362 research outputs found
Opacity in compact extragalactic radio sources and its effect on radio-optical reference frame alignment
Accurate alignment of the radio and optical celestial reference frames
requires detailed understanding of physical factors that may cause offsets
between the positions of the same object measured in different spectral bands.
Opacity in compact extragalactic jets (due to synchrotron self-absorption and
external free-free absorption) is one of the key physical phenomena producing
such an offset, and this effect is well-known in radio astronomy ("core
shift"). We have measured the core shifts in a sample of 29 bright compact
extragalactic radio sources observed using very long baseline interferometry
(VLBI) at 2.3 and 8.6 GHz. We report the results of these measurements and
estimate that the average shift between radio and optical positions of distant
quasars would be of the order of 0.1-0.2 mas. This shift exceeds positional
accuracy of GAIA and SIM. We suggest two possible approaches to carefully
investigate and correct for this effect in order to align accurately the radio
and optical positions. Both approaches involve determining a Primary Reference
Sample of objects to be used for tying the radio and optical reference frames
together.Comment: 4 pages, 1 figure; to appear in IAU Symposium 248 Proceedings, "A
Giant Step: from Milli- to Micro-arcsecond Astrometry", eds. W.-J. Jin, I.
Platais, M. Perryma
Discovery of the Fomalhaut C debris disc
Fomalhaut is one of the most interesting and well studied nearby stars,
hosting at least one planet, a spectacular debris ring, and two distant
low-mass stellar companions (TW PsA and LP 876-10, a.k.a. Fomalhaut B & C). We
observed both companions with Herschel, and while no disc was detected around
the secondary, TW PsA, we have discovered the second debris disc in the
Fomalhaut system, around LP 876-10. This detection is only the second case of
two debris discs seen in a multiple system, both of which are relatively wide
(3000 AU for HD 223352/40 and 158 kAU [0.77 pc] for Fomalhaut/LP
876-10). The disc is cool (24K) and relatively bright, with a fractional
luminosity , and represents the rare
observation of a debris disc around an M dwarf. Further work should attempt to
find if the presence of two discs in the Fomalhaut system is coincidental,
perhaps simply due to the relatively young system age of 440 Myr, or if the
stellar components have dynamically interacted and the system is even more
complex than it currently appears.Comment: Published in MNRAS Letters. Merry Xma
Role of brachytherapy in the treatment of cancers of the anal canal. Long-term follow-up and multivariate analysis of a large monocentric retrospective series.
BACKGROUND AND PURPOSE: There are few data on long-term clinical results and tolerance of brachytherapy in anal canal cancer. We present one of the largest retrospective analyses of anal canal cancers treated with external beam radiotherapy with/without (±) chemotherapy followed by a brachytherapy boost.
MATERIALS AND METHODS: We performed a retrospective analysis of clinical results in terms of efficacy and toxicity. The impact of different clinical and therapeutic variables on these outcomes was studied.
RESULTS: From May 1992 to December 2009, 209 patients received brachytherapy after external beam radiotherapy ± chemotherapy. Of these patients, 163 were stage II or stage IIIA (UICC 2002) and 58 were N1-3. According to age, ECOG performance status (PS), and comorbidities, patients received either radiotherapy alone (58/209) or radiochemotherapy (151/209). The median follow-up was 72.8 months. The 5- and 10-year local control rates were 78.6 and 73.9 %, respectively. Globally, severe acute and late G3-4 reactions (NCI-CTC scale v. 4.0) occurred in 11.2 and 6.3 % of patients, respectively. Univariate analysis showed the statistical impact of the pelvic treatment volume (p = 0.046) and of the total dose (p = 0.02) on the risk of severe acute and late toxicities, respectively. Only six patients required permanent colostomy because of severe late anorectal toxicities.
