We observed the pre–stellar core L1521F in dust emission at 1.2mm and in two transitions each of { N } _ { 2 } { H } ^ { + } , { N } _ { 2 } { D } ^ { + } , { C } ^ { 18 } { O } and { C } ^ { 17 } { O } in order to increase the sample of well studied centrally concentrated and chemically evolved starless cores , likely on the verge of star formation , and to determine the initial conditions for low–mass star formation in the Taurus Molecular Cloud .
The dust observation allows us to infer the density structure of the core and together with measurements of CO isotopomers gives us the CO depletion . { N } _ { 2 } { H } ^ { + } and { N } _ { 2 } { D } ^ { + } lines are good tracers of the dust continuum and thus they give kinematic information on the core nucleus .
We derived in this object a molecular hydrogen number density n ( { H } _ { 2 } ) \sim 10 ^ { 6 } \hbox { { cm } } ^ { -3 } and a CO depletion factor , integrated along the line of sight , f _ { D } \equiv 9.5 \times 10 ^ { -5 } / x _ { obs } ( CO ) \sim 15 in the central 20″ , similar to the pre–stellar core L1544 .
However , the N ( \hbox { $ { N } _ { 2 } { D } ^ { + } $ } ) / N ( \hbox { $ { N } _ { 2 } { H } ^ { + } $ } ) column density ratio is \sim 0.1 , a factor of about 2 lower than that found in L1544 .
The observed relation between the deuterium fractionation and the integrated CO depletion factor across the core can be reproduced by chemical models if { N } _ { 2 } { H } ^ { + } is slightly ( factor of \sim 2 in fractional abundance ) depleted in the central 3000 AU .
The { N } _ { 2 } { H } ^ { + } and { N } _ { 2 } { D } ^ { + } linewidths in the core center are \sim 0.3 \hbox { { km s } } ^ { -1 } , significantly larger than in other more quiescent Taurus starless cores but similar to those observed in the center of L1544 .
The kinematical behaviour of L1521F is more complex than seen in L1544 , and a model of contraction due to ambipolar diffusion is only marginally consistent with the present data .
Other velocity fields , perhaps produced by accretion of the surrounding material onto the core and/or unresolved substructure , are present .
Both chemical and kinematical analyses suggest that L1521F is less evolved than L1544 , but , in analogy with L1544 , it is approaching the “ critical ” state .