Supernovae are NOT spherical!

Tomography by the Subaru and VLT Telescopes

An international collaboration of researchers including the Max-Planck-Institute for Astrophysics (MPA) has uncovered geometric property of core-collapse supernovae. The team, led by Keiichi Maeda (a former postdoctoral researcher at MPA and currently an assistant professor at the Institute for the Physics and Mathematics of the Universe; IPMU) and Paolo A. Mazzali (MPA, Italian National Institute for Astrophysics; INAF), used the Subaru and VLT Telescopes to discover that "supernovae are NOT spherical". The result sheds light on actively debated unsolved topics in astrophysics, namely, the explosion mechanisms of supernovae and gamma-ray bursts.

Fig 1,2: If the explosion is more or less spherical, the oxygen-rich core of a supernova progenitor star is ejected as a shell, which results in a single-peaked oxygen emission line, irrespective the viewing direction.
On the other hand, if the explosion is aspherical and bipolar, the oxygen-rich materials are ejected mostly along the equatorial direction as a torus (Fig. 1). The oxygen emission line profile is then predicted to be single-peaked for an on-axis observer (Fig. 2, left), but double-peaked for an observer looking at the supernova sideways (Fig. 2, right). This stems from different Doppler shift - longer/shorter wavelengths detected for materials moving away from/toward the observer.

Fig. 3: Images of supernovae at the late-phase observed by the Subaru telescope. A few supernovae were also observed with the VLT. Spectroscopy was performed for these objects and for others not shown here.

Massive stars (more than 10 times the Sun) end their lives with a bang. When their inner core collapses under its own gravity, energy is released and the outer part of the star explodes as a Supernova. Astronomers have not yet identified the process which turns the collapse into the explosion. Various possibilities have been proposed from the theoretical side (this is one of the main research topics at MPA): It might be essential to break spherical symmetry, by either hydrodynamic instability or stellar rotation, possibly via a magnetic field. These scenarios should produce bipolar explosions.

Supernovae are usually discovered in external galaxies, and are too distant for direct imaging of their geometry. Keiichi Maeda and Paolo Mazzali worked on theoretical predictions which connect the geometry of a supernova with an oxygen emission line profile in observed spectra. They realized that "it is possible to derive the geometric property of supernova explosions, by performing spectroscopy of a supernova at late phases, more than about 200 days after the explosion" (Figs. 1, 2). They proceeded to collect a large sample of data in order to investigate the general properties of SN explosions. In particular, they concentrated on Type Ibc SNe, which are those explosions that disrupt massive stars that have lost their outer hydrogen and helium shells before their core collapses. This offers a more direct view of the deepest parts of the star, nearer the site of collapse. Also, these SN are particularly interesting because some of them, those with the highest energy, are sometimes connected to Gamma-Ray Bursts (GRB).

The research team collected late-time spectra of 15 Type Ibc supernovae using the 8m Subaru (operated by National Astronomical Observatory of Japan) and VLT (European Southern Observatory) telescopes (Fig. 3). This effort was soon to prove fruitful: one of their early results was that the oxygen line can indeed show different profiles: in the case of SN2003jd (Mazzali et al. 2003, Science) they argued that they viewed an event similar to the GRB-Supernova 1998bw, but which in this case was seen off-axis.

A large sample is essential in their investigation. While a variety of profiles were observed from different SNe, it is difficult to distinguish clearly between two cases - a spherical explosion and an aspherical explosion viewed from the polar direction - for a single object. With more than 10 supernovae, however, it becomes possible to derive the detailed, generic geometry, since the random distribution of viewing angles allows the uncertainty to be removed using a statistic argument. They found 5 supernovae showing a clear signature of an aspherical explosion viewed sideways, like SN2003jd, and 4 with a marginal detection of such a feature. Considering that there should also have been cases of supernovae observed from the polar direction, the statistics indicate that "ALL supernovae from stripped stars are aspherical".

This is the first observational evidence that at least those supernovae that originate in the collapse of those massive stars that are deprived of their outer envelopes are generally aspherical. Looking at their data in detail, Maeda and collaborators find that "normal" Type Ibc supernovae are moderately aspherical, while GRB-SNe are intrinsically more aspherical, as may be expected since GRBs are thought to be highly beamed events.

The result supports recent theoretical scenarios of the supernova explosion, which suggest that an important role in the collapse is played by hydrodynamic instability, rotation, or magnetic fields. "On the other hand," says Maeda, "the fact that the deviation from spherical symmetry appears to be smaller in "normal" supernovae than in the extreme ones that are associated with GRBs suggests that the explosion mechanisms of the two groups are intrinsically different." "Our next stage is to look into more details of individual scenarios and compare those with the observations", argues Mazzali. "This is even more challenging than the present study, both in theory and observation, but we believe that it is within reach."

Keiichi Maeda, Paolo A. Mazzali

Original publication:

Keiichi Maeda, Koji Kawabata, Paolo A. Mazzali, et al.: Asphericity in Supernova Explosions from Late-Time Spectroscopy,
Science Express (online edition of Science), 31 January 2008