In the news this month... a possible new class of supernova
SN 2005E discovery image, discovered by the Lick Observatory Supernova Search CREDIT: LOSS
Most supernovae are classified as one of two different types of explosion: single massive short-lived stars that explode when their cores run out of fuel at the end of their lives and undergo gravitational collapse, and old evolved white dwarfs in binary systems which accrete hydrogen from a companion star before exploding catastrophically. Core collapse supernovae are generally seen only in regions of ongoing star formation since, by stellar standards, their supergiant progenitors do not live for very long. In contrast, type Ia supernovae in binary systems are produced by old, evolved stars and so are seen in all galaxy types, even those which show no signs of recent star formation. However, in the May 20th issue of the journal Nature, two groups of astronomers report stellar explosions with characteristics that do not fit into existing categories of supernovae, and come to very different conclusions about their progenitors.
The first event, SN 2005E, was observed in the edge-on spiral galaxy NGC 1032 in 2005 and was initially classified as a type Ib (core collapse) supernova based on the chemical elements detected in its optical spectrum soon after explosion. Located in the halo, rather than the disk of the galaxy, the surrounding environment is composed of an old stellar population with no recent star formation, an unlikely location for a core collapse supernova. While some of its properties show similarities to type Ia explosions, the lightcurve shows a much faster decline than is expected for the thermonuclear explosion of a white dwarf. The mass ejected in the explosion, less than a third of a solar mass, is also low for this class of supernova, and analysis of the spectra showed significant differences from what is expected from either explosion mechanism. This evidence lead Dr Perets' team to conclude that the progenitor was something unusual, likely to be a helium-rich low mass star, probably a helium-accreting white dwarf, making 2005E the first example of a new class of supernova.
However, 2005E is not the only supernova with these unusual characteristics; several other calcium-rich, subluminous supernovae, spectroscopically classified as type Ib/c events, have also been observed. One such event is SN 2005cz, reported by Professor Kawabata and colleagues in the same issue of Nature. While 2005cz shares many properties with 2005E, Kawabata's team reaches a different conclusion on the cause of the explosion. According to their study, supernovae in this class are more likely to originate via the core collapse mechanism, but from stars with masses at the lower end of the range of those that explode. Unlike 2005E, SN2005cz is located in an elliptical galaxy. These galaxies are generally made up of old stellar populations, but NGC4589 has a relatively young stellar population (for an elliptical galaxy) so the explosion of a star by core collapse is not ruled out.
Since most supernova searches are more likely to detect bright events, the number of faint 2005E-like events currently known is small. More sensitive surveys are planned, however, and these should result in many more examples and further insights into this non-standard class of supernova.
This blog post is a news story from the Jodcast, aired in the June 2010 edition.
Perets, H., Gal-Yam, A., Mazzali, P., Arnett, D., Kagan, D., Filippenko, A., Li, W., Arcavi, I., Cenko, S., Fox, D., Leonard, D., Moon, D., Sand, D., Soderberg, A., Anderson, J., James, P., Foley, R., Ganeshalingam, M., Ofek, E., Bildsten, L., Nelemans, G., Shen, K., Weinberg, N., Metzger, B., Piro, A., 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 DOI: 10.1038/nature09056
Kawabata, K., Maeda, K., Nomoto, K., Taubenberger, S., Tanaka, M., Deng, J., Pian, E., Hattori, T., & Itagaki, K. (2010). A massive star origin for an unusual helium-rich supernova in an elliptical galaxy Nature, 465 (7296), 326-328 DOI: 10.1038/nature09055