arXiv subjects

Varsha Ramachandran

Publications and source records attributed to Varsha Ramachandran.

At least 19 recordsLinked to original sources

arXiv2026

The Treasury of Extremely Metal-Poor O Stars

The Treasury of Extremely Metal-Poor O Stars (TEMPOS) is a Hubble Space Telescope survey of hot and massive O-type stars in nearby, low-metallicity galaxies ($\lesssim$20% of the solar metallicity, $Z_\odot$). Understanding massive-star physics in this regime is essential to interpret observations of metal-poor galaxies, including both low-mass dwarf galaxies and chemically unevolved galaxies in the early Universe. Yet, few far-ultraviolet (FUV) spectra of O stars of sufficient quality to characterize their fundamental properties and stellar winds exist below 20% $Z_\odot$, and heterogeneous observation design and incomplete coverage of parameter space pose significant barriers to progress. To remedy this, TEMPOS obtained new Cosmic Origins Spectrograph (COS) FUV spectra of 12 very metal-poor O stars, building upon archival data to assemble a spectroscopic atlas of 29 homogeneously observed stars that efficiently samples a wide range of spectral types and luminosity classes. Here, we describe the motivation, sample selection, and observation design for TEMPOS and present the first data release of reduced and coadded COS spectra. We then present initial results on the empirical properties of FUV O-star spectra below 20% $Z_\odot$, including radial velocities, equivalent widths of photospheric lines, and terminal wind velocities ($v_\infty$). We show that $v_\infty$ correlates with host galaxy metallicity across $\sim$5-50% $Z_\odot$ and find tentative evidence of a steeper decline in wind strength below $\sim$10% $Z_\odot$. The combined dataset of FUV spectra and planned releases of photometry and optical spectra from the TEMPOS Treasury program will advance our understanding of both stellar astrophysics and the interstellar medium in the extremely metal-poor regime.

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arXiv2026

Ultraviolet spectroscopy reveals a hot and luminous companion to the Be star+black hole candidate MWC 656

The Galactic Be star binary MWC 656 was long considered the only known Be star+black hole (BH) system, making it a critical benchmark for models of massive binary evolution and for the expected X-ray emission of Be+BH binaries. However, recent dynamical measurements cast doubt on the presence of a BH companion. We present new multi-epoch ultraviolet spectroscopy from the Hubble Space Telescope, combined with high-resolution optical spectra, to reassess the nature of the companion. The far-ultraviolet spectra reveal high-ionisation features -- including prominent N v and He ii lines -- which are absent in the spectra of normal Be stars and are indicative of a hot, luminous companion. Spectral modelling shows that these features cannot originate from the Be star or from an accretion disc around a compact object. Instead, we find that the data are best explained by a hot ($T_\mathrm{eff} \approx 85$ kK), compact, hydrogen-deficient star with strong wind signatures, consistent with an intermediate-mass stripped star. Our revised orbital solution and composite spectroscopic modelling yield a companion mass of $M_2 = 1.48^{+0.55}_{-0.46}\,\mathrm{M}_\odot$, definitively ruling out a BH and disfavouring a white dwarf. MWC 656 thus joins the growing class of Be+stripped star binaries. The system's unusual properties -- including a high companion temperature and wind strength -- extend the known parameter space of such binaries. The continued absence of confirmed OBe+BH binaries in the Galaxy highlights a growing tension with population synthesis models.

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arXiv2026

The winds of OBA hypergiants and luminous blue variables: Dynamically-consistent atmosphere models reveal multiple wind regimes

