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.
