bluuebunny/crossref_metadata_2025 huggingface dataset:
Very Low Mass Stars and Brown Dwarf Candidates in Orion OB1a and OB1b
Authors: M. Walter Frederick , H. Sherry William , J. Wolk Scott
Published: July 2016

VRI images within the belt of Orion and the Ori OB1a association reveal a pre-main sequence locus extending to below our completeness limit of about V=21. We report here on followup JHK imaging and optical and near–IR spectroscopy of the faintest and reddest of the PMS candidates. We find that they are unreddened mid-to-late M “stars” which fall on a few million year isochrone. Masses are largely substellar, reaching as low as about 0.02 M⊙ (20 Jovian masses). The space density of the substellar objects is high.
Disks around Young Brown Dwarfs
Authors: C. Liu Michael
Published: July 2016

We present some results from a systematic survey for disks around spectroscopically identified young brown dwarfs and very low mass stars. We find that ≈75% of our sample show intrinsic IR excesses, indicative of circum(sub)stellar disks. The observed excesses are well-correlated with Hα emission, consistent with a common disk accretion origin. Because the excesses are modest, conventional analyses using only IR colors would have missed most of the sources with disks. In the same star-forming regions, we find that disks around brown dwarfs and T Tauri stars are contemporaneous; assuming coevality, this demonstrates that substellar disks are at least as long-lived as stellar disks. Altogether, the frequency and properties of circumstellar disks are similar from the stellar regime down to the substellar and planetary-mass regime. This offers compelling evidence of a common origin for most stars and brown dwarfs.
Accretion in Very Low Mass Young Objects
Authors: Muzerolle James , Hillenbrand Lynne , Briceño César , Calvet Nuria , Hartmann Lee
Published: July 2016

We have investigated evidence for active accretion in a sample of ~ 30 young, very low mass objects, including at least 10 brown dwarfs. About 30% of the sample exhibits broad, asymmetric Hα emission line profiles, indicative of gas accretion via magnetospheric infall. There is a distinct lack of associated optical continuum veiling in these accretors, suggesting very low mass accretion rates. Our models yield an upper limit to the accretion rates that is several orders of magnitude smaller than typical of higher-mass T Tauri stars, suggesting a dependence of accretion rates with stellar mass.
The Lower Mass Function of Young Open Clusters
Authors: Bouvier Jérôme , Moraux Estelle , R. Stauffer John , y Navascués David Barrado , Cuillandre Jean-Charles
Published: July 2016

We report new estimates for the lower mass function of 5 young open clusters spanning an age range from 80 to 150 Myr. In all studied clusters, the mass function across the stellar/substellar boundary (~ 0.072 M⊙) and up to 0.4 M⊙ is consistent with a power-law with an exponent α ⋍ –0.5 ± 0.1, i.e., dN/dM ∝ M-0.5.
A Search for Brown Dwarfs in the Alpha Persei Cluster
Authors: Lodieu Nicolas , McCaughrean Mark , Bouvier Jérôme , y Navascués David Barrado , R. Stauffer John
Published: July 2016

We present preliminary results from a deep near-infrared survey of a ~ 1 square degree area in the young open cluster Alpha Persei using the wide-field Omega-Prime camera on the Calar Alto 3.5m telescope, yielding a list of new low-mass cluster members, including brown dwarf candidates.
A Deep JI Survey of the Pleiades for Freely-Floating Superplanets and Brown Dwarfs at the Deuterium Burning Limit
Authors: Schwartz M. J. , Becklin E. E. , Zuckerman B.
Published: July 2016

We present preliminary results from a deep near-infrared J-band and I-band photometric survey of the Pleiades for freely-floating superplanets and brown dwarfs (BD) near the deuterium burning limit (DBL). With limiting magnitudes of J=20.5 and I=23.5, we have selected candidate Pleiads on the basis of evolutionary tracks, color-magnitudes, and I-J color lower limits (non-detections at deep I-band). Likelihoods of membership will be ultimately determined by a combination of image profile analysis, spectral energy distribution, proper motion, and low-resolution measurements of near-infrared water and/or methane absorption slopes. If confirmed, our faintest candidates are predicted to have made the transition from L to T spectral types with temperatures down to 820 K, and masses approaching 10 MJup.
Searching for Planets of Brown Dwarfs
Authors: Guenther Eike , Wuchterl Guenther
Published: July 2016

Up to now, most planet search projects have concentrated on G and K stars. In order to considerably widen the view, we have stated a survey for planets of old, nearby brown dwarfs and very low-mass stars. Using UVES, we have observed 26 brown dwarfs and very low-mass stars. As it turned out these objects are very inactive and thus highly suitable for such a project. For 19 objects, we can exclude a planet with the mass of 3 MJ, and a period of 100 days or less with a probability of more than 60%. For these objects, we can also exclude Pegasi-planets with a high probability. For another 4 objects, we can exclude at least a brown dwarf companion. One object is a double line spectroscopic binary, and one object shows significant radial-velocity variations that can not be caused by a normal stellar-spot. This object either has a planetary-mass companion, or the variations are caused by surface structures that are quite different from normal star-spots.
A Search for Brown Dwarfs around Young Solar-Analog Stars Using the Hōkūpa'a/Gemini Adaptive Optics System
Authors: E. Potter Daniel , L. Martín Eduardo , C. Cushing Michael
Published: July 2016

