The [Fe XIII] Infrared 10747 Angstrom and 10798 Angstrom Lines in Novae

TL;DR

The study addresses the lack of robust detections of the infrared [Fe XIII] coronal lines in novae by presenting two-epoch near-infrared spectra of the recurrent nova V3890 Sgr taken at 23.43 and 31.35 days after its 2019 eruption. They detect the [Fe XIII] 10,747 Å and 10,798 Å lines, deriving an electron density of $N_e \,=\ 1\times10^{10}$ cm^-3 at day 23.43 and a decline to about $3\times10^{8}$–$1\times10^{9}$ cm^-3 by day 31.35, with a coronal gas temperature of $T \,=\ (2.51\pm0.06)\times10^{6}$ K. The observed rapid density drop, well described by a steep $N_e \propto r^{-3}$ expansion and a Fe XIII production peak near day 16, implies an evolving, shock-driven, coronal-phase plasma in a system with a red-giant wind; this strengthens the case for coronal-line formation in RG-containing novae and guides future searches (eg, in T CrB). The work demonstrates the utility of infrared [Fe XIII] lines as diagnostics for density, temperature, and potentially magnetic-field information in nova ejecta, linking nova coronal physics to solar studies and informing observational strategies for similar systems.

Abstract

The forbidden lines of [Fe XIII] at 10,747 Angstrom and 10,798 Angsrtom 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 August 2019 outburst. From their line strengths, we derive values of 10^10 per cubic cm and 10^[8.5-9] per cubic cm for the electron density on the two. The decrease in density between epochs can be explained if the density decreased with a power law n ~ r**alpha with a alpha inferred to be -3. The average temperature of the coronal gas is estimated to be T = (2.51\pm0.06) x 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. epochs.

Figures (5)

• Figure 1: Term diagram for the [Fe13] lines of interest. The transition probabilities are shown to the right of the wavelengths. The $^3$P$_2$ and $^3$P$_1$ are 1.15 eV and 2.301 eV above the ground state respectively. Transition probabilities taken from NIST database https://physics.nist.gov/PhysRefData/ASD/lines_form.html
• Figure 2: The [Fe13] 10 747Å, 10 798Å forbidden lines, detected at two epochs, against the backdrop of the broad He1 10 830Å line. The spectra also clearly reveal Fe2 9 997Å (not shown here), 10 501, 10 863, and 11 126Å -- the so-called "1 micron Fe II lines" rudy00. These descend from a common upper multiplet and are routinely detected in novae and a variety of emission-line stars. The blue ticks, identifying the lines, are marked at identical wavelengths in both panels. The Y-axis in the bottom panel is on a linear scale to emphasize the weakening of the [Fe13] 10 798Å line between epochs.
• Figure 3: Variation of the electron density (N$_{\rm{e}}$) for V3890 Sgr from 2023AA...674A.139A; their values were binned and averaged over intervals of 2 days. The blue squares correspond to data from this infrared study. See Table \ref{['nete']} and text (section \ref{['sec:FeNe']}) for details.
• Figure 4: The continuous lines show the time at which the bulk of the [Fe13] should be produced to explain the drop in density of the ejecta by a factor of 10 or more in 8 days. The dashed line gives an example that a drop in density by 10, requires that the [Fe13] ion production maximize at around day 16 (for N$_{\rm{e}} \propto{r}^{-3}$ expansion law). The bottom panel, based on Swift observations, shows the evolution of the black body temperature of the white dwarf during the SSS phase (see Section \ref{['sec:FeNe']}, as well as page20).
• Figure 5: The [Fe13] 10 747Å line in SN 1987A. This figure was prepared using JWST/NIRSPEC data available from the Mikulski Archive (MAST). The original study is described in larsson23, who found several high-ionization coronal lines from the equatorial ring of SN 1987A, requiring a temperature $\ge2\times10^6$ K. As in Figure \ref{['coronals']}, we identify two of the "1 micron Fe lines" here to show the similarity with the profile in V3890 Sgr. A blue "X" shows the expected position of the 10 798Å Fe line, which however is not seen. The observed spectrum had to be shifted blueward by 13Å to make the H, He and Fe2 spectral features match with their rest wavelengths.