arXiv subjects

Andrea Antoni

Publications and source records attributed to Andrea Antoni.

16 recordsLinked to original sources

arXiv2026

The rise of the black hole X-ray binary AT2019wey observed with TESS

Black hole X-ray binaries (BHXRBs) have traditionally been discovered by X-ray surveys with cadences of hours to days. However, large optical time-domain surveys now provide novel avenues for early detection and insights into their elusive outburst triggering mechanisms. We present early-time light curves of the BHXRB AT 2019wey serendipitously observed by the Transiting Exoplanet Survey Satellite (TESS). The TESS images are sampled at 30 minute cadence from $\approx2$ d prior to $\approx25$ d after outburst, providing the highest time resolution optical rising phase observations of any known BHXRB. We fit a piece-wise power law to the rising light curve, finding an outburst onset time of MJD $58817.86\pm0.09$ and power-law rise index $n=0.74\pm0.04$. The onset time precedes all ground-based optical detections, and suggests that the optical rise began after the start of the faint X-ray brightening in MAXI data. We search for periodic high frequency modulation and detect none exceeding amplitude $\approx0.48$ mJy at periods of $\gtrsim1$ h at 90% confidence.

astro-ph.HE↗

arXiv2026

Fading into darkness: A weak mass ejection and low-efficiency fallback accompanying black hole formation in M31-2014-DS1

Stellar-mass black holes (BHs) can form from the near-complete collapse of massive stars, causing them to abruptly disappear. The star M31-2014-DS1 in the Andromeda galaxy was reported to exhibit such a disappearance between 2014 and 2022, with properties consistent with the failed explosion of a $\approx 12 - 13$ M$_\odot$ yellow supergiant leading to the formation of a $\approx 5$ M$_\odot$ BH. We present mid-infrared (MIR) observations of the remnant obtained with the James Webb Space Telescope (JWST) and X-ray observations from the Chandra X-ray Observatory in 2024. The JWST MIRI/NIRSpec data reveal an extremely red source, showing strong blueshifted absorption from molecular gas (CO, CO$_2$, H$_2$O, SO$_2$) and deep silicate dust features. Modeling the dust continuum confirms continued bolometric fading of the central source to $\log(L/L_\odot)\approx3.88$ ($\approx7-8$% of the progenitor luminosity), surrounded by a dust shell spanning $\approx40-200$ au. Modeling of the molecular gas indicates $\sim 0.1$ M$_\odot$ of gas expanding at $\approx 100$ km s$^{-1}$ near the inner edge of the dust shell. No X-ray source is detected down to a luminosity limit of $L_X\lesssim1.5\times10^{35}$ erg s$^{-1}$. We show that the panchromatic observations are explained by (i) a low-energy ($\approx10^{46}$ erg) ejection of the outer H-rich progenitor envelope and (ii) a fading central BH powered by inefficient ($\sim0.1$% in mass) accretion of loosely bound fallback material. The analysis robustly establishes the bolometric fading of M31-2014-DS1 and provides the first cohesive insights into BH formation via low-energy explosions and long-term fallback.

astro-ph.SR↗

arXiv2025

Super-Eddington Chimneys: On the Cooling Evolution of Tidal Disruption Event Envelopes

The formation of a compact accretion disk following a tidal disruption event (TDE) requires that the shocked stellar debris cool efficiently as it settles toward the black hole. While recent simulations suggest that stream dissipation occurs rapidly, how the weakly bound debris subsequently loses its thermal energy to assemble a compact disk near the circularization radius remains uncertain. We investigate this cooling process using axisymmetric radiation-hydrodynamic simulations of quasi-hydrostatic 'TDE envelopes', initialized with the total mass, angular momentum, and binding energy expected from a complete stellar disruption. The envelopes, supported by radiation pressure on large scales and rotation near the circularization radius, evolve through a combination of radiative diffusion, turbulent mixing, and polar outflows. In our fiducial model, a quasi-steady state is achieved in which a polar outflow radiates and expels matter at several times the Eddington luminosity. This enables the envelope to cool and contract, forming a dense, rotationally supported ring near the circularization radius, but on a timescale roughly ten times shorter than the naive photon-diffusion timescale. Comparative models without radiation transport confirm that cooling, not purely adiabatic evolution, is essential to driving this rapid inflow. Nevertheless, across a range of envelope masses, the effective envelope cooling time scales only weakly with its optical depth, implying that advective and wind-driven energy transport dominate over diffusion. Our results demonstrate the cooling-induced contraction, even absent viscosity and associated black hole accretion, can produce luminosities and large photosphere radii consistent with early UV/optical TDE emission. However, more quantitative light-curve predictions must incorporate self-consistent formation and feeding of the envelope by fall-back accretion.

