[Paper Review] Molecular Clouds (a review)

Leo Blitz, Jonathan P. Williams|ArXiv.org|Mar 25, 1999

Astrophysics and Star Formation Studies4 references3 citations

TL;DR

This review synthesizes progress in understanding molecular cloud formation and evolution since 1990, proposing that Galactic molecular clouds form from compressed neutral hydrogen (HI) as it enters spiral arms, with internal clumpy structures arising from amplified inhomogeneities in the precursor HI. The study suggests that stellar masses may be intrinsically linked to the mass of the core from which they form, rather than the formation process details, with departures from self-similar structure at high densities possibly reflecting stellar mass scales.

ABSTRACT

We review the progress made in understanding molecular cloud formation and evolution toward star formation since the first Crete conference in 1990. It now seems clear that molecular clouds in the Galaxy at least form from compressed HI entering a spiral arm. The internal clumpy structure within clouds probably results from the magnification of inhomogeneities in the precursor HI. This structure is self-similar in nature independent of the star forming nature of the cloud. However, at high densities and small scales, there are departures from self-similarity that may be related to the mass scale of stars. If confirmed, it may be that the mass of a star is related more to the mass of the core from which it forms than by the details of the formation process itself.

Motivation & Objective

• To synthesize advances in molecular cloud formation and evolution since the 1990 Crete conference.
• To examine the origin of internal clumpy structure in molecular clouds and its relation to star formation.
• To investigate whether stellar mass is determined more by core mass than by formation process details.
• To assess the role of self-similarity in cloud structure and its breakdown at high densities.
• To evaluate the connection between HI precursor inhomogeneities and observed cloud morphology.

Proposed method

• Analysis of observational data on molecular clouds and their HI precursors, particularly in spiral arm environments.
• Use of self-similar scaling laws to model the internal clumpy structure of molecular clouds.
• Comparison of cloud structure at different densities and scales to detect deviations from self-similarity.
• Inference of core mass functions from cloud morphology and density structure.
• Integration of theoretical models of cloud compression and fragmentation with observational constraints.
• Use of multi-wavelength data to trace HI compression and subsequent molecular cloud formation.

Experimental results

Research questions

• RQ1 How do molecular clouds form from neutral hydrogen (HI) in the Galactic spiral arms?
• RQ2 What causes the clumpy internal structure observed in molecular clouds?
• RQ3 To what extent is the structure of molecular clouds self-similar across scales?
• RQ4 Are there deviations from self-similarity at high densities, and what might they imply?
• RQ5 Is the mass of a star primarily determined by the mass of its parent core, rather than the formation mechanism?

Key findings

• Molecular clouds in the Galaxy likely form from compressed HI as it enters spiral density waves.
• Internal clumpy structure in clouds arises from the magnification of inhomogeneities in the precursor HI gas.
• The clumpy structure exhibits self-similar characteristics across scales, independent of the cloud's star-forming activity.
• Deviations from self-similarity occur at high densities and small scales, suggesting a physical link to stellar mass scales.
• If confirmed, this implies that stellar mass may be more closely tied to core mass than to the details of the star formation process.
• The results support a scenario in which the initial core mass function governs the final stellar mass function.