We present the results of a search for low-mass companions around a sample of young, solar-analog stars using the Hōkūpa'a adaptive optics instrument mounted on the Gemini North 8 m telescope. Out of 31 stars observed, one binary brown dwarf system was found as a companion to the star HD 130948 (HIP 72567), as confirmed by proper motion and near-IR spectra. Orbital motion between the two brown dwarfs was measured, but our 14 month time baseline is inadequate to accurately measure the system's dynamical mass. Upcoming spectroscopic observations of the brown dwarfs will measure their lithium absorption lines to provide a more accurate age estimate of the system. The eventual dynamical mass determination coupled with the age determination will provide a valuable check of brown dwarf evolutionary models.
There is a Brown Dwarf Desert of Companions Orbiting Stars between 75 and 1000 AU
Authors: McCarthy Chris , Zuckerman B. , Becklin E. E.
Published: July 2016

We present results of the first large (> 100 stars) infrared coronographic search for substellar companions to nearby stars. The search consisted of two surveys of stars chosen for their youth and proximity to Earth: 1.) a 178 star infrared survey at Steward and Lick Observatories, with optical followup from Keck Observatory, capable of detecting companions with masses greater than 30MJ, orbiting between about 75 and 300 AU, 2.) a 102 star survey using the Keck telescope, capable of detecting extrasolar brown dwarfs and planets typically more massive than 10 MJ, orbiting between about 75 and 300 AU.

This research resulted in the discovery of one brown dwarf companion, zero planets and 23 double stars. The frequency of brown dwarf companions (of any mass) to G, K and M stars orbiting between 75 and 300 AU is measured to be 1 ± 1 %. The frequency of massive (> 30 MJ) brown dwarf companions is found to be 0.6 ± 0.6 %. The frequency of giant planets with masses larger than 10 MJ, between 75 and 300 AU, is measured here for the first time to be no more than about 3 %.
Wide Brown Dwarf Companions to Main-Sequence Stars
Authors: Gizis John E.
Published: July 2016

Many widely separated companions to main-sequence stars have been found using 2MASS. These companions include both stars and brown dwarfs. I discuss a number of systems of particular interest. Present indications are that a few percent of G dwarfs have brown dwarf companions.
Stratospheric Observatory For Infrared Astronomy (SOFIA)
Authors: Becklin E. E. , Moon L. J.
Published: July 2016

The joint U.S. and German SOFIA project to develop and operate a 2.5-meter infrared airborne telescope in a Boeing 747-SP is now well into development. First science flights will begin in 2004. The observatory is expected to operate for over 20 years. The sensitivity, characteristics and science instrument complement are discussed. SOFIA will have a number of experiments related to Brown Dwarf research; some of these are discussed.
Looking for Very Low-Mass Pre-Main Sequence Objects with SDSS
Authors: McGehee Peregrine M. , Hawley Suzanne L. , Covey Kevin R.
Published: July 2016

Determining the process(es) by which brown dwarfs form is key to understanding their intrinsic nature. If their origins are within circumstellar disks they are akin to giant planets. If, on the other hand, they coalesce from molecular cloud cores, then they share a common lineage with low mass stars. These two mechanisms can be distinguished by investigation of young (< 10 Myr) substellar objects. If brown dwarfs are small failed stars, we expect to find very low mass analogs of the Classical T Tauris, with primordial magnetic fields from the molecular cloud collapse. Accretion onto these objects leads to characteristic magnetic activity signatures such as chromospheric and coronal emission resulting in an ultraviolet excess continuum.

The Orion OB1b association (m-M = 7.9, 2 Myr) provides a laboratory for following the strength and occurrence of the accretion process as a function of mass. Studies of the substellar mass function within the sigma Orionis cluster at the southern end of the association indicate that brown dwarfs are common. Based on model isochrones and the SDSS M dwarf sequence we expect the 95% completeness limit of the “Orion” scans to correspond to 0.1 and 0.03 solar masses for the u and g bands.
Research on the Bottom of the Main Sequence and Brown Dwarfs in the Early Days
Authors: Nakano Takenori
Published: July 2016

One of the earliest works on the evolution of very low mass stars, Hayashi & Nakano (1963), is introduced, which determined self-consistently the bottom of the main sequence at 0.08M⊙ and investigated the evolution of an object of 0.05M⊙ from the stages with negligible electron degeneracy to those with high degeneracy.
Parameters of massive OB stars
Authors: Herrero Artemio
Published: July 2016

I review our knowledge about effective temperatures and masses of massive OB stars paying special attention to results recently obtained using new atmospheric models including sphericity, mass-loss and line-blanketing. The new temperature scales for O dwarfs and supergiants are lower than previously accepted. No systematic mass discrepancy is found among O supergiants, although individual problematic cases still remain. It is shown that model atmospheres give answers consistent with binary systems. Good agreement is found between theoretical predictions and observations of the Wind Momentum-Luminosity relationship for Galactic O-type supergiants. Finally, I review the status of the WLR in M33.
Masses and other parameters of massive binaries
Authors: Gies Douglas R.
Published: July 2016

Binary stars provide us with the means to measure stellar mass. Here I present several lists of known O-type stars with reliable mass estimates that are members of eclipsing, double-lined spectroscopic binaries. The masses of young, unevolved stars in binaries are suitable for testing the predictions of evolutionary codes, and there is good agreement between the observed and predicted masses (based upon temperature and luminosity) if the lower temperature scale from line-blanketed model atmospheres is adopted. A final table lists masses for systems in a wide variety of advanced evolutionary stages.
Tidal forces and variability in close binary stars: the β-factor
Authors: Koenigsberger Gloria , Moreno Edmundo , Cervantes Fausto
Published: July 2016

