The [Fe <scp>xiii</scp> ] Infrared 10747 and 10798 Å Lines in Novae

Abstract The forbidden lines of [Fe xiii ] at 10747 and 10798 Å are among the most prominent lines in the near-infrared spectrum of the solar corona. They have been used routinely, both outside and during eclipses, as sensitive probes of the electron density and polarization in the solar corona. Many novae pass through a coronal phase, wherein the highly ionized nova ejecta have physical conditions that are remarkably similar to those of the solar corona. Many of the coronal emission lines that are seen are common to the spectra of both the Sun and novae. Yet, it appears that no robust detection of the [Fe xiii ] lines has been made in a nova. Here we report the detection of these two infrared [Fe xiii ] lines in the spectrum of the recurrent nova V3890 Sgr, taken 23.43 and 31.35 days after its 2019 August outburst. From their line strengths, we derive values of 10 10 and 10 [8.5−9] cm −3 for the electron density on the two epochs. The decrease in density between epochs can be explained if the density decreased with a power law n ( r ) ∝ r α , with α inferred to be −3. The average temperature of the coronal gas is estimated to be T = (2.51 ± 0.06) × 10 6 K. We find that recurrent novae with giant secondaries, including T CrB, whose eruption is imminent, are the most suitable sources for further detections of the [Fe xiii ] lines.