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Register a new userH-$alpha$ emission in the nebular spectrum of the Type Ia supernova ASASSN-18tb
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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.
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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.
We present photometric and spectroscopic observations of the unusual Type Ia supernova ASASSN-18tb, including a series of SALT spectra obtained over the course of nearly six months and the first observations of a supernova by the Transiting Exoplanet Survey Satellite (TESS). We confirm a previous observation by Kollmeier et al. (2019) showing that ASASSN-18tb is the first relatively normal Type Ia supernova to exhibit clear broad ($sim1000$ km s$^{-1}$) H$alpha$ emission in its nebular phase spectra. We find that this event is best explained as a sub-Chandrasekhar mass explosion with $M_{Ni} approx 0.3; rm{M}_odot$. Despite the strong H$alpha$ signature at late times, we find that the early rise of the supernova shows no evidence for deviations from a single-component power-law and is best fit with a moderately shallow power-law of index $1.69pm0.04$. We find that the H$alpha$ luminosity remains approximately constant after its initial detection at phase +37 d, and that the H$alpha$ velocity evolution does not trace that of the Fe~III$~lambda4660$ emission. These suggest that the H$alpha$ emission arises from circumstellar medium (CSM) rather than swept up material from a non-degenerate companion. However, ASASSN-18tb is strikingly different from other known CSM-interacting Type Ia supernovae in a number of significant ways. Those objects typically show an H$alpha$ luminosity two orders of magnitude higher than what is seen in ASASSN-18tb, pushing them away from the empirical light-curve relations that define normal Type Ia supernovae. Conversely, ASASSN-18tb exhibits a fairly typical light curve and luminosity for an underluminous or transitional SN Ia, with $M_R approx -18.1$ mag. Moreover, ASASSN-18tb is the only SN Ia showing H$alpha$ from CSM interaction to be discovered in an early-type galaxy.
We present optical photometry and spectroscopy of the Type Ia supernova SN2018cqj/ATLAS18qtd. The supernova exploded in an isolated region at $sim 65$~kpc from the S0 galaxy IC~550 at $z=0.0165$ ($Dapprox 74$~Mpc) and has a redshift consistent with a physical association to this galaxy. Multicolor photometry show that SN2018cqj/ATLAS18qtd is a low-luminosity ($M_{B_{max}}approx -17.9$ mag), fast-declining Type Ia with color stretch $s_{BV} approx 0.6$ and $B$-band decline rate $Delta m_{15}(B) approx 1.77$ mag. Two nebular-phase spectra obtained as part of the 100IAS survey at +193 and +307 days after peak show the clear detection of a narrow H$alpha$ line in emission that is resolved in the first spectrum with $rm FWHM approx 1200$ km s$^{-1}$ and $L_{Halpha} approx 3.8times 10^{37}$ erg s$^{-1}$. The detection of a resolved H$alpha$ line with a declining luminosity is broadly consistent with recent models where hydrogen is stripped from the non-degenerate companion in a single-degenerate progenitor system. However, the amount of hydrogen consistent with the luminosities of the H$alpha$ line would be $sim 10^{-3}$ M$_{odot}$, significantly less than theoretical model predictions in the classical single-degenerate progenitor systems. SN2018cqj/ATLAS18qtd is the second low-luminosity, fast-declining Type Ia SN after SN2018fhw/ASASSN-18tb that shows narrow H$alpha$ in emission in its nebular-phase spectra.
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.
We present photometric and spectroscopic observations of the 03fg-like type Ia supernova (SN Ia) ASASSN-15hy from the ultraviolet (UV) to the near-infrared (NIR). ASASSN-15hy shares many of the hallmark characteristics of 03fg-like SNe Ia, previously referred to as super-Chandrasekhar SNe Ia. It is bright in the UV and NIR, lacks a clear i-band secondary maximum, shows a strong and persistent C II feature, and has a low Si II $lambda$6355 velocity. However, some of its properties are also extreme among the subgroup. ASASSN-15hy is under-luminous (M$_{B,peak}=-19.14^{+0.11}_{-0.16}$ mag), red ($(B-V)_{Bmax}=0.18^{+0.01}_{-0.03}$ mag), yet slowly declining ($Delta{m_{15}}(B)=0.72 pm 0.04$ mag). It has the most delayed onset of the i-band maximum of any 03fg-like SN. ASASSN-15hy lacks the prominent H-band break emission feature that is typically present during the first month past maximum in normal SNe Ia. Such events may be a potential problem for high-redshift SN Ia cosmology. ASASSN-15hy may be explained in the context of an explosion of a degenerate core inside a non-degenerate envelope. The explosion impacting the non-degenerate envelope with a large mass provides additional luminosity and low ejecta velocities. An initial deflagration burning phase is critical in reproducing the low $^{56}$Ni mass and luminosity, while the large core mass is essential in providing the large diffusion time scales required to produce the broad light curves. The model consists of a rapidly rotating 1.47 $M_{odot}$ degenerate core and a 0.8 $M_{odot}$ non-degenerate envelope. This deflagration core-degenerate scenario may result from the merger between a white dwarf and the degenerate core of an asymptotic giant branch star.
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