Carla Cacciari

The RR Lyrae period–K-luminosity relation for globular clusters: an observational approach

The period-metallicity-K-band luminosity (PL K ) relation for RR Lyrae stars in 15 Galactic globular clusters and in the Large Magellanic Cloud (LMC) globular cluster Reticulum has been derived. It is based on accurate near-infrared (K) photometry combined with Two-Micron All-Sky Survey (2MASS) and other literature data. The PL K relation has been calibrated and compared with the previous empirical and theoretical determinations in literature. The zero point of the absolute calibration has been obtained from the K magnitude of RR Lyr whose distance modulus has been measured via trigonometric parallax with Hubble Space Telescope (HST). Using this relation, we obtain a distance modulus to the LMC of (m - M) 0 = 18.54 ± 0.15 mag, in good agreement with recent determinations based on the analysis of Cepheid variable stars.

Globular Cluster Distances from RR Lyrae Stars

The most common methods to derive the distance to globular clusters using RR Lyrae variables are reviewed, with a special attention to those that have experienced significant improvement in the past few years. From the weighted average of these most recent determinations the absolute magnitude of the RR Lyrae stars at [Fe/H] = −1.5 is M V = 0.59 ± 0.03, corresponding to a distance modulus for the LMC (m − M)0 = 18.48 ± 0.05.

The infrared JHK light curves of RR Lyr

We present infrared JHK time series photometry of the variable star RR Lyr, that allows us to construct the first complete and accurate infrared light curves for this star. The derived mean magnitudes areJ �= 6.74 ± 0.02, � H �= 6.60 ± 0.03 andK �= 6.50 ± 0.02. TheK� magnitude is used to estimate the reddening, the mass, the mean luminosity and the temperature of this variable star. The use of these RR Lyr data provides a more accurate absolute calibration of the P-LK-(Fe/H) relation, and a distance modulus (m − M)0 = 18.48 ± 0.11 to the globular cluster Reticulum in the Large Magellanic Cloud.

Gaia Data Release 1 - The photometric data

Context. This paper presents an overview of the photometric data that are part of the first Gaia data release. Aims. The principles of the processing and the main characteristics of the Gaia photometric data are presented. Methods. The calibration strategy is outlined briefly and the main properties of the resulting photometry are presented. Results. Relations with other broadband photometric systems are provided. The overall precision for the Gaia photometry is shown to be at the milli-magnitude level and has a clear potential to improve further in future releases.

Gaia Data Release 1 - Principles of the photometric calibration of the G band

Context. Gaia is an ESA cornerstone mission launched on 19 December 2013 aiming to obtain the most complete and precise 3D map of our Galaxy by observing more than one billion sources. This paper is part of a series of documents explaining the data processing and its results for Gaia Data Release 1, focussing on the G band photometry. Aims. This paper describes the calibration model of the Gaia photometric passband for Gaia Data Release 1. Methods. The overall principle of splitting the process into internal and external calibrations is outlined. In the internal calibration, a self-consistent photometric system is generated. Then, the external calibration provides the link to the absolute photometric flux scales. Results. The Gaia photometric calibration pipeline explained here was applied to the first data release with good results. Details are given of the various calibration elements including the mathematical formulation of the models used and of the extraction and preparation of the required input parameters (e.g. colour terms). The external calibration in this first release provides the absolute zero point and photometric transformations from the Gaia G passband to other common photometric systems. Conclusions. This paper describes the photometric calibration implemented for the first Gaia data release and the instrumental effects taken into account. For this first release no aperture losses, radiation damage, and other second-order effects have not yet been implemented in the calibration.

Gaia Data Release 2. Photometric content and validation

This work presents results from the European Space Agency (ESA) space mission Gaia. Gaia data are being processed by the Gaia Data Processing and Analysis Consortium (DPAC). Funding for the DPAC is provided by national institutions, in particular the institutions participating in the Gaia MultiLateral Agreement (MLA). The Gaia mission website is https://www.cosmos.esa.int/gaia. The Gaia Archive website is http://gea.esac.esa.int/archive/. This work has been supported by the United Kingdom Rutherford Appleton Laboratory, the United Kingdom Science and Technology Facilities Council (STFC) through grant ST/L006553/1, and the United Kingdom Space Agency (UKSA) through grant ST/N000641/1. This work was supported by the MINECO (Spanish Ministry of Economy) through grant ESP2016-80079-C2-1-R (MINECO/FEDER, UE) and ESP2014-55996-C2-1-R (MINECO/FEDER, UE) and MDM-2014-0369 of ICCUB (Unidad de Excelencia “Maria de Maeztu”). This work was supported by the Italian funding agencies Agenzia Spaziale Italiana (ASI) through grants I/037/08/0, I/058/10/0, 2014-025- R.0, and 2014- 025-R.1.2015 to INAF and contracts I/008/10/0 and 2013/030/I.0 to ALTEC S.p.A and Istituto Nazionale di Astrofisica (INAF). This research has made use of the APASS database, located at the AAVSO web site. Funding for APASS has been provided by the Robert Martin Ayers Sciences Fund. We thank A. Vallenari for supplying us with spectra for the validation of the external flux calibration and passband determination

