arXiv subjects

Gijs Nelemans

Publications and source records attributed to Gijs Nelemans.

At least 19 recordsLinked to original sources

arXiv2026

LISA and the LISA Science Team

LISA, the Laser Interferometer Space Antenna, due to launch mid-2035, is a large class space mission by the European Space Agency (ESA). In partnership with NASA and ESA-member states, ESA is on track to launch what is expected to be the first space-based gravitational wave detector. By hosting detectors in space, one gains access to a lower frequency band of gravitational wave sources and, with them, a plethora of new science. To maximise this scientific gain, ESA and NASA selected 20 scientists for the LISA Science Team to carry out and/or lead the necessary actions leading up to LISA's launch. We give a short overview and update of the LISA mission, its science objectives and related waveforms, as well as the work of the LISA Science Team as of April 2026.

astro-ph.IM↗

arXiv2025

ESO Expanding Horizons White Paper: Electromagnetic characterisation of millihertz gravitational-wave sources in the Milky Way

The millihertz band is densely populated by continuous gravitational-wave signals from Galactic compact binaries, dominated by double white dwarfs (DWDs; binaries of two white dwarfs) with contributions from systems containing neutron stars and black holes (Amaro-Seoane et al. 2023). As these binaries inspiral due to gravitational-wave radiation, they can reach contact and begin mass transfer in the millihertz band. Gravitational-wave detectors like LISA will survey such compact binaries across the Milky Way, yielding samples numbering in the tens of thousands, with essentially complete sensitivity to orbital periods shorter than ~10-20 min (e.g. Lamberts et al. 2019). Assessing the nature of the binary components - and deriving masses, temperatures and compositions - requires systematic electromagnetic characterisation that breaks gravitational-wave degeneracies and enables full atmospheric and orbital solutions. At present, no dedicated facility or coordinated survey is planned to deliver electromagnetic follow-up at the scale necessary to maximise the science return of the millihertz gravitational-wave data; this white paper discusses the need and requirements of such a capability.

astro-ph.IM↗

arXiv2025

Resolving white dwarf binaries within globular clusters with LISA

Context: Globular clusters (GCs) around the Milky Way (MW) are expected to host white dwarf (WD) binaries emitting gravitational waves that could be detectable by LISA. Aims: Our aim is to investigate whether LISA can resolve WD binaries in GCs well enough in terms of sky location and distance that they can be distinguished from binaries in the MW disc. Methods: We used a sample of 20 of the most massive GCs around the MW and simulated LISA's sky location and distance measurement errors for WD binaries in these GCs using the software package GWToolbox. We did this in the context of a model of the LISA-detectable binaries in the MW from the population synthesis code SeBa. Results: We find that for five of the GCs in our sample, binaries in the GC could be easily distinguished from MW disc binaries using the sky location alone; for another five, binaries in the GCs could be distinguished using a combination of LISA's sky location and distance measurements; and for the final ten, binaries in the GCs could not be distinguished from overlapping MW disc binaries. The results depend strongly on the sky locations of the GCs, with GCs far away from the Galactic plane being easy to resolve, while GCs close to the Galactic centre overlap with many MW disc binaries. The most promising GC for finding a WD binary that could be resolved to that GC, based on sky location and GC mass, is 47 Tucanae.

gr-qc↗

arXiv2025

The metallicity dependence of long-duration gamma-ray bursts

Both theoretical models and observations of collapsar created gamma-ray bursts -- typically long-duration gamma-ray bursts (LGRBs) -- suggest that these transients cannot occur at high metallicity, likely due to angular momentum losses via stellar winds for potential progenitor stars. However, the precise metallicity threshold (if it is a hard threshold) above which the formation of LGRBs is suppressed is still a topic of discussion. We investigated observed LGRBs and the properties of their host galaxies to constrain this metallicity dependence. In order to compute LGRB rates we modelled the cosmic history of star formation, as a function of host galaxy metallicity and stellar mass, and added a LGRB efficiency function that can include various shapes including abrupt cutoffs and more gradual variations in the GRB yield with metallicity. In contrast to previous work, this model includes scatters in the relations between mass, metallicity, and star formation rate, as well as a scatter in the metallicity distribution inside galaxies. We then varied both the threshold value and shape, and compared it to observed LGRBs and the properties of their host galaxies. In our model a sharp cutoff at an oxygen abundance $Z_{\text{O/H}}=12+\log(\text{O/H})=8.6\pm0.1$ (corresponding to $\sim0.6Z_{\odot}$) provides the best explanation for the observed LGRB data. In contrast, a lower threshold proposed in literature (i.e. at $Z_{\text{O/H}}=8.3$ or $\sim0.3Z_{\odot}$) fits observations poorly. We therefore conclude that, in contrast to most theoretical LGRB models, a relatively high metallicity threshold at near-solar values provides the best match between our model and observed LGRBs.

