A high mass progenitor for the Type Ic Supernova 2007gr inferred from its environment

We present an analysis of late-time Hubble Space Telescope Wide Field Camera 3 (WFC3) and Wide Field Planetary Camera 2 (WFPC2) observations of the site of the Type Ic supernova (SN) 2007gr in NGC 1058. The SN is barely recovered in the late-time WFPC2 observations, while a possible detection in the later WFC3 data is debatable. These observations were used to conduct a multiwavelength study of the surrounding stellar population. We fit spatial profiles to a nearby bright source that was previously proposed to be a host cluster. We find that, rather than being an extended cluster, it is consistent with a single point-like object. Fitting stellar models to the observed spectral energy distribution of this source, we conclude it is A1-A3 Yellow Supergiant, possibly corresponding to a star with MZAMS = 40 M⊙. SN 2007gr is situated in a massive star association, with diameter of ≈300 pc. We present a Bayesian scheme to determine the properties of the surrounding massive star population, in conjunction with the Padova isochrones. We find that the stellar population, as observed in either the WFC3 and WFPC2 observations, can be well fit by two age distributions with mean ages: ∼6.3 Myr and ∼50 Myr. The stellar population is clearly dominated by the younger age solution (by factors of 3.5 and 5.7 from the WFPC2 and WFC3 observations, respectively), which corresponds to the lifetime of a star with MZAMS ∼ 30 M⊙. This is strong evidence in favour of the hypothesis that SN 2007gr arose from a massive progenitor star, possibly capable of becoming a Wolf–Rayet star

VLT Spectropolarimetry of the optical transient in NGC300. Evidence for asymmetry in the circumstellar dust

AIMS: The main goal of this work is to study possible signs of asymmetry in the bright optical transient in NGC300, with the aim of getting independent information on the explosion mechanism, the progenitor star and its circumstellar environment. METHODS: Using VLT-FORS1 we have obtained low-resolution optical linear spectropolarimetry of NGC300 OT2008-1 on two epochs, 48 and 55 days after the discovery, covering the spectral range 3600--9330A. RESULTS: The data show a continuum polarization at a very significant level. At least two separate components are identified. The first is characterized by a strong wavelength dependency and a constant position angle (68.6+/-0.3 degrees), which is parallel to the local spiral arm of the host galaxy. The second shows a completely different position angle (151.3+/-0.4) and displays a mild but statistically significant evolution between the two epochs. While the former is identified as arising in the interstellar dust associated with NGC300, the latter is most likely due to continuum polarization by dust scattering in the circumstellar environment. No line depolarization is detected in correspondence of the most intense emission lines, disfavoring electron scattering as the source of intrinsic polarization. This implies a very small deviation from symmetry in the continuum-forming region. Given the observed level of intrinsic polarization, the transient must be surrounded by a significant amount of dust (>4x10^-5 Msun), asymmetrically distributed within a few thousand AU. This most likely implies that one or more asymmetric outflow episodes took place during the past history of the progenitor.Comment: Accepted for publication in Astronomy and Astrophysics. 16 pages, 16 figure

Whatever happened to the progenitors of supernovae 2008cn, 2009kr and 2009md?

We present new late-time, high-resolution observations of the sites of supernovae (SNe) 2008cn, 2009kr and 2009md, acquired with the Hubble Space Telescope. In all instances, significant flux from the SNe is still recovered at late times. We show that the previous identification of the progenitor of SN 2008cn was actually a blend of two sources, whose locations are resolved in these new observations. We suggest that the progenitor of SN 2008cn was actually a red supergiant with Minit < 16 M⊙. In the late-time observations of SN 2009kr, we find that the pre-explosion source (previously thought to be a yellow supergiant) is most probably a small compact cluster with mass ∼6000 M⊙. In late-time F814W observations of the site of SN 2009md, we find a single point source with identical brightness to the pre-explosion source, suggesting some caution in assuming that the pre-explosion source was the progenitor

