O Stars Research Articles

Discovery and analysis of three magnetic hot subdwarf stars: evidence for merger-induced magnetic fields

ABSTRACT Magnetic fields can play an important role in stellar evolution. Among white dwarfs, the most common stellar remnant, the fraction of magnetic systems is more than 20 per cent. The origin of magnetic fields in white dwarfs, which show strengths ranging from 40 kG to hundreds of MG, is still a topic of debate. In contrast, only one magnetic hot subdwarf star has been identified out of thousands of known systems. Hot subdwarfs are formed from binary interaction, a process often associated with the generation of magnetic fields, and will evolve to become white dwarfs, which makes the lack of detected magnetic hot subdwarfs a puzzling phenomenon. Here we report the discovery of three new magnetic hot subdwarfs with field strengths in the range 300–500 kG. Like the only previously known system, they are all helium-rich O-type stars (He-sdOs). We analysed multiple archival spectra of the three systems and derived their stellar properties. We find that they all lack radial velocity variability, suggesting formation via a merger channel. However, we derive higher than typical hydrogen abundances for their spectral type, which are in disagreement with current model predictions. Our findings suggest a lower limit to the magnetic fraction of hot subdwarfs of $0.147^{+0.143}_{-0.047}$ per cent, and provide evidence for merger-induced magnetic fields which could explain white dwarfs with field strengths of 50–150 MG, assuming magnetic flux conservation.

Probing the low-mass end of the companion mass function for O-type stars

Context. Past observations of O-type stars in the Galaxy have shown that almost all massive stars are part of a binary or higher-order multiple system. Given the wide range of separations at which these companions are found, several observational techniques have been adopted to characterize them. Despite the recent advancements in interferometric and adaptive optics observations, contrasts greater than 4 in the H band have never been reached between 100 and 1000 mas. Aims. Using new adaptive optics (AO) assisted coronagraphic observations, we aim to study the multiplicity properties of a sample of 18 dwarf (or sub-giant) O stars in the galactic field and in OB associations to probe the existence of stellar companions in the angular separation range from 0.″15 to 6″ down to very low mass ratios. Methods. We used VLT/SPHERE to observe simultaneously with the IRDIS and IFS sub-systems 18 O-type stars within 6 kpc and ages between 1 and 5 Myr. The IFS YJH band observations have allowed us to probe the presence of sub-solar companions in a 1.7″ × 1.7″ field-of-view down to magnitude limits of ΔH = 10 at 0.″4. In the wider 12″ × 12″ IRDIS field-of-view, we reached contrasts of ΔK = 12 at 1″, enabling us to look for even fainter companions at larger angular separations and to probe the source density of the surrounding portion of the sky. Results. This paper presents five newly discovered intermediate (< 1″) separation companions, three of which are smaller than 0.2 M⊙. If confirmed by future analyses of proper motions, these new detections represent the lowest-mass companions ever found around O-type stars. Additionally, 29 other sources are found in the IRDIS field-of-view with spurious association probabilities smaller than 5%. Assuming that all sources detected within 1″ are physically bound companions, the observed (uncorrected for bias) fraction of companions for O-type stars between 150 and 900 mas is 0.39 ± 0.15, whereas it increases to 1.6 ± 0.3 in the separation range from 0.″9 to 6″. Conclusions. These findings clearly support the notion that massive stars form almost exclusively in multiple systems, serving as proof of concept that supports the application of larger AO-assisted coronagraphic surveys as a crucial step in placing constraints on the multiplicity properties of massive star companions in regions of the parameter space that have previously gone unexplored. These results also demonstrate that the companion mass function is populated down to the lowest stellar masses.

