ALMA reveals sequential high-mass star formation in the G9.62+0.19
  complex
  Stellar feedback from high-mass stars (e.g., H{\sc ii} regions) can strongly
influence the surrounding interstellar medium and regulate star formation. Our
new ALMA observations reveal sequential high-mass star formation taking place
within one sub-virial filamentary clump (the G9.62 clump) in the G9.62+0.19
complex. The 12 dense cores (MM 1-12) detected by ALMA are at very different
evolutionary stages, from starless core phase to UC H{\sc ii} region phase.
Three dense cores (MM6, MM7/G, MM8/F) are associated with outflows. The
mass-velocity diagrams of outflows associated with MM7/G and MM8/F can be well
fitted with broken power laws. The mass-velocity diagram of SiO outflow
associated with MM8/F breaks much earlier than other outflow tracers (e.g., CO,
SO, CS, HCN), suggesting that SiO traces newly shocked gas, while the other
molecular lines (e.g., CO, SO, CS, HCN) mainly trace the ambient gas
continuously entrained by outflow jets. Five cores (MM1, MM3, MM5, MM9, MM10)
are massive starless core candidates whose masses are estimated to be larger
than 25 M$_{\sun}$, assuming a dust temperature of $\leq$ 20 K. The shocks from
the expanding H{\sc ii} regions ("B" \& "C") to the west may have great impact
on the G9.62 clump through compressing it into a filament and inducing core
collapse successively, leading to sequential star formation. Our findings
suggest that stellar feedback from H{\sc ii} regions may enhance the star
formation efficiency and suppress the low-mass star formation in adjacent
pre-existing massive clumps.
