arXiv subjects

F. Gran

Publications and source records attributed to F. Gran.

At least 19 recordsLinked to original sources

arXiv2026

Extinction law and stellar mass in the Nuclear Bulge from kinematically-selected red clump stars

The Nuclear Bulge of the Milky Way harbors stellar populations that provide crucial insights into galaxy formation processes and serve as a nearby analog for understanding bulge formation in external galaxies. However, detailed studies of this region are severely hampered by extreme and highly variable interstellar extinction, which obscures the intrinsic stellar properties and impedes accurate stellar mass determinations. Our goal is to measure the extinction law towards the Nuclear Bulge and to estimate its stellar density. We developed a method to determine the extinction law towards the Nuclear Bulge by kinematically selecting red clump stars belonging to this region. We created a high-spatial resolution reddening map, and computed stellar mass with completeness-corrected red clump star counts, scaled from empirical measurements. We find a total-to-selective extinction ratio of $\mathrm{A_K/{E_{H-K}} = 1.259 \pm 0.074}$, and an extinction ratio of $\mathrm{A_H/A_K = 1.794 \pm 0.046}$, which are consistent with previous works. The high-spatial resolution reddening map shows clear filamentary structures, and a gradient in the extinction over the giant molecular cloud G0.253+0.016 (i.e., the Brick). From the star counts, we measured a stellar mass of $\mathrm{12.2~\pm2.6\times10^8~M_{\odot}}$ for the Nuclear Bulge, in agreement with other mass estimates.

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arXiv2026

The metal-poor tail of the APOGEE survey II. Spectral analysis of Mg and Si in very metal-poor APOGEE spectra

H-band spectra contain very limited spectral information for stars at the most metal-poor tail ( Fe/H < -2.5) because the available Fe lines in FGK stars in this wavelength range are weak. The first paper in this series successfully identified a sample of 327 very metal-poor stars (with [Fe/H] < -2) from the APOGEE database, 289 of which are on the red giant branch. The spectra of these stars were not properly analysed by the APOGEE main pipeline because they are very metal poor. In this work, we measure metallicities for these stars using the abundances of the elements Mg and Si. We demonstrate that the absorption lines of the elements Mg and Si are of good quality despite the challenging combination of (low) metallicity, wavelength regime, spectral resolution, and signal-to-noise ratios available for these spectra. A specialised pipeline was designed to measure the abundance of Mg and Si in APOGEE spectra and yielded a robust estimate of the overall metallicity. In order to provide reliable measurements, we tested three different sets of assumptions for Mg and Si enhancement. We present Mg and Si abundances as well as overall metallicities for 327 stars, all of which had previously gotten null values from the main APOGEE pipeline for either the calibrated M/H or [Fe/H] . The typical uncertainties for our measurements are 0.2 dex. We found five stars in our sample with unusual [Si/Mg] abundances higher than 0.5, and we connect this signature to globular cluster stars, and this might be related to specific supernova events. Our data suggest a concentration of high [Si/Mg] stars in the Sextans dwarf galaxy. Other dwarf galaxies are found to agree well with results in the literature. Our derived metallicities range between -3.1 $\leq$ [M/H] $\leq$ -2.25, thereby pushing the metal-poor tail of APOGEE results down by 0.6 dex.

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arXiv2025

Survey of Surveys. II. Stellar parameters for 23 millions of stars

In the current panorama of large surveys, the vast amount of data obtained with different methods, data types, formats, and stellar samples, is making an efficient use of the available information difficult. The Survey of Surveys is a project to critically compile survey results in a single catalogue, facilitating the scientific use of the available information. In this second release, we present two new catalogs of stellar parameters (Teff, logg, and [Fe/H]). To build the first catalog, SoS-Spectro, we calibrated internally and externally stellar parameters from five spectroscopic surveys (APOGEE, GALAH, Gaia-ESO, RAVE, and LAMOST) and externally on the PASTEL database. The second catalog, SoS-ML catalog, is obtained by using SoS-Spectro as a reference to train a multi-layer perceptron, which predicts stellar parameters based on two photometric surveys, SDSS and SkyMapper. As a novel approach, we build on previous parameters sets, from Gaia DR3 and Andrae et al. (2023), aiming to improve their precision and accuracy. We obtain a catalog of stellar parameters for around 23 millions of stars, which we make publicly available. We validate our results with several comparisons with other machine learning catalogs, stellar clusters, and astroseismic samples. We find substantial improvements in the parameters estimates compared to other Machine Learning methods in terms of precision and accuracy, especially in the metal-poor range, as shown in particular when validating our results with globular clusters. We believe that there are two reasons behind our improved results at the low-metallicity end: first, our use of a reference catalog, the SoS-Spectro, which is calibrated using high-resolution spectroscopic data; and second, our choice to build on pre-existing parameter estimates from em Gaia and Andrae et al., rather than attempting to obtain our predictions from survey data alone.

