381 research outputs found
Chemical abundances in Galactic planetary nebulae with Spitzer spectra
We present new low-resolution (R~800) optical spectra of 22 Galactic PNe with
Spitzer spectra. These data are combined with recent optical spectroscopic data
available in the literature to construct representative samples of compact (and
presumably young) Galactic disc and bulge PNe with Spitzer spectra. Attending
to the nature of the dust features seen in their Spitzer spectra, Galactic disc
and bulge PNe are classified according to four major dust types (oxygen
chemistry or OC, carbon chemistry or CC, double chemistry or DC, featureless or
F) and subtypes (amorphous and crystalline, and aliphatic and aromatic).
Nebular gas abundances of He, N, O, Ne, S, Cl, and Ar, as well as plasma
parameters (e.g. Ne, Te) are homogeneously derived and we study the median
chemical abundances and nebular properties in Galactic disc and bulge PNe
depending on their Spitzer dust types and subtypes. A comparison of the derived
median abundance patterns with AGB nucleosynthesis predictions show mainly that
i) DC PNe, both with amorphous and crystalline silicates, display
high-metallicity (solar/supra-solar) and the highest He abundances and N/O
ratios, suggesting relatively massive (~3-5 M_sun) hot bottom burning AGB stars
as progenitors; ii) PNe with O-rich and C-rich unevolved dust (amorphous and
aliphatic) seem to evolve from subsolar metallicity (z~0.008) and lower mass
(<3 M_sun) AGB stars; iii) a few O-rich PNe and a significant fraction of
C-rich PNe with more evolved dust (crystalline and aromatic, respectively)
display chemical abundances similar to DC PNe, suggesting that they are related
objects. A comparison of the derived nebular properties with predictions from
models combining the theoretical central star evolution with a simple nebular
model is also presented. Finally, a possible link between the Spitzer dust
properties, chemical abundances, and evolutionary status is discussed.Comment: Accepted for publication in Astronomy & Astrophysics (45 pages, 17
figures, and 14 tables); final version (language corrected
Molecular orientational dynamics of the endohedral fullerene ScN@C as probed by C and Sc NMR
We measure 13C and 45Sc NMR lineshapes and spin-lattice relaxation times (T1)
to probe the orientational dynamics of the endohedral metallofullerene
Sc3N@C80. The measurements show an activated behavior for molecular
reorientations over the full temperature range with a similar behavior for the
temperature dependence of the 13C and 45Sc data. Combined with spectral data
from Magic Angle Spinning (MAS) NMR, the measurements can be interpreted to
mean the motion of the encapsulated Sc3N molecule is independent of that of the
C80 cage, although this requires the similar temperature dependence of the 13C
and 45Sc spin-lattice relaxation times to be coincidental. For the Sc3N to be
fixed to the C80 cage, one must overcome the symmetry breaking effect this has
on the Sc3N@C80 system since this would result in more than the observed two
13C lines.Comment: 6 pages, 5 figure
X-ray Imaging of Planetary Nebulae with Wolf-Rayet-type Central Stars: Detection of the Hot Bubble in NGC 40
We present the results of Chandra X-ray Observatory (CXO) observations of the
planetary nebulae (PNs) NGC 40 and Hen 2-99. Both PNs feature late-type
Wolf-Rayet central stars that are presently driving fast ~1000 km/s, massive
winds into denser, slow-moving (~10 km/s) material ejected during recently
terminated asymptotic giant branch (AGB) evolutionary phases. Hence, these
observations provide key tests of models of wind-wind interactions in PNs. In
NGC 40, we detect faint, diffuse X-ray emission distributed within a partial
annulus that lies nested within a ~40'' diameter ring of nebulosity observed in
optical and near-infrared images. Hen 2-99 is undetected. The inferred X-ray
temperature (T_X ~10^6 K) and luminosity (L_X ~ 2 X 10^30 ergs/s) of NGC 40 are
the lowest measured thus far for any PN displaying diffuse X-ray emission.
