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At least 487 records · Page 27
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NASA NTRS1992

The mass of spiral galaxy halos

A discussion is presented of previous and current work on the determination of the mass distribution of spiral galaxy halos. The two most common tools utilized to determine the mass of spiral galaxies, i.e., companion galaxies and rotation curves are discussed. The most recent research of companion galaxies, which probes the potential to larger distances and utilizes more accurate dynamic modeling, demonstrates that isolated late-type galaxies do have very large dark-matter halos.

Zaritsky, Dennis↗
NASA NTRS1991

UV absorption and emission lines from highly ionized gas in the Galactic halo

The detection of highly ionized gas in the Galactic halo through UV absorption and emission lines is reported. In absorption the species under consideration include Si IV, C IV, and N V. The presence of N V absorption implies the existence of some gas with a temperature near 200,000 K. The highly ionized absorbing gas toward distant stars in direction b of less than -50 deg has simple and relatively narrow line profiles and small average LSR velocities while the gas in the direction b of greater than 50 deg reveals a complex pattern of motions with substantial inflow and outflow velocities. C IV emission brightens toward the Galactic poles and has a polar intensity of 5000 photons/sq cm s ster. In the nonequilibrium cooling of a Galactic fountain, a flow rate of 4 M sub O/yr to each side of the Galaxy is required to produce the amount of N V absorption found in the halo, while a flow rate five times larger is required to produce the observed level of C IV emission.

Savage, Blair D.↗
NASA NTRS1992

The abundance of boron in three halo stars

B abundances for three halo stars: HD 140283, HD 19445, and HD 201891 are presented. Using recent determinations of the Be abundance in HD 140283, B/Be of 10 +5/-4 is found for this star, and similar ratios are inferred for HD 19445 and HD 201891. This ratio is equal to the minimum value of 10 expected from a synthesis of B and Be by high-energy cosmic-ray spallation reactions in the interstellar medium. It is shown that the accompanying synthesis of Li by alpha on alpha fusion reactions is probably a minor contributor to the observed 'primordial' Li of halo stars. The observed constant ratios of B/O and Be/O are expected if the principal channel of synthesis involves cosmic-ray CNO nuclei from the supernovae colliding with interstellar protons.

Duncan, Douglas K.↗
NASA NTRS1993

Calibration, testing and orbital performance of the halogen occultation experiment (HALOE) on the upper atmosphere research satellite

HALOE is an occultation viewing radiometer that requires part per million signal precision and characterization. Gas correlation radiometry is used to infer HF HCl, CH4 and NO concentrations, while broad band radiometry is used for temperature, pressure, O3, H2O and NO2. Validated results verify that goals have been met or exceeded in all channels. This was achieved through a comprehensive instrument analysis and calibration process on the ground and in orbit. Components and systems were put through a battery of tests in the lab. In orbit, a variety of exoatmospheric solar disk measurements are used for precise real time inferral of critical calibration parameters. In the end, technical success was based on the close interaction of scientist, engineer and data analysts. The accuracy and precision of HALOE species profiles are equal to any ever achieved from orbiting sensors.

Gordley, Larry L.↗
NASA NTRS1993

Kinematics of metal-poor giants in an inner-halo field, with implications for disk formation

A sample of approximately 100 predominantly metal-weak giants, identified in a high-latitude field towards the galactic center using an automated objective-prism survey technique, is presented. Abundances and radial velocities have been measured for these giants, whose distances from the Sun range from 1 to 18 kpc. While the extremely metal-weak stars in the field have halo kinematics, the majority of the stars with intermediate abundance have thick disk kinematics, despite the fact that their average distance from the galactic plane is 3 kpc. The most satisfactory explanation for this effect is that the inner halo is moderately flattened, and the metal-weak stars of the thick disk have a scale height of about 2 kpc. It is suggested that the thick disk may have formed in a dissipational collapse, rather than in a separate event such as the accretion of a small satellite galaxy.

Morrison, Heather L.↗
NASA NTRS1993

The boron-to-beryllium ratio in halo stars - A signature of cosmic-ray nucleosynthesis in the early Galaxy

We discuss Galactic cosmic-ray (GCR) spallation production of Li, Be, and B in the early Galaxy with particular attention to the uncertainties in the predictions of this model. The observed correlation between the Be abundance and the metallicity in metal-poor Population II stars requires that Be was synthesized in the early Galaxy. We show that the observations and such Population II GCR synthesis of Be are quantitatively consistent with the big bang nucleosynthesis production of Li-7. We find that there is a nearly model independent lower bound to B/Be of about 7 for GCR synthesis. Recent measurements of B/Be about 10 in HD 140283 are in excellent agreement with the predictions of Population II GCR nucleosynthesis. Measurements of the boron abundance in additional metal-poor halo stars is a key diagnostic of the GCR spallation mechanism. We also show that Population II GCR synthesis can produce amounts of Li-6 which may be observed in the hottest halo stars.

