arXiv subjects

Ylva Götberg

Publications and source records attributed to Ylva Götberg.

17 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.

astro-ph.SR↗

arXiv2026

A Galactic intermediate-mass stripped star with a Wolf-Rayet-like wind

Binary interaction in massive stars is expected to produce a large population of intermediate-mass ($2$-$8$ M$_\odot$) envelope-stripped stars, yet such objects have remained elusive in the Milky Way. We report the identification of an unambiguous Galactic example in a short-period ($P=5.94$ d), double-lined spectroscopic binary, discovered in the SDSS-V Milky Way Mapper survey. The system consists of a rapidly rotating O-type star and a hotter, lower-mass companion, which shows He II and N IV emission lines with large radial velocity variations, revealing its binary nature. Combined orbital constraints and joint spectroscopic and photometric modelling show that the companion is a hot ($T_\ast \approx 60$ kK), helium-rich star with a mass of $3.2$-$5.8$ M$_\odot$, placing it squarely in the intermediate-mass regime and below values typically inferred for classical Wolf-Rayet (WR) stars. The system's short period, negligible eccentricity, and rapidly rotating O-star point to a post-interaction configuration following efficient mass transfer and spin-up of the accretor. Comparison with binary evolution models suggests that the stripped star is observed in a brief inflated phase following mass transfer, which increases its optical flux contribution and facilitates its detection. The inferred mass-loss rate $\log \dot{M} = -6.3 \pm 0.1$ is in line with mass-loss rates observed for classical WR stars in the Milky Way and exceeds those measured for intermediate-mass stripped stars in the Magellanic Clouds, with the caveat that our target selection is biased towards systems with stronger emission features. As an unambiguous and well-characterised intermediate-mass stripped star, this system provides a key benchmark for models of binary evolution at solar metallicity, stripped-envelope supernova progenitors, and the formation of compact-object binaries.

astro-ph.SR↗

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.

astro-ph.SR↗

arXiv2026

A constant upper luminosity limit of cool supergiant stars down to the extremely low metallicity of I Zw 18

Stellar wind mass loss is often assumed to depend on their metallicity $Z$. Therefore, evolutionary models of massive stars at lower $Z$ are able to retain more of their H-rich layers and evolve into brighter cool supergiants (cool SGs; $T_\mathrm{eff} < 7$ kK). Surprisingly, in galaxies in the range $0.2 \lesssim Z / Z_\odot \lesssim 1.5$ previous studies did not find a $Z$-dependence of the upper luminosity limit $L_\mathrm{max}$ of cool SGs. Here, we study four extra galaxies with HST and JWST. Observations of the extremely low-$Z$ dwarf galaxy I Zw 18 from JWST allow us to go down to $Z / Z_\odot \approx 1/40$. For cool SGs in all studied galaxies including I Zw 18, we find a constant $L_\mathrm{max} \approx 10^{5.6}$L$_\odot$, similar to literature results for $0.2 \lesssim Z / Z_\odot \lesssim 1.5$. In I Zw 18 and the other studied galaxies, the presence of Wolf-Rayet stars has been claimed. Although we cannot rule out that some of them become intermediate-temperature objects, this paints a picture in which evolved stars with $L>10^{5.6}$L$_\odot$ burn He as hot He-rich stars down to extremely low $Z$. We argue that $Z$-independent late-phase mass loss would be the most likely mechanism responsible. Regardless of the exact stripping mechanism, for the Early Universe our results imply a limitation on black hole masses and a contribution of stars born with $M\gtrsim 30$M$_\odot$ to its surprisingly strong nitrogen enrichment. We propose a scenario in which single stars at low $Z$ emit sufficiently hard ionizing radiation to produce lines of He II and C IV. In this scenario, late-phase $Z$-independent mass loss produces hot He-rich stars. Due to the well-understood $Z$-dependence of radiation-driven winds of hot stars, a window of opportunity would open below 0.2Z$_\odot$, where self-stripped He-rich stars can exist without dense WR winds that absorb hard ionizing radiation.

astro-ph.SR↗

arXiv2024

Combining REBOUND and MESA: Dynamical Evolution of Planets Orbiting Interacting Binaries

