Which molecule traces what: chemical diagnostics of protostellar sources
  The physical and chemical conditions in Class 0/I protostars are fundamental
in unlocking the protostellar accretion process and its impact on planet
formation. The aim is to determine which physical components are traced by
different molecules at sub-arcsecond scales (100 - 400 au). We use a suite of
Atacama Large Millimeter/submillimeter Array (ALMA) datasets in Band 6 (1 mm),
Band 5 (1.8 mm) and Band 3 (3 mm) at spatial resolutions 0.5 - 3" for 16
protostellar sources. The protostellar envelope is well traced by C$^{18}$O,
DCO$^+$ and N$_2$D$^+$, with the freeze-out of CO governing the chemistry at
envelope scales. Molecular outflows are seen in classical shock tracers like
SiO and SO, but ice-mantle products such as CH$_3$OH and HNCO released with the
shock are also observed. The molecular jet is prominent not only in SiO and SO
but also occasionally in H$_2$CO. The cavity walls show tracers of
UV-irradiation such as C$_2$H c-C$_3$H$_2$ and CN. The hot inner envelope,
apart from showing emission from complex organic molecules (COMs), also
presents compact emission from small molecules like H$_2$S, SO, OCS and
H$^{13}$CN, most likely related to ice sublimation and high-temperature
chemistry. Sub-arcsecond millimeter-wave observations allow to identify those
(simple) molecules that best trace each of the physical components of a
protostellar system. COMs are found both in the hot inner envelope (high
excitation lines) and in the outflows (lower-excitation lines) with comparable
abundances. COMs can coexist with hydrocarbons in the same protostellar
sources, but they trace different components. In the near future, mid-IR
observations with JWST-MIRI will provide complementary information about the
hottest gas and the ice mantle content, at unprecedented sensitivity and at
resolutions comparable to ALMA for the same sources.
