arXiv subjects

F. De Angeli

Publications and source records attributed to F. De Angeli.

At least 55 records · Page 3Linked to original sources

arXiv2018

Gaia Data Release 2: Photometric content and validation

Aims. We describe the photometric content of the second data release of the Gaia project (Gaia DR2) and its validation along with the quality of the data. Methods. The validation was mainly carried out using an internal analysis of the photometry. External comparisons were also made, but were limited by the precision and systematics that may be present in the external catalogues used. Results. In addition to the photometric quality assessment, we present the best estimates of the three photometric passbands. Various colour-colour transformations are also derived to enable the users to convert between the Gaia and commonly used passbands. Conclusions. The internal analysis of the data shows that the photometric calibrations can reach a precision as low as 2 mmag on individual CCD measurements. Other tests show that systematic effects are present in the data at the 10 mmag level.

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arXiv2018

Gaia Data Release 2: processing of the photometric data

The second Gaia data release is based on 22 months of mission data with an average of 0.9 billion individual CCD observations per day. A data volume of this size and granularity requires a robust and reliable but still flexible system to achieve the demanding accuracy and precision constraints that Gaia is capable of delivering. The internal Gaia photometric system was initialised using an iterative process that is solely based on Gaia data. A set of calibrations was derived for the entire Gaia DR2 baseline and then used to produce the final mean source photometry. The photometric catalogue contains 2.5 billion sources comprised of three different grades depending on the availability of colour information and the procedure used to calibrate them: 1.5 billion gold, 144 million silver, and 0.9 billion bronze. These figures reflect the results of the photometric processing; the content of the data release will be different due to the validation and data quality filters applied during the catalogue preparation. The photometric processing pipeline, PhotPipe, implements all the processing and calibration workflows in terms of Map/Reduce jobs based on the Hadoop platform. This is the first example of a processing system for a large astrophysical survey project to make use of these technologies. The improvements in the generation of the integrated G-band fluxes, in the attitude modelling, in the cross-matching, and and in the identification of spurious detections led to a much cleaner input stream for the photometric processing. This, combined with the improvements in the definition of the internal photometric system and calibration flow, produced high-quality photometry. Hadoop proved to be an excellent platform choice for the implementation of PhotPipe in terms of overall performance, scalability, downtime, and manpower required for operations and maintenance.

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arXiv2017

Gaia Data Release 1. Testing the parallaxes with local Cepheids and RR Lyrae stars

Parallaxes for 331 classical Cepheids, 31 Type II Cepheids and 364 RR Lyrae stars in common between Gaia and the Hipparcos and Tycho-2 catalogues are published in Gaia Data Release 1 (DR1) as part of the Tycho-Gaia Astrometric Solution (TGAS). In order to test these first parallax measurements of the primary standard candles of the cosmological distance ladder, that involve astrometry collected by Gaia during the initial 14 months of science operation, we compared them with literature estimates and derived new period-luminosity ($PL$), period-Wesenheit ($PW$) relations for classical and Type II Cepheids and infrared $PL$, $PL$-metallicity ($PLZ$) and optical luminosity-metallicity ($M_V$-[Fe/H]) relations for the RR Lyrae stars, with zero points based on TGAS. The new relations were computed using multi-band ($V,I,J,K_{\mathrm{s}},W_{1}$) photometry and spectroscopic metal abundances available in the literature, and applying three alternative approaches: (i) by linear least squares fitting the absolute magnitudes inferred from direct transformation of the TGAS parallaxes, (ii) by adopting astrometric-based luminosities, and (iii) using a Bayesian fitting approach. TGAS parallaxes bring a significant added value to the previous Hipparcos estimates. The relations presented in this paper represent first Gaia-calibrated relations and form a "work-in-progress" milestone report in the wait for Gaia-only parallaxes of which a first solution will become available with Gaia's Data Release 2 (DR2) in 2018.

