arXiv subjects

Sivan Ginzburg

Publications and source records attributed to Sivan Ginzburg.

At least 19 recordsLinked to original sources

arXiv2026

Sinking and spreading of metal pollution in magnetic white dwarfs

Observations of magnetic polluted white dwarfs indicate that most of them have higher concentrations of metals near the poles compared to lower latitudes. Maintaining such abundance gradients requires gravitational sinking times $t_\downarrow$ below the convection zone that are shorter than the horizontal spreading time $t_\leftrightarrow$ across the surface by convective eddies. We show analytically that $t_\downarrow/t_\leftrightarrow\propto T^{8/3}$, where $T$ is the temperature at the base of the convection zone, which rises by more than an order of magnitude as convection penetrates deeper into the atmosphere. Correspondingly, $t_\downarrow/t_\leftrightarrow$ jumps by several orders of magnitude, clearly delineating between hot white dwarfs with abundance variations and cold ones with homogenous surfaces. We incorporate magnetic fields self-consistently into the stellar structure and compute for the first time the sinking to spreading time-scale ratio as a function of $B$. Magnetic white dwarfs have shallower convection zones at a given $T_{\rm eff}$, and the inward penetration of convection shifts to lower $T_{\rm eff}$. Quantitatively, $B\sim 10^5\textrm{ G}$ extends the range for enhanced abundance patches from $T_{\rm eff}\gtrsim 30\,000 \textrm{ K}$ ($13\,000\textrm{ K}$) to $T_{\rm eff}\gtrsim 15\,000 \textrm{ K}$ ($6000\textrm{ K}$) for helium (hydrogen) dominated atmospheres, which is insufficient to explain the observed patchy white dwarfs, which are even colder. A possible solution is variability in the accretion rate on a time-scale of a decade, as recently detected in another white dwarf.

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arXiv2026

Eccentricities of millisecond pulsars with intermediate-mass progenitors

One channel to form millisecond pulsars with CO white dwarf companions is through the stable Roche-lobe overflow of intermediate-mass ($3\,{\rm M}_\odot\lesssim M\lesssim 5\,{\rm M}_\odot$) stars at the end of the main sequence (Case A) or the beginning of the hydrogen shell burning phase (Case B). We reproduce previous numerical calculations of this channel and supplement them with a simple analytical model that relates the final orbital period $P(M,m_{\rm wd})$ to the white dwarf's mass and to its progenitor's initial mass $M$. We also theoretically calculate for the first time the eccentricity $e$ in this process, which is set by the fluctuating gravitational quadrupole moment of the progenitor's convective envelope during Roche-lobe detachment. Intermediate-mass progenitors detach when their non-degenerate cores ignite helium, in contrast to low-mass ($M\lesssim 2\,{\rm M}_\odot$) stars with degenerate cores that detach when their envelopes become too light to support a burning shell. Despite the order of magnitude higher envelope mass at detachment $m_{\rm e}$ in our case, the eccentricity is barely affected because $e\propto m_{\rm e}^{1/6}$, explaining why intermediate-mass ($m_{\rm wd}\lesssim 0.6\,{\rm M}_\odot)$ CO white dwarfs have similar eccentricities to lower mass helium white dwarfs. Massive CO and ONe white dwarfs ($m_{\rm wd}\gtrsim 0.6\,{\rm M}_\odot)$, on the other hand, probably formed through a different channel of unstable Roche-lobe overflow during helium shell burning (Case C), followed by common envelope inspiral. The measured eccentricities of these massive white dwarfs remain to be explained.

