arXiv subjects

Alexander Heger

Publications and source records attributed to Alexander Heger.

At least 19 recordsLinked to original sources

arXiv2026

Are most detected tidal disruption events partial?

During a tidal disruption event (TDE), a star loses mass due to the tidal gravitational forces of the black hole. In a partial tidal disruption event, a stellar remnant is left behind. Several dozen TDEs have been detected so far, including repeating partial events. We use the Phantom smoothed particle hydrodynamics code to model the disruption of a 1 Msun star around a 10^6 Msun black hole for impact parameters resulting in less than equal to 50 % mass loss. We only consider zero energy orbits. Our simulations show that the mass fallback rate can exceed the Eddington limit for beta greater than or equal to 0.8, allowing debris to obscure the accretion disc by forming a reprocessing layer, similar to full TDEs. The mass fallback rate is shallower than t^{-5/3}, tracking closer to t^{-9/4}. Assuming thermal emission from the debris, that shock heating is trapped, that electron scattering dominates the opacity, and a color correction f_{col} of 1.7, we find temperatures of ~10^4 K, optical bolometric luminosities of ~ 10^{42-44} erg/s and blackbody radii ranging from 10-100 au for our simulations. We compare our values with observations and find support for the previous argument that some TDEs classified as full disruptions might actually be partial. Moreover, our results explain the detected optical/UV TDEs. We also find that our zero energy partial TDEs have properties similar to the repeating partial TDEs such as ASSASN-19dj, ASSASN-14ko, ASSASN-18ul, ASSASN-22ci, AT2020vdq and AT2022dbl. In the beta=0.8, isentropic simulation where radiation is assumed to escape, we find X-ray luminosities of ~ 10^{44-45} erg/s and radii lower than the inner most stable circular orbit.

astro-ph.HE↗

arXiv2026

A benchmark for binary star interaction with a supermassive black hole in general relativity

Most galaxies have supermassive black holes (SMBH) at their centres, surrounded by stars with binary systems also present in this environment. We use two schemes - post-Newtonian (PN) and a scalar perturbation to a background metric to numerically solve the three-body problem of a binary with a SMBH. We test three different PN formulations for the PN scheme: The Einstein-Infeld-Hoffman equation, pair-wise implementation of two-body PN-terms for three bodies and the Arnowitt-Deser-Misner Hamiltonian. We compare these approaches for one million solar mass and one billion solar mass black holes, and find a statistical match between the two approximations for stellar mass binary interacting with a million solar mass black hole. We also perform a statistical study for encounters with this black hole, and find that the higher order PN formulation matches with metric-with-perturbation scheme. However, we find a decrease in separation of the binary, and eccentricity variations between different schemes around the billion solar mass black hole. This behaviour is not present if binary has a large separation or is further away from the black hole due to decreased general-relativistic effects. We find that the pair-wise PN method results in a decrease in separation at pericentre in all test cases irrespective of the distance from the black hole or mass of the black hole, making this the least reliable method for solving this problem. Our work highlights the need for caution when interpreting the results in different formulations around SMBHs. This also shows that when understanding extreme mass ratio inspirals (EMRIs) using simulations, one should beware as the binary gets closer to the black hole.

astro-ph.IM↗

arXiv2026

The deci-Hz gravitational wave signal from the collapse of rotating very massive stars

We calculate the gravitational wave signal from the collapse of a rotating 300 $M_\odot$ star at the upper end of the pair-instability regime. The large-scale asymmetries that develop during the collapse produce a strong signal in the deci-Hz range that has a characteristic shape which is likely amenable to a template-based search. The most ambitious designs for deci-Hz detectors could detect such signals out to distances of 200 Mpc, possibly at a rate of 0.5 per year.

astro-ph.HE↗

arXiv2026

Evolution of wide O star binaries through their LBV stage. Population synthesis with mass-ejection-driven orbital evolution

