Stars In OB Associations Research Articles

Outward migration in Nascent stellar groups

ABSTRACT As a stellar group forms within its parent molecular cloud, new members first appear in the deep interior. These overcrowded stars continually diffuse outward to the cloud boundary, and even beyond. Observations have so far documented only the interior drift. Those stars that actually leave the cloud form an expanding envelope that I call the ‘stellar mantle.’ Simple fluid models for the cloud and mantle illustrate their basic structure. The mantle’s expansion speed is subsonic with respect to the cloud’s dynamical temperature. I describe, in qualitative terms, how the expanding mantle and Galactic tidal radius might together shape the evolution of specific types of stellar groups. The massive stars in OB associations form in clouds that contract before extruding a substantial mantle. In contrast, the more slowly evolving clouds forming open clusters and T associations have extended mantles that encounter a shrinking tidal radius. These clouds are dispersed by internal stellar outflows. If the remaining group of stars is gravitationally bound, it appears as a long-lived open cluster, truncated by the tidal radius. If the group is unbound, it is a late-stage T association that will soon be torn apart by the tidal force. The ‘distributed’ populations of pre-main sequence stars observed in the outskirts of several star-forming regions are too distant to be stellar mantles. Rather, they could be the remnants of especially low-mass T associations.

Hα kinematics of superbubbles and supernova remnants of the dwarf galaxy NGC 4214

ABSTRACT We analysed the ionized gas kinematics of the dwarf galaxy NGC 4214 using high resolution Fabry–Perot interferometry observations and present a set of narrowband images in the Hα, [S ii] λ6717 Å, [N ii] λ6584 Å, and [O iii] λ5007 Å emission lines. The high-resolution Fabry–Perot observations of the Hα emission line, allowed us to derive the velocity field, the velocity dispersion σ, and the rotation curve of the galaxy. We also present for the first time, 3D kinematic maps of the complexes NGC 4214-I and NGC 4214-II and analysed the kinematics of the ionized gas of two new superbubbles, as well as the supernova remnants previously detected in this galaxy by other authors, in radio, optical, and X-ray emission. We computed the expansion velocities of the superbubbles and supernova remnants fitting their velocity profiles and obtained their respective physical parameters. We found that the superbubbles have an expansion velocity of ∼50 km s −1, dynamical age about ∼2 Myr and wind luminosity LW of ∼9 × 1038 erg s−1 produced probably by massive stars in OB associations. For supernova remnants, their expansion velocities are between ∼48 and ∼80 km s−1 with ages of about 104 yr and kinetic energy of about 1051 erg assuming they are in the radiative phase of evolution.

The structure and 3D kinematics of vela OB2

ABSTRACT The kinematics of stars in OB associations can provide insights into their formation, dynamical evolution, and eventual fate. The low-mass stellar content of OB associations are sufficiently numerous as to provide a detailed sampling of their kinematic properties, however spectroscopy is required to confirm the youth of individual stars and to get 3D kinematics. In this paper, we present and analyse results from a large spectroscopic survey of Vela OB2 conducted using 2dF/HERMES on the AAT. This spectroscopy is used to confirm the youth of candidate young stars and determine radial velocities, which are combined with proper motions and parallaxes from Gaia to measure 3D positions and velocities. We identify multiple separate kinematic groups in the region, for which we measure velocity dispersions and infer their virial states. We measure expansion rates for all these groups and find strong evidence for anisotropic expansion in the Vela OB2 association of at least 11σ significance in all three dimensions, as well as some evidence for expansion in the γ Vel and P Puppis clusters. We trace back the motions of these groups into the past and find that the open cluster NGC 2547 is an interloper in the Vela OB2 region and actually formed >100 pc away from the association. We conclude that Vela OB2 must have formed with considerable spatial and kinematic substructure over a time-scale of ∼10 Myr, with clear temporal substructure within the association, but no clear evidence for an age gradient.