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arXiv2025

Double-faced white dwarfs and the magnetic inhibition of convection

About one in five white dwarfs undergoes spectral evolution from a helium atmosphere to hydrogen and then back to helium. These short-lived hydrogen envelopes - the result of residual hydrogen diffusion - are eventually destroyed by either hydrogen or helium convection. An emerging class of double-faced white dwarfs seems to catch this process in the act, with varying amounts of hydrogen across regions of the stellar surface. Here, we quantitatively test the hypothesis that these inhomogeneities are the result of the magnetic inhibition of convection. We compute the critical magnetic field $B_{\rm crit}(M,T_{\rm eff})$ required to inhibit convection in both hydrogen and helium for $0.6-1.2\,M_\odot$ white dwarfs using two methods. Initially, we estimated $B_{\rm crit}\sim\sqrt{8πP}$ where $P$ is the pressure at the base of the convection zone, finding that most (three out of four) of the observed magnetic double-faced white dwarfs could potentially be explained by the magnetic inhibition of hydrogen convective energy transfer, with measured $B\gtrsim B_{\rm crit}^{\rm H}$. Then, we incorporated the magnetic field consistently into the stellar structure and directly computed the boundary of convective mixing. With this more appropriate method, we find that only half (two out of four) of the stars could be explained by the magnetic inhibition of helium convection, with $B\gtrsim B_{\rm crit}^{\rm He}$. Specifically, order of unity variations in the magnetic field's strength or orientation across the surface could account for the double-faced nature of these stars. Given our mixed results, other - including non-magnetic - scenarios should be considered as well.

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arXiv2025

Magnetic dynamos powered by white dwarf superficial convection

When the effective temperature of a cooling white dwarf $T_{\rm eff}$ drops below the ionization limit, it develops a surface convection zone that may generate a magnetic field $B$ through one of several dynamo mechanisms. We revisit this possibility systematically using detailed stellar evolution computations, as well as a simple analytical model that tracks the expansion of the convection zone. The magnetic field reaches a maximum of several kG (for a hydrogen atmosphere) shortly after a convection zone is established at a cooling time $t=t_{\rm conv}$. The field then declines as $B\propto T_{\rm eff}\propto t^{-7/20}$ until the convective envelope couples to the degenerate core at $t=t_{\rm coup}$. We compare the onset of convection $t_{\rm conv}\propto M^{25/21}$ to the crystallization of the white dwarf's core $t_{\rm cryst}\propto M^{-5/3}$, and find that in the mass range $0.5\,{\rm M}_\odot<M<0.9\,{\rm M}_\odot$ the order of events is $t_{\rm conv}<t_{\rm cryst}<t_{\rm coup}$. Specifically, surface dynamos are active for a period $Δt\approx t_{\rm cryst}-t_{\rm conv}$ of about a Gyr (shorter for higher masses), before the convection zone is overrun by a stronger magnetic field emanating from the crystallizing core. Our predicted magnetic fields are at the current detection limit, and we do not find any observed candidates that fit the theory. None the less, surface dynamos may be an inevitable outcome of white dwarf cooling, significantly affecting white dwarf accretion and seismology.

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arXiv2025

Seismology and diffusion of ultramassive white dwarf magnetic fields

Ultramassive white dwarfs (UMWDs; defined by masses $\gtrsim 1.1\,{\rm M}_\odot$) are prime targets for seismology, because they pass through the ZZ Ceti instability strip at the same time that their cores crystallize. Recent studies suggest that crystallization may magnetize white dwarf interiors with a strong magnetic field $B_0$ up to a radius $r_{\rm out}^0$, either through a magnetic dynamo or by transporting a pre-existing fossil field. We demonstrate that seismology can probe these buried fields before they break out at the surface, because even the weak exponential tail of the outwardly diffusing field can disrupt the propagation of gravity waves near the surface. Based on the observed oscillation modes of WD J0135+5722 - the richest pulsating UMWD to date - we constrain its surface field $B_{\rm surf}\lesssim 2\,\textrm{kG}$. We solve the induction equation and translate this to an upper limit on the internal field $B_0$. For a carbon-oxygen (CO) core we find $B_{\rm surf}\ll B_0\lesssim 0.6\,\textrm{MG}$, consistent with the crystallization dynamo theory. For an oxygen-neon (ONe) core, on the the other hand, $r_{\rm out}^0$ is larger, such that the magnetic field breaks out and $B_{\rm surf}\lesssim B_0\lesssim 7\,\textrm{kG}$. This low magnetic field rules out an ONe composition or, alternatively, an intense dynamo during crystallization or merger. Either way, the imprint of magnetic fields on UMWD seismology may reveal the uncertain composition and formation paths of these stars.