Star-to-Star Na and O Abundance Variations along the Red Giant Branch in NGC 2808*

We report for the first time Na and O abundances from high-resolution, high signal-to-noise ratio echelle spectra of 20 red giants in NGC 2808, taken as part of the Science Verification program of the FLAMES multiobject spectrograph at the ESO VLT. In these stars, spanning about 3 mag from the red giant branch (RGB) tip, large variations are detected in the abundances of oxygen and sodium, anticorrelated with each other; this is well-known evidence of proton-capture reactions at high temperatures in the ON and NeNa cycles. One star appears super-O-poor; if the extension of the Na-O anticorrelation is confirmed, NGC 2808 might reach O-depletion levels as large as those of M13. This result confirms our previous findings based on lower resolution spectra (Carretta et al.) of a large star-to-star scatter in proton-capture elements at all positions along the RGB in NGC 2808, with no significant evolutionary contribution. Finally, the average metallicity for NGC 2808 is [Fe/H] = -1.14 ± 0.01 dex (rms = 0.06) from 19 stars.

On the absolute magnitude of RR Lyrae stars : UU Ceti, RV Phoenicis, and W Tucanae

We present infrared JHK light curves for the RRab Lyrae stars UU Ceti, RV Phoenicis, and W Tucanae. These stars have similar periods, ∼0. d 6, and metallicities [Fe/H] ∼−1.00, −1.50, and −1.35, respectively. The infrared data, together with BVRI photometry and CORAVEL radial velocity data previously published by two of us, and Walraven photometry by Lub, are used to derive absolute magnitudes for the stars using two formulations of the Baade-Wesselink method: (1) the infrared flux version and (2) the surface brightness version

HST/ACS colour–magnitude diagrams of M 31 globular clusters

Aims. With the aim of increasing the sample of M 31 clusters for which a colour-magnitude diagram is available, we searched the HST archive for ACS images containing objects included in the Revised Bologna Catalogue of M 31 globular clusters***. Methods. Sixty-three such objects were found. We used the ACS images to confirm or revise their classification and were able to obtain useful CMDs for 11 old globular clusters and 6 luminous young clusters. We obtained simultaneous estimates of the distance, reddening, and metallicity of old clusters by comparing their observed field-decontaminated CMDs with a grid of template clusters of the Milky Way. We estimated the age of the young clusters by fitting with theoretical isochrones. Results. For the old clusters, we found metallicities in the range -0.4 ≤ [Fe/H] ≤ -1.9. The individual estimates generally agree with existing spectroscopic estimates. At least four of them display a clear blue horizontal branch, indicating ages ≳ 10 Gyr. All six candidate young clusters are found to have ages <1 Gyr. The photometry of the clusters is made publicly available through a dedicated web page. Conclusions. With the present work the total number of M 31 GCs with reliable optical CMD increases from 35 to 44 for the old clusters, and from 7 to 11 for the young ones. The old clusters show similar characteristics to those of the MW. We discuss the case of the cluster B407, with a metallicity [Fe/H] ≃ -0.6 and located at a large projected distance from the centre of M 31 (R p = 19.8 kpc) and from the major axis of the galaxy (Y = 11.3 kpc). Metal-rich globulars at large galactocentric distances are rare both in M 31 and in the Milky Way. B407, in addition, has a velocity in stark contrast with the rotation pattern shared by the bulk of M 31 clusters of similar metallicity. This, along with other empirical evidence, supports the hypothesis that the cluster (together with B403) is physically associated with a substructure in the halo of M 31 that has been interpreted as the relic of a merging event.

The population of variable stars in M54 (NGC 6715)

We present new B, V and I CCD time-series photometry for 177 variable stars in a 13 ′ × 13 ′ field centered on the globular cluster M54 (lying at the center of the Sagittarius dwarf spheroidal galaxy), 94 of which are newly identified variables. The total sample is composed of 2 anomalous Cepheids, 144 RR Lyrae stars (108 RR0 and 36 RR1), 3 SX Phoenicis, 7 eclipsing binaries (5 W UMA and 2 Algol binaries), 3 variables of uncertain classification and 18 long-period variables. The large majority of the RR Lyrae variables likely belong to M54. Ephemerides are provided for all the observed short-period variables. The pulsational properties of the M54 RR Lyrae variables are close to those of Oosterhoff I clusters, but a significant number of longperiod ab type RR Lyrae are present. We use the observed properties of the RR Lyrae to estimate the reddening and the distance modulus of M54, E(B-V)=0.16 ± 0.02 and (m-M)0=17.13 ± 0.11, respectively, in excellent agreement with the most recent estimates. The metallicity has been estimated for a subset of 47 RR Lyrae stars, with especially good quality light curves, from the Fourier parameters of the V light curve. The derived metallicity distribution has a symmetric bell shape, with a mean of h[Fe/H]i = −1.65 and a standard deviation � = 0.16 dex. Seven stars have been identified as likely belonging to the Sagittarius galaxy, based on their too high or too low metallicity. This evidence, if confirmed, might suggest that old stars in this galaxy span a wide range of metallicities.