[vsnet-rr 22] Re: [vsnet-chat 3765] RE: Delta Cep period analysis of visual observations

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

Hi Eric and Taichi,

I was interested in your comments about Cepheid and other periods. I've done
the same thing with a number of southern Cepheids which have visual
observations published. But whilst it's possible to get quite good
periodicities this way a better test is to get epochs of maxima (by fitting
seasonal measures to a master light curve) and fit them to slopes in an O-C
diagram. This is the true test of period determinations and is probably 10
to 100 times more demanding than using PDM or various other periodogram type
software. The latter are merely producers of the spread of periods which
Eric mentions.

Determining epochs by fitting visual measures to master light curves and
then plotting on O-C diagrams shows just the lack of time resolution that
you would expect. Visual measures with an accuracy of +/- 0.3 magnitudes
produce epochs about 20-50 times less accurate than pep measures which are
based on measures with accuracies of +/- 0.005 magnitudes.

In spite of the comments of Koen and Lombard the best test of periodicities
is still the fitting of epochs of maxima, minima or whatever to O-C
diagrams. All periodograms are merely indicative and are badly affected if
the period is changing, which is really what the game is all about.

But, if there are no other measures, visual measures still provide a nice
indication of what is happening and I am impressed by their accuracy, even
if it is not enough in many cases to make definitive statements. It would be
good to see the present analyses translated into O-C diagrams with seasonal
epochs.

Regards,
Stan


----- Original Message -----
From: Eric Broens <Eric.Broens@advalvas.be>
To: 'Taichi Kato' <tkato@kusastro.kyoto-u.ac.jp>;
<vsnet-chat@kusastro.kyoto-u.ac.jp>; <vsnet-rr@kusastro.kyoto-u.ac.jp>
Sent: Wednesday, November 15, 2000 11:57 AM
Subject: [vsnet-chat 3765] RE: Delta Cep period analysis of visual
observations


> Dear Taichi,
>
> I can confirm your findings.
> a few years ago I analysed the raw visual observations of BL Her made by
> observers of the Belgian VVS variable star section during the 80's. BL Her
> is a CWB star varying between magnitude 9.7V and 10.62V. In the literature
I
> found periods ranging from 1.307443 d - 1.3074604 d.
> The phase diagram of the raw data looked really bad, but using PDM, I
found
> a period of 1.307454! I didn't clean up the dataset for bad observations.
I
> don't have all the details of the analysis at hand (just a slide
summarising
> the result), but I remember the analysis was done on a few hundred
> observations spanning years, but with quite long interuptions in between.
> The nice thing besides the very well matched period was that all alias
> periods one can expect were shown and the periodogram showed very clearly
> 1.00273 c/d, the 23h56m4s sidereal rotation period!
>
> If one has a sufficient number of observations over a long time, one can
get
> extremely good results, even if the observations are not that accurate!
>
> Best Regards,
> Eric
>
>
> ************************************
> VVS Werkgroep Veranderlijke Sterren
>
> Eric Broens
> Wateringstraat 143
> B-2400 Mol
> BELGIUM
>
> e-mail: Eric.Broens@advalvas.be
> ************************************
>
>
>

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