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At least 595 records · Page 33
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

A study of the stellar population in the Lynds 1641 dark cloud. I - The IRAS catalog sources

The character of the sources identified in the IRAS Point Source Catalog and located within the boundaries of the nearest giant molecular cloud, Lynds 1641 is discussed. New optical and near-infrared photometry are combined to provide spectral energy distributions (SEDs) for these objects divided into three classes: class I objects with flat or rising spectra, class II objects with spectra intermediate in slope between a flat and blackbody spectrum, and class III objects with spectra similar to those of blackbodies. It is found that L1641 contains a much larger percentage of class I sources than does the nearby Taurus-Auriga star-forming complex. Spectral energy distributions for the IRAS-selected sample are examined and compared with SEDs for young stellar objects (YSOs) located in Taurus-Auriga. The IRAS-selected sources having optical counterparts in the H-R diagram are identified and discussed along with the distribution of masses and ages for these YSOs.

Strom, Karen M.↗
NASA NTRS1989

A study of the stellar population in the Lynds 1641 dark cloud - A possible dense cluster associated with IRAS 05338-0624

Sensitive images obtained at 1.6 and 2.2 microns reveal a possible dense cluster of stars associated with IRAS 05338-0624 and located within an optically thick molecular core; the visual extinction suffered by cluster members ranges from 7 to 40 mag. A circular region of diameter 70 arcsec (0.16 pc at a distance of 480 pc) contains 20 stellar objects or semistellar knots. Assuming that the depth of the region is comparable to its projected size leads to an estimated stellar density of 6800/cu pc. The ratio of stellar to molecular material contained within the core must lie between 4 percent and 43 percent. It is likely that this cluster will emerge from its parent core as a bound group.

Strom, Karen M.↗
NASA NTRS1989

Constraints on impact rates in the Pluto-Charon system and the population of the Kuiper comet belt

Impact rates in the Pluto-Charon system are dominated by comets from the proposed Kuiper Belt, 30 to 50 AU from the sun. Such collisions excite the eccentricity of Charon's orbit, which then decays due to tidal dissipation. Charon's eccentricity approaches a quasi-steady state, which can be used to constrain the total number and mass of comets in the Kuiper Belt. Unfortunately, the current upper limit on Charon's orbital eccentricity must be reduced by more than a factor of ten before useful constraints can be set.

Weissman, Paul R.↗
NASA NTRS1990

Dust obscuration by an evolving galaxy population

The effect of an evolving luminosity function (LF) on the ability of foreground galaxies to obscure background sources is discussed, using the Press-Schechter/CDM standard evolving LF model. Galaxies are modeled as simplified versions of local spirals and Poisson statistics are used to estimate the fraction of sky covered by intervening dusty galaxies and the mean optical depths due to these galaxies. The results are compared to those obtained in the case of nonevolving luminosity function in a low-density universe. It is found that evolution of the galaxy LF does not allow the quasar dust obscuration hypothesis to be sustained for dust disks with plausible sizes. Even in a low-density universe, where evolution at z = less than 10 is unimportant, large disk radii are needed to achieve the desired obscuring effect. The mean fraction of sky covered is presented as a function of the redshift z along with adequate diagram illustrations.

Najita, Joan↗
NASA NTRS1990

Multiple-wavelength analysis of energy release during a solar flare - Thermal and nonthermal electron populations

Collaborative solar investigations by Tufts University and the Observatoire de Paris have resulted in simultaneous radio observations with the Very Large Array (VLA) and the Nancay Radioheliograph (NR), comparisons of this radio data with X-ray observations, and theoretical interpretations of the dominant radiation mechanisms during a weak impulsive solar flare observed on May 28, 1988. The VLA has mapped the flaring structures at time intervals of 3.3 s, showing that the preflash and flash-phase components of the impulsive emission originate in spatially separated sources. The 20.7 cm preflash source is ascribed to thermal gyroresonance emission from coronal loops with typical magnetic field strengths of up to 270 G; this emission is associated with heating and exhibits no detectable hard X-ray radiation above 30 keV. The flash-phase 20.7 cm source and the hard X-ray emission are attributed to nonthermal electrons in the coronal and chromospheric portions of a magnetic loop. The combination of imaging observations at 20.7 and 91.6 cm excludes emission from a confined hot plasma during the flash phase.

