294 research outputs found
A non-LTE abundance analysis of the post-AGB star ROA 5701
An analysis of high-resolution Anglo-Australian Telescope (AAT)/ University
College London Echelle Spectrograph (UCLES) optical spectra for the ultraviolet
(UV)-bright star ROA 5701 in the globular cluster omega Cen (NGC 5139) is
performed, using non-local thermodynamic equilibrium (non-LTE) model
atmospheres to estimate stellar atmospheric parameters and chemical
composition. Abundances are derived for C, N, O, Mg, Si and S, and compared
with those found previously by Moehler et al. We find a general metal
underabundance relative to young B-type stars, consistent with the average
metallicity of the cluster. Our results indicate that ROA 5701 has not
undergone a gas-dust separation scenario as previously suggested. However, its
abundance pattern does imply that ROA 5701 has evolved off the AGB prior to the
onset of the third dredge-up.Comment: 9 pages, 2 figures. Accepted for publication in MNRAS (Online Early
Elevating crop disease resistance with cloned genes
Essentially all plant species exhibit heritable genetic variation for resistance to a variety of plant diseases caused by fungi, bacteria, oomycetes or viruses. Disease losses in crop monocultures are already significant, and would be greater but for applications of disease-controlling agrichemicals. For sustainable intensification of crop production, we argue that disease control should as far as possible be achieved using genetics rather than using costly recurrent chemical sprays. The latter imply CO2 emissions from diesel fuel and potential soil compaction from tractor journeys. Great progress has been made in the past 25 years in our understanding of the molecular basis of plant disease resistance mechanisms, and of how pathogens circumvent them. These insights can inform more sophisticated approaches to elevating disease resistance in crops that help us tip the evolutionary balance in favour of the crop and away from the pathogen. We illustrate this theme with an account of a genetically modified (GM) blight-resistant potato trial in Norwich, using the Rpi-vnt1.1 gene isolated from a wild relative of potato, Solanum venturii, and introduced by GM methods into the potato variety Desiree
The Origin of B-Type Runaway Stars: Non-LTE Abundances as a Diagnostic
There are two accepted mechanisms to explain the origin of runaway OB-type
stars: the Binary Supernova Scenario (BSS), and the Cluster Ejection Scenario
(CES). In the former, a supernova explosion within a close binary ejects the
secondary star, while in the latter close multi-body interactions in a dense
cluster cause one or more of the stars to be ejected from the region at high
velocity. Both mechanisms have the potential to affect the surface composition
of the runaway star. TLUSTY non-LTE model atmosphere calculations have been
used to determine atmospheric parameters and carbon, nitrogen, magnesium and
silicon abundances for a sample of B-type runaways. These same analytical tools
were used by Hunter et al. (2009) for their analysis of 50 B-type open cluster
Galactic stars (i.e. non-runaways). Effective temperatures were deduced using
the silicon-ionization balance technique, surface gravities from Balmer line
profiles and microturbulent velocities derived using the Si spectrum. The
runaways show no obvious abundance anomalies when compared with stars in the
open clusters. The runaways do show a spread in composition which almost
certainly reflects the Galactic abundance gradient and a range in the
birthplaces of the runaways in the Galactic disk. Since the observed Galactic
abundance gradients of C, N, Mg and Si are of a similar magnitude, the
abundance ratios (e.g., N/Mg) are, as obtained, essentially uniform across the
sample
CaII K observations of QSOs in the line-of-sight to the Magellanic Bridge
We describe medium-resolution spectroscopic observations taken with the ESO
Multi-Mode Instrument in the CaII K line (3933.661 Angstroms) towards 7 QSOs
located in the line-of-sight to the Magellanic Bridge. At a spectral resolution
R = 6,000, five of the sightlines have a signal-to-noise ratio of 20 or higher.