astro-ph.HE↗

arXiv2025

The DBL Survey II: towards a mass-period distribution of double white dwarf binaries

Double white dwarf binaries are an important remnant of binary evolution as they are possible type Ia supernova progenitors and strong sources of gravitational waves in the low-frequency regime. The double-lined double white dwarf (DBL) survey searches for compact double white dwarfs where both stars are spectrally disentangleable. Candidates are identified by being overluminous compared to the cooling sequence of a typical mass, single white dwarf. In this second DBL survey instalment, we present full orbital solutions of 15 double white dwarf binaries from our ongoing campaign to accurately measure a magnitude-limited mass-period distribution. 12 of these systems are fully solved for the first time. A long-standing bias in the full population has been evident, favouring systems with orbital periods up to a few hours, with little exploration of the majority of the compact double white dwarf population, whose orbital period distribution centres at approximately 20hr. The 15 systems in this study span the orbital period range 5-75hr, significantly augmenting the number of well-characterised systems over these periods, and in general have two similar mass stars combining to approximately 1.0 solar masses. We witness that the orbitally derived mass ratios generally show an excellent agreement with those deduced from atmospheric fits to double-lined spectra in previous work, emphasising the power of wide-scale spectroscopic surveys to efficiently locate the highest mass, double-lined double white dwarfs in the local Galaxy.

astro-ph.SR↗

arXiv2025

The Heavy Element Enrichment History of the Universe from Neutron Star Mergers with Habitable Worlds Observatory

Understanding where elements were formed has been a key goal in astrophysics for nearly a century, with answers involving cosmology, stellar burning, and cosmic explosions. Since 1957, the origin of the heaviest elements (formed via the rapid neutron capture process; r-process) has remained a mystery, identified as a key question to answer this century by the US National Research Council. With the advent of gravitational wave astronomy and recent measurements by the James Webb Space Telescope we now know that neutron star mergers are a key site of heavy element nucleosynthesis. We must now understand the heavy element yield of these events as well as mapping when these mergers occurred back through cosmic time, currently thought to peak when the universe was half its current age. This requires an extremely sensitive ultraviolet, optical, and infrared telescope which can respond rapidly to external discoveries of neutron star mergers. We here describe how the Habitable Worlds Observatory can provide the first complete answer to one of the questions of the century.

astro-ph.HE↗

arXiv2025

The first phase of mass transfer in low-mass binaries: neither stable nor a common envelope

The masses of the white dwarfs in a binary carry information about previous mass-transfer phases. The core mass -- radius relation of low-mass giants gives the size of the progenitor of a helium white dwarf at the moment it last filled its Roche lobe. Previously, we used this information for a few observed systems to propose a new mass-transfer type, based on an angular momentum balance. Our aim is to investigate if stable mass transfer instead of the angular momentum prescription is consistent with the observed double helium white dwarf masses. We reconstruct the progenitor evolution of observed double helium white dwarfs using the core mass -- radius relation and evaluate if the periods at the start of the second phases of mass transfer are consistent with the outcome of stable mass transfer. More generally, we calculate the mass distribution of double helium white dwarfs for three different progenitors scenarios: double common envelope (with parameter $\alpha \lambda$), angular momentum prescription (with parameter $\gamma$) and stable mass transfer. We find that the observed systems are generally not consistent with stable mass transfer. Stable mass transfer leads to a tight correlation between the two white dwarf masses in a binary that is not consistent with the observed mass distribution. Double common envelope evolution is a particularly poor fit to the observations. The angular momentum prescription can populate the observed mass distribution, but not perfectly. We conclude that the first phase of mass transfer initiated on the red giant branch in low-mass systems does not generally proceed as stable mass transfer nor as common envelope, and thus is poorly understood. This may be related to the fact that for many observed binaries that have finished the first phase of mass transfer the orbit is eccentric, which is an unexpected outcome of mass transfer.

astro-ph.SR↗

arXiv2025

Comparing population synthesis models of compact double white dwarfs to electromagnetic observations

