Abstract
We report spectropolarimetric observations of the Type Ia supernova (SN) SN 2021rhu at four epochs: −7, +0, +36, and +79 days relative to its
B
-band maximum luminosity. A ...wavelength-dependent continuum polarization peaking at 3890 ± 93 Å and reaching a level of
p
max
=
1.78
%
±
0.02
% was found. The peak of the polarization curve is bluer than is typical in the Milky Way, indicating a larger proportion of small dust grains along the sight line to the SN. After removing the interstellar polarization, we found a pronounced increase of the polarization in the Ca
ii
near-infrared triplet, from ∼0.3% at day −7 to ∼2.5% at day +79. No temporal evolution in high-resolution flux spectra across the Na
i
D and Ca
ii
H and K features was seen from days +39 to +74, indicating that the late-time increase in polarization is intrinsic to the SN as opposed to being caused by scattering of SN photons in circumstellar or interstellar matter. We suggest that an explanation for the late-time rise of the Ca
ii
near-infrared triplet polarization may be the alignment of calcium atoms in a weak magnetic field through optical excitation/pumping by anisotropic radiation from the SN.
The nearby SN 2017eaw is a Type II-P ("plateau") supernova (SN) showing early-time, moderate CSM interaction. We present a comprehensive study of this SN, including the analysis of high-quality ...optical photometry and spectroscopy covering the very early epochs up to the nebular phase, as well as near-ultraviolet and near-infrared spectra and early-time X-ray and radio data. The combined data of SNe 2017eaw and 2004et allow us to get an improved distance to the host galaxy, NGC 6946, of D ∼ 6.85 0.63 Mpc; this fits into recent independent results on the distance of the host and disfavors the previously derived (30% shorter) distances based on SN 2004et. From modeling the nebular spectra and the quasi-bolometric light curve, we estimate the progenitor mass and some basic physical parameters for the explosion and ejecta. Our results agree well with previous reports on a red supergiant progenitor star with a mass of ∼15-16 M . Our estimation of the pre-explosion mass-loss rate ( yr−1) agrees well with previous results based on the opacity of the dust shell enshrouding the progenitor, but it is orders of magnitude lower than previous estimates based on general light-curve modeling of Type II-P SNe. Combining late-time optical and mid-infrared data, a clear excess at 4.5 m can be seen, supporting the previous statements on the (moderate) dust formation in the vicinity of SN 2017eaw.
ABSTRACT
Detailed spectropolarimetric studies may hold the key to probing the explosion mechanisms and the progenitor scenarios of Type Ia supernovae (SNe Ia). We present multi-epoch ...spectropolarimetry and imaging polarimetry of SN 2019ein, an SN Ia showing high expansion velocities at early phases. The spectropolarimetry sequence spans from ∼−11 to +10 d relative to peak brightness in the B band. We find that the level of the continuum polarization of SN 2019ein, after subtracting estimated interstellar polarization, is in the range 0.0–0.3 per cent, typical for SNe Ia. The polarization position angle remains roughly constant before and after the SN light-curve peak, implying that the inner regions share the same axisymmetry as the outer layers. We observe high polarization (∼1 per cent) across both the Si ii λ6355 and Ca ii near-infrared triplet features. These two lines also display complex polarization modulations. The spectropolarimetric properties of SN 2019ein rule out a significant departure from spherical symmetry of the ejecta for up to a month after the explosion. These observations disfavour merger-induced and double-detonation models for SN 2019ein. The imaging polarimetry shows weak evidence for a modest increase in polarization after ∼20 d since the B-band maximum. If this rise is real and is observed in other SNe Ia at similar phases, we may have seen, for the first time, an aspherical interior similar to what has been previously observed for SNe IIP. Future polarization observations of SNe Ia extending to post-peak epochs will help to examine the inner structure of the explosion.
