arXiv subjects

Paul A. Crowther

Publications and source records attributed to Paul A. Crowther.

At least 19 recordsLinked to original sources

arXiv2026

Detection of new Galactic Wolf-Rayet stars with SPHEREx

We assess the potential for the NASA Explorer SPHEREx mission in adding to the Milky Way census of Wolf-Rayet (WR) stars by extracting 0.75--5micron spectrophotometry of 30 candidate WR stars previously identified using Gaia XP spectra. We confirm an additional Galactic 13 WR (8 WN and 5 WC) stars, identify 3 probable [WC]-type central stars of Planetary Nebulae, plus 2 He emission line sources. SPHEREx has the potential to detect additional relatively isolated WR stars on the far size of the Milky Way by combining Machine Learning techniques with a judicious choice of candidate selection criteria. Indeed SPHEREx is sufficiently sensitive to identify bright Magellanic Cloud Wolf-Rayet stars.

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arXiv2026

Binarity at LOw Metallicity (BLOeM): massive star variability revealed using a novel software tool for point-spread function fitting of TESS images

Massive stars, the progenitors of neutron stars and black holes, play a crucial role in shaping the chemical and radiative properties of entire galaxies through their winds and explosive deaths. Stellar pulsations are a common phenomenon in massive stars and asteroseismology -- the study of such pulsations -- provides crucial constraints on the physics of massive star interiors. The excitation of heat-driven pulsations in massive stars is expected to depend on a star's metallicity, but this remains largely uncalibrated in evolution models due to a lack of a sufficient observations. While TESS has dramatically improved the statistics for Galactic massive stars, obtaining TESS light curves for low-metallicity massive stars beyond the Milky Way is challenging, due to their faintness and heavy crowding. In this paper, we present a novel point-spread function (PSF) based light curve extraction method called {\sc Lemons}, which overcomes these challenges. We also demonstrate the limitations of the often-used simple aperture photometry (SAP) method that can provide heavily contaminated light curves. With this new technique, accurate light curves of 91 SMC massive stars in the BLOeM sample are extracted. They reveal a variety of variability types including indications of binarity (e.g. eclipses and ellipsoidal modulation) and stellar pulsations. They also enable us to investigate stochastic low-frequency (SLF) variability for massive stars in the SMC. Furthermore we demonstrate how the morphology of SLF variability probes a star's location in the Hertzsprung--Russell diagram, which appears similar to Galactic massive stars thus indicating that the underlying physical mechanism could be insensitive to metallicity.

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arXiv2026

Stellar winds of O-type stars traced by high ionization fine-structure emission lines with JWST/MIRI

We investigate the presence of high ionization fine-structure emission lines across a range of 22 OB-type stars observed with JWST-MIRI as part of calibration programmes and the WISCI and MEAD projects. MIR wind emission is detected in 4 late O-dwarfs (O8 V - O9 V), 1 early O-dwarf (O5 V) and there are tentative detections in an additional 3 stars (O8.5 II, O8.5 IV and O9 I). We measure the wind speeds and make estimates of lower limits on the mass-loss rates of 5 O-type stars from broad, flat-topped emission in the fine-structure line of [Ne V] 14.3micron. We find terminal wind speeds that are generally in agreement with empirical trends, but note that in some cases the wind speeds are surprisingly low. We highlight two main takeaways from this sample, which combine to establish an exciting new window into the winds of massive stars. First, a new diagnostic ability is gained from lines formed at much higher ionization, larger spatial extent, and longer wavelengths than typical wind diagnostics. Secondly, there is frequent incidence of MIR emission in O-type stars, even in the 'weak-wind' regime where wind emission is often not detected in the UV and optical.