Star formation in two irradiated globules around Cygnus OB2

Context. The ultraviolet irradiation and the action of stellar winds of newly formed massive stars on their parental molecular environment often produces isolated or small clouds, some of which become sites of star formation themselves. Aims. We investigate the young stellar populations associated with DR 18 and ECX 6-21, which are two isolated globules irradiated by the O-type stars of the Cygnus OB2 association. Both are HII regions containing obvious tracers of recent and ongoing star formation. We also study smaller isolated molecular structures in their surroundings. Methods. We combined near-infrared broad- and narrow-band imaging with broad-band imaging in the visible and with archive images obtained with the Spitzer Space Telescope. We used the joint photometry to select young stellar objects (YSOs), simultaneously estimating their intrinsic properties and classifying them according to the characteristics of their infrared excess. We also present low-resolution visible spectroscopy of selected sources. Results. We reproduce previous findings of an extended population of YSOs around both globules, dominated by the more evolved classes, associated with the general Cygnus OB2 population. Both globules contain their own embedded populations, with a higher fraction of the less-evolved classes. Masses and temperatures are estimated under the assumption of a common age of 1 Myr, which has been found to appropriately represent the general Cygnus OB2 YSO population but is most probably an overestimate for both globules, especially ECX 6-21. The early-B star responsible for the erosion of DR 18 is found to be part of a small aggregate of intermediate-mass stars still embedded in the cloud, which probably contains a second site of recent star formation, also with intermediate-mass stars. We confirm the two main star forming sites embedded in ECX 6-21 described in previous works, with the southern site being more evolved than the northern site. We also discuss the small globule ECX 6-21-W (=G79.8 + 1.2), and propose that its non thermal radio spectrum is due to synchrotron emission from an embedded jet, whose existence is suggested by our observations. Conclusions. The extreme youth of some of the YSOs suggests that star formation in both globules started after they became externally irradiated. The populations of both globules are not found to be particularly rich, but they contain stars with estimated masses similar or above that of the Sun in numbers that hint at some differences with respect to the star formation process taking place in more quiescent regions where low-mass stars dominate, which deeper observations may confirm.

FEEDBACK from the NGC 7538 H II region

Context. The interaction of expanding H II regions with their environmental clouds is one of the central questions driving the Stratospheric Observatory for Infrared Astronomy (SOFIA) legacy program FEEDBACK. Aims. We want to understand the interaction of the prototypical NGC 7538 H II region with the neighboring molecular cloud hosting several active star-forming regions. Methods. Using the SOFIA, we mapped an area of ~210′2 (~125 pc2) around NGC 7538 in the velocity-resolved ionized carbon fine-structure line [CII] at 1.9 THz (158 μm). Complementary observed atomic carbon [CI] at 492 GHz and high-J CO(8–7) data, as well as archival near- and far-infrared, cm continuum, CO(3–2), and HI data are folded into the analysis. Results. The ionized carbon [CII] data reveal rich morphological and kinematic structures. While the overall morphology follows the general ionized gas that is also visible in the radio continuum emission, the channel maps show multiple bubble-like structures with sizes on the order of ~80–100″ (~1.0–1.28 pc). While at least one of them may be an individual feedback bubble driven by the main exciting sources of the NGC 7538 H II region (the O3 and O9 stars IRS6 and IRS5), the other bubble-like morphologies may also be due to the intrinsically porous structure of the H II region. An analysis of the expansion velocities around 10 km s−1 indicates that thermal expansion is not sufficient but that wind-driving from the central O-stars is required. The region exhibits a general velocity gradient across, but we also identify several individual velocity components. The most blue-shifted [CII] component has barely any molecular or atomic counterparts. At the interface to the molecular cloud, we find a typical photon-dominated region (PDR) with a bar-shape. Ionized C+, atomic C0 and molecular carbon CO show a layered structure in this PDR. The carbon in the PDR is dominated by its ionized C+ form with atomic C0 and molecular CO masses of ~0.45 ± 0.1 M⊙ and ~1.2 ± 0.1 M⊙, respectively, compared to the ionized carbon C+ in the range of 3.6−9.7 M⊙. This bar-shaped PDR exhibits a velocity-gradient across, indicating motions along the line of sight toward the observer. Conclusions. Even if it is shown to be dominated by two nearby exciting sources (IRS6 and IRS5), the NGC 7538 H II region exhibits a diverse set of substructures that interact with each other as well as with the adjacent cloud. Compared to other recent [CII] observations of H II regions (e.g., Orion Veil, RCW120, RCW49), the bubble-shape morphologies revealed in [CII] emission that are indicative of expanding shells are recurring structures of PDRs.

High star cluster formation efficiency in the strongly lensed Sunburst Lyman-continuum galaxy at z = 2.37

