arXiv subjects

Stefano Benetti

Publications and source records attributed to Stefano Benetti.

At least 37 records · Page 2Linked to original sources

arXiv2019

Closing gaps to our origins. The UV window into the Universe

The investigation of the emergence of life is a major endeavour of science. Astronomy is contributing to it in three fundamental manners: (1) by measuring the chemical enrichment of the Universe, (2) by investigating planet formation and searching for exoplanets with signatures of life and, (3) by determining the abundance of aminoacids and the chemical routes to aminoacid and protein growth in astronomical bodies. This proposal deals with the first two. In the Voyage to 2050, the world-wide scientific community is getting equipped with large facilities for the investigation of the emergence of life in the Universe (i.e. VLT, JWST, ELT, GMT, TMT, ALMA, FAST, VLA, ATHENA, SKA) including the ESA's CHEOPS, PLATO and ARIEL missions. This white paper is a community effort to call for the development of a large ultraviolet optical observatory to gather fundamental data for this investigation that will not be accessible through other ranges of the electromagnetic spectrum. A versatile space observatory with UV sensitivity a factor of 50-100 greater than existing facilities will revolutionize our understanding of the pathway to life in the Universe.

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arXiv2019

Observations of Type Ia Supernova 2014J for Nearly 900 Days and Constraints on Its Progenitor System

We present extensive ground-based and $Hubble~Space~Telescope$ ($HST$) photometry of the highly reddened, very nearby type Ia supernova (SN Ia) 2014J in M82, covering the phases from 9 days before to about 900 days after the $B$-band maximum. SN 2014J is similar to other normal SNe Ia near the maximum light, but it shows flux excess in the $B$ band in the early nebular phase. This excess flux emission can be due to light scattering by some structures of circumstellar materials located at a few 10$^{17}$ cm, consistent with a single degenerate progenitor system or a double degenerate progenitor system with mass outflows in the final evolution or magnetically driven winds around the binary system. At t$\sim$+300 to $\sim$+500 days past the $B$-band maximum, the light curve of SN 2014J shows a faster decline relative to the $^{56}$Ni decay. Such a feature can be attributed to the significant weakening of the emission features around [Fe III] $λ$4700 and [Fe II] $λ$5200 rather than the positron escape as previously suggested. Analysis of the $HST$ images taken at t$>$600 days confirms that the luminosity of SN 2014J maintains a flat evolution at the very late phase. Fitting the late-time pseudo-bolometric light curve with radioactive decay of $^{56}$Ni, $^{57}$Ni and $^{55}$Fe isotopes, we obtain the mass ratio $^{57}$Ni/$^{56}$Ni as $0.035 \pm 0.011$, which is consistent with the corresponding value predicted from the 2D and 3D delayed-detonation models. Combined with early-time analysis, we propose that delayed-detonation through single degenerate scenario is most likely favored for SN 2014J.

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arXiv2019

Strongly Bipolar Inner Ejecta of the Normal Type IIP Supernova ASASSN-16at

We report distinctly double-peakedH-alpha and H-beta emission lines in the late-time, nebular-phase spectra (>~200 d) of the otherwise normal at early phases (<~ 100 d) Type IIP supernova ASASSN-16at (SN 2016X). Such distinctly double-peaked nebular Balmer lines have never been observed for a Type II SN. The nebular-phase Balmer emission is driven by the radioactive Co56 decay, so the observed line-profile bifurcation suggests a strong bipolarity in the Ni56 distribution or in the line-forming region of the inner ejecta. The strongly bifurcated blue- and red-shifted peaks are separated by ~3x10^3 km/s and are roughly symmetrically positioned with respect to the host-galaxy rest frame, implying that the inner ejecta are composed of two almost detached blobs. The red peak progressively weakens relative to the blue peak, and disappears in the 740 d spectrum. One possible reason for the line-ratio evolution is increasing differential extinction from continuous formation of dust within the envelope, which is also supported by the near-infrared flux excess that develops after ~100 d.

