From: Chris Lidman (clidman@eso.org)
Date: Thu Jul 20 2006 - 14:30:45 PDT
Dear Arjun,
I can answer the question about the FORS2 spectrum. The spectra were
not taken at the
paralactic angle because we were using the MOS mode nda we wanted to get
spectra of galaxies in the cluster. Hence, there certainly will be slit
losses, especially
in the blue. FORS2 does have an ADC, but it only fully corrects
for atmospheric refraction until an airmass of 1.6. The target had and
airmass of 2.1
when we observed it.
At some stage I will go back to the FORS2 data and correct the
spectrum for slit losses.
Regards
Chris.
> 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
>
> ===============================
> Arjun Dey
> National Optical Astronomy Observatory
> 950 N. Cherry Ave.
> Tucson, AZ 85719
> Tel: 520-318-8429
> Fax: 520-318-8360
> ===============================
>
>
>
European Southern Observatory
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