[vsnet-chat 2587] "Hot" Cool Carbon Stars

[JG <jgts@jgws.totalserve.co.uk>]

Further to VSNET-CHAT 2582 and 2579:

SAO 198860 [=FK Pup] and SAO 251015 are both flagged in the SAOJ2000 as
having their [F type] spectra taken from Henry Draper survey data: this
survey is nearly a century old now, was objective prism based, and was a
"one dimensional" classification scheme undertaken before some of the
subtleties of modern classification existed.  For example, most red
stars were simply classed as Ma or Mb.

Interestingly, although the above indicates they were surveyed as part
of the work for the Henry Draper catalogues, these stars appear to have
no HD number: presumably problems were noticed with the spectrum even
then.

"Hot" Cool Carbon Stars?

In A+AS 93, 151 (Fouque et al 1992), as noted in VS-Chat 2579, some IRAS
Point Source stars have their characteristics noted with respect to
three IR classification schemes (see that paper for details).  The new
red variable in Puppis, CGCS 1825, and R Vol being categorised as
LSR=45; VH=II; Val=c, which basically means that these stars show 11.3
micron SiC emission, occupy a region of the IRAS colour diagram normally
occupied by "blue" oxygen rich red variables, and have carbon rich
circumstellar dust envelopes.

Some oxygen rich stars can appear carbon rich due to a carbon rich
shell, apparently, hence appear to be carbon stars, but end up in zone
II of the IRAS colour diagram as they are actually oxygen stars.

HOWEVER, CGCS 1825 and R Vol definitely have carbon star spectra.  It
would therefore be more "normal" if they were in VH zone VII, more
suited to cool carbon stars.

Unfortunately the above paper deems SAO 198860 and SAO 251015 as having
IRAS colours of insufficient quality to rate a VH zone classification.
If anyone knows of such, it would be interesting. If a spectral survey
mistaking these two Carbon stars for type F can be used as
circumstantial evidence, it may be possible to include them with CGCS
1825 and R Vol.

It is also interesting to turn the situation upside down, and look in
table 2 of the above paper for carbon stars classified as LRS=4n;
VH=VII; Val=c (the more "likely" category), and those classified as
LRS=4n; VH=II; Val=c (the more "anomalous" category).  [where 'n' in
'LRS=4n' is a number between 1 and 9].

There are around 24 instances of the "likely" case (which includes
objects like R Lep and W Ori), and only 4 of the "anomalous" case,
_inclusive_ of the already mentioned CGCS 1825 and R Volantis!!!

The two other objects are IRAS_PSC 05418-3224 in Columba and IRAS_PSC
06230-0930=AFGL 935.
The former appears to have no visual counterpart known as yet (even in
the HST GSC), though CGCS 1045 lies at this position, whilst the latter
has been part of the GCVS since Namelist 73, and is V713 Mon (classed as
a Mira type LPV, but with only J Band variability known).

QUESTION:

So, whether it is possible or not to add SAO 198860 and SAO 251015 to
the above 4 to make 6, we still seem to have a subset of Carbon stars
with SiC (silicon carbide) emission at 11.3 micron and carbon rich
circumstellar dust envelopes that lie in a "hotter"/"bluer" than usual
position in an IRAS colour diagram.

All, bar R Vol, are barely known objects, difficult to observe visually,
and even non-existant on some survey plates (V713 Mon does not appear to
be in the HST GCS either), whilst many other carbon stars are well
known, and even have "early" discovery designations (eg R Lep, V Aql, W
Ori)

Are we on the edge of recognising a new subset of "hot/blue" carbon
stars?

Well, here's a little more circumstantial evidence.  Although occupying
a broad swath in the sky, these six stars all appear to lie pretty much
in a similar direction, and estimates for the bolometric distance of 5
of the 6 above mentioned objects lie in the range of 0.7 to 1.0 kpc
(with no known distance for V713 Mon, thus not precluding it).

Just some thoughts to chew on.  Southern Hemisphere observers interested
in taking up CGCS 1825 and SAOs 198860 and 251015 should consider adding
V713 Mon and the field of CGCS 1045 = IRAS_PSC 05418-3224 to their list.

[IMPORTANT NOTE: people are getting the CCCS and CGCS confused: these
are the first and second editions of Stephenson's carbon star
catalogues, respectively, and the numbering regimes _do not_ follow: for
instance the new red variable in Puppis is CGCS 1825 = CCCS 849 _and_
SAO 251015 is _CGCS 2792_ = CCCS 1705]


Cheers and clear skies [unlike the cloudy one that hid the lunar eclipse
this morning!]

John

John Greaves
UK

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