Abstract
We present observations of SN 2022joj, a peculiar Type Ia supernova discovered by the Zwicky Transient Facility. SN 2022joj exhibits an unusually red
g
ZTF
−
r
ZTF
color at early times and a ...rapid blueward evolution afterward. Around maximum brightness, SN 2022joj shows a high luminosity (
M
g
ZTF
,
max
≃
−
19.7
mag), a blue broadband color (
g
ZTF
−
r
ZTF
≃ −0.2 mag), and shallow Si
ii
absorption lines, consistent with those of overluminous, SN 1991T-like events. The maximum-light spectrum also shows prominent absorption around 4200 Å, which resembles the Ti
ii
features in subluminous, SN 1991bg-like events. Despite the blue optical-band colors, SN 2022joj exhibits extremely red ultraviolet minus optical colors at maximum luminosity (
u
−
v
≃ 0.6 mag and
uvw
1 −
v
≃ 2.5 mag), suggesting a suppression of flux at ∼2500–4000 Å. Strong C
ii
lines are also detected at peak. We show that these unusual spectroscopic properties are broadly consistent with the helium-shell double detonation of a sub-Chandrasekhar mass (
M
≃ 1
M
⊙
) carbon/oxygen white dwarf from a relatively massive helium shell (
M
s
≃ 0.04–0.1
M
⊙
), if observed along a line of sight roughly opposite to where the shell initially detonates. None of the existing models could quantitatively explain all the peculiarities observed in SN 2022joj. The low flux ratio of Ni
ii
λ
7378 to Fe
ii
λ
7155 emission in the late-time nebular spectra indicates a low yield of stable Ni isotopes, favoring a sub-Chandrasekhar mass progenitor. The significant blueshift measured in the Fe
ii
λ
7155 line is also consistent with an asymmetric chemical distribution in the ejecta, as is predicted in double-detonation models.
Abstract
The detonation of a thin (≲0.03
M
⊙
) helium shell (He-shell) atop a ∼1
M
⊙
white dwarf (WD) is a promising mechanism to explain normal Type Ia supernovae (SNe Ia), while thicker He-shells ...and less massive WDs may explain some recently observed peculiar SNe Ia. We present observations of SN 2020jgb, a peculiar SN Ia discovered by the Zwicky Transient Facility (ZTF). Near maximum brightness, SN 2020jgb is slightly subluminous (ZTF
g
-band absolute magnitude −18.7 mag ≲
M
g
≲ −18.2 mag depending on the amount of host-galaxy extinction) and shows an unusually red color (0.2 mag ≲
g
ZTF
−
r
ZTF
≲ 0.4 mag) due to strong line-blanketing blueward of ∼5000 Å. These properties resemble those of SN 2018byg, a peculiar SN Ia consistent with an He-shell double detonation (DDet) SN. Using detailed radiative transfer models, we show that the optical spectroscopic and photometric evolution of SN 2020jgb is broadly consistent with a ∼0.95–1.00
M
⊙
(C/O core + He-shell) progenitor ignited by a ≳0.1
M
⊙
He-shell. However, one-dimensional radiative transfer models without non-local-thermodynamic-equilibrium treatment cannot accurately characterize the line-blanketing features, making the actual shell mass uncertain. We detect a prominent absorption feature at ∼1
μ
m in the near-infrared (NIR) spectrum of SN 2020jgb, which might originate from unburnt helium in the outermost ejecta. While the sample size is limited, we find similar 1
μ
m features in all the peculiar He-shell DDet candidates with NIR spectra obtained to date. SN 2020jgb is also the first peculiar He-shell DDet SN discovered in a star-forming dwarf galaxy, indisputably showing that He-shell DDet SNe occur in both star-forming and passive galaxies, consistent with the normal SN Ia population.
Abstract
The fate of stars in the zero-age main-sequence (ZAMS) range ≈8–12
M
⊙
is unclear. They could evolve to form white dwarfs or explode as electron-capture supernovae (SNe) or iron ...core-collapse SNe (CCSNe). Even though the initial mass function indicates that this mass range should account for over 40% of all CCSN progenitors, few have been observationally confirmed, likely due to the faintness and rapid evolution of some of these transients. In this paper, we present a sample of nine Ca-rich/O-poor Type IIb SNe detected by the Zwicky Transient Facility with progenitors likely in this mass range. These sources have a Ca
ii
λ
λ
7291, 7324/O
i
λ
λ
6300, 6364 flux ratio of ≳2 in their nebular spectra. Comparing the measured O
i
luminosity (≲10
39
erg s
−1
) and derived oxygen mass (≈0.01
M
⊙
) with theoretical models, we infer that the progenitor ZAMS mass for these explosions is less than 12
M
⊙
. The ejecta properties (
M
ej
≲ 1
M
⊙
and
E
kin
∼ 10
50
erg) are also consistent. The low ejecta mass of these sources indicates a class of strongly-stripped SNe that is a transition between the regular stripped-envelope SNe and ultra-stripped SNe. The progenitor could be stripped by a main-sequence companion and result in the formation of a neutron star−main sequence binary. Such binaries have been suggested to be progenitors of neutron star−white dwarf systems that could merge within a Hubble time and be detectable with LISA.
