383 research outputs found
Radio detection of the young binary HD 160934
Precise determination of dynamical masses of pre-main-sequence (PMS) stars is
essential to calibrate stellar evolution models that are widely used to derive
theoretical masses of young low-mass objects. Binary stars in young, nearby
loose associations are particularly good candidates for this calibration since
all members share a common age. Interestingly, some of these young binaries
present a persistent and compact radio emission, which makes them excellent
targets for astrometric VLBI studies. We aim to monitor the orbital motion of
the binary system HD 160934, a member of the AB Doradus moving group. We
observed HD 160934 with the Very Large Array and the European VLBI Network at
8.4 and 5 GHz, respectively. The orbital information derived from these
observations was analyzed along with previously reported orbital measurements.
We show that the two components of the binary, HD 160934 A and HD 160934 c,
display compact radio emission at VLBI scales, providing precise information on
the relative orbit. Revised orbital elements were estimated. Future VLBI
monitoring of this pair should determine precise model-independent mass
estimates for the A and c components, which will serve as calibration tests for
PMS evolutionary models.Comment: 5 pages, 5 figures, accepted for publication in A&
82: Severe Hemorrhagic Cystitis (HC) After Allogeneic Hematopoeitic Stem Cell Transplantation (HSCT): Incidence and Risk Factors
With the purpose to investigate the radio emission of new ultracool objects,
we carried out a targeted search in the recently discovered system VHS
J125601.92125723.9 (hereafter VHS 12561257); this system is composed by
an equal-mass M7.5 binary and a L7 low-mass substellar object located at only
15.8\,pc. We observed in phase-reference mode the system VHS 12561257 with
the Karl G. Jansky Very Large Array at - and - band and with the European
VLBI Network at -band in several epochs during 2015 and 2016. We discovered
radio emission at -band spatially coincident with the equal-mass M7.5 binary
with a flux density of 60 Jy. We determined a spectral index between 8 and 12 GHz, suggesting that non-thermal,
optically-thin, synchrotron or gyrosynchrotron radiation is responsible for the
observed radio emission. Interestingly, no signal is seen at -band where we
set a 3- upper limit of 20 Jy. This might be explained by strong
variability of the binary or self-absorption at this frequency. By adopting the
latter scenario and gyrosynchrotron radiation, we constrain the turnover
frequency to be in the interval 5--8.5 GHz, from which we infer the presence of
kG-intense magnetic fields in the M7.5 binary. Our data impose a 3-
upper bound to the radio flux density of the L7 object of 9 Jy at 10\,GHz.Comment: 6 pages, 4 figures. Accepted for publication in A&
Homologous self-assembled superlattices: What causes their periodic polarity switching? Review, model, and experimental test
Quantum semiconductor structures are commonly achieved by bandgap engineering
that relies on the ability to switch from one semiconductor to another during
their growth. Growth of a superlattice is typically demanding technologically.
In contrast, accumulated evidence points to a tendency among a certain class of
multiple-cation binary oxides to self-assemble spontaneously as superlattice
structures. This class has been dubbed the homologous superlattices. For a
famous example, when a mixture of indium and zinc is oxidized, the phases of
In-O and ZnO separate in an orderly periodic manner, along the ZnO polar axis,
with polarity inversion taking place between consecutive ZnO sections. As we
review here, the same structure has been observed when the indium was replaced
with other metals, and perhaps even in ZnO alone. This peculiar self-assembled
structure has been attracting research over the past decade. The purpose of
this study is to gain understanding of the physics underlying the formation of
this unique structure. Here, we first provide an extensive review of the
accumulated literature on these spontaneously-formed structures and then
propose an explanation for the long-standing mystery of this intriguing
self-assembly in the form of an electrostatic growth phenomenon and test the
proposed model on experimental data
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 of the low-mass stellar binary AB Doradus B
Context. ABDoradus is the main system of the ABDoradus moving group. It is a quadruple system formed by two widely separated binaries of pre-main-sequence (PMS) stars: ABDorA/C and ABDor Ba/Bb. The pair ABDorA/C has been extensively studied and its dynamical masses have been determined with high precision, thus making of ABDorC a benchmark for calibrating PMS stellar models. If the orbit and dynamical masses of the pair ABDor Ba/Bb can be determined, they could not only play a similar role to that of ABDorC in calibrating PMS models, but would also help to better understand the dynamics of the whole ABDoradus system. Aims. We aim to determine the individual masses of the pair ABDor Ba/Bb using VLBI observations and archive infrared data, as part of a larger program directed to monitor binary systems in the ABDoradus moving group. Methods. We observed the system ABDor B between 2007 and 2013 with the Australian Long Baseline Array (LBA), at a frequency of 8.4 GHz in phase-reference mode. Results. We detected, for the first time, compact radio emission from both stars in the binary, ABDor Ba and ABDor Bb. This result allowed us to determine the orbital parameters of both the relative and absolute orbits and, consequently, their individual dynamical masses: 0.28±0.05M_sun and 0.25±0.05M_sun, respectively. Conclusions. Comparisons of the dynamical masses with the prediction of PMS evolutionary models show that the models underpredict the dynamical masses of the binary components Ba and Bb by ~30 and 40%, respectively, although they all still agree at the 2-sigma level. The same stellar models favour an age between 50 and 100 Myr for this system. We also discuss the evolutionary status of ABDor Ba/Bb in terms of an earlier double-double star scenario that might explain the strong radio emission detected in both components
