Abstract
We present a multiwavelength photometric and spectroscopic analysis of 13 super-Chandrasekhar-mass/2003fg-like Type Ia supernovae (SNe Ia). Nine of these objects were observed by the ...Carnegie Supernova Project. The 2003fg-like SNe have slowly declining light curves (Δ
m
15
(
B
) < 1.3 mag), and peak absolute
B
-band magnitudes of −19 <
M
B
< −21 mag. Many of the 2003fg-like SNe are located in the same part of the luminosity–width relation as normal SNe Ia. In the optical
B
and
V
bands, the 2003fg-like SNe look like normal SNe Ia, but at redder wavelengths they diverge. Unlike other luminous SNe Ia, the 2003fg-like SNe generally have only one
i
-band maximum, which peaks after the epoch of the
B
-band maximum, while their near-IR (NIR) light-curve rise times can be ≳40 days longer than those of normal SNe Ia. They are also at least 1 mag brighter in the NIR bands than normal SNe Ia, peaking above
M
H
= −19 mag, and generally have negative Hubble residuals, which may be the cause of some systematics in dark-energy experiments. Spectroscopically, the 2003fg-like SNe exhibit peculiarities such as unburnt carbon well past maximum light, a large spread (8000–12,000 km s
−1
) in Si
ii
λ
6355 velocities at maximum light with no rapid early velocity decline, and no clear
H
-band break at +10 days. We find that SNe with a larger pseudo-equivalent width of C
ii
at maximum light have lower Si
ii
λ
6355 velocities and more slowly declining light curves. There are also multiple factors that contribute to the peak luminosity of 2003fg-like SNe. The explosion of a C–O degenerate core inside a carbon-rich envelope is consistent with these observations. Such a configuration may come from the core-degenerate scenario.
Abstract
We present a JWST/MIRI low-resolution mid-infrared (MIR) spectroscopic observation of the normal Type Ia supernova (SN Ia) SN 2021aefx at +323 days past rest-frame
B
-band maximum light. The ...spectrum ranges from 4 to 14
μ
m and shows many unique qualities, including a flat-topped Ar
iii
8.991
μ
m profile, a strongly tilted Co
iii
11.888
μ
m feature, and multiple stable Ni lines. These features provide critical information about the physics of the explosion. The observations are compared to synthetic spectra from detailed non–local thermodynamic equilibrium multidimensional models. The results of the best-fitting model are used to identify the components of the spectral blends and provide a quantitative comparison to the explosion physics. Emission line profiles and the presence of electron capture elements are used to constrain the mass of the exploding white dwarf (WD) and the chemical asymmetries in the ejecta. We show that the observations of SN 2021aefx are consistent with an off-center delayed detonation explosion of a near–Chandrasekhar mass (
M
Ch
) WD at a viewing angle of −30° relative to the point of the deflagration to detonation transition. From the strengths of the stable Ni lines, we determine that there is little to no mixing in the central regions of the ejecta. Based on both the presence of stable Ni and the Ar velocity distributions, we obtain a strict lower limit of 1.2
M
⊙
for the initial WD, implying that most sub-
M
Ch
explosions models are not viable models for SN 2021aefx. The analysis here shows the crucial importance of MIR spectra in distinguishing between explosion scenarios for SNe Ia.
We present the H-band wavelength region of 37 postmaximum light near-infrared spectra of three normal, nine transitional, and four subluminous type Ia supernovae (SNe Ia), extending from +5 days to ...+20 days relative to the epoch of B-band maximum. We introduce a new observable, the blue-edge velocity, v edge, of the prominent Fe/Co/Ni-peak H-band emission feature, which is quantitatively measured. The v edge parameter is found to decrease over subtype ranging from around −14,000 km s−1 for normal SNe Ia, to −10,000 km s−1 for transitional SNe Ia, down to −5000 km s−1 for the subluminous SNe Ia. Furthermore, inspection of the +10 ± 3 days spectra indicates that v edge is correlated with the color-stretch parameter, s BV , and hence with peak luminosity. These results follow the previous findings that brighter SNe Ia tend to have 56Ni located at higher velocities as compared to subluminous objects. As v edge is a model-independent parameter, we propose it can be used in combination with traditional observational diagnostics to provide a new avenue to robustly distinguish between leading SNe Ia explosion models.