astro-ph.HE↗

arXiv2025

Radiation Hydrodynamic Simulations of Low-Energy Explosions of Red and Yellow Supergiants

A variety of physical processes leads to the low-energy ejection of material from the hydrogen-rich envelopes of red and yellow supergiants. These include common envelope events, stellar mergers, eruptive mass loss, and failed supernovae. These events may appear as luminous red novae, intermediate luminosity red transients, supernova imposters, or other transients with similar lightcurves and colors that are followed by the disappearance of the progenitor star (e.g. failed supernovae). The Vera C. Rubin Observatory will find these events in large numbers; detailed modeling of their lightcurves is essential for photometrically differentiating between these important physical processes in the lives of massive stars. We use one-dimensional, radiation hydrodynamic simulations to model the lightcurves of low-energy explosions of red and yellow supergiants. Red supergiant explosions have durations of 100-400 days, longer than Type IIp supernovae, while stripped, yellow supergiant explosions have durations of 10s of days. Our models probe the boundary between the radiation-pressure dominated and gas-pressure dominated regimes. We provide fitting formulae for the plateau luminosity and duration of the events. Finally, we show that the failed supernovae candidates in NGC 6946 and M31 are consistent with failed supernovae models for explosion energies of $\sim10^{47}-10^{49}$ erg.

astro-ph.SR↗

arXiv2024

Disappearance of a massive star in the Andromeda Galaxy due to formation of a black hole

When a massive star reaches the end of its lifetime, its core collapses and releases neutrinos that drive a shock into the outer layers (stellar envelope). A sufficiently strong shock ejects the envelope, producing a supernova. If the shock fails to eject it, the envelope is predicted to fall back onto the collapsing core, producing a stellar-mass black hole (BH) and causing the star to disappear. We report observations of M31-2014-DS1, a hydrogen-depleted supergiant in the Andromeda Galaxy. In 2014 it brightened in the mid-infrared. From 2017 to 2022 it faded by factors of $\gtrsim10^4$ in optical light, becoming undetectable, and $\gtrsim10$ in total light. We interpret these observations, and those of a previous event in NGC 6946, as evidence for failed supernovae forming stellar-mass BHs.

astro-ph.HE↗

arXiv2024

The Evolution of Binaries Embedded Within Common Envelopes

Triple stellar systems allow us to study stellar processes that cannot be attained in binary stars. The evolutionary phases in which the stellar members undergo mass exchanges can alter the hierarchical layout of these systems. Yet, the lack of a self-consistent treatment of common-envelope (CE) in triple star-systems hinders the comprehensive understanding of their long-term fate. This letter examines the conditions predicted around binaries embedded within CEs using local 3D hydrodynamical simulations. We explore varying the initial binary separation, the flow Mach number, and the background stellar density gradients as informed by a wide array of CE conditions, including those invoked to explain the formation of the triple system hosting PSR J0337+1715. We find that the stellar density gradient governs the gaseous drag force, which determines the final configuration of the embedded binary. We observe a comparable net drag force on the center of mass but an overall reduction in the accretion rate of the binary compared to the single object case. We find that for most CE conditions, and in contrast to the uniform background density case, the binary orbital separation increases with time, softening the binary and preventing it from subsequently merging. We conclude that binaries spiraling within CEs become more vulnerable to be disrupted by tidal interactions. This can have profound implications on the final outcomes of triple star-systems.

astro-ph.SR↗

arXiv2023

Numerical simulations of the random angular momentum in convection II: delayed explosions of red supergiants following "failed'' supernovae

When collapse of the iron core in a massive red or yellow supergiant does not lead to an energetic supernova, a significant fraction of the convective hydrogen envelope will fall in towards the black hole formed from the collapsing core. The random velocity field in the convective envelope results in finite specific angular momentum in each infalling shell. Using 3D hydrodynamical simulations, we follow the infall of this material to small radii, resolving the circularization radii of the flow. We show that infall of the convective envelope leads to nearly complete envelope ejection in a $\gtrsim$ 10$^{48}$ erg explosion with outflow speeds of $\gtrsim$ 200 km/s. The light curve of such an explosion would show a characteristic, red plateau as the ejecta cools and a hydrogen recombination front recedes through the expanding ejecta. Adopting supernova IIp scalings, the event would have a plateau luminosity of $\gtrsim$ 10$^{40}$ erg/s and a duration of several hundreds of days. These events would appear quite similar to luminous red novae with red or yellow supergiant progenitors; some luminous red novae may, in fact, be signposts of black hole formation. The mechanism studied here produces more energetic explosions than the weak shock generated from the radiation of neutrino energy during the proto-neutron star phase. Because we cannot simulate all the way to the horizon, our results are likely lower limits on the energy and luminosity of transients produced during the collapse of a red or yellow supergiant to form a black hole.

