L1506: a prestellar core in the making

Abstract

Context. Exploring the structure and dynamics of cold starless clouds is necessary to understand the different steps leading to the formation of protostars. Because clouds evolve slowly, many of them must be studied in detail to identify different moments in a cloud's lifetime. Aims. We study a fragment of the long filament L1506 in the Taurus region, which we name L1506C, a core with interesting dust properties observed by the PRONAOS balloon-borne telescope. Methods. To trace the mass content of L1506C and its kinematics, we mapped the dust emission, and the line emission of two key species, C 18 O and N 2 H + . We also observed 13 CO and C 17 O. We model the species emission using 1D Monte Carlo models.Results. This cloud is reminiscent of L1498 but also exhibits peculiar features: i) a large envelope emitting solely in 13 CO that contains a much smaller core with strong C 18 O depletion at its centre despite a low maximum opacity (A V ∼ 20 mag); ii) extremely narrow C 18 O lines indicating a low, non-measurable turbulence; iii) contraction traced by C 18 O itself (plus rotation); iv) unexpectedly, the kinematical signature of the external envelope is opposite to that of the core, the 13 CO and C 18 O velocity gradients having opposite directions and the C 18 O line profile being blue peaked in contrast to the 13 CO one which is red peaked. The core is large (r = 3 × 10 4 AU) and not very dense (n(H 2 ) ≤ 5 × 10 4 cm −3 , possibly less). This core is therefore not yet prestellar. Conclusions. All these properties imply that the core is kinematically detached from its envelope and in the process of forming a prestellar core. This is the first time that the dynamical formation of a prestellar core has been witnessed. The extremely low turbulence could explain the strong depletion of this core despite its relatively low density and opacity in contrast to undepleted cores such as L1521E, which exhibits a turbulence at least 4 times as high.

A method to measure CO and N2depletion profiles inside prestellar cores

“…If the edge of the depleted region is reached, we should be able to smoothly connect the CO abundance derived from our modeling with the undepleted CO abundance. However, N 2 D + usually disappears before reaching this edge, thus preventing the model to be extended that far, and it is possible that abundance jumps exist (Pagani et al 2010). Figure 3 shows the modeled CO and N 2 abundance profiles in the PSC as a function of distance.…”

“…Though depletion usually starts to appear in cores when extinction becomes higher than ∼8 A V and densities n > 3 × 10 4 cm −3 , when looking at the different papers cited above, there are also cases of strong depletion in cores that are not yet prestellar like L1498 (Willacy et al 1998), L1506C (Pagani et al 2010), and TMC2 (Brady-Ford & Shirley 2011). The reason is not yet clear.…”

Observation of rotation in star forming regions: clouds, cores, disks, and jets

“…L1506C is a large, low-density, starless core embedded in a filament in Taurus. Its inner parts (r < 0.15 pc) are characterized by a very low level of turbulence (σ turb < 47 m s −1 ) and a high level of C 18 O depletion, unexpected given the low density of the core (Pagani et al 2010). The authors proposed that this high level of depletion is related to the low level of turbulence promoting dust coagulation, which in turn may decrease the desorption efficiency.…”

“…The authors proposed that this high level of depletion is related to the low level of turbulence promoting dust coagulation, which in turn may decrease the desorption efficiency. An even more puzzling property of this core is its kinematical structure: Pagani et al (2010) report a velocity gradient in 13 CO with a direction opposite to the velocity gradients traced by C 18 O and N 2 H + . Their detailed radiative transfer analysis shows that the inner parts are contracting (v inf = 0.11 km s −1 ) and rotating with Ω ∝ r −1.5 , and the outer parts are expanding (v exp = 0.09 km s −1 ) and rotating in opposite direction.…”

CORESHINE : a tracer of grain growth in dark clouds

“…This is one example of the use of coreshine (or its absence) to help trace the evolution of a region. L1506C [18,22] is another intriguing case of cloud evolution where coreshine could bring some clues though a convincing solution remains to be found [23].…”