Ortho-H₂and the age of prestellar cores

Abstract

Prestellar cores form from the contraction of cold gas and dust material in dark clouds before they collapse to form protostars. Several concurrent theories exist to describe this contraction but they are currently difficult to distinguish. One major difference is the timescale involved in forming the prestellar cores: some theories advocate nearly free-fall speed via, e.g., rapid turbulence decay, while others can accommodate much longer periods to let the gas accumulate via, e.g., ambipolar diffusion. To tell the difference between these theories, measuring the age of prestellar cores could greatly help. However, no reliable clock currently exists. We present a simple chemical clock based on the regulation of the deuteration by the abundance of ortho-H 2 that slowly decays away from the ortho-para statistical ratio of 3 down to or less than 0.001. We use a chemical network fully coupled to a hydrodynamical model that follows the contraction of a cloud, starting from uniform density, and reaches a density profile typical of a prestellar core. We compute the N 2 D + /N 2 H + ratio along the density profile. The disappearance of ortho-H 2 is tied to the duration of the contraction and the N 2 D + /N 2 H + ratio increases in the wake of the ortho-H 2 abundance decrease. By adjusting the time of contraction, we obtain different deuteration profiles that we can compare to the observations. Our model can test fast contractions (from 10 4 to 10 6 cm −3 in ∼0.5 My) and slow contractions (from 10 4 to 10 6 cm −3 in ∼5 My). We have tested the sensitivity of the models to various initial conditions. The slowcontraction deuteration profile is approximately insensitive to these variations, while the fast-contraction deuteration profile shows significant variations. We found that, in all cases, the deuteration profile remains clearly distinguishable whether it comes from the fast collapse or the slow collapse. We also study the para-D 2 H + /ortho-H 2 D + ratio and find that its variation is not monotonic, so it does not discriminate between models. Applying this model to L183 (=L134N), we find that the N 2 D + /N 2 H + ratio would be higher than unity for evolutionary timescales of a few megayears independently of other parameters, such as cosmic ray ionization rate or grain size (within reasonable ranges). A good fit to the observations is only obtained for fast contraction (≤0.7 My from the beginning of the contraction and ≤4 My from the birth of the molecular cloud based on the need to keep a high ortho-H 2 abundance when the contraction starts -ortho-H 2 /para-H 2 ≥ 0.2 -to match the observations). This chemical clock therefore rules out slow contraction in L183 and steady-state chemical models, since steady state is clearly not reached here. This clock should be applied to other cores to help distinguish slow and fast contraction theories over a large sample of cases.

Chemical modeling of the L1498 and L1517B prestellar cores: CO and HCO+depletion

“…In addition, we note that the lower limits in the age we obtain here are comparable to shortest lifetime obtained by Lee & Myers (1999) (0.3 × 10 6 yr); it is therefore possible that two cores are at a relatively evolved stage, and may quickly evolve to form protostars. It is interesting to note that, using an independant technique, Pagani et al (2013) obtained an age of <0.7 My for the L183 core, based on its observed N 2 D + /N 2 H + ratio; this value is consistent with the chemical ages derived in this study. Figure 8 shows the predicted abundances for several species, as a function of the radius, for our best-fit models.…”

Dynamics of the D+ + H2 → HD + H+ reaction at the low energy regime by means of a statistical quantum method

“…Thermal rate constants for this ortho-para conversion 4,5 are thus employed as necessary input in astrophysical models to design chemical clocks to measure the age of molecular clouds and prestellar cores. 6,7 Several potential energy surfaces (PES) are nowadays available at the literature for the H + 3 system. [8][9][10][11][12][13] The disa) Electronic mail: t.gonzalez.lezana@csic.es tinct topology of these surfaces has been found to introduce some subtle effects on the dynamics of the corresponding reactive scattering processes.…”

Deuteration of ammonia in the starless core Ophiuchus/H-MM1

“…The fact that the average temperature exceeds 10 K has a favourable effect on deuteration, but its fast advancement is made possible by the low initial o/pH 2 ratio assumed in the simulation. A higher abundance of oH 2 would delay the deuterium peak by obstructing the primary deuteration through reaction H + 3 + HD ↔ H 2 D + + H 2 (Flower et al 2006b;Pagani et al 2011;Pagani et al 2013;Kong et al 2015).…”