Walker, T. P.↗
NASA NTRS1993

Evidence for a dispersion in the lithium abundances of extreme halo stars

Evidence is presented to the effect that there exists a small dispersion in the lithium abundances of extreme halo dwarfs. This dispersion cannot be accounted for by standard stellar models alone, particularly toward the turnoff, and would thus require early differential Galactic Li enrichment, perhaps independent of metallicity. The magnitude of the dispersion is also consistent with the predictions of evolutionary models of halo stars with rotation, which do not require, but do not rule out either, early Galactic enrichment. These rotational models also predict a significant depletion in the lithium abundance during the stars' lifetime. The rotational models predict that stars which formed with very low initial angular momentum will have lithium abundances measurably above the plateau.

Deliyannis, Constantine P.↗
NASA NTRS1993

The x ray halo of AM Her

The objective of this research was to study the halo surrounding the ROSAT image of the cataclysmic variable AM Her that is formed by scattering of x-rays by interstellar dust grains. AM Her was in a low state of x-ray emission during the 14,400 sec observation and thus an insufficient number of counts were obtained to detect the x-ray halo.

Catura, Richard C.↗
NASA NTRS1994

The soft x ray halo of the spiral galaxy NGC4631

ROSAT PSPC observations of the close to edge-on spiral galaxy NGC4631 are presented. This vigorously star forming galaxy shows extented x ray emission perpendicular to the plane, out to about 6 to 8 kpc. The spatial extent is largest at soft x ray energies. The total x ray luminosity of hot gas can be easily supplied by star formation in the disk, and it is likely that the halo is due to outflow of hot gas from the inner disk. Spectral analysis of the x ray data shows that part of the halo emission may be quite cool, well below 10(exp 6)K. Implications of these results are briefly discussed.

Walterbos, Rene A. M.↗
NASA NTRS1994

Do gamma-ray bursts originate from an extended Galactic Halo of high-velocity neutron stars?

The gamma-ray burst brightness distribution is inhomogeneous and the distribution on the sky is nearly isotropic. These features argue against an association of gamma-ray bursts with those Galactic objects that are known to exhibit a strong concentration toward the Galactic center or plane. The observed statistical properties indicate a cosmological origin. Circumstantial evidence suggests that neutron stars are involved in the burst phenomenon. Here we consider Population II neutron stars in an extended Galactic Halo (EGH) as an alternative to cosmological scenarios. The Burst and Transient Source Experiment (BATSE) data indicate a small deviation from isotropy near the 2 sigma level of statistical significance. If confirmed for an increasing number of bursts, these anisotropies could rule out cosmological scenarios. On the other hand, EGH models require small anisotropies like those observed by BATSE. We consider simple distribution models to determine the generic properties such halos must have to be consistent with the observations and discuss the implications of the corresponding distance scale on burst models.

Hartmann, Dieter H.↗
NASA NTRS1994

Stellar oxygen abundances. 3: The oxygen abundance of the very metal poor halo star BD -13 deg 3442

A spectrum of the very metal poor ((Fe/H) approximately -3) halo star BD -13 deg 3442 is presented and used to determine this star's oxygen abundance. Our determination makes BD -13 deg 3442 the most metal poor dwarf (though a somewhat evolved one) with an O abundance determination. The O abundance (determined from the 7774 A O I triped) and (O/Fe) ratio is compared to that of two other metal-poor stars. The (O/Fe) ratio of BD -13 deg 3442 is found to be approximately 0.35 dex larger than that of the other two halo stars. Possible implications of this result are discussed.

