Supernova Taxonomy

First published 1996 October 17; last updated 2002 February 12 by M. Montes.

A Supernovae Taxonomy Flow Chart

Description of the Flow Chart

The observational classes are in sharp-cornered boxes. Theoretical interpretations (i.e. possible progenitors) are listed in the boxes with rounded corners. Examples are listed underneath the observational classes. It should be noted that SN 1987A had a fairly odd behavior (for fairly well understood reasons) and is certainly not a prototypical SN IIP. It was rather sub-luminous, and while it may represent a certain population of sub-luminous SN II, we will probably not detect too many members of this population precisely because they are sub-luminous. SN 1987A may be better classified as an SN IIpec.

A theoretical interpretation of observations has driven the organization of this plot. At the left side, there is little or no hydrogen present in the ejecta; as one moves right, there is increasing evidence for hydrogen. SN IIP have a plateau in their post-maximum light curve, while SN IIL do not (Barbon et al 1979). SN IIL and IIP may differ in the amount of hydrogen present in their envelope (see Popov 1993); they may also differ spectroscopically (Schlegel 1996). I am not familiar with the light curves (as opposed to spectra) of SN IIn in general. In particular, the light curve of SN 1988Z is several magnitudes brighter (slower decline) than either IIP or IIL light curves at very late times (Stathakis & Sadler 1991; Aretxaga et al. 1999).

Current Classification Scheme

The current classification scheme has these broad divisions: SN I show no hydrogen lines in their early spectra; SN II do show hydrogen in their early spectra. Subclasses determined by spectral evidence are denoted by lower-case letters, Ia, Ib, Ic, IIb (Woosley et al. 1987; Filippenko 1988), and IIn (Schlegel 1990) . Subclasses determined by certain properties of the (usually) B or V light curves are denoted by upper-case letters, such as IIP and IIL (Barbon et al 1979; Doggett & Branch 1985). There is some suggestion (Schlegel 1996) that there are particular spectroscopic differences between SNe IIL and IIP; currently these SNe are most easily identified by their light curves.

Host Galaxies

SN Ia have been observed to occur in all types of galaxies. SNe Ib, Ic, and II have only been observed in various spirals, barred spirals, and irregular galaxies. In a recent paper, van den Bergh, Li, & Filippenko (2003) find that five SNe of types SNe Ib/c and II observed in their sample occurred in bright E or S0 galaxies, but please read their discussion carefully. In any case, they find no more 3% of SNe Ib/c and II occur in S/E0. Relevant graphs by other researchers are presented at the International Supernova Network.

Radio Emission

Radio emission has been observed for some supernovae in SN types Ib, Ic, IIb, IIn, IIL, and now IIP. Radio emission is an indicator of the presence of circumstellar medium (CSM). The SN IIpec 1987A was observed in the radio, but it probably would not have been observed at the distance of a few megaparsecs. Radio emission has not been observed from SNe Ia, but certain models of SN Ia propose radio emission at some level before maxima. The sole IIP observed near the time of its explosion in the radio is SN 1999em (Lacey et al. 1999); additionally the SN IIP 1923A has recently been detected in the radio (Eck et al. 1998). It is possible that SNe IIP are not usually observed in the radio since their usually tenuous CSM imply much weaker radio emission than we are able to detect with typical observations.

Distance estimates, observed B or V magnitudes, SN type, and age since optical maximum are used to help decide whether or not (and when) to observe SN in the radio. A few supernovae (SNe 1978K, 1981K, 1986J) have been serendipitously discovered in the radio before they were subsequently observed at other wavelengths.

A list of supernovae that have been observed (both detections and non-detections) in the radio may be found here.

X-Ray Emission

X-Ray emission is another indicator of the presence of reasonably dense circumstellar medium, and has been observed for a small number of supernovae at different ages. Many SN IIn (SNe 1988Z, 1995N, 1994W, ...), some IIL (1980K, 1979C), all the recent nearby objects ( 1987A, 1993J, and 1994I), and a IIP (1999em) have been at least detected in x-rays. To my knowledge, no SNe Ia have been detected in x-rays.

Frequently (but not always) radio detection is used to determine whether or not to observe at x-ray wavelengths.

A list of supernovae detected in X-Rays may be found here.

Photometry

Poznanski et al. (2002) have proposed a classification method based on photometry when spectroscopy is not available. The capabilities and limitations of this method are described in their article (see the references section below).

Supernova Progenitors

Supernova Types II, Ib, & Ic
All observations taken together imply that SNe II, Ib, and Ic arise from stars that are young, and were initially very massive. These supernovae are believed to form when their core collapses, that is, when nuclear burning cannot produce any more energy. Spectra and numerical models of SNe Ib & SNe Ic imply they have have lost much or all of their hydrogen envelopes; SNe Ic may also have lost much of their helium envelopes (see, for example, Nomoto et al. 1995; Nomoto et al. 1996; Filippenko 1997). Models and spectra of SNe IIn seem to imply the shock and ejecta of the supernova are interacting with a very dense circumstellar medium (see, for example, Cumming & Lundqvist 1997; Filippenko 1997).
Type Ia Supernovae
SNe Ia are believed to arise from accretion onto a white dwarf (a very old, dense, relatively low mass star that is not undergoing thermonuclear reactions in its core), although the exact population has yet to be identified. In this case, a detonation (supersonic burning front) or a deflagration (subsonic burning front) [I don't think the models are clear on this point yet] of the star occurs when it has accreted enough matter (the exact amount depends on the particular situation). Links to more detailed explanations may be found in the tutorial section of my Supernova and Supernova Remnants pages.

What is NOT Included in the Taxonomy Chart

I have not included SN IIn or SN IIpec (peculiar) supernovae. I made this chart awhile back, and cannot remember why I did not include them. Perhaps they will be included in a later version of this chart.

SN IIpec was proposed by Doggett & Branch (1985) to be a catch-all for all of Zwicky's SN III, SN IV, and SN V. I believe that the current class SN IIpec contains those SN II that are not SN IIL, IIP, IIb, or IIn.

SN IIn were introduced in Schlegel (1990). Members of the class typically have absent (or weak) H-alpha absorption, and narrow H-alpha emission on a broad base. Several members of this class have been observed in radio and X-Ray. Some recent ideas on the diversity of SN IIn are presented in Cumming & Lundqvist (1997). Some supernova classified as Zwicky's SN III, IV, and V were probably IIn. Some IIn (SN 1986J, SN 1978K) have been called SN V in the past.

For the curious, SN 1961I was listed as an SN III; SN 1961F is the prototype of Zwicky's SN IV; and SN 1961V was for quite some time the sole member of Zwicky's SN V (Zwicky 1964). (For a more recent discussion SN V, see Ball 1993.) SN V are a weird case, and many believe that SN 1961V was actually the outburst of a Luminous Blue Variable. For the latest on SN 1961V, see Stockdale et al (2001) and Filippenko et al (1995). SN III and SN IV have been reclassified as various SN IIpec (Doggett & Branch, 1985; Patat et al, 1993).

The Chart

This chart is also available as an EPS file at taxonomy.sne.ps.

Supernovae Taxonomy

This chart is also available as an EPS file at taxonomy.sne.ps.

Published References

These are more or less recent references. I hope I am not slighting anyone by leaving out even other references. Please consult the references of these references for more information.

Other Online Resources

Last updated 2002 February 12.
M. Montes
montes@rsd.nrl.navy.mil