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1. The progenitor of the Vela pulsar

   https://doi.org/10.1093/mnras/stac098  

   Kochanek, C. S. ( February 2022 , Monthly Notices of the Royal Astronomical Society)

   With Gaia parallaxes, it is possible to study the stellar populations associated with individual Galactic supernova remnants (SNRs) to estimate the mass of the exploding star. Here, we analyse the luminous stars near the Vela pulsar and SNR to find that its progenitor was probably ($\mathrel {\raise.3ex\rm{\gt }\lower0.6ex\rm{\sim }}90\rm \,per\,cent$) low mass (8.1–$10.3\, {\rm M}_\odot$). The presence of the O star γ2 Vel a little over 100 pc from Vela is the primary ambiguity, as including it in the analysis volume significantly increases the probability (to 5 per cent) of higher mass ($\gt 20\, {\rm M}_\odot$) progenitors. However, to be a high-mass star associated with γ2 Vel’s star cluster at birth, the progenitor would have to be a runaway star from an unbound binary with an unusually high velocity. The primary impediment to analysing large numbers of Galactic SNRs in this manner is the lack of accurate distances. This can likely be solved by searching for absorption lines from the SNR in stars as a function of distance, a method which yielded a distance to Vela in agreement with the direct pulsar parallax. If Vela was a $10\, {\rm M}_\odot$ supernova in an external galaxy, the 50-pc search region used in extragalactic studies would contain only $\simeq 10\rm \,per\,cent$ of the stars formed in a 50-pc region around the progenitor at birth and $\simeq 90\rm \,per\,cent$ of the stars in the search region would have been born elsewhere.

    

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2. Probing the Low-mass End of Core-collapse Supernovae Using a Sample of Strongly-stripped Calcium-rich Type IIb Supernovae from the Zwicky Transient Facility

   https://doi.org/10.3847/1538-4357/acfeeb  

   Das, Kaustav K. ; Kasliwal, Mansi M. ; Fremling, Christoffer ; Yang, Sheng ; Schulze, Steve ; Sollerman, Jesper ; Sit, Tawny ; De, Kishalay ; Tzanidakis, Anastasios ; Perley, Daniel A. ; et al ( November 2023 , The Astrophysical Journal)

   The fate of stars in the zero-age main-sequence (ZAMS) range ≈8–12M_⊙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 [Caii]λλ7291, 7324/[Oi]λλ6300, 6364 flux ratio of ≳2 in their nebular spectra. Comparing the measured [Oi] luminosity (≲10³⁹erg s⁻¹) and derived oxygen mass (≈0.01M_⊙) with theoretical models, we infer that the progenitor ZAMS mass for these explosions is less than 12M_⊙. The ejecta properties (M_ej≲ 1M_⊙andE_kin∼ 10⁵⁰erg) 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.

    

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3. Cool, Luminous, and Highly Variable Stars in the Magellanic Clouds. II. Spectroscopic and Environmental Analysis of Thorne–Żytkow Object and Super-AGB Star Candidates

   https://doi.org/10.3847/1538-4357/aca655  

   O‘Grady, Anna J. G. ; Drout, Maria R. ; Gaensler, B. M. ; Kochanek, C. S. ; Neugent, Kathryn F. ; Doherty, Carolyn L. ; Speagle 沈, Joshua S. 佳士 ; Shappee, B. J. ; Rauch, Michael ; Götberg, Ylva ; et al ( January 2023 , The Astrophysical Journal)

   In previous work, we identified a population of 38 cool and luminous variable stars in the Magellanic Clouds and examined 11 in detail in order to classify them as either Thorne–Żytkow objects (TŻOs; red supergiants with a neutron star cores) or super-asymptotic giant branch (sAGB) stars (the most massive stars that will not undergo core collapse). This population includes HV 2112, a peculiar star previously considered in other works to be either a TŻO or high-mass asymptotic giant branch (AGB) star. Here we continue this investigation, using the kinematic and radio environments and local star formation history of these stars to place constraints on the age of the progenitor systems and the presence of past supernovae. These stars are not associated with regions of recent star formation, and we find no evidence of past supernovae at their locations. Finally, we also assess the presence of heavy elements and lithium in their spectra compared to red supergiants. We find strong absorption in Li and s-process elements compared to RSGs in most of the sample, consistent with sAGB nucleosynthesis, while HV 2112 shows additional strong lines associated with TŻO nucleosynthesis. Coupled with our previous mass estimates, the results are consistent with the stars being massive (∼4–6.5M_⊙) or sAGB (∼6.5–12M_⊙) stars in the thermally pulsing phase, providing crucial observations of the transition between low- and high-mass stellar populations. HV 2112 is more ambiguous; it could either be a maximally massive sAGB star, or a TŻO if the minimum mass for stability extends down to ≲13M_⊙.

