Investigating the UV-excess in star clusters with $N$-body simulations: predictions for future CSST observations

TL;DR

This study uses N-body simulations to explore the origins of UV-excess in star clusters, predicting how future CSST observations can detect and analyze these features across different stellar populations and cluster ages.

Contribution

It provides new insights into the stellar populations contributing to UV-excess and offers predictive tools for future CSST observations, including color conversions and detection capabilities.

Findings

Identifies three stellar populations contributing to UV-excess: AGB stars, helium stars, and white dwarfs.

Predicts CSST's ability to detect UV-excess in clusters older than 200 Myr.

Establishes a conversion relation between CSST NUV-g and HST FUV-NUV colors.

Abstract

We study the origin of the UV-excess in star clusters by performing N-body simulations of six clusters with N=10k and N=100k (single stars & binary systems) and metallicities of Z=0.01, 0.001, and 0.0001, using PETAR. All models initially have a 50 percent primordial binary fraction. Using GalevNB we convert the simulated data into synthetic spectra and photometry for the China Space Station Telescope (CSST) and Hubble Space Telescope (HST). From the spectral energy distributions we identify three stellar populations that contribute to the UV-excess: (1) second asymptotic giant branch stars, which contribute to the UV flux at early times; (2) naked helium stars, and (3) white dwarfs, which are long-term contributors to the FUV spectra. Binary stars consisting of a white dwarf and a main-sequence star are cataclysmic variable (CV) candidates. The magnitude distribution of CV candidates is bimodal up to 2 Gyr. The bright CV population is particularly bright in FUV-NUV. The FUV-NUV color of our model clusters is 1-2 mag redder than the UV-excess globular clusters in M 87 and in the Milky Way. This discrepancy may be induced by helium enrichment in observed clusters. Our simulations are based on simple stellar evolution; we do not include the effects of variations in helium and light elements or multiple stellar populations. A positive radial color gradient is present in CSST NUV-y for main-sequence stars of all models with a color difference of 0.2-0.5 mag, up to 4 half-mass radii. The CSST NUV-g color correlates strongly with HST FUV-NUV for NUV-g>1 mag, with the linear relation $FUV-NUV=(1.09\pm0.12)\times(NUV-g)+(-1.01\pm0.22)$. This allows for conversion of future CSST NUV-g colors into HST FUV-NUV colors, which are sensitive to UV-excess features. We find that CSST will be able to detect UV-excess in galactic/extra-galactic star clusters with ages >200 Myr.