1,949 research outputs found
The very large G-protein coupled receptor VLGR1: a component of the ankle link complex required for the normal development of auditory hair bundles
Sensory hair bundles in the inner ear are composed of stereocilia that can be interconnected by a variety of different link types, including tip links, horizontal top connectors, shaft connectors, and ankle links. The ankle link antigen is an epitope specifically associated with ankle links and the calycal processes of photoreceptors in chicks. Mass spectrometry and immunoblotting were used to identify this antigen as the avian ortholog of the very large G-protein-coupled receptor VLGR1, the product of the Usher syndrome USH2C (Mass1) locus. Like ankle links, Vlgr1 is expressed transiently around the base of developing hair bundles in mice. Ankle links fail to form in the cochleae of mice carrying a targeted mutation in Vlgr1 (Vlgr1/del7TM), and the bundles become disorganized just after birth. FM1-43 [N-(3-triethylammonium)propyl)-4-(4-(dibutylamino)styryl) pyridinium dibromide] dye loading and whole-cell recordings indicate mechanotransduction is impaired in cochlear, but not vestibular, hair cells of early postnatal Vlgr1/del7TM mutant mice. Auditory brainstem recordings and distortion product measurements indicate that these mice are severely deaf by the third week of life. Hair cells from the basal half of the cochlea are lost in 2-month-old Vlgr1/del7TM mice, and retinal function is mildly abnormal in aged mutants. Our results indicate that Vlgr1 is required for formation of the ankle link complex and the normal development of cochlear hair bundles
Aldebaran's angular diameter: how well do we know it?
The bright, well-known K5 giant Aldebaran, alpha Tau, is probably the star
with the largest number of direct angular diameter determinations, achieved
over a long time by several authors using various techniques. In spite of this
wealth of data, or perhaps as a direct result of it, there is not a very good
agreement on a single angular diameter value. This is particularly unsettling
if one considers that Aldebaran is also used as a primary calibrator for some
angular resolution methods, notably for optical and infrared long baseline
interferometry. Directly connected to Aldebaran's angular diameter and its
uncertainties is its effective temperature, which also has been used for
several empirical calibrations. Among the proposed explanations for the
elusiveness of an accurate determination of the angular diameter of Aldebaran
are the possibility of temporal variations as well as a possible dependence of
the angular diameter on the wavelength. We present here a few, very accurate
new determinations obtained by means of lunar occultations and long baseline
interferometry. We derive an average value of 19.96+-0.03 milliarcseconds for
the uniform disk diameter. The corresponding limb-darkened value is 20.58+-0.03
milliarcseconds, or 44.2+-0.9 R(sun). We discuss this result, in connection
with previous determinations and with possible problems that may affect such
measurements.Comment: 8 pages, 4 figures, accepted for publication in A&
The Coevality of Young Binary Systems
Multiple star systems are commonly assumed to form coevally; they thus
provide the anchor for most calibrations of stellar evolutionary models. In
this paper we study the binary population of the Taurus-Auriga association,
using the component positions in an HR diagram in order to quantify the
frequency and degree of coevality in young binary systems. After identifying
and rejecting the systems that are known to be affected by systematic errors
(due to further multiplicity or obscuration by circumstellar material), we find
that the relative binary ages, |Delta log(tau)|, have an overall dispersion of
sigma~0.40 dex. Random pairs of Taurus members are coeval only to within
sigma~0.58 dex, indicating that Taurus binaries are indeed more coeval than the
association as a whole. However, the distribution of |Delta log(tau)| suggests
two populations, with ~2/3 of the sample appearing coeval to within the errors
(sigma~0.16 dex) and the other ~1/3 distributed in an extended tail reaching
|Delta log(tau)|~0.4-0.9 dex. To explain the finding of a multi-peaked
distribution, we suggest that the tail of the differential age distribution
includes unrecognized hierarchical multiples, stars seen in scattered light, or
stars with disk contamination; additional followup is required to rule out or
correct for these explanations. The relative coevality of binary systems does
not depend significantly on the system mass, mass ratio, or separation. Indeed,
any pair of Taurus members wider than ~10' (~0.7 pc) shows the full age spread
of the association.Comment: 23 pages, 11 figures; accepted to Ap
How large should whales be?
