On the nature of candidate luminous blue variables in M 33

Context. Luminous blue variables (LBVs) are a class of highly unstable stars that have been proposed to play a critical role in massive stellar evolution as well as being the progenitors of some of the most luminous supernovae known. However the physical processes underlying their characteristic instabilities are currently unknown. Aims. In order to provide observational constraints on this behaviour we have initiated a pilot study of the population of (candidate) LBVs in the Local Group galaxy M 33. Methods. To accomplish this we have obtained new spectra of 18 examples within M 33. These provide a baseline of ≥ 4 yr with respect to previous observations, which is well suited to identifying LBV outbursts. We also employed existing multi-epoch optical and mid-IR surveys of M 33 to further constrain the variability of the sample and search for the presence of dusty ejecta. Results. Combining the datasets reveals that spectroscopic and photometric variability appears common, although in the majority of cases further observations will be needed to distinguish between an origin for this behavour in short lived stochastic wind structure and low level photospheric pulsations or coherent long term LBV excursions. Of the known LBVs we report a hitherto unidentified excursion of M 33 Var C between 2001-5, while the transition of the WNLh star B517 to a cooler B supergiant phase between 1993−2010 implies an LBV classification. Proof-of-concept quantitative model atmosphere analysis is provided for Romano’s star; the resultant stellar parameters being consistent with the finding that the LBV excursions of this star are accompanied by changes in bolometric luminosity. The combination of temperature and luminosity of two stars, the BHG [HS80] 110A and the cool hypergiant B324, appear to be in violation of the empirical Humphreys-Davidson limit. Mid-IR observations demonstrate that a number of candidates appear associated with hot circumstellar dust, although no objects as extreme as η Car are identified. The combined dataset suggests that the criteria employed to identify candidate LBVs results in a heterogeneous sample, also containing stars demonstrating the B[e] phenomenon. Of these, a subset of optically faint, low luminosity stars associated with hot dust are of particular interest since they appear similar to the likely progenitor of SN 2008S and the 2008 NGC 300 transient (albeit suffering less intrinsic extinction). Conclusions. The results of such a multiwavelength observational approach, employing multiplexing spectrographs and supplemented with quantitative model atmosphere analysis, appears to show considerable promise in both identifying and characterising the physical properties of LBVs as well as other short lived phases of massive stellar evolution.

Infrared identification of 2XMM J191043.4+091629.4

Context. We report the infrared identification of the X-ray source 2XMM J191043.4+091629.4, which was detected by XMM-Newton/EPIC in the vicinity of the Galactic supernova remnant W49B. Aims. The aim of this work is to establish the nature of the X-ray source 2XMM J191043.4+091629.4 studying both the infrared photometry and spectroscopy of the companion. Methods. We analysed UKIDSS images around the best position of the X-ray source and obtained spectra of the best candidate using NICS in the Telescopio Nazionale Galileo (TNG) 3.5-m telescope. We present photometric and spectroscopic TNG analyses of the infrared counterpart of the X-ray source, identifying emission lines in the K-band. The H-band spectra does not present any significant feature. Results. We have shown that the Brackett γ H i at 2.165 μm, and He i at 2.184 μm and at 2.058 μm are significantly present in the infrared spectrum. The CO bands are also absent from our spectrum. Based on these results and the X-ray characteristics of the source, we conclude that the infrared counterpart is an early B-type supergiant star with an E(B − V) = 7.6 ± 0.3 at a distance of 16.0 ± 0.5 kpc. This would be, therefore, the first high-mass X-ray binary in the Outer Arm at galactic longitudes of between 30° and 60°.

Force-free twisted magnetospheres of neutron stars

Context. The X-ray spectra observed in the persistent emission of magnetars are evidence for the existence of a magnetosphere. The high-energy part of the spectra is explained by resonant cyclotron upscattering of soft thermal photons in a twisted magnetosphere, which has motivated an increasing number of efforts to improve and generalize existing magnetosphere models. Aims. We want to build more general configurations of twisted, force-free magnetospheres as a first step to understanding the role played by the magnetic field geometry in the observed spectra. Methods. First we reviewed and extended previous analytical works to assess the viability and limitations of semi-analytical approaches. Second, we built a numerical code able to relax an initial configuration of a nonrotating magnetosphere to a force-free geometry, provided any arbitrary form of the magnetic field at the star surface. The numerical code is based on a finite-difference time-domain, divergence-free, and conservative scheme, based of the magneto-frictional method used in other scenarios. Results. We obtain new numerical configurations of twisted magnetospheres, with distributions of twist and currents that differ from previous analytical solutions. The range of global twist of the new family of solutions is similar to the existing semi-analytical models (up to some radians), but the achieved geometry may be quite different. Conclusions. The geometry of twisted, force-free magnetospheres shows a wider variety of possibilities than previously considered. This has implications for the observed spectra and opens the possibility of implementing alternative models in simulations of radiative transfer aiming at providing spectra to be compared with observations.

