Low-mass Envelopes Research Articles

Rethinking Thorne–Żytkow Object Formation: Assembly via Common Envelope in Field Binaries

Thorne–Żytkow objects (TŻOs), hypothetical merger products in which a neutron star is embedded in a stellar core, are traditionally considered steady-state configurations. Their assembly, especially through dynamical channels, is not well understood. The predominant focus in the literature has been on the observational signatures related to the evolution and long-term fate of TŻOs, with their initial formation often treated as a given. However, the foundational calculations supporting the existence of TŻOs assume nonrotating spherically symmetric initial conditions that we find to be inconsistent with a binary merger scenario. In this work, we explore the implications of postmerger dynamics in TŻO formation scenarios with field binary progenitors, specifically the role that angular momentum transport during the common envelope phase plays in constraining the possible merger products, using the tools of stellar evolution and three-dimensional hydrodynamics. We also propose an alternative steady-state outcome for these mergers: the thin-envelope TŻO, an equilibrium solution consisting of a low-mass spherical envelope supported by the accretion disk luminosity of a central stellar-mass black hole. These configurations may be of interest to upcoming time-domain surveys as potential X-ray sources that may be preceded by a series of bright transient events.

Modelling CN Zeeman effect observations of the envelopes of a low-mass protostellar disc and a massive protostar

ABSTRACT We use the polaris radiative transfer code to produce simulated circular polarization Zeeman emission maps of the cyanide (CN) J = 1–0 molecular line transition for two types of protostellar envelope magnetohydrodynamic simulations. Our first model is a low-mass disc envelope system (box length L = 200 au), and our second model is the envelope of a massive protostar (L = 104 au) with a protostellar wind and a CN-enhanced outflow shell. We compute the velocity-integrated Stokes I and V, as well as the implied V/I polarization percentage, for each detector pixel location in our simulated emission maps. Our results show that both types of protostellar environments are in principle accessible with current circular polarization instruments, with each containing swaths of envelope area that yield percentage polarizations that exceed the 1.8 per cent nominal sensitivity limit for circular polarization experiments with the Atacama Large Millimeter/submillimeter Array. In both systems, high-polarization (≳1.8 per cent) pixels tend to lie at an intermediate distance away from the central star and where the line-centre opacity of the CN emission is moderately optically thin (τLC ∼ 0.1–1). Furthermore, our computed V/I values scale roughly with the density-weighted mean line-of-sight magnetic field strength, indicating that Zeeman observations can effectively diagnose the strength of envelope-scale magnetic fields. We also find that pixels with large V/I are preferentially co-located where the absolute value of the velocity-integrated V is also large, suggesting that locations with favourable percentage polarization are also favourable in terms of raw signal.

Early Planet Formation in Embedded Disks (eDisk). X. Compact Disks, Extended Infall, and a Fossil Outburst in the Class I Oph IRS43 Binary

We present the first results from the Early Planet Formation in Embedded Disks Atacama Large Millimeter/submillimeter Array Large Program toward Oph IRS43, a binary system of solar mass protostars. The 1.3 mm dust continuum observations resolve a compact disk, ∼6 au radius, around the northern component and show that the disk around the southern component is even smaller, ≲3 au. CO, 13CO, and C18O maps reveal a large cavity in a low-mass envelope that shows kinematic signatures of rotation and infall extending out to ∼2000 au. An expanding CO bubble centered on the extrapolated location of the source ∼130 yr ago suggests a recent outburst. Despite the small size of the disks, the overall picture is of a remarkably large and dynamically active region.

