2,838 research outputs found
Spectropolarimetry of Core-Collapse Supernovae
We briefly review the young field of spectropolarimetry of core-collapse
supernovae (SNe). Spectropolarimetry provides the only direct known probe of
early-time supernova (SN) geometry. The fundamental result is that asphericity
is a ubiquitous feature of young core-collapse SNe. However, the nature and
degree of the asphericity vary considerably. The best predictor of
core-collapse SN polarization seems to be the mass of the hydrogen envelope
that is intact at the time of the explosion: those SNe that arise from
progenitors with large, intact envelopes (e.g., Type II-plateau) have very low
polarization, while those that result from progenitors that have lost part (SN
IIb, SN IIn) or all (SN Ib) of their hydrogen (or even helium; SN Ic) layers
prior to the explosion tend to show substantial polarization. Thus, the deeper
we probe into core-collapse events, the greater the asphericity seems to be,
suggesting a fundamentally asymmetric explosion with the asymmetry damped by
the addition of envelope material.Comment: Invited review at Supernovae as Cosmological Lighthouses, 16-19 June,
Padua, IT. 6 page
Probing the Geometry of Supernovae with Spectropolarimetry
We present results from a spectropolarimetric survey of young supernovae
completed at the Keck Observatory, including at least one example from each of
the major supernova types: Ia (1997dt), Ib (1998T, 1997dq), Ib/c-pec (1997ef),
IIn (1997eg), and II-P (1997ds). All objects show evidence for intrinsic
polarization, suggesting that asphericity may be a common feature in young
supernova atmospheres.Comment: 4 pages, 2 figures. To appear in the Proceedings of the 10th Annual
October Astrophysics Conference in Maryland: Cosmic Explosion
A Spectropolarimetric Comparison of the Type II-Plateau Supernovae SN 2008bk and SN 2004dj
The Type II-Plateau supernova (SN II-P) SN 2004dj was the first SN II-P for
which spectropolarimetry data were obtained with fine temporal sampling before,
during, and after the fall off of the photometric plateau -- the point that
marks the transition from the photospheric to the nebular phase in SNe II-P.
Unpolarized during the plateau, SN 2004dj showed a dramatic spike in
polarization during the descent off of the plateau, and then exhibited a smooth
polarization decline over the next two hundred days. This behavior was
interpreted by Leonard et al. (2006) as evidence for a strongly non-spherical
explosion mechanism that had imprinted asphericity only in the innermost
ejecta. In this brief report, we compare nine similarly well-sampled epochs of
spectropolarimetry of the Type II-P SN 2008bk to those of SN 2004dj. In
contrast to SN 2004dj, SN 2008bk became polarized well before the end of the
plateau and also retained a nearly constant level of polarization through the
early nebular phase. Curiously, although the onset and persistence of
polarization differ between the two objects, the detailed spectropolarimetric
characteristics at the epochs of recorded maximum polarization for the two
objects are extremely similar, feature by feature. We briefly interpret the
data in light of non-Local-Thermodynamic Equilibrium, time-dependent
radiative-transfer simulations specifically crafted for SN II-P ejecta.Comment: 4 pages, 1 figure, to appear in AIP conference proceedings: Stellar
Polarimetry, From Birth to Death, eds. J. Hoffman, B. Whitney, and J.
Bjorkma
Evidence for Extremely High Dust Polarization Efficiency in NGC 3184
Recent studies have found the Type II-plateau supernova (SN) 1999gi to be
highly polarized (p_max = 5.8%, where p_max is the highest degree of
polarization measured in the optical bandpass; Leonard & Filippenko 2001) and
minimally reddened (E[B-V] = 0.21 +/- 0.09 mag; Leonard et al. 2002). From
multiple lines of evidence, including the convincing fit of a ``Serkowski''
interstellar polarization (ISP) curve to the continuum polarization shape, we
conclude that the bulk of the observed polarization is likely due to dust along
the line of sight (l-o-s), and is not intrinsic to SN 1999gi. We present new
spectropolarimetric observations of four distant Galactic stars close to the
l-o-s to SN 1999gi (two are within 0.02 degrees), and find that all are null to
within 0.2%, effectively eliminating Galactic dust as the cause of the high
polarization. The high ISP coupled with the low reddening implies an
extraordinarily high polarization efficiency for the dust along this l-o-s in
NGC 3184: ISP / E(B-V) = 31^{+22}_{-9} % mag^{-1}. This is inconsistent with
the empirical Galactic limit (ISP / E[B-V] < 9% mag^{-1}), and represents the
highest polarization efficiency yet confirmed for a single sight line in either
the Milky Way or an external galaxy.Comment: 27 pages, accepted for publication by the Astronomical Journa
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