A new code for the Hall-driven magnetic evolution of neutron stars

Over the past decade, the numerical modeling of the magnetic field evolution in astrophysical scenarios has become an increasingly important field. In the crystallized crust of neutron stars the evolution of the magnetic field is governed by the Hall induction equation. In this equation the relative contribution of the two terms (Hall term and Ohmic dissipation) varies depending on the local conditions of temperature and magnetic field strength. This results in the transition from the purely parabolic character of the equations to the hyperbolic regime as the magnetic Reynolds number increases, which presents severe numerical problems. Up to now, most attempts to study this problem were based on spectral methods, but they failed in representing the transition to large magnetic Reynolds numbers. We present a new code based on upwind finite differences techniques that can handle situations with arbitrary low magnetic diffusivity and it is suitable for studying the formation of sharp current sheets during the evolution. The code is thoroughly tested in different limits and used to illustrate the evolution of the crustal magnetic field in a neutron star in some representative cases. Our code, coupled to cooling codes, can be used to perform long-term simulations of the magneto-thermal evolution of neutron stars.

Unifying the observational diversity of isolated neutron stars via magneto-thermal evolution models

Observations of magnetars and some of the high magnetic field pulsars have shown that their thermal luminosity is systematically higher than that of classical radio-pulsars, thus confirming the idea that magnetic fields are involved in their X-ray emission. Here we present the results of 2D simulations of the fully coupled evolution of temperature and magnetic field in neutron stars, including the state-of-the-art kinetic coefficients and, for the first time, the important effect of the Hall term. After gathering and thoroughly re-analysing in a consistent way all the best available data on isolated, thermally emitting neutron stars, we compare our theoretical models to a data sample of 40 sources. We find that our evolutionary models can explain the phenomenological diversity of magnetars, high-B radio-pulsars, and isolated nearby neutron stars by only varying their initial magnetic field, mass and envelope composition. Nearly all sources appear to follow the expectations of the standard theoretical models. Finally, we discuss the expected outburst rates and the evolutionary links between different classes. Our results constitute a major step towards the grand unification of the isolated neutron star zoo.

The imprint of the crustal magnetic field on the thermal spectra and pulse profiles of isolated neutron stars

Isolated neutron stars (NSs) show a bewildering variety of astrophysical manifestations, presumably shaped by the magnetic field strength and topology at birth. Here, using state-of-the-art calculations of the coupled magnetic and thermal evolution of NSs, we compute the thermal spectra and pulse profiles expected for a variety of initial magnetic field configurations. In particular, we contrast models with purely poloidal magnetic fields to models dominated by a strong internal toroidal component. We find that, while the former displays double-peaked profiles and very low pulsed fractions, in the latter, the anisotropy in the surface temperature produced by the toroidal field often results in a single pulse profile, with pulsed fractions that can exceed the 50–60 per cent level even for perfectly isotropic local emission. We further use our theoretical results to generate simulated ‘observed’ spectra, and show that blackbody (BB) fits result in inferred radii that can be significantly smaller than the actual NS radius, even as low as ∼1–2 km for old NSs with strong internal toroidal fields and a high absorption column density along their line of sight. We compute the size of the inferred BB radius for a few representative magnetic field configurations, NS ages and magnitudes of the column density. Our theoretical results are of direct relevance to the interpretation of X-ray observations of isolated NSs, as well as to the constraints on the equation of state of dense matter through radius measurements.

The young open cluster Berkeley 55

We present UBV photometry of the highly reddened and poorly studied open cluster Berkeley 55, revealing an important population of B-type stars and several evolved stars of high luminosity. Intermediate-resolution far-red spectra of several candidate members confirm the presence of one F-type supergiant and six late supergiants or bright giants. The brightest blue stars are mid-B giants. Spectroscopic and photometric analyses indicate an age 50 ± 10 Myr. The cluster is located at a distance d ≈ 4 kpc, consistent with other tracers of the Perseus Arm in this direction. Berkeley 55 is thus a moderately young open cluster with a sizable population of candidate red (super)giant members, which can provide valuable information about the evolution of intermediate-mass stars.

