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Stephen Justham

Publications and source records attributed to Stephen Justham.

At least 55 records · Page 3Linked to original sources
arXiv2020

Looks can be deceiving: underestimating the age of single white dwarfs due to binary mergers

Context: White dwarfs (WDs) are important and abundant tools to study the structure and evolution of the Galactic environment. However, the multiplicity of WD progenitors is generally neglected. Specifically, a merger in a binary system can lead to a single WD, which could result in wrongly inferred quantities if only single stellar evolution (SSE) is considered. These mergers are linked to transients such as luminous red novae and Type Ia supernovae. Aims: We investigate the impact of binary evolution (BE) upon observable single WDs, and compare their properties to WDs formed through SSE. We assess the evolutionary channels and the age and mass distributions of the resulting single Wds. Methods: We employed \texttt{SeBa} to model the evolution of single star and binary populations. We synthesised the observable single WD population within $100$ pc, including cooling and observational selection effects. Additionally, we constructed models with different evolution and primordial population properties to study the effects on the properties of the resulting single WDs. Results: White dwarfs from binary mergers make up about $10-30\%$ of all observable single WDs and $30-50\%$ of massive WDs. On average, individual WDs take $3.1-5$ times longer to form through BE than SE, and so appear ${\sim} 1$ Gyr younger than they are if BE is ignored. In all models, the effect of mergers on the age distribution is clearly noticeable. The median age typically increases by $85-430$ Myr and $200-390$ Myr for massive WDs. Although abundant, we do not find evidence that WDs from mergers significantly alter the shape of the WD mass distribution. Conclusions: Assuming SSE for inferring properties of single WDs gives rise to intrinsic errors as single WDs can also be formed following a binary merger. Strategies for mitigating the effect of mergers on the WD age distributions are discussed.

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arXiv2020

Luminous Red Novae: population models and future prospects

A class of optical transients known as Luminous Red Novae (LRNe) have recently been associated with mass ejections from binary stars undergoing common-envelope evolution. We use the population synthesis code COMPAS to explore the impact of a range of assumptions about the physics of common-envelope evolution on the properties of LRNe. In particular, we investigate the influence of various models for the energetics of LRNe on the expected event rate and light curve characteristics, and compare with the existing sample. We find that the Galactic rate of LRNe is $\sim 0.2$ yr$^{-1}$, in agreement with the observed rate. In our models, the luminosity function of Galactic LRNe covers multiple decades in luminosity and is dominated by signals from stellar mergers, consistent with observational constraints from iPTF and the Galactic sample of LRNe. We discuss how observations of the brightest LRNe may provide indirect evidence for the existence of massive ($> 40$ M$_\odot$) red supergiants. Such LRNe could be markers along the evolutionary pathway leading to the formation of double compact objects. We make predictions for the population of LRNe observable in future transient surveys with the Large Synoptic Survey Telescope and the Zwicky Transient Facility. In all plausible circumstances, we predict a selection-limited observable population dominated by bright, long-duration events caused by common envelope ejections. We show that the Large Synoptic Survey Telescope will observe $20$--$750$ LRNe per year, quickly constraining the luminosity function of LRNe and probing the physics of common-envelope events.

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arXiv2019

A wide star-black-hole binary system from radial-velocity measurements

All stellar mass black holes have hitherto been identified by X-rays emitted by gas that is accreting onto the black hole from a companion star. These systems are all binaries with black holes below 30 M$_{\odot}$$^{1-4}$. Theory predicts, however, that X-ray emitting systems form a minority of the total population of star-black hole binaries$^{5,6}$. When the black hole is not accreting gas, it can be found through radial velocity measurements of the motion of the companion star. Here we report radial velocity measurements of a Galactic star, LB-1, which is a B-type star, taken over two years. We find that the motion of the B-star and an accompanying H$α$ emission line require the presence of a dark companion with a mass of $68^{+11}_{-13}$ M$_{\odot}$, which can only be a black hole. The long orbital period of 78.9 days shows that this is a wide binary system. The gravitational wave experiments have detected similarly massive black holes$^{7,8}$, but forming such massive ones in a high-metallicity environment would be extremely challenging to current stellar evolution theories$^{9-11}$.