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arXiv2025

Isolated massive star candidates in NGC 4242 with GULP

$\textit{Context.}$ There is considerable debate on how massive stars form, including whether a high-mass star must always form with a population of low-mass stars or whether it can also form in isolation. Massive stars found in the field are often considered to be runaways from star clusters or OB associations. However, there is evidence in the Milky Way and the Small Magellanic Cloud of high-mass stars that appear isolated in the field and cannot be related to any known star cluster or OB association. Studies of more distant galaxies have been lacking so far. $\textit{Aims.}$ In this work, we identified massive star candidates that appear isolated in the field of the nearby spiral galaxy NGC 4242 (distance: 5.3 Mpc), to explore how many candidates for isolated star formation we find in a galaxy outside the Local Group. $\textit{Methods.}$ We identified 234 massive ($M_{ini}\geq15M_{\odot}$) and young ($\leq 10$ Myr) field stars in NGC 4242 using the Hubble Space Telescope's Solar Blind Channel of the Advanced Camera for Surveys, the UVIS channel of the Wide Field Camera 3 from the Galaxy UV Legacy Project (GULP) and optical data from the Legacy ExtraGalactic UV Survey (LEGUS). We investigated the surroundings of our targets within the range of projected distances expected for runaway stars, $74$ pc and $204$ pc. $\textit{Results.}$ We find that between $9.8\%$ and $34.6\%$ of our targets have no young stellar groups or massive stars within the threshold radii, making them appear isolated. This fraction reduces to $3.2\%-11.5\%$ when we consider the total number of massive stars expected from the observed UV star formation rate. $\textit{Conclusions.}$ Our results show that there is a small population of young and massive, potentially isolated field stars in NGC 4242.

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arXiv2025

Binarity at LOw Metallicity (BLOeM): Pipeline-Determined Physical Properties of OB Stars

We aim to determine the physical properties of OB stars from the multi-epoch VLT/FLAMES BLOeM spectroscopic survey of the Small Magellanic Cloud. We apply a pipeline designed to analyse large spectroscopic samples of OB stars to the co-added, initial 9 epochs of the BLOeM survey, utilising grids of synthetic model spectra computed with the stellar atmosphere code FASTWIND. 69 OB stars are excluded from the analysis owing to disk emission or significant contamination by secondaries in SB2 binaries. We determine physical properties of 778 OB stars, including Teff, log g, log L/Lsun and v_e sin i. There appears to be a bimodality in v_e sin i of single O stars, while v_e sin i distributions of OB stars are strikingly different for single (median 78 km/s) and binary (median 200 km/s) systems. Inferred temperatures are broadly in agreement with literature results for stars in common, plus results from a grid-based automization tool for a subset of O and early B stars, although uncertainties are larger for surface gravities. Rotational velocities are broadly in line with an independent tool applied to the same subset. We recover the anticipated lower mass cutoff at 8 Msun from the survey design using a Bayesian inference method coupled with SMC metallicity evolutionary models, with median masses of 12.6 Msun (19.8 Msun) for B-type (O-type) stars. Spectroscopic masses exceed evolutionary masses, albeit with large uncertainties in surface gravities. We also provide an updated catalogue of O stars in the SMC since half of the 159 BLOeM O stars are newly classified as O-type stars.

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arXiv2025

The UV Legacy Library of Young Stars as Essential Standards (ULLYSES) Large Director's Discretionary Program with Hubble. I. Goals, Design, and Initial Results

Specifically selected to leverage the unique ultraviolet capabilities of the Hubble Space Telescope, the Hubble Ultraviolet Legacy Library of Young Stars as Essential Standards (ULLYSES) is a Director's Discretionary program of approximately 1000 orbits - the largest ever executed - that produced a UV spectroscopic library of O and B stars in nearby low metallicity galaxies and accreting low mass stars in the Milky Way. Observations from ULLYSES combined with archival spectra uniformly sample the fundamental astrophysical parameter space for each mass regime, including spectral type, luminosity class, and metallicity for massive stars, and the mass, age, and disk accretion rate for low-mass stars. The ULLYSES spectral library of massive stars will be critical to characterize how massive stars evolve at different metallicities; to advance our understanding of the production of ionizing photons, and thus of galaxy evolution and the re-ionization of the Universe; and to provide the templates necessary for the synthesis of integrated stellar populations. The massive star spectra are also transforming our understanding of the interstellar and circumgalactic media of low metallicity galaxies. On the low-mass end, UV spectra of T Tauri stars contain a plethora of diagnostics of accretion, winds, and the warm disk surface. These diagnostics are crucial for evaluating disk evolution and provide important input to assess atmospheric escape of planets and to interpret powerful probes of disk chemistry, as observed with ALMA and JWST. In this paper we motivate the design of the program, describe the observing strategy and target selection, and present initial results.

