O Stars Research Articles

Extreme ionizing properties of a metal-poor, M_UV ~ -12 star complex in the first gigayear

We report the serendipitous discovery of a faint (M$_ UV > -12.2$), low-metallicity (Z $ odot $) ionizing source, dubbed T2c, with a spectroscopic redshift of z$=$6.146. T2c is part of a larger structure amplified by the Hubble Frontier Field galaxy cluster MACSJ0416 and was observed with the James Webb Space Telescope ( NIRSpec integral field unit. Stacking the short-wavelength NIRCam data reveals no stellar continuum detection down to a magnitude limit of UV However, prominent and emissions are detected, with equivalent widths exceeding $200$ and $1300$ respectively. The corresponding intrinsic (magnification-corrected $ 3$) ultraviolet and optical rest-frame magnitudes exceed 34.4 and 33.9 (corresponding to UV $ and M$_ opt $ fainter than -12.2 and -12.8 at $ rest and $ respectively), suggesting a stellar mass lower than a few $10^4$ under an instantaneous burst scenario. The inferred ionizing photon production efficiency ($ ion $) is high: $ ion assuming no dust attenuation and no Lyman continuum leakage. This indicates the presence of massive stars despite the low mass of the object. The very poor sampling of the initial mass function in such a low-mass star-forming complex suggests that the formation of very massive stars might be favored in very low-metallicity environments. T2c is surrounded by Balmer and weak oxygen emission on a spatial scale of a few hundred parsecs, after correcting for lensing effects. This system resembles a region potentially powered by currently undetected, extremely efficient, low-metallicity star complexes or clusters. We propose that massive O-type stars populate these low-mass, low-metallicity, high-redshift satellites, likely observed in an early and short formation phase, and contribute to the ionization of the surrounding medium.

A Study of Stochastic Low-frequency Variability for Galactic O-type Stars

In order to explore how the ubiquitous stochastic low-frequency (SLF) variability of O-type stars is related to various stellar characteristics, we compiled a sample of 150 O-type stars observed via ground-based spectroscopic surveys, alongside photometric data obtained from the Transiting Exoplanet Survey Satellite (TESS). We analyzed 298 light curves obtained from TESS Sectors 1–65 for the stars in our sample. Leveraging the spectroscopic parameters, we used Bonnsai to determine masses, radii, fractional main-sequence ages, and mass-loss rates for stars of our sample. Subsequently, we identified possible correlations between the fitted parameters of SLF variability and stellar properties. Our analysis unveiled four significant correlations between the amplitude and stellar parameters, including mass, radius, fractional main-sequence ages, and mass-loss rate. For stars with ≳30 M ⊙, we observed a decrease in characteristic frequency and steepness with increasing radius. Finally, we compared various physical processes that may account for the SLF variability with our results. The observed SLF variability may arise from the combined effects of the iron convection zone (FeCZ) and internal gravity waves (IGWs), with IGWs potentially more dominant in the early stages of stellar evolution, and the contribution of FeCZ becoming more significant as stars evolve. Meanwhile, our results indicate that the SLF variability of O-type stars bears certain signatures of the line-driven wind instability and granulation.

Constraining the overcontact phase in massive binary evolution. III. Period stability of known B+B and O+B overcontact systems

Binary systems play a crucial role in massive star evolution. Systems composed of B-type and O-type stars are of particular interest due to their potential to lead to very energetic phenomena or the merging of exotic compact objects. We aim to determine the orbital period variations of a sample of B+B and O+B massive overcontact binaries, with the primary objectives of characterizing the evolutionary timescales of these systems and addressing the existing discrepancy between observational data and theoretical predictions derived from population synthesis models. We used Period04 to analyze archival photometric data going back a century for a sample of seven binary systems to measure their orbital periods. We then determine the period variations using a linear fit. We find that the period variation timescales of five truly overcontact binary systems align with the nuclear timescale, in agreement with previous findings for more massive overcontact binaries. Additionally, we noticed a clear distinction between the five systems that had been unambiguously classified as overcontact systems and both SV Cen and VFTS 066, which seem to be evolving on thermal timescales and might be misclassified as overcontact systems. In the case of the five overcontact binaries, our results indicate a noticeable mismatch between the observational data and the theoretical predictions derived from population synthesis models. Furthermore, our results suggest that additional physical mechanisms must be investigated to compare the observed variations more thoroughly with theoretical predictions.