Abstract
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
λ
6355 velocity. However, some of its properties are also extreme among the subgroup. ASASSN-15hy is underluminous (
M
B
,peak
=
−
19.14
−
0.16
+
0.11
mag), red (
(
B
−
V
)
B
max
=
0.18
−
0.03
+
0.01
mag), yet slowly declining (Δ
m
15
(
B
) = 0.72 ± 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 nondegenerate envelope. The explosion impacting the nondegenerate 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 timescales required to produce the broad light curves. The model consists of a rapidly rotating 1.47
M
⊙
degenerate core and a 0.8
M
⊙
nondegenerate 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.
ABSTRACT
We present ultraviolet (UV) to near-infrared (NIR) observations and analysis of the nearby Type Ia supernova SN 2021fxy. Our observations include UV photometry from Swift/UVOT, UV ...spectroscopy from HST/STIS, and high-cadence optical photometry with the Swope 1-m telescope capturing intranight rises during the early light curve. Early B − V colours show SN 2021fxy is the first ‘shallow-silicon’ (SS) SN Ia to follow a red-to-blue evolution, compared to other SS objects which show blue colours from the earliest observations. Comparisons to other spectroscopically normal SNe Ia with HST UV spectra reveal SN 2021fxy is one of several SNe Ia with flux suppression in the mid-UV. These SNe also show blueshifted mid-UV spectral features and strong high-velocity Ca ii features. One possible origin of this mid-UV suppression is the increased effective opacity in the UV due to increased line blanketing from high velocity material, but differences in the explosion mechanism cannot be ruled out. Among SNe Ia with mid-UV suppression, SNe 2021fxy and 2017erp show substantial similarities in their optical properties despite belonging to different Branch subgroups, and UV flux differences of the same order as those found between SNe 2011fe and 2011by. Differential comparisons to multiple sets of synthetic SN Ia UV spectra reveal this UV flux difference likely originates from a luminosity difference between SNe 2021fxy and 2017erp, and not differing progenitor metallicities as suggested for SNe 2011by and 2011fe. These comparisons illustrate the complicated nature of UV spectral formation, and the need for more UV spectra to determine the physical source of SNe Ia UV diversity.
Abstract
Unburned carbon is potentially a powerful probe of Type Ia supernova (SN) explosion mechanisms. We present comprehensive optical and near-infrared (NIR) data on the “transitional” Type Ia ...SN 2015bp. An early NIR spectrum (
days with respect to
B
-band maximum) displays a striking C
i
1.0693
μ
m line at 11.9 × 10
3
km s
−1
, distinct from the prominent Mg
ii
1.0927
μ
m feature, which weakens toward maximum light. SN 2015bp also displays a clear C
ii
6580 Å notch early (
days) at 13.2 × 10
3
km s
−1
, consistent with our NIR carbon detection. At
, SN 2015bp is less luminous than a normal SN Ia and, along with iPTF 13ebh, is the second member of the transitional subclass to display prominent early-time NIR carbon absorption. We find it unlikely that the C
i
feature is misidentified He
i
1.0830
μ
m because this feature grows weaker toward maximum light, while the helium line produced in some double-detonation models grows stronger at these times. Intrigued by these strong NIR carbon detections, but lacking NIR data for other SNe Ia, we investigated the incidence of optical carbon in the sample of nine transitional SNe Ia with early-time data (
t
≲ −4 days). We find that four display C
ii
6580 Å, while two others show tentative detections, in line with the SN Ia population as a whole. We conclude that at least ∼50% of transitional SNe Ia in our sample do not come from sub-Chandrasekhar-mass explosions due to the clear presence of carbon in their NIR and optical spectra.