Context. Long-period Wolf-Rayet (WR) star binaries produced by mass transfer are predicted to be abundant, but are observationally rare. This yields constraints on the evolution of initially wide O star binaries, including those potentially leading to the formation of gravitational-wave sources through the Common Envelope Channel. Aims. We investigate this issue in the light of a new type of orbital evolution for initially wide O star binaries, which is driven by mass ejection at periastron passage during the Luminous Blue Variable (LBV) phase. Methods. The assumption that the mass ejection occurs instantly at periastron passage allows us to analytically describe the orbital evolution. This approach is motivated by our understanding of an Eddington-limit driven LBV phase. We perform population synthesis calculations for the WR stars in the Small Magellanic Cloud (SMC), and compare them to the observed SMC WR star population. Results. Different from mass transfer, our mass ejection scenario leads to increased orbital periods and eccentricities. The Galactic system WR 140 (orbital period 2895 d, eccentricity 0.9) could be a typical result of this evolution scenario. Our models predict measurable binary space velocities, and allow for the disruption of the binary. Our SMC population synthesis model predicts statistically 5.3 close, 3.7 long-period, and further 2 runaway single WR stars. With largely increased orbital periods and eccentricities, such WR+O star binaries may not be ruled out by past radial-velocity searches. Applying our scenario to the Gaia BH1 and BH2 systems, we find that it provides viable progenitor evolution models. Conclusions. The mass-ejection-driven orbital evolution could explain why so few wide WR binaries are observed, and why some of the apparently single WR stars have high space velocities. We discuss implications for gravitational-wave sources.

astro-ph.SR↗

arXiv2025

GRBs and Relativistic Transients in the 2040s

Relativistic transients such as gamma-ray bursts (GRBs), jetted tidal disruption events, luminous fast blue optical transients, and fast X-ray transients, represent the brightest explosions in the Universe and serve dual roles as laboratories for extreme physics and as cosmic lighthouses probing the earliest epochs of the Universe. The 2040s will bring transformative capabilities: wide-field optical surveys discovering tens of thousands of optical transients nightly, proposed high-energy missions like THESEUS providing 10-100x improved high-energy monitoring, and third-generation gravitational wave detectors identifying $\mathcal{O}(10^5)$ compact object mergers annually, many accompanied by relativistic jets. This industrial-scale discovery rate will enable population studies addressing fundamental questions such as jet launching mechanisms, nucleosynthesis, the first stars, and how progenitor environments shape these transients across cosmic time. However, realizing this science requires overcoming a critical bottleneck: these transients evolve on timescales of seconds to days, with their physics encoded in rapidly-changing multi-wavelength signatures demanding immediate spectroscopic characterization down to m ~ 25. Current facilities, optimized for classical/queue scheduling, do not provide the rapid, flexible, multi-target response necessary for industrial-scale follow-up. This white paper demonstrates that without a dedicated large-aperture (10-30 m effective collecting area) time-domain facility with robotic scheduling and optical-NIR spectroscopic capabilities, the transformative potential of relativistic transient science in the 2040s will be considerably limited.

astro-ph.HE↗

arXiv2025

The Regulated GeAs Cycles with the New $^{63}$Ga(p,$\gamma$)$^{64}$Ge and $^{64}$Ge(p,$\gamma$)$^{65}$As Reaction Rates and Their Impact on the GS 1826$-$24 Clocked Bursts and SAX J1808.4$-$3658 Photospheric Radius Expansion Bursts

