In astronomy, acalcium-rich supernova (orCalcium-rich transient,Ca-rich SN) is a subclass ofsupernovae that, in contrast to more well-known traditionalsupernova classes, are fainter and produce unusually large amounts ofcalcium. Since theirluminosity is located in a gap between that ofnovae and other supernovae, they are also referred to as "gap"transients. Only around 15 events have been classified as a calcium-rich supernova (as of August 2017) – a combination of their intrinsic rarity and lowluminosity make new discoveries and their subsequent study difficult. This makes calcium-rich supernovae one of the most mysterious supernova subclasses currently known.
A peculiar group of supernova that were unusually rich in calcium were identified byAlexei Filippenko and collaborators. Although they appeared somewhat similar toType Ib and Ic supernovae, theirspectra were dominated by calcium, without other signatures often seen in Type Ib and Ic supernovae, and the termcalcium-rich was coined to describe them.[1] Subsequent discoveries led to the classification of empirically similar supernovae.[2][3] They share characteristics such as quickly rising and fadinglight curves that peak in luminosity between novae and supernovae, and spectra that are dominated by calcium 2–3 months after initial explosion.[4]
The exact nature of the stellar systems and their subsequent explosions that give rise to calcium-rich supernovae are unknown. Despite appearing similar to Type Ib supernovae, it was noted that a different explosion mechanism was likely to be responsible for calcium-rich supernovae.[2] Since a large proportion of thegalaxies from which they are thought to originate areearly-type galaxies, and thus composed of oldstellar populations, they are unlikely to contain many young, massivestars that give rise to Type Ib supernovae.[5] Supernova explosions in old stellar populations generally involved awhite dwarf since these are old systems that can undergo thermonuclear explosion under the right circumstances, as is the case forType Ia supernovae. However, because calcium-rich supernovae are much less luminous and fade more quickly than normal Type Ia supernovae, it is unlikely that the same mechanism is at play for both.
Another peculiarity of calcium-rich supernovae is that they appear to explode far away from galaxies, even reachingintergalactic space. Searches for faintdwarf galaxies at their locations have ruled that they are exploding in very low density environments, unlike other supernova types.[6][7]
There are several theories that attempt to explain this behaviour.Binary systems ofhigh-velocity stars, such as two white dwarfs or a white dwarf and aneutron star, that have been ejected from their galaxy either due to aneutron star kick[8][9] or interaction with thesupermassive black hole in their galaxy[10][11] could produce explosions when they eventually merge (due togravitational wave radiation) that would preferentially occur far from galaxies. Alternatively they have been suggested to be due to stars that reside in theintracluster medium within largegalaxy groups or clusters, having been expelled from their galaxy duringmergers or interactions.[7] The explosion would then be caused by the detonation of a low mass white dwarf during a merging event as part of a binary system, or the detonation of ahelium shell on a white dwarf.
A calcium-rich supernova event expels several tenths of asolar mass in material at thousands of kilometres per second and reaches a peak luminosity equal to around 100–200 million timesthat of the Sun. Despite calcium-rich supernovae being comparatively rare and diminutive compared to other supernova types, they are thought to make a significant contribution to the production of calcium in theUniverse.[12]
^Prentice, S. J.; Maguire, K.; Flörs, A.; Taubenberger, S.; Inserra, C.; Frohmaier, C.; Chen, T. W.; Anderson, J. P.; Ashall, C.; Clark, P.; Fraser, M.; Galbany, L.; Gal-Yam, A.; Gromadzki, M.; Gutiérrez, C. P.; James, P. A.; Jonker, P. G.; Kankare, E.; Leloudas, G.; Magee, M. R.; Mazzali, P. A.; Nicholl, M.; Pursiainen, M.; Skillen, K.; Smartt, S. J.; Smith, K. W.; Vogl, C.; Young, D. R. (2020). "The rise and fall of an extraordinary Ca-rich transient – The discovery of ATLAS19dqr/SN 2019bkc".Astronomy & Astrophysics.A186: 635.arXiv:1909.05567.Bibcode:2020A&A...635A.186P.doi:10.1051/0004-6361/201936515.S2CID202565575.
^Perets, H. B.; Gal-Yam, A.; Mazzali, P. A.; Arnett, D.; Kagan, D.; Filippenko, A. V.; Li, W.; Arcavi, I.; Cenko, S. B.; Fox, D. B.; Leonard, D. C.; Moon, D.-S.; Sand, D. J.;Soderberg, A. M.; Anderson, J. P.; James, P. A.; Foley, R. J.; Ganeshalingam, M.; Ofek, E. O.; Bildsten, L.; Nelemans, G.; Shen, K. J.; Weinberg, N. N.; Metzger, B. D.; Piro, A. L.; Quataert, E.; Kiewe, M.; Poznanski, D. (2010). "A faint type of supernova from a white dwarf with a helium-rich companion".Nature.465 (7296):322–325.arXiv:0906.2003.Bibcode:2010Natur.465..322P.doi:10.1038/nature09056.PMID20485429.S2CID4368207.