We present a spectroscopic analysis of the recently discovered fast-evolving Type I superluminous supernova (SLSN-I) SN 2019neq (at redshift z = 0.1059). We compare it to the well-studied slowly ...evolving SLSN-I SN 2010kd (z = 0.101). Our main goal is to search for spectroscopic differences between the two groups of SLSNe-I. Differences in the spectra may reveal different ejecta compositions and explosion mechanisms. Our investigation concentrates on optical spectra observed with the 10 m Hobby-Eberly Telescope Low Resolution Spectrograph-2 at McDonald Observatory during the photospheric phase. We apply the SYN++ code to model the spectra of SN 2019neq taken at −4 days, +5 days, and +29 days from maximum light. We examine the chemical evolution and ejecta composition of the SLSN by identifying the elements and ionization states in its spectra. We find that a spectral model consisting of O iii, Co iii, and Si iv gives a SYN++ fit that is comparable to the typical SLSN-I spectral model consisting of O ii, and conclude that the true identification of those lines, at least in the case of SN 2019neq, is ambiguous. Based on modeling the entire optical spectrum, we classify SN 2019neq as a fast-evolving SLSN-I having a photospheric velocity gradient of km s−1 day−1, which is among the highest velocity gradients observed for an SLSN-I. Inferring the velocity gradient from the proposed Fe ii λ5169 feature alone would result in km s−1 day−1, which is still within the observed range of fast-evolving SLSNe-I. In addition, we derive the number density of relevant ionization states for a variety of identified elements at the epoch of the three observations. Finally, we give constraints on the lower limit of the ejecta mass and find that both SLSNe have an ejecta mass at least one order of magnitude higher than normal SNe Ia, while the fast-evolving SN 2019neq has an ejecta mass a factor of two lower than the slowly evolving SN 2010kd. These mass estimates suggest the existence of a possible correlation between the evolution timescale and the ejected mass of SLSNe-I.
We present a study of the morphology of the ejecta in Supernova 1987A based on images and spectra from the Hubble Space Telescope (HST) as well as integral field spectroscopy from VLT/SINFONI. The ...HST observations were obtained between 1994 and 2011 and primarily probe the outer H-rich zones of the ejecta. The SINFONI observations were obtained in 2005 and 2011 and instead probe the SiI+FeII emission from the inner regions. We find a strong temporal evolution of the morphology in the HST images, from a roughly elliptical shape before ~5000 days, to a more irregular, edge-brightened morphology with a "hole" in the middle thereafter. Both the H alpha and the SiI+FeII line profiles show that the ejecta are distributed fairly close to the plane of the inner circumstellar ring, which is assumed to define the rotational axis of the progenitor star. We compare our results with explosion models and find some qualitative agreement, but note that the observations show a higher degree of large-scale asymmetry.
Abstract
The Hobby–Eberly Telescope Dark Energy Experiment (HETDEX) is an unbiased, massively multiplexed spectroscopic survey, designed to measure the expansion history of the universe through ...low-resolution (
R
∼ 750) spectra of Ly
α
emitters. In its search for these galaxies, HETDEX will also observe a few times 10
5
stars. In this paper, we present the first stellar value-added catalog within the internal second data release of the HETDEX Survey (HDR2). The new catalog contains 120,571 low-resolution spectra for 98,736 unique stars between 10 <
G
< 22 spread across the HETDEX footprint at relatively high (
b
∼ 60°) Galactic latitudes. With these spectra, we measure radial velocities (RVs) for ∼42,000 unique FGK-type stars in the catalog and show that the HETDEX spectra are sufficient to constrain these RVs with a 1
σ
precision of 28.0 km s
−1
and bias of 3.5 km s
−1
with respect to the Large Sky Area Multi-Object Fibre Spectroscopic Telescope surveys and 1
σ
precision of 27.5 km s
−1
and bias of 14.0 km s
−1
compared to the Sloan Extension for Galactic Understanding and Exploration survey. Since these RVs are for faint (
G
≥ 16) stars, they will be complementary to Gaia. Using t-Distributed Stochastic Neighbor Embedding (tSNE), we also demonstrate that the HETDEX spectra can be used to determine a star’s
T
eff
, and log
g
and its Fe/H. With the tSNE projection of the FGK-type stars with HETDEX spectra we also identify 416 new candidate metal-poor (Fe/H < −1 dex) stars for future study. These encouraging results illustrate the utility of future low-resolution stellar spectroscopic surveys.
