Engineering topics

Sweigart, Allen V.

Publications and source records attributed to Sweigart, Allen V..

24 records · Page 2

NASA NTRS1990

The development of the red giant branch. II - Astrophysical properties

Evolutionary sequences developed in another paper are used here to investigate the properties of the red giant branch (RGB) phase transition. Results are found for compositions in the range Y(MS) between 0.20 and 0.30 and Z between 0.004 and 0.04. The transition mass M(HeF) increases as either Y(MS) decreases or Z increases. The stellar population transition age t(HeF) is virtually independent of composition and close to 0.6 Gyr. The RGB phase transition occurs almost abruptly over a mass range of only a few tenths of a solar mass or, equivalently, over a time interval of about 0.2 Gyr in the life of a stellar population. During the RGB phase transition the core mass Mc at helium ignition increases very rapidly by about 0.15 solar mass, while the luminosity at the tip of the RGB increases by about one order of magnitude. Absolute minima are found for the values of Mc and the RGB tip luminosity.

Sweigart, Allen V.↗

NASA NTRS1990

Horizontal-branch stellar evolution

The results of canonical theory for the evolution of horizontal-branch (HB) stars are examined. Particular attention is given to how an HB star maintains the appropriate composition distribution within the semiconvective zone and how this composition is affected by the finite time-dependence with which convective boundaries actually move. Newly developed models based on time-dependent overshooting are presented for both the core-helium-exhaustion and main HB phases.

Sweigart, Allen V.↗

NASA NTRS1989

The development of the red giant branch. I - Theoretical evolutionary sequences

A grid of 100 evolutionary sequences extending from the zero-age main sequence to the onset of helium burning has been computed for stellar masses between 1.4 and 3.4 solar masses, helium abundances of 0.20 and 0.30, and heavy-element abundances of 0.004, 0.01, and 0.04. Using these computations the transition in the morphology of the red giant branch (RGB) between low-mass stars, which have an extended and luminous first RGB phase prior to helium ignition, and intermediate-mass stars, which do not, is investigated. Extensive tabulations of the numerical results are provided to aid in applying these sequences. The effects of the first dredge-up on the surface helium and CNO abundances of the sequences is discussed.

Sweigart, Allen V.↗

NASA NTRS1987

Evolutionary sequences for horizontal branch stars

A new grid of canonical evolutionary horizontal branch (HB) sequences is presented. Sequences are computed for each combination of the following helium and heavy-element abundances, respectively: Y(main sequence) = 0.20, 0.25, 0.30, and Z = 0.0001, 0.001, and 0.01. The results show that the bifurcation point at which the HB morphology changes from redward-evolving tracks to tracks with blueward loops shifts to higher effective temperatures with increasing helium abundance or metallicity. The sequences can be used to study in more detail how a number of HB properties such as the HB lifetime, the effective temperature at the bifurcation point in the track morphology, the luminosity dropoff of the blue HB, and the luminosity width of the red HB depend on the composition.

Sweigart, Allen V.↗

NASA NTRS1987

On the interpretation of the Sandage period-shift effect among globular-cluster RR Lyrae variables

A new grid of canonical zero-age horizontal branch (ZAHB) models is constructed to study the Sandage period-shift effect, and a number of possible explanations for the discrepancy between the observed and predicted period shifts are explored. Various factors which determine the period shift in ZAHB models are discussed in detail, showing that possible uncertainties in the input physics used for computing the preceding red-giant evolution are unlikely to resolve the discrepancy. Some test calculations for the ZAHB phase are presented which suggest than an increase in the metal opacity at temperatures around a million K might substantially increase the predicted period shift. It is found that a nonsolar CNO to Fe ratio is not a plausible explanation of the period-shift discrepancy, and the effects of evolution off the ZAHB in the period-effective temperature diagram are discussed. It is shown that evolution can increase the predicted period shift but not to the extent required by the Sandage effect. The possibility that rotation might explain the discrepancy is considered and rejected.

Sweigart, Allen V.↗

NASA NTRS1987

Theoretical horizontal-branch evolution

The general features of the theoretical evolution of canonical horizontal-branch (HB) stars are briefly reviewed with specific emphasis on the track morphology in the HR diagram and the determination of the globular cluster helium abundance. The observational evidence for the occurrence of semiconvection is discussed together with some remaining theoretical uncertainty.

Sweigart, Allen V.↗