Stellar Loci. IX. Estimation of Stellar Parameters from CSST-like Photometry

TL;DR

This paper tackles the challenge of deriving stellar parameters from CSST-like photometry by developing two complementary loci-based methods. The first method extends metallicity-dependent stellar loci to include both $[Fe/H]$ and $log\,g$ for simultaneous estimation, while the second uses a giant-dwarf loci approach to classify stars and infer metallicities with strong prior constraints. Tests on BOSZ-based synthetic data yield precisions of $[M/H]\approx 0.088$ dex and $log\,g\approx 0.083$ dex, with photometric errors degrading precision via $[Fe/H]$–$log\,g$ degeneracy, and observational data achieving $[Fe/H]_{phot}\approx 0.101$ dex and $log\,g_{phot}\approx 0.387$ dex, reflecting real-world color–gravity correlations. The giant-dwarf loci method delivers a slightly better metallicity precision of $\sim 0.084$ dex and excels at distinguishing giants from dwarfs, especially for metal-poor or red giants, with near-100% efficiency in favorable regimes. Overall, the study demonstrates CSST’s strong potential to deliver precise stellar parameters for billions of stars, advancing Galactic archaeology and exoplanet-related studies through wide, deep photometric surveys.

Abstract

The China Space Station Telescope (CSST) will conduct a deep and wide imaging survey in the NUV-, u-, g-, r-, i-, z-, and y-bands. In this work, using theoretical data synthesized from the BOSZ spectra of Bohlin et al. (2017), along with observational data constructed from different sources, we present two methods for estimating stellar parameters from CSST-like photometry. One approach is to estimate metallicity [M/H] and surface gravity log g simultaneously by using the metallicity- and log g-dependent stellar loci. Tests with theoretical data (without photometric errors) result in precisions of 0.088 dex and 0.083 dex for [M/H] and log g, respectively. With 0.01 mag photometric errors, precision is degraded by about a factor of two, due to degeneracy in [M/H] and log g. Tests with observational data, although with larger photometric errors, result in precisions of 0.10 dex and 0.39 dex for [Fe/H] and log g, respectively, thanks to the strong correlation between stellar colors and log g in real data. The other approach is the giant-dwarf loci method to obtain classifications and metallicity estimates. With the same observational data, it achieves a better [Fe/H] precision of 0.084 dex, due to the stronger constraints imposed on log g. The method also performs well in distinguishing giants from dwarfs, particularly for red or metal-poor giants. This work demonstrates the clear potential of the CSST data, paving the way for stellar-parameter estimates for many billions of stars.

Figures (15)

• Figure 1: H-R diagram for the observational data, shown as ($g-i$)$_0$ -- log g and (G$_{BP}$-G$_{RP}$)$_0$ -- M$_G$, color-coded by [Fe/H] shown in the color bar on the right. The BHB stars selected from the right panel are assigned a log g value of 2.5.
• Figure 2: Stellar loci of theoretical CSST colors, color-coded by [M/H] and log g shown in the color bars to the right of the upper and lower rows of panels, respectively.
• Figure 3: Fitting residuals of the [Fe/H]- and log g-dependent stellar loci. The left four panels are for the $NUV-g$ color, which contain the histogram distribution of fitting residuals, with the Gaussian fitting profile over-plotted in black, and fitting residuals as a function of $g-i$, [M/H] and log g, with the median values and standard deviations over-plotted in red. The right four panels are similar to the left ones but for the $u-g$ color. The points are color-coded by their number density.
• Figure 4: Top panels: The dependence of the $(NUV-g)_m$ (left) and $(u-g)_m$ (right) colors on [M/H], color-coded by [M/H] in the color bars to the right. Bottom panels: The dependence of the $(NUV-g)_m$ (left) and $(u-g)_m$ (right) colors on log g, color-coded by log g in the color bars to the right.
• Figure 5: Left panels: The result for photometric-metallicity estimates [M/H]. From top to bottom, the panels show histogram distributions of the residuals $\Delta [\rm M/H]$, with the Gaussian fitting profile over-plotted in black, comparison of the photometric-metallicity estimates with input values, $\Delta [\rm M/H]$ as a function of $g-i$ color, [M/H], and log g. Right panels: Similar to the left ones but for log g. The points are color-coded by their number density.