Engineering topics

Stahler, S. W.

Publications and source records attributed to Stahler, S. W..

5 records

NASA NTRS1986

Primordial stellar evolution - The pre-main-sequence phase

The quasi-static contraction of primordial stars composed of pure hydrogen and helium gas is studied by following numerically the evolution of a star of five solar masses from the end of protostellar accretion to the onset of hydrogen burning. Although the protostellar core of this mass is radiatively stable and undergoing nonhomologous contraction, its large surface area and luminosity force the star to a partially convective, homologously contracting state within only 100 yr. Deuterium later ignites at an off-center temperature maximum but fails to produce interior convection. The star follows a conventional premain sequence track in the HR diagram, reaching the ZAMS after 1.2 million yr, with a luminosity of 880 solar luminosities and a radius of 1.2 solar radii.

Stahler, S. W.↗

NASA NTRS1986

Primordial stellar evolution - The protostar phase

The structure and evolution of a protostar forming from a cloud composed of pure hydrogen and helium gas are calculated. Using an accretion rate of 0.0044 solar mass/yr, the collapse of the cloud is followed numerically as a sequence of steady state accretion flows onto the hydrostatic core, which grows from an initial mass of 0.01 solar mass to 10.5 solar masses. The core is surrounded by an optically thick radiative precursor for most of its evolution. The core radius reaches 47 solar radii when the mass is 1 solar mass. For sufficiently massive cores, the deep interior contracts strongly, driving out a 'luminosity wave' which reaches the surface when the mass is 8 solar masses. This results in a large increase in core radius, the establishment of surface convection, and the disappearance of the radiative precursor. The dependence of core radius on the mass and accretion rate is analytically derived, and a new table or Rosseland mean opacities for metal-free gas is presented.

Stahler, S. W.↗

NASA NTRS1985

The star-formation history of very young clusters

The popular idea that star formation has proceeded sequentially from lowest to highest mass members in open clusters is examined critically. For extremely young clusters, such as NGC 2264 and NGC 6530, this sequential hypothesis is a consequence of the assignment of pre-main-sequence contraction ages to all member stars. However, such ages yield a formation history which is implausible from a physical point of view, since the critical time for the onset of formation at any stellar mass is equal to the pre-main-sequence contraction time for that mass. Moreover, these ages are in conflict with the strong observational evidence that a substantial fraction of cluster members have already reached the main sequence. After reconsideration of the probable main-sequence members, the stellar ages in NGC 2264 and NGC 6530 are consistent with a variety of formation histories, and, in particular, with the view that all stellar masses form in approximately the same interval of time within a given cluster, i.e., that there is no mass-age correlation. A notion closely related to the sequential hypothesis, that the total star-formation rate increases exponentially with time, is subject to the same criticism.

Stahler, S. W.↗

NASA NTRS1984

The cyanopolyynes as a chemical clock for molecular clouds

A new method is proposed for determining the ages of molecular clouds. The method utilizes the properties of the long-chain organic molecules known as the cyanopolyynes (HC/2k + 1/N, k = 0, 1, 2, 3 ...), which are found in a variety of clouds. The observed regular abundance decline of these molecules as a function of chain length suggests that they are formed sequentially. If so, the age of the cloud can be read off as the time to grow the longest chain present. Although the creation process for the chains is still unknown, the age can be obtained from knowledge of the chain destruction mechanism. Destruction is probably due to adsorption onto the surfaces of interstellar grains. Using the known properties of grains, the age can be obtained from the cloud density and the abundance ratios of the cyanopolyynes. As a first application, minimum ages for the four dark clouds B335, TMC-1, TMC-2, and L183 are obtained. These minimum ages are 1.3 x 10 to the 6th yr, 9.7 x 10 to the 5th yr, 3.4 x 10 to the 5th yr, and 3.3 x 10 to the 5th yr, respectively. In each case, the ages are greater than or equal to the cloud's free-fall collapse time. These four clouds are therefore either in a state of hydrostatic balance or have only recently begun to collapse, following an earlier period of hydrostatic support.

Stahler, S. W.↗

NASA NTRS1983

The birthline for low-mass stars

Using the results of protostar theory, the locus in the Hertzsprung-Russell diagram is found where pre-main-sequence stars of subsolar mass should begin their quasi-static contraction phase and first appear as visible objects. This 'birthline' is in striking agreement with observations of T Tauri stars, providing a strong confirmation of the fact that these stars are indeed contracting along Hayashi tracks. The assumption that most T Tauri stars first appear along this line forces a recalibration of their ages. This recalibration removes the puzzling dip in present-day star formation seen in age histograms of several cloud complexes. Since the underlying protostar calculation assumes that the parent cloud was only thermally supported prior to its collapse, the observed location of the birthline places severe restrictions on the degree of extrathermal support provided by rotation, magnetic fields, or turbulence. In addition, the hypothesis that the collapse from thermally supported clouds to low-mass stars proceeds through protostellar disks appears untenable, since the disk accretion process almost certainly produces pre-main-sequence stars with radii well below the observed birthline.

Stahler, S. W.↗