The effect of rotation on the hydrodynamics of stellar collapse

Summary Using values of d, _min, and _max that Van Riper (1978) has found most promising for a hydrodynamic envelope ejection, we have shown that even a small amount of rotation in the initial core can stop its collapse before nuclear densities are reached. We expected _i > 0.02 to produce significant deviations from a spherically symmetric collapse, but have found that _i as much as ten times smaller than this will not allow the core to reach densities as high as in the spherical collapse. In no case, however, does the core flatten very much, nor does the value of become very large. Low final 's preclude the formation of an axisymmetric torus. They also indicate that deformation of an iron core into a triaxial configuration or fragmentation of the core during its collapse is an extremely unlikely event. (Note: Classically, must exceed 0.27 before a dynamic instability to non-axisymmetric perturbations is encountered.)The small degree of flattening of the core also suggests that the reduced moment of inertia I of the core will always be relatively small in magnitude and hence that the third time derivative of I, which is proportional to the energy emitted in gravity wave radiation, will not be very significant. Numerically calculated estimates of I- during some of these model evolutions supports this suspicion. If the _min and used here are found to be realistic values after the detailed physics of the core collapse is well understood, it is clear that gravitational radiation from a core collapse will be difficult to measure.Finally, we should point out that it is the relatively large values of Y_min (near 4/3) combined with values of d near unity that (a) prevented the core from flattening significantly in these models and (b) prevented the core from reaching high configurations. If realistic values of either one (or both) of these parameters are found to be much smaller in more complete models of the core collapse, then the core will have to become flatter (and denser) before pressure gradients will support it along the rotation axis. All of the conclusions drawn here would be modified accordingly under those circumstances. It should also be noted that in general relativistic models, the critical for spherical collapse is somewhat larger than 4/3 (Van Riper, 1979). Therefore, we predict that when fully general relativistic core collapses are performed including rotation, a given choice of _min and _i will produce a slightly flatter and slightly denser core than the corresponding model that has been presented here.     

Collapse,equilibrium, and fragmentation of rotating,adiabatic clouds

Numerical calculations of the collapse of adiabatic clouds from uniform density and rotation initial conditions show that when restricted to axisymmetry, the clouds form either near-equilibrium spheroids or rings. Rings form in the collapse of low thermal energy clouds and have = T/ ¦W¦ 0.43. When the axisymmetric constraint is removed and an initial m=2 density variation is introduced, clouds either collapse to form near-equilibrium ellipsoids or else fragment into binary systems through a bar phase. Ellipsoids form in the collapse of high thermal energy clouds and have 3 0.27. The results are consistent with the critical values of for instabilities in Maclaurin spheroids, and suggest that protostellar clouds may undergo a dynamic fragmentation in the nonisothermal collapse regime.National Academy of Sciences — National Research Council Resident Research Associate.     

The NASA Astrobiology Roadmap

The NASA Astrobiology Roadmap provides guidance for research and technology development across the NASA enterprises that encompass the space, Earth, and biological sciences. The ongoing development of astrobiology roadmaps embodies the contributions of diverse scientists and technologists from government, universities, and private institutions. The Roadmap addresses three basic questions: how does life begin and evolve, does life exist elsewhere in the universe, and what is the future of life on Earth and beyond? Seven Science Goals outline the following key domains of investigation: understanding the nature and distribution of habitable environments in the universe, exploring for habitable environments and life in our own Solar System, understanding the emergence of life, determining how early life on Earth interacted and evolved with its changing environment, understanding the evolutionary mechanisms and environmental limits of life, determining the principles that will shape life in the future, and recognizing signatures of life on other worlds and on early Earth. For each of these goals, Science Objectives outline more specific high priority efforts for the next three to five years. These eighteen objectives are being integrated with NASA strategic planning.     

Triggering Protostellar Collapse, Injection, and Disk Formation

Certain meteoritical inclusions contain evidence for the existence of short-lived radioactivities such as ²⁶Al and ⁴¹Ca at the time of their formation 4.566 billion years ago. Because the half-lives of these nuclides are so short, this evidence requires that no more than about a million years elapsed between their nucleosynthesis and their inclusion in cm-sized solids in the solar nebula. This abbreviated time span can be explained if these nuclides were synthesized in a stellar source such as a supernova, and were then transported across the interstellar medium by the resulting shock wave, which then triggered the gravitational collapse of the presolar molecular cloud core. Detailed 2D and 3D numerical hydrodynamical models are reviewed and show that such a scenario is consistent with the time scale constraint, and with the need to both trigger collapse and to inject shock-wave matter into the collapsing protostellar cloud and onto the protoplanetary disk formed by the collapse. This revised version was published online in June 2006 with corrections to the Cover Date.     

The NASA Astrobiology Roadmap

The NASA Astrobiology Roadmap provides guidance for research and technology development across the NASA enterprises that encompass the space, Earth, and biological sciences. The ongoing development of astrobiology roadmaps embodies the contributions of diverse scientists and technologists from government, universities, and private institutions. The Roadmap addresses three basic questions: How does life begin and evolve, does life exist elsewhere in the universe, and what is the future of life on Earth and beyond? Seven Science Goals outline the following key domains of investigation: understanding the nature and distribution of habitable environments in the universe, exploring for habitable environments and life in our own solar system, understanding the emergence of life, determining how early life on Earth interacted and evolved with its changing environment, understanding the evolutionary mechanisms and environmental limits of life, determining the principles that will shape life in the future, and recognizing signatures of life on other worlds and on early Earth. For each of these goals, Science Objectives outline more specific high-priority efforts for the next 3-5 years. These 18 objectives are being integrated with NASA strategic planning.     

A reappraisal of the habitability of planets around M dwarf stars

Stable, hydrogen-burning, M dwarf stars make up about 75% of all stars in the Galaxy. They are extremely long-lived, and because they are much smaller in mass than the Sun (between 0.5 and 0.08 M(Sun)), their temperature and stellar luminosity are low and peaked in the red. We have re-examined what is known at present about the potential for a terrestrial planet forming within, or migrating into, the classic liquid-surface-water habitable zone close to an M dwarf star. Observations of protoplanetary disks suggest that planet-building materials are common around M dwarfs, but N-body simulations differ in their estimations of the likelihood of potentially habitable, wet planets that reside within their habitable zones, which are only about one-fifth to 1/50th of the width of that for a G star. Particularly in light of the claimed detection of the planets with masses as small as 5.5 and 7.5 M(Earth) orbiting M stars, there seems no reason to exclude the possibility of terrestrial planets. Tidally locked synchronous rotation within the narrow habitable zone does not necessarily lead to atmospheric collapse, and active stellar flaring may not be as much of an evolutionarily disadvantageous factor as has previously been supposed. We conclude that M dwarf stars may indeed be viable hosts for planets on which the origin and evolution of life can occur. A number of planetary processes such as cessation of geothermal activity or thermal and nonthermal atmospheric loss processes may limit the duration of planetary habitability to periods far shorter than the extreme lifetime of the M dwarf star. Nevertheless, it makes sense to include M dwarf stars in programs that seek to find habitable worlds and evidence of life. This paper presents the summary conclusions of an interdisciplinary workshop (http://mstars.seti.org) sponsored by the NASA Astrobiology Institute and convened at the SETI Institute.