astro-ph.HE↗

arXiv2023

Left Ringing: Betelgeuse Illuminates the Connection Between Convective outbursts, Mode switching, and Mass Ejection in Red Supergiants

Betelgeuse, the nearest red supergiant, dimmed to an unprecedented level in early 2020. The star emerged from this Great Dimming episode with its typical, roughly 400-day pulsation cycle halved, and a new dominant period of around 200 days. The dimming event has been attributed to a surface mass ejection, in which rising material drove shocks through the stellar atmosphere and expelled some material, partially obscuring the star as it formed molecules and dust. In this paper, we use hydrodynamic simulations to reveal the connections between Betelgeuse's vigorously convective envelope, the surface mass ejection, and the pulsation mode switching that ensued. An anomalously hot convective plume, generated rarely but naturally in the star's turbulent envelope, can rise and break free from the surface, powering an upwelling that becomes the surface mass ejection. The rising plume also breaks the phase coherence of the star's pulsation, causing the surface to keep expanding even as the deeper layers contract. This drives a switch from the 400-day fundamental mode of pulsation, in which the whole star expands and contracts synchronously, to the 200-day first overtone, where a radial node separates the interior and exterior of the envelope moving in opposite phase. We predict that the star's convective motions will damp the overtone oscillation and Betelgeuse will return to its previous, 400-day fundamental mode pulsation in the next 5-10 years. With its resolved surface and unprecedentedly detailed characterization, Betelgeuse opens a window to episodic surface mass ejection in the late-stage evolution of massive stars.

astro-ph.SR↗

arXiv2023

Hydrodynamics and survivability during post-main-sequence planetary engulfment

The engulfment of substellar bodies (SBs, such as brown dwarfs and planets) by giant stars is a possible explanation for rapidly rotating giants, lithium-rich giants, and the presence of SBs in close orbits around subdwarfs and white dwarfs. We simulate the flow in the vicinity of an engulfed SB in three-dimensional hydrodynamics. We model the SB as a rigid body with a reflective surface because it cannot accrete. This reflective boundary changes the flow morphology to resemble that of engulfed compact objects with outflows. We measure the drag coefficients for the ram pressure and gravitational drag forces acting on the SB, and use them to integrate its trajectory inside the star. We find that engulfment can increase the luminosity of a $1M_\odot$ star by up to a few orders of magnitude. The time for the star to return to its original luminosity is up to a few thousand years when the star has evolved to $\approx10R_\odot$ and up to a few decades at the tip of the red giant branch. No SBs can eject the envelope of a $1M_\odot$ star before it evolves to $\approx10R_\odot$, if the orbit of the SB is the only energy source contributing to the ejection. In contrast, SBs as small as $\approx10M_\text{Jup}$ can eject the envelope at the tip of the red giant branch. The numerical framework we introduce here can be used to study planetary engulfment in a simplified setting that captures the physics of the flow at the scale of the SB.

astro-ph.SR↗

arXiv2023

The hydrodynamic evolution of binary black holes embedded within the vertically stratified disks of active galactic nuclei

Stellar-mass black holes can become embedded within the gaseous disks of active galactic nuclei (AGNs). Afterwards, their interactions are mediated by their gaseous surroundings. In this work, we study the evolution of stellar-mass binary black holes (BBHs) embedded within AGN disks using a combination of three-dimensional hydrodynamic simulations and analytic methods, focusing on environments in which the AGN disk scale height $H$ is $\gtrsim$ the BBH sphere of influence. We model the local surroundings of the embedded BBHs using a wind tunnel formalism and characterize different accretion regimes based on the local properties of the disk, which range from wind-dominated to quasi-spherical. We use our simulations to develop prescriptions for mass accretion and drag for embedded BBHs. We use these prescriptions, along with AGN disk models that can represent the Toomre-unstable outer regions of AGN disks, to study the long-term evolution of the BBHs as they migrate through the disk. We find that BBHs typically merge within $\lesssim 5-30\,{\rm Myr}$, increasing their mass significantly in the process, allowing BBHs to enter (or cross) the pair-instability supernova mass gap. The rate at which gas is supplied to these BBHs often exceeds the Eddington limit, sometimes by several orders of magnitude. We conclude that most embedded BBHs will merge before migrating significantly in the disk. Depending on the conditions of the ambient gas and the distance to the system, LISA can detect the transition between the gas-dominated and gravitational wave dominated regime for inspiraling BBHs that are formed sufficiently close to the AGN ($\lesssim$ 0.1 pc). We also discuss possible electromagnetic signatures during and following the inspiral, finding that it is generally unlikely but not inconceivable for the bolometric luminosity of the BBH to exceed that of the host AGN.

