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Register a new userNo Stripped Companion Material in the Nebular Spectrum of the Two-Component Type Ia Supernova ASASSN-18bt
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We analyze a KeckI/LRIS nebular spectrum taken 268 days after $B$-band maximum of ASASSN-18bt (SN~2018oh), a Type Ia supernova (SN Ia) observed by {it K2} at the time of explosion. ASASSN-18bt exhibited a two-component rise to peak brightness, possibly the signature of an interaction between the SN ejecta and a large ($gtrsim 20~R_odot$) nearby, non-degenerate companion. We search for emission signatures of stripped material from a non-degenerate companion in the nebular spectrum and find no evidence for any unbound material. We place an upper limit of $< 0.006~M_odot$ on the amount of stripped/ablated H-rich material that could go undetected in our spectrum, effectively ruling out all hydrogen-rich donor stars. Additionally, we place a more tentative upper limit on HeI emission in the observed spectrum of $lesssim 0.02~M_odot$ which also rules out helium star companions. Our deep limits rule out a non-degenerate companion as the explanation for the early-time feature in ASASSN-18bt.
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As part of the 100IAS survey, a program aimed to obtain nebular-phase spectra for a volume-limited and homogeneous sample of Type Ia supernovae (SNe Ia), we observed ASASSN-18tb (SN 2018fhw) at 139 days past maximum light. ASASSN-18tb was a fast-declining, sub-luminous event that fits well within the observed photometric and spectroscopic distributions of the SN Ia population. We detect a prominent H$alpha$ emission line of $L_{{rm H}alpha}=2.2pm0.2times10^{38}$ ergs s$^{-1}$ with FWHM $approx1100$ km s$^{-1}$ in the nebular-phase spectrum of this SN Ia. High luminosity H$alpha$ emission ($L_{{rm H}alpha}gtrsim 10^{40}$ ergs~s$^{-1}$) has previously been discovered in a rare class of SNe Ia-like objects showing CSM interactions (SNe Ia-CSM). They predominantly belong to over-luminous ($M_{rm max}<-19$ mag in optical) 1991T-like SNe Ia and are exclusively found in star-forming galaxies. By contrast, ASASSN-18tb is a sub-luminous SN Ia ($M_{B, {rm max}}sim -17.7$ mag) found in an early-type galaxy dominated by old stellar populations. We discuss possible origins for the observed hydrogen. Out of 75 SNe Ia for which we have so far obtained nebular spectra in 100IAS, no other SN shows a $sim 1000 ,{rm km s^{-1}}$ H$alpha$ emission line with comparable line luminosity as ASASSN-18tb, emphasizing the rarity of such emission in the nebular phase. Based on preliminary results from our survey, the rate for ASASSN-18tb-like nebular H$alpha$ emission could be as high as $sim 10%$ level among sub-luminous SNe Ia.
We present nebular phase optical and near-infrared spectroscopy of the Type Ia supernova (SN) 2017cbv. The early light curves of SN~2017cbv showed a prominent blue bump in the $U$, $B$ and $g$ bands lasting for $sim$5 d. One interpretation of the early light curve was that the excess blue light was due to shocking of the SN ejecta against a nondegenerate companion star -- a signature of the single degenerate scenario. If this is the correct interpretation, the interaction between the SN ejecta and the companion star could result in significant H$alpha$ (or helium) emission at late times, possibly along with other species, depending on the companion star and its orbital separation. A search for H$alpha$ emission in our +302 d spectrum yields a nondetection, with a $L_{Halpha}$$<$8.0$times$10$^{35}$ erg/s (given an assumed distance of $D$=12.3 Mpc), which we have verified by implanting simulated H$alpha$ emission into our data. We make a quantitative comparison to models of swept-up material stripped from a nondegenerate companion star, and limit the mass of hydrogen that might remain undetected to $M_{rm H} < 1 times 10^{-4}$ $rm M_{odot}$. A similar analysis of helium star related lines yields a $M_{rm He} < 5 times 10^{-4}$ $rm M_{odot}$. Taken at face value, these results argue against a nondegenerate H or He-rich companion in Roche lobe overflow as the progenitor of SN 2017cbv. Alternatively, there could be weaknesses in the envelope-stripping and radiative transfer models necessary to interpret the strong H and He flux limits.
