436 research outputs found
A search for hydrogenated fullerenes in fullerene-containing planetary nebulae
Detections of C60 and C70 fullerenes in planetary nebulae (PNe) of the
Magellanic Clouds and of our own Galaxy have raised the idea that other forms
of carbon such as hydrogenated fullerenes (fulleranes like C60H36 and C60H18),
buckyonions, and carbon nanotubes, may be widespread in the Universe. Here we
present VLT/ISAAC spectra (R ~600) in the 2.9-4.1 microns spectral region for
the Galactic PNe Tc 1 and M 1-20, which have been used to search for
fullerene-based molecules in their fullerene-rich circumstellar environments.
We report the non-detection of the most intense infrared bands of several
fulleranes around ~3.4-3.6 microns in both PNe. We conclude that if fulleranes
are present in the fullerene-containing circumstellar environments of these
PNe, then they seem to be by far less abundant than C60 and C70. Our
non-detections together with the (tentative) fulleranes detection in the
proto-PN IRAS 01005+7910 suggest that fulleranes may be formed in the short
transition phase between AGB stars and PNe but they are quickly destroyed by
the UV radiation field from the central star.Comment: Accepted for publication in Astronomy & Astrophysics (7 pages, 3
figures, and 3 Tables
The Kinematics of the Ionized and Molecular Hydrogen in the Starburst Galaxy NGC 253
Near-infrared H_2 1-0 S(1) and Br_gamma velocity curves along the major axis
of NGC 253 have revealed a central velocity gradient that is seven times
steeper than that shown by the optical velocity curve. This is interpreted as
an optical depth effect due to dust. Approximately 35 mag of visual extinction
in the center is required to match the SW side of the optical velocity curve.
The spatial variation of the ratio of these lines to the CO (J=1-0) line is
compared among starburst galaxies NGC 253, M82, and NGC 4945 to investigate the
excitation mechanism responsible for the H_2 1-0 S(1) line.Comment: Uuencoded postscript file, 10 pages (4 tables included), 8 figures
available on request to [email protected], Ap.J. (in press
A search for diffuse bands in fullerene planetary nebulae: evidence of diffuse circumstellar bands
Large fullerenes and fullerene-based molecules have been proposed as carriers
of diffuse interstellar bands (DIBs). The recent detection of the most common
fullerenes (C60 and C70) around some planetary nebulae (PNe) now enable us to
study the DIBs towards fullerene-rich space environments. We search DIBs in the
optical spectra towards three fullerene-containing PNe (Tc 1, M 1-20, and IC
418). Special attention is given to DIBs which are found to be unusually
intense towards these fullerene sources. In particular, an unusually strong
4428A absorption feature is a common charateristic of fullerene PNe. Similar to
Tc 1, the strongest optical bands of neutral C60 are not detected towards IC
418. Our high-quality (S/N > 300) spectra for PN Tc 1, together with its large
radial velocity, permit us to search for the presence of diffuse bands of
circumstellar origin, which we refer to as diffuse circumstellar bands (DCBs).
We report the first tentative detection of two DCBs at 4428 and 5780 A in the
fullerene-rich circumstellar environment around the PN Tc 1. Laboratory and
theoretical studies of fullerenes in their multifarious manifestations (carbon
onions, fullerene clusters, or even complex species formed by fullerenes and
other molecules like PAHs or metals) may help solve the mystery of some of the
diffuse band carriers.Comment: Accepted for publication in Astronomy & Astrophysics (16 pages, 10
figures, and 7 Tables); final version (changes regarding PN M 1-20 and
language corrected
Why are massive O-rich AGB stars in our Galaxy not S-stars?
We present the main results derived from a chemical analysis carried out on a
large sample of galactic O-rich AGB stars using high resolution optical
spectroscopy (R~40,000-50,000) with the intention of studying their lithium
abundances and/or possible s-process element enrichment. Our chemical analysis
shows that some stars are lithium overabundant while others are not. The
observed lithium overabundances are interpreted as a clear signature of the
activation of the so-called ``Hot Bottom Burning'' (HBB) process in massive
galactic O-rich AGB stars, as predicted by the models. However, these stars do
not show the zirconium enhancement (taken as a representative for the s-process
element enrichment) associated to the third dredge-up phase following thermal
pulses. Our results suggest that the more massive O-rich AGB stars in our
Galaxy behave differently from those in the Magellanic Clouds, which are both
Li- and s-process-rich (S-type stars). Reasons for this unexpected result are
discussed. We conclude that metallicity is probably the main responsible for
the differences observed and suggest that it may play a more important role
than generally assumed in the chemical evolution of AGB stars.Comment: 4 pages, 2 figures, to appear in the proceedings of the conference
"Planetary Nebulae as astronomical tools" held in Gdansk, Poland, jun 28/jul
02, 200
Sub-arcsecond Morphology of Planetary Nebulae
Planetary nebulae (PNe) can be roughly categorized into several broad
morphological classes. The high quality images of PNe acquired in recent years,
however, have revealed a wealth of fine structures that preclude simplistic
models for their formation. Here we present narrow-band, sub-arcsecond images
of a sample of relatively large PNe that illustrate the complexity and variety
of small-scale structures. This is especially true for bipolar PNe, for which
the images reveal multi-polar ejections and, in some cases, suggest turbulent
gas motions. Our images also reveal the presence or signs of jet-like outflows
in several objects in which this kind of component has not been previously
reported.Comment: 7 pages, 7 figures, Accepted for publication in PAS
Infrared Study of Fullerene Planetary Nebulae
We present a study of 16 PNe where fullerenes have been detected in their
Spitzer spectra. This large sample of objects offers an unique opportunity to
test conditions of fullerene formation and survival under different metallicity
environments as we are analyzing five sources in our own Galaxy, four in the
LMC, and seven in the SMC. Among the 16 PNe under study, we present the first
detection of C60 (possibly also C70) fullerenes in the PN M 1-60 as well as of
the unusual 6.6, 9.8, and 20 um features (possible planar C24) in the PN K
3-54. Although selection effects in the original samples of PNe observed with
Spitzer may play a potentially significant role in the statistics, we find that
the detection rate of fullerenes in C-rich PNe increases with decreasing
metallicity (5% in the Galaxy, 20% in the LMC, and 44% in the SMC). CLOUDY
photoionization modeling matches the observed IR fluxes with central stars that
display a rather narrow range in effective temperature (30,000-45,000 K),
suggesting a common evolutionary status of the objects and similar fullerene
formation conditions. The observed C60 intensity ratios in the Galactic sources
confirm our previous finding in the MCs that the fullerene emission is not
excited by the UV radiation from the central star. CLOUDY models also show that
line- and wind-blanketed model atmospheres can explain many of the observed
[NeIII]/[NeII] ratios by photoionization suggesting that possibly the UV
radiation from the central star, and not shocks, are triggering the
decomposition of the circumstellar dust grains. With the data at hand, we
suggest that the most likely explanation for the formation of fullerenes and
graphene precursors in PNe is that these molecular species are built from the
photo-chemical processing of a carbonaceous compound with a mixture of aromatic
and aliphatic structures similar to that of HAC dust.Comment: Accepted for publication in ApJ (43 pages, 11 figures, and 4 tables).
Small changes to fit the proof-corrected article to be published in Ap
- …