arXiv subjects

Bethany Ludwig

Publications and source records attributed to Bethany Ludwig.

4 recordsLinked to original sources

arXiv2025

Intermediate-Mass Stripped Stars in the Magellanic Clouds: Forward Modeling the Observed Population Discovered Via UV Excess

Stripped stars are hot, helium-rich stars formed when binary interactions remove a star's hydrogen envelope. While low-mass ($\lesssim 1 M_\odot$) and high-mass ($\gtrsim 8 M_\odot$) stripped stars are well studied as hot subdwarfs and Wolf-Rayet stars, their intermediate-mass counterparts ($1-8 M_\odot$) have only recently been discovered. The Stripped-Star Ultraviolet Magellanic Cloud Survey (SUMS) identified UV-excess sources (i.e., sources lying blueward of the main sequence) in the Magellanic Clouds using Swift-UVOT photometry and selected 820 photometric stripped-star candidates. However, the completeness and purity of this sample remain poorly understood. We forward model the population of stripped stars in the Magellanic Clouds using a binary population synthesis model combined with spatially resolved star formation histories and simulated UV photometry. To assess survey sensitivity, we inject simulated sources into real Swift-UVOT images and reproduce the SUMS selection process, including crowding, extinction, and photometric quality cuts. For the SMC and LMC, we respectively recover 31\% and 15\% of sources with intrinsic UV excess, and $11\%$ and $7\%$ of all stripped stars. The rest are missed due to dilution by luminous companions, crowding, high extinction, and limited survey coverage. The observed population is biased toward systems with low-mass companions formed by common envelope evolution and systems with compact object companions. We predict contamination of the observed stripped-star candidates by main-sequence stars with spurious UV excess due to crowding and provide guidelines for selecting higher-purity subsamples. Our population synthesis model matches the number and properties of observed stripped star candidates with masses of $(2-5) M_\odot$ well. It underpredicts the number of lower- and higher-mass candidates, perhaps due to contamination.

astro-ph.SR↗

arXiv2025

The Stripped-Star Ultraviolet Magellanic Cloud Survey (SUMS): The UV Photometric Catalog and Stripped Star Candidate Selection

Most massive stars will interact with a binary companion during their lifetimes. These interactions can remove the hydrogen-rich envelope, producing intermediate-mass ($\sim$2-8 M$_\odot$) and helium-rich stars. These "stripped stars" are predicted to emit predominantly in the ultraviolet (UV) and can therefore be identified via a UV excess provided they are not outshone by their companion. However, despite their importance to binary evolution, supernovae, and ionizing feedback, few stripped stars have been confirmed. This is likely due to the scarcity of wide-field, high angular resolution, UV surveys of stellar populations with reliable distances and extinction estimates. To address this, we present the Stripped-Star Ultraviolet Magellanic Clouds Survey (SUMS) catalog. We use the Tractor forward modeling software to perform PSF photometry on 2,420 Swift-UVOT images of the LMC and SMC. The resulting public catalog contains 734,869 sources in three UV filters to a depth of $\sim$20 Vega mag. We perform validation tests on the photometry pipeline and highlight the catalog's broad applicability. We then identify sources with excess UV light compared to main-sequence stars and apply a series of quality cuts. From this, we identify 522 candidate stripped stars in the LMC and 298 in the SMC. We assess the potential contamination from other UV excess systems and argue the dominant uncertainty to be dust: early main-sequence stars can mimic the colors of stripped star binaries when extinction is overcorrected. This survey lays the groundwork for the first systematic census of stripped stars and opens new windows into binary evolution and massive star populations.

astro-ph.SR↗

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.

astro-ph.SR↗

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$_\odot$.

astro-ph.SR↗