Physical and chemical modeling of the starless core L 1512

Abstract

Context. The deuterium fractionation in starless cores gives us a clue to estimate their lifetime scales, thus allowing us to distinguish between dynamical theories of core formation. Cores also seem to be subject to a differential N 2 and CO depletion, which was not expected from the models. Aims. We aim to create a survey of ten cores to estimate their lifetime scales and depletion profiles in detail. After describing L 183, located in Serpens, we present the second cloud of the series, L 1512, from the star-forming region Auriga. Methods. To constrain the lifetime scale, we performed chemical modeling of the deuteration profiles across L 1512 based on dust extinction measurements from near-infrared observations and nonlocal thermal equilibrium radiative transfer with multiple line observations of N 2 H + , N 2 D + , DCO + , C 18 O, and 13 CO, plus H 2 D + (1 10 -1 11 ). Results. We find a peak density of 1.1×10 5 cm −3 and a central temperature of 7.5±1 K, which are higher and lower, respectively, compared with previous dust emission studies. The depletion factors of N 2 H + and N 2 D + are 27 +17 −13 and 4 +2 −1 in L 1512, which are intermediate between the two other more advanced and denser starless core cases, L 183 and L 1544. These factors also indicate a similar freeze-out of N 2 in L 1512, compared to the two others despite a peak density one to two orders of magnitude lower. Retrieving CO and N 2 abundance profiles with the chemical model, we find that CO has a depletion factor of ∼430-870 and the N 2 profile is similar to that of CO unlike that toward L 183. Therefore, L 1512 has probably been living long enough so that N 2 chemistry has reached steady state. Conclusions. N 2 H + modeling is necessary to assess the precise physical conditions in the center of cold starless cores, rather than dust emission. L 1512 is presumably older than 1.4 Myr. Therefore, the dominating core formation mechanism should be ambipolar diffusion for this source.

Multi-wavelength observations and modelling of a quiescent cloud LDN1512

“…The values derived from observations are all averages over 5 × 5 pixels centred on point 1. The core temperature is based on the N 2 H + observations by Lin et al (2020). In addition to the models in Table 1, we tested three further modifications to the dust properties.…”

“…We have chosen the cloud LDN1512 (hereafter L1512) to study and model NIR light scattering and thermal dust emission at FIR wavelengths. The cloud has a simple cometary morphology, is nearby (140 ± 20 pc, Launhardt et al 2013), and based on the reported small line width of N 2 H + (Caselli et al 1995(Caselli et al , 2002Lin et al 2020), appears to be quiescent.…”

“…Based on the fitting of the 13 CO line, Lippok et al (2013) showed that the envelope of the core has a higher gas temperature compared to the central regions and their stability analysis shows that the core is thermally supercritical. The N 2 H + observations of Lin et al (2020) show a low kinetic temperature of ∼8 ± 1 K within the innermost ∼0.017 pc of the core, and based on their chemical modelling, the cloud is sufficiently evolved that the N 2 chemistry has reached a steady state. Their results suggest that L1512 is likely older than 1.4 Myr and that ambipolar diffusion has led to the formation of the core.…”

“…1, from the Wide InfraRed CAMera (WIRCAM) on the Canada-France-Hawaii Telescope (CFHT). The observations are presented in Lin et al (2020), where only the stellar content of the images has been exploited. in this paper we concentrate on the extended emission.…”

The Leiden Atomic and Molecular Database (LAMDA): Current Status, Recent Updates, and Future Plans

“…Other substitutions cause a minor change in the molecular symmetry, such as 12 C-13 C in C 3 H 2 ; in such cases, the potential energy surface of the interaction is basically unchanged. Significant effects on collisional properties occur mostly for H→D substitutions, especially for hydrides: DCO + -H 2 [138], ND-He [113], ND 3 -H 2 [137], D 2 O-H 2 [85], and N 2 D + -H 2 [139]. If isotopic substitution does not change the symmetry of the molecule at all, the effect on its collisional properties is small (<30%); this category includes 13 CN-H 2 and C 15 N-H 2 [140], and N 15 NH + -H 2 [141] This latter category is the least urgent to update because, for symmetry-conserving isotopic substitutions, simple scaling by (reduced) mass is quite accurate.…”