Discovery of a disrupting open cluster far into the Milky Way halo: a
  recent star formation event in the leading arm of the Magellanic stream?
  We report the discovery of a young (${\rm age} \sim 130~{\rm Myr}$), low-mass
($M \sim 1200~{\rm M}_\odot$), metal-poor ($[{\rm Fe}/{\rm H}] \sim -1.1$)
stellar association at a heliocentric distance $D \approx 29~{\rm kpc}$,
placing it far into the Milky Way halo. At its present Galactocentric position
$(R, z) \sim (23, 15)~{\rm kpc}$, the association is (on the sky) near the
leading arm of the gas stream emanating from the Magellanic cloud system, but
is located $\approx 60^\circ$ from the Large Magellanic Cloud (LMC) center on
the other side of the Milky Way disk. If we assume that the cluster is
co-located with HI gas in the stream, we directly measure the distance to the
leading arm of the Magellanic stream. The measured distance is inconsistent
with stream predictions from models of the LMC/SMC interaction and infall into
the Milky Way that do not account for ram pressure and gas interaction with
Milky Way disk. The estimated age of the cluster is consistent with the time of
last passage of the leading arm gas through the Galactic midplane, and we
therefore speculate that this star-formation event was triggered by its the
last disk midplane passage. Most details of this idea remain a puzzle: the
Magellanic stream has low column density, the Milky Way disk at this large
radius has low gas density, and the relative velocity of the leading arm gas
and Milky Way gas is large. However it formed, the discovery of a young stellar
cluster in the Milky Way halo presents an interesting opportunity for study.
This cluster was discovered with Gaia astrometry and photometry alone, but
folow-up DECam photometry was crucial for measuring its properties.
