10,349 research outputs found
Stars and brown dwarfs in the sigma Orionis cluster. III. OSIRIS/GTC low-resolution spectroscopy of variable sources
Context. Although many studies have been performed so far, there are still
dozens of low-mass stars and brown dwarfs in the young sigma Orionis open
cluster without detailed spectroscopic characterisation. Aims. We look for
unknown strong accretors and disc hosts that were undetected in previous
surveys. Methods. We collected low-resolution spectroscopy (R ~ 700) of ten
low-mass stars and brown dwarfs in sigma Orionis with OSIRIS at the Gran
Telescopio Canarias under very poor weather conditions. These objects display
variability in the optical, infrared, Halpha, and/or X-rays on time scales of
hours to years. We complemented our spectra with optical and near-/mid-infrared
photometry. Results. For seven targets, we detected lithium in absorption,
identified Halpha, the calcium doublet, and forbidden lines in emission, and/or
determined spectral types for the first time. We characterise in detail a
faint, T Tauri-like brown dwarf with an 18 h-period variability in the optical
and a large Halpha equivalent width of -125+/-15 AA, as well as two M1-type,
X-ray-flaring, low-mass stars, one with a warm disc and forbidden emission
lines, the other with a previously unknown cold disc with a large inner hole.
Conclusions. New unrevealed strong accretors and disc hosts, even below the
substellar limit, await discovery among the list of known sigma Orionis stars
and brown dwarfs that are variable in the optical and have no detailed
spectroscopic characterisation yet.Comment: A&A, in press (accepted for publication in section 14. Catalogs and
data of Astronomy and Astrophysics
Optimal generalized quantum measurements for arbitrary spin systems
Positive operator valued measurements on a finite number of N identically
prepared systems of arbitrary spin J are discussed. Pure states are
characterized in terms of Bloch-like vectors restricted by a SU(2 J+1)
covariant constraint. This representation allows for a simple description of
the equations to be fulfilled by optimal measurements. We explicitly find the
minimal POVM for the N=2 case, a rigorous bound for N=3 and set up the analysis
for arbitrary N.Comment: LateX, 12 page
Predictive learning, prediction errors, and attention: evidence from event-related potentials and eye tracking
Prediction error (‘‘surprise’’) affects the rate of learning: We learn more rapidly about cues for which we initially make incorrect predictions than cues for which our initial predictions are correct. The current studies employ electrophysiological measures to reveal early attentional differentiation of events that differ in their previous involvement in errors of predictive judgment.
Error-related events attract more attention, as evidenced by features of event-related scalp potentials previously implicated in selective visual attention (selection negativity, augmented anterior N1). The earliest differences detected occurred around 120 msec after stimulus onset, and distributed source localization (LORETA)
indicated that the inferior temporal regions were one source of the earliest differences. In addition, stimuli associated with the production of prediction errors show higher dwell times in an eyetracking procedure. Our data support the view that early attentional processes play a role in human associative learning
Minimal optimal generalized quantum measurements
Optimal and finite positive operator valued measurements on a finite number
of identically prepared systems have been presented recently. With physical
realization in mind we propose here optimal and minimal generalized quantum
measurements for two-level systems.
We explicitly construct them up to N=7 and verify that they are minimal up to
N=5. We finally propose an expression which gives the size of the minimal
optimal measurements for arbitrary .Comment: 9 pages, Late
Unconventional spin texture of a topologically nontrivial semimetal Sb(110)
The surfaces of antimony are characterized by the presence of spin-split
states within the projected bulk band gap and the Fermi contour is thus
expected to exhibit a spin texture. Using spin-resolved density functional
theory calculations, we determine the spin polarization of the surface bands of
Sb(110). The existence of the unconventional spin texture is corroborated by
the investigations of the electron scattering on this surface. The charge
interference patterns formed around single scattering impurities, imaged by
scanning tunneling microscopy, reveal the absence of direct backscattering
signal. We identify the allowed scattering vectors and analyze their bias
evolution in relation to the surface-state dispersion.Comment: 10 pages, 5 figure
Landau-gauge condensates from the quark propagator on the lattice
We compute the dimension-2 condensate, , and the dimension-4 mixed
condensate, , from the recent quenched lattice results for the
quark propagator in the Landau gauge. We fit the lattice data to the Operator
Product Expansion in the "fiducial" region 1.2 GeV < Q < 3 GeV. Our result for
the dynamical gluon mass at the scale of 10 GeV^2 is m_A=600-650 MeV, in
agreement with independent determinations. For the mixed Landau gauge
condensate of dimension-4 we get alpha_s = (-0.11 +/- 0.03)
GeV^4. This value is an order of magnitude larger than the gluon
condensate.Comment: 4 pages, 3 figures, references adde
Contact loci and Hironaka's order
We study contact loci sets of arcs and the behavior of Hironaka’s order function defined in constructive Resolution of singularities. We show that this function can be read in terms of the irreducible components of the contact loci sets at a singular point of an algebraic variety.The authors were partially supported by MTM2015-68524-P; The first author was partially supported from the Spanish Ministry of Economy and Competitiveness, through the "Severo Ochoa" Programme for Centres of Excellence in R&D (SEV-2015-0554)
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