A&A 410, 577-585 (2003)
A. Abergel 1 - D. Teyssier2,13 - J. P. Bernard 1 - F. Boulanger 1 - A. Coulais12 - D. Fosse2 - E. Falgarone 2 - M. Gerin 2 - M. Perault 2 - J.-L. Puget 1 - L. Nordh 3 - G. Olofsson 3 - M. Huldtgren 3 - A. A. Kaas 3 - P. André 4 - S. Bontemps 9 - M. M. Casali 10 - C. J. Cesarsky11 - E. Copet 5 - J. Davies 6 - T. Montmerle4 - P. Persi 7 - F. Sibille 8
1 - Institut d'Astrophysique Spatiale, Université Paris-Sud, Bât. 121, 91405 Orsay, France
2 - LERMA, École Normale Supérieure et Observatoire de Paris, 24 rue Lhomond, 75231 Paris Cedex 05, France
3 - Stockholm Observatory, 133 36 Saltsjöbaden, Sweden
4 - Service d'Astrophysique, Centre d'Études de Saclay, 91191 Gif-Sur-Yvette Cedex, France
5 - LESIA, Observatoire de Paris-Meudon, 5 place Jules Janssen, 92195 Meudon Cedex, France
6 - Joint Astronomy Center, 660 N. A'Ohoku Place, University Park, Hilo, HI 96720, USA
7 - Istituto Astrofisica Spaziale, Area di Ricerca Tor Vergata, via del Fosso del Cavaliere, 00133 Roma, Italy
8 - Observatoire de Lyon, 69230 Saint Genis Laval, France
9 - Observatoire de Bordeaux, BP 89, 33270 Floirac, France
10 - Royal Observatory, Blackford Hill, Edinburgh, UK
11 - ESO, Headquarters, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
12 - LERMA, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
13 - SRON - Low Energy Astrophysics Division Landleven 12, 9747 AD Groningen, The Netherlands
Received 2 August 2002 / Accepted 28 May 2003
We present ISOCAM observations (5-18 m) of the Horsehead nebula, together with observations of the (J=1-0) and (J=2-1) transitions of 12CO, 13CO and C18O taken at the IRAM 30-m telescope.
The Horsehead nebula presents a typical photodissociation region illuminated by the O9.5 V system Ori. The ISOCAM emission is due to very small particles transiently heated to high temperature each time they absorb a UV photon. A very sharp filament (width: or pc) is detected by ISOCAM at the illuminated edge of the nebula. This filament is due to a combined effect of steep increase of the column density and extinction of incident radiation, on typical sizes below pc.
Both the three-dimensional shape and the local density of the illuminated interface are strongly constrained. The dense material forming the edge of the Horsehead nebula appears illuminated edge-on by Ori, and the particles located beyond the border should not be affected by the incident radiation field. This structure may be due to dense filaments in the parental cloud which have shielded the material located in their shadow from the photo-dissociating radiations.
The measurement of the penetration depth of the incident radiation from the infrared data ( pc) gives a density of a few 104 cm-3 just behind the bright filament. This value is comparable to the estimate of the density beyond the edge and deduced from our molecular observations, and also to the density behind the ionization front calculated in the stationary case. The material behind the illuminated edge could also be non-homogeneous, with clump sizes significantly smaller than the observed penetration depth of pc. In that case no upper limit on the average density just behind the illuminated edge can be given.
Key words: ISM: individual objects: Horsehead - ISM: dust, extinction - ISM: clouds - infrared: ISM - radio lines: ISM
The observations of PDRs in atomic and molecular emission lines are generally conducted at angular resolutions larger than (or 0.1 pc at a distance of 400 pc), which do not allow a detailed analysis of the illuminated interfaces. However, they reveal a strong clumping of the emitting gas (see the review of Hollenbach & Tielens 1997). The observations of atomic lines ([OI], [CII], and [CI]) indicate emission scales much larger than expected for homogeneous material. The emission of many molecular species (e.g. 13CO, C18O, CS) appears to originate essentially from clumps ( cm-3), while the detection of emission due to molecules with high critical densities (e.g. HCO+, high level transitions of CS) reveals the presence of high-density molecular gas (n>105 cm-3). Molecular observations at higher angular resolution (10-20 ) have also been conducted for very excited PDRs, such as the Orion bar (van der Werf et al. 1996). High density clumps are detected, with size in the range from 0.02 to 0.1 pc. Recently, the 1-0 S(1) fluorescent line of H2 in the reflexion nebula NGC 2023 has been mapped by Field et al. (1998) & Mc Cartney et al. (1999) at an angular resolution of ( pc). The images reveal filamentary emission interpreted as evidence for density contrasts on a milliparsec scale. The presence in PDRs of density contrasts down to such small scales would dramatically affect the chemical structure and the energy balance, by permitting the radiation to penetrate deeper into the cloud.
