512 research outputs found
Joubert syndrome Arl13b functions at ciliary membranes and stabilizes protein transport in Caenorhabditis elegans
The small ciliary G protein Arl13b is required for cilium biogenesis and sonic hedgehog signaling and is mutated in patients with Joubert syndrome (JS). In this study, using Caenorhabditis elegans and mammalian cell culture systems, we investigated the poorly understood ciliary and molecular basis of Arl13b function. First, we show that Arl13b/ARL-13 localization is frequently restricted to a proximal ciliary compartment, where it associates with ciliary membranes via palmitoylation modification motifs. Next, we find that loss-of-function C. elegans arl-13 mutants possess defects in cilium morphology and ultrastructure, as well as defects in ciliary protein localization and transport; ciliary transmembrane proteins abnormally accumulate, PKD-2 ciliary abundance is elevated, and anterograde intraflagellar transport (IFT) is destabilized. Finally, we show that arl-13 interacts genetically with other ciliogenic and ciliary transport-associated genes in maintaining cilium structure/morphology and anterograde IFT stability. Together, these data implicate a role for JS-associated Arl13b at ciliary membranes, where it regulates ciliary transmembrane protein localizations and anterograde IFT assembly stability
A Study of Carbon Features in Type Ia Supernova Spectra
One of the major differences between various explosion scenarios of Type Ia
supernovae (SNe Ia) is the remaining amount of unburned (C+O) material and its
velocity distribution within the expanding ejecta. While oxygen absorption
features are not uncommon in the spectra of SNe Ia before maximum light, the
presence of strong carbon absorption has been reported only in a minority of
objects, typically during the pre-maximum phase. The reported low frequency of
carbon detections may be due to low signal-to-noise data, low abundance of
unburned material, line blending between C II 6580 and Si II 6355, ejecta
temperature differences, asymmetrical distribution effects, or a combination of
these. However, a survey of published pre-maximum spectra reveals that more SNe
Ia than previously thought may exhibit C II 6580 absorption features and relics
of line blending near 6300 Angstroms. Here we present new SN Ia observations
where spectroscopic signatures of C II 6580 are detected, and investigate the
presence of C II 6580 in the optical spectra of 19 SNe Ia using the
parameterized spectrum synthesis code, SYNOW. Most of the objects in our sample
that exhibit C II 6580 absorption features are of the low-velocity gradient
subtype. Our study indicates that the morphology of carbon-rich regions is
consistent with either a spherical distribution or a hemispheric asymmetry,
supporting the recent idea that SN Ia diversity may be a result of off-center
ignition coupled with observer line-of-sight effects.Comment: 10 papges, 9 figures, 3 table
On the modelling of the excesses of galaxy clusters over high-mass thresholds
In this work we present for the first time an application of the Pareto
approach to the modelling of the excesses of galaxy clusters over high-mass
thresholds. The distribution of those excesses can be described by the
generalized Pareto distribution (GPD), which is closely related to the
generalized extreme value (GEV) distribution. After introducing the formalism,
we study the impact of different thresholds and redshift ranges on the
distributions, as well as the influence of the survey area on the mean excess
above a given mass threshold. We also show that both the GPD and the GEV
approach lead to identical results for rare, thus high-mass and high-redshift,
clusters. As an example, we apply the Pareto approach to ACT-CL J0102-4915 and
SPT-CL J2106-5844 and derive the respective cumulative distribution functions
of the exceedance over different mass thresholds. We also study the possibility
to use the GPD as a cosmological probe. Since in the maximum likelihood
estimation of the distribution parameters all the information from clusters
above the mass threshold is used, the GPD might offer an interesting
alternative to GEV-based methods that use only the maxima in patches. When
comparing the accuracy with which the parameters can be estimated, it turns out
that the patch-based modelling of maxima is superior to the Pareto approach. In
an ideal case, the GEV approach is capable to estimate the location parameter
with a percent level precision for less than 100 patches. This result makes the
GEV based approach potentially also interesting for cluster surveys with a
smaller area.Comment: 10 pages, 8 figures, MNRAS accepted, minor modifications to match the
accepted versio
Hubble Space Telescope studies of low-redshift Type Ia supernovae: Evolution with redshift and ultraviolet spectral trends
We present an analysis of the maximum light, near ultraviolet (NUV; 2900-5500
A) spectra of 32 low redshift (0.001<z<0.08) Type Ia supernovae (SNe Ia),
obtained with the Hubble Space Telescope (HST). We combine this spectroscopic
sample with high-quality gri light curves obtained with robotic telescopes to
measure photometric parameters, such as stretch, optical colour, and
brightness. By comparing our data to a comparable sample of SNe Ia at
intermediate-z (0.4<z<0.9), we detect modest spectral evolution (3-sigma), in
the sense that our mean low-z NUV spectrum has a depressed flux compared to its
intermediate-z counterpart. We also see a strongly increased dispersion about
the mean with decreasing wavelength, confirming the results of earlier surveys.