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arXiv2025

A new mass estimate method with hydrodynamical atmospheres for very massive WNh stars

Very massive stars with masses over 100 Msun are key objects in the Universe for our understanding of chemical and energetic feedback in the Universe, but their evolution and fate are almost entirely determined by their wind mass loss. We aim to determine the mass of the most massive star known in the Local Group R136a1. For this we compute the first hydrodynamically consistent non-local thermodynamical equilibrium atmosphere models for both R136a1 (WN5h) as well as the binary system R144 (WN5/6h+WN6/7h) in the Tarantula nebula. Using the Potsdam Wolf-Rayet code, we simultaneously empirically derive and theoretically predict mass-loss rates and wind velocities. By fitting synthetic spectra derived from these models to multi-wavelength observations, we constrain the stellar and wind properties of R144 and R136a1. We first determine the clumping stratification required by our hydro-models to fit the spectra of R144 by using the available dynamical mass estimates for the two components. We then utilise this clumping stratification in hydrodynamic models of R136a1 and estimate a mass of $M_\mathrm{Hydro}$ of 233 Msun. Remarkably, the estimated mass is close to and entirely consistent with chemical homogeneous mass relations. This present-day mass of 233 Msun provides a lower limit to the initial stellar mass, that could be far higher due to previous wind mass loss.

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arXiv2025

X-Shooting ULLYSES: massive stars at low metallicity: XII. The clumped winds of O-type (super)giants in the Large Magellanic Cloud

Mass loss governs the evolution of massive stars and shapes the stellar surroundings. To quantify the impact of the stellar winds we need to know the exact mass-loss rates; however, empirical constraints on the rates are hampered by limited knowledge of their small-scale wind structure or 'wind clumping'. We aim to improve empirical constraints on the mass loss of massive stars by investigating the clumping properties of their winds, in particular the relation between stellar parameters and wind structure. We analyse the optical and ultraviolet spectra of 25 O-type (super)giants in the LMC, using the model atmosphere code Fastwind and a genetic algorithm. We derive stellar and wind parameters including detailed clumping properties, such as the amount of clumping, the density of the interclump medium, velocity-porosity of the medium, and wind turbulence. We obtain stellar and wind parameters for 24 of our sample stars and find that the winds are highly clumped, with an average clumping factor of $f_{\rm cl}=33\pm14$, an interclump density factor of $f_{\rm ic}=0.2\pm0.1$, and moderate to strong velocity-porosity effects. The scatter around the average values of the wind-structure parameters is large. With the exception of a significant, positive correlation between the interclump density factor and mass loss, we find no dependence of clumping parameters on either mass-loss rate or stellar properties. In the luminosity range that we investigate, the empirical and theoretical mass-loss rates both have a scatter of about 0.5~dex, or a factor 3. Within this uncertainty, the empirical rates and the theoretical predictions agree. The origin of the scatter of the empirical mass-loss rates requires further investigation. It is possible that our description of wind clumping is still not sufficient to capture effects of the structured wind; this could contribute to the scatter.

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arXiv2023

Extinction towards the cluster R136 in the Large Magellanic Cloud: An extinction law from the near-infrared to the ultraviolet

The cluster R136 in the giant star-forming region 30 Doradus in the Large Magellanic Cloud (LMC) offers a unique opportunity to resolve a stellar population in a starburst-like environment. We obtain the near-infrared to ultraviolet extinction towards 50 stars in the core of R136, employing the `extinction without standards' method. To assure good fits over the full wavelength range, we combine and modify existing extinction laws. We detect a strong spatial gradient in the extinction properties across the core of R136, coinciding with a gradient in density of cold gas that is part of a molecular cloud lying northeast of the cluster. In line with previous measurements of R136 and the 30 Doradus region, we obtain a high total-to-relative extinction ($R_V = 4.38 \pm 0.87$). However, the high values of $R_V$ are accompanied by relatively strong extinction in the ultraviolet, contrary to what is observed for Galactic sightlines. The relatively strong ultraviolet extinction suggests that the properties of the dust towards R136 differ from those in the Milky Way. For $R_{V} \sim 4.4$, about three times fewer ultraviolet photons can escape from the ambient dust environment relative to the canonical Galactic value of $R_{V} \sim 3.1$ at the same $A_{V}$. Therefore, if dust in the R136 star-bursting environment is characteristic for cosmologically distant star-bursting regions, the escape fraction of ultraviolet photons from such regions is overestimated by a factor of three relative to the standard Milky Way assumption for the total-to-selective extinction. Furthermore, a comparison with average curves tailored to other regions of the LMC shows that large differences in ultraviolet extinction exist within this galaxy. Further investigation is required in order to decipher whether or not there is a relation between $R_V$ and ultraviolet extinction in the LMC.