OBA hypergiants (OBAHGs) are evolved massive stars with notable wind features in their optical spectrum. Located at the cool edge of the line-driven wind regime, many are candidate luminous blue variables (LBVs) likely near the Eddington limit. Although brief, this evolutionary stage deeply affects their surroundings and subsequent evolution. We study the mechanisms behind OBAHG winds and spectra, covering the temperature range of non-eruptive LBVs. Using the PoWR atmosphere code, we compute models with an Eddington parameter Gamma_e ~ 0.4 and moderate turbulent pressure, typical for cool hypergiants, varying the effective temperature from ~12.5 to ~38.0 kK at solar metallicity. Our models show a complex temperature-dependent mass-loss pattern, with regions of higher/lower rates linked to two wind solutions: "dense" and "rarefied." Spectra of known OBAHGs and LBVs match models from all solution regions. We find bi-stability jumps -- with sharp mass-loss increases -- at temperatures where Fe IV recombines to Fe III (and Fe III to Fe II). "Drops" in mass loss also occur when the leading Fe ion changes at wind onset, signaling a switch to rarefied solutions under insufficient driving opacity. The resulting velocity fields also reflect these different regimes: rarefied solutions match the empirical terminal velocity vs temperature relation, while dense ones deviate. Turbulent pressure is crucial for wind acceleration at cooler temperatures. We demonstrate that the bi-stability jumps exist in OBAHGs but are part of a broader complex behavior not replicated by current mass-loss recipes. Combining our and other recent results, we suggest that the switch between rarefied and dense solutions only occurs within a certain proximity to the Eddington Limit. Testing this requires future models with broader parameters and advanced treatments of radiatively-driven turbulence.

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arXiv2026

Dynamically consistent analysis of Galactic WN4b stars

Many Wolf-Rayet (WR) stars have optically thick winds that cloak the hydrostatic layers of the underlying star. In these cases, traditional spectral analysis methods are plagued by degeneracies that make it difficult to constrain parameters such as the stellar radius and the deeper density and velocity structure of the atmosphere. Focussing on the regime of nitrogen-rich WN4-stars with strong emission lines, we employ hydrodynamically-consistent modelling using the PoWR-HD code branch to perform a next generation spectral analysis. The inherent coupling of the stellar and wind parameters enables us to break parameter degeneracies, constrain the wind structure, and get a mass estimate. With this information, we can draw evolutionary implications and test current mass-loss descriptions for WR stars. We selected a sample of six Galactic WN4b stars. Applying updated parallaxes from Gaia DR3 and calculating PoWR-HD models that sufficiently resemble most of their spectral appearance, we obtain new values for the stellar and wind parameters of the WN4b sample. We compare our results to previous studies employing grid models with a beta = 1 velocity structure and cross-check our derived parameters with stellar structure predictions from GENEC and FRANEC evolution tracks. For all six targets, we obtain a narrow range of stellar temperatures T~140 kK, in contrast to previous grid-model analyses. We confirm the existence of WRs with luminosities as low as log L/Lsol = 5.0 and M~5 Msol. All derived velocity fields include a plateau feature at ~85% of the terminal velocity. Both the distance updates and the switch to dynamically-consistent atmospheres lead to substantial parameter adjustments compared to earlier grid-based studies. A comparison of the derived mass-loss rates favours a different description for the WN4b sample than for WN2 stars analysed with the same methodology.

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arXiv2025

Metal-poor single Wolf-Rayet stars: The interplay of optically thick winds and rotation

The Small Magellanic Cloud (SMC) hosts 12 known Wolf-Rayet (WR) stars, seven of which are apparently single. Their formation is a challenge for current stellar evolution models because line-driven winds are generally assumed to be quenched at a metallicity of Z < 0.004. Here, we present a set of mesa models of single stars with zero-age main sequence masses of 20 - 80 Msun considering different initial rotation speeds (Ω = 0 - 0.7 Ω_c), metallicities (Z = 0.002 - 0.0045), and wind mass-loss models (optically thin and thick winds). We show that if we account for optically thick winds, fast rotating (Ω = 0.6 Ω_c) single metal-poor O-type stars (with M > 20 Msun) shed their envelope and become WR stars even at the low metallicity of the SMC. The luminosity, effective temperature, evolutionary timescale, surface abundance, and rotational velocity of our simulated WR stars are compatible to the WRs observed in the SMC. We speculate that this scenario can also alleviate the excess of giant stars across the Humphreys-Davidson limit. Our results have key implications for black hole masses, (pair instability) supernova explosions, and other observable signatures.