The $^{63}$Ga(p,$\gamma$)$^{64}$Ge and $^{64}$Ge(p,$\gamma$)$^{65}$As thermonuclear reactions connect the ZnGa and GeAs cycles by diverting the flow of the rapid proton capture process from $^{63}$Ga to $^{65}$As. Changes in these two reaction rates regulate the ZnGa and GeAs cycles and may affect the modeled properties matching with the observed counterparts of a type I X-ray burster. We implement the latest $^{63}$Ga(p,$\gamma$)$^{64}$Ge and $^{64}$Ge(p,$\gamma$)$^{65}$As reaction rates to the state-of-the-art self-consistent one-dimensional multi-zone thermo-hydrodynamic code, KEPLER, to study the influence of these new reaction rates on the models of the GS 1826$-$24 clocked burster and SAX J1808.4$-$3658 photospheric radius expansion burster. Both new reaction rates obtained by Lu et al. [Phys. Rev. C 110, 065804 (2024)] are determined from complementing the experimental input with the nuclear spectroscopic information deduced from the full pf-shell space configuration-interaction shell-model calculations. By constraining the models on reproducing the observed burst peak, light-curve profile, fluence, and recurrence time, we find that the impact of the newly measured proton thresholds and respective proton-capture reactions on the burst light-curve profile of the GS 1826$-$24 clocked burster is, in fact, not as significant as claimed by Zhou et al. [Nat. Phys. 19, 1091 (2023)]. With or without the inclusion of the newly determined reaction rate of the highly influential $^{22}$Mg($\alpha$,p)$^{25}$Al reaction, the impact of the new $^{63}$Ga(p,$\gamma$)$^{64}$Ge and $^{64}$Ge(p,$\gamma$)$^{65}$As reaction rates on SAX J1808.4$-$3658 photospheric radius expansion bursts is evident. Our finding indicates that the models reproducing the 2002 October epoch of SAX J1808.4$-$3658 photospheric radius expansion burster is more sensitive to the uncertainties of thermonuclear reaction rates.

astro-ph.HE↗

arXiv2025

Type Ibn supernovae from ultra-stripped supernova progenitors

Ultra-stripped supernovae are core-collapse supernovae from progenitors that lose a significant fraction of mass because of the binary interactions with their compact companion stars. Ultra-stripped supernovae have been connected to fast-evolving faint Type Ib or Ic supernovae. Here, we show that in some cases ultra-stripped supernovae can result in Type Ibn supernovae. Progenitors of ultra-stripped supernovae may trigger violent silicon burning shortly before the core collapse, leading to mass ejection that results in a dense circumstellar matter. By taking an ultra-stripped supernova progenitor that loses 0.2 Msun at 78 days before the core collapse, we compute the light-curve evolution of the ultra-stripped supernova within the dense circumstellar matter. The core collapse results in a supernova explosion with an ejecta mass of 0.06 Msun and an explosion energy of 9e49 erg. Because the dense circumstellar matter is more massive than the supernova ejecta, the ejecta are immediately decelerated and the light curve is powered mainly by the circumstellar interaction. Therefore, this ultra-stripped supernova is likely observed as a Type Ibn supernova. We suggest that some Type Ibn supernovae may originate from ultra-stripped supernova progenitors losing significant mass shortly before their explosion due to violent silicon burning.

astro-ph.HE↗

arXiv2025

1000-10,000 M$_\odot$ Primordial Stars Created the Nitrogen Excess in GS 3073 at $z = 5.55$

The advent of the James Webb Space Telescope has revealed a wealth of new galaxies just a few hundred Myr after the Big Bang. Some of these galaxies exhibit unusual N/O ratios that are difficult to explain with stellar populations today. While Wolf-Rayet stars in multiple-burst populations, very massive or rapidly-rotating primordial stars, general relativistic explosions of metal-enriched supermassive stars, or the precursors of globular clusters can in principle account for the nitrogen excess in the galaxies GN-z11 and CEERS 1019, no known stars or supernovae can explain the far higher N/O ratio of 0.46 in GS 3073 at redshift $z =$ 5.55. Here we show that the extreme nitrogen abundances in GS 3073 can be produced by 1000 - 10,000 M$_{\odot}$ primordial (Pop III) stars. We find that these are the only candidates that can account for its large N/O ratios and its C/O and Ne/O ratios. GS 3073 is thus the first conclusive evidence in the fossil abundance record of the existence of supermassive Pop III stars at cosmic Dawn.

