arXiv subjects

A. Riera

Publications and source records attributed to A. Riera.

At least 37 records · Page 2Linked to original sources

arXiv2010

Simulation of the Laughlin state in an optical lattice

We analyze the proposal of achieving a Mott state of Laughlin wave functions in an optical lattice [M. Popp {\it et al.}, Phys. Rev. A 70, 053612 (2004)] and study the consequences of considering the anharmonic corrections to each single site potential expansion that were not taken into account until now. Our result is that, although the anharmonic correction reduces the maximum frequency at which the system can rotate before the atoms escape from each site (centrifugal limit), the Laughlin state can still be achieved for a small number of particles and a realistic value of the laser intensity.

cond-mat.quant-gas↗

arXiv2009

A short review on entanglement in quantum spin systems

We review some of the recent progress on the study of entropy of entanglement in many-body quantum systems. Emphasis is placed on the scaling properties of entropy for one-dimensional multi-partite models at quantum phase transitions and, more generally, on the concept of area law. We also briefly describe the relation between entanglement and the presence of impurities, the idea of particle entanglement, the evolution of entanglement along renormalization group trajectories, the dynamical evolution of entanglement and the fate of entanglement along a quantum computation.

cond-mat.stat-mech↗

arXiv2009

Quantum algorithm for the Laughlin wave function

We construct a quantum algorithm that creates the Laughlin state for an arbitrary number of particles $n$ in the case of filling fraction one. This quantum circuit is efficient since it only uses $n(n-1)/2$ local qudit gates and its depth scales as $2n-3$. We further prove the optimality of the circuit using permutation theory arguments and we compute exactly how entanglement develops along the action of each gate. Finally, we discuss its experimental feasibility decomposing the qudits and the gates in terms of qubits and two qubit-gates as well as the generalization to arbitrary filling fraction.

quant-ph↗

arXiv2009

The nature of HH 223 from long-slit spectroscopy

HH 223 is a knotty, wiggling nebular emission of ~30" length found in the L723 star-forming region. It lies projected onto the largest blueshifted lobe of the cuadrupolar CO outflow powered by a low-mass YSO system embedded in the core of L723. We analysed the physical conditions and kinematics along HH 223 with the aim of disentangling whether the emission arises from shock-excited, supersonic gas characteristic of a stellar jet, or is only tracing the wall cavity excavated by the CO outflow. We performed long-slit optical spectroscopy along HH 223, crossing all the bright knots (A to E) and part of the low-brightness emission nebula (F filament). One spectrum of each knot, suitable to characterize the nature of its emission, was obtained. The physical conditions and the radial velocity of the HH 223 emission along the slits were also sampled at smaller scale (0.6") than the knot sizes. {The spectra of all the HH 223 knots appear as those of the intermediate/high excitation Herbig-Haro objects. The emission is supersonic, with blueshifted peak velocities ranging from -60 to -130 km/s. Reliable variations in the kinematics and physical conditions at smaller scale that the knot sizes are also found. The properties of the HH 223 emission derived from the spectroscopy confirm the HH nature of the object, the supersonic optical outflow most probably also being powered by the YSOs embedded in the L723 core.

astro-ph.GA↗

arXiv2008

Integral Field Spectroscopy of HH 262: The Spectral Atlas

HH 262 is a group of emitting knots displaying an "hour-glass" morphology in the Halpha and [SII] lines, located 3.5' to the northeast of the young stellar object L1551-IRS5, in Taurus. We present new results of the kinematics and physical conditions of HH 262 based on Integral Field Spectroscopy covering a field of 1.5'x3', which includes all the bright knots in HH 262. These data show complex kinematics and significant variations in physical conditions over the mapped region of HH 262 on a spatial scale of <3". A new result derived from the IFS data is the weakness of the [NII] emission (below detection limit in most of the mapped region of HH 262), including the brightest central knots. Our data reinforce the association of HH 262 with the redshifted lobe of the evolved molecular outflow L1551-IRS5. The interaction of this outflow with a younger one, powered by L1551 NE, around the position of HH 262 could give rise to the complex morphology and kinematics of HH 262.

