The changing-type SN 2014C may come from an 11-M⊙ star stripped by binary interaction and violent eruption

ABSTRACT SN 2014C was an unprecedented supernova (SN) that displayed a metamorphosis from Type Ib to Type IIn over ∼200 d. This transformation is consistent with a helium star having exploded in a cavity surrounded by a dense shell of the progenitor’s stripped hydrogen envelope. For at least 5 yr post-explosion, the ejecta continued to interact with an outer, extended component of circumstellar medium (CSM) that was ejected even before the dense shell. It is still unclear, however, what kind of progenitor could have undergone such a complicated mass-loss history before it produced this peculiar SN. In this paper, we report a new analysis of SN 2014C’s host star cluster based on data from the Hubble Space Telescope (HST). By carefully fitting its spectral energy distribution (SED), we derive a precise cluster age of 20.0$^{+3.5}_{-2.6}$ Myr, which corresponds to the progenitor’s lifetime assuming coevolution. Combined with binary stellar evolution models, we find that SN 2014C’s progenitor may have been an ∼11-M⊙ star in a relatively wide binary system. The progenitor’s envelope was partially stripped by Case C or Case BC mass transfer via binary interaction, followed by a violent eruption that ejected the last hydrogen layer before terminal explosion. Thus, SN 2014C, in common with SNe 2006jc and 2015G, may be a third example that violent eruptions, with mass-loss rates matching luminous blue variable (LBV) giant eruptions, can also occur in much lower mass massive stars if their envelopes are partially or completely stripped in interacting binaries.

Annular substructures in the transition disks around LkCa 15 and J1610

We present high-resolution millimeter continuum ALMA observations of the disks around the T Tauri stars LkCa 15 and 2MASS J16100501-2132318 (hereafter, J1610). These transition disks host dust-depleted inner regions, which have possibly been carved by massive planets, and they are of prime interest to the study of the imprints of planet-disk interactions. While at moderate angular resolution, they appear as a broad ring surrounding a cavity, the continuum emission resolves into multiple rings at a resolution of ~60 × 40 mas (~7.5 au for LkCa 15, ~6 au for J1610) and ~7 μJy beam−1 rms at 1.3 mm. In addition to a broad extended component, LkCa 15 and J1610 host three and two narrow rings, respectively, with two bright rings in LkCa 15 being radially resolved. LkCa 15 possibly hosts another faint ring close to the outer edge of the mm emission. The rings look marginally optically thick, with peak optical depths of ~0.5 (neglecting scattering), in agreement with high angular resolution observations of full disks. We performed hydrodynamical simulations with an embedded, sub-Jovian-mass planet and show that the observed multi-ringed substructure can be qualitatively explained as the outcome of the planet-disk interaction. We note, however, that the choice of the disk cooling timescale alone can significantly impact the resulting gas and dust distributions around the planet, leading to different numbers of rings and gaps and different spacings between them. We propose that the massive outer disk regions of transition disks are favorable places for planetesimals, and possibly second-generation planet formation of objects with a lower mass than the planets carving the inner cavity (typically few MJup), and that the annular substructures observed in LkCa 15 and J1610 may be indicative of planetary core formation within dust-rich pressure traps. Current observations are compatible with other mechanisms contributing to the origin of the observed substructures, in particular with regard to narrow rings generated (or facilitated) at the edge of the CO and N2 snowlines.

Spatially resolving the thermally inhomogeneous outer atmosphere of the red giant Arcturus in the 2.3 μm CO lines