astro-ph.GA↗

arXiv2024

The birth mass function of neutron stars

The birth mass function of neutron stars encodes rich information about supernova explosions, double star evolution, and properties of matter under extreme conditions. To date, it has remained poorly constrained by observations, however. Applying probabilistic corrections to account for mass accreted by recycled pulsars in binary systems to mass measurements of 90 neutron stars, we find that the birth masses of neutron stars can be described by a unimodal distribution that smoothly turns on at $1.1 M_{\odot}$, peaks at $1.27 M_{\odot}$, before declining as a steep power law. Such a ``turn-on" power-law distribution is strongly favoured against the widely-adopted empirical double-Gaussian model at the $3 \sigma$ level. The power-law shape may be inherited from the initial mass function of massive stars, but the relative dearth of massive neutron stars implies that single stars with initial masses greater than $\approx 18 M_{\odot}$ do not form neutron stars, in agreement with the absence of massive red supergiant progenitors to supernovae.

astro-ph.HE↗

arXiv2024

Synthetic Light Curves and Spectra for the Photospheric Phase of a 3D Stripped-Envelope Supernova Explosion Model

We present synthetic light curves and spectra from three-dimensional (3D) Monte Carlo radiative transfer simulations based on a 3D core-collapse supernova explosion model of an ultra-stripped $3.5\,\mathrm{M}_{\odot}$ progenitor. Our calculations predict a fast and faint transient with $\Delta m_{15} \sim 1\texttt{-} 2\,\mathrm{mag}$ and peak bolometric luminosity between $-15.3\,\mathrm{mag}$ and $-16.4\,\mathrm{mag}$. Due to a large-scale unipolar asymmetry in the distribution of $^{56}\mathrm{Ni}$, there is a pronounced viewing-angle dependence with about $1\,\mathrm{mag}$ difference between the directions of highest and lowest luminosity. The predicted spectra for this rare class of explosions do not yet match any observed counterpart. They are dominated by prominent Mg~II lines, but features from O, C, Si, and Ca are also found. In particular, the O~I line at \wl{7}{774} appears as a blended feature together with Mg~II emission. Our model is not only faster and fainter than the observed Ib/c supernova population, but also shows a correlation between higher peak luminosity and larger $\Delta m_{15}$ that is not present in observational samples. A possible explanation is that the unusually small ejecta mass of our model accentuates the viewing-angle dependence of the photometry. We suggest that the viewing-angle dependence of the photometry may be used to constrain asymmetries in explosion models of more typical stripped-envelope supernova progenitors in future.

astro-ph.HE↗

arXiv2024

The Impact of Initial Composition on Massive Star Evolution and Nucleosynthesis

We study the sensitivity of presupernova evolution and supernova nucleosynthesis yields of massive stars to variations of the initial composition. We use the solar abundances from Lodders (2009), and compute two different initial stellar compositions: i) scaled solar abundances, and ii) the isotopic galactic chemical history model (GCH) developed by West and Heger (2013b). We run a grid of models using the KEPLER stellar evolution code, with 7 initial stellar masses, 12 initial metallicities, and two for each scaling method to explore the effects on nucleosynthesis over a metallicity range of $-4.0\leq[Z]\leq+0.3$. We find that the compositions from the GCH model better reproduce the weak \emph{s}-process peak than the scaled solar models. The model yields are then used in the OMEGA Galactic Chemical Evolution (GCE) code to assess this result further. We find that initial abundances used in computing stellar structure have more of an impact on GCE results than initial abundances used in the burn network, with the GCH model again being favored when compared to observations. Lastly, a machine learning algorithm was used to verify the free parameter values of the GCH model, which were previously found by West and Heger (2013b) using a stochastic fitting process. The updated model is provided as an accessible tool for further nucleosynthesis studies.

astro-ph.SR↗

arXiv2024

Neutron star kicks plus rockets as a mechanism for forming wide low-eccentricity neutron star binaries

Recent neutron star surface observations corroborate a long-standing theory that neutron stars may be accelerated over extended periods after their birth. We analyze how these prolonged rocket-like accelerations, combined with rapid birth kicks, impact binary orbits. We find that even a small contribution of rocket kicks combined with instantaneous natal kicks can allow binaries to reach period--eccentricity combinations unattainable in standard binary evolution models. We propose these kick + rocket combinations as a new channel to form wide low-eccentricity neutron star binaries such as Gaia NS1, as well as inducing stellar mergers months to years after a supernova to cause peculiar high-energy transients.

