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At least 1,207 records · Page 67
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NASA NTRS1987

CO observations of nearby galaxies and the efficiency of star formation

The CO distributions and total molecular content of 160 galaxies were observed using the 14 meter millimeter telescope of the FCRAO. For the luminous, relatively face-on Sc galaxies, the azimuthally averaged CO distributions are centrally peaked, while for the Sb and Sa galaxies the Co distributions often exhibit central CO holes up to 5 kpc across. None of the Sc galaxies have CO distributions which resemble the Milky Way. A general correlation was found between total CO and IR luminosities in galaxies. The scatter in this relation is highly correlated with dust temperature. No strong correlation of IR luminosities was found with HI masses, and it was thereby concluded that the infrared emission is more directly tied to the molecular content of galaxies. It is suggested that galaxies which have high Star Formation Effiencies (SFEs) produce more stars per unit molecular mass, thereby increasing the average temperature of the dust in the star forming regions. Irregular galaxies and galaxies previously identified as mergers have the highest observed star formation efficiencies. For the mergers, evidence was found that the IR/CO luminosity ratio increases with the merger age estimated by Joseph and Wright (1985).

Young, Judith S.↗
NASA NTRS1987

Induced star formation in interacting galaxies

Measurements of H alpha emission line fluxes and FIR fluxes in approx. 100 interacting spirals were used to investigate the effects of close tidal interactions on the disk and nuclear star formation rates in galaxies. Two samples of interacting spirals were studied, a complete sample of close pairs, and a set of strongly perturbed systems from the Arp atlas. Both the integrated H alpha luminosities and FIR luminosities are enhanced in the interacting galaxies, indicating that the encounters indeed trigger massive star formation in many cases. The response of individual galaxies is highly variable, however. A majority of the interacting spirals exhibit normal star formation rates, while a small fraction are undergoing bursts with luminosities which are rarely, if ever, observed in noninteracting systems. Virtually all of the latter are in the Arp sample, indicating that the Arp atlas is heavily biased to the most active star forming systems.

Kennicutt, R. C.↗
NASA NTRS1986

Optical spectroscopy of HH-exciting stars from scattered light continua

Optical spectra of the reflected light continua visible in parts of several Herbig-Haro objects are presented. HH 100 unmistakably scatters the chromospheric spectrum of a strong emission-line T Tau star. HH 48S also reflects a T Tau stellar spectrum, as perhaps do HH 24 and HH 55, but less convincingly. HH 55 reveals photospheric absorption features too, corresponding to an M3.5 T Tau star. The continuum in HH 120 (= CG 30 HH) is unclassifiable while that in HH 46A has dimmed considerably (by a factor of order 20) although, only 7 yr ago, it clearly reflected a strong-line T Tau spectrum. It is concluded that at least some of the stars that excite Herbig-Haro nebulae, even when those stars are not directly visible, pass through a strong-line T Tau phase, and can undergo abrupt and dramatic changes in visual luminosity.

Cohen, M.↗
NASA NTRS1986

On star formation in stellar systems. I - Photoionization effects in protoglobular clusters

The progressive ionization and subsequent dynamical evolution of nonhomogeneously distributed low-metal-abundance diffuse gas after star formation in globular clusters are investigated analytically, taking the gravitational acceleration due to the stars into account. The basic equations are derived; the underlying assumptions, input parameters, and solution methods are explained; and numerical results for three standard cases (ionization during star formation, ionization during expansion, and evolution resulting in a stable H II region at its equilibrium Stromgren radius) are presented in graphs and characterized in detail. The time scale of residual-gas loss in typical clusters is found to be about the same as the lifetime of a massive star on the main sequence.

Tenorio-Tagle, G.↗
NASA NTRS1986

CNO abundances of some field horizontal-branch and Population I A stars

From observations of infrared lines, oxygen and nitrogen abundances of the prototype field horizontal-branch stars HD 109995 and HD 161817, the somewhat similar star HD 64488, and the sharp-lined Population I A stars Theta Leo and HR 6559 have been obtained, as well as carbon abundances of HD 161817 and Theta Leo. The results for HD 161817 may be in accord with the predictions of Sweigart and Mengel (1979) for dredge-up during the previous red-giant stage if the non-LTE effects for the O I lines have been properly accounted for, and those for the C I and N I lines are small. This star and HD 109995 are apparently both oxygen and nitrogen poor to the same degree although their iron abundances differ by 0.3 dex.

Adelman, S. J.↗
NASA NTRS1986

UBV photometry of ten southern hemisphere active-chromosphere stars

High-precision UBV photometry of ten southern hemisphere active-chromosphere stars with strong Ca II H and K and/or H-alpha emission has been obtained. Eight of these stars showed variability during June 1985. Complete or partial light curves are presented for the stars, and these data, as well as mean V magnitudes and colors, are compared with the results of other investigators. In a number of cases, significant changes in photometric amplitude are found, which may serve to track the formation and evolution of active regions on these stars.