Although planets have been found orbiting binary systems, whether they can survive binary interactions is debated. While the tightest-orbit binaries should host the most dynamically stable and long-lived circumbinary planetary systems, they are also the systems that are expected to experience mass transfer, common envelope evolution, or stellar mergers. In this study, we explore the effect of stable non-conservative mass transfer on the dynamical evolution of circumbinary planets. We present a new script that seamlessly integrates binary evolution data from the 1D binary stellar evolution code MESA into the N-body simulation code REBOUND. This integration framework enables a comprehensive examination of the dynamical evolution of circumbinary planets orbiting mass-transferring binaries, while simultaneously accounting for the detailed stellar structure evolution. In addition, we introduce a recalibration method to mitigate numerical errors from updates of binary properties during the system's dynamical evolution. We construct a reference binary model in which a $2.21\, M_\odot$ star loses its hydrogen-rich envelope through non-conservative mass transfer to the $1.76\, M_\odot$ companion star, creating a $0.38\,M_\odot$ subdwarf. We find the tightest stable orbital separation for circumbinary planets to be $\simeq 2.5$ times the binary separation after mass transfer. Accounting for tides by using the interior stellar structure, we find that tidal effects become apparent after the rapid mass transfer phase and start to fade away during the latter stage of the slow mass transfer phase. Our research provides a new framework for exploring circumbinary planet dynamics in interacting binary systems.

astro-ph.EP↗

arXiv2024

Tidal Disruption Events from Stripped Stars

Observations of tidal disruption events (TDEs) show signs of Nitrogen enrichment reminiscent of other astrophysical sources such as active galactic nuclei (AGN) and star-forming galaxies. Given that TDEs probe the gas from a single star, it is possible to test if the observed enrichment is consistent with expectations from the CNO cycle by looking at the observed Nitrogen/Carbon (N/C) abundance ratios. Given that $\approx 20\%$ of solar mass stars (and an even larger fraction of more massive stars) live in close binaries, it is worthwhile to also consider what TDEs from stars influenced by binary evolution would look like. We show here that TDEs from stars stripped of their Hydrogen-rich (and Nitrogen-poor) envelopes through previous binary-induced mass loss can produce much higher observable N/C enhancements than even TDEs from massive stars. Additionally, we predict that the time-dependence of the N/C abundance ratio in the mass fallback rate of stripped stars will follow the inverse behavior of main-sequence stars, enabling a more accurate characterization of the disrupted star.

astro-ph.HE↗

arXiv2024

X-Shooting ULLYSES: Massive Stars at Low Metallicity

The Hubble Space Telescope has devoted 500 orbits to observing 250 massive stars with low metallicity in the ultraviolet (UV) range within the framework of the ULLYSES program. The X-Shooting ULLYSES (XShootU) project enhances the legacy value of this UV dataset by providing high-quality optical and near-infrared spectra, which are acquired using the wide-wavelength-coverage X-shooter spectrograph at ESO's Very Large Telescope. XShootU emphasises the importance of combining UV with optical spectra for the consistent determination of key stellar parameters such as effective temperature, surface gravity, luminosity, abundances, and wind characteristics including mass-loss rates as a function of metallicity. Since uncertainties in these parameters have implications across various branches of astrophysics, the data and modelling generated by the XShootU project are poised to significantly advance our understanding of massive stars at low metallicity. This is particularly crucial for confidently interpreting JWST data of the earliest stellar generations, making XShootU a unique resource for comprehending individual spectra of low-metallicity stars.

astro-ph.SR↗

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.

astro-ph.SR↗

arXiv2022

A Panchromatic Study of Massive Stars in the Extremely Metal-Poor Local Group Dwarf Galaxy Leo A

We characterize massive stars (M>8 M_sun) in the nearby (D~0.8 Mpc) extremely metal-poor (Z~5% Z_sun) galaxy Leo A using Hubble Space Telescope ultra-violet (UV), optical, and near-infrared (NIR) imaging along with Keck/LRIS and MMT/Binospec optical spectroscopy for 18 main sequence OB stars. We find that: (a) 12 of our 18 stars show emission lines, despite not being associated with an H II region, suggestive of stellar activity (e.g., mass loss, accretion, binary star interaction), which is consistent with previous predictions of enhanced activity at low metallicity; (b) 6 are Be stars, which are the first to be spectroscopically studied at such low metallicity -- these Be stars have unusual panchromatic SEDs; (c) for stars well-fit by the TLUSTY non-local thermodynamic equilibrium (non-LTE) models, the photometric and spectroscopic values of T_eff and log(g) agree to within ~0.01 dex and ~0.18 dex, respectively, indicating that NUV/optical/NIR imaging can be used to reliably characterize massive (M ~ 8-30 M_sun) main sequence star properties relative to optical spectroscopy; (d) the properties of the most massive stars in H II regions are consistent with constraints from previous nebular emission line studies; and (e) 13 stars with M>8 M_sun are >40 pc from a known star cluster or H II region. Our sample comprises ~50% of all known massive stars at Z < 10% Z_sun with derived stellar parameters, high-quality optical spectra, and panchromatic photometry.

