Cas A Discussion

[jlaming@ssd5.nrl.navy.mil]

Dear All,

I've enjoyed reading the initial thoughts from various people on the Cas A
VLP. Here are some partial answers to points raised so far.

Atomic data needs? Try the Flexible Atomic Code available at
http://kipac-tree.stanford.ed/fac/. fac-1_0_5 seems to be the most recent
version. This can produce almost anything we will need in the distorted
wave approximation, and I suspect the anyone is really serious about x-ray
spectroscopy with Chandra, XMM or Astro-E2 will have to get to grips with
it sooner or later as and when the models in XSPEC are found to be
inadequate. It's freely available and easy to install, and we should all
thank Ming Fen Gu for it.

Electron-ion equilibration? A systematic study of T_e/T_i behind the
forward shock is crying out to be done, especially when you realize that
constraints on the magnetic field can come from the width of the
synchrotron emission "rim" to go with it (see Vink & Laming 202, ApJ, 584,
758). I want to try modeling this not just by transferring a certain
fraction of shock energy to the electrons at the shock by "magic" (i.e.
unspecified plasma processes), but by increasing the electron-ion
equilibration rate within a short distance of the shock front by an amount
appropriate to collisionless energy transfer by various means. I have
tried this for the fast solar wind (see my ApJ paper in the April 1 2004
issue), and with some modification a similar treatment could be applied to
the much stronger instabilities at SNR shocks. A bigger observational
challenge will be to try and identify the reverse shock and make similar
measurements there.

Small presentations for each item? That was our idea too. We'll probably
start the first morning off with a few short talks on the current status
of Cas A and other relevant SNR physics to bring everyone up to speed on
where things stand, before launching into more detailed discussion. Any
volunteers to make a presentation (10-15 mins, probably with plenty of
audience participation) on a relevant topic will be welcome, before we
start "volunteering" people ourselves.

Core collapse models and jets? There are some experts in these sorts of
models on this mailing list, and hopefully we can devote some of the time
at NRL to talking more about this. I have been thinking that a first
attempt to model the jet knots with Truelove/McKee models should probably
use an ejecta profile with outer envelope power law n<3, so that there is
no ejecta "core". Following talks and discussions at a meeting in Tucson a
few weeks ago, it seems there that ought to be Fe in the jet, which of
course we don't see. An interesting thought from Craig Wheeler was that
the Fe came out at the head of the jet and has since become thermally
unstable and cooled so we don't see it, but that material that followed it
out (Si and S) is still visible. Might this be because Fe collapses first
to the jet acceleration region and Si and S get there after the Fe, due to
starting further out? (A "first-in, first-out" model of the jet,
qualitatively different to R-T overturn which puts Fe ahead of Si
elsewhere in the remnant?). Clearly someone ought to take a long hard look
at the jet knots and see whether they can find any Fe.

That’s about it for now. Let's look forward to more ideas and seeing
everyone about a month from now at NRL.

Martin

Martin Laming
Naval Research Laboratory, Code 7674L
Washington DC 20375
tel: 202 767 4415
fax: 202 404 7997
jlaming@ssd5.nrl.navy.mil