Observations of Extremely Metal-poor O Stars: Weak Winds and Constraints for Evolution Models

Metal-poor massive stars drive the evolution of low-mass galaxies, both locally and at high redshift. However, quantifying the feedback they impart to their local surroundings remains uncertain because models of stellar evolution, mass loss, and ionizing spectra are unconstrained by observations below 20% solar metallicity (Z ⊙). We present new Keck Cosmic Web Imager optical spectroscopy of three O-type stars in the nearby dwarf galaxies Leo P, Sextans A, and WLM, which have gas-phase oxygen abundances of 3%–14% Z ⊙. To characterize their fundamental stellar properties and radiation-driven winds, we fit PoWR atmosphere models to the optical spectra simultaneously with Hubble Space Telescope far-ultraviolet (FUV) spectra and multiwavelength photometry. We find that all three stars have effective temperatures consistent with their spectral types and surface gravities typical of main-sequence dwarf stars. Yet, the combination of those inferred parameters and luminosity for the two lower-Z stars is not reproduced by stellar evolution models, even those that include rotation or binary interactions. The scenario of multiple-star systems is difficult to reconcile with all available data, suggesting that these observations pose a challenge to current evolution models. We highlight the importance of validating the relationship between stellar mass, temperature, and luminosity at very low Z for accurate estimates of ionizing photon production and spectral hardness. Finally, all three stars’ FUV wind profiles reveal low mass-loss rates and terminal wind velocities in tension with expectations from widely adopted radiation-driven wind models. These results provide empirical benchmarks for future development of mass-loss and evolution models for metal-poor stellar populations.

Binarity at LOw Metallicity (BLOeM)

Surveys in the Milky Way and Large Magellanic Cloud have revealed that the majority of massive stars will interact with companions during their lives. However, knowledge of the binary properties of massive stars at low metallicity, and therefore in conditions approaching those of the Early Universe, remain sparse. We present the Binarity at LOw Metallicity (BLOeM) campaign, an ESO large programme designed to obtain 25 epochs of spectroscopy for 929 massive stars in the Small Magellanic Cloud, allowing us to probe multiplicity in the lowest-metallicity conditions to date (Z = 0.2 Z⊙). BLOeM will provide (i) the binary fraction, (ii) the orbital configurations of systems with periods of P ≲ 3 yr, (iii) dormant black-hole binary candidates (OB+BH), and (iv) a legacy database of physical parameters of massive stars at low metallicity. Main sequence (OB-type) and evolved (OBAF-type) massive stars are observed with the LR02 setup of the GIRAFFE instrument of the Very Large Telescope (3960–4570 Å resolving power R = 6200; typical signal-to-noise ratio(S/N) ≈70–100). This paper utilises the first nine epochs obtained over a three-month time period. We describe the survey and data reduction, perform a spectral classification of the stacked spectra, and construct a Hertzsprung-Russell diagram of the sample via spectral-type and photometric calibrations. Our detailed classification reveals that the sample covers spectral types from O4 to F5, spanning the effective temperature and luminosity ranges 6.5 ≲ Teff/kK ≲ 45 and 3.7 < log L/L⊙ < 6.1 and initial masses of 8 ≲ Mini ≲ 80 M⊙. The sample comprises 159 O-type stars, 331 early B-type (B0–3) dwarfs and giants (luminosity classes V–III), 303 early B-type supergiants (II–I), and 136 late-type BAF supergiants. At least 82 stars are OBe stars: 20 O-type and 62 B-type (13% and 11% of the respective samples). In addition, the sample includes 4 high-mass X-ray binaries, 3 stars resembling luminous blue variables, 2 bloated stripped-star candidates, 2 candidate magnetic stars, and 74 eclipsing binaries.

