UVES observations of Nova Reticuli 2020 during Minimal Science Operations show it is entering the nebular phase
ATel #14048; L. Izzo (DARK/NBI), P. Molaro (INAF/Trieste), E. Aydi (MSU), P. Bonifacio (GEPI/O. Paris), G. Cescutti (INAF/Trieste), M. Della Valle (INAF/Naples), E. Harvey, (LJMU), M. Hernanz (ICE/CSIC), G. Schiulaz, P. Selvelli (INAF/Trieste)
on 28 Sep 2020; 15:46 UT
Credential Certification: Luca Izzo (luca.izzo@gmail.com)
With the re-opening of the ESO Paranal Observatory, it was possible to get UVES observations of Nova Ret 2020 (MGAB-V207). The nova was discovered on July 15, 2020, by R.H. McNaught with a mag of 5.3 mag (CBET #4811) and classified as a classical nova (ATEL #13867). The progenitor (EC 03572-5455, Gaia DR2 169291913) was of G = 16.2585 mag and distance of 2.7 +/-0.3 Kpc (Bailer-Jones et al., 2018, AJ 156, 58; ATEL #13868). High resolution (R ~ 80,000-120,000) spectra were acquired on September 18, 2020, with the nova of V ~ 8.5 mag. We used UVES with the DIC1 and DIC2 settings providing the range 310-946 nm.
The spectrum shows broad (FWZI of H_beta ~ 4000 km/s) and strong forbidden lines emission of [O III] 4959/5007, [O III] 4363 blended with H_gamma, and [N II] 5755. The [N II] 6548/6584 lines are blended within the bright H_alpha line. We also detect the doublet [O II] 7320/30 and faint lines with higher ionization potential, as [Fe VII] 6085. We detect high-ionization lines of He I (5876, 6678, 7035), N II 5679, C II 7235, and the Bowen blend structure that is also blended with He II 4686. In the near-UV region, we note the presence of a strong O III 3133 Bowen fluorescence line, whose flux ratio indicates a surprisingly high efficiency in the Bowen mechanism. This suggests an over-abundance of oxygen in the nova ejecta, which also justifies the presence of bright [O I] 6300/6363 and O VI 3435 lines. We remark the presence of [Ne III] 3342 and [Ne V] 3426, which show a line profile similar to Balmer and helium lines, and suggests a ONe white dwarf progenitor.
The high resolution permits disentangling fine structures (~ 100 km/s) on top of the emission lines. In particular, Balmer and He lines show a structured saddle-shaped profile suggesting non-spherical ejecta. Preliminary analysis shows that the observed structure can be explained by a regular symmetric equatorial and polar outflow at an inclination of ~60 degrees, assuming an axial ratio of 1.4. There appears to be a blue bump on all the unblended lines at ~ -1250km/s, and these seem to be matched in the red by a different overall slope. Thus the shell has not yet fully settled into its 'frozen' nebular spectral line structure. No evidence of shell absorptions was found in the Ca II or Fe II lines, suggesting that the nova is entering the nebular phase, as it is also supported by recent X-ray observations of the super-soft emission (ATEL #14043).
The spectrum shows weak Ca II and Na I interstellar lines at a heliocentric velocity of +13 km/s. The Ca II H & K lines have of EW = 42 and 20 mA, respectively, and the Na I D lines of EW = 13 and 6 mA, respectively. Three additional satellite components are detected in the Ca II K line at +31, -4, -17 km/s with EW of 7, 4, and 3 mA, respectively. The main component corresponds to column densities of N(Ca II) = 11.7 and N(Na I) = 10.7 cm-2. The N(Na I)/N(Ca II) ~ 0.1 is typical of warm gas (T ~ 7000 K) with most of the Ca in the gas phase. By using the NaI-H relation, we infer a log N(H+H2) = 19.0 cm-2 (Ferlet et al., 1985, ApJ, 298, 838), and an extinction A_V = 0.0045 mag (Guver & Ozel, 2009, MNRAS 400, 2050). With the typical A_V /E(B-V) = 3.1, we obtain E(B-V) = 0.0015 mag. We note that in this direction (l=265.4, b=-46.4) the Galactic extinction curve by Schlafly et al. (2017, ApJ 838, 36) reports E(B-V) = 0.015 mag, i.e. one order of magnitude higher. Thus, the low gas column density towards a considerable distance of 2.7 Kpc implies a vacant line of sight through the gas in the solar neighborhood (Welsh et al, 2010, A&A, 510, A54).
We gratefully thank the ESO staff for reopening the Paranal Observatory within the Minimal Science Operation scheme which made these observations possible. Based on observations collected at the ESO under the ToO program 105.20B6.001.
