2,963 research outputs found
The lithium depletion boundary and the age of NGC 2547
We present the results of a photometric and spectroscopic survey of cool M
dwarf candidates in the young open cluster NGC 2547. Using the 2dF fiber
spectrograph, we have searched for the luminosity at which lithium remains
unburned in an attempt to constrain the cluster age. The lack of a population
of individual lithium-rich objects towards the faint end of our sample places a
very strong lower limit to the cluster age of 35 Myr. However, the detection of
lithium in the averaged spectra of our faintest targets suggests that the
lithium depletion boundary lies at 9.5 < M(I) < 10.0 and that the cluster age
is < 54 Myr. The age of NGC 2547 judged from fitting isochrones to low-mass
pre-main-sequence stars in colour-magnitude diagrams is 20-35 Myr using the
same evolutionary models. The sense and size of the discrepancy in age
determined by these two techniques is similar to that found in another young
cluster, IC 2391, and in the low-mass pre main-sequence binary system, GJ
871.1AB. We suggest that the inclusion of rotation or dynamo-generated magnetic
fields in the evolutionary models could reconcile the two age determinations,
but only at the expense of increasing the cluster ages beyond that currently
indicated by the lithium depletion. Alternatively, some mechanism is required
that increases the rate of lithium depletion in young, very low-mass fully
convective stars.Comment: 14 pages, 7 figures, 3 tables, accepted by MNRA
Historical geography II: traces remain
The second report in this series turns to focus on the trace in relation to life-writing and biography in historical geography and beyond. Through attention to tracing journeys, located moments and listening to the presence of ghosts (Ogborn, 2005), this report seeks to highlight the range of different ways in which historical geographers have explored lives, deaths, and their transient traces through varied biographical terrains. Continuing to draw attention in historical geography to the darkest of histories, this piece will pivot on moments of discovering the dead to showcase the nuanced ways in which historical geography is opening doors into uncharted lives and unspoken histories
The stellar association around Gamma Velorum and its relationship with Vela OB2
We present the results of a photometric BVI survey of 0.9 square degrees
around the Wolf-Rayet binary gamma^2 Vel and its early-type companion gamma^1
Vel. Several hundred PMS stars are identified and the youth of a subset of
these is confirmed by the presence of lithium, H-alpha emission and X-ray
activity. We show that the PMS stars are kinematically coherent and spatially
concentrated around gamma Vel. The PMS stars have similar proper motions to
gamma Vel, to main-sequence stars around gammaVel and to early-type stars of
the wider Vela OB2 association of which gamma^2 Vel is the brightest member.
The ratio of main-sequence stars to low-mass (0.1-0.6 Msun) PMS stars is
consistent with a Kroupa mass function. Main-sequence fitting to stars around
gamma Vel gives a distance modulus of 7.76+/-0.07 mag, consistent with a
similarly-determined distance for Vela OB2 and with interferometric distances
to gamma^2 Vel. High-mass stellar models indicate an age of 3-4 Myr for gamma^2
Vel, but the low-mass PMS stars have ages of ~10 Myr according to low-mass
evolutionary models and 5-10 Myr by empirically placing them in an age sequence
with other clusters based on colour-magnitude diagrams and lithium depletion.
