Physical and chemical modeling of the starless core L 1512^★

¹ Institute of Astronomy, National Tsing Hua University, No. 101, Section 2, Kuang-Fu Road, Hsinchu 30013, Taiwan
e-mail: sj.lin@gapp.nthu.edu.tw
² LERMA & UMR8112 du CNRS, Observatoire de Paris, PSL University, Sorbonne Universités, CNRS, 75014 Paris, France
³ Institut de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, 38400 Saint-Martin d’Hères, France
⁴ LOMC – UMR 6294, CNRS-Université du Havre, 25 rue Philippe Lebon, BP 1123, 76063 Le Havre, France

Received: 9 October 2019
Accepted: 29 January 2020

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 N₂ 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 N₂H⁺, N₂D⁺, DCO⁺, C¹⁸O, and ¹³CO, plus H₂D⁺ (1₁₀–1₁₁).

Results. We find a peak density of 1.1 × 10⁵ 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 N₂H⁺ and N₂D⁺ are 27₋₁₃⁺¹⁷ and 4₋₁⁺² 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 N₂ in L 1512, compared to the two others despite a peak density one to two orders of magnitude lower. Retrieving CO and N₂ abundance profiles with the chemical model, we find that CO has a depletion factor of ~430–870 and the N₂ profile is similar to that of CO unlike that toward L 183. Therefore, L 1512 has probably been living long enough so that N₂ chemistry has reached steady state.

Conclusions. N₂H⁺ 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.