988 research outputs found
A Single Circumbinary Disk in the HD 98800 Quadruple System
We present sub-arcsecond thermal infrared imaging of HD 98800, a young
quadruple system composed of a pair of low-mass spectroscopic binaries
separated by 0.8'' (38 AU), each with a K-dwarf primary. Images at wavelengths
ranging from 5 to 24.5 microns show unequivocally that the optically fainter
binary, HD 98800B, is the sole source of a comparatively large infrared excess
upon which a silicate emission feature is superposed. The excess is detected
only at wavelengths of 7.9 microns and longer, peaks at 25 microns, and has a
best-fit black-body temperature of 150 K, indicating that most of the dust lies
at distances greater than the orbital separation of the spectroscopic binary.
We estimate the radial extent of the dust with a disk model that approximates
radiation from the spectroscopic binary as a single source of equivalent
luminosity. Given the data, the most-likely values of disk properties in the
ranges considered are R_in = 5.0 +/- 2.5 AU, DeltaR = 13+/-8 AU, lambda_0 =
2(+4/-1.5) microns, gamma = 0+/-2.5, and sigma_total = 16+/-3 AU^2, where R_in
is the inner radius, DeltaR is the radial extent of the disk, lambda_0 is the
effective grain size, gamma is the radial power-law exponent of the optical
depth, tau, and sigma_total is the total cross-section of the grains. The range
of implied disk masses is 0.001--0.1 times that of the moon. These results show
that, for a wide range of possible disk properties, a circumbinary disk is far
more likely than a narrow ring.Comment: 11 page Latex manuscript with 3 postscript figures. Accepted for
publication in Astrophysical Journal Letters. Postscript version of complete
paper also available at
http://www.hep.upenn.edu/PORG/web/papers/koerner00a.p
H_2 Emission From Disks Around Herbig Ae and T Tauri Stars
We present the initial results of a deep ISO-SWS survey
for the low J pure rotational emission lines of H2
toward a number of Herbig Ae and T Tauri stars.
The objects are selected to be as isolated as possible
from molecular clouds, with a spectral energy distribution
characteristic of a circumstellar disk. For most
of them the presence of a disk has been established
directly by millimeter interferometry. The S (1) line is
detected in most sources with a peak flux of 0.3-1 Jy.
The S(0) line is definitely seen in 2 objects: GG Tau
and HD 163296. The observations suggest the presence
of "warm" gas at T_(kin) ≈ 100 K with a mass of a
few % of the total gas+ dust mass, derived assuming
a gas-to-dust ratio of 100:1. The S(1) peak flux does
not show a strong correlation with spectral type of
the central star or continuum flux at 1.3 millimeter.
Possible origins for the warm gas seen in H_2 are discussed,
and comparisons with model calculations are
made
A circumstellar dust disk around T Tau N: Sub-arcsecond imaging at 3 mm
We present high-resolution imaging of the young binary T Tauri in 3 mm
continuum emission. Compact dust emission with integrated flux density 50 +/- 6
mJy is resolved in an aperture synthesis map at 0.5" resolution and is centered
at the position of the optically visible component, T Tau N. No emission above
a 3 sigma level of 9 mJy is detected 0.7" south of T Tau N at the position of
the infrared companion, T Tau S. We interpret the continuum detection as
arising from a circumstellar disk around T Tau N and estimate its properties by
fitting a flat-disk model to visibilities at wavelengths of 1 and 3 mm and to
the flux density at 7 mm. Given the data, probability distributions are
calculated for values of the free parameters, including the temperature,
density, dust opacity, and the disk outer radius. The radial variation in
temperature and density is not narrowly constrained by the data. The most
likely value of the frequency dependence of the dust opacity, beta =
0.53^{+0.27}_{-0.17}, is consistent with that of disks around other T Tauri
stars in which grain growth is believed to have taken place. The outer radius,
R = 41^{+26}_{-14} AU, is smaller than the projected binary separation, and may
indicate truncation of the disk. The total mass estimated for the disk,
log(M/M_sun) = {-2.4}^{+0.7}_{-0.6}, is similar to masses observed around many
young single sources and to the minimum nebular mass required to form a
planetary system like our own. This observation strongly suggests that the
presence of a binary companion does not rule out the formation of a sizeable
planetary system.Comment: Accepted by the Astrophysical Journal. 13 pages Latex (uses AASTeX
macros) including 3 postscript figures. Also at
http://astro.berkeley.edu/~rla
Crystal-field ground state of the non-centrosymmetric superconductor CePt3Si: a combined polarized soft X-ray absorption and polarized neutron study
We determined the crystal-field split Hund's rule ground state of the
non-centrosymmetric heavy fermion superconductor CePt3Si with polarization
dependent soft X-ray absorption spectroscopy (XAS) and polarized neutron
scattering. We are also able to give the sequence of the crystal-field states
from the temperature evolution of the linear dichroic signal in the XAS. The
quantitative analysis of the XAS temperature dependence together with the
neutron transition energies complete the identification of the crystal-field
level scheme.Comment: 6 pages, 5 figures, submitted to PR
HST/NICMOS Imaging of Disks and Envelopes Around Very Young Stars
We present HST/NICMOS observations with 0.1" (15 AU) resolution of six young
stellar objects in the Taurus star-formation region. The targets of our survey
are three Class I IRAS sources (IRAS 04016+2610, IRAS 04248+2612, and IRAS
04302+2247) and three low-luminosity stars (DG Tau B, Haro 6-5B, and CoKu
Tau/1) associated with Herbig Haro jets. The broad-band images show that the
near-infrared radiation from these sources is dominated by light scattered from
dusty circumstellar material distributed in a region 10 - 15 times the size of
our solar system. Although the detailed morphologies of the individual objects
are unique, the observed young stellar objects share common features. All of
the circumstellar reflection nebulae are crossed by dark lanes from 500 - 900
AU in extent and from less than 50 to 350 AU in apparent thickness. The
absorption lanes extend perpendicular to known optical and millimeter outflows
in these sources. We interpret the dark lanes as optically thick circumstellar
disks seen in silhouette against bright reflection nebulosity. The bipolar
reflection nebulae extending perpendicular to the dust lanes appear to be
produced by scattering from the upper and lower surfaces of the disks and from
dusty material within or on the walls of the outflow cavities. Out of five
objects in which the central source is directly detected, two are found to be
subarcsecond binaries. This mini-survey is the highest resolution near-infrared
study to date of circumstellar environments around solar-type stars with age <=
1 Myr.Comment: 34 pages, 4 figures; also available at
http://spider.ipac.caltech.edu/staff/brandner/topics/disks/disks.html ;
accepted for publication in AJ (March 1999 issue
On the errors-in-variables model with equality and inequality constraints for selected numerical examples
The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. IV. Lupus Observed with MIPS
We present maps of 7.78 square degrees of the Lupus molecular cloud complex
at 24, 70, and m. They were made with the Spitzer Space Telescope's
Multiband Imaging Photometer for Spitzer (MIPS) instrument as part of the
Spitzer Legacy Program, ``From Molecular Cores to Planet-Forming Disks'' (c2d).
