arXiv subjects

Anna J. G. O'Grady

Publications and source records attributed to Anna J. G. O'Grady.

14 recordsLinked to original sources

arXiv2026

Revealing Optically Hidden Companions to Intermediate-Mass Stripped Stars with Radial Velocities

The recent discovery of a population of intermediate-mass helium stars ($\sim$2-8 $M_\odot$) in the Magellanic Clouds has opened a new observational window into massive binary evolution. These hot, helium-rich stars are thought to be products of binary interaction, and some show no evidence for a luminous companion in their optical spectra. Here, we present multi-epoch radial velocity (RV) measurements for eight of these systems obtained with Magellan/MagE from 2018-2023. Five helium stars exhibit significant RV variability consistent with binary motion, while three do not. The five helium stars with evidence of binarity have orbital periods from 16 hours to 300 days, and eccentricities consistent with zero, implying companion mass lower limits from $\sim$0.6-2.9 $M_\odot$. We discuss implications of their orbits for the nature of the unseen companions, and find that low-mass late-pre/early-main sequence stars ($\lesssim$1.5-3.1 $M_\odot$), low-mass stripped stars and compact objects are currently viable. We identify an unexpected correlation between surface hydrogen abundance and binarity within this sample: helium stars in binaries are hydrogen-poor ($X_{\rm H,surf}$ $\sim$ 0.3-0.4), whereas helium stars without detected binary motion are nearly hydrogen-free ($X_{\rm H,surf}$ $\lesssim$ 0.05). This bimodality suggests that low RV variability in a subset of the sample is not simply due to inclination, but rather a distinct evolutionary pathway. We consider three possible explanations for these objects: wide binaries, disrupted binaries, and stellar mergers. The binary properties of this first sample of intermediate-mass helium stars is diverse, and provides important empirical constraints for binary evolution and compact object formation.

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arXiv2026

Hot blue progenitors of stellar-mass black holes

While the connection between massive stars and supernova explosions is well established observationally, the link between massive stars and black hole formation remains elusive. Some massive stars may collapse directly to black holes without a successful supernova, and may therefore be observed as disappearing stars. We investigate the expected photometric properties of such black hole progenitors by combining detailed single and binary stellar evolution models with physically motivated prescriptions linking pre-collapse core structure to explosion or direct collapse outcome, together with stellar atmosphere calculations, producing synthetic photometry across standard ultraviolet to near-infrared filters. Weighting by an initial mass function and empirical binary distributions, we predict both the observable distribution of progenitor brightness and colour and the rate of direct-collapse events, which we estimate to be about 0.4 per century for a galaxy forming stars at 1 Msun/yr. We find that black hole progenitors are predominantly hot and blue at the pre-collapse stage, with many in Wolf-Rayet phases and luminous in the ultraviolet, while only a minority are red supergiants. Consequently, searches that focus primarily on red supergiants are likely to miss a substantial fraction of direct-collapse events. Monitoring campaigns that include ultraviolet-sensitive observations of nearby star-forming galaxies therefore provide a promising route to detecting disappearing massive stars, offering a direct observational probe of black hole formation. Our results provide predictions to interpret such surveys and constrain the channels that lead to black hole formation.

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arXiv2026

Disco in the Dust: Reflected light at the bow shock around Betelgeuse's companion explains its observed luminosity

Recent detections of $α$ Orionis B, the putative companion to Betelgeuse, have been reported using multiple instruments and techniques. These include, most recently, a $>6σ$ detection using VLT/SPHERE reported by Montarges+2026. The authors infer a bright companion ($\sim10^{-3}\times$ Betelgeuse's luminosity) with a mass of $2-3\,M_\odot$ and $T_\text{eff}\approx10-12{,}000$\,K, loosely consistent with the uppermost bounds of Howell+2025's recent mass estimate from speckle-imaging. However, it is in tension by a factor of two with the reported mass-exclusion limits via non-detection from a recent HST far-UV campaign (Goldberg+2025). While mass identification from isochrone-fitting (conducted in all cases) is prone to uncertainty, the restrictive HST upper limit on FUV flux precludes the hot, blue emission of a $\gtrsim2\,M_\odot$ main-sequence star. This discrepancy is reconciled by the following hypothesis: these detections observe the reflection of Betelgeuse's own luminosity scattered off the companion's bow shock and wake as it traverses the dusty circumstellar medium. To evaluate the feasibility of this scenario, we draw from custom, filter-specific stellar models computed with MESA, as well as Athena++ simulations of the companion's bow shock in idealized conditions. Importantly, the expansive bow shock from a stellar-mass object necessarily subtends a sizable portion of Betelgeuse's outgoing flux. We find that optical luminosity ratios of $\sim10^{-3}-10^{-4}$ are straightforward to resolve within reasonable assumptions about the companion mass and circumstellar dust, and disfavor a hot $3\,M_\odot$ companion. We thereby reconcile competing mass hypotheses across observational campaigns through proper attribution of the observed brightness, motivating future multi-wavelength campaigns to further characterize this enigmatic system.

