arXiv subjects

C. J. K. Larkin

Publications and source records attributed to C. J. K. Larkin.

9 recordsLinked to original sources

arXiv2026

Multi-Dimensional MHD simulations of young Core-Collapse Supernova Remnants

Supernova remnants (SNRs) play a central role in shaping the interstellar medium. Core-Collapse Supernova (CCSN) progenitors are massive stars, which produce a dense circumstellar medium (CSM) through intense mass loss in post-main sequence evolution. The subsequent CCSN produces a strong shock which expands into a highly structured, complex magnetised environment. Magnetohydrodynamic (MHD) consideration of pre- and post-CCSN evolution in multi-D are desirable to further our understanding of non-thermal aspects. We aim to determine how detailed stellar evolution treatment influences the shock propagation, focusing on Red Supergiants (RSGs) and Wolf-Rayet (WR) stars. We use the PION code to perform 3D MHD simulations of these CCSN progenitors. We use a detailed stellar evolution prescription to accurately and self-consistently model the pre-SN CSM and initialise CCSN explosions to investigate the surrounding environment. Our 2D and 3D treatment, inclusion of radiative cooling and assumption of full photoionization produces CSM features not identified in previous work. In the WR model we produce a coherent set of fast reflected shocks. In both cases we find faster forward shocks than predicted by analytic theory due to additional wind acceleration from photoionization for the RSG case, and accounting for the CSM expansion in the WR case. Model predictions of slowly rotating RSG and WR stars results in weakly magnetised wind bubbles, limiting potential for their SNRs to become PeV particle accelerators. Detailed multi-D MHD treatment of the CSM is needed to account for SNR evolution beyond the wind termination shock, where dynamic instabilities can be important. Including self-consistent stellar evolution is important for determining the CSM density and magnetic field structure close to the star, which govern the shock properties and SNR evolution for the first few hundred yr. (Abridged)

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arXiv2026

EWOCS-V: Is Wd1-72 a recent post-interaction WR+O binary?

The evolutionary origin of Wolf-Rayet (WR) stars at Solar metallicity is unclear. Single-star evolution from massive O stars, possibly via a Luminous Blue Variable phase, is challenged by binary period distributions of different WR subtypes. Wd1-72 is a WN7b+O binary embedded in the collective wind of the Galactic young massive cluster Westerlund 1 (Wd 1). It is surrounded by highly structured nebulosity, with cometary tails pointing away from Wd 1 and quasi-spherical droplets towards it. In this letter, we demonstrate that this morphology can be qualitatively reproduced by a hydrodynamic simulation of non-conservative Roche Lobe Overflow (RLOF) mass-loss into a cluster wind. Our model is based on a detailed binary evolution track consistent with key known properties of Wd1-72. Our work suggests Wd1-72 could be only ~10 kyr post-RLOF, and the hydrogen-free nature of Wd1-72 favours this being a second or subsequent RLOF episode. Follow-up observations could make Wd1-72 a valuable benchmark for probing mass-loss and mass-transfer in forming gravitational-wave binary-progenitor systems.

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arXiv2025

Investigating dusty Red Supergiant outflows in Westerlund 1 with 3D Hydrodynamic simulations

Recent JWST observations towards Westerlund 1 revealed extensive nebular emission associated with the cluster. Given the age of the region and proximity of that material to massive stars it cannot be primordial star forming gas and the origin is uncertain. We aim to determine whether the nebular emission in Westerlund 1 could be due to ablation flows from Red Supergiant (RSG) stars embedded in the cluster wind driven by the Wolf-Rayet stars in the cluster core. We also aim to explore the efficiency of mass-loading for the RSG wind in this scenario. We use 3D hydrodynamic simulations with the \textsc{pion} code to study the interaction between the cluster and RSG winds. We compare with the JWST observations by generating synthetic dust-emission maps. We find that the ablation flow morphology is consistent with the observations towards Westerlund 1, with clumps and instabilities. Synthetic observations at 11 $μ$m predict fluxes in the ablation flow of $\sim1000-6000$ MJy ster$^{-1}$ which is consistent with the unsaturated components of the JWST F1130W observations in the vicinity of the red supergiants in the region. This good agreement is achieved without any consideration of polycyclic aromatic hydrocarbons (PAHs), which have a known 11.3 $μ$m feature that appears in the F1130W band. This suggests that the ablation flow is PAH depleted. Ablation of RSG winds can explain the observed nebulosity in Westerlund 1, at least in the vicinity of the RSGs. Further observations are encouraged to enable detailed studies of these interactions.