astro-ph↗

arXiv2008

Multiple and Precessing Collimated Outflows in the Planetary Nebula IC 4634

With its remarkable double-S shape, IC 4634 is an archetype of point-symmetric planetary nebulae (PN). In this paper, we present a detailed study of this PN using archival HST WFPC2 and ground-based narrow-band images to investigate its morphology, and long-slit spectroscopic observations to determine its kinematics and to derive its physical conditions and excitation. The data reveal new structural components, including a distant string of knots distributed along an arc-like feature 40"-60" from the center of the nebula, a skin of enhanced [O III]/H-alpha ratio enveloping the inner shell and the double-S feature, and a triple-shell structure. The spatio-kinematical study also finds an equatorial component of the main nebula that is kinematically independent from the bright inner S-shaped arc. We have investigated in detail the bow shock-like features in IC 4634 and found that their morphological, kinematical and emission properties are consistent with the interaction of a collimated outflow with surrounding material. Indeed, the morphology and kinematics of some of these features can be interpreted using a 3D numerical simulation of a collimated outflow precessing at a moderate, time-dependent velocity. Apparently, IC 4634 has experienced several episodes of point-symmetric ejections oriented at different directions with the outer S-shaped feature being related to an earlier point-symmetric ejection and the outermost arc-like string of knots being the relic of an even much earlier point-symmetric ejection. There is tantalizing evidence that the action of these collimated outflows has also taken part in the shaping of the innermost shell and inner S-shaped arc of IC 4634.

astro-ph↗

arXiv2007

Non-LTE dust nucleation in sub-saturated vapors

We use the kinetic theory of nucleation to explore the properties of dust nucleation in sub-saturated vapors. Due to radiation losses, the sub-critical clusters have a smaller temperature compared to their vapor. This alters the dynamical balance between attachment and detachment of monomers, allowing for stable nucleation of grains in vapors that are sub-saturated for their temperature. We find this effect particularly important at low densities and in the absence of a strong background radiation field. We find new conditions for stable nucleation in the n-T phase diagram. The nucleation in the non-LTE regions is likely to be at much slower rate than in the super-saturated vapors. We evaluate the nucleation rate, warning the reader that it does depend on poorly substantiated properties of the macro-molecules assumed in the computation. On the other hand, the conditions for nucleation depend only on the properties of the large stable grains and are more robust. We finally point out that this mechanism may be relevant in the early universe as an initial dust pollution mechanism, since once the interstellar medium is polluted with dust, mantle growth is likely to be dominant over non-LTE nucleation in the diffuse medium.

astro-ph↗

arXiv2006

The Structure of the Inner HH 34 Jet from Optical Integral Field Spectroscopy

We present high spatial resolution optical integral field spectroscopy of a collimated Herbig-Haro jet viewed nearly edge-on. Maps of the line emission, velocity centroid, and velocity dispersion were generated for the H$α$ and [S II] emission features from the inner collimated jet and exciting source region of the HH 34 outflow. The kinematic structure of the jet shows several maxima and minima in both velocity centroid value and velocity dispersion along the jet axis. Perpendicular to the flow direction the velocity decreases outward from the axis to the limb of the jet, but the velocity dispersion increases. Maps of the electron density structure were derived from the line ratio of [S II] 6731/6716 emission. We have found that the jet exhibits a pronounced ``striped'' pattern in electron density; the high $n_e$ regions are at the leading side of each of the emission knots in the collimated jet, and low $n_e$ regions in the down-flow direction. On average, the measured electron density decreases outward from the inner regions of the jet, but the highest $n_e$ found in the outflow is spatially offset from the nominal position of the exciting star. The results of our high spatial resolution optical integral field spectroscopy show very good agreement with the kinematics and electron density structure predicted by the existing internal working surface models of the HH~34 outflow.