Distance scale for high-luminosity stars in OB associations and in field with Gaia DR2. Spurious systematic motions

We calculated the median parallaxes for 47 OB associations including at least 10 stars with known Gaia DR2 parallaxes. A comparison between trigonometric and photometric parallaxes of OB associations reveals a zero-point offset of delta_pi=-0.11 +\- 0.04 mas indicating that Gaia DR2 parallaxes are, on average, underestimated and the distances derived from them are overestimated. The correction of delta_pi=-0.11 mas is consistent with the estimate that Arenou et al. (2018) obtained for bright stars. An analysis of parallaxes of OB associations and high-luminosity field stars confirms our previous conclusion (Dambis et al. 2001) that the distance scale for OB stars established by Blaha and Humphreys (1989) must be reduced by 10--20%. Spurious systematic motions of 10--20 km s-1 at the distances of 2--3 kpc from the Sun are found to arise from the use of the uncorrected Gaia DR2 parallaxes.

Fundamental parameters of B type stars

AbstractFundamental parameters of 26 well-studied sharp-lined single early B-type stars in OB associations and in the field within a distance of ≤400 pc from the Sun are compared to high-precision data from detached eclipsing binaries (DEBs). Fundamental parameters are derived from accurate and precise atmospheric parameters determined earlier by us from non-LTE analyses of high-quality spectra, utilising the new Geneva stellar evolution models in the mass-range ~6 to 18 M⊙ at metallicity Z = 0.014. Evolutionary masses, radii and luminosities are determined to better than typically 5%, 10%, and 20% uncertainty, respectively, facilitating the mass-radius and mass-luminosity relationships to be recovered for single core hydrogen-burning objects with a similar precision as derived from DEBs. Good agreement between evolutionary and spectroscopic masses is found. Absolute visual and bolometric magnitudes are derived to typically ~0.15-0.20 mag uncertainty. Metallicities are constrained to better than 15-20% uncertainty and tight constraints on evolutionary ages of the stars are provided. The spectroscopic distances and ages of individual sample stars agree with independently derived values for the host OB associations. The accuracy and precision achieved in the determination of fundamental stellar parameters from the quantitative spectroscopy of single early B-type stars comes close (within a factor 2-4) to data derived from DEBs.

Variability Survey of Massive Stars in OB-Associations: Preliminary Results on the Cygnus Region

Abstract We present V - and I-passband photometry of massive stars in the Cyg OB1 and Cyg OB2 associations, based on about 80 observing nights spanning 300 days in the 2011 season. The variability of 22 supergiants and 48 OB-stars with luminosity classes III-V is analyzed. We report two new variable OB stars and 15 variable supergiants of which four are new discoveries. The light variations of Schulte 12 are interpreted as microvariability. We also present light curves of the red supergiants BC Cyg and BI Cyg which exhibit brightness drop of more than 0.4 mag during the season.

Present-day cosmic abundances

Aims. A sample of early B-type stars in OB associations and the field within the solar neighbourhood is studied comprehensively. Present-day abundances for the astrophysically most interesting chemical elements are derived. Methods. High-resolution and high-S/N spectra of early B-type stars are analysed in NLTE. Atmospheric parameters are derived from the simultaneous establishment of independent indicators, from multiple ionization equilibria and the hydrogen Balmer lines. Results. Teff is constrained to 1-2% and logg to less than 15% uncertainty. Absolute values for metal abundances are determined to better than 25% uncertainty. The synthetic spectra match the observations reliably over almost the entire visual spectral range. Conclusions. A present-day cosmic abundance standard is established. Our results i) resolve the discrepancy between a chemical homogeneous local gas-phase ISM and a chemically inhomogeneous young stellar component, ii) facilitate the amount of heavy elements locked up in the interstellar dust to be constrained precisely: carbonaceous dust is largely destroyed inside the Orion HII region, unlike the silicates, and that graphite is only a minority species in interstellar dust -, iii) show that the mixing of CNO-burning products in the course of massive star evolution follows tightly the predicted nuclear path, iv) provide reliable present-day reference points for anchoring Galactic chemical evolution models to observation, and v) imply that the Sun has migrated outwards from the inner Galactic disk over its lifetime from a birthplace at a distance around 5-6 kpc from the Galactic Centre; a cancellation of the effects of Galactic chemical evolution and abundance gradients leads to the similarity of solar and present-day cosmic abundances in the solar neighbourhood, with a telltaling signature of the Sun's origin left in the C/O ratio. (ABRIDGED)