Abstract We present a sample of 34 normal Type II supernovae (SNe II) detected with the Zwicky Transient Facility, with multiband UV light curves starting at t ≤ 4 days after explosion, and X-ray ...observations. We characterize the early UV-optical color, provide empirical host-extinction corrections, and show that the t > 2 day UV-optical colors and the blackbody evolution of the sample are consistent with shock cooling (SC) regardless of the presence of “flash ionization” features. We present a framework for fitting SC models that can reproduce the parameters of a set of multigroup simulations up to 20% in radius and velocity. Observations of 15 SNe II are well fit by models with breakout radii <10 14 cm. Eighteen SNe are typically more luminous, with observations at t ≥ 1 day that are better fit by a model with a large >10 14 cm breakout radius. However, these fits predict an early rise during the first day that is too slow. We suggest that these large-breakout events are explosions of stars with an inflated envelope or with confined circumstellar material (CSM). Using the X-ray data, we derive constraints on the extended (∼10 15 cm) CSM density independent of spectral modeling and find that most SN II progenitors lose M ̇ < 10 − 4 M ⊙ yr − 1 up to a few years before explosion. We show that the overall observed breakout radius distribution is skewed to higher radii due to a luminosity bias. We argue that the 66 − 22 + 11 % of red supergiants (RSGs) explode as SNe II with breakout radii consistent with the observed distribution of RSGs, with a tail extending to large radii, likely due to the presence of CSM.
We present visible-light and ultraviolet (UV) observations of the supernova PTF 12glz. The SN was discovered and monitored in the near-UV and R bands as part of a joint GALEX and Palomar Transient ...Factory campaign. It is among the most energetic SNe IIn observed to date ( 1051 erg). If the radiated energy mainly came from the thermalization of the shock kinetic energy, we show that PTF 12glz was surrounded by ∼1 M of circumstellar material (CSM) prior to its explosive death. PTF 12glz shows a puzzling peculiarity: at early times, while the freely expanding ejecta are presumably masked by the optically thick CSM, the radius of the blackbody that best fits the observations grows at 7000 km s−1. Such a velocity is characteristic of fast moving ejecta rather than optically thick CSM. This phase of radial expansion takes place before any spectroscopic signature of expanding ejecta appears in the spectrum and while both the spectroscopic data and the bolometric luminosity seem to indicate that the CSM is optically thick. We propose a geometrical solution to this puzzle, involving an aspherical structure of the CSM around PTF 12glz. By modeling radiative diffusion through a slab of CSM, we show that an aspherical geometry of the CSM can result in a growing effective radius. This simple model also allows us to recover the decreasing blackbody temperature of PTF 12glz. SLAB-Diffusion, the code we wrote to model the radiative diffusion of photons through a slab of CSM and evaluate the observed radius and temperature, is made available online.
Abstract We present the discovery and analysis of SN 2022oqm, a Type Ic supernova (SN) detected <1 day after the explosion. The SN rises to a blue and short-lived (2 days) initial peak. Early-time ...spectral observations of SN 2022oqm show a hot (40,000 K) continuum with high ionization C and O absorption features at velocities of 4000 km s −1 , while its photospheric radius expands at 20,000 km s −1 , indicating a pre-existing distribution of expanding C/O material. After ∼2.5 days, both the spectrum and light curves evolve into those of a typical SN Ic, with line velocities of ∼10,000 km s −1 , in agreement with the evolution of the photospheric radius. The optical light curves reach a second peak at t ≈ 15 days. By t = 60 days, the spectrum of SN 2022oqm becomes nearly nebular, displaying strong Ca ii and Ca ii emission with no detectable O i , marking this event as Ca-rich. The early behavior can be explained by 10 −3 M ⊙ of optically thin circumstellar material (CSM) surrounding either (1) a massive compact progenitor such as a Wolf–Rayet star, (2) a massive stripped progenitor with an extended envelope, or (3) a binary system with a white dwarf. We propose that the early-time light curve is powered by both the interaction of the ejecta with the optically thin CSM and shock cooling (in the massive star scenario). The observations can be explained by CSM that is optically thick to X-ray photons, is optically thick in the lines as seen in the spectra, and is optically thin to visible-light continuum photons that come either from downscattered X-rays or from the shock-heated ejecta. Calculations show that this scenario is self-consistent.
