Peeking at a Puzzling Supernova with Spectropolarimetry

By measuring polarized light from an unusual exploding
star, an international team of astrophysicists and astronomers has
worked out the first detailed picture of a Type Ia supernova and the
distinctive star system in which it exploded.

Using the European Southern Observatory’s Very Large Telescope in Chile,
the researchers determined that supernova 2002ic exploded inside a flat,
dense, clumpy disk of dust and gas, previously blown away from a
companion star. Their work suggests that this and some other precursors
of Type Ia supernovae resemble the objects known as protoplanetary
nebulae, well known in our own Milky Way galaxy.

Lifan Wang of Lawrence Berkeley National Laboratory, Dietrich Baade of
the European Southern Observatory (ESO), Peter Hoeflich and J. Craig
Wheeler of the University of Texas at Austin, Koji Kawabata of the
National Astronomical Observatory of Japan, and Ken’ichi Nomoto of the
University of Tokyo report their findings in the 20 March 2004 issue of
Astrophysical Journal Letters.

Casting supernovae to type:

Supernovae are labeled according to the elements visible in their
spectra: Type I spectra lack hydrogen lines, while Type II spectra have
these lines. What makes SN 2002ic unusual is that its spectrum otherwise
resembles a typical Type Ia supernova but exhibits a strong hydrogen
emission line.

Type II and some other supernovae occur when the cores of very massive
stars collapse and explode, leaving behind extremely dense neutron stars
or even black holes. Type Ia supernovae, however, explode by a very
different mechanism.

“A Type Ia supernova is a metallic fireball,” explains Berkeley Lab’s
Wang, a pioneer in the field of supernova spectropolarimetry. “A Type Ia
has no hydrogen or helium but lots of iron, plus radioactive nickel,
cobalt, and titanium, a little silicon, and a bit of carbon and oxygen.
So one of its progenitors must be an old star that has evolved to leave
behind a carbon-oxygen white dwarf. But carbon and oxygen, as nuclear
fuels, do not burn easily. How can a white dwarf explode?”

The most widely accepted Type Ia models assume that the white dwarf —
roughly the size of Earth but packing most of the mass of the sun —
accretes matter from an orbiting companion until it reaches 1.4 solar
masses, known as the Chandrasekhar limit. The now superdense white dwarf
ignites in a mighty thermonuclear explosion, leaving behind nothing but
stardust.

Other schemes include the merger of two white dwarfs or even a lone
white dwarf that re-accretes the matter shed by its younger self.
Despite three decades of searching, however, until the discovery and
subsequent spectropolarimetric studies of SN 2002ic, there was no firm
evidence for any model.

In November of 2002, Michael Wood-Vasey and his colleagues in the
Department of Energy’s Nearby Supernova Factory based at Berkeley Lab
reported the discovery of SN 2002ic, shortly after its explosion was
detected almost a billion light-years away in an anonymous galaxy in the
constellation Pisces.

In August of 2003, Mario Hamuy from the Carnegie Observatories and his
colleagues reported that the source of the copious hydrogen-rich gas in
SN 2002ic was most likely a so-called Asymptotic Giant Branch (AGB)
star, a star in the final phases of its life, with three to eight times
the mass of the sun — just the sort of star that, after it has blown
away its outer layers of hydrogen, helium, and dust, leaves behind a
white dwarf.

Moreover, this seemingly self-contradictory supernova — a Type Ia with
hydrogen — was in fact similar to other hydrogen-rich supernovae
previously designated Type IIn. This in turn suggested that, while Type
Ia supernovae are indeed remarkably similar, there may be wide
differences among their progenitors.

Because Type Ia supernovae are so similar and so bright — as bright or
brighter than whole galaxies — they have become the most important
astronomical standard candles for measuring cosmic distances and the
expansion of the universe. Early in 1998, after analyzing dozens of
observations of distant Type Ia supernovae, members of the Department of
Energy’s Supernova Cosmology Project based at Berkeley Lab, along with
their rivals in the High-Z Supernova Search Team based in Australia,
announced the astonishing discovery that the expansion of the universe
is accelerating.

