Massive OB Stars Research Articles

The semicentennial binary system PSR J2032+4127 at periastron: X-ray photometry, optical spectroscopy and SPH modelling.

X-ray photometry and optical spectra are presented covering the periastron passage of the highly-eccentric, ~50 year binary system PSR J2032+4127 in November 2017. This system consists of a 143 ms pulsar in orbit around a massive OB star, MT 91-213. The data show dramatic changes during the encounter as the pulsar wind collided with the stellar wind. The X-ray flux rose on the approach to periastron, then underwent a major dip in the few days around periastron, and then gradually declined over the next few weeks. The optical spectroscopy revealed a steady decline in the H? line strength on the approach to periastron (from an Equivalent Width of -15A to -7A) implying a truncation of the OB star's circumstellar disk by the approaching neutron star. Smooth Particle Hydrodynamic (SPH) modelling is used here to model the system within the context of the observed behaviour and predict the geometrical configuration of the circumstellar disk with respect to the pulsar's orbit.

Chandra Observations of the Massive Star-forming Region Onsala 2

Abstract Previous radio and infrared observations have revealed an obscured region of high-mass star formation in Cygnus known as Onsala 2 (ON2). Within this region lies the optically revealed young stellar cluster Berkeley 87, which contains several OB stars and the rare oxygen-type Wolf–Rayet star WR 142. Previous radio studies of ON2 have also discovered masers and several H ii regions excited by embedded OB stars. Radio and Gaia parallaxes have now shown that the H ii regions are more distant than Berkeley 87. We summarize two Chandra X-ray observations of ON2, which detected more than 300 X-ray sources. Several optically identified stars in Berkeley 87 were detected, including massive OB stars and WR 142, the latter being a faint hard source whose X-ray emission likely arises in hot thermal plasma. Intense X-ray emission was detected near the compact H ii regions G75.77+0.34 and G75.84+0.40, consisting of numerous point sources and diffuse emission. Heavily absorbed X-ray sources and their near-IR counterparts that may be associated with the exciting OB stars of the H ii regions are identified. Shocked winds from embedded massive stars offer a plausible explanation of the diffuse emission. Young stellar object candidates in the ON2 region are identified using near-IR colors, but surprisingly few counterparts of X-ray sources have near-IR excesses typical of classical T Tauri stars.

WR + OB Binary Systems: Observational Evidence of Their Formation as a Result of Mass Exchange

An analysis of observational data shows that, in most cases,Wolf–Rayet (WR) stars in known WR+ OB binary systems were formed as a result of mass transfer in initial OB + OB systems, rather than through radial mass loss by the more massive OB star via its stellar wind.

Statistics of magnetic field measurements in OBA stars and the evolution of their magnetic fields

We review the measurements of magnetic fields of OBA stars. Based on these data, we confirm that magnetic fields are distributed according to a lognormal law with mean log B = − 0.5 (B in kG) with a standard deviation σ = 0.5. The shape of the magnetic field distribution (MFD) is similar to that for neutron stars. This finding favors the hypothesis that the magnetic field of a neutron star is determined mainly by the magnetic field of its predecessor, namely the massive OB star. Further, we model the evolution of an ensemble of magnetic massive stars in the Galaxy. We use our own population synthesis code to obtain the distribution of stellar radii, ages, masses, temperatures, effective magnetic fields, and magnetic fluxes from the pre‐main‐sequence (PMS) via zero‐age main sequence (ZAMS) up to the terminal‐age main sequence stages. A comparison of the MFD obtained in our model with that obtained from the recent measurements of the stellar magnetic field allows us to conclude that the evolution of magnetic fields of massive stars is slow if not absent. The shape of the real MFD shows no indications of the magnetic desert proposed previously. Based on this finding, we argue that the observed fraction of magnetic stars is determined by physical conditions at the PMS stage of stellar evolution.

The origin of discrete multiple stellar populations in globular clusters

Recent observations have revealed that at least several old globular clusters (GCs) in the Galaxy have discrete distributions of stars along the Mg-Al anti-correlation. In order to discuss this recent observation, we construct a new one-zone GC formation model in which the maximum stellar mass (m_max) in the initial mass function (IMF) of stars in a forming GC depends on the star formation rate (SFR), as deduced from independent observations. We investigate the star formation histories of forming GCs. The principal results are as follows. About 30 Myr after the formation of the first generation (1G) of stars within a particular GC, new stars can be formed from ejecta from asymptotic giant branch (AGB) stars of 1G. However, the formation of this second generation (2G) of stars can last only for [10-20] Myr, because the most massive SNe of 2G expel all of the remaining gas. The third generation (3G) of stars are then formed from AGB ejecta ~ 30$ Myr after the truncation of 2G star formation. This cycle of star formation followed by its truncation by SNe can continue until all AGB ejecta is removed from the GC by some physical process. Thus, it is inevitable that GCs have discrete multiple stellar populations in the [Mg/Fe]-[Al/Fe] diagram. Our model predicts that low-mass GCs are unlikely to have discrete multiple stellar populations, and young massive clusters may not have massive OB stars owing to low m_max ([20-30] M_sun) during the secondary star formation.

