Neutral carbon and CO emission in dark clouds / weak-fuv PDRs

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1 1 Neutral carbon and CO emission in dark clouds / weak-fuv PDRs Frank Bensch Radioastronomisches Institut, Universität Bonn (Germany) Submillimeter Astronomy in the era of the SMA, Harvard University, Cambridge, MA (USA); June 13-16, 2005

2 Introduction 2 CO, [CI] emission in dark/translucent clouds... why bother?. bulk of the molecular gas in the Galaxy is exposed to the ISRF (COBE: Fixsen, Bennett & Mather) modeling of the emission in normal galaxies examples: Kramer et al Jean Turner,.... CO, [CI] can be cosmological distances example: [CI] 810GHz line at z=6.4 example: Bertoldi et al. Fabian Walter, Jason Glenn et al.,... [CII]: Iono et al. (poster),.... Understand formation of [CI], CO lines in clouds closer to home: [CII],[CI], CO: dominant cooling lines for clouds in the ISRF Open questions still remain regarding the chemistry and the excitation of [CI],CO in Galactic dark & translucent clouds...

3 Example: Dark Cloud Barnard 5 3 B5: a dark cloud in the easternmost part of the Perseus complex... Data set: [CI] 3 P 1-3 P 0 SWAS 12 CO, 13 CO 1-0 FCRAO 12 CO, 13 CO 2-1, 3-2 KOSMA Mapping data: dark cloud (A v > 7) to halo (A v < 1) + literature data A v, UV-absorption data (density, temp & composition of the halo),...

4 Example: Dark Cloud Barnard 5 4 Spectral line data, smoothed to SWAS angular resolution (4 3): S-N cut: B5 core halo B5 halo (A v < 1) N C + N C 0 N CO n 70 cm 3, T 40 K; Wannier et al. CO detected no [CI] detected B5 core/cloud (7 > A v > 2) relatively constant [CI]/CO line ratios ratio similar to other d.c. no bright [CI] emission at cloud edge

5 Example: Dark Cloud Barnard 5 5 B5: [CI]/CO line integrated intensity ratios: B5 halo: CO emission in halo excitation by line radiation no [CI] emission detected excitation by collisions B5 core/molecular cloud: relatively constant [CI]/ 12 CO(1-0), [CI]/ 13 CO(1-0) line ratio line ratios similar to ratios in other dark, translucent clouds [CI],CO emission consistent with a weak-fuv PDR?

6 Spherical PDR Model 6 Spherically symmetric PDR model for clumpy molecular clouds: Compare observations to PDR model: (clump-averaged quantities) Line integrated intensity ratios (if clump < beam) Line peak temperature (if beam filling 1) Goal: derive the typical n, M of the clumps governing the line emission

7 Spherical PDR Model 7 PDR model: (Störzer, Stutzki & Sternberg 1996) spherical clump, density n r 1.5 Model parameter: n cl, M cl + other parameter: χ = 1, chem. network, initial abundances,... self-consistent solution for heating/cooling & chemistry temperature, abundance profiles Radiative transfer model: (Onion; Gierens et al. 1992) input: PDR model results line intensities, peak temperature,... Compare Onion results to observations

8 Spherical PDR Model 8 Compare (clump-av.) line ratios in the PDR models to observations model grid: M cl = M log(n cl ) = cm 3 B5 observations: six [CI]/CO line ratios χ 2 = Σ i (R i,obs R i,pdr ) 2 R 2 i,obs search minimum for χ 2 (n cl, M cl )

9 PDR Model: Barnard 5 9 Model grid # 1: FUV-field = ISRF (χ = 1) Results B5 core M cl : 10 4 M n cl : cm 3 BUT: to CO line excitation (+ literature) line ratios similar to those observed for other translucent, dark clouds

10 Solution: pre-shielding by H 2 in halo 10 The impact of the diffuse halo surrounding B5: (Pre-)shielding of CO by H 2,CO in a diffuse halo surrounding the cloud Properties of the halo: A v < 1 mag, N H cm 2 C + C>CO H>H 2 Impact on molec. cloud: CO abundance, C : [CI]/CO heating const.

11 Solution: pre-shielding by H 2 in halo 11 Model grid # 3: FUV-field: χ = 1, N H2 (halo)= cm 2 Results B5 core M cl : 1 M n cl : cm 3 CO line excitation peak temperatures clump size/extinction (literature)

12 Conclusions 12 PDR model for dark and translucent clouds in weak-fuv fields: Impact by molecular hydrogen in diffuse halo N H2 (halo) cm 2 ( B5 halo) substantially changes [CI], CO emission in cloud [CI]/CO ratios reduced by a factor of a few up to an order of magnitude! B5 cloud: model with pre-shielding gives line ratios consistent with observations CO, neutral carbon in cloud halo CO emission detected: excitation by line absorption [CI] not detected: ex. dominated by collisions (n halo n cr ) Future observations Test pre-shielding model Search for [CI] emission in cloud halo