Multiline spectral imaging of dense cores in the Lupus molecular cloud

The molecular clouds Lupus 1, 3 and 4 were mapped with the Mopra Telescope at 3 and 12 mm. Emission lines from high-density molecular tracers were detected, i.e. NH3 (1,1), NH3 (2,2), N2H+ (1-0), HC3N (3-2), HC3N (10-9), CS (2-1), CH3OH (20-10)A+ and CH3OH (2-1-1-1)E. Velocity gradients of more than 1 km s-1 are present in Lupus 1 and 3, and multiple gas components are present in these clouds along some lines of sight. Lupus 1 is the cloud richest in high-density cores: eight cores were detected in it, five cores were detected in Lupus 3 and only two in Lupus 4. The intensity of the three species HC3N, NH3 and N2H+ changes significantly in the various cores: cores that are brighter in HC3N are fainter or undetected in NH3 and N2H+ and vice versa. We found that the column density ratios HC3N/N2H+ and HC3N/NH3 change by 1 order of magnitude between the cores, indicating that also the chemical abundance of these species is different. The time-dependent chemical code that we used to model our cores shows that the HC3N/N2H+ and HC3N/NH3 ratios decrease with time, therefore the observed column density of these species can be used as an indicator of the chemical evolution of dense cores. On this basis we classified five out of eight cores in Lupus 1 and one out of five cores in Lupus 3 as very young protostars or pre-stellar cores. Comparing the millimetre core population with the population of the more evolved young stellar objects identified in the Spitzer surveys, we conclude that in Lupus 3 the bulk of the star formation activity has already passed and only a moderate number of stars are still forming. In contrast, in Lupus 1 star formation is ongoing and several dense cores are still in the pre-/protostellar phase. Lupus 4 is at an intermediate stage, with a smaller number of individual objects.