BONNSAI: correlated stellar observables in Bayesian methods

In an era of large spectroscopic surveys of stars and big data, sophisticated statistical methods become more and more important in order to infer fundamental stellar parameters such as mass and age. Bayesian techniques are powerful methods because they can match all available observables simultaneously to stellar models while taking prior knowledge properly into account. However, in most cases it is assumed that observables are uncorrelated which is generally not the case. Here, we include correlations in the Bayesian code BONNSAI by incorporating the covariance matrix in the likelihood function. We derive a parametrisation of the covariance matrix that, in addition to classical uncertainties, only requires the specification of a correlation parameter that describes how observables co-vary. Our correlation parameter depends purely on the method with which observables have been determined and can be analytically derived in some cases. This approach therefore has the advantage that correlations can be accounted for even if information for them are not available in specific cases but are known in general. Because the new likelihood model is a better approximation of the data, the reliability and robustness of the inferred parameters are improved. We find that neglecting correlations biases the most likely values of inferred stellar parameters and affects the precision with which these parameters can be determined. For example, we apply our technique to massive OB stars, but emphasise that it is valid for any type of stars. For effective temperatures and surface gravities determined from atmosphere modelling, we find that masses can be underestimated on average by $0.5\sigma$ and mass uncertainties overestimated by a factor of about 2 when neglecting correlations. At the same time, the age precisions are underestimated over a wide range of stellar parameters. [abridged]

Modeling the early evolution of massive OB stars with an experimental wind routine

Stellar evolution models of massive stars are very sensitive to the adopted mass-loss scheme. The magnitude and evolution of mass-loss rates significantly affect the main sequence evolution, and the properties of post-main sequence objects, including their rotational velocities. Driven by potential discrepancies between theoretically predicted and observationally derived mass-loss rates in the OB star range, we particularly aim to investigate the response to mass-loss rates that are lower than currently adopted, in parallel with the mass-loss behavior at the "first" bi-stability jump. We perform 1D hydrodynamical model calculations of single $20 - 60 \, M_{\odot}$ Galactic ($Z = 0.014$) stars where the effects of stellar winds are already significant during the main sequence phase. We develop an experimental wind routine to examine the behavior and response of the models under the influence of different mass-loss rates. This observationally guided, simple and flexible wind routine is not a new mass-loss description but a useful tool based on the Wind-momentum Luminosity Relation and other scaling relations, and provides a meaningful base for various tests and comparisons. The main result of this study indicates a dichotomy when accounting for currently debated problems regarding mass-loss rates of hot massive stars. In a fully diffusive approach, and for commonly adopted initial rotational velocities, lower mass-loss rates than theoretically predicted require to invoke an additional source of angular momentum loss (either due to bi-stability braking, or yet unidentified) to brake down surface rotational velocities. [...]

The deep OB star population in Carina from the VST Photometric Hα Survey (VPHAS+)

Massive OB stars are critical to the ecology of galaxies, and yet our knowledge of OB stars in the Milky Way, fainter than $V \sim 12$, remains patchy. Data from the VST Photometric H$\alpha$ Survey (VPHAS+) permit the construction of the first deep catalogues of blue excess-selected OB stars, without neglecting the stellar field. A total of 14900 candidates with 2MASS cross-matches are blue-selected from a 42 square-degree region in the Galactic Plane, capturing the Carina Arm over the Galactic longitude range $282^{\circ} \lesssim \ell \lesssim 293^{\circ}$. Spectral energy distribution fitting is performed on these candidates' combined VPHAS+ $u,g,r,i$ and 2MASS $J,H,K$ magnitudes. This delivers: effective temperature constraints, statistically separating O from early-B stars; high-quality extinction parameters, $A_0$ and $R_V$ (random errors typically $< 0.1$). The high-confidence O-B2 candidates number 5915 and a further 5170 fit to later B spectral type. Spectroscopy of 276 of the former confirms 97% of them. The fraction of emission line stars among all candidate B stars is 7--8% . Greyer ($R_V > 3.5$) extinction laws are ubiquitous in the region, over the distance range 2.5--3 kpc to $\sim$10~kpc. Near prominent massive clusters, $R_V$ tends to rise, with particularly large and chaotic excursions to $R_V \sim 5$ seen in the Carina Nebula. The data reveal a hitherto unnoticed association of 108 O-B2 stars around the O5If$+$ star LSS 2063 ($\ell = 289.77^{\circ}$, $b = -1.22^{\circ}$). Treating the OB star scale-height as a constant within the thin disk, we find an orderly mean relation between extinction ($A_0$) and distance in the Galactic longitude range, $287.6^{\circ} < \ell < 293.5^{\circ}$, and infer the subtle onset of thin-disk warping. A halo around NGC 3603, roughly a degree in diameter, of $\sim$500 O-B2 stars with $4 < A_0 (\rm{mag}) < 7$ is noted.

