TWO PROTOTYPICAL GALACTIC PDRS: THE ORION BAR AND THE HORSEHEAD MANE

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1 Title : will be set by the publisher Editors : will be set by the publisher EAS Publications Series, Vol.?, 2008 TWO PROTOTYPICAL GALACTIC PDRS: THE ORION BAR AND THE HORSEHEAD MANE J. Pety 1, 2, J. R. Goicoechea 2 and M. Gerin 2 Abstract. Photodissociation region (PDR) models are used to understand the evolution of the FUV illuminated matter both in our Galaxy and in external galaxies. To prepare for the unprecedented spatial and spectroscopic capabilities of ALMA and Herschel, two different kinds of progresses are currently taking place in the field. First, numerical models describing the chemistry of a molecular cloud are being benchmarked against each other to ensure that all models agree not only qualitatively but also quantitatively on at least simple cases (Röllig et al. 2007). Second, new or improved chemical rates are being calculated/measured by several theoretical and experimental groups. However, the difficulty of this last effort implies that only a few reactions (among the thousand ones used in chemical networks) can be thoroughly studied. New numerical tools are thus being developed for taking into account the impact of the uncertainties of the chemical rates on the chemical model predictions, and their comparison with observed abundances (e.g. Wakelam et al. 2005, 2006). In view of the intrinsic complexity of building reliable chemical networks and models, there is an obvious need of well-defined observations that can serve as basic references. PDRs are particularly well suited to serve as references because they make the link between diffuse and dark clouds, thus enabling to probe a large variety of physical and chemical processes. We describe the merit of two prototypical PDRs: the Orion bar and the Horsehead mane. PDR stands for Photon-Dominated Region or Photo-Dissocation Region. The study of Galactic PDRs is an extremely large field of activity as it concerns the physics and chemistry of gas from many astrophysical objects: diffuse or translucent clouds, proto-stellar disks, AGB circumstellar envelopes and planetary nebulae, and of course the interface between HII regions and dense molecular clouds. Even when restricted to the latter environment, the subject is quite large as many 1 IRAM, France; pety@iram.fr 2 LERMA, Obs. de Paris and CNRS, France c EDP Sciences 2008 DOI: (will be inserted later)

2 2 Title : will be set by the publisher Orion Bar Horsehead Mane Location Orion Molecular Cloud, in M42 Orion Molecular Cloud, near IC434 Distance from Earth 450 pc ( pc) 400 pc ( pc) (RA,Dec) J2000 (05h35m24.00s, ) (05h40m54.27s, ) Main exciting star θ 1 C Ori (O6) at 2 (0.26 pc), PA 45 σori (O9.5V) at 0.5 (3.5 pc), PA 76 Far-UV intensity G 0 (1 4) 10 4 (Habing) G (Habing) Environment Southern lip of a bowl-shaped nebula Edge of a pillar Geometry Cylindrical filament 1D, almost edge-on (Maybe cylinder?) Density High density clump: cm 3 Steep gradient up to 10 5 cm 3 in 10 Inter clump gas: (1 4) 10 4 cm 3 Surrounding halo: (5 10) 10 3 cm 3 Thermal pressure Roughly uniform at K cm 3 Table 1. Identity card of the Orion bar and the Horsehead mane PDRs. such regions are still studied in detail today: e.g. NGC 2023, NGC 2024, NGC 7023, S106, S140, Carina, Monoceros, etc... We will here concentrate on relatively recent (> 2000) results on two well-studied PDRs: The Orion bar and the Horsehead mane. After a short description of the environment and of the physical properties of both PDRs, we will describe complementary aspects of their chemical properties. The reader is refereed to the PDR everywhere review by Ossenkopf for a larger view on Galactic PDRs (Ossenkopf et al. 2007). 1 Environment and physical properties Geometry Both the Orion bar and the Horsehead mane are at the edges of the Orion giant molecular cloud, implying a typical distance from Earth of pc. On optical and IR images, the Orion bar appears as a bright elongated structure which seems to be viewed edge-on. Walmsley et al. (2000) showed that the easiest geometry model to explain the emission profile across the bar of the OI 1.3 µm line is a cylindrical filament. The western edge of the Horsehead nebulae presents one of the sharpest infrared filament (width: 10 or 0.02 pc) detected in our Galaxy by ISOCAM. The most straightforward explanation given by Abergel et al. (2003) is that most of the dense material is within a flat structure viewed edge-on and illuminated in the plane of the sky by its exciting star. The H 2 fluorescent emission observed by Habart et al. (2005) is even sharper (width: 5 ), implying the inclination of the PDR on the plane-of-sky to be less than 5. The possibility of a cylindrical geometry for the Horsehead PDR remains an open question. Both sources thus offer the opportunity to study at small linear scales (1 corresponds to pc at 400 pc) the physics and chemistry of a PDR with a simple geometry. Both are used as references for chemical models. Far UV illumination One of the main difference between both regions is the distance between the PDR and its exciting star. Indeed, both PDR are excited by an O star but σori is more than an order of magnitude further away from the Horsehead mane than θ 1 Ori is from the Orion bar. This implies that the far-uv intensity reaching the PDR is 2 orders of magnitude larger for the Orion bar than for the Horsehead mane. The environment of both regions is also very different. Horsehead mane environment, density and temperature Pound et al. (2003) were the first to cast the Horsehead nebula in the category of the pillars like the well-known examples in the Eagle nebula, implying a sharp transition

