A CANDIDATE FOR THE MOST LUMINOUS OB ASSOCIATION IN THE GALAXY

We investigate the morphology and kinematics of the Galactic spiral structure based on a new sample of O- and early B-type stars. We select 6,858 highly confident OB star candidates from the combined data of the VST Photometric H$\alpha$ Survey Data Release 2 (VPHAS+ DR2) and the Gaia Data Release 2 (Gaia DR2). Together with the O-B2 stars from the literature, we build a sample consisting of 14,880 O- and early B-type stars, all with Gaia parallax uncertainties smaller than 20 per cent. The new sample, hitherto the largest one of O- and early B-type stars with robust distance and proper motion estimates, covers the Galactic plane of distances up to $\sim$ 6 kpc from the Sun. The sample allows us to examine the morphology of the Scutum, Sagittarius, Local and Perseus Arms in great detail. The spiral structure of the Milky Way as traced by O- and early B-type stars shows flocculent patterns. Accurate structure parameters, as well as the means and dispersions of the vertical velocity distributions of the individual Spiral Arms are presented.

We present a comprehensive study of X-ray emission by, and wind properties of, massive magnetic early B-type stars. Dedicated XMM–Newton observations were obtained for three early-type B-type stars, ξ1 CMa, V2052 Oph and ζ Cas, with recently discovered magnetic fields. We report the first detection of X-ray emission from V2052 Oph and ζ Cas. The latter is one the softest X-ray sources among the early-type stars, while the former is one of the X-ray faintest. The observations show that the X-ray spectra of our programme stars are quite soft with the bulk of X-ray emitting material having a temperature of about 1 MK. We compile the complete sample of early B-type stars with detected magnetic fields to date and existing X-ray measurements, in order to study whether the X-ray emission can be used as a general proxy for stellar magnetism. We find that the X-ray properties of early massive B-type magnetic stars are diverse, and that hard and strong X-ray emission does not necessarily correlate with the presence of a magnetic field, corroborating similar conclusions reached earlier for the classical chemically peculiar magnetic Bp–Ap stars. We analyse the ultraviolet (UV) spectra of five non-supergiant B stars with magnetic fields (τ Sco, β Cep, ξ1 CMa, V2052 Oph and ζ Cas) by means of non-local thermodynamic equilibrium (non-LTE) iron-blanketed model atmospheres. The latter are calculated with the Potsdam Wolf–Rayet (PoWR) code, which treats the photosphere as well as the wind, and also accounts for X-rays. With the exception of τ Sco, this is the first analysis of these stars by means of stellar wind models. Our models accurately fit the stellar photospheric spectra in the optical and the UV. The parameters of X-ray emission, temperature and flux are included in the model in accordance with observations. We confirm the earlier findings that the filling factors of X-ray emitting material are very high. Our analysis reveals that the magnetic early-type B stars studied here have weak winds with velocities not significantly exceeding vesc. The mass-loss rates inferred from the analysis of UV lines are significantly lower than predicted by hydrodynamically consistent models. We find that, although the X-rays strongly affect the ionization structure of the wind, this effect is not sufficient in reducing the total radiative acceleration. When the X-rays are accounted for at the intensity and temperatures observed, there is still sufficient radiative acceleration to drive a stronger mass-loss than we empirically infer from the UV spectral lines.

We have used a Galactic sample of OB stars and associations to test the performance of an automatic grouping algorithm designed to identify extragalactic OB associations. The algorithm identifies the known Galactic OB associations correctly when the search radius (78 pc) is defined by the observed stellar surface density. Galactic OB associations identified with a 78 pc search radius have diameters that are $\sim$3 times larger than OB associations identified with a 22 pc search radius in M33. Applying the smaller search radius to the Galactic data matches both the sizes and the number of member stars between the two galaxies quite well. Thus, we argue that this and similar algorithms should be used with a constant physical search radius, rather than one which varies with the stellar surface density. Such an approach would allow the identification of differences in the giant molecular cloud populations and star formation efficiency under most circumstances.

