Abstract
We present observations and analysis of the hostless and luminous Type Ia supernova 2022ilv, illustrating it is part of the 2003fg-like family, often referred to as super-Chandrasekhar ...(Ia-SC) explosions. The Asteroid Terrestrial-impact Last Alert System light curve shows evidence of a short-lived, pulse-like early excess, similar to that detected in another luminous Type Ia supernova (SN 2020hvf). The light curve is broad, and the early spectra are remarkably similar to those of SN 2009dc. Adopting a redshift of
z
= 0.026 ± 0.005 for SN 2022ilv based on spectral matching, our model light curve requires a large
56
Ni mass in the range 0.7–1.5
M
⊙
and a large ejecta mass in the range 1.6–2.3
M
⊙
. The early excess can be explained by fast-moving SN ejecta interacting with a thin, dense shell of circumstellar material close to the progenitor (∼10
13
cm) a few hours after the explosion. This may be realized in a double-degenerate scenario, wherein a white dwarf merger is preceded by the ejection of a small amount (∼10
−3
–10
−2
M
⊙
) of hydrogen and helium-poor tidally stripped material. A deep pre-explosion Pan-STARRS1 stack indicates no host galaxy to a limiting magnitude of
r
∼ 24.5. This implies a surprisingly faint limit for any host of
M
r
≳ −11, providing further evidence that these types of explosions occur predominantly in low-metallicity environments.
We present CN and CH indices and Ca ii triplet metallicities for 34 giant stars and chemical abundances for 33 elements in 14 giants in the globular cluster M2. Assuming that the programme stars are ...cluster members, our analysis reveals (i) an extreme variation in CN and CH line strengths, (ii) a metallicity dispersion with a dominant peak at Fe/H ≈ −1.7 and smaller peaks at −1.5 and −1.0, (iii) star-to-star abundance variations and correlations for the light elements O, Na, Al and Si and (iv) a large (and possibly bimodal) distribution in the abundances of all elements produced mainly via the s-process in Solar system material. Following Roederer, Marino & Sneden, we define two groups of stars, ‘r + s’ and ‘r-only’, and subtract the average abundances of the latter from the former group to obtain an ‘s-process residual’. This s-process residual is remarkably similar to that found in M22 and in M4 despite the range in metallicity covered by these three systems. With recent studies identifying a double subgiant branch in M2 and a dispersion in Sr and Ba abundances, our spectroscopic analysis confirms that this globular cluster has experienced a complex formation history with similarities to M22, NGC 1851 and ω Centauri.
Abstract
We present 3D radiative transfer calculations for the ejecta from a neutron star merger that include line-by-line opacities for tens of millions of bound–bound transitions, composition from ...an
r
-process nuclear network, and time-dependent thermalization of decay products from individual
α
and
β
−
decay reactions. In contrast to expansion opacities and other wavelength-binned treatments, a line-by-line treatment enables us to include fluorescence effects and associate spectral features with the emitting and absorbing lines of individual elements. We find variations in the synthetic observables with both the polar and azimuthal viewing angles. The spectra exhibit blended features with strong interactions by Ce
iii
, Sr
ii
, Y
ii
, and Zr
ii
that vary with time and viewing direction. We demonstrate the importance of wavelength calibration of atomic data using a model with calibrated Sr, Y, and Zr data, and find major differences in the resulting spectra, including a better agreement with AT2017gfo. The synthetic spectra for a near-polar inclination show a feature at around 8000 Å, similar to AT2017gfo. However, they evolve on a more rapid timescale, likely due to the low ejecta mass (0.005
M
☉
) as we take into account only the early ejecta. The comparatively featureless spectra for equatorial observers gives a tentative prediction that future observations of edge-on kilonovae will appear substantially different from AT2017gfo. We also show that 1D models obtained by spherically averaging the 3D ejecta lead to dramatically different direction-integrated luminosities and spectra compared to full 3D calculations.
ABSTRACT
The detection of GW170817 and the accompanying electromagnetic counterpart, AT2017gfo, have provided an important set of observational constraints for theoretical models of neutron star ...mergers, nucleosynthesis, and radiative transfer for kilonovae. We apply the three-dimensional (3D) Monte Carlo radiative transfer code artis to produce synthetic light curves of the dynamical ejecta from a neutron star merger, which has been modelled with 3D smooth particle hydrodynamics and included neutrino interactions. Nucleosynthesis calculations provide the energy released from radioactive decays of r-process nuclei, and radiation transport is performed using grey opacities given as functions of the electron fraction. We present line-of-sight-dependent bolometric light curves, and find the emission along polar lines of sight to be up to a factor of ∼2 brighter than that along equatorial lines of sight. Instead of a distinct emission peak, our bolometric light curve exhibits a monotonic decline, characterized by a shoulder at the time when the bulk ejecta becomes optically thin. We show approximate band light curves based on radiation temperatures and compare these to the observations of AT2017gfo. We find that the rapidly declining temperatures lead to a blue to red colour evolution similar to that shown by AT2017gfo. We also investigate the impact of an additional, spherically symmetric secular ejecta component, and we find that the early light curve remains nearly unaffected, while after about $1\,$ d the emission is strongly enhanced and dominated by the secular ejecta, leading to the shift of the shoulder from ∼1–2 to 6–10 d.