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arXiv2019

Observations of A Fast-Expanding and UV-Bright Type Ia Supernova SN 2013gs

In this paper, we present extensive optical and ultraviolet (UV) observations of the type Ia supernova (SN Ia) 2013gs discovered during the Tsinghua-NAOC Transient Survey. The photometric observations in the optical show that the light curves of SN 2013gs is similar to that of normal SNe Ia, with an absolute peak magnitude of $M_{B}$ = $-$19.25 $\pm$ 0.15 mag and a post-maximum decline rate $Δ$m$_{15}$(B) = 1.00 $ \pm $ 0.05 mag. \emph{Gehrels Swift} UVOT observations indicate that SN 2013gs shows unusually strong UV emission (especially in the $uvw1$ band) at around the maximum light (M$_{uvw1}$ $\sim$ $-$18.9 mag). The SN is characterized by relatively weak Fe~{\sc ii} {\sc iii} absorptions at $\sim$ 5000Å in the early spectra and a larger expansion velocity ($v_{Si}$ $\sim$ 13,000 km s$^{-1}$ around the maximum light) than the normal-velocity SNe Ia. We discuss the relation between the $uvw1-v$ color and some observables, including Si~{\sc ii} velocity, line strength of Si~{\sc ii} $λ$6355, Fe~{\sc ii}/{\sc iii} lines and $Δm_{15}$(B). Compared to other fast-expanding SNe Ia, SN 2013gs exhibits Si and Fe absorption lines with similar strength and bluer $uvw1-v$ color. We briefly discussed the origin of the observed UV dispersion of SNe Ia.

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arXiv2018

Signatures of an eruptive phase before the explosion of the peculiar core-collapse SN 2013gc

We present photometric and spectroscopic analysis of the peculiar core-collapse SN 2013gc, spanning seven years of observations. The light curve shows an early maximum followed by a fast decline and a phase of almost constant luminosity. At +200 days from maximum, a brightening of 1 mag is observed in all bands, followed by a steep linear luminosity decline after +300 d. In archival images taken between 1.5 and 2.5 years before the explosion, a weak source is visible at the supernova location, with mag$\approx$20. The early supernova spectra show Balmer lines, with a narrow ($\sim$560 km s$^{-1}$) P-Cygni absorption superimposed on a broad ($\sim$3400 km s$^{-1}$) component, typical of type IIn events. Through a comparison of colour curves, absolute light curves and spectra of SN 2013gc with a sample of supernovae IIn, we conclude that SN 2013gc is a member of the so-called type IId subgroup. The complex profile of the H$α$ line suggests a composite circumstellar medium geometry, with a combination of lower velocity, spherically symmetric gas and a more rapidly expanding bilobed feature. This circumstellar medium distribution has been likely formed through major mass-loss events, that we directly observed from 3 years before the explosion. The modest luminosity ($M_I\sim-16.5$ near maximum) of SN 2013gc at all phases, the very small amount of ejected $^{56}$Ni (of the order of $10^{-3}$ M$_\odot$), the major pre-supernova stellar activity and the lack of prominent [O I] lines in late-time spectra support a fall-back core-collapse scenario for the massive progenitor of SN~2013gc.

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arXiv2018

ASASSN-15nx: A luminous Type II supernova with a "perfect" linear decline

We report a luminous Type II supernova, ASASSN-15nx, with a peak luminosity of M_V=-20 mag, that is between typical core-collapse supernovae and super-luminous supernovae. The post-peak optical light curves show a long, linear decline with a steep slope of 2.5 mag/100 days (i.e., an exponential decline in flux), through the end of observations at phase ~260 days. In contrast, the light curves of hydrogen rich supernovae (SNe II-P/L) always show breaks in their light curves at phase ~100 days, before settling onto Co56 radioactive decay tails with a decline rate of about 1 mag/100 days. The spectra of ASASSN-15nx do not exhibit the narrow emission-line features characteristic of Type IIn SNe, which can have a wide variety of light-curve shapes usually attributed to strong interactions with a dense circumstellar medium (CSM). ASASSN-15nx has a number of spectroscopic peculiarities, including a relatively weak and triangularly-shaped H-alpha emission profile with no absorption component. The physical origin of these peculiarities is unclear, but the long and linear post-peak light curve without a break suggests a single dominant powering mechanism. Decay of a large amount of Ni56 (M_Ni56 = 1.6 +/- 0.2 M_sun) can power the light curve of ASASSN-15nx, and the steep light-curve slope requires substantial gamma-ray escape from the ejecta, which is possible given a low-mass hydrogen envelope for the progenitor. Another possibility is strong CSM interactions powering the light curve, but the CSM needs to be sculpted to produce the unique light-curve shape and to avoid producing SN IIn-like narrow emission lines.

