A Population of Heavily Reddened, Optically Missed Novae from Palomar Gattini-IR: Constraints on the Galactic Nova Rate

Abstract

The nova rate in the Milky Way remains largely uncertain, despite its vital importance in constraining models of Galactic chemical evolution as well as understanding progenitor channels for Type Ia supernovae. The rate has been previously estimated to be in the range of ≈10-300 yr −1 , either based on extrapolations from a handful of very bright optical novae or the nova rates in nearby galaxies; both methods are subject to debatable assumptions. The total discovery rate of optical novae remains much smaller (≈5-10 yr −1 ) than these estimates, even with the advent of all-sky optical time-domain surveys. Here, we present a systematic sample of 12 spectroscopically confirmed Galactic novae detected in the first 17 months of Palomar Gattini-IR (PGIR), a wide-field near-infrared time-domain survey. Operating in the J band (≈1.2 μm), which is significantly less affected by dust extinction compared to optical bands, the extinction distribution of the PGIR sample is highly skewed to a large extinction values (>50% of events obscured by A V  5 mag). Using recent estimates for the distribution of Galactic mass and dust, we show that the extinction distribution of the PGIR sample is commensurate with dust models. The PGIR extinction distribution is inconsistent with that reported in previous optical searches (null-hypothesis probability <0.01%), suggesting that a large population of highly obscured novae have been systematically missed in previous optical searches. We perform the first quantitative simulation of a 3π time-domain survey to estimate the Galactic nova rate using PGIR, and derive a rate of » -+ 43.7 8.719.5 yr −1 . Our results suggest that all-sky near-infrared timedomain surveys are well poised to uncover the Galactic nova population.

The impact of metallicity on nova populations

“…We compute a current Galactic nova rate of approximately 33 novae per year, broadly consistent with observational estimates making use of extra-Galactic nova data. This prediction is inconsistent with observational estimates making use of Galactic nova data, including the recent De et al (2021) estimate of 43.7 +19.5 −8.7 novae per year, which used Galactic novae observed in the near-infrared. However, we note that our theoretical rate estimate could potentially vary by as much as 30 novae per year under different combinations of physical assumptions.…”

“…This work's estimate of 33 novae per year 3 can be regarded as consistent with observational estimates deriving from extra-Galactic nova data. Relative to observational estimates deriving from Galactic optical nova data, as well as the De et al (2021) estimate, our result can be considered an under-prediction, but not an excessive one. However, it should be noted that the nature of our methods means that our nova rates are inherently sensitive to modelling choices.…”

“…Estimates using this method typically arrive at numbers ranging from approximately 30-80 novae per year (Shafter 1997;Hatano et al 1997 De et al 2020) in the near-infrared, where extinction caused by interstellar dust is far less prevalent. Figure 10 of De et al (2021) presents a nice summary of the current state of the field regarding Galactic nova rates. This work's estimate of 33 novae per year 3 can be regarded as consistent with observational estimates deriving from extra-Galactic nova data.…”

“…. However, observationally estimating the Galactic nova rate is made difficult by complications such as extinction caused by interstellar dust, source crowding, systematic biases caused by our place in the Galaxy, and the selection functions of the surveys in which novae are detected(Shafter & Irby 2001;Darnley et al 2006;Shafter 2017;Della Valle & Izzo 2020;De et al 2021). These issues are mitigated substantially by observing M31, where it is far easier to estimate completeness and account for a wide range of observational systematic uncertainties.…”

A Speckle-Imaging Search for Close Triple Companions of Cataclysmic Binaries

“…Five to ten erupting Galactic classical novae are usually detected in any given year... often by amateur astronomers. A germinal wide-area, infrared survey of the Milky Way has recently shown that the Galactic nova eruption rate is 46 ±12 novae/yr (De et al 2021). This confirms that ∼ 80-90% E-mail: mshara@amnh.org of Galactic novae have been missed over the past century because they are distant and reddened.…”