612 research outputs found
An investigation of the formation and line properties of MgH in 3D hydrodynamical model stellar atmospheres
Studies of the isotopic composition of magnesium in cool stars have so far
relied upon the use of one-dimensional (1D) model atmospheres. Since the
isotopic ratios derived are based on asymmetries of optical MgH lines, it is
important to test the impact from other effects affecting line asymmetries,
like stellar convection. Here, we present a theoretical investigation of the
effects of including self-consistent modeling of convection. Using spectral
syntheses based on 3D hydrodynamical COBOLD models of dwarfs
(4000K, log(g),
) and giants (K,
log(g), ), we perform a detailed
analysis comparing 3D and 1D syntheses.
We describe the impact on the formation and behavior of MgH lines from using
3D models, and perform a qualitative assessment of the systematics introduced
by the use of 1D syntheses.
Using 3D model atmospheres significantly affect the strength of the MgH
lines, especially in dwarfs, with 1D syntheses requiring an abundance
correction of up to +0.69 dex largest for our 5000K models. The corrections are
correlated with and are also affected by the metallicity. The
shape of the strong MgH component in the 3D syntheses is poorly
reproduced in 1D. This results in 1D syntheses underestimating MgH by up
to percentage points and overestimating MgH by a similar amount
for dwarfs. This discrepancy increases with decreasing metallicity. MgH
is recovered relatively well, with the largest difference being
percentage points. The use of 3D for giants has less impact, due to smaller
differences in the atmospheric structure and a better reproduction of the line
shape in 1D.Comment: 20 pages, 15 figures, accepted for publication in Ap
Unconventional Josephson Effect in Hybrid Superconductor-Topological Insulator Devices
We report on transport properties of Josephson junctions in hybrid
superconducting-topological insulator devices, which show two striking
departures from the common Josephson junction behavior: a characteristic energy
that scales inversely with the width of the junction, and a low characteristic
magnetic field for suppressing supercurrent. To explain these effects, we
propose a phenomenological model which expands on the existing theory for
topological insulator Josephson junctions
Deep Hubble Space Telescope Imaging of IC 1613 II. The Star Formation History
We present deep HST WFPC2 imaging of the Local Group dwarf irregular galaxy
IC 1613. The photometry is the deepest to date for an isolated dwarf irregular
galaxy. The resulting color-magnitude diagram (CMD) is analyzed using three
different methods to derive a star formation history (SFH). All three find an
enhanced star formation rate (SFR), from 3 to 6 Gyr ago, and similar
age-metallicity relationships (AMR). A comparison of the newly observed outer
field with an earlier studied central field of IC 1613 shows that the SFR in
the outer field has been significantly depressed during the last Gyr. This
implies that the optical scale length of the galaxy has been decreasing with
time and that comparison of galaxies at intermediate redshift with present day
galaxies should take this effect into account. We find strong similarities
between IC 1613 and the more distant Milky Way dSph companions in that all are
dominated by star formation at intermediate ages. In particular, the SFH and
AMR for IC 1613 and Leo I are indistinguishable. This implies that dIrr
galaxies cannot be distinguished from dSphs by their intermediate age stellar
populations. This type of a SFH may also be evidence for slower or suppressed
early star formation in dwarf galaxies due to photoionization after the
reionization of the universe by background radiation. Assuming that IC 1613 is
typical of a dIrr evolving in isolation, since most of the star formation
occurs at intermediate ages, these dwarf systems cannot be responsible for the
fast chemical enrichment of the IGM which is seen at high redshift. There is no
evidence for any large amplitude bursts of star formation in IC 1613, and we
find it highly unlikely that analogs of IC 1613 have contributed to the excess
of faint blue galaxies in existing galaxy redshift surveys.Comment: 32 pages, including 1 table and 17 figures, accepted for publication
in the Astrophysical Journal October 10, 2003 issu
Molecular brakes regulating mTORC1 activation in skeletal muscle following synergist ablation
The goal of the current work was to profile positive (mTORC1 activation, autocrine/paracrine growth factors) and negative [AMPK, unfolded protein response (UPR)] pathways that might regulate overload-induced mTORC1 (mTOR complex 1) activation with the hypothesis that a number of negative regulators of mTORC1 will be engaged during a supraphysiological model of hypertrophy. To achieve this, mTORC1- IRS-1/2 signaling, BiP/CHOP/IRE1, and AMPK activation were determined in rat plantaris muscle following synergist ablation (SA). SA resulted in significant increases in muscle mass of 4% per day throughout the 21 days of the experiment. The expression of the insulin-like growth factors (IGF) were high throughout the 21st day of overload. However, IGF signaling was limited, since IRS-1 and -2 were undetectable in the overloaded muscle from day 3 to day 9. The decreases in IRS-1/2 protein were paralleled by increases in GRB10 Ser501/503 and S6K1 Thr389 phosphorylation, two mTORC1 targets that can destabilize IRS proteins. PKB Ser473 phosphorylation was higher from 3– 6 days, and this was associated with increased TSC2 Thr939 phosphorylation. The phosphorylation of TSC2 Thr1345 (an AMPK site) was also elevated, whereas phosphorylation at the other PKB site, Thr1462, was unchanged at 6 days. In agreement with the phosphorylation of Thr1345, SA led to activation of AMPK1 during the initial growth phase, lasting the first 9 days before returning to baseline by day 12. The UPR markers CHOP and BiP were elevated over the first 12 days following ablation, whereas IRE1 levels decreased. These data suggest that during supraphysiological muscle loading at least three potential molecular brakes engage to downregulate mTORC1. m
Deep HST Imaging of Sextans A I. The Spatially Resolved Recent Star Formation History
We have measured stellar photometry from deep Cycle 7 Hubble Space
