Affiliation
Vereniging Voor Sterrenkunde, Werkgroep Veranderlijke Sterren (Belgium) (VVS)
Tue, 03/12/2013 - 20:30
Hi, I just have made a new chart for X Cep. There is a big black dot at the place where X Cep must be. Is this X Cep or a close companion? I used to observe X Cep years ago but I don't have my old charts anymore to check this out.
Usually is down the chart a warning if there is a close companion but not at this chart.
Can someone of the chart team check this one? The chartcode is 10837IGP
Yes, it must be X Cep. See the DSS image. (Btw. IMHO it's not a big dot.)
Clear skies,
Robert Fidrich (FRF)
Robert, thanks but why is it visible on the chart?
Since it is a star and it has been listed in the UCAC3 or UCAC4 catalogues with a certain brightness (~12-13th magnitudes according to the size of the dot) it is printed by VSP.
Some miras are printed as a big dot, some as a small dot (or no dot at all), depending on the phase when they brighness was measured.
Robert, I think that this
Robert, I think that this is no longer the case. It was this way a few years ago but I had the impression that the AAVSO has found a way to remove the variable and replace it by a circle.
I even had a discussion years ago on the previous AAVSO email discussion list about the problems with visibility of the variables.
I have made a lot of charts the last years and the variables where always replaced by a circle. I think in the case of X Cep something has gone wrong.
A bug in the software?
VSP now deletes the variable during the initial plotting phase, then inserts the open_circle_plus_crosshair symbol at the location of the variable. Any remaining dot is usually a nearby companion, as is the case for X Cep. If you use a 5arcmin field on your chart, for example, you will see that the black dot is offset from the crosshair symbol. Also, the dot is small, not big as described in the original posting.
We did this because many of the Miras were caught at different phases and with red light on UCAC, and so had a wide variety of dot sizes. We felt it was better to identify the location rather than confuse the observer with a dot size that might not reflect what they would see tonight.
Note that this process only works if the location of the variable is well-known in VSX, and matches an object in VSP. Computers are only as good as the data they work with!
Arne
Dear Arne,
"Any remaining dot is usually a nearby companion, as is the case for X Cep."
Well, looking at the color DSS image, it seems to me there is no nerby companion for X Cep - the red color of the variable clearly shows this. The variable is printed slightly off to the star on a "g" scale chart, so I assume probably this is why the dot was actually printed by VSP.
Clear skies,
Robert Fidrich (FRF)
Hi Arne, I think this is X Cep because if this was a close companion this must reflect in the observations. This dot is comparable to magnitude 13 and in the lightcurve there are no mis identifications visible. I can imagine that some observers would take that dot for X Cep and estimate it like a lot of observers do with Z Tau.
Hi Robert,
UCAC4 has a single star, X Cep, at
20:56:10.137 +83:03:25.37 J2000, Rmag=12.0
USNO-A2.0 shows a second star at
20:56:06.792 +83:03:09.3 J2000, Rmag=18
about 15arcsec to the southwest, not showing up on most VSP charts unless you set the mag limit very faint.
VSX has X Cep at
20:56:09.39 +83:03:26.2 J2000
So the difference is about an arcsec. Most likely, VSP thinks there is no star at the VSX position, and just plots the symbol there. Then the UCAC4 "star" is plotted just to the southeast of the assumed position of X Cep, and with a dot size equivalent to a 12th magnitude star.
Therefore, you are correct - there is no companion, at least to relatively faint magnitudes and with the available catalogs. There is a mismatch between the UCAC4 position and the VSX position for the variable. Perhaps someone can submit a revision for X Cep's position to VSX.
Arne
Hi, all,
the problem comes from the GCVS where X Cep's position is 1.6" off.
There is indeed no star at the GCVS position so there is no companion.
VSX has been updated with the UCAC4 position for X Cep so no black dot should appear now.
Cheers,
Sebastian.
I have reported it to CHET.
Indeed the
Hi Sebastian,
Indeed the new chart doesn't display the star anymore.
Thanks for the update!