The interstellar Ca II distance scale

Aims. We attempt to extend the relation between the strengths of the interstellar Ca ii lines and the distances to early-type stars to objects beyond 1 kiloparsec, with the line saturation taken into account. Methods. We measure the Ca ii Ka nd Caii H equivalent widths, and compute Ca ii column densities for 262 lines of sight towards early-type stars with available Hipparcos parallaxes (π). The targets are located within a few hundred parsecs of the Galactic plane, and span all the range of Galactic longitudes. We fit the NCaii – parallax relation with a function of the form π = 1/(a · NCaii + b), using a maximum-likelihood approach to take account of errors in both variables. We use the resultant formula to estimate distances to stars in OB associations and clusters, and compare them to those found in the literature, usually estimated by spectrophotometric methods. Results. For lines of sight with EW(K)/EW(H) > 1.3, we obtain the following approximate formula for the distance: DCaii = 77 + (2.78 + 2.60 EW(K) EW(H) −0.932 )EW(H), where the equivalent widths EW(K )a ndEW(H) are in mA, and the distance D Caii in parsecs. The errors in DCaii, resulting from the uncertainty in the fit parameters and errors in the equivalent widths, are typically about 15% of the distance. We can also expect the equation not to hold for objects situated farther than a few hundred parsecs from the Galactic plane. We find several cases of significant column density differences between association or cluster members, especially notable in the Trumpler 16 cluster, indicating either a local contribution to the Ca ii column density, or background/foreground stars being confused with members. The ratio DCaii/Dassoc appears to depend on the Galactic longitude, being highest in the range 70 ◦ < l < 120 ◦ and lowest for 200 ◦ < l < 300 ◦ .T his effect may be due to large-scale structure being present in the Ca ii layer, or to the nonmember

Triggered Star Formation by Massive Stars

We present our diagnosis of the role that massive stars play in the formation of low- and intermediate-mass stars in OB associations (the Lambda Ori region, Ori OB1, and Lac OB1 associations). We find that the classical T Tauri stars and Herbig Ae/Be stars tend to line up between luminous O stars and bright-rimmed or comet-shaped clouds; the closer to a cloud the progressively younger they are. Our positional and chronological study lends support to the validity of the radiation-driven implosion mechanism, where the Lyman continuum photons from a luminous O star create expanding ionization fronts to evaporate and compress nearby clouds into bright-rimmed or comet-shaped clouds. Implosive pressure then causes dense clumps to collapse, prompting the formation of low-mass stars on the cloud surface (i.e., the bright rim) and intermediate-mass stars somewhat deeper in the cloud. These stars are a signpost of current star formation; no young stars are seen leading the ionization fronts further into the cloud. Young stars in bright-rimmed or comet-shaped clouds are likely to have been formed by triggering, which would result in an age spread of several megayears between the member stars or star groups formed in the sequence.

Origins of field O-type stars

About 20% of the high mass O-type stars in the Galaxy are found outside stellar clusters and associations. In the solar neighbourhood this fraction amounts to 43 O-type stars. In the framework of high-mass star formation, we search for the origins of these stars. We aim at separating the O-type field stars from runaway O-type stars, where the former does not betray any indication of past dynamical interactions resulting in their present field location. We specifically search for the presence of stellar clusters, but also evaluate spatial velocities, distance from Galactic plane, locations in the Galaxy with respect to young stellar clusters. Among our sample of 43, we find that 5 stars are in fact observed in the K-band to be members of previously unnoticed small clusters. On the other hand it is plausible that nearly half the sample (22 objects) could have undergone a dynamical ejection from a young cluster. Based on the current available data, of the Galactic O-type stars cannot be associated with a stellar cluster. Finally by assuming that stars form in clusters with a power law in membership number with index −1.7 down to “clusters” consisting of single stars, we calculate the expected statistics regarding isolated O stars and O stars in OB-associations. We conclude that the results of the calculations are consistent with the observed statistics of O-type stars.