Using the results of the most recent stellar atmosphere models applied to a sample of hot stars, we construct calibrations of effective temperature (T(sub eff)), and gravity (log(sub g)) with a spectral type and luminosity class for Galactic 0-type and early B-type stars. From the model results we also derive an empirical relation between the bolometric correction and T(sub eff) and log g. Using a sample of stars with known distances located in OB associations in the Galaxy and the Large Magellanic Cloud, we derive a new calibration of M(sub v) with spectral class. With these new calibrations and the stellar atmosphere models of Kurucz, we calculate the physical parameters and ionizing photon luminosities in the H(0) and He(0) continua for O and early B-type stars. We find substantial differences between our values of the Lyman- continuum luminosity and those reported in the literature. We also discuss the systematic discrepancy between O-type stellar masses derived from spectroscopic models and those derived from evolutionary tracks. Most likely, the cause of this 'mass discrepancy' lies primarily in the atmospheric models, which are plane parallel and hydrostatic and therefore do not account for an extended atmosphere and the velocity fields in a stellar wind. Finally, we present a new computation of the Lyman-continuum luminosity from 429 known O stars located within 2.5 kpc of the Sun. We find the total ionizing luminosity from this population ((Q(sub 0)(sup T(sub ot))) = 7.0 x 10(exp 51) photons/s) to be 47% larger than that determined using the Lyman continuum values tabulated by Panagia.

Massive stars are of interest as progenitors of supernovae, i.e. neutron stars and black holes, which can be sources of gravitational waves. Recent population synthesis models can predict neutron star and gravitational wave observations but deal with a fixed supernova rate or an assumed initial mass function for the population of massive stars. Here we investigate those massive stars, which are supernova progenitors, i.e. with O‐ and early B‐type stars, and also all supergiants within 3 kpc. We restrict our sample to those massive stars detected both in 2MASS and observed by Hipparcos, i.e. only those stars with parallax and precise photometry. To determine the luminosities we calculated the extinctions from published multi‐colour photometry, spectral types, luminosity class, all corrected for multiplicity and recently revised Hipparcos distances. We use luminosities and temperatures to estimate the masses and ages of these stars using different models from different authors. Having estimated the luminosities of all our stars within 3 kpc, in particular for all O‐ and early B‐type stars, we have determined the median and mean luminosities for all spectral types for luminosity classes I, III, and V. Our luminosity values for supergiants deviate from earlier results: Previous work generally overestimates distances and luminosities compared to our data, this is likely due to Hipparcos parallaxes (generally more accurate and larger than previous ground‐based data) and the fact that many massive stars have recently been resolved into multiples of lower masses and luminosities. From luminosities and effective temperatures we derived masses and ages using mass tracks and isochrones from different authors. From masses and ages we estimated lifetimes and derived a lower limit for the supernova rate of ≈20 events/Myr averaged over the next 10 Myr within 600 pc from the sun. These data are then used to search for areas in the sky with higher likelihood for a supernova or gravitational wave event (like OB associations) (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The R136 cluster in 30 Doradus is the prototype "super star cluster," and the only example sufficiently close that its massive star content can be studied directly. We have used the Hubble Space Telescope to obtain spectra of 65 of the bluest, most luminous stars in R136 and find that the majority of these stars are of type O3, the hottest, most luminous, and most massive stars known. The total number of O3 stars in this one cluster exceeds the total number known elsewhere in the Milky Way or Magellanic Clouds. The highest luminosity stars found are O3 If*, O4 If+, O3 If/WN6-A, and H-rich WN stars, with masses in excess of 120 ☉, the highest masses for which appropriate evolutionary tracks are currently available. In accord with de Koter, Heap, & Hubeny, we conclude that these WN stars must be core H-burning stars whose spectra are WR-like because of high luminosity, and we find that their individual luminosities are a factor of 10 higher than is normal for WN stars of similar type but are like those found in the Galactic cluster NGC 3603, which they also resemble spectroscopically. Our spectroscopy does include stars as late as B0 V and samples most stars in the core of the R136 cluster with masses >50 M☉. The spectroscopy has been combined with HST photometry to study the star formation history and initial mass function of the R136 cluster. The young age (<1-2 Myr) for the highest mass stars, combined with what was previously known for the intermediate-mass populations, suggests that the lower mass stars began forming 4-5 Myr ago and continued forming until the high mass stars formed, consistent with the paradigm in which the formation of massive stars shuts down further star formation in the molecular cloud. Despite the unique preponderance of the highest mass and luminosity stars ever seen, the initial mass function (IMF) is found to be completely normal, with a slope Γ = -1.3 to -1.4. The number of high-mass stars is in good accord with that predicted by the IMF of the intermediate-mass stars, suggesting that a Salpeter-like IMF holds over the mass range 2.8-120 ☉ within the R136 cluster. The fact that the IMF slope in R136 is indistinguishable from those of Galactic and Magellanic Cloud OB associations suggests that star formation produces the same distribution of masses over a range of ~200 times in stellar density, from that of sparse OB associations to that typical of globular clusters. The large number of O3 stars in R136 is then simply a consequence of its youth (<1-2 Myr) and its richness, suggesting that the upper mass "cutoff" to the IMF seen in OB associations may simply be the result of their sparcity.