This paper presents ISOCAM observations (Cesarsky et al. 1996) of the Horsehead nebula, together with observations of the (J=1-0) and (J=2-1) transitions of 12CO, 13CO and C18O taken at the IRAM 30-m telescope. The edge of the Horsehead nebula is illuminated edge-on by the O9.5 V system Ori and is ideal to study the penetration of radiation and the density structure of a PDR. The ISOCAM observations are taken from the large scale mapping of the southern part of Orion B conducted in the two broad-band filters LW2 and LW3 (5-8.5 m and 12-18 m respectively) and presented in Abergel et al. (1999, 2002). The emission detected in the LW2 filter is dominated by the aromatic features around 7.7 m, while the emission in the LW3 filter is continuum-like (Abergel et al. 2002). In the interstellar medium, the intensity of the aromatic bands is found to scale with the UV radiation field (for ranging between 1 and 104, where is the intensity of the UV radiation field normalised to the local interstellar radiation field, see Habing 1968), while the shape and relative amplitude of their profiles appear remarkably constant (Boulanger et al. 1998; Uchida et al. 2000). This is expected for emission from very small particles transiently heated to high temperature each time they absorb a UV photon (Léger & Puget 1984; Sellgren et al. 1985).
The angular resolution of the ISOCAM observations (6 ) is 10 times better than previous infrared or molecular and atomic lines observations conducted in this region. Thus, unprecedented constraints both on the shape of the illuminated interface and the penetration of the incident radiation can be derived. The paper is organised as follows. We present the Horsehead nebula in Sect. 2. The ISOCAM and molecular observations are described in Sects. 3 and 4, respectively. We show in Sect. 5 that these observations bring new constraints on the three-dimensional shape, the local density and the clumpiness of the material at the illuminated edge of the nebula. The main conclusions are given in the last section.
In front of the western illuminated edge of the molecular cloud L1630, the visible plates are dominated by extended red emission due to H emission line emerging from the HII region IC 434 (e.g. Malin 1987). In the visible, the Horsehead nebula (B33 in the catalogue of Barnard 1913) emerges from the edge of L1630 as a dark cloud in the near side of IC 434 (Fig. 1). L1630 is located at a distance of 400 pc (from the study of the distances to B stars in the Orion association by Anthony-Twarog 1982). At this distance, 1 corresponds to 0.12 pc.
At first one expects the two OB-systems Ori to the north and Ori to the west to illuminate the western edge of L1630 (Reipurth & Bouchet 1984). But, as suggested by Malin (1987), Ori is located closer than Ori, which has been confirmed by Hipparcos estimates of the distance (Perryman et al. 1997): pc for Ori, pc for Ori. Located at a projected distance of (or 3.5 pc) from the Horsehead nebula, Ori is a O9.5 V binary system (Warren & Hesser 1977), with an effective temperature of 33 000 K (Panagia 1973). Assuming that Ori and the Horsehead nebula are in a common plane perpendicular to the line of sight, the far-UV intensity of the incident radiation field illuminating the Horsehead nebula is , which is moderate compared to those of classical PDRs illuminated by O stars (generally , e.g. Tielens et al. 1993). For such a low intensity, the ISOCAM emission is due to transiently heated particles, and scales with the incident radiation field.