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arXiv2024

The Pristine survey. XXVI. Chemical abundances of subgiant stars of the extremelymetal-poor stream C-19

Context: The C-19 stellar stream is the most metal-poor stream known to date. While its width and velocity dispersion indicate a dwarf galaxy origin, its metallicity spread and abundance patterns are more similar to those of globular clusters (GCs). If it is indeed of GC origin, its extremely low metallicity ([Fe/H]=-3.4, estimated from giant stars) implies that these stellar systems can form out of gas that is as extremely poor in metals as this. Previously, only giant stream stars were observed spectroscopically, although the majority of stream stars are unevolved stars. Aims: We pushed the spectroscopic observations to the subgiant branch stars ($G\approx 20$) in order to consolidate the chemical and dynamical properties of C-19. Methods: We used the high-efficiency spectrograph X-shooter fed by the ESO 8.2 m VLT telescope to observe 15 candidate subgiant C-19 members. The spectra were used to measure radial velocities and to determine chemical abundances using the \mygi\ code. Results; We developed a likelihood model that takes metallicity and radial velocities into account. We conclude that 12 stars are likely members of C-19, while 3 stars (S05, S12, and S13) are likely contaminants. When these 3 stars are excluded, our model implies a mean metallicity $\rm \langle [Fe/H]\rangle = -3.1\pm 0.1$, the mean radial velocity is $\langle v_r\rangle = -192\pm3$ kms$^{-1}$, and the velocity dispersion is $\sigma_{vr} = 5.9^{+3.6}_{-5.9}$ kms$^{-1}$. This all agrees within errors with previous studies. The A(Mg) of a sample of 15 C-19 members, including 6 giant stars, shows a standard deviation of 0.44 dex, and the mean uncertainty on Mg is 0.25 dex. Conclusions: Our preferred interpretation of the current data is that C-19 is a disrupted GC. We cannot completely rule out the possibility that the GC could have belonged to a dwarf galaxy that contained more metal-rich stars, however. This scenario would explain the radial velocity members at higher metallicity, as well as the width and velocity dispersion of the stream. In either case, a GC formed out of gas as poor in metals as these stars seems necessary to explain the existence of C-19. The possibility that no GC was associated with C-19 cannot be ruled out either.

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arXiv2024

Unveiling VVV/WISE Mira variables on the far side of the Galactic disk: Distances, kinematics and a new extinction law

The structure and kinematics of the Milky Way disk are largely inferred from the solar vicinity. To gain a comprehensive understanding, it is essential to find reliable tracers in less-explored regions like the bulge and the far side of the disk. Mira variables, which are well-studied and bright standard candles, offer an excellent opportunity to trace intermediate and old populations in these complex regions. We aim to isolate a clean sample of Miras in the Vista Variables in the V\'ia L\'actea survey using Gaussian process algorithms. This sample will be used to study intermediate and old age populations in the Galactic bulge and far disk. Near- and mid-infrared time-series photometry were processed using Gaussian Process algorithms to identify Mira variables and model their light curves. We calibrated selection criteria with a visually inspected sample to create a high-purity sample of Miras, integrating multi-band photometry and kinematic data from proper motions. We present a catalog of 3602 Mira variables. By analyzing photometry, we classify them by O-rich or C-rich surface chemistry and derive selective-to-total extinction ratios of $A_{K_{s}}/E(J - K_{s}) = 0.471 \pm 0.01$ and $A_{K_{s}}/E(H - K_{s}) = 1.320 \pm 0.020$. Using the Mira period-age relation, we find evidence supporting the inside-out formation of the Milky Way disk. The distribution of proper motions and distances aligns with the Galactic rotation curve and disk kinematics. We extend the rotation curve up to R$_{\rm GC} \sim 17 \ \rm{kpc}$ and find no strong evidence of the nuclear stellar disk in our Mira sample. This study constitutes the largest catalog of variable stars on the far side of the Galactic disk to date.