astro-ph↗

arXiv2006

Symmetry breaking in small rotating cloud of trapped ultracold Bose atoms

We study the signatures of rotational and phase symmetry breaking in small rotating clouds of trapped ultracold Bose atoms by looking at rigorously defined condensate wave function. Rotational symmetry breaking occurs in narrow frequency windows, where the ground state of the system has degenerated with respect to the total angular momentum, and it leads to a complex wave function that exhibits vortices clearly seen as holes in the density, as well as characteristic vorticity. Phase symmetry (or gauge symmetry) breaking, on the other hand, is clearly manifested in the interference of two independent rotating clouds.

cond-mat.mes-hall↗

arXiv2006

Area law and vacuum reordering in harmonic networks

We review a number of ideas related to area law scaling of the geometric entropy from the point of view of condensed matter, quantum field theory and quantum information. An explicit computation in arbitrary dimensions of the geometric entropy of the ground state of a discretized scalar free field theory shows the expected area law result. In this case, area law scaling is a manifestation of a deeper reordering of the vacuum produced by majorization relations. Furthermore, the explicit control on all the eigenvalues of the reduced density matrix allows for a verification of entropy loss along the renormalization group trajectory driven by the mass term. A further result of our computation shows that single-copy entanglement also obeys area law scaling, majorization relations and decreases along renormalization group flows.

quant-ph↗

arXiv2006

Shock excitation of the knots of Hen 3-1475

We present new optical STIS HST spectroscopic observations of the jets of the proto-planetary nebula Hen 3-1475. The excitation conditions of the knots of Hen 3-1475 are derived from the observed optical spectra, confirming that the knots are shock excited. The shocked spectra are qualitatively reproduced by simple ``3/2''D bow shock models. We present a set of bow shock models devoted to planetary nebulae, and discuss the effects of the pre-ionization conditions, the bow shock velocity, the bow shock shape and the chemical abundances on the predicted spectra. To explore the reliability of the ``3/2''D bow shock models, we also compare the observed spectra of other three proto-planetary nebulae (M 1-92, M 2-56 and CRL 618) to the predicted spectra.

astro-ph↗

arXiv2006

Optical imaging of L723: the structure of HH 223

We imaged the Lynds 723 dark nebula (L723) with the aim of studying the morphology of the Herbig-Haro object HH 223 and other line-emission nebula detected in the region. We obtained deep narrow-band images in the Halpha and [SII] lines and in the continuum nearby Halpha of a field of ~5' of the L723 dark nebula centered on HH 223. The Halpha and [SII] images reveal the detailed morphology of HH 223, unresolved in previous optical images. Both images show a quite complex knotty, wiggling structure embedded in a low-emission nebula. Comparison between the [SII] and Halpha fluxes of the knots are indicative of variations in the excitation conditions through HH 223. In addition, several other faint nebula are detected in Halpha a few arcmin to the SE and to the NW of HH 223, all of them lying projected onto the east-west pair of lobes of the quadrupolar CO outflow. Comparison between the Halpha and the continuum images confirms the HH-like nature of the Vrba object V83, while the Vrba objects V84 and V85 are identified as faint field stars.

astro-ph↗

arXiv2005

3D Spectroscopy of Herbig-Haro objects

HH 110 and HH 262 are two Herbig-Haro jets with rather peculiar, chaotic morphology. In the two cases, no source suitable to power the jet has been detected along the outflow, at optical or radio wavelengths. Both, previous data and theoretical models, suggest that these objects are tracing an early stage of an HH jet/dense cloud interaction. We present the first results of the integral field spectroscopy observations made with the PMAS spectrophotometer (with the PPAK configuration) of these two turbulent jets. New data of the kinematics in several characteristic HH emission lines are shown. In addition, line-ratio maps have been made, suitable to explore the spatial excitation an density conditions of the jets as a function of their kinematics.

astro-ph↗

arXiv2005

The interaction of a jet-like pulse with a wind from a stellar companion

We present numerical simulations of the interaction between a collimated, bipolar ``pulse'' ejected from a star and a continuous wind ejected from a stellar companion. We explore the characteristics of the predicted H$α$ intensity maps by varying selected input parameters. We find that the asymmetry (in size and strength) between the two lobes of the proto-planetary nebula OH 231.8+4.2 is reproduced in this scenario if the wind ejected by the companion star has a strong latitude dependence.

