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J. A. Caballero

Publications and source records attributed to J. A. Caballero.

At least 343 records · Page 19Linked to original sources
arXiv2005

Proper motion Pleiades candidate L-type brown dwarfs

We present results of an optical and near-infrared (IR) 1.8 deg^2 survey in the Pleiades open cluster to search for substellar objects. From optical I-band images from the CFHT and J-band images from the 3.5 m CAHA Telescope, we identify 18 faint and very red L brown dwarf candidates, with I> 20.9 and I-J> 3.2. The follow-up observations of nine objects in the H- and Ks-bands confirm that eight belong to the IR sequence of the cluster and the proper motion measurements of seven candidates confirm that they are Pleiades members. A preliminary estimation of the substellar mass spectrum dN/dM in the form of a power law M^-alpha provides alpha=0.57+-0.14. We extrapolate this function to estimate the number of very low-mass brown dwarfs and planetary mass objects that could be present in the cluster down to 1 M_Jup. Sensitive searches combining far red and near infrared observations may unveal these objects in a near future.

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arXiv2005

Ultra low-mass star and substellar formation in sigma Orionis

The nearby young sigma Orionis cluster (~360 pc, ~3 Ma) is becoming one of the most important regions for the study of ultra low-mass star formation and its extension down to the mass regimes of the brown dwarfs and planetary-mass objects. Here, I introduce the sigma Orionis cluster and present three studies that the JOVIAN group is developing: a pilot programme of near-infrared adaptive-optics imaging, intermediate-resolution optical spectroscopy of a large sample of stars of the cluster and a study of the mass function down to the planetary-mass domain. This paper is a summary of the content of four posters that I presented in the Ultra low-mass star formation and evolution Workshop, as single author or on behalf of different collaborations.

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arXiv2005

Superscaling in charged current neutrino quasielastic scattering in the relativistic impulse approximation

Superscaling of the quasielastic cross section in charged current neutrino-nucleus reactions at energies of a few GeV is investigated within the framework of the relativistic impulse approximation. Several approaches are used to describe final state interactions and comparisons are made with the plane wave approximation. Superscaling is very successful in all cases. The scaling function obtained using a relativistic mean field for the final states shows an asymmetric shape with a long tail extending towards positive values of the scaling variable, in excellent agreement with the behavior presented by the experimental scaling function.

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arXiv2005

S Ori J053825.4-024241: A Classical T Tauri-like object at the substellar boundary

We present a spectrophotometric analysis of S Ori J053825.4-024241, a candidate member close to the substellar boundary of the young (1-8 Myr), nearby (~360 pc) sigma Orionis star cluster. Our optical and near-infrared photometry and low-resolution spectroscopy indicate that S Ori J053825.4-024241 is a likely cluster member with a mass estimated from evolutionary models at 0.06+0.07-0.02 Msol, which makes the object a probable brown dwarf. The radial velocity of S Ori J053825.4-024241 is similar to the cluster systemic velocity. This target, which we have classified as an M 6.0+-1.0 low-gravity object, shows excessemission in the near-infrared and anomalously strong photometric variability for its type (from the blue to the J band), suggesting the presence of a surrounding disc. The optical spectroscopic observations show a continuum excess at short wavelengths and a persistent and resolved Halpha emission (pseudo-equivalent width of ~-250 AA) in addition to the presence of other forbidden and permitted emission lines, which we interpret as indicating accretion from the disc and possibly mass loss. We conclude that despite the low mass of S Ori J053825.4-024241, this object exhibits some of the properties typical of active classical T Tauri stars.

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arXiv2005

A search for planetary-mass objects and brown dwarfs in the Upper Scorpius association

We report the results of a deep photometric search for planets and brown dwarfs in the nearby young OB Upper Scorpius association. We obtained optical (I) and near-infrared (JKs) images around nine very low-mass stars and brown dwarf member candidates of the association, covering a total area of 113 arcmin^2. Using a point spread function subtraction technique, we have searched for planetary-mass companions (0.002 Msol < M < 0.013 Msol) at separations as close as 3 arcsec from the targets. We have not found any brown dwarfs more massive than 0.030 Msol at projected distances larger than 70AU, or planets more massive than 0.007 Msol at projected distances larger than 600AU. We set an upper limit of 20% (confidence level 68.3%) to the fraction of very low-mass stars and massive brown dwarfs with planetary mass companions (M > 0.007 Msol) at physical distances larger than 600AU. From the I, I-J colour-magnitude diagrams and follow-up Ks-band photometry, we identify four very red objects (I-J > 2.5) in the area of the survey. According to their positions in the diagram, below the expected theoretical sequence of the cluster, three are probable late M-dwarfs in the field. The faintest and reddest object (J-Ks = 2.39 +/- 0.06) may be either a field intermediate L-dwarf with anomalously red J-Ks colour at a distance of approx. 135pc, an extremely red distant galaxy, or a reddened planetary-mass object in the Upper Scorpius association.

