2019
Taxonomy of asteroid spectra using density-based clustering methods in the Gaia era

L. Galluccio; Marco Delbò; A. Cellino; P. Tanga; F. De Angeli

HAL (Le Centre pour la Communication Scientifique Directe)

International audience

2019
The distance, rotation, and physical parameters of ζ Pup

Ian D. Howarth; F. van Leeuwen

Monthly Notices of the Royal Astronomical Society · DOI ↗

We scrutinize the Hipparcos parallax for the bright O supergiant ζ Pup, and confirm that the implied distance of 332 ± 11 pc appears to be reliable. We then review the implications for the star’s physical parameters, and the consequences for the interpretation of Pphot, the 1.78-d photometric period. The equatorial rotation period is <3.7 d (with 95% confidence), ruling out a proposed ∼5.1-d value. If the photometric period is the rotation period then i, the inclination of the rotation axis to the line of sight, is 33.⁰2 ± 1.⁰8. The inferred mass, radius, and luminosity are securely established to be less than canonical values for the spectral type, and are not in agreement with single-star evolution models. The runaway status, rapid rotation, and anomalous physical properties are all indicative of an evolutionary history involving binary (or multiple-star) interaction. We perform simple starspot modelling to show that the low axial inclination required if Prot = 1.78 d has testable spectroscopic consequences, which have not been identified in existing time series. If Pphot is directly related to drivers of systematic, high-velocity stellar-wind variability (‘discrete absorption components’) in ζ Pup, antisolar differential rotation is required. Model line profiles calculated on that basis are at variance with observations.

2019
The outer halo globular cluster system of M31 – III. Relationship to the stellar halo

Dougal Mackey; A. M. N. Ferguson; Avon Huxor; J. Veljanoski; Geraint F. Lewis; Alan W. McConnachie; Nicolas F. Martin; Rodrigo Ibata; M. J. Irwin; Patrick Côté; Michelle Collins; N. R. Tanvir; N. F. Bate

Monthly Notices of the Royal Astronomical Society · DOI ↗

We utilize the final catalogue from the Pan-Andromeda Archaeological Survey to investigate the links between the globular cluster system and field halo in M31 at projected radii Rproj = 25–150 kpc. In this region the cluster radial density profile exhibits a power-law decline with index Γ = −2.37 ± 0.17, matching that for the stellar halo component with [Fe/H] < −1.1. Spatial density maps reveal a striking correspondence between the most luminous substructures in the metal-poor field halo and the positions of many globular clusters. By comparing the density of metal-poor halo stars local to each cluster with the azimuthal distribution at commensurate radius, we reject the possibility of no correlation between clusters and field overdensities at $99.95{{\ \rm per\ cent}}$ significance. We use our stellar density measurements and previous kinematic data to demonstrate that ${\approx }35\hbox{--}60{{\ \rm per\ cent}}$ of clusters exhibit properties consistent with having been accreted into the outskirts of M31 at late times with their parent dwarfs. Conversely, at least ${\sim }40{{\ \rm per\ cent}}$ of remote clusters show no evidence for a link with halo substructure. The radial density profile for this subgroup is featureless and closely mirrors that observed for the apparently smooth component of the metal-poor stellar halo. We speculate that these clusters are associated with the smooth halo; if so, their properties appear consistent with a scenario where the smooth halo was built up at early times via the destruction of primitive satellites. In this picture the entire M31 globular cluster system outside Rproj = 25 kpc comprises objects accumulated from external galaxies over a Hubble time of growth.

