Antarctic Infrared Binocular Telescope: Early Data Release of observations in the 1.4 μm water-vapor-absorption band

TL;DR

This study presents an Early Data Release of AIRBT observations in the 1.4 $\mu$m water-vapor-absorption band using a custom $W'$ filter ($1.34$–$1.48\,\mu$m) at Dome A. It demonstrates that Dome A's extremely low PWV yields high and stable transmission, enabling 2 s exposures that reach $J\sim11.5$ mag and $W'\sim9.9$ mag, and shows that water-vapor features can be identified via a $J-W'$ vs $J-H$ color diagram and by variability in $W'$ relative to $J$. The work provides a preliminary $W'$ calibration based on $J$ and $H$ measurements, introduces an optical-depth proxy $\tau$ for absorption strength, and finds that $\tau$ grows with later spectral types, supporting use of $W'$ to estimate spectral types of ultracool stars. This positions Dome A as a powerful site for 1.4 $\mu$m studies and outlines plans for deeper co-additions, mount upgrades, and a larger telescope to enable time-domain observations of water-vapor phenomena.

Abstract

Ground-based observations around 1.4 $μ$m are normally limited by strong absorption of telluric water-vapor. However, Dome A, Antarctica has exceptionally dry conditions that offer a unique opportunity for observations in this band. We designed a new filter covering 1.34--1.48 $μ$m, namely $W'$, and installed it on the Antarctic Infrared Binocular Telescope (AIRBT) at Dome A in 2025. AIRBT comprises two identical 15 cm optical tube assemblies and two InGaAs cameras equipped with $J$ and $W'$ filters, respectively. With this Early Data Release (EDR), we aim to evaluate the performance of the $W'$ band at Dome A to observe objects with water-vapor features. This EDR covers $\thicksim 20 \ \mathrm{deg^2}$ in the Galactic plane using $\thicksim 20,000$ images in three nights. For 2 s exposures, the 5 $σ$ limiting magnitude histogram peaks at $J \thicksim 11.5$ mag (Vega) and $W' \thicksim 9.9$ mag, respectively. The $J-W'$ vs $J-H$ color-color diagram distinguishes ultracool candidates with water-vapor-absorption features from reddened early type stars. Furthermore, later-type stars tend to exhibit stronger water-vapor absorption. Some sources show larger $ΔW'$ than $ΔJ$ across the three nights, which we attribute to variations of their water-vapor-absorption depth. We conclude that it will be efficient to search for ultracool stars and estimate their spectral subtypes using $W'$ band imaging at Dome A, where the atmospheric transmission is high and stable.

Figures (13)

• Figure 1: Theoretical atmosphere transmission at Dome A (blue, median PWV = 0.141 mm, airmass = 1.0) and Mauna Kea (gray, median PWV = 1.6 mm, airmass = 1.0) at a 4 nm resolution, and filter transmission curves of AIRBT $J$, $W'$ filters and 2MASS $J$, $H$ filters. All filter transmission curves include the effect of atmosphere transmission.
• Figure 2: Upper Left: The Antarctica Infrared Binocular Telescope (AIRBT) at Dome A, Antarctica. Upper Right: The FoVs of the $J$ (shown in yellow) and $W'$ (shown in red) bands. The overlapping area is about $\mathrm{512 \times 360 \ pixel}$ and covers $\delta=-61.7^\circ \thicksim -61.0^\circ$. Lower: The sky coverage of the AIRBT Early Data Release (red) and the full data set (gray) in 2025. The blue line is the Galactic plane.
• Figure 3: An example of raw (left), sampled sky background residuals from SEP after first background subtraction (middle), and background-subtracted (right) image.
• Figure 4: The upper (lower) panels show the twilight flat, photometric flat, and distribution map of the FWHM, respectively, for $J$ ($W'$) band. The dark spots in the $J$ band photometric flat are likely caused by ice crystals, while the striping in the $W'$ band photometric flat is correlated with the striping in the map of FWHM and may be introduced by the $W'$ filter, which will be inspected on the next expedition.
• Figure 5: Transformation for $W'$ band. The left upper (lower) panel is the spectrum of F5III HD17918 (K7V Gl388), the orange, green and purple points are fluxes converted from 2MASS $J$, $H$ and $K_s$; the red triangle is the AIRBT $W'$ flux. The right panel is $J-W'$ vs $J-H$ color-color diagram for stars earlier than K7 from the IRTF spectral library, and the black line is a linear fit of it, as the transformation for $W'$ band.