The maps cover three separate regions in Lupus, denoted I, III, and IV. We
discuss the c2d pipeline and how our data processing differs from it. We
compare source counts in the three regions with two other data sets and
predicted star counts from the Wainscoat model. This comparison shows the
contribution from background galaxies in Lupus I. We also create two color
magnitude diagrams using the 2MASS and MIPS data. From these results, we can
identify background galaxies and distinguish them from probable young stellar
objects. The sources in our catalogs are classified based on their spectral
energy distribution (SED) from 2MASS and Spitzer wavelengths to create a sample
of young stellar object candidates. From 2MASS data, we create extinction maps
for each region and note a strong corresponence between the extinction and the
m emission. The masses we derived in each Lupus cloud from our
extinction maps are compared to masses estimated from CO and CO
and found to be similar to our extinction masses in some regions, but
significantly different in others. Finally, based on our color-magnitude
diagrams, we selected 12 of our reddest candidate young stellar objects for
individual discussion. Five of the 12 appear to be newly-discovered YSOs.Comment: 15 pages, 17 figures, uses emulateapj.cls. Accepted for publication
in ApJ. A version with high-quality figures can be found at
http://peggysue.as.utexas.edu/SIRTF
Nurses' perceptions and experiences of communication in the operating theatre: a focus group interview
UNLABELLED: Nurses' perceptions and experiences of communication in the operating theatre: a focus group interview BACKGROUND: Communication programmes are well established in nurse education. The focus of programmes is most often on communicating with patients with less attention paid to inter-professional communication or skills essential for working in specialised settings. Although there are many anecdotal reports of communication within the operating theatre, there are few empirical studies. This paper explores communication behaviours for effective practice in the operating theatre as perceived by nurses and serves as a basis for developing training. METHODS: A focus group interview was conducted with seven experienced theatre nurses from a large London teaching hospital. The interview explored their perceptions of the key as well as unique features of effective communication skills in the operating theatre. Data was transcribed and thematically analysed until agreement was achieved by the two authors. RESULTS: There was largely consensus on the skills deemed necessary for effective practice including listening, clarity of speech and being polite. Significant influences on the nature of communication included conflict in role perception and organisational issues. Nurses were often expected to work outside of their role which either directly or indirectly created barriers for effective communication. Perceptions of a lack of collaborative team effort also influenced communication. CONCLUSION: Although fundamental communication skills were identified for effective practice in the operating theatre, there were significant barriers to their use because of confusion over clarity of roles (especially nurses' roles) and the implications for teamwork. Nurses were dissatisfied with several aspects of communication. Future studies should explore the breadth and depth of this dissatisfaction in other operating theatres, its impact on morale and importantly on patient safety. Interprofessional communication training for operating theatre staff based in part on the key issues identified in this study may help to create clarity in roles and focus attention on effective teamwork and promote clinical safety
Quasiperiodic spin-orbit motion and spin tunes in storage rings
We present an in-depth analysis of the concept of spin precession frequency
for integrable orbital motion in storage rings. Spin motion on the periodic
closed orbit of a storage ring can be analyzed in terms of the Floquet theorem
for equations of motion with periodic parameters and a spin precession
frequency emerges in a Floquet exponent as an additional frequency of the
system. To define a spin precession frequency on nonperiodic synchro-betatron
orbits we exploit the important concept of quasiperiodicity. This allows a
generalization of the Floquet theorem so that a spin precession frequency can
be defined in this case too. This frequency appears in a Floquet-like exponent
as an additional frequency in the system in analogy with the case of motion on
the closed orbit. These circumstances lead naturally to the definition of the
uniform precession rate and a definition of spin tune. A spin tune is a uniform
precession rate obtained when certain conditions are fulfilled. Having defined
spin tune we define spin-orbit resonance on synchro--betatron orbits and
examine its consequences. We give conditions for the existence of uniform
precession rates and spin tunes (e.g. where small divisors are controlled by
applying a Diophantine condition) and illustrate the various aspects of our
description with several examples. The formalism also suggests the use of
spectral analysis to ``measure'' spin tune during computer simulations of spin
motion on synchro-betatron orbits.Comment: 62 pages, 1 figure. A slight extension of the published versio
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