Subject: Comments from Nick Suntzeff on SN discussion

From: Michael Strauss

Submitted: Sun, 30 Jan 2005 17:20:30 -0500

Message number: 296 (previous: 295, next: 297 up: Index)

    Supernovae:
>
>    The first thing I learned from the discussion was that spectroscopy
>  is much less important for SN physics than I had previously thought.
>  Spectroscopic features come from the SN photosphere, which is due to
>  the material in the outer "8 teaspoons" (in Phil Pinto's memorable
>  phrase) of the exploding star, while the full light curve is due to
>  the physics of the star as a whole.  LST will of course be a light

An absorption line is seen as a very broad feature representing a
large range in velocity. The line actually forms over a very large
range in radius. Early on (well before maximum light) the line forming
region is thinner. There are a number of Type Ia's now where we see
banding at the CaII triplet, presumably due to layering in the
proto-SN.

By the time LSST is a reality, we will probably be very dissatified
with a one parameter correction to luminosity distance. A second
important parameter will (maybe) be the KE of the photosphere or its
polarization. The latter will be very difficult to measure in a large
number of objects. But the KE is easy, just the line velocities. We
have no relationships yet, but I bet we will have them in the future.

My hope is that with a large number of SNe from LSST, we will not need
spectroscopy. We will be able to get the redshift and the luminosity
corrections from precision photometry. But how precise is needed? And
how many filters? Do we want to argue that we should discard the SDSS
filter system and create our own filter system? I see little advantage
to adopting SDSS at this point. It is poorly matched to K-corrections,
with the g filter being way too wide. By going to log-lambda sets, we
can reduce the K-correction errors to the systematic error in the Vega
spectrophotometric scales at "magic" redshifts. Is this an important
science driver?

To me, the key is still - how precise does the photometry have to be?
I see no reason why the LSST will not produce natural system
photometry to 0.005mag accuracy and maybe even better - 0.002mag. We,
as astronomers, are stuck on 0.02mag errors and are not challenging
ourselves to do better. In 10 years, 0.02mag errors will look really
lame, as the photo-z people push on the photometry to extract more
precise redshifts. 

>  curve factory. (Spectroscopy of course remains important for
>  determining redshifts, and the adequacy of photometric redshifts for
>  cosmological studies has not yet been explored thoroughly).  As
>  discussed in the DRM document, even an etendue of 250-300 is not
>  adequate to get good cadence SN light curves in all filters at high
>  redshift as part of the routine survey; we proposed there a special
>  campaign of longer exposures to yield this (the routine survey will do
>  an adequate job for SN up to z~0.7).  In Pan-STARRS, the problem is
>  correspondingly worse, and they plan a focussed campaign on SN over a
>  limited area of sky, 1300 deg^2 (indeed, their design reference
>  mission refers, to a much greater extent than for LSST, to a whole
>  series of different observing modes).  Full-sky coverage for SN
>  doesn't have an obvious strong science driver (although perhaps
>  directional dependence in w would suggest otherwise; see below), and

Full-sky coverage is needed for any science dealing with local
peculiar velocities and convergence depth. I am not sure these will be 
highly topical in 10 years but they still are today.

>  the relationship between PS4 and LSST comes down to shear numbers of
>  objects.  Phil emphasized the importance in supernova physics of
>  finding new and unanticipated types of supernovae; here, the larger
>  one's survey (i.e., the larger the etendue), the larger one's
>  sensitivity to rare finds.  To use SN for cosmology, one wants to make
>  sure that there are enough objects in each bin of redshift and
>  supernova type to reach (and presumably explore) the systematics floor
>  (perhaps caused in part by the unusual types of SN mentioned above).
>  The problem, of course, is that we do not yet have a clear idea of
>  what that floor is, making it difficult to estimate a priori how many
>  SN with excellent light curves are "enough".
>
>    Phil Pinto will explore these ideas further for the SWG.
>

cheers nick


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