882 research outputs found
A rival for Babcock's star: the extreme 30-kG variable magnetic field in the Ap star HD 75049
The extraordinary magnetic Ap star HD 75049 has been studied with data obtained with the European Southern Observatory Very Large Telescope and 2.2-m telescopes. Direct measurements reveal that the magnetic field modulus at maximum reaches 30 kG. The star shows photometric, spectral and magnetic variability with a rotation period of 4.049 d. Variations of the mean longitudinal magnetic field can be described to first order by a centred dipole model with an inclination i= 25°, an obliquity β= 60° and a polar field Bp= 42 kG. The combination of the longitudinal and surface magnetic field measurements implies a radius of R= 1.7 R⊙, suggesting that the star is close to the zero-age main sequence. HD 75049 displays moderate overabundances of Si, Ti, Cr, Fe and large overabundances of rare earth elements. This star has the second strongest magnetic field of any main-sequence star after Babcock's star, HD 215441, which it rivals
Spectropolarimetry of the Classical T Tauri Star TW Hydrae
We present high resolution (R ~ 60,000) circular spectropolarimetry of the
classical T Tauri star TW Hydrae. We analyze 12 photospheric absorption lines
and measure the net longitudinal magnetic field for 6 consecutive nights. While
no net polarization is detected the first five nights, a significant
photospheric field of Bz = 149 \pm 33 G is found on the sixth night. To rule
out spurious instrumental polarization, we apply the same analysis technique to
several non-magnetic telluric lines, detecting no significant polarization. We
further demonstrate the reality of this field detection by showing that the
splitting between right and left polarized components in these 12 photospheric
lines shows a linear trend with Lande g-factor times wavelength squared, as
predicted by the Zeeman effect. However, this longitudinal field detection is
still much lower than that which would result if a pure dipole magnetic
geometry is responsible for the mean magnetic field strength of 2.6 kG
previously reported for TW Hya. We also detect strong circular polarization in
the He I 5876 and the Ca II 8498 emission lines, indicating a strong field in
the line formation region of these features. The polarization of the Ca II line
is substantially weaker than that of the He I line, which we interpret as due
to a larger contribution to the Ca II line from chromospheric emission in which
the polarization signals cancel. However, the presence of polarization in the
Ca II line indicates that accretion shocks on Classical T Tauri stars do
produce narrow emission features in the infrared triplet lines of Calcium.Comment: One tar file. The paper has 22 pages, 5 figures. Accepted by AJ on
Sep 10, 200
Constraining the fundamental parameters of the O-type binary CPD-41degr7733
Using a set of high-resolution spectra, we studied the physical and orbital
properties of the O-type binary CPD-41 7733, located in the core of \ngc. We
report the unambiguous detection of the secondary spectral signature and we
derive the first SB2 orbital solution of the system. The period is 5.6815 +/-
0.0015 d and the orbit has no significant eccentricity. CPD-41 7733 probably
consists of stars of spectral types O8.5 and B3. As for other objects in the
cluster, we observe discrepant luminosity classifications while using
spectroscopic or brightness criteria. Still, the present analysis suggests that
both components display physical parameters close to those of typical O8.5 and
B3 dwarfs. We also analyze the X-ray light curves and spectra obtained during
six 30 ks XMM-Newton pointings spread over the 5.7 d period. We find no
significant variability between the different pointings, nor within the
individual observations. The CPD-41 7733 X-ray spectrum is well reproduced by a
three-temperature thermal mekal model with temperatures of 0.3, 0.8 and 2.4
keV. No X-ray overluminosity, resulting e.g. from a possible wind interaction,
is observed. The emission of CPD-41 7733 is thus very representative of typical
O-type star X-ray emission.Comment: Accepted by ApJ, 15 pages, 9 figure
Hierarchical Triggering of Star Formation by Superbubbles in W3/W4
It is generally believed that expanding superbubbles and mechanical feedback
from massive stars trigger star formation, because there are numerous examples
of superbubbles showing secondary star formation at their edges. However, while
these systems show an age sequence, they do not provide strong evidence of a
causal relationship. The W3/W4 Galactic star-forming complex suggests a
three-generation hierarchy: the supergiant shell structures correspond to the
oldest generation; these triggered the formation of IC 1795 in W3, the
progenitor of a molecular superbubble; which in turn triggered the current
star-forming episodes in the embedded regions W3-North, W3-Main, and W3-OH. We
present UBV photometry and spectroscopic classifications for IC 1795, which
show an age of 3 - 5 Myr. This age is intermediate between the reported 6 - 20
Myr age of the supergiant shell system, and the extremely young ages (10^4 -
10^5 yr) for the embedded knots of ultracompact HII regions, W3-North, W3-Main,
and W3-OH. Thus, an age sequence is indeed confirmed for the entire W3/W4
hierarchical system. This therefore provides some of the first convincing
evidence that superbubble action and mechanical feedback are indeed a
triggering mechanism for star formation.Comment: 10 pages, 6 figures; accepted to the Astronomical Journal. Figure 2
included in this submission as JPE
Les conventions-programmes : un nouvel outil pour la collaboration verticale en Suisse
Le présent cahier offre une description de cinq conventions-programmes appliquées dans six cantons (Genève, Vaud, Valais, Berne, Zürich et Uri). Introduits suite à l'acceptation par le peuple du projet de réforme de la péréquation financière et de la répartition des tâches (RPT) le 28 novembre 2004, puis à son application au 1er janvier 2008, ces contrats de droit administratif permettent à la Confédération et aux cantons de gérer conjointement des tâches publiques dites « communes » dont la répartition s'est révélée impossible. Les conventions-programmes ont pour objectif de clarifier les compétences et les charges financières afin d'améliorer l'efficacité du système d'accomplissement de ces tâches communes et d'instaurer une nouvelle forme de partenariat. Après une revue détaillée de chaque convention-programme, cette étude examine comment les responsables cantonaux perçoivent ces contrats à l'aide d'un questionnaire, soumis à chacun lors d'entretiens semi-directifs. Cette évaluation permet ainsi une vue d'ensemble de l'application de ces contrats, notamment pour les deux premières périodes contractuelles : 2008-11 et 2012-15
Phase-resolved far-ultraviolet HST spectroscopy of the peculiar magnetic white dwarf RE J0317-853
We present phase resolved FUV HST FOS spectra of the rapidly rotating, highly
magnetic white dwarf RE J0317-853. Using these data, we construct a new model
for the magnetic field morphology across the stellar surface. From an expansion
into spherical harmonics, we find the range of magnetic field strengths present
is 180-800MG. For the first time we could identify an absorption feature
present at certain phases at 1160A as a ``forbidden'' 1s_0 -> 2s_0 component,
due to the combined presence of an electric and magnetic field.Comment: 15 pages including 4 figures. Accepted for publication in ApJ Letter
Theoretical light curves of dipole oscillations in roAp stars
Context.
