Yin Wu

The escape velocity is a key probe of the Milky Way (MW) potential, but standard estimates can be biased by unrelaxed substructures. Using MW analogs from the TNG50 cosmological simulation, we investigate the impact of the dominant, highly radial Gaia–Sausage–Enceladus (GSE) debris on escape-velocity measurements. We introduce an adaptive selection strategy that balances statistical reliability with sample size to identify the power-law decline regime of the local halo velocity distribution. Applying this strategy to 16 MW analogs with prominent GSE-like structures and continuous high-velocity tails, we find that retaining GSE-like stars yields a systematic underestimate, with a recovery fraction of 97 . 5 % − 2.9 % + 2.1 % , while removing GSE-like stars alone increases the recovery fraction to 99 . 4 % − 2.5 % + 2.1 % . The ∼2% underestimation of the escape velocity corresponds to an ∼6% underestimation of the total mass. This improvement persists in LAMOST-like magnitude and footprint selection. GSE-like debris is therefore a key contributor to the systematic underestimation of escape-velocity measurements, with direct implications for the inferred MW mass.

Escape velocity has long been used to constrain the mass of the dark matter (DM) halo in the Milky Way (MW). Here, we present a study of the escape velocity curve using a sample of high-velocity K giants with full 6D phase-space information and of relatively good quality, selected from LAMOST DR8 and cross-matched with Gaia DR3. To expand the high-velocity stars to larger distances, we used radius-dependent criteria of total velocity, i.e., v _GC > 300 km s⁻¹ for the solar neighborhood and v GC > v min ∼ 0.6 × v esc ( r GC ) for the outer region. We also selected halo stars based on v _ϕ − [Fe/H] information to ensure that the sample is isotropic. We modeled the velocity distribution with traditional power-law models to determine the escape velocity in each radial bin. For the first time, we have directly measured a relatively continuous escape velocity curve that can extend to Galactocentric radii of ∼50 kpc, finding a decline in agreement with previous studies. The escape velocity at the solar position yielded by our measurements is 523.7 4 − 13.47 + 12.83 km s − 1 . Combined with the local circular velocity, we estimated the mass of the MW assuming a Navarro–Frenk–White DM profile, which resulted in a total mass of M 200 , total = 0.9 0 − 0.07 + 0.06 × 1 0 12 M ⊙ , with a concentration of c 200 = 13.4 7 − 1.70 + 1.85 . The small uncertainty implies that including the escape velocities beyond the solar neighborhood can result in a more precise mass estimate. Our derived MW mass is consistent with some recent studies using the escape velocity as well as other tracers, which may support a lower mass of the DM halo than in the past.

The main-sequence turnoff (MSTO) stars well preserve the chemical properties where they were born, making them ideal tracers for studying the stellar population. We perform a detailed chemo-dynamical analysis on moderately metal-poor (−2.0 < [Fe/H] < −1.0) MSTO stars to explore the early accretion history of the Milky Way. Our sample includes four stars observed with high-resolution spectroscopy using ESPaDOnS at the Canada–France–Hawaii Telescope and 163 nearby MSTO stars selected from the SAGA database with high-resolution results. Within the action-angle spaces, we identified Gaia–Sausage–Enceladus (GSE, 35 objects), stars born in the Milky Way (in situ, 31 objects), and other substructures (21 objects). We find that both GSE and in situ stars present a similar Li plateau around A(Li) ∼ 2.17. GSE shows a clear α-knee feature in Mg at [Fe/H] ∼ −1.60 ± 0.06, while the α-elements of in situ stars remain nearly constant within this metallicity range. The iron-peak elements show little difference between GSE and in situ stars except for Zn and Ni, which decrease in GSE at [Fe/H] > −1.6, while they remain constant for in situ stars. Among heavy elements, GSE shows overall enhancement in Eu, with [Ba/Eu] increasing with the metallicity, while this ratio remains almost constant for in situ stars, suggesting the contribution of longer timescale sources to the s-process in GSE. Moreover, for the first time, we present the r-process abundance pattern for an extremely r-process enhanced (r-II) GSE star, which appears consistent with the solar r-process pattern except for Pr. Further investigation of larger GSE samples using high-resolution spectra is required to explore the reason for the significantly higher Pr in the GSE r-II star.