124 research outputs found
Does formal workplace based assessment add value to informal feedback?
Feedback is a key component of learning but effective feedback is a complex process with many aspects. One aspect may be a written summary which is passed to the learner but this may not be valued by learners. We examined the role of written feedback in the feedback process to determine whether it does more than provide a simple summary of the interaction. We conducted a secondary analysis of data gathered for a study of formative workplace based assessment. Interview data from 24 interviews with students and written summaries of workplace based assessments for 23 of them were reanalysed by two researchers who were already immersed in the data and examined all references to verbal, informal feedback and written, formal feedback or the assessment tool used. We found that students valued the verbal feedback discussion highly and that they often considered the written summaries superfluous. We also found that the act of preparing written feedback augmented the feedback discussion and tutors had adopted the language of the formal instrument in the verbal feedback and free text written feedback.
What this study adds to existing research is evidence that there may be a secondary faculty development effect of requiring the preparation of written feedback which has served to enhance the educational content of feedback. Although this is not proof of causality (the requirement to provide written feedback alone producing the positive effects), we consider that the likelihood is sufficiently strong to continue the practice
Colors and taxonomy of Centaurs and Trans-Neptunian Objects
The study of the surface properties of Centaurs and Trans-Neptunian Objects
(TNOs) provides essential information about the early conditions and evolution
of the outer Solar System. Due to the faintness of most of these distant and
icy bodies, photometry currently constitutes the best technique to survey a
statistically significant number of them. Our aim is to investigate color
properties of a large sample of minor bodies of the outer Solar System, and set
their taxonomic classification. We carried out visible and near-infrared
photometry of Centaurs and TNOs, making use, respectively, of the FORS2 and
ISAAC instruments at the Very Large Telescope (European Southern Observatory).
Using G-mode analysis, we derived taxonomic classifications according to the
Barucci et al. (2005a) system. We report photometric observations of 31
objects, 10 of them have their colors reported for the first time ever. 28
Centaurs and TNOs have been assigned to a taxon. We combined the entire sample
of 38 objects taxonomically classified in the framework of our programme (28
objects from this work; 10 objects from DeMeo et al. 2009a) with previously
classified TNOs and Centaurs, looking for correlations between taxonomy and
dynamics. We compared our photometric results to literature data, finding hints
of heterogeneity for the surfaces of 4 objects.Comment: 7 pages, 4 figures. To be published in Astronomy and Astrophysic
JHK Observations of Faint Standard Stars in the Mauna Kea Near-Infrared Photometric System
JHK photometry in the Mauna Kea Observatory (MKO) near-IR system is presented
for 115 stars. Of these, 79 are UKIRT standards and 42 are LCO standards. The
average brightness is 11.5 mag, with a range of 10 to 15. The average number of
nights each star was observed is 4, and the average of the internal error of
the final results is 0.011 mag. These JHK data agree with those reported by
other groups to 0.02 mag. The measurements are used to derive transformations
between the MKO JHK photometric system and the UKIRT, LCO and 2MASS systems.
The 2MASS-MKO data scatter by 0.05 mag for redder stars: 2MASS-J includes H2O
features in dwarfs and MKO-K includes CO features in giants. Transformations
derived for stars whose spectra contain only weak features cannot give accurate
transformations for objects with strong absorption features within a filter
bandpasses. We find evidence of systematic effects at the 0.02 mag level in the
photometry of stars with J<11 and H,K<10.5. This is due to an underestimate of
the linearity correction for stars observed with the shortest exposure times;
very accurate photometry of stars approaching the saturation limits of infrared
detectors which are operated in double-read mode is difficult to obtain. Four
stars in the sample, GSPC S705-D, FS 116 (B216-b7), FS 144 (Ser-EC84) and FS 32
(Feige 108), may be variable. 84 stars in the sample have 11< J< 15 and
10.5<H,K<15, are not suspected to be variable, and have magnitudes with an
estimated error <0.027 mag; 79 of these have an error of <0.020 mag. These
represent the first published high-accuracy JHK stellar photometry in the MKO
photometric system; we recommend these objects be employed as primary standards
for that system [abridged].Comment: Accepted for publication in MNRAS, 14 pages, 5 Figure
The Vela Pulsar in the Near-Infrared
We report on the first detection of the Vela pulsar in the near-infrared with
the VLT/ISAAC in the Js and H bands. The pulsar magnitudes are Js=22.71 +/-
0.10 and H=22.04 +/- 0.16. We compare our results with the available
multiwavelength data and show that the dereddened phase-averaged optical
spectrum of the pulsar can be fitted with a power law F_nu propto
