55 research outputs found
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 . Multiplying the PM uncertainties by brings the
Gaia (VIRAC2) PMs into a agreement with HST PMs. The factor 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, and are underestimated by up to a factor of 4
in fields with more than 300 Gaia DR3 sources per arcmin. 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 value. At the faint end (G>19), is
close to 1, meaning that the PMs already fully agree with the HST measurements
within . 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 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
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