A massive association around the obscured open cluster RSGC3

Context. Four clusters of red supergiants have been discovered in a region of the Milky Way close to base of the Scutum-Crux Arm and the tip of the Long Bar. Population synthesis models indicate that they must be very massive to harbour so many supergiants. If the clusters are physically connected, this Scutum Complex would be the largest and most massive star-forming region ever identified in the Milky Way. Aims. The spatial extent of one of these clusters, RSGC3, has not been investigated. In this paper we explore the possibility that a population of red supergiants could be located in its vicinity. Methods. We utilised 2MASS JHKS photometry to identify candidate obscured luminous red stars in the vicinity of RSGC3. We observed a sample of candidates with the TWIN spectrograph on the 3.5-m telescope at Calar Alto, obtaining intermediate-resolution spectroscopy in the 8000−9000 Å range. We re-evaluated a number of classification criteria proposed in the literature for this spectral range and found that we could use our spectra to derive spectral types and luminosity classes. Results. We measured the radial velocity of five members of RSGC3, finding velocities similar to the average for members of Stephenson 2. Among the candidates observed outside the cluster, our spectra revealed eight M-type supergiants at distances <18′ from the centre of RSGC3, distributed in two clumps. The southern clump is most likely another cluster of red supergiants, with reddening and age identical to RSGC3. From 2MASS photometry, we identified four likely supergiant members of the cluster in addition to the five spectroscopically observed. The northern clump may be a small cluster with similar parameters. Photometric analysis of the area around RSGC3 suggests the presence of a large (>30) population of red supergiants with similar colours. Conclusions. Our data suggest that the massive cluster RSGC3 is surrounded by an extended association, which may be very massive ( ≳ 105 M⊙). We also show that supergiants in the Scutum Complex may be characterised via a combination of 2MASS photometry and intermediate-to-high-resolution spectroscopy in the Z band.

A VLT/FLAMES survey for massive binaries in Westerlund 1. III. The WC9d binary W239 and implications for massive stellar evolution

Context. There is growing evidence that a treatment of binarity amongst OB stars is essential for a full theory of stellar evolution. However the binary properties of massive stars – frequency, mass ratio & orbital separation – are still poorly constrained. Aims. In order to address this shortcoming we have undertaken a multiepoch spectroscopic study of the stellar population of the young massive cluster Westerlund 1. In this paper we present an investigation into the nature of the dusty Wolf-Rayet star and candidate binary W239. Methods. To accomplish this we have utilised our spectroscopic data in conjunction with multi-year optical and near-IR photometric observations in order to search for binary signatures. Comparison of these data to synthetic non-LTE model atmosphere spectra were used to derive the fundamental properties of the WC9 primary. Results. We found W239 to have an orbital period of only ~5.05 days, making it one of the most compact WC binaries yet identified. Analysis of the long term near-IR lightcurve reveals a significant flare between 2004-6. We interpret this as evidence for a third massive stellar component in the system in a long period (>6 yr), eccentric orbit, with dust production occuring at periastron leading to the flare. The presence of a near-IR excess characteristic of hot (~1300 K) dust at every epoch is consistent with the expectation that the subset of persistent dust forming WC stars are short (<1 yr) period binaries, although confirmation will require further observations. Non-LTE model atmosphere analysis of the spectrum reveals the physical properties of the WC9 component to be fully consistent with other Galactic examples. Conclusions. The simultaneous presence of both short period Wolf-Rayet binaries and cool hypergiants within Wd 1 provides compelling evidence for a bifurcation in the post-Main Sequence evolution of massive stars due to binarity. Short period O+OB binaries will evolve directly to the Wolf-Rayet phase, either due to an episode of binary mediated mass loss – likely via case A mass transfer or a contact configuration – or via chemically homogenous evolution. Conversely, long period binaries and single stars will instead undergo a red loop across the HR diagram via a cool hypergiant phase. Future analysis of the full spectroscopic dataset for Wd 1 will constrain the proportion of massive stars experiencing each pathway; hence quantifying the importance of binarity in massive stellar evolution up to and beyond supernova and the resultant production of relativistic remnants.