Stellar populations in the Carina region

Previous studies of the Carina region have revealed its complexity and richness as well as a significant number of early-type stars. In many cases, these studies only concentrated on the central region or were not homogeneous. This latter aspect, in particular, is crucial because very different ages and distances for key clusters have been claimed in recent years. The aim of this work is to study in detail an area of the Galactic plane in Carina. We analyze the properties of different stellar populations and focus on a sample of open clusters and their population of YSOs and highly reddened early stars. We also studied the stellar mass distribution in these clusters and the possible scenario of their formation. Finally, we outline the Galactic spiral structure in this direction. We obtained photometric data for six young open clusters located in Carina at l = 291, and their adjacent stellar fields, which we complemented with spectroscopic observations of a few selected targets. We also culled additional information from the literature. Our results provide more reliable estimates of distances, color excesses, masses, and ages of the stellar populations in this direction. We estimate the basic parameters of the studied clusters and find that they identify two overdensities of young stellar populations. We find evidence of PMS populations inside them, with an apparent coeval stellar formation in the most conspicuous clusters. We also discuss apparent age and distance gradients in the direction NW-SE. We study the mass distributions of several clusters in the region. They consistently show a canonical IMF slope. We discover and characterise an abnormally reddened massive stellar population. Spectroscopic observations of ten stars of this latter population show that all selected targets were massive OB stars. Their location is consistent with the position of the Car-Sag spiral arm.

Photometry and spectroscopy of the luminous red nova PSNJ14021678+5426205 in the galaxy M101

We present the results of the study of the red nova PSN J14021678+5426205 based on the observations carried out with the Russian 6-m telescope (BTA) along with other telescopes of SAO RAS and SAI MSU. To investigate the nova progenitor, we used the data from the Digital Sky Survey and amateur photos available on the internet. In the period between April 1993 and July 2014, the brightness of the progenitor gradually increased by 2.2 mag in the V band. At the peak of the first outburst in mid-November of 2014, the star reached an absolute visual magnitude of -12.75 mag but was discovered later, in February 2015, in a repeated outburst at the absolute magnitude of -11.65 mag. The amplitude of the outburst was minimum among the red novae, only 5.6 mag in the V band. The H alpha emission line and the continuum of a cool supergiant with a gradually decreasing surface temperature were observed in the spectra. Such process is typical for red novae, although the object under study showed extreme parameters: maximum luminosity, maximum outburst duration, minimum outburst amplitude, unusual shape of the light curve. This event is interpreted as a massive OB star system components merging accompanied by the formation of a common envelope and then the expansion of this envelope with minimal energy losses.

Relating turbulent pressure and macroturbulence across the HR diagram with a possible link toγDoradus stars

A significant fraction of the envelope of low- and intermediate-mass stars is unstable to convection, leading to sub-surface turbulent motion. Here, we consider and include the effects of turbulence pressure in our stellar evolution calculations. In search of an observational signature, we compare the fractional contribution of turbulent pressure to the observed macroturbulent velocities in stars at different evolutionary stages. We find a strong correlation between the two quantities, similar to what was previously found for massive OB stars. We therefore argue that turbulent pressure fluctuations of finite amplitude may excite high-order, high-angular degree stellar oscillations, which manifest themselves at the surface an additional broadening of the spectral lines, i.e., macroturbulence, across most of the HR diagram. When considering the locations in the HR diagram where we expect high-order oscillations to be excited by stochastic turbulent pressure fluctuations, we find a close match with the observational γ Doradus instability strip, which indeed contains high-order, non-radial pulsators. We suggest that turbulent pressure fluctuations on a percentual level may contribute to the γ Dor phenomenon, calling for more detailed theoretical modeling in this direction.