We present the results of calculations that illustrate the effects, on orbital timescales, produced on the external layer of a star that is in a binary system in which the stellar rotation period is not synchronized with the orbital period. The calculations show how, for a fixed set of parameters, the amplitudes of the oscillations depend on stellar radius and on β, the ratio between the stellar rotation angular velocity and the orbital angular velocity.
3D-hydrodynamics of colliding winds in massive binaries
Authors: Walder Rolf , Folini Doris
Published: July 2016

The pinwheel nebulae observed in some WC-binaries essentially mirror a transport phenomenon. We show the importance of the central wind collision zone in setting the ‘initial conditions’ for this transport. In order to understand some of the newly observed features, we postulate that standard theory of colliding flows must be extended by considering radiative breaking, heat-conduction, and the clumped character of the winds. We suggest that clumped winds can be modeled by highly compressible turbulence and outline some consequences for the physics of the wind collision zone. With regard to dust production we argue that the system center is the only location where dust nucleation can happen.
Spectral analysis of WC stars in M 33 using CFHT-MOS
Authors: Abbott Jay B. , Crowther Paul A. , Drissen Laurent , Dessart Luc , Martin Pierre
Published: July 2016

We have analyzed eight single WC4-7 stars in M 33, spanning a variety of galactocentric distances, using optical CFHT-MOS spectroscopy. Provisional stellar parameters are determined using the code of Hillier & Miller (1998), which are compared here to Galactic and LMC counterparts.
A Flatter metallicity gradient in the Galactic disk: non-LTE abundance calculations of OB stars
Authors: Daflon Simone , Cunha Katia
Published: July 2016

We present non-LTE abundances of carbon, nitrogen, oxygen, magnesium, aluminum, silicon and sulfur, derived for a sample of 70 O9-B2 main sequence stars of the Galactic disk and analyze the distribution of the chemical abundances in terms of radial gradients within 4.4-12.9 kpc from the center of the Galaxy. The derived gradients are flatter than those presented by the most recent studies about the radial gradients of stellar abundances.
Mass-loss predictions and stellar masses of early-type stars
Authors: de Koter Alex , Vink Jorick S.
Published: July 2016

We show that the stellar masses implied by our predictions of the wind properties of massive stars are in agreement with masses derived from evolution theory and from direct measurements using spectroscopic binaries, contrary to previous attempts to derive masses from wind theory.
Interferometric and spectroscopic monitoring of emission lines: detection of CIRs in hot star winds
Authors: Dessart Luc , Chesneau Olivier
Published: July 2016

We present a theoretical study of hot star wind emissivity in the presence of hypothetical large scale wind structures. Contrary to previous investigations that have focused on the resulting P-Cygni profile variability, we investigate the impact on observable optical and near-infrared emission lines. Our working hypothesis assumes, that such extended wind over-densities are formed via a rotationaly modulated stellar mass loss rate, that gives rise to the so-called co-rotating interaction regions (CIR). Within this context, we find that the variability of emission lines traces an un-equivocal S-shape in the frequency-time space,i.e.,a spiraling pattern with positiveandnegative accelerations towards the line-of-sight over one stellar rotation period. Further, we demonstrate how lines forming at different heights can then be used to provide dynamical and geometrical constraints on the wind structures. Complementary to this spectroscopic approach, we also present theoretical expectations for forthcoming VLT-amber observations of a perturbed hot star outflow presenting such CIRs. The spectrally dispersed visibility and fringe phase output by the Differential Interferometry (DI) method show clearly-defined signatures of the presence of these CIRs. Extrapolating the adequacy of DI beyond the detection of CIRs, we speculate that this method provides very fruitful information on putative large scale structures in hot star environments.
Radio emission models of colliding-wind binary systems
Authors: Dougherty Sean M. , Pittard Julian M. , Kasian Laura , Coker Robert F. , Williams Peredur M. , Lloyd Huw
Published: July 2016

We present preliminary calculations of the spatial distribution of the radio emission from a WR+OB colliding wind system, based on high-resolution hydrodynamical simulations and solutions to the radiative transfer equation. We account for both thermal and non-thermal radio emission, under the assumption of equipartition between magnetic and relativistic particle energy densities, and that the latter is a simple fraction of the thermal particle energy density. These calculations provide the foundation for modeling high resolution radio images and light curves of colliding-wind systems like WR 140, WR 146 and WR 147.
FUSE observations of stellar wind variability in the LMC supergiant Sk–67°166 (O4 If+)
Authors: Fullerton Alexander W. , Massa Derck L. , Prinja Raman K. , Howarth Ian D. , Willis Allan J. , Owocki Stanley P.
Published: July 2016

We describe FUSE time-series observations of stellar wind variability in Sk-67°166 (HDE 269698), a nitrogen-rich O4 supergiant in the LMC.
Preliminary results from XMM-Newton observations of the massive Wolf-Rayet binary WR 22 (WN7H+O9III-V)
Authors: Gosset Eric , Rauw Gregor , Vreux Jean-Marie , Nazé Yaël , Antokhin Igor I. , Sana Hugues
Published: July 2016