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arXiv2017

Gaia Data Release 1. Open cluster astrometry: performance, limitations, and future prospects

Context. The first Gaia Data Release contains the Tycho-Gaia Astrometric Solution (TGAS). This is a subset of about 2 million stars for which, besides the position and photometry, the proper motion and parallax are calculated using Hipparcos and Tycho-2 positions in 1991.25 as prior information. Aims. We investigate the scientific potential and limitations of the TGAS component by means of the astrometric data for open clusters. Methods. Mean cluster parallax and proper motion values are derived taking into account the error correlations within the astrometric solutions for individual stars, an estimate of the internal velocity dispersion in the cluster, and, where relevant, the effects of the depth of the cluster along the line of sight. Internal consistency of the TGAS data is assessed. Results. Values given for standard uncertainties are still inaccurate and may lead to unrealistic unit-weight standard deviations of least squares solutions for cluster parameters. Reconstructed mean cluster parallax and proper motion values are generally in very good agreement with earlier Hipparcos-based determination, although the Gaia mean parallax for the Pleiades is a significant exception. We have no current explanation for that discrepancy. Most clusters are observed to extend to nearly 15 pc from the cluster centre, and it will be up to future Gaia releases to establish whether those potential cluster-member stars are still dynamically bound to the clusters. Conclusions. The Gaia DR1 provides the means to examine open clusters far beyond their more easily visible cores, and can provide membership assessments based on proper motions and parallaxes. A combined HR diagram shows the same features as observed before using the Hipparcos data, with clearly increased luminosities for older A and F dwarfs.

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arXiv2017

Gaia Data Release 1: The variability processing & analysis and its application to the south ecliptic pole region

The ESA Gaia mission provides a unique time-domain survey for more than one billion sources brighter than G=20.7 mag. Gaia offers the unprecedented opportunity to study variability phenomena in the Universe thanks to multi-epoch G-magnitude photometry in addition to astrometry, blue and red spectro-photometry, and spectroscopy. Within the Gaia Consortium, Coordination Unit 7 has the responsibility to detect variable objects, classify them, derive characteristic parameters for specific variability classes, and provide global descriptions of variable phenomena. We describe the variability processing and analysis that we plan to apply to the successive data releases, and we present its application to the G-band photometry results of the first 14 months of Gaia operations that comprises 28 days of Ecliptic Pole Scanning Law and 13 months of Nominal Scanning Law. Out of the 694 million, all-sky, sources that have calibrated G-band photometry in this first stage of the mission, about 2.3 million sources that have at least 20 observations are located within 38 degrees from the South Ecliptic Pole. We detect about 14% of them as variable candidates, among which the automated classification identified 9347 Cepheid and RR Lyrae candidates. Additional visual inspections and selection criteria led to the publication of 3194 Cepheid and RR Lyrae stars, described in Clementini et al. (2016). Under the restrictive conditions for DR1, the completenesses of Cepheids and RR Lyrae stars are estimated at 67% and 58%, respectively, numbers that will significantly increase with subsequent Gaia data releases. Data processing within the Gaia Consortium is iterative, the quality of the data and the results being improved at each iteration. The results presented in this article show a glimpse of the exceptional harvest that is to be expected from the Gaia mission for variability phenomena. [abridged]

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arXiv2017

Gaia Data Release 1: Validation of the photometry

Aims. The photometric validation of the Gaia DR1 release of the ESA Gaia mission is described and the quality of the data shown. Methods. This is carried out via an internal analysis of the photometry using the most constant sources. Comparisons with external photometric catalogues are also made, but are limited by the accuracies and systematics present in these catalogues. An analysis of the quoted errors is also described. Investigations of the calibration coefficients reveal some of the systematic effects that affect the fluxes. Results. The analysis of the constant sources shows that the early-stage photometric calibrations can reach an accuracy as low as 3 mmag.

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arXiv2016

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.

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arXiv2016

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.

