Feasibility of detecting Be star outbursts using
Star Analyser grating (1)
Be Star QR Vul in outburst
Sept 2008
An outburst of the Be Star QR Vul was
recently reported http://arasbeam.free.fr/spip.php?article12&lang=en
A spectrum of QR Vul was recorded using a Star Analyser on 4th Sept 2008. Dispersion was 28.67A/pixel which is low even for the Star Analyser. (It was set up at the time with a smaller than normal distance to the CCD to record a faint mag 14 Nova.)
The spectrum was reduced as normal and then normalised to a fit to the continuum to make any emission/absorption lines more clear. The same was done with a spectrum of Vega as a comparison.
Some Balmer absorption lines are evident in the in
QR Vul spectrum (H beta and below) but the H alpha wavelength does not
show any obvious evidence of emission or absorption.
To estimate what should be seen in a low resolution
spectrum, I looked in the BeSS
database for high resolution spectra of the H alpha region from before
and after the outburst (Buil Sept 07 and Mauclaire Aug 08). I
normalised them to the continuum and measured the average level relative to the
continuum in an 80A window (approximately the same resolution as the Star
Analyser spectra)
The pre outburst average level was -0.064, an
absorption region which would have easily been detected in a low
resolution spectrum taken using the Star Analyser.
The post outburst average level was only +0.005. ie
essentially flat in an 80A window. This is because the very wide absorption
wings cancel out the emission region in an 80A window. This explains the lack of
absorption or emission seen at the H alpha wqvelength in the post outburst Star
Analyser spectrum.
The conclusion is that the QR Vul outburst could have been
detected using the Star Analyser.
This could provide an efficient way to
survey many Be Stars to fainter magnitudes than is possible at higher
resolution, looking for large changes.