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At least 181 records · Page 10
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NASA NTRS1990

Empirical relationships between gas abundances and UV selective extinction

Several studies of gas-phase abundances in lines of sight through the outer edges of dense clouds are summarized. These lines of sight have 0.4 less than E(B-V) less than 1.1 and have inferred spatial densities of a few hundred cm(-3). The primary thrust of these studies has been to compare gaseous abundances in interstellar clouds that have various types of peculiar selective extinction. To date, the most notable result has been an empirical relationship between the CN/Fe I abundance ratio and the depth of the 2200 A extinction bump. It is not clear at the present time, however, whether these two parameters are linearly correlated or the data are organized into two discrete ensembles. Based on 19 samples and assuming the clouds form discrete ensembles, lines of sight that have a CN/Fe I abundance ratio greater than 0.3 (dex) appear to have a shallow 2.57 plus or minus 0.55 bump compared to 3.60 plus or minus 0.36 for other dense clouds and compared to the 3.6 Seaton (1979) average. The difference in the strength of the extinction bump between these two ensembles is 1.03 plus or minus 0.23. Although a high-resolution IUE survey of dense clouds is far from complete, the few lines of sight with shallow extinction bumps all show preferential depletion of certain elements, while those lines of sight with normal 2200 A bumps do not. Ca II, Cr II, and Mn II appear to exhibit the strongest preferential depletion compared to S II, P II, and Mg II. Fe II and Si II depletions also appear to be enhanced somewhat in the shallow-bump lines of sight. It should be noted that Copernicus data suggest all elements, including the so-called nondepletors, deplete in diffuse clouds (Snow and Jenkins 1980, Joseph 1988). Those lines of sight through dense clouds that have normal 2200 A extinction bumps appear to be extensions of the depletions found in the diffuse interstellar medium. That is, the overall level of depletion is enhanced, but the element-to-element abundances are similar to those in diffuse clouds. In a separate study, the abundances of neutral atoms were studied in a dense cloud having a shallow 2200 A bump and in one with a normal strength bump.

Joseph, Charles L.↗
NASA NTRS1989

The detection of a broad interstellar extinction feature near 1700 A

A statistical examination of 126 extinction curves has revealed the presence of a second broad absorption feature similar in nature to the 2200 A feature. The feature is centered on wavelength 1706 A, has a full half-width of 350 A, and a mean central height of 0.21 magnitudes. The strength of the feature increases with E(B-V) supporting an interstellar origin, and on average it is 18 times weaker than the 2200 A feature.

Carnochan, David J.↗
NASA NTRS1989

The dependence of UV extinction properties on dust environment

UV extinction data, derived from the Savage et al. (1985) ANS extinction catalog, are analyzed. The data include the normalized extinction at 1550 A, the strength of the 2175 A bump, and a crude estimate of the bump width. The results confirm the systematic increase of far-UV extinction with galactic altitude first uncovered by Kiszkurno-Koziej and Lequeux (1987) and verify that this effect is in fact a result of the dust being away from the plane, and not a generalized density dependence. It is also shown that the width of the 2175 A bump is systematically broader in denser regions (defined by large values of E(B-V) per Kpc), implying that a similar galactic altitude effect seen in this parameter may only be a reflection of the lower densities encountered away from the plane. The dependence of bump width upon bump strength is also examined. It is shown that a relationship between these two parameters is expected for certain models of the bump, but none is found. However, two factors which could be complicating a straightforward interpretation of the observations are identified and discussed.

Massa, Derck↗
NASA NTRS1989

Does CO trace H2 at high galactic latitude

A CO survey of 342 Infrared Excess Clouds (IRECs) distributed uniformly across the sky is presented. Following comparison of the integrated CO brightness with the 100 micron infrared brightness B(sub 4) obtained from the IRAS data, evidence was found for a threshold in B(sub 4) of 4-5 MJy sr(exp -1) below which CO does not form. Evidence is also presented that the threshold effect can be seen within an individual cloud, providing evidence for a phase transition between atomic and molecular gas. While the main thrust was to examine the CO content of the IRECs, it was also attempted to detect CO toward a number of UV stars so that CO brightness could be correlated with direct measurements of H2 column density and E(B-V). Of the 26 observed stars CO was detected toward 6. It is consistent with the results obtained using infrared data.