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arXiv2025

Characterizing the Low-Mass Pre-Main-Sequence Population in the Low-Metallicity Star-Forming Region Dolidze 25 Using VLT-MUSE

The metallicity of the star-forming environment is a fundamental parameter shaping the evolution of protoplanetary disks and the formation of planetary systems, yet its influence remains poorly constrained. We present a spectroscopic study of low-mass pre-main sequence (PMS) stars ($M < 1 \, M_\odot$) in the exceptionally metal-poor cluster Dolidze~25 ($Z \approx 0.2 \, Z_\odot$), using VLT/MUSE observations to probe accretion processes and disk evolution in a subsolar environment. We identify 132 cluster members using a combination of \textit{Gaia} astrometry and spectroscopic youth indicators, including lithium absorption and Balmer emission. The stellar parameters are derived using low-metallicity BT-Settl models yielding effective temperatures, extinctions, luminosities enabling robust estimates of stellar masses and ages. Mass accretion rates ($\dot{M}_\mathrm{acc}$) derived from H$α$ emission span $10^{-10}$--$10^{-8} \, M_\odot\,\mathrm{yr}^{-1}$ with a median value of $8 \times 10^{-10}\,M_\odot\,\mathrm{yr}^{-1}$. These rates are comparable to those in solar-metallicity regions of similar age, such as Lupus and Orion, indicating minimal metallicity dependence in accretion processes. Our analysis shows that using solar-metallicity templates to fit low-metallicity stars leads to systematic overestimations of $T_\mathrm{eff}$ (by approximately $300\,\mathrm{K}$) and $A_V$ (by around $0.5\,\mathrm{mag}$), underscoring the importance of employing metallicity-matched models for reliable characterization in low-$Z$ environments. We present flux-calibrated, extinction-corrected spectra of these metal-poor PMS stars as a valuable resource for future investigations of disk evolution in subsolar regimes.

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arXiv2025

Discovery of a new transitional type of evolved massive stars with hard ionizing flux

Wolf-Rayet (WR) stars are the evolved descendants of the most massive stars and show emission-line dominated spectra formed in their powerful stellar winds. Marking the final evolution stage before core collapse, the standard picture of WR stars has been that they evolve through three well-defined spectral subtypes known as WN, WC, and WO. Here, we present a detailed analysis of five objects that defy this scheme, demonstrating that WR stars can also evolve directly from the WN to the WO stage. Our study reveals that this direct transition is connected to low metallicity and weaker winds. The WN/WO stars and their immediate WN precursors are hot and emit a high flux of photons capable of fully ionizing helium. The existence of these stages unveil that high mass stars which manage to shed off their outer hydrogen layers in a low-metallicity environment can spend a considerable fraction of their lifetime in a stage that is difficult to detect in integrated stellar populations, but at the same time yields hard ionizing flux. The identification of the WN to WO evolution path for massive stars has significant implications for understanding the chemical enrichment and ionizing feedback in star-forming galaxies, in particular at earlier cosmic times.

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arXiv2025

Minimum and maximum mass-luminosity relations for stripped stars

Envelope stripping, whether through single-star wind mass loss or binary mass transfer, is a key evolutionary pathway for the formation of classical Wolf-Rayet stars and lower-mass stripped helium (He) stars. However, to study the evolution of these objects into black holes, neutron stars, and stripped-envelope supernovae, we need appropriate input models for the core-He burning phase without relying on the uncertain evolution into this evolved phase. Reliable mass-luminosity relations (MLRs) for He stars are needed for stellar wind and evolution studies, but the MLRs currently in literature are either for fully-stripped or chemically homogeneous stars, neither of which reflect the important and recently also observationally confirmed stage of partial stripping. We alleviate this drawback by computing sets of MESA synthetic structure models with partially-stripped chemical profiles, consisting of a pure-He core and a hydrogen (H)-depleted envelope with an H/He chemical gradient left behind from the receding convective core during the main sequence. As the H slope increases from 0 (full chemical homogeneity) to $\infty$ (pure-He stars) in our synthetic models, we find the luminosity to initially increase before eventually decreasing. The maximum luminosity for a given mass is reached for an intermediate H-profile slope corresponding to a partially-stripped structure, exceeding even the values documented for pure-He stars, primarily due to the H shell disproportionately dominating the total luminosity budget. We also provide convenient mass-luminosity fit relations to predict the minimum, maximum, and pure-He luminosities for a given mass -- and vice versa -- while accounting for structures achievable through partial stripping. We also explore the impact of the higher luminosity on the wind properties of partially-stripped configurations using hydrodynamically consistent atmosphere models.

