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A thermonuclear explosion triggered by a He-shell detonation on a carbonâ€"oxygen white-dwarf core has been predicted to have strong UV line blanketing at early times due to the iron-group elements produced during He-shell burning. We present the photometric and spectroscopic observations of SN 2016dsg, a subluminous peculiar Type I supernova consistent with a thermonuclear explosion involving a thick He shell. With a redshift of 0.04, the i -band peak absolute magnitude is derived to be around âˆ'17.5. The object is located far away from its host, an early-type galaxy, suggesting it originated from an old stellar population. The spectra collected after the peak are unusually red, show strong UV line blanketing and weak O i λ 7773 absorption lines, and do not evolve significantly over 30 days. An absorption line around 9700â€"10500 Ă... is detected in the near-infrared spectrum and is likely from the unburnt He in the ejecta. The spectroscopic evolution is consistent with the thermonuclear explosion models for a sub-Chandrasekhar-mass white dwarf with a thick He shell, while the photometric evolution is not well described by existing models. [ABSTRACT FROM AUTHOR]

We study the production of very light elements (Z < 20) in the dynamical and spiral-wave wind ejecta of binary neutron star mergers by combining detailed nucleosynthesis calculations with the outcome of numerical relativity merger simulations. All our models are targeted to GW170817 and include neutrino radiation. We explore different finite-temperature, composition-dependent nuclear equations of state, and binary mass ratios, and find that hydrogen and helium are the most abundant light elements. For both elements, the decay of free neutrons is the driving nuclear reaction. In particular, ∼0.5â€"2 Ă— 10âˆ'6 M ⊙ of hydrogen are produced in the fast expanding tail of the dynamical ejecta, while ∼1.5â€"11 Ă— 10âˆ'6 M ⊙ of helium are synthesized in the bulk of the dynamical ejecta, usually in association with heavy r -process elements. By computing synthetic spectra, we find that the possibility of detecting hydrogen and helium features in kilonova spectra is very unlikely for fiducial masses and luminosities, even when including nonlocal thermodynamic equilibrium effects. The latter could be crucial to observe helium lines a few days after merger for faint kilonovae or for luminous kilonovae ejecting large masses of helium. Finally, we compute the amount of strontium synthesized in the dynamical and spiral-wave wind ejecta, and find that it is consistent with (or even larger than, in the case of a long-lived remnant) the one required to explain early spectral features in the kilonova of GW170817. [ABSTRACT FROM AUTHOR]

We present ultraviolet (UV) spectroscopy and photometry of four Type Ia supernovae (SNe 2004dt, 2004ef, 2005M, and 2005cf) obtained with the UV prism of the Advanced Camera for Surveys on the Hubble Space Telescope. This data set provides unique spectral time series down to 2000Å. Significant diversity is seen in the near-maximum-light spectra (~2000-3500Å) for this small sample. The corresponding photometric data, together with archival data from Swift Ultraviolet/Optical Telescope observations, provide further evidence of increased dispersion in the UV emission with respect to the optical. The peak luminosities measured in the uvw1/F250W filter are found to correlate with the B-band light-curve shape parameter Δm15(B), but with much larger scatter relative to the correlation in the broadband B band (e.g., ~0.4 mag versus ~0.2 mag for those with 0.8 mag <Δm 15(B) < 1.7 mag). SN 2004dt is found as an outlier of this correlation (at>3σs), being brighter than normal SNe Ia such as SN 2005cf by ~0.9m a g and ~2.0 mag in the uvw1/F250W and uvm2/F220W filters, respectively. We show that different progenitor metallicity or line-expansion velocities alone cannot explain such a large discrepancy. Viewing-angle effects, such as due to an asymmetric explosion, may have a significant influence on the flux emitted in the UV region. Detailed modeling is needed to disentangle and quantify the above effects. [ABSTRACT FROM AUTHOR]