Bazell, David↗
NASA NTRS1990

Observations and models of star formation in the tidal features of interacting galaxies

Multi-color surface photometry (BVri) is presented for the tidal features in a sample of interacting galaxies. Large color variations are found between the morphological components and within the individual components. The blue colors in the primary and the tidal features are most dramatic in B-V, and not in V-i, indicating that star formation instead of metallicity or age dominates the colors. Color variations between components is larger in systems shortly after interaction begins and diminishes to a very low level in systems which are merged. Photometric models for interacting systems are presented which suggest that a weak burst of star formation in the tidal features could cause the observed color distributions. Dynamical models indicate that compression occurs during the development of tidal features causing an increase in the local density by a factor of between 1.5 and 5. Assuming this density increase can be related to the star formation rate by a Schmidt law, the density increases observed in the dynamical models may be responsible for the variations in color seen in some of the interacting systems. Limitations of the dynamical models are also discussed.

Wallin, John F.↗
NASA NTRS1990

Cepheid companions? FM Aquilae, FN Aquilae, RX Aurigae, Y Lacertae, and RS Orionis

IUE spectra have been used to search for companions of FN Aql and RX Aur, two classical Cepheids which have been tentatively identified as binary stars on the basis of cyclic variations in their pulsation periods. Three other Cepheids, FM Aql, Y Lac, and RS Ori, suspected of having blue companions from the (U-B), (B-V) diagram, have also been investigated. There is no indication of a companion of FM Aql, FN Aql, and RX Aur. If these stars have companions, they must be at least as cool as A0 V, Al V, and A1 V, respectively.

Evans, Nancy Remage↗
NASA NTRS1991

Stellar Lyman-alpha emission and the local interstellar medium

The UV spectra now available in the IUE archives have been systematically searched for the presence of Lyman-alpha emission in late-type stars. The results provide the first survey of the behavior of the Ly-alpha emission line across most of the cool half of the H-R diagram. The normalized Ly-alpha emission flux is strongest in the chromospherically active RS CVn stars and in the dMe stars. The range of normalized flux values is much smaller among F-type stars than in stars of later spectral type. A dropoff appears in the flux levels of stars at B-V less than 0.30 mag. The measurements are used to search for evidence of possible high column density clouds in the local ISM. The cloud previously identified toward Alpha Oph may be seen in the reduced flux observed toward Beta Oph.

Landsman, Wayne↗
NASA NTRS1991

Ultraviolet and optical extinction for the Herbig-Haro objects HH1 and HH2

Based on a comparison of reddening-corrected line ratios with predictions from bow-shock models, an attempt is made to determine which type of reddening curve is the most appropriate for HH1 and HH2. Specifically, the extinction curves for R = 3.1 and R = 5.4 are considered. In order to discriminate between these two curves, the flux ratios I(C IV 1550)/I(semiforbidden C III 1909) and I(C IV 1550)/I(H-beta) are used together with information on E(B-V). The suprising result is that the R = 3.1 curve leads to a better agreement between predicted and observed line flux ratios than the R = 5.4 curve.

Bohm, K. H.↗
NASA NTRS1991

DD 13 - A very young and heavily reddened early O star in the Large Magellanic Cloud

This paper investigates the Large Magellanic Cloud star DD 13, which is likely the major ionizing source of the nebula N159A. New optical spectroscopy and new estimates of the broadband photometric properties of DD 13 are obtained. A spectral type of O3-O6 V, E(B-V) = 0.64, and M(V) = -6.93 is found. The spectral type cannot be more precisely defined due to contamination of the spectral data by nebular emission, obliterating the important He I classification lines. These results, plus a published estimate of the Lyman continuum photon injection rate into N159A, suggest that DD 13 actually consists of about 2-4 young, early O stars still enshrouded by their natal dust cloud. The star DD 13 may be a younger example of the type of tight cluster represented by the LMC 'star' Sk-66 deg 41, recently revealed to be composed of six or more components.