Abstract Multipeaked supernovae with precursors, dramatic light-curve rebrightenings, and spectral transformation are rare, but are being discovered in increasing numbers by modern night-sky ...transient surveys like the Zwicky Transient Facility. Here, we present the observations and analysis of SN 2023aew, which showed a dramatic increase in brightness following an initial luminous (−17.4 mag) and long (∼100 days) unusual first peak (possibly precursor). SN 2023aew was classified as a Type IIb supernova during the first peak but changed its type to resemble a stripped-envelope supernova (SESN) after the marked rebrightening. We present comparisons of SN 2023aew’s spectral evolution with SESN subtypes and argue that it is similar to SNe Ibc during its main peak. P-Cygni Balmer lines are present during the first peak, but vanish during the second peak’s photospheric phase, before H α resurfaces again during the nebular phase. The nebular lines (O i , Ca ii , Mg i , H α ) exhibit a double-peaked structure that hints toward a clumpy or nonspherical ejecta. We analyze the second peak in the light curve of SN 2023aew and find it to be broader than that of normal SESNe as well as requiring a very high 56 Ni mass to power the peak luminosity. We discuss the possible origins of SN 2023aew including an eruption scenario where a part of the envelope is ejected during the first peak and also powers the second peak of the light curve through interaction of the SN with the circumstellar medium.
Abstract We present SN 2023zaw—a subluminous ( M r = −16.7 mag) and rapidly evolving supernova ( t 1/2, r = 4.9 days), with the lowest nickel mass (≈0.002 M ⊙ ) measured among all stripped-envelope ...supernovae discovered to date. The photospheric spectra are dominated by broad He i and Ca near-infrared emission lines with velocities of ∼10,000−12,000 km s −1 . The late-time spectra show prominent narrow He i emission lines at ∼1000 km s −1 , indicative of interaction with He-rich circumstellar material. SN 2023zaw is located in the spiral arm of a star-forming galaxy. We perform radiation-hydrodynamical and analytical modeling of the lightcurve by fitting with a combination of shock-cooling emission and nickel decay. The progenitor has a best-fit envelope mass of ≈0.2 M ☉ and an envelope radius of ≈50 R ⊙ . The extremely low nickel mass and low ejecta mass (≈0.5 M ⊙ ) suggest an ultrastripped SN, which originates from a mass-losing low-mass He-star (zero-age main-sequence mass < 10 M ⊙ ) in a close binary system. This is a channel to form double neutron star systems, whose merger is detectable with LIGO. SN 2023zaw underscores the existence of a previously undiscovered population of extremely low nickel mass (<0.005 M ☉ ) stripped-envelope supernovae, which can be explored with deep and high-cadence transient surveys.
Abstract
Superluminous supernovae (SLSNe) are the most luminous supernovae in the Universe. They are found in extreme star-forming galaxies and are probably connected with the death of massive stars. ...One hallmark of very massive progenitors would be a tendency to explode in very dense, UV-bright and blue regions. In this paper, we investigate the resolved host galaxy properties of two nearby hydrogen-poor SLSNe, PTF 11hrq and PTF 12dam. For both galaxies Hubble Space Telescope multifilter images were obtained. Additionally, we perform integral field spectroscopy of the host galaxy of PTF 11hrq using the Very Large Telescope Multi Unit Spectroscopic Explorer (VLT/MUSE), and investigate the line strength, metallicity and kinematics. Neither PTF 11hrq nor PTF 12dam occurred in the bluest part of their host galaxies, although both galaxies have overall blue UV-to-optical colours. The MUSE data reveal a bright starbursting region in the host of PTF 11hrq, although far from the SN location. The SN exploded close to a region with disturbed kinematics, bluer colour, stronger O iii and lower metallicity. The host galaxy is likely interacting with a companion. PTF 12dam occurred in one of the brightest pixels, in a starbursting galaxy with a complex morphology and a tidal tail, where interaction is also very likely. We speculate that SLSN explosions may originate from stars generated during star formation episodes triggered by interaction. High-resolution imaging and integral field spectroscopy are fundamental for a better understanding of SLSNe explosion sites and how star formation varies across their host galaxies.
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