3 The Orion bar vs the Horsehead mane 3 Fig. 1. Emission maps obtained with the IRAM Plateau de Bure Interferometer or 30m single-dish, except for the H 2 v=1-0 S(1) emission observed with the NTT/SOFI and the PAH mid IR emission observed with ISO-LW2. The maps have been rotated by 14 counter clockwise as this eases the comparison of the PDR tracer stratifications. Maps have also been horizontally shifted by 20 (compared to the projection center: RA = 05h40m54.27s, Dec = , J2000) to set the horizontal zero at the PDR edge delineated as the vertical red line. Either the synthesized beam or the single dish beam is plotted in the bottom left corner. The emission of all the lines is integrated between 10.1 and 11.1 km s 1. Values of contour level are shown on each image lookup table (contours of the H 2 image have been computed on an image smoothed to 5 resolution). The green crosses display the positions where we derived column densities and abundances (see Table 2). between the HII region and the molecular cloud. Through the modelling of the H 2 and CO emission, Habart et al. (2005) showed that the PDR has a very steep density gradient, rising to n H 10 5 cm 3 in less than 10 (i.e pc), at a roughly constant pressure of P K cm 3. Recently, Pety et al. (2007) observed bright DCO + lines (up to 4 K) less than 40 from the H 2 /PAH emission peak (PDR illuminated edge). The modelling of this gas implied that the gas at the DCO + peak must be cold (10 20 K) and dense (n H cm 3 ). This opens up the interesting possibility to probe at high resolution the chemical transition from far-uv photodominated, hot gas to dark cloud, shielded and cold gas in a small field of view. Finally, through a detailed radiative transfer modelling of several transitions of CS and C 34 S, Goicoechea et al. (2006) showed that the Horsehead mane is also surrounded by a more diffuse halo (n(h 2 ) (5 10) 10 3 cm 3 ).