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arXiv2024

White dwarf eccentricity fluctuation and dissipation by AGB convection

Millisecond pulsars with white dwarf companions have typical eccentricities $e\sim 10^{-6}-10^{-3}$. The eccentricities of helium white dwarfs are explained well by applying the fluctuation-dissipation theorem to convective eddies in their red giant progenitors. We extend this theory to more massive carbon-oxygen (CO) white dwarfs with asymptotic giant branch (AGB) progenitors. Due to the radiation pressure in AGB stars, the dominant factor in determining the remnant white dwarf's eccentricity is the critical residual hydrogen envelope mass $m_{\rm env}$ required to inflate the star to giant proportions. Using a suite of MESA stellar evolution simulations with $Δm_{\rm c}=10^{-3}\,{\rm M}_\odot$ core-mass intervals, we resolved the AGB thermal pulses and found that the critical $m_{\rm env}\propto m_{\rm c}^{-6}$. The resulting eccentricity $e\sim 3\times 10^{-3}$ is almost independent of the remnant CO white dwarf's mass $m_{\rm c}$. Nearly all of the measured eccentricities lie below this robust theoretical limit, indicating that the eccentricity is damped during the common-envelope inspiral that follows the unstable Roche-lobe overflow of the AGB star. Specifically, we focused on white dwarfs with median masses $m_{\rm c}>0.6\,{\rm M}_\odot$. These massive white dwarfs begin their inspiral with practically identical orbital periods and eccentricities, eliminating any dependence on the initial conditions. For this sub-sample, we find an empirical relation $e\propto P^{3/2}$ between the final period and eccentricity that is much tighter than previous studies - motivating theoretical work on the eccentricity evolution during the common envelope phase. The eccentricities of lower mass CO white dwarfs may be explained by alternative formation channels.

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arXiv2024

Younger age for the oldest magnetic white dwarfs

Sufficiently old white dwarfs cool down through a convective envelope that directly couples their degenerate cores to the surface. Magnetic fields may inhibit this convection by stiffening the criterion for convective instability. We consistently implemented the modified criterion in the stellar evolution code MESA, and computed the cooling of white dwarfs as a function of their mass and magnetic field $B$. In contrast to previous estimates, we find that magnetic fields can significantly change the cooling time $t$ even if they are relatively weak $B^2\ll 8πP$, where $P$ is the pressure at the edge of the degenerate core. Fields $B\gtrsim 1\textrm{ MG}$ open a radiative window that decouples the core from the convective envelope, effectively lowering the luminosity to that of a fully radiative white dwarf. We identified a population of observed white dwarfs that are younger by $Δt\sim$ Gyr than currently thought due to this magnetic inhibition of convective energy transfer - comparable to the cooling delay due to carbon-oxygen phase separation. In volume-limited samples, the frequency and strength of magnetic fields increase with age. Accounting for magnetic inhibition is therefore essential for accurate cooling models for cosmic chronology and for determining the origin of the magnetic fields.

