[tel-00726959, v1] Caractériser le milieu interstellaire ... - HAL - INRIA

A&A 534, A49 (2011)tel-00726959, version 1 - 31 Aug 2012Fig. 3. Radiative transfer modeling of H 2 CO lines for two positions toward the Horsehead. Two left columns: the PDR position (T kin = 60 K,n(H 2 ) = 6 × 10 4 cm −3 , N(o-H 2 CO) = 7.2 × 10 12 cm −2 )andtwo right columns: the dense-core position (T kin = 20 K, n(H 2 ) = 10 5 cm −3 ,N(o-H 2 CO) = 9.6 × 10 12 cm −2 ). The two top rows display the ortho lines, for which we varied the column density around the best match (redcurve) by a factor of 1.5 (blue curve) and 1/1.5 (green curve). The two bottom rows display the para lines, for which we kept the column densityof the best match for o-H 2 CO (red curves) constant and varied the ortho-to-para ratio of H 2 CO: o/p = 1.5 (dashed blue), o/p = 2 (dashed red) ando/p = 3 (dashed green).estimate the optical depth of the H 2 CO lines. Then, we made a where β is the escape probability function, which in the case ofused rotational diagram analysis (Goldsmith & Langer 1999).F H2 CO= [12 C]F H132 CO [ 13 C] β (1) To do this, we assume that the gas is under LTE, and thereforeall excitation temperatures are the same, and the energy levelsfirst estimate of the column densities and excitation temperatures a homogeneous slab of gas (de Jong et al. 1980) is equal toas an input for a detailed nonlocal non-LTE excitation and radiativetransfer analysis to compute the H 2 CO abundances. Thespectroscopic parameters for the detected transitions (shown inusing rotational diagrams. Finally, we used these first estimatesβ = 1 − exp(−3τ) ·3τ(2)Fig. 2) aregiveninTable3. We assumed that the emission is The isotopic abundance ratio 12 C/ 13 C ≃ 60 (Langer & Penziasextended and fills the 30 m beam, as shown by the map of the 1990; Savage et al. 2002) is almost twice the line intensity ratio3 03 −2 02 transition (see Fig. 1).between formaldehyde and its isotopologue, and therefore theH 2 CO lines have moderate opacities. From the observations we3.2.1. Opacity of the H 2 CO linesWe detected two transitions of the formaldehyde isotopologueestimate τ 212 −2 11∼ 1.6andτ 202 −1 01∼ 1.9forH 2 CO in the densecore.H 132CO in the dense-core position (see upper panels in Fig. 4).By comparing the flux between H 2 CO and H 132CO for the same 3.2.2. Rotational diagram analysistransition it is possible to estimate the opacity of the H 2 CO line,assuming that the H 132CO line is optically thin, as follows: First-order estimates of the beam-averaged column densities andthe rotational temperatures can be found by means of the widelyA49, page 4 of 9