Bopp, Bernard W.↗
NASA NTRS1986

Star counts and galactic structure

Star counts are discussed as an aid in studies of galactic structure. Iterative computations using a method-of-color-bins to match numerical models with observational data are described. The star counts provide bounds for the luminosity function and the color-magnitude diagram and allow spheroid normalization in a copycat Galaxy model which posits a galaxy with a Population I disk and a Population II spheroid. Good agreement has been obtained between star counts predicted by the model and observed distributions, including color and magnitude ranges and direction. Modifications being considered for the model include consideration of the halo, a galactic bulge and a thick disk. Several galactic structural features which have yet to be modeled and which may be tractable to the copycat model corrected with star counts are identified.

Bahcall, John N.↗
NASA NTRS1987

Ground-based observations of IRAS candidates for late asymptotic giant branch stars

Ground-based IR photometric observations were obtained for 16 of the IRAS Circular No. 9 sources which have been suggested to be candidates of OH/IR stars. Five of these sources are confirmed to be OH/IR stars and three others as carbon stars. Two are young objects in an early stage of star formation and one, possibly two, are planetary nebulae. Two sources of very low color temperatures (about 180 K) are also found, and may represent objects in transition between the asymptotic giant branch and planetary nebula phases.

Kwok, Sun↗
NASA NTRS1986

High resolution spectroscopy of six new extreme helium stars

High resolution spectra of six newly discovered extreme helium stars are presented. LSS 5121 is shown to be a spectroscopical twin of the hot extreme helium star HD 160641. A preliminary LTE analysis of LSS 3184 yielded an effective temperature of 22,000 K and a surface gravity of log g = 3.2. Four stars form a new subgroup, classified by sharp-lined He I spectra and pronounced O II spectra, and it is conjectured that these lie close to the Eddington limit. The whole group of extreme helium stars apparently is inhomogeneous with respect to luminosity to mass ratio and chemical composition.

Heber, U.↗
NASA NTRS1987

Polarization of light scattered from the winds of early-type stars

There is evidence from visible wavelength polarimetry that the matter in the winds of early-type stars is not distributed uniformly about the stars. In this paper equations for the observable polarization from an axisymmetric distribution of electrons are derived first from the basic point source model of Brown and McLean (1977) and then from extended atmosphere radiative transfer theory. A factor is derived which corrects the Brown and McLean model for the finite size of stars. This finite disk factor is found to reduce the magnitude of the predicted polarization to less than half of the point source prediction. The effects of having the mass outflow concentrated in a plume are considered. The effects of absorption in an unocculted plume are shown to give rise to a wavelength-dependent polarization that is described using a 'polarization color' parameter. A continuum opacity index is defined that allows for a straightforward interpretation of the polarization of stars of differing effective temperatures.

Cassinelli, J. P.↗
NASA NTRS1987

The distribution of maser emission in OH/IR stars

The 1612 MHz OH emission from five OH/IR stars has been mapped at three epochs over a 2.5 yr period of time. Although the stars were observed at very different phases in the radio light curve of each, there were no remarkable changes in the appearance of the maps. This probably implies that the properties of the masers do not range widely throughout a maser shell. The maps have been used to produce star-centered surface maps of the distribution of maser emission from each star. The surface maps generally are sparsely filled with OH emission and are dominated by relatively few (about 10) major clumps of emission. The presence of large regions of low intensity in the surface maps suggests that the number of individual emitting elements is relatively small or that there are a larger number of elements which are distributed in the shell in a distinctly nonrandom manner.

Welty, Alan D.↗
NASA NTRS1987

New CO and HCN sources associated with IRAS carbon stars

Emission of CO and HCN was detected in 22 out of a sample of 53 IRAS sources classified as unidentified carbon-rich objects. The sample was selected according to the presence of the silicon carbide feature as revealed by low-resolution spectra. The molecular line widths indicate that the CO and HCN emission arises from the circumstellar envelopes of very highly evolved stars undergoing mass loss. The visible stars tend to be deficient in CO as compared with unidentified sources. Most the detected CO and HCN IRAS stars are distinct and thick-shelled objects, but their infrared and CO luminosities are similar to those of IRC + 102156 AFGL and IRC-CO evolved stars. The 12 micron flux seems to be a good indicator of the distance, hence a guide for molecular searches.