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arXiv2024

Magnetic braking below the cataclysmic variable period gap and the observed dearth of period bouncers

Period bouncers are cataclysmic variables (CVs) that have evolved past their orbital period minimum. The strong disagreement between theory and observations of the relative fraction of period bouncers is a severe shortcoming in the understanding of CV evolution. We test the implications of the hypothesis that magnetic braking (MB), which is suggested to be an additional angular momentum loss (AML) mechanism for CVs below the period gap ($P_\mathrm{orb}\lesssim 120$ min), weakens around their period minimum. We compute the evolution of CV donors below the period gap using the MESA code, assuming that the evolution of the system is driven by AML by gravitational wave radiation (GWR) and MB. We parametrize the MB strength as $\mathrm{AML_{MB}}=κ\mathrm{AML_{GWR}}$. We compute two qualitatively different sets of models, one where $κ$ is a constant and the other where $κ$ depends on stellar parameters. We find that two crucial effects drive the latter set of models. (1) A decrease in $κ$ as CVs approach the period minimum stalls their evolution so that they spend a long time in the observed period minimum spike ($80\lesssim P_\mathrm{orb}/\,\mathrm{min}\lesssim 86$). Here, they become difficult to distinguish from pre-bounce systems in the spike. (2) A strong decrease in the mass-transfer rate makes them virtually undetectable as they evolve further. So, the CV stalls around the period minimum and then `disappears'. This reduces the number of detectable bouncers. Physical processes, such as dynamo action, white dwarf magnetism, and dead zones, may cause such a weakening of MB at short orbital periods. The weakening magnetic braking formalism provides a possible solution to the problem of the lack of period bouncers in CV observational surveys.

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arXiv2024

Magnetic field breakout in ultramassive crystallizing white dwarfs

Ultramassive white dwarfs with masses $M\gtrsim 1.1\,{\rm M}_\odot$ probe extreme physics near the Chandrasekhar limit. Despite the rapid increase in observations, it is still unclear how many harbour carbon-oxygen (CO) versus oxygen-neon (ONe) cores. The origin of these white dwarfs and their strong magnetic fields - single stellar evolution or a stellar merger - is another open question. The steep mass-radius relation of the relativistic ultramassive white dwarfs shortens their crystallization time $t_{\rm cryst}$, such that the recently proposed crystallization dynamo mechanism may present an alternative to mergers in explaining the early appearance of magnetism in the observed population. However, the magnetic diffusion time from the convective dynamo to the white dwarf's surface delays the magnetic field's breakout time $t_{\rm break}>t_{\rm cryst}$. We compute $t_{\rm break}(M)$ for CO and ONe ultramassive white dwarfs and compare it to the local 40 pc volume-limited sample. We find that the breakout time from CO cores is too long to account for the observations. ONe crystallization dynamos remain a viable option, but their surrounding non-convective envelopes comprise only a few per cent of the total mass, such that $t_{\rm break}$ is highly sensitive to the details of stellar evolution.

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arXiv2024

Magnetic field breakout from white dwarf crystallization dynamos

A convective dynamo operating during the crystallization of white dwarfs is one of the promising channels to produce their observed strong magnetic fields. Although the magnitude of the fields generated by crystallization dynamos is uncertain, their timing may serve as an orthogonal test of this channel's contribution. The carbon-oxygen cores of $M\approx 0.5-1.0\,{\rm M}_\odot$ white dwarfs begin to crystallize at an age $t_{\rm cryst}\propto M^{-5/3}$, but the magnetic field is initially trapped in the convection zone - deep inside the CO core. Only once a mass of $m_{\rm cryst}$ has crystallized, the convection zone approaches the white dwarf's helium layer, such that the magnetic diffusion time through the envelope shortens sufficiently for the field to break out to the surface, where it can be observed. This breakout time is longer than $t_{\rm cryst}$ by a few Gyr, scaling as $t_{\rm break}\propto t_{\rm cryst}f^{-1/2}$, where $f\equiv 1-m_{\rm cryst}/M$ depends on the white dwarf's initial C/O profile before crystallization. The first appearance of strong magnetic fields $B\gtrsim 1\textrm{ MG}$ in volume-limited samples approximately coincides with our numerically computed $t_{\rm break}(M)$ - potentially signalling crystallization dynamos as a dominant magnetization channel. However, some observed magnetic white dwarfs are slightly younger, challenging this scenario. The dependence of the breakout process on the white dwarf's C/O profile implies that magnetism may probe the CO phase diagram, as well as uncertainties during the core helium burning phase in the white dwarf's progenitor, such as the $^{12}{\rm C}(α,γ)^{16}{\rm O}$ nuclear reaction.

