Engineering topics

Mathis, J. S.

Publications and source records attributed to Mathis, J. S..

12 records

NASA NTRS1992

The origin of the extremely metal-poor post-AGB stars

Certain A-F supergiants show almost solar photospheric abundances of C, N, O, S, and Zn, while Fe, Mg, Ca, Si, Cr, and other elements are more than two orders of magnitude below solar. We suggest that the present photosphere was originally solar in composition; the missing elements were incorporated into dust grains that were separated from the atmosphere. The heavy-element-depleted gas comprises the present photosphere. There are two possible mechanisms by which this process might take place: capture by the presently visible star of the depleted gas from a binary companions, or the rapid termination of a vigorous stellar wind in a single star, so that the grains are blown outward through the expanding envelope. Besides the abundance peculiarities, the hypothesis explains why some objects have carbon-rich dust outside of an oxygen rich photosphere, why these stars have the rather narrow range of (T(eff), log g) near (7000 K, 1.0), and why some might have a great deal of circumstellar dust while others have little. Perhaps the most serious objection to the hypothesis is the possible presence of stellar winds observed in the winds of the peculiar post-AGB stars themselves.

Mathis, J. S.↗

NASA NTRS1988

Summary and future prospects

The 'known' properties of interstellar dust are reviewed. Silicates are present because of the 9.7-micron feature in both circumstellar and interstellar dust. The near-infrared emission bands (3.3-11.4 microns, with continuum extending to longer wavelengths) strongly suggest that aromatic materials of some kind are present. A variety of considerations suggest that large graphite grains are probably absent, but smaller graphite particles are probably the most viable explanation for the 2175 A 'bump'. Considerations mentioned at this meeting regarding several properties of grains are then discussed: the UV continuum, the 2175 A 'bump', and various grain theories.

Mathis, J. S.↗

NASA NTRS1986

The alignment of interstellar grains

The magnetic-field alignment of interstellar silicate grains containing superparamagnetic (SPM) inclusions is investigated theoretically. It is shown that the observed dependence of polarization on wavelength can be fully explained in terms of the alignment mechanisms and the size distribution of the SPM grains, at least for interstellar polarizations peaking at wavelength 550 nm or higher. The implications of this finding for models of grain coagulation and shattering are considered.

Mathis, J. S.↗

NASA NTRS1986

The photoionization of the diffuse galactic gas

In a study of the diffuse ionized gas (DIG) component of the interstellar medium, it is attempted to see if the general properties of dilute gas ionized by O stars are similar to observations and to what extent the observations of the DIG can be used to determine the nature of the ionizing radiation field at great distances above the plane of the Galaxy. It has been suggested by Reynolds (1985) that either shocks or photoionization might be responsible for the DIG. The photoionization model seems required by the observations.

Mathis, J. S.↗

NASA NTRS1986

Observations and theories of interstellar dust

The observational properties of dust are summarized based on: the extinction over a factor of 100 in wavelength (0.1 to 10 microns); polarization, both linear and circular; narrow emission and absorption features in the spectrom; and reflection nebulae.

Mathis, J. S.↗

NASA NTRS1986

Grain alignment by ferromagnetic impurities

The observed wavelength dependence of linear polarization, and its variation from region to region can be explained by the following assumptions. Interstellar grains resemble interplanetary grains, in that they are composed of collections of small particles coagulated together into elongated masses. A fraction of the small particles are ferromagnetic. Presumably these are either metallic Fe or magnetite, Fe3O4. If and only if a large grain contains one or more magnetic particles is the grain aligned in the galactic magnetic field. The magnetic particles stick only to silicate grains because of chemical similarities, or (equivalently) any pure carbon grains in the diffuse interstellar medium (ISM) are too spherical to produce polarization. Grains in dense regions, such as the outer parts of molecular clouds, are larger than those in the diffuse ISM because of coagulation of the grains rather than accretion of icy mantles. These regions are known to have larger than normal values of lambda (max), the wavelength of the maximum of linear polarization. The above assumptions are sufficient to allow the calculation of the wavelength dependence of the polarization.