NGUYEN-Q-RIEU↗
NASA NTRS1987

Rotational modulation and flares on RS CVn and BY Dra stars. VI - Physical parameters of the chromospheres/transition regions of V 711 Tau (HR 1099), II Peg and AR Lac during October 1981

Ground-based optical and IUE satellite-ultraviolet observations of three RS CVn stars are combined with density sensitive line ratios and differential emission measure curves to describe the physical conditions in their outer atmospheres. Solar-like densities are found to be representative of average conditions on two of the stars, V 711 Tau and AR Lac. The total radiative losses from these two stars are estimated and found to be larger than the sun by at least two orders of magnitude. Consideration of the volume emitting in two of the principal transition region lines suggests a possible relation between the disk 'filling factor' for these two lines and the dynamo-related Rossby number. Only one hemisphere of the star II Peg, the one showing least evidence of starspots, is similar. On the opposite hemisphere the presence of a discrete emitting region is deduced which is almost coincident in phase with the passage of the dominant optical spot group across the visible disk. The dimensions of this region, and an illustrative interpretation in terms of a large emitting magnetic loop, are discussed. It is compared to large active region loops on the sun.

Byrne, P. B.↗
NASA NTRS1988

Apparent brightness of stars and lasers

Foremost among issues affecting the potential use of astrometric techniques to locate and track laser-carrying spacecraft is the apparent brightness of a laser relative to reference stars. Broadband detectors offer improved sensitivity to stars, but not to lasers. The important and fundamental step of calibrating laser and star brightnesses according to detector spectral sensitivity is performed for four representative kinds of broadband detectors, located above and below the atmosphere. Stars are identified according to parameters traditionally used by astronomers: temperature and apparent brightness at visible wavelengths. These are translated into an energy and photon flux for each kind of broadband detector and are then compared with the corresponding flux from a laser. The comparisons are also given as magnitude correction factors. Astrometrists typically characterize the sensitivity of their instruments in terms of the precision with which they can make a relative measurement of position and the minimum brightness needed to achieve that precision. Given this information and the instruments's spectral sensitivity, the calculations described can be used to infer the detectability of a laser and the precision with which it could be located and tracked. Since the limiting brightness quoted for a particular instrument is typically firm to within 4.0 dB, judging a laser's detectability by its visible brightness alone could lead to serious underestimation of the requirements on its effective radiated power. The laser brightness corrections given solve this problem.

Schumaker, B. L.↗
NASA NTRS1987

Physical parameters for 12 planetary nebulae and their central stars in the Magellanic Clouds

Nebular and central star parameters and elemental abundances of C, N, O, Ne, S, and Ar are presented for the planetary nebulae N2, N5, N43, N54, and N67 in the SMC and P2, P7, P9, P25, P33, and P40 in the LMC. The nebular chemical compositions are affected by nuclear processes in the precursor stars, which may not have been sufficiently massive to synthesize Ne, S, or Ar, which appear to be deficient with respect to their solar abundances by factors of roughly four and five for the LMC and SMC, respectively. Even after excluding nebulae formed by stars in which O apparently was destroyed by nuclear processes, O depletion in the LMC and SMC nebulae is significantly greater than in galactic planetaries. The estimated masses of the 12 remnant central stars range from 0.58 to 0.71 solar mass.

Aller, Lawrence H.↗
NASA NTRS1987

Detection of X-ray emission from the young low-mass star Rossiter 137B

Rst 137B, a close M-dwarf companion to the active K-star HD 36705, has been detected in a High Resolution Image in the Einstein Observatory Archive. The X-ray surface fluxes (0.2-4 keV) from both stars are close to the empirical saturation level, F(x)/F(bol) of about 0.001, defined by rapid rotators and very young stars. This supports the earlier results of the youthfulness of the system. This young couple is an excellent subject for studies of dependence of early evolution on stellar mass. Rst 137B is one of the latest spectral types and thus lowest-mass premain-sequence stars yet detected as an X-ray source.

Vilhu, O.↗
NASA NTRS1988

LkH-alpha 101 - The stellar wind, the surrounding nebula, and an associated radio star cluster

Radio observations of LkH-alpha 101 have been taken to determine the characteristics of the stellar wind from the central star as well as to image the surrounding nebulosity. They also revealed the presence of a high concentration of weak compact radio sources in the neighborhood of LkH-alpha 101, four of which have optical stellar counterparts. Spectra of three of the stars indicate two T Tauri stars and a highly obscured B star.

Becker, Robert H.↗
NASA NTRS1987

Spectral differential imaging detection of planets about nearby stars

Direct ground-based optical imaging of planets in orbit about nearby stars may be accomplished by spectral differential imaging using multiple passband acoustooptic filters with a CCD. This technique provides two essential results. First, it provides a means to modulate the stellar flux reflected from a planet while leaving the flux from the star and other sources in the same field of view unmodulated. Second, spectral differential imaging enables the CCD detector to achieve a sufficiently high dynamic range to locate planets near a star in spite of an integrated brightness differential of 5 x 10 to the 8th. Spectral differential imaging at nearby diffraction limited imaging conditions with telescope apodization can reduce the time to conduct a sensitive planetary search to a few hours in some cases. The feasibility of this idea is discussed here and shown to provide, in principle, the discrimination and sensitivity to detect a Jovian-class planet about stars at distances of about 10 parsecs. The detection of brown dwarfs is shown to be feasible as well.

Smith, W. Hayden↗