Conti, Peter S.↗
NASA NTRS1991

IUE spectra of the eclipsing binary NN Serpentis

Low-resolution SWP and LWP IUE spectra are used to fit the temperature and angular radius of the white dwarf in the detached eclipsing binary NN Ser. It is found that the redenning to the system has E(B-V) of 0.05 +/-0.05, the white dwarf temperature is 60,000 +/-10,000 K, and the age of the white dwarf is less than 10 exp 7. The shape of eclipse and the K-magnitude of the secondary star are used to constrain the inclination of the binary and the masses and radii of the two stars. The size of the secondary star relative to its Roche lobe and the age of the white dwarf indicate that mass transfer has not yet occurred and that the system is a precataclysmic variable rather than a cataclysmic variable which has entered the period gap. Fitting the observed magnitude of the sinusoidal modulation with a reprocessing model shows that only when i is approximately equal to 90 deg is the required temperature of the secondary star consistent with these results. For this solution the white dwarf temperature is also consistent with those obtained from the IUE spectra.

Wood, Janet H.↗
NASA NTRS1992

Systematic differences between the field and cluster elliptical galaxies

Multivariate statistical techniques and fundamental plane fits are used here to study possible systematic differences between field ellipticals (FEs) and cluster ellipticals (CEs). The FEs show more intrinsic scatter in their properties, especially when stellar population variables are included. Pairwise correlations for the two samples are different; the correlations are systematically better for the cluster sample, meaning that ellipticals in the two samples populate their fundamental planes in different ways. Bivariate correlations are different for the two samples, implying that they have different fundamental planes. This is especially true for the correlations which include the population variables Mg2 and (B-V), which are sensitive both to the enrichment history and the storm formation history.

De Carvalho, R. R.↗
NASA NTRS1992

Carbon and nitrogen abundances determined from transition layer lines

The possibility of determining relative carbon, nitrogen, and silicon abundances from the emission-line fluxes in the lower transition layers between stellar chromospheres and coronae is explored. Observations for main-sequence and luminosity class IV stars with presumably solar element abundances show that for the lower transition layers Em = BT sup -gamma. For a given carbon abundance the constants gamma and B in this relation can be determined from the C II and C IV emission-line fluxes. From the N V and S IV lines, the abundances of these elements relative to carbon can be determined from their surface emission-line fluxes. Ratios of N/C abundances determined in this way for some giants and supergiants agree within the limits of errors with those determined from molecular bands. For giants, an increase in the ratio of N/C at B-V of about 0.8 is found, as expected theoretically.

Boehm-Vitense, Erika↗
NASA NTRS1992

Ly-alpha emission from cooling flows and measures of the dust content of rich clusters of galaxies

Ly-alpha and UV continuum fluxes from measurements across the emission-line regions of 10 'cooling flow' clusters of galaxies are tabulated. Comparison of Ly-alpha emission fluxes obtained with the IUE satellite with optical Balmer emission fluxes is used to demonstrate that the lines of sight to the centers of these clusters show an average excess reddening E(B-V) about 0.19 above foreground extinction. While some fraction of this may derive from localized extinction in the central (cooling flow) region of the cluster, a substantial portion of this effect should arise from an extended cluster component. The case where this extinction originates in the hot intracluster gas of the cluster halo is considered. Estimates for the corresponding 100-micron infrared emission, assuming grain properties similar to those of interstellar grains, place the expected emission at the about 1/2 Jy level for nearby clusters. Measurements from IRAS pointed mode observations of clusters are consistent with the expected values. These results are used to show that ISO and SIRTF may detect clusters to cosmological distances.

Hu, Esther M.↗
NASA NTRS1992

A prediction of the gamma-ray flux from Nova Herculis 1991

UV observations made with the IUE satellite of Nova Her 1991 are presented and discussed. A distance of 3.4 kpc and reddening of 0.6 E(B-V) to Nova Her 1991 are determined by comparing its UV and optical outburst behavior with those of the extragalactic ONeMg nova LMC 1990 No. 1. Her 1991 was also detected by Rosat five days into its outburst, and it is argued that a small fraction of the X-rays may be the product of Comptonized 1.2 and 0.5 MeV emission from the decay of Na-22. If the mass of ejected shell is indeed as high as was estimated by Woodward et al. (1992), namely 0.0001 solar mass, then the present results predicted that Na-22 gamma-ray emission from the ejecta of Nova Her 1991 would still be detectable by GRO in the spring of 1992. The detection level was expected to be about 5 sigma in a two-week observation. Such a detection would strongly guide and constrain the hydrodynamic and nucleosynthesis simulations of nova outbursts on massive white dwarfs.