We investigate the strongly lensed (μ ≃ ×10 − 100) Lyman continuum (LyC) galaxy, dubbed Sunburst, at z = 2.37, taking advantage of a new accurate model of the lens. A characterization of the intrinsic (delensed) properties of the system yields a size of ≃3 sq. kpc, a luminosity of MUV = −20.3, and a stellar mass of M ≃ 109 M⊙; 16% of the ultraviolet light is located in a 3 Myr old gravitationally bound young massive star cluster (YMC), with an effective radius of ∼8 pc (corresponding to 1 milliarcsec without lensing) and a dynamical mass of ∼107 M⊙ (similar to the stellar mass) – from which LyC radiation is detected (λ < 912 Å). The star formation rate and stellar mass surface densities for the YMC are Log10(ΣSFR[M⊙ yr−1 kpc−2]) ≃ 3.7 and Log10(ΣM[M⊙ pc−2]) ≃ 4.1, with sSFR > 330 Gyr−1, consistent with the values observed in local young massive star clusters. The inferred outflowing gas velocity (> 300 km s−1) exceeds the escape velocity of the cluster. The resulting relative escape fraction of the ionizing radiation emerging from the entire galaxy is higher than 6−12%, whilst it is ≳46 − 93% if inferred from the YMC multiple line of sights. At least 12 additional unresolved star-forming knots with radii spanning the interval 3 − 20 pc (the majority of them likely gravitationally bound star clusters) are identified in the galaxy. A significant fraction (40−60%) of the ultraviolet light of the entire galaxy is located in such bound star clusters. In adopting a formation timescale of the star clusters of 20 Myr, a cluster formation efficiency Γ ≳ 30%. The star formation rate surface density of the Sunburst galaxy (Log10(ΣSFR) = 0.5−0.2+0.3) is consistent with the high inferred Γ, as observed in local galaxies experiencing extreme gas physical conditions. Overall, the presence of a bursty event (i.e., the 3 Myr old YMC with large sSFR) significantly influences the morphology (nucleation), photometry (photometric jumps), and spectroscopic output (nebular emission) of the entire galaxy. Without lensing magnification, the YMC would be associated to an unresolved 0.5 kpc–size star-forming clump. The delensed LyC and UV magnitude m1600 (at 1600 Å) of the YMC are ≃30.6 and ≃26.9, whilst the entire galaxy has m1600 ≃ 24.8. The Sunburst galaxy shows a relatively large rest-frame equivalent width of EWrest(Hβ + [O III]λλ4959, 5007) ≃ 450 Å, with the YMC contributing to ∼30% (having a local EWrest ≃ 1100 Å) and ∼1% of the total stellar mass. If O-type (ionizing) stars are mainly forged in star clusters, then such engines were the key ionizing agents during reionization and the increasing occurrence of high equivalent width lines (Hβ + [O III]) observed at z > 6.5 might be an indirect signature of a high frequency of forming massive star clusters (or high Γ) at reionization. Future facilities, which will perform at few tens milliarcsec resolution (e.g., VLT/MAVIS or ELT), will probe bound clusters on moderately magnified (μ < 5 − 10) galaxies across cosmic epochs up to reionization.

New Massive Contact Twin Binary in a Radio-quiet H ii Region Associated with the M17 Complex

Early-B stars, much less energetic than O stars, may create an H ii region that appears as radio-quiet. We report the identification of new early-B stars associated with the radio-quiet H ii region G014.645–00.606 in the M17 complex. The radio-quiet H ii region G014.645–00.606 is adjacent to three radio-quiet WISE H ii region candidates. The ionizing sources of the radio-quiet H ii regions are expected to be later than B1V, given the sensitivity about 1–2 mJy of the MAGPIS 20 cm survey. The stars were first selected if their parallaxes of Gaia EDR3 match that of the 22 GHz H2O maser source within the same region. We used the color–magnitude diagram made from the Zwicky Transient Facility photometric catalog to select the candidates for massive stars because the intrinsic g − r colors of massive stars change little from B-type to O-type stars. Five stars lie in the areas of the color–magnitude diagram where either reddened massive stars or evolved post-main sequence stars of lower masses are commonly found. Three of the five stars, sources 1, 2, and 3, are located at the cavities of the three IR bubbles, and extended Hα emission is detected around the three IR bubbles. We suggest that sources 1, 2, and 3 are candidates for early-B stars associated with the radio-quiet region G014.645–00.606. Particularly, source 1 is an EW type eclipsing binary with a short period of 0.825 day, while source 2 is an EA type eclipsing binary with a short period of 0.919 day. The physical parameters of the two binary systems have been derived through the PHOEBE model. Source 1 is a twin binary of two stars with T eff ≈ 23,500 K, and source 2 contains a hotter component (T eff ≈ 20,100 K) and a cooler one (T eff ≈ 15,500 K). The O − C values of source 1 show a trend of decline, implying that the period of the source is deceasing. Source 1 is likely a contact early-B twin binary, for which mass transfer might cause its orbit to shrink.