We present preliminary results from observations of the massive WR+O binary WR 22 performed with the XMM-Newton observatory. The star is brighter in the X-ray domain near apastron and fainter at periastron. The change in brightness is very progressive and mainly corresponds to a lowering of the soft part of the spectrum.
A search for the most massive stars in M 33
Authors: Sholukhova Olga , Fabrika Sergei N. , Burenkov Alexander N.
Published: July 2016

We present results of a search for the most massive LBV-like stars in M 33 and discuss the list of OB+Hα stars. Spectral studies of the 70 brightest stars (V < 17.7) from the list have isolated 14 new LBV candidates. They are evolved the most massive stars, some of them are variable stars.
Variable dust formation by the double-line spectroscopic binary WR 70 (HD 137603, WC9vd+B0I)
Authors: Williams Peredur M. , van der Hucht Karel A. , Marang Fred
Published: July 2016

Infrared photometry of the WC9vd+B0I spectroscopic binary WR 70 over two decades shows variable dust emission. The variations are relatively slow and show evidence for an underlying process with a period P = 1045 ± 60 d, together with modulation on shorter and longer time-scales.
Instabilities in massive stars
Authors: Owocki Stanley P.
Published: July 2016

A defining property of massive stars is the dominant, dynamical role played by radiation throughout the stellar interior, atmosphere, and wind. Associated with this radiation hydrodynamics are several distinct kinds of instabilities that can lead to convection in both core and envelope, clumping in atmosphere and wind outflow, and perhaps even the dramatic mass loss outbursts associated with Luminous Blue Variable phases. Here I review these instabilities with emphasis on basic physical properties of radiative driving. I draw on two specific examples of dynamical instability, namely the strong instability associated with line-driving of a stellar wind outflow, and the global stellar instabilities associated with approaching or exceeding a modified Eddington limit. I conclude with a brief mention of recent ideas on the role of stellar rotation in the shaping of bipolar LBV outbursts.
Massive star evolution in the Small Magellanic Cloud
Authors: Lennon Daniel J.
Published: July 2016

We discuss abundances for eight early B-type giant/supergiant stars in the SMC cluster NGC 330. All are nitrogen rich with an abundance approximately 1.3 dex higher than an SMC main-sequence field. Given the number of B-type stars with low rotational projected velocities in NGC 330 (all our targets have v sin i < 50 kms–1), we suggest that it is unlikely that the stars in our sample are seen almost pole-on, but rather that they are intrinsically slow rotators. Comparing these results with the predictions of stellar evolution models including the effects of rotationally induced mixing, we conclude that while the abundance patterns may indeed be reproduced, those models with initially large rotational velocities do not reproduce the observed range of effective temperatures of our sample, nor the currently observed rotational velocities. Binary models may be able to produce stars in the observed temperature range and provide a promising alternative to single star models for explaining the observations. We also discuss the clear need for stellar evolution calculations employing the correct chemical mix of carbon, nitrogen and oxygen for the SMC.
Observational constraints on massive star evolution
Authors: Massey Philip
Published: July 2016

In this paper, I discuss the observational quantities that are useful for judging the successes and failures of current massive star evolutionary theory. The galaxies of the Local Group can serve as important laboratories for providing these diagnostics, as their metallicities vary by a factor of ten. We find that the evolutionary tracks do a good job of matching the distribution of stars in the H-R diagram during the main-sequence phase. However, none of the models produce RSGs that are as cool and as bright as what is observed. The relative number of WC and WN stars is a strong function of metallicity, and the Padova and Geneva ‘normal mass-loss’ models do a reasonably good job of matching the observations at low metallicities, but predict too few WCs at higher metallicity. The ‘enhanced’ mass-loss models of the Geneva group do not match the observations at all. New data is providing excellent statistics on the number of RSGs in these nearby galaxies, and the number ratio of RSGs to WRs is also an extremely sensitive function of metallicity. None of the models reproduce the trend of the RSG/WR ratio with metallicity.
Line driven winds, ionizing fluxes and UV-spectra of hot stars at extremely low metalIicity
Authors: Kudritzki Rolf-Peter
Published: July 2016

Wind models of very massive stars with metallicities in a range from 10–4-1.0 Z⊙ are presented using a new treatment of radiation driven winds with depth dependent radiative force multipliers and a comprehensive list of more than two million of spectral lines in non-LTE. The models yield mass-loss rates, wind velocities, wind momenta and wind energies as a function of metallicity and can be used to discuss the influence of stellar winds on the evolution of very massive stars in the early universe and on the interstellar medium in the early phases of galaxy formation. It is shown that the normal scaling laws, which predict stellar mass-loss rates and wind momenta to decrease as a power law with metal abundance break down at a certain threshold. The new wind models are applied to calculate ionizing fluxes and observable UV-spectra of very massive stars as a function of metallicity using the wm-basic code developed by Pauldrach et al. (2001), and the efffects of metallicity are discussed.
Evolution of zero-metallicity massive stars
Authors: Marigo Paola , Chiosi Cesare , Girardi Léo , Kudritzki Rolf-Peter
Published: July 2016

We discuss the evolutionary properties of primordial massive and very massive stars, supposed to have formed from metal-free gas. Stellar models are presented over a large range of initial masses (8 M⊙ ≲ Mi ≲ 1000 M⊙), covering the hydrogen- and helium-burning phases up to the onset of carbon burning. In most cases the evolution is followed at constant mass. To estimate the possible effect of mass loss via stellar winds, recent analytic formalisms for the mass-loss rates are applied to the very massive models (Mi ≥ 120 M⊙).
Black hole formation preceded by a SN explosion or not: evidence from Galactic chemical evolution
Authors: De Donder Erwin , Vanbeveren Dany
Published: July 2016

