66 research outputs found
Diagnostic Line Emission from EUV and X-ray Illuminated Disks and Shocks around Low Mass stars
Extreme ultraviolet (EUV, 13.6 eV < h\nu \lta 100 eV) and X-rays in the
0.1-2 keV band can heat the surfaces of disks around young, low mass stars to
thousands of degrees and ionize species with ionization potentials greater than
13.6 eV. Shocks generated by protostellar winds can also heat and ionize the
same species close to the star/disk system. These processes produce diagnostic
lines (e.g., [NeII] 12.8 m and [OI] 6300 \AA) that we model as functions
of key parameters such as EUV luminosity and spectral shape, X-ray luminosity
and spectral shape, and wind mass loss rate and shock speed. Comparing our
models with observations, we conclude that either internal shocks in the winds
or X-rays incident on the disk surfaces often produce the observed [NeII] line,
although there are cases where EUV may dominate. Shocks created by the oblique
interaction of winds with disks are unlikely [NeII] sources because these
shocks are too weak to ionize Ne. Even if [NeII] is mainly produced by X-rays
or internal wind shocks, the neon observations typically place upper limits of
\lta 10^{42} s on the EUV photon luminosity of these young low mass
stars. The observed [OI] 6300 \AA line has both a low velocity component (LVC)
and a high velocity component. The latter likely arises in internal wind
shocks. For the former we find that X-rays likely produce more [OI] luminosity
than either the EUV layer, the transition layer between the EUV and X-ray
layer, or the shear layer where the protostellar wind shocks and entrains disk
material in a radial flow across the surface of the disk. Our soft X-ray models
produce [OI] LVCs with luminosities up to L, but may not be
able to explain the most luminous LVCs.Comment: 51 pages, 10 figures, accepted to Ap
Exchange Instabilities in Semiconductor Double Quantum Well Systems
We consider various exchange-driven electronic instabilities in semiconductor
double-layer systems in the absence of any external magnetic field. We
establish that there is no exchange-driven bilayer to monolayer charge transfer
instability in the double-layer systems. We show that, within the unrestricted
Hartree-Fock approximation, the low density stable phase (even in the absence
of any interlayer tunneling) is a quantum ``pseudospin rotated'' spontaneous
interlayer phase coherent spin-polarized symmetric state rather than the
classical Ising-like charge-transfer phase. The U(1) symmetry of the double
quantum well system is broken spontaneously at this low density quantum phase
transition, and the layer density develops quantum fluctuations even in the
absence of any interlayer tunneling. The phase diagram for the double quantum
well system is calculated in the carrier density--layer separation space, and
the possibility of experimentally observing various quantum phases is
discussed. The situation in the presence of an external electric field is
investigated in some detail using the
spin-polarized-local-density-approximation-based self-consistent technique and
good agreement with existing experimental results is obtained.Comment: 24 pages, figures included. Also available at
http://www-cmg.physics.umd.edu/~lzheng/preprint/ct.uu/ . Revised final
version to appear in PR
Sheared Flow As A Stabilizing Mechanism In Astrophysical Jets
It has been hypothesized that the sustained narrowness observed in the
asymptotic cylindrical region of bipolar outflows from Young Stellar Objects
(YSO) indicates that these jets are magnetically collimated. The j cross B
force observed in z-pinch plasmas is a possible explanation for these
observations. However, z-pinch plasmas are subject to current driven
instabilities (CDI). The interest in using z-pinches for controlled nuclear
fusion has lead to an extensive theory of the stability of magnetically
confined plasmas. Analytical, numerical, and experimental evidence from this
field suggest that sheared flow in magnetized plasmas can reduce the growth
rates of the sausage and kink instabilities. Here we propose the hypothesis
that sheared helical flow can exert a similar stabilizing influence on CDI in
YSO jets.Comment: 13 pages, 2 figure
Photoevaporation of Circumstellar Disks by FUV, EUV and X-ray Radiation from the Central Star
We calculate the rate of photoevaporation of a circumstellar disk by
energetic radiation (FUV, 6eV 13.6eV; EUV, 13.6eV 0.1keV; and
Xrays, keV) from its central star. We focus on the effects of FUV and
X-ray photons since EUV photoevaporation has been treated previously, and
consider central star masses in the range . Contrary to
the EUV photoevaporation scenario, which creates a gap at about $r_g\sim 7\
(M_*/1{\rm M}_{\odot})\sim 2\sim 10_* \sim 1{\rm M}_{\odot}\sim 10^60.3-3_{\odot}\gtrsim 7 {\rm M}_{\odot}\sim 10^5$ years)
due to their high EUV and FUV fields. Disk lifetimes are shorter for shallow
surface density distributions and when the dust opacity in the disk is reduced
by processes such as grain growth or settling. The latter suggests that the
photoevaporation process may accelerate as the dust disk evolves.Comment: Astrophysical Journal, Accepte
Docking of LDCVs Is Modulated by Lower Intracellular [Ca2+] than Priming
Many regulatory steps precede final membrane fusion in neuroendocrine cells. Some parts of this preparatory cascade, including fusion and priming, are dependent on the intracellular Ca2+ concentration ([Ca2+]i). However, the functional implications of [Ca2+]i in the regulation of docking remain elusive and controversial due to an inability to determine the modulatory effect of [Ca2+]i. Using a combination of TIRF-microscopy and electrophysiology we followed the movement of large dense core vesicles (LDCVs) close to the plasma membrane, simultaneously measuring membrane capacitance and [Ca2+]i. We found that a free [Ca2+]i of 700 nM maximized the immediately releasable pool and minimized the lateral mobility of vesicles, which is consistent with a maximal increase of the pool size of primed LDCVs. The parameters that reflect docking, i.e. axial mobility and the fraction of LDCVs residing at the plasma membrane for less than 5 seconds, were strongly decreased at a free [Ca2+]i of 500 nM. These results provide the first evidence that docking and priming occur at different free intracellular Ca2+ concentrations, with docking efficiency being the most robust at 500 nM
