7,063 research outputs found
A.E. Sloan to T.L. Treadwell, 3 January 1859
https://egrove.olemiss.edu/aldrichcorr_c/1172/thumbnail.jp
Force and energy dissipation variations in non-contact atomic force spectroscopy on composite carbon nanotube systems
UHV dynamic force and energy dissipation spectroscopy in non-contact atomic
force microscopy were used to probe specific interactions with composite
systems formed by encapsulating inorganic compounds inside single-walled carbon
nanotubes. It is found that forces due to nano-scale van der Waals interaction
can be made to decrease by combining an Ag core and a carbon nanotube shell in
the Ag@SWNT system. This specific behaviour was attributed to a significantly
different effective dielectric function compared to the individual
constituents, evaluated using a simple core-shell optical model. Energy
dissipation measurements showed that by filling dissipation increases,
explained here by softening of C-C bonds resulting in a more deformable
nanotube cage. Thus, filled and unfilled nanotubes can be discriminated based
on force and dissipation measurements. These findings have two different
implications for potential applications: tuning the effective optical
properties and tuning the interaction force for molecular absorption by
appropriately choosing the filling with respect to the nanotube.Comment: 22 pages, 6 figure
The low wind expansion velocity of metal-poor carbon stars in the Halo and the Sagittarius stream
We report the detection, from observations using the James Clerk Maxwell
Telescope, of CO J 3 2 transition lines in six carbon stars, selected
as members of the Galactic Halo and having similar infrared colors. Just one
Halo star had been detected in CO before this work. Infrared observations show
that these stars are red (J-K 3), due to the presence of large dusty
circumstellar envelopes. Radiative transfer models indicates that these stars
are losing mass with rather large dust mass-loss rates in the range 1--3.3
Myr, similar to what can be observed in the
Galactic disc. We show that two of these stars are effectively in the Halo, one
is likely linked to the stream of the Sagittarius Dwarf Spheroidal galaxy (Sgr
dSph), and the other three stars certainly belong to the thick disc. The wind
expansion velocities of the observed stars are low compared to carbon stars in
the thin disc and are lower for the stars in the Halo and the Sgr dSph stream
than in the thick disc. We discuss the possibility that the low expansion
velocities result from the low metallicity of the Halo carbon stars. This
implies that metal-poor carbon stars lose mass at a rate similar to metal-rich
carbon stars, but with lower expansion velocities, as predicted by recent
theoretical models. This result implies that the current estimates of mass-loss
rates from carbon stars in Local Group galaxies will have to be reconsidered.Comment: 10 pages, 7 figures, accepted for publication in MNRA
Carbon-rich dust production in metal-poor galaxies in the Local Group
We have observed a sample of 19 carbon stars in the Sculptor, Carina, Fornax,
and Leo I dwarf spheroidal galaxies with the Infrared Spectrograph on the
Spitzer Space Telescope. The spectra show significant quantities of dust around
the carbon stars in Sculptor, Fornax, and Leo I, but little in Carina. Previous
comparisons of carbon stars with similar pulsation properties in the Galaxy and
the Magellanic Clouds revealed no evidence that metallicity affected the
production of dust by carbon stars. However, the more metal-poor stars in the
current sample appear to be generating less dust. These data extend two known
trends to lower metallicities. In more metal-poor samples, the SiC dust
emission weakens, while the acetylene absorption strengthens. The bolometric
magnitudes and infrared spectral properties of the carbon stars in Fornax are
consistent with metallicities more similar to carbon stars in the Magellanic
Clouds than in the other dwarf spheroidals in our sample. A study of the carbon
budget in these stars reinforces previous considerations that the dredge-up of
sufficient quantities of carbon from the stellar cores may trigger the final
superwind phase, ending a star's lifetime on the asymptotic giant branch.Comment: ApJ, in press, 21 pages, 12 figures. Replaced Fig 12, corrected two
reference
Predoctoral Interns\u27 Nondisclosure in Supervision
In interviews with 14 counseling center predoctoral interns regarding a significant nondisclosure in supervision, eight interns reported good supervisory relationships and six indicated that they experienced problematic supervisory relationships. Nondisclosures for the interns in good supervisory relationships related to personal reactions to clients, whereas nondisclosures for interns in problematic supervisory relationships related to global dissatisfaction with the supervisory relationship. In both groups, interns mentioned concerns about evaluation and negative feelings as typical reasons for nondisclosure. Additional reasons for nondisclosure for interns in problematic supervision were power dynamics, inhibiting demographic or cultural variables, and the supervisor\u27s theoretical orientation. Both groups described negative effects of nondisclosure on themselves and their relationships with clients. Interns in problematic supervision also reported that nondisclosures had negative effects on the supervisory relationship
Luminosities and mass-loss rates of SMC and LMC AGB stars and Red Supergiants
(Abridged) Dust radiative transfer models are presented for 101 carbon stars
and 86 oxygen-rich evolved stars in the Magellanic Clouds for which 5-35 \mum\
{\it Spitzer} IRS spectra are available. The spectra are complemented with
available optical and infrared photometry to construct the spectral energy
distribution. A minimisation procedure is used to fit luminosity, mass-loss
rate and dust temperature at the inner radius. Different effective temperatures
and dust content are also considered. Periods from the literature and from new
OGLE-III data are compiled and derived. The O-rich stars are classified in
foreground objects, AGB stars and Red Super Giants.
