44 research outputs found
X-ray irradiated protoplanetary disk atmospheres II: Predictions from models in hydrostatic equilibrium
We present new models for the X-ray photoevaporation of circumstellar discs
which suggest that the resulting mass loss (occurring mainly over the radial
range 10-40 AU) may be the dominant dispersal mechanism for gas around low mass
pre-main sequence stars, contrary to the conclusions of previous workers. Our
models combine use of the MOCASSIN Monte Carlo radiative transfer code and a
self-consistent solution of the hydrostatic structure of the irradiated disc.
We estimate the resulting photoevaporation rates assuming sonic outflow at the
surface where the gas temperature equals the local escape temperature and
derive mass loss rates of ~10^{-9} M_sun/yr, typically a factor 2-10 times
lower than the corresponding rates in our previous work (Ercolano et al., 2008)
where we did not adjust the density structure of the irradiated disc. The
somewhat lower rates, and the fact that mass loss is concentrated towards
slightly smaller radii, result from the puffing up of the heated disc at a few
AU which partially screens the disc at tens of AU. (.....) We highlight the
fact that X-ray photoevaporation has two generic advantages for disc dispersal
compared with photoevaporation by extreme ultraviolet (EUV) photons that are
only modestly beyond the Lyman limit: the demonstrably large X-ray fluxes of
young stars even after they have lost their discs and the fact that X-rays are
effective at penetrating much larger columns of material close to the star
(abridged).Comment: Accepted for publication in ApJ, 12 pages, 11 figure
Separase prevents genomic instability by controlling replication fork speed
Proper chromosome segregation is crucial for preserving genomic integrity, and errors in this process cause chromosome mis-segregation, which may contribute to cancer development. Sister chromatid separation is triggered by Separase, an evolutionary conserved protease that cleaves the cohesin complex, allowing the dissolution of sister chromatid cohesion. Here we provide evidence that Separase participates in genomic stability maintenance by controlling replication fork speed. We found that Separase interacted with the replication licensing factors MCM2-7, and genome-wide data showed that Separase co-localized with MCM complex and cohesin. Unexpectedly, the depletion of Separase increased the fork velocity about 1.5-fold and caused a strong acetylation of cohesin's SMC3 subunit and altered checkpoint response. Notably, Separase silencing triggered genomic instability in both HeLa and human primary fibroblast cells. Our results show a novel mechanism for fork progression mediated by Separase and thus the basis for genomic instability associated with tumorigenesis
Separase prevents genomic instability by controlling replication fork speed
Proper chromosome segregation is crucial for preserving genomic integrity, and errors in this process cause chromosome mis-segregation, which may contribute to cancer development. Sister chromatid separation is triggered by Separase, an evolutionary conserved protease that cleaves the cohesin complex, allowing the dissolution of sister chromatid cohesion. Here we provide evidence that Separase participates in genomic stability maintenance by controlling replication fork speed. We found that Separase interacted with the replication licensing factors MCM2-7, and genome-wide data showed that Separase co-localized with MCM complex and cohesin. Unexpectedly, the depletion of Separase increased the fork velocity about 1.5-fold and caused a strong acetylation of cohesin's SMC3 subunit and altered checkpoint response. Notably, Separase silencing triggered genomic instability in both HeLa and human primary fibroblast cells. Our results show a novel mechanism for fork progression mediated by Separase and thus the basis for genomic instability associated with tumorigenesis
X-ray Photoevaporation-starved T Tauri Accretion
X-ray luminosities of accreting T Tauri stars are observed to be
systematically lower than those of non-accretors. There is as yet no widely
accepted physical explanation for this effect, though it has been suggested
that accretion somehow suppresses, disrupts or obscures coronal X-ray activity.