Empirical mass-loss rates and clumping properties of O-type stars in the Large Magellanic Cloud

Empirical mass-loss rates and clumping properties of O-type stars in the Large Magellanic Cloud

Unraveling the birthplaces of NGC 2070’s massive stars, tracked with MUSE and revealed with JWST

The formation of massive O-type stars cannot be simply explained as a scaled-up version of the accretion mechanisms observed in lower-mass stars. Understanding these processes necessitates systematic studies of their early stages, which are challenging to identify. Forming massive stars remain embedded in their dense nursery clouds, and IR instruments with high spatial resolution capabilities are needed to better observe them. Despite these challenges, MUSE optical observations of the massive cluster NGC 2070 successfully detected potential star-forming regions through spatially resolved electron density maps. To further explore these regions, the James Webb Space Telescope (JWST) utilized its NIRCam and MIRI instruments to penetrate optically obscured areas. This study examines two specific regions in the southeast part of the NGC 2070 MUSE density map, where tracks of highly dense point sources were identified. NIRCam, partially overlapped with MIRI, resolved these MUSE findings, revealing a procession of stellar point sources in the projected images. The detections are associated with elongated clouds, suggesting greater proper motions compared to the surrounding interstellar medium. These findings may indicate the presence of runaway candidates in the early stages of their evolution that are following common escape routes. This would support the notion that dynamical ejection is an efficient mechanism for the formation of massive runaway stars during early stages and likely plays a significant role in the origin of O-type field stars. However, additional data are required to confirm this scenario and rule out other ionizing feedback mechanisms, such as those observed in the formation of pillar-like structures around HII regions in the Milky Way. MUSE electron density mapping effectively captures the complexity of NGC 2070’s interstellar medium and highlights targets for subsequent spectroscopic follow-ups, as demonstrated by the JWST data in the two fields studied.

Unveiling very young O stars

Context. O-type stars are known to significantly contribute to both the dynamics and evolution of galaxies. Massive and luminous, they probably control and regulate the galaxies star formation rates. The stellar feedback generated by such cosmic beasts can strongly affect the local star formation rate, with effects in the current (and future) generations of low and intermediate mass stars, and possibly also in the disruption process of the galaxies’ giant gas reservoirs. Aims. For this work I performed a redetermination of the spectral types and effective temperatures of the Galactic O-type stars MSP182, MSP183, MSP199, VPHAS-01338, and VPHAS-01273. Methods. From a careful examination of the spectral features present in the blue optical spectral region, it was possible to identify several nitrogen lines usually only seen in the blue optical spectra of O2–O3 stars. From the nitrogen ionic equivalent width ratios measured in the spectra of MSP182, MSP183, MSP199, VPHAS-01338, and VPHAS-01273, and in those of standard stars of the O2–O4 spectral types, earlier spectral types and hotter effective temperature values were derived. Results. Two O2V((f*))z, together with three new O3 V stars are now firmly identified in the Westerlund 2 region. Besides RFS1 in NGC 3603, the O2 V stars found in Westerlund 2 are the only other exemplars known to date in the Milky Way. From the nitrogen equivalent width line ratios measured in the spectra of standard stars of the O2–O4 spectral types, linear relations between the N IVλ4058/N IIIλ4640 ratio and the effective temperature in the 47 000–51 000 K range were derived. Based on my spectroscopic analysis of the science targets and the use of a HRD, a mean heliocentric distance of 5 kpc to Westerlund 2 was computed, a result that is in line with the mean heliocentric distance of 5.3 ± 1.5 kpc obtained from the associated Gaia DR3 parallaxes and distances. Conclusions. The Westerlund 2 massive stars studied in this work probably share a common evolutionary process that might be representative of the evolutionary ages of a large fraction of the cluster’s O-type stellar population, which seems to be much younger than 1 Myr.

Runaway OB Stars in the Small Magellanic Cloud. III. Updated Kinematics and Insights into Dynamical versus Supernova Ejections

