arXiv subjects

Stefano Valenti

Publications and source records attributed to Stefano Valenti.

At least 91 records · Page 5Linked to original sources

arXiv2021

The electron-capture origin of supernova 2018zd

In the transitional mass range ($\sim$ 8-10 solar masses) between white dwarf formation and iron core-collapse supernovae, stars are expected to produce an electron-capture supernova. Theoretically, these progenitors are thought to be super-asymptotic giant branch stars with a degenerate O+Ne+Mg core, and electron capture onto Ne and Mg nuclei should initiate core collapse. However, no supernovae have unequivocally been identified from an electron-capture origin, partly because of uncertainty in theoretical predictions. Here we present six indicators of electron-capture supernovae and show that supernova 2018zd is the only known supernova having strong evidence for or consistent with all six: progenitor identification, circumstellar material, chemical composition, explosion energy, light curve, and nucleosynthesis. For supernova 2018zd, we infer a super-asymptotic giant branch progenitor based on the faint candidate in the pre-explosion images and the chemically-enriched circumstellar material revealed by the early ultraviolet colours and flash spectroscopy. The light-curve morphology and nebular emission lines can be explained with the low explosion energy and neutron-rich nucleosynthesis produced in an electron-capture supernova. This identification provides insights into the complex stellar evolution, supernova physics, cosmic nucleosynthesis, and remnant populations in the transitional mass range.

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arXiv2021

SN 2017fgc: A Fast-Expanding Type Ia Supernova Exploded in Massive Shell Galaxy NGC 474

We present extensive optical photometric and spectroscopic observations of the high-velocity (HV) Type Ia supernova (SN Ia) 2017fgc, covering the phase from $\sim$ 12 d before to $\sim 389$ d after maximum brightness. SN 2017fgc is similar to normal SNe Ia, with an absolute peak magnitude of $M_{\rm max}^{B} \approx$ $-19.32 \pm 0.13$ mag and a post-peak decline of $Δm_{15}(B)$ = $1.05 \pm 0.07$ mag. Its peak bolometric luminosity is derived as $1.32 \pm 0.13) \times 10^{43} $erg s$^{-1}$, corresponding to a $^{56}$Ni mass of $0.51 \pm 0.03 M_{\odot}$. The light curves of SN 2017fgc are found to exhibit excess emission in the $UBV$ bands in the early nebular phase and pronounced secondary shoulder/maximum features in the $RrIi$ bands. Its spectral evolution is similar to that of HV SNe Ia, with a maximum-light Si II velocity of $15,000 \pm 150 $km s$^{-1}$ and a post-peak velocity gradient of $\sim$ $120 \pm 10 $km s$^{-1} $d$^{-1}$. The Fe II and Mg II lines blended near 4300 Å and the Fe II, Si II, and Fe III lines blended near 4800 Å are obviously stronger than those of normal SNe Ia. Inspecting a large sample reveals that the strength of the two blends in the spectra, and the secondary peak in the $i/r$-band light curves, are found to be positively correlated with the maximum-light Si II velocity. Such correlations indicate that HV SNe~Ia may experience more complete burning in the ejecta and/or that their progenitors have higher metallicity. Examining the birthplace environment of SN 2017fgc suggests that it likely arose from a stellar environment with young and high-metallicity populations.

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arXiv2021

The Gravity Collective: A Search for the Electromagnetic Counterpart to the Neutron Star-Black Hole Merger GW190814

