3,169 research outputs found
Low-mass Star Formation: Observations
I briefly review recent observations of regions forming low mass stars. The
discussion is cast in the form of seven questions that have been partially
answered, or at least illuminated, by new data. These are the following: where
do stars form in molecular clouds; what determines the IMF; how long do the
steps of the process take; how efficient is star formation; do any theories
explain the data; how are the star and disk built over time; and what chemical
changes accompany star and planet formation. I close with a summary and list of
open questions.Comment: Proceedings of Computational Star Formation Conference, Barcelon
Feshbach resonances with large background scattering length: interplay with open-channel resonances
Feshbach resonances are commonly described by a single-resonance Feshbach
model, and open-channel resonances are not taken into account explicitly.
However, an open-channel resonance near threshold limits the range of validity
of this model. Such a situation exists when the background scattering length is
much larger than the range of the interatomic potential. The open-channel
resonance introduces strong threshold effects not included in the
single-resonance description. We derive an easy-to-use analytical model that
takes into account both the Feshbach resonance and the open-channel resonance.
We apply our model to Rb, which has a large background scattering
length, and show that the agreement with coupled-channels calculations is
excellent. The model can be readily applied to other atomic systems with a
large background scattering length, such as Li and Cs. Our approach
provides full insight into the underlying physics of the interplay between
open-channel (or potential) resonances and Feshbach resonances.Comment: 16 pages, 12 figures, accepted for publication in Phys. Rev. A; v2:
added reference
Gender-specific changes in quality of life following cardiovascular disease: a prospective study
Gender-specific changes in Quality of Life (QoL) following cardiovascular disease (CVD) were studied in 208 patients to determine whether gender-related differences in postmorbid QoL result from differences in disease severity, premorbid QoL, or different CVD-related recovery. Premorbid data were available from a community-based survey. Follow-ups were done at 6 weeks, 6 months, and 12 months after diagnosis. Results showed that females had worse QoL at all three postmorbid assessments compared to males. However, multivariate analyses adjusting for premorbid gender differences and disease severity showed no significant gender-related differences for physical and psychologic, functioning. Therefore, gender differences in QoL following CVD mainly result from premorbid differences in QoL, age, comorbidity, and disease severity at the time of diagnosis, and do not appear to be the consequence of gender-specific recovery. However, in clinical practice it is important to acknowledge the poorer QoL of females following CVD. (C) 2002 Elsevier Science. All rights reserve
Far infrared CO and HO emission in intermediate-mass protostars
Intermediate-mass young stellar objects (YSOs) provide a link to understand
how feedback from shocks and UV radiation scales from low to high-mass star
forming regions. Aims: Our aim is to analyze excitation of CO and HO in
deeply-embedded intermediate-mass YSOs and compare with low-mass and high-mass
YSOs. Methods: Herschel/PACS spectral maps are analyzed for 6 YSOs with
bolometric luminosities of . The maps
cover spatial scales of AU in several CO and HO lines located
in the m range. Results: Rotational diagrams of CO show two
temperature components at K and
K, comparable to low- and high-mass protostars
probed at similar spatial scales. The diagrams for HO show a single
component at K, as seen in low-mass protostars, and
about K lower than in high-mass protostars. Since the uncertainties in
are of the same order as the difference between the
intermediate and high-mass protostars, we cannot conclude whether the change in
rotational temperature occurs at a specific luminosity, or whether the change
is more gradual from low- to high-mass YSOs. Conclusions: Molecular excitation
in intermediate-mass protostars is comparable to the central AU of
low-mass protostars and consistent within the uncertainties with the high-mass
protostars probed at AU scales, suggesting similar shock
conditions in all those sources.Comment: Accepted to Astronomy & Astrophysics. 4 pages, 5 figures, 3 table
Recommended from our members
Microstructure and Mechanical Properties of Maraging Steel 300 After Selective Laser Melting
Selective laser melting (SLM) is an additive manufacturing process for the direct fabrication of
prototypes, tools and functional parts. The process uses a high intensity laser beam to selectively
fuse fine metal powder particles together in a layer-wise manner by scanning cross-sections
generated from a three-dimensional CAD model. The SLM process is capable of producing near
fully dense functional products without almost any geometrical limitation and having mechanical
properties comparable to those produced by conventional manufacturing techniques. There is a
wide range of materials that are suitable to be processed by SLM including various steels, Ti, Al
and CoCr alloys. Being one of these materials, maraging steel 300 (18Ni-300) is an iron-nickel
steel alloy which is often used in applications where high fracture toughness and strength are
required or where dimensional changes have to remain at a minimal level, e.g. aircraft and
aerospace industries for rocket motor castings and landing gear or tooling applications. To
achieve its superior strength and hardness, maraging steel, of which the name is derived from
‘martensite aging’, should be treated with an aging heat treatment. In this study, the effect of the
SLM parameters (scan speed and layer thickness) on the obtained density, surface quality and
hardness of maraging steel 300 parts is investigated. Moreover, various aging heat treatments
(different combinations of duration and maximum temperature) are applied on the SLM parts to
achieve high hardness values. The mechanical testing of maraging steel 300 specimens produced
by SLM and treated with an appropriate aging treatment is accomplished by impact toughness
and tensile tests and compared to the results obtained using conventional production techniques.