These results, combined with the ring-like morphology of the X-ray emission
from NGC 40, suggest that its X-ray emission arises from a ``hot bubble'' that
is highly evolved and is generated by a shocked, quasi-spherical fast wind from
the central star, as opposed to AGB or post-AGB jet activity. In constrast, the
lack of detectable X-ray emission from Hen 2-99 suggests that this PN has yet
to enter a phase of strong wind-wind shocks.Comment: 15 pages, 5 figures to appear in The Astrophysical Journa
Longitudinal Study of Primary HIV-1 Isolates in Drug-Naïve Individuals Reveals the Emergence of Variants Sensitive to Anti-HIV-1 Monoclonal Antibodies
To study how virus evolution affects neutralization sensitivity and to determine changes that occur in and around epitopes, we tested the ability of 13 anti-HIV-1 gp120 (anti-V2, anti-V3, anti-CD4bd and anti-carbohydrate) human monoclonal antibodies (mAbs) to neutralize sequential viruses obtained from five HIV-1 chronically infected drug naïve individuals. Overall, primary viruses collected from patients at first visit were resistant to neutralization by all anti-HIV-1 mAbs with the exception of one virus sensitive to IgG1b12. Four of the five patients' viruses evolved increased sensitivity to neutralization by anti-V3 mAbs. Virus collected from a patient obtained 31 months later, evolved increased sensitivity to anti-V2, anti-V3, and anti-CD4bd mAbs. Furthermore, the anti-V2 and anti-CD4bd mAbs also exhibited increased neutralization capacities against virus collected from a patient 29 months later. Of the seven anti-V3 mAbs, five showed increased potency to neutralize the evolved virus from a patient collected after 11 months, and three exhibited increased potency against viruses from two patients collected 29 and 36 months later. Anti-V3 mAbs exhibited the most breadth and potency in neutralizing the evolving viruses. Sequence analysis of the envelope regions revealed amino acid conservation within the V3 loop, while most of the changes identified occurred outside the core epitopes and in particular within the C3 region; these may account for increased neutralization sensitivity. These studies demonstrate that in vivo, HIV-1 can evolve increased neutralization sensitivity to mAbs and that the spectrum of neutralization capacities by mAbs can be broader when studied in longitudinal analysis
C/C ratio in planetary nebulae from the IUE archives
We investigated the abundance ratio of C/C in planetary nebulae
by examining emission lines arising from \ion{C}{3} 2s2p ^3P_{2,1,0} \to 2s^2
^1S_0. Spectra were retrieved from the International Ultraviolet Explorer
archives, and multiple spectra of the same object were coadded to achieve
improved signal-to-noise. The C hyperfine structure line at 1909.6 \AA
was detected in NGC 2440. The C/C ratio was found to be
1.2. In all other objects, we provide an upper limit for the flux
of the 1910 \AA line. For 23 of these sources, a lower limit for the
C/C ratio was established. The impact on our current
understanding of stellar evolution is discussed.
