Recalibration of the Landolt UBVRI Standard Stars and the Generation of 5.4 Million New UBVRI Standard Stars using LAMOST and Gaia

TL;DR

The paper tackles the challenge of achieving milli-magnitude precision in optical photometry by recalibrating Landolt UBVRI standards with BEST XPSP and SCR methods, identifying field-by-field zero-point offsets and spatial structures, and producing a 5.4 million-star LAMOST–Gaia-based UBVRI standard catalog. It implements a three-stage calibration (BEST→L13/L16 standardization, field ZP corrections, spatial-flat-field corrections) and validates Landolt L92 stars, finding correlated band residuals rooted in historical measurement practices. Additionally, it derives temperature- and extinction-dependent extinction coefficients using a star-pair approach and presents a robust, large-scale standard-star catalog that outperforms XPSP in U-band precision for many stars. The resulting resources, publicly accessible through the BEST database, significantly improve the accuracy and density of standard stars for wide-field surveys and Gaia-era photometric calibration. The work highlights residual systematics tied to extinction-law variations and Gaia XP corrections but demonstrates strong overall consistency and substantial gains in photometric precision across the UBVRI system.

Abstract

We present an independent validation and recalibration of the Landolt 2013 (celestial equator and $δ\sim -50^\circ$) and 2016 ($δ\sim -50^\circ$) standard stars in the Johnson $UBV$ and Kron-Cousins $RI$ systems, using tens of thousands of XPSP data from the BEst STar (BEST) database. Our analysis reveals an overall zero-point offset between the 2016 and 2013 datasets. We further identify zero-point offsets for each standard field, ranging from 5 -- 14 mmag across all $UBVRI$ bands, with correlations between offsets in different bands. Additionally, we confirm the spatial structures up to 7 -- 10 mmag in the $BVRI$ bands. We also find that spatial structures are similar across bands for the same field, and similar across different fields for the same band. These similarities may arise from the averaged flat-fields from each observing run. The recalibrated results are consistent with the XPSP data within 48 mmag in the $U$ band, 11 mmag in the $B$ band, and 5 -- 6 mmag in the $VRI$ bands in the brightness $G<16$. Furthermore, based on stellar atmospheric parameters from LAMOST DR12 and Gaia DR3 photometry, along with the XPSP data, we derive temperature- and extinction-dependent extinction coefficients for the $UBVRI$ bands as well as a LAMOST \& Gaia-based catalog of 5.4 million standard stars in the $UBVRI$ bands, for which the U-band photometry of the vast majority of sources exhibits significantly higher precision than XPSP. The recalibrated Landolt standard stars and LAMOST \& Gaia-based standard stars will be available on the BEST website (https://nadc.china-vo.org/data/best/) and (https://doi.org/10.12149/101704).

Figures (13)

• Figure 1: The differences between the BEST standard stars and the L13/L16 datasets in the third calibration iteration, shown as a function of $BP-RP$ color and $G$ magnitude. The top two rows correspond to the original BEST standard stars, while the bottom two rows show the standardized BEST standard stars.
• Figure 2: The $\delta m_{x}^{ZP}$ values for each field are plotted against Right Ascension (R.A.). The panels from top to bottom correspond to the UBVRI bands. Blue points with error bars represent fields from L13, while red points with error bars represent those from L16. The panels on the right show the distribution of $\delta m_{x}^{ZP}$, along with the maximum likelihood estimation of the intrinsic dispersion for the UBVRI bands, based on the distribution and uncertainties of $\delta m_{x}^{ZP}$ in L13.
• Figure 3: The pairwise comparisons of $\delta m_{x}^{ZP}$ across the UBVRI bands. The solid red lines represent linear regression results based solely on fields from L13 and accounting for measurement uncertainties. The linear regression coefficients and coefficients of determination ($R^2$) are indicated in each panel.
• Figure 4: A sample spatial structure correction for field LSE259. The first row illustrates the spatial distribution (relative to the field center) of magnitude differences between BEST standard stars and L13/L16 in UBVRI bands before spatial structure correction; the second row depicts these distributions after correction; the third row displays the characterized flat-field structure $\delta m_{x}^{SS}$, with red boxes marking the edges of the Y4KCam CCD.
• Figure 5: Comparison of spatial structure pattern across different bands within LSE259 and between LSE259 and other fields. The first row presents an internal comparison of spatial structure pattern among different bands within LSE259. The second and third rows show inter-field comparisons of corresponding bands between LSE259 and WD0830-535, and between LSE259 and LSE44, respectively. The solid red line marks the line of equality. The color of each point represents its position relative to the field center, as defined by the spatial mapping shown the panel at top row, second column.