[Message Prev][Message Next][Thread Prev][Thread Next][Message Index][Thread Index]
[vsnet-gcvs 112] Re: [vsnet-chat 4739] Upsilon Sagittarii status
Dear Thom:
Sorry for the delay in answering.
> 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.
I never said that.
RCB are stars that show deep minima. Most are HdC stars. But when people
says "HdC" stars mean they are stars that don't show deep minima. None of
the HdB stars have shown this. Furthermore, no RCB star has been found to be
binary.
However, there are three stars that are hotter (they are EHe - extreme
helium stars-) and also show deep minima. The most known example is V348
Sgr.
What I found interesting is that RCB stars, HdC stars, some EHe stars, and
HdB stars pulsate with periods of tens of days.
This relation in the cause of the periodic variability is mentioned in
Morrison (1988) where pulsation periods of RY Sgr (RCB), BD +1º4831 (EHe)
and ups Sgr (HdB) are compared: "...The consistency of the Q-estimates
suggests that the pulsational characteristics are similar in these stars..."
> 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.
V4152 Sgr is a HdC star (Hydrogen-deficient carbon star). Cottrell and
Lawson (1998) refer to these stars as possible "low activity RCB stars on
account of their lower pulsational activity and evidence for limited mass
loss"
However V4152 Sgr has been found to be rather active. Its pulsational range
is 9.24 - 9.58. The B-V color index is 1.11 - 1.19. The period is near 40
days.
Both amplitude and period are similar to the active RCB stars.
No way it can be considered a mira variable. Mira variables show several
magnitudes of amplitude and are red giants.
Information on V4152 Sgr (HD 175893) can be found in "Discovery of
variability in the 'non-variable' hydrogen-deficient carbon stars" (Kilkenny
et al, 1988). They also mention the need of looking for deep extinction
minima in this star.
You can also find the star in Drilling's Catalog of Hydrogen-Deficient Stars
(1986).
> 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.
The LBV is a very confusing type that appeared in the GCVS recently and that
ignored the existence of the very known meaning of LBV as "Luminous Blue
Variables"...
All members show amplitudes less than 0.1 magnitudes (excepting one or two
cases as V969 Cen)
LBV is a provisional type for stars that are not well-known.
But periods are also short (although greater than 1 day).
I think hydrogen-deficient stars (RCB, EHe, HdC, HdB) that are variable
should be classified as another group, maybe according to their
temperatures.
For instance, we could create the SRHD class with subtypes SRHDa for hot
stars (ups Sgr) and SRHDb for cooler stars (RY Sgr, V4152 Sgr).
Also the hotter stars appear to have shorter periods (9-20 days) while the
cooler stars show periods greater than 20 days (most near 40 days).
Then some RCB's would appear as "RCB + SRHDb", offering more information on
the star.
And we would know that V4152 Sgr hasn't shown any deep minima when we read
"SRHDb" only.
V348 Sgr wasn't found to be a pulsating star at maximum, so it would only be
an "RCB" star.
Upsilon Sgr could be classified as an "SRHDa" star, a hot hydrogen-deficient
semirregular star with a period close to 20 days.
If binarity proves to be playing a role in the variations, then another type
(I don't know which...) could be added.
Well, it is getting too long....
Cheers,
Sebastian.
---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://vsnet.grisoft.com).
Version: 6.0.265 / Virus Database: 137 - Release Date: 18/07/01
vsnet-adm@kusastro.kyoto-u.ac.jp
Return to Daisaku Nogami
vsnet-adm@kusastro.kyoto-u.ac.jp
VSNET Home Page