Affiliation
American Association of Variable Star Observers (AAVSO)
Mon, 05/09/2016 - 04:05
Hi,
Occasionally I find errors or missing data when I build new sequences in VPhot from my long-exposure images. For example, VPhot and VSX try to place fast eclipser CM Dra at 16:34:20.22 +57:09:43.9 while my image puts it squarely at 16:34:18.07 +57:10:05.17 (I traced this to large proper motion). Then I found that the same star CM Dra (12.9-13.6V) has a companion star 5 arcsec to the SSE at ~17 mag in V, not mentioned in VSX but which I've seen in multiple V images.
I find such things once in a while--how or to whom should I best report them (this does not seem like the place)? What level of image verification is needed? Any guidance appreciated.
Thanks!
Hi Eric:
I would recommend an email to Sebastian Otero. He can advise on supporting info.
Since the companion is about 4 mags fainter it will have a small impact on your measurement but perhaps still nice to know about.
Ken
Follow-up: I've
Thanks, Ken.
Follow-up: I've since calculated that, because of CM Dra's rapid proper motion (>1500 arcsec/year), the close companion would not have been close when the eclipser's RA,Dec was published. And ~2019 it will again be 10 arcseconds away. But until then it's a photometric challenge, at least for absolute magnitudes.
Various catalogs show the companion at about 17th magnitude. Let's assume an exact 17.00, and the magnitude range of CM Dra. At maximum, the magnitude difference is 4.13mags, which corresponds to a 0.02mag brighter measurement if the companion is included. At minimum, the magnitude difference is 3.37mags, which corresponds to a 0.05mag brighter measurement.
So the two problems with including companions in your measurement aperture are that they lead to a systematic offset in the measurement of the variable, always in the sense that the measurement is too bright; and the offset is itself variable, with a bigger effect as the variable declines in brightness.
As CCD observers get more proficient and start doing things like transformation to recover a few percent of accuracy, these offsets from companions become more important. Always keep this in mind, and if in doubt, include a comment with your observations that a companion is present in the photometry.
Eric is to be commended on noticing that proper motion has caused a current situation that wasn't there in the past! This could also lead to incorrect analysis, for example a report of a variable getting systematically brighter over decades. Photometry is always a challenge - you have to carefully analyze your data and make your report as detailed as possible to help the researcher. I think this is half of the fun!
Arne
Hi Eric,
I have added a remark about the companion to be displayed on the charts to warn observers.
Also updated VSX with your position.
Interesting case. I hadn't thought about the "transient companions" of these large proper motion stars!
Cheers,
Sebastian