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arXiv2024

VVV catalog of ab-type RR Lyrae in the inner Galactic bulge

Context. Observational evidence has accumulated in the past years, showing that the Galactic bulge includes two populations, a metal poor and a metal rich one that, in addition to a different metallicity, show different alpha over iron abundances, spatial distribution, and kinematics. While the metal rich, barred component has been fairly well characterized, the metal poor, spheroidal component has been more elusive and harder to describe. RR Lyrae variables are clean tracers of the old bulge component, and they are, on average, more metal poor than red clump stars. Aims. In the present paper, we provide a new catalog of 16488 ab type RR Lyrae variables in the bulge region within -10<l<10 and -2.8<b<2.8, extracted from multi epoch PSF photometry performed on VISTA Variable in the Via Lactea survey data. We used the catalog to constrain the shape of the old, metal poor, bulge stellar population. Methods. The identification of ab type RR Lyrae among a large sample of candidate variables of different types has been performed via a combination of a Random Forest classifier and visual inspection... [abridged] Results. We use the present catalog to derive the shape of their distribution around the Galactic Center, resulting in an elongated spheroid with projected axis ratio b/a~0.7 and inclination angle ~20 degrees. We discuss how observational biases such as errors on the distances and a non-uniform sampling in longitude, affect both the present measurements and previous ones, especially those based on red clump stars... [abridged] Conclusions. We publish a high purity RRab sample for future studies of the oldest Galactic bulge population, close to the midplane. We explore different choices for the period-luminosity-metallicity relation, highlighting how some of them introduce spurious trends of distances with either period or metallicity, or both... [abridged]

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arXiv2024

The VISTA Variables in the V\'ia L\'actea eXtended (VVVX) ESO public survey: Completion of the observations and legacy

The ESO public survey VISTA Variables in the V\'ia L\'actea (VVV) surveyed the inner Galactic bulge and the adjacent southern Galactic disk from $2009-2015$. Upon its conclusion, the complementary VVV eXtended (VVVX) survey has expanded both the temporal as well as spatial coverage of the original VVV area, widening it from $562$ to $1700$ sq. deg., as well as providing additional epochs in $JHK_{\rm s}$ filters from $2016-2023$. With the completion of VVVX observations during the first semester of 2023, we present here the observing strategy, a description of data quality and access, and the legacy of VVVX. VVVX took $\sim 2000$ hours, covering about 4% of the sky in the bulge and southern disk. VVVX covered most of the gaps left between the VVV and the VISTA Hemisphere Survey (VHS) areas and extended the VVV time baseline in the obscured regions affected by high extinction and hence hidden from optical observations. VVVX provides a deep $JHK_{\rm s}$ catalogue of $\gtrsim 1.5\times10^9$ point sources, as well as a $K_{\rm s}$ band catalogue of $\sim 10^7$ variable sources. Within the existing VVV area, we produced a $5D$ map of the surveyed region by combining positions, distances, and proper motions of well-understood distance indicators such as red clump stars, RR Lyrae, and Cepheid variables. In March 2023 we successfully finished the VVVX survey observations that started in 2016, an accomplishment for ESO Paranal Observatory upon 4200 hours of observations for VVV+VVVX. The VVV+VVVX catalogues complement those from the Gaia mission at low Galactic latitudes and provide spectroscopic targets for the forthcoming ESO high-multiplex spectrographs MOONS and 4MOST.