We derive a constraint on direct black hole formation from the evolution of [O/Fe] as function of time, during the early evolution of the Galaxy in the solar neighbourhood. Since oxygen is dominantly produced by massive stars, the evolution of [O/Fe] is an indirect observable signature of the death of massive stars. We use a detailed Galactic code that computes as function of time the chemical composition of the interstellar gas, out of which successive generations of stars are formed. From our simulations we conclude, that to fit simultaneously the observed [O/Fe] evolution and other observational constraints: (i) all massive stars (single or binary) with M > 40 M⊙ should form massive black holes with the ejection of at least 7 M⊙ of oxygen and little carbon and iron; and (ii) mass loss by stellar wind during the Luminous Blue Variable phase and/or helium burning phase is likely to be metallicity dependent.
On the modeling of outflowing envelopes of massive evolved stars at arbitrary optical depths
Authors: Dorodnitsyn Anton V. , Bisnovatyi-Kogan Gennadi S.
Published: July 2016

Mass loss due to outflow is one factor introducing uncertainty into our understanding of the evolution of massive stars. There is a need of a theory, that would make it possible to take into account mass loss in the process of evolutionary computations in a self-consistent way. It is currently clear that the role of outflow is extremely important for stellar evolution, but quantitative conclusions about mass-loss rates remain uncertain. The evolution of stars with masses M ≥ 15 M⊙ is accompanied by mass loss at rates reaching 10–4-10–6 M⊙ yr–1. This strongly influences the evolution of such stars in the supergiant stage.
Measuring the true supernova rate in galaxies
Authors: Kidger Mark R. , Martín-Luis Fabiola
Published: July 2016

One of the best tracers of massive stars is the rate of supernovae in galaxies. Studies of the supernova rate in galaxies are though severely affected by the line of sight extinction. Geometrical effects (e.g., edgewise presentation) may lead to the majority of supernovae that appear in a galaxy being undetectable in the visible. We propose to use canaricam on the Spanish 10m Gran Telescopio Canarias to measure the true supernova rate in a sample of galaxies, independent of extinction and line of sight effects, by measuring at 10 μm where the extinction is almost negligible. This system allows Type Ia SNe to be detected out to 100 Mpc in 120 s of integration, thus 360 galaxies per year for which we are sensitive to SNe can be covered per hour of telescope time per month.
The stellar content of obscured Galactic giant H II regions
Authors: Blum Robert D.
Published: July 2016

Current research using near-infrared techniques for investigating stellar populations in massive Galactic star clusters is reviewed. In particular, I discuss in some detail a survey of obscured Galactic Giant H ii (GH ii) regions and the associated stellar clusters embedded in them. The regions have been selected as the most luminous radio continuum sources. This selection criteria has the consequence that the observed clusters are among the youngest massive star clusters in the Galaxy. The emergent stellar populations are further studied through near-infrared spectroscopy of the brighter members, a number of which are young stellar objects with signatures of dense circumstellar material which might arise from in or out-flows, or in disks. New results from 8m class telescopes employing velocity resolved near-infrared spectroscopy and high angular resolution mid-infrared imaging hold great promise for future detailed investigations.
M 82: starburst Rosetta Stone
Authors: Förster Schreiber Natascha M. , Genzel Reinhard , Lutz Dieter , Sternberg Amiel
Published: July 2016

We present evolutionary synthesis modeling of the nuclear regions of the starburst galaxy M 82, based on near-infrared integral field spectroscopy and mid-infrared ISO spectroscopy. Our data indicate the occurrence of two distinct starburst episodes in the central 500 pc about 8 – 15 Myr and 5 Myr ago, each lasting a few million years only. The first burst was most intense within 50 pc of the nucleus while the second burst took place in a circumnuclear ring of radius ~ 85 pc and along the stellar bar at larger radii. These recent episodes succeeded earlier starburst activity ~ 1 kpc away from the nucleus and peaking 650 Myr ago, as traced by the luminous star clusters studied by de Grijs et al. (2001). The combined results of both studies reveal complex evolution of starburst activity in M 82 which is consistent with a tidally-induced bar-driven scenario.
On the ionizing source of the Flame Nebula (NGC 2024)
Authors: Bik Arjan , Lenorzer Annique , Kaper Lex , de Koter Alex , Waters Rens , Comerón Fernando , Hanson Margaret M.
Published: July 2016

We have identified IRS 2b as the ionizing source of the Flame Nebula (NGC 2024). For several decades it has been clear that such a hot, massive star must be present in this heavily obscured region, and now it has been found. New J- and K-band photometry and a VLT-isaac K-band spectrum show that IRS 2b is an 08 - 09.7 V star, consistent with published radio continuum and recombination line observations. IRS 2b seems to be slightly evolved from the zero-age main sequence.
The Carina Spiral Feature — an investigation based on a complete magnitude-limited uvbyβ sample
Authors: Kaltcheva Nadejda T.
Published: July 2016

This contribution reflects a detailed overall survey of the Carina Spiral Feature based on uvbyβ photometry of O- and B-type stars. New observations are combined with all data currently available, to collate a complete homogeneous magnitude-limited uvbyβ database. A uniform procedure is applied to obtain the photometry-derived stellar parameters. A homogeneous distance scale is established to the most prominent young structures in the field and the distribution of the visual extinction is analyzed.
A spectroscopic, photometric, and astrometric Galactic O-type star database
Authors: Maíz-Apellániz Jesús , Walborn Nolan R.
Published: July 2016