The resulting high signal-to-noise \ion{C}{3} spectrum helps constrain the
atomic physics of the line formation process. Some objects have the measured
1907/1909 flux ratio outside the low-electron density theoretical limit for
C. A mixture of C with C helps to close the gap somewhat.
Nevertheless, some observed 1907/1909 flux ratios still appear too high to
conform to the presently predicted limits. It is shown that this limit, as well
as the 1910/1909 flux ratio, are predominantly influenced by using the standard
partitioning among the collision strengths for the multiplet --
according to the statistical weights. A detailed calculation for the fine
structure collision strengths between these individual levels would be
valuable.Comment: ApJ accepted: 19 pages, 3 Figures, 2 Table
The effects of magnetic field on the d-density wave order in the cuprates
We consider the effects of a perpendicular magnetic field on the d-density
wave order and conclude that if the pseudogap phase in the cuprates is due to
this order, then it is highly insensitive to the magnetic field in the
underdoped regime, while its sensitivity increases as the gap vanishes in the
overdoped regime. This appears to be consistent with the available experiments
and can be tested further in neutron scattering experiments. We also
investigate the nature of the de Haas- van Alphen effect in the ordered state
and discuss the possibility of observing it.Comment: 5 pages, 4 eps figures, RevTex4. Corrected a silly but important typo
in the abstrac
Recommended from our members
Anti-V3 Monoclonal Antibodies Display Broad Neutralizing Activities against Multiple HIV-1 Subtypes
Background: The V3 loop of the HIV-1 envelope (Env) glycoprotein gp120 was identified as the “principal neutralizing domain” of HIV-1, but has been considered too variable to serve as a neutralizing antibody (Ab) target. Structural and immunochemical data suggest, however, that V3 contains conserved elements which explain its role in binding to virus co-receptors despite its sequence variability. Despite this evidence of V3 conservation, the ability of anti-V3 Abs to neutralize a significant proportion of HIV-1 isolates from different subtypes (clades) has remained controversial. Methods: HIV-1 neutralization experiments were conducted in two independent laboratories to test human anti-V3 monoclonal Abs (mAbs) against pseudoviruses (psVs) expressing Envs of diverse HIV-1 subtypes from subjects with acute and chronic infections. Neutralization was defined by 50% inhibitory concentrations (IC50), and was statistically assessed based on the area under the neutralization titration curves (AUC). Results: Using AUC analyses, statistically significant neutralization was observed by ≥1 anti-V3 mAbs against 56/98 (57%) psVs expressing Envs of diverse subtypes, including subtypes A, AG, B, C and D. Even when the 10 Tier 1 psVs tested were excluded from the analysis, significant neutralization was detected by ≥1 anti-V3 mAbs against 46/88 (52%) psVs from diverse HIV-1 subtypes. Furthermore, 9/24 (37.5%) Tier 2 viruses from the clade B and C standard reference panels were neutralized by ≥1 anti-V3 mAbs. Each anti-V3 mAb tested was able to neutralize 28–42% of the psVs tested. By IC50 criteria, 40/98 (41%) psVs were neutralized by ≥1 anti-V3 mAbs. Conclusions: Using standard and new statistical methods of data analysis, 6/7 anti-V3 human mAbs displayed cross-clade neutralizing activity and revealed that a significant proportion of viruses can be neutralized by anti-V3 Abs. The new statistical method for analysis of neutralization data provides many advantages to previously used analyses
V605 Aql: The Older Twin of Sakurai's Object
New optical spectra have been obtained with VLT/FORS2 of the final helium
shell flash (FF) star, V605 Aql, which peaked in brightness in 1919. New models
suggest that this star is experiencing a very late thermal pulse. The evolution
to a cool luminous giant and then back to a compact hot star takes place in
only a few years. V605 Aql, the central star of the Planetary Nebula (PN), A58,
has evolved from T5000 K in 1921 to 95,000 K today. There are
indications that the new FF star, Sakurai's Object (V4334 Sgr), which appeared
in 1996, is evolving along a similar path. The abundances of Sakurai's Object
today and V605 Aql 80 years ago mimic the hydrogen deficient R Coronae Borealis
(RCB) stars with 98% He and 1% C. The new spectra show that V605 Aql has
stellar abundances similar to those seen in Wolf-Rayet [WC] central stars of
PNe with ~55% He, and ~40% C. The stellar spectrum of V605 Aql can be seen even
though the star is not directly detected. Therefore, we may be seeing the
spectrum in light scattered around the edge of a thick torus of dust seen
edge-on. In the present state of evolution of V605 Aql, we may be seeing the
not too distant future of Sakurai's Object.Comment: 12 pages, 1 figure, ApJ Letters in pres
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