Engineering topics

Jura, M.

Publications and source records attributed to Jura, M..

At least 37 records · Page 2

NASA NTRS1990

The absence of circumstellar dust debris around G giants

The IRAS data base has been searched for evidence for circumstellar dust around luminosity class III G giants, stars whose progenitors are mostly main-sequence A stars. While 20 percent of all main-sequence A dwarfs have dust which absorbs at least 5 x 10 to the -6th of the light from the star, less than 3 percent of all G giants have such clouds. One possible explanation for the absence of detectable dust debris around the G giants is that the Poynting-Robertson effect leads to the decay of the dust around the main-sequence A stars, and that the supply of these grains is not renewed indefinitely. In this case, the derived upper limit to the grain radius of about 0.2 cm for the bulk of the grains emitting the far-infrared emission is consistent with data derived from ground-based submillimeter observations. Another possible explanation for the lack of grains around at least some G giants is that the dust around the original A dwarf is mainly composed of relatively volatile material like water ice which thermally evaporates in a relatively short time during the giant phase of higher luminosity.

Jura, M.↗

NASA NTRS1990

Very dusty carbon-rich asymptotic giant branch stars between about 1 and about 2.5 kiloparsecs from the sun

Combining IRAS, Two Micron Sky Survey, and ground-based optical and radio data, carbon-rich asymptotic giant branch (AGB) stars within about 2.5 kpc of the sun, in the zone delta between 81 deg and -33 deg, that are typically losing about 0.00001 solar mass/yr were identified. Distances are derived assuming a luminosity of 10,000 solar luminosities; there are 126 stars in this zone that are between about 1 and about 2.5 kpc from the sun. By including the 29 very dusty carbon stars that were previously identified to lie within 1 kpc of the sun, it is found that there is no Galactocentric gradient in the space distribution of the very dusty carbon stars, in contrast to the general population of stars which is more concentrated toward the Galactic center. The surface density of very dusty carbon stars in the Galactic plane is about 10/kpc sq. In the solar neighborhood, carbon stars return roughly half of the material from all AGB stars into the interstellar medium; in the outer Galaxy they dominate the mass return, while they are probably not so important in the inner Galaxy.

Jura, M.↗

NASA NTRS1990

Mass-losing M supergiants in the solar neighborhood

A list of the 21 mass-losing red supergiants (20 M type, one G type; L greater than 100,000 solar luminosities) within 2.5 kpc of the sun is compiled. These supergiants are highly evolved descendants of main-sequence stars with initial masses larger than 20 solar masses. The surface density is between about 1 and 2/sq kpc. As found previously, these stars are much less concentrated toward the Galactic center than W-R stars, which are also highly evolved massive stars. Although with considerable uncertainty, it is estimated that the mass return by the M supergiants is somewhere between 0.00001 and 0.00003 solar mass/sq kpc yr. In the hemisphere facing the Galactic center there is much less mass loss from M supergiants than from W-R stars, but, in the anticenter direction, the M supergiants return more mass than do the W-R stars. The duration of the M supergiant phase appears to be between 200,000 and 400,000 yr. During this phase, a star of initially at least 20 solar masses returns perhaps 3-10 solar masses into the interstellar medium.

Jura, M.↗

NASA NTRS1990

Astronomical observations of solid phase carbon

In the outer envelopes of red giants, when the gas cools sufficiently, molecules and solids form. Thermodynamically, the most stable molecule is CO, and it is usually assumed that all the available carbon and oxygen are consumed in the formation of this molecule (Salpeter 1977). If the carbon abundance is greater than the oxygen abundance, then the carbon left over after the formation of CO is available for solid grains. Because carbon is by far the most abundant species available for making solids in these environments, researchers anticipate that the grains are composed of nearly pure carbon in some form. The observations which can be used to infer the nature of this solid phase carbon are discussed. The observations of the dust around carbon-rich red giants are discussed. These results are then placed into their broader astrophysical context.

Jura, M.↗

NASA NTRS1990

The probable dust formation episode around Rho Cassiopeiae

By comparing IRAS photometry with previous 10-micron data, it appears that dust formed in the circumstellar envelope around the supergiant G star, Rho Cas, sometime between 1973 and 1983. This dust formation may have been the consequence of the gas outflowing and cooling from the intense mass outburst of 1946. In 1983, the dust was detected at a temperature between 600 and 800 K at a distance between 10 to the 15th cm and 4 x 10 to the 15th cm from the star. While the dust-to-gas ratio in the circumstellar envelope may be as low as 0.00001 and thus much lower than in other mass-losing stars, the luminosity of Rho Cas is sufficiently large that radiation pressure on this dust might be dynamically important in helping to drive mass loss.