ABSTRACT
The double detonation is a widely discussed mechanism to explain Type Ia supernovae from explosions of sub-Chandrasekhar mass white dwarfs. In this scenario, a helium detonation is ignited ...in a surface helium shell on a carbon/oxygen white dwarf, which leads to a secondary carbon detonation. Explosion simulations predict high abundances of unburnt helium in the ejecta, however, radiative transfer simulations have not been able to fully address whether helium spectral features would form. This is because helium can not be sufficiently excited to form spectral features by thermal processes, but can be excited by collisions with non-thermal electrons, which most studies have neglected. We carry out a full non-local thermodynamic equilibrium radiative transfer simulation for an instance of a double detonation explosion model, and include a non-thermal treatment of fast electrons. We find a clear He i λ10830 feature which is strongest in the first few days after explosion and becomes weaker with time. Initially this feature is blended with the Mg ii λ10927 feature but over time separates to form a secondary feature to the blue wing of the Mg ii λ10927 feature. We compare our simulation to observations of iPTF13ebh, which showed a similar feature to the blue wing of the Mg ii λ10927 feature, previously identified as C i. Our simulation shows a good match to the evolution of this feature and we identify it as high velocity He i λ10830. This suggests that He i λ10830 could be a signature of the double detonation scenario.
ABSTRACT
The progenitor systems and explosion mechanism of Type Ia supernovae are still unknown. Currently favoured progenitors include double-degenerate systems consisting of two carbon-oxygen white ...dwarfs with thin helium shells. In the double-detonation scenario, violent accretion leads to a helium detonation on the more massive primary white dwarf that turns into a carbon detonation in its core and explodes it. We investigate the fate of the secondary white dwarf, focusing on changes of the ejecta and observables of the explosion if the secondary explodes as well rather than survives. We simulate a binary system of a $1.05\, \mathrm{M_\odot }$ and a $0.7\, \mathrm{M_\odot }$ carbon-oxygen white dwarf with $0.03\, \mathrm{M_\odot }$ helium shells each. We follow the system self-consistently from inspiral to ignition, through the explosion, to synthetic observables. We confirm that the primary white dwarf explodes self-consistently. The helium detonation around the secondary white dwarf, however, fails to ignite a carbon detonation. We restart the simulation igniting the carbon detonation in the secondary white dwarf by hand and compare the ejecta and observables of both explosions. We find that the outer ejecta at $v~\gt ~15\, 000$ km s−1 are indistinguishable. Light curves and spectra are very similar until $\sim ~40 \ \mathrm{d}$ after explosion and the ejecta are much more spherical than violent merger models. The inner ejecta differ significantly slowing down the decline rate of the bolometric light curve after maximum of the model with a secondary explosion by ∼20 per cent. We expect future synthetic 3D nebular spectra to confirm or rule out either model.
ABSTRACT
The nebular spectra of Type Ia supernovae (⪆100 d after explosion) consist mainly of emission lines from singly and doubly ionized Fe-group nuclei. However, theoretical models for many ...scenarios predict that non-thermal ionization leads to multiply ionized species whose recombination photons ionize and deplete Fe+, resulting in negligible Fe ii emission. We investigate a method to determine the collisional excitation conditions from Fe ii line ratios independently from the ionization state and find that it cannot be applied to highly ionized models due to the influence of recombination cascades on Fe+ level populations. When the ionization state is artificially lowered, the line ratios (and excitation conditions) are too similar to distinguish between explosion scenarios. We investigate changes to the treatment of non-thermal energy deposition as a way to reconcile overionized theoretical models with observations and find that a simple work function approximation provides closer agreement with the data for sub-Mch models than a detailed Spencer–Fano treatment with widely used cross-section data. To quantify the magnitude of additional heating processes that would be required to sufficiently reduce ionization from fast leptons, we artificially boost the rate of energy loss to free electrons. We find that the equivalent of as much as an eight times increase to the plasma loss rate would be needed to reconcile the sub-Mch model with observed spectra. Future studies could distinguish between reductions in the non-thermal ionization rates and increased recombination rates, such as by clumping.
