Search NASA⌕ Search

Engineering topics

Gerola, H.

Publications and source records attributed to Gerola, H..

10 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Stochastic self-propagating star formation in the Large Magellanic Cloud

The considered investigation provides a unifying theoretical basis for an understanding of a variety of observations on the Large Magellanic Cloud (LMC). It is demonstrated that an intrinsically statistical theory can supply a model for individual objects. With the aid of the model of stochastic self-propagating star formation described by Gerola and Seiden (1978), many structural features, both in the stars and in the gas are related to the process of star formation and the evolution of the LMC. It is found that the pattern of massive star formation in the LMC is partially ordered. The stars occur in large clumps and their global pattern is basically irregular and asymmetric with respect to the rotation center. The mean rate of chemical evolution of the LMC is smaller than that of the Milky Way. This fact is largely explained by the model in terms of the difference in shear for these galaxies. The formation of large, dense regions of massive stars has a great effect on the gas.

Feitzinger, V.↗
NASA NTRS1978

Molecular evolution of contracting clouds - Basic methods and initial results

The relationship between the dynamics of the interstellar gas and the thermal and chemical effects associated with interstellar molecules and dust is investigated. The evolution of a rather massive isolated initially diffuse cloud under self-gravity is studied, using the equations of hydrodynamics; only radial motions are considered, and the heat, chemical, and radiative-transfer equations are solved simultaneously with the hydrodynamic equations. The relevant chemistry is described along with the thermal model, the radiative-transfer process, and the numerical methods employed. Results for a contracting cloud are discussed in terms of the problem of initial conditions, the dynamical evolution of the cloud, its chemical and thermal evolution, time scales, and column densities. It is shown that the chemical evolution of a massive contracting diffuse cloud is sensitive to such physical properties as temperature and ion abundances, that warm and cool versions of a typical cloud evolve differently, and that the physical origin of this effect is the level of heating due to H2 formation on interstellar dust grains.

Gerola, H.↗
NASA NTRS1977

Contraction of the Orion Nebula cluster-molecular cloud A complex

Observational evidence is presented in support of the view that the width of the molecular lines in Orion is produced by a contraction of the cloud with a velocity directly proportional to the distance from the cloud center. The main support is provided by close agreement between the motions of the cloud and that of the stars embedded in it. A further implication of this agreement is that star formation occurred throughout the Orion cloud, rather than only in the dense center as predicted by the conventional picture of star formation.

Fallon, F. W.↗
NASA NTRS1976

Enhanced effects of starlight on the interstellar medium

The photodesorption of molecules and atoms from the surfaces of interstellar grains can be an important source of heating for the interstellar medium and the origin of instabilities which may separate grains and gas. For low densities, the force exerted on the grains is proportional to the gas density and independent of the radiation intensity; for high densities, it is proportional to the radiative flux and independent of the gas density. This force may act differently on grains of different sizes. The photoelectric effect may also be an efficient mechanism for the separation of gas and dust in diffuse clouds.

Gerola, H.↗
NASA NTRS1975

Diffusion effects on the line intensities of He I and He II in the solar transition region

A heuristic treatment of diffusion in the solar chromosphere-corona transition region is developed. It is shown that diffusion becomes increasingly important with steeper temperature gradients, in active and quiet regions relative to coronal holes, and with increasing excitation potential. Numerical calculations are made for the resonance lines of He I and He II and show that diffusion can enhance these lines. Thus the helium lines may appear relatively weak in coronal holes due to a weakening of the enhancement mechanism. Most transition region lines will be less affected by diffusion than He I or He II.

Shine, R.↗
NASA NTRS1975

Enhanced effects of starlight on the interstellar medium

The photodesorption of molecules and atoms from the surfaces of interstellar grains can be an important source of heating for the interstellar medium and the origin of instabilities which may separate grains and gas. For low densities, the force exerted on the grains is proportional to the gas density and independent of the radiation intensity; for high densities, it is proportional to the radiative flux and independent of the gas density. This force may act differently on grains of different sizes. The photoelectric effect may also be an efficient mechanism for the separation of gas and dust in diffuse clouds.

Gerola, H.↗
NASA NTRS1974

Evidence for a corona of beta Geminorum

A spectrometer was used on the satellite Copernicus to observe a chromospheric L alpha emission from the K0 giant beta Gem at 1218.4 A. This emission appears to be in the corona at temperatures near 260,000 deg K, since the ion it is identified with requires 77.4 eV to be produced.

Gerola, H.↗
NASA NTRS1974

Statistical time-dependent model for the interstellar gas

We present models for temperature and ionization structure of low, uniform-density (approximately 0.3 per cu cm) interstellar gas in a galactic disk which is exposed to soft X rays from supernova outbursts occurring randomly in space and time. The structure was calculated by computing the time record of temperature and ionization at a given point by Monte Carlo simulation. The calculation yields probability distribution functions for ionized fraction, temperature, and their various observable moments. These time-dependent models predict a bimodal temperature distribution of the gas that agrees with various observations. Cold regions in the low-density gas may have the appearance of clouds in 21-cm absorption. The time-dependent model, in contrast to the steady-state model, predicts large fluctuations in ionization rate and the existence of cold (approximately 30 K), ionized (ionized fraction equal to about 0.1) regions.

Gerola, H.↗