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arXiv2016

Gaia Data Release 1 - The Cepheid & RR Lyrae star pipeline and its application to the south ecliptic pole region

We present an overview of the Specific Objects Study (SOS) pipeline developed within the Coordination Unit 7 (CU7) of the Gaia Data Processing and Analysis Consortium (DPAC), the coordination unit charged with the processing and analysis of variable sources observed by Gaia, to validate and fully characterise Cepheids and RR Lyrae stars observed by the spacecraft. We describe how the SOS for Cepheids and RR Lyrae stars (SOS Cep&RRL) was specifically tailored to analyse Gaia's G-band photometric time-series with a South Ecliptic Pole (SEP) footprint, which covers an external region of the Large Magellanic Cloud (LMC). G-band time-series photometry and characterization by the SOS Cep&RRL pipeline (mean magnitude and pulsation characteristics) are published in Gaia Data Release 1 (Gaia DR1) for a total sample of 3,194 variable stars, 599 Cepheids and 2,595 RR Lyrae stars, of which 386 (43 Cepheids and 343 RR Lyrae stars) are new discoveries by Gaia. All 3,194 stars are distributed over an area extending 38 degrees on either side from a point offset from the centre of the LMC by about 3 degrees to the north and 4 degrees to the east. The vast majority, but not all, are located within the LMC. The published sample also includes a few bright RR Lyrae stars that trace the outer halo of the Milky Way in front of the LMC.

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arXiv2015

Total eclipse of the heart: The AM CVn Gaia14aae / ASSASN-14cn

We report the discovery and characterisation of a deeply eclipsing AM CVn-system, Gaia14aae (= ASSASN-14cn). Gaia14aae was identified independently by the All-Sky Automated Survey for Supernovae (ASAS-SN; Shappee et al. 2014) and by the Gaia Science Alerts project, during two separate outbursts. A third outburst is seen in archival Pan-STARRS-1 (PS1; Schlafly et al. 2012; Tonry et al. 2012; Magnier et al. 2013) and ASAS-SN data. Spectroscopy reveals a hot, hydrogen-deficient spectrum with clear double-peaked emission lines, consistent with an accreting double degenerate classification. We use follow-up photometry to constrain the orbital parameters of the system. We find an orbital period of 49.71 min, which places Gaia14aae at the long period extremum of the outbursting AM CVn period distribution. Gaia14aae is dominated by the light from its accreting white dwarf. Assuming an orbital inclination of 90 degrees for the binary system, the contact phases of the white dwarf lead to lower limits of 0.78 M solar and 0.015 M solar on the masses of the accretor and donor respectively and a lower limit on the mass ratio of 0.019. Gaia14aae is only the third eclipsing AM CVn star known, and the first in which the WD is totally eclipsed. Using a helium WD model, we estimate the accretor's effective temperature to be 12900+-200 K. The three out-burst events occurred within 4 months of each other, while no other outburst activity is seen in the previous 8 years of Catalina Real-time Transient Survey (CRTS; Drake et al. 2009), Pan-STARRS-1 and ASAS-SN data. This suggests that these events might be rebrightenings of the first outburst rather than individual events.

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arXiv2007

Is our Sun a Singleton?

Most stars are formed in a cluster or association, where the number density of stars can be high. This means that a large fraction of initially-single stars will undergo close encounters with other stars and/or exchange into binaries. We describe how such close encounters and exchange encounters can affect the properties of a planetary system around a single star. We define a singleton as a single star which has never suffered close encounters with other stars or spent time within a binary system. It may be that planetary systems similar to our own solar system can only survive around singletons. Close encounters or the presence of a stellar companion will perturb the planetary system, often leaving planets on tighter and more eccentric orbits. Thus planetary systems which initially resembled our own solar system may later more closely resemble some of the observed exoplanet systems.

