117 research outputs found
Telomere structure and shortening in telomerase-deficient Trypanosoma brucei
Telomerase consists of a reverse transcriptase (TERT) and an RNA that contains a template for telomere-repeat extension. Telomerase is required to prevent telomere erosion and its activity or lack thereof is important for tumorigenesis and ageing. Telomerase has been identified in numerous organisms but it has not been studied in kinetoplastid protozoa. Trypanosoma brucei, the causative agent of African sleeping sickness, evades the host immune response by frequently changing its variant surface glycoprotein (VSG). The single expressed VSG is transcribed from one of âŒ20 subtelomeric âExpression Sitesâ, but the role telomeres might play in regulating VSG transcription and switching is unknown. We identified and sequenced the T.brucei TERT gene. Deleting TERT resulted in progressive telomere shortening of 3â6 bp per generation. In other organisms, the rate of telomere shortening is proportional to the length of the terminal 3âČ single-strand overhang. In T.brucei, G-overhangs were undetectable (<30 nt) by in-gel hybridization. The rate of telomere shortening therefore, agrees with the predicted shortening due to the end replication problem, and is consistent with our observation that G-overhangs are short. Trypanosomes whose telomere length can be manipulated provide a new tool to investigate the role of telomeres in antigenic variation
Blue Variable Stars from the MACHO database I: Photometry and Spectroscopy of the LMC sample
We present the photometric properties of 1279 blue variable stars within the
LMC. Photometry is derived from the MACHO database. The lightcurves of the
sample exhibit a variety of quasi-periodic and aperiodic outburst behavior. A
characteristic feature of the photometric variation is that the objects are
reddest when at maximum outburst. A subset of 102 objects were examined
spectroscopically. Within this subset, 91% exhibited Balmer emission in at
least one epoch, in some cases with spectacular spectral variability. The
variability observed in the sample is consistent with the establishment and
maintenance of the Be phenomenon.Comment: 19 pages, AJ accepte
Explaining UXOR variability with self-shadowed disks
In this Letter we propose a new view on UX Orionis type variability. The idea
is based on the earlier proposal by various authors that UXORs are
nearly-edge-on disks in which hydrodynamic fluctuations could cause clumps of
dust and gas to cross the line of sight. However, because the standard disk
models have a flaring geometry, it is mostly the outer regions of the disk that
obscure the star. The time scales for such obscuration events would be too long
to match the observed time scales of weeks to months. Recent 2-D
self-consistent models of Herbig Ae/Be protoplanetary disks (Dullemond et al.
2002,2003 henceforth D02/DD03), however, have indicated that for Herbig Ae/Be
star disks there exists, in addition to the usual flared disks, also a new
class of disks: those that are fully self-shadowed. Only their puffed-up inner
rim (at the dust evaporation radius) is directly irradiated by the star, while
the disk at larger radius resides in the shadow of the rim. For these disks
there exist inclinations at which the line of sight towards the star skims the
upper parts of the puffed-up inner rim, while passing high over the surface of
outer disk regions. Small hydrodynamic fluctuations in the puffed-up inner rim
could then be held responsible for the extinction events seen in UXORs. If this
idea is correct, it makes a prediction for the shape of the SEDs of these
stars. It was shown by D02/DD03 that flared disks have a strong far-IR excess
and can be classified as `group I' (in the classification of Meeus et al.
2001), while self-shadowed disks have a relatively weak far-IR excess and are
classified as `group II'. Our model therefore predicts that UXORs belong to the
`group II' sources. We show that this correlation is indeed found within a
sample of 86 Herbig Ae/Be stars.Comment: Accepted for publication in ApJ Letters (a few lines added to
original version to accommodate comments of referee
Chiral and herringbone symmetry breaking in water-surface monolayers
We report the observation from monolayers of eicosanoic acid in the LâČ2 phase of three distinct out-of-plane first-order diffraction peaks, indicating molecular tilt in a nonsymmetry direction and hence the absence of mirror symmetry. At lower pressures the molecules tilt in the direction of their nearest neighbors. In this region we find a structural transition, which we tentatively identify as the rotator-herringbone transition L2dâL2h
Phase Behaviour of Amphiphilic Monolayers: Theory and Simulation
Coarse grained models of monolayers of amphiphiles (Langmuir monolayers) have
been studied theoretically and by computer simulations. We discuss some of the
insights obtained with this approach, and present new simulation results which
show that idealised models can successfully reproduce essential aspects of the
generic phase behaviour of Langmuir monolayers.Comment: To appear in J. Phys.: Cond. Matte
Relation between the luminosity of young stellar objects and their circumstellar environment
We present a new model-independent method of comparison of NIR visibility
data of YSOs. The method is based on scaling the measured baseline with the
YSO's distance and luminosity, which removes the dependence of visibility on
these two variables. We use this method to compare all available NIR visibility
data and demonstrate that it distinguishes YSOs of luminosity >1000L_sun
(low-L) from YSOs of <1000L_sun (high-L). This confirms earlier suggestions,
based on fits of image models to the visibility data, for the difference
between the NIR sizes of these two luminosity groups. When plotted against the
``scaled'' baseline, the visibility creates the following data clusters: low-L
Herbig Ae/Be stars, T Tauri stars, and high-L Herbig Be stars. The T Tau
cluster is similar to the low-L Herbig Ae/Be cluster, which has ~7 times
smaller ``scaled'' baselines than the high-L Herbig Be cluster. We model the
shape and size of clusters with different image models and find that low-L
Herbig stars are the best explained by the uniform brightness ring and the halo
model, T Tauri stars with the halo model, and high-L Herbig stars with the
accretion disk model. However, the plausibility of each model is not well
established. Therefore, we try to build a descriptive model of the
circumstellar environment consistent with various observed properties of YSOs.