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arXiv2025

Mass loading of outflows from evolving Young Massive Clusters

Feedback from Young Massive Clusters (YMCs) is an important driver of galaxy evolution. In the first few Myr, mechanical feedback is dominated by collective effects of the massive stellar winds in the YMC. The mass-loss rates and terminal wind velocities of these stars change by orders of magnitude over pre-SN timescales as the massive stars evolve, and mass-loss rates of Cool Supergiant (CSG) stars in particular are uncertain by a factor $\sim~20$ or more. In this work we perform a first study of the time evolution of average cluster wind velocity $\bar{V}_{\mathrm{cl}}$ as a function of stellar metallicity $Z$, assuming single star evolution. We also check the validity of assuming Wolf-Rayet stars dominate the feedback effects of a YMC, as often done when interpreting X-ray and $γ$-ray observations, and test how sensitive $\bar{V}_{\mathrm{cl}}$ is to current uncertainties in mass-loss rates. We use pySTARBURST99 to calculate integrated properties of YMCs for $Z$ in the range of $0.0004-0.02$, representing a range of environments from IZw18 to the Galactic Centre. We find that $\bar{V}_{\mathrm{cl}}$ drops off rapidly for sub-LMC $Z$, and we recommend a value of $500-1000\,~\textrm{km~s}^{-1}$ be used in this regime. We show accounting only for WR stars can overestimate $\bar{V}_{\mathrm{cl}}$ by $500-2000\,~\textrm{km~s}^{-1}$ at $Z \geq Z_\text{LMC}$. We also find that different RSG mass-loss assumptions can change the inferred $\bar{V}_{\mathrm{cl}}$ by $\sim1000\,~\textrm{km~s}^{-1}$, highlighting the need for improved observational constraints for RSGs in YMCs.

astro-ph.GA↗

arXiv2025

Deciphering the gamma-ray emission in the Cygnus region

The Cygnus region is a vast star-forming complex harbouring a population of powerful objects, including massive star clusters and associations, Wolf-Rayet stars, pulsars, and supernova remnants. The multi-wavelength picture is far from understood, in particular the recent LHAASO detection of multi-degree scale diffuse gamma-ray emission up to PeV energies. We aim to model the broadband gamma-ray data, discriminating plausible scenarios amongst all candidate accelerators. We consider in particular relic hadronic emission from a supernova remnant expanding in a low-density environment and inverse Compton emission from stellar-wind termination shocks in the Cygnus OB2 stellar association. We first estimate the maximum particle energy from a 3D hydrodynamical simulation of the supernova remnant scenario. The transport equation is then solved numerically to determine the radial distribution of non-thermal protons and electrons. In order to compute synthetic gamma-ray spectra and emission maps, we develop a 3D model of the gas distribution. This includes, firstly, a HI component with a low-density superbubble around Cygnus OB2 and, secondly, molecular clouds lying at the edge of the superbubble and in the foreground. We find that a powerful, ~50 kyr-old supernova remnant can account for both the morphology and spectrum from 10 TeV-PeV. At PeV energies, the microquasar Cygnus X-3 and diffuse Galactic cosmic rays might also contribute to the flux. Below about 10 TeV, hadronic models are incompatible with the expected existence of a superbubble centred on Cygnus OB2. Instead, the spectrum is well fitted with inverse Compton emission from electrons accelerated at stellar-wind termination shocks in Cygnus OB2 in line with existing multi-wavelength limits.

astro-ph.HE↗

arXiv2025

EWOCS-IV: 1Ms ACIS Chandra observation of the supergiant B[e] star Wd1-9

Supergiant B[e] (sgB[e]) stars are exceptionally rare objects, with only a handful of confirmed examples in the Milky Way. The evolutionary pathways leading to the sgB[e] phase remain largely debated, highlighting the need for additional observations. The sgB[e] star Wd1-9, located in the massive cluster Westerlund 1 (Wd1), is enshrouded in a dusty cocoon--likely the result of past eruptive activity--leaving its true nature enigmatic. We present the most detailed X-ray study of Wd1-9 to date, using X-rays that pierce through its cocoon with the aim to uncover its nature and evolutionary state. We utilize 36 Chandra observations of Wd1 from the 'Extended Westerlund 1 and 2 Open Clusters Survey' (EWOCS), plus eight archival datasets, totalling 1.1 Ms. This dataset allows investigation of long-term variability and periodicity in Wd1-9, while X-ray colours and spectra are analysed over time to uncover patterns that shed light on its nature. Wd1-9 exhibits significant long-term X-ray variability, within which we identify a strong 14-day periodic signal. We interpret this as the orbital period, marking the first period determination for the system. The X-ray spectrum of Wd1-9 is thermal and hard (kT approximately 3.0 keV), resembling the spectra of bright Wolf-Rayet (WR) binaries in Wd1, while a strong Fe emission line at 6.7 keV indicates hot plasma from a colliding-wind X-ray binary. Wd1-9, with evidence of past mass loss, circumbinary material, a hard X-ray spectrum, and a newly detected 14-day period, displays all the hallmarks of a binary--likely a WR+OB--that recently underwent early Case B mass transfer. Its sgB[e] classification is likely phenomenological reflecting emission from the dense circumbinary material. This places Wd1-9 in a rarely observed phase, possibly revealing a newly formed WN star, bridging the gap between immediate precursors and later evolutionary stages in Wd1.

