[vsnet-chat 3496] Re: [vsnet-be 27] Re: Delta Sco

[no name <crawl@zoom.co.uk>]

astroman wrote:

> The present discussion on delta Scorpii as a Be or gamma Cas star prompts me
> to ask about the latter. Quite a number of blue stars seem listed as gamma
> Cas variables yet they don't do anything much. Some we've used as UBV
> comparisons for decades without seeing any variation greater than 1%. Now
> I'm not querying delta Sco - a good effort by Sebastian and my
> congratulations - but I wonder how these others receive their
> classifications. Is this from spectral measures, or what? And I wonder how
> many blue stars are classed as low amplitude variables purely because of
> poor extinction corrections or transformations to the standard UBV system.

Hi Stan

This one reminds me of an old favourite of mine : ie how come so many
UGZ stars that have barely been observed, let alone caught in
standstill, are classed UGZ?

Re Be and gCas stars... ...I've been reading a couple of books, so here
goes:

I too had noticed a group of so called "gamma Cas" [gCas henceforth]
stars in the GCVS that had very low amplitude ranges.  The "well known"
gCas stars such as gamma Cas, BU Tau [Pleione] and X Per have pretty
characteristic lightcurves, which so as not to overly bias folk I'll
just say have outburst increases of about half to one mag, larger full
ranges, and aren't overnight, or overweek for that matter, events.

So, as you say, objects of barely 0.2 mag range being called gamma Cas
is a bit strange.

On the spectroscopic side, there is a distinguishing feature.  Be stars
in general show hydrogen alpha in emission [hence the 'e' part!]: Be
stars undergoing gamma Cas outburst often have selfabsorption in the
cores, and the V/R wings are usually asymmetric, and possibly varying
[V/R = violet and red wings].  This can also vary over time, & normal Be
profiles return.

I have seen one scan on one of Christian Buil's webpages that showed
this gCas effect for the current delta Sco event, though I've seen
others that show no selfabsorption [such can just be a matter of
dispersion used mind : delta Sco is quite low from northern climes
also].

Maybe try and think of it as almost but not quite a "baby" P Cygni
profile effect : it's quite a bit off from being P Cyg like, but the
underlying cause of outflowing material is the same.

So maybes these other gCas stars, though not exhibiting optical
outbursts to speak of, may well have exhibited these "gamma Cas
profiles" in their Halpha emission on occasion?

Which if true is actually a bit naughty GCVS wise, cos the defining
criteria in that should primarily be photometric.

The thing with both B, Be, betaCep=betaCMa and gamma Cas stars, however,
is that they are quite willing to swap class at any time, and also seem
somewhat dependent on how, when and why you are studying them!

Cheers

John

JG, UK

PS, on your final point re true variation or "seeing" or poor
photometric transformations: I must admit to a constant worry that folk
don't take photometry serious enough. I also sometimes worry that the
relative ease people have with astrometry using CCDs and home computers
may make them think that photometry is just as easy, when it is a
totally different barrel of kettles.

I believe you've done "old fashioned" photomultiplier photometry in the
past, and to good effect! I personally think folk should be forced to do
some of this afore even being let near their CCDs if their aim is to do
serious Johnson type photometry! Too much reliance on the machine I feel
: if the software chucks out numbers to 3 decimals, for eg, it don't
mean the qualities at that level!!!!! "Old style" PEP should get them in
the right mindset... ...maybes...

VSNET Home Page

Return to Daisaku Nogami

---

vsnet-adm@kusastro.kyoto-u.ac.jp