UW Dra CST???

Sun, 01/05/2020 - 15:51


Looking through the bright wrong stars in the AAVSO database I accidentally came across UW Dra's observations in the "V" band 7.3-7.74, then I looked in the VSX and saw that the star is constant! I didn't believe it, I looked at the KWS (Kamogata/Kiso/Kyoto Wid-fieldSurvey  http://kws.cetus-net.org/~maehara/VSdata.py ) data and saw brightness fluctuations in the "V" band 7.3-7.5. A contradiction occurs. On the one hand, Hipparcos satellite data show that it is a constant star https://www.aavso.org/vsx/index.php?view=detail.top&oid=13708, on the other hand, AAVSO data and KWS data show that it is a low-amplitude star with an amplitude of 0.2 or 0.3 mag. So what happens to this star in this case?

All the best, Ivan

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good question

In response to your question I went to VizieR http://vizier.u-strasbg.fr/viz-bin/VizieR-4 and checked out the data on the star from Tycho2 (Hoeg 2000), Hipparcos (2007 reduction and Gaia DR2.  All show very small standard deviations, consistent with the star being constant.  I trust those sources far more than any earth-bound observatory, for the smple reasons that the data masses are much larger and they are not plagued by the vagaries of the atmosphere.

As to what happens next, your guess is as good as mine.  There are numerous examples, both within the AAVSO's list (VSX) and the more generally accepted GCVS (accessible via VizieR), and I am not aware of any effort to remove stars from those lists.  On the contrary, as you can see for yourself looking at posts on this forum, lots of people seem to be interested in discovering new ones, but I know of no examples of someone actually trying to remove stars from the catalogs.  (But my experience is limited, and I am sure that if I am wrong, one of the cognoscenti will jump in to set the record straight.).

Dear Stephen !

Data from the TESS telescope show low-amplitude luminance oscillations with an amplitude of about 0.023 magnitudes and cyclic brightness irregularities. These observations are incontrovertible proof that the star is a variable. I bring three graphs. I show three graphics  according to TESS from July to September 2019. Probably all permanent stars (CSTs) in the VSX are low-amplitude variables. The question here is, which star can be considered a variable and which constant? Aren't all bright permanent stars that we see are very small amplitude variables?

All the best, I.Sergey

Did you analyze the data?

I looked at your three plots; not being familiar with TESS I don't know the reason why some of the data points appear to have been excluded.  I have no reason to reach any conclusion based on the apparent contradiction, but, especially considering the tiny amplitude, wonder if the curves are not driven by noise.  Have you done a Lomb-Scargle periodogram of the data?  If so, at what significance level does periodicity exist?

In answer to your second question, I think the terminology "variable star" is somewhat ambiguous.  It appears to me that if you set the threshold low enough, virtually all stars are variable on a short term basis, and certainly all stars are variable on a very long term basis, regardless of the threshold set.  For instance, the 11 year or so sunspot cycle induces variability in solar luminosity of about one part in 1360, but usually the sun is not said to be a variable star.  (Not to mention the end stage of solar evolution.)  Again, perhaps the cognoscenti have arrived at some sort of convention regarding their taxonomy of variable star types, which would declare some sorts of variability to be outside the bounds of their taxonomical construct.  I really don't know.



Hello, Stephen!

I'm just looking at the documentation related to the TESS telescope data. I can say that the amplitude of white variable star oscillations in the TESS photometric system should be multiplied by about 1.7 to get an approximate amplitude of variable star oscillations in the "V" band. No, what you see in the graphs is not noise. These are real low amplitude brightness changes. I have not yet done a detailed analysis of the TESS data for this star.  But I have looked at some possible eclipsed stars as well as possible and unconfirmed EPs, as well as known EAs and known EPs and TESS data show very good curves! For this star, the amplitude in band "V" is around 0.04.

All the best, Ivan

p.s. I use a Google translator to translate and hope I can be understood.

American Association of Variable Star Observers (AAVSO)
Constant or low amplitude variable stars

Hi Ivan,

You need to take scatter into account when you determine a variable star amplitude. You say this star shows an amplitude of 0.2-0.3 mag. and that is just noise in the KWS data.
It is important to know the limitations of each source. HIPPARCOS is much better than KWS and TESS is much better than HIPPARCOS.
The star was classified as CST based on the known evidence at the time of the revision. You can revise it again. You need to be aware that we have found some artifacts in TESS data too, both in the SAP_FLUX and the corrected PDCSAP_FLUX data. Ups and downs, false eclipses. So be careful.

About the definition of a variable star, for VSX, we set a limit of 0.001 mag. I agree with both of you that any star might be variable if we can detect variations below that level. We had to set a limit.

Stephen, I do not think that the GCVS is the most generally accepted source of data nowadays. We have 20x the number of variable stars in VSX and we update and revise the data continuously. Even then we are behind with the most recent papers and catalogues but we do our best to include the most recent information. VSX is also available through VizieR.

So we will accept a revision for this object once is properly analyzed.