Linking ice and gas in the Coronet cluster in Corona Australis

Context. During the journey from the cloud to the disc, the chemical composition of the protostellar envelope material can be either preserved or processed to varying degrees depending on the surrounding physical environment. Aims. This works aims to constrain the interplay of solid (ice) and gaseous methanol (CH3OH) in the outer regions of protostellar envelopes located in the Coronet cluster in Corona Australis (CrA), and assess the importance of irradiation by the Herbig Ae/Be star R CrA. CH3OH is a prime test case as it predominantly forms as a consequence of the solid-gas interplay (hydrogenation of condensed CO molecules onto the grain surfaces) and it plays an important role in future complex molecular processing. Methods. We present 1.3 mm Submillimeter Array (SMA) and Atacama Pathfinder Experiment (APEX) observations towards the envelopes of four low-mass protostars in the Coronet cluster. Eighteen molecular transitions of seven species were identified. We calculated CH3OH gas-to-ice ratios in this strongly irradiated cluster and compared them with ratios determined towards protostars located in less irradiated regions such as Serpens SVS 4 in Serpens Main and the Barnard 35A cloud in the λ Orionis region. Results. The CH3OH gas-to-ice ratios in the Coronet cluster vary by one order of magnitude (from 1.2 × 10−4 to 3.1 × 10−3) which is similar to less irradiated regions as found in previous studies. We find that the CH3OH gas-to-ice ratios estimated in these three regions are remarkably similar despite the different UV radiation field intensities and formation histories. Conclusions. This result suggests that the overall CH3OH chemistry in the outer regions of low-mass envelopes is relatively independent of variations in the physical conditions and hence that it is set during the prestellar stage.

Terminating a common envelope jets supernova impostor event with a super-Eddington blue supergiant

ABSTRACT We conducted one-dimensional stellar evolutionary numerical simulations to build blue supergiant stellar models with a very low-envelope mass and a super-Eddington luminosity of 107L⊙ that mimic the last phase of a common envelope evolution (CEE) where a neutron star (NS) accretes mass from the envelope and launches jets that power the system. Common envelope jets supernovae (CEJSNe) are CEE transient events where an NS spirals-in inside the envelope and then the core of a red supergiant (RSG) star accretes mass and launches jets that power the transient event. In case the NS (or black hole) does not enter the core of the RSG, the event is a CEJSN-impostor. We propose that in some cases a CEJSN-impostor event might end with such a phase of a blue supergiant lasting for several years to few tens of years. The radius of the blue supergiant is about tens to few hundreds’ solar radii. We use a simple prescription to deposit the jets’ energy into the envelope. We find that the expected accretion rate of envelope mass onto the NS at the end of the CEE allows the power of the jets to be as we assume, 107L⊙. Such a low-mass envelope might be the end of the RSG envelope or a rebuilt envelope from mass fallback. Our study of a blue supergiant at the termination of a CEJSN-impostor event adds to the rich variety of transients that CEJSNe and CEJSN-impostors might form.

Convection reconciles the difference in efficiencies between low-mass and high-mass common envelopes

ABSTRACT The formation pathways for gravitational-wave merger sources are predicted to include common envelope (CE) evolution. Observations of high-mass post-common envelope binaries suggest that energy transfer to the envelope during the CE phase must be highly efficient. In contrast, observations of low-mass post-CE binaries indicate that energy transfer during the CE phase must be highly inefficient. Convection, a process present in low-mass and high-mass stars naturally explains this dichotomy. Using observations of Wolf–Rayet binaries, we study the effects of convection and radiative losses on the predicted final separations of high-mass common envelopes. Despite robust convection in massive stars, the effect is minimal as the orbit decays well before convection can transport the liberated orbital energy to the surface. In low-mass systems, convective transport occurs faster then the orbit decays, allowing the system to radiatively cool, thereby lowering the efficiency. The inclusion of convection reproduces observations of low-mass and high-mass binaries and remains a necessary ingredient for determining outcomes of common envelopes.