We present optical follow-up imaging obtained with the Katzman Automatic Imaging Telescope, Las Cumbres Observatory Global Telescope Network, Nickel Telescope, Swope Telescope, and Thacher Telescope of the LIGO/Virgo gravitational wave (GW) signal from the neutron star-black hole (NSBH) merger GW190814. We searched the GW190814 localization region (19 deg$^{2}$ for the 90th percentile best localization), covering a total of 51 deg$^{2}$ and 94.6% of the two-dimensional localization region. Analyzing the properties of 189 transients that we consider as candidate counterparts to the NSBH merger, including their localizations, discovery times from merger, optical spectra, likely host-galaxy redshifts, and photometric evolution, we conclude that none of these objects are likely to be associated with GW190814. Based on this finding, we consider the likely optical properties of an electromagnetic counterpart to GW190814, including possible kilonovae and short gamma-ray burst afterglows. Using the joint limits from our follow-up imaging, we conclude that a counterpart with an $r$-band decline rate of 0.68 mag day$^{-1}$, similar to the kilonova AT 2017gfo, could peak at an absolute magnitude of at most $-17.8$ mag (50% confidence). Our data are not constraining for ''red'' kilonovae and rule out ''blue'' kilonovae with $M>0.5 M_{\odot}$ (30% confidence). We strongly rule out all known types of short gamma-ray burst afterglows with viewing angles $<$17$^{\circ}$ assuming an initial jet opening angle of $\sim$$5.2^{\circ}$ and explosion energies and circumburst densities similar to afterglows explored in the literature. Finally, we explore the possibility that GW190814 merged in the disk of an active galactic nucleus, of which we find four in the localization region, but we do not find any candidate counterparts among these sources.

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arXiv2021

Luminous Type II Short-Plateau Supernovae 2006Y, 2006ai, and 2016egz: A Transitional Class from Stripped Massive Red Supergiants

The diversity of Type II supernovae (SNe II) is thought to be driven mainly by differences in their progenitor's hydrogen-rich (H-rich) envelope mass, with SNe IIP having long plateaus ($\sim100$ days) and the most massive H-rich envelopes. However, it is an ongoing mystery why SNe II with short plateaus (tens of days) are rarely seen. Here, we present optical/near-infrared photometric and spectroscopic observations of luminous Type II short-plateau SNe 2006Y, 2006ai, and 2016egz. Their plateaus of about $50$-$70$ days and luminous optical peaks ($\lesssim-18.4$ mag) indicate significant pre-explosion mass loss resulting in partially stripped H-rich envelopes and early circumstellar material (CSM) interaction. We compute a large grid of MESA+STELLA single-star progenitor and light-curve models with various progenitor zero-age main-sequence (ZAMS) masses, mass-loss efficiencies, explosion energies, $^{56}$Ni masses, and CSM densities. Our model grid shows a continuous population of SNe IIP-IIL-IIb-like light-curve morphology in descending order of H-rich envelope mass. With large $^{56}$Ni masses ($\gtrsim0.05\,M_\odot$), short-plateau SNe II lie in a confined parameter space as a transitional class between SNe IIL and IIb. For SNe 2006Y, 2006ai, and 2016egz, our findings suggest high-mass red supergiant (RSG) progenitors ($M_{\rm ZAMS} \simeq 18$-$22\,M_{\odot}$) with small H-rich envelope masses ($M_{\rm H_{\rm env}} \simeq 1.7\,M_{\odot}$) that have experienced enhanced mass loss ($\dot{M} \simeq 10^{-2}\,M_{\odot}\,{\rm yr}^{-1}$) for the last few decades before the explosion. If high-mass RSGs result in rare short-plateau SNe II, then these events might ease some of the apparent underrepresentation of higher-luminosity RSGs in observed SN II progenitor samples.

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arXiv2021

Dynamical Modeling of the CIV Broad Line Region of the $z=2.805$ Multiply Imaged Quasar SDSS J2222+2745

We present CIV BLR modeling results for the multiply imaged $z=2.805$ quasar SDSS J2222+2745. Using data covering a 5.3 year baseline after accounting for gravitational time delays, we find models that can reproduce the observed emission-line spectra and integrated CIV fluctuations. The models suggest a thick disk BLR that is inclined by $\sim$40 degrees to the observer's line of sight and with a emissivity weighted median radius of $r_{\rm median} = 33.0^{+2.4}_{-2.1}$ light days. The kinematics are dominated by near-circular Keplerian motion with the remainder inflowing. The rest-frame lag one would measure from the models is $τ_{\rm median} = 36.4^{+1.8}_{-1.8}$ days, which is consistent with measurements based on cross-correlation. We show a possible geometry and transfer function based on the model fits and find that the model-produced velocity-resolved lags are consistent with those from cross-correlation. We measure a black hole mass of $\log_{10}(M_{\rm BH}/M_\odot) = 8.31^{+0.07}_{-0.06}$, which requires a scale factor of $\log_{10}(f_{{\rm mean},σ}) = 0.20^{+0.09}_{-0.07}$.