We use the kinematics of field OB stars to estimate the frequencies of runaway stars generated by the dynamical ejection scenario (DES), the binary supernova scenario (BSS), and the combined two-step mechanism. We update the proper motions for field OB and OBe stars in the Small Magellanic Cloud (SMC) using Gaia DR3. Our sample now contains 336 stars from the Runaways and Isolated O-Type Star Spectroscopic Survey of the SMC, and we update our algorithm to calculate more accurate velocities compared to those obtained previously from DR2. We find a decrease in median velocity from 39 to 29 km s−1, implying that the proper motions from our previous work were systematically overestimated. We present the velocity distribution for OBe stars and quantitatively compare it to those of noncompact binaries and high-mass X-ray binaries. We confirm that OBe stars appear to be dominated by the BSS and are likely post-SN binary systems, further supporting the mass-transfer model to explain the origin of their emission-line disks. In contrast, normal OB stars may show a bimodal velocity distribution, which may be expected from different processes that occur with dynamical ejections. The kinematics of fast-rotating OB stars are similar to those of normal OB stars rather than OBe stars, suggesting that the origin of their high vrsini is different from that of OBe stars. We update our model parameters describing the kinematic origins of the SMC field population, still confirming that for runaway stars, the DES mechanism dominates, and two-step ejections seem comparable in frequency to pure BSS ejections.

A comprehensive X-ray analysis of the massive O-type binary HD 93250 over two decades

Massive star winds are known to be responsible for X-ray emission arising from wind plasma heated by the strong shocks up to temperatures of 10$^6$--10$^7$ K in the case of colliding wind binaries. The investigation of X-ray emission from massive stars thus constitutes a valuable tool for identifying binaries, which is otherwise a difficult task using classical techniques. We investigated thermal and nonthermal X-ray emission from the massive O-type star HD\,93250 to unveil its binary orbital parameters independently. To meet our goal, we analyzed X-ray data obtained with European Photon Imaging Camera on board XMM-Newton spanning over sim 19 years. Additionally, we analyzed NuSTAR observations of HD\,93250 taken at various epochs. We determine the variability timescale of the X-ray emission to be 193.8pm 1.3\,d, in full agreement with the 194.3pm 0.4\,d period derived from the astrometric orbit. The X-ray spectrum of HD\,93250 is well explained by a three-temperature thermal plasma emission model with temperatures of 0.26, 1.0, and 3.3 keV. The resulting X-ray flux varies in compliance with the typical colliding wind emission from eccentric massive binaries where it enhances near periastron passage and decreases gradually close to apastron in proportion to the inverse of the binary separation. The periastron-to-apastron X-ray emission ratio points to an eccentricity range of 0.20-0.25, once again in agreement with the previously determined astrometric orbit. Finally, we do not detect any hard X-ray emission attributable to nonthermal emission above 10 keV. Given the derived plasma temperature, the strong phase-locked variability, and the significant over-luminosity in X-rays, we establish that the X-ray emission from HD\,93250 is dominated by the colliding-wind region. Our results lend support to the idea that X-ray time analysis of massive stars constitutes a relevant tool for investigating their multiplicity and for extracting relevant information on their basic orbital parameters ---such as period and eccentricity--- independently of any orbital solution derived from classical techniques.

2D unified atmosphere and wind simulations of O-type stars

Context. Massive and luminous O-type star (O star) atmospheres with winds have been studied primarily using one-dimensional (1D), spherically symmetric, and stationary models. However, observations and theory have suggested that O star atmospheres are highly structured, turbulent, and time-dependent. As such, when making comparisons to observations, present-day 1D modeling tools require the introduction of ad hoc quantities such as photospheric macro- and microturbulence, wind clumping, and other relevant properties. Aims. We present a series of multi-dimensional, time-dependent, radiation-hydrodynamical (RHD) simulations for O stars that encapsulate the deeper sub-surface envelope (down to T ~ 450 kK), as well as the supersonic line-driven wind outflow in one unified approach. Our overarching aim is to develop a framework that is free from the ad-hoc prescriptions that plague present-day 1D models. Here, we start with an analysis of a small set of such multi-dimensional simulations and then compare them to atmospheric structures predicted by their 1D counterparts. Methods. We performed time-dependent, two-dimensional (2D) simulations of O star atmospheres with winds using a flux-limiting RHD finite volume modelling technique. Opacities are computed using a hybrid approach combining tabulated Rosseland means with calculations (based on the Sobolev approximation) of the enhanced line opacities expected for supersonic flows. The initial conditions and comparison models were derived using similar procedures as those applied in standard 1D stationary model atmosphere with wind codes. Results. Structure starts appearing in our simulations just below the iron-opacity peak at ~200 kK. Local pockets of gas with radiative accelerations that exceed gravity then shoot up from these deep layers into the upper atmosphere, where they interact with the line-driven wind outflow initiated around or beyond the variable photosphere. This complex interplay creates large turbulent velocities in the photospheric layers of our simulations, on the order of ~30–100km s−1, with higher values for models with higher luminosity-to-mass ratios. This offers a generally good agreement with observations of large photospheric ‘macroturbulence’ in O stars. When compared to 1D models, the average structures in the 2D simulations display less envelope expansion and no sharp density-inversions, along with density and temperature profiles that are significantly less steep around the photosphere, and a strong anti-correlation between velocity and density in the supersonic wind. Although the wind initiation region is complex and highly variable in our simulations, our average mass-loss rates agree well with stationary wind models computed by means of full co-moving frame radiative transfer solutions. Conclusions. The different atmospheric structures found in 2D and 1D simulations are likely to affect the spectroscopic determination of fundamental stellar and wind parameters for O stars as well as the empirical derivation of their chemical abundance patterns. To qualitatively match the different density and temperature profiles seen in our multi-dimensional and 1D models, we need to add a modest amount of convective energy transport in the deep sub-surface layers and a large turbulent pressure around the photosphere to the 1D models.

