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ABSTRACT We present six epochs of optical spectropolarimetry of the Type IIP supernova (SN) 2021yja ranging from ∼25 to 95 d after the explosion. An unusually high continuum linear polarization of $$p \approx 0.9~{{\ \rm per\ cent}}$$ is measured during the early photospheric phase, followed by a steady decrease well before the onset of the nebular phase. This behaviour has not been observed before in Type IIP supernovae (SNe IIP). The observed continuum polarization angle does not change significantly during the photospheric phase. We find a pronounced axis of symmetry in the global ejecta that is shared in common with the Hα and Ca ii near-infrared triplet lines. These observations are consistent with an ellipsoidal geometry. The temporal evolution of the continuum polarization is also compatible with the SN ejecta interacting with aspherical circumstellar matter (CSM), although no spectroscopic features that may be associated with strong interaction can be identified. Alternatively, we consider the source of the high polarization to be an extended hydrogen envelope that is indistinguishable from low-density CSM. 

ABSTRACT Long gamma-ray bursts (LGRBs) are associated to the collapse of a massive star and the formation of a relativistic jet. As the jet propagates through the star, it forms an extended, hot cocoon. The dynamical evolution of the jet/cocoon system and its interaction with the environment has been studied extensively both analytically and numerically. On the other hand, the role played by the supernova (SN) explosion associated with LGRBs in determining the outcome of the system has been barely considered. In this paper, we discuss the large landscape of outcomes resulting from the interaction of the SN, jet, and cocoon. We show that the outcome depends mainly on three time-scales: the times for the cocoon and SN shock wave to break through the surface of the progenitor star, and the time needed for the cocoon to engulf completely the progenitor star. The delay between the launch of the SN shock moving through the progenitor star and the jet can be related to these three time-scales. Depending on the ordering of these time-scales, the jet-cocoon might propagate inside the SN ejecta or the other way around, and the outcome for the properties of the explosion would be different. We discuss the imprint of the complex interaction between the jet-cocoon and the SN shock on the emergent thermal and non-thermal radiation. 

Abstract We present a study of the influence of magnetic field strength and morphology in Type Ia supernovae and their late-time light curves and spectra. In order to both capture self-consistent magnetic field topologies and evolve our models to late times, a two-stage approach is taken. We study the early deflagration phase (∼1 s) using a variety of magnetic field strengths and find that the topology of the field is set by the burning, independent of the initial strength. We study late-time (∼1000 days) light curves and spectra with a variety of magnetic field topologies and infer magnetic field strengths from observed supernovae. Lower limits are found to be 10 6 G. This is determined by the escape, or lack thereof, of positrons that are tied to the magnetic field. The first stage employs 3D MHD and a local burning approximation and uses the code Enzo. The second stage employs a hybrid approach, with 3D radiation and positron transport and spherical hydrodynamics. The second stage uses the code HYDRA. In our models, magnetic field amplification remains small during the early deflagration phase. Late-time spectra bear the imprint of both magnetic field strength and morphology. Implications for alternative explosion scenarios are discussed. 

ABSTRACT Optical spectropolarimetry of the normal thermonuclear supernova (SN) 2019np from −14.5 to +14.5 d relative to B-band maximum detected an intrinsic continuum polarization (pcont) of 0.21 ± 0.09 per cent at the first epoch. Between days −11.5 and  +0.5, pcont remained ∼0 and by day +14.5 was again significant at 0.19 ± 0.10 per cent. Not considering the first epoch, the dominant axis of $${\rm Si\, {\small II}}$$ λ6355 was roughly constant staying close the continuum until both rotated in opposite directions on day  +14.5. Detailed radiation-hydrodynamical simulations produce a very steep density slope in the outermost ejecta so that the low first-epoch pcont ≈ 0.2 per cent nevertheless suggests a separate structure with an axis ratio ∼2 in the outer carbon-rich (3.5–4) × 10−3 M⊙. Large-amplitude fluctuations in the polarization profiles and a flocculent appearance of the polar diagram for the $${\rm Ca\, {\small II}}$$ near-infrared triplet (NIR3) may be related by a common origin. The temporal evolution of the polarization spectra agrees with an off-centre delayed detonation. The late-time increase in polarization and the possible change in position angle are also consistent with an aspherical 56Ni core. The pcont and the absorptions due to $${\rm Si\, {\small II}}$$ λ6355 and $${\rm Ca\, {\small II}}$$ NIR3 form in the same region of the extended photosphere, with an interplay between line occultation and thermalization producing p. Small-scale polarization features may be due to small-scale structures, but many could be related to atomic patterns of the quasi-continuum; they hardly have an equivalent in the total-flux spectra. We compare SN 2019np to other SNe and develop future objectives and strategies for SN Ia spectropolarimetry. 

Abstract Dust associated with various stellar sources in galaxies at all cosmic epochs remains a controversial topic, particularly whether supernovae play an important role in dust production. We report evidence of dust formation in the cold, dense shell behind the ejecta–circumstellar medium (CSM) interaction in the Type Ia-CSM supernova (SN) 2018evt three years after the explosion, characterized by a rise in mid-infrared emission accompanied by an accelerated decline in the optical radiation of the SN. Such a dust-formation picture is also corroborated by the concurrent evolution of the profiles of the Hα emission line. Our model suggests enhanced CSM dust concentration at increasing distances from the SN as compared to what can be expected from the density profile of the mass loss from a steady stellar wind. By the time of the last mid-infrared observations at day +1,041, a total amount of 1.2 ± 0.2 × 10⁻² M_⊙of new dust has been formed by SN 2018evt, making SN 2018evt one of the most prolific dust factories among supernovae with evidence of dust formation. The unprecedented witness of the intense production procedure of dust may shed light on the perceptions of dust formation in cosmic history. 