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arXiv2026

Hubble as a Unique Discovery Engine of the Fate of Massive Stars and Black Hole Formation

How stellar-mass black holes are formed is an open question in astrophysics, with very limited observational constraints. It is not known which types of stars are more likely to produce black holes, and whether the formation process is accompanied by strong or weak electromagnetic transients - or none at all - and this issue remains a critical missing piece in the puzzle of the fate of massive stars. Recent theoretical work predicts that many stellar-mass black holes form from hot, UV-luminous massive stars, including Wolf-Rayet-like progenitors, and searches focused primarily on luminous cool supergiants may therefore miss a substantial fraction of black-hole formation events. While the coming decade will bring major advances in time-domain astronomy through Rubin/LSST, Roman, JWST, and wide-field transient surveys, none of these combines UV sensitivity, sub-arcsecond imaging, and decade-long continuity. HST uniquely enables direct searches for disappearing hot massive stars associated with black-hole formation. We outline a roadmap for extending HST's role in this area into the 2030s through a dedicated, large program to re-image nearby galaxies in the UV and identify candidate disappearing stars and unusual low-luminosity transients identified by complementary surveys. Theoretical event rates imply that the nearby galaxy population accessible to HST should yield of order one detectable black-hole-forming disappearance event per year. Extending HST operations into the 2030s would therefore provide crucial insights into the unsolved problem of black hole formation.

astro-ph.HE↗

arXiv2026

The Kinematic Properties of TŻO Candidate HV 11417 with Gaia DR3

HV 11417 is a candidate Thorne-Żytkow Object, a red supergiant with a neutron star core, located within the Small Magellanic Cloud (SMC). Previous studies have questioned, using Gaia DR2 data, whether HV 11417 was truly located at the distance of the SMC or was instead a foreground star. However, the proper motion measurement uncertainties for HV 11417 in DR2 were high. In this work, we use Gaia DR3 data to show that HV 11417 is very likely to be a true member of the SMC. We further analyze the kinematics of HV 11417 relative to its local environment, and compare it to populations of massive and evolved stars in the SMC. We find HV 11417 has a local transverse velocity of $52\pm15$ km/s, and thus qualifies as a runaway star (v$_\mathrm{loc}\geq$ 30 km/s). This runaway classification does not conclusively prove its nature as a TŻO, particularly given results from recent TŻO models, but does indicate that HV 11417 experienced a kinematic disruption in its evolution.

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arXiv2025

Betelgeuse's Buddy: X-Ray Constraints on the Nature of $α$ Ori B

The $\sim$$2100$d Long Secondary Period of Betelgeuse's optical lightcurve and radial velocity motivated the prediction of a low-mass stellar companion, expected to be at maximal apparent separation from Betelgeuse around December 2024. We carried out Director's Discretionary Time observations with the Chandra X-ray Observatory to identify any X-ray emission from the companion and constrain its nature as either a compact object or young stellar object (YSO). Past X-ray observations occurred at the wrong phase of the companion's orbit for optimal detection prospects and/or lacked the deep exposure required to constrain the typical X-ray luminosities of YSOs. In our 41.85 ks exposure with Chandra, we do not detect an X-ray source at the position of Betelgeuse. For an estimated hydrogen column density $N_H$$=$$6\times10^{22}$ cm$^{-2}$, we place a limit on the X-ray luminosity of $L_X$$\lesssim$$2\times10^{30}$ erg s$^{-1}$ ($\lesssim$$4.7\times10^{-4}L_\odot$) in $0.5$$-$$8$ keV for a 10 MK plasma temperature spectral model, or $L_X$$\lesssim$$5\times10^{29}$ erg s$^{-1}$ ($\lesssim$$1.2\times10^{-4}L_\odot$) for an absorbed power law with photon index $Γ$$=$$2$. These limits robustly exclude an accreting compact object (white dwarf or neutron star) as the companion. Solar mass YSOs with an age similar to Betelgeuse ($\sim$10 Myr) display a range of X-ray luminosities ($10^{28-32}$ erg s$^{-1}$), and we can place upper bounds within this range for most absorbing columns. Based on these considerations, we conclude that the companion to Betelgeuse is likely a low-mass YSO.