The Measurement of Masses of OB-type Stars from LAMOST DR5

The measurements of masses and luminosities of massive stars play an important role in understanding the formation and evolution of their host galaxies. In this work, we present the measurement of masses and luminosities of 2946 OB-type stars, including 78 O-type stars and 2868 B-type stars, based on their stellar parameters (effective temperature, surface gravity, and metallicity) and the Padova and Trieste Stellar Evolution Code isochrones model. Our results show that the median mass and luminosity of the 2946 OB-type stars are 5.4 M ⊙ and log(L/L ⊙) = 3.2 with median relative errors of 21.4% and 71.1%, respectively. A good agreement between our results estimated by using our method and those derived by using the orbital motions of binary stars from the literature is found for some B-type stars. In addition, we also fit the mass–luminosity relation of B-type stars by using our derived mass and the luminosity from Gaia Data Release 3.

Hot bubbles of planetary nebulae with hydrogen-deficient winds. III. Formation and evolution in comparison with hydrogen-rich bubbles

We seek to understand the evolution of Wolf-Rayet central stars by comparing the diffuse X-ray emission from their wind-blown bubbles with that from their hydrogen-rich counterparts with predictions from hydrodynamical models. We simulate the dynamical evolution of heat-conducting wind-blown bubbles using our 1D radiation-hydrodynamics code NEBEL/CORONA . We use a post-AGB-model of 0.595 but allow for variations of its evolutionary timescale and wind power. We follow the evolution of the circumstellar structures for different post-AGB wind prescriptions: for O-type central stars and for Wolf-Rayet central stars where the wind is hydrogen-poor, more dense, and slower. We use the CHIANTI software to compute the X-ray properties of bubble models along the evolutionary paths. We explicitly allow for non-equilibrium ionisation of key chemical elements. A sample of 12 planetary nebulae with diffuse X-ray emission ---seven harbouring an O-type and five a Wolf-Rayet nucleus--- is used to test the bubble models. The properties of most hydrogen-rich bubbles (X-ray temperature, X-ray luminosity, size) and their central stars (photon and wind luminosity) are fairly well represented by bubble models of our 0.595 AGB remnant. The bubble evolution of Wolf-Rayet objects is different, thanks to the high radiation cooling of their carbon- and oxygen-rich winds. The bubble formation is delayed, and the early evolution is dominated by condensation instead of evaporation. Eventually, evaporation begins and leads to chemically stratified bubbles. The bubbles of the youngest Wolf-Rayet objects appear chemically uniform, and their X-ray properties can be explained by faster-evolving nuclei. The bubbles of the evolved Wolf-rayet objects have excessively low characteristic temperatures that cannot be explained by our modelling. The formation of nebulae with O-type nuclei follows mainly a single path, but the formation pathways leading to the Wolf-Rayet-type objects appear diverse. Bubbles with a pure Wolf-Rayet composition can exist for some time after their formation despite the presence of heat conduction.

