Mock Observations of Multiple Stellar Populations in Tidal Streams of Palomar 5 for the Chinese Space Station Survey Telescope

Abstract

Observations show that multiple stellar populations (MPs) are ubiquitous in globular clusters. The Hubble Space Telescope (HST) has been a pivotal tool for previous photometric studies of MPs. The Chinese Space Station Survey Telescope (CSST) is a two-meter telescope scheduled for launch. One of its imaging instruments, the Survey Camera (SC), combines ultraviolet sensitivity comparable to that of HST with a significantly larger field of view, making it well-suited for conducting large-scale photometric surveys of MPs within extensive stellar stream structures. In this work, we perform mock observations of the stellar stream Palomar 5 to assess the feasibility of detecting MPs with the CSST/SC. The results indicate that the CSST/SC cannot resolve MPs in stellar streams at distances comparable to Palomar 5 ($\gtrsim 20$ kpc) with one or ten 150 s exposures. This fundamental limitation arises from the absence of the precise proper motions required to disentangle stream members. We estimate that successful resolution would require the target stream to be $\lesssim$ 8 kpc under a 150 s exposure. Furthermore, using theoretical color-magnitude diagrams, we find that the CSST/SC $g$-band provides an optimal balance between contamination rate and completeness rate for member identification in the cluster's core. However, this approach fails in the stream due to severe field star contamination. Therefore, future CSST observations of Palomar 5 and its tidal tails will employ multiple epochs across several bands to obtain the deep photometry and proper motion data for a definitive MP analysis.

Figures (13)

• Figure 1: Dartmouth isochrones for Palomar 5 are displayed in the plane of $\log{(L/L_\odot})$ versus $\log{(T_{\mathrm{eff}})}$. The isochrones were linearly interpolated at [Fe/H] = –1.41 and ages from 11.5 Gyr. Red and blue dots denote 1P ($Y=0.2461$) and 2P ($Y=0.3300$) stars, respectively.
• Figure 2: The total transmission curves for CSST/SC filter bands 2025arXiv251106970W.
• Figure 3: Spatial distribution of the entire sky region of Palomar 5 derived with CSST/SC g limiting magnitude. The boxes in different colors represent the final selected simulation regions, each with an identical area of $0.7^\circ$$\times$$0.5^\circ$. The cyan, green, and blue boxes correspond to the core region of the Palomar 5 cluster, the high-density segment, and the low-density segment of the tidal tail, respectively.
• Figure 4: Partial g-band mosaic images of simulated region 1 (upper panel), region 2 (middle panel), and region 3 (lower panel).
• Figure 5: Photometric results for region 1 presented as CMDs. Top two rows: CMDs with stars classified into three populations. Red, blue, and gray dots represent 1P, 2P, and field stars, respectively. Bottom two rows: the same CMDs with stars color-coded by normalized number density (color bar on the right). Panels from left to right and top to bottom in each block show: $g$ vs. $NUV - i$, $u - i$, $g - i$, $r - i$, $z - i$, and $y - i$.