The Horsehead nebula coincides with a CS clump (Lada et al. 1991; Zhou et al. 1993). From multi-transition CS observations (resolution: 11-24 or 0.02-0.04 pc) and using a Large Velocity Gradient model, Zhou et al. have derived a density of cm-3. A local density of cm-3 is also found by Kramer et al. (1996) from 12CO and 13CO observations taken with a resolution of 1.5-2 (or 0.2 pc). On the other hand, the penetration depth of UV radiation measured by Zhou et al. (1993) using the spatial extent of the [CII] 158 m emission (resolution: 55 or 0.1 pc) indicates homogeneous material with a density of cm-3. Clumpiness at scales around 0.01 pc or below cannot be excluded, however.
|Figure 1: Composite colour image of the Horsehead nebula with the VLT (ESO). The seeing was about 0.75 .|
|Open with DEXTER|
|Figure 2: ISOCAM map of the Horsehead nebula in the LW2 filter (5-8.5 m).|
|Open with DEXTER|
|Figure 3: ISOCAM contours of the LW2 emission (5-8.5 m, levels are 0 to 10 by 2.5, then 10 to 25 by 5 MJy/sr) plotted over the 12CO(2-1) peak temperature map ( scale, corrected for error beam pick-up).|
|Open with DEXTER|
In complement to the ISOCAM data, we conducted molecular observations at the IRAM 30-m telescope. On-the-fly maps of the (J=1-0) and (J=2-1) transitions of 12CO, 13CO and C18O were obtained in a region of 2.53.5 for the main isotopomer, and in a more restricted band of 1.51 for the latter ones. These maps were centred at the brightest position of the infrared filament. The data were acquired in January 1999. With a system temperature of order 150-250 K (400-500 K) and a resolution element of 80 kHz, the noise rms per regridded pixel is 0.20 K (0.35 K) at 2.6 mm (1.3 mm respectively). All coverages were performed in two orthogonal directions and combined using the PLAIT algorithm developed by Emerson & Gräve (1988), reducing significantly the spurious stripes in the maps. The calibration and conversion of the data into a corrected main beam temperature scale ( ) is detailed in Appendix A.
|Figure 4: Emission profiles at constant declination ( 284 ). The offset origin (at 4053.7) corresponds to the ISOCAM peak on the edge of the Horsehead nebula. Upper panel: ISOCAM emission in the LW2 filter (5-8.5 m, solid thick line). The thin line shows the profile for a point source focused at the center of a pixel and observed with the same filter and the same field of view per pixel (6 ). Lower panel: Peak temperature for the (J=2-1) transitions of 12CO (solid line), 13CO (dashed line) and C18O (dash-dotted line). The thin line shows the beam profile of the 30-m telescope at 1.3 mm (from Greve et al. 1998).|
|Open with DEXTER|
The bright filament detected with ISOCAM (Fig. 2) is precisely at the position of the edge of the dark nebula seen in the visible (Fig. 1) and at the edge of the molecular emission (Figs. 3, 4, 6). The border of the 13CO and C18O emissions is slightly shifted to the east (Fig. 4), as expected from selective photodissociation due to self-shielding in the outermost layers of molecular structures (e.g. Fuente et al. 1993; White & Sandell 1995). Outside the molecular cloud, the CO, 13CO and C18O molecules are photodissociated, then the CO emission appears first, followed by the 13CO emission and finally the C18O emission. However the steep edge of the cloud ( or 0.01 pc) is not resolved by the molecular observations (spatial resolutions are 10.5 and 21 , or 0.04 and 0.02 pc, at 2.6 mm and 1.3 mm respectively).
We have seen in the introduction that, in the interstellar medium and on large angular scales, the ISOCAM emission in the LW2 filter scales, to a first order, with the incident radiation field. Unfortunately, we have no observation of an independant tracer of the column density taken at a sufficient angular resolution to relate the spatial variations of the infrared emission observed at small angular scale to variations of the abundance or the optical properties of the aromatic emitters. Therefore, in this paper we work in the hypothesis that the LW2 emission is proportional to the column density and the incident radiation field. The infrared extinction is only significant for very large column densities (typically
|Figure 5: Spectra of the three isotopemers in the two observed positions (J=1-0 thick, J=2-1 thin) at the position corresponding to the offset origin of Fig. 4.|
|Open with DEXTER|
|Figure 6: Mosaic of spectra in the 12CO(2-1) line. The offset origin is the same as Fig. 4. Each spectrum is shown on a 4 grid with 8.5 km s 13.5 km s-1 and -3 K 35 K.|
|Open with DEXTER|
To the west of the Horsehead nebula the LW2 emission is fairly uniform ( MJy sr) and corresponds to the minimal emission measured by ISOCAM within the field taken in the southern part of Orion B. This emission could be due to particles in the low density HII region facing the illuminated edge of the nebula (Sect. 5.4). In the LW3 filter, the detection of faint uniform emission is not possible because of the amplitude of the zodiacal emission.