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arXiv2018

Spectropolarimetry of Galactic stars with anomalous extinction sightlines

Highly reddened type Ia Supernovae (SNe Ia) with low total-to-selective visual extinction ratio values, $R_V$, also show peculiar linear polarization wavelength dependencies with peak polarizations at short wavelengths ($λ_{max} \lesssim 0.4 μm$). It is not clear why sightlines to SNe Ia display such different continuum polarization profiles from interstellar sightlines in the Milky Way with similar $R_V$ values. We investigate polarization profiles of a sample of Galactic stars with low $R_V$ values, along anomalous extinction sightlines, with the aim to find similarities to the polarization profiles that we observe in SN Ia sightlines. We undertook spectropolarimetry of 14 stars, and used archival data for three additional stars, and run dust extinction and polarization simulations to infer a simple dust model that can reproduce the observed extinction and polarization curves. Our sample of Galactic stars with low $R_V$ values and anomalous extinction sightlines displays normal polarization profiles with an average $λ_{max} \sim 0.53 {μm}$, and is consistent within 3$σ$ to a larger coherent sample of Galactic stars from literature. Despite the low $R_V$ values of dust towards the stars in our sample, the polarization curves do not show any similarity to the continuum polarization curves observed towards SNe Ia with low $R_V$ values. There is a correlation between the best-fit Serkowski parameters $K$ and $λ_{max}$, but we did not find any significant correlation between $R_V$ and $λ_{max}$. Our simulations show that the $K-λ_{max}$ relationship is an intrinsic property of polarization. Furthermore, we have shown that in order to reproduce polarization curves with normal $λ_{max}$ and low $R_V$ values, a population of large (a $\geq 0.1 μm$) interstellar silicate grains must be contained in the dust's composition.

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arXiv2018

SN 2017dio: a type-Ic supernova exploding in a hydrogen-rich circumstellar medium

SN 2017dio shows both spectral characteristics of a type-Ic supernova (SN) and signs of a hydrogen-rich circumstellar medium (CSM). Prominent, narrow emission lines of H and He are superposed on the continuum. Subsequent evolution revealed that the SN ejecta are interacting with the CSM. The initial SN Ic identification was confirmed by removing the CSM interaction component from the spectrum and comparing with known SNe Ic, and reversely, adding a CSM interaction component to the spectra of known SNe Ic and comparing them to SN 2017dio. Excellent agreement was obtained with both procedures, reinforcing the SN Ic classification. The light curve constrains the pre-interaction SN Ic peak absolute magnitude to be around $M_g = -17.6$ mag. No evidence of significant extinction is found, ruling out a brighter luminosity required by a SN Ia classification. These pieces of evidence support the view that SN 2017dio is a SN Ic, and therefore the first firm case of a SN Ic with signatures of hydrogen-rich CSM in the early spectrum. The CSM is unlikely to have been shaped by steady-state stellar winds. The mass loss of the progenitor star must have been intense, $\dot{M} \sim 0.02$ $(ε_{Hα}/0.01)^{-1}$ $(v_\textrm{wind}/500$ km s$^{-1}$) $(v_\textrm{shock}/10 000$ km s$^{-1})^{-3}$ $M_\odot$~yr$^{-1}$, peaking at a few decades before the SN. Such a high mass loss rate might have been experienced by the progenitor through eruptions or binary stripping.