Star Formation Triggered by the Expanding Bubble S111

Abstract This paper investigates the impact of radiative and mechanical feedback from O-type stars on their parent molecular clouds and the triggering of formation of a future generation of stars. We study the infrared bubble S111 created by the embedded massive stellar cluster G316.80–0.05. A significant fraction of gas in shells created due to the compression of the ambient medium by expanding bubbles is photodissociated by the stellar radiation. The kinematics of the shells are thus best studied using spectroscopic observations of singly ionized carbon, the most dominant species. We have used velocity-resolved maps of the 2P3/2 → 2P1/2 transition of [C ii] at 158 μm, the J = 2–1 transition of 13CO and C18O, and the J = 1–0 transition of HCO+ to study the rim of the bubble S111 that partly coincides with the southern part of the infrared dark ridge G316.75. The [C ii] spectra conclusively show evidence of a shell expanding with a moderate velocity of ∼7 km s−1, which amounts to a kinetic energy that is ∼0.5–40 times the thermal energy of the H ii region. The pressure causing the expansion of the H ii region arises mainly from hydrogen ionization and dust-processed radiation. Among the far-infrared sources located in compressed shells, we find the core G316.7799–0.0942 to show broad spectral features consistent with outflow activity and conclude that it is a site of active star formation. Based on the age of the H ii region we conclude that this expanding H ii region is responsible for triggering the current star formation activity in the region.

The observed multiplicity properties of B-type stars in the Galactic young open cluster NGC 6231

Context. It is well known that massive O stars are frequently (if not always) found in binary or higher-order multiple systems, but this fact has been less robustly investigated for the lower mass range of the massive stars, represented by B-type stars. Obtaining the binary fraction and orbital parameter distributions of B-type stars is crucial to understand the impact of multiplicity on the archetypal progenitor of core-collapse supernovae as well as to properly investigate formation channels for gravitational wave progenitors. Aims. This work aims to characterise the multiplicity of the B star population of the young open cluster NGC 6231 through multi-epoch optical spectroscopy of 80 B-type stars. Methods. We analyse 31 FLAMES/GIRAFFE observations of 80 B-type stars, monitoring their radial velocities (RVs) and performing a least-squares spectral analysis (Lomb-Scargle) to search for periodicity in those stars with statistically significant variability in their RVs. Results. We constrained an observed spectroscopic binary fraction of 33 ± 5% for the B-type stars of NGC 6231, with a first order bias correction giving a true spectroscopic binary fraction of 52 ± 8%. Out of 27 B-type binary candidates, we obtained orbital solutions for 20 systems: 15 single-lined (SB1) and five double-lined spectroscopic binaries (SB2s). We present these orbital solutions and the orbital parameter distributions associated with them. Conclusions. Our results indicate that Galactic B-type stars are less frequently found in binary systems than their more massive O-type counterparts, but their orbital properties generally resemble those of B- and O-type stars in both the Galaxy and Large Magellanic Cloud.

Carina High-contrast Imaging Project for massive Stars (CHIPS)

Context.Most massive stars belong to multiple systems, yet the formation process leading to such high multiplicity remains insufficiently understood. To help constrain the different formation scenarios that exist, insights into the low-mass end of the companion mass function of such stars is crucial. However, this is a challenging endeavour as (sub-)solar mass companions at angular separations (ρ) below 1″ (corresponding to 1000–3000 au in nearby young open clusters and OB associations) are difficult to detect due to the large brightness contrast with the central star.Aims.With the Carina High-contrast Imaging Project of massive Stars (CHIPS), we aim to obtain statistically significant constraints on the presence and properties of low-mass companions around massive stars in a previously unreachable observing window (Δmag ≳ 10 atρ ≲ 1″). In the second paper of the series, we focus on the Trumpler 14 cluster, which harbours some of the youngest and most massive O-type stars in the Milky Way.Methods.We obtained VLT-SPHERE observations of seven O-type objects in Trumpler 14 using IRDIFS_EXT mode. These provide us with a 12″ × 12″ field of view (approximately ((3 × 3)×104au) centred on each O star and allow us to search for companions at separations larger than 0″​​.15 (approx. 360 au) and down to magnitude contrast > 10 mag in the near-infrared. We used angular and spectral differential imaging along with Point Spread Function (PSF) fitting to detect sources and measure their flux relative to that of the central object. We then used grids of ATLAS9 and PHOENIX Local Thermodynamic Equilibrium (LTE) atmosphere models combined with (pre-)main-sequence evolutionary tracks to estimate the mass of the detected candidate companions.Results.We detected 211 sources with near-infrared magnitude contrast in the range of 2–12. Given the large surface number density of stars in Trumpler 14, one cannot reliably distinguish between cluster members and genuine companions for most of the detected sources. The closest companion, at only 0″​​.26, is characterised as a 1.4M⊙star with an age of 0.6 Myr, in excellent agreement with previous age estimates for Tr 14. The mass function peaks at about 0.4M⊙and presents a dearth of stars in the 0.5–0.8M⊙mass range compared to previous estimates of the initial mass function in Tr 14. While statistically significant, part of these differences may result from contamination of theK-band fluxes by circumstellar material.Conclusions.SPHERE is clearly suitable to probe the low-mass end of the mass function in the vicinity of massive stars. Follow-up SPHERE observations to obtain the full Y to K spectral energy distribution would allow for better constraints on the masses of the detected sources, and to confirm (or invalidate) the curious mass function that we derived for low-mass stars in the vicinity of the O-type objects in Trumpler 14.