A Morphokinematic Study of the Enigmatic Emission Nebula NGC 6164/5 Surrounding the Magnetic O-type Star HD 148937

HD 148937 is a peculiar massive star (Of?p) with a strong magnetic field (1 kG). The hourglass-shaped emission nebula NGC 6164/5 surrounds this star. This nebula is presumed to originate from episodic mass-loss events of the central O-type star, but the detailed formation mechanism is not yet well understood. Grasping its three-dimensional structure is essential to uncovering the origin of this nebula. Here we report the high-resolution multiobject spectroscopic observations of NGC 6164/5 using the GIRAFFE on the 8.2 m Very Large Telescope. Integrated intensity maps constructed from several spectral lines delineate well the overall shape of this nebula, such as the two bright lobes and the inner gas region. The position–velocity diagrams show that the two bright lobes are found to be redshifted and blueshifted, respectively, while the inner region has multiple layers. We consider a geometric model composed of a bilateral outflow harboring nitrogen-enriched knots and expanding inner shells. Its spectral features are then simulated by using a Monte Carlo radiative transfer technique for different sets of velocities. Some position–velocity diagrams from simulations are very similar to the observed ones. According to the model that best reproduces the observational data, the two bright lobes and the nitrogen-enriched knots are moving away from HD 148937 at about 120 km s−1. Their minimum kinematic age is estimated to be about 7500 yr. We discuss possible formation mechanisms of this nebula in the context of binary interaction.

NIR spectral classification of the companion in the gamma-ray binary HESS J1832−093 as an O6 V star

ABSTRACT HESS J1832−093 is a member of the rare class of gamma-ray binaries, as recently confirmed by the detection of orbitally modulated X-ray and gamma-ray emission with a period of ∼86 d. The spectral type of the massive companion star has been difficult to retrieve as there is no optical counterpart, but the system is coincident with a near-infrared source. Previous results have shown that the infrared counterpart is consistent with an O or B-type star, but a clear classification is still lacking. We observed the counterpart twice, in 2019 and 2021, with the X-Shooter spectrograph operating on the Very Large Telescope (VLT). The obtained spectra classify the counterpart as an O6 V-type star. We estimate a distance to the source of 6.7 ± 0.5 kpc, although this estimate can be severely affected by the high extinction towards the source. This new O6 V classification for the companion star in HESS J1832−093 provides further support to an apparent grouping around a given spectral type for all discovered gamma-ray binaries that contain an O-type star. This may be due to the interplay between the initial mass function and the wind momentum–luminosity relation.

Fast and Robust Mid-Air Gesture Typing for AR Headsets using 3D Trajectory Decoding.

We present a fast mid-air gesture keyboard for head-mounted optical see-through augmented reality (OST AR) that supports users in articulating word patterns by merely moving their own physical index finger in relation to a virtual keyboard plane without a need to indirectly control a visual 2D cursor on a keyboard plane. To realize this, we introduce a novel decoding method that directly translates users' three-dimensional fingertip gestural trajectories into their intended text. We evaluate the efficacy of the system in three studies that investigate various design aspects, such as immediate efficacy, accelerated learning, and whether it is possible to maintain performance without providing visual feedback. We find that the new 3D trajectory decoding design results in significant improvements in entry rates while maintaining low error rates. In addition, we demonstrate that users can maintain their performance even without fingertip and gesture trace visualization.

Galactic runaway O and Be stars found usingGaiaDR3

Context.A relevant fraction of massive stars are runaway stars. These stars move with a significant peculiar velocity with respect to their environment.Aims.We aim to discover and characterize the population of massive and early-type runaway stars in the GOSC and BeSS catalogs usingGaiaDR3 astrometric data.Methods.We present a two-dimensional method in the velocity space to discover runaway stars as those that deviate significantly from the velocity distribution of field stars. Field stars are considered to follow the Galactic rotation curve.Results.We found 106 O runaway stars, 42 of which were not previously identified as runaways. We found 69 Be runaway stars, 47 of which were not previously identified as runaways. The dispersion of runaway stars is a few times higher inZandbthan that of field stars. This is explained by the ejections they underwent when they became runaways. The percentage of runaways is 25.4% for O-type stars, and it is 5.2% for Be-type stars. In addition, we conducted simulations in three dimensions for our catalogs. They revealed that these percentages could increase to ∼30% and ∼6.7%, respectively. Our runaway stars include seven X-ray binaries and one gamma-ray binary. Moreover, we obtain velocity dispersions of ∼5 km s−1perpendicular to the Galactic plane for O- and Be-type field stars. These values increase in the Galactic plane to ∼7 km s−1for O-type stars due to uncertainties and to ∼9 km s−1for Be-type stars due to Galactic velocity diffusion.Conclusions.The excellentGaiaDR3 astrometric data have allowed us to identify a significant number of O-type and Be-type runaways in the GOSC and BeSS catalogs. The higher percentages and higher velocities found for O-type compared to Be-type runaways underline that the dynamical ejection scenario is more likely than the binary supernova scenario. Our results open the door to identifying new high-energy systems among our runaways by conducting detailed studies.

