2,372 research outputs found
On the consistency of magnetic field measurements of Ap stars: lessons learned from the FORS1 archive
CONTEXT. The ESO archive of FORS1 spectropolarimetric observations may be
used to create a homogeneous database of magnetic field measurements. However,
no systematic comparison of FORS field measurements to those obtained with
other instruments has been undertaken so far. AIMS. We exploit the FORS archive
of circular spectropolarimetric data to examine in a general way how reliable
and accurate field detections obtained with FORS are. METHODS. We examine the
observations of Ap and Bp stars, on the grounds that almost all of the
unambiguous detections of magnetic fields in the FORS1 archive are in these
kinds of stars. We assess the overall quality of the FORS1 magnetic data by
examining the consistency of field detections with what is known from previous
measurements obtained with other instruments, and we look at patterns of
internal consistency. RESULTS. FORS1 magnetic measurements are fully consistent
with those made with other instruments, and the internal consistency of the
data is excellent. However, it is important to recognise that each choice of
grism and wavelength window constitutes a distinct instrumental measuring
system, and that simultaneous field measurements in different instrumental
systems may produce field strength values that differ up to 20 %, or more.
Furthermore, we found that field measurements using hydrogen lines only yield
results that meaningfully reflect the field strength as sampled specifically by
lines of hydrogen for stars with effective temperatures above about 9000 K.
CONCLUSIONS. In general the magnetic field measurements of Ap and Bp stars
obtained with FORS1 are of excellent quality, accuracy and precision, and FORS1
provides an extremely useful example that offers valuable lessons for field
measurements with other low- resolution Cassegrain spectropolarimeters.Comment: 14 pages, 8 figures. Accepted for publication in section 13 of
Astronomy & Astrophysics on 13 October 201
The FORS1 catalogue of stellar magnetic field measurements
The FORS1 instrument on the ESO Very Large Telescope was used to obtain
low-resolution circular polarised spectra of nearly a thousand different stars,
with the aim of measuring their mean longitudinal magnetic fields. A catalogue
of FORS1 magnetic measurements would provide a valuable resource with which to
better understand the strengths and limitations of this instrument and of
similar low-dispersion, Cassegrain spectropolarimeters. However, FORS1 data
reduction has been carried out by a number of different groups using a variety
of reduction and analysis techniques. Our understanding of the instrument and
our data reduction techniques have both improved over time. A full re-analysis
of FORS1 archive data using a consistent and fully documented algorithm would
optimise the accuracy and usefulness of a catalogue of field measurements.
Based on the ESO FORS pipeline, we have developed a semi-automatic procedure
for magnetic field determinations, which includes self-consistent checks for
field detection reliability. We have applied our procedure to the full content
of circular spectropolarimetric measurements of the FORS1 archive. We have
produced a catalogue of spectro-polarimetric observations and magnetic field
measurements for about 1400 observations of about 850 different objects. The
spectral type of each object has been accurately classified. We have also been
able to test different methods for data reduction is a systematic way. The
resulting catalogue has been used to produce an estimator for an upper limit to
the uncertainty in a field strength measurement of an early type star as a
function of the signal-to-noise ratio of the observation. While FORS1 is not
necessarily an optimal instrument for the discovery of weak magnetic fields, it
is very useful for the systematic study of larger fields, such as those found
in Ap/Bp stars and in white dwarfs.Comment: Accepted for publication by A&
Detection of magnetic field in the B2 star Oph A with ESO FORS2
Circumstantial evidence suggests that magnetism and enhanced X-ray emission
are likely correlated in early B-type stars: similar fractions of them (
10 %) are strong and hard X-ray sources and possess strong magnetic fields. It
is also known that some B-type stars have spots on their surface. Yet up to now
no X-ray activity associated with spots on early-type stars was detected. In
this Letter we report the detection of a magnetic field on the B2V star
Oph A. Previously, we assessed that the X-ray activity of this star is
associated with a surface spot, herewith we establish its magnetic origin. We
analyzed FORS2 ESO VLT spectra of Oph A taken at two epochs and detected
a longitudinal component of the magnetic field of order of G in one
of the datasets. The detection of the magnetic field only at one epoch can be
explained by stellar rotation which is also invoked to explain observed
periodic X-ray activity. From archival HARPS ESO VLT high resolution spectra we
derived the fundamental stellar parameters of Oph A and further
constrained its age. We conclude that Oph A provides strong evidence for
the presence of active X-ray emitting regions on young magnetized early type
stars.Comment: 4 pages, 1 figure, 2 tables, accepted as a "Letter to the Editor" to
Astronomy & Astrophysic
Signs of strong Na and K absorption in the transmission spectrum of WASP-103b
Context: Transmission spectroscopy has become a prominent tool for
characterizing the atmospheric properties on close-in transiting planets.
