Supernova Polarization and the Type IIn Classification
Supernova Polarization
and the Type IIn Classification
Jennifer L. Hoffman
Department of Astronomy, UC Berkeley, 601 Campbell Hall, Berkeley, CA 94720-3411
Abstract. While the members of the Type IIn category of supernovae are united by the presence
of strong multicomponent Balmer emission lines in their spectra, they are quite heterogeneous with
respect to other properties such as Balmer line profiles, light curves, strength of radio emission,
and intrinsic brightness. We are now beginning to see variety among SNe IIn in their polarimetric
characteristics as well, some but not all of which may be due to inclination angle effects. The
increasing number of known “hybrid” SNe with IIn-like emission lines suggests that circumstellar
material may be more common around all types of SNe than previously thought. Investigation of the
correlations between spectropolarimetric signatures and other IIn attributes will help us address the
question of classification of “interacting SNe” and the possibility of distinguishing different groups
within the diverse IIn subclass.
Keywords: supernovae, type IIn, supernova classification
PACS: 97.60.Bw
Type IIn (“narrow-line”) supernovae (SNe IIn) are Type II events whose primary
distinction is the existence of strong narrow hydrogen Balmer emission lines in the
spectra [1, 2]. These lines indicate the presence of circumstellar material ejected by the
progenitor star and excited by the UV and X-ray photons from the supernova explosion.
The IIn subclass includes 2–5% of all Type II supernovae [3].
Turatto [4] categorized the SNe IIn as a special class of core-collapse supernovae
related to the SNe Ib/c and hypernovae. However, in recent years several new discoveries
have blurred the boundaries between the IIn category and other types of supernovae. It
should be noted that since the label of IIn is assigned based on spectral characteristics,
while that of II-L or II-P is based on features of the light curve, these three categories are
not mutually exclusive, and some objects may be given more than one classification in
the literature. But observations of “hybrid” supernovae such as SN 2002ic [5, 6] and
SN 2005gj [7], which showed IIn-like Hα lines superposed on type Ia-like spectra,
and of “chameleon” supernovae such as SN 2001em [8, 9], which evolved from a
Type Ic to a Type IIn over the span of a few years, suggest that not only SNe II
but potentially all supernovae may show signatures of interaction with circumstellar
material. Consequently, Turatto’s revised classification scheme [10] includes a broad
category of “interacting SNe” that spans all SN types and includes the SNe IIn.
The IIn category has always been heterogeneous, as noted by Filippenko [2], whose
Figure 14 presented a non-coeval collection of SN IIn spectra. Figure 1 shows that even
when compared at similar ages, SNe IIn have quite diverse spectral characteristics. The
primary feature of SNe IIn spectra, the strong Hα emission line, also varies substantially
in strength and profile between objects of comparable age (Figure 2) and even for a given
object over time. In particular, the very narrow (FWHM < 200 km/s) component of Hα
http://arxiv.org/abs/0704.0266v2
FIGURE 1: Deredshifted optical
spectra of four Type IIn super-
novae at similar epochs, showing
the diversity of spectral character-
istics within the subtype. All dates
are post-discovery. Detailed analy-
ses of SN 1997eg and SN 2000P
have been conducted by [11] and
[12], respectively; the other data
are presented courtesy of A. Fil-
ippenko (priv. comm.). For the
unpublished spectra, redshift cor-
rections are based on information
in the Asiago Supernova Catalog
[13]. Hydrogen Balmer lines are
labeled.
does not necessarily exist at all times in the evolution of a SN IIn. In SN 1997eg (Figure
3; [11]), this narrow Hα component disappeared after 100 days post-discovery, was
replaced by a small symmetric absorption feature, and then reappeared around day 400.
The Hα line in SN 1998S ([12]; their Figure 7) also lost its narrow emission component
early on, but was very asymmetric and changed much more dramatically over time.
Because of interaction with circumstellar material, the light curves of SNe IIn often
decline quite slowly in comparison with other core-collapse objects, but not all members
of the subclass show this slow decline [2]. The canonical IIn SN 1988Z decreased in
brightness by only 5 magnitudes in 1000 days [14], but others decline more quickly
(e.g., SN 1998S; [15]). SN 1994W faded by ∼6 magnitudes in B in only 100 days [16].
Circumstellar interaction can also cause some SNe IIn to become strong radio and
X-ray emitters; SN 1988Z and SN 1986J are among the brightest radio supernovae
ever observed [17]. Nearly all the SNe IIn detected in X-rays have also been strong
radio sources [18, 19]. However, not all SNe IIn become radio-loud [18], and there
is considerable heterogeneity even among the “radio-quiet” subset [2]. The overlap
between the IIn, II-P, and II-L categories makes it difficult to identify trends in radio
brightness with core-collapse subtype.
FIGURE 2. As in Fig. 1, but for the Hα region of the spectrum.
FIGURE 3. Time evolution of the multi-component Hα profile of SN 1997eg [11]. Spectra are normal-
ized to day 16 at v =−10,000 km s−1.
FIGURE 4. Polarized flux spectra (percent polarization multiplied by flux) for SN 1998S [12] and SN
1997eg [11], binned to 10 Angstroms in each case. Dashed lines show total flux spectra for comparison,
scaled by 0.037 and 0.036, respectively. Polarization data are corrected for estimated interstellar polariza-
tion (ISP) effects; these estimates are somewhat uncertain in each case, but a simple correction for ISP
could not account for all the differences between these two polarized flux spectra.