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arXiv2021

The fast evolving type Ib Supernova SN 2015dj in NGC 7371

We present the detailed optical evolution of a type Ib SN 2015dj in NGC 7371, using data spanning up to $\sim$ 170 days after discovery. SN 2015dj shares similarity in light curve shape with SN 2007gr and peaks at M$_{V}$ = $-17.37\pm$0.02 mag. Analytical modelling of the quasi bolometric light curve yields 0.06$\pm$0.01 M$_{\odot}$ of $^{56}$Ni, ejecta mass $M_{\rm ej} = 1.4^{+1.3}_{-0.5}$ \msol\, and kinetic energy $E_{\rm k} = 0.7^{+0.6}_{-0.3} \times 10^{51}$ erg. The spectral features show a fast evolution and resemble those of spherically symmetric ejecta. The analysis of nebular phase spectral lines indicate a progenitor mass between 13-20 M$_{\odot}$ suggesting a binary scenario.

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arXiv2021

SN 2017hpa: A Nearby Carbon-Rich Type Ia Supernova with a Large Velocity Gradient

We present extensive, well-sampled optical and ultraviolet photometry and optical spectra of the Type Ia supernova (SN Ia) 2017hpa. The light curves indicate that SN 2017hpa is a normal SN Ia with an absolute peak magnitude of $M_{\rm max}^{B} \approx$ -19.12$\pm$0.11 mag and a post-peak decline rate \mb\ = 1.02$\pm$0.07 mag. According to the quasibolometric light curve, we derive a peak luminosity of 1.25$\times$10$^{43}$ erg s$^{-1}$ and a $^{56}$Ni mass of 0.63$\pm$0.02 $M_{\odot}$. The spectral evolution of SN 2017hpa is similar to that of normal SNe Ia, while it exhibits unusually rapid velocity evolution resembling that of SN 1991bg-like SNe Ia or the high-velocity subclass of SNe Ia, with a post-peak velocity gradient of $\sim$ 130$\pm$7 km s$^{-1}$ d$^{-1}$. Moreover, its early spectra ($t < -7.9$ d) show prominent \CII~$λ$6580 absorption feature, which disappeared in near-maximum-light spectra but reemerged at phases from $t \sim +8.7$ d to $t \sim +11.7$ d after maximum light. This implies that some unburned carbon may mix deep into the inner layer, and is supported by the low \CII~$λ$6580 to \SiII~$λ$6355 velocity ratio ($\sim 0.81$) observed in SN 2017hpa. The \OI~$λ$7774 line shows a velocity distribution like that of carbon. The prominent carbon feature, low velocity seen in carbon and oxygen, and large velocity gradient make SN 2017hpa stand out from other normal SNe Ia, and are more consistent with predictions from a violent merger of two white dwarfs. Detailed modelling is still needed to reveal the nature of SN 2017hpa.

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arXiv2020

SN 2017gci: a nearby Type I Superluminous Supernova with a bumpy tail

We present and discuss the optical spectro-photometric observations of the nearby (z=0.087) Type I superluminous supernova (SLSN I) SN 2017gci, whose peak K-corrected absolute magnitude reaches Mg=-21.5 mag. Its photometric and spectroscopic evolution includes features of both slow and of fast evolving SLSN I, thus favoring a continuum distribution between the two SLSN-I subclasses. In particular, similarly to other SLSNe I, the multi-band light curves of SN 2017gci show two rebrightenings at about 103 and 142 days after the maximum light. Interestingly, this broadly agrees with a broad emission feature emerging around 6520 A after 51 days from the maximum light, which is followed by a sharp knee in the light curve. If we interpret this feature as Halpha, this could support the fact that the bumps are the signature of late interactions of the ejecta with a (hydrogen rich) circumstellar material. Then we fitted magnetar and CSM-interaction powered synthetic light curves onto the bolometric one of SN 2017gci. In the magnetar case, the fit suggests a polar magnetic field Bp = 6 x 1e14 G, an initial period of the magnetar Pinitial=2.8 ms, an ejecta mass Mejecta=9 Msun and an ejecta opacity k = 0.08 cm g^{-1} . A CSM interaction scenario would imply a CSM mass of 5 Msun and an ejecta mass of 12 Msun. Finally, the nebular spectrum of phase 187 days was modeled, deriving a mass of 10 Msun for the ejecta. Our models suggest that either a magnetar or CSM interaction might be the power sources for SN 2017gci and that its progenitor was a massive (40 Msun) star.