2018
<i>Gaia</i> Data Release 2

D. W. Evans; M. Riello; F. De Angeli; J. M. Carrasco; P. Montegriffo; C. Fabricius; C. Jordi; L. Palaversa; C. Diener; G. Busso; C. Cacciari; F. van Leeuwen; P. W. Burgess; M. Davidson; D. L. Harrison; S. T. Hodgkin; E. Pancino; P. J. Richards; G. Altavilla; L. Balaguer-Núñez; M. A. Barstow; M. Bellazzini; A. G. A. Brown; M. Castellani; G. Cocozza; F. De Luise; A. Delgado; C. Ducourant; S. Galleti; G. Gilmore; G. Giuffrida; B. Holl; A. Kewley; S. E. Koposov; S. Marinoni; P. M. Marrese; P. Osborne; A. M. Piersimoni; J. Portell; L. Pulone; S. Ragaini; N. Sanna; D. Terrett; N. A. Walton; T. Wevers; Ł. Wyrzykowski

Astronomy and Astrophysics · DOI ↗

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.

2018
<i>Gaia</i> Data Release 2

A. G. A. Brown; A. Vallenari; T. Prusti; J. H. J. de Bruijne; C. Babusiaux; C. A. L. Bailer-Jones; M. Biermann; D. W. Evans; L. Eyer; F. Jansen; et al.

Astronomy and Astrophysics · DOI ↗

Context. We present the second Gaia data release, Gaia DR2, consisting of astrometry, photometry, radial velocities, and information on astrophysical parameters and variability, for sources brighter than magnitude 21. In addition epoch astrometry and photometry are provided for a modest sample of minor planets in the solar system. Aims. A summary of the contents of Gaia DR2 is presented, accompanied by a discussion on the differences with respect to Gaia DR1 and an overview of the main limitations which are still present in the survey. Recommendations are made on the responsible use of Gaia DR2 results. Methods. The raw data collected with the Gaia instruments during the first 22 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium (DPAC) and turned into this second data release, which represents a major advance with respect to Gaia DR1 in terms of completeness, performance, and richness of the data products. Results. Gaia DR2 contains celestial positions and the apparent brightness in G for approximately 1.7 billion sources. For 1.3 billion of those sources, parallaxes and proper motions are in addition available. The sample of sources for which variability information is provided is expanded to 0.5 million stars. This data release contains four new elements: broad-band colour information in the form of the apparent brightness in the G BP (330–680 nm) and G RP (630–1050 nm) bands is available for 1.4 billion sources; median radial velocities for some 7 million sources are presented; for between 77 and 161 million sources estimates are provided of the stellar effective temperature, extinction, reddening, and radius and luminosity; and for a pre-selected list of 14 000 minor planets in the solar system epoch astrometry and photometry are presented. Finally, Gaia DR2 also represents a new materialisation of the celestial reference frame in the optical, the Gaia -CRF2, which is the first optical reference frame based solely on extragalactic sources. There are notable changes in the photometric system and the catalogue source list with respect to Gaia DR1, and we stress the need to consider the two data releases as independent. Conclusions. Gaia DR2 represents a major achievement for the Gaia mission, delivering on the long standing promise to provide parallaxes and proper motions for over 1 billion stars, and representing a first step in the availability of complementary radial velocity and source astrophysical information for a sample of stars in the Gaia survey which covers a very substantial fraction of the volume of our galaxy.

2018
<i>Gaia</i> Data Release 2

M. Riello; F. De Angeli; D. W. Evans; G. Busso; N. C. Hambly; M. Davidson; P. W. Burgess; P. Montegriffo; P. Osborne; A. Kewley; J. M. Carrasco; C. Fabricius; C. Jordi; C. Cacciari; F. van Leeuwen; G. Holland

Astronomy and Astrophysics · DOI ↗

Context. 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. Aims. We aim to describe the input data, the treatment of blue photometer/red photometer (BP/RP) low-resolution spectra required to produce the integrated G BP and G RP fluxes, the process used to establish the internal Gaia photometric system, and finally, the generation of the mean source photometry from the calibrated epoch data for Gaia DR2. Methods. 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. Results. 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.