First systematic high-precision survey of bright supernovae

Rapid variability before and near the maximum brightness of supernovae has the potential to provide a better understanding of nearly every aspect of supernovae, from the physics of the explosion up to their progenitors and the circumstellar environment. Thanks to modern time-domain optical surveys, which are discovering supernovae in the early stage of their evolution, we have the unique opportunity to capture their intraday behavior before maximum. We present high-cadence photometric monitoring (on the order of seconds-minutes) of the optical light curves of three Type Ia and two Type II SNe over several nights before and near maximum light, using the fast imagers available on the 2.3 m Aristarchos telescope at Helmos Observatory and the 1.2 m telescope at Kryoneri Observatory in Greece. We applied differential aperture photometry techniques using optimal apertures and we present reconstructed light curves after implementing a seeing correction and the Trend Filtering Algorithm (TFA, Kovács et al. 2005, MNRAS, 356, 557). TFA yielded the best results, achieving a typical precision between 0.01 and 0.04 mag. We did not detect significant bumps with amplitudes greater than 0.05 mag in any of the SNe targets in the VR-, R-, and I-bands light curves obtained. We measured the intraday slope for each light curve, which ranges between −0.37−0.36 mag day−1 in broadband VR, −0.19−0.31 mag day−1 in R band, and −0.13−0.10 mag day−1 in I band. We used SNe light curve fitting templates for SN 2018gv, SN 2018hgc and SN 2018hhn to photometrically classify the light curves and to calculate the time of maximum. We provide values for the maximum of SN 2018zd after applying a low-order polynomial fit and SN 2018hhn for the first time. We conclude that optimal aperture photometry in combination with TFA provides the highest-precision light curves for SNe that are relatively well separated from the centers of their host galaxies. This work aims to inspire the use of ground-based, high-cadence and high-precision photometry to study SNe with the purpose of revealing clues and properties of the explosion environment of both core-collapse and Type Ia supernovae, the explosion mechanisms, binary star interaction and progenitor channels. We suggest monitoring early supernovae light curves in hotter (bluer) bands with a cadence of hours as a promising way of investigating the post-explosion photometric behavior of the progenitor stars

A new precise mass for the progenitor of the Type IIP SN 2008bk

The progenitor of the Type IIP supernova (SN) 2008bk was discovered in pre-explosion g&#39;r&#39;i&#39;IYJHK(s) images, acquired with European Southern Observatory Very Large Telescope FOcal Reducer and low dispersion Spectrograph, High Acuity Wide field K-band Imager and Infrared Spectrometer and Array Camera instruments and the Gemini Multi-Object Spectrograph-South instrument. The wealth of pre-explosion observations makes the progenitor of this SN one of the best studied, since the detection of the progenitor of SN 1987A. Previous analyses of the properties of the progenitor were hampered by the limited quality of the photometric calibration of the pre-explosion images and the crowded nature of the field containing the SN. We present new late-time observations of the site of SN 2008bk acquired with identical instrument and filter configurations as the pre-explosion observations, and confirm that the previously identified red supergiant (RSG) star was the progenitor of this SN and has now disappeared. Image subtraction techniques were used to conduct precise photometry of the now missing progenitor, independently of blending from any nearby stars. The nature of the surrounding stellar population and their contribution to the flux attributed to the progenitor in the pre-explosion images are probed using Hubble Space Telescope Wide Field Camera 3 Ultraviolet-Visible/Infrared observations. In comparison with MARCS synthetic spectra, we find the progenitor was a highly reddened RSG with luminosity log(L/L-circle dot) = 4.84(-0.12)(+0.10), corresponding to an initial mass of M-init = 12.9(-1.8)(+1.6)M(circle dot). The temperature of the progenitor was hotter than previously expected for RSGs (T similar to 4330 K), but consistent with new temperatures derived for RSGs using spectral energy distribution fitting techniques. We show that there is evidence for significant extinction of the progenitor, possibly arising in the circumstellar medium, but that this dust yields a similar reddening law to dust found in the interstellar medium (E(B - V) = 0.77 with R-V = 3.1). Our improved analysis, which carefully accounts for the systematics, results in a more precise and robust mass estimate, making the progenitor of SN 2008bk the most well understood progenitor of a Type IIP SN from pre-explosion observations.</p

Properties of extragalactic dust inferred from linear polarimetry of Type Ia Supernovae