The Disk–Outflow System around the Rare Young O-type Protostar W42-MME

Abstract We present line and continuum observations (resolution ∼0.″3–3.″5) made with the Atacama Large Millimeter/submillimeter Array (ALMA), Submillimeter Array, and Very Large Array of a young O-type protostar W42-MME (mass: 19 ± 4 M ⊙). The ALMA 1.35 mm continuum map (resolution ∼1″) shows that W42-MME is embedded in one of the cores (i.e., MM1) located within a thermally supercritical filament-like feature (extent ∼0.15 pc) containing three cores (mass ∼1–4.4 M ⊙). Several dense/hot gas tracers are detected toward MM1, suggesting the presence of a hot molecular core with a gas temperature of ∼38–220 K. The ALMA 865 μm continuum map (resolution ∼0.″3) reveals at least five continuum sources/peaks (A–E) within a dusty envelope (extent ∼9000 au) toward MM1, where shocks are traced in the SiO (8–7) emission. Source A associated with W42-MME is seen almost at the center of the dusty envelope and is surrounded by other continuum peaks. The ALMA CO (3–2) and SiO (8–7) line observations show the bipolar outflow extended below 10,000 au, which is driven by source A. The ALMA data hint at the episodic ejections from W42-MME. A disk-like feature (extent ∼2000 au, mass ∼1 M ⊙) with velocity gradients is investigated in source A (dynamical mass ∼9 M ⊙) using the ALMA H13CO+ emission, and it is perpendicular to the CO outflow. A small-scale feature (below 3000 au), probably heated by UV radiation from the O-type star, is also investigated toward source A. Overall, W42-MME appears to gain mass from its disk and the dusty envelope.

The influence of initial orbital period on helium and carbon-oxygen core masses in massive case a binary systems with low accretion efficiency

The evolutionary models of 33 massive Case A binary systems in mass range from 30M? to 40M? with initial orbital periods of 3, 4 and 5 days, accretion efficiency of 10% and at the solar metallicity are presented. The models are obtained with the MESA (Modules for Experiments in Stellar Astrophysics) numerical code. The evolution is followed from a double O-type star, through Case A and Case mass transfer, to the formation of a carbon-oxygen (CO) core in the primary. The evolution of the secondary star in each binary system is further modeled with the same numerical code in an approximation of a single star, also to the formation of a carbon-oxygen core. The resulting helium core masses are in the range of 7.94M? - 13.19M? and 12.30M? and 19.12M? for primary and secondary stars, respectively. The carbon-oxygen core masses are between 5.26M? and 10M? for primaries and between 8.96M? and 15.32M? for secondaries. A clear influence of the initial orbital period on the resulting helium and CO core masses is demonstrated: primary stars in binary systems with initial orbital periods of 3, 4 and 5 days have on average about 15%, 8% and 2.5% smaller CO cores than single stars with the same initial masses. On the other hand, it was found that the correlation between the CO and helium core mass does not depend on the initial orbital period and can be approximated with the same linear fit for all binary systems. The CO/helium core mass ratio is found to be larger in binary systems than for single stars. It is also shown that the black hole formation limit for primary stars depends on the initial orbit and is between 33M? - 34M?, 32M? - 33M? and 30M? - 31M?, for the initial orbital periods of 3, 4 and 5 days, respectively. The resulting double compact objects are of two types: mixed neutron star - black hole systems (6 models) and double black holes (27 models). The resulting black hole masses are estimated to be in the range of 5M? to 17M?.

Untangling magnetic massive star properties with linear polarization variability and the analytic dynamical magnetosphere model

ABSTRACT Light scattered off particles can become linearly polarized. Stars surrounded by oblique, co-rotating envelopes are therefore expected to manifest periodic linear polarimetric variations. The electron scattering magnetospheres of magnetic massive stars are expected to be suitable candidates to observe this effect. In this paper, we present the first semi-analytical model capable of synthesizing the continuum polarimetric signatures of magnetic O-type stars in an optically thin, single electron scattering limit. The purpose of this investigation is to improve our general understanding of magnetic hot stars by characterizing their polarimetric behaviour. Our linear polarization model is constructed by combining the analytical expressions for the polarimetric variations of an obliquely rotating envelope with the analytic dynamical magnetosphere model to represent a physical model for the envelope density structure. We compute grids of model Stokes Q and U curves and show that their shapes are unique to the choice of inclination and obliquity angles. We apply our model to HD 191612, a prototypical Of?p-type star, having both polarimetric and photometric observations. We find that the polarimetric modulations are best reproduced with $i=19^{+12}_{-3}$○, $\beta =71^{+3}_{-9}$○, and $\log \dot{M}_{B=0}=-6.11^{+0.12}_{-0.06}$ [M⊙ yr−1]. These results agree with previous investigations of this star. By combining both polarimetric and photometric synthesis tools, we simultaneously model the observations thus adding further refinement of the wind and magnetic properties of HD 191612.

