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Testing Lorentz symmetry in strong gravitational fields provides a unique probe of extensions to standard model. The orbiting motions of the S-stars around the supermassive black hole Sgr~A* provide a natural laboratory for such tests. In this paper, we analyze the S2 orbital data focusing on a static and spherically symmetric black hole within Kalb--Ramond gravity, where the deviations from general relativity are encoded in a single Lorentz-violating parameter $\ell$ introduced by the Kalb--Ramond tensor field. Using a full 14-dimensional Markov Chain Monte Carlo analysis under uniform and Gaussian priors, we obtain $\ell = {1.60 \times 10^{-5}}^{+1.38 \times 10^{-4}}_{-1.76 \times 10^{-4}} $ and $\ell = {-1.02 \times 10^{-5}}^{+1.26 \times 10^{-4}}_{-1.19 \times 10^{-4}} $ at $1σ$ confidence level, respectively. These constraints are about three orders of magnitude tighter than those from Event Horizon Telescope imaging of Sgr~A*. We also perform MCMC simulation by fitting data of S38 and S55 stars, as well as their joint analysis. Our results show that the best fit values of $\ell$ in all simulations are always of $10^{-5}$ order, but S2 star provides the most stringent constraints on the parameters because S2 star has higher precision observational data comparing to the fewer public data for other two stars.

We present a homogeneous multiwavelength analysis of 35 blue straggler star (BSS) candidates in nine open clusters, combining Swift/UVOT near-ultraviolet data with Gaia DR3 astrometry and optical-to-infrared photometry. We construct spectral energy distributions (SEDs) to search for signatures of hot companions associated with past mass transfer. Among the sample, 15 BSSs (∼43%) show ultraviolet excesses that are better described by two-component SED fits. The inferred companions are consistent with hot white dwarfs and pre-extremely low-mass (pre-ELM) white dwarf candidates, suggesting systems observed at different stages following mass transfer. We examine the radial distribution of the BSSs and find evidence for mass segregation in dynamically evolved clusters, a result that is broadly consistent with the estimated half-mass relaxation timescales of the host systems. To place the clusters in a Galactic context, we compute their orbits using galpy, obtaining low eccentricities (e ≤ 0.1) and disk-like trajectories. We also find a positive relation between the half-number radius of the BSS population (r<SUB>50</SUB>) and the total number of BSSs. Overall, our results are consistent with a scenario in which the BSS population in these clusters is dominated by binary evolution. The systems identified here provide observational constraints on post-mass-transfer evolutionary phases. While the number of robust detections is limited and intrinsic degeneracies remain in SED-based decomposition, these results provide a useful foundation for future spectroscopic confirmation.

The neutron lifetime τ<SUB>n</SUB> is a fundamental input to Big Bang Nucleosynthesis (BBN) that controls the neutron-to-proton ratio at the onset of element production and thus the primordial helium mass fraction Y<SUB>P</SUB>. A persistent ≃ 4σ tension exists between beam-based experiments (<mml:math><mml:mrow><mml:msubsup><mml:mi>τ</mml:mi><mml:mi>n</mml:mi><mml:mrow><mml:mrow><mml:mi>b</mml:mi></mml:mrow><mml:mi>e</mml:mi><mml:mi>a</mml:mi><mml:mi>m</mml:mi></mml:mrow></mml:msubsup><mml:mo>≈</mml:mo><mml:mn>888.0</mml:mn><mml:mo>±</mml:mo><mml:mn>2.0</mml:mn></mml:mrow></mml:math> s) and cryogenic bottle/trap experiments (<mml:math><mml:mrow><mml:msubsup><mml:mi>τ</mml:mi><mml:mi>n</mml:mi><mml:mrow><mml:mrow><mml:mi>b</mml:mi></mml:mrow><mml:mi>o</mml:mi><mml:mi>t</mml:mi><mml:mi>t</mml:mi><mml:mi>l</mml:mi><mml:mi>e</mml:mi></mml:mrow></mml:msubsup><mml:mo>≈</mml:mo><mml:mn>879.4</mml:mn><mml:mo>±</mml:mo><mml:mn>0.6</mml:mn></mml:mrow></mml:math> s). The discrepancy ∆τ<SUB>n</SUB> ≈ 8.6 s translates into a predicted shift in the primordial helium mass fraction of ∆Y<SUB>P</SUB> ≈ 0.0018, or roughly 0.7% of the Standard-Model value. We critically assess whether next-generation astrophysical observations of Y<SUB>P</SUB>, achievable with integral-field-unit (IFU) spectroscopy of metal-poor Blue Compact Dwarf (BCD) galaxies combined with precision spectrophotometric calibration, can resolve this tension. Using a Fisher-matrix forecast over the parameter space {τ<SUB>n</SUB>, Ω<SUB>b</SUB>h<SUP>2</SUP>, N<SUB>eff</SUB>}, we show that a 0.1% determination of Y<SUB>P</SUB>, when combined with a sub-percent measurement of (D/H)<SUB>P</SUB> and Planck-quality CMB data, yields an astrophysical constraint on τ<SUB>n</SUB> whose precision depends critically on the treatment of the effective number of neutrino species N<SUB>eff</SUB>. If N<SUB>eff</SUB> is fixed to its Standard Model value 3.044 --- motivated by the absence of new light species --- a 0.1% determination of Y<SUB>P</SUB> combined with current D/H data already reaches σ(τ<SUB>n</SUB>) ≃ 1.3 s, providing &gt; 6σ discrimination. If N<SUB>eff</SUB> is treated as a free parameter, Planck-quality CMB data are insufficient to break the N<SUB>eff</SUB>--τ<SUB>n</SUB> degeneracy in Y<SUB>P</SUB>, and a next-generation CMB experiment (CMB-S4) with σ(N<SUB>eff</SUB>) ≃ 0.03 is required, yielding σ(τ<SUB>n</SUB>) ≃ 4.3 s at the ~ 2σ threshold. In all cases an independent measurement of the baryon density --- from either primordial D/H or the CMB --- is an essential prerequisite, since τ<SUB>n</SUB> and Ω<SUB>b</SUB>h<SUP>2</SUP> are completely degenerate in Y<SUB>P</SUB> alone.