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arXiv2023

Convective dynamos of black widow companions

Black widows and redbacks are binary millisecond pulsars with close low-mass companions that are irradiated and gradually ablated by the pulsar's high-energy luminosity $L_{\rm irr}$. These binaries evolve primarily through magnetic braking, which extracts orbital angular momentum and pushes the companion to overflow its Roche lobe. Here, we use the stellar evolution code MESA to examine how the irradiation modifies the companion's structure. Strong $L_{\rm irr}$ inhibits convection to the extent that otherwise fully convective stars become almost fully radiative. By computing the convective velocities and assuming a dynamo mechanism, we find that the thin convective envelopes of such strongly irradiated companions ($L_{\rm irr}\gtrsim 3\,{\rm L}_\odot$) generate much weaker magnetic fields than previously thought - halting binary evolution. With our improved magnetic braking model, we explain most observed black widow and redback companions as remnants of main-sequence stars. We also apply our model (with $L_{\rm irr}$) to evolved companions that overflow their Roche lobe close to the end of their main-sequence phase. The evolutionary tracks of such companions bifurcate, explaining the shortest period systems (which are potential gravitational wave sources) as well as the longest period ones (which are the progenitors of common pulsar-white dwarf binaries). The variety of black widow structures and evolutionary trajectories may be utilized to calibrate the dependence of magnetic braking on the size of the convective layer and on the existence of a radiative-convective boundary, with implications for single stars as well as other binaries, such as cataclysmic variables and AM Canum Venaticorum stars.

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arXiv2022

AU Microscopii in the FUV: Observations in Quiescence, During Flares, and Implications for AU Mic b and c

High energy X-ray and ultraviolet (UV) radiation from young stars impacts planetary atmospheric chemistry and mass loss. The active $\sim 22$ Myr M dwarf AU Mic hosts two exoplanets orbiting interior to its debris disk. Therefore, this system provides a unique opportunity to quantify the effects of stellar XUV irradiation on planetary atmospheres as a function of both age and orbital separation. In this paper we present over 5 hours of Far-UV (FUV) observations of AU Mic taken with the Cosmic Origins Spectrograph (COS; 1070-1360 Angstrom) on the Hubble Space Telescope (HST). We provide an itemization of $120$ emission features in the HST/COS FUV spectrum and quantify the flux contributions from formation temperatures ranging from $10^4-10^7$ K. We detect 13flares in the FUV white-light curve with energies ranging from $10^{29} - 10^{31}$ ergs. The majority of the energy in each of these flares is released from the transition region between the chromosphere and the corona. There is a 100$\times$ increase in flux at continuum wavelengths $λ< 1100$ Angstrom in each flare which may be caused by thermal Bremsstrahlung emission. We calculate that the baseline atmospheric mass-loss rate for AU Mic b is $\sim 10^8$ g s$^{-1}$, although this rate can be as high as $\sim 10^{14}$ g s$^{-1}$ during flares with $L_\textrm{flare} \simeq 10^{33}$ erg s$^{-1}$. Finally, we model the transmission spectra for AU Mic b and c with a new panchromatic spectrum of AU Mic c and motivate future JWST observations of these planets.