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arXiv2017

Gaia17biu/SN 2017egm in NGC 3191: The closest hydrogen-poor superluminous supernova to date is in a "normal", massive, metal-rich spiral galaxy

Hydrogen-poor superluminous supernovae (SLSNe-I) have been predominantly found in low-metallicity, star-forming dwarf galaxies. Here we identify Gaia17biu/SN 2017egm as an SLSN-I occurring in a "normal" spiral galaxy (NGC 3191) in terms of stellar mass (several times 10^10 M_sun) and metallicity (roughly Solar). At redshift z=0.031, Gaia17biu is also the lowest redshift SLSN-I to date, and the absence of a larger population of SLSNe-I in dwarf galaxies of similar redshift suggests that metallicity is likely less important to the production of SLSNe-I than previously believed. With the smallest distance and highest apparent brightness for an SLSN-I, we are able to study Gaia17biu in unprecedented detail. Its pre-peak near-ultraviolet to optical color is similar to that of Gaia16apd and among the bluest observed for an SLSN-I while its peak luminosity (M_g = -21 mag) is substantially lower than Gaia16apd. Thanks to the high signal-to-noise ratios of our spectra, we identify several new spectroscopic features that may help to probe the properties of these enigmatic explosions. We detect polarization at the ~0.5% level that is not strongly dependent on wavelength, suggesting a modest, global departure from spherical symmetry. In addition, we put the tightest upper limit yet on the radio luminosity of an SLSN-I with <5.4x10^26 erg/s/Hz (at 10 GHz), which is almost a factor of 40 better than previous upper limits and one of the few measured at an early stage in the evolution of an SLSN-I. This limit largely rules out an association of this SLSNe-I with known populations of gamma-ray burst (GRB) like central engines.

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arXiv2017

Type Ibn Supernovae Show Photometric Homogeneity and Spectral Diversity at Maximum Light

Type Ibn supernovae (SNe) are a small yet intriguing class of explosions whose spectra are characterized by low-velocity helium emission lines with little to no evidence for hydrogen. The prevailing theory has been that these are the core-collapse explosions of very massive stars embedded in helium-rich circumstellar material (CSM). We report optical observations of six new SNe Ibn: PTF11rfh, PTF12ldy, iPTF14aki, iPTF15ul, SN 2015G, and iPTF15akq. This brings the sample size of such objects in the literature to 22. We also report new data, including a near-infrared spectrum, on the Type Ibn SN 2015U. In order to characterize the class as a whole, we analyze the photometric and spectroscopic properties of the full Type Ibn sample. We find that, despite the expectation that CSM interaction would generate a heterogeneous set of light curves, as seen in SNe IIn, most Type Ibn light curves are quite similar in shape, declining at rates around 0.1 mag/day during the first month after maximum light, with a few significant exceptions. Early spectra of SNe Ibn come in at least two varieties, one that shows narrow P Cygni lines and another dominated by broader emission lines, both around maximum light, which may be an indication of differences in the state of the progenitor system at the time of explosion. Alternatively, the spectral diversity could arise from viewing-angle effects or merely from a lack of early spectroscopic coverage. Together, the relative light curve homogeneity and narrow spectral features suggest that the CSM consists of a spatially confined shell of helium surrounded by a less dense extended wind.

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arXiv2017

Revisiting the Lick Observatory Supernova Search Volume-Limited Sample: Updated Classifications and Revised Stripped-Envelope Supernova Fractions

We re-examine the classifications of supernovae (SNe) presented in the Lick Observatory Supernova Search (LOSS) volume-limited sample with a focus on the stripped-envelope SNe. The LOSS volume-limited sample, presented by Leaman et al. (2011) and Li et al. (2011b), was calibrated to provide meaningful measurements of SN rates in the local universe; the results presented therein continue to be used for comparisons to theoretical and modeling efforts. Many of the objects from the LOSS sample were originally classified based upon only a small subset of the data now available, however, and recent studies have both updated some subtype distinctions and improved our ability to perform robust classications, especially for stripped-envelope SNe. We re-examine the spectroscopic classifications of all events in the LOSS volume-limited sample (180 SNe and SN impostors) and update them if necessary. We discuss the populations of rare objects in our sample including broad-lined Type Ic SNe, Ca-rich SNe, SN 1987A-like events (we identify SN 2005io as SN 1987A-like here for the first time), and peculiar subtypes. The relative fractions of Type Ia SNe, Type II SNe, and stripped-envelope SNe in the local universe are not affected, but those of some subtypes are. Most significantly, after discussing the often unclear boundary between SNe Ib and Ic when only noisy spectra are available, we find a higher SN Ib fraction and a lower SN Ic fraction than calculated by Li et al. (2011b): spectroscopically normal SNe Ib occur in the local universe $1.7 \pm 0.9$ times more often than do normal SNe Ic.