In a previous paper on the Magellanic Clouds, we demonstrated that coeval clusters provide a powerful tool for probing the progenitor masses of Wolf-Rayet (W-R) stars and luminous blue variables (LBVs). Here we extend this work to the higher metallicity regions of the Milky Way, studying 12 Galactic clusters. We present new spectral types for the unevolved stars and use these, plus data from the literature, to construct H-R diagrams. We find that all but two of the clusters are highly coeval, with the highest mass stars having formed over a period of less than 1 Myr. The turnoff masses show that at Milky Way metallicities some W-R stars (of early WN type) come from stars with masses as low as 20–25 M⊙. Other early-type WN stars appears to have evolved from high masses, suggesting that a large range of masses evolve through an early WN stage. On the other hand, WN7 stars are found only in clusters with very high turnoff masses, over 120 M⊙. Similarly, the LBVs are only found in clusters with the highest turnoff masses, as we found in the Magellanic Clouds, providing very strong evidence that LBVs are a normal stage in the evolution of the most massive stars. Although clusters containing WN7 stars and LBVs can be as young as 1 Myr, we argue that these objects are evolved, and that the young age simply reflects the very high masses that characterize the progenitors of such stars. In particular, we show that the LBV η Car appears to be coeval with the rest of the Trumpler 14/16 complex. Although the WC stars in the Magellanic Clouds were found in clusters with turnoff masses as low as 45 M⊙, the three Galactic WC stars in our sample are all found in clusters with high turnoff masses (>70 M⊙); whether this difference is significant or due to small number statistics remains to be seen. The bolometric corrections of Galactic W-R stars are hard to establish using the cluster turnoff method but are consistent with the "standard model" of Hillier.

Radio continuum observations of 88 O-type and early B-type stars are examined. Results are presented for a subset of these stars which forms a distance-limited sample of all OB stars within 2.5 kpc of the sun. Mass loss rates are derived for HD 15570, HD 166734, HD 151804, HD 152408, Alpha Cam, HD 169454, and Zeta Sco. All of these very luminous OB stars are found to be losing mass at a rate on the order of 10 to the -5 solar masses/yr. Multifrequency observations confirm the free-free interpretation for Zeta Pup and provide evidence for variability in the free-free sources P Cygni and Cyg OB2 No. 12.

Fil: Corti, Mariela Alejandra. Universidad Nacional de La Plata. Facultad de Ciencias Astronomicas y Geofisicas; Argentina. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - La Plata. Instituto de Astrofisica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronomicas y Geofisicas. Instituto de Astrofisica La Plata; Argentina

Early B-type stars with initial masses between 8 and 15 M⊙ are frequently found in multiple systems, as is evidenced by multi-epoch spectroscopic campaigns in the Milky Way and the Large Magellanic Cloud (LMC). Previous studies have shown no strong metallicity dependence in the close-binary (a &lt; 10 au) fraction or orbital-period distributions between the Milky Way’s solar metallicity (Z⊙) and that of the LMC (Z = 0.5 Z⊙). However, similar analyses for a large sample of massive stars in more metal-poor environments are still scarce. We focus on 309 early B-type stars (luminosity classes III-V) from the Binarity at LOw Metallicity (BLOeM) campaign, which targeted nearly 1000 massive stars in the Small Magellanic Cloud (SMC, Z = 0.2 Z⊙) using VLT/FLAMES multi-epoch spectroscopy. By applying binary detection criteria consistent with previous works on Galactic and LMC samples, we identify 153 stars (91 SB1, 59 SB2, 3 SB3) exhibiting significant radial-velocity (RV) variations, resulting in an observed multiplicity fraction of fmultobs = 50 ± 3%. Using Monte Carlo simulations to account for observational biases, we infer an intrinsic close-binary fraction of fmult = 80 ± 8%. This fraction reduces to ∼55% when increasing our RV threshold from 20 to 80 km s−1; however, an independent Markov chain Monte Carlo analysis of the peak-to-peak RV distribution (ΔRVmax) confirms a high multiplicity fraction of fmult = 79 ± 5%. These findings suggest a possible anti-correlation between metallicity and the fraction of close B-type binaries, with the SMC multiplicity fraction significantly exceeding previous measurements in the LMC and the Galaxy. The enhanced fraction of close binaries at SMC’s low metallicity may have broad implications for massive-star evolution in the early Universe. More frequent mass transfer and envelope stripping could boost the production of exotic transients, stripped supernovae, gravitational-wave progenitors, and sustained UV ionising flux, potentially affecting cosmic reionisation. Theoretical predictions of binary evolution under metal-poor conditions will provide a key test of our results.