4 4 Title : will be set by the publisher Orion bar environment, density and temperature The Orion bar is the southern lip of a bowl shaped interface between the HII region and the molecular cloud. This is examplified on the CO J=7 6 map (Fig. 3 Wilson et al. 2001), which clearly shows emission ( 10 2 K km s 1 ) between the PDR and the Trapezium cluster. This image also shows that the Orion bar is connected (at least in projection) to the famous Orion KL region by a structure known as the Orion ridge. The usual modelling of the Orion bar emission uses a combination of clump and interclump gas. The standard value for the inter-clump gas is about cm 3. Lis & Schilke (2003) deduced from IRAM/PdBI observations of H 13 CO + J=1 0 and H 13 CN J=1 0 a median clump density as high as cm 3 and a typical temperature of 50 K. Batrla & Wilson (2003) deduced from 1 cm rotation-inversion lines of ammonia a kinetic temperature of about 150 K for the inter-clump medium. These authors suggest that ammonia is formed in dense clumps and then released into the inter-clump gas by evaporation of the dense clumps. 2 Chemical properties Deuteration chemistry Pety et al. (2007) studied the deuterium fractionation in the Horsehead edge from observations of several H 13 CO + and DCO + lines. A large [DCO + ]/[HCO + ] abundance ratio ( 0.02) is inferred at the DCO + emission peak, a condensation shielded from the illuminating far-uv radiation field where the gas is cold (10 20 K) and dense (n H cm 3 ). DCO + is not detected in the warmer photodissociation front, implying a lower [DCO + ]/[HCO + ] ratio (< 10 3 ). This situation is well-understood by cold deuteration, i.e. through deuteration of H + 3. Recent, still unpublished observations shows faint lines of DCN and DNC at the DCO + peak. The deuteration situation of HCO + and HCN is reverse in the Orion bar where DCN is easily detected (Leurini et al. 2006), implying [DCN]/[HCN] = 0.01 in the dense clumps, while the DCO + line are faint, implying [DCO + ]/[HCO + ] < (Parise, priv. comm.). Roueff et al. (2007) showed that the HCN deuteration in the Orion bar can be understood in the framework of warm (i.e K) deuteration, which proceeds through the deuteration of CH + 3 and C 2H + 2. An interesting follow-up would be to search for those ions in the Orion bar. Small hydrocarbon, sulfur chemistry and depletion Pety et al. (2005) showed that the abundances of the hydrocarbons (CCH, c-c 3 H 2, C 4 H) in the Horsehead mane are higher than the predictions based on pure gas phase chemical models (Le Petit et al. 2002, and reference therein). These results could be explained either by the photoerosion of the large aromatic molecules and/or small carbon grains or by a turbulent mixing which would transport in the illuminated part of the PDR, molecules produced in the dark part (Lesaffre et al. 2007). Indeed Compiègne et al. (2007) have detected neutral PAHs in the ionised gas ahead of the horsehead mane. The presence of PAHs can be explained by the continuous production of fresh matter by the photoevaporation of the Horsehead, and the long survival time of these particules in the HII region created by an O9.5 star. Another interesting result come from the comparison of the small hydrocarbon

5 The Orion bar vs the Horsehead mane 5 Species (δra,δdec) (δx,δy) Angular Res. Col. Dens. Abundance [arcsec] [arcsec] [arcsec] N(X) [ cm 2 n ] X n H +2n H2 H 2 ( 12, 4) (+7.4, 1.0) 12 (3.6 ± 1.7) C 18 O (1.0 ± 0.3) CCH (1.1 ± 0.3) c-c 3 H (9.5 ± 5.0) C 4 H (3.7 ± 1.0) H 2 ( 6, 4) (+12.2, 2.4) 12 (1.1 ± 0.4) C 18 O (4.0 ± 0.5) CCH (3.0 ± 0.5) c-c 3 H (2.4 ± 1.0) C 4 H (4.0 ± 1.0) H 2 (+6, 4) (+24.9, 5.3) 12 (2.7 ± 1.0) C 18 O (5.8 ± 0.5) CCH (5.5 ± 1.0) c-c 3 H (2.3 ± 0.7) C 4 H (2.0 ± 1.0) CS (+4, 0) (+23.9, 1.0) 10 (8.1 ± 1.0) C 34 S 16 (3.7 ± 0.5) HCS + 29 (6.8 ± 0.5) H 13 CO + (+19.7, +20.4) (+44.0, +15.0) (0.5 1) (0.5 1) DCO + 12 (0.5 1) (0.5 1) CS (+21, +15) (+44.0, +9.5) 10 (1.2 ± 1.0) C 34 S 16 (5.3 ± 0.5) HCS + 29 (6.8 ± 0.5) CS (+36, +25) (+61.0, +15.5) 10 (1.7 ± 1.0) C 34 S 16 (7.9 ± 0.5) HCS + 29 (9.0 ± 0.5) Table 2. Column densities and abundances of several chemical species from the UV illuminated PDR (upper part) to the shielded region (lower part) of the Horsehead edge. Coordinates offsets are given both in the Equatorial system from RA = 05h40m54.27s, Dec = (J2000) and in the coordinate system adapted to the source geometry and used in Figure 1. Abundances are computed with respect to the density of protons (n H). and DCO + maps. At the DCO + peak, CCH emission is faint while c-c 3 H 2 emission is bright, suggesting a different depletion behavior of the two species on this particularly cold and dense region. Goicoechea et al. (2006) linked the PDR model predictions with detailed non- LTE, nonlocal excitation and radiative transfer models adapted to the Horsehead geometry. They showed that the gas sulfur depletion invoked to account for CS and HCS + abundances is orders of magnitude lower than in previous studies of the sulfur chemistry. Reactive ions, SiO and HCO chemistry In the Orion bar, Savage & Ziurys (2004) observed that the [HOC + ]/[HCO + ] ratio increases by about one order of magnitude when going from the dark to the illuminated part of the PDR. They also found a good spatial correlation between HOC + and CO +, which they explain as resulting from the fact that both specie share C + as chemical precursor. Fuente et al. (2003) found that CO + arises from PDR layers with A v < 2 mag while SO + arises from more far-uv shielded layers. They also note that the SO + abundance is quite uniform while [CO + ] and [HOC + ] strongly increase in the illuminated side of the PDR. Schilke et al. (2001) found that SiO emission is relatively extended all over the Orion bar. The ratio of integrated emission of two SiO transition implies a large thermal pressure and thus a formation in the dense clumps. The authors