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arXiv2024

X-Shooting ULLYSES: Massive stars at low metallicity. IV. Spectral analysis methods and exemplary results for O stars

CONTEXT: The spectral analysis of hot, massive stars is a fundamental astrophysical method to obtain their intrinsic properties and their feedback. Quantitative spectroscopy for hot, massive stars requires detailed numerical modeling of the atmosphere and an iterative treatment to obtain the best solution within a given framework. AIMS: We present an overview of different techniques for the quantitative spectroscopy of hot stars employed within the X-Shooting ULLYSES collaboration, from grid-based approaches to tailored fits. By performing a blind test, we gain an overview about the similarities and differences of the resulting parameters. Our study aims to provide an overview of the parameter spread caused by different approaches. METHODS: For three different stars from the sample (SMC O5 star AzV 377, LMC O7 star Sk -69 50, and LMC O9 star Sk -66 171), we employ different atmosphere codes (CMFGEN, Fastwind, PoWR) and strategies to determine their best-fitting model. For our analyses, UV and optical spectra are used to derive the properties with some methods relying purely on optical data for comparison. To determine the overall spectral energy distribution, we further employ additional photometry from the literature. RESULTS: Effective temperatures for each of three sample stars agree within 3 kK while the differences in log g can be up to 0.2 dex. Luminosity differences of up to 0.1 dex result from different reddening assumptions, which seem to be larger for the methods employing a genetic algorithm. All sample stars are nitrogen-enriched. CONCLUSIONS: We find a reasonable agreement between the different methods. Tailored fitting tends to be able to minimize discrepancies obtained with more course or automatized treatments. UV spectral data is essential for the determination of realistic wind parameters. For one target (Sk -69 50), we find clear indications of an evolved status.

astro-ph.SR↗

arXiv2024

X-Shooting ULLYSES: Massive Stars at low metallicity II. DR1: Advanced optical data products for the Magellanic Clouds

Using the medium resolution spectrograph X-shooter, spectra of 235 OB and Wolf-Rayet (WR) stars in sub-solar metallicity environments have been secured. [...]This second paper focuses on the optical observations of 232 Magellanic Clouds targets. It describes the uniform reduction of the UVB (300 - 560 nm) and VIS (550 - 1020 nm) XShootU data as well as the preparation of advanced data products [...] . The data reduction of the raw data is based on the ESO CPL X-shooter pipeline. We paid particular attention to the determination of the response curves [...] We implemented slit-loss correction, absolute flux calibration, (semi-)automatic rectification to the continuum, and a correction for telluric lines. The spectra of individual epochs were corrected for the barycentric motion, re-sampled and co-added, and the spectra from the two arms were merged into a single flux calibrated spectrum covering the entire optical range with maximum signal-to-noise ratio. [...] We provide three types of data products: (i) two-dimensional spectra for each UVB and VIS exposure; (ii) one-dimensional UVB and VIS spectra before and after response-correction, as well as after applying various processing, including absolute flux calibration, telluric removal, normalisation and barycentric correction; and (iii) co-added flux-calibrated and rectified spectra over the full optical range, for which all available XShootU exposures were combined. For many of the targets, the final signal-to-noise ratio per resolution element is above 200 in both the UVB and the VIS co-added spectra. The reduced data and advanced scientific data products will be made available to the community upon publication of this paper. [...]

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arXiv2023

X-Shooting ULLYSES: massive stars at low metallicity. I. Project Description

Observations of individual massive stars, super-luminous supernovae, gamma-ray bursts, and gravitational-wave events involving spectacular black-hole mergers, indicate that the low-metallicity Universe is fundamentally different from our own Galaxy. Many transient phenomena will remain enigmatic until we achieve a firm understanding of the physics and evolution of massive stars at low metallicity (Z). The Hubble Space Telescope has devoted 500 orbits to observe 250 massive stars at low Z in the ultraviolet (UV) with the COS and STIS spectrographs under the ULLYSES program. The complementary ``X-Shooting ULLYSES'' (XShootU) project provides enhanced legacy value with high-quality optical and near-infrared spectra obtained with the wide-wavelength coverage X-shooter spectrograph at ESO's Very Large Telescope. We present an overview of the XShootU project, showing that combining ULLYSES UV and XShootU optical spectra is critical for the uniform determination of stellar parameters such as effective temperature, surface gravity, luminosity, and abundances, as well as wind properties such as mass-loss rates in function of Z. As uncertainties in stellar and wind parameters percolate into many adjacent areas of Astrophysics, the data and modelling of the XShootU project is expected to be a game-changer for our physical understanding of massive stars at low Z. To be able to confidently interpret James Webb Space Telescope (JWST) spectra of the first stellar generations, the individual spectra of low Z stars need to be understood, which is exactly where XShootU can deliver.

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