The low-mass companion of GQ Lup

Using NACO on the VLT in the imaging mode we have detected an object at a distance of only 0.7 arcsec from GQ Lup. The object turns out to be co-moving. We have taken two K-band spectra with a resolution of lambda /Delta lambda=700. In here, we analyze the spectra in detail. We show that the shape of spectrum is not spoiled by differences in the Strehl ratio in the blue and in the red part, as well as differential refraction. We reanalyze the spectra and derive the spectral type of the companion using classical methods. We find that the object has a spectral type between M9V and L4V, which corresponds to a Teff between 1600 and 2500 K. Using GAIA-dusty models, we find that the spectral type derivation is robust against different log(g)-values. The Teff derived from the models is again in the range between 1800 and 2400 K. While the models reproduce nicely the general shape of the spectrum, the 12CO-lines in the spectrum have about half the depth as those in the model. We speculate that this difference might be caused by veiling, like in other objects of similar age, and spectral class. We also find that the absolute brightness of the companion matches that of other low-mass free-floating objects of similar age and spectral type. A comparison with the objects in USco observed by Mohanty et al. (2004) shows that the companion of GQ Lup has a lower mass than any of these, as it is of later spectral type, and younger. The same is as true, for the companion of AB Pic. To have a first estimate of the mass of the object we compare the derived Teff and luminosity with those calculated from evolutionary tracks. We also point out that future instruments, like NAHUAL, will finally allow us to derive the masses of such objects more precisely.Comment: 6 figures, proceeding of the workshop on Ultralow-mass star formation and evolution held in La Palama June 28 to July 1, 200

A new approach to partial synchronization in globally coupled rotators

We develop a formalism to analyze the behaviour of pulse--coupled identical phase oscillators with a specific attention devoted to the onset of partial synchronization. The method, which allows describing the dynamics both at the microscopic and macroscopic level, is introduced in a general context, but then the application to the dynamics of leaky integrate-and-fire (LIF) neurons is analysed. As a result, we derive a set of delayed equations describing exactly the LIF behaviour in the thermodynamic limit. We also investigate the weak coupling regime by means of a perturbative analysis, which reveals that the evolution rule reduces to a set of ordinary differential equations. Robustness and generality of the partial synchronization regime is finally tested both by adding noise and considering different force fields.Comment: 5 pages, 3 eps figure

Radial Flow from Electromagnetic Probes and Signal of Quark Gluon Plasma

A first attempt has been made to extract the evolution of radial flow from the analysis of the experimental data on electromagnetic probes experimentally measured at SPS and RHIC energies. The $p_T$ spectra of photons and dileptons measured by WA98 and NA60 collaborations respectively at CERN-SPS and the photon spectra obtained by PHENIX collaboration at BNL-RHIC have been used to constrain the theoretical models, rendering the outcome of the analysis largely model independent. We argue that the variation of the radial velocity with invariant mass is indicative of a phase transition from initially produced partons to hadrons at SPS and RHIC energies.Comment: One LaTeX and 9 eps files, to appear in Phys. Rev.

A mid-infrared study of very low mass stars and brown dwarfs in Upper Scorpius

We report the results of mid-IR observations with VISIR at the VLT of 10 ultracool dwarfs members of the nearby Upper Scorpius OB association in four filters ranging between 8.59 (PAH1) to 12.8 $\mu$m (Ne II), and one brown dwarf with Spitzer between 3.6 and 24 $\mu$m. Seven of our targets are detected in at least one of the bands, and we derive upper limits on the fluxes of the remaining 4. These results combined with previous studies from the literature lead to an improved disk frequency of 50$\pm$12%. This frequency is significantly higher than that of accretors (16.3%$\pm$6.2%). Only one object showing mid-IR excess also has H$\alpha$ emission at a level indicating that it must be accreting. Four of the detected targets are multiple system candidates. The observed disk frequency for sub-stellar objects in the Upper Scorpius association is similar to that of stars, consistent with a common formation scenario. It is also similar to the disk fractions observed in younger clusters, suggesting that the disk lifetimes might be longer for ultracool dwarfs than for higher-mass stars.Comment: 10 pages, 4 figures, accepted for A&