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arXiv2024

SpectroTranslator: a deep-neural network algorithm to homogenize spectroscopic parameters

The emergence of large spectroscopic surveys requires homogenising on the same scale the quantities they measure in order to increase their scientific legacy. We developed the SpectroTranslator, a data-driven deep neural network algorithm that can convert spectroscopic parameters from the base of one survey to another. The algorithm also includes a method to estimate the importance that the various parameters play in the conversion from base A to B. As a showcase, we apply the algorithm to transform effective temperature, surface gravity, metallicity, [Mg/Fe] and los velocity from the base of GALAH into the APOGEE base. We demonstrate the efficiency of the SpectroTranslator algorithm to translate the spectroscopic parameters from one base to another using parameters directly by the survey teams, and are able to achieve a similar performance than previous works that have performed a similar type of conversion but using the full spectrum rather than the spectroscopic parameters, allowing to reduce the computational time, and to use the output of pipelines optimized for each survey. By combining the transformed GALAH catalogue with the APOGEE catalogue, we study the distribution of [Fe/H] and [Mg/Fe] across the Galaxy, and we find that the median distribution of both quantities present a vertical asymmetry at large radii. We attribute it to the recent perturbations generated by the passage of a dwarf galaxy across the disc or by the infall of the Large Magellanic Cloud. Although several aspects still need to be refined, in particular how to deal in an optimal manner with regions of the parameter space meagrely populated by stars in the training sample, the SpectroTranslator already shows its capability and promises to play a crucial role in standardizing various spectroscopic surveys onto a unified basis.

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arXiv2023

The treasure behind the haystack: MUSE analysis of five recently discovered globular clusters

After the second data release of Gaia, the number of new globular cluster candidates has increased importantly. However, most of them need to be properly characterised, both spectroscopically and photometrically, by means of radial velocities, metallicities, and deeper photometric observations. Our goal is to provide an independent confirmation of the cluster nature of Gran 4, a recently discovered globular cluster, with follow-up spectroscopic observations. The derived radial velocity for individual stars, coupled with proper motions, allows us to isolate cluster members from field stars, while the analysis of their spectra allows us to derive metallicities. By including in the analysis the recently confirmed clusters Gran 1, 2, 3, and 5, we aim to completely characterise the sample presented in Gran et al. 2022. Using Gaia DR3 and VVV catalogue data and MUSE@VLT observations, we performed a selection of cluster members based on their proper motions, radial velocities and their position in colour-magnitude diagrams. Furthermore, full spectral synthesis was performed on the cluster members, extracting surface parameters and metallicity from MUSE spectra. Finally, a completeness estimation was performed on the total globular cluster population of the Milky Way. We confirm the nature of Gran 4, a newly discovered globular cluster behind the Galactic bulge, with a mean radial velocity of ${\rm RV} = -265.28 \pm 3.92$ km s$^{-1}$ and a mean metallicity of ${\rm [Fe/H] = -1.72 \pm 0.32}$ dex. Additionally, independent measurements of the metallicities were derived for Gran 1, 2, 3, and 5. We also revise the observational lower mass limit for a globular cluster to survive in the bulge/disk environment. We estimate that $\sim 12-26$ globular clusters have still to be discovered on the other side of the Galaxy (i.e., behind the bulge/bar/disk), up to 20 kpc.

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arXiv2021

Hidden in the Haystack: Low-luminosity globular clusters towards the Milky Way bulge

Recent wide-area surveys have enabled us to study the Milky Way with unprecedented detail. Its inner regions, hidden behind dust and gas, have been partially unveiled with the arrival of near-IR photometric and spectroscopic datasets. Among recent discoveries, there is a population of low-mass globular clusters, known to be missing, especially towards the Galactic bulge. In this work, five new low-luminosity globular clusters located towards the bulge area are presented. They were discovered by searching for groups in the multi-dimensional space of coordinates, colours, and proper motions from the Gaia EDR3 catalogue and later confirmed with deeper VVV survey near-IR photometry. The clusters show well-defined red-giant branches and, in some cases, horizontal branches with their members forming a dynamically coherent structure in proper motion space. Four of them were confirmed by spectroscopic follow-up with the MUSE instrument on the ESO VLT. Photometric parameters were derived, and when available, metallicities, radial velocities and orbits were determined. The new clusters Gran 1 and 5 are bulge globular clusters, while Gran 2, 3, and 4 present halo-like properties. Preliminary orbits indicate that Gran 1 might be related to the Main Progenitor, or the so-called ''low-energy'' group, while Gran 2, 3 and 5 appear to follow the Gaia-Enceladus-Sausage. This study demonstrates that the Gaia proper motions, combined with the spectroscopic follow-up and colour-magnitude diagrams, are required to confirm the nature of cluster candidates towards the inner Galaxy. High stellar crowding and differential extinction may hide other low-luminosity clusters.

