Predicting the detection yields of giant planets and brown dwarfs with CSST astrometry

TL;DR

This study estimates how many giant planets and brown dwarfs CSST astrometry could detect around M-dwarfs and brown dwarfs by generating realistic CSST source catalogs and running companion injection-recovery simulations with three occurrence-rate models. The approach combines a differential astrometry framework, an astrometric model with Thiele-Innes elements, and Bayesian model comparison to identify detectable signals, applying stringent criteria (ln BF > 5 and n ≥ 6). The main findings are that FGK-dwarf hosts yield near-zero detections, M-dwarfs within 300 pc could host ~83^{+84}_{-62} substellar companions (≈77^{+78}_{-58} BD and 6^{+6}_{-4} giants), and brown dwarfs within 600 pc could reveal ~420^{+153}_{-115} BD companions (≈418^{+152}_{-114} BD and 2^{+1}_{-1} planets), with BD binaries dominating the BD-host yield. Overall, CSST astrometry is poised to significantly enrich the nearby substellar companion sample, especially around low-mass hosts, and will complement Gaia while offering unique access to fainter targets and wider spatial coverage.

Abstract

Chinese Space Station Telescope (CSST), which will begin its scientific operations around 2027, is going to survey the sky area of the median-to-high Galactic latitude and median-to-high ecliptic latitude. The high astrometric precision of the CSST Survey Camera for faint objects enables the detection of a number of giant planets and brown dwarfs around M-dwarfs and brown dwarfs via differential astrometry in its optical survey. In this paper, we predict the number of giant planets and brown dwarfs around stars and brown dwarfs detectable with CSST astrometry. We generate synthetic samples of CSST stellar and substellar sources, and carry out companion injection-recovery simulations in the samples using different occurrence rates for FGK-dwarfs, M-dwarfs, and brown dwarfs. We calculate companion yields based on CSST astrometric precision. Our analysis reveals that over its 10-year mission, the CSST Survey Camera could barely discover giant planets and low-mass BDs around FGK-dwarfs, but is projected to detect 20 - 170 giant planets and low-mass brown dwarfs around M-dwarfs within 300 pc, and 300 - 570 brown dwarf binaries within 600 pc. Therefore, CSST astrometry is likely to significantly increase the current sample of substellar companions around M-dwarfs and brown dwarfs. This sample will deepen our understanding of planet formation and evolution around low-mass stars and brown dwarfs.

Figures (8)

• Figure 1: Comparison of the astrometric precision of one single measurement by Gaia and CSST. The astrometric precision in the R.A. and Decl. directions are shown with different colors. The horizontal axis is either the Gaia G band or the CSST g band. Gaia position uncertainties are derived from equation (1-4) in Perryman2014 and equation (7) in Gaia2016 while CSST position uncertainties are obtained from figure 4 in Fu2023.
• Figure 2: CSST filter transmission curves corresponding to NUV, u, g, r, i, z and y band. The limiting magnitude of the wide field survey for each band is 25.4, 25.4, 26.3, 26.0, 25.9, 25.2, 24.4 mag, shown under corresponding curves. This figure is adapted from figure 1 in Lu2024.
• Figure 3: Workflow for stars from constructing stellar synthetic samples of M-dwarfs and FGK-dwarfs to yield predictions.
• Figure 4: Workflow for brown dwarfs from constructing substellar synthetic samples to yield predictions.
• Figure 5: Distribution of predicted companion yield in period-companion mass space for M-dwarfs.