The evolution and distribution of species body sizes for terrestrial mammals
is well-explained by a macroevolutionary tradeoff between short-term selective
advantages and long-term extinction risks from increased species body size,
unfolding above the 2g minimum size induced by thermoregulation in air. Here,
we consider whether this same tradeoff, formalized as a constrained
convection-reaction-diffusion system, can also explain the sizes of fully
aquatic mammals, which have not previously been considered. By replacing the
terrestrial minimum with a pelagic one, at roughly 7000g, the terrestrial
mammal tradeoff model accurately predicts, with no tunable parameters, the
observed body masses of all extant cetacean species, including the 175,000,000g
Blue Whale. This strong agreement between theory and data suggests that a
universal macroevolutionary tradeoff governs body size evolution for all
mammals, regardless of their habitat. The dramatic sizes of cetaceans can thus
be attributed mainly to the increased convective heat loss is water, which
shifts the species size distribution upward and pushes its right tail into
ranges inaccessible to terrestrial mammals. Under this macroevolutionary
tradeoff, the largest expected species occurs where the rate at which
smaller-bodied species move up into large-bodied niches approximately equals
the rate at which extinction removes them.Comment: 7 pages, 3 figures, 2 data table
Evolutionary models for low-mass stars and brown dwarfs: uncertainties and limits at very young ages
We analyse pre-Main Sequence evolutionary tracks for low mass stars with
masses m \le 1.4 \msol based on the Baraffe et al. (1998) input physics. We
also extend the recent Chabrier et al. (2000) evolutionary models based on
dusty atmosphere to young brown dwarfs down to one mass of Jupiter. We analyse
current theoretical uncertainties due to molecular line lists, convection and
initial conditions. Simple tests on initial conditions show the high
uncertainties of models at ages \simle 1 Myr. We find a significant
sensitivity of atmosphere profiles to the treatment of convection at low
gravity and \te < 4000 K, whereas it vanishes as gravity increases. This
effect adds another source of uncertainty on evolutionary tracks at very early
phases. We show that at low surface gravity (\log g \simle 3.5,) the common
picture of vertical Hayashi lines with constant \te is oversimplified. The
effect of a variation of initial deuterium abundance is studied. We compare our
models with evolutionary tracks available in the literature and discuss the
main differences. We finally analyse to which extent current observations of
young systems provide a good test for pre-Main Sequence tracks.Comment: 12 pages, Latex file, uses aa.cls, accepted for publication in A&
HST/STIS observations of the RW Aurigae bipolar jet: mapping the physical parameters close to the source
We present the results of new spectral diagnostic investigations applied to
high-resolution long-slit spectra of the RW Aur bipolar jet obtained with
HST/STIS. The spectra include the forbidden doublets [O I] 6300,6363 \AA, [S
II] 6716,6731 \AA, and [N II] 6548, 6583 \AA that we utilized to determine
electron density, electron temperature, hydrogen ionisation fraction, total
hydrogen density, radial velocity and the mass outflow rate. We were able to
extract the parameters as far as 3".9 in the red- and 2".1 in the blueshifted
beam. The RW Aur jet appears to be the second densest outflow from a T Tauri
star studied so far, but its other properties are quite similar to those found
in other jets from young stars. The overall trend of the physical parameters
along the first few arcseconds of the RW Aur jet is similar to that of HH 30
and DG Tau and this can reflect analogies in the mechanisms operating in that
region, suggesting the same engine is accelerating the jets in the T Tauri
stars with outflows. Our study of the RW Aur jet indicates for the first time
that, despite the detected marked asymmetries in physical and kinematic
properties between the two lobes, the mass outflow rates in the two lobes are
similar. This appears to indicate that the central engine has constraining
symmetries on both sides of the system, and that the observed asymmetries are
probably due to different environmental conditions.Comment: 24 pages, 10 figures, accepted for publication in the Astronomy and
Astrophysic
A Test of Pre-Main Sequence Evolutionary Models Across the Stellar/Substellar Boundary Based on Spectra of the Young Quadruple GG Tau
We present spatially separated optical spectra of the components of the young
hierarchical quadruple GG Tau. Spectra of GG Tau Aa and Ab (separation 0".25 ~
35 AU) were obtained with the Faint Object Spectrograph aboard the Hubble Space
Telescope. Spectra of GG Tau Ba and Bb (separation 1".48 ~ 207 AU) were
obtained with both the HIRES and the LRIS spectrographs on the W. M. Keck
telescopes. The components of this mini-cluster, which span a wide range in
spectral type (K7 - M7), are used to test both evolutionary models and the
temperature scale for very young, low mass stars under the assumption of coeval
formation. Of the evolutionary models tested, those of Baraffe et al. (1998,
A&A, 337, 403) yield the most consistent ages when combined with a temperature
scale intermediate between that of dwarfs and giants. The version of the
Baraffe et al. models computed with a mixing length nearly twice the pressure
scale height is of particular interest as it predicts masses for GG Tau Aa and
Ab that are in agreement with their dynamical mass estimate.
Using this evolutionary model and a coeval (at 1.5 Myrs) temperature scale,
we find that the coldest component of the GG Tau system, GG Tau Bb, is
substellar with a mass of 0.044 +/- 0.006 Msun. This brown dwarf companion is
especially intriguing as it shows signatures of accretion, although this
accretion is not likely to alter its mass significantly. GG Tau Bb is currently
the lowest mass, spectroscopically confirmed companion to a T Tauri star, and
is one of the coldest, lowest mass T Tauri objects in the Taurus-Auriga star
forming region.Comment: 25 pages, 6 figures, accepted for publication in The Astrophysical
Journa
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