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arXiv2025

First JWST/NIRSpec Spectroscopy of O Stars in the Small Magellanic Cloud

Determining how much mass is removed by stellar winds is crucial to understanding massive star evolution and feedback. However, traditional spectroscopic diagnostics in the UV and optical are not sensitive enough to characterize weak stellar winds of OB stars in low-metallicity environments. A new tool to access weak stellar winds is provided by spectroscopy in the infrared (IR). Stellar atmosphere models indicate that the hydrogen Br$α$ line at $λ$\,4.05\,$μ$m is a useful mass-loss rate indicator, particularly at low metallicity. The unprecedented capabilities of the NIRSpec spectrograph on board of the \emph{James Webb Space Telescope} (JWST) allow us to measure this line in spectra of massive stars in other galaxies. In this work, we present the first NIRSpec spectra of O-type stars in the Small Magellanic Cloud (SMC), which has a metallicity of only 20\% Solar. Our sample consists of thirteen stars with spectral types ranging from O2 to O9.5 including supergiants, giants, and dwarfs. The stars belong to NGC\,346, the most massive young cluster in the SMC. We describe the observing strategy and data reduction, highlighting the treatment of the nebular background emission. The spectra cover the 2.8--5.1 $μ$m wavelength range, and we detect the Br$α$ line in emission in each of our sample stars. Using a combination of spectral and photometric data ranging from the UV to the IR, we improve the measurements of stellar luminosity and reddening. A first qualitative comparison of the observed Br$α$ line with stellar atmosphere models shows its potential as a wind diagnostic for weak-winded stars.

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arXiv2025

Exploring the connection between atmosphere models and evolution models of very massive stars

Very massive stars (VMS) dominate the light of young stellar populations and are sources of intense stellar feedback. Their evolution is mainly driven by strong wind mass loss, yet current evolution models make simplistic assumptions on their atmospheric physics which are incompatible with the nature of VMS. In this work, we aim to understand VMS atmospheres throughout their evolution by supplementing structure models (computed with GENEC) with detailed atmosphere models (computed with PoWR) capable of capturing the physics of a radially-expanding medium in non-LTE. An important aspect is the computation of atmosphere models reaching into deeper layers of the star, notably including the iron-opacity peak as an important source of radiative driving. In this study, we compute atmosphere models at 16 snapshots along the main sequence of a 150 $M_\odot$ star. For each snapshot, we compute two atmosphere models connected to the underlying structure model at different depths (below and above the hot iron bump). We perform a detailed spectroscopic and structural comparison of the two sequences of model atmospheres, and present a generalized method for the correction of the effective temperature in evolution models with strong winds. The choice of connection point between structure and atmosphere models has a severe influence on the predicted spectral appearance, which constitutes a previously unexplored source of uncertainty in quantitative spectroscopy. The simplified atmosphere treatment of current stellar structure codes likely leads to an overestimation of the spatial extension of very massive stars, caused by opacity-induced sub-surface inflation. This inflation does not occur in our deep atmosphere models, resulting in a discrepancy in predicted effective temperatures of up to 20 kK. Future improvements with turbulence and dynamically-consistent models may resolve these discrepancies.

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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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arXiv2024

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 ($Z_\odot$). We present new Keck Cosmic Web Imager optical spectroscopy of three O stars in the nearby dwarf galaxies Leo P, Sextans A, and WLM, which have gas-phase oxygen abundances of 3-14% $Z_\odot$. 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 multi-wavelength 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.

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arXiv2024

Hydrodynamic simulation of Cygnus OB2: the absence of a cluster wind termination shock

We perform a large-scale hydrodynamic simulation of a massive star cluster whose stellar population mimics that of the Cygnus OB2 association. The main-sequence stars are first simulated during 1.6 Myr, until a quasi-stationary state is reached. At this time the three Wolf-Rayet stars observed in Cygnus OB2 are added to the simulation, which continues to 2 Myr. Using a high-resolution grid in the centre of the domain, we can resolve the most massive stars individually, which allows us to probe the kinetic structures at small (parsec) scales. We find that, although the cluster excavates a spherical "superbubble" cavity, the stellar population is too loosely distributed to blow a large-scale cluster wind termination shock, and that collective effects from wind-wind interactions are much less efficient than usually assumed. This challenges our understanding of the ultra-high energy emission observed from the region.