nu^(-alpha_nu) with alpha_nu = 0.12 +/- 0.05, assuming the color excess
E(B-V)=0.055 +/-0.005 based on recent spectral fits of the emission of the Vela
pulsar and its supernova remnant in X-rays. The negative slope of the pulsar
spectrum is different from the positive slope observed over a wide optical
range in the young Crab pulsar spectrum. The near-infrared part of the Vela
spectrum appears to have the same slope as the phase-averaged spectrum in the
high energy X-ray tail, obtained in the 2-10 keV range with the RXTE. Both of
these spectra can be fitted with a single power law suggesting their common
origin. Because the phase-averaged RXTE spectrum in this range is dominated by
the second X-ray peak of the pulsar light curve, coinciding with the second
main peak of its optical pulse profile, we suggest that this optical peak can
be redder than the first one. We also detect two faint extended structures in
the 1.5''-3.1'' vicinity of the pulsar, projected on and aligned with the
south-east jet and the inner arc of the pulsar wind nebula, detected in X-rays
with Chandra. We discuss their possible association with the nebula.Comment: 12 pages, 8 figures, accepted for publication in A&A, the associated
near-infrared images in the fits format are available at
http://www.ioffe.ru/astro/NSG/obs/vela-ir
Water maser emission and the parsec-scale jet in NGC 3079
We have conducted VLBI observations of water maser and radio continuum
emission in the nucleus of NGC 3079. The 22 GHz maser emission arises in
compact clumps, distributed along an axis that is aligned with the major axis
of the galactic disk. The velocities of the masers are consistent with their
lying in the inner parsec of a molecular disk rotating in the same sense as the
rest of the galaxy. However, the velocity field has a significant
non-rotational component, which may indicate supersonic turbulence. The bright
maser emission is not coincident with any detected compact 22 GHz continuum
source, suggesting the high apparent luminosity of the maser may not due to
beamed amplification of continuum emission. We observed two compact continuum
sources that have inverted spectra between 5 and 8 GHz, and steep spectra
between 8 and 22 GHz. NGC 3079 may be a nearby, low-luminosity example of the
class of compact symmetric gigahertz-peaked spectrum (GPS) radio sources. We
detected a third continuum component that lies along the same axis as the other
two, strongly suggesting that this galaxy possesses a nuclear jet. Faint maser
emission was detected near this axis, which may indicate a second population of
masers associated with the jet.Comment: 31 pages, includes 8 figures. To appear in ApJ, Vol 495, 10 March
1998. Full-resolution figures and color plate available at
http://cfa-www.harvard.edu/~trotter/ngc3079.htm
High Energy gamma-rays From FR I Jets
Thanks to Hubble and Chandra telescopes, some of the large scale jets in
extragalactic radio sources are now being observed at optical and X-ray
frequencies. For the FR I objects the synchrotron nature of this emission is
surely established, although a lot of uncertainties - connected for example
with the particle acceleration processes involved - remain. In this paper we
study production of high energy gamma-rays in FR I kiloparsec-scale jets by
inverse-Compton emission of the synchrotron-emitting electrons. We consider
different origin of seed photons contributing to the inverse-Compton
scattering, including nuclear jet radiation as well as ambient, stellar and
circumstellar emission of the host galaxies. We discuss how future detections
or non-detections of the evaluated gamma-ray fluxes can provide constraints on
the unknown large scale jet parameters, i.e. the magnetic field intensity and
the jet Doppler factor. For the nearby sources Centaurus A and M 87, we find
measurable fluxes of TeV photons resulting from synchrotron self-Compton
process and from comptonisation of the galactic photon fields, respectively. In
the case of Centaurus A, we also find a relatively strong emission component
due to comptonisation of the nuclear blazar photons, which could be easily
observed by GLAST at energy ~10 GeV, providing important test for the
unification of FR I sources with BL Lac objects.Comment: 39 pages, 6 figures included. Modified version, accepted for
publication in Astrophysical Journa
Variable stars in the field of the old open cluster Melotte 66
We report the results of photometric monitoring of the Melotte 66 field in
BVI filters. Ten variables were identified with nine being new discoveries. The
sample includes eight eclipsing binaries of which four are W UMa type stars,
one star is a candidate blue straggler. All four contact binaries are likely
members of the cluster based on their estimated distances. Ten blue stars with
U-B<-0.3 were detected inside a 14.8 x 22.8 arcmin^2 field centred on the
cluster. Time series photometry for 7 of them showed no evidence for any
variability. The brightest object in the sample of blue stars is a promising
candidate for a hot subdwarf belonging to the cluster. We show that the
anomalously wide main sequence of the cluster, reported in some earlier
studies, results from a combination of two effects: variable reddening occuring
across the cluster field and the presence of a rich population of binary stars