The single-degenerate scenario for Type Ia supernovae (SNe Ia) should yield metal-rich ejecta that enclose some stripped material from the non-degenerate H-rich companion star. We present a large grid of non-local thermodynamic equilibrium steady-state radiative transfer calculations for such hybrid ejecta and provide analytical fits for the Halpha luminosity and equivalent width. Our set of models covers a range of masses for 56Ni and the ejecta, for the stripped material (Mst), and post-explosion epochs from 100 to 300d. The brightness contrast between stripped material and metal-rich ejecta challenges the detection of HI and HeI lines prior to ~100d. Intrinsic and extrinsic optical depth effects also influence the radiation emanating from the stripped material. This inner denser region is marginally thick in the continuum and optically thick in all Balmer lines. The overlying metal-rich ejecta blanket the inner regions, completely below about 5000A, and more sparsely at longer wavelengths. As a consequence, Hbeta should not be observed for all values of Mst through at least 300 days, while Halpha should be observed after ~100d for all Mst >= 0.01Msun. This contrasts with the case of circumstellar (CSM) interaction, not subject to external blanketing, which should produce Halpha and Hbeta lines with a strength dependent primarily on CSM density. We confirm previous analyses that suggest low values of order 0.001Msun for Mst to explain the observations of the two SNe Ia with nebular-phase Halpha detection, in conflict with the much greater stripped mass predicted by hydrodynamical simulations for the single-degenerate scenario. A more likely solution is the double-degenerate scenario, together with CSM interaction, or enclosed material from a tertiary star in a triple system or from a giant planet. [Abridged]
On 2018 Feb. 4.41, the All-Sky Automated Survey for SuperNovae (ASAS-SN) discovered ASASSN-18bt in the K2 Campaign 16 field. With a redshift of z=0.01098 and a peak apparent magnitude of B_{max}=14.31, ASASSN-18bt is the nearest and brightest SNe Ia yet observed by the Kepler spacecraft. Here we present the discovery of ASASSN-18bt, the K2 light curve, and pre-discovery data from ASAS-SN and the Asteroid Terrestrial-impact Last Alert System (ATLAS). The K2 early-time light curve has an unprecedented 30-minute cadence and photometric precision for an SN~Ia light curve, and it unambiguously shows a ~4 day nearly linear phase followed by a steeper rise. Thus, ASASSN-18bt joins a growing list of SNe Ia whose early light curves are not well described by a single power law. We show that a double-power-law model fits the data reasonably well, hinting that two physical processes must be responsible for the observed rise. However, we find that current models of the interaction with a non-degenerate companion predict an abrupt rise and cannot adequately explain the initial, slower linear phase. Instead, we find that existing, published models with shallow 56Ni are able to span the observed behavior and, with tuning, may be able to reproduce the ASASSN-18bt light curve. Regardless, more theoretical work is needed to satisfactorily model this and other early-time SNe~Ia light curves. Finally, we use Swift X-ray non-detections to constrain the presence of circumstellar material (CSM) at much larger distances and lower densities than possible with the optical light curve. For a constant density CSM these non-detections constrain rho<4.5 * 10^5 cm^-3 at a radius of 4 *10^15 cm from the progenitor star. Assuming a wind-like environment, we place mass-loss limits of Mdot< 8 * 10^-6 M_sun yr^-1 for v_w=100 km s^-1, ruling out some symbiotic progenitor systems.
As the closest Type Ia supernova in decades, SN 2014J provides a unique opportunity for detailed investigation into observational signatures of the progenitor system and explosion mechanism in addition to burning product distribution. We present a late-time near-infrared spectral series from Gemini-N at $307-466$ days after the explosion. Following the $H$-band evolution probes the distribution of radioactive iron group elements, the extent of mixing, and presence of magnetic fields in the expanding ejecta. Comparing the isolated $1.6440$ $mu$m [Fe II] emission line with synthetic models shows consistency with a Chandrasekhar-mass white dwarf of $rho_c=0.7times10^9$ g cm${}^{-3}$ undergoing a delayed detonation. The ratio of the flux in the neighboring $1.54$ $mu$m emission feature to the flux in the $1.6440$ $mu$m feature shows evidence of some limited mixing of stable and radioactive iron group elements in the central regions. Additionally, the evolution of the $1.6440$ $mu$m line shows an intriguing asymmetry. When measuring line-width of this feature, the data show an increase in line width not seen in the evolution of the synthetic spectra, corresponding to $approx1{,}000$ km s${}^{-1}$, which could be caused by a localized transition to detonation producing asymmetric ionization in the ejecta. Using the difference in width between the different epochs, an asymmetric component in the central regions, corresponding to approximately the inner $2times10^{-4}$ of white dwarf mass suggests an off-center ignition of the initial explosion and hence of the kinematic center from the chemical center. Several additional models investigated, including a He detonation and a merger, have difficulty reproducing the features seen these spectra.
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