Formation of Glass with Embedded Metal and Sulfides from Shock‐accelerated Crystalline Dust in Superbubbles

Interplanetary dust particles (IDPs) contain enigmatic submicron components called GEMS (glass with embedded metal and sulfides). The compositions and structures of GEMS indicate that they have been processed by exposure to ionizing radiation, but details of the actual irradiation environment(s) have remained elusive. Here we propose a mechanism and astrophysical site for GEMS formation that explains for the first time the following key properties of GEMS: they are stoichiometrically enriched in oxygen and systematically depleted in S, Mg, Ca, and Fe (relative to solar abundances); most have normal (solar) oxygen isotopic compositions; they exhibit a strikingly narrow size distribution (0.1-0.5 μm diameter); and some of them contain crystals within their silicate glass matrices. We show that the compositions, size distribution, and survival of relict crystals are inconsistent with amorphization by particles accelerated by diffusive shock acceleration. Instead, we propose that GEMS are formed from crystalline grains that condense in stellar outflows from massive stars in OB associations, are accelerated in encounters with frequent supernova shocks inside the associated superbubble (SB), and are implanted with atoms from the hot gas in the SB interior. We thus reverse the usual roles of target and projectile. Rather than being bombarded at rest by energetic ions, grains are accelerated and bombarded by a nearly monovelocity beam of atoms as viewed in their rest frame. Meyer, Drury, and Ellison have proposed that Galactic cosmic rays (GCRs) originate from ions sputtered from such accelerated dust grains. We suggest that GEMS are surviving members of a population of fast grains that constitute the long-sought source material for GCRs. Thus, representatives of the GCR source material may have been awaiting discovery in cosmic dust labs for the last 30 yr.

A CS J = 5 → 4 Mapping Survey Toward High‐Mass Star‐forming Cores Associated with Water Masers

We have mapped 63 regions forming high-mass stars in CS J = 5 → 4 using the CSO. The CS peak position was observed in C34S J = 5 → 4 toward 57 cores and in 13CS J = 5 → 4 toward the nine brightest cores. The sample is a subset of a sample originally selected toward water masers; the selection on maser sources should favor sources in an early stage of evolution. The cores are located in the first and second Galactic quadrants with an average distance of 5.3 ± 3.7 kpc and were well detected with a median peak signal-to-noise ratio in the integrated intensity of 40. The integrated intensity of CS J = 5 → 4 correlates very well with the dust continuum emission at 350 μm. For 57 sufficiently isolated cores, a well-defined angular size (FWHM) was determined. The core radius (RCS), aspect ratio [(a/b)obs], virial mass (Mvir), surface density (Σ), and the luminosity in the CS J = 5 → 4 line (L(CS54)) are calculated. The distributions of size, virial mass, surface density, and luminosity are all peaked with a few cores skewed toward much larger values than the mean. The median values, μ1/2, are as follows: μ1/2 (RCS) = 0.32 pc, μ1/2 ((a/b)obs) = 1.20, μ1/2 (Mvir) = 920 M⊙, μ1/2 (Σ) = 0.60 g cm-2, μ1/2 (L(CS54)) = 1.9 × 10-2 L⊙, and μ1/2 (Lbol/Mvir) = 165 (L/M)⊙. We find a weak correlation between C34S line width and size, consistent with Δv ∼ R0.3. The line widths are much higher than would be predicted by the usual relations between line width and size determined from regions of lower mass. These regions are very turbulent. The derived virial mass agrees within a factor of 2-3 with mass estimates from dust emission at 350 μm after corrections for the density structure are accounted for. The resulting cumulative mass spectrum of cores above 1000 M⊙ can be approximated by a power law with a slope of about -0.9, steeper than that of clouds measured with tracers of lower density gas and close to that for the total masses of stars in OB associations. The median turbulent pressures are comparable to those in UCH II regions, and the pressures at small radii are similar to those in hypercompact H II regions (P/k ∼ 1010 K cm-3). The filling factors for dense gas are substantial, and the median abundance of CS is about 10-9. The ratio of bolometric luminosity to virial mass is much higher than the value found for molecular clouds as a whole, and the correlation of luminosity with mass is tighter.