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arXiv2005

Semi-relativistic description of quasielastic neutrino reactions and superscaling in a continuum shell model

The so-called semi-relativistic expansion of the weak charged current in powers of the initial nucleon momentum is performed to describe charge-changing, quasielastic neutrino reactions $(ν_μ,μ^-)$ at intermediate energies. The quality of the expansion is tested by comparing with the relativistic Fermi gas model using several choices of kinematics of interest for ongoing neutrino oscillation experiments. The new current is then implemented in a continuum shell model together with relativistic kinematics to investigate the scaling properties of $(e,e')$ and $(ν_μ,μ^-)$ cross sections.

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arXiv2004

Using Electron Scattering Superscaling to predict Charge-changing Neutrino Cross Sections in Nuclei

Superscaling analyses of few-GeV inclusive electron scattering from nuclei are extended to include not only quasielastic processes, but now also into the region where $Δ$-excitation dominates. It is shown that, with reasonable assumptions about the basic nuclear scaling function extracted from data and information from other studies of the relative roles played by correlation and MEC effects, the residual strength in the resonance region can be accounted for through an extended scaling analysis. One observes scaling upon assuming that the elementary cross section by which one divides the residual to obtain a new scaling function is dominated by the $N\toΔ$ transition and employing a new scaling variable which is suited to the resonance region. This yields a good representation of the electromagnetic response in both the quasielastic and $Δ$ regions. The scaling approach is then inverted and predictions are made for charge-changing neutrino reactions at energies of a few GeV, with focus placed on nuclei which are relevant for neutrino oscillation measurements. For this a relativistic treatment of the required weak interaction vector and axial-vector currents for both quasielastic and $Δ$-excitation processes is presented.

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arXiv2004

Final-state interaction and recoil polarization in (e,e'p) reactions: comparison with the polarized target case

A study of the total cross section for polarized proton knockout in $(e,e'{\vec p})$ reactions is carried out for the closed-shell nucleus $^{40}$Ca. The dependence of FSI effects on polarization observables viewed as functions of the nucleon polarization angles is analyzed and interpreted within the basis of a semi-classical model for the orbit of the struck nucleon and trajectory of the ejected nucleon. A comparison with the case of a $^{39}$K polarized target and unpolarized protons is performed.

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arXiv2004

Optical Linear Polarization of Late M- and L-Type Dwarfs

(Abridged). We report on the linear polarimetric observations in the Johnson I filter of 44 M6-L7.5 ultracool dwarfs (2800-1400 K). Eleven (10 L and 1 M) dwarfs appear to have significant linear polarization (P = 0.2-2.5%). We have compared the M- and L-dwarf populations finding evidence for a larger frequency of high I-band polarization in the coolest objects, supporting the presence of significant amounts of dust in L-dwarfs. The probable polarizing mechanism is related to the presence of heterogeneous dust clouds nonuniformly distributed across the visible photospheres and the asymmetric shape of the objects. In some young ultracool dwarfs, surrounding dusty disks may also yield polarization. For polarimetric detections, a trend for slightly larger polarization from L0 to L6.5 may be present in our data, suggesting changes in the distribution of the grain properties, vertical height of the clouds, metallicity, age, and rotation speed. One of our targets is the peculiar brown dwarf (BD) 2MASS J2244+20 (L6.5), which shows the largest I-band polarization degree. Its origin may lie in a surrounding dusty disk or rather large photospheric dust grains. The M7 young BD CFHT-BD-Tau 4 and the L3.5 field dwarf 2MASS J0036+18 were also observed in the Johnson R filter. Our data support the presence of a circum(sub)stellar disk around the young accreting BD. Our data also support a grain growth in the submicron regime in the visible photosphere of J0036+18 (1900 K). The polarimetric data do not obviously correlate with activity or projected rotational velocity. Three polarized early- to mid-L dwarfs display I-band light curves with amplitudes below 10 mmag.

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arXiv2004

Skewed recoil polarization in (e,e'p) reactions from polarized nuclei

The general formalism describing $\vec{A}(\vec{e},e'\vec{p})B$ reactions, involving polarization of the electron beam, target and ejected proton, is presented within the framework of the relativistic plane wave impulse approximation for medium and heavy nuclei. It is shown that the simultaneous measurement of the target and ejected proton polarization can provide new information which is not contained in the separate analysis of the $\vec{A}(\vec{e},e'p)B$ and $A(\vec{e},e'\vec{p})B$ reactions. The polarization transfer mechanism in which the electron interacts with the initial nucleon carrying the target polarization, making the proton exit with a fractional polarization in a different direction, is referred to here as ``skewed polarization''. The new observables characterizing the process are identified, and written in terms of polarized response functions and asymmetries which are of tensor nature. The corresponding half-off-shell single-nucleon responses are analyzed using different prescriptions for the electromagnetic vertex and for different kinematics. Numerical predictions are presented for selected perpendicular and parallel kinematics in the case of $^{39}$K as polarized target.