The Electrical-Thermal Switching in Carbon Black-Polymer Composites as a Local Effect
Following the lack of microscopic information about the intriguing well-known
electrical-thermal switching mechanism in Carbon Black-Polymer composites, we
applied atomic force microscopy in order to reveal the local nature of the
process and correlated it with the characteristics of the widely used
commercial switches. We conclude that the switching events take place in
critical interparticle tunneling junctions that carry most of the current. The
macroscopic switched state is then a result of a dynamic-stationary state of
fast switching and slow reconnection of the corresponding junctions.Comment: 14 pages, 5 figures,Typographic correctio
The AIQ Meta-Testbed: Pragmatically Bridging Academic AI Testing and Industrial Q Needs
AI solutions seem to appear in any and all application domains. As AI becomes
more pervasive, the importance of quality assurance increases. Unfortunately,
there is no consensus on what artificial intelligence means and interpretations
range from simple statistical analysis to sentient humanoid robots. On top of
that, quality is a notoriously hard concept to pinpoint. What does this mean
for AI quality? In this paper, we share our working definition and a pragmatic
approach to address the corresponding quality assurance with a focus on
testing. Finally, we present our ongoing work on establishing the AIQ
Meta-Testbed.Comment: Accepted for publication in the Proc. of the Software Quality Days
2021, Vienna, Austri
Radio emission in a nearby, ultra-cool dwarf binary: A multifrequency study
Context. The substellar triple system VHS J125601.92â125723.9 (hereafter VHS 1256â1257) is composed of an equal-mass M7.5 brown dwarf binary and an L7 low-mass substellar object. In Guirado et al. (2018, A&A, 610, A23) we published the detection of radio emission at 8.4 GHz coming from the central binary and making it an excellent target for further observations. Aims. We aim to identify the origin of the radio emission occurring in the central binary of VHS 1256â1257 while discussing the expected mechanisms involved in the radio emission of ultra-cool dwarfs. Methods. We observed this system with the Karl G. Jansky Very Large Array, the European very-long-baseline interferometry (VLBI) Network, the enhanced Multi-Element Remotely Linked Interferometer Network, the NOrthern Extended Millimeter Array, and the Atacama Large Millimetre Array at frequencies ranging from 5 GHz up to 345 GHz in several epochs during 2017, 2018, and 2019. Results. We found radio emission at 6 GHz and 33 GHz coincident with the expected position of the central binary of VHS 1256â1257. The Stokes I density fluxes detected were 73â
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4 ÎŒJy and 83â
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13 ÎŒJy, respectively, with no detectable circular polarisation or pulses. No emission is detected at higher frequencies (230 GHz and 345 GHz), nor at 5 GHz with VLBI arrays. The emission appears to be stable over almost three years at 6 GHz. To explain the constraints obtained both from the detections and non-detections, we considered multiple scenarios including thermal and nonthermal emission, and different contributions from each component of the binary.
Conclusions. Our results can be well explained by nonthermal gyrosynchrotron emission originating at radiation belts with a low plasma density (neâ=â300â700 cmâ3), a moderate magnetic field strength (Bâââ140 G), and an energy distribution of electrons following a power-law (dN/dEâââEâÎŽ) with ÎŽ fixed at 1.36. These radiation belts would need to be present in both components and also be viewed equatorially. © ESO 2022.We sincerely thank the anonymous referee for his/her very useful and constructive criticisms and suggestions. This paper is based on observations carried out with the IRAM NOEMA interferometer and the IRAM 30-m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain). JBC and JCG were partially supported by the Spanish MINECO projects AYA2015-63939-C2-2-P, PGC2018-098915-B-C22 and by the Generalitat Valenciana project GVPROMETEO2020â080. MPT acknowledges financial support from the State Agency for Research of the Spanish MCIU through the âCenter of Excellence Severo Ochoaâ award to the Instituto de AstrofĂsica de AndalucĂa (SEV-2017-0709) and through grants PGC2018-098915-B-C21 and PID2020-117404GB-C21 (MCI/AEI/FEDER, UE). RA was supported by the Generalitat Valenciana postdoctoral grant APOSTD/2018/177. BG acknowledges support from the UK Science and Technology Facilities Council (STFC) via the Consolidated Grant ST/R000905/1. MRZO and VJSB acknowledge the financial support from PID2019-109522GB-C51 and PID2019-109522GB-C53, respectively.Peer reviewe
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