227 research outputs found
Gelsolin superfamily proteins: key regulators of cellular functions
Abstract.: Cytoskeletal rearrangement occurs in a variety of cellular processes and involves a wide spectrum of proteins. Among these, the gelsolin superfamily proteins control actin organization by severing filaments, capping filament ends and nucleating actin assembly [1]. Gelsolin is the founding member of this family, which now contains at least another six members: villin, adseverin, capG, advillin, supervillin and flightless I. In addition to their respective role in actin filament remodeling, these proteins have some specific and apparently non-overlapping particular roles in several cellular processes, including cell motility, control of apoptosis and regulation of phagocytosis (summarized in table 1). Evidence suggests that proteins belonging to the gelsolin superfamily may be involved in other processes, including gene expression regulation. This review will focus on some of the known functions of the gelsolin superfamily proteins, thus providing a basis for reflection on other possible and as yet incompletely understood roles for these protein
Modeling Spitzer observations of VV Ser. I. The circumstellar disk of a UX Orionis star
We present mid-infrared Spitzer-IRS spectra of the well-known UX Orionis star
VV Ser. We combine the Spitzer data with interferometric and spectroscopic data
from the literature covering UV to submillimeter wavelengths. The full set of
data are modeled by a two-dimensional axisymmetric Monte Carlo radiative
transfer code. The model is used to test the prediction of (Dullemond et al.
2003) that disks around UX Orionis stars must have a self-shadowed shape, and
that these disks are seen nearly edge-on, looking just over the edge of a
puffed-up inner rim, formed roughly at the dust sublimation radius. We find
that a single, relatively simple model is consistent with all the available
observational constraints spanning 4 orders of magnitude in wavelength and
spatial scales, providing strong support for this interpretation of UX Orionis
stars. The grains in the upper layers of the puffed-up inner rim must be small
(0.01-0.4 micron) to reproduce the colors (R_V ~ 3.6) of the extinction events,
while the shape and strength of the mid-infrared silicate emission features
indicate that grains in the outer disk (> 1-2 AU) are somewhat larger (0.3-3.0
micron). From the model fit, the location of the puffed-up inner rim is
estimated to be at a dust temperature of 1500 K or at 0.7-0.8 AU for small
grains. This is almost twice the rim radius estimated from near-infrared
interferometry. A best fitting model for the inner rim in which large grains in
the disk mid-plane reach to within 0.25 AU of the star, while small grains in
the disk surface create a puffed-up inner rim at ~0.7-0.8 AU, is able to
reproduce all the data, including the near-infrared visibilities. [Abstract
abridged]Comment: 12 pages, accepted for publication in Ap
Probing protoplanetary disks with silicate emission: Where is the silicate emission zone?
Recent results indicate that the grain size and crystallinity inferred from observations of silicate features may be correlated with the spectral type of the central star and/or disk geometry. In this paper, we show that grain size, as probed by the 10 ÎŒm silicate feature peak-to-continuum and 11.3 to 9.8 ÎŒm flux ratios, is inversely proportional to log Lsstarf. These trends can be understood using a simple two-layer disk model for passive irradiated flaring disks, CGPLUS. We find that the radius, R10, of the 10 ÎŒm silicate emission zone in the disk goes as (L*/Lâ)^0.56, with slight variations depending on disk geometry (flaring angle and inner disk radius). The observed correlations, combined with simulated emission spectra of olivine and pyroxene mixtures, imply a dependence of grain size on luminosity. Combined with the fact that R10 is smaller for less luminous stars, this implies that the apparent grain size of the emitting dust is larger for low-luminosity sources. In contrast, our models suggest that the crystallinity is only marginally affected, because for increasing luminosity, the zone for thermal annealing (assumed to be at T > 800 K) is enlarged by roughly the same factor as the silicate emission zone. The observed crystallinity is affected by disk geometry, however, with increased crystallinity in flat disks. The apparent crystallinity may also increase with grain growth due to a corresponding increase in contrast between crystalline and amorphous silicate emission bands
Protostellar holes: Spitzer Space Telescope observations of the protostellar binary IRAS16293-2422
Mid-infrared (23-35 micron) emission from the deeply embedded "Class 0"
protostar IRAS16293-2422 is detected with the Spitzer Space Telescope infrared
spectrograph. A detailed radiative transfer model reproducing the full spectral
energy distribution (SED) from 23 micron to 1.3 mm requires a large inner
cavity of radius 600 AU in the envelope to avoid quenching the emission from
the central sources. This is consistent with a previous suggestion based on
high angular resolution millimeter interferometric data. An alternative
interpretation using a 2D model of the envelope with an outflow cavity can
reproduce the SED but not the interferometer visibilities. The cavity size is
comparable to the centrifugal radius of the envelope and therefore appears to
be a natural consequence of the rotation of the protostellar core, which has
also caused the fragmentation leading to the central protostellar binary. With
a large cavity such as required by the data, the average temperature at a given