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arXiv2020

The black hole mass of the $z=2.805$ multiply imaged quasar SDSS J2222+2745 from velocity-resolved time lags of the CIV emission line

We present the first results of a 4.5 year monitoring campaign of the three bright images of multiply imaged $z=2.805$ quasar SDSS J2222+2745 using the Gemini North Multi-Object Spectrograph (GMOS-N) and the Nordic Optical Telescope (NOT). We take advantage of gravitational time delays to construct light curves surpassing 6 years in duration and achieve average spectroscopic cadence of 10 days during the 8 months of visibility per season. Using multiple secondary calibrators and advanced reduction techniques, we achieve percent-level spectrophotometric precision and carry out an unprecedented reverberation mapping analysis, measuring both integrated and velocity-resolved time lags for CIV. The full line lags the continuum by $τ_{\rm cen} = 36.5^{+2.9}_{-3.9}$ rest-frame days. We combine our measurement with published CIV lags and derive the $r_{\rm BLR}-L$ relationship $\log_{10}( τ/ {\rm day}) = (1.00\pm 0.08) + (0.48\pm 0.04) \log_{10}[λL_λ(1350{Å})/10^{44}~{\rm erg ~s}^{-1}]$ with 0.32$\pm$0.06 dex intrinsic scatter. The velocity-resolved lags are consistent with circular Keplerian orbits, with $τ_{\rm cen} = 86.2^{+4.5}_{-5.0}$, $25^{+11}_{-15}$, and $7.5^{+4.2}_{-3.5}$ rest-frame days for the core, blue wing, and red wing, respectively. Using $σ_{\rm line}$ with the mean spectrum and assuming $\log_{10} (f_{{\rm mean},σ}) = 0.52 \pm 0.26$, we derive $\log_{10}(M_{\rm BH}/M_{\odot}) = 8.63 \pm 0.27$. Given the quality of the data, this system represents a unique benchmark for calibration of $M_{\rm BH}$ estimators at high redshift. Future work will present dynamical modeling of the data to constrain the virial factor $f$ and $M_{\rm BH}$.

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arXiv2020

Optical and Near-Infrared Observations of the Nearby Type Ia Supernova 2017cbv

Supernova (SN) 2017cbv in NGC 5643 is one of a handful of type Ia supernovae (SNe~Ia) reported to have excess blue emission at early times. This paper presents extensive $BVRIYJHK_s$-band light curves of SN 2017cbv, covering the phase from $-16$ to $+125$ days relative to $B$-band maximum light. SN 2017cbv reached a $B$-band maximum of 11.710$\pm$0.006~mag, with a post-maximum magnitude decline $Δm_{15}(B)$=0.990$\pm$0.013 mag. The supernova suffered no host reddening based on Phillips intrinsic color, Lira-Phillips relation, and the CMAGIC diagram. By employing the CMAGIC distance modulus $μ=30.58\pm0.05$~mag and assuming $H_0$=72~$\rm km \ s^{-1} \ Mpc^{-1}$, we found that 0.73~\msun $^{56}$Ni was synthesized during the explosion of SN 2017cbv, which is consistent with estimates using reddening-free and distance-free methods via the phases of the secondary maximum of the NIR-band light curves. We also present 14 near-infrared spectra from $-18$ to $+49$~days relative to the $B$-band maximum light, providing constraints on the amount of swept-up hydrogen from the companion star in the context of the single degenerate progenitor scenario. No $Paβ$ emission feature was detected from our post-maximum NIR spectra, placing a hydrogen mass upper limit of 0.1 $M_{\odot}$. The overall optical/NIR photometric and NIR spectral evolution of SN 2017cbv is similar to that of a normal SN~Ia, even though its early evolution is marked by a flux excess no seen in most other well-observed normal SNe~Ia. We also compare the exquisite light curves of SN 2017cbv with some $M_{ch}$ DDT models and sub-$M_{ch}$ double detonation models.