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arXiv2025

Betelgeuse, Betelgeuse, Betelgeuse, Betel-buddy? Constraints on the dynamical companion to $α$ Orionis from HST

Recently, two independent analyses have asserted that the cause of the Long Secondary Period (LSP) observed in the variability spectrum of our nearest red supergiant, Betelgeuse ($α$ Ori), is an as-yet undetected, low-mass binary companion dubbed $α$ Ori B. In this paper, we present the results of a far-UV observational campaign using the STIS echelle spectrograph on the Hubble Space Telescope aimed at detecting spectral signatures of the companion. The four-quadrant tiling pattern and timing of the observations were optimized to isolate the companion, with observations taking place during a period of maximum angular and velocity separation between Betelgeuse and the putative companion. Spectral differencing between quadrants recovers no spectral features at the companion's velocity in excess of the background or Betelgeuse's chromosphere, i.e. a non-detection. Having determined that $α$ Ori B is most likely a Young Stellar Object (YSO) thanks to constraints from a complementary X-ray campaign with the Chandra X-ray Observatory in a companion paper, comparison of our data against canonical spectra from YSOs in the ULLYSES database allows us to confidently exclude masses above $\gtrsim1.5M_\odot$ and companion continuum or line emission in excess of $\approx10^{-14}$ erg s$^{-1}$ cm$^{-2}$ angstrom$^{-1}$ in the FUV ($\approx1200-1700$ angstroms). Future observational campaigns aware of the LSP phase are needed to place deeper constraints on the spectroscopic nature of $α$ Ori B.

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arXiv2025

A Spectroscopic Hunt for Post-Red Supergiants in the Large Magellanic Cloud II: Turbulent Line Broadening in the Spectra of LMC Yellow Supergiants

Massive stars in the Hertzsprung gap are a mixed population of objects in short-lived evolutionary phases: yellow supergiants (YSGs) evolving towards the red supergiant (RSG) phase, partially-stripped post-RSGs, and other, rarer outcomes of stellar evolution. Studies of sufficiently large samples of these objects can constrain massive star structure and evolution during these poorly-understood phases. As part of our ongoing program searching for post-RSGs, we characterized the spectral line profiles of 32 YSGs in the Large Magellanic Cloud using high-resolution spectra obtained with the MIKE spectrograph on the Magellan 2/Clay telescope at Las Campanas Observatory. We find that the line profiles are strongly broadened by turbulent photospheric motion. After fitting the profiles to measure microturbulent and macroturbulent velocities, we identify two groups within our sample that are separated by the ratio of the two velocity scales. In both groups, the macroturbulent velocity scales with stellar properties such as effective temperature. Additionally, we find statistically-significant correlations between the macroturbulent velocity and other possible probes of large-scale photospheric motions: line profile asymmetry, as well as the amplitude and quality factor of the stochastic low frequency variability measured from {\it Transiting Exoplanet Survey Satellite} lightcurves. These correlations differ between the two groups of YSGs. Finally, we construct 1D evolutionary models of YSGs in both pre- and post-RSG phases, and find reasonable agreement between the convective velocities in these models and our measured microturbulent velocities. However, the macroturbulent velocities are much higher than the convective velocities in the models.

