1,094 research outputs found
Fluorine in the solar neighborhood - is it all produced in AGB-stars?
The origin of 'cosmic' fluorine is uncertain, but there are three proposed
production sites/mechanisms: AGB stars, nucleosynthesis in Type II
supernovae, and/or the winds of Wolf-Rayet stars. The relative importance of
these production sites has not been established even for the solar
neighborhood, leading to uncertainties in stellar evolution models of these
stars as well as uncertainties in the chemical evolution models of stellar
populations.
We determine the fluorine and oxygen abundances in seven bright, nearby
giants with well-determined stellar parameters. We use the 2.3 m
vibrational-rotational HF line and explore a pure rotational HF line at 12.2
m. The latter has never been used before for an abundance analysis. To be
able to do this we have calculated a line list for pure rotational HF lines. We
find that the abundances derived from the two diagnostics agree.
Our derived abundances are well reproduced by chemical evolution models only
including fluorine production in AGB-stars and therefore we draw the conclusion
that this might be the main production site of fluorine in the solar
neighborhood. Furthermore, we highlight the advantages of using the 12 m
HF lines to determine the possible contribution of the -process to the
fluorine budget at low metallicities where the difference between models
including and excluding this process is dramatic
A computer program for hydrostatic bearings including the effects of non-uniform film thickness and relative velocity for various methods of lubricant supply final technical report
Computer program for hydrostatic bearing - effects of nonuniform film thickness and lubricant suppl
GaAs interfacial self-cleaning by atomic layer deposition
The reduction and removal of surface oxides from GaAs substrates by atomic layer deposition (ALD) of Al2O3 and HfO2 are studied using in situ monochromatic x-ray photoelectron spectroscopy. Using the combination of in situ deposition and analysis techniques, the interfacial "self-cleaning" is shown to be oxidation state dependent as well as metal organic precursor dependent. Thermodynamics, charge balance, and oxygen coordination drive the removal of certain species of surface oxides while allowing others to remain. These factors suggest proper selection of surface treatments and ALD precursors can result in selective interfacial bonding arrangements
Frequency dispersion reduction and bond conversion on n-type GaAs by in situ surface oxide removal and passivation
The method of surface preparation on n-type GaAs, even with the presence of an amorphous-Si interfacial passivation layer, is shown to be a critical step in the removal of accumulation capacitance frequency dispersion. In situ deposition and analysis techniques were used to study different surface preparations, including NH4OH, Si-flux, and atomic hydrogen exposures, as well as Si passivation depositions prior to in situ atomic layer deposition of Al2O3. AsâO bonding was removed and a bond conversion process with Si deposition is observed. The accumulation capacitance frequency dispersion was removed only when a Si interlayer and a specific surface clean were combined
The Evolution of Oxygen and Magnesium in the Bulge and Disk of the Milky Way
We show that the Galactic bulge and disk share a similar, strong, decline in
[O/Mg] ratio with [Mg/H]. The similarity of the [O/Mg] trend in these two,
markedly different, populations suggests a metallicity-dependent modulation of
the stellar yields from massive stars, by mass loss from winds, and related to
the Wolf-Rayet phenomenon, as proposed by McWilliam & Rich (2004). We have
modified existing models for the chemical evolution of the Galactic bulge and
the solar neighborhood with the inclusion of metallicity-dependent oxygen
yields from theoretical predictions for massive stars that include mass loss by
stellar winds. Our results significantly improve the agreement between
predicted and observed [O/Mg] ratios in the bulge and disk above solar
metallicity; however, a small zero-point normalization problem remains to be
resolved. The zero-point shift indicates that either the semi-empirical yields
of Francois et al. (2004) need adjustment, or that the bulge IMF is not quite
as flat as found by Ballero et al. (2007); the former explanation is preferred.
Our result removes a previous inconsistency between the interpretation of
[O/Fe] and [Mg/Fe] ratios in the bulge, and confirms the conclusion that the
bulge formed more rapidly than the disk, based on the over-abundances of
elements produced by massive stars. We also provide an explanation for the
long-standing difference between [Mg/Fe] and [O/Fe] trends among disk stars
more metal-rich than the sun.Comment: 22 pages including 5 figures. Submitted to the Astronomical Journa
CO observations of symbiotic stellar systems
We have performed mm-wave observations with the IRAM 30m telescope of the
12CO J=2-1 and J=1-0, 13CO J=2-1 and J=1-0, and SiO J=5-4 transitions in the
symbiotic stars R Aqr, CH Cyg, and HM Sge. The data were analyzed by means of a
simple analytical description of the general properties of molecular emission
from the inner shells around the cool star. Numerical calculations of the
expected line profiles, taking into account the level population and radiative
transfer under such conditions, were also performed. Weak emission of 12CO
J=2-1 and J=1-0 was detected in R Aqr and CH Cyg; a good line profile of 12CO
J=2-1 in R Aqr was obtained. The intensities and profile shapes of the detected
lines are compatible with emission coming from a very small shell around the
Mira-type star, with a radius comparable to or slightly smaller than the
distance to the hot dwarf companion, 10 - 2 10 cm. We argue that
other possible explanations are improbable. This region probably shows
properties similar to those characteristic of the inner shells around standard
AGB stars: outwards expansion at about 5 - 25 km/s, with a significant
acceleration of the gas, temperatures decreasing with radius between about 1000
and 500 K, and densities ~ 10 - 3 10 cm. Our model calculations
are able to explain the asymmetric line shape observed in 12CO J=2-1 from R
Aqr, in which the relatively weaker red part of the profile would result from
selfabsorption by the outer layers (in the presence of a velocity increase and
a temperature decrease with radius). The mass-loss rates are somewhat larger
than in standard AGB stars, as often happens for symbiotic systems. In R Aqr,
we find that the total mass of the CO emitting region is ~ 2 - 3 10 Mo,
corresponding to M' ~ 5 10 - 10 Mo/yr, and compatible with
results obtained from dust emission. Taking into account other existing data on
molecular emission, we suggest that the small extent of the molecule-rich gas
in symbiotic systems is mainly due to molecule photodissociation by the
radiation of the hot dwarf star.Comment: 11 pages, 4 figure
Lessons from a Minimal Genome: What Are the Essential Organizing Principles of a Cell Built from Scratch?