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arXiv2021

APOGEE view of the globular cluster NGC 6544

The second phase of the APOGEE survey is providing near-infrared, high-resolution, high signal-to-noise spectra of stars in the halo, disk, bar and bulge of the Milky Way. The near-infrared spectral window is especially important in the study of the Galactic bulge, where stars are obscured by the dust and gas of the disk in its line-of-sight. We present a chemical characterisation of the globular cluster NGC 6544 with high-resolution spectroscopy. The characterisation of the cluster chemical fingerprint, given its status of "interloper" towards the Galactic bulge and clear signatures of tidal disruption in its core is crucial for future chemical tagging efforts. Cluster members were selected from the DR16 of the APOGEE survey, using chemo-dynamical criteria of individual stars. A sample of 23 members of the cluster was selected. An analysis considering the intra-cluster abundance variations, known anticorrelations is given. According to the RGB content of the cluster, the iron content and $\alpha$-enhancement are [Fe/H] $= -1.44 \pm 0.04$ dex and [$\alpha$/Fe] $= 0.20 \pm 0.04$ dex, respectively. Cluster members show a significant spread in [Fe/H] and [Al/Fe] that is larger than expected based on measurement errors. An [Al/Fe] spread, signal of an Mg-Al anticorrelation is observed and used to constraint the cluster mass budget, along with C, N, Mg, Si, K, Ca, and Ce element variations are discussed. Across all the analysed evolutionary stages (RGB and AGB), about $\sim2/3$ (14 out of 23) show distinct chemical patterns, possibly associated with second-generation stars.

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arXiv2019

Globular cluster candidates in the Galactic bulge: Gaia and VVV view of the latest discoveries

Thanks to the recent wide-area photometric surveys, the number of star cluster candidates have risen exponentially in the last few years. Most detections, however, are based only on the presence of an overdensity of stars in a given region, or an overdensity of variable stars, regardless of their distance. As candidates, their detection has not been dynamically confirmed. Therefore, it is currently unknown how many, and which ones, of the published candidates, are true clusters, and which ones are chance alignments. We present a method to detect and confirm star clusters based on the spatial distribution, coherence in motion and appearance on the color-magnitude diagram. We explain and apply it to one new star cluster, and several candidate star clusters published in the literature. The presented method is based on data from the Second Data Release of Gaia complemented with data from the VISTA Variables in the V\'ia L\'actea survey for the innermost bulge regions. It consists of a nearest neighbors algorithm applied simultaneously over spatial coordinates, star color, and proper motions, in order to detect groups of stars that are close in the sky, move coherently and define narrow sequences in the color-magnitude diagram, such as a young main sequence or a red giant branch. When tested in the bulge area ($-10<\ell\ {\rm (deg)}<+10$; $-10<b\ {\rm (deg)}<+10$) the method successfully recovered several known young and old star clusters. We report here the detection of one new, likely old star cluster, while deferring the others to a forthcoming paper. Additionally, the code has been applied to the position of 93 candidate star clusters published in the literature. As a result, only two of them are confirmed as coherently moving groups of stars at their nominal positions.

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arXiv2017

Proper motions in the VVV Survey: Results for more than 15 million stars across NGC 6544