The line-of-sight velocities of OB associations and molecular clouds in a wide solar neighborhood: The streaming motions of stars and gas in the Perseus arm

We compare the line-of-sight velocities of stars in OB associations within 3 kpc of the Sun and the molecular clouds closest to them. The Perseus arm is used as an example to show that the line-of-sight velocity ranges in which OB-association stars, molecular clouds, and H II regions ionized by these association stars are observed overlap. The streaming motions produced by density waves in the (l, V LSR) plane were found to lead to an inversion of the distances to objects of the Perseus arm and the interarm space beyond this arm.

Ultraviolet Imaging Telescope Observations of the Magellanic Clouds

We present an analysis of wide-field, far-ultraviolet images of the LMC and SMC obtained by the Ultraviolet Imaging Telescope. The photometric catalog of over 37,000 stars allows us to make large-scale, statistical studies of massive star formation in OB associations and in the field population. Our results show that: (1) the most probable slope for the initial mass function (IMF) of field stars is Γ = −1.80, slightly steeper than the Salpeter slope; and (2) there doesn't seem to be a single, unique IMF slope for stars in OB associations, with a range of values from Γ = −1.0 to −2.0. We also analyze the stellar vs. diffuse UV flux, and the population of OB star candidates in the field.

What the galaxies of the Local Group tell us about massive star evolution

We consider what we've learned about massive star evolution from observations of the resolved stellar content of Local Group galaxies. Studies of mixed-age (galaxy-wide) and coeval (single associations) populations reveal much about massive star evolution, and how it is controlled by metallicity, demonstrating the ‘Conti scenario’ in action! The number of WC stars to WN stars increases with increasing metallicity, as expected: in regions of higher metallicity stars of somewhat lower luminosity can evolve all the way to the WC stage. The exception is the starburst galaxy IC 10, for which I speculate that the IMF may be weighted towards high mass stars. The highest luminosity red supergiants are lacking in galaxies of higher metallicity, suggesting that the stars that would have become these RSGs are spending more of their time as WRs. The presence of luminous RSGs is highly correlated with the presence of WC and WN stars in OB associations, suggesting that many massive stars evolve through both a RSG and WR stage. The relative number of RSGs and WRs does decrease strongly with increasing metallicity, again consistent with higher metallicity systems leading to increased time in the WR phase. The various WC subclasses appear to be the result of the influence of metallicity on stellar wind structure in these stars, and are not due to to differences in mass or luminosity. Data on the field population in the Magellanic Clouds suggest that stars more massive than 30 become WRs in the LMC, while the limit may be more like 50 in the SMC, again as expected. Studies of the turn-off masses in clusters and associations in the MCs and Milky Way are nearing completion, while investigations in the more distant galaxies of the Local Group are just getting underway. For the LMC we find the following: WNE stars come from a large mass range of progenitor (30–100 ), and have very large (negative) bolometric corrections (−6 to −8 mag). The Ofpe/WN9 stars seem to come from lower mass progenitor (20–30 ), and have more modest BCs (−1 to −3 mag). WC stars come from stars with masses &gt; 60–70 , and have BCs of −3 to −4 mag. Both ‘B2I+WN3’ systems and LBV stars like S Doradus are found only in clusters containing very high turn off masses (&gt;70–90 ).

Wolf-Rayet stars in OB associations

We present first results of our search for Wolf-Rayet members of OB associations, aiming in determination of distances and absolute magnitudes for WR stars.

Ultraviolet Extinction by Interstellar Dust in External Galaxies: M31

Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) spectra of stars in OB associations of M31 are used to derive the UV extinction by interstellar dust in M31 by three different methods: (1) comparing spectra of M31 star pairs, (2) comparing spectra of M31 stars to those of Galactic standard stars, and (3) comparing M31 star spectra to atmosphere models. The derived intrinsic M31 extinction curve has an overall wavelength dependence very similar to that of the average Galactic extinction curve but possibly has a weaker 2175 Å bump, however, with a significance of only 1 σ. This result is different from the LMC (30 Dor)-like curves published earlier, which contained both intrinsic M31 extinction and "foreground" extinction, and were based either on low-signal IUE spectra, or on FOS data affected by inaccuracy in the preliminary flux calibration, and were not computed with the pair method used in this work.

