484 research outputs found
Regulation of Repair by the 26S Proteasome
Cellular processes such as transcription and DNA repair may be regulated through diverse mechanisms, including RNA synthesis, protein synthesis, posttranslational modification and protein degradation. The 26S proteasome, which is responsible for degrading a broad spectrum of proteins, has been shown to interact with several nucleotide excision repair proteins, including xeroderma pigmentosum B protein (XPB), Rad4, and Rad23. Rad4 and Rad23 form a complex that binds preferentially to UV-damaged DNA. The 26S proteasome may regulate repair by degrading DNA repair proteins after repair is completed or, alternatively, the proteasome may act as a molecular chaperone to promote disassembly of the repair complex. In either case, the interaction between the proteasome and nucleotide excision repair depends on proteins like Rad23 that bind ubiquitin-conjugated proteins and the proteasome. While the iteration between Rad4 and Rad23 is well established, it will be interesting to determine what other proteins are regulated in a Rad23-dependent manner
Detection of the compressed primary stellar wind in eta Carinae
A series of three HST/STIS spectroscopic mappings, spaced approximately one
year apart, reveal three partial arcs in [Fe II] and [Ni II] emissions moving
outward from eta Carinae. We identify these arcs with the shell-like
structures, seen in the 3D hydrodynamical simulations, formed by compression of
the primary wind by the secondary wind during periastron passages.Comment: Accepted for publication in the Astrophysical Journal Letter
The three-dimensional structure of the Eta Carinae Homunculus
We investigate, using the modeling code SHAPE, the three-dimensional
structure of the bipolar Homunculus nebula surrounding Eta Carinae, as mapped
by new ESO VLT/X-Shooter observations of the H2 micron
emission line. Our results reveal for the first time important deviations from
the axisymmetric bipolar morphology: 1) circumpolar trenches in each lobe
positioned point-symmetrically from the center and 2) off-planar protrusions in
the equatorial region from each lobe at longitudinal (~55 degrees) and
latitudinal (10-20 degrees) distances from the projected apastron direction of
the binary orbit. The angular distance between the protrusions (~110 degrees)
is similar to the angular extent of each polar trench (~130 degrees) and nearly
equal to the opening angle of the wind-wind collision cavity (~110 degrees). As
in previous studies, we confirm a hole near the centre of each polar lobe and
no detectable near-IR H2 emission from the thin optical skirt seen prominently
in visible imagery. We conclude that the interaction between the outflows
and/or radiation from the central binary stars and their orientation in space
has had, and possibly still has, a strong influence on the Homunculus. This
implies that prevailing theoretical models of the Homunculus are incomplete as
most assume a single star origin that produces an axisymmetric nebula. We
discuss how the newly found features might be related to the Homunculus
ejection, the central binary and the interacting stellar winds. We also include
a 3D printable version of our Homunculus model.Comment: 14 pages, 7 color figures, 1 interactive 3D figure (Figure 5,
requires Adobe Reader), published in MNRAS. A 3D printable version of our
Homunculus model can be downloaded from
http://svs.gsfc.nasa.gov/vis/a010000/a011500/a011568/Eta_Car_Homunuculus_3D_model.zip
or from the 'Supporting Information' link in the electronic version of the
MNRAS articl
Effects of the LBV Primary's Mass-loss Rate on the 3D Hydrodynamics of eta Carinae's Colliding Winds
At the heart of eta Carinae's spectacular "Homunculus" nebula lies an extremely luminous (L(sub Total) greater than approximately 5 10(exp 6) solar luminosity) colliding wind binary with a highly eccentric (e approximately 0.9), 5.54-year orbit (Figure 1). The primary of the system, a Luminous Blue Variable (LBV), is our closest (D approximately 2.3 kpc) and best example of a pre-hypernova or pre-gamma ray burst environment. The remarkably consistent and periodic RXTE X-ray light curve surprisingly showed a major change during the system's last periastron in 2009, with the X-ray minimum being approximately 50% shorter than the minima of the previous two cycles1. Between 1998 and 2011, the strengths of various broad stellar wind emission lines (e.g. Halpha, Fe II) in