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arXiv2020

The Exotic Type Ic Broad-Lined Supernova SN 2018gep: Blurring the Line Between Supernovae and Fast Optical Transients

In the last decade a number of rapidly evolving transients have been discovered that are not easily explained by traditional supernovae models. We present optical and UV data on onee such object, SN 2018gep, that displayed a fast rise with a mostly featureless blue continuum around maximum light, and evolved to develop broad features more typical of a SN Ic-bl while retaining significant amounts of blue flux throughout its observations. The blue excess is most evident in its near-UV flux that is over 4 magnitudes brighter than other stripped envelope supernovae, but also visible in optical g$-$r colors at early times. Its fast rise time of $t_{\rm rise,V} \lesssim 6.2 \pm 0.8$ days puts it squarely in the emerging class of Fast Evolving Luminous Transients, or Fast Blue Optical Transients. With a peak absolute magnitude of M$_r=-19.49 \pm 0.23 $ mag it is on the extreme end of both the rise time and peak magnitude distribution for SNe Ic-bl. Only one other SN Ic-bl has similar properties, iPTF16asu, for which less of the important early time and UV data have been obtained. We show that the objects SNe 2018gep and iPTF16asu have similar photometric and spectroscopic properties and that they overall share many similarities with both SNe Ic-bl and Fast Evolving Transients. We obtain IFU observations of the SN 2018gep host galaxy and derive a number of properties for it. We show that the derived host galaxy properties for both SN 2018gep and iPTF16asu are overall consistent with the SNe Ic-bl and GRB/SNe sample while being on the extreme edge of the observed Fast Evolving Transient sample. These photometric observations are consistent with a simple SN Ic-bl model that has an additional form of energy injection at early times that drives the observed rapid, blue rise, and we speculate that this additional power source may extrapolate to the broader Fast Evolving Transient sample.

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arXiv2020

Robotic reverberation mapping of the broad-line radio galaxy 3C 120

We carried out photometric and spectroscopic observations of the well-studied broad-line radio galaxy 3C 120 with the Las Cumbres Observatory (LCO) global robotic telescope network from 2016 December to 2018 April as part of the LCO AGN Key Project on Reverberation Mapping of Accretion Flows. Here, we present both spectroscopic and photometric reverberation mapping results. We used the interpolated cross-correlation function (ICCF) to perform multiple-line lag measurements in 3C 120. We find the H$γ$, He II $λ4686$, H$β$ and He I $λ5876$ lags of $τ_{\text{cen}} = 18.8_{-1.0}^{+1.3}$, $2.7_{-0.8}^{+0.7}$, $21.2_{-1.0}^{+1.6}$, and $16.9_{-1.1}^{+0.9}$ days respectively, relative to the V-band continuum. Using the measured lag and rms velocity width of the H$β$ emission line, we determine the mass of the black hole for 3C 120 to be $M=\left(6.3^{+0.5}_{-0.3}\right)\times10^7\,(f/5.5)$ M$_\odot$. Our black hole mass measurement is consistent with similar previous studies on 3C 120, but with small uncertainties. In addition, velocity-resolved lags in 3C 120 show a symmetric pattern across the H$β$ line, 25 days at line centre decreasing to 17 days in the line wings at $\pm4000$ km s$^{-1}$. We also investigate the inter-band continuum lags in 3C 120 and find that they are generally consistent with $τ\proptoλ^{4/3}$ as predicted from a geometrically-thin, optically-thick accretion disc. From the continuum lags, we measure the best fit value $τ_{\rm 0} = 3.5\pm 0.2$ days at $λ_{\rm 0} = 5477$A. It implies a disc size a factor of $1.6$ times larger than prediction from the standard disc model with $L/L_{\rm Edd} = 0.4$. This is consistent with previous studies in which larger than expected disc sizes were measured.