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arXiv2004

Electromagnetic (${\vec e},e'{\vec p}$) Observables & Relativistic Nucleon Dynamics

At present there exists a great interest in the search for evidence of possible modification of the nucleon form factors inside the nuclear medium. Recent theoretical work predict changes in the form factors within the experimental limits. Importantly, the longitudinal to sideways transferred polarization ratio has been identified as being ideally suited for such studies, as these polarization observables are believed to be the least sensitive to most standard nuclear structure uncertainties while their ratio shows a high sensitivity to the ratio of the electric to magnetic form factors. The kinematic regime where the measurements have been undertaken is at relatively high energy and it is clear that relativistic effects in wave functions and operators are essential. In this work we focus on the analysis of polarized $^{16}O(\vec{e},e'\vec{p})^{15}N$ observables. Our aim is to explore a selected set of model dependences that could contaminate any attempt to infer medium modifications, mainly related to the description of FSI and to the role played by relativity.

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arXiv2004

Final state interaction effects in neutrino-nucleus quasielastic scattering

We consider the charged-current quasielastic scattering of muon neutrinos on an Oxygen 16 target, described within a relativistic shell model and, for comparison, the relativistic Fermi gas. Final state interactions are described in the distorted wave impulse approximation, using both a relativistic mean field potential and a relativistic optical potential, with and without imaginary part. We present results for inclusive cross sections at fixed neutrino energies in the range $E_ν=$ 200 MeV - 1 GeV, showing that final state interaction effects can remain sizable even at large energies.

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arXiv2004

A$(\vec{e},e'\vec{p})$B responses: from bare nucleons to complex nuclei

We study the occurrence of factorization in polarized and unpolarized observables in coincidence quasi-elastic electron scattering. Starting with the relativistic distorted wave impulse approximation, we reformulate the effective momentum approximation and show that the latter leads to observables which factorize under some specific conditions. Within this framework, the role played by final state interactions and, in particular, by the spin-orbit term is explored. Connection with the nonrelativistic formalism is studied in depth. Numerical results are presented to illustrate the analytical derivations and to quantify the differences between factorized and unfactorized approaches.

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arXiv2004

Photometric variability of young brown dwarfs in the sigma Orionis open cluster

We have carried out multi-epoch, time-series differential I-band photometry of a large sample of objects in the south-east region of the young (~3 Myr), nearby (~350 pc) sigma Orionis open cluster. A field of ~1000 arcmin^2 was monitored during four nights over a period of two years. Using this dataset, we have studied the photometric variability of twenty-eight brown dwarf cluster candidates with masses ranging from the stellar-substellar boundary down to the planetary-mass domain. We have found that about 50% of the sample show photometric variability on timescales from less than one hour to several days and years. The amplitudes of the I-band light curves range from less than 0.01 up to ~0.4 magnitudes. A correlation between the near-infrared excess in the K_s band, strong Halpha emission and large-amplitude photometric variation is observed. We briefly discuss how these results may fit the different scenarios proposed to explain the variability of cool and ultracool dwarfs (i.e. magnetic spots, patchy obscuration by dust clouds, surrounding accretion discs and binarity). Additionally, we have determined tentative rotational periods in the range 3 to 40 h for three objects with masses around 60 M_Jup, and the rotational velocity of 14+/-4 km/s for one of them.

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arXiv2004

Uncorrelated scattering approximation revisited

The formalism to describe the scattering of a weakly bound projectile nucleus by a heavy target is investigated, using the Uncorrelated Scattering Approximation. The main assumption involved is to neglect the correlation between the fragments of the projectile in the region where the interaction with the target is important. It is shown that the angular momentum of each fragment with respect to the target is conserved. Moreover, when suitable approximations are assumed, the kinetic energy of each fragment is also shown to be conserved. The S-matrix for the scattering of the composite system can be written as a combination of terms, each one being proportional to the product of the S-matrices of the fragments.

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arXiv2004

Induced Nucleon Polarization and Meson-Exchange Currents in (e,e'p) Reactions

Nucleon recoil polarization observables in $(e,e'\vec{p})$ reactions are investigated using a semi-relativistic distorted-wave model which includes one- and two-body currents with relativistic corrections. Results for the induced polarization asymmetry are shown for closed-shell nuclei and a comparison with available experimental data for $^{12}$C is provided. A careful analysis of meson exchange currents shows that they may affect significantly the induced polarization for high missing momentum.

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arXiv2004

Inelastic electron-nucleus scattering and scaling at high inelasticity

Highly inelastic electron scattering is analyzed within the context of the unified relativistic approach previously considered in the case of quasielastic kinematics. Inelastic relativistic Fermi gas modeling that includes the complete inelastic spectrum - resonant, non-resonant and Deep Inelastic Scattering - is elaborated and compared with experimental data. A phenomenological extension of the model based on direct fits to data is also introduced. Within both models, cross sections and response functions are evaluated and binding energy effects are analyzed. Finally, an investigation of the second-kind scaling behavior is also presented.

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