V3890 Sgr: no deceleration of the ejecta after all ?

ATel #13099; U. Munari (INAF Padova) and F. M. Walter (Stony Brook University)
on 11 Sep 2019; 18:54 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)

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We are continuing the daily spectroscopic monitoring of the eruption of V3890 Sgr with Asiago 1.22m and SMARTS 1.5m telescopes (cf. ATel #13069, #13081). This symbiotic star and recurrent nova is currently the subject of an all-out campaign from X-rays/UV (ATel #13050, #13072, #13083, #13084, #13086), to Infrared (ATel #13088, #13096), and Radio (ATel #13089, #13092).

One recurrent comment about the eruption of V3890 Sgr is the supposed sharpening of emission lines with time. This is expected in the case of nova ejecta expanding within and decelerated by the wind of the red giant companion, as manifestly evident in the 2010 outburst of V407 Cyg (Munari et al. 2011, MNRAS 410, L52), the first nova observed in gamma-rays (ATel #2487).

We have compared all our high-resolution, daily spectra of V3890 Sgr, including the last one obtained on Sept 11.77 UT with the Asiago 1.22m telescope, and the conclusion we reached is quite surprising: there is no evident deceleration in the broad pedestal of emission lines, in particular Halpha which is the line best showing it at this advanced decline stage (we are now already past t_3). The FWZI of Halpha was ~8300 km/s on Aug 29 (when it fully emerged in emission), and it is still 7900 km/s on Sept 11. Initially, the main body of emission lines was broadly triangular, and it has carefully retained its shape (including all superimposed wiggles) while reducing in flux with passing days, and in the process progressively revealing a narrower (~500 km/s) component, which gave the impression of an overall sharpening of the profile. The initial broad, triangular main body has now declined to be the pedestal of the narrow component.

There are at least other two novae that have show the initial flash-ionization of the wind of the evolved companion but at the same time negligible deceleration of the nova ejecta: V2944 Oph (Nova Oph 2015; Munari and Walter 2016, MNRAS 455, L57) and V1534 Sco (Nova Sco 2014; Munari and Banerjee 2018, MNRAS 475, 508).

The initial flash-ionization of the RG wind in V3890 Sgr has produced a very sharp emission component (with superimposed absorption from external neutral material not ionized by the flash) that completed recombination by Aug 30.8, or about 3.4 days past initial TNR, estimated in ATel #13047 to have occoured on Sept 27.4+/-0.2. Following Osterbrock and Ferland (2006, "Astrophysics of gaseous nebulae and active galactic nuclei") the corresponding electron density in the flash-ionized portion of the RG wind is therefore 4e+7 cm-3.