Telescope/WFPC2 imaging of the dwarf irregular galaxy Sextans A. The imaging
was taken in three filters: F555W (; 8 orbits), F814W (; 16 orbits), and
F656N (H; 1 orbit). Combining these data with Cycle 5 WFPC2
observations provides nearly complete coverage of the optically visible portion
of the galaxy. The Cycle 7 observations are nearly 2 magnitudes more sensitive
than the Cycle 5 observations, which provides unambiguous separation of the
faint blue helium burning stars (BHeB stars) from contaminant populations. The
depth of the photometry allows us to compare recent star formation histories
recovered from both the main sequence (MS) stars and the BHeB stars for the
last 300 Myr. The excellent agreement between these independent star formation
rate (SFR) calculations is a resounding confirmation for the legitimacy of
using the BHeB stars to calculate the recent SFR. Using the BHeB stars we have
calculated the global star formation history over the past 700 Myr. The history
calculated from the Cycle 7 data is remarkably identical to that calculated
from the Cycle 5 data, implying that both halves of the galaxy formed stars in
concert. We have also calculated the spatially resolved star formation history,
combining the fields from the Cycle 5 and Cycle 7 data. Our interpretation of
the pattern of star formation is that it is an orderly stochastic process.Comment: 27 pages, 14 figures, 2 mpeg movies, accepted in the Astronomical
Journa
Observations and Implications of the Star Formation History of the LMC
We present derivations of star formation histories based on color-magnitude
diagrams of three fields in the LMC from HST/WFPC2 observations. A significant
component of stars older than 4 Gyr is required to match the observed
color-magnitude diagrams. Models with a dispersion-free age-metallicity
relation are unable to reproduce the width of the observed main sequence;
models with a range of metallicity at a given age provide a much better fit.
Such models allow us to construct complete ``population boxes'' for the LMC
based entirely on color-magnitude diagrams; remarkably, these qualitatively
reproduce the age-metallicity relation observed in LMC clusters. We discuss
some of the uncertainties in deriving star formation histories. We find,
independently of the models, that the LMC bar field has a larger relative
component of older stars than the outer fields. The main implications suggested
by this study are: 1) the star formation history of field stars appears to
differ from the age distribution of clusters, 2) there is no obvious evidence
for bursty star formation, but our ability to measure bursts shorter in
duration than 25% of any given age is limited by the statistics of the
observed number of stars, 3) there may be some correlation of the star
formation rate with the last close passage of the LMC/SMC/Milky Way, but there
is no dramatic effect, and 4) the derived star formation history is probably
consistent with observed abundances, based on recent chemical evolution models.Comment: Accepted by AJ, 36 pages including 12 figure
Stellar Populations at the Center of IC 1613
We have observed the center of the Local Group dwarf irregular galaxy IC 1613
with WFPC2 aboard the Hubble Space Telescope in the F439W, F555W, and F814W
filters. We find a dominant old stellar population (aged ~7 Gyr), identifiable
by the strong red giant branch (RGB) and red clump populations. From the (V-I)
color of the RGB, we estimate a mean metallicity of the intermediate-age
stellar population [Fe/H] = -1.38 +/- 0.31. We confirm a distance of 715 +/- 40
kpc using the I-magnitude of the RGB tip. The main-sequence luminosity function
down to I ~25 provides evidence for a roughly constant SFR of approximately
0.00035 solar masses per year across the WFPC2 field of view (0.22 square kpc)
during the past 250-350 Myr. Structure in the blue loop luminosity function
implies that the SFR was ~50% higher 400-900 Myr ago than today. The mean heavy
element abundance of these young stars is 1/10th solar. The best explanation
for a red spur on the main-sequence at I = 24.7 is the blue horizontal branch
component of a very old stellar population at the center of IC 1613. We have
also imaged a broader area of IC 1613 using the 3.5-meter WIYN telescope under
excellent seeing conditions. The AGB-star luminosity function is consistent
with a period of continuous star formation over at least the age range 2-10
Gyr. We present an approximate age-metallicity relation for IC 1613, which
appears similar to that of the Small Magellanic Cloud. We compare the Hess
diagram of IC 1613 to similar data for three other Local Group dwarf galaxies,
and find that it most closely resembles the nearby, transition-type dwarf
galaxy Pegasus (DDO 216).Comment: To appear in the September 1999 Astronomical Journal. LaTeX, uses
AASTeX v4.0, emulateapj style file, 19 pages, 12 postscript figures, 2
tables. 5 of the figures available separately via the WW
Towards species-level forecasts of drought-induced tree mortality risk
Predicting species-level responses to drought at the landscape scale is critical to reducing uncertainty in future terrestrial carbon and water cycle projections.
We embedded a stomatal optimisation model in the Community Atmosphere Biosphere Land Exchange (CABLE) land surface model and parameterised the model for 15 canopy dominant eucalypt tree species across South-Eastern Australia (mean annual precipitation range: 344–1424 mm yr−1). We conducted three experiments: applying CABLE to the 2017–2019 drought; a 20% drier drought; and a 20% drier drought with a doubling of atmospheric carbon dioxide (CO2).
The severity of the drought was highlighted as for at least 25% of their distribution ranges, 60% of species experienced leaf water potentials beyond the water potential at which 50% of hydraulic conductivity is lost due to embolism. We identified areas of severe hydraulic stress within-species’ ranges, but we also pinpointed resilience in species found in predominantly semiarid areas. The importance of the role of CO2 in ameliorating drought stress was consistent across species.
Our results represent an important advance in our capacity to forecast the resilience of individual tree species, providing an evidence base for decision-making around the resilience of restoration plantings or net-zero emission strategies
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