Context: Studies of the Galactic population of double white dwarfs (DWDs) that would be detectable in gravitational waves by LISA have found differences in the number of predicted detectable DWDs of more than an order of magnitude, depending on the binary stellar evolution model used. Particularly, the binary population synthesis code BPASS predicts 20 to 40 times fewer detectable DWDs than the codes SeBa or BSE, which relates to differing treatments of mass transfer and common-envelope events (CEEs). Aims: We aimed to investigate which of these models are closer to reality by comparing their predictions to the DWDs known from electromagnetic observations. Methods: We compared the DWDs predicted by a BPASS galaxy model and a SeBa galaxy model to a DWD catalogue and the sample of DWDs observed by the Zwicky Transient Facility (ZTF), taking into account the observational limits and biases of the ZTF survey. Results: We found that BPASS underpredicts the number of short-period DWDs by at least an order of magnitude compared to the observations, while the SeBa galaxy model is consistent with the observations for DWDs more distant than 500 pc. These results highlight how LISA's observations of DWDs will provide invaluable information on aspects of stellar evolution such as mass transfer and CEEs, which will allow theoretical models to be better constrained.

astro-ph.SR↗

arXiv2025

21 years of Astronomy at Warwick: celebrating the legacy of Prof. Tom Marsh

Between the 4th and 6th of September 2024, the Astronomy & Astrophysics group at the University of Warwick held a meeting to celebrate 21 years of astronomy at Warwick and the scientific legacy of the late Prof. Tom Marsh, the group founder. More than a hundred people attended the meeting, with about half of the attendees being external delegates and coming from as far afield as the USA and South Africa. Tom Marsh moved to the University of Warwick from Southampton in 2003, after the Department of Physics decided to expand the scope of its research. From its humble beginnings with only two staff members, Tom himself and Boris G\"ansicke, one postdoc and a couple of PhD students, the group has now grown to more than 95 members, including 25 staff. Tom pioneered the development of Doppler tomography, led key discoveries in the field of double-degenerate binary systems and made extensive contributions to instrumentation, primarily to developing the high-speed imaging photometers ULTRACAM, ULTRASPEC and HiPERCAM. This article provides a summary of Tom's legacy and Warwick's history as presented in the 21 years of Astronomy at Warwick meeting.

astro-ph.SR↗

arXiv2025

A super-Chandrasekhar mass type Ia supernova progenitor at 49 pc set to detonate in 23 Gyr

Double white dwarf binaries are a leading explanation to the origin of type Ia supernovae, but no system exceeding the Chandrasekhar mass limit (1.4 M$_\odot$) has been found that will explode anywhere close to a Hubble time. Here, we present the super-Chandrasekhar mass double white dwarf WDJ181058.67+311940.94 whose merger time ($22.6\pm1.0$ Gyr) is of the same order as a Hubble time. The mass of the binary is large, combining to $1.555\pm0.044$ M$_\odot$, while being located only 49 pc away. We predict that the binary will explode dynamically via a double detonation destroying both stars just before they merge, appearing as a subluminous type Ia supernova with a peak apparent magnitude of about $m_V=-16$ (200,000 times brighter than Jupiter). The observationally-derived birthrate of super-Chandrasekhar mass double white dwarfs is now at least $6.0\times10^{-4}$ yr$^{-1}$ and the observed rate of type Ia supernovae in the Milky Way from such systems is approximately $4.4\times10^{-5}$ yr$^{-1}$, while the predicted type Ia supernova rate in the Milky Way from all progenitor channels is about sixty times larger. Hence, WDJ181058.67+311940.94 mitigates the observed deficit of massive double white dwarfs witnessed in volume-complete populations, but further evidence is required to determine the majority progenitors of type Ia supernovae.

astro-ph.SR↗

arXiv2024

WR + O binaries as probes of the first phase of mass transfer

Context. Wolf-Rayet (WR) and O-star binaries can be the progenitors of X-ray binaries and double black hole binaries. Their formation is not yet fully understood, however. For 21 observed WR+O systems, we aim to infer whether the mass transfer started on the main sequence (Case A) or later (Case B). We also calculated (limits on) the mass-transfer efficiency {\beta}, that is, the fraction of transferred mass that is accreted, and the parameter {\gamma}, which denotes the fraction of angular momentum of the binary that is lost per unit mass in units of the average angular momentum of the binary per unit mass. Aims. We inferred the possible values for the initial masses based on the observed WR masses and models for WR from the literature. With these initial primary masses, we created a grid of possible periods and secondary masses for which we determined the values that {\beta} and {\gamma} would have taken for either Case A or Case B mass transfer. Based on this, we also determined the case of mass transfer that is most likely for each system. Methods. Taking into account the progenitor distribution of WR+O binaries, we find that highly non-conservative Case A mass transfer seems to be the most likely scenario for the majority of systems as this can explain 14 out of 21 systems. The angular momentum loss is likely relatively high (typically {\gamma} > 1). Our finding that most systems in our sample experienced Case A mass transfer contradicts the expectation that most massive binaries go through Case B mass transfer. This suggests that post-case-B systems are significantly underrepresented in the observed WR+O binary population, either intrinsically or due to severe selection effects.

