  The Ophiuchus clouds, in particular L~1688, are an excellent region to study
the embedded phases of star formation, due to the relatively large number of
protostars. However, the standard method of finding and characterizing embedded
young stellar objects (YSOs) through just their infrared spectral slope does
not yield a reliable sample. This may affect the age determinations, often
derived from the statistics on the total number of embedded YSOs and pre-main
sequence stars within a cloud.Our aim is to characterize the structure of
protostellar envelopes on an individual basis and to correctly identify the
embedded YSO population of L1688. Spectral maps of the HCO+ J=4--3 and C18O
J=3--2 lines using the HARP-B array on the James Clerk Maxwell Telescope and
SCUBA 850 micron dust maps are obtained of all sources in the L1688 region with
infrared spectral slopes consistent with, or close to, that of embedded YSOs.
Selected 350 micron maps obtained with the Caltech Submillimeter Observatory
are presented as well. The properties, extent and variation of dense gas,
column density and dust on scalesup to 1' are probed at 15" resolution. Using
the spatial variation of the gas and dust, together with the intensity of the
HCO+ J=4--3 line, we are able to accurately identify the truly embedded YSOs
and determine their properties. RESULTS The protostellar envelopes range from
0.05 to 0.5 Msun in mass. The concentration of HCO+ emission (~0.5 to 0.9) is
generally higher than that of the dust concentration. Combined with absolute
intensities, HCO+ proves to be a better tracer of protostellar envelopes than
dust, which can contain disk and cloud contributions. Our total sample of 45
sources, including all previously classified Class I sources, several
flat-spectrum sources and some known disks, was re-classified using the ....