Detection of anomalous element distribution in the extremely slowly rotating magnetic O9.7 V star HD 54879

ABSTRACT The O9.7 V star HD 54879 is currently the only massive magnetic star whose magnetic field geometry and rotation period are not constrained. Over the last 3 yr, we gathered additional observations of this star, obtained using various instruments at several astronomical facilities, with the aim to constrain the rotation period and the magnetic field geometry. The new data include the first full Stokes vector observations with the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI), installed at the Large Binocular Telescope. The acquired spectropolarimetric observations show a very slow magnetic field variability related to the extremely slow rotation of HD 54879, which is also indicated in a dynamical spectrum, displaying variability of the Hα line. The most intriguing result of our study is the discovery of differences in longitudinal magnetic field strengths measured using different least-squares deconvolution (LSD) masks containing lines belonging to different elements. It is the first time that such a differential analysis of the field strength in dependence of the used lines is carried out for a magnetic O-type star. Since the LSD Stokes I profiles of the studied O, Si, and He line masks remain stable over all observing epochs, we conclude that the detection of different field strengths using lines belonging to these elements is related to the different formation depths, with the He lines formed much higher in the stellar atmosphere compared to the silicon and the oxygen lines, and non-local thermodynamic equilibrium (NLTE) effects. Our numerical magnetospherical model suggests the presence of enhanced gas density that fills the volume inside the field lines close to the star.

Multiplicity of Galactic luminous blue variable stars

Context.Luminous blue variables (LBVs) are characterised by strong photometric and spectroscopic variability. They are thought to be in a transitory phase between O-type stars on the main sequence and the Wolf-Rayet stage. Recent studies also evoked the possibility that they might be formed through binary interaction. Only a few are known in binary systems so far, but their multiplicity fraction is still uncertain.Aims.We derive the binary fraction of the Galactic LBV population. We combine multi-epoch spectroscopy and long-baseline interferometry to probe separations from 0.1 to 120 mas around confirmed and candidate LBVs.Methods.We used a cross-correlation technique to measure the radial velocities of these objects. We identified spectroscopic binaries through significant radial velocity variability with an amplitude larger than 35 km s−1. We also investigated the observational biases to take them into account when we established the intrinsic binary fraction. We usedCANDIDto detect interferometric companions, derive their flux fractions, and their positions on the sky.Results.From the multi-epoch spectroscopy, we derive an observed spectroscopic binary fraction of 26−10+16%. Considering period and mass ratio ranges from log(Porb) = 0 − 3 (i.e. from 1 to 1000 days),q = 0.1 − 1.0, and a representative set of orbital parameter distributions, we find a bias-corrected binary fraction of 62−24+38%. Based on data of the interferometric campaign, we detect a binary fraction of 70 ± 9% at projected separations between 1 and 120 mas. Based on the derived primary diameters and considering the distances of these objects, we measure for the first time the exact radii of Galactic LBVs to be between 100 and 650 R⊙. This means that it is unlikely that short-period systems are included among LBV-like stars.Conclusions.This analysis shows for the first time that the binary fraction in the Galactic LBV population is large. If they form through single-star evolution, their orbit must be large initially. If they form through a binary channel, the implication is that either massive stars in short binary systems must undergo a phase of fully non-conservative mass transfer to be able to sufficiently widen the orbit to form an LBV, or that LBVs form through merging in initially binary or triple systems. Interferometric follow-up would provide the distributions of orbital parameters at more advanced stages and would serve to quantitatively test the binary evolution in massive stars.

Probing Gas Kinematics and PDR Structure around O-type Stars in the Sh 2-305 H ii Region

Abstract We report an observational study of the Galactic H ii region Sh 2-305/S305 using the [C ii] 158 μm line data, which are used to examine the gas dynamics and structure of photodissociation regions. The integrated [C ii] emission map at [39.4, 49.5] km s−1 spatially traces two shell-like structures (i.e., inner and outer neutral shells) having a total mass of ∼565 M ⊙. The inner neutral shell encompasses an O9.5V star at its center and has a compact ring-like appearance. However, the outer shell is seen with more extended and diffuse [C ii] emission, hosting an O8.5V star at its center, and surrounds the inner neutral shell. The velocity channel maps and position–velocity diagrams confirm the presence of a compact [C ii] shell embedded in the diffuse outer shell, and both the shells seem to expand with v exp ∼ 1.3 km s−1. The outer shell appears to be older than the inner shell, hinting that these shells are formed sequentially. The [C ii] profiles are examined toward S305, which are either double peaked or blue skewed and have the brighter redshifted component. The redshifted and blueshifted components spatially trace the inner and outer neutral shell geometry, respectively. The ionized, neutral, and molecular zones in S305 are seen adjacent to one another around the O-type stars. The regularly spaced dense molecular and dust clumps (mass ∼10–103 M ⊙) are investigated around the neutral shells, which might have originated as a result of gravitational instability in the shell of collected materials.