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arXiv2022

Slow convection and fast rotation in crystallization-driven white dwarf dynamos

It has been recently suggested that white dwarfs generate magnetic fields in a process analogous to the Earth. The crystallization of the core creates a compositional inversion that drives convection, and combined with rotation, this can sustain a magnetic dynamo. We reanalyse the dynamo mechanism, arising from the slow crystallization of the core, and find convective turnover times $t_{\rm conv}$ of weeks to months - longer by orders of magnitude than previously thought. With white dwarf spin periods $P\ll t_{\rm conv}$, crystallization-driven dynamos are almost always in the fast rotating regime, where the magnetic field $B$ is at least in equipartition with the convective motion and is possibly further enhanced by a factor of $B\propto (t_{\rm conv}/P)^{1/2}$, depending on the assumed dynamo scaling law. We track the growth of the crystallized core using MESA and compute the magnetic field $B(T_{\rm eff})$ as a function of the white dwarf's effective temperature $T_{\rm eff}$. We compare this prediction with observations and show that crystallization-driven dynamos can explain some - but not all - of the $\sim$MG magnetic fields measured for single white dwarfs, as well as the stronger fields measured for white dwarfs in cataclysmic variables, which were spun up by mass accretion to short $P$. Our $B(T_{\rm eff})$ curves might also explain the clustering of white dwarfs with Balmer emission lines around $T_{\rm eff}\approx 7500\textrm{ K}$.

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arXiv2021

Eccentric millisecond pulsars by resonant convection

Eccentric millisecond pulsars (eMSPs) with white dwarf companions exhibit orbital eccentricities orders of magnitude larger than predicted by turbulent convection in the white dwarfs' red giant progenitors. The orbital periods of eMSPs cluster around $P=20-30$ d, remarkably close to the red giant convective eddy turnover time $t_{\rm eddy}$. We propose that the anomalously large eccentricities are resonantly driven by convective flows somehow made coherent when the turnover time matches the tidally locked red giant's spin period, which is also the tidal forcing period. Numerical simulations of rotating red giants and magnetic field studies of stars show some evidence for especially ordered flow patterns when the convective Rossby number $P/t_{\rm eddy}$ is of order unity. We show that resonant convection boosts eccentricities by a factor of $(t_{\rm nuc}/P)^{1/2}\approx 3\times 10^3$ over the random-walk values that characterize conventional MSPs, in good agreement with observations ($t_{\rm nuc}$ is the giant's nuclear burning time-scale). We also show how variations in the eddy turnover time arising from red giant metallicity variations can reproduce the observed effective width of the resonance, $ΔP/P\approx 0.4$.

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arXiv2021

Novae heat their food: mass transfer by irradiation

A nova eruption irradiates and heats the donor star in a cataclysmic variable to high temperatures $T_{\rm irr}$, causing its outer layers to expand and overflow the Roche lobe. We calculate the donor's heating and expansion both analytically and numerically, under the assumption of spherical symmetry, and find that irradiation drives enhanced mass transfer from the donor at a rate $\dot{m}\propto T_{\rm irr}^{5/3}$, which reaches $\dot{m}\sim 10^{-6}\textrm{ M}_\odot\textrm{ yr}^{-1}$ at the peak of the eruption - about a thousand times faster than during quiescence. As the nova subsides and the white dwarf cools down, $\dot{m}$ drops to lower values. We find that under certain circumstances, the decline halts and the mass transfer persists at a self-sustaining rate of $\dot{m}\sim 10^{-7}\textrm{ M}_\odot\textrm{ yr}^{-1}$ for up to $\sim 10^3$ yr after the eruption. At this rate, irradiation by the white dwarf's accretion luminosity is sufficient to drive the mass transfer on its own. The self-sustaining rate is close to the white dwarf's stable burning limit, such that this bootstrapping mechanism can simultaneously explain two classes of puzzling binary systems: recurrent novae with orbital periods $\approx 2$ h (T Pyxidis and IM Normae) and long-lived supersoft X-ray sources with periods $\approx 4$ h (RX J0537.7-7034 and 1E 0035.4-7230). Whether or not a system reaches the self-sustaining state is sensitive to the donor's chromosphere structure, as well as to the orbital period change during nova eruptions.