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arXiv2006

Search and analysis of blue straggler stars in open clusters

Aims: This paper presents a new homogeneous catalogue of blue straggler stars (BSS) in Galactic open clusters. Methods: Photometric data for 216 clusters were collected from the literature and 2782 BSS candidates were extracted from 76 of them. Results: We found that the anticorrelation of BSS frequency vs. total magnitude identified in similar studies conducted on Galactic globular clusters extends to the open cluster regime: clusters with smaller total magnitude tend to have higher BSS frequencies. Moreover, a clear correlation between the BSS frequency (obtained normalising the total number of BSS either to the total cluster mass or, for the older clusters, to the total number of clump stars) and the age of the clusters was found. A simple model is developed here to try to explain this last and new result. The model allows us to ascertain the important effect played by mass loss in the evolution of open clusters.

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arXiv2006

Outer structure of the Galactic warp and flare: explaining the Canis Major over-density

(Abridged) We derive the structure of the Galactic stellar Warp and Flare using 2MASS RC and RGB stars, selected at mean heliocentric distances of 3, 7 and 17 kpc. Our results are: (i) a clear stellar warp signature is derived for the 3 selected rings; (ii) the derived stellar warp is consistent (both in amplitude and phase-angle) with that for the Galactic interstellar dust and HI gas; (iii) the Sun seems not to fall on the line of nodes. The stellar warp phase-angle orientation (+15 degrees) is close to the orientation angle of the Galactic bar and this produces an asymmetric warp for the inner rings; (iv) a Northern/Southern warp symmetry is observed only for the ring at 17 kpc; (v) treating a mixture of thin and thick disk populations we trace the disk flaring and derive a constant scale-height (~0.65 kpc) within R(GC)~15 kpc. Further out, the disk flaring increase gradually reaching a mean scale-height of ~1.5 kpc at R(GC)~23 kpc; and (vi) these results provide further robust evidence that there is no disk radial truncation at R(GC)~14 kpc. In the particular case of the Canis Major over-density we confirm its coincidence with the Southern stellar maximum warp occurring near l=240. We present evidence to conclude that all observed parameters (e.g. number density, radial velocities, proper motion etc) of CMa are consistent with it being a normal Milky Way outer-disk population, thereby leaving no justification for a more complex interpretations of its origin. The present analysis does not provide a conclusive test of the structure or origin of the Monoceros Ring. Nevertheless, we show that a warped flared Milky Way contributes significantly at the locations of the Monoceros Ring.

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arXiv2005

Multivariate Analysis of Globular Cluster's Horizontal Branch Morphology: searching for the second parameter

The interpretation of globular cluster horizontal branch (HB) morphology is a classical problem that can significantly blur our understanding of stellar populations. In this paper, we present a new multivariate analysis connecting the effective temperature extent of the HB with other cluster parameters. The work is based on Hubble Space Telescope photometry of 54 Galactic globular clusters. The present study reveals an important role of the total mass of the globular cluster on its HB morphology. More massive clusters tend to have HBs more extended to higher temperatures. For a set of three input variables including the temperature extension of the HB, [Fe/H] and M_V, the first two eigenvectors account for the 90% of the total sample variance. Possible effects of cluster self-pollution on HB morphology, eventually stronger in more massive clusters, could explain the results here derived.

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arXiv2005

Precise reddening and metallicity of NGC6752 from FLAMES spectra

(abridged) Accurate reddenings for Globular Clusters could be obtained by comparing the colour-temperature obtained using temperatures from reddening-free indicator (Halpha), with that given by standard colour-temperature calibrations. From a single 1300 seconds exposure with FLAMES at VLT2 we obtained spectra centred on Halpha (R=6000, 5<S/N<50) for 120 stars near the turn-off of NGC6752. This set of spectra was used to derive effective temperatures from fittings of Halpha profiles with typical errors of about +/- 200 K and reddening estimates with individual errors of 0.05 mag. Averaging all individual reddenings, a high precision reddening estimate has been obtained for the cluster: E(B-V)=0.046 +/- 0.005. The same exposure provided UVES spectra of seven stars near the red giant branch bump (R=40000, 20<S/N<40) that provided Fe abundances with internal errors of 0.026 dex, and with average metallicity [Fe/H]= -1.48 +/- 0.01 +/- 0.06 dex (random + systematic). Abundances were obtained for several other elements, allowing e.g. an accurate estimate of the ratio between the alpha-elements and Fe ([alpha/Fe]=+0.27 +/- 0.01). The O-Na anticorrelation is evident from our UVES data, in agreement with past results. This analysis shows the power of FLAMES for analysis of globular clusters: the accurate reddenings and metal abundances obtained by a procedure like that described here, combined with distance determinations from cluster dynamics or main sequence fitting, and high quality colour-magnitude diagrams, could allow derivation of ages with errors below 1 Gyr for individual globular clusters.

