arXiv subjects

O. A. Gonzalez

Publications and source records attributed to O. A. Gonzalez.

43 records · Page 3Linked to original sources

arXiv2011

VVV DR1: The First Data Release of the Milky Way Bulge and Southern Plane from the Near-Infrared ESO Public Survey VISTA Variables in the Via Lactea

The ESO Public Survey VISTA Variables in the Via Lactea (VVV) started in 2010. VVV targets 562 sq. deg in the Galactic bulge and an adjacent plane region and is expected to run for ~5 years. In this paper we describe the progress of the survey observations in the first observing season, the observing strategy and quality of the data obtained. The observations are carried out on the 4-m VISTA telescope in the ZYJHKs filters. In addition to the multi-band imaging the variability monitoring campaign in the Ks filter has started. Data reduction is carried out using the pipeline at the Cambridge Astronomical Survey Unit. The photometric and astrometric calibration is performed via the numerous 2MASS sources observed in each pointing. The first data release contains the aperture photometry and astrometric catalogues for 348 individual pointings in the ZYJHKs filters taken in the 2010 observing season. The typical image quality is ~0.9-1.0". The stringent photometric and image quality requirements of the survey are satisfied in 100% of the JHKs images in the disk area and 90% of the JHKs images in the bulge area. The completeness in the Z and Y images is 84% in the disk, and 40% in the bulge. The first season catalogues contain 1.28x10^8 stellar sources in the bulge and 1.68x10^8 in the disk area detected in at least one of the photometric bands. The combined, multi-band catalogues contain more than 1.63x10^8 stellar sources. About 10% of these are double detections due to overlapping adjacent pointings. These overlapping multiple detections are used to characterise the quality of the data. The images in the JHKs bands extend typically ~4 mag deeper than 2MASS. The magnitude limit and photometric quality depend strongly on crowding in the inner Galactic regions. The astrometry for Ks=15-18 mag has rms ~35-175 mas.

astro-ph.GA↗

arXiv2011

The inner Galactic bar traced by the VVV survey

We use the VVV survey observations in bulge regions close to the Galactic plane to trace the bar inclination at the Galactic latitude b\sim\pm1° and to investigate a distinct structure in the inner regions of the bar that was previously detected at positive latitude (b=+1°). We use the (J-Ks) colors of the red clump stars to obtain reddening values on 6 x 6 arcmin scale, minimizing the problems arising from differential extinction. Dereddened magnitudes are then used to build the luminosity function of the bulge in regions of ~0.4 sq deg to obtain the mean red clump magnitudes. These are used as distance indicators to trace the bar structure. The luminosity function clearly shows the red clump mean magnitude variation with longitude, as expected from a large scale bar oriented towards us at positive Galactic longitude, with a dereddened magnitude varying from Ks_0=13.4 at l=-10° to Ks_0=12.4 at l=+10°. We detect a change in the orientation of the bar in the central regions with |l|<4° at b=\pm1°, in agreement with results obtained at positive latitudes by other authors. Our results are based on a different dataset and at different latitude, which shows that this change in the bar orientation is real. This suggests that there is an inner structure distinct to the large-scale Galactic bar, with a different orientation angle. This inner structure could be a secondary, inner bar, with a semi-major axis of ~500 pc that is symmetric with respect to the Galactic plane.

astro-ph.GA↗

arXiv2011

Reddening and metallicity maps of the Milky Way Bulge from VVV and 2MASS. I.The method and minor axis maps

We present a method to obtain reddening maps and to trace structure and metallicity gradients of the bulge using data from the recently started ESO public survey Vista Variables in the Via Lactea (VVV). We derive the mean J-Ks color of the red clump (RC) giants in 1835 subfields in the Bulge region with -8<b<-0.4 and 0.2<l<1.7, and compare it to the color of RC stars in Baade's Window for which we adopt E(B-V)=0.55. This allows us to derive the reddening map on a small enough scale to minimize the problems arising from differential extinction. The dereddened magnitudes are then used to build the bulge luminosity function in regions of 0.4 x 0.4 deg to obtain the mean RC magnitudes. These are used as distance indicator in order to trace the bulge structure. Finally, for each subfield we derive photometric metallicities through interpolation of red giant branch colors on a set of empirical ridge lines. The photometric metallicity distributions are compared to metallicity distributions obtained from high resolution spectroscopy in the same regions. The reddening determination is sensitive to small scale variations which are clearly visible in our maps. The luminosity function clearly shows the double RC recently discovered in 2MASS and OGLE III datasets, hence allowing to trace the X-shape morphology of the bulge. Finally, the mean of the derived photometric metallicity distributions are in remarkable agreement with those obtained from spectroscopy. The VVV survey presents a unique tool to map the bulge properties by means of the consistent method presented here. The remarkable agreement between our results and those presented in literature for the minor axis allows us to safely extend our method to the whole region covered by the survey.

astro-ph.GA↗

arXiv2011

Alpha element abundances and gradients in the Milky Way bulge from FLAMES-GIRAFFE spectra of 650 K giants