CONCLUSION: After a long follow-up time, brachytherapy showed an acceptable toxicity profile and high local control rates in patients with anal canal cancer
Analysis of the Herschel DEBRIS Sun-like star sample
This paper presents a study of circumstellar debris around Sun-like stars using data from the Herschel DEBRIS Key Programme. DEBRIS is an unbiased survey comprising the nearest ∼90 stars of each spectral type A-M. Analysis of the 275 F-K stars shows that excess emission from a debris disc was detected around 47 stars, giving a detection rate of 17.1
+2.6−2.3
per cent, with lower rates for later spectral types. For each target a blackbody spectrum was fitted to the dust emission to determine its fractional luminosity and temperature. The derived underlying distribution of fractional luminosity versus blackbody radius in the population showed that most detected discs are concentrated at f ∼ 10−5 and at temperatures corresponding to blackbody radii 7–40 au, which scales to ∼40 au for realistic dust properties (similar to the current Kuiper belt). Two outlying populations are also evident; five stars have exceptionally bright emission ( f > 5 × 10−5), and one has unusually hot dust <4 au. The excess emission distributions at all wavelengths were fitted with a steady-state evolution model, showing that these are compatible with all stars being born with a narrow belt that then undergoes collisional grinding. However, the model cannot explain the hot dust systems – likely originating in transient events – and bright emission systems – arising potentially from atypically massive discs or recent stirring. The emission from the present-day Kuiper belt is predicted to be close to the median of the population, suggesting that half of stars have either depleted their Kuiper belts (similar to the Solar system) or had a lower planetesimal formation efficiency.This work was supported by the European Union through European Research Council grant number 279973 (MCW, GMK). GMK was also supported by the Royal Society as a Royal Society University Research Fellow
Radio Scintillation due to Discontinuities in the Interstellar Plasma Density
We develop the theory of interstellar scintillation as caused by an irregular
plasma having a power-law spatial density spectrum with a spectral exponent of
4 corresponding to a medium with abrupt changes in its density. An ``outer
scale'' is included in the model representing the typical scale over which the
density of the medium remains uniform. Such a spectrum could be used to model
plasma shock fronts in supernova remnants or other plasma discontinuities. We
investigate and develop equations for the decorrelation bandwidth of
diffractive scintillations and the refractive scintillation index and compare
our results with pulsar measurements. We consider both a medium concentrated in
a thin layer and an extended irregular medium. We conclude that the
discontinuity model gives satisfactory agreement for many diffractive
measurements, in particular the VLBI meaurements of the structure function
exponent between 5/3 and 2. However, it gives less satisfactory agreement for
the refractive scintillation index than does the Kolmogorov turbulence
spectrum. The comparison suggests that the medium consists of a pervasive
background distribution of turbulence embedded with randomly placed discrete
plasma structures such as shocks or HII regions. This can be modeled by a
composite spectrum following the Kolmogorov form at high wavenumbers and
steepening at lower wavenumbers corresponding to the typical (inverse) size of
the discrete structures. Such a model can also explain the extreme scattering
events. However, lines of sight through the enhanced scattering prevalent at
low galactic latitudes are accurately described by the Kolmogorov spectrum in
an extended medium and do not appear to have a similar low-wavenumber
steepening.Comment: Accpeted for ApJ vol 531, March 200
Radio Interferometric Planet Search II: Constraints on sub-Jupiter-Mass Companions to GJ 896A
We present results from the Radio Interferometric Planet (RIPL) search for
compan- ions to the nearby star GJ 896A. We present 11 observations over 4.9
years. Fitting astrometric parameters to the data reveals a residual with
peak-to-peak amplitude of ~ 3 mas in right ascension. This residual is well-fit
by an acceleration term of 0.458 \pm 0.032 mas/y^2. The parallax is fit to an
accuracy of 0.2 mas and the proper motion terms are fit to accuracies of 0.01
mas/y. After fitting astrometric and acceleration terms residuals are 0.26 mas
in each coordinate, demonstrating that stellar jitter does not limit the
ability to carry out radio astrometric planet detection and characterization.
The acceleration term originates in part from the companion GJ 896B but the
amplitude of the acceleration in declination is not accurately predicted by the
orbital model. The acceleration sets a mass upper limit of 0.15 MJ at a
semi-major axis of 2 AU for a planetary companion to GJ 896A. For semi-major
axes between 0.3 and 2 AU upper limits are determined by the maximum angular
separation; the upper limits scale from the minimum value in proportion to the
inverse of the radius. Upper limits at larger radii are set by the acceleration
and scale as the radius squared. An improved solution for the stellar binary
system could improve the exoplanet mass sensitivity by an order of magnitude.Comment: Accepted for publication in Ap
The milliarcsecond-scale radio structure of AB Dor A
Context: The fast rotator, pre-main sequence star AB Dor A is a strong and
persistent radio emitter. The extraordinary coronal flaring activity is thought
to be the origin of compact radio emission and other associated phenomena as
large slingshot prominences. Aim: We aim to investigate the radio emission
mechanism and the milliarcsecond radio structure around AB Dor A. Methods: We
performed phase-referenced VLBI observations at 22.3 GHz, 8.4 GHz, and 1.4 GHz
over more than one decade using the Australian VLBI array. Results: Our 8.4 GHz
images show a double core-halo morphology, similar at all epochs, with emission
extending at heights between 5 and 18 stellar radii. Furthermore, the sequence
of the 8.4 GHz maps shows a clear variation of the source structure within the
observing time. However, images at 1.4 GHz and 22.3 GHz are compatible with a
compact source. The phase-reference position at 8.4 GHz and 1.4 GHz are
coincident with those expected from the well-known milliarcsecond-precise
astrometry of this star, meanwhile the 22.3 GHz position is 4 off the
prediction in the north-west direction. The origin of this offset is still
unclear. Conclusions: We have considered several models to explain the
morphology and evolution of the inner radio structure detected in AB Dor A
which include emission from the stellar polar caps, a flaring,
magnetically-driven loop structure, and the presence of helmet streamers. A
possible close companion to AB Dor A has been also investigated. Our results
confirm the extraordinary coronal magnetic activity of this star, able to
produce compact radio structures at very large heights, so far only seen in
binary interacting systems.Comment: 11 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
Dynamical Masses for Pre-Main Sequence Stars: A Preliminary Physical Orbit for V773 Tau A
We report on interferometric and radial-velocity observations of the
double-lined 51-d period binary (A) component of the quadruple pre-main
sequence (PMS) system V773 Tau. With these observations we have estimated
preliminary visual and physical orbits of the V773 Tau A subsystem. Among other
parameters, our orbit model includes an inclination of 66.0 2.4 deg, and
allows us to infer the component dynamical masses and system distance. In
particular we find component masses of 1.54 0.14 and 1.332 0.097
M_{\sun} for the Aa (primary) and Ab (secondary) components respectively.