Finally, SNe IIn show considerable variety in their spectropolarimetric characteristics
(Figure 4). Supernovae of all types are known to be polarized due to intrinsic asphericity
of the ejecta. The circumstellar material surrounding SNe IIn can produce its own po-
larization signature in addition to that arising from the ejecta. Some of the polarization
variations between SNe IIn are likely due to inclination; that is, they may have similar
geometrical distributions of circumstellar material, but have different polarization sig-
natures due to different viewing angles. To study this effect, I am constructing a grid of
Monte Carlo radiative transfer models of the Hα line polarization produced by circum-
stellar matter distributions with various geometries and seen at various viewing angles
[20]. Preliminary results suggest that differences in viewing angle may account for some
of the polarimetric variety in SNe IIn. However, such geometric effects cannot explain
all of the IIn diversity; for example, the transformation of SN 2001em from a Ic into a
IIn [8, 9] is very unlikely to be due to a sudden change in inclination!
These examples show that the IIn classification is currently something of a “catchall”
for a number of related but distinct objects, all with close connections to Turatto’s
[10] new category of “interacting SNe”. Now that circumstellar interaction has been
recognized to occur for a broad range of supernova types, and now that high-quality
time-dependent data are more readily available, we are in a good position to reconsider
the categorization of SNe IIn and other interacting supernovae. An initiative underway
within the Berkeley supernova group will quantify the spectral and spectropolarimetric
characteristics of SNe IIn and search for correlations between these features and other
properties such as light curve shape and radio/X-ray behavior. If found, such correlations
will illuminate the relationships between diverse members of the IIn category and
perhaps ultimately argue for the category’s subdivision.
In the future, full understanding and correct classification of these important objects
will depend critically on obtaining a broad range of observational information, including
multiwavelength and polarimetric data, with as much time coverage as possible. In
addition, supernova researchers should expand their collaborations with the evolved star
community. If the circumstellar media around SNe IIn and related objects are indeed
created by winds and outflows from massive stars, we can use nearby evolved star
nebulae as analogous cases to the circumstellar envelopes of supernovae. Such an effort
could greatly improve our understanding of supernova progenitors and help shed light
on the physical causes of the diversity we observe among interacting supernovae.
ACKNOWLEDGMENTS
This research was funded by an NSF Astronomy & Astrophysics Postdoctoral Fellow-
ship, AST-0302123; by NSF grant AST-0607485 to A. V. Filippenko; and by the Na-
tional Energy Research Scientific Computing Center, US DOE Contract #DE-AC03-
76SF00098. I thank Alex Filippenko at UC Berkeley and Peter Nugent at the Lawrence
Berkeley Laboratory for their support and invaluable contributions.
REFERENCES
1. E. M. Schlegel, MNRAS 244, 269 (1990).
2. A. V. Filippenko, ARAA 35, 309 (1997).
3. E. Cappellaro, M. Turatto, D. Y. Tsvetkov, et al., A&A 322, 431 (1997).
4. M. Turatto, S. Benetti, and E. Cappellaro, “Variety in Supernovae,” in From Twilight to Highlight:
The Physics of Supernovae, Springer, Berlin, 2003, p. 200.
5. J. Deng, K. S. Kawabata, Y. Ohyama, et al., ApJ 605, L37 (2004).
6. M. Hamuy, M. M. Phillips, N. B. Suntzeff, et al., Nature 424, 651 (2003).
7. G. Aldering, P. Antilogus, S. Bailey, et al., ApJ 650, 510 (2006).
8. A. M. Soderberg, A. Gal-Yam, and S. R. Kulkarni, GCN 2586, 1 (2004).
9. N. N. Chugai, and R. A. Chevalier, ApJ 641, 1051–1059 (2006).
10. M. Turatto, “Classification of Supernovae,” in This Volume, AIP, New York, 2007.
11. J. L. Hoffman, D. C. Leonard, R. Chornock, et al. (2007), in prep.
12. D. C. Leonard, A. V. Filippenko, A. J. Barth, et al., ApJ 536, 239 (2000).
13. R. Barbon, V. Buondí, E. Cappellaro, et al., A&AS 139, 531 (1999).
14. M. Turatto, E. Cappellaro, I. J. Danziger, et al., MNRAS 262, 128 (1993).
15. A. Fassia, W. P. S. Meikle, W. D. Vacca, et al., MNRAS 318, 1093 (2000).
16. R. J. Cumming, and P. Lundqvist, “Supernova Progenitor Constraints from Circumstellar Interaction:
Type II,” in Advances in Stellar Evolution, Cambridge University Press, Cambridge, 1997, p. 297.
17. R. A. Chevalier, C. Fransson, and T. K. Nymark, ApJ 641, 1029 (2006).
18. S. D. van Dyk, M. Hamuy, and A. V. Filippenko, AJ 111, 2017 (1996).
19. D. Pooley, W. H. G. Lewin, D. W. Fox, et al., ApJ 572, 932 (2002).
20. J. L. Hoffman, “Polarized Line Profiles as Diagnostics of Circumstellar Geometry in Type IIn
Supernovae,” in Circumstellar Media and Late Stages of Massive Stellar Evolution, Rev. Mex. AA
Ser. Conf., 2007, in press.