From pc-1, we finally infer n cm-3, which is comparable to the value estimated from our molecular observations beyond the illuminated edge (for distances of from the peak position of the infrared filament). In our simple model, we have assumed an infinite gradient of column density at the illuminated edge, which is obviously a simplifying assumption. The infrared profile (Fig. 4) is compatible with a steep edge ( or 0.01 pc), but geometrical effects can obviously smooth the emerging profile. In any case, the value of cm-3 determined from the extinction depth of UV radiations can only be considered as a lower limit for homogeneous material.
The density of the neutral region exposed to the ionizing radiations, just behind the ionization front, can be estimated from the flux of ionizing photons. Details are given in Appendix B. We find a value ( cm-3) comparable to the density measured behind the infrared filament from the penetration depth of UV radiation ( cm-3). However, these two densities have not to be equal, since the ionization front corresponds to the most external regions of the cloud, while UV radiations heating the aromatic particles are absorbed deeper inside the cloud.
Let n1 and n2 (and T1 and T2) be the densities (and temperatures) in the neutral and HII regions behind and ahead the ionization front, respectively. From Eqs. (B.2) and (B.4), and using K and K (Appendix B), we have: . From this ratio and the value of the peak emission of the LW2 profile (MJy sr), we can estimate the LW2 emission from the HII region ahead the ionization front assuming constant abundance and emissivity of the emitting particles across the interface. We obtain a value of MJy sr, which is comparable to the emission detected in front of the edge ( MJy sr). We see that we cannot evidence any decrease of the abundance or the emissivity of the aromatic particles from the neutral to the HII regions. The same conclusion was obtained by Habart et al. (2003) for the interface of the Ophiuchi main cloud illuminated, as the Horsehead nebula, by a moderate radiation field ( ). We have also shown in Abergel et al. (2002) that the contribution of the aromatic bands relative to intensity of the underlying continuum emission is systematically higher in low density HII regions than at the illuminated edges of dense structures (for ). These results indicate that the aromatic particles are not systematically destroyed in HII regions when the radiation field is limited ().
Clumping reduces the effective extinction and thus increases the observed penetration depth. This is quantitatively illustrated in Fig. C.3. For example, for an average density of cm-3, the presence of clumps with typical sizes of 10 -4-10-3 pc with a volume filling factor of about 0.1, clumping reduces the extinction by a factor about 100 (Fig. C.3) and the penetration depth is the same that for homogeneous medium with a density of cm-3. This is due to the decreasing contribution of the inter-clump medium to the extinction. Thus, the presence of clumps with sizes smaller than pc cannot be excluded. In that case the density derived from our C18O observations (a few 104 cm-3, see Sect. 5.2) could correspond to the inter-clump material. A comparable "interclump'' density is also observed in the PDR of the Orion bar together with dense clumps with size in the range 0.02 to 0.1 pc (van der Werf et al. 1996).
At the illuminated edge of the Horsehead nebula the incident radiation appears absorbed within a thin layer (thickness below or pc). Within this layer located just behind the infrared filament, and assuming homogeneous material, the density must be at least of cm-3, which corresponds to the density deduced beyond the edge from our C18O observations. However the absorbing material may also be non homogeneous, with clump sizes significantly smaller than 10-2 pc. In that case, the density derived from the C18O observations could correspond to the "interclump'' material. The average density behind the infrared filament must in any case be higher than cm-3. It is interesting to note that this lower limit is also comparable to the density of the neutral material just behind the ionization front, estimated to cm-3 in the stationary case. Finally we cannot evidence any drecrease of the abundance or the emissivity of the aromatic particles from the neutral to the HII regions of the Horsehead nebula, suggesting that the aromatic particles are not systematically destroyed in HII regions when the radiation field is limited ().