The magnetic field and magnetosphere of Plaskett’s star: a fundamental shift in our understanding of the system

ABSTRACT Plaskett’s ‘star’ appears to be one of a small number of short-period binary systems known to contain a hot, massive, magnetic star. We combine an extensive spectropolarimetric (Stokes V) data set with archival photometry and spectropolarimetry to establish the essential characteristics of the magnetic field and magnetosphere of the rapidly rotating, broad-line component of the system. We apply least-squares deconvolution (LSD) to infer the longitudinal magnetic field from each Stokes V spectrum. Using the time series of longitudinal field measurements, in combination with CoRoT photometry and equivalent width measurements of magnetospheric spectral lines, we infer the rotation period of the magnetic star to be equal to $1.21551^{+0.00028}_{-0.00034}$ d. Modelling the Stokes V LSD profiles with Zeeman–Doppler Imaging, we produce the first reliable magnetic map of an O-type star. We find a magnetic field that is predominantly dipolar, with an obliquity near 90° and a polar strength of about 850 G. We update the calculations of the theoretical magnetospheric parameters, and in agreement with their predictions we identify clear variability signatures of the H α, H β, and He ii λ4686 lines confirming the presence of a dense centrifugal magnetosphere surrounding the star. Finally, we report a lack of detection of radial velocity (RV) variations of the observed Stokes V profiles, suggesting that historical reports of the large RV variations of the broad-line star’s spectral lines may be spurious. This discovery may motivate a fundamental revision of the historical model of the Plaskett’s star as a near-equal mass O + O binary system.

Massive stars dying alone: the remote environment of supernova 2010jp and its associated late-time source

ABSTRACT We present late-time images of the site of the peculiar jet-driven Type IIn supernova (SN) 2010jp, including Hubble Space Telescope images taken 2–5 yr post-explosion and deep ground-based images over a similar time. These are used to characterize its unusually remote environment and to constrain the progenitor’s initial mass and age. The position of SN 2010jp is found to reside along a chain of diffuse starlight that is probably an outer spiral arm or tidal tail of the interacting galaxy pair NGC 2207/IC 2163. There is one bright H ii region projected within 1 kpc, and there is faint extended H α emission immediately surrounding the continuum source at the position of SN 2010jp, which has MF555W = −7.7 ± 0.2 mag. In principle, the lingering light could arise from late-time circumstellar material (CSM) interaction, an evolved supergiant, a host star cluster, or some combination of these. Steady flux over 3 yr and a lack of strong, spatially unresolved H α emission make ongoing CSM interaction unlikely. If an evolved supergiant dominates, its observed luminosity implies an initial mass $\lesssim\!{22}$ M⊙ and an age $\gtrsim\!{8}$ Myr. If the source is a star cluster, then its colour and absolute magnitude imply an age of 8–13 Myr and a modest cluster initial mass of log(M/M⊙) = 3.6–3.8. Extended H α emission out to a radius of ∼30 pc reveals a faint evolved H ii region, pointing to recent star formation with at least one late O-type star. Based on these various clues, we conclude that the progenitor of SN 2010jp had a likely initial mass of 18–22 M⊙.

LBV phenomenon and binarity: The environment of HR Car

Luminous blue variable stars (LBVs) are of great interest in massive-star evolution as they experience very high mass-loss episodes within short periods of time. HR Car is a famous member of this class in the Galaxy. It has a large circumstellar nebula and has also been confirmed as being in a binary system. One means of gaining information about the evolutionary status and physical nature of LBVs is studying their environments. We investigated the stellar content within ∼100 pc of HR Car and also its circumstellar nebula. Very Large Telescope Multi Unit Spectroscopic Explorer (MUSE) observations of a 2′×2′ region around the star highlight the incompleteness of stellar classification for stars with magnitudes of V > 13 mag. Eight B0 to B9 stars have been identified which may lie in close spatial vicinity to HR Car. For a region with a radius of r = 1.2° (∼100 pc at a distance of 4.8 kpc) around HR Car, existing catalogs list several late O-type and early B-type stars, but only one early O-type star. Given the relatively low stellar and nebular masses in the HR Car system, no early O-type stars and only a few late O-type stars would be expected in association with HR Car. Instead, HR Car’s location in a point vector diagram suggests that HR Car is not isolated, but is part of a moving group with a population of B-type stars in a spiral arm, and it has not received a strong kick from a supernova explosion of a companion star or a merger event. Potential binary evolution pathways for the HR Car system cannot be fully explored because of the unknown nature of the companion star. Furthermore, the MUSE observations reveal the presence of a fast outflow and “bullets” that have been ejected at intervals of about 400 years. These features may have been caused by recurrent mass transfer in the system.

