RX And B-V anomoly?

Affiliation
British Astronomical Association, Variable Star Section (BAA-VSS)
Fri, 03/15/2024 - 12:38

Wikipedia (Ref Tycho 2)  has this       B−V color index                -0.4556

AAVSO (VSX) has  B-V   0.28

Obviously relevant to choosing Comp. and Check stars

Can anyone explain please?

Affiliation
American Association of Variable Star Observers (AAVSO)
Just to be clear, you're…

Just to be clear, you're talking about  RX And, right?

The B-V listed in VSX comes from APASS

APASS-DR9    B = 13.71   V = 13.43   g' = 13.53   r' = 13.31   i' = 12.65   B - V = 0.28

Affiliation
American Association of Variable Star Observers (AAVSO)
RX And color

     This is a cv, so one could expect a fairly blue color.  But the problem with the Tycho-2 measure quoted by Wikipedia is that the uncertainty is omitted.  The star is mag 12-ish, so is right at the limit of detectability for Tycho-2.  The uncertainty on the B value is nominally 0.09 mag, while that for V is 0.21 mag.  'Real' Tycho-2 errors for faint stars (fainter than 9th) are in fact about 50% larger than this, so the implied B-V color is very uncertain.  VSX quotes the APASS B-V, whose uncertainty is 2.14 mag(!), which surely comes from the mean value including outburst as well as quiescent data (says 14 observations on three nights).  The Mermilliod UBV compilation quotes B-V = 0.00, but you'd have to look up the source for that.  If there are some A- or F-type stars in the field, you might select those as comps, but they are relatively uncommon, so you'll have to use whatever's available.  Since this is a well-observed object, I am surprised there is no 'official' AAVSO sequence, or that one exists in the literature.  SIMBAD shows no Henden biblio item for RX And, so evidently Arne did not get to this one as part of cv calibration papers he did 20+ years ago.

\Brian

Affiliation
American Association of Variable Star Observers (AAVSO)
RX And B-V

"SIMBAD shows the B-V as .283, coming from the UCAC4 catalog."  ...which comes from an early version of APASS.

     Meanwhile, there is a B,V sequence from long ago by Lenouvel & Daguillon available with a chart here:

https://articles.adsabs.harvard.edu/pdf/1956JO.....39....1L

\Brian

Affiliation
British Astronomical Association, Variable Star Section (BAA-VSS)
Thanks Brian,

I usually…

Thanks Brian,

I usually measure B and V so I wondered after your comments how wise it would be to use my own calculated.

Thanks for the guidance.

Regards

Kevin

Affiliation
American Association of Variable Star Observers (AAVSO)
RX And B-V

     Certainly if you have transformation coefficients for your system, by all means try it.  There is a modest range in color in the Lenouvel sequence, perhaps other stars in the field, too.  So you may be able to get a fairly good value from stars just in that area. 

\Brian