The TESS-Keck Survey. II. An Ultra-short-period Rocky Planet and Its Siblings Transiting the Galactic Thick-disk Star TOI-561

Abstract We report the discovery of TOI-561, a multiplanet system in the galactic thick disk that contains a rocky, ultra-short-period planet. This bright (V = 10.2) star hosts three small transiting planets identified in photometry from the NASA TESS mission: TOI-561 b (TOI-561.02, P = 0.44 days, R p = 1.45 ± 0.11 R ⊕), c (TOI-561.01, P = 10.8 days, R p = 2.90 ± 0.13 R ⊕), and d (TOI-561.03, P = 16.3 days, R p = 2.32 ± 0.16 R ⊕). The star is chemically ([Fe/H] = −0.41 ± 0.05, [α/Fe] = +0.23 ± 0.05) and kinematically consistent with the galactic thick-disk population, making TOI-561 one of the oldest (10 ± 3 Gyr) and most metal-poor planetary systems discovered yet. We dynamically confirm planets b and c with radial velocities from the W. M. Keck Observatory High Resolution Echelle Spectrometer. Planet b has a mass and density of 3.2 ± 0.8 M ⊕ and g cm−3, consistent with a rocky composition. Its lower-than-average density is consistent with an iron-poor composition, although an Earth-like iron-to-silicates ratio is not ruled out. Planet c is 7.0 ± 2.3 M ⊕ and 1.6 ± 0.6 g cm−3, consistent with an interior rocky core overlaid with a low-mass volatile envelope. Several attributes of the photometry for planet d (which we did not detect dynamically) complicate the analysis, but we vet the planet with high-contrast imaging, ground-based photometric follow-up, and radial velocities. TOI-561 b is the first rocky world around a galactic thick-disk star confirmed with radial velocities and one of the best rocky planets for thermal emission studies.

Observations of the Onset of Complex Organic Molecule Formation in Interstellar Ices

Abstract Isolated dense molecular cores are investigated to study the onset of complex organic molecule formation in interstellar ice. Sampling three cores with ongoing formation of low-mass stars (B59, B335, and L483) and one starless core (L694-2), we sample lines of sight to nine background stars and five young stellar objects (YSOs; A K ∼ 0.5–4.7). Spectra of these stars from 2 to 5 μm with NASA’s Infrared Telescope Facility (IRTF) simultaneously display signatures from the cores of H2O (3.0 μm), CH3OH (C–H stretching mode, 3.53 μm), and CO (4.67 μm) ices. The CO ice is traced by nine stars, in which five show a long-wavelength wing due to a mixture of CO with polar ice (CO r ), presumably CH3OH. Two of these sightlines also show independent detections of CH3OH. For these we find the ratio of the CH3OH:CO r is 0.55 ± 0.06 and 0.73 ± 0.07 from L483 and L694-2, respectively. The detections of both CO and CH3OH for the first time through lines of sight toward background stars observationally constrains the conversion of CO into CH3OH ice. Along the lines of sight, most of the CO exists in the gas phase and ≤15% of the CO is frozen out. However, CH3OH ice is abundant with respect to CO (∼50%) and exists mainly as a CH3OH-rich CO ice layer. Only a small fraction of the lines of sight contains CH3OH ice, presumably those with the highest density. The high conversion of CO to CH3OH can explain the abundances of CH3OH ice found in later stage Class 1 low-mass YSO envelopes (CH3OH:CO r ∼ 0.5–0.6). For high-mass YSOs and one Class 0 YSO, this ratio varies significantly, implying local variations can affect ice formation. The large CH3OH ice abundance indicates that the formation of complex organic molecules is likely during the prestellar phase in cold environments without higher energy particle interactions (e.g., cosmic rays).