Cosmologists subsequently determined that over two-thirds of the
universe consists of a mysterious something dubbed “dark energy,” which
stretches space and drives the accelerating expansion. But learning more
about dark energy will depend on careful study of many more distant Type
Ia supernovae, including a better knowledge of what kind of star systems
trigger them.

Picturing structure with spectropolarimetry:

The spectropolarimetry of SN 2002ic has provided the most detailed
picture of a Type Ia system yet. Polarimetry measures the orientation of
light waves; for example, Polaroid sunglasses “measure” horizontal
polarization when they block some of the light reflected from flat
surfaces. In an object like a cloud of dust or a stellar explosion,
however, light is not reflected from surfaces but scattered from
particles or from electrons.

If the dust cloud or explosion is spherical and uniformly smooth, all
orientations are equally represented and the net polarization is zero.
But if the object is not spherical — shaped like a disk or a cigar, for
example — more light will oscillate in some directions than in others.

Even for quite noticeable asymmetries, net polarization rarely exceeds
one percent. Thus it was a challenge for the ESO spectropolarimetry
instrument to measure faint SN 2002ic, even using the powerful Very
Large Telescope. It took several hours of observation on four different
nights to acquire the necessary high-quality polarimetry and
spectroscopy data.

The team’s observations came nearly a year after SN 2002ic was first
detected. The supernova had grown much fainter, yet its prominent
hydrogen emission line was six times brighter. With spectroscopy the
astronomers confirmed the observation of Hamuy and his associates, that
ejecta expanding outward from the explosion at high velocity had run
into surrounding thick, hydrogen-rich matter.

Only the new polarimetric studies, however, could reveal that most of
this matter was shaped as a thin disk. The polarization was likely due
to the interaction of high-speed ejecta from the explosion with the dust
particles and electrons in the slower-moving surrounding matter. Because
of the way the hydrogen line had brightened long after the supernova was
first observed, the astronomers deduced that the disk included dense
clumps and had been in place well before the white dwarf exploded.

“These startling results suggest that the progenitor of SN 2002ic was
remarkably similar to objects that are familiar to astronomers in our
own Milky Way, namely protoplanetary nebulae,” says Wang. Many of these
nebulae are the remnants of the blown-away outer shells of Asymptotic
Giant Branch stars. Such stars, if rotating rapidly, throw off thin,
irregular disks.

A matter of timing:

For a white dwarf to collect enough material to reach the Chandrasekhar
limit takes a million years or so. By contrast, an AGB star loses
copious amounts of matter relatively quickly; the protoplanetary-nebula
phase is transitory, lasting only a few hundreds or thousands of years
before the blown-off matter dissipates. “It’s a small window,” says
Wang, not a long enough time for the leftover core (itself a white
dwarf) to re-accrete enough material to explode.

Thus it’s more likely that a white dwarf companion in the SN 2002ic
system was already busily collecting matter long before the nebula
formed. Because the protoplanetary phase lasts only a few hundred years,
and assuming a Type Ia supernova typically takes a million years to
evolve, only about a thousandth of all Type Ia supernovae are expected
to resemble SN 2002ic. Fewer still will exhibit its specific spectral
and polarimetric features, although “it would be extremely interesting
to search for other Type Ia supernovae with circumstellar matter,” Wang
says.

Nevertheless, says Dietrich Baade, principal investigator of the
polarimetry project that used the VLT, “it’s the assumption that all
Type Ia supernovae are basically the same that permits the observations
of SN 2002ic to be explained.”

Binary systems with different orbital characteristics and different
kinds of companions at different stages of stellar evolution can still
give rise to similar explosions, through the accretion model. Notes
Baade, “The seemingly peculiar case of SN 2002ic provides strong
evidence that these objects are in fact very much alike, as the stunning
similarity of their light curves suggests.”

By showing the distribution of the gas and dust, spectropolarimetry has
demonstrated why Type Ia supernovae are so much alike even though the
masses, ages, evolutionary states, and orbits of their precursor systems
may differ so widely.

“On the hydrogen emission from the Type Ia supernova 2002ic,” by Lifan
Wang, Dietrich Baade, Peter Hoeflich, J. Craig Wheeler, Koji Kawabata,
and Ken’ichi Nomoto, appears in the 20 March 2004 issue of Astrophysical
Journal Letters (vol 604, no 1, part 2, p L53).

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