Guess What I Was Doing New Year’s Eve?
It was clear here on New Year’s Eve and I decided to try something a bit different. I installed the 150 line/mm grating in the spectrograph and tried to acquire a spectrum of a newly-discovered transient object called ASAS-SN14mv. It appears to be a dwarf nova in outburst and since I gather lots of photometric data on cataclysmic variables, and dwarf novae in particular, it made sense to try. The object was at about 12th magnitude so I knew it would be a challenge. Figure #1 is the resulting spectrum from the combination of three 1800-second exposures.
Note the absence of hydrogen lines. In fact, when compared to the spectrum of an A0V star (like Vega) the spectrum is fairly similar except for this absence. Outbursts of dwarf novae originate in the accretion disk surrounding the white dwarf star in a very close double-star system. During the early stages of the outburst the disk reaches a temperature similar to the photosphere of a star like Vega, around 10,000 K, so not surprising that it’s continuum shape would mimic that of Vega. Prior to outburst (in quiescence) these objects typically have obvious emission lines of hydrogen and helium. But the light from the outburst overwhelms the brightness of the emission lines and, conversely, usually show broad lines of hydrogen absorption, sometimes with small emission peaks in their centers. At the time these data were acquired ASAS-SN14mv was likely very close to it’s maximum brightness (around 12th magnitude). The spectrum shows hints at a number of lines other than the two mentioned in the caption, but being a fairly novice spectroscopist I’m hesitant to say for sure what they are. But comparing to A-type stars (objects with similar surface temperatures) I could guess that Fe II λ4926 is present, as well as badly subtracted sodium night sky lines at λ5683-5688, mercury at λ5460, and another sodium doublet at λ5890-5896. In fact the biggest challenge with the extraction of these spectra from the 2-d images was to get accurate subtractions of night sky lines. Figure 2 shows what one of the spectra looked like before being extracted. Figure 3 is a spectrum of the night skies here. Pretty dreadful!
The fact is that some of these lines might actually be useful for wavelength calibration since my only reference lamp, at present is Neon – and there are no bright lines blueward of about 5400 Angstroms. Note that the values in the Figure 3 annotations are based on a similar plot that appears on Christian Buil’s site HERE. Christian’s site is a wealth of information for anyone interested in the topic of astronomical spectroscopy and is very highly recommended! Clearly he’s dealing with the same bright sky problems that I do and achieves great results!