Spectroscopy of the Young Stellar Association Price-Whelan 1: Origin in the Magellanic Leading Arm and Constraints on the Milky Way Hot Halo

We report spectroscopic measurements of stars in the recently discovered young stellar association Price-Whelan 1 (PW 1), which was found in the vicinity of the Leading Arm (LA) of the Magellanic Stream. We obtained Magellan+MIKE high-resolution spectra of the 28 brightest stars in PW 1 and used The Cannon to determine their stellar parameters. We find that the mean metallicity of PW 1 is [Fe/H]=-1.23 with a small scatter of 0.06 dex and the mean radial velocity is Vhelio=276.7 km/s with a dispersion of 11.0 km/s. Our results are consistent in Teff, logg, and [Fe/H] with the young and metal-poor characteristics (116 Myr and [Fe/H]=-1.1) determined for PW 1 from our discovery paper. We find a strong correlation between the spatial pattern of the PW 1 stars and the LA II gas with an offset of -10.15 deg in L_MS and +1.55 deg in B_MS. The similarity in metallicity, velocity, and spatial patterns indicates that PW 1 likely originated in LA II. We find that the spatial and kinematic separation between LA II and PW 1 can be explained by ram pressure from Milky Way gas. Using orbit integrations that account for the LMC and MW halo and outer disk gas, we constrain the halo gas density at the orbital pericenter of PW 1 to be n_halo (17 kpc) = 2.7 (3) x 10^-3 atoms/cm^3 and the disk gas density at the midplane at 20 kpc to be n_disk (20 kpc,0) = 6.0 (1.8) x 10^-2 atoms/cm^3. We, therefore, conclude that PW 1 formed from the LA II of the Magellanic Stream, making it a powerful constraint on the Milky Way-Magellanic interaction.