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arXiv2019

STROOPWAFEL: Simulating rare outcomes from astrophysical populations, with application to gravitational-wave sources

Gravitational-wave observations of double compact object (DCO) mergers are providing new insights into the physics of massive stars and the evolution of binary systems. Making the most of expected near-future observations for understanding stellar physics will rely on comparisons with binary population synthesis models. However, the vast majority of simulated binaries never produce DCOs, which makes calculating such populations computationally inefficient. We present an importance sampling algorithm, STROOPWAFEL, that improves the computational efficiency of population studies of rare events, by focusing the simulation around regions of the initial parameter space found to produce outputs of interest. We implement the algorithm in the binary population synthesis code COMPAS, and compare the efficiency of our implementation to the standard method of Monte Carlo sampling from the birth probability distributions. STROOPWAFEL finds $\sim$25-200 times more DCO mergers than the standard sampling method with the same simulation size, and so speeds up simulations by up to two orders of magnitude. Finding more DCO mergers automatically maps the parameter space with far higher resolution than when using the traditional sampling. This increase in efficiency also leads to a decrease of a factor $\sim$3-10 in statistical sampling uncertainty for the predictions from the simulations. This is particularly notable for the distribution functions of observable quantities such as the black hole and neutron star chirp mass distribution, including in the tails of the distribution functions where predictions using standard sampling can be dominated by sampling noise.

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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.

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arXiv2019

Clues on the Origin and Evolution of Massive Contact Binaries: Atmosphere Analysis of VFTS 352

The massive O4.5 V + O5.5 V binary VFTS 352 in the Tarantula nebula is one of the shortest-period and most massive overcontact binaries known. Recent theoretical studies indicate that some of these systems could ultimately lead to the formation of gravitational waves via black hole binary mergers through the chemically homogeneous evolution pathway. By analyzing ultraviolet-optical phase-resolved spectroscopic data, we aim to constrain atmospheric and wind properties that could be later used to confront theoretical predictions from binary evolution. In particular, surface abundances are powerful diagnostics of the evolutionary status, mass transfer and the internal mixing processes. From a set of 32 VLT/FLAMES visual and 8 HST/COS ultraviolet spectra, we used spectral disentangling to separate the primary and secondary components. Using a genetic algorithm wrapped around the NLTE model atmosphere and spectral synthesis code FASTWIND, we perform an 11-parameter optimization to derive the atmospheric and wind parameters of both components, including the surface abundances of He, C, N, O and Si. We find that both components are hotter than expected compared to single-star evolutionary models indicating that additional mixing processes may be at play. However the derived chemical abundances do not show significant indications of mixing when adopting baseline values typical for the system environment.

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arXiv2019

Massive Stellar Mergers as Precursors of Hydrogen-rich Pulsational Pair Instability Supernovae

Interactions between massive stars in binaries are thought to be responsible for much of the observed diversity of supernovae. As surveys probe rarer populations of events, we should expect to see supernovae arising from increasingly uncommon progenitor channels. Here we examine a scenario in which massive stars merge after they have both formed a hydrogen-exhausted core. We suggest this could produce stars which explode as pair-instability supernovae (PISNe) with significantly more hydrogen, at a given metallicity, than in single-star models with the same pre-explosion oxygen-rich core mass. We investigate the subset of those stellar mergers which later produce pulsational PISNe, and estimate that the rate of such post-merger, hydrogen-rich pulsational PISNe could approach a few in a thousand of all core-collapse supernovae. The nature and predicted rate of such hydrogen-rich pulsational PISNe are reminiscent of the very unusual supernova iPTF14hls. For plausible assumptions, PISNe from similar mergers might dominate the rate of PISNe in the local Universe.

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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)

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arXiv2018

On the formation history of Galactic double neutron stars

Double neutron stars (DNSs) have been observed as Galactic radio pulsars, and the recent discovery of gravitational waves from the DNS merger GW170817 adds to the known DNS population. We perform rapid population synthesis of massive binary stars and discuss model predictions, including formation rates, mass distributions, and delay time distributions. We vary assumptions and parameters of physical processes such as mass transfer stability criteria, supernova kick distributions, remnant mass distributions and common-envelope energetics. We compute the likelihood of observing the orbital period-eccentricity distribution of the Galactic DNS population under each of our population synthesis models, allowing us to quantitatively compare the models. We find that mass transfer from a stripped post-helium-burning secondary (case BB) onto a neutron star is most likely dynamically stable. We also find that a natal kick distribution composed of both low (Maxwellian $σ=30\rm~km~s^{-1}$) and high ($σ=265\rm~km~s^{-1}$) components is preferred over a single high-kick component. We find that the observed DNS mass distribution can place strong constraints on model assumptions.

