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

980 M. Gerin et al.: HCO mapping of the Horsehead: tracing the illuminated dense molecular cloud surfacestel-00726959, version 1 - 31 Aug 2012Fig. 2. IRAM-30 m observations (histograms) of several HCO hyperfine components of the 1 01 –0 00 ,2 02 –1 01 and 3 03 –2 02 rotational transitionstowards the PDR (“HCO peak”) and towards the dense core (“DCO + peak”) Pety et al. (2007a). Solid lines are single-T ex radiative transfer modelsof the PDR-filament (red curves) and line-of-sight cloud surface (blue curves). A sketch of the HCO rotational energy levels is also shown (rightcorner).Table 3. Einstein coefficients and upper level energies.Molecule Transition Frequency A ij E upJ, F−J ′ , F ′ GHz (s −1 ) (K)HCO 1 01 −0 003/2, 2−1/2, 1 86.670760 4.69 × 10 −6 4.23/2, 1−1/2, 0 86.708360 4.60 × 10 −6 4.21/2, 1, 1/2, 0 86.777460 4.61 × 10 −6 4.21/2, 0−1/2, 1 86.805780 4.71 × 10 −6 4.22 02 −1 015/2, 3−3/2, 2 173.3773770 4.51 × 10 −5 12.55/2, 2−3/2, 1 173.4060816 4.43 × 10 −5 12.53/2, 2−1/2, 1 173.4430648 3.39 × 10 −5 12.53 03 −2 027/2, 4−5/2, 3 260.0603290 1.63 × 10 −4 25.07/2, 3−5/2, 2 260.0821920 1.61 × 10 −4 25.05/2, 3−3/2, 2 260.1335860 1.45 × 10 −4 25.05/2, 2−3/2, 1 260.1557690 1.37 × 10 −4 25.0H 13 CO + J = 1–0 86.754288 3.2 × 10 −5 4.2J = 3–2 260.2553390 1.3 × 10 −3 25.0The line frequencies and intensities were extracted from the JPL Pickettet al. (1998) andCDMS(Müller et al. 2001, 2005) molecular spectroscopydata bases for HCO and H 13 CO + respectively.3.2.2. Structure of the PDR in HCO and H 13 CO +For more insight into the spatial variation of the HCO andH 13 CO + column densities and abundances, we have analyzed acut through the PDR, centered on the “HCO peak” at δy = 0 ′′(see Fig. 3). The cut clearly shows that HCO is brighter thanH 13 CO + in the PDR and vice-versa in the dense core. Takinginto account the different level degeneracies of both transitions(a factor of 2.4) and the fact that the associated EinsteinFig. 3. Observations along a horizontal cut through “the HCO peak”(histograms). The H 13 CO + J = 1–0 and HCO 1 01 −0 00 lines weremapped with the PdBI at an angular resolution of 6.8 ′′ , whereas theH 13 CO + J = 3–2 line was mapped with HERA-30 m (and smoothed toa spatial resolution of 13.5 ′′ ). Radiative transfer models of an edge-oncloud with a line of sight extinction of A V = 20, inclined 5 ◦ relativeto the line of sight for HCO (red curve), and H 13 CO + (blue curves) areshown. The single-T ex HCO model assumes a 12 ′′ width filament witha column density of 3.2 × 10 13 cm −2 , while N(HCO) is 4.6 × 10 12 cm −2behind the filament. The H 13 CO + model assumes a constant density ofn(H 2 ) = 5×10 4 cm −3 with T k = 60 K and N(H 13 CO + ) = 5.8×10 11 cm −2for δx < 35 ′′ ;andT k = 10 K and N(H 13 CO + ) = 7.6 × 10 11 cm −2 forδx > 35 ′′ . Modeled line profiles have been convolved with an appropriateGaussian beam corresponding to each PdBI synthesized beam or30 m main beam resolution.coefficients A ij differ by a factor ∼8 (due to the different permanentdipole moments, see Table 3), N(H 13 CO + ) must be significantlylower than N(HCO) towards the PDR.We modeled the PDR as an edge-on cloud inclined by∼5 ◦ relative to the line-of-sight. We have chosen a cloud depth