Additionally, the microstructures of as-built and heat treated parts are investigated.Mechanical Engineerin
Dust, Ice and Gas in Time (DIGIT) Herschel program first results: A full PACS-SED scan of the gas line emission in protostar DK Cha
DK Cha is an intermediate-mass star in transition from an embedded
configuration to a star plus disk stage. We aim to study the composition and
energetics of the circumstellar material during this pivotal stage. Using the
Range Scan mode of PACS on the Herschel Space Observatory, we obtained a
spectrum of DK Cha from 55 to 210 micron as part of the DIGIT Key Program.
Almost 50 molecular and atomic lines were detected, many more than the 7 lines
detected in ISO-LWS. Nearly the entire ladder of CO from J=14-13 to 38-37
(E_u/k = 4080 K), water from levels as excited as E_u/k = 843 K, and OH lines
up to E_u/k = 290 K were detected. The continuum emission in our PACS SED scan
matches the flux expected from a model consisting of a star, a surrounding disk
of 0.03 Solar mass, and an envelope of a similar mass, supporting the
suggestion that the object is emerging from its main accretion stage.
Molecular, atomic, and ionic emission lines in the far-infrared reveal the
outflow's influence on the envelope. The inferred hot gas can be photon-heated,
but some emission could be due to C-shocks in the walls of the outflow cavity.Comment: 4 Page letter, To appear in A&A special issue on Hersche
Predicting scattering properties of ultracold atoms: adiabatic accumulated phase method and mass scaling
Ultracold atoms are increasingly used for high precision experiments that can
be utilized to extract accurate scattering properties. This calls for a
stronger need to improve on the accuracy of interatomic potentials, and in
particular the usually rather inaccurate inner-range potentials. A boundary
condition for this inner range can be conveniently given via the accumulated
phase method. However, in this approach one should satisfy two conditions,
which are in principle conflicting, and the validity of these approximations
comes under stress when higher precision is required. We show that a better
compromise between the two is possible by allowing for an adiabatic change of
the hyperfine mixing of singlet and triplet states for interatomic distances
smaller than the separation radius. A mass scaling approach to relate
accumulated phase parameters in a combined analysis of isotopically related
atom pairs is described in detail and its accuracy is estimated, taking into
account both Born-Oppenheimer and WKB breakdown. We demonstrate how numbers of
singlet and triplet bound states follow from the mass scaling.Comment: 14 pages, 9 figure
Atomic size oscillations in conductance histograms for gold nanowires and the influence of work hardening
Nanowires of different nature have been shown to self-assemble as a function
of stress at the contact between two macroscopic metallic leads. Here we
demonstrate for gold wires that the balance between various metastable nanowire
configurations is influenced by the microstructure of the starting materials
and we discover a new set of periodic structures, which we interpret as due to
the atomic discreteness of the contact size for the three principal crystal
orientations.Comment: This version corrects an error in attributing the three observed
periods, and includes a comparison with recent model calculation
- …