Aims: The aim of this paper is twofold: 1) to investigate the properties of extragalactic dust and compare them to what is seen in the Galaxy; 2) to address in an independent way the problem of the anomalous extinction curves reported for reddened Type Ia Supernovae (SN) in connection to the environments in which they explode. Methods: The properties of the dust are derived from the wavelength dependence of the continuum polarization observed in four reddened Type Ia SN: 1986G, 2006X, 2008fp, and 2014J. [...] Results: All four objects are characterized by exceptionally low total-to-selective absorption ratios (R_V) and display an anomalous interstellar polarization law, characterized by very blue polarization peaks. In all cases the polarization position angle is well aligned with the local spiral structure. While SN~1986G is compatible with the most extreme cases of interstellar polarization known in the Galaxy, SN2006X, 2008fp, and 2014J show unprecedented behaviours. The observed deviations do not appear to be connected to selection effects related to the relatively large amounts of reddening characterizing the objects in the sample. Conclusions: The dust responsible for the polarization of these four SN is most likely of interstellar nature. The polarization properties can be interpreted in terms of a significantly enhanced abundance of small grains. The anomalous behaviour is apparently associated with the properties of the galactic environment in which the SN explode, rather than with the progenitor system from which they originate. For the extreme case of SN2014J, we cannot exclude the contribution of light scattered by local material; however, the observed polarization properties require an ad hoc geometrical dust distribution.Comment: 10 pages, 6 figures, accepted for publication in Astronomy and Astrophysic

SN 1999ga: a low-luminosity linear type II supernova?

Type II-linear supernovae are thought to arise from progenitors that have lost most of their H envelope by the time of the explosion, and they are poorly understood because they are only occasionally discovered. It is possible that they are intrinsically rare, but selection effects due to their rapid luminosity evolution may also play an important role in limiting the number of detections. In this context, the discovery of a subluminous type II-linear event is even more interesting. We investigate the physical properties and characterise the explosion site of the type II SN 1999ga, which exploded in the nearby spiral galaxy NGC 2442. Spectroscopic and photometric observations of SN 1999ga allow us to constrain the energetics of the explosion and to estimate the mass of the ejected material, shedding light on the nature of the progenitor star in the final stages of its life. The study of the environment in the vicinity of the explosion site provides information on a possible relation between these unusual supernovae and the properties of the galaxies hosting them. Despite the lack of early-time observations, we provide reasonable evidence that SN 1999ga was probably a type II-linear supernova that ejected a few solar masses of material, with a very small amount of radioactive elements of the order of 0.01 solar masses.Comment: 11 pages, 9 figures. Accepted for publication in A&A (March 28, 2009

Analyzing SN2003Z with PHOENIX

We present synthetic spectra around maximum for the type II supernova SN 2003Z, which was first detected on January 29.7 2003. Comparison with observed spectra aim at the determination of physical parameters for SN 2003Z. Synthetic spectra are calculated with our stellar atmosphere code PHOENIX. It solves the special relativistic equation of radiative transfer, including large NLTE-calculations and line blanketing by design, in 1-dimensional spherical symmetry. The observed spectra were obtained at the 3.5 meter telescope at Calar Alto. The TWIN instrument was used so that a spectral range from about 3600 to 7500 Angstroem was covered. The spectra were taken on Feb. 4, 5, 9, and 11, 2003. The physical parameters of the models give the luminosities, a range of possible velocity profiles for the SN, an estimate of the colour excess, and the observed metalicity.Comment: 8 figure

Constraining the mass of the GRB 030329 progenitor

The long-duration gamma-ray burst (GRB) 030329, associated with supernova (SN) 2003dh, occurred inside a star-forming dwarf galaxy at redshift $z=0.1685$. The low redshift, and a rich set of archival Hubble Space Telescope (HST) images, makes this GRB well-suited for a detailed study of the stellar population in the immediate vicinity of the explosion. Since the lifetime of a star is directly tied to its mass, the age of the stellar population can be used to put constraints on the GRB and SN progenitor mass. From the HST images we extract the colours of the precise site from which the GRB originated, and find that the colours are significantly different from those of the overall host galaxy and the surrounding starburst environment. We have used spectral evolutionary models, including nebular emission, to carefully constrain the age of the stellar population, and hence the progenitor, at the very explosion site. For instantaneous burst models we find that a population age of 5 Myr best matches the data, suggesting a very massive (M > 50 M_sun) star as the progenitor, with an upper limit of 8 Myr (M > 25 M_sun). For more extended star formation scenarios, the inferred progenitor age is in most cases still very young (age 25 M_sun), with an upper limit of 20 Myr (M > 12 M_sun). These age estimates are an order of magnitude lower than the ages inferred from the overall host galaxy colours, indicating that progenitor mass estimates based on data for spatially unresolved GRB host galaxies will in general be of limited use. Our results are consistent with the collapsar scenario.Comment: Accepted to MNRA