arXiv subjects

Jakob Stegmann

Publications and source records attributed to Jakob Stegmann.

30 records · Page 2Linked to original sources

arXiv2025

Prompt stellar and binary black hole mergers in tight triples: Insights from chemically homogeneous evolution

Short-period massive binary stars are predicted to undergo chemically homogeneous evolution (CHE), making them prime candidates for producing binary black holes (BBHs) that may merge within the age of the Universe. Most of these binaries have a tertiary companion, and here we explore how a nearby third body possibly influences this evolutionary channel. Our analysis combines analytic treatments of triple dynamics with insights from detailed stellar evolution models, focusing on the role of the von Zeipel-Lidov-Kozai mechanism while also accounting for tidal and general relativistic apsidal precession. We examine the dynamics of triples at three critical evolutionary stages: the zero-age main sequence, shortly after the main sequence, and at the time of BBH formation. We find that, for triples having outer orbital periods less than 70 d(120 d), the inner binary can merge during(or after) the main-sequence stage, leading to a hydrogen-rich(helium-rich) stellar merger. If a stellar merger is avoided, the inner binary may eventually form a BBH. In mildly hierarchical triples, with outer periods of around 100 d, the tertiary component can trigger a rapid merger of the BBH on timescales comparable to the outer orbital period. Stellar tides play a crucial role in determining the fate of the inner binary in such tight triple systems, as they can suppress the perturbative effects of the third star. When tidal forces damp the oscillations induced by the tertiary, the BBH merger may occur soon after stellar collapse. Notably, these outcomes are not restricted to CHE binaries but can also apply to any BBH formed from stars in tight orbits. Mergers in these systems are characterized by the proximity of a tertiary companion and the presence of recently ejected gas, making them promising candidates for electromagnetic counterparts and gravitational-wave signals influenced by nearby tertiary objects.

astro-ph.SR↗

arXiv2025

Supermassive black holes stripping a subgiant star down to its helium core: a new type of multi-messenger source for LISA

Some stars orbiting supermassive black holes (SMBH) are expected to undergo a gravitational-wave (GW)-driven inspiral and initiate mass transfer on nearly circular orbits. However, the stability and duration of such phases remain unexplored. In this work, we focus on the evolution of a low-mass, radiative-envelope subgiant star being stripped by an SMBH. We find that such systems can undergo a long-lasting, stable mass-transfer phase, even if none of the angular momentum of the transferred material returns to the orbit to counterbalance the GW-driven decay. We show an example where a 2 Msun subgiant is stripped before entering the LISA band and loses almost its entire hydrogen envelope. The remaining helium core undergoes a prolonged GW-driven inspiral, becoming a loud LISA source. If formed in our galaxy, the system would be detectable for several hundred thousand years, ultimately reaching extreme signal-to-noise ratios of a million. Hydrogen shell flashes in the residual envelope cause temporary radial expansions of the stripped star. As a result, a few additional phases of rapid mass transfer occur at orbital periods of 20 - 30 hours. Eventually, the core possibly undergoes circular partial tidal disruption at an orbital period of ~10 minutes, corresponding to a GW emission frequency of a few mHz. We estimate a chance of about 1% that such a detectable LISA source exists in our own galactic center. The loud final GW transient may lead to a few detections reaching as far as ~1 Gpc, including, e.g., the Abell clusters.

astro-ph.HE↗

arXiv2025

Planet formation and long-term stability in a very eccentric stellar binary

Planets orbiting one of the two stars in a binary are vulnerable to gravitational perturbations from the other star. Particularly, highly eccentric companion stars risk disrupting planetary orbits, such as in the extreme system TOI 4633 where close encounters between the companion and a gas giant planet in the habitable zone make it one of the most fragile systems discovered so far. Here, we report that TOI 4633's planet likely survived these encounters throughout the system's age by orbiting retrograde relative to the binary, stabilised by the Coriolis force. Using direct $N$-body simulations, we show it otherwise tends to collide with the binary stars or becomes free-floating after getting ejected. A retrograde planetary orbit has profound implications for TOI 4633's formation and evolution, suggesting an extraordinary history where its eccentric companion was likely randomly captured after planet formation in a single-star system. Alternatively, if stars and planet are born in situ from the same gas clump, we show the planet must have formed at sub-snow-line distances, contrary to the conventional core-accretion model. Our study highlights the importance of considering the long-term stability ($\gtrsim\rm Gyr$) of planets in eccentric binaries and demonstrates that the mere existence in such dynamically hostile environments places strong constraints on their orbital configuration and formation.

