Engineering topics

Shu, Frank H.

Publications and source records attributed to Shu, Frank H..

25 records · Page 2

NASA NTRS1988

The evolution of molecular clouds

The problem of the structure and evolution of molecular clouds is reviewed, with particular emphasis given to the relationship with star formation. The basic hypothesis is that magnetic fields are the primary agents for supporting molecular clouds, although damped Alfven waves may play an important role in the direction parallel to the field lines. This picture naturally leads to a conception of 'bimodal star formation'. It is proposed that high-mass stars form from the overall gravitational collapse of a supercritical cloud, whereas low-mass stars form from small individual cores that slowly condense by ambipolar diffusion from a more extended envelope until they pass the brink of graviational instability and begin to collapse dynamically from 'inside-out'. The evidence that the infall stage of protostellar evolution is terminated by the development of a powerful stellar wind is reviewed.

Shu, Frank H.↗

NASA NTRS1988

SIRTF and star formation

Four problems in the field of star formation that can be attacked to advantage with SIRTF are discussed: (1) the patterns of star formation in spiral galaxies, (2) the physical mechanism for bimodal star formation, (3) the nature of bipolar outflows from young stellar objects, and (4) the birth of brown dwarfs. In each case, SIRTF can provide the crucial combination of high angular resolution with great sensitivity over a broad range of wavelengths that is needed to address the relevant issues.

Shu, Frank H.↗

NASA NTRS1987

Planetary ring dynamics and morphology

Evidence for a moonlet belt in the region between Saturn's close-in moonrings Pandora and Prometheus is discussed. It is argued that little-known observations of magnetospheric electron density by Pioneer 11 imply substantial, ongoing injections of mass into the 2000 km region which surrounds the F ring. A hypothesis is presented that these events result naturally from interparticle collisions between the smaller members of an optically thin belt of moonlets. Also discussed is work on Uranus ring structure and photometry, image processing and analysis of the Jonian ring strucure, photometric and structural studies of the A ring of Saturn, and improvements to an image processing system for ring studies.

Cuzzi, Jeffrey N.↗

NASA NTRS1987

Summary of symposium: Low luminosity sources

The author summarized certain aspects of the conference. He shares this task with another colleague thereby breaking the task into more manageable proportions. The author covers the low luminosity sources. He begins his review with a summary of some major themes of the conference and ends with a few speculations on possible theoretical mechanisms.

Shu, Frank H.↗

NASA NTRS1987

Spectral evolution of young stellar objects

An evolutionary sequence, from protostars to pre-main sequence stars, for the classification of young stellar objects is derived by comparing the predictions of the theoretical protostar models of Adams and Shu (AS, 1986) with the morphological classification scheme of Lada and Wilking (1984). It is shown that the AS models adequately explain the emergent spectral energy distributions of unidentified objects with negative spectral indices in the mid-IR and near-IR in both Taurus and Ophiuchus. If the infalling dust envelope is then completely removed, the spectra of the underlying stars and nebular disks used by AS provide a natural explanation for the near-IR and mid-IR excesses and the positive spectral indices of embedded T Tauri stars. It is found that the addition of a simple physical model for residual dust envelopes can reproduce the far-IR excesses found in some of these T Tauri stars.

Adams, Fred C.↗

NASA NTRS1987

Star formation in molecular clouds - Observation and theory

Star-formation (SF) processes occurring on the scale of giant molecular clouds (10 to the 6th solar masses and 10 to the 20th cm) or smaller are discussed, reviewing the results of recent theoretical and observational investigations. Topics examined include the origin of stellar masses; bimodal SF; initial mass functions; binary stars, bound clusters, and hierarchical fragmentation; and the efficiency of SF. The properties of molecular clouds and the origin of substructures in molecular clumps are explored in detail, and consideration is given to gravitational collapse and protostars, bipolar outflows from young stellar objects, visible young stellar objects, and the implications for binary-star and planetary-system formation.

Shu, Frank H.↗

NASA NTRS1987

Formation and heating of molecular cloud cores

The origin of molecular cloud cores and their evolution into star-forming regions are discussed, reviewing the results of recent theoretical investigations. Numerical results are presented graphically, and it is argued that the existence and properties of self-gravitating clouds can be explained most easily by the action of magnetic fields. Supercritical and subcritical cloud regimes (where the cloud mass is greater or less, respectively, than the mass at which cloud self-gravity can overcome magnetic-field support) are defined and related to two types of star formation: (1) highly efficient star formation and (2) inefficient formation of low-mass stars. Mechanism (1) is shown to favor the formation of high-mass stars whenever significant core heating occurs (e.g., via ambipolar diffusion).

Lizano, Susana↗