Jura, M.↗

NASA NTRS1990

Dust around AFGL 2688, molecular shielding, and the production of carbon chain molecules

The molecular, IR, and optical maps of the evolved carbon star AFGL 2688 (the 'Egg' Nebula) are all consistent with a model of a bipolar outflow of approximately 0.0001 solar masses/yr that stopped as this object evolved beyond the asymptotic giant branch about 200 years ago. In order to explain the extended HC7N emission around this star, it is proposed that carbon grains are collisionally fragmented as they supersonically steam through the circumstellar envelope.

Jura, M.↗

NASA NTRS1990

The space distribution of AGB stars

The AGB stars can be classified into three main types on the basis of their atmospheric composition: carbon-rich, oxygen-rich, and S-type. The carbon-rich stars typically have 1.5-solar-mass main sequence stars as progenitors. There are about 40 of these stars per sq kpc projected onto the plane of the Milky Way galaxy in the neighborhood of the sun with an exponential scale height above the galactic disk of about 200 pc. Contrary to the general distribution of mass, there is no decrease with galactocentric radius of the surface density of these stars for at least 3 kpc beyond the solar circle. The S-type stars appear to have the same spatial distribution as the carbon stars; there are about 1/3 as many 'pure' S stars as there are luminous carbon stars. One major class of oxygen-rich AGB stars is the Mira variables. There are approximately between 1 and 2 times as many of these stars per sq kpc projected onto the galactic plane of the Milky Way as there are carbon stars.

Jura, M.↗

NASA NTRS1989

Dust-enshrouded asymptotic giant branch stars in the solar neighborhood

Using available infrared catalogs, an inventory is taken of the AGB star losing large amounts of mass within about 1 kpc of the sun. A surface density of these stars is estimated of about 25/sq kpc projected onto the plane of the Galaxy. Of these stars, about one-half are oxygen-rich while the other half are carbon-rich. The total mass-loss rate from AGB stars into the interstellar medium is probably between 3 and 6 x 10 to the -4th solar mass/sq kpc/yr. Within the uncertainties, this is in reasonable agreement with an estimated net loss rate of about 8 x 10 to the -4th solar mass/sq kpc/yr for main-sequence stars with initial masses between 1 and 5 solar masses as they evolve to white dwarfs. However, it is possible that there are important sources of mass loss which have not yet been identified. In the solar neighborhood, about one-half of all about 1.2 solar mass main-sequence stars spend greater than 30,000 yr in a carbon-star phase where they lose 1-2 x 10 to the -5th solar mass/yr and then become white dwarfs with about 0.7 solar mass.

Jura, M.↗

NASA NTRS1989

High-luminosity carbon stars in the galactic anticenter

K-band magnitudes of 211 carbon stars in the Galactic anticenter are presented. The surface density, brightness range, and observed colors of stars in the sample are discussed. It is suggested that the K-band extinction gradient in the Galactic plane near l = 180 deg is between 0.15 and 0.3/kpc. In contrast to the total density of stars, it is found that the density of high-luminosity carbon stars does not decrease in the anticenter region out to at least 3 kpc from the sun. It is proposed that this is due to the fact that lower metallicity stars spend more time as carbon stars compared to stars of solar metallicity. Also, it is shown that the average mass-loss rates from high-luminosity carbon stars in the anticenter direction appear to be lower by a factor of 1.7 than those for carbon stars in the solar neighborhood.

Jura, M.↗

NASA NTRS1989

Discovery of CO emission from NGC 1275

CO radio emission has been discovered in both the J=1-0 and J=2-1 rotational lines from the elliptical galaxy NGC 1275. The CO, which may condense from the cooling flow by means of which mass is thought to be accumulating, exhibits such a small velocity dispersion that it may collapse into stars before it virializes in the gravitational potential of this galaxy. If the star formation rate is as high as presently calculated, the initial mass function will be weighted toward the production of low-mass stars.

Lazareff, B.↗

NASA NTRS1989

Mass-losing peculiar red giants - The comparison between theory and observations

The mass loss from evolved red giants is considered. It seems that red giants on the Asymptotic Giant Branch (AGB) are losing between 0.0003 and 0.0006 solar mass/sq kpc yr in the solar neighborhood. If all the main sequence stars between 1 and 5 solar masses ultimately evolve into white dwarfs with masses of 0.7 solar mass, the predicted mass loss rate in the solar neighborhood from these stars is 0.0008 solar mass/sq kpc yr. Although there are still uncertainties, it appears that there is no strong disagreement between theory and observation.