Abstract
We present a JWST mid-infrared (MIR) spectrum of the underluminous Type Ia Supernova (SN Ia) 2022xkq, obtained with the medium-resolution spectrometer on the Mid-Infrared Instrument (MIRI) ...∼130 days post-explosion. We identify the first MIR lines beyond 14
μ
m in SN Ia observations. We find features unique to underluminous SNe Ia, including the following: isolated emission of stable Ni, strong blends of Ti
ii
, and large ratios of singly ionized to doubly ionized species in both Ar and Co. Comparisons to normal-luminosity SNe Ia spectra at similar phases show a tentative trend between the width of the Co
iii
11.888
μ
m feature and the SN light-curve shape. Using non-LTE-multi-dimensional radiation hydro simulations and the observed electron capture elements, we constrain the mass of the exploding WD. The best-fitting model shows that SN 2022xkq is consistent with an off-center delayed-detonation explosion of a near-Chandrasekhar mass WD (
M
WD
≈1.37
M
⊙
) of high central density (
ρ
c
≥ 2.0 × 10
9
g cm
−3
) seen equator-on, which produced
M
(
56
Ni) =0.324
M
⊙
and
M
(
58
Ni) ≥0.06
M
⊙
. The observed line widths are consistent with the overall abundance distribution; and the narrow stable Ni lines indicate little to no mixing in the central regions, favoring central ignition of subsonic carbon burning followed by an off-center deflagration-to-detonation transition beginning at a single point. Additional observations may further constrain the physics revealing the presence of additional species including Cr and Mn. Our work demonstrates the power of using the full coverage of MIRI in combination with detailed modeling to elucidate the physics of SNe Ia at a level not previously possible.
We present an early-phase g-band light curve and visual-wavelength spectra of the normal Type Ia supernova (SN) 2013gy. The light curve is constructed by determining the appropriate S-corrections to ...transform KAIT natural-system B- and V-band photometry and Carnegie Supernova Project natural-system g-band photometry to the Pan-STARRS1 g-band natural photometric system. A Markov chain Monte Carlo calculation provides a best-fit single power-law function to the first ten epochs of photometry described by an exponent of 2.16+0.06−0.06 2 . 16 − 0.06 + 0.06 $ 2.16^{+0.06}_{-0.06} $ and a time of first light of MJD 56629.4+0.1−0.1 56629 . 4 − 0.1 + 0.1 $ 56629.4^{+0.1}_{-0.1} $ , which is 1.93+0.12−0.13 1 . 93 − 0.13 + 0.12 $ 1.93^{+0.12}_{-0.13} $ days (i.e., < 48 h) before the discovery date (2013 December 4.84 UT) and −19.10+0.12−0.13 − 19 . 10 − 0.13 + 0.12 $ -19.10^{+0.12}_{-0.13} $ days before the time of B-band maximum (MJD 56648.5 ± 0.1). The estimate of the time of first light is consistent with the explosion time inferred from the evolution of the Si IIλ6355 Doppler velocity. Furthermore, discovery photometry and previous nondetection limits enable us to constrain the companion radius down to Rc ≤ 4 R⊙. In addition to our early-time constraints, we used a deep +235 day nebular-phase spectrum from Magellan/IMACS to place a stripped H-mass limit of < 0.018 M⊙. Combined, these limits effectively rule out H-rich nondegenerate companions.
ABSTRACT
We present detailed investigation of a specific i-band light-curve feature in Type Ia supernovae (SNe Ia) using the rapid cadence and high signal-to-noise ratio light curves obtained by the ...Carnegie Supernova Project. The feature is present in most SNe Ia and emerges a few days after the i-band maximum. It is an abrupt change in curvature in the light curve over a few days and appears as a flattening in mild cases and a strong downward concave shape, or a ‘kink’, in the most extreme cases. We computed the second derivatives of Gaussian Process interpolations to study 54 rapid-cadence light curves. From the second derivatives we measure: (1) the timing of the feature in days relative to i-band maximum; tdm2(i) and (2) the strength and direction of the concavity in mag d−2; dm2(i). 76 per cent of the SNe Ia show a negative dm2(i), representing a downward concavity – either a mild flattening or a strong ‘kink’. The tdm2(i) parameter is shown to correlate with the colour-stretch parameter sBV, a SN Ia primary parameter. The dm2(i) parameter shows no correlation with sBV and therefore provides independent information. It is also largely independent of the spectroscopic and environmental properties. Dividing the sample based on the strength of the light-curve feature as measured by dm2(i), SNe Ia with strong features have a Hubble diagram dispersion of 0.107 mag, 0.075 mag smaller than the group with weak features. Although larger samples should be obtained to test this result, it potentially offers a new method for improving SN Ia distance determinations without shifting to more costly near-infrared or spectroscopic observations.
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