    

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4. The Diverse Properties of Type Icn Supernovae Point to Multiple Progenitor Channels

   https://doi.org/10.3847/1538-4357/ac8ff6  

   Pellegrino, C. ; Howell, D. A. ; Terreran, G. ; Arcavi, I. ; Bostroem, K. A. ; Brown, P. J. ; Burke, J. ; Dong, Y. ; Gilkis, A. ; Hiramatsu, D. ; et al ( October 2022 , The Astrophysical Journal)

   We present a sample of Type Icn supernovae (SNe Icn), a newly discovered class of transients characterized by their interaction with H- and He-poor circumstellar material (CSM). This sample is the largest collection of SNe Icn to date and includes observations of two published objects (SN 2019hgp and SN 2021csp) and two objects not yet published in the literature (SN 2019jc and SN 2021ckj). The SNe Icn display a range of peak luminosities, rise times, and decline rates, as well as diverse late-time spectral features. To investigate their explosion and progenitor properties, we fit their bolometric light curves to a semianalytical model consisting of luminosity inputs from circumstellar interaction and radioactive decay of⁵⁶Ni. We infer low ejecta masses (≲2M_⊙) and⁵⁶Ni masses (≲0.04M_⊙) from the light curves, suggesting that normal stripped-envelope supernova (SESN) explosions within a dense CSM cannot be the underlying mechanism powering SNe Icn. Additionally, we find that an estimate of the star formation rate density at the location of SN 2019jc lies at the lower end of a distribution of SESNe, in conflict with a massive star progenitor of this object. Based on its estimated ejecta mass,⁵⁶Ni mass, and explosion site properties, we suggest a low-mass, ultra-stripped star as the progenitor of SN 2019jc. For other SNe Icn, we suggest that a Wolf–Rayet star progenitor may better explain their observed properties. This study demonstrates that multiple progenitor channels may produce SNe Icn and other interaction-powered transients.

    

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5. s- process enrichment of ultrafaint dwarf galaxies

   https://doi.org/10.1093/mnras/stab1487  

   Tarumi, Yuta ; Suda, Takuma ; van de Voort, Freeke ; Inoue, Shigeki ; Yoshida, Naoki ; Frebel, Anna ( June 2021 , Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT We study the production of barium (Ba) and strontium (Sr) in ultrafaint dwarf (UFDs) galaxies. Both r- and s- processes produce these elements, and one can infer the contribution of the r-process from the characteristic r-process abundance pattern, whereas the s-process contribution remains largely unknown. We show that the current s-process yield from asymptotic giant branch (AGB) stars is not sufficient to explain the Ba and Sr abundances observed in UFDs. Production of these elements would need to be efficient from the beginning of star formation in the galaxies. The discrepancy of nearly or more than 1 dex is not reconciled even if we consider s-process in super-AGB stars. We consider a possible resolution by assuming rotating massive stars (RMSs) and electron-capture supernovae (ECSNe) as additional contributors. We find that the RMSs could be the origin of Ba in UFDs if ∼10 per cent of massive stars are rotating at 300 km s−1. As for ECSNe, we argue that their fraction is less than 2 per cent of core-collapse supernova. It narrows the progenitor mass-range to ${\lesssim}0.1\, \mathrm{M}_\odot$ at −3 ≲ [Fe/H] ≲ −2. We also explore another resolution by modifying the stellar initial mass function (IMF) in UFDs and find a top-light IMF model that reproduces the observed level of Ba-enrichment. Future observations that determine or tightly constrain the europium and nitrogen abundances are crucial to identify the origin of Ba and Sr in UFDs. 

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