astro-ph.EP↗

arXiv2024

Imprints of massive black-hole binaries on neighbouring decihertz gravitational-wave sources

The most massive black holes in our Universe form binaries at the centre of merging galaxies. The recent evidence for a gravitational-wave (GW) background from pulsar timing may constitute the first observation that these supermassive black hole binaries (SMBHBs) merge. Yet, the most massive SMBHBs are out of reach of interferometric {GW} detectors and are exceedingly difficult to resolve individually with pulsar timing. These limitations call for unexplored strategies to detect individual SMBHBs in the uncharted frequency band $\lesssim10^{-5}\,\rm Hz$ in order to establish their abundance and decipher the coevolution with their host galaxies. Here we show that SMBHBs imprint detectable long-term modulations on GWs from stellar-mass binaries residing in the same galaxy. We determine that proposed deci-Hz GW interferometers sensitive to numerous stellar-mass binaries could uncover modulations from $\sim\mathscr{O}(10^{-1}$ - $10^4)$ SMBHBs with masses $\sim\mathscr{O}(10^7$ - $10^8)\,\rm M_\odot$ out to redshift $z\sim3.5$. This offers a unique opportunity to map the population of SMBHBs through cosmic time, which might remain inaccessible otherwise.

astro-ph.HE↗

arXiv2024

Detecting the heterodyning of gravitational waves

Gravitational waves modulate the apparent frequencies of other periodic signals. Low-frequency gravitational waves could therefore be detected by observing frequency modulations in signals from higher-frequency sources, e.g., those from binary white dwarfs detected with the LISA gravitational-wave detector. We propose a concrete method to extract these modulations by coherently adding the cross-spectra of a large number of well-resolved quasi-monochromatic signals. We apply this method to the case of LISA, and find this method would enable the detection of background gravitational wave strain amplitudes of, e.g., $A\simeq10^{-10}$ at a frequency $F\simeq10^{-8}\,\rm Hz$, given current projections for the number and properties of Galactic binary white dwarfs and the sensitivity of the instrument. We also estimate (to within an order of magnitude) that this method could potentially compete with that of current Pulsar Timing Arrays when using signals from binary neutron stars such as those expected to be observed with proposed detectors like DECIGO. Our results show that gravitational-wave detectors could be sensitive at frequencies outside of their designed bandwidth using the same infrastructure, which has the potential to open up unexplored and otherwise inaccessible parts of the gravitational wave spectrum.

gr-qc↗

arXiv2024

Close Encounters of Wide Binaries Induced by the Galactic Tide: Implications for Stellar Mergers and Gravitational-Wave Sources

A substantial fraction of stars can be found in wide binaries with projected separations between $\sim10^2$ and $10^5\,\rm AU$. In the standard lore of binary physics, these would evolve as effectively single stars that remotely orbit one another on stationary Keplerian ellipses. However, embedded in their Galactic environment their low binding energy makes them exceptionally prone to perturbations from the gravitational potential of the Milky Way and encounters with passing stars. Employing a fully relativistic $N$-body integration scheme, we study the impact of these perturbations on the orbital evolution of wide binaries along their trajectory through the Milky Way. Our analysis reveals that the torques exerted by the Galaxy can cause large-amplitude oscillations of the binary eccentricity to $1-e\lesssim10^{-8}$. As a consequence, the wide binary members pass close to each other at periapsis, which, depending on the type of binary, potentially leads to a mass transfer or collision of stars or to an inspiral and subsequent merger of compact remnants due to gravitational-wave radiation. Based on a simulation of $10^5$ wide binaries across the Galactic field, we find that this mechanism could significantly contribute to the rate of stellar collisions and binary black hole mergers as inferred from observations of Luminous Red Novae and gravitational-wave events by LIGO/Virgo/Kagra. We conclude that the dynamics of wide binaries, despite their large mean separation, can give rise to extreme interactions between stars and compact remnants.