Willson, Robert F.↗
NASA NTRS1990

Velocity dispersion and the stellar population in the central 1.2 parsecs of the Galaxy

New observations of the 2.3 micron absorption band head of CO in integrated stellar light in the central 1.2 pc of the Galaxy are presented. These data confirm that the velocity dispersion of the stars increases toward the Galactic center and totally dominates their systematic rotation at small radii. The velocity dispersion is constant an average value of 125 km/s within about 30 arcsec projected diameter central region. The strength of the CO features decreases in the integrated faint light within the same region. These observations are all consistent with the hypothesis that both the diffuse 2 micron light and the bright 2 micron point sources in the central region are dominated by sources that have little or no CO absorption feature. The observed CO feature at projected distances less than 0.6 pc thus actually arises in stars that are at true radii greater than 0.6 pc. The mass distribution at radius between 3.7 and 0.6 pc from IRS 16 is strong evidence for an increasing mass/2 micron radiation ratio toward the Galactic center.

Sellgren, K.↗
NASA NTRS1990

The Wolf-Rayet star population in the most massive giant H II regions of M33

Narrow-band images of NGC 604, NGC 595, and NGC 592, the most massive giant H II regions (GHRs) in M33 have been obtained, in order to study their Wolf-Rayet content. These images reveal the presence of nine candidates in NGC 604 (seven WN, two WC), 10 in NGC 595 (nine WN, one WC), and two in NGC 592 (two WN). Precise positions and estimated magnitudes are given for the candidates, half of which have so far been confirmed spectroscopically as genuine W-R stars. The flux in the emission lines of all candidates is comparable to that of normal Galactic W-R stars of similar subtype. A few of the putative superluminous W-R stars are shown to be close visual double or multiple stars; their newly estimated luminosities are now more compatible with those of normal W-R stars. NGC 595 seems to be overabundant in W-R stars for its mass compared to other GHRs, while NGC 604 is normal. Factors influencing the W-R/O number ratio in GHRs are discussed: metallicity and age appear to be the most important.

Drissen, Laurent↗
NASA NTRS1991

Fanaroff-Riley I galaxies as the parent population of BL Lacertae objects. II - Optical constraints

The hypothesis is investigated that BL Lacertae objects are a subset of Fanaroff-Riley (FR) I galaxies dominated by beamed emission from a relativistic jet aligned with the line of sight. It is found that FR I galaxies, although of low radio luminosity, are bright optical galaxies. If the beaming hypothesis is correct, the optical luminosity function of BL Lac objects, including the effect of the host galaxy, can be approximated by a double power law with a given differential index. New estimates of the number counts of BL Lac objects are made which constrain their local number density for L(B) greater than about 6 x 10 to the 43rd ergs/s to be in the range 110-720/cu Gpc. The corresponding ratio between the total number densities of BL Lac objects and FR I galaxies is 0.003 to 0.02. This limits the Lorentz factor of the optical emission to the range 8-20, larger than that of the X-ray emitting plasma and roughly of the same order as the radio Lorentz factor.

Padovani, Paolo↗
NASA NTRS1991

Population studies in groups and clusters of galaxies. IV - Comparison of the luminosity functions and morphological-type distributions in seven nearby groups

Published observational data on the Leo, Dorado, NGC 1400, NGC 5044, Antlia, Fornax, and Virgo groups of galaxies are analyzed in terms of the luminosity functions and morphological types of their members. The data sets employed are characterized, and the results are presented in extensive tables and graphs and discussed in detail. While the fractions of early and late galaxies in the groups are similar, the ratio of dwarfs to giants (D/G) in the early galaxies varies monotonically with the richness of the cluster, leading to artificial flattening at the faint end of the total luminosity function in environments with low D/G. The luminosity function for dwarfs in all environments is found to have a slope of about -1.3.

Ferguson, Henry C.↗
NASA NTRS1992

Nature and evolution of the early Martian atmosphere: Evidence from highland crater populations

Release of water in a CO2 rich atmosphere by precipitation and channel forming processes has led to speculation on the creation of Martian carbonate deposits. On Mars water probably was not on the surface long enough to allow eroded material to concentrate, raise the pH, and induce the formation of carbonates. This suggests that the Martian primordial atmosphere could be thinner (approximately 5 bars) and still allow highland degradation to occur over a long period of time (.45 to 1.2 billion years).

Craddock, Robert A.↗
NASA NTRS1991

Metal opacities and convective core overshooting in Population I stars

New stellar opacities computed by Rogers and Iglesias are found to lead to revised evolutionary models for stars of intermediate-to-high mass that show remarkably close agreement with observed stars, under otherwise standard physical assumptions. Most or all of the earlier discrepancies, except for the problem of predicted apsidal motion constants that are perhaps still too large, appear to have been resolved by using these new opacities with their increased metal line contribution. Thus, the implied interior metallicity of massive stars in the solar neighborhood now becomes 'normal'. Furthermore, the effective distance of convective overshooting beyond the classical convective core boundary is confirmed to be small and probably less than 0.2 times the local pressure scale height.

