Subaru light-echo spectroscopy of historic Galactic Supernovae:
Revealing the nature of Tycho Brahe's SN 1572 and Cassiopeia A

Oliver Krause (Max-Planck-Institut for Astronomy, Heidelberg)
Masaomi Tanaka (IPMU; University of Tokyo)
Tomonori Usuda (Subaru Telescope, National Astronomical Observatory of Japan),
Takashi Hattori (Subaru Telescope, NAOJ),
Miwa Goto (MPIA),
Stephan Birkmann (European Space Agency)
Ken'ichi Nomoto (IPMU; University of Tokyo)


Young Galactic supernova remnants such as Cassiopeia A and Tycho are unique laboratories 
for supernova physics. Because of their proximity they provide the most detailed view on
the outcome of a supernova. However, the exact types of their original explosions
have been unknown so far - hindering to fully relate the wealth of knowledge about their
remnants to the diverse population of supernovae.

By observing light-echoes with the Subaru telescope we have now secured the missing precise
spectroscopic classification for two of the most important historic supernovae - Tycho Brahe's
SN 1572 and the Cassiopeia A supernova. A light echo is generated when the brilliant wave
of light from a supernova propagates into space and is scattered off dust particles in
interstellar clouds. The time lag of these echoes makes it possible to still study the same
light witnessed on Earth in the 16th and 17th century - shortly before the invention of the
telescope- with the powerful scientific instruments of the 21st century available at Subaru.

We found that SN 1572 was a normal type Ia SN and is the closest confirmed example of a
Ia cosmological standard candle. The presence of a high-velocity Ca II feature suggests
some asphericity for the SN 1572 explosion and provides a new constraint to the models.
We have identified Cassiopeia A as originating from a rare IIb supernova. This puts an
important new constraint on conclusions based on Cas A as prototype of a core-collapse supernova.

Subaru observations of light echoes at different position angles from the SN will provide a
direct three-dimensional view of the explosion - an unprecedented opportunity which is
impossible with distant supernovae in galaxies outside the Milky Way.

References:  Krause et al. 2008a, Science 320, 1195
             Krause et al. 2008b, Nature 456, 617


We would be very happy to present these results during a talk.