One of the primary challenges facing synthetic biology is reconstituting a living system from its component parts. A particularly difficult landmark is reconstituting a selfâorganizing system that can undergo autonomous chromosome compaction, segregation, and cell division. Here, we discuss how the syn3.0 minimal genome can inform us of the core selfâorganizing principles of a living cell and how these selfâorganizing processes can be built from the bottom up. The review underscores the importance of fundamental biology in rebuilding life from its molecular constituents.A primary challenge in synthetic biology is reconstituting selfâorganizing systems that can undergo autonomous chromosome compaction, segregation, and cell division. Here, we discuss how the syn3.0 minimal genome sheds light on the core selfâorganizing principles of living cells and how these selfâorganizing processes can be built from the bottom up.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/152011/1/cbic201900249.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/152011/2/cbic201900249_am.pd
The stellar population histories of early-type galaxies. III. The Coma Cluster
We present stellar population parameters of twelve early-type galaxies (ETGs)
in the Coma Cluster based on spectra obtained using the Low Resolution Imaging
Spectrograph on the Keck II Telescope. Our data allow us to examine in detail
the zero-point and scatter in their stellar population properties. Our ETGs
have SSP-equivalent ages of on average 5-8 Gyr with the models used here, with
the oldest galaxies having ages of ~10 Gyr old. This average age is identical
to the mean age of field ETGs. Our ETGs span a large range in velocity
dispersion but are consistent with being drawn from a population with a single
age. Specifically, ten of the twelve ETGs are consistent within their formal
errors of having the same age, 5.2+/-0.2 Gyr, over a factor of more than 750 in
mass. We therefore find no evidence for downsizing of the stellar populations
of ETGs in the core of the Coma Cluster. We suggest that Coma Cluster ETGs may
have formed the majority of their mass at high redshifts but suffered small but
detectable star formation events at z~0.1-0.3. Previous detections of
'downsizing' from stellar populations of local ETGs may not reflect the same
downsizing seen in lookback studies of RSGs, as the young ages of the local
ETGs represent only a small fraction of their total masses. (abridged)Comment: 49 pages, 20 figures (19 EPS, 1 JPEG). MNRAS, in press. For version
with full resolution of Fig. 1 see
http://www.astro.rug.nl/~sctrager/coma.pdf; for Table 2, see
http://www.astro.rug.nl/~sctrager/coma_table2.pdf; for Table B3, see
http://www.astro.rug.nl/~sctrager/coma_tableB3.pd
Towards More Precise Survey Photometry for PanSTARRS and LSST: Measuring Directly the Optical Transmission Spectrum of the Atmosphere
Motivated by the recognition that variation in the optical transmission of
the atmosphere is probably the main limitation to the precision of ground-based
CCD measurements of celestial fluxes, we review the physical processes that
attenuate the passage of light through the Earth's atmosphere. The next
generation of astronomical surveys, such as PanSTARRS and LSST, will greatly
benefit from dedicated apparatus to obtain atmospheric transmission data that
can be associated with each survey image. We review and compare various
approaches to this measurement problem, including photometry, spectroscopy, and
LIDAR. In conjunction with careful measurements of instrumental throughput,
atmospheric transmission measurements should allow next-generation imaging
surveys to produce photometry of unprecedented precision. Our primary concerns
are the real-time determination of aerosol scattering and absorption by water
along the line of sight, both of which can vary over the course of a night's
observations.Comment: 41 pages, 14 figures. Accepted PAS
Near-Infrared Classification Spectroscopy: H-band Spectra of Fundamental MK Standards
We present a catalogue of H-band spectra for 85 stars of approximately solar
abundance observed at a resolving power of 3000 with the KPNO Mayall 4m FTS.
The atlas covers spectral types O7-M5 and luminosity classes I-V as defined on
the MK system. We identify both atomic and molecular indices and line-ratios
which are temperature and luminosity sensitive allowing spectral classification
to be carried out in the H-band. The line ratios permit spectral classification
in the presence of continuum excess emission, which is commonly found in
pre-main sequence and evolved stars. We demonstrate that with spectra of R =
1000 obtained at SNR > 50 it is possible to derive spectral types within +- 2
subclasses for late-type stars. These data are available electronically through
the Astronomical Data Center in addition to being served on the World-Wide-Web.Comment: To appear in the November 20, 1998 issue of ApJ (Volume 508, #1
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