been derived by means of calibrations based on most recent spectroscopic NLTE analyses and models of Galactic stars by Repolust et al. (2003) and Martins et al. (2002). Comparing (i) the derived wind- densities to those determined by Repolust et al. (2003) for eleven stars in common and (ii) the Wind-momentum Luminosity Relationship (WLR) for our sample stars to those derived by other investigations, we conclude that our approximate approach is actually able to provide consistent results. Additionally, we studied the consequences of fine tuning some of the direct and indirect parameters entering the WLR, especially by accounting for different possible values of stellar reddening and distances. Combining our data set with the corresponding data provided by Herrero et al. (2002) and Repolust et al. (2003) we finally study the WLR for the largest sample of Galactic O-type stars gathered so far, including an elaborate error treatment. The established disagreement between the theoretical predictions and the observed WLRs being a function of luminosity class is suggested to be a result of wind clumping. Different strategies to check this hypothesis are discussed, particularly by comparing the Hα mass-loss rates with the ones derived from radio observations.

The association surrounding NGC 2439

The structure of the field surrounding the Galactic cluster NGC 2439 is studied utilizing photometry of bright OB stars. We collate all photometric and kinematic data available to identify possible groupings. The stars of our sample show a large scatter in their distances, radial velocities, proper motions and reddenings. We conclude that they do not belong to a single stellar association. We find evidence of the existence of three coherent structures at distances of 370 pc, 1 kpc, and 2.6-3.2 kpc. The high stellar density toward NGC 2439 is very likely due to a decreased absorption in this direction, which poses some doubt on the reality of the cluster. A comprehensive study of NGC 2439 is required to clarify its nature. The spatial distribution of the stars and their reddening are used to characterise the spatial distribution of the visual extinction in the region. The results obtained confirm previously determined constraints on the formation mechanism of interstellar CH+ towards the NGC 2439 field.

The distribution of bright OB stars in the Canis Major-Puppis-Vela region of the Milky Way

The picture of the young stellar groups in the Canis Major–Puppis–Vela (215°<l<275°) section of the Milky Way is studied and updated utilizing uvbyβ photometry of intrinsically luminous OB stars. We use all data from the literature to create a sample with 98 per cent completeness to 9.5 mag. The very dense low reddened OB association CMa OB1 is confirmed at a distance of 0.99 (±0.05 s.e.) kpc. Towards Puppis the brightest intrinsically luminous stars do not reveal Pup OB1 and Pup OB2. In the same direction, we separate two small groups, previously related to the association surrounding NGC 2439 at 3.5–4.5 kpc. The first one contains four highly reddened B-type supergiants situated in front of the cluster at 1.03 (±0.14 s.e.) kpc – much closer to the Sun than has been estimated before. The second one lies north-west from the cluster at 3.2 (±0.23 s.e.) kpc according to our estimate. In the direction to Vela, the bright OB stars are apparently embedded in a dust cloud and spread out between 0.3 and 2.5 kpc, forming clumps over this distance range. In general, the prominent apparent young structures delineated by the brightest intrinsically luminous OB stars in the directions of Canis Major and Vela are some 20–25 per cent closer to the Sun than has previously been thought. This is in agreement with the Hipparcos results for the Galactic OB associations, and is highly likely to be caused by the overestimation of the spectroscopic distances used in the previous studies.

The ROSAT all-sky survey catalogue of optically bright OB-type stars

For the detailed statistical analysis of the X-ray emission of hot stars we selected all stars of spectral type O and B listed in the Yale Bright Star Catalogue and searched for them in the ROSAT All-Sky Survey. In this paper we describe the selection and preparation of the data and present a compilation of the derived X-ray data for a complete sample of bright OB stars.

Have stellar and supernovae radiations altered the pristine nature of comets?

Effect of stellar and supernova radiations on cometary nuclei in the Oort cloud is investigated. Radiation dose received by a comet is calculated and compared with the one which Halley's comet receives by one perihelion passage. Stellar radiation provides 10 to 50% of Halley unit over 4 billion years. Inclusion of sublimation of volatile molecules such as CO or N2 does not allow the temperature to rise to 30 K by irradiation of bright OB stars, contrary to the claim of Stern and Shull. A chance encounter with a SN provides radiation dose which is just sufficient to raise to 30 K the surface layer which is 1 m thick on the assumption that the radiation is wholly communicated to the interior. Thus, the comets remain pristine under the effect of stellar and SN radiations.

The role of H II regions during star formation and chemical enrichment in globular clusters

view Abstract Citations (11) References (37) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The role of H II regions during star formation and chemical enrichment in globular clusters Smith, G. H. Abstract The shapes of the H II Stroemgren 'spheres' produced by stars located within massive gas clouds are determined by solving the equation of ionization equilibrium. The cloud is taken to be spherical, with an r exp -2 internal density distribution. The minimum distance that a star can be situated from the cloud center and still ionize gas out to the surface has been calculated, as well as the solid angle subtended by such H II regions at the cloud center. It is concluded that bright OB stars can form quite deep within the interiors of open cluster gas clouds and still ionize a large fraction of them. Globular cluster protoclouds are not susceptible to this form of disruption provided that they were more massive than their stellar progeny. The requirement for a greater initial gas mass is particularly stringent if the first generation of stars were supermassive (approximately 500 solar masses). Publication: The Astrophysical Journal Pub Date: August 1982 DOI: 10.1086/160196 Bibcode: 1982ApJ...259..607S Keywords: Gas Ionization; Globular Clusters; H Ii Regions; Interstellar Chemistry; Protostars; Star Formation; Astronomical Models; B Stars; Density Distribution; O Stars; Open Clusters; Supermassive Stars; Supernovae; Astrophysics full text sources ADS |