Context: In the last six years, the VVV survey mapped 562 sq. deg. across the bulge and southern disk of the Galaxy. However, a detailed study of these regions, which includes $\sim 36$ globular clusters (GCs) and thousands of open clusters is by no means an easy challenge. High differential reddening and severe crowding along the line of sight makes highly hamper to reliably distinguish stars belonging to different populations and/or systems. Aims: The aim of this study is to separate stars that likely belong to the Galactic GC NGC 6544 from its surrounding field by means of proper motion (PM) techniques. Methods: This work was based upon a new astrometric reduction method optimized for images of the VVV survey. Results: Photometry over the six years baseline of the survey allowed us to obtain a mean precision of $\sim0.51$ mas/yr, in each PM coordinate, for stars with Ks < 15 mag. In the area studied here, cluster stars separate very well from field stars, down to the main sequence turnoff and below, allowing us to derive for the first time the absolute PM of NGC 6544. Isochrone fitting on the clean and differential reddening corrected cluster color magnitude diagram yields an age of $\sim$ 11-13 Gyr, and metallicity [Fe/H] = -1.5 dex, in agreement with previous studies restricted to the cluster core. We were able to derive the cluster orbit assuming an axisymmetric model of the Galaxy and conclude that NGC 6544 is likely a halo GC. We have not detected tidal tail signatures associated to the cluster, but a remarkable elongation in the galactic center direction has been found. The precision achieved in the PM determination also allows us to separate bulge stars from foreground disk stars, enabling the kinematical selection of bona fide bulge stars across the whole survey area. Our results show that VVV data is perfectly suitable for this kind of analysis.

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arXiv2017

Near-IR period-luminosity relations for pulsating stars in $\omega$ Centauri (NGC 5139)

$\omega$ Centauri (NGC 5139) hosts hundreds of pulsating variable stars of different types, thus representing a treasure trove for studies of their corresponding period-luminosity (PL) relations. Our goal in this study is to obtain the PL relations for RR Lyrae, and SX Phoenicis stars in the field of the cluster, based on high-quality, well-sampled light curves in the near-infrared (IR). $\omega$ Centauri was observed using VIRCAM mounted on VISTA. A total of 42 epochs in $J$ and 100 epochs in $K_{\rm S}$ were obtained, spanning 352 days. Point-spread function photometry was performed using DoPhot and DAOPHOT in the outer and inner regions of the cluster, respectively. Based on the comprehensive catalogue of near-IR light curves thus secured, PL relations were obtained for the different types of pulsators in the cluster, both in the $J$ and $K_{\rm S}$ bands. This includes the first PL relations in the near-IR for fundamental-mode SX Phoenicis stars. The near-IR magnitudes and periods of Type II Cepheids and RR Lyrae stars were used to derive an updated true distance modulus to the cluster, with a resulting value of $(m-M)_0 = 13.708 \pm 0.035 \pm 0.10$ mag, where the error bars correspond to the adopted statistical and systematic errors, respectively. Adding the errors in quadrature, this is equivalent to a heliocentric distance of $5.52\pm 0.27$ kpc.

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arXiv2016

Discovery of RR Lyrae Stars in the Nuclear Bulge of the Milky Way

Galactic nuclei, like the one of the Milky Way, are extreme places with high stellar densities and, in most cases, hosting a supermassive black hole. One of the scenarios proposed for the formation of the Galactic nucleus is by merging of primordial globular clusters (Capuzzo-Dolcetta 1993). An implication of this model is that this region should host stars characteristically found in old Milky Way globular clusters. RR Lyrae stars are primary distance indicators, well known representatives of old and metal-poor stellar populations, and therefore regularly found in globular clusters. Here we report the discovery of a dozen RR Lyrae ab-type stars in the vicinity of the Galactic center, i.e. in the so-called nuclear stellar bulge of the Milky Way. This discovery provides the first direct observational evidence that the Galactic nuclear stellar bulge contains ancient stars (>10 Gyr old). Based on this we conclude that merging globular clusters likely contributed to building-up the high stellar density in the nuclear stellar bulge of the Milky Way.

