Gaseous nebulae and massive stars in the giant HI ring in Leo
  Chemical abundances in the Leo ring, the largest HI cloud in the local
Universe, have recently been determined to be close or above solar,
incompatible with a previously claimed primordial origin of the ring. The gas,
pre-enriched in a galactic disk and tidally stripped, did not manage to form
stars very efficiently in intergalactic space. We map nebular lines in 3 dense
HI clumps of the Leo ring and complement these data with archival stellar
continuum observations to investigate the slow building up of a sparse
population of stars in localized areas of the ring. Individual young stars as
massive as O7-types are powering some HII regions. The average star formation
rate density is of order of 10^{-5} Msun/yr/kpc^2 and proceeds with local
bursts a few hundred parsecs in size, where loose stellar associations of
500-1000 Msun occasionally host massive outliers. The far ultraviolet-to-Halpha
emission ratio in nebular regions implies recent stellar bursts, from 2 to 7
Myr ago. The relation between the local HI gas density and the star formation
rate in the ring is similar to what is found in dwarfs and outer disks with gas
depletion times as long as 100~Gyrs. We find a candidate planetary nebula in a
compact and faint Halpha region with [OIII]/Halpha line enhancement, consistent
with the estimated mean stellar surface brightness of the ring. The presence of
1 kpc partial ring emitting weak Halpha lines around the brightest and youngest
HII region suggests that local shocks might be the triggers of new star forming
events.