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arXiv2023

A UV census of the environments of stripped-envelope supernovae

This paper reports an environmental analysis of 41 uniformly-selected stripped-envelope supernovae (SESNe) based on deep ultraviolet-optical images acquired by the Hubble Space Telescope. Young stellar populations are detected in most SN environments and their ages are derived with a hierarchical Bayesian approach. The age distributions are indistinguishable between Type IIb and Type Ib while that for Type Ic is systematically younger. This suggests that the Type Ic SN progenitors are more massive while the Type IIb and Type Ib SNe have very similar progenitor masses. Our result supports a hybrid envelope-stripping mechanism, in which the hydrogen envelopes of the SESN progenitors are stripped via a mass-insensitive process (e.g. binary interaction) while the helium envelopes are stripped via a mass-sensitive process (e.g. stellar wind of the post-binary interaction progenitor). We also provide progenitor constraints for three Type Ibn SNe and two broad-lined Type Ic SNe. All these results demonstrate the importance of the very diverse mass-loss processes in the origins of SESNe.

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arXiv2023

Line Luminosities of Galactic and Magellanic Cloud Wolf-Rayet stars

We provide line luminosities and spectroscopic templates of prominent optical emission lines of 133 Galactic Wolf-Rayet stars by exploiting Gaia DR3 parallaxes and optical spectrophotometry, and provide comparisons with 112 counterparts in the Magellanic Clouds. Average line luminosities of the broad blue (He II 4686, C III 4647,51, N III 4634,41, N V 4603,20) and yellow (C IV 5801,12) emission features for WN, WN/C, WC and WO stars have application in characterising the Wolf-Rayet populations of star-forming regions of distant, unresolved galaxies. Early-type WN stars reveal lower line luminosities in more metal poor environments, but the situation is less clear for late-type WN stars. LMC WC4-5 line luminosities are higher than their Milky Way counterparts, with line luminosities of Magellanic Cloud WO stars higher than Galactic stars. We highlight other prominent optical emission lines, N IV 3478,85 for WN and WN/C stars, O IV 3403,13 for WC and WO stars and O VI 3811,34 for WO stars. We apply our calibrations to representative metal-poor and metal-rich WR galaxies, IC 4870 and NGC 3049, respectively, with spectral templates also applied based on a realistic mix of subtypes. Finally, the global blue and C IV 5801,12 line luminosities of the Large (Small) Magellanic Clouds are 2.6e38 erg/s (9e36 erg/s) and 8.8e37 erg/s (4e36 erg/s), respectively, with the cumulative WR line luminosity of the Milky Way estimated to be an order of magnitude higher than the LMC.

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arXiv2023

Bringing Stellar Evolution & Feedback Together: Summary of proposals from the Lorentz Center Workshop, 2022

Stars strongly impact their environment, and shape structures on all scales throughout the universe, in a process known as ``feedback''. Due to the complexity of both stellar evolution and the physics of larger astrophysical structures, there remain many unanswered questions about how feedback operates, and what we can learn about stars by studying their imprint on the wider universe. In this white paper, we summarize discussions from the Lorentz Center meeting `Bringing Stellar Evolution and Feedback Together' in April 2022, and identify key areas where further dialogue can bring about radical changes in how we view the relationship between stars and the universe they live in.