Starrfield, Sumner↗
NASA NTRS1992

Halo common proper motion stars - Subdwarf distance scale, halo binary fraction, and UBVRI colors

Common proper motion stars identified by Luyten (1979, 1980) were selected according to reduced proper motion and photometric criteria to produce a list of candidate halo objects. UBVRI photometry was obtained to identify genuine halo parts. About 20 percent of the stars with (Fe/H) less than -1.0 were found to have random errors not less than several x 0.1 mag in distance modulus, in addition to the photometric errors, thought to be due to the existence of close binary companions to some of the common proper motion stars. The implications of these errors for kinematic studies of field Population II stars are considered. Systematic errors of order 16 percent were also detected in distances derived from the U-B,B-V deblanketing technique. It is suggested that Population II field stars did not originate in globular clusters which subsequently disintegrated but formed in less dense environments. Several K subdwarfs were found with very high UV excesses, comparable with those predicted by some synthetic colors.

Ryan, Sean G.↗
NASA NTRS1993

Nuclear activity and the environments of nearby radio galaxies

Much of our present understanding of galaxy evolution over a large redshift range is based on the study of samples selected on the basis of non-thermal radio emission. It is therefore necessary to understand the relationship between radio source activity and the host galaxy. Recent observations suggest that there is a connection between radio galaxy (RG) activity and radio galaxy evolution. For example, high-redshift RGs (z approx. greater than 0.7) show evidence for significant populations of young stars, and have optical continuum morphologies nearly always aligned with the radio axis (McCarthy et al. 1987; Chambers et al. 1987). This phenomenon is generally attributed to radio jet induced star formation (DeYoung 1989), but the lack of high S/N spectra of the galaxy continua, and recent detections of polarized light in a few objects make it hard to rule out other processes such as scattering or synchrotron radiation. A detailed study of the continuum light in the distant RGs is difficult as they are optically very faint. However, nearby RGs (z approx. less than 0.1) have bluer B-V colors than radio-quiet ellipticals, presumably due to the presence of young stellar populations (Smith and Heckman 1989) and several have extended UV continuum emitting regions along their radio axes (van Bruegel et al. 1985a, b, di Serego Alighieri et al. 1989), reminiscent of the alignment effect seen in the high redshift RGs. We have almost completed a continuum imaging survey of nearby (and therefore optically brighter), powerful RGs to study any possible relationships between the optical continuum light and radio source activity. In particular we are interested in (1) whether these lower redshift RGs shown any evidence of the alignment effect (in their rest-frame UV light) that is seen in the distant RGs, and (2) the effects that the radio source has on the environment of the host galaxy.

Dey, Arjun↗
NASA NTRS1992

Large infrared and optical color gradients in the Cartwheel ring galaxy - Evidence for the first epoch of star formation in the wake of an expanding ring

Substantial near-infrared and optical color gradients have been observed within the disk of the classical ring galaxy A0035-324 (the 'Cartwheel'). The slope of the radial B-V, V-K color gradient coincides with starburst color evolution predictions, indicating that the age of the disk population is a function of radius. Observations indicate that the youngest stars are on the leading edge of the outer ring, and the stellar population grows increasingly older toward the nucleus. The radial color gradient, taken together with other evidence, strongly support the collisional picture for the formation of ring galaxies. Strong IR emission observed in the outer ring appears to be directly associated with the young star-forming regions rather than an evolved stellar population. We conclude that star formation is occurring in a mainly gaseous expanding density wave and that the Cartwheel is undergoing its first epoch of star formation since its formation.

Marcum, P. M.↗
NASA NTRS1993

Spectrophotometry of Wolf-Rayet stars. I - Continuum energy distributions

All available low-resolution IUE spectra are assembled for Galactic, LMC, and SMC W-R stars and are merged with ground-based optical and NIR spectra in order to collate in a systematic fashion the shapes of these energy distributions over the wavelength range 0.1-1 micron. They can be consistently fitted by a power law of the form F(lambda) is approximately equal to lambda exp -alpha over the range 1500-9000 A to derive color excesses E(B-V) and spectral indices by removing the 2175-A interstellar absorption feature. The WN star color excesses derived are found to be in good agreement with those of Schmutz and Vacca (1991) and Koesterke et al. (1991). Significant heterogeneity in spectral index values was generally seen with any given subtype, but the groups consisting of the combined set of Galactic and LMC W-R stars, the separate WN and WC sequences, and the Galactic and LMC W-R stars all showed a striking and consistent Gaussian-like frequency distribution of values.

Morris, Patrick W.↗