Because mass loss is a fundamental phenomenon in massive stars, an interaction with circumstellar material (CSM) should be universal in core-collapse supernovae (SNe). Leaving aside the extreme CSM ...density, extent, or mass typically encountered in Type IIn SNe, we investigate the diverse long-term radiative signatures of an interaction between a Type II SN ejecta and CSM corresponding to mass-loss rates up to 10
−3
M
⊙
yr
−1
. Because these CSM are relatively tenuous and optically thin to electron scattering beyond a few stellar radii, radiation hydrodynamics is not essential and one may treat the interaction directly as an additional power source in the non-local thermodynamic equilibrium radiative transfer problem. The CSM accumulated since shock breakout forms a dense shell in the outer ejecta and leads to high-velocity absorption features in spectral lines, even for a negligible shock power. In addition to Balmer lines, such features may appear in Na
I
D and He
I
lines, among others. A stronger interaction strengthens the continuum flux (preferentially in the UV), quenches the absorption of P-Cygni profiles, boosts the Mg
II
λλ
2795, 2802 doublet, and fosters the production of a broad-boxy H
α
emission component. The rise in ionization in the outer ejecta may quench some lines (e.g., the Ca
II
near-infrared triplet). The interaction power emerges preferentially in the UV, in particular at later times, shifting the optical color to the blue, but increasing the optical luminosity modestly. Strong thermalization and clumping seem to be required to make an interaction superluminous in the optical. The UV range contains essential signatures that provide critical constraints to infer the mass-loss history and inner workings of core-collapse SN progenitors at death.
Abstract
We present panchromatic observations and modeling of supernova (SN) 2020tlf, the first normal Type II-P/L SN with confirmed precursor emission, as detected by the Young Supernova Experiment ...transient survey. Pre-SN activity was detected in
riz
-bands at −130 days and persisted at relatively constant flux until first light. Soon after discovery, “flash” spectroscopy of SN 2020tlf revealed narrow, symmetric emission lines that resulted from the photoionization of circumstellar material (CSM) shed in progenitor mass-loss episodes before explosion. Surprisingly, this novel display of pre-SN emission and associated mass loss occurred in a red supergiant (RSG) progenitor with zero-age main-sequence mass of only 10–12
M
⊙
, as inferred from nebular spectra. Modeling of the light curve and multi-epoch spectra with the non-LTE radiative-transfer code CMFGEN and radiation-hydrodynamical code HERACLES suggests a dense CSM limited to
r
≈ 10
15
cm, and mass-loss rate of 10
−2
M
⊙
yr
−1
. The luminous light-curve plateau and persistent blue excess indicates an extended progenitor, compatible with an RSG model with
R
⋆
= 1100
R
⊙
. Limits on the shock-powered X-ray and radio luminosity are consistent with model conclusions and suggest a CSM density of
ρ
< 2 × 10
−16
g cm
−3
for distances from the progenitor star of
r
≈ 5 × 10
15
cm, as well as a mass-loss rate of
M
̇
<
1.3
×
10
−
5
M
☉
yr
−
1
at larger distances. A promising power source for the observed precursor emission is the ejection of stellar material following energy disposition into the stellar envelope as a result of gravity waves emitted during either neon/oxygen burning or a nuclear flash from silicon combustion.
There is both observational and theoretical evidence that the ejecta of core-collapse supernovae (SNe) are structured. Rather than being smooth and homogeneous, the material is made of over-dense and ...under-dense regions of distinct composition. Here, we have explored the effect of clumping on the SN radiation during the photospheric phase using 1D non-local thermodynamic equilibrium radiative transfer and an ejecta model arising from a blue-supergiant explosion (yielding a Type II-peculiar SN). Neglecting chemical segregation, we adopted a velocity-dependent volume-filling factor approach that assumes that the clumps are small but does not change the column density along any sightline. We find that clumping boosts the recombination rate in the photospheric layers, leading to a faster recession of the photosphere, an increase in bolometric luminosity, and a reddening of the SN colors through enhanced blanketing. The SN bolometric light curve peaks earlier and transitions faster to the nebular phase. On the rise to maximum, the strongest luminosity contrast between our clumped and smooth models is obtained at the epoch when the photosphere has receded to ejecta layers where the clumping factor is only 0.5 – this clumping factor may be larger in nature. Clumping is seen to have a similar influence in a Type II-Plateau SN model. As we neglected both porosity and chemical segregation, our models underestimate the true impact of clumping. These results warrant further study of the influence of clumping on the observables of other SN types during the photospheric phase.