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arXiv2024

Thorne-Żytkow Objects

Interacting binary star systems play a critical role in many areas of astrophysics. One interesting example of a binary merger product are Thorne-Żytkow Objects (TŻOs), stars that look like red supergiants but contain neutron stars at their cores. TŻOs were theorized nearly five decades ago, and significant work has gone into understanding the physics of their formation, evolution, and stability. Several searches for TŻO candidates have also been carried out. Whether or not TŻOs could even exist or if they would be stable after formation has also been investigated. Understanding the existence and possible prevalence of TŻOs would have important effects on our understanding of binary evolution, stellar mergers, and inform binary population synthesis models. In this chapter, we review the formation channels, evolution and structure, final fates, and observable signatures of TŻOs, as well as candidates in the literature, from the inception of TŻO theory to recent progress in the field.

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arXiv2024

Binary Yellow Supergiants in the Magellanic Clouds I: Photometric Candidate Identification

Recent works have constrained the binary fraction of evolved populations of massive stars in local galaxies such as red supergiants and Wolf-Rayet stars, but the binary fraction of yellow supergiants (YSGs) in the Hertzsprung Gap remains unconstrained. Binary evolution theory predicts that the Hertzsprung Gap is home to multiple populations of binary systems with varied evolutionary histories. In this paper, we develop a method to distinguish single YSGs from YSG plus O- or B-type main sequence binaries using optical and ultraviolet photometry, and then apply this method to identify candidate YSG binaries in the Magellanic Clouds. After constructing a set of combined stellar atmosphere models, we find that optical photometry is, given typical measurement and reddening uncertainties, sufficient to discern single YSGs from YSG+OB binaries if the OB-star is at least $\sim5$M$_{\odot}$ for T$_{\mathrm{eff,YSG}}\sim$ 4000 K, but requires a $\sim$20M$_{\odot}$ OB star for YSGs up to T$_{\mathrm{eff,YSG}}\sim$ 9000 K. For these hotter YSG temperatures, ultraviolet photometry allows binaries with OB companions as small as $\sim$7M$_{\odot}$ to be identified. We use color-color spaces developed from these models to search for evidence of excess blue or ultraviolet light in a set of $\sim$1000 YSG candidates in the Magellanic Clouds. We identify hundreds of candidate YSG binary systems and report a preliminary fraction of YSGs that show a blue/UV color excess of 20-60\%. Spectroscopic follow-up is now required to confirm the true nature of this population.

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arXiv2023

Type II-P Supernova Progenitor Star Initial Masses and SN 2020jfo: Direct Detection, Light Curve Properties, Nebular Spectroscopy, and Local Environment

We present optical, ultraviolet, and infrared data of the type II supernova (SN II) 2020jfo at 14.5 Mpc. This wealth of multiwavelength data allows to compare different metrics commonly used to estimate progenitor masses of SN II for the same object. Using its early light curve, we infer SN 2020jfo had a progenitor radius of $\approx$700 $R_{\odot}$, consistent with red supergiants of initial mass $M_{\rm ZAMS}=$11-13 $M_{\odot}$. The decline in its late-time light curve is best fit by a ${}^{56}$Ni mass of 0.018$\pm$0.007 $M_{\odot}$ consistent with that ejected from SN II-P with $\approx$13 $M_{\odot}$ initial mass stars. Early spectra and photometry do not exhibit signs of interaction with circumstellar matter, implying that SN 2020jfo experienced weak mass loss within the final years prior to explosion. Our spectra at $>$250 days are best fit by models from 12 $M_{\odot}$ initial mass stars. We analyzed integral field unit spectroscopy of the stellar population near SN 2020jfo, finding its massive star population had a zero age main sequence mass of 9.7$\substack{+2.5\\-1.3} M_{\odot}$. We identify a single counterpart in pre-explosion imaging and find it has an initial mass of at most $7.2\substack{+1.2\\-0.6} M_{\odot}$. We conclude that the inconsistency between this mass and indirect mass indicators from SN 2020jfo itself is most likely caused by extinction with $A_{V}=2$-3 mag due to matter around the progenitor star, which lowered its observed optical luminosity. As SN 2020jfo did not exhibit extinction at this level or evidence for interaction with circumstellar matter between 1.6-450 days from explosion, we conclude that this material was likely confined within $\approx$3000 $R_{\odot}$ from the progenitor star.