2018
An automated search for transiting exocomets

Grant M. Kennedy; Gregory A. Hope; S. T. Hodgkin; M. C. Wyatt

Monthly Notices of the Royal Astronomical Society · DOI ↗

This paper discusses an algorithm for detecting single transits in photometric time-series data. Specifically, we aim to identify asymmetric transits with ingress that is more rapid than egress, as expected for cometary bodies with a significant tail. The algorithm is automated, so can be applied to large samples and only a relatively small number of events need to be manually vetted. We applied this algorithm to all long-cadence light curves from the Kepler mission, finding 16 candidate transits with significant asymmetry, 11 of which were found to be artefacts or symmetric transits after manual inspection. Of the five remaining events, four are the 0.1 per cent depth events previously identified for KIC 3542116 and 11084727. We identify HD 182952 (KIC 8027456) as a third system showing a potential comet transit. All three stars showing these events have H–R diagram locations consistent with ∼100 Myr-old open cluster stars, as might be expected given that cometary source regions deplete with age, and giving credence to the comet hypothesis. If these events are part of the same population of events as seen for KIC 8462852, the small increase in detections at 0.1 per cent depth compared to 10 per cent depth suggests that future work should consider whether the distribution is naturally flat, or if comets with symmetric transits in this depth range remain undiscovered. Future searches relying on asymmetry should be more successful if they focus on larger samples and young stars, rather than digging further into the noise.

2018
Discovery of new dipper stars with K2: a window into the inner disc region of T Tauri stars

Christina Hedges; S. T. Hodgkin; Grant M. Kennedy

Monthly Notices of the Royal Astronomical Society · DOI ↗

In recent years a new class of Young Stellar Object has been defined, referred to as dippers, where large transient drops in flux are observed. These dips are too large to be attributed to stellar variability, last from hours to days and can reduce the flux of a star by 10-50%. This variability has been attributed to occultations by warps or accretion columns near the inner edge of circumstellar disks. Here we present 95 dippers in the Upper Scorpius association and Ophiuchus cloud complex found in K2 Campaign 2 data using supervised machine learning with a Random Forest classifier. We also present 30 YSOs that exhibit brightening events on the order of days, known as bursters. Not all dippers and bursters are known members, but all exhibit infrared excesses and are consistent with belonging to either of the two young star forming regions. We find 21.0 5.5% of stars with disks are dippers for both regions combined. Our entire dipper sample consists only of late-type (KM) stars, but we show that biases limit dipper discovery for earlier spectral types. Using the dipper properties as a proxy, we find that the temperature at the inner disk edge is consistent with interferometric results for similar and earlier type stars.

2018
Gaia Data Release 2: All-sky classification of high-amplitude pulsating stars

L. Rimoldini; B. Holl; M. Audard; N. Mowlavï; K. Nienartowicz; D. W. Evans; L. P. Guy; Lecoeur-Taïbi, I.; G. Jévardat de Fombelle; O. Marchal; M. Roelens; J. De Ridder; L. M. Sarro; S. Regibo; M. López; G. Clementini; V. Ripepi; R. Molinaro; A. Garofalo; L. Molnár; E. Plachy; Á. L. Juhász; L. Szabados; T. Lebzelter; D. Teyssier; L. Eyer

Repository of the Academy's Library (Library of the Hungarian Academy of Sciences) · DOI ↗

More than half a million of the 1.69 billion sources in Gaia Data Release 2 (DR2) are published with photometric time series that exhibit light variations during the 22 months of observation. An all-sky classification of common high-amplitude pulsators (Cepheids, long-period variables, Delta Scuti / SX Phoenicis, and RR Lyrae stars) is provided for stars with brightness variations greater than 0.1 mag in G band. A semi-supervised classification approach was employed, firstly training multi-stage random forest classifiers with sources of known types in the literature, followed by a preliminary classification of the Gaia data and a second training phase that included a selection of the first classification results to improve the representation of some classes, before the improved classifiers were applied to the Gaia data. Dedicated validation classifiers were used to reduce the level of contamination in the published results. A relevant fraction of objects were not yet sufficiently sampled for reliable Fourier series decomposition, consequently classifiers were based on features derived from statistics of photometric time series in the G, BP, and RP bands, as well as from some astrometric parameters. The published classification results include 195,780 RR Lyrae stars, 150,757 long-period variables, 8550 Cepheids, and 8882 Delta Scuti / SX Phoenicis stars. All of these results represent candidates whose completeness and contamination are described as a function of variability type and classification reliability. Results are expressed in terms of class labels and classification scores, which are available in the vari_classifier_result table of the Gaia archive.