Magnetically confined wind shocks in X-rays – A review

A subset (∼10%) of massive stars present strong, globally ordered (mostly dipolar) magnetic fields. The trapping and channeling of their stellar winds in closed magnetic loops leads to magnetically confined wind shocks (MCWS), with pre-shock flow speeds that are some fraction of the wind terminal speed. These shocks generate hot plasma, a source of X-rays. In the last decade, several developments took place, notably the determination of the hot plasma properties for a large sample of objects using XMM and Chandra, as well as fully self-consistent MHD modeling and the identification of shock retreat effects in weak winds. Despite a few exceptions, the combination of magnetic confinement, shock retreat and rotation effects seems to be able to account for X-ray emission in massive OB stars. Here we review these new observational and theoretical aspects of this X-ray emission and envisage some perspectives for the next generation of X-ray observatories.

Massive OB stars at varying Z

AbstractMassive stars play a key role in environments with very different metallicities. To interpret the role of massive stars in these systems we have to know their properties at different metallicities. The Local Group offers an excellent laboratory to this aim.

Asteroseismic Diagnostics for Semi-Convection in B Stars in the Era of K2

AbstractSemi-convection is a slow mixing process in chemically-inhomogeneous radiative interiors of stars. In massive OB stars, it is important during the main sequence. However, the efficiency of this mixing mechanism is not properly gauged yet. Here, we argue that asteroseismology of β Cep pulsators is capable of distinguishing between models of varying semi-convection efficiencies. We address this in the light of upcoming high-precision space photometry to be obtained with theKeplertwo-wheel mission for massive stars along the ecliptic.

X-rays From Centrifugal Magnetospheres in Massive Stars

AbstractIn the subset of massive OB stars with strong global magnetic fields, X-rays arise from magnetically confined wind shocks (Babel &amp; Montmerle 1997). However, it is not yet clear what the effect of stellar rotation and mass-loss rate is on these wind shocks and resulting X-rays. Here, we present results from a grid of Arbitrary Rigid-Field Hydrodynamic simulations (ARFHD) of a B-star centrifugal magnetosphere with an eye towards quantifying the effect of stellar rotation and mass-loss rate on the level of X-ray emission. The results are also compared to a generalized XADM model for X-rays in dynamical magnetospheres (ud-Doula et al. 2014).

A HIGH ANGULAR RESOLUTION SURVEY OF MASSIVE STARS IN CYGNUS OB2: RESULTS FROM THEHUBBLE SPACE TELESCOPEFINE GUIDANCE SENSORS

We present results of a high angular resolution survey of massive OB stars in the Cygnus OB2 association that we conducted with the Fine Guidance Sensor 1R (FGS1r) on the Hubble Space Telescope. FGS1r is able to resolve binary systems with a magnitude difference delta-V < 4 down to separations as small as 0.01 arcsec. The sample includes 58 of the brighter members of Cyg OB2, one of the closest examples of an environment containing a large number of very young and massive stars. We resolved binary companions for 12 targets and confirmed the triple nature of one other target, and we offer evidence of marginally resolved companions for two additional stars. We confirm the binary nature of 11 of these systems from complementary adaptive optics imaging observations. The overall binary frequency in our study is 22% to 26% corresponding to orbital periods ranging from 20 - 20,000 years. When combined with the known short-period spectroscopic binaries, the results supports the hypothesis that the binary fraction among massive stars is > 60%. One of the new discoveries is a companion to the hypergiant star MT 304 = Cyg OB2-12, and future measurements of orbital motion should provide mass estimates for this very luminous star.

Galactic kinematics from a sample of young massive stars

Based on published sources, we have created a kinematic database on 220 massive (> 10 M ⊙) young Galactic star systems located within ≤3 kpc of the Sun. Out of them, ≈100 objects are spectroscopic binary and multiple star systems whose components are massive OB stars; the remaining objects are massive Hipparcos B stars with parallax errors of no more than 10%. Based on the entire sample, we have constructed the Galactic rotation curve, determined the circular rotation velocity of the solar neighborhood around the Galactic center at R 0 = 8kpc, V 0 = 259±16 km s−1, and obtained the following spiral density wave parameters: the amplitudes of the radial and azimuthal velocity perturbations f R = −10.8 ± 1.2 km s−1 and f θ = 7.9 ± 1.3 km s−1, respectively; the pitch angle for a two-armed spiral pattern i = −6.0° ± 0.4°, with the wavelength of the spiral density wave near the Sun being λ = 2.6 ± 0.2 kpc; and the radial phase of the Sun in χ ⊙ = −120° ± 4°. We show that such peculiarities of the Gould Belt as the local expansion of the system, the velocity ellipsoid vertex deviation, and the significant additional rotation can be explained in terms of the density wave theory. All these effects decrease noticeably once the influence of the spiral density wave on the velocities of nearby stars has been taken into account. The influence of Gould Belt stars on the Galactic parameter estimates has also been revealed. Eliminating them from the kinematic equations has led to the following new values of the spiral density wave parameters: f θ = 2.9 ± 2.1 km s−1 and χ ⊙ = −104° ± 6°.