For the O-rich stars silicates based on laboratory optical constants are
compared to "astronomical silicates". Overall, the grain type by Volk & Kwok
(1988) fit the data best. However, the fit based on laboratory optical
constants for the grains can be improved by abandoning the small-particle
limit. The influence of grain size, core-mantle grains and porosity are
explored.
Relations between mass-loss rates and luminosity and pulsation period are
presented and compared to the predictions of evolutionary models, those by
Vassiliadis & Wood (1993) and their adopted mass-loss recipe, and those based
on a Reimers mass-loss law with a scaling of a factor of five. The Vassiliadis
& Wood models describe the data better, although there are also some
deficiencies, in particular to the maximum adopted mass-loss rate.
The OGLE-III data reveal an O-rich star in the SMC with a period of 1749
days. Its absolute magnitude of makes it a good candidate
for a super-AGB star.Comment: A&A accepte
On the metallicity dependence of crystalline silicates in oxygen-rich asymptotic giant branch stars and red supergiants
We investigate the occurrence of crystalline silicates in oxygen-rich evolved
stars across a range of metallicities and mass-loss rates. It has been
suggested that the crystalline silicate feature strength increases with
increasing mass-loss rate, implying a correlation between lattice structure and
wind density. To test this, we analyse Spitzer IRS and Infrared Space
Observatory SWS spectra of 217 oxygen-rich asymptotic giant branch stars and 98
red supergiants in the Milky Way, the Large and Small Magellanic Clouds and
Galactic globular clusters. These encompass a range of spectral morphologies
from the spectrally-rich which exhibit a wealth of crystalline and amorphous
silicate features to 'naked' (dust-free) stars. We combine spectroscopic and
photometric observations with the GRAMS grid of radiative transfer models to
derive (dust) mass-loss rates and temperature. We then measure the strength of
the crystalline silicate bands at 23, 28 and 33 microns. We detect crystalline
silicates in stars with dust mass-loss rates which span over 3 dex, down to
rates of ~10^-9 solar masses/year. Detections of crystalline silicates are more
prevalent in higher mass-loss rate objects, though the highest mass-loss rate
objects do not show the 23-micron feature, possibly due to the low temperature
of the forsterite grains or it may indicate that the 23-micron band is going
into absorption due to high column density. Furthermore, we detect a change in
the crystalline silicate mineralogy with metallicity, with enstatite seen
increasingly at low metallicity.Comment: Accepted for publication in MNRAS, 24 pages, 16 figure
Size dependence of volume and surface nucleation rates for homogeneous freezing of supercooled water droplets
The relative roles of volume and surface nucleation were investigated for the homogeneous freezing of pure water droplets. Experiments were carried out in a cryogenic laminar aerosol flow tube using supercooled water aerosols with maximum volume densities at radii between 1 and 3 μm. Temperature- and size-dependent values of volume- and surface-based homogeneous nucleation rates between 234.8 and 236.2 K were derived using a microphysical model and aerosol phase compositions and size distributions determined from infrared extinction measurements in the flow tube. The results show that the contribution from nucleation at the droplet surface increases with decreasing droplet radius and dominates over nucleation in the bulk droplet volume for droplets with radii smaller than approximately 5 μm. This is interpreted in terms of a lowered free energy of ice germ formation in the surface-based process. The implications of surface nucleation for the parameterization of homogeneous ice nucleation in numerical models are considered
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