102 research outputs found
Composite Accretion Disk and White Dwarf Photosphere Analyses of the FUSE and HST Observations of EY Cygni
We explore the origin of FUSE and HST STIS far UV spectra of the dwarf nova,
EY Cyg, during its quiescence using \emph{combined} high gravity photosphere
and accretion disk models as well as model accretion belts. The best-fitting
single temperature white dwarf model to the FUSE plus HST STIS spectrum of EY
Cygni has TK, log , with an Si abundance of 0.1 x
solar and C abundance of 0.2 x solar but the distance is only 301 pc. The
best-fitting composite model consists of white dwarf with TK,
log , plus an accretion belt with TK covering 27% of
the white dwarf surface with V km/s. The accretion belt
contributes 63% of the FUV light and the cooler white dwarf latitudes
contribute 37%. This fit yields a distance of 351 pc which is within 100 pc of
our adopted distance of 450 pc. EY Cyg has very weak C {\sc iv} emission and
very strong N {\sc v} emission, which is atypical of the majority of dwarf
novae in quiescence. We also conducted a morphological study of the
surroundings of EY Cyg using direct imaging in narrow nebular filters from
ground-based telescopes. We report the possible detection of nebular material^M
associated with EY Cygni. Possible origins of the apparently large N {\scv}/C
{\sc iv} emission ratio are discussed in the context of nova explosions,
contamination of the secondary star and accretion of nova abundance-enriched
matter back to the white dwarf via the accretion disk or as a descendant of a
precursor binary that survived thermal timescale mass transfer. The scenario
involving pollution of the secondary by past novae may be supported by the
possible presence of a nova remnant-like nebula around EY Cyg.Comment: To appear in AJ, Oct. 2004. 5 figures, including 2 color ones (2D
pictures
Cliff Roosting by Migrant Semipalmated Sandpipers, Calidris pusilla, at Farrier's Cove, Shepody Bay, New Brunswick
An observation of Semipalmated Sandpipers roosting on a cliff face in Shepody Bay, New Brunswick, suggests changes from âtraditionalâ roosting sites. Sandpipers may be altering their roosting patterns due to pressures from avian predators such as the recent, and successful, re-introduction of the Peregrine Falcon
Far Ultraviolet Observations of the Dwarf Novae SS Aur and RU Peg in Quiescence
We have analyzed the Far Ultraviolet Spectrocopic Explorer (FUSE) spectra of
two U Gem-Type dwarf novae, SS Aur and RU Peg, observed 28 days and 60 days
(respectively) after their last outburst. In both systems the FUSE spectra (905
- 1182 A) reveal evidence of the underlying accreting white dwarf exposed in
the far UV. Our grid of theoretical models yielded a best-fitting photosphere
to the FUSE spectra with Teff=31,000K for SS Aur and Teff=49,000K for RU Peg.
This work provides two more dwarf nova systems with known white dwarf
temperatures above the period gap where few are known. The absence of CIII
(1175 A) absorption in SS Aur and the elevation of N above solar suggests the
possibility that SS Aur represents an additional accreting white dwarf where
the surface C/N ratio derives from CNO processing. For RU Peg, the modeling
uncertainties prevent any reliable conclusions about the surface abundances and
rotational velocity.Comment: AJ, Oct. 200
A Novel Substrate-Based HIV-1 Protease Inhibitor Drug Resistance Mechanism
BACKGROUND: HIV protease inhibitor (PI) therapy results in the rapid selection of drug resistant viral variants harbouring one or two substitutions in the viral protease. To combat PI resistance development, two approaches have been developed. The first is to increase the level of PI in the plasma of the patient, and the second is to develop novel PI with high potency against the known PI-resistant HIV protease variants. Both approaches share the requirement for a considerable increase in the number of protease mutations to lead to clinical resistance, thereby increasing the genetic barrier. We investigated whether HIV could yet again find a way to become less susceptible to these novel inhibitors. METHODS AND FINDINGS: We have performed in vitro selection experiments using a novel PI with an increased genetic barrier (RO033-4649) and demonstrated selection of three viruses 4- to 8-fold resistant to all PI compared to wild type. These PI-resistant viruses did not have a single substitution in the viral protease. Full genomic sequencing revealed the presence of NC/p1 cleavage site substitutions in the viral Gag polyprotein (K436E and/or I437T/V) in all three resistant viruses. These changes, when introduced in a reference strain, conferred PI resistance. The mechanism leading to PI resistance is enhancement of the processing efficiency of the altered substrate by wild-type protease. Analysis of genotypic and phenotypic resistance profiles of 28,000 clinical isolates demonstrated the presence of these NC/p1 cleavage site mutations in some clinical samples (codon 431 substitutions in 13%, codon 436 substitutions in 8%, and codon 437 substitutions in 10%). Moreover, these cleavage site substitutions were highly significantly associated with reduced susceptibility to PI in clinical isolates lacking primary protease mutations. Furthermore, we used data from a clinical trial (NARVAL, ANRS 088) to demonstrate that these NC/p1 cleavage site changes are associated with virological failure during PI therapy. CONCLUSIONS: HIV can use an alternative mechanism to become resistant to PI by changing the substrate instead of the protease. Further studies are required to determine to what extent cleavage site mutations may explain virological failure during PI therapy