A 3 mm Chemical Exploration of Small Organics in Class I YSOs

Abstract There is mounting evidence that the composition and structure of planetary systems are intimately linked to their birth environments. During the past decade, several spectral surveys probed the chemistry of the earliest stages of star formation and late planet-forming disks. However, very little is known about the chemistry of intermediate protostellar stages, i.e., class I young stellar objects (YSOs), where planet formation may have already begun. We present here the first results of a 3 mm spectral survey performed with the IRAM 30 m telescope to investigate the chemistry of a sample of seven class I YSOs located in the Taurus star-forming region. These sources were selected to embrace the wide diversity identified for low-mass protostellar envelope and disk systems. We present detections and upper limits of 13 small (N atoms ≤ 3) C, N, O, and S carriers—namely, CO, HCO+, HCN, HNC, CN, N2H+, C2H, CS, SO, HCS+, C2S, SO2, and OCS—and some of their D, 13C, 15N, 18O, 17O, and 34S isotopologues. Together, these species provide constraints on gas-phase C/N/O ratios, D and 15N fractionation, source temperature, and UV exposure, as well as the overall S chemistry. We find substantial evidence of chemical differentiation among our source sample, some of which can be traced back to class I physical parameters, such as the disk-to-envelope mass ratio (proxy for class I evolutionary stage), source luminosity, and UV field strength. Overall, these first results allow us to start investigating the astrochemistry of class I objects; however, interferometric observations are needed to differentiate envelope versus disk chemistry.

Parametric strategy on passive design affordances in cool-humid climate zones of North-East India

Buildings often have fixed function spaces that are complimentary or incompatible with thermal comfort (18–28 °C). Synergetic relationships ameliorates energy shortage and affords comfort. Galvanized iron roof two-storey houses of North-East India were studied to develop a theory-and-strategy to optimize design process and energy conservation. Methods include affordance theory criticism, surveys, simulations, synergy analysis. Parametric strategy on passive design affordances examines human comfort and temperature on diurnal time scales: Daytime (08–17 h), Evening (17–22 h), Night (22–08 h) in various seasons. Under flexible ventilation, living-dining space (S1) shows optimum temperature ranges: 20–28 °C in autumn (M1), 17–22 °C in winter (M2), and 20–31 °C in summer (M3) due to the complementary combination of passive design elements and can function as bedroom, living-room, kitchen, and social space in most seasons. In the attic-space flexible ventilation shows peak temperatures of 42 °C (autumn) and 48 °C (summer) due to low thermal mass but high thermal conductivity envelopes, and low air-changes rate (0.5 ACR) above 28 °C. Normal ventilations with 30 ACR in autumn, and a combination of 30 ACR (night) and 0.5 ACR (day) in summer reduced maximum temperature to ≤35 °C in autumn, and ≤41 °C in summer. Attic-space (S2) shows ≤29 °C in winter daytime and ≥20 °C in summer nights due to the envelope’s high heat emissivity (0.8) and function as day space in winter and summer bedroom. Shaded veranda (S3) shows low temperature (18–28 °C) in summer evening and afternoon and can function as shaded space for light work and enjoying fresh air. Passive design connotes responsiveness of spaces to the climate, and affordance theory’s complementarity lifestyle adds novelty, and it is critical to energy and space efficiency. Climate analysis affords perceptions of space and climate relationship. Parametric strategy straddles differences between space, climate, and functions to ameliorate energy needs and optimize design process.

Light-curve Modeling of Fast-evolving Supernova KSN 2015K: Explosion in Circumstellar Matter of a Super-AGB Progenitor

Abstract Recent supernova searches revealed a number of fast-evolving luminous transients. We perform radiation-hydrodynamic simulations of light curves of several models of supernova from super-asymptotic giant branch (super-AGB, SAGB) stars with low-mass envelopes (M env ∼ 0.05–1.25 M ⊙). The differences in the light curves found among the models are used to link the observed events to the most appropriate models. In particular, we propose that KSN 2015K is an electron-capture supernova. We assume an “optically” thick circumstellar medium (CSM) around SAGB and the circumstellar interaction powers the peak luminosity of the light curve with a short rise time. The faint tail might be influenced by the spin-down luminosity of a newborn Crab-like pulsar. Our fits indicate an ejecta mass of 0.02–0.05 M ⊙, a CSM mass of 0.10–0.12 M ⊙, a radius of the CSM photosphere ∼1014 cm, a kinetic energy of ∼3 × 1050 erg, a photospheric velocity v ph ≳ 10,000 km s−1, and a pulsar total spin energy (2.5–4) × 1049 erg.