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arXiv2005

Galactic Globular Cluster Relative Ages. II

We present accurate relative ages for a sample of 55 Galactic globular clusters. The ages have been obtained by measuring the difference between the horizontal branch and the turnoff in two, internally photometrically homogeneous databases. The mutual consistency of the two data sets has been assessed by comparing the ages of 16 globular clusters in common between the two databases. We have also investigated the consistency of our relative age determination within the recent stellar model framework. All clusters with [Fe/H]<-1.7 are found to be old, and coeval, with the possible exception of two objects, which are marginally younger. The age dispersion for the metal poor clusters is 0.6 Gyr (rms), consistent with a null age dispersion. Intermediate metallicity clusters (-1.7<[Fe/H]<-0.8) are on average 1.5 Gyr younger than the metal poor ones, with an age dispersion of 1.0 Gyr (rms), and a total age range of ~3 Gyr. About 15% of the intermediate metallicity clusters are coeval with the oldest clusters. All the clusters with [Fe/H]>-0.8 are ~1 Gyr younger than the most metal poor ones, with a relatively small age dispersion, though the metal rich sample is still too small to allow firmer conclusions. There is no correlation of the cluster age with the Galactocentric distance. We briefly discuss the implication of these observational results for the formation history of the Galaxy.

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arXiv2004

A Homogenous Set of Globular Cluster Relative Distances and Reddenings

We present distance modulus and reddening determinations for 72 Galactic globular clusters from the homogeneous photometric database of Piotto et al. (2002), calibrated to the HST flight F439W and F555W bands. The distances have been determined by comparison with theoretical absolute magnitudes of the ZAHB. For low and intermediate metallicity clusters, we have estimated the apparent Zero Age Horizontal Branch (ZAHB) magnitude from the RR Lyrae level. For metal rich clusters, the ZAHB magnitude was obtained from the fainter envelope of the red HB. Reddenings have been estimated by comparison of the HST colour-magnitude diagrams (CMD) with ground CMDs of low reddening template clusters. The homogeneity of both the photometric data and the adopted methodological approach allowed us to obtain highly accurate relative cluster distances and reddenings. Our results are also compared with recent compilations in the literature.

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arXiv2004

Probing the Canis Major stellar over-density as due to the Galactic warp

Proper-motion, star counts and photometric catalog simulations are used to explain the detected stellar over-density in the region of Canis Major (CMa), claimed to be the core of a disrupted dwarf galaxy (Martin et al. 2004, Bellazzini et al. 2003), as due to the Galactic warp and flare in the external disk. We compare the kinematics of CMa M-giant selected sample with surrounding Galactic disk stars in the UCAC2 catalog and find no peculiar proper motion signature: CMa stars mimic thick disk kinematics. Moreover, when taking into account the Galactic warp and flare of the disk, 2MASS star count profiles reproduce the CMa stellar over-density. This star count analysis is confirmed by direct comparison with synthetic color-magnitude diagrams simulated with the Besancon models (Robin et al. 2003) that include the warp and flare of the disk. The presented evidence casts doubt on the identification of the CMa over-density as the core of a disrupted Milky Way satellite. This however does not make clear the origin of over-densities responsible for the ring structure in the anticenter direction of the Galactic halo (Newberg et al. 2002; Yanny et al. 2003; Zaggia et al. 2004, in preparation).

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