astro-ph.HE↗

arXiv2022

Numerical Simulations of the Random Angular Momentum in Convection: Implications for Supergiant Collapse to Form Black Holes

During the core collapse of massive stars that do not undergo a canonical energetic explosion, some of the hydrogen envelope of a red supergiant (RSG) progenitor may infall onto the newborn black hole (BH). Within the Athena++ framework, we perform three-dimensional, hydrodynamical simulations of idealized models of supergiant convection and collapse in order to assess whether the infall of the convective envelope can give rise to rotationally-supported material, even if the star has zero angular momentum overall. Our dimensionless, polytropic models are applicable to the optically-thick hydrogen envelope of non-rotating RSGs and cover a factor of 20 in stellar radius. At all radii, the specific angular momentum due to random convective flows implies associated circularization radii of 10 - 1500 times the innermost stable circular orbit of the BH. During collapse, the angular momentum vector of the convective flows is approximately conserved and is slowly varying on the timescale relevant to forming disks at small radii. Our results indicate that otherwise failed explosions of RSGs lead to the formation of rotationally-supported flows that are capable of driving outflows to large radii and powering observable transients. When the BH is able to accrete most of the hydrogen envelope, the final BH spin parameter is $\sim$ 0.5, even though the star is non-rotating. For fractional accretion of the envelope, the spin parameter is generally lower and never exceeds 0.8. We discuss the implications of our results for transients produced by RSG collapse to a black hole.

astro-ph.HE↗

arXiv2019

Common Envelope Wind Tunnel: The Effects of Binary Mass Ratio and Implications for the Accretion-Driven Growth of LIGO Binary Black Holes

We present three-dimensional local hydrodynamic simulations of flows around objects embedded within stellar envelopes using a "wind tunnel" formalism. Our simulations model the common envelope dynamical inspiral phase in binary star systems in terms of dimensionless flow characteristics. We present suites of simulations that study the effects of varying the binary mass ratio, stellar structure, equation of state, relative Mach number of the object's motion through the gas, and density gradients across the gravitational focusing scale. For each model, we measure coefficients of accretion and drag experienced by the embedded object. These coefficients regulate the coupled evolution of the object's masses and orbital tightening during the dynamical inspiral phase of the common envelope. We extrapolate our simulation results to accreting black holes with masses comparable to that of the population of LIGO black holes. We demonstrate that the mass and spin accrued by these black holes per unit orbital tightening are directly related to the ratio of accretion to drag coefficients. We thus infer that the mass and dimensionless spin of initially non-rotating black holes change by of order $1\%$ and 0.05, respectively, in a typical example scenario. Our prediction that the masses and spins of black holes remain largely unmodified by a common envelope phase aids in the interpretation of the properties of the growing observed population of merging binary black holes. Even if these black holes passed through a common envelope phase during their assembly, features of mass and spin imparted by previous evolutionary epochs should be preserved.

astro-ph.SR↗

arXiv2019

The Evolution of Binaries in a Gaseous Medium: Three-Dimensional Simulations of Binary Bondi-Hoyle-Lyttleton Accretion

Binary stars are common. While only those with small separations may exchange gas with one another, even the widest binaries interact with their gaseous surroundings. Drag forces and accretion rates dictate how these systems are transformed by these interactions. We perform three-dimensional hydrodynamic simulations of Bondi-Hoyle-Lyttleton flows, in which a binary moves supersonically relative to a homogeneous medium, using the adaptive mesh refinement code FLASH. We simulate a range of values of the initial semi-major axis of the orbit relative to the gravitational focusing impact parameter of the pair. When the binary separation is less than the gravitational focusing impact parameter, the pair orbits within a shared bow shock. When the pair is wider, each object has an individual bow-shock structure. The long-term evolution of the binary is determined by the timescales for accretion, slowing of the center of mass, and orbital inspiral. We find a clear hierarchy of these timescales; a binary's center-of-mass motion is slowed over a shorter timescale than the pair inspirals or accretes. In contrast to previous analytic predictions, which assume an unperturbed background medium, we find that the timescale for orbital inspiral is proportional to the semi-major axis to the $0.19 \pm 0.01$ power. This positive scaling indicates that gaseous drag forces can drive binaries either to coalescence or to the critical separation at which gravitational radiation dominates their further evolution. We discuss the implications of our results for binaries embedded in the interstellar medium, active galactic nuclei disks, and common envelope phases.

