The first detection of the Raman scattered O VI 1032 A line in classical novae - the case of Nova Del 2013 and Nova Cyg 2014

ATel #6132; A. Skopal (Astronomical Institute of the Slovak Academy of Sciences, Tatranska Lomnica), M. Wolf (Astronomical Institute, Charles University, Prague),M. Slechta (Astronomical Institute, Academy of Sciences of the Czech Republic, Ondrejov), F. Teyssier, J. Montier, T. Lester, O. Garde, C. Buil, T. Lemoult, S. Charbonnel (contributing participants, ARAS)
on 12 May 2014; 14:44 UT
Credential Certification: Augustin Skopal (skopal@ta3.sk)

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We report on a transient emergence of the Raman scattered O VI 1032 A line in the spectrum of classical novae V339 Del (Nova Del 2013) and V2659 Cyg (Nova Cyg 2014). The result of this scattering process - a faint, broad emission feature located around 6825 A - has never been reported for a classical nova to date.

The Raman emission feature was indicated on the spectra obtained within the Astronomical Ring for Access to Spectroscopy initiative (ARAS) with resolutions ranging from 580 to 11000. In the case of Nova Cyg 2014, the presence of a faint Raman emission was confirmed by a spectrum obtained on 2014 April 17.043 UT at the Ondrejov Observatory using a single-order spectrograph attached to the coude focus of the 2-m Perek telescope (resolution of 12000 and the wavelength range 6406-6879 A).

In our spectra of V339 Del, the Raman 6825 A line was seen continuously for more than 1 month, from Aug. 19 (day 5 after the nova discovery; see CBET No. 3628) to around the end of Sept. 2013 (around day 40, at the end of the first plateau phase in the light curve; see Munari et al. 2013, IBVS 6080). In the spectrum of Nova Cyg 2014, we first indicated the Raman line on April 14, around day 4 after the V-maximum (see the AAVSO light curve) and lastly on April 24. On the following spectra, we obtained at the beginning of May 2014 around a secondary optical maximum, no Raman 6825 emission was detectable. The profile of the present lines (mostly of H I, He I and Fe II) changed again into the P Cyg-type. Such the evolution of nova V2659 Cyg and conditions for emergence of the Raman scattered 6825 A line suggest its re-appearance during following transition of the nova to a harder spectrum. Examples of the Raman 6825 A line in the spectrum of classical novae V339 Del and V2659 Cyg are available at http://www.ta3.sk/~astrskop/atel_052014/v339_ncyg_ra.png .

On 2014 May 06.057 UT, the absorption component in the H-alpha profile was located at around -1200 km/s as we measured at the Ondrejov Observatory spectrum of V2659 Cyg. This value is more than a factor of 2 larger than was measured a few days after the nova discovery (see Munari et al., Arai and Ayani, CBET 3842; Tomov et al. ATel #6060) and still indicated in our spectra from April 10, taken during the first optical maximum.

The Raman conversion of the original 1032 A line to 6825 A emission requires the column density of the neutral H I atoms on the line of the incident O VI photons to be as large as 1E+24/cm**2. Therefore, the presence of the Raman scattered O VI line in the spectrum of classical novae puts constraints in determining the ionization structure of the nova ejecta during its early period of evolution.