V1405 Cas (= Nova Cas 2021) has entered a recombination phase
ATel #14622; S. N. Shore (Univ. of Pisa), F. Teyssier, O. Garde, S. Charbonnel, M. Bajer, C. Boussin, H. Allen (ARAS Group)
on 13 May 2021; 03:09 UT
Credential Certification: S. N. Shore (shore@df.unipi.it)
As a result of our continuing spectroscopic monitoring, described in ATel #14577 (with resolutions ranging from 560 to 12568), we report that the slow nova V1405 Cas has now entered a new phase with the development of a strong recombination event coincident with the start of the optical brightening reported in ATel #14614.
On 2021 May 9.0 there was no detectable absorption on the Na I D lines other than interstellar (EW < 0.06 A, R ~ 11000) while on May 12.1 the two D lines showed strong, nearly identical absorption compoennts (EW = 0.6A) at -670 km/s with FWZI ~ 250 km/s (the line depths, 0.25 of the continuum, suggests partial covering). Lower resolution (R~1500) show the absorption was already present on May 10.8 at a similar intensity. The He I lines (e.g. 4471, 5876, 6678, 7065 A) are all still present but with very weak P Cyg profiles; the absorption extends to -1100 km/s (width of 400 km/s, minimum at -800 km/s). Ca II H and K now display extremely strong absorption, extending to -1400 km/s, with weak emission wings extending to +1200 km/s. All of the Balmer series continue to display strong P Cyg profiles but the visibility of the narrow emission at around 0 km/s has greatly diminished relative to the broader emission component, and is absent on Hgamma and beyond (maximum absorption velocity, -1200 km/s, maximum emission velociity -1300 km/s). These velocities are quite similar to those on the Ca II H and K lines that now display extremely strong absorption, extending to -1400 km/s, with weak emission wings extending to +1200 km/s. The same applies to Si II 6347, 6670 A and to all of the principal Fe II transitions whose emission component is now contiguous with the absorption. The He I lines (e.g. 4471, 5876, 6678, 7065 A) are all present but with very weak P Cyg profiles; the absorption extends to -1100 km/s (width of 400 km/s, minimum at -800 km/s).
The May 10.8 spectrum was flux calibrated with a measured integrated value of 8.E-8 erg/s/cm^2 (3900-7300A). We confirm the DIB equivalent width measurements reported in ATel #14476 (3780 A: 0.32A; 3790: 0.08A; 6613: 0.11A) with a de-reddened flux of 5.1E-7 erg/s/cm^2 applying E(B-V)=0.51 (for standard Galactic extinction). It appears the rapid brightening reported in ATel #14614 #14615 is the signal of a recombination wave driven by the development of strong UV Fe-curtain absorption in the expanding ejecta.
Further high cadence observations arte strongly advised, ours will continue as weather permits.
ARAS Database: Novae
