arXiv subjects

Semih Tuna

Publications and source records attributed to Semih Tuna.

6 recordsLinked to original sources

arXiv2026

Circumbinary Discs as the Origin of Circumstellar Material around Interacting H-poor Supernovae and Fast Blue Optical Transients

Around 10 % of hydrogen-poor supernovae explode inside compact ($\sim 10^{15}$ cm), massive ($\sim 0.1 \ \mathrm{M_\odot}$) circumstellar material (CSM), signalling an episode of enhanced pre-explosion mass loss whose mechanism remains unclear. The extreme members of this population are considered to constitute some of the Fast Blue Optical Transients (FBOTs), which exhibit rapid rise times of $\sim$ few days and high peak luminosity $\sim 10^{44} \ \mathrm{erg}$. Recent binary evolution calculations show that the expansion of helium stars during their latest evolutionary stages can trigger a rapid but stable mass-transfer episode that can form a dense circumbinary disc (CBD) that may explain the observed dense CSM. However, a detailed, quantitative analysis of this process and the resulting CBD properties such as its mass, radius and density profile has not yet been undertaken. We present a set of models that solve the viscous evolution of such a CBD under time-dependent mass injection. We find that although the injected mass is initially sub-Keplerian, a lower ``accretion eigenvalue'' $\chi$ prevents more mass from falling back onto the central binary. For our fiducial set of models, the CBD immediately prior to the explosion reaches a mass of $0.07-0.20 \ \mathrm{M}_\odot$, a half-mass radius of $640 - 4000 \ \mathrm{R}_\odot$, and an aspect ratio of $\theta = H/R \sim 0.1$. We also show that the interaction between SN ejecta and the CBD can power some of the fastest-evolving interacting Type Ibc SNe that can be classified as FBOTs, such as SN 2018gep or SN 2019jc. Despite uncertainties in the model parameters, our results demonstrate that CBD formation triggered by rapid, stable mass transfer is a viable mechanism to explain the dense circumstellar environments observed around rapid, hydrogen-poor interacting SNe. (abridged)

astro-ph.HE↗

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

Nature of phase transitions and metastability in scalar-tensor theories

Compact stars above a critical stellar mass develop large scalar fields in some scalar-tensor theories. This scenario, called spontaneous scalarization, has been an intense topic of study since it passes weak-field gravity tests naturally while providing clear observables in the strong-field regime. The underlying mechanism for the onset of scalarization is often depicted as a second-order phase transition. Here, we show that a first-order phase transition is in fact the most common mechanism. This means metastability and transitions between locally stable compact object configurations are much more likely than previously believed, opening vast new avenues for observational prospects.

gr-qc↗

arXiv2025

Time-Dependent Radiation Transport Simulations of Infrared Echoes from Dust-Shrouded Luminous Transients

A wide range of stellar explosions, including supernovae (SNe), tidal disruption events (TDE), and fast blue optical transients (FBOT), can occur in dusty environments initially opaque to the transient's optical/UV light, becoming visible only once the dust is destroyed by the transient's rising luminosity. We present axisymmetric time-dependent radiation transport simulations of dust-shrouded transients with \texttt{Athena++} and tabulated gray opacities, which predict the light-curves of the dust-reprocessed infrared (IR) radiation. The luminosity and timescale of the IR light-curve depends on whether the transient rises rapidly or slowly compared to the light crossing-time of the photosphere, $t_{\rm lc}$. For slow-rising transients ($t_{\rm rise} \gg t_{\rm lc}$) such as SNe, the reprocessed IR radiation diffuses outwards through the dust shell faster than the sublimation front expands; the IR light-curve therefore begins rising prior to the escape of UV/optical light, but peaks on a timescale $\sim t_{\rm rise}$ shorter than the transient duration. By contrast, for fast-rising transients ($t_{\rm rise} \ll t_{\rm lc}$) such as FBOTs and some TDEs, the finite light-travel time results in the reprocessed radiation arriving as an ``echo'' lasting much longer than the transient itself (despite the dust photosphere having already being destroyed by peak light). We explore the effects of the system geometry by considering a torus-shaped distribution of dust. The IR light-curves seen by observers in the equatorial plane of the torus resemble those for a spherical dust shell, while polar observers see faster-rising, brighter and shorter-lived emission. We successfully model the IR excess seen in AT2018cow as a dust echo, supporting the presence of an opaque dusty medium surrounding FBOTs prior to explosion.

astro-ph.HE↗

arXiv2023

Long-Term Evolution of Massive-Star Post-Common Envelope Circumbinary Disks and the Environments of Fast Luminous Transients

If the envelope of a massive star is not entirely removed during common envelope (CE) interaction with an orbiting compact (e.g., black hole [BH] or neutron star [NS]) companion, the residual bound material eventually cools, forming a centrifugally-supported disk around the binary containing the stripped He core. We present a time-dependent height-integrated model for the long-term evolution of post-CE circumbinary disks (CBD), accounting for mass and angular momentum exchange with the binary and irradiation heating by the He core and photoevaporation wind mass-loss. A large fraction of the CBD's mass is accreted prior to its outwards viscous spreading and wind-dispersal on a timescale ~10^{4}-10^{5} yr, driving significant changes in the binary separation, even for disks containing ~ 10% of the original envelope mass. Insofar that the CBD lifetime is comparable to the thermal (and, potentially, nuclear) timescale of the He core, over which a second mass-transfer episode onto the companion can occur, the presence of the CBD could impact the stability of this key phase. Disruption of the He core by the BH/NS would result in a jetted energetic explosion into the dense gaseous CBD (<~10^{15} cm) and its wind (>~ 10^{16} cm), consistent with the environments of luminous fast blue optical transients like AT2018cow. Evolved He cores which undergo core-collapse still embedded in their CBD could generate Type Ibn/Icn supernovae. Thousands of dusty wind-shrouded massive-star CBD may be detectable as extragalactic luminous infrared sources with the Roman Space Telescope; synchrotron radio nebulae powered by the CBD-fed BH/NS may accompany these systems.

astro-ph.HE↗

arXiv2022

Constraining scalar-tensor theories using neutron star mass and radius measurements

We use neutron star mass and radius measurements to constrain the spontaneous scalarization phenomenon in scalar-tensor theories using Bayesian analysis. Neutron star structures in this scenario can be significantly different from the case of general relativity, which can be used to constrain the theory parameters. We utilize this idea to obtain lower bounds on the coupling parameter $β$ for the case of massless scalars. These constraints are currently weaker than the ones coming from binary observations, and they have relatively low precision due to the approximations in our method. Nevertheless, our results clearly demonstrate the power of the mass-radius data in testing gravity, and can be further improved with future observations. The picture is different for massive scalars, for which the same data is considerably less effective in constraining the theory parameters in an unexpected manner. We identify the main reason for this to be a large high-likelihood region in the parameter space where deviations from general relativity are relatively small. We hope this initial study to be an invitation to use neutron star structure measurements more commonly to test alternative theories in general.

gr-qc↗