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arXiv2015

The spectacular evolution of Supernova 1996al over 15 years: a low energy explosion of a stripped massive star in a highly structured environment

Spectrophotometry of SN 1996al carried out throughout 15 years is presented. The early photometry suggests that SN 1996al is a Linear type-II supernova, with an absolute peak of Mv ~ -18.2 mag. Early spectra present broad, asymmetric Balmer emissions, with super-imposed narrow lines with P-Cygni profile, and He I features with asymmetric, broad emission components. The analysis of the line profiles shows that the H and He broad components form in the same region of the ejecta. By day +142, the Halpha profile dramatically changes: the narrow P-Cygni profile disappears, and the Halpha is fitted by three emission components, that will be detected over the remaining 15 yrs of the SN monitoring campaign. Instead, the He I emissions become progressively narrower and symmetric. A sudden increase in flux of all He I lines is observed between 300 and 600 days. Models show that the supernova luminosity is sustained by the interaction of low mass (~1.15 Msun) ejecta, expelled in a low kinetic energy (~ 1.6 x 10^50 erg) explosion, with highly asymmetric circumstellar medium. The detection of Halpha emission in pre-explosion archive images suggests that the progenitor was most likely a massive star (~25 Msun ZAMS) that had lost a large fraction of its hydrogen envelope before explosion, and was hence embedded in a H-rich cocoon. The low-mass ejecta and modest kinetic energy of the explosion are explained with massive fallback of material into the compact remnant, a 7-8 Msun black hole.

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arXiv2015

SN 2009ip at late times - an interacting transient at +2 years

We present photometric and spectroscopic observations of the interacting transient SN 2009ip taken during the 2013 and 2014 observing seasons. We characterise the photometric evolution as a steady and smooth decline in all bands, with a decline rate that is slower than expected for a solely $^{56}$Co-powered supernova at late phases. No further outbursts or eruptions were seen over a two year period from 2012 December until 2014 December. SN 2009ip remains brighter than its historic minimum from pre-discovery images. Spectroscopically, SN 2009ip continues to be dominated by strong, narrow ($\lesssim$2000 km~s$^{-1}$) emission lines of H, He, Ca, and Fe. While we make tenuous detections of [Fe~{\sc ii}] $λ$7155 and [O~{\sc i}] $λλ$6300,6364 lines at the end of 2013 June and the start of 2013 October respectively, we see no strong broad nebular emission lines that could point to a core-collapse origin. In general, the lines appear relatively symmetric, with the exception of our final spectrum in 2014 May, when we observe the appearance of a redshifted shoulder of emission at +550 km~s$^{-1}$. The lines are not blue-shifted, and we see no significant near- or mid-infrared excess. From the spectroscopic and photometric evolution of SN 2009ip until 820 days after the start of the 2012a event, we still see no conclusive evidence for core-collapse, although whether any such signs could be masked by ongoing interaction is unclear.

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arXiv2014

High Density Circumstellar Interaction in the Luminous Type IIn SN 2010jl: The first 1100 days

HST and ground based observations of the Type IIn SN 2010jl are analyzed, including photometry, spectroscopy in the ultraviolet, optical and NIR bands, 26-1128 days after first detection. At maximum the bolometric luminosity was $\sim 3\times10^{43}$ erg/s and even at 850 days exceeds $10^{42}$ erg/s. A NIR excess, dominating after 400 days, probably originates in dust in the circumstellar medium (CSM). The total radiated energy is $> 6.5\times10^{50}$ ergs, excluding the dust component. The spectral lines can be separated into one broad component due to electron scattering, and one narrow with expansion velocity $\sim 100$ km/s from the CSM. The broad component is initially symmetric around zero velocity but becomes blueshifted after $\sim 50$ 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 $\sim 0.1$ M_sun/yr. The total mass lost is $> 3$ M_sun. These properties are consistent with the SN expanding into a CSM characteristic of an LBV progenitor with a bipolar geometry. The apparent absence of nuclear processing is attributed to a CSM still opaque to electron scattering.