We argue that low-L YSOs have optically thick disks with the optically thin
inner dust sublimation cavity and an optically thin dusty outflow above the
inner disk regions. High-L YSOs have optically thick accretion disks with high
accretion rates enabling gas to dominate the NIR emission over dust. Although
observations would favor such a description of YSOs, the required dust
distribution is not supported by our current understanding of dust dynamics.Comment: 20 pages, 12 figures, Accepted for publication in the Astrophysical
Journa
VLT/NACO adaptive optics imaging of the TY CrA system - A fourth stellar component candidate detected
We report the detection of a possible subsolar mass companion to the triple
young system TY CrA using the NACO instrument at the VLT UT4 during its
commissioning. Assuming for TY CrA a distance similar to that of the close
binary system HD 176386, the photometric spectral type of this fourth stellar
component candidate is consistent with an ~M4 star. We discuss the dynamical
stability of this possible quadruple system as well as the possible location of
dusty particles inside or outside the system.Comment: 4 pages, 2 figures postscrip
Inner disc rearrangement revealed by dramatic brightness variations in the young star PV Cep
Young Sun-like stars at the beginning of the pre-main sequence (PMS)
evolution are surrounded by accretion discs and remnant protostellar envelopes.
Photometric and spectroscopic variations of these stars are driven by
interactions of the star with the disc. Time scales and wavelength dependence
of the variability carry information on the physical mechanisms behind these
interactions. We conducted multi-epoch, multi-wavelength study of PV Cep, a
strongly variable, accreting PMS star. By combining our own observations from
2004-2010 with archival and literature data, we show that PV Cep started a
spectacular fading in 2005, reaching an I_C-band amplitude of 4 mag. Analysis
of variation of the optical and infrared fluxes, colour indices, and emission
line fluxes suggests that the photometric decline in 2005-2009 resulted from an
interplay between variable accretion and circumstellar extinction: since the
central luminosity of the system is dominated by accretion, a modest drop in
the accretion rate could induce the drastic restructuring of the inner disc.
Dust condensation in the inner disc region might have resulted in the
enhancement of the circumstellar extinction.Comment: 11 pages, 4 figures, accepted for publication by MNRAS. 3 online
tables adde
Magnetospheres and Disk Accretion in Herbig Ae/Be Stars
We present evidence of magnetically-mediated disk accretion in Herbig Ae/Be
stars. Magnetospheric accretion models of Balmer and sodium profiles calculated
with appropriate stellar and rotational parameters are in qualitative agreement
with the observed profiles of the Herbig Ae star UX Ori, and yield a mass
accretion rate of ~ 10^{-8} Msun/yr. If more recent indications of an extremely
large rotation rate for this object are correct, the magnetic field geometry
must deviate from that of a standard dipole in order to produce line emission
consistent with observed flux levels. Models of the associated accretion shock
qualitatively explain the observed distribution of excess fluxes in the Balmer
discontinuity for a large ensemble of Herbig Ae/Be stars, and imply typically
small mass accretion rates, < 10^{-7} Msun/yr. In order for accretion to
proceed onto the star, significant amounts of gas must exist inside the dust
destruction radius, which is potentially problematic for recently advocated
scenarios of "puffed" inner dust wall geometries. However, our models of the
inner gas disk show that for the typical accretion rates we have derived, the
gas should be generally optically thin, thus allowing direct stellar
irradiation of the inner dust edge of the disk.Comment: 32 pages, 12 figures, accepted by Ap
Herringbone ordering and lattice distortions in a planar-molecule model for Langmuir monolayers
A model of planar molecules, made up of "atoms" interacting by Lennard-Jones potentials and arranged to mimic the cross section of alkyl chains, is used to study the problem of backbone plane ordering in Langmuir monolayers. It is shown that two minima of the interaction energy are reached if molecules lie on the sites of a centered rectangular lattice in a herringbone configuration with two different dihedral angles. These orientationally ordered phases can be related to the so-called herringbone and pseudoherringbone structures, whose lattice distortions qualitatively agree with those determined by means of grazing incidence x-ray diffraction experiments on Langmuir monolayers. A third energy minimum is obtained for a configuration of parallel molecules on an oblique lattice, which has also been observed in some experiments. The competition between the three phases is investigated, upon varying geometric parameters of the model molecules and surface pressure. The effect of temperature is analyzed in a mean field approximation, by taking into account the orientational entropy contribution on a lattice system with variable unit cell parameters. In this framework the transition to an orientationally disordered phase is also pointed out
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