radius does not increase above 60-80 K and although hot spots with higher
temperatures may be present close to each protostar, these constitute a small
fraction of the material in the inner envelope. The proposed cavity will also
have consequences for the interpretation of molecular line data, especially of
complex species probing high temperatures in the inner regions of the envelope.Comment: Accepted for publication in ApJ Letter
Recommended from our members
Hydrolysable tannin-based diet rich in gallotannins has a minimal impact on pig performance but significantly reduces salivary and bulbo-urethral gland size
Tannins have long been considered âanti-nutritionalâ factors in monogastric nutrition, shown to reduce feed intake and palatability. However, recent studies revealed that compared to condensed tannins, hydrolysable tannins (HT) appear to have far less impact on growth performance but may be inhibitory to the total activity of caecal bacteria. This in turn could reduce microbial synthesis of skatole and indole in the hindgut of entire males (EM). Thus, the objective of this study was to determine the impact of a group of dietary HT on growth performance, carcass traits and boar taint compounds of group housed EM. For the study, 36 Swiss Large White boars were assigned within litter to three treatment groups. Boars were offered ad libitum one of three finisher diets supplemented with 0 (C), 15 (T15) or 30 g/kg (T30) of HT from d 105 to 165 of age. Growth performance, carcass characteristics, boar taint compounds in the adipose tissue and CYP2E1, CYP1A2, and CYP2A19 gene expression in the liver was assessed. Compared to C, feed efficiency but not daily gain and daily feed intake was lower (P < 0.05) in T15 and T30 boars. Except for the percent carcass weight loss during cooling, which tended (P < 0.10) to be greater in T30 than C and T15, carcass characteristics were not affected by the diets. In line with the numerically lower androstenone level, bulbo-urethral and salivary glands of T30 boars were lighter (P < 0.05) than of T15 with intermediate values for C. Indole level was lower (P < 0.05) in the adipose tissue of T30 than C pigs with intermediate levels in T15. Skatole levels tended (P < 0.10) to be lower in T30 and C than T15 pigs. Hepatic gene expression of CYP isoenzymes did not differ between treatment groups but was negatively correlated (P < 0.05) with androstenone (CYP2E1 and CYP1A2), skatole (CYP2E1, CYP2A) and indole (CYP2A) level. In line with the numerically highest androstenone and skatole concentrations, boar taint odour but not flavour was detected by the panelists in loins from T15 compared with loins from C and T30 boars. These results provide evidence that HT affected metabolism of indolic compounds and androstenone and that they affected the development of accessory sex glands. However, the effects were too small to be detected by sensory evaluation
Dust retention in protoplanetary disks
Context: Protoplanetary disks are observed to remain dust-rich for up to
several million years. Theoretical modeling, on the other hand, raises several
questions. Firstly, dust coagulation occurs so rapidly, that if the small dust
grains are not replenished by collisional fragmentation of dust aggregates,
most disks should be observed to be dust poor, which is not the case. Secondly,
if dust aggregates grow to sizes of the order of centimeters to meters, they
drift so fast inwards, that they are quickly lost.
Aims: We attempt to verify if collisional fragmentation of dust aggregates is
effective enough to keep disks 'dusty' by replenishing the population of small
grains and by preventing excessive radial drift.
Methods: With a new and sophisticated implicitly integrated coagulation and
fragmentation modeling code, we solve the combined problem of coagulation,
fragmentation, turbulent mixing and radial drift and at the same time solve for
the 1-D viscous gas disk evolution.
Results: We find that for a critical collision velocity of 1 m/s, as
suggested by laboratory experiments, the fragmentation is so effective, that at
all times the dust is in the form of relatively small particles. This means
that radial drift is small and that large amounts of small dust particles
remain present for a few million years, as observed. For a critical velocity of
10 m/s, we find that particles grow about two orders of magnitude larger, which
leads again to significant dust loss since larger particles are more strongly
affected by radial drift.Comment: Letter accepted 3 July 2009, included comments of language edito
Hot Organic Molecules Toward a Young Low-Mass Star: A Look at Inner Disk Chemistry
Spitzer Space Telescope spectra of the low mass young stellar object (YSO)
IRS 46 (L_bol ~ 0.6 L_sun) in Ophiuchus reveal strong vibration-rotation
absorption bands of gaseous C2H2, HCN, and CO2. This is the only source out of
a sample of ~100 YSO's that shows these features and the first time they are
seen in the spectrum of a solar-mass YSO. Analysis of the Spitzer data combined
with Keck L- and M-band spectra gives excitation temperatures of > 350 K and
abundances of 10(-6)-10(-5) with respect to H2, orders of magnitude higher than
those found in cold clouds. In spite of this high abundance, the HCN J=4-3 line
is barely detected with the James Clerk Maxwell Telescope, indicating a source
diameter less than 13 AU. The (sub)millimeter continuum emission and the
absence of scattered light in near-infrared images limits the mass and
temperature of any remnant collapse envelope to less than 0.01 M_sun and 100 K,
respectively. This excludes a hot-core type region as found in high-mass YSO's.