astro-ph.SR↗

arXiv2024

Expanding the ultracompacts: gravitational wave-driven mass transfer in the shortest-period binaries with accretion disks

We report the discovery of three ultracompact binary white dwarf systems hosting accretion disks, with orbital periods of 7.95, 8.68, and 13.15 minutes. This significantly augments the population of mass-transferring binaries at the shortest periods, and provides the first evidence that accretors in ultracompacts can be dense enough to host accretion disks even below 10 minutes (where previously only direct-impact accretors were known). In the two shortest-period systems, we measured changes in the orbital periods driven by the combined effect of gravitational wave emission and mass transfer; we find $\dot{P}$ is negative in one case, and positive in the other. This is only the second system measured with a positive $\dot{P}$, and it the most compact binary known that has survived a period minimum. Using these systems as examples, we show how the measurement of $\dot{P}$ is a powerful tool in constraining the physical properties of binaries, e.g. the mass and mass-radius relation of the donor stars. We find that the chirp masses of ultracompact binaries at these periods seem to cluster around $\mathcal{M}_c \sim 0.3 M_\odot$, perhaps suggesting a common origin for these systems or a selection bias in electromagnetic discoveries. Our new systems are among the highest-amplitude known gravitational wave sources in the millihertz regime, providing exquisite opportunity for multi-messenger study with future space-based observatories such as \textit{LISA} and TianQin; we discuss how such systems provide fascinating laboratories to study the unique regime where the accretion process is mediated by gravitational waves.

astro-ph.HE↗

arXiv2024

Two waves of massive stars running away from the young cluster R136

Massive stars are predominantly born in stellar associations or clusters. Their radiation fields, stellar winds, and supernovae strongly impact their local environment. In the first few million years of a cluster's life, massive stars are dynamically ejected running away from the cluster at high speed. However, the production rate of dynamically ejected runaways is poorly constrained. Here we report on a sample of 55 massive runaway stars ejected from the young cluster R136 in the Large Magellanic Cloud. Astrometric analysis with Gaia reveals two channels of dynamically ejected runaways. The first channel ejects massive stars in all directions and is consistent with dynamical interactions during and after the birth of R136. The second channel launches stars in a preferred direction and may be related to a cluster interaction. We find that 23-33% of the most luminous stars initially born in R136 are runaways. Model predictions have significantly underestimated the dynamical escape fraction of massive stars. Consequently, their role in shaping and heating the interstellar and galactic medium, along with their role in driving galactic outflows, is far more important than previously thought.

astro-ph.SR↗

arXiv2024

Evaluating the gravitational wave detectability of globular clusters and the Magellanic Clouds for LISA

We use the stellar evolution code BPASS and the gravitational wave simulation code LEGWORK to simulate populations of compact binaries that may be detected by the in-development space-based gravitational wave (GW) detector LISA. Specifically, we simulate the Magellanic Clouds and binary populations mimicking several globular clusters, neglecting dynamical effects. We find that the Magellanic Clouds would have a handful of detectable sources each, but for globular clusters the amount of detectable sources would be less than one. We compare our results to earlier research and find that our predicted numbers are several tens of times lower than calculations using the stellar evolution code BSE that take dynamical effects into account, but also calculations using the stellar evolution code SeBa for the Magellanic Clouds. This correlates with earlier research which compared BPASS models for GW sources in the Galactic disk with BSE models and found a similarly sized discrepancy. We analyse and explain this discrepancy as being caused by differences between the stellar evolution codes, particularly in the treatment of mass transfer and common-envelope events in binaries, where in BPASS mass transfer is more likely to be stable and tends to lead to less orbital shrinkage in the common-envelope phase than in other codes. This difference results in fewer compact binaries with periods short enough to be detected by LISA existing in the BPASS population. For globular clusters, we conclude that the impact of dynamical effects is uncertain from the literature, but the differences in stellar evolution have an effect of a factor of a few tens.

gr-qc↗

arXiv2024

On the uncertainty of the White Dwarf Astrophysical Gravitational Wave Background