6 6 Title : will be set by the publisher also observed bright HCO lines whose origin is not well-understood. Gerin et al (in prep.) will present IRAM/PdBI observation of HCO in the Horsehead mane, showing that this molecule correlates well with others tracers of the PDR gas (see also Fig. 1). 3 Towards observational benchmarks An ideal observational benchmark would deliver to chemists a set of abundances (with the associated uncertainties) as a function of the distance (or extinction) to the illuminating star. This goal is difficult to achieve for several reasons: 1) The geometry of the source is never as simple as wished when it is known at all; 2) The spectra produced by the instruments must be inverted to obtain abundances; 3) The spectra are often measured at very different angular resolutions, implying beam dilution and/or mixing of different gas components. While the Orion bar is probably the most studied PDR of the sky, no consistent data set is available for this source. For several years now, we have started to systematically study the western edge of the Horsehead nebula. Many observations have been published. The on-going steps are a biased line survey of sulfur molecules, the study of nitrogen chemistry (CN, HCN, HNC, NH 3, N 2 H +,...), the study of more complex molecules (methanol, formaldehyde, ketene,...) and the study of their deuterated counterparts. All those observations are done by the same team, using the same instruments (mainly IRAM Plateau de Bure and 30m) and the same methods both of data reduction and data analysis. For each species, we are trying to observe several transitions at similar angular resolutions (from 5 to 15 ) to constrain properly the excitation conditions and derive accurate column densities and abundances. Obtaining emission maps when possible has proved essential to understand the spatial distribution of the species. We are preparing the public release of spectral data so that the radiative transfer analysis can be refined as the knowledge of collisional rates progresses. In the meantime, Table 2 is our first attempt at quantitatively summarizing the results obtained up to now. In the future, the zoom capacity and resolving power of ALMA will enable to map many different species at a resolution of 1, enabling to resolve all the physical and chemical gradients. References Abergel, A., Teyssier, D., Bernard, J. P., et al. 2003, A&A, 410, 577 Batrla, W. & Wilson, T. L. 2003, A&A, 408, 231 Compiègne, M., Abergel, A., Verstraete, L., et al. 2007, A&A, 471, 205 Fuente, A., Rodrıguez-Franco, A., Garcıa-Burillo, S., Martın-Pintado, J., & Black, J. H. 2003, A&A, 406, 899 Goicoechea, J. R., Pety, J., Gerin, M., et al. 2006, A&A, 456, 565

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