Towards inferring the geometry of kilonovae Collins, Christine E; Shingles, Luke J; Bauswein, Andreas ...
Monthly Notices of the Royal Astronomical Society,
03/2024, Letnik:
529, Številka:
2
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
Recent analysis of the kilonova, AT2017gfo, has indicated that this event was highly spherical. This may challenge hydrodynamics simulations of binary neutron star mergers, which usually ...predict a range of asymmetries, and radiative transfer simulations show a strong direction dependence. Here we investigate whether the synthetic spectra from a 3D kilonova simulation of asymmetric ejecta from a hydrodynamical merger simulation can be compatible with the observational constraints, suggesting a high degree of sphericity in AT2017gfo. Specifically, we determine whether fitting a simple P-Cygni line profile model leads to a value for the photospheric velocity that is consistent with the value obtained from the expanding photosphere method. We would infer that our kilonova simulation is highly spherical at early times, when the spectra resemble a blackbody distribution. The two independently inferred photospheric velocities can be very similar, implying a high degree of sphericity, which can be as spherical as inferred for AT2017gfo, demonstrating that the photosphere can appear spherical even for asymmetrical ejecta. The last-interaction velocities of radiation escaping the simulation show a high degree of sphericity, supporting the inferred symmetry of the photosphere. We find that when the synthetic spectra resemble a blackbody, the expanding photosphere method can be used to obtain an accurate luminosity distance (within 4–7 per cent).
Motivated by unexplained observations of low sulphur abundances in planetary nebulae (PNe) and the PG1159 class of post-asymptotic giant branch (AGB) stars, we investigate the possibility that ...sulphur may be destroyed by nucleosynthetic processes in low-to-intermediate mass stars during stellar evolution. We use a 3 M, Z = 0.01 evolutionary sequence to examine the consequences of high and low reaction rate estimates of neutron captures on to sulphur and neighbouring elements. In addition, we have also tested high and low rates for the neutron producing reactions 13C(α,n)16O and 22Ne(α,n)25Mg. We vary the mass width of a partially mixed zone (PMZ), which is responsible for the formation of a 13C pocket and is the site of the 13C(α,n)16O neutron source. We test PMZ masses from zero up to an extreme upper limit of the entire He-intershell mass at 10−2 M. We find that the alternative reaction rates and variations to the PMZ have almost no effect on surface sulphur abundances and do not reproduce the anomaly. To understand the effect of initial mass on our conclusions, 1.8 and 6 M evolutionary sequences are also tested with similar results for sulphur abundances. We are able to set a constraint on the size of the PMZ, as PMZ sizes that are greater than half of the He-intershell mass (in the 3 M model) are excluded by comparison with neon abundances in PNe. We compare the 1.8 M model's intershell abundances with observations of PG1159−035, whose surface abundances are thought to reflect the intershell composition of a progenitor AGB star. We find general agreement between the patterns of F, Ne, Si, P and Fe abundances and a very large discrepancy for sulphur where our model predicts abundances that are 30-40 times higher than those observed in the star.
There is now strong evidence that some stars have been born with He mass fractions as high as Y ≈ 0.40 (e.g. in ω Centauri). However, the advanced evolution, chemical yields, and final fates of ...He-rich stars are largely unexplored. We investigate the consequences of He-enhancement on the evolution and nucleosynthesis of intermediate-mass asymptotic giant branch (AGB) models of 3, 4, 5, and 6 M⊙ with a metallicity of Z = 0.0006 (Fe/H ≈ −1.4). We compare models with He-enhanced compositions (Y = 0.30, 0.35, 0.40) to those with primordial-He (Y = 0.24). We find that the minimum initial mass for C burning and super-AGB stars with CO(Ne) or ONe cores decreases from above our highest mass of 6 to ∼4–5 M⊙ with Y = 0.40. We also model the production of trans-Fe elements via the slow neutron-capture process (s-process). He-enhancement substantially reduces the third dredge-up efficiency and the stellar yields of s-process elements (e.g. 90 per cent less Ba for 6 M⊙, Y = 0.40). An exception occurs for 3 M⊙, where the near-doubling in the number of thermal pulses with Y = 0.40 leads to ∼50 per cent higher yields of Ba-peak elements and Pb if the 13C neutron source is included. However, the thinner intershell and increased temperatures at the base of the convective envelope with Y = 0.40 probably inhibit the 13C neutron source at this mass. Future chemical evolution models with our yields might explain the evolution of s-process elements among He-rich stars in ω Centauri.
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