Partial-Wave Amplitudes and Resonances in pbar + p -> pi + pi
Partial wave amplitudes have been extracted from accurate data on pbar + p ->
pi + pi by a method which incorporates the theoretical constraints of
analyticity and crossing symmetry. The resulting solution gives a good fit to
the annihilation data and is also consistent with the wealth of information in
the crossed channel pi + N -> pi + N. The partial wave amplitudes show evidence
for resonances in all partial waves with J < 6, at least one of which, a J = 0+
state, (and possibly another with J = 1-) is unlikely to have a simple
quark-antiquark structure.Comment: 17 pages, Revtex, 21 postscript figure
The Dwarf Novae During Quiescence
We present a synthetic spectral analysis of nearly the entire FUV IUE archive
of spectra of DNe in or near quiescence. We have examined all of the systems
for which S/N permitted an analysis. The study includes 53 systems of all DN
subtypes both above and below the period gap. The spectra were uniformly
analyzed using synthetic spectral codes for optically thick accretion disks and
stellar photospheres along with the best-available distance measurements or
estimates. We present newly determined approximate WD temperatures or upper
limits and estimated accretion rates. The average temperature of WDs in DNe
below the period gap is ~18,000K. For WDs in DNe above the period gap, the
average WD temperature is ~26,000K. There is a flux component, in addition to a
WD photosphere, which contributes >60% of the flux in the FUV in 53% of the
quiescent DNe in this study. We find that for 41% of the DNe in our sample, a
WD photosphere provides >60% of the FUV flux. Accretion rates estimated from
the FUV alone for the sample of DNe during quiescence ranged from 10^-12
Msun/yr to 10^-10 Msun/yr.The additional flux component is almost certainly not
an optically thick accretion disk since, according to the disk instability
model, the disk should be optically thin and too cool during DN quiescence to
be a significant FUV continuum emitter. Among the candidates for the second
component of FUV light are the quiescent inner disk, a hot equatorial accretion
belt, and a hot rotating ring. The implications of our study for disk accretion
physics and CV evolution are discussed.Comment: 36 pages, 3 tables, 8 figures, final accepted version of manuscrip
A Quantitative Study of the Mechanisms behind Thymic Atrophy in Gαi2-Deficient Mice during Colitis Development
Mice deficient for the G protein subunit Gαi2 spontaneously develop colitis, a chronic inflammatory disease associated with dysregulated T cell responses. We and others have previously demonstrated a thymic involution in these mice and an aberrant thymocyte dynamics. The Gαi2â/â mice have a dramatically reduced fraction of double positive thymocytes and an increased fraction of single positive (SP) thymocytes. In this study, we quantify a number of critical parameters in order to narrow down the underlying mechanisms that cause the dynamical changes of the thymocyte development in the Gαi2â/â mice. Our data suggest that the increased fraction of SP thymocytes results only from a decreased number of DP thymocytes, since the number of SP thymocytes in the Gαi2â/â mice is comparable to the control littermates. By measuring the frequency of T cell receptor excision circles (TRECs) in the thymocytes, we demonstrate that the number of cell divisions the Gαi2â/â SP thymocytes undergo is comparable to SP thymocytes from control littermates. In addition, our data show that the mature SP CD4+ and CD8+ thymocytes divide to the same extent before they egress from the thymus. By estimating the number of peripheral TREC+ T lymphocytes and their death rate, we could calculate the daily egression of thymocytes. Gαi2â/â mice with no/mild and moderate colitis were found to have a slower export rate in comparison to the control littermates. The quantitative measurements in this study suggest a number of dynamical changes in the thymocyte development during the progression of colitis
Annexin A2 Binds RNA and Reduces the Frameshifting Efficiency of Infectious Bronchitis Virus
Annexin A2 (ANXA2) is a protein implicated in diverse cellular functions, including exocytosis, DNA synthesis and cell proliferation. It was recently proposed to be involved in RNA metabolism because it was shown to associate with some cellular mRNA. Here, we identified ANXA2 as a RNA binding protein (RBP) that binds IBV (Infectious Bronchitis Virus) pseudoknot RNA. We first confirmed the binding of ANXA2 to IBV pseudoknot RNA by ultraviolet crosslinking and showed its binding to RNA pseudoknot with ANXA2 protein in vitro and in the cells. Since the RNA pseudoknot located in the frameshifting region of IBV was used as bait for cellular RBPs, we tested whether ANXA2 could regulate the frameshfting of IBV pseudoknot RNA by dual luciferase assay. Overexpression of ANXA2 significantly reduced the frameshifting efficiency from IBV pseudoknot RNA and knockdown of the protein strikingly increased the frameshifting efficiency. The results suggest that ANXA2 is a cellular RBP that can modulate the frameshifting efficiency of viral RNA, enabling it to act as an anti-viral cellular protein, and hinting at roles in RNA metabolism for other cellular mRNAs
- âŠ