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arXiv2016

Mapping the outer bulge with RRab stars from the VVV Survey

The VISTA Variables in the V\'ia L\'actea (VVV) is a near-IR time-domain survey of the Galactic bulge and southern plane. One of the main goals of this survey is to reveal the 3D structure of the Milky Way through their variable stars. Particularly the RR Lyrae stars have been massively discovered in the inner regions of the bulge ($-8^\circ \lesssim b \lesssim -1^\circ$) by optical surveys such as OGLE and MACHO but leaving an unexplored window of more than $\sim 47$ sq deg ($-10.0^\circ \lesssim \ell \lesssim +10.7^\circ$ and $-10.3^\circ \lesssim b \lesssim -8.0^\circ$) observed by the VVV Survey. Our goal is to characterize the RR Lyrae stars in the outer bulge in terms of their periods, amplitudes, Fourier coefficients, and distances, in order to evaluate the 3D structure of the bulge in this area. The distance distribution of RR Lyrae stars will be compared to the one of red clump stars that is known to trace a X-shaped structure in order to determine if these two different stellar populations share the same Galactic distribution. We report the detection of more than 1000 RR Lyrae ab-type stars in the VVV Survey located in the outskirts of the Galactic bulge. Some of them are possibly associated with the Sagittarius Dwarf Spheroidal Galaxy. We calculated colors, reddening, extinction, and distances of the detected RR Lyrae stars in order to determine the outer bulge 3D structure. Our main result is that, at the low galactic latitudes mapped here, the RR Lyrae stars trace a centrally concentrated spheroidal distribution. This is a noticeably different spatial distribution to the one traced by red clump stars known to follow a bar and X-shape structure. We estimate the completeness of our RRab sample in $80\%$ for $K_{\rm s}\lesssim15$ mag.

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arXiv2015

VVV Survey Observations of a Microlensing Stellar Mass Black Hole Candidate in the Field of the Globular Cluster NGC 6553

We report the discovery of a large timescale candidate microlensing event of a bulge stellar source based on near-infrared observations with the VISTA Variables in the Via Lactea Survey (VVV). The new microlensing event is projected only 3.5 arcmin away from the center of the globular cluster NGC 6553. The source appears to be a bulge giant star with magnitude Ks = 13.52, based on the position in the color-magnitude diagram. The foreground lens may be located in the globular cluster, which has well-known parameters such as distance and proper motions. If the lens is a cluster member, we can directly estimate its mass simply following Paczynski et al. (1996) which is a modified version of the more general case due to Refsdal. In that case, the lens would be a massive stellar remnant, with M = 1.5-3.5 Msun. If the blending fraction of the microlensing event appears to be small, and this lens would represent a good isolated black hole (BH) candidate, that would be the oldest BH known. Alternative explanations (with a larger blending fraction) also point to a massive stellar remnant if the lens is located in the Galactic disk and does not belong to the globular cluster.

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arXiv2015

An Updated Census of RR~Lyrae Stars in the Globular Cluster $\omega$~Centauri (NGC\,5139)

[ABRIDGED] $\omega$ Centauri (NGC 5139) contains large numbers of variable stars of different types and, in particular, more than a hundred RR Lyrae stars. We have conducted a variability survey of $\omega$ Cen in the NIR, using ESO's 4.1m Visible and Infrared Survey Telescope for Astronomy (VISTA). This is the first paper of a series describing our results. $\omega$ Cen was observed using VIRCAM mounted on VISTA. A total of 42 and 100 epochs in $J$ and $K_{\rm S}$, respectively, were obtained, distributed over a total timespan of 352 days. PSF photometry was performed, and periods of the known variable stars were improved when necessary using an ANOVA analysis. An unprecedented homogeneous and complete NIR catalogue of RR Lyrae stars in the field of $\omega$ Cen was collected, allowing us to study, for the first time, all the RR Lyrae stars associated to the cluster, except 4 located far away from the cluster center. Membership status, subclassifications between RRab and RRc subtypes, periods, amplitudes, and mean magnitudes were derived for all the stars in our sample. Additionally, 4 new RR Lyrae stars were discovered, 2 of them with high probability of being cluster members. The distribution of $\omega$ Cen stars in the Bailey (period-amplitude) diagram is also discussed. Reference lines in this plane, for both Oosterhoff type I (OoI) and II (OoII) components, are provided. In the present paper, we clarify the status of many (candidate) RR Lyrae stars that had been unclear in previous studies. This includes stars with anomalous positions in the color-magnitude diagram, uncertain periods or/and variability types, and possible field interlopers. We conclude that $\omega$ Cen hosts a total of 88 RRab and 101 RRc stars, for a grand total of 189 likely members. We confirm that most RRab stars in the cluster belong to an OoII component, as previously found using visual data.

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