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1. The extraplanar type II supernova ASASSN-14jb in the nearby edge-on galaxy ESO 467-G051

   https://doi.org/10.1051/0004-6361/201834972  

   Meza, Nicolás ; Prieto, J. L. ; Clocchiatti, A. ; Galbany, L. ; Anderson, J. P. ; Falco, E. ; Kochanek, C. S. ; Kuncarayakti, H. ; Sánchez, S. F. ; Brimacombe, J. ; et al ( September 2019 , Astronomy & Astrophysics)

   We present optical photometry and spectroscopy of the Type II supernova ASASSN-14jb, together with Very Large Telescope (VLT) Multi Unit Spectroscopic Explorer (MUSE) integral field observations of its host galaxy and a nebular-phase spectrum. This supernova, in the nearby galaxy ESO 467-G051 ( z  = 0.006), was discovered and followed-up by the all-sky automated survey for supernovae (ASAS-SN). We obtained well-sampled las cumbres network (LCOGTN) B V g r i and Swift w 2 m 1 w 1 u b v optical, near-UV/optical light curves, and several optical spectra in the early photospheric phases. The transient ASASSN-14jb exploded ∼2 kpc abovemore »the star-forming disk of ESO 467-G051, an edge-on disk galaxy. The large projected distance from the disk of the supernova position and the non-detection of any H II region in a 1.4 kpc radius in projection are in conflict with the standard environment of core-collapse supernova progenitors and suggests the possible scenario that the progenitor received a kick in a binary interaction. We present analysis of the optical light curves and spectra, from which we derived a distance of 25 ± 2 Mpc using state-of-the-art empirical methods for Type II SNe, physical properties of the SN explosion ( 56 Ni mass, explosion energy, and ejected mass), and properties of the progenitor; namely the progenitor radius, mass, and metallicity. Our analysis yields a 56 Ni mass of 0.0210  ±  0.0025  M ⊙ , an explosion energy of ≈0.25 × 10 51 ergs, and an ejected mass of ≈6  M ⊙ . We also constrained the progenitor radius to be R *  = 580  ±  28  R ⊙ which seems to be consistent with the sub-Solar metallicity of 0.3  ±  0.1  Z ⊙ derived from the supernova Fe II λ 5018 line. The nebular spectrum constrains strongly the progenitor mass to be in the range 10–12 M ⊙ . From the Spitzer data archive we detect ASASSN-14jb ≈330 days past explosion and we derived a total dust mass of 10 −4   M ⊙ from the 3.6 μ m and 4.5 μ m photometry. Using the F U V , N U V , B V g r i , K s , 3.6 μ m, and 4.5 μ m total magnitudes for the host galaxy, we fit stellar population synthesis models, which give an estimate of M *  ≈ 1 × 10 9   M ⊙ , an age of 3.2 Gyr, and a SFR ≈0.07  M ⊙ yr −1 . We also discuss the low oxygen abundance of the host galaxy derived from the MUSE data, having an average of 12 + log(O/H) = 8.27 +0.16 −0.20 using the O 3 N 2 diagnostic with strong line methods. We compared it with the supernova spectra, which is also consistent with a sub-Solar metallicity progenitor. Following recent observations of extraplanar H II regions in nearby edge-on galaxies, we derived the metallicity offset from the disk, being positive, but consistent with zero at 2 σ , suggesting enrichment from disk outflows. We finally discuss the possible scenarios for the unusual environment for ASASSN-14jb and conclude that either the in-situ star formation or runaway scenario would imply a low-mass progenitor, agreeing with our estimate from the supernova nebular spectrum. Regardless of the true origin of ASASSN-14jb, we show that the detailed study of the environment roughly agree with the stronger constraints from the observation of the transient.« less
2. Constraining Type Iax supernova progenitor systems with stellar population age dating

   https://doi.org/10.1093/mnras/staa294  

   Takaro, Tyler ; Foley, Ryan J ; McCully, Curtis ; Fong, Wen-fai ; Jha, Saurabh W ; Narayan, Gautham ; Rest, Armin ; Stritzinger, Maximilian ; McKinnon, Kevin ( March 2020 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Type Iax supernovae (SNe Iax) are the most common class of peculiar SNe. While they are thought to be thermonuclear white-dwarf (WD) SNe, SNe Iax are observationally similar to, but distinct from SNe Ia. Unlike SNe Ia, where roughly 30 per cent occur in early-type galaxies, only one SN Iax has been discovered in an early-type galaxy, suggesting a relatively short delay time and a distinct progenitor system. Furthermore, one SN Iax progenitor system has been detected in pre-explosion images with its properties consistent with either of two models: a short-lived (<100 Myr) progenitor system consisting of a WD primary and a He-star companion, or a singular Wolf–Rayetmore »progenitor star. Using deep Hubble Space Telescope images of nine nearby SN Iax host galaxies, we measure the properties of stars within 200 pc of the SN position. The ages of local stars, some of which formed with the SN progenitor system, can constrain the time between star formation and SN, known as the delay time. We compare the local stellar properties to synthetic photometry of single-stellar populations, fitting to a range of possible delay times for each SN. With this sample, we uniquely constrain the delay-time distribution for SNe Iax, with a median and 1σ confidence interval delay time of $63_{- 15}^{+ 58} \times 10^{6}$ yr. The measured delay-time distribution provides an excellent constraint on the progenitor system for the class, indicating a preference for a WD progenitor system over a Wolf–Rayet progenitor star.« less
3. Swift UVOT grism observations of nearby Type Ia supernovae – II. Probing the progenitor metallicity of SNe Ia with ultraviolet spectra

   https://doi.org/10.1093/mnras/stz3391  

   Pan, Y-C ; Foley, R. J. ; Jones, D. O. ; Filippenko, A. V. ; Kuin, N. P. M. ( December 2019 , Monthly Notices of the Royal Astronomical Society)