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arXiv2022

The Tarantula Massive Binary Monitoring VI: Characterisation of hidden companions in 51 single-lined O-type binaries, a flat mass-ratio distribution, and black-hole binary candidates

We aim to hunt for massive binaries hosting a black hole companion (OB+BH) and establish the natal mass-ratio distribution of massive stars at the subsolar metallicity environment of the Large Magellanic Cloud (LMC). We use the shift-and-add grid disentangling technique to characterize the hidden companions in 51 SB1 O-type and evolved B-type binaries in the LMC monitored in the framework of the Tarantula Massive Binary Monitoring (TMBM). Out of the 51 SB1 systems, 43 (84%) are found to have non-degenerate stellar companions, of which 28 are confident detections, and 15 are less certain (SB1: or SB2:). Of these 43 targets, one is found to be a triple (VFTS 64), and two are found to be quadruples (VFTS 120, 702). The remaining eight targets (16%) retain an SB1 classification. Aside from the unambiguous case of VFTS 243, analysed in detailed in a separate paper, we identify two additional OB+BH candidates: VFTS 514 and VFTS 779. Additional black holes may be present in the sample but at a lower probability. Our study firmly establishes a virtually flat natal mass-ratio distribution for O-type stars at LMC metallicity, covering the entire mass-ratio range (0.05 < q < 1) and periods in the range 0 < log P < 3 [d]. The nature of the OB+BH candidates should be verified through future monitoring, but the frequency of OB+BH candidates is generally in line with recent predictions at LMC metallicity.

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arXiv2022

An X-ray quiet black hole born with a negligible kick in a massive binary within the Large Magellanic Cloud

Stellar-mass black holes are the final remnants of stars born with more than 15 solar masses. Billions are expected to reside in the Local Group, yet only few are known, mostly detected through X-rays emitted as they accrete material from a companion star. Here, we report on VFTS 243: a massive X-ray faint binary in the Large Magellanic Cloud. With an orbital period of 10.4-d, it comprises an O-type star of 25 solar masses and an unseen companion of at least nine solar masses. Our spectral analysis excludes a non-degenerate companion at a 5-sigma confidence level. The minimum companion mass implies that it is a black hole. No other X-ray quiet black hole is unambiguously known outside our Galaxy. The (near-)circular orbit and kinematics of VFTS 243 imply that the collapse of the progenitor into a black hole was associated with little or no ejected material or black-hole kick. Identifying such unique binaries substantially impacts the predicted rates of gravitational-wave detections and properties of core-collapse supernovae across the Cosmos.

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arXiv2022

A hot and luminous source at the site of the fast transient AT2018cow at 2-3 years after its explosion

We report the discovery of a luminous late-time source at the position of the fast blue optical transient (FBOT) AT2018cow on images taken by the Hubble Space Telescope (HST) at 714 d and 1136 d after its explosion. This source is detected at both UV and optical wavelengths and has prominent H$α$ emission. It has a very stable brightness between the two epochs and a very blue spectral energy distribution (SED) consistent with $f_λ$ $\propto$ $λ^{-4.1 \pm 0.1}$, i.e. the Rayleigh-Jeans tail of a hot blackbody with a very high temperature of log($T$/K) $>$ 4.6 and luminosity of log($L$/$L_\odot$) $>$ 7.0. This late-time source is unlikely to be an unrelated object in chance alignment, or due to a light echo of AT2018cow. Other possible scenarios also have some difficulties in explaining this late-time source, including companion star(s), star cluster, the survived progenitor star, interaction with circumstellar medium (CSM), magnetar, or delayed accretion in a tidal disruption event (TDE). Long-term and multi-wavelength monitoring will help to resolve its nature and finally reveal the origin of the "Cow".