astro-ph.GA↗

arXiv2024

Stupendously Large Black Hole Coalescence and Hubble Tensions

Local and distant measurements of the Hubble constant are in significant tension: local measurements of the Hubble constant appear to show a Universe that is significantly contracted when compared to distant measurements. From the point of view of an observer, a passing gravitational wave could cause the Universe to appear locally contracted and expanded in a quadrupolar pattern. The inspiral of a pair of stupendously large black holes (SLABs) with a chirp mass of $\sim10^{21}~\rm{M}_\odot$ may produce gravitational radiation with sufficiently large amplitude and wavelength to increase $H_0$ in one direction, and decrease it by the same amount in the other. These tensions would then oscillate with a period corresponding to half the orbital period of the binary. If such a gravitational wave were aligned with the plane of the Milky Way, most readily visible galaxies would appear closer than they actually are, thereby causing the apparent Hubble tension. Due to the long binary period, we would be in the same phase of the gravitational wave for the complete history of astronomical observation. The opposite tension would be visible in the orthogonal directions, thus giving the opportunity to falsify the existence of inspiraling SLAB binaries. As a corollary, the Hubble tension may place an upper limit on the maximum mass of inspiraling black holes in the Universe.

gr-qc↗

arXiv2023

Double black hole mergers in nuclear star clusters: eccentricities, spins, masses, and the growth of massive seeds

We investigate the formation of intermediate mass black holes (IMBHs) through hierarchical mergers of stellar origin black holes (BHs), as well as BH mergers formed dynamically in nuclear star clusters. Using a semi-analytical approach which incorporates probabilistic mass-function-dependent double BH (DBH) pairing, binary-single encounters, and a mass-ratio-dependent prescription for energy dissipation in hardening binaries, we find that IMBHs with masses of $O(10^2)$-$O(10^4)\rm M_\odot$ can be formed solely through hierarchical mergers in timescales of a few $100$ Myrs to a few Gyrs. Clusters with escape velocities $\gtrsim400$ km s$^{-1}$ inevitably form high-mass IMBHs. The spin distribution of IMBHs with masses $\gtrsim 10^3$ M$_\odot$ is strongly clustered at $χ\sim 0.15$; while for lower masses, it peaks at $χ\sim 0.7$. Eccentric mergers are more frequent for equal-mass binaries containing first-and/or second-generation BHs. Metal-rich, young, dense clusters can produce up to $20\%$ of their DBH mergers with eccentricity $\geq0.1$ at $10\,\rm Hz$, and $\sim2$-$9\%$ of all in-cluster mergers can form at $>10$ Hz. Nuclear star clusters are therefore promising environments for the formation of highly-eccentric DBH mergers, detectable with current gravitational-wave detectors. Clusters of extreme mass ($\sim10^8$ M$_\odot$) and density ($\sim10^8$ M$_\odot$pc$^{-3}$) can have about half of all of their DBH mergers with primary masses $\geq100$ M$_\odot$. The fraction of in-cluster mergers increases rapidly with increasing cluster escape velocity, being nearly unity for $v_{\rm esc}\gtrsim 200$ km s$^{-1}$. Cosmological merger rate of DBHs from nuclear clusters varies $\lessapprox0.01-1$ Gpc$^{-3}$yr$^{-1}$.

astro-ph.HE↗

arXiv2022

Evolution of massive stellar triples and implications for compact object binary formation

Most back hole and neutron star progenitors are found in triples or higher multiplicity systems. Here, we present a new triple stellar evolution code, ${\tt TSE}$, which simultaneously takes into account the physics of the stars and their gravitational interaction. ${\tt TSE}$ is used to simulate the evolution of massive stellar triples in the galactic field from the zero-age-main-sequence until they form compact objects. To this end, we implement initial conditions that incorporate the observed high correlation between the orbital parameters of early-type stars. We show that the interaction with a tertiary companion can significantly impact the evolution of the inner binary. High eccentricities can be induced by the third-body dynamical effects, leading to a Roche lobe overflow or even to a stellar merger from initial binary separations $~10^3$ - $10^5\,\rm R_\odot$. In $\sim5\,\%$ of the systems the tertiary companion itself fills its Roche lobe, while $\sim 10\,\%$ of all systems become dynamically unstable. We find that between $0.3\%$ and $5\%$ of systems form a stable triple with an inner compact object binary, where the exact fraction depends on metallicity and the natal kick prescription. The vast majority of these triples are binary black holes with black hole companions. We find no binary neutron star in any surviving triple, unless zero natal kicks are assumed. About half of all black hole binaries formed in our models are in triples, where in the majority the tertiary black hole is able to perturb their long-term evolution. Our results show that triple interactions are key to a full understanding of massive star evolution and compact object binary formation.

