Affiliation
American Association of Variable Star Observers (AAVSO)
Mon, 06/01/2020 - 13:11
As a bright star monitor participant, I have been observing an EA that is listed as an A0 type in VSX. Over the course of the past 12 months+, I have noted that the star is consistenly much brighter in the Johnson V, Counsins R, and Cousins I bands than Johnson B. My limited understanding of the typical A0 spectrum would suggest that B should be the brightest band of the four. Could there be something about this EA geometry that would have the other three bands brighter than B in this case?
Your data in AID since 2017 show B-V color of +0.05-0.10 with error bars often encompassing B-V=0. This is quite blue and similar to B-V color of other A stars.
This is a good question! When one looks at the textbook spectrum for an A0 V star, it has greater flux in B wavelengths than V wavelengths. So, one would expect B mags to be more negative than V mags, and thus B-V would be less than zero.
But, the zeropoints of the the UBV (and extended to UBVRI) magnitude system were **defined** to be such that B-V = 0, and U-B = 0 for an average of six A0 V stars. See Henden and Kaitchuk 1982, page 36.
Further, when one looks at B-V for a bunch of stars spectrally classified as A0, one finds there is some scatter... e.g. one of the A0 stars listed in above book, Appendix C, "UBV Standard Field stars", has B-V = 0.20
Leading to, as Eric said, your star not being peculiar in that regard.
But, all of that ignores the binary nature of GO Cnc. A quick search (probably incomplete) suggests it has never been modelled, and ASAS-SN data isn't sufficient to show a full lightcurve, thus I can't say anything about the temperature (spectral) type of the secondary. No spectral type for the companion is shown in simbad, but if the hottest star of the pair is A0, and the companion is cooler, that would lead so a small excess of longer wavelengths... but that is as much as I would want to say without more careful work on the question!
Gary Billings
Hi Charles,
B-V 0.05-0.10 is pretty normal for early A-type stars so there is nothing unusual.
I updated the information in VSX and added a recent A2V spectral type in the LAMOST catalogues.
The published B-V is 0.08 (consistent with your measures) and the published E(B-V) is 0.03. The expected B-V for an A2V star is 0.05 so everything fits.
If there is some contribution from the secondary, it is negligible.
Gary, there are no useful measurements in ASAS-SN because the star is too bright for that survey.
There is a complete light curve, but only in V, in ASAS-3.
It is an eccentric system.
Cheers,
Sebastian