Context. It has recently been discovered that the Galactic bulge is X-shaped, with the two southern arms of the X both crossing the lines of sight at l = 0 and |b| > 4, hence producing a double red clump in the bulge color magnitude diagram. Dynamical models predict the formation of X-shaped bulges as extreme cases of boxy-peanut bulges. However, since X-shaped bulges were known to be present only in external galaxies, models have never been compared to 3D kinematical data for individual stars. Aims. We study the orbital motion of Galactic bulge stars in the two arms (overdensities) of the X in the southern hemisphere. The goal is to provide observational constraints to bulge formation models that predict the formation of X-shapes through bar dynamical instabilities. Methods. Radial velocities have been obtained for a sample of 454 bulge giants, roughly equally distributed between the bright and the faint red clump, in a field at (l, b) = (0, −6). Proper motions were derived for all red clump stars in the same field by combining images from two epochs, which were obtained 11 years apart, with WFI at the 2.2 m at La Silla. The observed field contains the globular cluster NGC 6558, whose member stars were used to assess the accuracy of the proper motion measurement. At the same time, as a by-product, we provide the first proper motion measurement of NGC 6558. The proper motions for the spectroscopic subsample are analyzed for a subsample of 352 stars, taking into account the radial velocities and metallicities measured from near-infrared calcium triplet lines. Results. The radial velocity distribution of stars in the bright red clump, which traces the closer overdensity of bulge stars, shows an excess of stars moving towards the Sun. Similarly, an excess of stars receding from the Sun is seen in the far overdensity, which is traced by faint red clump stars. This is explained by the presence of stars on elongated orbits, which are most likely streaming along the arms of the X-shaped bulge. Proper motions for these stars are consistent with qualitative predictions of dynamical models of peanut-shaped bulges. Surprisingly, stars on elongated orbits have preferentially metal-poor (subsolar) metallicities, while the metal rich ones, in both overdensities, are preferentially found in more axisymmetric orbits. The observed proper motion of NGC 6558 has been measured as (μ l cos (b), μ b ) = (0.30 ± 0.14, −0.43 ± 0.13), with a velocity dispersion of (σ l cos(b), σ b ) = (1.8, 1.7) mas/yr. This is the first proper motion measurement for this cluster.Key words. Galaxy: bulge -Galaxy: kinematics and dynamics -Galaxy: structure -galaxies: kinematics and dynamics Kinematics for IMACS and FLAMES spectra are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via

We present PUCHEROS, the high-resolution echelle spectrograph, developed at the Center of Astro-Engineering of Pontificia Universidad Catolica de Chile to provide an effective tool for research and teaching of astronomy. The instrument is fed by a single-channel optical fibre and it covers the visible range from 390 to 730 nm in one shot, reaching a spectral resolution of about 20 000. In the era of extremely large telescopes our instrument aims to exploit the capabilities offered by small telescopes in a cost-effective way, covering the observing needs of a community of astronomers, in Chile and elsewhere, which do not necessarily need large collecting areas for their research. In particular the instrument is well suited for long-term spectroscopic monitoring of bright variable and transient targets down to a V magnitude of about 10. We describe the instrument and present a number of text case examples of observations obtained during commissioning and early science.

Recent analytical work has shown that when an acoustic plane wave propagates through a rotational flow field there is a linear relationship between the Fourier component of the scattered acoustic pressure and the Fourier transform in space and time of the vorticity component that is normal to the plane defined by the wave vectors of the incident and scattered acoustic waves. Hence, ultrasound scattering can be used as a non-intrusive spectral probe of vorticity and potentially as a tool for direct measurements of vorticity distributions. Some aspects of this technique have been tested in a swirling air jet emanating from a 2.54 cm diameter nozzle where the swirl is generated upstream of the jet nozzle by a rotating paddle. For a given exit volume flow rate, swirl numbers up to 0.4 are realized. Radial distributions of the streamwise and tangential velocity components downstream of the jet exit plane are measured using two-component hot-wire anemometry and the corresponding distributions of streamwise vorticity are computed. A nominally plane ultrasonic wave field is generated normal to the jet axis by a transmitter having a 16 cm square aperture. The scattered ultrasound in the radial direction is measured at a number of streamwise and azimuthal stations. In accord with the theory, the normalized amplitude of the scattered acoustic wave is a linear function of the magnitude of the centerline vorticity at the exit plane of the jet, and is independent of the intensity of the incident wave field. Fourier components of the vorticity distribution are directly measured by varying the scattering angle and are in good agreement with theoretical predictions.

We study the coherent propagation of an acoustic wave through a disordered flow with zero mean velocity. The flow is modeled as an assembly of vortices randomly distributed. The source term of the linear wave equation satisfied by the acoustic pressure must be expanded up to terms of order Mach number of the background flow squared. The complex wave number of the coherent wave is calculated analytically and related to average properties of the flow using multiple scattering theory in a Bourret approximation. The perturbations induced by the fluid motion on the index of refraction and on the attenuation length of the wave are of order Mach number squared. The role of the finite compressibility of the fluid is considered in detail, as well as the need, or lack thereof, to consider widely different time scales for acoustic propagation and flow evolution. For a gas and for long wavelengths, the phase velocity of the coherent wave may become higher than in the fluid at rest.

Analytic expressions are found for the index of refraction and attenuation length of an acoustic wave that propagates in a flow that can be modeled in terms of many slender vortices in three dimensions. This is done to second order in the Mach number, within a multiple scattering scheme and a Born approximation also carried to second order. As an example, their dependence on wavelength is explicitly computed in the case of the vortices being circular rings, uniformly distributed in space but randomly oriented. [S0031-9007(98)