Sun, 01/06/2013 - 05:17
I notice several U measurements on U Gem (no pun intended) over the last few days, which do not "look right". They are in the 13-mag range, while the V is in the 14 mag range and B is near 15.
I thought CV's were pretty normal sun-like color in quiescence? So, I would think the brightness in these passbands should be U < B < V ?
Mike LMK
I don't see any
Mike,
I don't see any observations in the LCG plot now for U Gem in the mag 13 range except for a "fainter than obs.). My two CCDV observations from last night are now there and in the mid 14s range.
Dave Lane (LDJ)
Mike, these are the first U-band observations of U Gem that I see in the database; at least over the last 15 years. These new observations may be right; we would need to investigate further and perhaps look at some blue spectra for complete confidence.
There are several things to remember. In quiescence, you are primarily seeing the white dwarf. For U Gem, this is a very hot star - typical temperature around 30,000 degrees. So it is far hotter than an A0 star, which typically has U=B=V=R=I due to the definition of the Johnson/Cousins system. Because the white dwarf is hotter than A0 (10,000K), U can be brighter than B for these stars. Quiescent and outburst temperatures and photometric colors of CVs can be quite different, and the colors of these systems in general are complex since they are formed from the white dwarf, the red donor star and the accretion disk, and depend on which dominates at which wavelength.
So I'd consider these U-band points as interesting, and a good starting point for an inquiry into the ultravoilet nature of this system. As we've mentioned many times, filtered photometry of just about any star gives you much more information for a variety of research projects.
Arne
Hello Mike and Arne
I am at the AAS meeting in Long Beach, but will make a point of observing U Gem in U when I get back in a week, if others have not done so. I have been observing it in B and V for the past few weeks.
Gary
Well it still seems un-physical for U to be brighter than B yet B is fainter than V ? Wonder what comp stars were used and how? Do we even have good U comp stars for the field?
Mike LMK
Hi, all
I found UBV photometry paper of U Gem in ADS.
Title: Photometric Observations of U Geminorum
Authors: Paczyński, B.
Journal: Acta Astronomica, Vol. 15, p.305
Bibliographic Code: 1965AcA....15..305P
In this paper U-B color is around -0.8 to -1.0. And B-V color is around +0.2 to +0.4
Well Planck's/Wien's Law for black bodies says there is a single peak in the luminosity curve and the general shape of the curve is independent of Temp. So, these measurements do seem odd. Stars generally follow black body spectrum very closely. You would expect as the wavelength goes from V->B->U you would see a general increase in intensity for a very hot white dwarf, not a dip in the blue?
I would be curious about how these measurements can be reconciled with this basic theory?
Mike LMK
Mike, as I said in an earlier post, U Gem is a COMPOSITE spectrum. You have three objects whose flux is combined. Don't think of this as a single black body. At U, the white dwarf dominates.
Arne