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arXiv2020

The low-luminosity type II SN\,2016aqf: A well-monitored spectral evolution of the Ni/Fe abundance ratio

Low-luminosity type II supernovae (LL SNe~II) make up the low explosion energy end of core-collapse SNe, but their study and physical understanding remain limited. We present SN\,2016aqf, a LL SN~II with extensive spectral and photometric coverage. We measure a $V$-band peak magnitude of $-14.58$\,mag, a plateau duration of $\sim$100\,days, and an inferred $^{56}$Ni mass of $0.008 \pm 0.002$\,\msun. The peak bolometric luminosity, L$_{\rm bol} \approx 10^{41.4}$\,erg\,s$^{-1}$, and its spectral evolution is typical of other SNe in the class. Using our late-time spectra, we measure the [\ion{O}{i}] $λ\lambda6300, 6364$ lines, which we compare against SN II spectral synthesis models to constrain the progenitor zero-age main-sequence mass. We find this to be 12 $\pm$ 3\,\msun. Our extensive late-time spectral coverage of the [\ion{Fe}{ii}] $\lambda7155$ and [\ion{Ni}{ii}] $\lambda7378$ lines permits a measurement of the Ni/Fe abundance ratio, a parameter sensitive to the inner progenitor structure and explosion mechanism dynamics. We measure a constant abundance ratio evolution of $0.081^{+0.009}_{-0.010}$, and argue that the best epochs to measure the ratio are at $\sim$200 -- 300\,days after explosion. We place this measurement in the context of a large sample of SNe II and compare against various physical, light-curve and spectral parameters, in search of trends which might allow indirect ways of constraining this ratio. We do not find correlations predicted by theoretical models; however, this may be the result of the exact choice of parameters and explosion mechanism in the models, the simplicity of them and/or primordial contamination in the measured abundance ratio.

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arXiv2019

Flash ionization signatures in the type Ibn supernova SN 2019uo

We present photometric and spectroscopic observations of the type Ibn supernova (SN) 2019uo, the second ever SN Ibn with flash ionization (He II, C III, N III) features in its early spectra. SN 2019uo displays a rapid post-peak luminosity decline of 0.1 mag d$^{-1}$ similar to most of the SNe Ibn, but is fainter ($M^V_{max} = -18.30 \pm 0.24$ mag) than a typical SN Ibn and shows a color evolution that places it between SNe Ib and the most extreme SNe Ibn. SN 2019uo shows P-cygni He I features in the early spectra which gradually evolves and becomes emission dominated post peak. It also shows faster evolution in line velocities as compared to most other members of the type Ibn subclass. The bolometric light curve is fairly described by a $^{56}$Ni + circumstellar interaction model.

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arXiv2019

SN2018kzr: a rapidly declining transient from the destruction of a white dwarf

We present SN2018kzr, the fastest declining supernova-like transient, second only to the kilonova, AT2017gfo. SN2018kzr is characterized by a peak magnitude of $M_r = -17.98$, peak bolometric luminosity of ${\sim} 1.4 \times 10^{43}$erg s$^{\mathrm{-1}}$ and a rapid decline rate of $0.48 \pm 0.03$ mag day$^{\textrm{-1}}$ in the $r$ band. The bolometric luminosity evolves too quickly to be explained by pure $^{\mathrm{56}}$Ni heating, necessitating the inclusion of an alternative powering source. Incorporating the spin-down of a magnetized neutron star adequately describes the lightcurve and we estimate a small ejecta mass of $M_\mathrm{ej} = 0.10 \pm 0.05$ $\textrm{M}_{\odot}$. Our spectral modelling suggests the ejecta is composed of intermediate mass elements including O, Si and Mg and trace amounts of Fe-peak elements, which disfavours a binary neutron star merger. We discuss three explosion scenarios for SN2018kzr, given the low ejecta mass, intermediate mass element composition and the high likelihood of additional powering - core collapse of an ultra-stripped progenitor, the accretion induced collapse of a white dwarf and the merger of a white dwarf and neutron star. The requirement for an alternative input energy source favours either the accretion induced collapse with magnetar powering or a white dwarf - neutron star merger with energy from disk wind shocks.

