534 research outputs found
Planetary Nebula Abundances and Morphology: Probing the Chemical Evolution of the Milky Way
This paper presents a homogeneous study of abundances in a sample of 79
northern galactic planetary nebulae whose morphological classes have been
uniformly determined. Ionic abundances and plasma diagnostics were derived from
selected optical line strengths in the literature, and elemental abundances
were estimated with the Ionization Correction Factor developed by Kingsbourgh &
Barlow (1994). We compare the elemental abundances to the final yields obtained
from stellar evolution models of low-and intermediate-mass stars, and we
confirm that most Bipolar planetary nebulae have high nitrogen and helium
abundance, and are the likely progeny of stars with main-sequence mass larger
than 3 solar masses. We derive =0.27, and discuss the implication of such
a high ratio in connection with the solar neon abundance. We determine the
galactic gradients of oxygen and neon, and found Delta log (O/H)/Delta R=-0.01
dex/kpc$ and Delta log (Ne/H)/Delta R=-0.01 dex/kpc. These flat PN gradients do
not reconcile with galactic metallicity gradients flattening with time.Comment: The Astrophysical Journal, in pres
Clarification of the pH-dependent kinetic behaviour of papain by using reactivity probes and analysis of alkylation and catalysed acylation reactions in terms of multihydronic state models: implications for electrostatics calculations and interpretation of the consequences of site-specific mutations such as Asp-158-Asn and Asp-158-Glu
Internal Dynamics of the Hypercompact H II Region G28.20-0.04N
High resolution (0.15") Very Large Array observations of 7 mm continuum and
H53a line emission toward the hypercompact H II region G28.20-0.04N reveal the
presence of large-scale ordered motions. We find a velocity gradient of 1000
km/s/pc along the minor axis of the continuum source. Lower resolution
(1.0"-2.3") radio recombination line observations indicate a systematic
increase of line width from H30alpha to H92alpha. Under the assumption that the
H30alpha line does not suffer significant pressure broadening, we have
deconvolved the contributions of statistical broadening (thermal, turbulent,
and pressure) from large-scale motions. The pressure broadening of the
H53alpha, H76alpha, and H92alpha lines implies an electron density of 6.9E+06,
8.5E+05, and 2.8E+05 cm^(-3), respectively.Comment: 12 pages, 2 figures, 1 table, Accepted for publication in Ap
A Search for "Dwarf" Seyfert Nuclei. III. Spectroscopic Parameters and Properties of the Host Galaxies
We have completed an optical spectroscopic survey of the nuclear regions (r <
200 pc) of a large sample of nearby galaxies. Although the main objectives of
the survey are to search for low-luminosity active galactic nuclei and to
quantify their luminosity function, the database can be used for a variety of
other purposes. This paper presents measurements of the spectroscopic
parameters for the 418 emission-line nuclei, along with a compilation of the
global properties of all 486 galaxies in the survey. Stellar absorption
generally poses a serious obstacle to obtaining accurate measurement of
emission lines in nearby galactic nuclei. We describe a procedure for removing
the starlight from the observed spectra in an efficient and objective manner.
The main parameters of the emission lines (intensity ratios, fluxes, profile
widths, equivalent widths) are measured and tabulated, as are several stellar
absorption-line and continuum indices useful for studying the stellar
population. Using standard nebular diagnostics, we determine the probable
ionization mechanisms of the emission-line objects. The resulting spectral
classifications provide extensive information on the demographics of
emission-line nuclei in the local universe. This new catalog contains over 200
objects showing spectroscopic evidence for recent star formation and an equally
large number of active galactic nuclei, including 46 which show broad H-alpha
emission. These samples will serve as the basis of future studies of nuclear
activity in nearby galaxies.Comment: To appear in the Astrophysical Journal Supplements. LaTex, 31 pages,
plus an additional 23 figures and 5 tables. AASTex macro aaspp4.st
Improving Predictions for Helium Emission Lines
We have combined the detailed He I recombination model of Smits with the
collisional transitions of Sawey & Berrington in order to produce new accurate
helium emissivities that include the effects of collisional excitation from
both the 2 (3)S and 2 (1) S levels. We present a grid of emissivities for a
range of temperature and densities along with analytical fits and error
estimates.