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arXiv2022

The R136 star cluster dissected with Hubble Space Telescope/STIS. III. The most massive stars and their clumped winds

Context: The star cluster R136 inside the LMC hosts a rich population of massive stars, including the most massive stars known. The strong stellar winds of these very luminous stars impact their evolution and the surrounding environment. We currently lack detailed knowledge of the wind structure that is needed to quantify this impact. Aims: To observationally constrain the stellar and wind properties of the massive stars in R136, in particular the parameters related to wind clumping. Methods: We simultaneously analyse optical and UV spectroscopy of 53 O-type and 3 WNh-stars using the FASTWIND model atmosphere code and a genetic algorithm. The models account for optically thick clumps and effects related to porosity and velocity-porosity, as well as a non-void interclump medium. Results: We obtain stellar parameters, surface abundances, mass-loss rates, terminal velocities and clumping characteristics and compare these to theoretical predictions and evolutionary models. The clumping properties include the density of the interclump medium and the velocity-porosity of the wind. For the first time, these characteristics are systematically measured for a wide range of effective temperatures and luminosities. Conclusions: We confirm a cluster age of 1.0-2.5 Myr and derive an initial stellar mass of $\geq 250 {\rm M}_\odot$ for the most massive star in our sample, R136a1. The winds of our sample stars are highly clumped, with an average clumping factor of $f_{\rm cl}=29\pm15$. We find tentative trends in the wind-structure parameters as a function of mass-loss rate, suggesting that the winds of stars with higher mass-loss rates are less clumped. We compare several theoretical predictions to the observed mass-loss rates and terminal velocities and find that none satisfactorily reproduces both quantities. The prescription of Krtička & Kubát (2018) matches best the observed mass-loss rates.

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arXiv2022

Grids of stellar models with rotation VII: Models from 0.8 to 300 M$_\odot$ at super-solar metallicity (Z = 0.020)

We present a grid of stellar models at super-solar metallicity (Z = 0.020) extending the previous grids of Geneva models at solar and sub-solar metallicities. A metallicity of Z = 0.020 was chosen to match that of the inner Galactic disk. A modest increase of 43% (=0.02/0.014) in metallicity compared to solar models means that the models evolve similarly to solar models but with slightly larger mass loss. Mass loss limits the final total masses of the super-solar models to 35 M$_\odot$ even for stars with initial masses much larger than 100 M$_\odot$. Mass loss is strong enough in stars above 20 M$_\odot$ for rotating stars (25 M$_\odot$ for non-rotating stars) to remove the entire hydrogen-rich envelope. Our models thus predict SNII below 20 M$_\odot$ for rotating stars (25 M$_\odot$ for non-rotating stars) and SNIb (possibly SNIc) above that. We computed both isochrones and synthetic clusters to compare our super-solar models to the Westerlund 1 (Wd1) massive young cluster. A synthetic cluster combining rotating and non-rotating models with an age spread between log10 (age/yr) = 6.7 and 7.0 is able to reproduce qualitatively the observed populations of WR, RSG and YSG stars in Wd1, in particular their simultaneous presence at log10(L/L$_\odot$) = 5-5.5. The quantitative agreement is imperfect and we discuss the likely causes: synthetic cluster parameters, binary interactions, mass loss and their related uncertainties. In particular, mass loss in the cool part of the HRD plays a key role.

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arXiv2021

An environmental analysis of the Type Ib SN 2019yvr and the possible presence of an inflated binary companion

SN 2019yvr is the second Type Ib supernova (SN) with a possible direct detection of its progenitor (system); however, the spectral energy distribution (SED) of the pre-explosion source appears much cooler and overluminous than an expected helium-star progenitor. Using Hubble Space Telescope (HST) images and MUSE integral-field-unit (IFU) spectroscopy, we find the SN environment contains three episodes of star formation; the low ejecta mass suggests the SN progenitor is most likely from the oldest population, corresponding to an initial mass of 10.4$^{+1.5}_{-1.3}$ $M_\odot$. The pre-explosion SED can be reproduced by two components, one for the hot and compact SN progenitor and one for a cool and inflated yellow hypergiant (YHG) companion that dominates the brightness. Thus, SN 2019yvr could possibly be the first Type Ib/c SN for which the progenitor's binary companion is directly detected on pre-explosion images. Both the low progenitor mass and the YHG companion suggest significant binary interaction during their evolution. Similar to SN 2014C, SN 2019yvr exhibits a metamorphosis from Type Ib to Type IIn, showing signatures of interaction with hydrogen-rich circumstellar material (CSM) at >150 days; our result supports enhanced pre-SN mass loss as an important process for hydrogen-poor stars at the low mass end of core-collapse SN progenitors.

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