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arXiv2019

SN 2015an: a normal luminosity type II supernova with low expansion velocity at early phases

We present the photometry and spectroscopy of SN 2015an, a Type II Supernova (SN) in IC 2367. The recombination phase of the SN lasts up to $\sim$120 d, with a decline rate of 1.24 mag/100d, higher than the typical SNe IIP. The SN exhibits bluer colours than most SNe II, indicating higher ejecta temperatures. The absolute $V$-band magnitude of SN 2015an at 50 d is $-$16.83$\pm$0.04 mag, pretty typical for SNe II. However, the $^{56}$Ni mass yield, estimated from the tail $V$-band light curve to be 0.021$\pm$0.010 M$_\odot$, is comparatively low. The spectral properties of SN 2015an are atypical, with low H$α$ expansion velocity and presence of high velocity component of H$α$ at early phases. Moreover, the continuum exhibits excess blue flux up to $\sim$50 d, which is interpreted as a progenitor metallicity effect. The high velocity feature indicates ejecta-circumstellar material interaction at early phases. The semi-analytical modelling of the bolometric light curve yields a total ejected mass of $\sim$12 M$_\odot$, a pre-supernova radius of $\sim$388~R$_\odot$ and explosion energy of $\sim$1.8 foe.

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arXiv2019

On the Ca-strong 1991bg-like type Ia supernova 2016hnk: evidence for a Chandrasekhar-mass explosion

We present a comprehensive dataset of optical and near-infrared photometry and spectroscopy of type~Ia supernova (SN) 2016hnk, combined with integral field spectroscopy (IFS) of its host galaxy, MCG -01-06-070, and nearby environment. Properties of the SN local environment are characterized by means of single stellar population synthesis applied to IFS observations taken two years after the SN exploded. SN 2016hnk spectra are compared to other 1991bg-like SNe Ia, 2002es-like SNe Ia, and Ca-rich transients. In addition, abundance stratification modelling is used to identify the various spectral features in the early phase spectral sequence and the dataset is also compared to a modified non-LTE model previously produced for the sublumnious SN 1999by. SN 2016hnk is consistent with being a sub-luminous (M$_{\rm B}=-16.7$ mag, s$_{\rm BV}$=0.43$\pm$0.03), highly reddened object. IFS of its host galaxy reveals both a significant amount of dust at the SN location, as well as residual star formation and a high proportion of old stellar populations in the local environment compared to other locations in the galaxy, which favours an old progenitor for SN 2016hnk. Inspection of a nebular spectrum obtained one year after maximum contains two narrow emission lines attributed to the forbidden [Ca II] $λλ$7291,7324 doublet with a Doppler shift of 700 km s$^{-1}$. Based on various observational diagnostics, we argue that the progenitor of SN 2016hnk was likely a near Chandrasekhar-mass ($M_{\rm Ch}$) carbon-oxygen white dwarf that produced 0.108 $M_\odot$ of $^{56}$Ni. Our modeling suggests that the narrow [Ca II] features observed in the nebular spectrum are associated with $^{48}$Ca from electron capture during the explosion, which is expected to occur only in white dwarfs that explode near or at the $M_{\rm Ch}$ limit.

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arXiv2019

The Young and Nearby Normal Type Ia Supernova 2018gv: UV-Optical Observations and the Earliest Spectropolarimetry

The non-detection of companion stars in Type Ia supernova (SN) progenitor systems lends support to the notion of double-degenerate (DD) systems and explosions triggered by the merging of two white dwarfs. This very asymmetric process should lead to a conspicuous polarimetric signature. By contrast, observations consistently find very low continuum polarization as the signatures from the explosion process largely dominate over the pre-explosion configuration within several days. Critical information about the interaction of the ejecta with a companion and any circumstellar matter is encoded in the early polarization spectra. In this study, we obtain spectropolarimetry of SN\,2018gv with the ESO Very Large Telescope at $-$13.6 days relative to the $B-$band maximum light, or $\sim$5 days after the estimated explosion --- the earliest spectropolarimetric observations to date of any Type Ia SN. These early observations still show a low continuum polarization ($\lesssim$0.2\%) and moderate line polarization (0.30$\pm$0.04\% for the prominent \ion{Si}{2} $λ$6355 feature and 0.85$\pm$0.04\% for the high-velocity Ca component). The high degree of spherical symmetry implied by the low line and continuum polarization at this early epoch is consistent with explosion models of delayed detonations and is inconsistent with the merger-induced explosion scenario. The dense UV and optical photometry and optical spectroscopy within the first $\sim$100 days after the maximum light indicate that SN\,2018gv is a normal Type Ia SN with similar spectrophotometric behavior to SN\,2011fe.

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