Multigene Phylogeny of Choanozoa and the Origin of Animals
Animals are evolutionarily related to fungi and to the predominantly unicellular protozoan phylum Choanozoa, together known as opisthokonts. To establish the sequence of events when animals evolved from unicellular ancestors, and understand those key evolutionary transitions, we need to establish which choanozoans are most closely related to animals and also the evolutionary position of each choanozoan group within the opisthokont phylogenetic tree. Here we focus on Ministeria vibrans, a minute bacteria-eating cell with slender radiating tentacles. Single-gene trees suggested that it is either the closest unicellular relative of animals or else sister to choanoflagellates, traditionally considered likely animal ancestors. Sequencing thousands of Ministeria protein genes now reveals about 14 with domains of key significance for animal cell biology, including several previously unknown from deeply diverging Choanozoa, e.g. domains involved in hedgehog, Notch and tyrosine kinase signaling or cell adhesion (cadherin). Phylogenetic trees using 78 proteins show that Ministeria is not sister to animals or choanoflagellates (themselves sisters to animals), but to Capsaspora, another protozoan with thread-like (filose) tentacles. The Ministeria/Capsaspora clade (new class Filasterea) is sister to animals and choanoflagellates, these three groups forming a novel clade (filozoa) whose ancestor presumably evolved filose tentacles well before they aggregated as a periciliary collar in the choanoflagellate/sponge common ancestor. Our trees show ichthyosporean choanozoans as sisters to filozoa; a fusion between ubiquitin and ribosomal small subunit S30 protein genes unifies all holozoa (filozoa plus Ichthyosporea), being absent in earlier branching eukaryotes. Thus, several successive evolutionary innovations occurred among their unicellular closest relatives prior to the origin of the multicellular body-plan of animals
Telomerase promoter mutations in cancer: an emerging molecular biomarker?
João Vinagre, Vasco Pinto and Ricardo Celestino contributed equally to
the manuscript.Cell immortalization has been considered for a long time as a classic hallmark of cancer cells. Besides telomerase reactivation, such immortalization could be due to telomere maintenance through the “alternative mechanism of telomere lengthening” (ALT) but the mechanisms underlying both forms of reactivation remained elusive. Mutations in the coding region of telomerase gene are very rare in the cancer setting, despite being associated with some degenerative diseases. Recently, mutations in telomerase (TERT) gene promoter were found in sporadic and familial melanoma and subsequently in several cancer models, notably in gliomas, thyroid cancer and bladder cancer. The importance of these findings has been reinforced by the association of TERT mutations in some cancer types with tumour aggressiveness and patient survival. In the first part of this review, we summarize the data on the biology of telomeres and telomerase, available methodological approaches and non-neoplastic diseases associated with telomere dysfunction. In the second part, we review the information on telomerase expression and genetic alterations in the most relevant types of cancer (skin, thyroid, bladder and central nervous system) on record, and discuss the value of telomerase as a new biomarker with impact on the prognosis and survival of the patients and as a putative therapeutic target
Connecting Planetary Composition with Formation
The rapid advances in observations of the different populations of
exoplanets, the characterization of their host stars and the links to the
properties of their planetary systems, the detailed studies of protoplanetary
disks, and the experimental study of the interiors and composition of the
massive planets in our solar system provide a firm basis for the next big
question in planet formation theory. How do the elemental and chemical
compositions of planets connect with their formation? The answer to this
requires that the various pieces of planet formation theory be linked together
in an end-to-end picture that is capable of addressing these large data sets.
In this review, we discuss the critical elements of such a picture and how they
affect the chemical and elemental make up of forming planets. Important issues
here include the initial state of forming and evolving disks, chemical and dust
processes within them, the migration of planets and the importance of planet
traps, the nature of angular momentum transport processes involving turbulence
and/or MHD disk winds, planet formation theory, and advanced treatments of disk
astrochemistry. All of these issues affect, and are affected by the chemistry
of disks which is driven by X-ray ionization of the host stars. We discuss how
these processes lead to a coherent end-to-end model and how this may address
the basic question.Comment: Invited review, accepted for publication in the 'Handbook of
Exoplanets', eds. H.J. Deeg and J.A. Belmonte, Springer (2018). 46 pages, 10
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Telomere length and telomerase activity in non-small cell lung cancer prognosis: clinical usefulness of a specific telomere status
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