42 research outputs found
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
Comparative Absorption and Emission Abundance Analyses of Nebulae: Ion Emission Densities for IC 418
Recent analyses of nebular spectra have resulted in discrepant abundances
from CNO forbidden and recombination lines. We consider independent methods of
determining ion abundances for emission nebulae, comparing ion emission
measures with column densities derived from resonance absorption lines viewed
against the central star continuum. Separate analyses of the nebular emission
lines and the stellar UV absorption lines yield independent abundances for
ions, and their ratio can be expressed in terms of a parameter n_e_{em}, the
``emission density'' for each ion. Adequate data for this technique are still
scarce, but separate analyses of spectra of the planetary nebula and central
star of IC 418 do show discrepant abundances for several ions, especially Fe
II. The discrepancies are probably due to the presence of absorbing gas which
does not emit and/or to uncertain atomic data and excitation processes, and
they demonstrate the importance of applying the technique of combining
emission- and absorption-line data in deriving abundances for nebulae.Comment: 25 pages, 3 figures, accepted for publication in PAS
X-ray Destruction of Dust Along the Line of Sight to Gamma-Ray Bursts
We show that if all gamma-ray bursts emit X-rays in a way similar to those
observed by BeppoSax, much of the extinction along the line of sight in the
host galaxy of the burst can be destroyed. Two mechanisms are principally
responsible for dust destruction: grain heating and grain charging. The latter,
which can lead to electrostatic stresses greater than the tensile strength of
the grains, is often the more important. Grains may regularly be destroyed at
distances as large as 100 parsecs. This dust destruction can permit us to see
the UV/optical afterglow even when the burst is embedded deep within a
highly-obscured region. Because the destruction rate depends on grain
composition and size, it may be possible to observe the amount and
wavelength-dependence of extinction change during the course of the burst and
first few minutes of the afterglow. It may also be possible to detect
interstellar absorption lines in the afterglow spectrum that would not exist
but for the return of heavy elements to the gas phase.Comment: Accepted for publication in The Astrophysical Journal. Uses AASTeX.
31 pages, including a six-panel figur
Independent Emission and Absorption Abundances for Planetary Nebulae
Emission-line abundances have been uncertain for more than a decade due to
unexplained discrepancies in the relative intensities of the forbidden lines
and weak permitted recombination lines in planetary nebulae (PNe) and H II
regions. The observed intensities of forbidden and recombination lines
originating from the same parent ion differ from their theoretical values by
factors of more than an order of magnitude in some of these nebulae. In this
study we observe UV resonance line absorption in the central stars of PNe
produced by the nebular gas, and from the same ions that emit optical forbidden
lines. We then compare the derived absorption column densities with the
emission measures determined from ground-based observations of the nebular
forbidden lines. We find for our sample of PNe that the collisionally excited
forbidden lines yield column densities that are in basic agreement with the
column densities derived for the same ions from the UV absorption lines. A
similar comparison involving recombination line column densities produces
poorer agreement, although near the limits of the formal uncertainties of the
analyses. An additional sample of objects with larger abundance discrepancy
factors will need to be studied before a stronger statement can be made that
recombination line abundances are not correct.Comment: 19 pages, 13 figures, accepted by ApJ. Preprint utilizes
emulateapj.cls v. 12/01/06 (included
The coronal line regions of planetary nebulae NGC6302 and NGC6537: 3-13um grating and echelle spectroscopy
We report on advances in the study of the cores of NGC6302 and NGC6537 using
infrared grating and echelle spectroscopy. In NGC6302, emission lines from
species spanning a large range of ionization potential, and in particular
[SiIX]3.934um, are interpreted using photoionization models (including CLOUDY),
which allow us to reestimate the central star's temperature to be about
250000K. All of the detected lines are consistent with this value, except for
[AlV] and [AlVI]. Aluminium is found to be depleted to one hundredth of the
solar abundance, which provides further evidence for some dust being mixed with
the highly ionized gas (with photons harder than 154eV). A similar depletion
pattern is observed in NGC6537. Echelle spectroscopy of IR coronal ions in
NGC6302 reveals a stratified structure in ionization potential, which confirms
photoionization to be the dominant ionization mechanism. The lines are narrow
(< 22km/s FWHM), with no evidence of the broad wings found in optical lines
from species with similar ionization potentials, such as [NeV]3426A. We note
the absence of a hot bubble, or a wind blown bipolar cavity filled with a hot
plasma, at least on 1'' and 10km/s scales. We also provide accurate new
wavelengths for several of the infrared coronal lines observed with the
echelle.Comment: Accepted for publication in MNRA
Interacting Supernovae: Types IIn and Ibn
Supernovae (SNe) that show evidence of strong shock interaction between their
ejecta and pre-existing, slower circumstellar material (CSM) constitute an
interesting, diverse, and still poorly understood category of explosive
transients. The chief reason that they are extremely interesting is because
they tell us that in a subset of stellar deaths, the progenitor star may become
wildly unstable in the years, decades, or centuries before explosion. This is
something that has not been included in standard stellar evolution models, but
may significantly change the end product and yield of that evolution, and
complicates our attempts to map SNe to their progenitors. Another reason they
are interesting is because CSM interaction is an efficient engine for making
bright transients, allowing super-luminous transients to arise from normal SN
explosion energies, and allowing transients of normal SN luminosities to arise
from sub-energetic explosions or low radioactivity yield. CSM interaction
shrouds the fast ejecta in bright shock emission, obscuring our normal view of
the underlying explosion, and the radiation hydrodynamics of the interaction is
challenging to model. The CSM interaction may also be highly non-spherical,
perhaps linked to binary interaction in the progenitor system. In some cases,
these complications make it difficult to definitively tell the difference
between a core-collapse or thermonuclear explosion, or to discern between a
non-terminal eruption, failed SN, or weak SN. Efforts to uncover the physical
parameters of individual events and connections to possible progenitor stars
make this a rapidly evolving topic that continues to challenge paradigms of
stellar evolution.Comment: Final draft of a chapter in the "SN Handbook". Accepted. 25 pages, 3
fig
Efficient surface formation route of interstellar hydroxylamine through no hydrogenation. Ii. The multilayer regime in interstellar relevant ices
Contains fulltext :
103328.pdf (publisher's version ) (Open Access
Electric Field Transients Observed by the HUYGENS Probe in the Atmosphere of Titan: Atmospheric Electricity Phenomena or Artefacts? (abstract). Planetary Radio Emissions| PLANETARY RADIO EMISSIONS VII 7|
During the first 35 close Titan flybys the Radio and Plasma Wave Science instrument (RPWS) aboard the CASSINI orbiter did not observe radio signals possibly associated with lightning in the atmosphere of Titan [Fischer et al., 2007, Geophys. Res. Lett., 34, L22104). The electric field sensors of the HUYGENS PWA instrument (permittivity, waves and altimetry) observed smooth variations as well as impulsive events varying with altitude during the descent of the probe in the atmosphere of Titan. While a part of the low frequency signals was explained as externally driven Schumann resonances, there is still a debate on the origin of the impulsive events. In order to differentiate natural atmospheric discharges from sources on the parachute or the probe the HUYGENS electric field data have been re-evaluated und combined with probe attitude and velocity. The correlation results indicate that atmospheric electricity phemonena are present in the atmosphere of Titan