Type Ibn supernovae (SNe) are a mysterious class of transients whose spectra exhibit persistently narrow He
I
lines, and whose bolometric light curves are typically fast evolving and overluminous at ...peak relative to standard Type Ibc SNe. We explore the interaction scenario of such Type Ibn SNe by performing radiation-hydrodynamics and radiative-transfer calculations. We find that standard-energy helium-star explosions within dense wind-like circumstellar material (CSM) can reach a peak luminosity of a few 10
44
erg s
−1
on day timescales, which is reminiscent of exceptional events such as AT 2018cow. Similar interactions but with weaker winds can lead to Type Ibc SNe with double-peak light curves and peak luminosities in the range ∼10
42.2
to ∼10
43
erg s
−1
. In contrast, the narrow spectral lines and modest peak luminosities of most Type Ibn SNe are suggestive of a low-energy explosion in an initially ≲5
M
⊙
helium star, most likely arising from interacting binaries and colliding with a massive helium-rich, probably ejecta-like, CSM at ∼10
15
cm. Nonlocal thermodynamic equilibrium radiative-transfer simulations of a slow-moving dense shell born out and powered by the interaction compare favorably to Type Ibn SNe such as 2006jc, 2011hw, or 2018bcc at late times and suggest a composition made of about 50% helium, a solar metallicity, and a total ejecta and CSM mass of 1–2
M
⊙
. A lower fractional helium abundance leads to weak or absent He
I
lines and thus excludes more massive configurations for observed Type Ibn SNe. Further, the dominance of Fe
II
emission below 5500 Å seen in Type Ibn SNe at late times is not predicted at low metallicity. Hence, despite their promising properties, Type Ibn SNe from a pulsational-pair instability in very massive stars, requiring low metallicity, probably have not been observed yet.
We present VULCAN/2D multigroup flux-limited-diffusion radiation-hydrodynamics simulations of binary neutron star mergers, using the Shen equation of state, covering 100 ms, and starting from ...azimuthal-averaged two-dimensional slices obtained from three-dimensional smooth-particle-hydrodynamics simulations of Rosswog & Price for 1.4 M (baryonic) neutron stars with no initial spins, co-rotating spins, or counter-rotating spins. Snapshots are post-processed at 10 ms intervals with a multiangle neutrino-transport solver. We find polar-enhanced neutrino luminosities, dominated by and ' Delta is a subset of ' neutrinos at the peak, although Delta e emission may be stronger at late times. We obtain typical peak neutrino energies for Delta e , , and ' Delta is a subset of ' of ~ 12, ~ 16, and ~ 22 MeV, respectively. The supermassive neutron star (SMNS) formed from the merger has a cooling timescale of 1 s. Charge-current neutrino reactions lead to the formation of a thermally driven bipolar wind with 10-3 M s-1 and baryon-loading in the polar regions, preventing any production of a gamma -ray burst prior to black hole formation. The large budget of rotational free energy suggests that magneto-rotational effects could produce a much-greater polar mass loss. We estimate that 10-4 M of material with an electron fraction in the range 0.1-0.2 becomes unbound during this SMNS phase as a result of neutrino heating. We present a new formalism to compute the annihilation rate based on moments of the neutrino-specific intensity computed with our multiangle solver. Cumulative annihilation rates, which decay as ~t -1.8, decrease over our 100 ms window from a few X1050 to ~ 1049 erg s-1, equivalent to a few X1054 to ~ 1053 e-e+ pairs per second.
