High ionization conditions finally emerge as Nova Cas 2021 (V1405 Cas) ends the plateau and embraces a steady decline
ATel #15093; U. Munari (INAF Padova), P. Valisa, S. Dallaporta, and A. Maitan (ANS Collaboration)
on 8 Dec 2021; 16:37 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)
Nova Cas 2021 (= PNV J23244760+6111140 = V1405 Cas) was discovered
on 2021 March 18.424 by Yuji Nakamura and measured at
B=9.58, V=9.27, R=8.78 on March 18.684 UT (CBET #4945). At that time the
nova was still rising in brightness and high ionization features
(HeII, Bowen blend) were briefly visible on its spectra (ATel #14471
#14472). The high ionization were all gone by the following day when the nova completed the rise in brightness to B=8.22, V=7.72,
R=7.24, I=6.94 and the spectra displayed only Balmer and HeI emission lines,
as we reported for March 19.76 UT in ATel #14476. We are since then keeping a constant
photometric and spectroscopic watch of Nova Cas 2021 with various ANS
Collaboration photometric instruments (primarily ID 0310 and ID 2203), and
with Varese 0.84m+Echelle, Asiago 1.22m+B&C, and Asiago 1.82m+Echelle
telescopes.
For the first seven months, Nova Cas 2021 lingered around maximum brightness
in a sort of plateau, with V mag varying between 6.4 and 8.4 mag with no
less than eight distinct rebrightenings, as the ANS Collaboration BVRI
lightcurve presented below well highlights, attaining maximum brightness on
May 9.03 UT at V=5.449, B-V=+0.652, V-R=+0.452, and V-I=+0.833. The passage
through such short-lived maximum turned the spectrum of Nova Cas 2021 to a
text-book example of a FeII-type event (ATel #14614), and it was soon
followed by detection of the nova in gamma-rays by Fermi-LAT (ATel #14658).
Such a detection sprinted forward the expectation for synchrotron signatures
of particle-accelerating shocks, but none was found in VLA
radio observations during June that recorded only normal thermal emission
from the nova (ATel #14731).
ANS Collaboration BVRI lightcurve shows that Nova Cas 2021 left the
seven-months-long plateau around the end of October/beginning of November 2021,
ending the incessant up-and-downs in brightness and embracing instead a
linear decline at a rate of 0.036 mag/day which is continuing well into
December, with B-V and V-I colors that are becoming progressively bluer,
while V-R remains flat. Our latest measurement for Dec 7.761 UT provides
V=9.184 B-V=+0.276, V-R=+0.939, and V-I=+0.828, with the nova now close to 4
magnitudes below the peak brightness of May 9.
In parallel with the end of the plateau and the beginning of the linear
photometric decline, also the spectra transitioned to an entirely new
state, as illustrated by the figure below where a small portion of the
latest Echelle spectra we obtained with the Varese 0.84m is presented: (1)
the ionization conditions are rapidly increasing as illustrated by the
appearance and steady growth in intensity of HeII and the 4640 Bowen blend (and
parallel disappearance of low-ionization features like FeII), (2) the P-Cyg
absorptions that have always been rather strong during the plateau (cf. ATel
#14478, #14482, #14577, #14622, #14665) are now much reduced in intensity, (3) the
emission line profiles are developing a distinctive three-component profile, best
described for the Hbeta shown below as a double-peaked central component (640 km/s
in width at half maximum), superimposed to a trapezoidal and broader component
(WHM=1780 km/s), on top of a wider pedestal at the bottom (WHM=2860 km/s,
FWZI=3680 km/s).
It is interesting to note on the figures below how a weak and narrow P-Cyg absorption
(at 2085 km/s from the photocenter of Hbeta emission) has reappeared on the
last two Echelle spectra for 2021 Dec 6 and 7, simultaneous with an halt in
the linear photometric decline from the plateau. It seems be forming in a
new/reinforced wind blowing off the central star, internal to the larger
ejecta where the 3-components emission line profiles originate.
Echelle spectra and lightcurve of Nova Cas 2021