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arXiv2017

Episodic mass ejections from common-envelope objects

After the initial fast spiral-in phase experienced by a common-envelope binary, the system may enter a slow, self-regulated phase, possibly lasting 100s of years, in which all the energy released by orbital decay can be efficiently transported to the surface, where it is radiated away. If the remaining envelope is to be removed during this phase, this removal must occur through some as-yet-undetermined mechanism. We carried out 1-d hydrodynamic simulations of a low-mass red giant undergoing a synthetic common-envelope event in such a slow spiral-in phase, using the stellar evolutionary code MESA. We simulated the heating of the envelope due to frictional dissipation from a binary companion's orbit in multiple configurations and investigated the response of the giant's envelope. We find that our model envelopes become dynamically unstable and develop large-amplitude pulsations, with periods in the range 3-20 years and very short growth time-scales of similar order. The shocks and associated rebounds that emerge as these pulsations grow are in some cases strong enough to dynamically eject shells of matter of up to 0.1 $\mathrm{M}_{\odot}$, $\sim 10$ % of the mass of the envelope, from the stellar surface at above escape velocity. These ejections are seen to repeat within a few decades, leading to a time-averaged mass-loss rate of order $10^{-3}$ $\mathrm{M}_{\odot} \: \mathrm{yr}^{-1}$ which is sufficiently high to represent a candidate mechanism for removing the entire envelope over the duration of the slow spiral-in phase.

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arXiv2016

Time-Variable Aluminum Absorption in the Polar AR Ursae Majoris, and an Updated Estimate for the Mass of the White Dwarf

We present spectra of the extreme polar AR Ursae Majoris (AR UMa) which display a clear Al I absorption doublet, alongside spectra taken less than a year earlier in which that feature is not present. Re-examination of earlier SDSS spectra indicates that the Al I absorption doublet was also present $\approx$8 years before our first non-detection. We conclude that this absorbing material is unlikely to be on the surface of either the white dwarf (WD) or the donor star. We suggest that this Al I absorption feature arises in circumstellar material, perhaps produced by the evaporation of asteroids as they approach the hot WD. The presence of any remaining reservoir of rocky material in AR UMa might help to constrain the prior evolution of this unusual binary system. We also apply spectral decomposition to find the stellar parameters of the M dwarf companion, and attempt to dynamically measure the mass of the WD in AR UMa by considering both the radial velocity curves of the H$_β$ emission line and the Na I absorption line. Thereby we infer a mass range for the WD in AR UMa of 0.91 $M_{\odot}$ $<$ $M_{\mathrm{WD}}$ $<$ 1.24 $M_{\odot}$.

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arXiv2015

Relativistic baryonic jets from an ultraluminous supersoft X-ray source

The formation of relativistic jets by an accreting compact object is one of the fundamental mysteries of astrophysics. While the theory is poorly understood, observations of relativistic jets from systems known as microquasars have led to a well-established phenomenology. Relativistic jets are not expected from sources with soft or supersoft X-ray spectra, although two such systems are known to produce relatively low-velocity bipolar outflows. Here we report optical spectra of an ultraluminous supersoft X-ray source (ULS) in the nearby galaxy M81 (M81 ULS-1) showing blueshifted broad Hα emission lines, characteristic of baryonic jets with relativistic speeds. The time variable jets have projected velocities ~17 per cent of the speed of light, and seem similar to those in the prototype microquasar SS 433. Such relativistic jets are not expected to be launched from white dwarfs, but an origin from a black hole or neutron star in M81 ULS-1 is hard to reconcile with its constant soft X-rays. The completely unexpected presence of relativistic jets in a ULS challenges the canonical theories for jet formation, but may possibly be explained by a long speculated super-critically accreting black hole with optically thick outflows

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arXiv2015

Globular cluster formation efficiencies from black-hole X-ray binary feedback

We investigate a scenario in which feedback from black-hole X-ray binaries (BHXBs) sometimes begins inside young star clusters before strong supernova feedback. Those BHXBs could reduce the gas fraction inside embedded young clusters whilst maintaining virial equilibrium, which may help globular clusters (GCs) to stay bound when supernova-driven gas ejection subsequently occurs. Adopting a simple toy model with parameters guided by BHXB population models, we produce GC formation efficiencies consistent with empirically-inferred values. The metallicity dependence of BHXB formation could naturally explain why GC formation efficiency is higher at lower metallicity. For reasonable assumptions about that metallicity dependence, our toy model can produce a GC metallicity bimodality in some galaxies without a bimodality in the field-star metallicity distribution.