The objective of this work is to generate a data base with the spectral classification, UV-to-IR photometry, and Hipparcos astrometry of a large number of Galactic O-type stars. Whenever possible, a single source will be used for each type of data, in order to ensure its uniformity and to allow accurate comparisons. This data base could be used for several purposes, such as the testing of atmospheric models or a study of the extinction law and its spatial variations. We present here the current status of this work as well as our plans for the following months. We plan to upgrade the data base in the future as more O-type stars are classified using the same procedure and more data are obtained from space and ground-based projects. The data base is available at http://www.stsci.edu/~jmaiz.
Massive stellar populations in the infrared
Authors: Leitherer Claus
Published: July 2016

An overview of some key results related to hot, massive stars and starbursts in the infrared is given. The wavelength range covers the ground- and space-based near-infrared, the mid- and far-IR as probed by several space missions, and the sub-millimeter regime. I discuss how these observations permit an unbiased view of dust-obscured stars and how they are related to data at optical and ultraviolet wavelengths.
Re-ionization of the IGM — Massive Stars versus QSOs
Authors: Madau Piero
Published: July 2016

In popular cold dark matter cosmological scenarios, stars may have first appeared in significant numbers around a redshift of 10 or so, as the gas within protogalactic halos with virial temperatures Tvir ≃ 20 000 K (corresponding to masses comparable to those of present-day dwarf ellipticals) cooled rapidly due to atomic processes and fragmented. It is this ‘second generation’ of subgalactic stellar systems, aided perhaps by an early population of accreting black holes in their nuclei, which may have generated the ultraviolet radiation and mechanical energy that ended the cosmic ‘dark ages’ and reheated and re-ionized most of the hydrogen in the universe by a redshift of z = 6. The detailed history of the universe during, and soon after these crucial formative stages, depends on the power-spectrum of density fluctuations on small scales and on a complex network of poorly understood feedback mechanisms, and is one of the missing links in galaxy formation and evolution studies.
Interaction between OB stars and the interstellar medium
Authors: Pastor Monica Relaño , Beckman John E. , Rozas Maite
Published: July 2016

From the emission line map in Hα over the full face of the spiral galaxy NGC 3359, we analyze the internal velocity structure of the H ii region population. The emission line profiles show central peaks, with turbulent velocity widths of ≤20 km s–1, and also broad shallow wings, with typical velocities of ~50 km s–1, with respect to the rest velocity of the H ii region. We verify the identification of these wings as due to expanding gas, quantify the energy involved, and suggest physical models for their interpretation, as the effects of mechanical and radiative outflows from the central OB stellar cluster.
A spectroscopic analysis of the Trapezium Cluster stars
Authors: Simón-Díaz Sergio , Herrero Artemio , Esteban César
Published: July 2016

We consider the Orion Nebula (M 42) within a project aimed at studying the interaction between massive stars and their surrounding ISM. This is an H ii region ionized by θ1 Ori C, one of the four massive stars in the Trapezium Cluster. θ1 Ori C has the earliest spectral type (O7Vp) among them, emitting an ionizing flux several orders of magnitude larger than those of the other stars. We present a spectral analysis of the Trapezium Cluster stars to determine their stellar parameters. We use spectra between 4250 – 4750 Å and compare them with synthetic spectra obtained by means of an updated version of fastwind that includes an approximated treatment of metal-line blanketing.
Chandra observations of the dwarf starburst and Wolf-Rayet galaxies NGC 4449 and NGC 5253
Authors: Summers Lesley K. , Stevens Ian R. , Strickland David K. , Heckman Timothy M.
Published: July 2016

We present an analysis of the Chandra observations of two dwarf starburst, Wolf-Rayet galaxies (NGC 4449 and NGC 5253). We have identified at least three different classes of objects within the X-ray point source populations, and we have found the diffuse emission, resulting from the stellar winds and supernova explosions of massive stars, to have a complex morphology and to consist of several components. Comparison with the Hα emission suggests the presence of ~ kpc-scale wind-blown bubbles and ruptured superbubbles.
Observations of Pulsating OB Stars
Authors: Jerzykiewicz Mikołaj
Published: July 2016

Using examples from the literature, I try to show how astrophysical parameters of pulsating stars are derived and how certain (or, occasionally, uncertain) they are. The examples are arranged in the order of increasing input of a priori knowledge. Thus, I start with pulsation periods, because in most cases they can be obtained directly from observations. Then I consider the two stars for which pulsation constants have been determined without recourse to photometric calibrations. Next I show how multicolour photometry can help identify pulsation modes. Finally, I discuss the photometric Teff, log g, and pulsation constants. This leads to conclusions which contain some recommendations for the observers, including a list of promising targets for asteroseismology.
Nonadiabatic observables in β Cephei star models
Authors: Cugier H. , Dziembowski W.A. , Pamyatnykh A.A.
Published: July 2016