ASASSN-15nx: A Luminous Type II Supernova with a “Perfect” Linear Decline

Abstract We report a luminous Type II supernova, ASASSN-15nx, with a peak luminosity of mag that is between those of typical core-collapse supernovae and super-luminous supernovae. The post-peak optical light curves show a long, linear decline with a steep slope of 2.5 mag (100 day)−1 (i.e., an exponential decline in flux) through the end of observations at phase . In contrast, the light curves of hydrogen-rich supernovae (SNe II-P/L) always show breaks in their light curves at phase ∼100 day, before settling onto 56Co radioactive decay tails with a decline rate of about 1 mag (100 day)−1. The spectra of ASASSN-15nx do not exhibit the narrow emission-line features characteristic of Type IIn SNe, which can have a wide variety of light-curve shapes usually attributed to strong interactions with a dense circumstellar medium (CSM). ASASSN-15nx has a number of spectroscopic peculiarities, including a relatively weak and triangular-shaped Hα emission profile with no absorption component. The physical origin of these peculiarities is unclear, but the long and linear post-peak light curve without a break suggests a single dominant powering mechanism. Decay of a large amount of (M Ni = 1.6 ± 0.2 ) can power the light curve of ASASSN-15nx, and the steep light-curve slope requires substantial γ-ray escape from the ejecta, which is possible given a low-mass hydrogen envelope for the progenitor. Another possibility is strong CSM interactions powering the light curve, but the CSM needs to be sculpted to produce the unique light-curve shape and avoid producing SN IIn-like narrow emission lines.

Fast and Luminous Transients from the Explosions of Long-lived Massive White Dwarf Merger Remnants

Abstract We study the evolution and final outcome of long-lived ( years) remnants from the merger of an He white dwarf (WD) with a more massive C/O or O/Ne WD. Using Modules for Experiments in Stellar Astrophysics ( ), we show that these remnants have a red giant configuration supported by steady helium burning, adding mass to the WD core until it reaches . At that point, the base of the surface convection zone extends into the burning layer, mixing the helium-burning products (primarily carbon and magnesium) throughout the convective envelope. Further evolution depletes the convective envelope of helium and dramatically slows the mass increase of the underlying WD core. The WD core mass growth re-initiates after helium depletion, as then an uncoupled carbon-burning shell is ignited and proceeds to burn the fuel from the remaining metal-rich extended envelope. For large enough initial total merger masses, O/Ne WD cores would experience electron-capture triggered collapse to neutron stars (NSs) after growing to near Chandrasekhar mass ( ). Massive C/O WD cores could suffer the same fate after a carbon-burning flame converts them to ONe. The NS formation would release erg into the remaining extended low mass envelope. Using the STELLA radiative transfer code, we predict the resulting optical light curves from these exploded envelopes. Reaching absolute magnitudes of , these transients are bright for about one week and have many features of the class of luminous, rapidly evolving transients studied by Drout and collaborators.

Reduced gas accretion on super-Earths and ice giants

A large fraction of giant planets have gaseous envelopes that are limited to about 10 % of their total mass budget. Such planets are present in the Solar System (Uranus, Neptune) and are frequently observed in short periods around other stars (the so-called Super-Earths). In contrast to these observations, theoretical calculations based on the evolution of hydrostatic envelopes argue that such low mass envelopes cannot be maintained around cores exceeding five Earth masses. Instead, under nominal disc conditions, these planets would acquire massive envelopes through runaway gas accretion within the lifetime of the protoplanetary disc. In this work, we show that planetary envelopes are not in hydrostatic balance, which slows down envelope growth. A series of 3-dimensional, global, radiative hydrodynamical simulations reveal a steady state gas flow, which enters through the poles and exits in the disc midplane. Gas is pushed through the outer envelope in about 10 orbital timescales. In regions of the disc that are not significantly dust-depleted, envelope accretion onto cores of about five Earth masses can get stalled as the gas flow enters the deep interior. Accreted solids sublimate deep in the convective interior, but small opacity-providing grains are trapped in the flow and do not settle, which further prevents rapid envelope accretion. The transition to runaway gas accretion can however be reached when cores grow larger than typical Super-Earths, beyond 15 Earth masses, and preferably when disc opacities are below kappa=1 cm^2/g. These findings offer an explanation for the typical low-mass envelopes around the cores of Super-Earths.

