From: Arjun Dey (dey@noao.edu)
Date: Thu Jul 20 2006 - 11:58:59 PDT
Hi Tony, Kyle and Saul,
I am very puzzled by your mystery object; although it is plausible
that Pat Hall's identification of it as a DQ white dwarf may make
sense (at least based on the spectrum), i am not sure how this is
likely given the huge variability. The thing that puzzles me is why a
stand-alone DQ white dwarf would vary in brightness. The variability
seen in other magnetic WDs (like AM Her or DQ Her stars) is because
there is a companion that fills its Roche lobe. However, companions
would generally outshine the WD at red wavelengths. Since we do not
see this in our spectrum, i don't think there is evidence for an
obvious stellar companion.
My impressions are of course just based on the spectra on the Twiki
page. I had some questions about these (apologies if you have already
gone through all this yourselves!):
- was the VLT FORS2 spectrum obtained using the parallactic angle?
(another way of asking this is: should i believe the relative flux
calibration?) I am puzzled as to why the VLT/FORS spectrum looks
different from the Subaru/FOCAS spectrum in the overlap region. This
pertains to the question of whether or not there is really any
variability in color - the spectra seem to suggest that there is. It
would make sense for a red variable if the star got redder as it got
brighter (i.e., lower Teff).
- Did you have to scale the Subaru and VLT spectra to match around
6300A? By how much?
- are we confident about the wavelength calibration? To my eye, the A
and B telluric bands are in the right spots. But the thing that
puzzles me is the absorption line at ~6300A. It is not possible that
this is OI as suggested by the SDSS spectrum of the DQ: - this is a
forbidden line: it would be amazing to see it in absorption, right?
Is the corresponding line of OI 6363 present? I am also puzzled that
i do not see Ha. Smoothing might help. I suspect the line might be
CaI, with possibly some contribution from TiO (although i do not see
redder TiO bands in the unsmoothed version of the spectrum). It would
be a bit surprising to see CaI without Ha, or NaI (5893).
- smoothing the VLT spectrum may also help. Do you see evidence for
the Ca triplet lines? There seems to be some features at the right
wavelengths, but these are obscured in the current plot by the noise
from the Subaru spectrum.
- do you have a calibrated version (say in mag or AB mag) of the i
and z brightness variations as a function of time? I took a look at
our Bw/R/I data and there is no object at the correct location. This
would suggest that the source is likely to have been in a low state
for many years prior to the current brightening.
Would it be possible to get a 1d version of the calibrated spectrum
to play with? You also mentioned that you had more recent
spectroscopy - it would be great to see that.
Cheers
Arjun
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Arjun Dey
National Optical Astronomy Observatory
950 N. Cherry Ave.
Tucson, AZ 85719
Tel: 520-318-8429
Fax: 520-318-8360
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