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arXiv2022

Cool, Luminous, and Highly Variable Stars in the Magellanic Clouds. II: Spectroscopic and Environmental Analysis of Thorne-Żytkow Object and Super-AGB Star Candidates

In previous work we identified a population of 38 cool and luminous variable stars in the Magellanic Clouds and examined 11 in detail in order to classify them as either Thorne-Żytkow Objects (TŻOs, red supergiants with a neutron star cores) or super-AGB stars (the most massive stars that will not undergo core collapse). This population includes HV\,2112, a peculiar star previously considered in other works to be either a TŻO or high-mass AGB star. Here we continue this investigation, using the kinematic and radio environments and local star formation history of these stars to place constraints on the age of the progenitor systems and the presence of past supernovae. These stars are not associated with regions of recent star formation, and we find no evidence of past supernovae at their locations. Finally, we also assess the presence of heavy elements and lithium in their spectra compared to red supergiants. We find strong absorption in Li and s-process elements compared to RSGs in most of the sample, consistent with super-AGB nucleosynthesis, while HV\,2112 shows additional strong lines associated with TŻO nucleosynthesis. Coupled with our previous mass estimates, the results are consistent with the stars being massive (~4-6.5M$_{\odot}$) or super-AGB (~6.5-12M$_{\odot}$) stars in the thermally pulsing phase, providing crucial observations of the transition between low- and high-mass stellar populations. HV\,2112 is more ambiguous; it could either be a maximally massive sAGB star, or a TŻO if the minimum mass for stability extends down to <13 M$_\odot$.

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arXiv2021

Searching for Anomalies in the ZTF Catalog of Periodic Variable Stars

Periodic variables illuminate the physical processes of stars throughout their lifetime. Wide-field surveys continue to increase our discovery rates of periodic variable stars. Automated approaches are essential to identify interesting periodic variable stars for multi-wavelength and spectroscopic follow-up. Here, we present a novel unsupervised machine learning approach to hunt for anomalous periodic variables using phase-folded light curves presented in the Zwicky Transient Facility Catalogue of Periodic Variable Stars by \citet{Chen_2020}. We use a convolutional variational autoencoder to learn a low dimensional latent representation, and we search for anomalies within this latent dimension via an isolation forest. We identify anomalies with irregular variability. Most of the top anomalies are likely highly variable Red Giants or Asymptotic Giant Branch stars concentrated in the Milky Way galactic disk; a fraction of the identified anomalies are more consistent with Young Stellar Objects. Detailed spectroscopic follow-up observations are encouraged to reveal the nature of these anomalies.

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arXiv2020

Cool, Luminous, and Highly Variable Stars in the Magellanic Clouds from ASAS-SN: Implications for Thorne-Żytkow Objects and Super-Asymptotic Giant Branch Stars

Stars with unusual properties can provide a wealth of information about rare stages of stellar evolution and exotic physics. However, determining the true nature of peculiar stars is often difficult. In this work, we conduct a systematic search for cool and luminous stars in the Magellanic Clouds with extreme variability, motivated by the properties of the unusual SMC star and Thorne-Żytkow Object (TŻO) candidate HV2112. Using light curves from ASAS-SN we identify 38 stars with surface temperatures T $<$ 4800K, luminosities $\log$(L/L$_\odot$) $>$ 4.3, variability periods $>$ 400 days, and variability amplitudes $Δ$V $>$ 2.5 mag. Eleven of these stars possess the distinctive double-peaked light curve morphology of HV2112. We use the pulsation properties and derived occurrence rates for these 12 objects to constrain their nature. From comparisons to stellar populations and models, we find that one star may be a red supergiant with large amplitude pulsations. For the other 11 stars we derive current masses of $\sim$5-10 M$_{\odot}$, below the theoretical minimum mass of $\sim$15 M$_{\odot}$ for TŻOs to be stable, casting doubt on this interpretation. Instead, we find that the temperatures, luminosities, mass-loss rates, and periods of these stars are consistent with predictions for super-Asymptotic Giant Branch (s-AGB) stars that have begun carbon burning but have not reached the superwind phase. We infer lifetimes in this phase of $\sim($1$-$7) $\times$ 10$^{4}$ years, also consistent with an s-AGB interpretation. If confirmed, these objects would represent the first identified population of s-AGB stars, illuminating the transition between low- and high-mass stellar evolution.

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