MONOS: Multiplicity Of Northern O-type Spectroscopic systems

Context. Massive stars are a key element for understanding the chemical and dynamical evolution of galaxies. Stellar evolution is conditioned by many factors: Rotation, mass loss, and interaction with other objects are the most important ones for massive stars. During the first evolutionary stages of stars with initial masses (i.e., MZAMS) in the MZAMS ∼ 18–70 M⊙ range, they are of spectral type O. Given that stars in this mass range spend roughly 90% of their lifetime as O-type stars, establishing the multiplicity frequency and binary properties of O-type stars is crucial for many fields of modern astrophysics. Aims. The aim of the MONOS project is to collect information to study northern Galactic O-type spectroscopic binaries. In this second paper, we tackle the study of the 35 single-line spectroscopic binary (SB1) systems identified in the previous paper of the series, analyze our data, and review the literature on the orbits of the systems. Methods. We have measured ∼4500 radial velocities for a selection of diagnostic lines for the ∼700 spectra of the studied systems in our database, for which we have used two different methods: a Gaussian fit for several lines per object and cross-correlation with synthetic spectra computed with the FASTWIND stellar atmospheric code. We have also explored the photometric data delivered by the TESS mission to analyze the light curve (LC) of the systems, extracting 31 of them. We have explored the possible periods with the Lomb-Scargle method and, whenever possible, calculated the orbital solutions using the SBOP and GBART codes. For those systems in which an improved solution was possible, we merged our radial velocities with those in the literature and calculated a combined solution. Results. As a result of this work, of the 35 SB1 systems identified in our first paper we have confirmed 21 systems as SB1 with good orbits, discarded the binary nature of six stars (9 Sge, HD 192 281, HDE 229 232 AB, 68 Cyg, HD 108, and α Cam), and left six stars as inconclusive due to a lack of data. The remaining two stars are 15 Mon Aa, which has been classified as SB2, and Cyg OB2-22 C, for which we find evidence that it is most likely a triple system where the O star is orbiting an eclipsing SB1. We have also recalculated 20 new orbital solutions, including the first spectroscopic orbital solution for V747 Cep. For Cyg OB2-22 C, we have obtained new ephemerides but no new orbit.

Detecting the intrinsic X-ray emission from the O-type donor star and the residual accretion in a supergiant fast X-ray transient in its faintest state

We report on the results of an XMM–Newton observation of the supergiant fast X-ray transient (SFXT) IGR J08408-4503 performed in June 2020. The source is composed of a compact object (likely a neutron star) orbiting around an O8.5Ib-II(f)p star, LM Vel. The X-ray light curve shows a very low level of emission, punctuated by a single, faint flare. We analysed spectra measured during the flare and during quiescence. The quiescent state shows a continuum spectrum that is well deconvolved to three spectral models: two components are from a collisionally ionized plasma (with temperatures of kT1 = 0.24 keV and kT2 = 0.76 keV), together with a power-law model (photon index, Γ, of ∼2.55), dominating above ∼2 keV. The X-ray flux emitted at this lowest level is 3.2 × 10−13 erg cm−2 s−1 (0.5–10 keV, corrected for the interstellar absorption), implying an X-ray luminosity of 1.85 × 1032 erg s−1 (at 2.2 kpc). The two-temperature collisionally ionized plasma is intrinsic to the stellar wind of the donor star, while the power-law can be interpreted as emission due to residual, low-level accretion onto the compact object. The X-ray luminosity contributed by the power-law component only, in the lowest state, is (4.8 ± 1.4)×1031 erg s−1, which is the lowest quiescent luminosity detected from the compact object in an SFXT. Thanks to this very faint X-ray state caught by XMM–Newton, X-ray emission from the wind of the donor star LM Vel could be well-established and studied in detail for the first time, along with a very low level of accretion onto the compact object. The residual accretion rate onto the compact object in IGR J08408-4503 can be interpreted as the Bohm diffusion of (possibly magnetized) plasma entering the neutron star magnetosphere at low Bondi capture rates from the supergiant donor wind at the quasi-spherical, radiation-driven settling accretion stage.