Engineering topics

Natta, Antonella

Publications and source records attributed to Natta, Antonella.

5 records

NASA NTRS1995

Time-Dependent Photodissociation Regions

We present theoretical models of the time-dependent thermal and chemical structure of molecular gas suddenly exposed to far-ultraviolet (FUV) (6 eV less than hv less than 13.6 eV) radiation fields and the consequent time- dependent infrared emission of the gas. We focus on the response of molecular hydrogen for cloud densities ranging from n = 10(exp 3) to 10(exp 6)/cu cm and FUV fluxes G(sub 0) = 10(exp 3)-10(exp 6) times the local FUV interstellar flux. For G(sub 0)/n greater than 10(exp -2) cu cm, the emergent H(sub 2) vibrational line intensities are initially larger than the final equilibrium values. The H(sub 2) lines are excited by FUV fluorescence and by collisional excitation in warm gas. Most of the H(sub 2) intensity is generated at a characteristic hydrogen column density of N approximately 10(exp 21)/sq cm, which corresponds to an FUV optical depth of unity caused by dust opacity. The time dependence of the H(sub 2) intensities arises because the initial abundances of H(sub 2) at these depths is much higher than the equilibrium values, so that H(sub 2) initially competes more effectively with dust in absorbing FUV photons. Considerable column densities of warm (T approximately 1000) K H(sub 2) gas can be produced by the FUV pumping of H(sub 2) vibrational levels followed by collisional de-excitation, which transfers the energy to heat. In dense (n greater than or approximately 10(exp 5)/cu cm) gas exposed to high (G(sub 0) greater than or approximately 10(exp 4)) fluxes, this warm gas produces a 2-1 S(1)/1-0 S(l) H(sub 2) line ratio of approximately 0.1, which mimics the ratio found in shocked gas. In lower density regions, the FUV pumping produces a pure-fluorescent ratio of approximately 0.5. We also present calculations of the time dependence of the atomic hydrogen column densities and of the intensities of 0 I 6300 A, S II 6730 A, Fe II 1.64 microns, and rotational OH and H20 emission. Potential applications include star-forming regions, clouds near active galactic nuclei, and planetary nebulae. We apply our models to five planetary nebulae and conclude that only BD +30deg3639 shows evidence of enhanced H(sub 2) emission due to (high) nonequilibrium H(sub 2) abundances.

Hollenbach, David↗

NASA NTRS1994

'Spherical' disks: Moving toward a unified source model for L1551

To predict the effects of a disk on the spectral energy distribution of a deeply embedded protostar, we construct disk models with power-law temperature distributions T is proportional to r(exp -q). We then use the spherically averaged disk emission as the central source for a spherical envelope, hence the term, 'spherical' disk. We then calculate the predicted spectral energy distribution of the disk and envelope, using a spherically symmetric radiative transport code. Applying this procedure to L1551 IRS 5, we find that the predicted far-infrared flux is not very sensitive to the nature of the central source. The best source model is consistent with the far-infrared emission arising from the infalling region in an 'inside-out' collapse model, independent of the nature of the central source. Disk models are superior to the star-only model when we try to match millimeter interferometer data. While disks with various q can reproduce the observed 2.7 mm interferometer flux, only an active disk (q = 0.5) can produce enough emission in a region small enough to match the observed 2.7 mm visibilities. However, if the disk is backwarmed by the envelope, even purely reprocessing disks can meet this constraint. All types of backwarmed disks are virtually in distinguishable in their millimeter properties. We find that all reasonable envelope models are sufficiently opaque in the mid-infrared to attenuate any disk model to a level well below the observations, unless the ratio of the mid-infrared to far-infrared dust opacities is similar to that of the dust opacities advocated by Mathis, Mezger, & Panagia (1983).

Butner, Harold M.↗

NASA NTRS1993

Infrared studies of circumstellar matter around Herbig Ae/Be and related stars

The study presents high spatial resolution data at 50 and 100 microns for seven young intermediate-mass stars with flat or rising spectral energy distributions. Five stars, namely, LkH-alpha 198, R Mon, CD -42 deg 11721, R CrA, and V645 Cyg, are resolved at 100 microns. The emission comes from spatially extended envelopes, with sizes varying between 0.025 and 0.4 pc. The wavelengths are optically thin at mid-IR and FIR wavelengths but become optically thick in the visual where the extinction amounts to 5-10 mag. In two cases, R CrA and V 645 Cyg, dust density distributions with steep density gradients are obtained, alpha is approximately equal to 2, while for LkH-alpha, R Mon, and CD -42 deg 11721, alpha is approximately equal to 0.5. Model fitting to the spectral energy distributions, combined with the constraints on the size of the emitting regions, indicates that inside the circumstellar envelopes a large fraction of the stellar radiation is already degraded to longer wavelengths.

Natta, Antonella↗

NASA NTRS1992

Infrared studies of pre-main-sequence intermediate-mass stars - LkH-Alpha 198

The paper presents FIR scans of LkH-Alpha 198 at 50 and 100 microns along several directions; the observations reveal an extended, roughly spherical source diameter of 33 arcsec, or 20,000 AU, at 100 microns and not more than 10 arcsec, or 15,000 AU, at 50 microns. A luminosity of 250 solar luminosities at a distance of 600 pc is estimated on the basis of a comparison of the observed properties at optical, IR, and millimeter wavelengths with the predictions of radiation transfer models. The optical depth at 100 microns is 0.004-0.006, implying a mass of about 2 solar masses with a radius of 10,000 AU. It is argued that the inner radius of the dusty envelope is rather small, less than about 300 AU (0.5 arcsec), and it may be as small as the dust destruction front: in LkH-Alpha 198 there is no evidence of a large region devoid of grains.

Natta, Antonella↗

NASA NTRS1990

High-resolution, far-infrared observations of NGC 2071

The far-IR emission of the visible reflection nebula NGC 2071 has been resolved at both 50 and 100 microns along several directions. The observations reveal an extended, roughly spherical source with an average source diameter of about 12 arcsec or 4700 AU at 50 microns and about 16 arcsec or 6200 AU at 100 microns. The source is modeled using a radiative transport code to match scans of the source and previous photometry. The luminosity of the source is 520 solar at a distance of 390 pc. The optical depth at 100 microns is 0.20, implying a mass of 1.2-10 solar within a radius of 5900 AU. The density gradient is in good agreement with theoretical models for infalling envelopes around protostars and in reasonable agreement with other observational determinations.

Butner, Harold M.↗