ASYMMETRIC ASHES: Astronomers Study Shape of Stellar Candles

Texas A&M University astrophysicist Dr. Lifan Wang and colleagues Drs. Dietrich Baade and Ferdinando Patat from the renowned European Southern Observatory (ESO) in Munich, Germany, are reporting remarkable new findings that help shed light on a decade-long debate about one kind of supernova, the explosion that marks a star’s final demise: Does the star die in a slow burn or with a fast bang?

After more than a decade spent observing Type Ia supernovae — extraordinarily bright explosions used as “standard candles” in measuring cosmological distances — Wang and his ESO collaborators have determined that the matter ejected by these explosions shows significant peripheral asymmetry but a nearly spherical interior, most likely implying that the powerful explosions of the star finally propagate at supersonic speed.

Their findings are reported today in Science Express, the online version of the research journal Science.

“Our results strongly suggest a two-stage explosion process in this type of supernova,” Wang explains. “This is an important finding with important implications in cosmology.”

Using observations of 17 supernovae made with the ESO’s Very Large Telescope and the McDonald Observatory’s Otto Struve Telescope, the astronomers inferred the shape and structure of the debris cloud thrown out from Type Ia supernovae, which are thought to be the result of the explosion of a small and dense star, a white dwarf, within a binary system. As its companion star continuously spills matter onto the white dwarf, the white dwarf reaches a critical mass, leading to a fatal instability and the supernova.

According to Wang, holder of the Mitchell-Heep-Munnerlyn Endowed Career Enhancement Professorship in Physics at Texas A&M, what sparks the initial explosion and how the blast travels through the star have long been thorny issues — ones with major repercussions in the ongoing investigation of dark energy and the mysterious role it plays in the Universe.

The supernovae Wang and his colleagues observed occurred in distant galaxies and, because of the vast cosmic distances, could not be studied in detail using conventional imaging techniques, including interferometry. Instead, the team determined the shape of the exploding cocoons by recording the polarization of the light from the dying stars.

Wang explains that polarimetry relies on the fact that light is composed of electromagnetic waves that oscillate in certain directions. Reflection, or scattering, of light favors certain orientations of the electric and magnetic fields over others. This is why polarizing sunglasses can filter out the glint of sunlight reflected off a pond. When light scatters through the expanding debris of a supernova, it retains information about the orientation of the scattering layers. If the supernova is spherically symmetric, all orientations will be present equally and will average out, resulting in no net polarization. If, however, the gas shell is not round, a slight net polarization will be imprinted on the light.

If additional spectral information is available (spectro-polarimetry), Wang notes that one can determine whether the asymmetry is in the continuum light or in some spectral lines. In the case of the Type Ia supernovae, the team found that the continuum polarization is very small, resulting in the explosion’s crudely spherical overall shape. However, the much larger polarization in strongly blue-shifted spectral lines indicates the presence in the outer regions of fast-moving clumps with peculiar chemical composition.

“Our study reveals that explosions of Type Ia supernovae are really three-dimensional phenomena,” Baade notes. “The outer regions of the blast cloud are asymmetric, with different materials found in ‘clumps,’ while the inner regions are smooth.”

The researchers credit the discovery in large part to vast resources of the ESO, one of the world’s biggest and most prestigious observatories.

“This study was possible because polarimetry could unfold its full strength, thanks to the light-collecting power of the Very Large Telescope and the very precise calibration of the FORS [FOcal Reducer/low dispersion Spectrograph] instrument,” Baade adds.

Although the team first spotted this asymmetry in 2003, their new, more extensive results show that the degree of polarization — and, hence, the asphericity — correlates with the intrinsic brightness of the explosion. The brighter the supernova, the smoother, or less clumpy, it is.

“This has some impact on the use of Type Ia supernovae as standard candles,” Patat says. “This kind of supernova is used to measure the rate of acceleration of the expansion of the Universe, assuming these objects behave in a uniform way. But asymmetries can introduce dispersions in the quantities observed.”

“Our discovery puts strong constraints on any successful models of thermonuclear supernova explosions,” Wang adds.

Popular models of Type Ia supernovae have suggested that the clumpiness may be caused by a slow-burn process, called deflagration, which leaves an irregular trail of ashes. The smoothness of the inner regions of the exploding star implies that at a given stage, the deflagration gives way to a more violent process, a “detonation” that travels at supersonic speeds — so fast that it erases all the asymmetries in the central region of the ashes left behind by the slower burning of the first stage, resulting in a smoother, more homogeneous residue.

To learn more about Wang and his research, visit http://www.physics.tamu.edu/people/showpeople.php?name=Lifan%20Wang&userid=wang.

For more information on Baade and Patat or the ESO, visit http://www.eso.org/.

-aTm-

The results presented here are reported in “Spectropolarimetric diagnostics of thermonuclear explosions” by Lifan Wang, Dietrich Baade and Ferdinando Patat, Science Express, 30 November 2006.

Contacts: Dr. Lifan Wang, (979) 845-7717 or wang@physics.tamu.edu; Dr. Dietrich Baade, +49 89 3200 6388 or dbaade@eso.org; Dr. Ferdinando Patat, +49 89 3200 6388 6744 or fpatat@eso.org