Mathis, J. S.↗

NASA NTRS1982

Results from the Orbiting Astronomical Observatory. XLI - Photometry of M31 and M33

Filter photometry observations from the Orbiting Astronomical Observatory, in six wavelength bands between 1550 and 4250 A, are reported for several positions along the major and minor axes in M31 and at some positions in M33. The spatial resolution is 10 arcmin. The colors along the major axis of M31 within about 20 arcmin of the nucleus are similar to the nucleus; at 45 arcmin the colors are much bluer, indicating a spiral arm population. This behavior is expected from other observations of H I, CO, and early-type stars, which are all concentrated in a ring of material whose inner edge is about 7 kpc (= 35 arcmin) from the nucleus. The center of M33 is bluer than the ring of M31, and the arms of M33 even bluer. There are no colors too blue to be explained (within observational error) by unreddened B stars.

Davis, J.↗

NASA NTRS1981

The ultraviolet properties of dust in the Orion Nebula

The continuum in the Orion Nebula was observed with the IUE at 16 positions varying from 30 arcsec to 5 arcmin in angular separation from the central star Theta(1) Orionis C. The atomic continuum was estimated from the H-beta brightnesses. The remaining scattered starlight was analyzed by multiple-scattering models in which the main parameters are: (1) the albedo of the dust; (2) the scattering phase function parameter, the averaged cosine of the angle of scattering; and (3) the density of dust grains at small distances from the star. It is found that (a) dust must be depleted near the star; (b) the albedo is fairly well determined and is constant across the 2200 A extinction feature; and (c) grains are quite forward-throwing at 1300 A, as they have been found at 4600 A earlier. The scattering is more isotropic at 2400 A. However, the absolute value of the scattering phase function parameters at all wavelengths is quite uncertain because it depends sensitively upon the density of grains close to the star.

Mathis, J. S.↗

NASA NTRS1981

Central object of the 30 Doradus Nebula, a supermassive star

Observations by the International Ultraviolet Explorer of R136(HD 38268), the central object of the 30 Doradus Nebula, at high and low spectral resolutions show a peculiar hot object with a massive stellar wind; spectra indicating an outflow speed of 3500 kilometers/sec and a temperature of 60,000 K. The peculiar object R136a, providing most of the ionizations, is interpreted as being dominated by a single superluminous object with the following, approximate properties: a radius 100 times that of the sun, a mass 2500 times that of the sun, and a loss rate of 10 to the -3.5 solar masses per year. The instability expected to be typical of such stars may account for the massive stellar wind.

Cassinelli, J. P.↗

NASA NTRS1981

Ultraviolet slit spectroscopy in the core of 30 Doradus with the International Ultraviolet Explorer

Low resolution (about 7 A) IUE observations of the core of the 30 Doradus Nebula in the Large Magellanic Cloud are presented. Data were obtained through the large entrance aperture at 11 different positions in both the long wavelength (1900-3200 A) and short wavelength (1150-2000 A) cameras. The aperture was stepped in 10-arcsec intervals, in the direction of the long axis of the aperture, effectively yielding a 2-arcmin long slit with the central object, R136, close to the center. The brightness distribution is strongly peaked at the position of R136, but it is found that the far-ultraviolet fluxes are dominated by luminous early-type stars at any position along the slit. The behavior of the ultraviolet colors along the slit implies that the core of 30 Dor lies behind a cloud of dust extending over at least 2 arcmin = 30 pc and suggests that the reddening law changes with distance from R136.

Koornneef, J.↗

NASA NTRS1979

Observed properties of interstellar dust

Recent information on the observed properties of interstellar dust is reviewed, with an attempt made to clarify some of the observational uncertainties associated with obtaining dust parameters. Attention is given to interstellar extinction, the interstellar dust distribution, the dust-to-gas ratio, and light scattering by dust grains. Interstellar polarization is also examined, along with heavy-element depletion in the interstellar medium, thermal emission from interstellar dust grains, diffuse interstellar features, and the composition and origin of interstellar grains.

Savage, B. D.↗

NASA NTRS1975

Photometry of a bright filament in the Orion nebula

The spectrum of a bright filament in the outer portion of the Orion nebula, 14 min from the Trapezium, was studied. The temperature of the filament, derived from /N II/ lambda 5755/6583 ratio, is 9560 K. The electron density (from /S II/ lambda 6717, 6731) is low: 360 plus or minus 150 per cu cm. The ionic abundances of He(+), N(+), O(++), and S(-) are about the same as Peimbert and Costero (1969) found in regions in Orion closer to the Trapezium. It is concluded that the filament is photoionized, presumably by the Trapezium, rather than a shock front, as had originally been suspected.

Dufour, R. J.↗