The interaction between the expanding supernova (SN) ejecta with the circumstellar material (CSM) that was expelled from the progenitor prior to explosion is a long-sought phenomenon, yet ...observational evidence is scarce. Here we confirm a new example: SN 2004dk, originally a hydrogen-poor, helium-rich Type Ib SN that reappeared as a strong -emitting point source on narrowband images. We present follow-up optical spectroscopy that reveals the presence of a broad component with full width at half maximum of ∼ 290 in addition to the narrow +N ii emission features from the host galaxy. Such a broad component is a clear sign of an ejecta-CSM interaction. We also present observations with the XMM-Newton Observatory, the Swift satellite, and the Chandra X-ray Observatory that span 10 days to 15 years after discovery. The detection of strong radio, X-ray, and emission years after explosion allows various constraints to be put on pre-SN mass-loss processes. We present a wind-bubble model in which the CSM is "pre-prepared" by a fast wind interacting with a slow wind. Much of the outer density profile into which the SN explodes corresponds to no steady-state mass-loss process. We estimate that the shell of compressed slow wind material was ejected ∼1400 yr prior to explosion, perhaps during carbon burning, and that the SN shock had swept up about 0.04 of material. The region emitting the has a density of order .
Abstract
We present a sequence of eight spectropolarimetric observations monitoring the geometric evolution of the late phase of the major 2012 outburst of SN 2009ip. These were acquired with the ...Focal Reducer and Low Dispersion Spectrograph polarimeter mounted on European Southern Observatory VLT. The continuum was polarized at 0.3–0.8 per cent throughout the observations, showing that the photosphere deviated substantially from spherical symmetry by 10–15 per cent. Significant line polarization is detected for both hydrogen and helium at high velocities. The similarity in the polarized signal between these elements indicates that they form in the same location in the ejecta. The line polarization (p ∼ 1–1.5 per cent) at low velocities revealed the presence of a highly aspherical hydrogen- and helium-rich circumstellar medium (CSM). Monte Carlo simulations of the observed polarimetry were performed in an effort to constrain the shape of the CSM. The simulations imply that the polarimetry can be understood within the framework of a disc-like CSM inclined by 14° ± 2° out of the line of sight, obscuring the photosphere only at certain epochs. The varying temporal evolution of polarization at high and low velocities indicated that the fast-moving ejecta expanded with a preferred direction orthogonal to that of the CSM.
We present imaging polarimetry of the superluminous supernova SN 2015bn, obtained over nine epochs between −20 and +46 days with the Nordic Optical Telescope. This was a nearby, slowly evolving Type ...I superluminous supernova that has been studied extensively and for which two epochs of spectropolarimetry are also available. Based on field stars, we determine the interstellar polarization in the Galaxy to be negligible. The polarization of SN 2015bn shows a statistically significant increase during the last epochs, confirming previous findings. Our well-sampled imaging polarimetry series allows us to determine that this increase (from ∼0.54% to 1.10%) coincides in time with rapid changes that took place in the optical spectrum. We conclude that the supernova underwent a "phase transition" at around +20 days, when the photospheric emission shifted from an outer layer, dominated by natal C and O, to a more aspherical inner core, dominated by freshly nucleosynthesized material. This two-layered model might account for the characteristic appearance and properties of Type I superluminous supernovae.
ABSTRACT We present the first polarimetric observations of a Type I superluminous supernova (SLSN). LSQ14mo was observed with VLT/FORS2 at five different epochs in the V band, with the observations ...starting before maximum light and spanning 26 days in the rest frame (z = 0.256). During this period, we do not detect any statistically significant evolution ( ) in the Stokes parameters. The average values we obtain, corrected for interstellar polarization in the Galaxy, are Q = −0.01% ( 0.15%) and U = −0.50% ( 0.14%). This low polarization can be entirely due to interstellar polarization in the SN host galaxy. We conclude that, at least during the period of observations and at the optical depths probed, the photosphere of LSQ14mo does not present significant asymmetries, unlike most lower-luminosity hydrogen-poor SNe Ib/c. Alternatively, it is possible that we may have observed LSQ14mo from a special viewing angle. Supporting spectroscopy and photometry confirm that LSQ14mo is a typical SLSN I. Further studies of the polarization of Type I SLSNe are required to determine whether the low levels of polarization are a characteristic of the entire class and to also study the implications for the proposed explosion models.
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