Red supergiants around the obscured open cluster Stephenson 2

Context. Several clusters of red supergiants have been discovered in a small region of the Milky Way close to the base of the Scutum-Crux Arm and the tip of the Long Bar. Population synthesis models indicate that they must be very massive to harbour so many supergiants. Amongst these clusters, Stephenson 2, with a core grouping of 26 red supergiants, is a strong candidate to be the most massive young cluster in the Galaxy. Aims. Stephenson 2 is located close to a region where a strong over-density of red supergiants had been found. We explore the actual cluster size and its possible connection to this over-density. Methods. Taking advantage of Virtual Observatory tools, we have performed a cross-match between the DENIS, USNO-B1 and 2MASS catalogues to identify candidate obscured luminous red stars around Stephenson 2, and in a control nearby region. More than 600 infrared bright stars fulfill our colour criteria, with the vast majority having a counterpart in the I band and >400 being sufficiently bright in I to allow observation with a 4-m class telescope. We observed a subsample of ~250 stars, using the multi-object, wide-field, fibre spectrograph AF2 on the WHT telescope in La Palma, obtaining intermediate-resolution spectroscopy in the 7500–9000 Å range. We derived spectral types and luminosity classes for all these objects and measured their radial velocities. Results. Our targets turned out to be G and K supergiants, late (≥ M4) M giants, and M-type bright giants (luminosity class II) and supergiants. We found ~35 red supergiants with radial velocities similar to Stephenson 2 members, spread over the two areas surveyed. In addition, we found ~40 red supergiants with radial velocities incompatible in principle with a physical association. Conclusions. Our results show that Stephenson 2 is not an isolated cluster, but part of a huge structure likely containing hundreds of red supergiants, with radial velocities compatible with the terminal velocity at this Galactic longitude (and a distance ~6 kpc). In addition, we found evidence of several populations of massive stars at different distances along this line of sight.

A Be-type star with a black-hole companion

Stellar-mass black holes have all been discovered through X-ray emission, which arises from the accretion of gas from their binary companions (this gas is either stripped from low-mass stars or supplied as winds from massive ones). Binary evolution models also predict the existence of black holes accreting from the equatorial envelope of rapidly spinning Be-type stars1, 2, 3 (stars of the Be type are hot blue irregular variables showing characteristic spectral emission lines of hydrogen). Of the approximately 80 Be X-ray binaries known in the Galaxy, however, only pulsating neutron stars have been found as companions2, 3, 4. A black hole was formally allowed as a solution for the companion to the Be star MWC 656 (ref. 5; also known as HD 215227), although that conclusion was based on a single radial velocity curve of the Be star, a mistaken spectral classification6 and rough estimates of the inclination angle. Here we report observations of an accretion disk line mirroring the orbit of MWC 656. This, together with an improved radial velocity curve of the Be star through fitting sharp Fe ii profiles from the equatorial disk, and a refined Be classification (to that of a B1.5–B2 III star), indicates that a black hole of 3.8 to 6.9 solar masses orbits MWC 656, the candidate counterpart of the γ-ray source AGL J2241+4454 (refs 5, 6). The black hole is X-ray quiescent and fed by a radiatively inefficient accretion flow giving a luminosity less than 1.6 × 10−7 times the Eddington luminosity. This implies that Be binaries with black-hole companions are difficult to detect in conventional X-ray surveys.

IRAS 18357-0604 – an analogue of the galactic yellow hypergiant IRC +10420?

Context. Yellow hypergiants represent a short-lived evolutionary episode experienced by massive stars as they transit to and from a red supergiant phase. As such, their properties provide a critical test of stellar evolutionary theory, while recent observations unexpectedly suggest that a subset may explode as Type II supernovae. Aims. The galactic yellow hypergiant IRC +10420 is a cornerstone system for understanding this phase since it is the strongest post-RSG candidate known, has demonstrated real-time evolution across the Hertzsprung-Russell diagram and been subject to extensive mass loss. In this paper we report on the discovery of a twin of IRC +10420 - IRAS 18357-0604. Methods. Optical and near-IR spectroscopy are used to investigate the physical properties of IRAS 18357-0604 and also provide an estimate of its systemic velocity, while near- to mid-IR photometry probes the nature of its circumstellar environment. Results. These observations reveal pronounced spectral similarities between IRAS 18357-0604 and IRC +10420, suggesting comparable temperatures and wind geometries. IR photometric data reveals a similarly dusty circumstellar environment, although historical mass loss appears to have been heavier in IRC +10420. The systemic velocity implies a distance compatible with the red supergiant-dominated complex at the base of the Scutum Crux arm; the resultant luminosity determination is consistent with a physical association but suggests a lower initial mass than inferred for IRC +10420 (≲20 M⊙ versus ~40 M⊙). Evolutionary predictions for the physical properties of supernova progenitors derived from ~18–20 M⊙ stars – or ~12–15 M⊙ stars that have experienced enhanced mass loss as red supergiants – compare favourably with those of IRAS 18357-0604, which in turn appears to be similar to the the progenitor of SN2011dh; it may therefore provide an important insight into the nature of the apparently H-depleted yellow hypergiant progenitors of some Type IIb SNe.

