Infrared Spectroscopy of YZ Reticuli (Nova Reticuli 2020)

YZ Reticuli (Nova Reticuli 2020) (hereafter YZ Ret) was the visually brightest nova since 2013 but was also notable for other reasons. It was a rare nova for which the precursor was previously identified and had a measured parallax. This gave immediate information concerning its distance and thus its luminosity. Also, because it was located at a relatively high Galactic latitude, it was both devoid of nearby neighbors, that might have confused the identification of the precursor, and had little or no interstellar reddening, that would have attenuated its ultraviolet and X-ray emission. Moreover, YZ Ret was actually detected in gamma rays (Li et al. 2020) prior to its optical discovery (McNaught 2020), and was studied extensively at X-ray wavelengths (Pei et al. 2020; Sokolovsky et al. 2020).

Early optical spectroscopy (Aydi et al. 2020) showed broad lines and P-Cygni profiles extending out to 2700 km s⁻¹. Emission lines of H i, O i, and, most notably, Fe ii, were present suggesting that YZ Ret was an "Fe ii" type nova. However, spectroscopy from a half day later by Carr et al. (2020) showed a spectrum that had already evolved and led to their classification of YZ Ret as a "He/N" nova. That classification was consistent with the lines widths, rapid spectral development, and later light curve: AAVSO data showed the time to decline by two magnitudes was t₂ = 12 days, placing YZ Ret in the "fast" speed class (Warner 1995).

This paper supplements the X-ray, UV, and optical observations of YZ Ret with infrared spectroscopy from three epochs. A initial summary of the first and second epoch measurements was reported by Sitko et al. (2020).

The infrared spectroscopic data presented here were obtained on the nights of 2020 October 7, 2020 November 9, and 2021 February 13 using the Spex instrument of the Infrared Telescope Facility (Rayner et al. 2003). The October and February observations covered the wavelength range 0.7–2.5 μm; the November measurements included the same range but with additional coverage extending out beyond 4 μm. The raw measurements were converted to absolute flux using the Spextools software package (Cushing et al. 2004) together with observations of the A3 V calibration star HD 25590, which was observed close in time and at a similar airmass to YZ Ret.

The three epochs of spectroscopy are presented in the panels of Figure 1. The individual panels each span a different wavelength range and include the different epochs of data. The spectra are scaled and offset to better exhibit the data. Midwave infrared (MWIR) data were obtained during the 2020 November observations and attempted in 2021 February, but the object had faded beyond detection by that time. Selected emission lines are labeled.

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By the time of our first measurement, YZ Ret was already well into its coronal phase. The coronal lines we detect in the 2020 October 7 spectrum are: [S viii] λ0.9914, [Si vi] λ1.9645, [Ca viii] λ2.3210, and [Si vii] λ2.4833. [Mg viii] λ3.0279 was observed in the November MWIR spectrum. High excitation permitted lines of N v seen frequently in novae are present in YZ Ret at 1.1112, 1.5540, and 2.0996 μm. He ii and C iv lines are detected and C vi lines are probably present. He i λ1.0830 was the strongest emission feature in the wavelength range spanned by our observations for all three epochs.

Despite the presence of such high excitation lines, YZ Ret showed a number of low excitation features as well. These included lines of N i and O i and forbidden lines of [N i] and [O ii]. These lines displayed a triangular profile, markedly different from the broad, double-peaked line shape that characterized the H i and high excitation lines. Perhaps the most interesting is O i λ0.8446. In the October data, its shape is much like the hydrogen lines and like O i λ1.1287, since both lines are produced predominantly by Lyβ fluorescence. That process occurs in the H ii region where hydrogen is predominantly ionized and recombining. However, there is a small central peak in O i λ0.8446 that is produced by continuum fluorescence and originates in the zone where oxygen is mostly neutral. The O i λ1.3165 line is almost completely produced by this process and it has a triangular appearance that characterizes line emission in the neutral region of YZ Ret. By November, however, the Lyβ fluorescence had weakened considerably (as seen from the near disappearance of O i λ1.1287), and O i λ0.8446 is mostly produced by continuum fluorescence and shows a much more triangular profile.

The three YZ Ret spectra show the expected decline in brightness after outburst. The spectrum itself hardened between the 2020 October (day 84 after maximum light) and November (day 115) measurements. This is best illustrated by the strengthening of the [Si vi] and [Si vii] lines relative to the nearby lines of H i and He i. Conversely, the excitation fell between 2020 November and 2021 February (day 213). This is seen most clearly by the near disappearance of [Si vii] line. This reflects a drop in, or even the cessation of, nuclear burning. YZ Ret retained its largely neutral gas even for our latest observations, still exhibiting lines like [N i] λ1.0400 and O i λ1.3165.

In none of our spectra covering the three epochs was there any evidence for the presence of dust intrinsic to the nova. This best seen in the mid-wave infrared spectrum in the bottom panel of Figure 1. Even a small amount of dust would emit significantly is this wavelength range and none is seen.

Nor do the spectra of YZ Ret show measurable effects from dust in the interstellar medium. This is not surprising given the high Galactic latitude of YZ Ret (−464). Estimating the reddening using the technique of Rudy et al. (1991) as applied to the O i lines λ0.8446, λ1.1287, λ1.3165, and the reddening law of Schlafly & Finkbeiner (2011), gives E(B–V) = 0.02 and −0.06 for the October and November data, respectively, consistent with no reddening and an error of about E(B–V) = 0.1.

We thank Drs. J. Rayner and B. Bus of the Infrared Telescope Facility for making observing time available, and B. Cabreira, D. Griep, and G. Engh for critical assistance in acquiring the data.

This work also made use of the Atomic Line List (van Hoof 2018).

Facilities: IRTF - Infrared Telescope Facility, AAVSO. -