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arXiv2021

Re-inflation of warm and hot Jupiters

Understanding the anomalous radii of many transiting hot gas giant planets is a fundamental problem of planetary science. Recent detections of re-inflated warm Jupiters orbiting post-main-sequence stars and the re-inflation of hot Jupiters while their host stars evolve on the main-sequence may help constrain models for the anomalous radii of hot Jupiters. In this work, we present evolution models studying the re-inflation of gas giants to determine how varying the depth and intensity of deposited heating affects both main-sequence re-inflation of hot Jupiters and post-main-sequence re-inflation of warm Jupiters. We find that deeper heating is required to re-inflate hot Jupiters than is needed to suppress their cooling, and that the timescale of re-inflation decreases with increasing heating rate and depth. We find a strong degeneracy between heating rate and depth, with either strong shallow heating or weak deep heating providing an explanation for main-sequence re-inflation of hot Jupiters. This degeneracy between heating rate and depth can be broken in the case of post-main-sequence re-inflation of warm Jupiters, as the inflation must be rapid to occur within post-main-sequence evolution timescales. We also show that the dependence of heating rate on incident stellar flux inferred from the sample of hot Jupiters can explain re-inflation of both warm and hot Jupiters. TESS will obtain a large sample of warm Jupiters orbiting post-main-sequence stars, which will help to constrain the mechanism(s) causing the anomalous radii of gas giant planets.

astro-ph.EP↗

arXiv2020

Black widow formation by pulsar irradiation and sustained magnetic braking

Black widows are millisecond pulsars with low-mass companions, a few per cent the mass of the sun, on orbits of several hours. These companions are presumably the remnants of main sequence stars that lost their mass through a combination of Roche-lobe overflow and ablation by the host pulsar's high-energy radiation. While ablation itself is too weak to significantly reduce the mass of the companion star, the ablated wind couples to its magnetic field, removes orbital angular momentum, and thus maintains stable Roche-lobe overflow. We use the MESA stellar evolution code, complemented by analytic estimates, to track initially main sequence companions as they are reduced to a fraction of their original mass by this ablation-driven magnetic braking. We argue that magnetic braking remains effective even for low-mass companions. A key ingredient of our model is that the irradiating luminosity of the pulsar $L_{\rm irr}$ deposits energy in the companion's atmosphere and thereby slows down its Kelvin-Helmholtz cooling. We find that the high-energy luminosities measured by Fermi $L_{\rm irr}=0.1-3\,{\rm L}_\odot$ can explain the span of black widow orbital periods. The same $L_{\rm irr}$ range reproduces the companions' night-side temperatures, which cluster around 3000 K, as inferred from optical light curves.

astro-ph.HE↗

arXiv2020

Supernova PTF12glz: a possible shock breakout driven through an aspherical wind

We present visible-light and ultraviolet (UV) observations of the supernova PTF12glz. The SN was discovered and monitored in near-UV and R bands as part of a joint GALEX and Palomar Transient Factory campaign. It is among the most energetic Type IIn supernovae observed to date (~10^{51} erg). If the radiated energy mainly came from the thermalization of the shock kinetic energy, we show that PTF12glz was surrounded by ~1 solar mass of circumstellar material (CSM) prior to its explosive death. PTF12glz shows a puzzling peculiarity: at early times, while the freely expanding ejecta are presumably masked by the optically thick CSM, the radius of the blackbody that best fits the observations grows at ~7000 km/s. Such a velocity is characteristic of fast moving ejecta rather than optically thick CSM. This phase of radial expansion takes place before any spectroscopic signature of expanding ejecta appears in the spectrum and while both the spectroscopic data and the bolometric luminosity seem to indicate that the CSM is optically thick. We propose a geometrical solution to this puzzle, involving an aspherical structure of the CSM around PTF12glz. By modelling radiative diffusion through a slab of CSM, we show that an aspherical geometry of the CSM can result in a growing effective radius. This simple model also allows us to recover the decreasing blackbody temperature of PTF12glz. SLAB-Diffusion, the code we wrote to model the radiative diffusion of photons through a slab of CSM and evaluate the observed radius and temperature, is made available on-line.

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