Fits accurate to within 1% are given for the emissivities of the brightest
lines over a restricted range for estimates of primordial helium abundance. We
characterize the analysis uncertainties associated with uncertainties in
temperature, density, fitting functions, and input atomic data. We estimate
that atomic data uncertainties alone may limit abundance estimates to an
accuracy of 1.5%; systematic errors may be greater than this. This analysis
uncertainty must be incorporated when attempting to make high accuracy
estimates of the helium abundance. For example, in recent determinations of the
primordial helium abundance, uncertainties in the input atomic data have been
neglected.Comment: ApJ, accepte
A re-appraisal of the structural basis of stereochemical recognition in papain. Insensitivity of binding-site-catalytic-site signalling to P2-chirality in a time-dependent inhibition
A persistent Norwegian Atlantic Current through the Pleistocene glacials
Changes in oceanâcirculation regimes in the northern North Atlantic and the Nordic Seas may affect not only the Arctic but potentially hemispheric or even global climate. Therefore, unraveling the longâterm evolution of the North Atlantic CurrentâNorwegian Atlantic Current system through the Pleistocene glaciations could yield useful information and climatological context for understanding contemporary changes. In this work, ~50,000 km2 of 3âD seismic reflection data are used to investigate the Pleistocene stratigraphy for evidence of paleoâoceanographic regimes on the midâNorwegian margin since 2.58 Ma. Across 33 semicontinuous regional paleoâseafloor surfaces ~17,500 iceberg scours have been mapped. This mapping greatly expands our spatiotemporal understanding of currents and iceberg presence in the eastern Nordic Seas. The scours display a dominant southwestânortheast trend that complements previous sedimentological and numerical modeling studies that suggest northwardâflowing currents in the Norwegian Sea during the Pleistocene. This paleoâoceanographic study suggests that through many of the Pleistocene glaciations, the location of surface ocean currents in the Norwegian Sea and, by extension, the eastern North Atlantic, were broadly similar to the present
Physical Conditions in the Narrow-Line Region of M51
We have investigated the physical conditions in the narrow-line region (NLR)
of M51 using long-slit spectra obtained with the Space Telescope Imaging
Spectrograph (STIS) aboard the Hubble Space Telescope (HST) and 3.6 cm radio
continuum observations obtained with the Very Large Array (VLA). Emission-line
diagnostics were employed for nine NLR clouds, which extend 2.5" (102 pc) from
the nucleus, to examine the electron density, temperature, and ionization state
of the NLR gas. The emission-line ratios are consistent with those typically
found in Seyfert nuclei and indicate that within the inner near-nuclear region
(r ~< 1") the ionization decreases with increasing radius. Upper-limits to the
[O III] electron temperature (T ~< 11,000 K) for the inner NLR clouds indicate
that photoionization is the dominant ionization mechanism close to the nucleus.
The emission-line fluxes for most of the NLR clouds can be reproduced
reasonably well by simple photoionization models using a central power-law
continuum source and supersolar nitrogen abundances. Shock+precursor models,
however, provide a better fit to the observed fluxes of an NLR cloud ~2.5"
south of the nucleus that is identified with the extra-nuclear cloud (XNC). The
large [O III] electron temperature of this cloud (T = 24,000 K) further
suggests the presence of shocks. This cloud is straddled by two radio knots and
lies near the location where a weak radio jet, ~2.5" (102pc) in extent,
connects the near-nuclear radio emission with a diffuse lobe structure spanning
\~4" (163 pc). It is plausible that this cloud represents the location where
the radio jet impinges on the disk ISM.Comment: 25 pages, 26 figures (9 color), 7 tables. Accepted for publication in
the Astrophysical Journa
Broad Recombination Line Objects in W49N on 600 AU Scales
High resolution 7 mm observations of the W49N massive star forming region
have detected recombination line emission from the individual ultracompact (UC)
HII regions on 50 milliarcsecond (600 AU) scales. These line observations,
combined with multifrequency, high-resolution continuum imaging of the region
at 7 mm (VLA) and at 3 mm and 1 mm (BIMA), indicate that five to seven of the
eighteen ultracompact sources in W49N are broad recombination line objects
(BRLOs) as described by Jaffe & Martin-Pintado (1999). BRLOs have both broad
radio recombination lines (V60 \kms) and rising spectra
(S), with values greater than 0.4. The broad
line widths of the H52 line are probably related to motions in the
ionized gas rather than pressure broadening. A number of models have been
proposed to explain the long lifetime of UC HII regions, including the
photoevaporated disk model proposed by Hollenbach et al. (1994). This model can
also explain the broad lines, rising spectra and bipolar morphologies of some
sources. We suggestbased on line and continuum observations as well as
source morphologythat in a subset of the W49N ultracompact sources we may be
observing ionized winds that arise from circumstellar disks.Comment: 15 pages, 2 figures, to appear in The Astrophysical Journal (v. 600,
no. 1), 1 January 200
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