Jura, M.↗

NASA NTRS1988

Detection of C-13O radio emission from C-13-rich carbon stars

A high ratio of C-13O radio emission in the J = 1-0 rotational line has been detected from three mass-losing carbon stars which optical data indicate have high C-13/C12 ratios. Since chemical fractionation, isotope-dependent photodissociation and opacity in the rotational and vibrational lines may not raise significantly the C-13O ratio above the actual C-13/C-12 ratio in these circumstellar envelopes, the relative abundance of C-13 in these stars might be even greater by perhaps a factor of two than previously believed. About 15 percent of all luminous carbon stars are C-13-rich, and these stars may play a significant role in the enhancement in the C-13/C12 ratio that has occurred during the past 4.6 billion years since the formation of the sun.

Jura, M.↗

NASA NTRS1988

The bipolar outflow from the rotating carbon star, V Hydrae

A high-resolution optical spectrum of the mass-losing red giant carbon star, V Hya, has been obtained, and the (C-12)O (J = 1-0) millimeter emission in the circumstellar envelope around this star has been mapped. It is found that the CO emission is extended, clearly anisotropic and can be interpreted as the superposition of an isotropic emission with that of a bipolar flow. The optical spectrum of the photosphere suggests that this star is rotating with v sin i between 10 and 20 km/s. These data are interpreted, together, to suggest that the bipolar nature of the outflow results from the flattening of the star induced by its rapid rotation.

Kahane, C.↗

NASA NTRS1988

Mass loss from S stars

The mass-loss process in S stars is studied using 65 S stars from the listing of Wing and Yorka (1977). The role of pulsations in the mass-loss process is examined. It is detected that stars with larger mass-loss rates have a greater amplitude of pulsations. The dust-to-gas ratio for the S stars is estimated as 0.002 and the average mass-loss rate is about 6 x 10 to the -8th solar masses/yr. Some of the properties of the S stars, such as scale height, surface density, and lifetime, are measured. It is determined that scale height is 200 pc; the total duration of the S star phase is greater than or equal to 30,000 yr; and the stars inject 3 x 10 to the -6th solar masses/sq kpc yr into the interstellar medium.

Jura, M.↗

NASA NTRS1987

A flux-limited sample of Galactic carbon stars

Published observational data (including IRAS observations) for a flux-limited sample of 215 Galactic carbon stars (CSs) selected from the 2-micron sky survey of Neugebauer and Leighton (1969) are compiled in extensive tables and graphs and analyzed statistically. The sample is found to penetrate a volume of radius 1.5 kpc, and the local CS space density and surface density are calculated as log rho0 (per cu kpc) = 2.0 + or - 0.4 and log N (per sq kpc) = 1.6 + or - 0.2, respectively. The total Galactic mass-return rate from these CSs is estimated as 0.013 solar mass/yr, implying a time scale of 0.1-1 Myr for the CS evolutionary phase and a mass of 1.2-1.6 solar mass for the (probably F-type) main-seqence progenitors of CSs.

Claussen, M. J.↗

NASA NTRS1987

The relative amounts of stars and interstellar matter in the local Milky Way

This paper considers the balance between star formation and mass loss from evolved stars in the region within 1 kpc of the sun. There is considerably more mass in stars than in the interstellar medium, and more material is being incorporated into new stars than is being returned by evolved stars. In the simplest interpretation of the data, it appears that unless there is some infall of new interstellar gas, the era of substantial star formation out of interstellar gas will be over in a few (perhaps 3) billion years.

Jura, M.↗

NASA NTRS1987

Mass-losing red giants in open clusters

Mass-losing stars in open clusters with main-sequence turn-offs at intermediate mass have been searched for by using the IRAS data base. The absence of many strong 60 micron sources in open clusters implies that intermediate-mass stars lose much of their mass during an intense wind phase of rather short duration. For stars of about seven solar masses, this phase, if it exists at all, lasts for not much more than 100,000 yr. For stars of about four solar masses, the intense wind phase appears to last considerably less than 10 million yr; it may well last for less than a million yr.

Jura, M.↗

NASA NTRS1987

The Milky Way as a Galaxy

The physical conditions in the interstellar medium of the Milky Way are briefly reviewed. It is noted that the local interstellar medium shows relatively uniform abundances and dust/gas ratios; in fact recent measurements of the C-12/C-13 ratio indicate that the gas is homogeneous to better than 15 percent. Observations of external galaxies similar to the Milky Way can provide useful insight into the nature of the interstellar medium of the Galaxy.

Jura, M.↗