These trends are consistent with changes in metallicity as predicted by
contemporary SN Ia spectral models. We also examine the properties of various
NUV spectral diagnostics in the individual spectra. We find a general
correlation between stretch and the velocity (or position) of many NUV spectral
features. In particular, we observe that higher stretch SNe have larger Ca II
H&K velocities, that also correlate with host galaxy stellar mass. This latter
trend is probably driven by the well-established correlation between stretch
and stellar mass. We find no trends between UV spectral features and optical
colour. Mean spectra constructed according to whether the SN has a positive or
negative Hubble residual show very little difference at NUV wavelengths,
indicating that the NUV evolution and variation we identify do not directly
correlate with Hubble residuals. Our work confirms and strengthens earlier
conclusions regarding the complex behaviour of SNe Ia in the NUV spectral
region, but suggests the correlations we find are more useful in constraining
progenitor models than improving the use of SNe Ia as cosmological probes.Comment: 22 pages, 14 figures, accepted in MNRAS with minor changes - Spectra
are available on WISeREP, http://www.weizmann.ac.il/astrophysics/wiserep
The Subluminous and Peculiar Type Ia Supernova PTF09dav
PTF09dav is a peculiar subluminous type Ia supernova (SN) discovered by the
Palomar Transient Factory (PTF). Spectroscopically, it appears superficially
similar to the class of subluminous SN1991bg-like SNe, but it has several
unusual features which make it stand out from this population. Its peak
luminosity is fainter than any previously discovered SN1991bg-like SN Ia (M_B
-15.5), but without the unusually red optical colors expected if the faint
luminosity were due to extinction. The photospheric optical spectra have very
unusual strong lines of Sc II and Mg I, with possible Sr II, together with
stronger than average Ti II and low velocities of ~6000 km/s. The host galaxy
of PTF09dav is ambiguous. The SN lies either on the extreme outskirts (~41kpc)
of a spiral galaxy, or in an very faint (M_R>-12.8) dwarf galaxy, unlike other
1991bg-like SNe which are invariably associated with massive, old stellar
populations. PTF09dav is also an outlier on the light-curve-width--luminosity
and color--luminosity relations derived for other sub-luminous SNe Ia. The
inferred 56Ni mass is small (0.019+/-0.003Msun), as is the estimated ejecta
mass of 0.36Msun. Taken together, these properties make PTF09dav a remarkable
event. We discuss various physical models that could explain PTF09dav. Helium
shell detonation or deflagration on the surface of a CO white-dwarf can explain
some of the features of PTF09dav, including the presence of Sc and the low
photospheric velocities, but the observed Si and Mg are not predicted to be
very abundant in these models. We conclude that no single model is currently
capable of explaining all of the observed signatures of PTF09dav.Comment: Accepted for publication in Ap
The strongest gravitational lenses: II. Is the large Einstein radius of MACS J0717.5+3745 in conflict with LCDM?
Can the standard cosmological model be questioned on the basis of a single
observed extreme galaxy cluster? Usually, the word extreme refers directly to
cluster mass, which is not a direct observable and thus subject to substantial
uncertainty. Hence, it is desirable to extend studies of extreme clusters to
direct observables, such as the Einstein radius (ER). We aim to evaluate the
occurrence probability of the large observed ER of MACS J0717.5 within the
standard LCDM cosmology. In particular, we want to model the distribution
function of the single largest ER in a given cosmological volume and to study
which underlying assumptions and effects have the strongest impact on the
results. We obtain this distribution by a Monte Carlo approach, based on the
semi-analytic modelling of the halo population on the past lightcone. After
sampling the distribution, we fit the results with the general extreme value
(GEV) distribution which we use for the subsequent analysis. We find that the
distribution of the maximum ER is particularly sensitive to the precise choice
of the halo mass function, lens triaxiality, the inner slope of the halo
density profile and the mass-concentration relation. Using the distributions so
obtained,we study the occurrence probability of the large ER of MACS J0717.5,
finding that this system is not in tension with LCDM. We also find that the GEV
distribution can be used to fit very accurately the sampled distributions and
that all of them can be described by a Frechet distribution. With a multitude
of effects that strongly influence the distribution of the single largest ER,
it is more than doubtful that the standard LCDM cosmology can be ruled out on
the basis of a single observation. If, despite the large uncertainties in the
underlying assumptions, one wanted to do so, a much larger ER (> 100 arcsec)
than that of MACS J0717.5 would have to be observed.Comment: 15 pages, 11 figures, accepted for publication in Astronomy and
Astrophysics, minor corrections to match the accepted version, added
discussion of the distribution of the largest Einstein radii for the MACS
survey area, extended Fig.
Gender Differences in Russian Colour Naming
In the present study we explored Russian colour naming in a web-based psycholinguistic experiment
(http://www.colournaming.com). Colour singletons representing the Munsell Color Solid (N=600 in total) were presented on a computer monitor and named using an unconstrained colour-naming method. Respondents were
Russian speakers (N=713). For gender-split equal-size samples (NF=333, NM=333) we estimated and compared (i)
location of centroids of 12 Russian basic colour terms (BCTs); (ii) the number of words in colour descriptors; (iii) occurrences of BCTs most frequent non-BCTs. We found a close correspondence between females’ and males’
BCT centroids. Among individual BCTs, the highest inter-gender agreement was for seryj ‘grey’ and goluboj
‘light blue’, while the lowest was for sinij ‘dark blue’ and krasnyj ‘red’. Females revealed a significantly richer repertory of distinct colour descriptors, with great variety of monolexemic non-BCTs and “fancy” colour names; in comparison, males offered relatively more BCTs or their compounds. Along with these measures, we gauged
denotata of most frequent CTs, reflected by linguistic segmentation of colour space, by employing a synthetic
observer trained by gender-specific responses. This psycholinguistic representation revealed females’ more
refined linguistic segmentation, compared to males, with higher linguistic density predominantly along the redgreen axis of colour space
On Solving the Coronal Heating Problem
This article assesses the current state of understanding of coronal heating,
outlines the key elements of a comprehensive strategy for solving the problem,
and warns of obstacles that must be overcome along the way.Comment: Accepted by Solar Physics; Published by Solar Physic
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