Astrophysicists have gradually been refining a system of classification for supernovae. The most basic characteristic of interest is whether or not there are spectral lines that indicate the presence of hydrogen. If these lines are absent, the supernova is Type I, which implies the progenitor star was rather old and had used up all of its hydrogen. If there are lines for hydrogen, the supernova is Type II. In this case, the supernova more likely was a very massive star which collapsed because nuclear fusion in its interior was no longer sufficient to support the weight.
Further subdivisions within each type are possible, especially in the Type I case. If there is a certain type of line from ionized silicon, the supernova is Type Ia. If there is no silicon line, but a line from helium, it’s Type Ib. Otherwise, it’s Type Ic. It is hoped that these subdivisions reflect characteristics of the progenitor that culminated in the supernova, but there’s still a lot of uncertainty about the mechanisms involved in various cases.
Type Ia supernovae are of particular interest, since observations of relatively nearby examples show that most events have a predictable intrinsic luminosity. This makes them very useful as “standard candles“. Most models of Type Ia supernova ascribe the event to hydrogen-depleted white dwarf stars that have exceeded the Chandrasekhar limit of about 1.38 solar masses because of accretion of matter from a companion star. It’s not clear how ofter this results from mergers between two white dwarfs, or simply from matter drawn from another star that survives the event, though the latter model has generally been preferred.
To date there have been no verified observations of progenitors that could clarify the situation, at least for the particular case at hand. However, detailed comparisons of the shape of the light curve after the explosion and the luminosity at maximum have made it possible to use Type Ia supernovae more accurately as standard candles. This is important, for example, in studies that use these supernovae to gauge the expansion rate of the universe as a function of time, in order to better understand dark energy.
Anomalous cases still remain, in which the luminosity curves or spectral data do not fit the usual patterns. For example, there are “subluminous” cases that have a curve or the normal width but subnormal luminosity. Often such cases occur in events at relatively large distances from an apparent host galaxy. Recently published research presents a case of a subluminous Type Ia supernova at the farthest distance yet observed from a possible host galaxy – in this case almost 500 thousand light-years distant, effectively in the middle of nowhere.
PTF10ops is a Type Ia supernova (SN Ia), whose lightcurve and spectral properties place it outside the current SN Ia subtype classifications. Its spectra display the characteristic lines of subluminous SNe Ia, but it has a normal-width lightcurve with a long rise-time, typical of normal luminosity SNe Ia. The early-time optical spectra of PTF10ops were modelled using a spectral fitting code and found to have all the lines typically seen in subluminous SNe Ia, without the need to invoke more uncommon elements. The host galaxy environment of PTF10ops is also unusual with no galaxy detected at the position of the SN down to an absolute limiting magnitude of r \geq -12.0 mag, but a very massive galaxy is present at a separation of ~148 kpc and at the same redshift as suggested by the SN spectral features. The progenitor of PTF10ops is most likely a very old star, possibly in a low metallicity environment, which affects its explosion mechanism and observational characteristics. PTF10ops does not easily fit into any of the current models of either subluminous or normal SN Ia progenitor channels.
The authors of the research paper offer reassurance that such anomalous cases won’t bias the results of using Type Ia supernovae in cosmological studies:
Understanding why these objects exist with normal stretch lightcurves but very different spectral properties to normal SNe Ia, may also help in our understanding of the bulk of the SN Ia population. These unusual events such as PTF10ops and SN 2006bt should not bias SN Ia cosmological samples as long as spectra are obtained to identify their subluminous features, regardless of their lightcurve properties.
|Maguire, K., Sullivan, M., Thomas, R., Nugent, P., Howell, D., Gal-Yam, A., Arcavi, I., Ben-Ami, S., Blake, S., Botyanszki, J., Buton, C., Cooke, J., Ellis, R., Hook, I., Kasliwal, M., Pan, Y., Pereira, R., Podsiadlowski, P., Sternberg, A., Suzuki, N., Xu, D., Yaron, O., Bloom, J., Cenko, S., Kulkarni, S., Law, N., Ofek, E., Poznanski, D., & Quimby, R. (2011). PTF10ops – a subluminous, normal-width light curve Type Ia supernova in the middle of nowhere Monthly Notices of the Royal Astronomical Society DOI: 10.1111/j.1365-2966.2011.19526.x|