New Insights in the Bubble Wall of NGC 3324: Intertwined Substructures and a Bipolar Morphology Uncovered by JWST

We report the discovery of intertwined/entangled substructures toward the bubble wall of NGC 3324 below a physical scale of 4500 au, which is the sharp edge/ionization front/elongated structure traced at the interface between the H ii region and the molecular cloud. The sharp edge appears wavy in the Spitzer 3.6–8.0 μm images (resolution ∼2″). Star formation signatures have mostly been traced on one side of the ionization front, which lies on the molecular cloud’s boundary. The James Webb Space Telescope’s (JWST) near- and mid-infrared images (resolution ∼0.″07—0.″7) are employed to resolve the sharp edge, which has a curvature facing the exciting O-type stars. The elongated structures are associated with the 3.3 μm polycyclic aromatic hydrocarbon (PAH) emission, the 4.05 μm ionized emission, and the 4.693 μm H2 emission. However, the PAH-emitting structures are depicted between the other two. The H2 emission reveals numerous intertwined substructures that are not prominently traced in the 3.3 μm PAH emission. The separation between two substructures in the H2 emission is ∼1.″1 or 2420 au. The intertwined substructures are traced in the spatial areas associated with the neutral to H2 transition zone, suggesting the origin of these structures by “thin-shell” instability. Furthermore, an arc-like feature traced in the Spitzer 3.6–8.0 μm images is investigated as a bipolar H ii region (extent ∼0.35 pc) at T d ∼25–28 K using the JWST images. A massive-star candidate VPHAS-OB1 #03518 seems to be responsible for the bipolar H ii region.

The Chandra Cygnus OB2 Legacy Survey: Design and X-Ray Point-source Catalog

The Cygnus OB2 association is the largest concentration of young and massive stars within 2 kpc of the Sun, including an estimated ∼65 O-type stars and hundreds of OB stars. The Chandra Cygnus OB2 Legacy Survey is a large imaging program undertaken with the Advanced CCD Imaging Spectrometer on board the Chandra X-ray Observatory. The survey has imaged the central 0.5 deg2 of the Cyg OB2 association with an effective exposure of ∼120 ks and an outer 0.35 deg2 area with an exposure of ∼60 ks. Here we describe the survey design and observations, discuss the data reduction and source detection, and present a catalog of ∼8000 X-ray point sources. The survey design employs a grid of 36 heavily (∼50%) overlapping pointings, a method that overcomes Chandra's low off-axis sensitivity and produces a highly uniform exposure over the inner 0.5 deg2. The full X-ray catalog is described here and is made available online.

Stellar parameter estimation in O-type stars using artificial neural networks

This work presents the results of the implementation of a deep learning system capable of estimating the effective temperature and surface gravity of O-type stars. The proposed system was trained with a database of 5,557 synthetic spectra computed with the stellar atmosphere code CMFGEN that covers stars with Teff from ∼20,000 K to ∼58,000 K, log(L/L⊙) from 4.3 to 6.3 dex, logg from 2.4 to 4.2 dex, and mass from 9 to 120 M⊙. Important advantages proposed in this paper include using a set of equivalent width measurements over the optical region of the stellar spectra, which avoids processing the full spectra with the inherent computational cost and allows it to apply the same trained system over different spectra resolutions. The validation of the system was performed by processing a sample of twenty O-type stars taken from the IACOB database, and a subgroup of eleven stars of those twenty taken from The Galactic O-Star Spectroscopic Catalog (GOSC) with lower resolution. As complementary work, we show the results of a synthetic spectra fitting process with the aim of simplifying the comparison with other estimations and parameter fitting from the literature.