line-of-sight (l.o.s.) also decreased by factors of 1.5 - 3 relative to the continuum2. The current interpretation for these changes is that they are due to a gradual factor of 2 - 4 drop in the primary's mass-loss rate over the last approximately 15 years1, 2. However, while a secular change is seen for a direct view of the central source, little to no change is seen in profiles at high stellar latitudes or reflected off of the dense, circumbinary material known as the "Weigelt blobs"2, 3. Moreover, model spectra generated with CMFGEN predict that a factor of 2 - 4 drop in the primary's mass-loss rate should lead to huge changes in the observed spectrum, which thus far have not been seen. Here we present results from large- (plus or minus 1620 AU) and small- (plus or minus 162 AU) domain, full 3D smoothed particle hydrodynamics (SPH) simulations of eta Car's massive binary colliding winds for three different primary-star mass-loss rates (2.4, 4.8, and 8.5 10(exp -4) solar mass/yr). The goal is to investigate how the mass-loss rate affects the 3D geometry and dynamics of eta Car's optically-thick wind and spatially-extended wind-wind collision (WWC) regions, both of which are known sources of observed X-ray, optical, UV, and near-IR emission and absorption. We use two domain sizes in order to better understand how the primary's mass-loss rate influences the various observables that form at different length scales. The 3D simulations provide information important for helping constrain Car's recent mass-loss history and future state
He II 4686 emission from the massive binary system in Car: constraints to the orbital elements and the nature of the periodic minima
{\eta} Carinae is an extremely massive binary system in which rapid spectrum
variations occur near periastron. Most notably, near periastron the He II
line increases rapidly in strength, drops to a minimum value,
then increases briefly before fading away. To understand this behavior, we
conducted an intense spectroscopic monitoring of the He II
emission line across the 2014.6 periastron passage using ground- and
space-based telescopes. Comparison with previous data confirmed the overall
repeatability of EW(He II ), the line radial velocities, and the
timing of the minimum, though the strongest peak was systematically larger in
2014 than in 2009 by 26%. The EW(He II ) variations, combined
with other measurements, yield an orbital period d. The observed
variability of the EW(He II ) was reproduced by a model in which
the line flux primarily arises at the apex of the wind-wind collision and
scales inversely with the square of the stellar separation, if we account for
the excess emission as the companion star plunges into the hot inner layers of
the primary's atmosphere, and including absorption from the disturbed primary
wind between the source and the observer. This model constrains the orbital
inclination to -, and the longitude of periastron to
-. It also suggests that periastron passage occurred on
d. Our model also reproduced EW(He II )
variations from a polar view of the primary star as determined from the
observed He II emission scattered off the Homunculus nebula.Comment: The article contains 23 pages and 17 figures. It has been accepted
for publication in Ap
A Luminous Be+White Dwarf Supersoft Source in the Wing of the SMC: MAXI J0158-744
We present a multi-wavelength analysis of the very fast X-ray transient MAXI
J0158-744, which was detected by MAXI/GSC on 2011 November 11. The subsequent
exponential decline of the X-ray flux was followed with Swift observations, all
of which revealed spectra with low temperatures (~100eV) indicating that MAXI
J0158-744 is a new Supersoft Source (SSS). The Swift X-ray spectra near maximum
show features around 0.8 keV that we interpret as possible absorption from
OVIII, and emission from O, Fe, and Ne lines. We obtained SAAO and ESO optical
spectra of the counterpart early in the outburst and several weeks later. The
early spectrum is dominated by strong Balmer and HeI emission, together with
weaker HeII emission. The later spectrum reveals absorption features that
indicate a B1/2IIIe spectral type, and all spectral features are at velocities
consistent with the Small Magellanic Cloud. At this distance, it is a luminous
SSS (>10^37 erg/s) but whose brief peak luminosity of >10^39 erg/s in the 2-4
keV band makes it the brightest SSS yet seen at "hard" X-rays. We propose that
