Astrometry in crowded fields towards the Galactic Bulge

The astrometry towards the Galactic Bulge is hampered by high stellar crowding and patchy extinction. This effect is particularly severe for optical surveys such as Gaia. In this study, we assess the consistency of proper motions (PMs) between optical (Gaia DR3) and near-infrared (VIRAC2) catalogues in comparison with PMs measured with the Hubble Space Telescope (HST) in several crowded fields towards the Galactic Bulge and in Galactic globular clusters. Assuming that the PMs are well characterised, the uncertainty-normalised PM differences between pairs of catalogues are expected to follow a normal distribution. A deviation from a normal distribution defines the inflation factor $r$. Multiplying the PM uncertainties by $r$ brings the Gaia (VIRAC2) PMs into a $1\sigma$ agreement with HST PMs. The factor $r$ has a dependence on stellar surface density and for the brightest stars in our sample (G<18), there is a strong dependence on G-band magnitude. Assuming that the HST PMs are well determined and free from systematic errors, we find that Gaia DR3 PM uncertainties are better characterised, having r<1.5, in fields under 200 Gaia DR3 sources per arcmin$^2$, and are underestimated by up to a factor of 4 in fields with more than 300 Gaia DR3 sources per arcmin$^2$. For the most crowded fields in VIRAC2, the PM uncertainties are underestimated by a factor of 1.1 up to 1.5, with a dependence on J-band magnitude. In all fields, the brighter sources have the larger $r$ value. At the faint end (G>19), $r$ is close to 1, meaning that the PMs already fully agree with the HST measurements within $1\sigma$. In the crowded fields with both catalogues in common, VIRAC2 PMs agree with HST PMs and do not need an inflation factor for their uncertainties. Given the depth and completeness of VIRAC2 in such fields, it is an ideal complement to Gaia DR3 for proper motion studies towards the Galactic Bulge.Comment: Accepted for publication in A&A. 11 pages (+6 in Appendix), 9 figures (+18 in Appendix), 4 table

A new near-IR window of low extinction in the Galactic plane

Reproduced with permission from Astronomy & Astrophysics. © 2018 ESO. Comments: 6 pages, 6 figures. Accepted for publication in Astronomy & AstrophysicsAims. The windows of low extinction in the Milky Way (MW) plane are rare but important because they enable us to place structural constraints on the opposite side of the Galaxy, which has hither to been done rarely. Methods. We use the near-infrared (near-IR) images of the VISTA Variables in the Vía Láctea (VVV) Survey to build extinction maps and to identify low extinction windows towards the Southern Galactic plane. Here we report the discovery of VVV WIN 1713-3939, a very interesting window with relatively uniform and low extinction conveniently placed very close to the Galactic plane. Results. The new window of roughly 30 arcmin diameter is located at Galactic coordinates (l, b) = (347.4, -0.4) deg. We analyse the VVV near-IR colour-magnitude diagrams in this window. The mean total near-IR extinction and reddening values measured for this window are A Ks = 0.46 and E(J - K s) = 0.95. The red clump giants within the window show a bimodal magnitude distribution in the K s band, with peaks at K s = 14.1 and 14.8 mag, corresponding to mean distances of D = 11.0 ± 2.4 and 14.8 ± 3.6 kpc, respectively. We discuss the origin of these red clump overdensities within the context of the MW disk structure.Peer reviewe

A high-reflectivity, ambient-stable graphene mirror for neutral atomic and molecular beams

The following article appeared in Applied Physics Letters Volume 99, Issue 21, 21 November 2011, Article number 211907 and may be found at http://scitation.aip.org/content/aip/journal/apl/99/21/10.1063/1.3663866We report a He and H2 diffraction study of graphene-terminated Ru(0001) thin films grown epitaxially on c-axis sapphire. Even for samples exposed for several weeks to ambient conditions, brief annealing in ultrahigh vacuum restored extraordinarily high specular reflectivities for He and H 2 beams (23 and 7 of the incident beam, respectively). The quality of the angular distributions recorded with both probes exceeds the one obtained from in-situ prepared graphene on Ru(0001) single crystals. Our results for graphene-terminated Ru thin films represent a significant step toward ambient tolerant, high-reflectivity curved surface mirrors for He-atom microscopyWork performed in part at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which issupported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. Work supported by the Ministerio de Educacion y Ciencia through projects CONSOLIDER-INGENIO 2010 on Molecular Nanoscience (CSD 2007-00010), FIS2010-18847, and byComunidad de Madrid through the program NANOBIOMAGNET (S2009/MAT-1726)