astro-ph.HE↗

arXiv2019

Bondi-Hoyle-Lyttleton Accretion onto Star Clusters

An isolated star moving supersonically through a uniform gas accretes material from its gravitationally-induced wake. The rate of accretion is set by the accretion radius of the star and is well-described by classical Bondi-Hoyle-Lyttleton theory. Stars, however, are not born in isolation. They form in clusters where they accrete material that is influenced by all the stars in the cluster. We perform three-dimensional hydrodynamic simulations of clusters of individual accretors embedded in a uniform-density wind in order to study how the accretion rates experienced by individual cluster members are altered by the properties of the ambient gas and the cluster itself. We study accretion as a function of number of cluster members, mean separation between them, and size of their individual accretion radii. We determine the effect of these key parameters on the aggregate and individual accretion rates, which we compare to analytic predictions. We show that when the accretion radii of the individual objects in the cluster substantially overlap, the surrounding gas is effectively accreted into the collective potential of the cluster prior to being accreted onto the individual stars. We find that individual cluster members can accrete drastically more than they would in isolation, in particular when the flow is able to cool efficiently. This effect could potentially modify the luminosity of accreting compact objects in star clusters and could lead to the rejuvenation of young star clusters as well as globular clusters with low-inclination and low-eccentricity.

astro-ph.HE↗

arXiv2017

Accretion Disk Assembly During Common Envelope Evolution: Implications for Feedback and LIGO Binary Black Hole Formation

During a common envelope episode in a binary system, the engulfed companion spirals to tighter orbital separations under the influence of drag from the surrounding envelope material. As this object sweeps through material with a steep radial gradient of density, net angular momentum is introduced into the flow, potentially leading to the formation of an accretion disk. The presence of a disk would have dramatic consequences for the outcome of the interaction because accretion might be accompanied by strong, polar outflows with enough energy to unbind the entire envelope. Without a detailed understanding of the necessary conditions for disk formation during common envelope, therefore, it is difficult to accurately predict the population of merging compact binaries. This paper examines the conditions for disk formation around objects embedded within common envelopes using the `wind tunnel' formalism developed by MacLeod et al. (2017). We find that the formation of disks is highly dependent on the compressibility of the envelope material. Disks form only in the most compressible of stellar envelope gas, found in envelopes' outer layers in zones of partial ionization. These zones are largest in low-mass stellar envelopes, but comprise small portions of the envelope mass and radius in all cases. We conclude that disk formation and associated accretion feedback in common envelope is rare, and if it occurs, transitory. The implication for LIGO black hole binary assembly is that by avoiding strong accretion feedback, common envelope interactions should still result in the substantial orbital tightening needed to produce merging binaries.

astro-ph.SR↗

arXiv2017

Common Envelope Wind Tunnel: Coefficients of Drag and Accretion in a Simplified Context for Studying Flows Around Objects Embedded Within Stellar Envelopes

This paper examines the properties of flows around objects embedded within common envelopes in the simplified context of a "wind tunnel." We establish characteristic relationships between key common envelope flow parameters like the Mach number and density scale height. Our wind tunnel is a three-dimensional, cartesian geometry hydrodynamic simulation setup that includes the gravity of the primary and secondary stars and allows us to study the coefficients of drag and accretion experienced by the embedded object. Accretion and drag lead to a transformation of an embedded object and its orbit during a common envelope phase. We present two suites of simulations spanning a range of density gradients and Mach numbers -- relevant for flow near the limb of a stellar envelope to the deep interior. In one suite, we adopt an ideal gas adiabatic exponent of $γ=5/3$, in the other, $γ=4/3$. We find that coefficients of drag rise in flows with steeper density gradients and that coefficients of drag and accretion are consistently higher in the more compressible, $γ=4/3$ flow. We illustrate the impact of these newly derived coefficients by integrating the inspiral of a secondary object through the envelopes of $3M_\odot$ ($γ\approx5/3$) and $80M_\odot$ ($γ\approx4/3$) giants. In these examples, we find a relatively rapid initial inspiral because, near the stellar limb, dynamical friction drag is generated mainly from dense gas focussed from deeper within the primary-star's envelope. This rapid initial inspiral timescale carries potential implications for the timescale of transients from early common envelope interaction.

astro-ph.SR↗