[vsnet-chat 3499] Re: [vsnet-be 27] Re: Delta Sco

["Stan Walker" <astroman@voyager.co.nz>]

Hi John,

Thanks for the comments. We've tended to avoid B stars for comparisons as
they seem a bit erratic, but some are quite stable. But in 1968 when we were
just beginning pep and didn't know about extinction they all seemed
variable. So, as you say, there should be a prescribed couse for all CCD
users to understand what the altitude and colour do to stars. It's not as
noticeable with the red response of CCDs but it's still there. As is a large
variation between responses of different CCDs. The ST6 is different from the
ST6B and the ST7. And the reduction software can also lead to different
results due to such aspects as the aperture size and the sky annulus sizes.
Some of these problems persisit even with BVRI filters.

I'm in the process of getting back to BVRI photometry using a Meade 4"
piggybacked on the larger telescope. It'll be interesting to see how good
the results come out.

Regards,
Stan
----- Original Message -----
From: no name <crawl@zoom.co.uk>
To: <vsnet-chat@kusastro.kyoto-u.ac.jp>
Sent: Saturday, July 29, 2000 1:06 AM
Subject: [vsnet-chat 3496] Re: [vsnet-be 27] Re: Delta Sco


> 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...
>

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