Science Program

Issues to be addressed include (but not limited to):

1. Is filamentary structure (FS) in molecular clouds universal?

It will be important to establish the universality of filamentary structure (FS) in clouds from both the perspectives of whether they are always present and whether they have the same physical properties (e.g. mass/length, radial width).

We can't say how "universal" they are because we can't sufficiently resolve such structure in distant (> 5 kpc) regions with requisite spatial dynamic range.  But certainly where we can resolve cold dense gas there are lots of FS.  Quantifying the degree of filamentary structure for a large number of regions may establish the conditions that regulate the development of filaments.  Also it's interesting to describe morphological types of FS - single filaments without cores, single filaments with cores, filaments with nodes and branches, systems with parallel filaments, networks with many nodes and branches, hub-filament systems, etc.

2. What are the kinematic characteristics of filaments and their local environment?

Are filaments ribbons or cylinders, or sheets viewed edge-on?  Are they braided fibers?  Do they rotate about their long axes? What is the magnetic field strength in and around the filaments?  How is the magnetic field oriented relative to the filaments?

Do the filaments we observe arise from converging, super-Alfvenic flows, stretching and shearing of pre-existing features, or sub-Alfvenic flows along magnetic fields driven by self-gravity?

Does filamentary gas arise mostly in a formation event?  Is it sustained by accretion from the low-density atomic or molecular substrate of the cloud?  Does the accreting gas flow axially, radially or both?

3. How important is filamentary structure for star formation?

Would the IMF be different if filaments were different?  Do clusters begin as filaments that fragment and subsequently coalesce?  If so, do they retain a memory of the filamentary structure from which they formed?  Do cores grow at the expense of their host filaments, or do they both grow together?  Do filaments feed gas to proto-clusters as the CDM cosmic web feeds galaxies?  What fraction of a GMC mass resides in filamentary structures?  Does this fraction regulate the star formation rate in GMCs and in turn, in galaxies?

4.  What is the connection between the filamentary structure in molecular clouds and the shell like structure of the HI medium?

The HI medium in the Galaxy and LMC are observed to be in shell like structures, presumably formed from various forms of feedbacks from stars.  Molecular clouds are presumably formed from the HI medium.  The formation of filamentary structure in molecular clouds is therefore likely to be physically connected to how the HI medium is shaped. (If observed at comparable resolution, which is difficult, the HI medium may show the same filamentary structure in molecular clouds.)  A review of the observational status and possibilities of further observations of the HI medium at high resolution will inform us on perhaps the same underlying formation mechanisms of HI and molecular cloud structures.

5.  What further observations would be needed to fully characterize the internal (filamentary) structure of molecular clouds in various environments?

Further studies would require high spatial dynamic range (with smallest scale £ 0.1 pc) spectroscopic imaging observations of tracers of dense gas in molecular clouds in the Galaxy, especially with kinematic information that is lacking in the Herschel dust continuum observations.  Measurements of magnetic field configuration and strength with ALMA and other facilities are important as well.  Discussions to formulate key observing projects will be encouraged at the workshop.

6.  The viability of similar studies of molecular clouds in the Magellanic Clouds, nearby Local Group galaxies and perhaps the nearest starburst galaxies, such as NGC253.

Angular resolution needed to achieve linear resolution of 0.05 pc is ~10”/D_Kpc , where D_kpc is the distance to the cloud in Kpc unit.  To resolve molecular clouds at the same linear resolution of 0.1 pc would require 1” at 10 Kpc, 0.2” for the LMC (D = 50 Kpc), ~10 mas for the local group galaxies (D = 1 Mpc), and the highest ALMA resolution at ~5 mas in nearby starburst galaxies (such as NGC253 at 4 Mpc).