Following our recent work on Type II supernovae (SNe), we present a set of 1D nonlocal thermodynamic equilibrium radiative transfer calculations for nebular-phase Type Ibc SNe starting from ...state-of-the-art explosion models with detailed nucleosynthesis. Our grid of progenitor models is derived from He stars that were subsequently evolved under the influence of wind mass loss. These He stars, which most likely form through binary mass exchange, synthesize less oxygen than their single-star counterparts with the same zero-age main sequence (ZAMS) mass. This reduction is greater in He-star models evolved with an enhanced mass loss rate. We obtain a wide range of spectral properties at 200 d. In models from He stars with an initial mass > 6
M
⊙
, the O
I
λ
λ
6300, 6364 is of a comparable or greater strength than Ca
II
λ
λ
7291, 7323 – the strength of O
I
λ
λ
6300, 6364 increases with the He-star initial mass. In contrast, models from lower mass He stars exhibit a weak O
I
λ
λ
6300, 6364, strong Ca
II
λ
λ
7291, 7323, and also strong N
II
lines and Fe
II
emission below 5500 Å. The ejecta density, which is modulated by the ejecta mass, the explosion energy, and clumping, has a critical impact on gas ionization, line cooling, and spectral properties. We note that Fe
II
dominates the emission below 5500 Å and is stronger at earlier nebular epochs. It ebbs as the SN ages, while the fractional flux in O
I
λ
λ
6300, 6364 and Ca
II
λ
λ
7291, 7323 increases with a similar rate as the ejecta recombine. Although the results depend on the adopted wind mass loss rate and pre-SN mass, we find that He-stars of 6–8
M
⊙
initially (ZAMS mass of 23–28
M
⊙
) match the properties of standard SNe Ibc adequately. This finding agrees with the offset in progenitor masses inferred from the environments of SNe Ibc relative to SNe II. Our results for less massive He stars are more perplexing since the predicted spectra are not seen in nature. They may be missed by current surveys or associated with Type Ibn SNe in which interaction power dominates over decay power.
We present here the first 2D rotating, multigroup, radiation magnetohydrodynamics (RMHD) simulations of supernova core collapse, bounce, and explosion. In the context of rapid rotation, we focus on ...the dynamical effects of magnetic stresses and the creation and propagation of MHD jets. We find that a quasi-steady state can be quickly established after bounce, during which a well-collimated MHD jet is maintained by continuous pumping of power from the differentially rotating core. If the initial spin period of the progenitor core is unk2 s, the free energy reservoir in the secularly evolving proto-neutron star is adequate to power a supernova explosion and may be enough for a hypernova. The jets are well collimated by the infalling material and magnetic hoop stresses and maintain a small opening angle. We see evidence of sausage instabilities In the emerging jet stream. Neutrino heating is subdominant in the rapidly rotating models we explore but can contribute 10%-25% to the final explosion energy. Our simulations suggest that even in the case of modest or slow rotation, a supernova explosion might be followed by a secondary, weak MHD jet explosion, which, because of its weakness, may to date have gone unnoticed in supernova debris. Furthermore, we suggest that the generation of a nonrelativistic MHD precursor jet during the early proto-neutron star/supernova phase is implicit in both the collapsar and "millisecond magnetar" models of GRBs. The multidimensional, multigroup, rapidly rotating RMHD simulations we describe here are a start along the path toward more realistic simulations of the possible role of magnetic fields in some of nature's most dramatic events.
Abstract
A non-local-thermodynamic-equilibrium (NLTE) level population model of the first and second ionisation stages of iron, nickel and cobalt is used to fit a sample of XShooter optical + ...near-infrared (NIR) spectra of Type Ia supernovae (SNe Ia). From the ratio of the NIR lines to the optical lines limits can be placed on the temperature and density of the emission region. We find a similar evolution of these parameters across our sample. Using the evolution of the Fe ii 12 570 Å to 7 155 Å line as a prior in fits of spectra covering only the optical wavelengths we show that the 7200 Å feature is fully explained by Fe ii and Ni ii alone. This approach allows us to determine the abundance of Ni ii/Fe ii for a large sample of 130 optical spectra of 58 SNe Ia with uncertainties small enough to distinguish between Chandrasekhar mass (MCh) and sub-Chandrasekhar mass (sub-MCh) explosion models. We conclude that the majority (85%) of normal SNe Ia have a Ni/Fe abundance that is in agreement with predictions of sub-MCh explosion simulations of ∼Z⊙ progenitors. Only a small fraction (11%) of objects in the sample have a Ni/Fe abundance in agreement with MCh explosion models.
