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Double star as comp star for CH Cyg. Is the companion variable?

JimK's picture
JimK
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Joined: 2013-02-19

I noticed that SAO 31586, which is used as the comparison star for PEP observations of CH Cyg is a double star with a component about 7 arc seconds away, and a magnitude of about 10.  The companion seems bright enough that there could be some error depending upon where in the apeture the star is placed.  Is it possible the companion has brightened since it SAO 31586 was choosen as the comp star?

Jim

Good question
Matthew Templeton's picture
Matthew Templeton
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Joined: 2010-03-12

Hi Jim,

It's an interesting thought -- the companion could be an active M or K dwarf. 

The PEP comp star was changed to SAO 31586 in 1991 according to our chart archives.  (I haven't yet found why it was changed -- 1991 was well before my time here.)  This star isn't found in the GCPD which is where we sourced the magnitudes for the PEP program update in 2006.  However, the V magnitude of 7.333 is exactly what's found in Kharchenko's V-band catalog, so we probably used that value.  I don't have any more insight beyond that at this time, but you might be able to check out the Kharchenko reference and figure out whether the magnitude is a combination.  It's a close enough pair that the 7.333 might be a combined magnitude.

Matthew

Companion
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Sebastian Otero
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Joined: 2010-09-19

Hi Jim, Matthew,
The best magnitude comes from HIPPARCOS and is V= 7.32 (derived from the Hp magnitude using Bessell's tables).
There is a 0.02 mag. scatter but no prrof of variability. A close companion might cause such scatter in HIPPARCOS data.
There is no companion detection in the satellite's data. This suggests the companion is faint. The WDS gives V= 11.1. If its magnitude is indeed 11.1, the contribution to the blend would be 0.03 mag. HIPPARCOS magnitude, considering a separation of 7", is the combined magnitude of the two so A= 7.35, B= 11.1. But the 11.1 is not a photometric value.
If the companion has V= 10 as you suggest (what is your source?), the contribution would be 0.1 mag. But again, a 10 mag. companion at 7" would be easy to detect for HIPPARCOS so I don't think the companion is that bright.
Also the 2MASS values (J= 9.64; K= 9.00) are not consistent with a 10th mag. strar in V. The star would be even fainter than 11 in the visual.

Cheers,
Sebastian
 

Companion magnitude
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tcalderw
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Joined: 2012-11-12

 

I have been making CH Cyg PEP mesurements over the past month or so.  On Friday night I was startled to see the companion, because I had not noticed it before.  Just looking through the eyepiece, it seemed brighter than EE Cep that night.

Tom

 

Catalog magnitudes of the companion to SAO 31586
JimK's picture
JimK
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I did a quick search in Vizier of the "All Sky Compiled Catalog of 2.5 Million Stars'.  It contained the following information:

 

SAO 31586  B=8.88 V=7.33 J=5.026 H=4.093 K=3.895

Companion B=11.48  V=10.07 J=7.745 H=6.999 K=6.849

The magnitude 10 reference in my original post was just my visual estimate, but as there were no comparison stars nearby it should not be given any weight.  I have seen the companion during my other PEP observations of CH Cyg. I thought it might be a little brighter recently, but I don't have a good way to measure the brightness independant of SAO 31586.  May be worth someone with the right equipment making a quick measurement given the difference in magnitudes given by the various sources quoted by those who responded to this thread.

Jim

 

 

http://vizier.u-strasbg.fr/viz-bin/VizieR-4

Correction to previous post
JimK's picture
JimK
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The information I just posted for the companion was for a nearby field star.  I have not been able to find a separate entry for the companion in the catalog.

Jim

Recommend comparison to check star
JimK's picture
JimK
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I will go back to my observations over the previous month and see if there is any variation in the delta magnitude between SAO 31586 and the check star SAO 31737.  Based upon Sebastian's input we would expect to see about a 0.1 magnitude variation if the companion moved from 11V to 10V.  I will also check the AAVSO database to see the delta from other PEP observers. 

Jim

comp star companion
HQA's picture
HQA
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Hi Jim,

The comp star companion can be found in the Washington Double Star catalog (WDS19205+5020):

http://vizier.u-strasbg.fr/viz-bin/VizieR-S?WDS%20J19205%2b5020A

where the SAO star is listed at 7.51 and the companion at 11.10.  These are approximate magnitudes, but indicate about 3.6mag difference, so the inclusion/exclusion of the companion would influence the photometry for a couple of hundredths of a magnitude at most if the ratio is correct.  With a 7arcsec separation (in 2000, and less than that today), most PEP systems would include both stars.  What is the diameter of your measuring aperture in arcsec?

Arne

 

comp star companion
JimK's picture
JimK
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My aperature is 50 arc seconds so the double is definately in the aperature during measurements.  At mag 11.1 it only has a small impact on measurements. (Sebastion calculated 0.03 contribution)  It seemed a bit brighter the last time I looked at it, but there really iis no way to comapre it to a star of similar magnitude in my setup.  If it brightened a whole magnitude to 10 we might be able to detect the change. (Sebastion calculated a 0.1 contribuition in this case.).  I examined the reported PEP check star measures and don't see a statistically significant  trend (at least when I eyeball the data).  I will do a more thorough analysis to be sure.  

In any case it is a very pretty double star.  

Jim

AAVSO 49 Bay State Rd. Cambridge, MA 02138 aavso@aavso.org 617-354-0484