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arXiv2023

Clumping and X-Rays in cooler B supergiant stars

B supergiants (BSGs) are evolved stars with effective temperatures between 10 to 30 kK and are important to understand massive star evolution. Located on the edge of the line-driven wind regime, the study of their atmospheres is helpful to understand phenomena such as the bi-stability jump. Key UV features of their spectra have so far not been reproduced by models for types later than B1. Here, we aim to remedy this situation via spectral analysis that accounts for wind clumping and X-rays. In addition, we investigate the evolutionary status of our sample stars based on the obtained stellar parameters. We determined parameters via quantitative spectroscopy using CMFGEN and PoWR codes. The models were compared to UV and optical data of four BSGs: HD206165, HD198478, HD53138, and HD164353. We also study the evolutionary status of our sample using GENEC and MESA tracks. When including clumping and X-rays, we find good agreements between synthetic and observed spectra for our sample stars. For the first time, we reproduced key lines in the UV. For that, we require a moderately clumped wind (f_infty > ~0.5). We also infer relative X-ray luminosities of ~10^-7.5 to 10^-8 -- lower than the typical ratio of 10^-7. Moreover, we find a possible mismatch between evolutionary and spectroscopic masses, which could be related to the mass-discrepancy problem present in other OB stars. Our results provide evidence that X-rays and clumping are needed to describe the winds of cool BSGs. However, their winds seem less structured than in earlier type stars. This aligns with observational X-rays and clumping constraints as well as recent hydrodynamical simulations. The BSGs' evolutionary status appears diverse: some objects are potentially post-red supergiants or merger products. The wind parameters provide evidence for a moderate mass-loss rate increase around the bi-stability jump. Abstract abridged

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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

Winds of OB stars: impact of metallicity, rotation and binary interaction

Winds of massive stars are an important ingredient in determining their evolution, final remnant mass, and feedback to the surrounding interstellar medium. We compare empirical results for OB star winds at low metallicity with theoretical predictions. Observations suggest very weak winds at SMC metallicity, but there are exceptions. We identified promising candidates for rotationally enhanced mass-loss rates with two component wind and partially stripped stars hiding among OB stars with slow but dense wind in the SMC. A preliminary analysis of these systems, derived parameters, and their implications are discussed. Finally, we briefly discuss the interaction of OB winds near black holes in X-ray binaries.

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arXiv2022

Massive star feedback in the Magellanic Clouds and the tidal Bridge

Massive stars have far-reaching feedback effects that alter the surrounding environment on local, global, and cosmic scales. Spectral analyses of massive stars with adequate stellar-atmosphere models are important to study massive star feedback in detail. We discuss the most recent UV and optical studies of massive metal-poor stars, including those with metallicities ranging from half to one twentieth of solar, connected with large-scale ISM structures in the Magellanic Clouds and the tidal Magellanic Bridge. We present ionizing fluxes from massive stars with low metallicity along with mechanical energy, and we further compare these to the observed energetics in the ISM. The results give hints on the leakage of hot gas and ionizing photons in the Magellanic Clouds. The paper outlines feedback from individual massive stars to population-level collective feedback, the significance of various feedback mechanisms (radiation, wind, supernova), and the influence by the physical conditions of the ISM

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arXiv2021

The excess of cool supergiants from contemporary stellar evolution models defies the metallicity-independent Humphreys-Davidson limit

The Humphreys-Davidson (HD) limit empirically defines a region of high luminosities (log L > 5.5) and low effective temperatures (T < 20kK) on the Hertzsprung-Russell Diagram in which hardly any supergiant stars are observed. Attempts to explain this limit through instabilities arising in near- or super-Eddington winds have been largely unsuccessful. Using modern stellar evolution we aim to re-examine the HD limit, investigating the impact of enhanced mixing on massive stars. We construct grids of stellar evolution models appropriate for the Small and Large Magellanic Clouds (SMC, LMC), as well as for the Galaxy, spanning various initial rotation rates and convective overshooting parameters. Significantly enhanced mixing apparently steers stellar evolution tracks away from the region of the HD limit. To quantify the excess of over-luminous stars in stellar evolution simulations we generate synthetic populations of massive stars, and make detailed comparisons with catalogues of cool (T < 12.5kK) and luminous (log L > 4.7) stars in the SMC and LMC. We find that adjustments to the mixing parameters can lead to agreement between the observed and simulated red supergiant populations, but for hotter supergiants the simulations always over-predict the number of very luminous (log L > 5.4) stars compared to observations. The excess of luminous supergiants decreases for enhanced mixing, possibly hinting at an important role mixing has in explaining the HD limit. Still, the HD limit remains unexplained for hotter supergiants.

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