MAXI J0158-744 is a Be-WD binary, and the first example to possibly enter ULX
territory. The brief hard X-ray flash could possibly be a result of the
interaction of the ejected nova shell with the B star wind in which the white
dwarf (WD) is embedded. This makes MAXI J0158-744 only the third Be/WD system
in the Magellanic Clouds, but it is by far the most luminous. The properties of
MAXI J0158-744 give weight to previous suggestions that SSS in nearby galaxies
are associated with early-type stellar systems.Comment: 15 pages, 8 figures; ApJ accepte
Detection of high-velocity material from the wind-wind collision zone of Eta Carinae across the 2009.0 periastron passage
This is the author accepted manuscript. The final version is available from EDP Sciences via the DOI in this record.We report near-infrared spectroscopic observations of the Eta Carinae massive binary system during 2008–2009 using the CRIRES spectrograph mounted on the 8 m UT 1 Very Large Telescope (VLT Antu). We detect a strong, broad absorption wing in He i λ10833 extending up to -1900 km s-1 across the 2009.0 spectroscopic event. Analysis of archival Hubble Space Telescope/Space Telescope Imaging Spectrograph ultraviolet and optical data identifies a similar high-velocity absorption (up to -2100 km s-1) in the ultraviolet resonance lines of Si iv λλ1394, 1403 across the 2003.5 event. Ultraviolet resonance lines from low-ionization species, such as Si ii λλ1527, 1533 and C ii λλ1334, 1335, show absorption only up to -1200 km s-1, indicating that the absorption with velocities -1200 to -2100 km s-1 originates in a region markedly more rapidly moving and more ionized than the nominal wind of the primary star. Seeing-limited observations obtained at the 1.6 m OPD/LNA telescope during the last four spectroscopic cycles of Eta Carinae (1989–2009) also show high-velocity absorption in He i λ10833 during periastron. Based on the large OPD/LNA dataset, we determine that material with velocities more negative than -900 km s-1 is present in the phase range 0.976 ≤ ϕ ≤ 1.023 of the spectroscopic cycle, but absent in spectra taken at ϕ ≤ 0.94 and ϕ ≥ 1.049. Therefore, we constrain the duration of the high-velocity absorption to be 95 to 206 days (or 0.047 to 0.102 in phase). We propose that the high-velocity absorption component originates in shocked gas in the wind-wind collision zone, at distances of 15 to 45 AU in the line-of-sight to the primary star. With the aid of three-dimensional hydrodynamical simulations of the wind-wind collision zone, we find that the dense high-velocity gas is along the line-of-sight to the primary star only if the binary system is oriented in the sky such that the companion is behind the primary star during periastron, corresponding to a longitude of periastron of ω ~ 240°–270°. We study a possible tilt of the orbital plane relative to the Homunculus equatorial plane and conclude that our data are broadly consistent with orbital inclinations in the range i = 40°–60°.JHG thanks the Max-Planck-Gesellschaft for financial support
for this work. AD and MT thanks the FAPESP foundation for continuous support.
TIM is supported by a NASA GSRP fellowshi
A Conditional Yeast E1 Mutant Blocks the Ubiquitin–Proteasome Pathway and Reveals a Role for Ubiquitin Conjugates in Targeting Rad23 to the Proteasome
E1 ubiquitin activating enzyme catalyzes the initial step in all ubiquitin-dependent processes. We report the isolation of uba1-204, a temperature-sensitive allele of the essential Saccharomyces cerevisiae E1 gene, UBA1. Uba1-204 cells exhibit dramatic inhibition of the ubiquitin–proteasome system, resulting in rapid depletion of cellular ubiquitin conjugates and stabilization of multiple substrates. We have employed the tight phenotype of this mutant to investigate the role ubiquitin conjugates play in the dynamic interaction of the UbL/UBA adaptor proteins Rad23 and Dsk2 with the proteasome. Although proteasomes purified from mutant cells are intact and proteolytically active, they are depleted of ubiquitin conjugates, Rad23, and Dsk2. Binding of Rad23 to these proteasomes in vitro is enhanced by addition of either free or substrate-linked ubiquitin chains. Moreover, association of Rad23 with proteasomes in mutant and wild-type cells is improved upon stabilizing ubiquitin conjugates with proteasome inhibitor. We propose that recognition of polyubiquitin chains by Rad23 promotes its shuttling to the proteasome in vivo
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