Hubble Space Telescope and ground-based observations of the Type IIn supernova (SN) 2010jl are analyzed, including photometry and spectroscopy in the ultraviolet, optical, and near-IR bands, 26-1128 days after first detection. At maximum, the bolometric luminosity was ∼3 × 1043 erg s–1 and even at 850 days exceeds 1042 erg s–1. A near-IR excess, dominating after 400 days, probably originates in dust in the circumstellar medium (CSM). The total radiated energy is ≳ 6.5 × 1050 erg, excluding the dust component. The spectral lines can be separated into one broad component that is due to electron scattering and one narrow with expansion velocity ∼100 km s–1 from the CSM. The broad component is initially symmetric around zero velocity but becomes blueshifted after ∼50 days, while remaining symmetric about a shifted centroid velocity. Dust absorption in the ejecta is unlikely to explain the line shifts, and we attribute the shift instead to acceleration by the SN radiation. From the optical lines and the X-ray and dust properties, there is strong evidence for large-scale asymmetries in the CSM. The ultraviolet lines indicate CNO processing in the progenitor, while the optical shows a number of narrow coronal lines excited by the X-rays. The bolometric light curve is consistent with a radiative shock in an r–2 CSM with a mass-loss rate of . The total mass lost is ≳ 3 M☼. These properties are consistent with the SN expanding into a CSM characteristic of a luminous blue variable progenitor with a bipolar geometry. The apparent absence of nuclear processing is attributed to a CSM that is still opaque to electron scattering. [ABSTRACT FROM AUTHOR]

A set of hydrodynamical models based on stellar evolutionary progenitors is used to study the nature of SN 2011dh. Our modeling suggests that a large progenitor star—with R ∼ 200 R☼—is needed to reproduce the early light curve (LC) of SN 2011dh. This is consistent with the suggestion that the yellow super-giant star detected at the location of the supernova (SN) in deep pre-explosion images is the progenitor star. From the main peak of the bolometric LC and expansion velocities, we constrain the mass of the ejecta to be ≈2 M☼, the explosion energy to be E = (6-10) × 1050 erg, and the 56Ni mass to be approximately 0.06 M☼. The progenitor star was composed of a helium core of 3-4 M☼ and a thin hydrogen-rich envelope of ≈0.1M☼ with a main-sequence mass estimated to be in the range of 12-15 M☼. Our models rule out progenitors with helium-core masses larger than 8 M☼, which correspond to MZAMS ≳ 25M☼. This suggests that a single star evolutionary scenario for SN 2011dh is unlikely. [ABSTRACT FROM AUTHOR]

We present the spectroscopic and photometric evolution of the nearby (z = 0.059) spectroscopically confirmed Type Ic supernova, SN 2010bh, associated with the soft, long-duration gamma-ray burst (X-ray flash) GRB 100316D. Intensive follow-up observations of SN 2010bh were performed at the ESO Very Large Telescope (VLT) using the X-shooter and FORS2 instruments. Thanks to the detailed temporal coverage and the extended wavelength range (3000-24800 Å), we obtained an unprecedentedly rich spectral sequence among the hypernovae, making SN 2010bh one of the best studied representatives of this SN class. We find that SN 2010bh has a more rapid rise to maximum brightness (8.0 ± 1.0 rest-frame days) and a fainter absolute peak luminosity (Lbol ≈ 3 × 1042 erg s–1) than previously observed SN events associated with GRBs. Our estimate of the ejected 56Ni mass is 0.12 ± 0.02 M☼. From the broad spectral features, we measure expansion velocities up to 47,000 km s–1, higher than those of SNe 1998bw (GRB 980425) and 2006aj (GRB 060218). Helium absorption lines He I λ5876 and He I 1.083 μm, blueshifted by ∼20,000-30,000 km s–1 and ∼28,000-38,000 km s–1, respectively, may be present in the optical spectra. However, the lack of coverage of the He I 2.058 μm line prevents us from confirming such identifications. The nebular spectrum, taken at ∼186 days after the explosion, shows a broad but faint [O I] emission at 6340 Å. The light curve shape and photospheric expansion velocities of SN 2010bh suggest that we witnessed a highly energetic explosion with a small ejected mass (Ek ≈ 1052 erg and Mej ≈ 3 M☼). The observed properties of SN 2010bh further extend the heterogeneity of the class of GRB SNe. [ABSTRACT FROM AUTHOR]