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EE Cep comparison stars

HQA's picture
HQA
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Joined: 2010-05-10

I've now been able to get calibration photometry from APASS, SRO35 and coker30.  I'd like to lead you through the process that I use when determining what magnitudes to use, when multiple sources are involved.

The Polish campaign is being run by Piotr Wychudzki (adyrbyh@gmail.com) and Cezary Galan at the Nicolaus Copernicus Astronomical Center, Poland.  They have a nice website at:

https://sites.google.com/site/eecep2014campaign/home

EE Cep itself has a magnitude out of eclipse of around V=10.84, (B-V) = 0.23, so a pretty blue object.  The nearby comps are also blue, which is a good sign.  I looked close to the comparison stars, and they are all reasonably "clean" and so as long as you use apertures in the sub-20arcsec diameter regime, your magnitudes should be similar to others.

Brian Skiff (AAVSO discussion 2003-02-12) that there was no official chart for EE Cep, and suggested the following photometry for the comparison stars, using the 53cm Lowell telescope on two dates in October, 1994, and the Stromren filter set:

Star

a (GSC 3973-1177; BD+55 2690)   10.386 +/- 0.11

b  (GSC 3973-2150; NSV 25842) 11.248 +/- 0.006

c (GSC 3973-1103; BD+55 2691; NSV 25843)   11.232 +/- 0.008

Star a has been used as the primary comparison star for earlier campaigns.  Meinunger (1976, MVS 7, 97) calibrated it photoelectrically, with values

V = 10.37

B-V = 0.31

U-B = 0.16

but without giving details of the equipment (what aperture size, for example) and without listing photometric errors.

Star a was also calibrated by Mikolajewski et al. (2003, IBVS 5412), with values

V=10.38

B-V = 0.30

U-B = 0.18

again using PEP with no errors or equipment information.

The Polish group give their own measures for star b:

V = 11.23

B-V = 0.25

U-B = 0.12

Star c was calibrated by Mikolajewski et al. (2003, IBVS 5412), with values

V = 11.22

B-V = 0.25

U-B = 0.22

Barbier et al.  (1973, A&A 27, 421) measured star c in the course of studying galactic structure:

V = 11.32

B-V = 0.25

U-B = -0.08

This value is 0.1mag different than other measures; I'm willing to bet that Barbier transposed digits, and the true value is 11.23.

I also see no evidence of variability for comparison stars b and c, even though they are New Suspected Variable entries.  The variability suspicion was due to photographic estimates; even though researchers said that they were continuing to observe the comparison stars to determine the variability, no papers have been published in the intervening decades. Let's assume that they are constant for now, and use them as normal.

Note that I've avoided the Polish R,I measures of the three comparison stars.  Those bandpasses are Johnson, not Cousins, and so are of no value to us.  Please do not use them.

If someone wants to create a table with the magnitude/colors from the various observers for each comp star, that would be great.

Now, on to the calibrations from AAVSOnet/APASS.  Here are the numbers (the errors for the SRO data are in the VSP table).

Star  Value  SRO    C30    APASS

a        V   10.397  10.332  10.405

a       B-V  0.307  0.261  0.268

a        V-R  0.179  0.160    -

a        R-I  0.201  0.180  -

a       V-I  0.382  0.340  -

b       V  11.245  11.187  11.249

b       B-V  0.200  0.157  0.176

b       V-R  0.124  0.113  -

b        R-I  0.152  0.131  -

b        V-I  0.277  0.244  -

c          V 11.249  11.188  11.267

c        B-V  0.251  0.207  0.182

c        V-R  0.165  0.150  -

c        R-I  0.198  0.168  -

c         V-I  0.365  0.319  -

Where I have not used the APASS r'/i' conversion to Rc/Ic.  The SRO35 data comes from 17 nights of calibration, so I trust it the most.  C30 (coker30) comes from the first calibration results with this telescope, with 4 nights of calibration.  It seems consistent, but 0.06mag too bright at V, +0.04 with respect to (B-V) and perhaps -0.02 with respect to (V-Rc) and (Rc-Ic).  I haven't checked to see whether these offsets are similar for other fields, but for now, I'm going to use C30 just to confirm SRO/APASS values.

The end result?  SRO/APASS are about 0.02mag fainter at V for the three comparison stars than other researchers; (B-V) is about identical in comparison with other observers.  Since the Polish group do not report errors, and only give their measures to the nearest 0.01mag, I'm happier with SRO/VSP.  So as long as you identify what set of magnitudes/colors you are using, I think the SRO/APASS/VSP photometry table numbers are very close to the Polish numbers, and are easier for us to use in VPHOT, Canopus, etc.  Let's stick with the VSP values.

Arne

typo?
FRF's picture
FRF
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Joined: 2010-08-09

Arne, you wrote:

Star  Value  SRO    C30    APASS

a        V   10.397  10.332  11.405

 

I think the APASS photometry 11.405V was a typo. Seqplot  gives 10.405V APASS photometry for star 'a'.

Clear skies,

Robert

 

typo? == yes
HQA's picture
HQA
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Joined: 2010-05-10

Thanks, Robert, for finding that.  I've corrected the post.

Arne

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