We conclude that the low-mass PMS stars form a genuine association with gamma
Vel and that this is a subcluster within the larger Vela OB2 association. We
speculate that gamma^2 Vel formed after the low-mass stars, expelling gas,
terminating star formation and unbinding the association. The velocity
dispersion of the PMS stars is too low for this star forming event to have
produced all the stars in Vela OB2. Instead, star formation must have started
at several sites within a molecular cloud, either sequentially or,
simultaneously after some triggering event [abridged].Comment: Accepted for publication in MNRA
Mass segregation in the young open cluster NGC 2547
We present a study of mass segregation of the young (20-35 Myr isochronal
age), open cluster NGC 2547. We find good evidence that mass segregation exists
in NGC 2547 down to 3 Msun, and weak evidence for mass segregation down to 1
Msun. Theoretical models of an initially unsegregated model of NGC 2547 using
the NBODY2 code show weaker mass segregation, implying that at least some of
the observed mass segregation has a primordial origin. We also report the
discovery of three possible escaped cluster members, which share the proper
motion and colours of the cluster, but lie nearly a degree from the cluster
centre.Comment: 7 pages, 7 figures. To appear in Monthly Notices (in press
Increasing the Value of Health Care: The Role of Nurses
Increasing health care value has become a central objective of payment policies, insurance design and purchasing, and patient and provider decision-making. The word âvalueâ appeared in the title of seven sections of the Affordable Care Act (ACA), and earlier this year CMS set a goal of having 50 percent of reimbursement based on value. This brief reviews nursesâ contribution to value, highlighting evidence published by researchers in the Interdisciplinary Nursing Quality Research Initiative (INQRI), an 8-year program funded by the Robert Wood Johnson Foundation. It also looks at interventions designed to address outcomes targeted by ACA- or Medicare-related payment policies
Correcting the extended-source calibration for the <i>Herschel</i>-SPIRE Fourier-transform spectrometer
We describe an update to the Herschel-Spectral and Photometric Imaging Receiver (SPIRE) Fourier-transform spectrometer (FTS) calibration for extended sources, which incorporates a correction for the frequency-dependent far-field feedhorn efficiency, ηff. This significant correction affects all FTS extended-source calibrated spectra in sparse or mapping mode, regardless of the spectral resolution. Line fluxes and continuum levels are underestimated by factors of 1.3â2 in thespectrometer long wavelength band (447â1018 GHz; 671â294 ÎŒm) and 1.4â1.5 in the spectrometer short wavelength band (944â1568 GHz; 318â191 ÎŒm). The correction was implemented in the FTS pipeline version 14.1 and has also been described in the SPIRE Handbook since 2017 February. Studies based on extended-source calibrated spectra produced prior to this pipeline version should be critically reconsidered using the current products available in the Herschel Science Archive. Once the extended-source calibrated spectra are corrected for ηff, the synthetic photometry and the broad-band intensities from SPIRE photometer maps agree within 2â4âperâcent â similar levels to the comparison of point-source calibrated spectra and photometry from point-source calibrated maps. The two calibration schemes for the FTS are now self-consistent: the conversion between the corrected extended-source and point-source calibrated spectra can be achieved with the beam solid angle and a gain correction that accounts for the diffraction loss
Systematic characterisation of the Herschel SPIRE Fourier Transform Spectrometer
A systematic programme of calibration observations was carried out to monitor
the performance of the SPIRE FTS instrument on board the Herschel Space
Observatory. Observations of planets (including the prime point-source
calibrator, Uranus), asteroids, line sources, dark sky, and cross-calibration
sources were made in order to monitor repeatability and sensitivity, and to
improve FTS calibration. We present a complete analysis of the full set of
calibration observations and use them to assess the performance of the FTS.
Particular care is taken to understand and separate out the effect of pointing
uncertainties, including the position of the internal beam steering mirror for
sparse observations in the early part of the mission. The repeatability of
spectral line centre positions is <5km/s, for lines with signal-to-noise ratios
>40, corresponding to <0.5-2.0% of a resolution element. For spectral line
flux, the repeatability is better than 6%, which improves to 1-2% for spectra
corrected for pointing offsets. The continuum repeatability is 4.4% for the SLW
band and 13.6% for the SSW band, which reduces to ~1% once the data have been
corrected for pointing offsets. Observations of dark sky were used to assess
the sensitivity and the systematic offset in the continuum, both of which were
found to be consistent across the FTS detector arrays. The average point-source
calibrated sensitivity for the centre detectors is 0.20 and 0.21 Jy [1 sigma; 1
hour], for SLW and SSW. The average continuum offset is 0.40 Jy for the SLW
band and 0.28 Jy for the SSW band.Comment: 41 pages, 37 figures, 32 tables. Accepted for publication in MNRA
Calibration of <i>Herschel</i> SPIRE FTS observations at different spectral resolutions
The SPIRE Fourier Transform Spectrometer on-board the Herschel Space Observatory had two standard spectral resolution modes for science observations: high resolution (HR) and low resolution (LR), which could also be performed in sequence (H+LR). A comparison of the HR and LR resolution spectra taken in this sequential mode revealed a systematic discrepancy in the continuum level. Analysing the data at different stages during standard pipeline processing demonstrates that the telescope and instrument emission affect HR and H+LR observations in a systematically different way. The origin of this difference is found to lie in the variation of both the telescope and instrument response functions, while it is triggered by fast variation of the instrument temperatures. As it is not possible to trace the evolution of the response functions using housekeeping data from the instrument subsystems, the calibration cannot be corrected analytically. Therefore, an empirical correction for LR spectra has been developed, which removes the systematic noise introduced by the variation of the response functions
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