Context: The astrophysical gravitational wave background (AGWB) is a stochastic gravitational wave (GW) signal that is emitted by different populations of inspiralling binary systems containing compact objects throughout the Universe. In the frequency range between 0.1 and 100 mHz it will be detected by future space-based gravitational wave detectors like the Laser Interferometer Space Antenna (LISA). Recently, we concluded that the white dwarf (WD) contribution to the AGWB dominates over that of black holes (BHs) and neutron stars (NSs). Aims: We aim to investigate the uncertainties of the WD AGWB that arise from the use of different stellar metallicities, different star formation rate density (SFRD) models, and different binary evolution models. Methods: We use the code developed before to determine the WD component of the AGWB. We use a metallicity dependent SFRD based on earlier work to construct five different SFRD models. We use four different population models that use different common-envelope treatment and six different metallicities for each model. Results: For all possible combinations, the WD component of the AGWB is dominant over other populations of compact objects. The effects of metallicity and population model are smaller than the effect of a (metallicity dependent) SFRD model. We find a range of about a factor of 5 in the level of the WD AGWB around a mid value of $\Omega_{\rm WD} = 4\times10^{-12}$ at 1 mHz and a shape that depends weakly on the model. Conclusions: We find an uncertainty for the WD component of the AGWB of about a factor 5. We note that there exist other uncertainties that have an effect on this signal as well. We discuss whether the turnover of the WD AGWB at 10 mHz will be detectable by LISA, and find that this is likely. We confirm the previous finding that the WD component of the AGWB dominates over other populations, in particular BHs.

astro-ph.HE↗

arXiv2024

The DBL Survey I: discovery of 34 double-lined double white dwarf binaries

We present the first discoveries of the double-lined double white dwarf (DBL) survey that targets over-luminous sources with respect to the canonical white dwarf cooling sequence according to a set of well-defined criteria. The primary goal of the DBL survey is to identify compact double white dwarf binary star systems from a unique spectral detection of both stars, which then enables a precise quantification of the atmospheric parameters and radial velocity variability of a system. Our search of 117 candidates that were randomly selected from a magnitude limited sample of 399 yielded a 29% detection efficiency with 34 systems exhibiting a double-lined signature. A further 38 systems show strong evidence of being single-lined or potentially-double-lined double white dwarf binaries and 7 single-lined sources from the full observed sample are radial velocity variable. The 45 remaining candidates appear as a single WD with no companion or a non-DA white dwarf, bringing the efficiency of detecting binaries to 62%. Atmospheric fitting of all double-lined systems reveals a large fraction that have two similar mass components that combine to a total mass of 1.0-1.3 solar masses - a class of double white dwarf binaries that may undergo a sub-Chandrasekhar mass type Ia detonation or merge to form a massive O/Ne WD, although orbital periods are required to infer on which timescales. One double-lined system located 49pc away, WDJ181058.67+311940.94, is super-Chandrasekhar mass, making it the second such double white dwarf binary to be discovered.

astro-ph.SR↗

arXiv2024

LISA Definition Study Report

The Laser Interferometer Space Antenna (LISA) is the first scientific endeavour to detect and study gravitational waves from space. LISA will survey the sky for Gravitational Waves in the 0.1 mHz to 1 Hz frequency band which will enable the study of a vast number of objects ranging from Galactic binaries and stellar mass black holes in the Milky Way, to distant massive black-hole mergers and the expansion of the Universe. This definition study report, or Red Book, presents a summary of the very large body of work that has been undertaken on the LISA mission over the LISA definition phase.

astro-ph.CO↗

arXiv2023

The Astrophysical Gravitational Wave Background in the mHz band is likely dominated by White Dwarf binaries

Context. The Astrophysical Gravitational Wave Background (AGWB) is a collective signal of astrophysical gravitational wave sources and is dominated by compact binaries. Its measurement is one of the science goals of current and future gravitational wave detectors. Aims. We aim to determine what population of compact binaries dominates the AGWB in the mHz band. Methods. We revisit and update earlier work by Farmer & Phinney (2003) to model the astrophysical gravitational wave background sourced by extragalactic white dwarf binaries in the mHz frequency band. We calculate the signal using a single-metallicity model for the white dwarf population in the Universe using a global star formation history. Results. We estimate the white dwarf AGWB amplitude to be $\sim$ 60% larger than the earlier estimate and find that the overall shape of the white dwarf AGWB is well fitted by a broken power law combined with an exponential cut-off. Conclusions. We compare the results to the present-day best estimates for the background due to black hole and neutron star binaries, and find that the white dwarf component likely dominates in the mHz band. We provide an order of magnitude estimate that explains this hierarchy, and comment on the implications for future missions that aim to detect the AGWB. The black hole AGWB may only be detectable at high frequency. We outline several improvements that can be made to our estimate, but this is unlikely to change our main conclusion that the white dwarf AGWB dominates in the mHz band.

astro-ph.HE↗