Definite Ca absorption due to Bridge material is detected towards 3 objects,
with probable detection towards two other sightlines. Gas-phase CaII K Bridge
and Milky Way abundances or lower limits for the all sightlines are estimated
by the use of Parkes 21-cm HI emission line data. These data only have a
spatial resolution of 14 arcminutes compared with the optical observations
which have milli-arcsecond resolution. With this caveat, for the three objects
with sound CaII K detections, we find that the ionic abundance of CaII K
relative to HI, A=log(N(CaK)/N(HI)) for low-velocity Galactic gas ranges from
-8.3 to -8.8 dex, with HI column densities varying from 3-6x10^20 cm^-2. For
Magellanic Bridge gas, the values of A are 0.5 dex higher, ranging from -7.8 to
-8.2 dex, with N(HI)=1-5x10^20 cm^-2. Higher values of A correspond to lower
values of N(HI), although numbers are small. For the sightline towards
B0251--675, the Bridge gas has two different velocities, and in only one of
these is CaII K tentatively detected, perhaps indicating gas of a different
origin or present-day characteristics (such as dust content), although this
conclusion is uncertain and there is the possibility that one of the components
could be related to the Magellanic Stream. Higher signal-to-noise CaII K data
and higher resolution HI data are required to determine whether A changes with
N(HI) over the Bridge and if the implied difference in the metalicity of the
two Bridge components towards B0251-675 is real.Comment: Astronomy and Astrophysics, in pres
FUSE Observations of the Magellanic Bridge Gas toward Two Early-Type Stars: Molecules, Physical Conditions, and Relative Abundance
We discuss FUSE observations of two early-type stars, DI1388 and DGIK975, in
the low density and low metallicity gas of Magellanic Bridge (MB). Toward
DI1388, the FUSE observations show molecular hydrogen, O VI, and numerous other
atomic or ionic transitions in absorption, implying the presence of multiple
gas phases in a complex arrangement. The relative abundance pattern in the MB
is attributed to varying degrees of depletion onto dust similar to that of halo
clouds. The N/O ratio is near solar, much higher than N/O in damped Ly-alpha
systems, implying subsequent stellar processing to explain the origin of
nitrogen in the MB. The diffuse molecular cloud in this direction has a low
column density and low molecular fraction. H2 is observed in both the
Magellanic Stream and the MB, yet massive stars form only in the MB, implying
significantly different physical processes between them. In the MB some of the
H2 could have been pulled out from the SMC via tidal interaction, but some also
could have formed in situ in dense clouds where star formation might have taken
place. Toward DGIK975, the presence of neutral, weakly and highly ionized
species suggest that this sight line has also several complex gas phases. The
highly ionized species of O VI, C IV, and Si IV toward both stars have very
broad features, indicating that multiple components of hot gas at different
velocities are present. Several sources (a combination of turbulent mixing
layer, conductive heating, and cooling flows) may be contributing to the
production of the highly ionized gas in the MB. Finally, this study has
confirmed previous results that the high-velocity cloud HVC 291.5-41.2+80 is
mainly ionized composed of weakly and highly ions. The high ion ratios are
consistent with a radiatively cooling gas in a fountain flow model.Comment: Accepted for publication in the ApJ (October 10, 2002). Added
reference (Gibson et al. 2000
Metallicity and Physical Conditions in the Magellanic Bridge
We present a new analysis of the diffuse gas in the Magellanic Bridge (RA>3h)
based on HST/STIS E140M and FUSE spectra of 2 early-type stars lying within the
Bridge and a QSO behind it. We derive the column densities of HI (from
Ly\alpha), NI, OI, ArI, SiII, SII, and FeII of the gas in the Bridge. Using the
atomic species, we determine the first gas-phase metallicity of the Magellanic
Bridge, [Z/H]=-1.02+/-0.07 toward one sightline, and -1.7<[Z/H]<-0.9 toward the
other one, a factor 2 or more smaller than the present-day SMC metallicity.
Using the metallicity and N(HI), we show that the Bridge gas along our three
lines of sight is ~70-90% ionized, despite high HI columns, logN(HI)=19.6-20.1.
Possible sources for the ongoing ionization are certainly the hot stars within
the Bridge, hot gas (revealed by OVI absorption), and leaking photons from the
SMC and LMC. From the analysis of CII*, we deduce that the overall density of
the Bridge must be low (<0.03-0.1 cm^-3). We argue that our findings combined
with other recent observational results should motivate new models of the
evolution of the SMC-LMC-Galaxy system.Comment: Accepted for publication in the Ap
Making FORS2 fit for exoplanet observations (again)
For about three years, it was known that precision spectrophotometry with
FORS2 suffered from systematic errors that made quantitative observations of
planetary transits impossible. We identified the Longitudinal Atmospheric
Dispersion Compensator (LADC) as the most likely culprit, and therefore engaged
in a project to exchange the LADC prisms with the uncoated ones from FORS1.
This led to a significant improvement in the depth of FORS2 zero points, a
reduction in the systematic noise, and should make FORS2 again competitive for
transmission spectroscopy of exoplanets.Comment: To appear in the March issue of the ESO Messenge
Multiwavelength observations of the M15 intermediate velocity cloud
We present Westerbork Synthesis Radio Telescope HI images, Lovell Telescope
multibeam HI wide-field mapping, Wisconsin H-alpha Mapper facility images,
William Herschel Telescope longslit echelle CaII observations, and IRAS ISSA 60
and 100 micron coadded images towards the intermediate velocity cloud located
in the general direction of the M15 globular cluster. When combined with
previously-published Arecibo data, the HI gas in the IVC is found to be clumpy,
with peak HI column density of 1.5x10^(20) cm^(-2), inferred volume density
(assuming spherical symmetry) of 24 cm^(-3)/(D kpc), and maximum brightness
temperature at a resolution of 81x14 arcsec of 14 K. The HI gas in the cloud is
warm, with a minimum FWHM value of 5 km/s, corresponding to a kinetic
temperature, in the absence of turbulence, of 540 K. There are indications in
the HI data of 2-component velocity structure in the IVC, indicative of
cloudlets. This velocity structure is also tentatively seen in the CaK spectra,
although the SNR is low. The main IVC condensation is detected by WHAM in
H-alpha with intensities uncorrected for Galactic absorption of upto 1.3
Rayleigh, indicating that the cloud is partially ionised. The FWHM of the
ionised component, at a resolution of 1 degree, exceeds 30 km/s. The spatial
and velocity coincidence of the H-alpha and HI peaks in emission is
qualitatively good. Finally, the 100 and 60 micron IRAS images show spatial
coincidence over a 0.7 degree field, with low and intermediate-velocity gas,
respectively, indicating that the IVC may contain dust.Comment: MNRAS, in pres
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