An HST Study of OB Associations and Star Clusters in M101

view Abstract Citations (36) References (54) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS An HST Study of OB Associations and Star Clusters in M101 Bresolin, Fabio ; Kennicutt, Robert C., Jr. ; Stetson, Peter B. Abstract The massive stellar content, the OB associations and the star clusters in an HST field in M101 = NGC 5457 are investigated. A clustering algorithm yields 79 putative associations. Their size distribution is similar to that found in the Magellanic Clouds, M31, and M33, with an average size around 90 pc. The V luminosity function for the stars contained within the associations has a slope dlogN/dV = 0.60+/-0.05, while an average reddening E(B - V) = 0.21 mag is measured. The stellar content is further discussed by means of color-magnitude and color-color diagrams. Ages are estimated using theoretical isochrones, and range between 3 and 14 Myr (+/-2 Myr). We find a suggestion that the upper mass limit of the IMF for stars in OB associations in M101 may be quite high, contrary to some theoretical expectations that the mass limit should be lower in a high metallicity environment. Forty-one star cluster candidates and two H II region core clusters are identified in the M101 field, and their integrated photometric properties are compared with the cluster system of the Large Magellanic Cloud (LMC) and M33. Most of the M101 clusters probably belong to the class of young, populous star clusters such as are found in the LMC. Red clusters are rare in this field. In the Appendix the objective finding algorithm is applied to the brightest stars in the LMC. Publication: The Astronomical Journal Pub Date: September 1996 DOI: 10.1086/118073 arXiv: arXiv:astro-ph/9606050 Bibcode: 1996AJ....112.1009B Keywords: STARS: FORMATION; GALAXIES: EVOLUTION; Astrophysics E-Print: 25 pages + 6tables Figures also available via anonymous ftp at ftp://pollux.as.arizona.edu/pub/fabio/ full text sources arXiv | ADS | data products SIMBAD (8) NED (6) MAST (1) ESA (1)

High-Velocity H I in the Spiral Galaxy NGC 5668

The nature and origin of the high-velocity clouds (HVCs) of neutral hydrogen in our Galaxy are not well understood, in part because of the uncertain distances to the clouds. As a result it is difficult to measure the mass of neutral hydrogen in the HVCs in the Galaxy, but further insight into the nature of HVCs can be obtained by finding and studying such gas in other galaxies. The Sc-galaxy NGC 5668 was found to have high-velocity wings beyond the double-horned 21 cm profile in observations performed with the Arecibo 305 m telescope; these wings are the expected signature of HVCs in a disk galaxy. In this paper, Very Large Array observations of NGC 5668 are presented that confirm the high- velocity H I wings detected with the Arecibo telescope. About 60% of the material in the high-velocity wings is distinct in position-velocity diagrams, located primarily outside the optical disk of the galaxy, and may be infalling material comparable to the Magellanic Stream in our own Galaxy. The total mass of this kinematically distinct neutral hydrogen is 4 x 10^8^ M_sun_. A natural source for the rest of the wing material is halo gas produced by a galactic fountain. Such a fountain would be driven by supernovae and energetic stellar winds from massive stars in OB associations that would also transfer kinetic energy to the neutral hydrogen disk. This would result in a larger H I velocity dispersion within the optical disk than outside of it, as is suggested by these observations.

A search for Lambda Bootis stars in OB associations

view Abstract Citations (61) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Search for Lambda Bootis Stars in OB Associations Gray, R. O. ; Corbally, C. J. Abstract A search for Lambda Bootis stars in two OB associations, Orion OB1 and Lacerta OB1, has been carried out. One Lambda Bootis star has been discovered in the Orion OB1 association. In addition, nine A-type stars which appear to be slightly metal weak have been found in the two associations. The Lambda Bootis star is situated on the Zero-Age Main Sequence (ZAMS). The discovery of these stars strengthens the hypothesis that the Lambda Bootis stars are related to a short-lived evolutionary phase associated with the arrival of the star on the ZAMS. Publication: The Astronomical Journal Pub Date: August 1993 DOI: 10.1086/116668 Bibcode: 1993AJ....106..632G Keywords: B Stars; Main Sequence Stars; O Stars; Stellar Evolution; Stellar Spectra; Charge Coupled Devices; Metallicity; Astrophysics; STARS: CHEMICALLY PECULIAR; STARS: ATMOSPHERES full text sources ADS | data products SIMBAD (68)