V2670 - Nova Oph 2008


Above is an average combine of 7x10s uncalibrated, unfiltered, unguided images taken with an ST8XME camera on a C11f10 Schmidt-Cassegrain telescope in Carp, Ontario, Canada.
The nova V2670 is at the coordinates 17h39m50.91s -23d50m00.6s (J2000) in the constellation of Ophiuchus just east of the globular cluster NGC6401. The approximate magnitude of the nova on May 31, 2008 was calculated using the circled reference star on the left, I estimated the uncertainty to be on the order of 0.2 magnitudes.





Spectrum of the nova V2670 with the sky background subtracted. Recorded with a SBIG ST7XME camera coupled to a LHIRES3 spectrograph with a SBIG Remote Guide Head used to keep the target on the entrance slit. The telescope was a Celestron 11"f/10 SCT on a Losmandy G11 equatorial mount with a Gemini goto system housed in my backyard SkyShed POD observatory. The slit width was 29 microns and the instrument dispersion is 0.3 nm/pixel. The spectrum wavelength was calibrated from a linear fit of the known spectral peaks of the built-in Neon lamp. The spectrum was smoothed by an 11pt, 4th order Savitzky-Golay filter, this type of filter preserve the peak shapes while reducing the high frequency noise.
The vertical axis is plotted in ADU/min since I used different exposure lengths. The blue trace was 3 x 5 min exposures, the pink trace 1 x 10 min and the dark red trace 4 x 10 min. The subexposures were individually dark frame subtracted and median combined to remove gamma ray artifacts. Each subexposure was acquired in 1x3 binning mode to reduce the effect of read noise as much as possible. Each spectral point is the vertical sum of 7 signal rows, which is actually 21 CCD rows due to in camera binning, minus 7 sky background rows. Variations in flux from one spectra to the next are only due to changes in sky transparency, target star seeing disk size and guiding quality. Perhaps in the future I will record with the guiding camera the integrated star intensity on and off the slit to estimate system efficiency and therefore calibrate the flux for repeated measurements of the same object.
The two peaks at 615.5 nm and 623.9 nm are due to Fe-II (multiplet 74) emission. There were also early Fe-II (multiplet 49) emissions at 527.8 nm and 531.7 nm. The Na-I doublet at 589nm was very weak but has strengthened considerably over the course of two weeks. The absorption peak near 688 nm is due to the oxygen in the Earth's atmosphere, it has weakened as the continuum level has faded.





The mean expansion velocity of a nova can be approximated from the Doppler broadening the spectral lines. The radial velocity is computed from the expression

where c is the speed of light, lambda is the center wavelength and delta lambda is the full width at half maximum (FWHM). The factor of 2 accounts for the expansion away and towards the observer, presumably at the same velocity for a symmetric line. From the Halpha emission line at 656.3 nm I estimate the radial velocity to be 740 km/s on May 30th, and 685 km/s on June 10th. I believe this nova would be classified as a Fe-II class "slow" nova. The triple peaked line on June 12th may indicate that the expansion has changed from a continuum of velocities to a slower shell (central narrow peak) and a separate thin shell (outer peaks) expanding at greater velocity. The strength of the Halpha line relative to the continuum has changed considerably over the measurement period. The Halpha/continuum ratio is largely unaffected by changes in system efficiency. Unfortunately clouds and rain nearly every day in June, which is unusual for this location, have curtailed further observations of this intriguing object.