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↗

arXiv2022

Successful Common Envelope Ejection and Binary Neutron Star Formation in 3D Hydrodynamics

A binary neutron star merger has been observed in a multi-messenger detection of gravitational wave (GW) and electromagnetic (EM) radiation. Binary neutron stars that merge within a Hubble time, as well as many other compact binaries, are expected to form via common envelope evolution. Yet five decades of research on common envelope evolution have not yet resulted in a satisfactory understanding of the multi-spatial multi-timescale evolution for the systems that lead to compact binaries. In this paper, we report on the first successful simulations of common envelope ejection leading to binary neutron star formation in 3D hydrodynamics. We simulate the dynamical inspiral phase of the interaction between a 12$M_\odot$ red supergiant and a 1.4$M_\odot$ neutron star for different initial separations and initial conditions. For all of our simulations, we find complete envelope ejection and final orbital separations of $a_{\rm f} \approx 1.3$-$5.1 R_\odot$ depending on the simulation and criterion, leading to binary neutron stars that can merge within a Hubble time. We find $α_{\rm CE}$-equivalent efficiencies of $\approx 0.1$-$2.7$ depending on the simulation and criterion, but this may be specific for these extended progenitors. We fully resolve the core of the star to $\lesssim 0.005 R_\odot$ and our 3D hydrodynamics simulations are informed by an adjusted 1D analytic energy formalism and a 2D kinematics study in order to overcome the prohibitive computational cost of simulating these systems. The framework we develop in this paper can be used to simulate a wide variety of interactions between stars, from stellar mergers to common envelope episodes leading to GW sources.

astro-ph.HE↗

arXiv2021

Pre-Explosion Properties of Helium Star Donors to Thermonuclear Supernovae

Helium star - carbon-oxygen white dwarf (CO WD) binaries are potential single-degenerate progenitor systems of thermonuclear supernovae. Revisiting a set of binary evolution calculations using the stellar evolution code $\texttt{MESA}$, we refine our previous predictions about which systems can lead to a thermonuclear supernova and then characterize the properties of the helium star donor at the time of explosion. We convert these model properties to NUV/optical magnitudes assuming a blackbody spectrum and support this approach using a matched stellar atmosphere model. These models will be valuable to compare with pre-explosion imaging for future supernovae, though we emphasize the observational difficulty of detecting extremely blue companions. The pre-explosion source detected in association with SN 2012Z has been interpreted as a helium star binary containing an initially ultra-massive WD in a multi-day orbit. However, extending our binary models to initial CO WD masses of up to $1.2\,M_{\odot}$, we find that these systems undergo off-center carbon ignitions and thus are not expected to produce thermonuclear supernovae. This tension suggests that, if SN 2012Z is associated with a helium star - WD binary, then the pre-explosion optical light from the system must be significantly modified by the binary environment and/or the WD does not have a carbon-rich interior composition.

astro-ph.SR↗

arXiv2021

Spectral Signatures of Population III and Envelope-stripped Stars in Galaxies at the Epoch of Reionization

While most simulations of the epoch of reionization have focused on single-stellar populations in star-forming dwarf galaxies, products of binary evolution are expected to significantly contribute to emissions of hydrogen-ionizing photons. Among these products are stripped stars (or helium stars), which have their envelopes stripped from interactions with binary companions, leaving an exposed helium core. Previous work has suggested these stripped stars can dominate the LyC photon output of high-redshift low luminosity galaxies. Other sources of hard radiation in the early universe include zero-metallicity Population III stars, which may have similar SED properties to galaxies with radiation dominated by stripped star emissions. Here, we use two metrics (the power-law exponent over wavelength intervals 240-500 Å, 600-900 Å, and 1200-2000 Å, and the ratio of total luminosity in FUV wavelengths to LyC wavelengths) to compare the SEDs of simulated galaxies with only single-stellar evolution, galaxies containing stripped stars, and galaxies containing Population III stars, with four different IMFs. We find that stripped stars significantly alter the SEDs in the LyC range of galaxies at the epoch of reionization. SEDs in galaxies with stripped stars present have lower power-law indices in the LyC range and lower FUV to LyC luminosity ratios. These differences in SEDs are present at all considered luminosities ($M_{UV} > -15$, AB system), and are most pronounced for lower luminosity galaxies. We also find that SEDs of galaxies with stripped stars and Pop III stars are distinct from each other for all tested IMFs.