The dipole modes are the most common geometry of oscillations in roAp stars inferred from photometric measurements and are therefore of special interest for asteroseismic purposes.
Aims.
We present a theoretical and analytical study of the light curves associated with dipole (ℓ = 1) pulsations of roAp stars in the framework of the revisited oblique pulsator model.
Methods.
We describe the light curves in terms of the inclination and polarization of the elliptical displacement vector of the dipole modes. We study the influence of the magnetic field and rotation on the shape of these light curves for both amplitudes and phases.
Results.
Despite the inclination of dipole mode with respect to the magnetic axis, we find that the dipole mode can have maxima that are in phase with the magnetic maxima. We apply our formalism to the well-known roAp star HR 3831 (HD 83368) to derive its mode properties. Our results are similar to those obtained by time-series spectroscopy. We also consider the cases of three other roAp stars, HD 6532, HD 99563, and HD 128898 (α Cir).
Conclusions.
We demonstrate that the formalism of the revisited oblique pulsator model is adequate to explain the properties of the photometric light curves associated with dipole modes in roAp stars. In addition, we show that the coincidence of pulsation and magnetic extrema can also occur for inclined modes with respect to the magnetic axis. With the stars considered in this paper, we conclude that the polarization of the modes present in roAp stars are quasi linearly polarized
Bayesian modelling captures inter-individual differences in social belief computations in the putamen and insula
Computational models of social learning and decision-making provide mechanistic tools to investigate the neural mechanisms that are involved in understanding other people. While most studies employ explicit instructions to learn from social cues, everyday life is characterized by the spontaneous use of such signals (e.g., the gaze of others) to infer on internal states such as intentions. To investigate the neural mechanisms of the impact of gaze cues on learning and decision-making, we acquired behavioural and fMRI data from 50 participants performing a probabilistic task, in which cards with varying winning probabilities had to be chosen. In addition, the task included a computer-generated face that gazed towards one of these cards providing implicit advice. Participants\u2019 individual belief trajectories were inferred using a hierarchical Gaussian filter (HGF) and used as predictors in a linear model of neuronal activation. During learning, social prediction errors were correlated with activity in inferior frontal gyrus and insula. During decision-making, the belief about the accuracy of the social cue was correlated with activity in inferior temporal gyrus, putamen and pallidum while the putamen and insula showed activity as a function of individual differences in weighting the social cue during decision-making. Our findings demonstrate that model-based fMRI can give insight into the behavioural and neural aspects of spontaneous social cue integration in learning and decision-making. They provide evidence for a mechanistic involvement of specific components of the basal ganglia in subserving these processes
HD 213258: a new rapidly oscillating, super-slowly rotating, strongly magnetic Ap star in a spectroscopic binary
We report about HD 213258, an Ap star that we recently identified as
presenting a unique combination of rare, remarkable properties. Our study of
this star is based on ESPaDOnS Stokes I and V data obtained at 7 epochs
spanning a time interval slightly shorter than 2 years, on TESS data, and on
radial velocity measurements from the CORAVEL data base. We confirm that HD
213258 is definitely an Ap star. We found that, in its spectrum, the Fe II
{\lambda}6149.2 {\AA} line is resolved into its two magnetically split
components. The mean magnetic field modulus of HD 213258, ~ 3.8 kG does not
show significant variations over ~2 years. Comparing our mean longitudinal
field determinations with a couple of measurements from the literature, we show
that the stellar rotation period must likely be of the order of 50 years, with
a reversal of the field polarity. Moreover, HD 213258 is a rapidly oscillating
Ap (roAp) star, in which high overtone pulsations with a period of 7.58 min are
detected. Finally, we confirm that HD 213258 has a mean radial velocity
exceeding (in absolute value) that of at least 99% of the Ap stars. The radial
velocity shows low amplitude variations, which suggests that the star is a
single-line spectroscopic binary. It is also a known astrometric binary. While
its orbital elements remain to be determined, its orbital period likely is one
of the shortest known for a binary roAp star. Its secondary is close to the
borderline between stellar and substellar objects. There is a significant
probability that it may be a brown dwarf. While most of the above-mentioned
properties, taken in isolation, are observed in a small fraction of the whole
population of Ap stars, the probability that a single star possesses all of
them is extremely low. This makes HD 213258 an exceptionally interesting object
that deserves to be studied in detail in the future.Comment: Accepted for publication in A&A; 6 pages, 4 figure
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