We obtained FLAMES-GIRAFFE spectra (R=22,500) at the ESO Very Large Telescope for 650 bulge red giant branch (RGB) stars and performed spectral synthesis to measure Mg, Ca, Ti, and Si abundances. This sample is composed of 474 giant stars observed in 3 fields along the minor axis of the Galactic bulge and at latitudes b=-4, b=-6, b=-12. Another 176 stars belong to a field containing the globular cluster NGC 6553, located at b=-3 and 5 degrees away from the other three fields along the major axis. Our results confirm, with large number statistics, the chemical similarity between the Galactic bulge and thick disk, which are both enhanced in alpha elements when compared to the thin disk. In the same context, we analyze [alpha/Fe] vs. [Fe/H] trends across different bulge regions. The most metal rich stars, showing low [alpha/Fe] ratios at b=-4 disappear at higher Galactic latitudes in agreement with the observed metallicity gradient in the bulge. Metal-poor stars ([Fe/H]<-0.2) show a remarkable homogeneity at different bulge locations. We have obtained further constrains for the formation scenario of the Galactic bulge. A metal-poor component chemically indistinguishable from the thick disk hints for a fast and early formation for both the bulge and the thick disk. Such a component shows no variation, neither in abundances nor kinematics, among different bulge regions. A metal-rich component showing low [alpha/Fe] similar to those of the thin disk disappears at larger latitudes. This allows us to trace a component formed through fast early mergers (classical bulge) and a disk/bar component formed on a more extended timescale.

astro-ph.GA↗

arXiv2011

Lithium-rich giants in the Galactic thick disk

Context: Lithium is a fragile element, which is easily destroyed in the stellar interior. The existence of lithium-rich giants still represents a challenge for stellar evolution models. Aims: We have collected a large database of high-resolution stellar spectra of 824 candidate thick-disk giants having 2\,MASS photometry and proper motions measured by the Southern Proper-Motion Program (SPM). In order to investigate the nature of Li-rich giants, we searched this database for giants presenting a strong Li\,I resonance line. Methods: We performed a chemical abundance analysis on the selected stars with the MOOG code along with proper ATLAS-9 model atmospheres. The iron content and atmospheric parameters were fixed by using the equivalent width of a sample of Fe lines. We also derive abundances for C, N, and O and measure or derive lower limits on the $^{12}$C/$^{13}$C isotopic ratios, which is a sensible diagnostic of the stars evolutionary status. Results: We detected five stars with a lithium abundance higher than 1.5, i.e. Li-rich according to the current definition. One of them (SPM-313132) has A(Li)$>$3.3 and, because of this, belongs to the group of the rare super Li-rich giants. Its kinematics makes it a likely thin-disk member and its atmospheric parameters are compatible with it being a 4\,M$_\odot$ star either on the red giant branch (RGB) or the early asymptotic giant branch. This object is the first super Li-rich giant detected at this phase. The other four are likely low-mass thick-disk stars evolved past the RGB luminosity bump, as determined from their metallicities and atmospheric parameters. The most evolved of them lies close to the RGB-tip. It has A(Li)$>$2.7 and a low $^{12}$C/$^{13}$C isotopic ratio, close to the cool bottom processing predictions.

astro-ph.SR↗

arXiv2009

VISTA Variables in the Via Lactea (VVV): The public ESO near-IR variability survey of the Milky Way

We describe the public ESO near-IR variability survey (VVV) scanning the Milky Way bulge and an adjacent section of the mid-plane where star formation activity is high. The survey will take 1929 hours of observations with the 4-metre VISTA telescope during five years (2010-2014), covering ~10^9 point sources across an area of 520 deg^2, including 33 known globular clusters and ~350 open clusters. The final product will be a deep near-IR atlas in five passbands (0.9-2.5 microns) and a catalogue of more than 10^6 variable point sources. Unlike single-epoch surveys that, in most cases, only produce 2-D maps, the VVV variable star survey will enable the construction of a 3-D map of the surveyed region using well-understood distance indicators such as RR Lyrae stars, and Cepheids. It will yield important information on the ages of the populations. The observations will be combined with data from MACHO, OGLE, EROS, VST, Spitzer, HST, Chandra, INTEGRAL, WISE, Fermi LAT, XMM-Newton, GAIA and ALMA for a complete understanding of the variable sources in the inner Milky Way. This public survey will provide data available to the whole community and therefore will enable further studies of the history of the Milky Way, its globular cluster evolution, and the population census of the Galactic Bulge and center, as well as the investigations of the star forming regions in the disk. The combined variable star catalogues will have important implications for theoretical investigations of pulsation properties of stars.

astro-ph.GA↗

arXiv2009

Li-rich RGB stars in the Galactic Bulge

We present Lithium abundance determination for a sample of K giant stars in the galactic bulge. The stars presented here are the only 13 stars with detectable Lithium line (6767.18 A) among ~400 stars for which we have spectra in this wavelength range, half of them in Baade's Window (b=-4) and half in a field at b=-6. The stars were observed with the GIRAFFE spectrograph of FLAMES@VLT, with a spectral resolution of R~20,000. Abundances were derived via spectral synthesis and the results are compared with those for stars with similar parameters, but no detectable Li line. We find 13 stars with a detectable Li line, among which 2 have abundances A(Li)>2.7. No clear correlations were found between the Li abundance and those of other elements. With the exception of the two most Li rich stars, the others follow a fairly tight A(Li)-T_eff correlation. It would seems that there must be a Li production phase during the red giant branch (RGB), acting either on a very short timescale, or selectively only in some stars. The proposed Li production phase associated with the RGB bump cannot be excluded, although our targets are significantly brighter than the predicted RGB bump magnitude for a population at 8 kpc

astro-ph.SR↗