Context. The features in the light curves and spectra of many Type I and Type II supernovae (SNe) can be understood by assuming an interaction of the SN ejecta with circumstellar matter (CSM) ...surrounding the progenitor star. This suggests that many massive stars may undergo various degrees of envelope stripping shortly before exploding, and may therefore produce a considerable diversity in their pre-explosion CSM properties. Aims. We explore a generic set of about 100 detailed massive binary evolution models in order to characterize the amount of envelope stripping and the expected CSM configurations. Methods. Our binary models were computed with the MESA stellar evolution code, considering an initial primary star mass of 12.6 M ⊙ and secondaries with initial masses of between ∼12 M ⊙ and ∼1.3 M ⊙ , and focus on initial orbital periods above ∼500 d. We compute these models up to the time of iron core collapse in the primary. Results. Our models exhibit varying degrees of stripping due to mass transfer, resulting in SN progenitor models ranging from fully stripped helium stars to stars that have not been stripped at all. We find that Roche lobe overflow often leads to incomplete stripping of the mass donor, resulting in a large variety of pre-SN envelope masses. In many of our models, the red supergiant (RSG) donor stars undergo core collapse during Roche lobe overflow, with mass transfer and therefore system mass-loss rates of up to 0.01 M ⊙ yr −1 at that time. The corresponding CSM densities are similar to those inferred for Type IIn SNe, such as SN 1998S . In other cases, the mass transfer becomes unstable, leading to a common-envelope phase at such late time that the mass donor explodes before the common envelope is fully ejected or the system has merged. We argue that this may cause significant pre-SN variability, as witnessed for example in SN 2020tlf . Other models suggest a common-envelope ejection just centuries before core collapse, which may lead to the strongest interactions, as observed in superluminous Type IIn SNe, such as SN 1994W and SN 2006gy . Conclusions. Wide massive binaries exhibit properties that may not only explain the diverse envelope stripping inferred in Type Ib, IIb, IIL, and IIP SNe, but also offer a natural framework to understand a broad range of hydrogen-rich interacting SNe. On the other hand, the flash features observed in many Type IIP SNe, such as SN 2013fs , may indicate that RSG atmospheres are more extended than currently assumed; this could enhance the parameter space for wide binary interaction.
Context.
At present, there are strong indications that white dwarf (WD) stars with masses well below the Chandrasekhar limit (
M
Ch
≈ 1.4
M
⊙
) contribute a significant fraction of SN Ia progenitors. ...The relative fraction of stable iron-group elements synthesized in the explosion has been suggested as a possible discriminant between
M
Ch
and sub-
M
Ch
events. In particular, it is thought that the higher-density ejecta of
M
Ch
WDs, which favours the synthesis of stable isotopes of nickel, results in prominent Ni
II
lines in late-time spectra (≳150 d past explosion).
Aims.
We study the explosive nucleosynthesis of stable nickel in SNe Ia resulting from
M
Ch
and sub-
M
Ch
progenitors. We explore the potential for lines of Ni
II
in the optical an near-infrared (at 7378 Å and 1.94 μm) in late-time spectra to serve as a diagnostic of the exploding WD mass.
Methods.
We reviewed stable Ni yields across a large variety of published SN Ia models. Using 1D
M
Ch
delayed-detonation and sub-
M
Ch
detonation models, we studied the synthesis of stable Ni isotopes (in particular,
58
Ni) and investigated the formation of Ni
II
lines using non-local thermodynamic equilibrium radiative-transfer simulations with the CMFGEN code.
Results.
We confirm that stable Ni production is generally more efficient in
M
Ch
explosions at solar metallicity (typically 0.02–0.08
M
⊙
for the
58
Ni isotope), but we note that the
58
Ni yield in sub-
M
Ch
events systematically exceeds 0.01
M
⊙
for WDs that are more massive than one solar mass. We find that the radiative proton-capture reaction
57
Co(
p
,
γ
)
58
Ni is the dominant production mode for
58
Ni in both
M
Ch
and sub-
M
Ch
models, while the
α
-capture reaction on
54
Fe has a negligible impact on the final
58
Ni yield. More importantly, we demonstrate that the lack of Ni
II
lines in late-time spectra of sub-
M
Ch
events is not always due to an under-abundance of stable Ni; rather, it results from the higher ionization of Ni in the inner ejecta. Conversely, the strong Ni
II
lines predicted in our 1D
M
Ch
models are completely suppressed when
56
Ni is sufficiently mixed with the innermost layers, which are rich in stable iron-group elements.
Conclusions.
Ni
II
lines in late-time SN Ia spectra have a complex dependency on the abundance of stable Ni, which limits their use in distinguishing among
M
Ch
and sub-
M
Ch
progenitors. However, we argue that a low-luminosity SN Ia displaying strong Ni
II
lines would most likely result from a Chandrasekhar-mass progenitor.
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