Recent observations have revealed a remarkable diversity in exoplanet spectra,
which show absorption signatures of Na, K and , in some cases
partially or fully attenuated by atmospheric aerosols. Aerosols (clouds and
hazes) themselves have been detected in the transmission spectra of several
planets thanks to wavelength-dependent slopes caused by the particles'
scattering properties. Aims: We present an optical 550 - 960 nm transmission
spectrum of the extremely irradiated hot Jupiter WASP-103b, one of the hottest
(2500 K) and most massive (1.5 ) planets yet to be studied with this
technique. WASP-103b orbits its star at a separation of less than 1.2 times the
Roche limit and is predicted to be strongly tidally distorted. Methods: We have
used Gemini/GMOS to obtain multi-object spectroscopy hroughout three transits
of WASP-103b. We used relative spectrophotometry and bin sizes between 20 and 2
nm to infer the planet's transmission spectrum. Results: We find that WASP-103b
shows increased absorption in the cores of the alkali (Na, K) line features. We
do not confirm the presence of any strong scattering slope as previously
suggested, pointing towards a clear atmosphere for the highly irradiated,
massive exoplanet WASP-103b. We constrain the upper boundary of any potential
cloud deck to reside at pressure levels above 0.01 bar. This finding is in line
with previous studies on cloud occurrence on exoplanets which find that clouds
dominate the transmission spectra of cool, low surface gravity planets while
hot, high surface gravity planets are either cloud-free, or possess clouds
located below the altitudes probed by transmission spectra.Comment: Accepted for publication in A&
The Contribution of Blazars to the Extragalactic Diffuse Gamma-ray Background and Their Future Spatial Resolution
We examine the constraints on the luminosity-dependent density evolution
model for the evolution of blazars given the observed spectrum of the diffuse
gamma-ray background (DGRB), blazar source-count distribution, and the blazar
spectral energy distribution sequence model, which relates the observed the
blazar spectrum to its luminosity. We show that the DGRB observed by the Large
Area Telescope (LAT) aboard the Fermi Gamma Ray Space Telescope can be produced
entirely by gamma-ray emission from blazars and nonblazar active galactic
nuclei, and that our blazar evolution model is consistent with and constrained
by the spectrum of the DGRB and flux source-count distribution function of
blazars observed by Fermi-LAT. Our results are consistent with previous work
that used EGRET spectral data to forecast the Fermi-LAT DGRB. The model
includes only three free parameters, and forecasts that >~ 95% of the flux from
blazars will be resolved into point sources by Fermi-LAT with 5 years of
observation, with a corresponding reduction of the flux in the DGRB by a factor
of ~2 to 3 (95% confidence level), which has implications for the Fermi-LAT's
sensitivity to dark matter annihilation photons.Comment: 13 pages, 7 figures; v3: minor changes, matches version to appear in
Phys. Rev.
On the influence of environment on star forming galaxies
We use our state-of-the-art semi analytic model for GAlaxy Evolution and
Assembly (GAEA), and observational measurements of nearby galaxies to study the
influence of the environment on the gas content and gaseous/stellar disc sizes
of star-forming galaxies. We analyse the origin of differences between physical
properties of satellites and those of their central counterparts, identified by
matching the Vmax of their host haloes at the accretion time of the satellites.
Our model reproduces nicely the differences between centrals and satellites
measured for the HI mass, size of the star-forming region, and stellar radii.
In contrast, our model predicts larger differences with respect to data for the
molecular gas mass and star formation rate. By analysing the progenitors of
central and satellite model galaxies, we find that differences in the gas
content arise after accretion, and can be entirely ascribed to the
instantaneous stripping of the hot gas reservoir. The suppression of cold gas
replenishment via cooling and star formation leads to a reduction of the cold
gas and of its density. Therefore, more molecular gas is lost than lower
density HI gas, and model satellites have less molecular gas and lower star
formation rates than observed satellites. We argue that these disagreements
could be largely resolved with the inclusion of a proper treatment for
ram-pressure stripping of cold gas and a more gradual stripping of the hot gas
reservoir. A more sophisticated treatment of angular momentum exchanges,
accounting for the multi-phase nature of the gaseous disc is also required.Comment: 15 pages, 9 figures, accepted for publication in MNRA
Weak magnetic fields in white dwarfs and their direct progenitors?
We have carried out a re-analysis of polarimetric data of central stars of
planetary nebulae, hot subdwarfs, and white dwarfs taken with FORS1 (FOcal
Reducer and low dispersion Spectrograph) on the VLT (Very Large Telescope), and
added a large number of new observations in order to increase the sample. A
careful analysis of the observations using only one wavelength calibration for
the polarimetrically analysed spectra and for all positions of the retarder
plate of the spectrograph is crucial in order to avoid spurious signals. We
find that the previous detections of magnetic fields in subdwarfs and central
stars could not be confirmed while about 10% of the observed white dwarfs have
magnetic fields at the kilogauss level.Comment: 6 pages, Proceedings of the 18th European White Dwarf Workshop, ASP
Conference Serie
- A tool for multiband light curve modeling of planetary transits and stellar spots
Several studies have shown that stellar activity features, such as occulted
and non-occulted starspots, can affect the measurement of transit parameters
biasing studies of transit timing variations and transmission spectra. We
present , which we designed to model multiband transit
light curves showing starspot anomalies, inferring both transit and spot
parameters. The code follows a pixellation approach to model the star with its
corresponding limb darkening, spots, and transiting planet on a two dimensional
Cartesian coordinate grid. We combine with an MCMC
framework to study and derive exoplanet transmission spectra, which provides
statistically robust values for the physical properties and uncertainties of a
transiting star-planet system. We validate 's performance
by analyzing eleven synthetic light curves of four different star-planet
systems and 20 transit light curves of the well-studied WASP-41b system. We
also investigate the impact of starspots on transit parameters and derive
wavelength dependent transit depth values for WASP-41b covering a range of
6200-9200 , indicating a flat transmission spectrum.Comment: 17 pages, 22 figures; accepted for publication in Astronomy &
Astrophysic
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