in the cluster itself. The density profile of the cluster field is derived and
the total number of member stars with 16<V<21 or 2.8<M_{V}<7.8 is estimated
conservatively at about 1100.Comment: 8 pages, 12 figures, accepted to MNRAS - 29 June 200
Central Structural Parameters of Early-Type Galaxies as Viewed with HST/NICMOS
We present surface photometry for the central regions of a sample of 33
early-type (E, S0, and S0/a) galaxies observed at 1.6 microns (H band) using
the Hubble Space Telescope (HST). We employ a new technique of two-dimensional
fitting to extract quantitative parameters for the bulge light distribution and
nuclear point sources, taking into consideration the effects of the
point-spread function. Parameterizing the bulge profile with a ``Nuker'' law,
we confirm that the central surface-brightness distributions largely fall into
two categories, each of which correlates with the global properties of the
galaxies. ``Core'' galaxies tend to be luminous ellipticals with boxy or pure
elliptical isophotes, whereas ``power-law'' galaxies are preferentially lower
luminosity systems with disky isophotes. Unlike most previous studies, however,
we do not find a clear gap in the distribution of inner cusp slopes; several
objects have inner cusp slopes (0.3 < gamma < 0.5) which straddle the regimes
conventionally defined for core and power-law type galaxies. The nature of
these intermediate objects is unclear. We draw attention to two objects in the
sample which appear to be promising cases of galaxies with isothermal cores
that are not the brightest members of a cluster. Unresolved nuclear point
sources are found in about 50% of the sample galaxies, roughly independent of
profile type, with magnitudes in the range m^{nuc}_H = 12.8 to 17.4 mag, which
correspond to M_H^{nuc} = -12.8 to -18.4 mag. (Abridged)Comment: To appear in The Astronomical Journal. Latex, 24 pages and 17 JPEG
image
NMA CO (J=1-0) Observations of the Halpha/Radio Lobe Galaxy NGC 3079: Gas Dynamics in a Weak Bar Potential and Central Massive Core
within 24 lines with 80 characters) We present ^12CO (1-0) observations in
the central 4.5 kpc (1 arcmin) of the Halpha/Radio lobe galaxy NGC 3079 with
the Nobeyama Millimeter Array. The molecular gas shows four components: a main
disk, spiral arms, a nuclear disk, and a nuclear core. The main disk extends
beyond our spatial coverage. The spiral arms are superimposed on the main disk.
The nuclear disk with about 600 pc radius has an intense concentration of
molecular gas. Its appearance on PV diagrams is indicative of oval motions of
the gas, rather than circular. The nuclear core is more compact than our
resolution. Though it is unresolved, the nuclear core shows a very high
velocity about 200 km/s even at the radius of about 100 pc on the PV diagram.
We propose a model that NGC 3079 contains a weak bar. This model explains the
observed features: the main disk and spiral arms result from gaseous x1-orbits
and associated crowding respectively. The nuclear disk arises from gaseous
x2-orbits. From the appearance of the spiral arms on the PV diagram, the
pattern speed of the bar is estimated to be 55+-10 km/s/kpc. The high velocity
of the nuclear core cannot be explained by our model for a bar. Thus we
attribute it to a central massive core with a dynamical mass of 10^9 Msun
within the central 100 pc. This mass is three orders of magnitude more massive
than that of a central black hole in this galaxy.Comment: 43 pages, 17 figures; ApJ, 573, 105, 200
Interstellar reddening towards six small areas in Puppis-Vela
We investigate the distribution of the interstellar dust towards six small
volumes of the sky in the region of the Gum nebula. New high-quality
four-colour uvby and H\beta\ Str\"omgren photometry obtained for 352 stars in
six selected areas of Kapteyn, complemented with data obtained in a previous
investigation for two of these areas, were used to estimate the colour excess
and distance to these objects. The obtained colour excess versus distance
diagrams, complemented with other information, when available, were analysed in
order to infer the properties of the interstellar medium permeating the
observed volumes. On the basis of the overall standard deviation in the
photometric measurements, we estimate that colour excesses and distances are
determined with an accuracy of 0.010 mag and better than 30%, respectively, for
a sample of 520 stars. A comparison with 37 stars in common with the new
Hipparcos catalogue attests to the high quality of the photometric distance
determination. The obtained colour excess versus distance diagrams testify to
the low density volume towards the observed lines-of-sight. Very few stars out
to distances of 1 kpc from the Sun have colour excesses larger than E(b-y) =
0.1 mag. In spite of the low density character of the interstellar medium
towards the Puppis-Vela direction, the obtained reddening as a function of the
distance indicates that two or more interstellar structures are crossed towards
the observed lines-of-sight. One of these structures may be associated with the
very low density wall of the Local Cavity, which has a distance of 100-150 pc
from the Sun. Another structure might be related to the Gum nebula, and if so,
its front face would be located at about 350 pc from the Sun.Comment: Accepted by Astronomy and Astrophysic
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