astro-ph.GA↗

arXiv2021

Binary-Stripped Stars as Core-Collapse Supernovae Progenitors

Most massive stars experience binary interactions in their lifetimes that can alter both the surface and core structure of the stripped star with significant effects on their ultimate fate as core-collapse supernovae. However, core-collapse supernovae simulations to date have focused almost exclusively on the evolution of single stars. We present a systematic simulation study of single and binary-stripped stars with the same initial mass as candidates for core-collapse supernovae (11 - 21 M$_{\odot}$). Generally, we find that binary-stripped stars core tend to be less compact, with a more prominent, deeper silicon/oxygen interface, and explode preferentially to the corresponding single stars of the same initial mass. Such a dichotomy of behavior between these two modes of evolution would have important implications for supernovae statistics, including the final neutron star masses, explosion energies, and nucleosynthetic yields. Binary-stripped remnants are also well poised to populate the possible mass gap between the heaviest neutron stars and the lightest black holes. Our work presents an improvement along two fronts, as we self-consistently account for the pre-collapse stellar evolution and the subsequent explosion outcome. Even so, our results emphasize the need for more detailed stellar evolutionary models to capture the sensitive nature of explosion outcome.

astro-ph.SR↗

arXiv2021

The Detection and Characterization of Be+sdO Binaries from HST/STIS FUV Spectroscopy

The B-emission line stars are rapid rotators that were probably spun up by mass and angular momentum accretion through mass transfer in an interacting binary. Mass transfer will strip the donor star of its envelope to create a small and hot subdwarf remnant. Here we report on Hubble Space Telescope/STIS far-ultraviolet spectroscopy of a sample of Be stars that reveals the presence of the hot sdO companion through the calculation of cross-correlation functions of the observed and model spectra. We clearly detect the spectral signature of the sdO star in 10 of the 13 stars in the sample, and the spectral signals indicate that the sdO stars are hot, relatively faint, and slowly rotating as predicted by models. A comparison of their temperatures and radii with evolutionary tracks indicates that the sdO stars occupy the relatively long-lived, He-core burning stage. Only one of the ten detections was a known binary prior to this investigation, which emphasizes the difficulty of finding such Be+sdO binaries through optical spectroscopy. However, these results and others indicate that many Be stars probably host hot subdwarf companions.

astro-ph.SR↗

arXiv2019

The diverse lives of progenitors of hydrogen-rich core-collapse supernovae: the role of binary interaction

Hydrogen-rich supernovae, known as Type II (SNe II), are the most common class of explosions observed following the collapse of the core of massive stars. We use analytical estimates and population synthesis simulations to assess the fraction of SNe II progenitors that are expected to have exchanged mass with a companion prior to explosion. We estimate that 1/3 to 1/2 of SN II progenitors have a history of mass exchange with a binary companion before exploding. The dominant binary channels leading to SN II progenitors involve the merger of binary stars. Mergers are expected to produce a diversity of SN II progenitor characteristics, depending on the evolutionary timing and properties of the merger. Alternatively, SN II progenitors from interacting binaries may have accreted mass from their companion, and subsequently been ejected from the binary system after their companion exploded. We show that the overall fraction of SN II progenitors that are predicted to have experienced binary interaction is robust against the main physical uncertainties in our models. However, the relative importance of different binary evolutionary channels is affected by changing physical assumptions. We further discuss ways in which binarity might contribute to the observed diversity of SNe II by considering potential observational signatures arising from each binary channel. For supernovae which have a substantial H-rich envelope at explosion (i.e., excluding Type IIb SNe), a surviving non-compact companion would typically indicate that the supernova progenitor star was in a wide, non-interacting binary. We argue that a significant fraction of even Type II-P SNe are expected to have gained mass from a companion prior to explosion.

astro-ph.HE↗

arXiv2018

No surviving stellar companion for Cassiopeia A

Massive stars in binaries can give rise to extreme phenomena such as X-ray binaries and gravitational wave sources after one or both stars end their lives as core-collapse supernovae. Stars in close orbit around a stellar or compact companion are expected to explode as "stripped-envelope supernovae", showing no (Type Ib/c) or little (Type IIb) signs of hydrogen in the spectra, because hydrogen-rich progenitors are too large to fit. The physical processes responsible for the stripping process and the fate of the companion are still very poorly understood. Aiming to find new clues, we investigate Cas~A, which is a very young ($\sim$340 \,yr) and near ($\sim$3.4\,kpc) remnant of a core collapse supernova. Cas~A has been subject to several searches for possible companions, all unsuccessfully. We present new measurements of the proper motions and photometry of stars in the vicinity based on deep HST ACS/WFC and WFC3-IR data. We identify stellar sources that are close enough in projection, but using their proper motions we show that none are compatible with being at the location of center at the time of explosion, in agreement with earlier findings. Our photometric measurements allow us to place much deeper (order of magnitude) upper limits on the brightness of possible undetected companions. We systematically compare them with model predictions for a wide variety of scenarios. We can confidently rule out the presence of any stellar companion of any reasonable mass and age (main sequence, pre main sequence or stripped). Although this finding is not in direct conflict with model predictions, it does rule out what many considered to be the most likely evolutionary scenario for type IIb supernova. (abstract abbreviated)

astro-ph.SR↗