King, Jeremy R.↗
NASA NTRS1994

Highly ionized gas in the Galactic halo

We reexamine the values of electron density n(sub e) and gas pressure P/k in the interstellar medium (ISM) of the Galactic halo, as inferred from C IV emission and absorption lines and using current C IV atomic data. In a homogeneous model with 4.7 less than or equal to log T less than or equal to 5.3, the data are consistent with 0.01 less than or equal to n(sub e) less than or equal to 0.02/cu cm and 2200 less than or equal to P/k less than or equal to 3700/cu cm K, a factor of 2-3 higher than advocated by Martin & Bowyer (1990) and comparable to the thermal pressure in the disk. If some of the C IV absorption arises from nonemitting, photoionized gas, then the inferred density and pressure will increase accordingly. The volume filling factor for homogeneous models ranges from 0.5% to 5%. Because of the constraints arising from filling factor and radiated power, most of the C IV must arise from gas near the peak of the cooling curve, at log t less than or equal to 5.6. We relate both emission-line and absorption-line observations to recent models in which turbulent mixing layers and isobarically cooling supernova remnants (SNRs) provide significant amounts of halo gas at approximately 10(exp 5.3) K and process 20-40 solar mass/yr with a power of approximately 10(exp 41) ergs/sec. Since the observed C IV and N V absorption scale heights have been reported to differ, at 4.9 kpc and 1.6 kpc, respectively, we examine inhomogeneous models with different exponential scale heights of T, P, and SN energy input. The ISM may change its character with distance above the Galactic plane, as superbubbles and mixing layers dominate over isolated SNRs as the source of the C IV. For appropiate scale heights, the midplane pressure is twice the homogeneous values quoted above. The O IV lambda 1034 diffuse emission line, which can be used as a temperature diagnostic of the hot gas, is predicted to be comparable in strength to that of C IV lambda 1549 (approximately 6000 photons/cu cm/s/sr. The ions C IV, N V, and O VI are predicted to show a decreasing trend in vertical extent, with the O IV scale height perhaps as low as approximately 1 kpc.

Shull, J. Michael↗
NASA NTRS1995

Gas phase abundances and conditions along the sight line to the low-halo, inner galaxy star HD 167756

We present high-resolution (3.5 km/s) Goddard High Resolution Spectrograph (GHRS) measurements of the Mg II, Si II, Cr II, Fe II, and Zn II lines toward HD 167756, a low-latitude halo star at a distance of 4 kpc in the direction l = 351.5 deg, b = -12.3 and at a Galactic altitude of z = -0.85 kpc. Supplemental Na I, Ca II, and H I data are also presented for comparison with the UV lines. Our analysis centers on converting the observed absoprtion-line data into measures of the apparent column density per unit velocity. N(sub a)(v), over the velocity range -25 less than or = v(sub lsr) less than 30 km/s for each species observed. We use these N(sub a)(v) profiles to construct logarithmic abundance ratios of Mg II, Si II, Cr II, Fe II, and Ca II relative to Zn II, normalized to solar abundances, as a function of velocity. Compared to Zn, these species show an underabundance relative to their solar values, with the largest underabundances occurring in the v(sub lsr) approximately equals 5 km/s component(s), for which we find logarithmic abundances A(sub Si/Zn) greater than -0.38, A(Mg/Zn) = -0.82, A(sub Cr/Zn) = -1.18, and A(sub Fe/Zn) greater than 1.40 dex. We show that ionization effects, abundance gradients, or intrinsic abundance variability cannot be significant sources for the underabundances observed. The most likely explanation is gas phase depletion of elements onto dust grains. Comparisons with the gas phase abundances along other diffuse, warm gas sight lines, like the halo sight line to HD 93521, support this interpretation as do the derived physical properties of the sight line.

Cardelli, Jason A.↗
NASA NTRS1994

A study of ultraviolet absorption lines through the complete Galactic halo by the analysis of HST faint object spectrograph spectra of active Galactic nuclei, 1

The ultraviolet (1150 - 2850 A) spectra of a number of active galactic nuclei (AGNs) observed with the Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) have been used to study the properties of the Galactic halo. The objects that served as probes are 3C 273, PKS 0454-220, Pg 1211+143, CSO 251, Ton 951, and PG 1351+640. The equivalent widths of certain interstellar ions have been measured, with special attention paid to the C IV/C II and Si IV/Si II ratios. These ratios have been intercompared, and the highest values are found in the direction of 3C 273, where C IV/C II = 1.2 and Si IV/Si II greater than 1. These high ratios may be due to a nearby supernova remnant, rather than to ionized gas higher up in the Galactic halo. Our data give some support to the notion that QSO metal-line systems may arise from intervening galaxies which contain high supernova rates, galactic fountains, and turbulent mixing layers.

Burks, Geoffrey S.↗
NASA NTRS1995

A search for an X-ray scattering halo around Scorpius X-1

Results are presented of an experiment to detect the presence of X-ray scattering by interstellar dust grains in the form of a halo around Sco X-1. We utilize te principle that X-ray scattering off an optical is reduced by 1/sin theta for reflections out of the plane of incidence, thus reducing instrumental scattering off our moderate quality (1 arcminute) X-ray optic. We find an upper limit X-ray flux from Sco X-1 in the form of a halo at a mean energy of 0.69 keV of 7.6% of the point source flux at the 1 sigma confidence level. From this we derive an upper limit of E(B-V) = 0.12 towards Sco X-1. This is about half the value (E(B-V) approximately 0.3) derived toward Sco X-1 using the 2200 A interstellar absorption feature, indicating probable circumstellar origin to the 2200 A feature.