The most plausible origin of this hot gas rich in organic molecules is in the
inner (<6 AU radius) region of the disk around IRS 46, either the disk itself
or a disk wind. A nearly edge-on 2-D disk model fits the spectral energy
distribution (SED) and gives a column of dense warm gas along the line of sight
that is consistent with the absorption data. These data illustrate the unique
potential of high-resolution infrared spectroscopy to probe organic chemistry,
gas temperatures and kinematics in the planet-forming zones close to a young
star.Comment: 4 pages, 4 figures; To appear in Astrophysical Journal Letter
Stellar-Mass-Dependent Disk Structure in Coeval Planet-Forming Disks
Previous studies suggest that the planet-forming disks around very-low-mass
stars/brown dwarfs may be flatter than those around more massive stars, in
contrast to model predictions of larger scale heights for gas-disks around
lower-mass stars. We conducted a statistically robust study to determine
whether there is evidence for stellar-mass-dependent disk structure in
planet-forming disks. We find a statistically significant difference in the
Spitzer/IRAC color distributions of disks around very-low-mass and low-mass
stars all belonging to the same star-forming region, the Chamaeleon I
star-forming region. We show that self consistently calculated disk models
cannot fit the median spectral energy distributions (SEDs) of the two groups.
These SEDs can be only explained by flatter disk models, consistent with the
effect of dust settling in disks. We find that relative to the disk structure
predicted for flared disks the required reduction in disk scale height is
anti-correlated with the stellar mass, i.e. disks around lower-mass stars are
flatter. Our results show that the initial and boundary conditions of planet
formation are stellar-mass-dependent, an important finding that must be
considered in planet formation models.Comment: Astrophysical Journal, in pres
Coagulation of small grains in disks: the influence of residual infall and initial small-grain content
Turbulent coagulation in protoplanetary disks is known to operate on
timescale far shorter than the lifetime of the disk. In the absence of
mechanisms that replenish the small dust grain population, protoplanetary disks
would rapidly lose their continuum opacity-bearing dust. This is inconsistent
with infrared observations of disks around T Tauri stars and Herbig Ae/Be
stars, which are usually optically thick at visual wavelengths and show
signatures of small (a<~ 3um) grains. A plausible replenishing mechanism of
small grains is collisional fragmentation or erosion of large dust aggregates,
which model calculations predict to play an important role in protoplanetary
disks. If optically thick disks are to be seen as proof for ongoing
fragmentation or erosion, then alternative explanations for the existence of
optically thick disks must be studied carefully. In this study we explore two
scenarios. First, we study the effect of residual, low-level infall of matter
onto the disk surface. We find that infall rates as low as 10^{-11} Msun/yr
can, in principle, replenish the small grain population to a level that keeps
the disk marginally optically thick. However, it remains to be seen if the
assumption of such inflow is realistic for star+disk systems at the age of
several Myrs, at which winds and jets are expected to have removed any residual
envelope. In summary, fragmentation or erosion still appear to be the most
promising processes to explain the abundant presence of small grains in old
disks.Comment: 10 pages, 4 figures, A&A in pres
Ices in the edge-on disk CRBR 2422.8-3423: Spitzer spectroscopy and Monte Carlo radiative transfer modeling
We present 5.2-37.2 micron spectroscopy of the edge-on circumstellar disk
CRBR 2422.8-3423 obtained using the InfraRed Spectrograph (IRS) of the Spitzer
Space Telescope. The IRS spectrum is combined with ground-based 3-5 micron
spectroscopy to obtain a complete inventory of solid state material present
along the line of sight toward the source. We model the object with a 2D
axisymmetric (effectively 3D) Monte Carlo radiative transfer code. It is found
that the model disk, assuming a standard flaring structure, is too warm to
contain the very large observed column density of pure CO ice, but is possibly
responsible for up to 50% of the water, CO2 and minor ice species. In
particular the 6.85 micron band, tentatively due to NH4+, exhibits a prominent
red wing, indicating a significant contribution from warm ice in the disk. It
is argued that the pure CO ice is located in the dense core Oph-F in front of
the source seen in the submillimeter imaging, with the CO gas in the core
highly depleted. The model is used to predict which circumstances are most
favourable for direct observations of ices in edge-on circumstellar disks. Ice
bands will in general be deepest for inclinations similar to the disk opening
angle, i.e. ~70 degrees. Due to the high optical depths of typical disk
mid-planes, ice absorption bands will often probe warmer ice located in the
upper layers of nearly edge-on disks. The ratios between different ice bands
are found to vary by up to an order of magnitude depending on disk inclination
due to radiative transfer effects caused by the 2D structure of the disk.
Ratios between ice bands of the same species can therefore be used to constrain
the location of the ices in a circumstellar disk. [Abstract abridged]Comment: 49 pages, accepted for publication in Ap
- âŠ