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arXiv2014

Abundance stratification in Type Ia Supernovae - IV: the luminous, peculiar SN 1991T

The abundance distribution of the elements in the ejecta of the peculiar, luminous Type Ia supernova (SN Ia) 1991T is obtained modelling spectra from before maximum light until a year after the explosion, with the method of "Abundance Tomography". SN 1991T is different from other slowly declining SNe Ia (e.g. SN 1999ee) in having a weaker Si II 6355 line and strong features of iron group elements before maximum. The distance to the SN is investigated along with the abundances and the density profile. The ionization transition that happens around maximum sets a strict upper limit on the luminosity. Both W7 and the WDD3 delayed detonation model are tested. WDD3 is found to provide marginally better fits. In this model the core of the ejecta is dominated by stable Fe with a mass of about 0.15 solar masses, as in most SNe Ia. The layer above is mainly 56Ni up to v~10000 km/s (~0.78 solar masses). A significant amount of 56Ni (~3 %) is located in the outer layers. A narrow layer between 10000 km/s and ~12000 km/s is dominated by intermediate mass elements (IME), ~0.18 solar masses. This is small for a SN Ia. The high luminosity and the consequently high ionization, and the high 56Ni abundance at high velocities explain the peculiar early-time spectra of SN 1991T. The outer part is mainly of oxygen, ~0.3 solar masses. Carbon lines are never detected, yielding an upper limit of 0.01 solar masses for C. The abundances obtained with the W7 density model are qualitatively similar to those of the WDD3 model. Different elements are stratified with moderate mixing, resembling a delayed detonation.

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arXiv2014

Asiago Supernova classification program: blowing out the first two hundred candles

We present the compilation of the first 221 supernovae classified during the Asiago Classification Program (ACP). The details of transients classification and the preliminarily reduced spectra, in fits format, are immediately posted on the Padova-Asiago SN group web site. The achieved performances for the first 2 years of the ACP are analysed, showing that half of all our classifications were made within 5 days from transient detection. The distribution of the supernova types of this sample resembles the distribution of the general list of all the supernovae listed in the Asiago SN catalog (ASNC, Barbon et al. 1999). Finally, we use our sub-sample of 78 core-collapse supernovae, for which we retrieve the host-galaxy morphology and r-band absolute magnitudes, to study the observed subtype distribution in dwarf compared to giant galaxies. This ongoing program will give its contribution to the classification of the large number of transients that will be soon delivered by the Gaia mission.

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arXiv2013

The Disappearance of the Red Supergiant Progenitor of Supernova 2008bk

Massive stars end their lives in spectacular supernova explosions. Identifying the progenitor star is a test of stellar evolution and explosion models. Here we show that the progenitor star of the supernova SN 2008bk has now disappeared, which provides conclusive evidence that this was the death of a red supergiant star.

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arXiv2013

On the progenitor of the Type IIP SN 2013ej in M74

We use natural seeing imaging of SN 2013ej in M74 to identify a progenitor candidate in archival {\it Hubble Space Telescope} + ACS images. We find a source coincident with the SN in the {\it F814W}-filter, however the position of the progenitor candidate in contemporaneous {\it F435W} and {\it F555W}-filters is significantly offset. We conclude that the "progenitor candidate" is in fact two physically unrelated sources; a blue source which is likely unrelated to the SN, and a red source which we suggest exploded as SN 2013ej. Deep images with the same instrument onboard {\it HST} taken when the supernova has faded (in approximately two years time) will allow us to accurately characterise the unrelated neighbouring source and hence determine the intrinsic flux of the progenitor in three filters. We suggest that the {\it F814W} flux is dominated by the progenitor of SN 2013ej, and assuming a bolometric correction appropriate to an M-type supergiant, we estimate that the mass of the progenitor of SN 2013ej was between 8 -- 15.5 M$_{\odot}$.

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