X-rays from magnetic massive OB stars

AbstractThe magnetic activity of solar-type and low-mass stars is a well known source of coronal X-ray emission. At the other end of the main sequence, X-rays emission is instead associated with the powerful, radiatively driven winds of massive stars. Indeed, the intrinsically unstable line-driving mechanism of OB star winds gives rise to shock-heated, soft emission (~0.5 keV) distributed throughout the wind. Recently, the latest generation of spectropolarimetric instrumentation has uncovered a population of massive OB-stars hosting strong, organized magnetic fields. The magnetic characteristics of these stars are similar to the apparently fossil magnetic fields of the chemically peculiar ApBp stars. Magnetic channeling of these OB stars' strong winds leads to the formation of large-scale shock-heated magnetospheres, which can modify UV resonance lines, create complex distributions of cooled Halpha emitting material, and radiate hard (~2-5 keV) X-rays. This presentation summarizes our coordinated observational and modelling efforts to characterize the manifestation of these magnetospheres in the X-ray domain, providing an important contrast between the emission originating in shocks associated with the large-scale fossil fields of massive stars, and the X-rays associated with the activity of complex, dynamo-generated fields in lower-mass stars.

Analysis of far-UV data of central stars of planetary nebulae: Occurrence and variability of stellar winds

The occurrence of stellar wind in the central star of a planetary nebula (CSPN) can be revealed by the presence of P Cygni profiles of high-excitation lines overimposed on its stellar continuum. We examined the entire Far-Ultraviolet Spectroscopic Explorer (FUSE) archive and merged all useful spectroscopic observations of CSPNe to produce the highest quality spectra that can be used to assess the frequency of stellar winds. Furthermore, the individual spectra of each CSPN were compared to search for variability in the P Cygni profile. P Cygni profiles of high-excitation lines have been found in 44 CSPNe, with a clear correlation between the ionization potential of the lines and the effective temperature of the star. We introduce a prescription to derive the terminal wind velocity (v∞) from saturated and unsaturated P Cygni profiles and provide new values of v∞ for these stars. Another 23 CSPNe do not show P Cygni profiles, or else their data in the FUSE archive are not conclusive enough to determine the occurrence of P Cygni profiles. Variability in the P Cygni profile of high-excitation, far-UV lines is found for the first time in six CSPNe, namely Hen 2-131, NGC 40, NGC 1535, NGC 2392, Sp 3, and SwSt 1. This increases up to 13 the number of CSPNe with variable P Cygni profiles in the UV, including those previously reported using IUE or FUSE observations. Variability is seen primarily in the unsaturated P v and Si iv lines, but also in saturated C iii and O vi lines. The CSPNe with variable P Cygni profiles have similar stellar properties (relatively low log (g) and Teff) that suggest they are less evolved CSPNe. Some of the CSPNe with variable P Cygni profiles show O vi lines, while their effective temperature is insufficient to produce this ion. We suggest that this ion is produced by Auger ionization from X-rays associated to shocks in their stellar winds, as is the case in massive OB stars of high ionization potential ions that cannot be abundantly produced by photoionizations.

Population synthesis of young neutron stars

AbstractWe investigate the fortune of young neutron stars (NS) in the whole volume of the Milky Way with new code for population synthesis. We start our modeling from the birth of massive OB stars and follow their motion in the Galaxy up to the Supernova explosion. Next we integrate the equations of motion of NS in the averaged gravitational potential of the Galaxy. We estimate the mean kick velocities from a comparison the model Z and R-distributions of radio emitting NS with that for galactic NS accordingly ATNF pulsar catalog. We follow the history of the rotational velocity and the surface magnetic field of NS taking into account the significant magnetic field decay during the first million year of a neutron star's life. The derived value for the mean time of ohmic decay is 2.3ċ105 years. We model the subsample of galactic radio pulsars which can be detected with available radio telescopes, using a radio beaming model with inhomogeneous distribution of the radio emission in the cone. The distributions functions of the pulsar periods P, period derivatives Ṗ and surface magnetic fields B appear to be in a close agreement with those obtained from an ensemble of neutron stars in the ATNF catalogue.