   Ultraviolet (UV) observations of Type Ia supernovae (SNe Ia) are crucial for constraining the properties of their progenitor systems. Theoretical studies predicted that the UV spectra, which probe the outermost layers of an SN, should be sensitive to the metal content of the progenitor. Using the largest SN Ia UV (λ < 2900 Å) spectroscopic sample obtained from Neil Gehrels Swift Observatory, we investigate the dependence of UV spectra on metallicity. For the first time, our results reveal a correlation (∼2σ) between SN Ia UV flux and host-galaxy metallicities, with SNe in more metal-rich galaxies (which are likely to havemore »higher progenitor metallicities) having lower UV flux level. We find that this metallicity effect is only significant at short wavelengths (λ ≲ 2700 Å), which agrees well with the theoretical predictions. We produce UV spectral templates for SNe Ia at peak brightness. With our sample, we could disentangle the effect of light-curve shape and metallicity on the UV spectra. We also examine the correlation between the UV spectra and SN luminosities as parametrized by Hubble residuals. However, we do not see a significant trend with Hubble residuals. This is probably due to the large uncertainties in SN distances, as the majority of our sample members are extremely nearby (redshift z ≲ 0.01). Future work with SNe discovered in the Hubble flow will be necessary to constrain a potential metallicity bias on SN Ia cosmology.

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4. Investigating the Unusual Spectroscopic Time Evolution in SN 2012fr

   Cain, Christopher ; Baron, E. ; Phillips, M. M. ; Contreras, Carlos ; Ashall, Chris ; Stritzinger, Maximilian D. ; Burns, Christopher R. ; Piro, Anthony L. ; Hsiao, Eric Y. ; Hoeflich, P. ; et al ( December 2018 , Astrophysical journal)

   The type Ia supernova (SN) 2012fr displayed an unusual combination of its Si II λλ5972, 6355 features. This includes the ratio of their pseudo-equivalent widths, placing it at the border of the shallow silicon (SS) and core normal (CN) spectral subtype in the Branch diagram, while the Si II λ6355 expansion velocities place it as a high-velocity (HV) object in the Wang et al. spectral type that most interestingly evolves slowly, placing it in the low-velocity gradient (LVG) typing of Benetti et al. Only 5% of SNe Ia are HV and located in the SS+CN portion of the Branch diagram,more »and fewer than 10% of SNe Ia are both HV and LVG. These features point toward SN 2012fr being quite unusual, similar in many ways to the peculiar SN 2000cx. We modeled the spectral evolution of SN 2012fr to see if we could gain some insight into its evolutionary behavior. We use the parameterized radiative transfer code SYNOW to probe the abundance stratification of SN 2012fr at pre-maximum, maximum, and post-maximum light epochs. We also use a grid of W7 models in the radiative transfer code PHOENIX to probe the effect of different density structures on the formation of the Si II λ6355 absorption feature at post-maximum epochs. We find that the unusual features observed in SN 2012fr are likely due to a shell-like density enhancement in the outer ejecta. We comment on possible reasons for atypical Ca II absorption features, and suggest that they are related to the Si II features. This paper includes data gathered with the 6.5 m Magellan Baade Telescope, located at Las Campanas Observatory, Chile.« less
5. Origin of metals in old Milky Way halo stars based on GALAH and Gaia

   https://doi.org/10.1093/mnras/stab1982  

   Ishigaki, Miho N ; Hartwig, Tilman ; Tarumi, Yuta ; Leung, Shing-Chi ; Tominaga, Nozomu ; Kobayashi, Chiaki ; Magg, Mattis ; Simionescu, Aurora ; Nomoto, Ken’ichi ( August 2021 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Stellar and supernova nucleosynthesis in the first few billion years of the cosmic history have set the scene for early structure formation in the Universe, while little is known about their nature. Making use of stellar physical parameters measured by GALAH Data Release 3 with accurate astrometry from the Gaia EDR3, we have selected ∼100 old main-sequence turn-off stars (ages ≳12 Gyr) with kinematics compatible with the Milky Way stellar halo population in the Solar neighbourhood. Detailed homogeneous elemental abundance estimates by GALAH DR3 are compared with supernova yield models of Pop III (zero-metal) core-collapse supernovae (CCSNe), normal (non-zero-metal) CCSNe,more »and Type Ia supernovae (SN Ia) to examine which of the individual yields or their combinations best reproduce the observed elemental abundance patterns for each of the old halo stars (‘OHS’). We find that the observed abundances in the OHS with [Fe/H] > −1.5 are best explained by contributions from both CCSNe and SN Ia, where the fraction of SN Ia among all the metal-enriching SNe is up to 10–20 per cent for stars with high [Mg/Fe] ratios and up to 20–27 per cent for stars with low [Mg/Fe] ratios, depending on the assumption about the relative fraction of near-Chandrasekhar-mass SNe Ia progenitors. The results suggest that, in the progenitor systems of the OHS with [Fe/H] > −1.5, ∼ 50–60 per cent of Fe mass originated from normal CCSNe at the earliest phases of the Milky Way formation. These results provide an insight into the birth environments of the oldest stars in the Galactic halo.« less