Recent developments in optical spectra of nova AT 2019qwf (Nova V2891 Cyg)
ATel #13340; U. Munari (INAF Padova), K. Cotar (Univ of Ljubljana), V. Andreoli, S. Dallaporta (ANS Collaboration), L. N. Yalyalieva (Sternberg, Moscow)
on 9 Dec 2019; 16:36 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)
AT 2019qwf (=PGIR 19brv=ZTF 19abyukuy) was discovered on Sept 17, 2019 and
proposed as a highly reddened Galactic nova by De et al. (ATel #13130),
later classified of the FeII type by Munari (ATel #13283). Further optical
spectroscopic observations were reported by Lee et al. (ATel #13149) and
Srivastava et al. (ATel 13258), and near-IR data by Joshi et al. (ATel
#13301). All these reports concur on AT 2019qwf being very reddened, still
lingering around maximum brightness more than two months after its
discovery, showing emission lines typical of the FeII class and
characterized by narrow profiles.
We are regularly acquiring low resolution spectra of AT 2019qwf with the
Asiago 1.22m telescope + B&C spectrograph, covering the 3200-8000 Ang range
at a dispersion 2.31 Ang/pix, and with the Asiago + Echelle spectrograph,
over the 3600-7300 Ang range at a resolving power R(P)=20,000.
Our B&C spectra for November 5, 13 and 27 resemble each other closely, with
strong signatures of interstellar absorption (NaI, KI and various DIBs), and
emission lines from Balmer, OI 7772, NaI, FeII multiplets 38, 42, 48, 49,
and 74, and NII multiplet 3. All emission lines presented P-Cyg absorption
components, that became particularly pronounced on Nov 27 (E.W.=8 Ang for
OI 7772).
The most recent B&C spectra that we have obtained on Dec 6 and 7 display a
series of marked changes compared with the previous months: all P-Cyg
absorptions have now disappeared, the equivalent width of all emission lines
has greatly increased (from 6 to 31 Ang for FeII 5018 Ang multiplet 42),
their FWHM widened (from 350-500 km/s to 900/1000 km/s for FeII and Balmer
lines; values corrected for instrumental resolution), and Balmer lines now
clearly show structured profiles. In particular, strong [OI] has now
emerged, both the auroral 5577 and the nebular 6300, 6364 lines, in addition
to permitted multiplets (like #21 at 7002 Ang). The [OI] are usually the
first forbidden lines to appear in FeII novae thanks to their low ionization
and high critical densities. The [OI] 6300/6364 flux ratio was 2.0 on Dec 6
and 2.4 on Dec 7, much lower than the intrinsic 3.0 ratio and suggesting a
large optical depth (as usual in early stages of FeII novae).
Our latest Echelle spectrum has been obtained on Dec 8.75 UT. It shows the
Halpha profile being composed by a broad emission component with a Voigt
profile and FWHM=930 km/s, centered at an heliocentric velocity of -28
km/s. Superimposed to it, two absorption components are seen: one at an
heliocentric velocity of -300 km/s and the other at -650 km/s, that we
identify as the "pre-maximum" and "principal" components as typical of FeII
novae (cf. Munari 2014, ASPC 490, 183).
The acceleration in the spectroscopic evolution seen over the last few days
may signal the end of the protracted plateau at maximum brightness
and the start of the decline from it. Clearly a phase worth further
observations.