astro-ph.HE↗

arXiv2024

The minimum neutron star mass in neutrino-driven supernova explosions

Supernova theory has struggled to explain the lightest known neutron star candidate with an accurate mass determination, the $1.174\mathrm{M}_\odot$ companion in the eccentric compact binary system J0453+1559. To improve the theoretical lower limit for neutron star birth masses, we perform 3D supernova simulations for five stellar models close to the minimum mass for iron core collapse. We obtain a record-low neutron star mass of $1.192\mathrm{M}_\odot$ and a substantial kick of $\mathord{\sim} 100\,\mathrm{km}\,\mathrm{s}^{-1}$. Given residual uncertainties in stellar evolution, a neutron star origin for the $1.174\mathrm{M}_\odot$ object remains plausible.

astro-ph.HE↗

arXiv2024

New $^{63}$Ga(p,$\gamma$)$^{64}$Ge and $^{64}$Ge(p,$\gamma$)$^{65}$As reaction rates corresponding to the temperature regime of thermonuclear X-ray bursts

We compute the $^{63}$Ga(p,$\gamma$)$^{64}$Ge and $^{64}$Ge(p,$\gamma$)$^{65}$As thermonuclear reaction rates using the latest experimental input supplemented with theoretical nuclear spectroscopic information. The experimental input consists of the latest proton thresholds of $^{64}$Ge and $^{65}$As, and the nuclear spectroscopic information of $^{65}$As, whereas the theoretical nuclear spectroscopic information for $^{64}$Ge and $^{65}$As are deduced from the full pf-shell space configuration-interaction shell-model calculations with the GXPF1A Hamiltonian. Both thermonuclear reaction rates are determined with known uncertainties at the energies that correspond to the Gamow windows of the temperature regime relevant to Type I X-ray bursts, covering the typical temperature range of the thermonuclear runaway of the GS 1826$-$24 periodic bursts and SAX J1808.4$-$3658 photospheric radius expansion bursts.

nucl-th↗

arXiv2024

Convection and the Core $g$-mode in Proto-Compact Stars -- A detailed analysis

We present a detailed analysis of the dynamics of proto-compact star (PCS) convection and the core ${}^2\!g_1$-mode in core-collapse supernovae based on general relativistic 2D and 3D neutrino hydrodynamics simulations. Based on 2D simulations, we derive a mode relation for the core $g$-mode frequency in terms of PCS and equation of state parameters, and discuss its limits of accuracy. This relation may prove useful for parameter inference from future supernova gravitational wave (GW) signals if the core $g$-mode or an emission gap at the avoided crossing with the fundamental mode can be detected. The current 3D simulation does not show GW emission from the core $g$-mode due to less power in high-frequency convective motions to excite the mode, however. Analysing the dynamics of PCS convection in 3D, we find that simple scaling laws for convective velocity from mixing-length theory (MLT) do not apply. Energy and lepton number transport is instead governed by a more complex balance between neutrino fluxes and turbulent fluxes that results in roughly uniform rates of change of entropy and lepton number in the PCS convection zone. Electron fraction and enthalpy contrasts in PCS convection are not well captured by the MLT gradient approximation. We find distinctly different spectra for the turbulent kinetic energy and turbulent fluctuations in the electron fraction, which scale approximately as $l^{-1}$ without a downturn at low $l$. We suggest that the different turbulence spectrum of the electron fraction is naturally expected for a passive scalar quantity.