Aim. The outer atmosphere of K giants shows thermally inhomogeneous structures consisting of the hot chromospheric gas and the cool molecular gas. We present spectro-interferometric observations of the multicomponent outer atmosphere of the well-studied K1.5 giant Arcturus (α Boo) in the CO first overtone lines near 2.3 μm. Methods. We observed Arcturus with the AMBER instrument at the Very Large Telescope Interferometer (VLTI) at 2.28–2.31 μm with a spectral resolution of 12 000 and at projected baselines of 7.3, 14.6, and 21.8 m. Results. The high spectral resolution of the VLTI/AMBER instrument allowed us to spatially resolve Arcturus in the individual CO lines. Comparison of the observed interferometric data with the MARCS photospheric model shows that the star appears to be significantly larger than predicted by the model. It indicates the presence of an extended component that is not accounted for by the current photospheric models for this well-studied star. We found out that the observed AMBER data can be explained by a model with two additional CO layers above the photosphere. The inner CO layer is located just above the photosphere, at 1.04 ± 0.02 R⋆, with a temperature of 1600 ± 400 K and a CO column density of 1020 ± 0.3 cm−2. On the other hand, the outer CO layer is found to be as extended as to 2.6 ± 0.2 R⋆ with a temperature of 1800 ± 100 K and a CO column density of 1019 ± 0.15 cm−2. Conclusions. The properties of the inner CO layer are in broad agreement with those previously inferred from the spatially unresolved spectroscopic analyses. However, our AMBER observations have revealed that the quasi-static cool molecular component extends out to 2–3 R⋆, within which region the chromospheric wind steeply accelerates.

Ubiquitous ram pressure stripping in the Coma cluster of galaxies

We report the detection of Hα trails behind three new intermediate-mass irregular galaxies in the NW outskirts of the nearby cluster of galaxies Abell 1656 (Coma). Hints that these galaxies possess an extended component were found in earlier, deeper Hα observations carried out with the Subaru telescope. However the lack of a simultaneous r-band exposure, together with the presence of strong stellar ghosts in the Subaru images, prevented us from quantifying the detections. We therefore devoted one full night of Hα observation to each of the three galaxies using the San Pedro Martir 2.1 m telescope. One-sided tails of Hα emission of 10–20 kpc projected size were detected, suggesting an ongoing ram pressure stripping event. We added these 3 new sources of extended ionized gas to the 12 previously found, NGC 4848, and NGC 4921 whose ram pressure stripping is certified by HI asymmetry. This brings the number sources with Hα trails to 17 gaseous tails out of 27 (63%) late-type galaxies (LTG) members of the Coma cluster with direct evidence of ram pressure stripping. The 27 LTG galaxies, among these the 17 with extended Hα tails, have kinematic properties that are different from the rest of the early-type galaxy population of the core of the Coma cluster, as they deviate in the phase-space diagram |ΔV|/σ versus r/R200.

Extended Dust Emission from Nearby Evolved stars

We derive azimuthally-averaged surface-brightness profiles of 16 AGB stars in the far-IR and sub-mm with the aim of studying the resolved historic mass loss in the extended circumstellar envelope. The PSF-subtracted extended component fluxes were found to be ∼40% of the total source flux. By fitting SEDs at each radial point we derive the dust temperature, column density and spectral index of emissivity via Bayesian inference. The measured dust-to-gas ratios were somewhat consistent with canonical values however with a large scatter.

Nature of highly extended CS(J=7-6) emission around low-mass protostar L483

Single-dish observations in CS(J=7-6) using the Atacama Submillimeter Telescope Experiment (ASTE) reveal emission extending out to thousands of AU from low-mass protostars, much larger than is expected based on simple models for their envelopes. Hypotheses for this emission invoke gas dispersed from the envelope surfaces facing the bipolar outflow cavities. Here, we combine interferometric data from the Submillimeter Array (SMA) with the previous single-dish data from ASTE for the low-mass protostar L483 to study the spatial-kinematic structure of its CS(J=7-6) emission on projected scales ≳600 AU. In addition to providing more detailed information for the extended component, our combined maps reveal a compact central component in CS(J=7-6) having a steeper velocity gradient. Both the compact and extended components exhibit a velocity gradient in the opposite sense to that of a bipolar molecular outflow traced in CO(J=2-1). Finding that previous models make a number of wrong predictions for the observed features, we propose that both CS(J=7-6) components are produced by rotating and infalling gas along the envelope surfaces exposed by the bipolar outflow and therefore subjected to stellar irradiation and outflow compression.