Title Small scale structure of molecular clouds Pi J. Pety Time 225 hrs 1. Name of program and authors Name: Small scale structure of molecular clouds Authors: J. Pety and E. Falgarone 2. One short paragraph with science goal(s) Characterize the threshold of the self-similar hierarchy of non star forming molecular clouds. This hierarchy is a (multi)fractal structure, resulting of the interplay of gravity and turbulence. Its threshold is determined by the dissipative processes of the turbulent cascade (shocks, coherent vortices, see Pety & Falgarone 2000 AA 356 279). Molecular viscosity sets this threshold at about 10 AU (0.1" at 100pc) in molecular clouds. The formation and structuration of dense cores is linked to dissipation processes of the turbulent and magnetic energy of the molecular clouds. Studying the small-scale structure of molecular clouds in the environment of a non-star forming dense core (in line, continuum) will thus shed light on the underlying dissipation processes. A mosaic of 13 fields obtained with the PdBI in 12CO(1-0) uncovers a set of elongated and straight structures in a high latitude cloud. Some are unresolved in their transverse direction (resolution 500 AU) calling for higher resolution. Other extend further than the field of the mosaic (2' by 1'), calling for large spatial coverage. Several steps are required in the observational strategy, given the fractal distribution of these structures. 3. Number of sources 1 field of 4'x4' at 1.0" resolution 1 field of 2'x2' at 0.3" resolution 4. Coordinates: 4.1. Rough RA and DEC 1 source in Chamaeleon (RA=12:15, DEC=-82, 1950) 4.2. Moving target: no 4.3. Time critical: no 4.4. Scheduling constraints: Avoid windy periods to ensure high precision mosaicing. 5. Spatial scales: 5.1. Angular resolution (arcsec): 1", then 0.3" 5.2. Range of spatial scales/FOV (arcsec): We propose to make Nyquist sampled mosaics following an hexagonal compact pattern. This implies: 240 (59-fields mosaic at 1.0" resolution) 400 (13-fields mosaic at 0.3" resolution) In view of the total number of fields, such a program could benefit from the On-The-Fly interferometric mode that IRAM will try to prototype for ALMA in the coming three years, but this observing mode is not optional for this project. 5.3. Required pointing accuracy: (arcsec) 0.6" rms to ensure high precission mosaicing. 6. Observational setup 6.1. Single dish total power data: required Observing modes for single dish total power: * On-The-Fly is required by the field of view. * The narrow zero-power linewidth (6 km/s) makes frequency switch suitable to this project, which will improve single dish signal-to-noise ratio by sqrt(2). 6.2. Stand-alone ACA: Required 6.3. Cross-correlation of 7m ACA and 12m baseline-ALMA antennas: This question is difficult to answer. We do not know of any public studies that shows that cross-correlation between ACA and ALMA would be beneficial. Several points need to be clarified: 1) Does the collecting surface of ACA is enough to ensure alone (i.e. without a correction for the integrating time) good sensitivity at spatial frequencies around 7m relatively to the sensitivity at frequencies measured by ALMA alone? 2) Cross correlating 2 different interferometers implies a multiplication of the respective primary beams in the measurement equation: Does this implies in practice a limitation of the field of view of the small antennas to the field of view of the large antennas? 6.4. Subarrays of 12m baseline-ALMA antennas: No 7. Frequencies: 7.1. Receiver band: Band 3 7.2. Lines and Frequencies (GHz): 12CO(1-0) at 115 GHz 13CO(1-0) at 110 GHz 7.3. Spectral resolution (km/s): 0.1 km/s at 1.0" resolution 0.3 km/s at 0.3" resolution 7.4. Bandwidth or spectral coverage (km/s or GHz): 2 to 6 km/s for the line 8. Continuum flux density: 8.1. Typical value (Jy): 55 microJy at 1.0" resolutions 8.2. Required continuum rms (Jy or K): 8.2 microJy or 0.7 mK at 1.0" resolution 2.6 microJy or 2.6 mK at 0.3" resolution 8.3. Dynamic range within image: Unknown 8.4. Calibration requirements: Absolute: n/a Repeatability: n/a Relative: 10% 9. Line intensity: 9.1. Typical value (K or Jy): 5K for 12CO(1-0) 2K for 13CO(1-0) 9.2. Required rms per channel (K or Jy): 0.35K at 115 GHz and 1.0" resolution 0.70K at 115 GHz and 0.3" resolution 9.3. Spectral dynamic range: 10 9.4. Calibration requirements: Absolute: n/a Repeatability: n/a Relative: 10% 10. Polarization: No 11. Integration time for each observing mode/receiver setting (hr): 70h for the 4'x4' mosaic at 1.0" resolution 155h for the 2'x2' mosaic at 0.3" resolution 12. Total integration time for program (hr): 225h 13. Comments on observing strategy: This program would probably be submitted 2 times to the proposal committee over the 2 year span of the design reference mission. The idea is to survey a large enough area at 1" resolution and then to zoom on a particularly interesting region to obtain a factor 3 order--of--magnitude increase in resolution over the molecular structures observed by the current generation of mm interferometers. The possibility to have 0.3" structure is the most highly prospective assumption here. This is however the goal of this project to find the threshold of the self-similar hierarchy in molecular clouds. The typical line intensity values come from Plateau de Bure observation of Polaris. We expect that beam dilution will make up for the loss of sensitivity to extended regions when the resolution increase. The large range of spatial scales sampled at the three resolution will enable the meaningful computation of statistical measures like structrure functions to improve the comparison with Magneto-Hydro-Dynamics simulations, which have made huge progresses in the past decade. -------------------------------------------------- Review v2.0: init_1 = 2.1.1 Name: Small scale structure of molecular clouds Authors: J. Pety and E. Falgarone This program will really drive the mosaicing capabilities of ALMA to their limits in terms of pointing and calibration. ACA is essential, but I share the authors view that the 7m x 12m correlations are probably not necessary. The gain in sensitivity is not essential here and the calibration complexity makes me shudder. The authors say that they plan to first get the 1" data and then zoom in on interesting regions at 0.3". Doesn't this introduce a bias that this study should try to avoid? In other words, should one try to get the entire region at both resolution?