The IRAM key-project: Small-scale structure of pre-star
  forming regions

Bensch, F.; Panis, J. F.; Stutzki, J.; Heithausen, A.; Falgarone, É.

doi:10.1051/0004-6361:20000475

Cited by 11 publications

(12 citation statements)

References 10 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…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 H 2 . the error beam removal technique presented by Falgarone et al (1998) and Bensch et al (2001). This method allows for each spatial pixel and each velocity channel an independent correction of the main beam temperature (T mb ).…”

Section: Discussionmentioning

confidence: 99%

“…This also shows that our assumption of a comparable 13 CO and C 18 O spatial distribution could lead to underestimate the C 18 O(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 T mb,c . clumps and inter-clump material, respectively (p c + p ic = 1).…”

Section: Discussionmentioning

confidence: 99%

See 1 more Smart Citation

ISOCAM and molecular observations of the edge of the Horsehead nebula

Abergel

Teyssier

Bernard

et al. 2003

A&A

Self Cite

121

View full text Add to dashboard Cite

Abstract. We present ISOCAM observations (5-18 µm) of the Horsehead nebula, together with observations of the (J = 1−0) and (J = 2−1) transitions of 12 CO, 13 CO and C 18 O taken at the IRAM 30-m telescope. The Horsehead nebula presents a typical photodissociation region illuminated by the O9.5 V system σ 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: ∼10 or ∼0.02 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 ∼0.01 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 σ 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 (∼0.01 pc) gives a density of a few 10 4 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 ∼0.01 pc. In that case no upper limit on the average density just behind the illuminated edge can be given.

show abstract

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

ISOCAM and molecular observations of the edge of the Horsehead nebula

Abergel

Teyssier

Bernard

et al. 2003

A&A

Self Cite

121

View full text Add to dashboard Cite

show abstract

“…Bensch et al 2001b). Without extremely high signal to noise data it is impossible to obtain reliable constraints on the spatial variation even for the next higher velocity moments.…”

Section: Centroid Velocity Pdfsmentioning

confidence: 99%

Turbulent velocity structure in molecular clouds

Ossenkopf

Low

2002

A&A

246

313

View full text Add to dashboard Cite

Abstract.We compare velocity structure observed in the Polaris Flare molecular cloud at scales ranging from 0.015 pc to 20 pc to the velocity structure of a suite of simulations of supersonic hydrodynamic and MHD turbulence computed with the ZEUS MHD code. We examine different methods of characterising the structure, including a scanning-beam method that provides an objective measurement of Larson's size-linewidth relation, structure functions, velocity and velocity difference probability distribution functions (PDFs), and the ∆-variance wavelet transform, and use them to compare models and observations. The ∆-variance is most sensitive to characteristic scales and scaling laws, but is limited in its application by a lack of intensity weighting so that its results are easily dominated by observational noise in maps with large empty areas. The scanning-beam size-linewidth relation is more robust with respect to noisy data. Obtaining the global velocity scaling behaviour requires that large-scale trends in the maps not be removed but treated as part of the turbulent cascade. We compare the true velocity PDF in our models to simulated observations of velocity centroids and average line profiles in optically thin lines, and find that the line profiles reflect the true PDF better unless the map size is comparable to the total line-of-sight thickness of the cloud. Comparison of line profiles to velocity centroid PDFs can thus be used to measure the line-of-sight depth of a cloud. The observed density and velocity structure is consistent with supersonic turbulence with a driving scale at or above the size of the molecular cloud and dissipative processes below 0.05 pc. Ambipolar diffusion could explain the dissipation. Over most of the observed range of scales the velocity structure is that of a shock-dominated medium driven from large scale. The velocity PDFs exclude small-scale driving such as that from stellar outflows as a dominant process in the observed region. In the models, large-scale driving is the only process that produces deviations from a Gaussian PDF shape consistent with observations, almost independent of the strength of driving or magnetic field. Strong magnetic fields impose a clear anisotropy on the velocity field, reducing the velocity variance in directions perpendicular to the field.

show abstract

“…Note, however, that an error-beam pick also modifies the line profile, which is not corrected with this treatment (see Bensch et al 2001b for a more detailed discussion).…”

Section: Ground-state Co Transitionsmentioning

confidence: 99%

Neutral carbon and CO emission in the core and the halo of dark cloud Barnard 5

Bensch

2006

A&A

Self Cite

View full text Add to dashboard Cite

Aims. The physical conditions and chemical structure in the dark cloud of Barnard 5 and its surrounding atomic halo is studied. The impact of the halo on the line emission emerging from the molecular cloud is investigated. Methods. We present observations of the [CI] 3 P 1 → 3 P 0 transition of neutral carbon and the low-J transitions of 12 CO and 13 CO. The CO maps extend from the core (A v > ∼ 7) to the northern cloud edge and into the halo (A v < ∼ 1). They are complemented by deeply integrated [CI] spectra made along a 1D cut of similar extent. Escape probability and photon-dominated region (PDR) models are employed to interpret the observations. Results. 12 CO and 13 CO are detected in the cloud and the halo, while [CI] is detected only toward the molecular cloud. This occurs even though the neutral carbon column density is > ∼ 5 times larger than the CO column density in the halo, but it can be understood in terms of excitation. The [CI] excitation is governed by collisions even at the low halo densities, while the CO excitation is dominated by the absorption of line photons emitted by the nearby molecular cloud. The upper limit on the neutral carbon column density in the halo is 6 × 10 15 cm −2 . The PDR studies show that even small column densities of H 2 and CO, such as those in the B5 halo, can significantly change the [CI] and CO line emission (pre-shielding). Since this effect decreases the [CI] intensity and increases the CO intensity, the largest impact is noted for the [CI]/CO line ratios. For the B5 cloud, a PDR model with a molecular hydrogen column density of ∼6 × 10 19 cm −2 in the halo matches the observed [CI]/CO line ratios best. Models with no pre-shielding, in contrast, suggest high gas densities that are in conflict with independently derived densities. The PDR models with a χ < 1 demonstrate that the [CI]/CO ratios cannot be attributed solely to a reduced FUV field.Key words. ISM: abundances -astrochemistry -ISM: clouds -ISM: individual: B5 -radio lines: ISM IntroductionDark clouds are noted at optical wavelengths by their absorption of the stellar background, showing that they have a column density of at least several magnitudes in visible extinction (A v ). Many of the dark clouds are relatively quiescent with either no active star formation or low-mass star formation proceeding in a few isolated cores. Gas heating is provided by the diffuse interstellar radiation field (ISRF) via photo-electric heating and by cosmic rays for the FUV-shielded parts of the cloud. Protostars embedded in dense cores can heat the gas locally, and the interaction of a linear jet emerging from the protostar with the parental cloud can give rise to shock-heated molecular gas and drive a molecular outflow. However, these processes have generally not had a significant impact on the global thermal balance of the gas, while the cloud is recognized as a dark cloud.Far ultraviolet photons of the ISRF dominate the chemistry at the dark cloud surface, dissociating and ionizing the molecular gas. This resu...

show abstract

The IRAM key-project: Small-scale structure of pre-star forming regions

Cited by 11 publications

References 10 publications

ISOCAM and molecular observations of the edge of the Horsehead nebula

ISOCAM and molecular observations of the edge of the Horsehead nebula

Turbulent velocity structure in molecular clouds

Neutral carbon and CO emission in the core and the halo of dark cloud Barnard 5

Contact Info

Product

Resources

About