Kinematic Evidence for an Old Stellar Halo in the Large Magellanic Cloud

The oldest and most metal-poor Milky Way stars form a kinematically hot halo, which motivates the two major formation scenarios for our galaxy: extended hierarchical accretion and rapid collapse. RR Lyrae stars are excellent tracers of old and metal-poor populations. We measure the kinematics of 43 RR Lyrae stars in the inner regions of the nearby Large Magellanic Cloud (LMC) galaxy. The velocity dispersion, \sigma_{true}=53\pm10 km/s, indicates that a kinematically hot metal-poor old halo also exists in the LMC. This suggests that our galaxy and smaller late-type galaxies like the LMC have similar early formation histories.Comment: 8 pages, 2 figures; to be published in Science on Sept. 12, 200

Helium reflectivity and Debye temperature of graphene grown epitaxially on Ru(0001)

It is shown that the surface of an epitaxial graphene monolayer grown on Ru(0001) could be used as a quite efficient external mirror for He-atom microscopy, with a specular reflectivity of 20% of the incident beam. Furthermore, the system is stable up to 1150 K, and the He reflectivity remains almost unchanged after exposure to air. Additionally, the high reflectivity for H2 molecules (11%) opens up the development of a H2 microprobe suitable for lithography. The Debye temperature for this epitaxial graphene monolayer has been determined from a study of the temperature dependence of the He specular intensity as a function of incident parameters. A value of 1045 K has been obtained, which is much higher than the 590 K reported for graphite under similar conditions, and close to the value of 1287 K calculated for isolated grapheneThis work was supported by the Ministerio de Educación y Ciencia through the program CONSOLIDER-INGENIO on Molecular Nanoscience (Project No. CSD 2007-00010), Project No. FIS2010-18847, and a Juan de la Cierva grant (A.P.), and by Comunidad de Madrid through the program NANOBIOMAGNE

Optical Counterparts of X-Ray Point Sources Observed by CHANDRA in NGC5128: 20 New Globular Cluster X-Ray Sources

VLT images in $BVI$ are used to identify the optical counterparts to bright CHANDRA X-ray points sources discovered by Kraft et al. (2001, ApJ, 560, 675) in NGC5128. Of a total of 111 X-ray point sources with L_X>2*10^{36} ergs/s present in a 56 arcmin^2 field centered on this galaxy, 58 have optical counterparts. Based on the sizes, optical magnitudes and colors, 20 new globular cluster counterparts of X-ray sources are identified, plus 3 identified based on their sizes. This brings the total number of globular cluster X-ray sources in this galaxy to 33, and establishes that 30% of the X-ray point sources in NGC5128 are associated with globular clusters. These X-ray globular clusters occupy the brightest end of the globular cluster luminosity function, indicating that bright low-mass X-ray binaries are preferentially found in massive clusters. Most of the globular clusters with X-ray sources have red colors, with 1.0<V-I<1.5, indicating that low-mass X-ray binaries are preferentially formed in metal-rich clusters. The NGC5128 X-ray globular cluster sources are brighter in comparison with the Milky Way sources: there are 24 globular clusters with X-ray sources of L_x>10^{37} erg/sec. There is, however, no globular cluster X-ray source in NGC5128 as bright as expected for an accreting black hole. In addition, 31 optical counterparts of X-ray point sources that are not associated with globular clusters are identified. Finally, 53 X-ray point sources (48% of the population), do not have any optical counterparts down to the faintest magnitude limits (B=25).Comment: 32 pages, 8 figures; replaced with the accepted version; to appear in ApJ, January 200

Electronic and Geometric Corrugation of Periodically Rippled, Self-nanostructured Graphene Epitaxially Grown on Ru(0001)

Graphene epitaxially grown on Ru(0001) displays a remarkably ordered pattern of hills and valleys in Scanning Tunneling Microscopy (STM) images. To which extent the observed "ripples" are structural or electronic in origin have been much disputed recently. A combination of ultrahigh resolution STM images and Helium Atom diffraction data shows that i) the graphene lattice is rotated with respect to the lattice of Ru and ii) the structural corrugation as determined from He diffraction is substantially smaller (0.015 nm) than predicted (0.15 nm) or reported from X-Ray Diffraction or Low Energy Electron Diffraction. The electronic corrugation, on the contrary, is strong enough to invert the contrast between hills and valleys above +2.6 V as new, spatially localized electronic states enter the energy window of the STM. The large electronic corrugation results in a nanostructured periodic landscape of electron and holes pockets.Comment: 16 pages, 6 figure