L1498 is a classic example of a dense cold pre-protostellar core. To s
tudy the evolutionary status, the structure, dynamics, and chemical pr
operties of this core we have obtained high spatial and high spectral
resolution observations of molecules tracing densities of 10(3)-10(5)
cm. We observed CCS, NH3, C3H2, and HC7N with NASA's DSN 70 m antennas
. We also present large-scale maps of (CO)-O-18 and (CO)-C-13 observed
with the AT&T 7 m antenna. For the high spatial resolution maps of se
lected regions within the core we used the VLA for CCS at 22 GHz, and
the Owens Valley Radio Observatory (OVRO) MMA for CCS at 94 GHz and CS
(2-1). The 22 GHz CCS emission marks a high-density [n(H-2)>10(4) cm(
-3)] core, which is elongated with a major axis along the SE-NW direct
ion. NH, and C,H, emissions are located inside the boundary of the CCS
emission. (CO)-O-18 emission traces a lower density gas extending bey
ond the CCS boundary. Along the major axis of the dense core, CCS, NH3
and C3H2 emission show evidence of limb brightening. The observations
are consistent with a chemically differentiated onion-shell structure
for the L1498 core, with NH3 in the inner and CCS in the outer parts
of the core. The high angular resolution (9 ''-12 '') spectral line ma
ps obtained by combining NASA Goldstone 70 m and VLA data resolve the
CCS 22 GHz emission in the southeast and northwest boundaries into arc
like enhancements, supporting the picture that CCS emission originates
in a shell outside the NH3 emitting legion. Interferometric maps of C
CS at 94 GHz and CS at 98 GHz show that their emitting regions contain
several small-scale dense condensations. We suggest that the differen
ces between the CCS, CS, C3H2, and NH3 emission are caused by a time-d
ependent effect as the core evolves slowly. We interpret the chemical
and physical properties of L1498 in terms of a quasi-static (or slowly
contracting) dense core in which the outer envelope is still growing.
The growth rate of the core is determined by the density increase in
the CCS shell resulting from the accretion of the outer low-density ga
s traced by (CO)-O-18. We conclude that L1498 could become unstable to
rapid collapse to form a protostar in less than 5 x 10(6) yr.