Stothers, Richard B.↗
NASA NTRS1991

The discrete X-ray source population in M31

Results of an analysis of the complete set of Einstein imaging observations of M31 are reported. One-hundred-eight individual X-ray sources are detected, with (0.2-4.0 keV) luminosities ranging from 5 x 10 exp 36 ergs/s to above 10 exp 38 ergs/s at the distance of M31. Fourteen of these sources are luminous enough to perform a spectral fit of the X-ray data. Fourteen sources are found to vary between Einstein observations, in addition to the two sources already reported by Collura, Reale, and Peres (1990). Contrary to earlier reports, no substantial difference is found between the luminosity distributions of X-ray sources in the bulge and in the disk. Contour maps of the X-ray emission in the central region is presented, and the radial distribution of the surface brightness is compared with the optical profiles. It is inferred that the disk is relatively underluminous in X-rays, relative to the stellar content. The spatial distribution of bulge sources is compared with that of optical novae and with emission-like images of the bulge. These comparisons constrain models of the formation of low-mass X-ray binaries.

Trinchieri, G.↗
NASA NTRS1992

A catalog of co-added IRAS fluxes of Orion population stars

A catalog of co-added IRAS fluxes for the pre-main-sequence objects in the Herbig-Bell catalog (HBC) is presented. This catalog doubles the number of HBC stars with detected IRAS fluxes and provides improved flux values for the previously known sources. Noise level are given for all HBC fields in each band, permitting upper limits to be estimated for all undetected sources.

Weaver, Wm. B.↗
NASA NTRS1992

The effect of electron collisions on rotational populations of cometary water

The e-H2O collisional rate for exciting rotational transitions in cometary water is evaluated for conditions found in Comet Halley during the Giotto spacecraft encounter. In the case of the 0(00)-1(11) rotational transition, the e-H2O collisional rate exceeds that for excitation by neutral-neutral collisions at distances exceeding 3000 km from the cometary nucleus. The estimates are based on theoretical and experimental studies of e-H2O collisions, on ion and electron parameters acquired in situ by instruments on the Giotto and Vega spacecraft, and on results obtained from models of the cometary ionosphere. Thus, the rotational temperature of the water molecule in the intermediate coma may be controlled by collisions with electrons rather than with neutral molecules, and the rotational temperature retrieved from high-resolution IR spectra of water in Comet Halley may reflect electron temperatures rather than neutral gas temperatures in the intermediate coma.

Xie, Xingfa↗
NASA NTRS1992

Two-population model for the sources of gamma-ray bursts

It is shown here that no single class of gamma-ray bursts, either Galactic or cosmological in origin can explain recent observations which suggest that the distribution of gamma-ray burst sources is isotropic about the earth's position but also radially nonuniform. It is argued that the constraints can be reconciled if there are at least two distinct classes of bursts. Two kinds of burst are considered which both originate on Galactic neutron stars but which differ in intrinsic luminosity by a factor of 10 exp 5.

Lingenfelter, R. E.↗
NASA NTRS1992

Stellar evolution in blue populous clusters of the Small Magellanic Cloud and the problems of envelope semiconvection and convective core overshooting

New theoretical evolutionary sequences of models for stars with low metallicities, appropriate to the Small Magellanic Cloud, are derived with both standard Cox-Stewart opacities and the new Rogers-Iglesias opacities. Only those sequences with little or no convective core overshooting are found to be capable of reproducing the two most critical observations: the maximum effective temperature displayed by the hot evolved stars and the difference between the average bolometric magnitudes of the hot and cool evolved stars. An upper limit to the ratio of the mean overshoot distance beyond the classical Schwarzschild core boundary to the local pressure scale height is set at 0.2. It is inferred from the frequency of cool supergiants in NGC 330 that the Ledoux criterion, rather than the Schwarzschild criterion, for convection and semiconvection in the envelopes of massive stars is strongly favored. Residuals from the fitting for NGC 330 suggest the possibility of fast interior rotation in the stars of this cluster. NGC 330 and NGC 458 have ages of about 3 x 10 exp 7 and about 1 x 10 exp 8 yr, respectively.

Stothers, Richard B.↗
NASA NTRS1991

The Rho Ophiuchi cloud young stellar population - Results of a near-infrared study

An imaging survey of 1.4 sq pc of the Rho Ophiuchi dark cloud in the J, H, and K near-infrared photometric bands was conducted. Approximately 332 of the 481 detected sources are associated with the cloud, and 30 percent of these have near-infrared excesses, indicative of disks or circumstellar material surrounding these young stellar objects (YSOs). The K-band luminosity function is significantly different in different regions of the survey area, suggesting that YSOs in these regions have different ages or mass functions. The entire survey area has a high star formation efficiency of about 23 percent. Finally, many newly detected sources provide a relatively large, uniformly sensitive sample of objects for study at longer wavelengths to better determine true source luminosities and evolutionary lifetimes.

Greene, Thomas P.↗