[vsnet-chat 4749] Re: Upsilon Sagittarii status

[Thom Gandet <tgandet@mindspring.com>]

Dear Sebastian,

   Photometric types are often assigned purely on the photometric 
behaviour, often because the spectral type or behaviour are unknown.
There is some overlap in assigning photometric types, but the
H-deficient B-stars behave differently from the RCB stars - while
there are some similarities, there is no good reason to assign the
H-deficient B stars to the RCB class.

   Gamma Herculis has been classified as A9III and as F0III by
different observers.  It is splitting hairs as to what exactly is a
"yellow" star, but I believe an F0 star could be considered "yellow".
(The U-B color must also be used when interpreting spectral types
and colors.)

   I'm sorry if I didn't understand you, but I'm not sure which
stars you believe should now be classified as SR.  V4152 Sgr's SR
photometric type is SR:, so it's not certain if it's an SR type - 
it could be a Mira variable.  Its spectral type is R, which is
a late-G to early-K star with increased Carbon and Oxygen - 
Hydrogen and He are normal (or at least that's what I recall from
15 years ago! <G>).

   The R stars (usually called C stars now) are very different
spectroscopically from RCB stars.  A variable C-star belongs in
one of the SR subclasses on the basis of the H and He abundances.

   If you meant that semi-regular blue variables should have a
new SR subtype, there are already photometric types for them,
e.g. LBV and the various subtypes of the I type.


Cordially,
Thom Gandet

Sebastian Otero wrote:

> Actually the semiregular variations detected seem to belong to a special
> group made up by most of the hydrogen-deficient stars. Most of the RCB stars
> also show that behaviour (well, they are hydrogen-deficient too...).
> However, when these stars show semiregular variations and have a redder
> color (without showing RCB fading events) they have been classified as SR's:
> it is the case of V4152 Sgr for instance.
> I think they should compose another group of SR stars.
> Gamma Herculis is another star that has been classified as SRd (yellow F to
> K variables) when its spectral type is A9III.

-- 
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                      Thomas L. Gandet, Director
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