Our modeling of the subsystem component spectral energy distributions finds
temperatures and luminosities consistent with previous studies, and coupled
with the component mass estimates allows for comparison with PMS stellar models
in the intermediate-mass range. We compare V773 Tau A component properties with
several popular solar-composition models for intermediate-mass PMS stars. All
models predict masses consistent to within 2-sigma of the dynamically
determined values, though some models predict values that are more consistent
than others.Comment: ApJ in press; 25 pages, 6 figures; data tables available in journal
versio
The Ultraviolet Spectrum and Physical Properties of the Mass Donor Star in HD 226868 = Cygnus X-1
We present an examination of high resolution, ultraviolet spectroscopy from
Hubble Space Telescope of the photospheric spectrum of the O-supergiant in the
massive X-ray binary HD 226868 = Cyg X-1. We analyzed this and ground-based
optical spectra to determine the effective temperature and gravity of the O9.7
Iab supergiant. Using non-local thermodynamic equilibrium (non-LTE), line
blanketed, plane parallel models from the TLUSTY grid, we obtain T_eff = 28.0
+/- 2.5kK and log g > 3.00 +/- 0.25, both lower than in previous studies. The
optical spectrum is best fit with models that have enriched He and N
abundances. We fit the model spectral energy distribution for this temperature
and gravity to the UV, optical, and IR fluxes to determine the angular size of
and extinction towards the binary. The angular size then yields relations for
the stellar radius and luminosity as a function of distance. By assuming that
the supergiant rotates synchronously with the orbit, we can use the radius -
distance relation to find mass estimates for both the supergiant and black hole
as a function of the distance and the ratio of stellar to Roche radius. Fits of
the orbital light curve yield an additional constraint that limits the
solutions in the mass plane. Our results indicate masses of 23^{+8}_{-6} M_sun
for the supergiant and 11^{+5}_{-3} M_sun for the black hole.Comment: ApJ in pres
Opacity in compact extragalactic radio sources and its effect on astrophysical and astrometric studies
The apparent position of the "core" in a parsec-scale radio jet (a compact,
bright emitting region at the narrow end of the jet) depends on the observing
frequency, owing to synchrotron self-absorption and external absorption. While
providing a tool probing physical conditions in the vicinity of the core, this
dependency poses problems for astrometric studies using compact radio sources.
We investigated the frequency-dependent shift in the positions of the cores
(core shift) observed with very long baseline interferometry (VLBI) in
parsec-scale jets. We discuss related physics, as well as its effect on radio
astrometry and the connection between radio and optical positions of
astrometric reference objects. We searched for the core shift in a sample of
277 radio sources imaged at 2.3 GHz (13 cm) and 8.6 GHz (4 cm) frequency bands
using VLBI observations made in 2002 and 2003. The core shift was measured by
referencing the core position to optically thin jet features whose positions
are not expected to change with frequency. We present here results for 29
selected active galactic nuclei (AGN) with bright distinct VLBI jet features
that can be used in differential measurements and that allow robust
measurements of the shift to be made. In these AGN, the magnitude of the
measured core shift between 2.3 and 8.6 GHz reaches 1.4 mas, with a median
value for the sample of 0.44 mas. Nuclear flares result in temporal variability
of the shift. An average shift between the radio (4 cm) and optical (6000
Angstrom) bands is estimated to be approximately 0.1 mas, and it should be
taken into account in order to provide the required accuracy of the
radio-optical reference frame connection. This can be accomplished with
multi-frequency VLBI measurements... (abridged)Comment: 11 pages, 8 figures, 2 tables, accepted for publication in the
Astronomy and Astrophysics; minor corrections to the manuscript are mad
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