Photometry shown in Figure Extended Data 4 (a) of Wang, Lingzhi, et al. 2024, Nature Astronomy, https://doi.org/10.1038/s41550-024-02197-9.Phase is days since B-band maximum MJD 58352.BVgri-band photometry from 1-m network at Las Cumbres Observatory.SN2018evt_lcogt_lc.datBVgri-band photometry from 2.4-m LiJiang Telescope (LJT) and 60/90-cm XingLong Schmidt Telescope (XLST)SN2018evt_xlt_ljt_lc.datOptical and NIR spectra data shown in Figures Extended Data 2, 3, and Table Extended Data 2 of Wang, Lingzhi, et al. 2024, Nature Astronomy, NIR spectraSN2018evt_181224_spex.txt SN2018evt_190511_spex.txtSN2018evt_190617_spex.txtSN2018evt_200119_spex.txtSN2018evt_20190101_gnirs.txtSN2018evt_20190108_gnirs.txtSN2018evt_20190516_fire.datSN2018evt_20190712_fire.datOptical spectraOptical spectra observed with 2.4-m LiJiang Telescope (LJT)SN2018evt_190104_LJT_G3.datSN2018evt_190131_LJT_G3.datSN2018evt_190328_LJT_G3.datSN2018evt_190520_LJT_G3.datOptical spectra observed with 2.16-m XingLong Telescope (XLT)SN2018evt_20190208_2458551.3570_bao_bfosc.txtSN2018evt_20190220_2458563.3588_bao-bfosc.txtSN2018evt_20190413_2458587.2169_bao-bfosc.txtOptical spectra observed with 3.6-m ESO New Technology Telescope (NTT)SN2018evt_20180812_NTT_Gr13_Free_slit1.0_58346_1_e.asciSN2018evt_20190425_NTT_Gr13_Free_slit1.0_58599_1_e.asciSN2018evt_20190512_NTT_Gr13_Free_slit1.0_58616_1_e.asciSN2018evt_20190608_NTT_Gr13_Free_slit1.0_58643_1_e.asciSN2018evt_20200218_NTT_Gr13_Free_slit1.0_58899_1_e.asciSN2018evt_20200322_NTT_Gr13_Free_slit1.0_58931_1_e.asciOptical spectrum observed with WiFes mounted on 2.3-m telescope at the Siding Spring Observatory (WiFeS)SN2018evt_20190624_ANU_Wifes.datOptical spectrum observed with 2.0-m Faulkes Telescope North (FTN)/FLOYDSSN2018evt_20191224_FTN-floyds-redblu_145742.306.asciiSN2018evt_20200119_FTN-floyds-redblu_133856.906.asciiSN2018evt_20200203_FTN-floyds-redblu_125905.990.ascii 

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. 

Abstract We present near-infrared (NIR) and optical observations of the Type Ic supernova (SN Ic) SN 2021krf obtained between days 13 and 259 at several ground-based telescopes. The NIR spectrum at day 68 exhibits a rising K -band continuum flux density longward of ∼2.0 μ m, and a late-time optical spectrum at day 259 shows strong [O i ] 6300 and 6364 Å emission-line asymmetry, both indicating the presence of dust, likely formed in the SN ejecta. We estimate a carbon-grain dust mass of ∼2 × 10 −5 M ⊙ and a dust temperature of ∼900–1200 K associated with this rising continuum and suggest the dust has formed in SN ejecta. Utilizing the one-dimensional multigroup radiation-hydrodynamics code STELLA, we present two degenerate progenitor solutions for SN 2021krf, characterized by C–O star masses of 3.93 and 5.74 M ⊙ , but with the same best-fit 56 Ni mass of 0.11 M ⊙ for early times (0–70 days). At late times (70–300 days), optical light curves of SN 2021krf decline substantially more slowly than those expected from 56 Co radioactive decay. Lack of H and He lines in the late-time SN spectrum suggests the absence of significant interaction of the ejecta with the circumstellar medium. We reproduce the entire bolometric light curve with a combination of radioactive decay and an additional powering source in the form of a central engine of a millisecond pulsar with a magnetic field smaller than that of a typical magnetar. 

ABSTRACT A rare class of supernovae (SNe) is characterized by strong interaction between the ejecta and several solar masses of circumstellar matter (CSM) as evidenced by strong Balmer-line emission. Within the first few weeks after the explosion, they may display spectral features similar to overluminous Type Ia SNe, while at later phase their observation properties exhibit remarkable similarities with some extreme case of Type IIn SNe that show strong Balmer lines years after the explosion. We present polarimetric observations of SN 2018evt obtained by the ESO Very Large Telescope from 172 to 219 d after the estimated time of peak luminosity to study the geometry of the CSM. The non-zero continuum polarization decreases over time, suggesting that the mass-loss of the progenitor star is aspherical. The prominent H α emission can be decomposed into a broad, time-evolving component and an intermediate-width, static component. The former shows polarized signals, and it is likely to arise from a cold dense shell (CDS) within the region between the forward and reverse shocks. The latter is significantly unpolarized, and it is likely to arise from shocked, fragmented gas clouds in the H-rich CSM. We infer that SN 2018evt exploded inside a massive and aspherical circumstellar cloud. The symmetry axes of the CSM and the SN appear to be similar. SN 2018evt shows observational properties common to events that display strong interaction between the ejecta and CSM, implying that they share similar circumstellar configurations. Our preliminary estimate also suggests that the circumstellar environment of SN 2018evt has been significantly enriched at a rate of ∼0.1 M⊙ yr−1 over a period of >100 yr.