Gallagher, Dennis↗
NASA NTRS1995

Composition of interstellar clouds in the disk and halo. 3: HD 149881

The spectrum of HD 149881, a halo star 1300 pc above the Galactic plane, has been analyzed, based on high-resolution GHRS data from Hubble Space Telescope (HST) together with 21 cm emission observations and profiles of Na I and Ca II features. The results reveal 11 absorption components, of which nine are produced in H I regions, with log N(H(sup 0) in each region mostly approximately 19.5; one component clearly originates in an H II region. Detailed column densities show that (D(Zn)), the absolute value of the depletion D, for Zn, like that for S, does not much exceed 0.1 dex. With Zn used as a standard of reference, D(Fe) varies from -1.1 to -0.6 dex; (D(Cr)) and (D(Mn)) average less than (D(Fe)) by about 0.1 and 0.2 dex, respectively, with dispersions of about 0.1 dex in this difference. As in Papers I and II, the measured D(Si) is consistent with a 2 to 1 ratio of Fe to Si atoms in grains. The kinetic temperature is these H I regions varies widely among components. Maximum temperatures, found from the widths of 21 cm emission components, range from about 100 to 10,000 K, while actual temperatures, found by comparing the H I widths with the b-values for the ultraviolet features, are about equal or less than 1000 K for six of the H I components. In contrast, toward the halo star HD 93521, temperatures of about 6000 K were found for most of the components. With use of these HD 149881 temperatures, values of n(sub e) were obtained from the ionization equilibrium of Ca(+) and also of Na(0). These values range over an order of magnitude, from about 4.9 x 10(exp -2) to 1.3 x 10(exp -3)/cu cm, systematically smaller (but also less certain) than in HD 93521. For the low values of n(sub e) found in the coldest components, the electrons can be accounted for by ionization of C atoms, if n(sub H) approximately equals 10/cu cm. For the warmest component, an ionization probability of beta(H(sup 0)) approximately equals 3 x 10(exp -15)/s is required, with lower values for other H I components.

Spitzer, Lyman, Jr.↗
NASA NTRS1999

HALOE Algorithm Improvements for Upper Tropospheric Sounding

This report details the ongoing efforts by GATS, Inc., in conjunction with Hampton University and University of Wyoming, in NASA's Mission to Planet Earth UARS Science Investigator Program entitled "HALOE Algorithm Improvements for Upper Tropospheric Soundings." The goal of this effort is to develop and implement major inversion and processing improvements that will extend HALOE measurements further into the troposphere. In particular, O3, H2O, and CH4 retrievals may be extended into the middle troposphere, and NO, HCl and possibly HF into the upper troposphere. Key areas of research being carried out to accomplish this include: pointing/tracking analysis; cloud identification and modeling; simultaneous multichannel retrieval capability; forward model improvements; high vertical-resolution gas filter channel retrievals; a refined temperature retrieval; robust error analyses; long-term trend reliability studies; and data validation. The current (first-year) effort concentrates on the pointer/tracker correction algorithms, cloud filtering and validation, and multi-channel retrieval development. However, these areas are all highly coupled, so progress in one area benefits from and sometimes depends on work in others.

McHugh, Martin J.↗
NASA NTRS2000

The Age of the Inner Halo Globular Cluster NGC 6652

Hubble Space Telescope (HST) (V,I) photometry has been obtained for the inner halo globular cluster NGC 6652. The photometry reaches approximately 4 mag below the turn-off and includes a well populated horizontal branch (HB). This cluster is located close to the Galactic center at RGC approximately equal to 2.0 kpc with a reddening of E(V-I) = 0.15 +/- 0.02 and has a metallicity of [Fe/H] approximately equal to -0.85. Based upon DELTA V (sup SGB) (sub HB), NGC 6652 is 11.7 plus or minus 1.6 Gyr old. Using A HB precise differential ages for 47 Tuc (a thick disk globular), M107 and NGC 1851 (both halo clusters) were obtained. NGC 6652 appears to be the same age as 47 Tuc and NGC 1851 (within +/- 1.2 Gyr), while there is a slight suggestion that M107 is older than NGC 6652 by 2.3 +/- 1.5 Gyr. As this is a less than 2 sigma result, this issue needs to be investigated further before a definitive statement regarding the relative age of M107 and NGC 6652 may be made.

Chaboyer, Brian↗