astro-ph.SR↗

arXiv2022

Binary black hole mergers from merged stars in the Galactic field

The majority of massive stars are found in close binaries which: (i) are prone to merge and (ii) are accompanied by another distant tertiary star (triples). Here, we study the evolution of the stellar post-merger binaries composed of the merger product and the tertiary companion. We find that post-merger binaries originating from compact stellar triples with outer semi-major axes $a_{\rm out,init}\lesssim10^1\,-\,10^2\,\rm AU$ provide a new way to form binary black hole mergers in the galactic field. By means of a population synthesis, we estimate their contribution to the total black hole merger rate to be $\mathcal{R}(z=0)=0.3\,-\,25.2\,\rm Gpc^{-3}\,yr^{-1}$. Merging binary black holes that form from stellar post-merger binaries have exceptionally low mass ratios. We identify a critical mass ratio $q\simeq0.5$ below which they dominate the total black hole merger rate in the field. We show that after including their additional contribution, the mass ratio distribution of binary black hole mergers in the galactic field scenario is in better agreement with that inferred from gravitational wave detections.

astro-ph.GA↗

arXiv2021

Flipping spins in mass transferring binaries and origin of spin-orbit misalignment in binary black holes

Close stellar binaries are prone to undergo a phase of stable mass transfer in which a star loses mass to its companion. Assuming that the donor star loses mass along the instantaneous interstellar axis, we derive the orbit-averaged equations of motion describing the evolution of the donor rotational angular momentum vector (spin) which accompanies the transfer of mass. We consider: (i) a model in which the mass transfer rate is constant within each orbit and (ii) a phase-dependent rate in which all mass per orbit is lost at periapsis. In both cases, we find that the ejection of $\gtrsim 30$ per cent of the donor's initial mass causes its spin to nearly flip onto the orbital plane of the binary, independently of the initial spin-orbit alignment. Moreover, we show that the spin flip due to mass transfer can easily dominate over tidal synchronisation in any giant stars and main-sequence stars with masses $\sim1.5$ to $5\,\rm M_\odot$. Finally, the general equations of motion, including tides, are used to evolve a realistic population of massive binary stars leading to the formation of binary black holes. Assuming that the stellar core and envelope are fully coupled, the resulting tilt of the first-born black hole reduces its spin projection onto the orbit normal by a factor $\sim\mathcal{O}(0.1)$. This result supports previous studies in favour of an insignificant contribution to the effective spin projection, $χ_{\rm eff}$, in binary black holes formed from the evolution of field binaries.

astro-ph.HE↗

arXiv2020

Improved constraints from ultra-faint dwarf galaxies on primordial black holes as dark matter

Soon after the recent first ever detection of gravitational waves from merging black holes it has been suggested that their origin is primordial. Appealingly, a sufficient number of primordial black holes (PBHs) could also partially or entirely constitute the dark matter (DM) in the Universe. However, recent studies on PBHs in ultra-faint dwarf galaxies (UFDGs) suggest that they would dynamically heat up the stellar component due to two-body relaxation processes. From the comparison with the observed stellar velocity dispersions and the stellar half-light radii it was claimed that only PBHs with masses $\lesssim10\,M_\odot$ can significantly contribute to the DM. In this work, we improve the latter constraints by considering the largest observational sample of UFDGs and by allowing the PBH masses to follow an extended (log-normal) distribution. By means of collisional Fokker-Planck simulations, we explore a wide parameter space of UFDGs containing PBHs. The analysis of the half-light radii and velocity dispersions resulting from the simulations leads to three general findings that exclude PBHs with masses $\sim\mathcal{O}(1$-$100)\,M_\odot$ from constituting all of the DM: (i) We identify a critical sub-sample of UFDGs that only allows for $\sim\mathcal{O}(1)\,M_\odot$ PBH masses; (ii) for any PBH mass, there is an UFDG in our sample that disfavours it; (iii) the spatial extensions of a majority of simulated UFDGs containing PBHs are too large to match the observed.

astro-ph.GA↗