Secondary standards in the UKIRT faint standard fields

TL;DR

This study addresses the need for dense, faint infrared standard stars to improve photometric calibrations. It presents a decade-long, multi-night campaign (2008–2018) with the NTT/SOFI at La Silla to produce 128 secondary standards in 19 UKIRT/MKO faint standard fields, calibrated in the MKO system for the $J$ and $K$ bands. The authors implement rigorous photometry and a differential-star analysis to ensure stability, achieving magnitudes from $J$ to $K$ with precision better than $0.01$ mag and median values of $ ilde{J}=13.5$ and $ ilde{K}=13.0$, while providing a robust standardization scheme including a two-period color-term treatment due to instrumental changes. The resulting catalog enables improved calibration accuracy for UKIRT and other infrared observations without extra observing overhead, benefiting precision photometry in deep surveys and distance-scale work.

Abstract

We present precise J- and K-band photometric measurements for 128 near-infrared secondary standard stars, located in the 19 UKIRT/MKO primary faint standard fields. The data were collected over more than 50 nights, covering a decade of observations between 2008 and 2018 at the ESO La Silla Observatory, using the New Technology Telescope (NTT) equipped with the SOFI NIR camera. Presented magnitudes are calibrated onto the MKO photometric system. The J- and K-band magnitudes range from 10 to 15.8 mag, with median values of $\tilde{J}$ = 13.5 and $\tilde{K}$ = 13 mag. The selection process ensured high photometric quality, with a precision better than 0.01 mag for all stars. The catalog excludes stars with close neighbors, high proper motion, or variable stars. Using these fields for standardization can improve the precision and accuracy of photometric calibrations without increasing the observational time cost.

Figures (3)

• Figure 1: RMS versus the average DAOPHOT error for all stars in the exemplary field FS001. The upper panels display the results of the first iteration of the differential correction, while the lower panels present the results of the second iteration. The red solid lines represent the fitted models (eq. \ref{['eq:err']}). Comparison stars (green points) were selected if their RMS value did not exceed the corresponding value of the fitted function by more than 0.01 mag, and if its formal DAOPHOT error was below 0.04 mag (dashed red vertical line). The green shaded area indicates the region where both criteria are satisfied.
• Figure 2: Equation \ref{['eq:transformation']} color-term coefficient (upper panel), airmass coefficient (middle panel), and zero-point (lower panel) values obtained for all nights (observing epochs) using the free-fit approach. Blue and red points represent values for the J- and K-band, respectively. The blue horizontal lines in the upper panel mark the average values of the J-band color-term coefficient with their corresponding uncertainty (blue shaded area) for the periods up to and including 12 December 2013, and starting from 8 December 2014. Notably, the difference in the mean coefficient value reaches a significance level of $3\sigma$. The dotted blue and red lines in the middle panel mark the $3\sigma$ range for the J- and K-bands, respectively. The green dashed vertical lines in the lower panel indicate epochs where consecutive observations were separated by more than one week. The red vertical line between epochs 27 and 28 marks the transition corresponding to the change occurring between 12 December 2013 and 8 December 2014, as discussed in the text. The correspondence between observing epochs and calendar dates is given in Table \ref{['tab:coefficients']}.
• Figure 3: Equation \ref{['eq:transformation']} zero-point values obtained for all nights (observing epochs) using the general least-squares fitting using a single value of the airmass coefficient for the entire 49-epoch observational period, and two values of the color coefficient separated into two periods: up to and including 12 December 2013, and starting from 8 December 2014, for the J- and K- bands (blue and red points, respectively). The green dashed vertical lines indicate epochs where consecutive observations were separated by more than one week. The red vertical line between epochs 27 and 28 marks the transition corresponding to the change of the color-term coefficient occurring between 12 December 2013 and 8 December 2014. The correspondence between observing epochs and calendar dates is given in Table \ref{['tab:coefficients']}. It can be noted that zero-point variations are much smaller compared to the free-fit results presented in Figure \ref{['fig:coefficients_free']}.