Here, we suggest that the opposite might be the case: coronal X-rays modulate
the accretion flow. We re-examine the X-ray luminosities of T Tauri stars in
the Orion Nebula Cluster and find that not only are accreting stars
systematically fainter, but that there is a correlation between mass accretion
rate and stellar X-ray luminosity. We use the X-ray heated accretion disk
models of Ercolano et al. to show that protoplanetary disk photoevaporative
mass loss rates are strongly dependent on stellar X-ray luminosity and
sufficiently high to be competitive with accretion rates. X-ray disk heating
appears to offer a viable mechanism for modulating the gas accretion flow and
could be at least partially responsible for the observed correlation between
accretion rates and X-ray luminosities of T Tauri stars.Comment: 4 pages 3 figures, ApJ Letters, in pres
X-ray irradiated protoplanetary disk atmospheres I: Predicted emission line spectrum and photoevaporation
We present MOCASSIN 2D photoionisation and dust radiative transfer models of
a prototypical T Tauri disk irradiated by X-rays from the young pre-main
sequence star. The calculations demonstrate a layer of hot gas reaching
temperatures of ~10^6 K at small radii and ~10^4 K at a distance of 1 AU. The
gas temperatures decrease sharply with depth, but appear to be completely
decoupled from dust temperatures down to a column depth of ~5*10^21 cm^-2.
We predict that several fine-structure and forbidden lines of heavy elements,
as well as recombination lines of hydrogen and helium, should be observable
with current and future instrumentation, although optical lines may be
smothered by the stellar spectrum. Predicted line luminosities are given for
the the brightest collisionally excited lines (down to ~10^-8L_sun, and for
recombination transitions from several levels of HI and HeI.
The mass loss rate due to X-ray photoevaporation estimated from our models is
of the order of 10^-8 M_sun yr^-1, implying a dispersal timescale of a few Myr
for a disk of mass 0.027 M_sun, which is the mass of the disk structure model
we employed. We discuss the limitations of our model and highlight the need for
further calculations that should include the simultaneous solution of the 2D
radiative transfer problem and the 1D hydrostatic equilibrium in the polar
direction.Comment: 11 pages, 6 figures. Accepted for publication in Ap
OXavidin for Tissue Targeting Biotinylated Therapeutics
Avidin is a glycoprotein from hen egg white that binds biotin with very high affinity. Here we describe OXavidin, a product containing aldehyde groups, obtained by ligand-assisted sugar oxidation of avidin by sodium periodate. OXavidin chemically reacts with cellular and tissue proteins through Schiff's base formation thus residing in tissues for weeks while preserving the biotin binding capacity. The long tissue residence of OXavidin as well as that of OXavidin/biotinylated agent complex occurs in normal and neoplastic tissues and immunohistochemistry shows a strong and homogenous stromal localization. Once localized in tissue/tumor, OXavidin becomes an “artificial receptor” for intravenous injected biotin allowing tumor targeting with biotinylated therapeutics like radioisotopes or toxins. Moreover, present data also suggest that OXavidin might be useful for the homing of biotinylated cells. Overall, OXavidin exhibits a remarkable potential for many different therapeutic applications
Protoplanetary disc evolution and dispersal: the implications of X-ray photoevaportion
(Abridged) We explore the role of X-ray photoevaporation in the evolution and
dispersal of viscously evolving T-Tauri discs. We show that the X-ray
photoevaporation wind rates scale linearly with X-ray luminosity, such that the
observed range of X-ray luminosities for solar-type T-Tauri stars (10e28-10e31
erg\s) gives rise to vigorous disc winds with rates of order 10e-10-10e-7
M_sun/yr. We use the wind solutions from radiation-hydrodynamic models, coupled
to a viscous evolution model to construct a population synthesis model so that
we may study the physical properties of evolving discs and so-called
`transition discs'. Current observations of disc lifetimes and accretion rates
can be matched by our model assuming a viscosity parameter alpha = 2.5e-3. Our
models confirm that X-rays play a dominant role in the evolution and dispersal
of protoplanetary discs giving rise to the observed diverse population of inner
hole `transition' sources which include those with massive outer discs, those
with gas in their inner holes and those with detectable accretion signatures.