ISOCAM observations bring new constraints on the shape of the Horsehead nebula, but the angular resolution of 6 is still unsufficient to resolve properly the density structure. We will present in forthcoming papers a combination of interferometric observations taken at the Plateau de Bures with the 30-m data, and high resolution mapping of the 1-0 S(1) fluorescent line of H2.
ISO is an ESA project with instruments funded by ESA Member States and with the participation of ISAS and NASA. The authors would like to thank F. Bensch and J.-F. Panis for constructive discussions on the error beam removal technique.
In comparison to what would be obtained in a scale, the final temperatures are reduced by 20% for 12CO(2-1), 12% for 13CO(2-1) and C18O(2-1), and 5% for the 2.6 mm transitions. This also shows that our assumption of a comparable 13CO and C18O spatial distribution could lead to underestimate the C18O(2-1) line intensities by at most 10%, which has no significant consequences on the results presented thereafter. Following Bensch et al. (2001), we call the corrected main beam temperature scale .
We assume that Ori is at a distance
pc from the edge of the Horsehead nebula, corresponding to the angular distance of
As explained by Spitzer (1978), the protons produced at the ionization front are at a higher pressure than the gas outside the cloud. Thus the ionized gas expands toward the star, forming a thin layer of dense ionized gas wrapping the neutral cloud. The flux of ionizing photons reaching this layer is: J0=S0 e
cm-2 s-1, where S0 is the number of ionizing photons emitted by the star (1048.25 for a O9.5 V star, from Schaerer & de Koter 1997), and e
the extinction of the Lyman photons by dust in the HII region, taken equal to 0.5.
Electrons will recombine with protons in the layer of dense ionized gas, and the resultant H atoms reionized, which reduce the flux of photons reaching the ionization front, J. In a spherical geometry, Spitzer (1978) has shown that the ratio J/J0 can be estimated with:
Following Kaplan (1966), the velocity and the density in the neutral region (V1 and n1) can be estimated in the stationary case using the mass and momentum conservation equations across the ionization front which write:
We follow the approach of Meixner & Tielens (1993) based on the radiative transfer formalism developped by Boissé (1990) for two-phase clumpy regions. The physical processes considered here are the absorption and isotropic scattering by dust grains. The radiative transfer in a clumpy region is reduced to that of a homogeneous region characterized by an effective extinction coefficient
and an albedo
(Eqs. (26) and (27) of Boissé 1990), both of them decreasing with the clumpiness. Moreover, the spatial variations of the incident intensity
with the depth z are nearly exponential:
|Figure C.1: Clump and inter-clump relative densities ( and , respectively) vs. the density contrast for different filling factors: (solid line), 0.1 (dotted line), 0.05 (dashed line), 0.025 (dashed-dotted line).|
|Figure C.2: Relative value of the effective extinction coefficient vs. the density contrast for different filling factors: (solid line), 0.1 (dotted line), 0.05 (dashed line), 0.025 (dashed-dotted line). The point at (1, 1) corresponds to the homogeneous case. The average density is cm-3. Top: For , Clump diameters: - pc. Bottom: For , Clump diameters: - pc. The presence of very small clumps does not significantly affect the effective extinction coefficient.|
|Figure C.3: Same as Fig. C.2 for an average density cm-3. Top: For , Clump diameters: - pc. Bottom: For , Clump diameters: - pc.|
In practice, we first assign the average density ( cm-3 or cm-3), the average clump mass (10-3 or 10 ) and the filling factor (from 0.2 to 0.025). For an average density cm-3, an average clump mass of 10 (resp. 10 ) implies clump diameters in the range pc (resp. pc) depending on the density contrast. We then compute the effective extinction coefficient for different values of the density contrast. In Boissé (1990), the clumps have an exponential size distribution, exp(-l/l0), where l0 is the average length through a clump (for spherical clumps, , being the average clump radius).
Figure C.2 (resp. Fig. C.3) presents a set of results for an average density of cm-3 (resp. cm-3) and average clump masses of and . We see that, for density constrats below 100, the extinction coefficient () significantly decreases for increasing density contrast and filling factor, as expected. For very large density contrasts ( ), the extinction is purely geometrical and essentially due to optically thick clumps. Therefore the computed values of converge towards an asymptotic value which decreases with decreasing filling factors .