281 research outputs found
Infrared emission lines in planetary nebulae
Infrared spectroscopy was used to detect many forbidden fine structure emission lines in planetary nebulae. Measurements of these lines offer sensitive probes of the physical conditions and ionization structure, and lead to improved abundance determinations
The Production Rate and Employment of Ph.D. Astronomers
In an effort to encourage self-regulation of the astronomy job market, I
examine the supply of, and demand for, astronomers over time. On the supply
side, I document the production rate of Ph.D. astronomers from 1970 to 2006
using the UMI Dissertation Abstracts database, along with data from other
independent sources. I compare the long-term trends in Ph.D. production with
federal astronomy research funding over the same time period, and I demonstrate
that additional funding is correlated with higher subsequent Ph.D. production.
On the demand side, I monitor the changing patterns of employment using
statistics about the number and types of jobs advertised in the AAS Job
Register from 1984 to 2006. Finally, I assess the sustainability of the job
market by normalizing this demand by the annual Ph.D. production. The most
recent data suggest that there are now annual advertisements for about one
postdoctoral job, half a faculty job, and half a research/support position for
every new domestic Ph.D. recipient in astronomy and astrophysics. The average
new astronomer might expect to hold up to 3 jobs before finding a steady
position.Comment: 5 pages, 4 figures, 2 tables, PASP accepte
The Abundances of Light Neutron-Capture Elements in Planetary Nebulae III. The Impact of New Atomic Data on Nebular Selenium and Krypton Abundance Determinations
The detection of neutron(n)-capture elements in several planetary nebulae
(PNe) has provided a new means of investigating s-process nucleosynthesis in
low-mass stars. However, a lack of atomic data has inhibited accurate
trans-iron element abundance determinations in astrophysical nebulae. Recently,
photoionization and recombination data were determined for Se and Kr, the two
most widely detected n-capture elements in nebular spectra. We have
incorporated these new data into the photoionization code Cloudy. To test the
atomic data, numerical models were computed for 15 PNe that exhibit emission
lines from multiple Kr ions. We found systematic discrepancies between the
predicted and observed emission lines that are most likely caused by inaccurate
photoionization and recombination data. These discrepancies were removed by
adjusting the Kr--Kr photoionization cross sections within their
cited uncertainties and the dielectronic recombination rate coefficients by
slightly larger amounts. From grids of models spanning the physical conditions
encountered in PNe, we derive new, broadly applicable ionization correction
factor (ICF) formulae for calculating Se and Kr elemental abundances. The ICFs
were applied to our previous survey of near-infrared [Kr III] and [Se IV]
emission lines in 120 PNe. The revised Se and Kr abundances are 0.1-0.3 dex
lower than former estimates, with average values of [Se/(O, Ar)]=0.120.27
and [Kr/(O, Ar)]=0.820.29, but correlations previously found between their
abundances and other nebular and stellar properties are unaffected. We also
find a tendency for high-velocity PNe that can be associated with the Galactic
thick disk to exhibit larger s-process enrichments than low-velocity PNe
belonging to the thin disk population.Comment: 73 pages, 6 figures, 18 tables, accepted for publication in ApJ
Far-infrared line observations of planetary nebulae. 1: The O 3 spectrum
Observations of the far-infrared fine structure lines of O III have been obtained for six planetary nebulae. The infrared measurements are combined with optical O III line fluxes to probe physical conditions in the gas. From the observed line intensity ratios, a simultaneous solution was obtained for electron temperature and density, as well as means of evaluating the importance of inhomogeneities. Densities determined from the far-infrared O III lines agree well density diagnostics from other ions, indicating a fairly homogeneous density in the emitting gas. Temperatures are determined separately from the O III 4363/5007 A and 5007 A/52 micron intensity ratios and compared. Systematically higher values are derived from the former ratio, which is expected from a nebula which is not isothermal. Allowance for the presence of temperature variations within these nebulae raises their derived oxygen abundances, determinations to be reconciled with the solar value
A far-infrared study of N/O abundance ratio in galactic H 2 regions
Far-infrared lines of N++ and O++ in several galactic H II regions were measured in an effort to probe the abundance ratio N/O. New measurements are presented for W32 (630.8-0.0), Orion A, and G75.84+0.4. The combination of (N III) 57.3 millimicrons and (O III) 88.4 and 51.8 millimicrons yields measurements of N++/O++ that are largely insensitive to electron temperature, density uncertainties, and to clumping of the ionized gas, due to the similarity of the critical densities for these transitions. In the observed nebulae, N++/O++ should be indicative of N/O, a ratio that is of special importance in nucleosynthesis theory. Measurements are compared with previous measurements of M17 and W51. For nebulae in the solar circle, N++/O++ is greater than the N/O values derived from optical studies of N+/O+ in low ionization zones of the same nebulae. We find that N++/O++ in W43 is significantly higher than for the other H II regions in the sample. Since W43 is located at R = 5 kpc, which is the smallest galactocentric distance in our sample, our data appear consistent with the presence of a negative abundance gradient d(N/O)dR
Discovery of Enhanced Germanium Abundances in Planetary Nebulae with FUSE
We report the discovery of Ge III 1088.46 in the planetary nebulae
(PNe) SwSt 1, BD+303639, NGC 3132, and IC 4593, observed with the Far
Ultraviolet Spectroscopic Explorer. This is the first astronomical detection of
this line and the first measurement of Ge (Z = 32) in PNe. We estimate Ge
abundances using S and Fe as reference elements, for a range of assumptions
about gas-phase depletions. The results indicate that Ge, which is synthesized
in the initial steps of the s-process and therefore can be self-enriched in
PNe, is enhanced by factors of > 3-10. The strongest evidence for enrichment is
seen for PNe with Wolf-Rayet central stars, which are likely to contain heavily
processed material.Comment: 11 pages, 1 figure, accepted for publication in ApJ Letter
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