Query : 2005A&A...437..177H

2005A&A...437..177H - Astronomy and Astrophysics, volume 437, 177-188 (2005/7-1)

Density structure of the Horsehead nebula photo-dissociation region.


Abstract (from CDS):

We present high angular resolution images of the H2 1-0 S(1) line emission obtained with the Son of ISAAC (SOFI) at the New Technology Telescope (NTT) of the Horsehead nebula. These observations are analysed in combination with Hα line emission, aromatic dust, CO and dust continuum emissions. The Horsehead nebula illuminated by the O9.5V star σ Ori (χ∼60) presents a typical photodissociation region (PDR) viewed nearly edge-on and offers an ideal opportunity to study the gas density structure of a PDR. The H2 fluorescent emission observations reveal extremely sharp and bright filaments associated with the illuminated edge of the nebula which spatially coincides with the aromatic dust emission. Analysis of the H2 fluorescent emission, sensitive to both the far-UV radiation field and the gas density, in conjunction with the aromatic dust and Hα line emission, brings new constraints on the illumination conditions and the gas density in the outer PDR region. Furthermore, combination of this data with millimeter observations of CO and dust continuum emission allows us to trace the penetration of the far-UV radiation field into the cloud and probe the gas density structure throughout the PDR. From comparison with PDR model calculations, we find that i) the gas density follows a steep gradient at the cloud edge, with a scale length of 0.02pc (or 10'') and nH∼104 and 105cm–3 in the H2 emitting and inner cold molecular layers respectively; and ii) this density gradient model is essentially a constant pressure model, with P∼4x106K/cm3. The constraints derived here on the gas density profile are important for the study of physical and chemical processes in PDRs and provide new insight into the evolution of interstellar clouds. Also, this work shows the strong influence of the density structure on the PDR spatial stratification and illustrates the use of different tracers to determine this density structure.

Abstract Copyright:

Journal keyword(s): ISM: clouds - ISM: dust, extinction - atomic processes - molecular processes - radiative transfer - ISM: individual objects: Horsehead nebula

CDS comments: Paragr. 2.2: standard stars from the NICMOS list: N9115, N9116 and N9118 not identified.

Simbad objects: 5

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Number of rows : 5
N Identifier Otype ICRS (J2000)
ICRS (J2000)
Mag U Mag B Mag V Mag R Mag I Sp type #ref
1850 - 2024
1 * sig Ori Y*O 05 38 44.7653828520 -02 36 00.283179888 2.54 3.58 3.79 3.87 4.11 O9.5V+B0.2V(n) 802 0
2 NAME Horsehead Nebula DNe 05 40 59.0 -02 27 30           ~ 512 0
3 IC 434 HII 05 41 -02.5           ~ 89 1
4 LDN 1630 DNe 05 45.5 -00 59           ~ 381 0
5 NAME Ophiuchus Molecular Cloud SFR 16 28 06 -24 32.5           ~ 3636 1

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