To help understand the nature of observed transition discs we present a
diagnostic diagram based on accretion rates versus inner hole sizes that
demonstrate that, contrary to recent claims, many of the observed accreting and
non accreting transition discs can easily be explained by X-ray
photoevaporation. Finally, we confirm the conjecture of Drake et al. (2009),
that accretion is suppressed by the X-rays through `photoevaporation starved
accretion' and predict this effect can give rise to a negative correlation
between X-ray luminosity and accretion rate, as reported in the Orion data.Comment: Figure 12 and 13 have been updated. In the original version the
results from an unused model run were plotted by mistak
Theoretical spectra of photoevaporating protoplanetary discs: An atlas of atomic and low-ionisation emission lines
We present a calculation of the atomic and low-ionisation emission line
spectra of photoevaporating protoplanetary discs. Line luminosities and
profiles are obtained from detailed photoionisation calculations of the disc
and wind structures surrounding young active solar-type stars. The disc and
wind density and velocity fields were obtained from the recently developed
radiation-hydrodynamic models of Owen et al., that include stellar X-ray and
EUV irradiation of protoplanetary discs at various stages of clearing, from
primordial sources to inner hole sources of various hole sizes.
Our models compare favourably with currently available observations, lending
support to an X-ray driven photoevaporation model for disc dispersal. In
particular, we find that X-rays drive a warm, predominantly neutral flow where
the OI 6300A line can be produced by neutral hydrogen collisional excitation.
Our models can, for the first time, provide a very good match to both
luminosities and profiles of the low-velocity component of the OI 6300A line
and other forbidden lines observed by Hartigan et al., which covered a large
sample of T-Tauri stars.
We find that the OI 6300A and the NeII 12.8um lines are predominantly
produced in the X-ray-driven wind and thus appear blue-shifted by a few km/s
for some of the systems when observed at non-edge-on inclinations. We note
however that blue-shifts are only produced under certain conditions: X-ray
luminosity, spectral shape and inner hole size all affect the location of the
emitting region and the physical conditions in the wind. We caution therefore
that while a blueshifted line is a tell-tale sign of an outflow, the lack of a
blueshift should not be necessarily interpreted as a lack of outflow.Comment: 18 pages, 7 figures, accepted to be published in MNRAS - changes in
the revised version: reference list update
A novel TMPRSS6 mutation that prevents protease auto-activation causes IRIDA
IRIDA (iron-refractory iron-deficiency anaemia) is a rare autosomal-recessive disorder hallmarked by hypochromic microcytic anaemia, low transferrin saturation and high levels of the iron-regulated hormone hepcidin. The disease is caused by mutations in the transmembrane serine protease TMPRSS6 (transmembrane protease serine 6) that prevent inactivation of HJV (haemojuvelin), an activator of hepcidin transcription. In the present paper, we describe a patient with IRIDA who carries a novel mutation (Y141C) in the SEA domain of the TMPRSS6 gene. Functional characterization of the TMPRSS6(Y141C) mutant protein in cultured cells showed that it localizes to similar subcellular compartments as wild-type TMPRSS6 and binds HJV, but fails to auto-catalytically activate itself. As a consequence, hepcidin mRNA expression is increased, causing the clinical symptoms observed in this IRIDA patient. The present study provides important mechanistic insight into how TMPRSS6 is activated
The Structure of Brown Dwarf Circumstellar Disks
We present synthetic spectra for circumstellar disks that are heated by
radiation from a central brown dwarf. Under the assumption of vertical
hydrostatic equilibrium, our models yield scaleheights for brown dwarf disks in
excess of three times those derived for classical T Tauri (CTTS) disks. If the
near-IR excess emission observed from brown dwarfs is indeed due to
circumstellar disks, then the large scaleheights we find could have a
significant impact on the optical and near-IR detectability of such systems.
Our radiation transfer calculations show that such highly flared disks around
brown dwarfs will result in a large fraction of obscured sources due to
extinction of direct starlight by the disk over a wide range of sightlines. The
obscured fraction for a 'typical' CTTS is less than 20%. We show that the
obscured fraction for brown dwarfs may be double that for CTTS, but this
depends on stellar and disk mass. We also comment on possible confusion in
identifying brown dwarfs via color-magnitude diagrams: edge-on CTTS display
similar colors and magnitudes as a face-on brown dwarf plus disk systems.Comment: 13 pages, 13 figures, accepted for publication in MNRA