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

Abstract During a common envelope (CE) 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 CE, 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 CEs using the “wind tunnel” formalism developed by MacLeod et al. 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 CE is rare, and if it occurs, transitory. The implication for LIGO black hole binary assembly is that by avoiding strong accretion feedback, CE interactions should still result in the substantial orbital tightening needed to produce merging binaries.

A Hubble Space Telescope Survey for Novae in M87. II. Snuffing out the Maximum Magnitude–Rate of Decline Relation for Novae as a Non-standard Candle, and a Prediction of the Existence of Ultrafast Novae*

Abstract The extensive grid of numerical simulations of nova eruptions from the work of Yaron et al. first predicted that some classical novae might significantly deviate from the Maximum Magnitude–Rate of Decline (MMRD) relation, which purports to characterize novae as standard candles. Kasliwal et al. have announced the observational detection of a new class of faint, fast classical novae in the Andromeda galaxy. These objects deviate strongly from the MMRD relationship, as predicted by Yaron et al. Recently, Shara et al. reported the first detections of faint, fast novae in M87. These previously overlooked objects are as common in the giant elliptical galaxy M87 as they are in the giant spiral M31; they comprise about 40% of all classical nova eruptions and greatly increase the observational scatter in the MMRD relation. We use the extensive grid of the nova simulations of Yaron et al. to identify the underlying causes of the existence of faint, fast novae. These are systems that have accreted, and can thus eject, only very low-mass envelopes, of the order of 10−7–10−8 M ⊙, on massive white dwarfs. Such binaries include, but are not limited to, the recurrent novae. These same models predict the existence of ultrafast novae that display decline times, t 2, to be as short as five hours. We outline a strategy for their future detection.

Evolution Models of Helium White Dwarf–Main-sequence Star Merger Remnants

Abstract It is predicted that orbital decay by gravitational-wave radiation and tidal interaction will cause some close binary stars to merge within a Hubble time. The merger of a helium-core white dwarf with a main-sequence (MS) star can produce a red giant branch star that has a low-mass hydrogen envelope when helium is ignited and thus become a hot subdwarf. Because detailed calculations have not been made, we compute post-merger models with a stellar evolution code. We find the evolutionary paths available to merger remnants and find the pre-merger conditions that lead to the formation of hot subdwarfs. We find that some such mergers result in the formation of stars with intermediate helium-rich surfaces. These stars later develop helium-poor surfaces owing to diffusion. Combining our results with a model population and comparing to observed stars, we find that some observed intermediate helium-rich hot subdwarfs can be explained as the remnants of the mergers of helium-core white dwarfs with low-mass MS stars.