astro-ph↗

arXiv2004

K 4-47: a planetary nebula excited by photons and shocks

K 4-47 is an unusual planetary nebula composed of a compact high-ionization core and a pair of low-ionization knots. Long-slit medium-resolution spectra of the knots and core are analyzed in this paper. Assuming photoionization from the central star, we have derived physical parameters for all the nebular components, and the (icf) chemical abundances of the core, which appear similar to Type-I PNe for He and N/O but significantly deficient in oxygen. The nebula has been further modelled using both photoionization (CLOUDY) and shock (MAPPINGS) codes. From the photoionization modelling of the core, we find that both the strong auroral [O III] 4363A and [N II] 5755A emission lines observed and the optical size of the core cannot be accounted for if a homogeneus density is adopted. We suggest that a strong density stratification, matching the high-density core detected at radio wavelengths and the much lower density of the optical core, might solve the problem. From the bow-shock modelling of the knots, on the other hand, we find that knots' chemistry is also represented by Type-I PN abundances, and that they would move with velocities of 250 - 300 km/s.

astro-ph↗

arXiv2004

Time-dependent ejection velocity model for the outflow of Hen 3--1475

We present 2D axisymmetric and 3D numerical simulations of the proto-planetary nebula Hen 3-1475, which is characterized by a remarkably highly collimated optical jet, formed by a string of shock-excited knots along the axis of the nebula. It has recently been suggested that the kinematical and morphological properties of the Hen 3-1475 jet could be the result of an ejection variability of the central source (Riera et al. 2003). The observations suggest a periodic variability of the ejection velocity superimposed on a smoothly increasing ejection velocity ramp. From our numerical simulations, we have obtained intensity maps (for different optical emission lines) and position-velocity diagrams, in order to make a direct comparison with the HST observations of this object. Our numerical study allows us to conclude that a model of a precessing jet with a time-dependent ejection velocity, which is propagating into an ISM previously perturbed by an AGB wind, can succesfully explain both the morphological and the kinematical characteristics of this proto-planetary nebula.

astro-ph↗

arXiv2003

The 3-Dimensional Structure of HH 32 from GMOS IFU Spetroscopy

We present new high resolution spectroscopic observations of the Herbig-Haro object HH 32 from System Verification observations made with the GMOS IFU at Gemini North Observatory. The 3D spectral data covers a 8''.7 x 5''.85 spatial field and 4820 - 7040 Angstrom spectral region centered on the HH~32 A knot complex. We show the position-dependent line profiles and radial velocity channel maps of the Halpha line, as well as line ratio velocity channel maps of [OIII]5007/Halpha, [OI]6300/Halpha, [NII]6583/Halpha, [SII](6716+6730)/Halpha and [SII]6716/6730. We find that the line emission and the line ratios vary significantly on spatial scales of ~1'' and over velocities of ~50 km/s. A ``3/2-D'' bow shock model is qualitatively successful at reproducing the general features of the radial velocity channel maps, but it does not show the same complexity as the data and it fails to reproduce the line ratios in our high spatial resolution maps. The observations of HH 32 A show two or three superimposed bow shocks with separations of ~3'', which we interpret as evidence of a line of sight superposition of two or three working surfaces located along the redshifted body of the HH 32 outflow.

astro-ph↗

arXiv2003

A model for the cross section of a turbulent, radiative jet or wake

We present an analytical model for the cross section of a turbulent, radiative jet or wake. This model is appropriate for modeling HH jets, or "wakes" left behind by "astrophysical bullets". Even though the model is very simple, it has the benign property of only having four free parameters (the outer radius of the beam, the axial velocity, the velocity at the edge of the beam, and the turbulent velocity width), which can be derived by fitting the radial velocity and line width cross sections of an observed outflow. We illustrate how to do such fits using previously published spectroscopic data of the HH 110 jet.

astro-ph↗