Qualitative and quantitative assessment of microstructure in Al-B4C metal matrix composite processed by modified stir casting technique

In this study, we present a detailed abundance analysis of 12 OB stars in the Orion Nebula (M42), a star-forming region. High- and medium-resolution spectra of these target stars were obtained from the Ankara University Kreiken Observatory, TÜBİTAK National Observatory, and the archives of the European Southern Observatory. The atmospheric parameters of each star were estimated from the hydrogen Balmer, ionized silicon, and/or helium lines. Non-local thermodynamic equilibrium model atmosphere calculations were employed to derive precise elemental abundances of helium, carbon, nitrogen, oxygen, neon, magnesium, aluminum, silicon, sulphur, and iron from the stellar spectra of each target. The analysis revealed that early B-type stars within the M42 star-forming region are chemically homogeneous. The identification of chlorine spectral lines in HD 37356 further motivated a broader investigation into chlorine abundances in early B-type stars beyond the M42 region–specifically in $\epsilon$ CMa, $\gamma$ Peg, and HD 135485, all of which were found to exhibit chlorine overabundance relative to the solar value. Additionally, the masses and ages of all target stars were estimated using evolutionary tracks and isochrones appropriate to the physical characteristics of each star.

Simple though admittedly speculative considerations explain why most of our galaxy’s stellar nurseries are highly fragile but a few survive for a remarkably long time.

Massive O-type and early B-type (OB) stars in the nearby Galaxy remain incompletely cataloged due to high extinction, bright visible and infrared nebular emission in H ii regions, and high field star contamination. These difficulties are alleviated by restricting the search to stars with X-ray emission. Using the X-ray point sources from the Massive Young Star-forming Complex Study in Infrared and X-Rays (MYStIX) survey of OB-dominated regions, we identify 98 MYStIX candidate OB (MOBc) stars by fitting their 1–8 μm spectral energy distributions (SEDs) with reddened stellar atmosphere models. We identify 27 additional MOBc stars based on JHK S photometry of X-ray stars lacking SED fitting. These candidate OB stars indicate that the current census of stars earlier than B1, taken across the 18 MYStIX regions studied, is less than 50% complete. We also fit the SEDs of 239 previously published OB stars to measure interstellar extinction and bolometric luminosities, revealing six candidate massive binary systems and five candidate O-type (super)giants. As expected, candidate OB stars have systematically higher extinction than previously published OB stars. Notable results for individual regions include identification of the OB population of a recently discovered massive cluster in NGC 6357, an older OB association in the M17 complex, and new massive luminous O stars near the Trifid Nebula. In several relatively poorly studied regions (RCW 38, NGC 6334, NGC 6357, Trifid, and NGC 3576), the OB populations may increase by factors of .

The B0.2 V magnetic star τ Sco stands out from the larger population of massive magnetic OB stars due to its remarkable, superionized wind, apparently related to its peculiar magnetic field – a field which is far more complex than the mostly-dipolar fields usually observed in magnetic OB stars. τ Sco is therefore a puzzling outlier in the larger picture of stellar magnetism – a star that still defies interpretation in terms of a physically coherent model.Recently, two early B-type stars were discovered as τ Sco analogues, identified by the striking similarity of their UV spectra to that of τ Sco, which was – until now – unique among OB stars. We present the recent detection of their magnetic fields by the MiMeS collaboration, reinforcing the connection between the presence of a magnetic field and a superionized wind. We will also present ongoing observational efforts undertaken to establish the precise magnetic topology, in order to provide additional constrains for existing models attempting to reproduce the unique wind structure of τ Sco-like stars.