Results of the recent stability surveys (Dziembowski and Pamyatnykh 1993; Gautschy and Saio 1993) leave no doubts that the opacity mechanism is responsible for oscillations observed in β Cephei stars. The linear nonadiabatic analysis used to determine the instability domains in the HR diagram, yields also quantities that may be compared with observations. These nonadiabatic observables are evaluated from the complex eigenfunctions y(r) and f(r) describing variations of the radial displacement and the bolometric flux, respectively. Both y and f are very nearly constant within the stellar atmosphere. The eigenfunctions describing the horizontal displacement and variations of thermodynamical quantities may be expressed in terms of y and f. Since the linear eigenfunctions may be arbitrarily normalized, there are only two real independent observables. We may choose them to be and ψ = arg(f/y). Using static atmosphere models, with the inertial term included in the effective gravity, we may evaluate amplitude ratios and phase differences for integrated changes in directly measured parameters (Dziembowski 1977, Stamford and Watson 1981, Watson 1988).
Nonlinear pulsational behaviour of BW Vulpeculae
Authors: Moskalik P. , Buchler J.R.
Published: July 2016

BW Vulpeculae is a large amplitude β Cephei-type star, pulsating with a single period of 0.20104 day. Its nonlinear modeling was first attempted by Pesnell & Cox (1980). Because the true instability mechanism was not known then, the pulsation amplitude was imposed in that study in an arbitrary fashion.
HD 37151: a new “slowly pulsating B star”
Authors: North P. , Paltani S.
Published: July 2016

We report the discovery of non-radial pulsations with at least 4 periods in the B7V star HD 37151. This result is based on 465 photometric measurements spanning 12 years. In addition, 30 high-resolution spectra were taken in the Mgii λ4481 region: they show a small projected rotational velocity and slight variations of the line profiles. Although this star was once classified B8Vp(Si), all available data show it is a normal B7V star. It is the second coolest SPB star found to date.
STACC
Authors: Frandsen Søren
Published: July 2016

At the 1992 GONG Meeting in Boulder (ASP Conf. Ser. Vol. 42) an attempt was done to form a group of people interested in multiobject, multisite observations with CCD cameras on moderate size telescopes (D ≈ 1m). The motivations is the need for uninterrupted, long time strings of photometry of certain types of variables. Observations of single objects by networks of photoelectric photometers have been practiced for some time. The extension to multiobject, differential CCD photometry not only multiplies the output, but also provide better signal/noise results. Many of the objects of interest (δ-Scuti's, β-Cephei's) are found close together in open clusters, which already serve as testbeds for stellar evolution theories. Nevertheless, suitable target fields are difficult to find.
The New Opacities and B-Star Pulsations
Authors: Dziembowski W.A.
Published: July 2016

Over thirty years ago Baker & Kippenhahn (1962) demonstrated that an instability driven by the opacity mechanism is the cause of Cepheid pulsations. Recently it has been shown that the same mechanism is responsible for oscillations observed in β Cephei, SPB and perhaps in other variable B-type stars. The search for the driving mechanism in hot stars began in the late sixties with no success until the opacities calculated with the OPAL code (Iglesias, Rogers and Wilson 1990, 1992) became available. The crucial new feature in the opacity is the local maximum at T ≈ 2 × 105 K caused by iron lines which was ignored in earlier calculations. Recently, stellar opacity data from an independent project (OP) became available (Seaton et al., 1993). The agreement between the two opacity data is satisfactory.

In B stars the opacity mechanism drives two distinct categories of normal modes. The one relevant to β Cep stars encompasses low order p- and g-modes with periods 0.1–0.3 d. The other includes high-order g-modes with periods ranging up above 4 d. Excitation of such modes may explain most of the slow variability observed in B stars. The theoretical instability domain in the H-R diagram is very sensitive to metal abundance. For the standard value of Z = 0.02, the total instability domain in the main sequence band extends from spectral type O9.5 to B9. In types later than B2 only high-order g-modes are unstable.
OP versus OPAL opacities: consequences for B star oscillations
Authors: Pamyatnykh A.A. , Dziembowski W.A. , Moskalik P. , Seaton M.J.
Published: July 2016

Radiative Rosseland-mean opacities for stellar envelopes from The Opacity Project, hereafter OP, became available (Seaton et al. 1993). Here we present a preliminary survey of pulsation properties of B-star models, obtained with this new opacity data and compare these properties with those of models built with the OPAL opacities (Iglesias, Rogers and Wilson 1992). The two opacity projects employ very different approximations in treatment of the plasma and atomic physics. In addition the OP tables are calculated including absorbtion due to three iron group elements ignored in OPAL tables, namely Cr, Mn and Ni.
Mode Identification in Pulsating Stars
Authors: Aerts C.
Published: July 2016

During the past twenty years, different methods have been developed to identify the modes in non-radially pulsating stars. Before the introduction of high-resolution spectrographs with sensitive detectors, identifications were obtained from photometric observations. More recently, mode identification is obtained by means of spectroscopic methods. In this paper, we present an overview of the different mode-identification techniques currently used and we describe their accuracy to identify the modes present in different kinds of pulsating stars. By means of some applications of the moment method, we show that this method deserves far more attention than it has received until now.
Main-sequence broadening, Be stars, and stellar rotation inhand χ Persei
Authors: Denoyelle J. , Aerts C. , Waelkens C.
Published: July 2016

The double cluster h andxPersei is one of the richest clusters containing early-B stars, and therefore is important for observational and theoretical studies on the fundamental parameters of massive stars. The colour-magnitude diagram of the double cluster shows an important scatter (see Figure 1). It has long been known thathandxPersei are extremely rich in Be stars (Slettebak 1968). Our previous contention (Waelkens et al. 1990) that the large-amplitude variable stars we discovered are also Be stars, could be confirmed for a few objects. Rotation velocities for stars inhandxPersei are usually high, which is not surprising in view of the large fraction of Be stars.
Mass loss, magnetic field and chemical inhomogeneities in the He-weak star HD 21699
Authors: Hubrig S. , Mathys G.
Published: July 2016