IPTF15dtg: a double-peaked Type Ic supernova from a massive progenitor

Type Ic supernovae (SNe Ic) arise from the core-collapse of H (and He) poor stars, which could be either single WR stars or lower-mass stars stripped of their envelope by a companion. Their light curves are radioactively powered and usually show a fast rise to peak ($\sim$10-15 d), without any early (first few days) emission bumps (with the exception of broad-lined SNe Ic) as sometimes seen for other types of stripped-envelope SNe (e.g., Type IIb SN 1993J and Type Ib SN 2008D). We have studied iPTF15dtg, a spectroscopically normal SN Ic with an early excess in the optical light curves followed by a long ($\sim$30 d) rise to the main peak. It is the first spectroscopically-normal double-peaked SN Ic observed. We aim to determine the properties of this explosion and of its progenitor star. Optical photometry and spectroscopy of iPTF15dtg was obtained with multiple telescopes. The resulting light curves and spectral sequence are analyzed and modelled with hydrodynamical and analytical models, with particular focus on the early emission. Results. iPTF15dtg is a slow rising SN Ic, similar to SN 2011bm. Hydrodynamical modelling of the bolometric properties reveals a large ejecta mass ($\sim$10 $M_{\odot}$) and strong $^{56}$Ni mixing. The luminous early emission can be reproduced if we account for the presence of an extended ($\sim$500 R$_{\odot}$), low-mass ($\sim$0.045 M$_{\odot}$) envelope around the progenitor star. Alternative scenarios for the early peak, such as the interaction with a companion, a shock-breakout (SBO) cooling tail from the progenitor surface, or a magnetar-driven SBO are not favored. The large ejecta mass and the presence of H and He free extended material around the star suggest that the progenitor of iPTF15dtg was a massive ($\gtrsim$ 35 M$_{\odot}$) WR star suffering strong mass loss.

450 d of Type II SN 2013ej in optical and near-infrared

We present optical and near-infrared photometric and spectroscopic observations of SN 2013ej, in galaxy M74, from 1 to 450 days after the explosion. SN 2013ej is a hydrogen-rich supernova, classified as a Type IIL due to its relatively fast decline following the initial peak. It has a relatively high peak luminosity (absolute magnitude M$_\rm{V}$ = -17.6) but a small $^{56}$Ni production of ~0.023 M$_\odot$. Its photospheric evolution is similar to other Type II SNe, with shallow absorption in the H{\alpha} profile typical for a Type IIL. During transition to the radioactive decay tail at ~100 days, we find the SN to grow bluer in B - V colour, in contrast to some other Type II supernovae. At late times, the bolometric light curve declined faster than expected from $^{56}$Co decay and we observed unusually broad and asymmetric nebular emission lines. Based on comparison of nebular emission lines most sensitive to the progenitor core mass, we find our observations are best matched to synthesized spectral models with a M$_\rm{ZAMS}$ = 12 - 15 M$_\odot$ progenitor. The derived mass range is similar to but not higher than the mass estimated for Type IIP progenitors. This is against the idea that Type IIL are from more massive stars. Observations are consistent with the SN having a progenitor with a relatively low-mass envelope.

Jet or shock breakout? The low-luminosity GRB 060218

We consider a model for the low-luminosity gamma-ray burst GRB 060218 that plausibly accounts for multiwavelength observations to day 20. The model components are: (1) a long-lived ($t_j \sim 3000$ s) central engine and accompanying low-luminosity ($L_j \sim 10^{47}$ erg s$^{-1}$), semirelativistic ($\gamma \sim 10$) jet; (2) a low-mass ($ \sim 10^{-2} M_\odot$) envelope surrounding the progenitor star; and (3) a modest amount of dust ($A_V \sim 0.1$ mag) in the interstellar environment. Blackbody emission from the transparency radius in a low-power jet outflow can fit the prompt thermal X-ray emission, and the nonthermal X-rays and gamma-rays may be produced via Compton scattering of thermal photons from hot leptons in the jet interior or the external shocks. The later mildly relativistic phase of this outflow can produce the radio emission via synchrotron radiation from the forward shock. Meanwhile, interaction of the associated SN 2006aj with a circumstellar envelope extending to $\sim10^{13}$ cm can explain the early optical emission. The X-ray afterglow can be interpreted as a light echo of the prompt emission from dust at $\sim 30$ pc. Our model is a plausible alternative to that of Nakar, who recently proposed shock breakout of a jet smothered by an extended envelope as the source of prompt emission. Both our results and Nakar's suggest that bursts such as GRB 060218 may originate from unusual progenitors with extended circumstellar envelopes, and that a jet is necessary to decouple the prompt emission from the supernova.