Strong Neon Emission in CSS081007:030559+054715: A Possible Oxygen/Neon Nova

ATel #1835; J. L. Prieto, K. Denney, O. Pejcha, & R. M. Wagner (Ohio State University)
on 7 Nov 2008; 21:54 UT
Credential Certification: Jose Prieto (prieto@astronomy.ohio-state.edu)

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On 2008 October 30.4 we obtained low-resolution optical spectra (range 3350-4800 Angstroms) of the transient CSS081007:030559+054715 using the Boller & Chivens CCD Spectrograph on the 1.3m McGraw-Hill telescope at MDM Observatory. The most striking feature in this spectral region is very strong, double-peaked [Ne V] at 3426 Angstroms. The two peaks are offset by -1700 and 1100 km/s with respect to the rest wavelength. The equivalent width of this line is about five times higher than of Halpha in our previous spectrum (ATEL #1825). The HWZI of this line is 3100 km/s. Similar double-peaked structure with the same wavelength shifts are observed in substantially weaker lines of [Ne III] at 3869 and 3968 Angstroms. The bright emission line between 4650 and 4760 Angstroms, that we observed in ATEL #1825, is due to [Ne IV] 4721 blended with He II 4686.

Apart from Catalina Real-time Transient Survey, CSS081007:030559+054715 was observed by several other surveys. The All Sky Automated Survey (Pojmanski, 2002, Acta Astronomica, 52, 397) detected the object four times, specifically on 2008 Jan 14.19 UT at V=16.09(7), 2008 Jul 17.4 UT at V=14.26(5), 2008 Jul 24.4 UT at V=14.37(5), and 2008 Aug 3.4 UT at V=15.06(5). John Greaves brought to our attention that CSS081007:030559+054715 was measured by SDSS on 2004 Dec 14.2 UT to have u=18.61(2), g=18.56(1), r=18.52(1), i=18.57(1) and z=18.68(5).

Photometry on our RETROCAM images from 2008 Oct 27.35 UT give g=15.94(5), r=16.12(5) and i=16.28(5). Two other recent photometric measurements are provided by CRTS followup and indicate that CSS081007:030559+054715 is slowly fading, but with fairly rapid decline in the g-band.

The ASAS measurements from 2008 Jul and Aug are very close to the limiting magnitude of that survey, but seem to indicate that the object was brighter during the seasonal gap. Our blue spectrum is similar to the late nebular phase spectrum of the very fast Nova Sgr 1991 (V4160 Sgr, see Figure 10 of Williams et al. 1994, ApJS, 90, 297), which developed very strong and broad forbidden neon emission lines. The exceptional strength of the forbidden Ne emission lines in the spectrum of CSS081007:030559+054715 suggests that it is a member of the class of neon novae which may originate on massive ONeMg white dwarfs and the outburst occurred during its seasonal gap, i.e., around JD 2454600.

The plot of the MDM spectrum and the light curve can be seen at http://www.astronomy.ohio-state.edu/~pejcha/crts_transient.

We thank K. Z. Stanek for useful discussions and suggestions.

Webpage with spectra and light curve of the object.