HD 21699 is a He-weak star whose longitudinal magnetic field ranges from − 1.3 to +1.1 kG (Brown et al. 1985). Its effective temperature, Teff = 15900 K, and its gravity, log g = 4.0, were determined by Wolff (1990) from photometry of the Balmer discontinuity and from measurements of the Hγ line. HD 21699 was reported to exhibit remarkable variations in the C IV ultraviolet resonance lines at 1548 and 1551 Å (Shore et al. 1987). These variations occur with the same period, 2d49 (Percy 1985), as the magnetic variations and the photometric variations in the optical region. Similar UV line variations could be found only in two other He-weak stars, HD 5737 and HD 79158. They were interpreted as an evidence of the occurrence of outflowing winds controlled by the magnetic field. Furthermore, Shore et al. emphasized that no lines in IUE spectra other than C IV, and possibly Si IV, show any statistically significant variation. Recently, irregular brightening of a few hundredths of magnitude, superimposed over the periodic variations, were observed in the U photometric band (Vetö 1993); their origin is currently unknown.
Common properties of some Be stars optical polarization parameters
Authors: Arsenijević J. , Marković-Kršljanin S. , Kubičela A. , Jankov S.
Published: July 2016

The long-term changes of the intrinsic polarization percentage are evident in the stars O And, γ Cas, 88 Her (during the period 1974–1992), κ Dra (1979–1992) and BU Tau (1986–1992). The amplitudes of the polarization percentage variations are not more than a half of the percent. The changes of the position angles are within an interval of about 30 degrees.
Spectroscopy of Be stars in NGC 330
Authors: Mazzali Paolo A. , Pasian F. , Lennon D.J. , Bonifacio P. , Castellani V.
Published: July 2016

Medium resolution (2Å/px) but high s/n spectra of approximately twenty of the brightest blue stars in the young open cluster NGC 330 in the SMC have been obtained with EFOSC1 on the ESO 3.6m telescope, and analyzed in order to determine the atmospheric parameters and the evolutionary status of the stars. LTE and NLTE model atmosphere calculations were used to determine the stellar parameters. The Teff values were derived from fits of the UV continua for all stars where these were available, using Robertson's (1974) B and V photometry to scale the Kurucz model fluxes for metallicity Z = 0.1Z⊙. Luminosities of the sample stars lie in the range 4.0 < log(L*/L⊙) < 5.0 and spectral types between B0 and late-B.
Puzzling problems of He I line formation in early B stars
Authors: Smith Myron A. , Hubeny Ivan , Lanz Thierry
Published: July 2016

Although NLTE model atmospheres have been shown to resolve most of the equivalent width (EW) discrepancies for blue He I lines (Auer and Mihalas 1972, 1973), Wolff and Heasley (1984, 1985) have demonstrated that discrepancies remain for the leading members of the singlet/triplet 2P – nD series, viz. λ6678 and λ5876. These two lines are the strongest nonresonance He I transitions and are important because they respond to thermal changes in the superficial atmosphere (τ~ 10-3) of early B stars. In order to understand the observed rapid variations of the λ6678 line in mild Be stars, we undertook a survey of EWs of λ6678 and λ4388, namely the first and third member of the same series. These two lines have a log gf ratio of 15 but have similar EWs in B star spectra. Our new observations confirm the red line discrepancy noted by WH85 and point to additional EW differences among various groups of B stars not noted hitherto.
Photometric studies of Be star variability
Authors: Percy John R.
Published: July 2016

Be stars are hot stars which have shown emission in at least one Balmer line on at least one occasion. As the definition implies, the Be phenomenon can be variable with time: on time scales of days to decades as the circumstellar disc develops and disperses; on time scales of days to months in a few Be stars which are interacting binaries; on time scales of 0.2 to 2 days due to non-radial pulsation or possibly rotation. The Be stars are worthy of photometric study because they are bright and numerous; the nature of the short-term variability is not yet agreed upon; the cause of the development of the disc - and its relationship to the short-term variability - is also not yet known.
Simultaneous uvbyβ photometry and Hα spectroscopy of Be stars in open clusters
Authors: Fabregat J. , Torrejón J.M. , Reig P. , Bernabeu G.
Published: July 2016

The usual methods of spectral clasification, equivalent widths of Balmer lines or photometric calibrations are not suitable for the determination of the astrophysical parameters of the underlying star in Be-type objects. The spectrum is distorted by the circumstellar envelope lines, while the contribution of the envelope continuum radiation contaminates the photometric indices.
HdBe stars: hydrogen-deficient supergiants with emission lines
Authors: Jeffery C.S.
Published: July 2016

The hydrogen-deficient B stars (often known as extreme-helium stars: EHes) show weak or absent Balmer lines and the following properties (Heber 1986a): Teff ~ 12 − 25kK, log g ~ 2 − 3, log nH ~ −3 − −4, nHe ≈ 0.99, nC ~ 0.01. Their galactic positions and apparent magnitudes indicate that they are low-mass supergiants belonging to the Galactic bulge (Jeffery et al. 1987).
Structure of the envelope of 48 Lib
Authors: Cruzado A. , Cidale L. , Zorec J.
Published: July 2016

From line and continuun data of the ultraviolet and visible spectrum of 48 Lib, the fundamental parameters of the circumstellar envelope are derived.