Where are all the super-AGB stars?

I've been following with interest the progress that has been made at
understanding the fate of stars in a certain mass range, probably
within 7-10 Msun, which burn Carbon but never proceed farther. It is
apparent that at least some of these stars (except possibly at the
lowest metallicities, which no longer exist) will lose enough mass to
become an ONe WD (and many WDs in that mass range are known, all 1.1
Msun must be ONe, unless they later accreted additional mass), and at
high enough metallicities, essentially all.

There have been several supernovae attributed to such stars but no
unambiguous detections of the stars themselves. Their track before
carbon burning has no qualitative difference with those of lower or of
higher mass, so we must not look there. It is the later, mass-losing
phase that we must consider. Theoretically, these should be
distinguishable by their place in the HR diagram: they are redder than
any supergiant (the tip is ~M8, compared to the reddest SGs at M5),
but brighter than ordinary AGB stars - the tip of the AGB is ~30,000
Lsun or Mbol ~ -7, while a super AGB star can reach up to ~100,000
Lsun or Mbol ~ -8 (from memory, anyway - it's something like that).

However, it seems these thermally pulsing stars will be obscured by
dust. I'm not entirely sure why, given that lower-mass giants are not,
but that seems to be from what I've picked up. Is there any hope of
detection? One indirect method would be seeing a SN with a known
progenitor which was enshrouded in dust (which SGs are not), but that
very rarely happens.

Andrew Usher

In article ,
Andrew Usher writes:
... while a super AGB star can reach up to ~100,000
Lsun or Mbol ~ -8 (from memory, anyway - it's something like that).

However, it seems these thermally pulsing stars will be obscured by
dust. I'm not entirely sure why, given that lower-mass giants are not,

Any star that's losing mass will likely have dust condense in the
outflow. The exception would be if the outflow is low density and
very fast, so density becomes too low at the radius where dust would
be cool enough to condense. However, even novae and supernovae (some
of them, anyway) show dust condensation, though the solar wind, for
example, does not.

There are plenty of "extreme carbon stars" with luminosities around
10^5 solar. Think of CW Leo or LL Peg, for example. (Other names
that might be more familiar to some are IRC +10216 and AFGL 3068,
respectively.) One recent paper is by Speck et al. (2009 ApJ 691,
1202). References in it will probably lead you to the extensive
literature on the subject, or you could just put the phrase quoted
above into ADS and see what appears.

--
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Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA

On Jan 29, 3:44*pm, (Steve Willner) wrote:

Any star that's losing mass will likely have dust condense in the
outflow. *

Our Sun is losing mass. Obviously the rate matters!

There are plenty of "extreme carbon stars" with luminosities around
10^5 solar.

No, there aren't. According to the paper you cite, there are only
about 30 known - also it seems none are near that luminosity.

A paper I found, http://arxiv.org/PS_cache/arxiv/pdf/...908.3087v1.pdf
, says that no carbon stars exceed about 35,000 Lsun; therefore there
is an upper as well as a lower limit to the mass at which AGBs become
C stars.
It also gives one super-AGB candidate (Mbol = -8.0); it is dust-
enshrouded.

*One recent paper is by Speck et al. (2009 ApJ 691,
1202). *References in it will probably lead you to the extensive
literature on the subject, or you could just put the phrase quoted
above into ADS and see what appears.

That paper says that the 'extreme carbon star' phase is very short-
lived. Anyway, I think I've found it; the models of Poelarends (http://
http://www.iop.org/EJ/article/0004-6...3-54fa94eb3701)
show that the superwind phase (where the star will be obscured), which
is still fairly short at 5 Msun compared to the whole TP-AGB, grows to
encompass almost all of it by the limit. So optically visible late-
SAGB stars should indeed be rare - but not unknown.

Andrew Usher