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arXiv2015

Another thread in the tapestry of stellar feedback: X-ray binaries

We consider X-ray binaries (XBs) as potential sources of stellar feedback. XBs observationally appear able to deposit a high fraction of their power output into their local interstellar medium, which may make them a non-negligible source of energy input. The formation rate of the most luminous XBs rises with decreasing metallicity, which should increase their significance during galaxy formation in the early universe. We also argue that stochastic effects are important to XB feedback (XBF) and may dominate the systematic changes due to metallicity in many cases. Large stochastic variation in the magnitude of XBF at low absolute star formation rates provides a natural reason for diversity in the evolution of dwarf galaxies which were initially almost identical, with several percent of such halos experiencing energy input from XBs roughly two orders of magnitude above the most likely value. These probability distributions suggest that the effect of XBF is most commonly significant for total stellar masses between ~10^7 and 10^8 Msun, which might resolve a current problem with modelling populations of such galaxies. We explain how XBs might inject energy before luminous supernovae (SNe) contribute significantly to feedback and how XBs can assist in keeping gas hot long after the last core-collapse SN has exploded. [...] XBF could be especially important to some dwarf galaxies, potentially heating gas without expelling it; the properties of XBF also match those previously derived as allowing episodic star formation. We also argue that the efficiency of SN feedback (SNF) might be reduced when XBF has had the opportunity to act first. In addition, we note that the effect of SNF is unlikely to be scale-free; galaxies smaller than ~100 pc might well experience less effective SNF. (Slightly abbreviated to fit arXiv size limit.)

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arXiv2014

Luminous Blue Variables and superluminous supernovae from binary mergers

Evidence suggests that the direct progenitor stars of some core-collapse supernovae (CCSNe) are luminous blue variables (LBVs), perhaps including some `superluminous supernovae' (SLSNe). We examine models in which massive stars gain mass soon after the end of core hydrogen burning. These are mainly intended to represent mergers following a brief contact phase during early Case B mass transfer, but may also represent stars which gain mass in the Hertzsprung Gap or extremely late during the main-sequence phase for other reasons. The post-accretion stars spend their core helium-burning phase as blue supergiants (BSGs), and many examples are consistent with being LBVs at the time of core collapse. Other examples are yellow supergiants at explosion. We also investigate whether such post-accretion stars may explode successfully after core collapse. The final core properties of post-accretion models are broadly similar to those of single stars with the same initial mass as the pre-merger primary star. More surprisingly, when early Case B accretion does affect the final core properties, the effect appears likely to favour a successful SN explosion, i.e., to make the core properties more like those of a lower-mass single star. However, the detailed structures of these cores sometimes display qualitative differences to any single-star model we have calculated. The rate of appropriate binary mergers may match the rate of SNe with immediate LBV progenitors; for moderately optimistic assumptions we estimate that the progenitor birthrate is ~1% of the CCSN rate.

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arXiv2014

On the evolution of rotating accreting white dwarfs and type Ia supernovae

The potential importance of the angular momentum which is gained by accreting white dwarfs (WDs) has been increasingly recognized in the context of type Ia supernova (SN Ia) single-degenerate model. The expectation that the spin of the WD can delay the explosion should help the single-degenerate model to be consistent with the observed properties of most SNe Ia, in particular by avoiding hydrogen contamination. In this article, we attempt to study the most prominent single-degenerate supersoft (WD + MS) channel when the rotation of accreting WDs is considered. We present a detailed binary population synthesis study to examine the predicted population of SNe Ia for this channel. For our standard model, we find that 77% of these SNe Ia explode with WD masses which are low enough to be supported by solid-body rotation (1.378-1.5Msun); this is a substantially higher proportion than found by previous work. Only 2% have WD explosion masses >2.0Msun; these require the initial WD mass to be larger than 1.0 Msun. We further discuss the possible origin of the diversity of SNe Ia from the pre- and post- accretion properties of the WDs in this population. We also suggest that some SN Ia progenitors with substantial circumstellar hydrogen, including some apparent type IIn SNe, might be related to WDs which required support from differential rotation to avoid explosion, since these can still be accreting from hydrogen-rich donors with a relatively high mass-transfer rate at the time of the SN explosion.

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arXiv2014

The binarity of Milky Way F,G,K stars as a function of effective temperature and metallicity

We estimate the fraction of F,G,K stars with close binary companions by analysing multi-epoch stellar spectra from SDSS and LAMOST for radial velocity (RV) variations. We employ a Bayesian method to infer the maximum likelihood of the fraction of binary stars with orbital periods of 1000 days or shorter, assuming a simple model distribution for a binary population with circular orbits. The overall inferred fraction of stars with such a close binary companion is 43.0% \pm 2.0% for a sample of F, G, K stars from SDSS SEGUE, and 30% \pm 8.0% in a similar sample from LAMOST. The apparent close binary fraction decreases with the stellar effective temperature. We divide the SEGUE and LEGUE data into three subsamples with different metallicity ([Fe/H] < -1.1; -1.1 < [Fe/H] < -0.6; -0.6 < [Fe/H]), for which the inferred close binary fractions are 56% \pm 5.0%, 56.0% \pm 3%, and 30% \pm 5.7%. The metal-rich stars from our sample are therefore substantially less likely to possess a close binary companion than otherwise similar stars drawn from metal-poor populations. The different ages and formation environments of the Milky Way's thin disk, thick disk and halo may contribute to explaining these observations. Alternatively metallicity may have a significant effect on the formation and/or evolution of binary stars.

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