astro-ph.HE↗

arXiv2024

Partial tidal disruption events: The elixir of life

In our Galactic Center, about 10,000 to 100,000 stars are estimated to have survived tidal disruption events, resulting in partially disrupted remnants. These events occur when a supermassive black hole (SMBH) tidally interacts with a star, but not enough to completely disrupt the star. We use the 1D stellar evolution code Kepler and the 3D smoothed particle hydrodynamics code Phantom to model the tidal disruption of 1, 3, and 10 solar mass stars at zero-age (ZAMS), middle-age (MAMS), and terminal-age main-sequence (TAMS). We map the disruption remnants into Kepler in order to understand their post-distribution evolution. We find distinct characteristics in the remnants, including increased radius, rapid core rotation, and differential rotation in the envelope. The remnants undergo composition mixing that affects their stellar evolution. Whereas the remnants formed by disruption of ZAMS models evolve similarly to unperturbed models of the same mass, for MAMS and TAMS stars, the remnants have higher luminosity and effective temperature. Potential observational signatures include peculiarities in nitrogen and carbon abundances, higher luminosity, rapid rotation, faster evolution, and unique tracks in the Hertzsprung-Russell diagram

astro-ph.HE↗

arXiv2024

Spectacular nucleosynthesis from early massive stars

Stars formed with initial mass over 50 Msun are very rare today, but they are thought to be more common in the early universe. The fates of those early, metal-poor, massive stars are highly uncertain. Most are expected to directly collapse to black holes, while some may explode as a result of rotationally powered engines or the pair-creation instability. We present the chemical abundances of J0931+0038, a nearby low-mass star identified in early followup of SDSS-V Milky Way Mapper, which preserves the signature of unusual nucleosynthesis from a massive star in the early universe. J0931+0038 has relatively high metallicity ([Fe/H] = -1.76 +/- 0.13) but an extreme odd-even abundance pattern, with some of the lowest known abundance ratios of [N/Fe], [Na/Fe], [K/Fe], [Sc/Fe], and [Ba/Fe]. The implication is that a majority of its metals originated in a single extremely metal-poor nucleosynthetic source. An extensive search through nucleosynthesis predictions finds a clear preference for progenitors with initial mass > 50 Msun, making J0931+0038 one of the first observational constraints on nucleosynthesis in this mass range. However the full abundance pattern is not matched by any models in the literature. J0931+0038 thus presents a challenge for the next generation of nucleosynthesis models and motivates study of high-mass progenitor stars impacted by convection, rotation, jets, and/or binary companions. Though rare, more examples of unusual early nucleosynthesis in metal-poor stars should be found in upcoming large spectroscopic surveys.

astro-ph.SR↗

arXiv2023

On the Core-Collapse Supernova Explanation for LAMOST J1010+2358

Low-metallicity very massive stars with an initial mass of $\sim 140$--$260\, {\rm M_\odot}$ are expected to end their lives as pair-instability supernovae (PISNe). The abundance pattern resulting from a PISN differs drastically from regular core-collapse supernova (CCSN) models and is expected to be seen in very metal-poor (VMP) stars of ${\rm[Fe/H]}\lesssim -2$. Despite the routine discovery of many VMP stars, the unique abundance pattern expected from PISNe has not been unambiguously detected. The recently discovered VMP star LAMOST J1010+2358, however, shows a peculiar abundance pattern that is remarkably well fit by a PISN, indicating the potential first discovery of a bonafide star born from gas polluted by a PISN. In this paper, we study the detailed nucleosynthesis in a large set of models of CCSN of Pop III and Pop II star of metallicity ${\rm[Fe/H]}=-3$ with masses ranging from $12$--$30\,{\rm M_\odot}$. We find that the observed abundance pattern in LAMOST J1010+2358 can be fit at least equally well by CCSN models of $\sim 12$--$14\,{\rm M_\odot}$ that undergo negligible fallback following the explosion. The best-fit CCSN models provide a fit that is even marginally better than the best-fit PISN model. We conclude the measured abundance pattern in LAMOST J1010+2358 could have originated from a CCSN and therefore cannot be unambiguously identified with a PISN given the set of elements measured in it to date. We identify key elements that need to be measured in future detections in stars like LAMOST J1010+2358 that can differentiate between CCSN and PISN origin.

astro-ph.SR↗