1,308 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. As–O 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
Water vapor on supergiants. The 12 micron TEXES spectra of mu Cephei
Several recent papers have argued for warm, semi-detached, molecular layers
surrounding red giant and supergiant stars, a concept known as a MOLsphere.
Spectroscopic and interferometric analyses have often corroborated this general
picture. Here, we present high-resolution spectroscopic data of pure rotational
lines of water vapor at 12 microns for the supergiant mu Cephei. This star has
often been used to test the concept of molecular layers around supergiants.
Given the prediction of an isothermal, optically thick water-vapor layer in
Local Thermodynamic Equilibrium around the star (MOLsphere), we expected the 12
micron lines to be in emission or at least in absorption but filled in by
emission from the molecular layer around the star. Our data, however, show the
contrary; we find definite absorption. Thus, our data do not easily fit into
the suggested isothermal MOLsphere scenario. The 12 micron lines, therefore,
put new, strong constraints on the MOLsphere concept and on the nature of water
seen in signatures across the spectra of early M supergiants. We also find that
the absorption is even stronger than that calculated from a standard,
spherically symmetric model photosphere without any surrounding layers. A cool
model photosphere, representing cool outer layers is, however, able to
reproduce the lines, but this model does not account for water vapor emission
at 6 microns. Thus, a unified model for water vapor on mu Cephei appears to be
lacking. It does seem necessary to model the underlying photospheres of these
supergiants in their whole complexity. The strong water vapor lines clearly
reveal inadequacies of classical model atmospheres.Comment: Accepted for publication in the Astrophysical Journa
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
SN 2006bp: Probing the Shock Breakout of a Type II-P Supernova
HET optical spectroscopy and unfiltered ROTSE-III photometry spanning the
first 11 months since explosion of the Type II-P SN 2006bp are presented. Flux
limits from the days before discovery combined with the initial rapid
brightening suggest the supernova was first detected just hours after shock
breakout. Optical spectra obtained about 2 days after breakout exhibit narrow
emission lines corresponding to HeII 4200, HeII 4686, and CIV 5805 in the rest
frame, and these features persist in a second observation obtained 5 hours
later; however, these emission lines are not detected the following night nor
in subsequent observations. We suggest that these lines emanate from material
close to the explosion site, possibly in the outer layers of the progenitor
that have been ionized by the high energy photons released at shock breakout. A
P-Cygni profile is observed around 4450 A in the +2 and +3 day spectra.
Previous studies have attributed this feature to high velocity H-beta, but we
discuss the possibility that this profile is instead due to HeII 4687. Further
HET observations (14 nights in total) covering the spectral evolution across
the photometric plateau up to 73 days after breakout and during the nebular
phase around day +340 are presented, and expansion velocities are derived for
key features. The measured decay slope for the unfiltered light curve is 0.0073
+/- 0.0004 mag/day between days +121 and +335, which is significantly slower
than the decay of rate 56Co. We combine our HET measurements with published
X-ray, UV, and optical data to obtain a quasi-bolometric light curve through
day +60. We see a slow cooling over the first 25 days, but no sign of an early
sharp peak; any such feature from the shock breakout must have lasted less than
~1 day.[ABRIDGED]Comment: ApJ accepted, 43 page
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 self‐organizing 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 self‐organizing principles of a living cell and how these self‐organizing 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 self‐organizing 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 self‐organizing principles of living cells and how these self‐organizing 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
- …