Intrinsic dissipation in high-frequency micromechanical resonators

We report measurements of intrinsic dissipation in micron-sized suspended resonators machined from single crystals of galium arsenide and silicon. In these experiments on high-frequency micromechanical resonators, designed to understand intrinsic mechanisms of dissipation, we explore dependence of dissipation on temperature, magnetic field, frequency, and size. In contrast to most of the previous measurements of acoustic attenuation in crystalline and amorphous structures in this frequency range, ours is a resonant measurement; dissipation is measured at the natural frequencies of structural resonance, or modes of the structure associated with flexural and torsional motion. In all our samples we find a weakly temperature dependent dissipation at low temperatures. We compare and contrast our data to various probable mechanisms, including thermoelasticity, clamping, anharmonic mode-coupling, surface anisotropy and defect motion, both in bulk and on surface. The observed parametric dependencies indicate that the internal defect motion is the dominant mechanism of intrinsic dissipation in our samples

Spectroscopy of Very Low Mass Stars and Brown Dwarfs in the Lambda Orionis Star Forming Region

Context. Most observational studies so far point towards brown dwarfs sharing a similar formation mechanism as the one accepted for low mass stars. However, larger databases and more systematic studies are needed before strong conclusions can be reached. Aims. In this second paper of a series devoted to the study of the spectroscopic properties of the members of the Lambda Orionis Star Forming Region, we study accretion, activity and rotation for a wide set of spectroscopically confirmed members of the central star cluster Collinder 69 to draw analogies and/or differences between the brown dwarf and stellar populations of this cluster. Moreover, we present comparisons with other star forming regions of similar and different ages to address environmental effects on our conclusions. Methods. We study prominent photospheric lines to derive rotational velocities and emission lines to distinguish between accretion processes and chromospheric activity. In addition, we include information about disk presence and X-ray emission. Results. We report very large differences in the disk fractions of low mass stars and brown dwarfs (~58%) when compared to higher mass stars (26+4-3%) with 0.6 Msun being the critical mass we find for this dichotomy. As a byproduct, we address the implications of the spatial distribution of disk and diskless members in the formation scenario of the cluster itself. We have used the Halpha emission to discriminate among accreting and non-accreting sources finding that 38+8-7% of sources harboring disks undergo active accretion and that his percentage stays similar in the substellar regime. For those sources we have estimated accretion rates. Finally, regarding rotational velocities, we find a high dispersion in vsin(i) which is even larger among the diskless population.Comment: Accepted for publication in A&A. 18 figs including the Appendix and an online tabl

Numerical investigation of the Somali jet interaction with the Western Ghat Mountains

Several major features of the interaction of the Somali jet with the Western Ghat Mountains have been observed. These include a pressure ridge, strong vertical motions, and occurrences of highly reflective cloud and heavy rainfall rates along the west coast of India. A triple nested regional weather prediction model has been used to investigate the dynamic interaction between the Somali jet and the Western Ghat Mountains. Two numerical experiments were conducted; one with the topography of western India and the other without. In the experiment without topography, the Western Ghat Mountains were removed from the innermost domain. The results for the innermost domain in the two experiments were analyzed and compared. The results from the simulation with topography captured several of the observed features of the Somali jet interaction with the Western Ghat Mountains. The simulation without topography failed to reveal these features. The results suggest that the blocking effect of the Western Ghats plays an important role in the prediction of the rainfall over the west coast of India

Flared Disks and Silicate Emission in Young Brown Dwarfs

We present mid-infrared photometry of three very young brown dwarfs located in the $\rho$ Ophiuchi star-forming region -- GY5, GY11 and GY310 --obtained with the Subaru 8-meter telescope. All three sources were detected at 8.6 and 11.7$\mu$m, confirming the presence of significant mid-infrared excess arising from optically thick dusty disks. The spectral energy distributions of both GY310 and GY11 exhibit strong evidence of flared disks; flat disks can be ruled out for these two brown dwarfs. The data for GY5 show large scatter, and are marginally consistent with both flared and flat configurations. Inner holes a few substellar radii in size are indicated in all three cases (and especially in GY11), in agreement with magnetospheric accretion models. Finally, our 9.7$\mu$m flux for GY310 implies silicate emission from small grains on the disk surface (though the data do not completely preclude larger grains with no silicate feature). Our results demonstrate that disks around young substellar objects are analogous to those girdling classical T Tauri stars, and exhibit a similar range of disk geometries and dust properties.Comment: submitted to Astrophysical Journal Letter

Equilibration in Quark Gluon Plasma

The hydrodynamic expansion rate of quark gluon plasma (QGP) is evaluated and compared with the scattering rate of quarks and gluons within the system. Partonic scattering rates evaluated within the ambit of perturbative Quantum Choromodynamics (pQCD) are found to be smaller than the expansion rate evaluated with ideal equation of state (EoS) for the QGP. This indicate that during the space-time evolution the system remains out of equilibrium. Enhancement of pQCD cross sections and a more realistic EoS keep the partons closer to the equilibrium.Comment: To be published in the Quark Matter 2008 poster proceeding