VdBH 222: a starburst cluster in the inner Milky Way

Context. It has been suggested that the compact open cluster VdBH 222 is a young massive distant object. Aims. We set out to characterise VdBH 222 using a comprehensive set of multi-wavelength observations. Methods. We obtained multi-band optical (UBVR) and near-infrared (JHKS) photometry of the cluster field, as well as multi-object and long-slit optical spectroscopy for a large sample of stars in the field. We applied classical photometric analysis, as well as more sophisticated methods using the CHORIZOS code, to determine the reddening to the cluster. We then plotted dereddened HR diagrams and determined cluster parameters via isochrone fitting. Results. We have identified a large population of luminous supergiants confirmed as cluster members via radial velocity measurements. We find nine red supergiants (plus one other candidate) and two yellow supergiants. We also identify a large population of OB stars. Ten of them are bright enough to be blue supergiants. The cluster lies behind ≈7.5 mag of extinction for the preferred value of RV = 2.9. Isochrone fitting allows for a narrow range of ages between 12 and 16 Ma. The cluster radial velocity is compatible with distances of ~6 and ~10 kpc. The shorter distance is inconsistent with the age range and Galactic structure. The longer distance implies an age ≈ 12 Ma and a location not far from the position where some Galactic models place the far end of the Galactic bar. Conclusions. VdBH 222 is a young massive cluster with a likely mass >20 000 M⊙. Its population of massive evolved stars is comparable to that of large associations, such as Per OB1. Its location in the inner Galaxy, presumably close to the end of the Galactic bar, adds to the increasing evidence for vigorous star formation in the inner regions of the Milky Way.

Spectral features in isolated neutron stars induced by inhomogeneous surface temperatures

The thermal X-ray spectra of several isolated neutron stars display deviations from a pure blackbody. The accurate physical interpretation of these spectral features bears profound implications for our understanding of the atmospheric composition, magnetic field strength and topology, and equation of state of dense matter. With specific details varying from source to source, common explanations for the features have ranged from atomic transitions in the magnetized atmospheres or condensed surface, to cyclotron lines generated in a hot ionized layer near the surface. Here, we quantitatively evaluate the X-ray spectral distortions induced by inhomogeneous temperature distributions of the neutron star surface. To this aim, we explore several surface temperature distributions, we simulate their corresponding general relativistic X-ray spectra (assuming an isotropic, blackbody emission), and fit the latter with a single blackbody model. We find that, in some cases, the presence of a spurious ‘spectral line’ is required at a high significance level in order to obtain statistically acceptable fits, with central energy and equivalent width similar to the values typically observed. We also perform a fit to a specific object, RX J0806.4−4123, finding several surface temperature distributions able to model the observed spectrum. The explored effect is unlikely to work in all sources with detected lines, but in some cases it can indeed be responsible for the appearance of such lines. Our results enforce the idea that surface temperature anisotropy can be an important factor that should be considered and explored also in combination with more sophisticated emission models like atmospheres.

Population synthesis of isolated neutron stars with magneto-rotational evolution

We revisit the population synthesis of isolated radio-pulsars incorporating recent advances on the evolution of the magnetic field and the angle between the magnetic and rotational axes from new simulations of the magneto-thermal evolution and magnetosphere models, respectively. An interesting novelty in our approach is that we do not assume the existence of a death line. We discuss regions in parameter space that are more consistent with the observational data. In particular, we find that any broad distribution of birth spin periods with P0 ≲ 0.5 s can fit the data, and that if the alignment angle is allowed to vary consistently with the torque model, realistic magnetospheric models are favoured compared to models with classical magneto-dipolar radiation losses. Assuming that the initial magnetic field is given by a lognormal distribution, our optimal model has mean strength 〈log B0[G]〉 ≈ 13.0–13.2 with width σ(log B0) = 0.6–0.7. However, there are strong correlations between parameters. This degeneracy in the parameter space can be broken by an independent estimate of the pulsar birth rate or by future studies correlating this information with the population in other observational bands (X-rays and γ-rays).