2018
Gaia Data Release 2: Observational Hertzsprung-Russell diagrams

Gaia Collaboration; C. Babusiaux; F. van Leeuwen; M. A. Barstow; C. Jordi; A. Vallenari; D. Bossini; A. Bressan; T. Cantat-Gaudin; M. van Leeuwen; et al.

RECERCAT (Consorci de Serveis Universitaris de Catalunya) · DOI ↗

We highlight the power of the Gaia DR2 in studying many fine structures of the Hertzsprung-Russell diagram (HRD). Gaia allows us to present many different HRDs, depending in particular on stellar population selections. We do not aim here for completeness in terms of types of stars or stellar evolutionary aspects. Instead, we have chosen several illustrative examples. We describe some of the selections that can be made in Gaia DR2 to highlight the main structures of the Gaia HRDs. We select both field and cluster (open and globular) stars, compare the observations with previous classifications and with stellar evolutionary tracks, and we present variations of the Gaia HRD with age, metallicity, and kinematics. Late stages of stellar evolution such as hot subdwarfs, post-AGB stars, planetary nebulae, and white dwarfs are also analysed, as well as low-mass brown dwarf objects. The Gaia HRDs are unprecedented in both precision and coverage of the various Milky Way stellar populations and stellar evolutionary phases. Many fine structures of the HRDs are presented. The clear split of the white dwarf sequence into hydrogen and helium white dwarfs is presented for the first time in an HRD. The relation between kinematics and the HRD is nicely illustrated. Two different populations in a classical kinematic selection of the halo are unambiguously identified in the HRD. Membership and mean parameters for a selected list of open clusters are provided. They allow drawing very detailed cluster sequences, highlighting fine structures, and providing extremely precise empirical isochrones that will lead to more insight in stellar physics. Gaia DR2 demonstrates the potential of combining precise astrometry and photometry for large samples for studies in stellar evolution and stellar population and opens an entire new area for HRD-based studies.

2018
Gaia Data Release 2: processing of the photometric data

M. Riello; F. De Angeli; D. W. Evans; G. Busso; N. C. Hambly; M. Davidson; P. W. Burgess; P. Montegriffo; P. Osborne; A. Kewley; J. M. Carrasco; C. Fabricius; C. Jordi; C. Cacciari; F. van Leeuwen; G. Holland

RECERCAT (Consorci de Serveis Universitaris de Catalunya) · DOI ↗

CONTEXT. 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. AIMS. We aim to describe the input data, the treatment of blue photometer/red photometer (BP/RP) low–resolution spectra required to produce the integrated GBP and GRP fluxes, the process used to establish the internal Gaia photometric system, and finally, the generation of the mean source photometry from the calibrated epoch data for Gaia DR2. METHODS. 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. RESULTS. 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.

2018
Gaia Photometric Catalogue: the calibration of the DR2 photometry

D. W. Evans; M. Riello; F. De Angeli; J. M. Carrasco; P. Montegriffo; C. Fabricius; C. Jordi; L. Palaversa; C. Diener; G. Busso; C. Cacciari; E. Pancino; F. van Leeuwen

Proceedings of the International Astronomical Union · DOI ↗

Abstract Gaia DR2 was released in April 2018 and contains a photometric catalogue of more than 1 billion sources. This release contains colour information in the form of integrated BP and RP photometry in addition to the latest G-band photometry. The level of uncertainty can be as good as 2 mmag with some residual systematics at the 10 mmag level. The addition of colour information greatly enhances the value of the photometric data for the scientific community. A high level overview of the photometric processing, with a focus on the improvements with respect to Gaia DR1, was given. The definition of the Gaia photometric system, a crucial part of the calibration of the photometry, was also explained. Finally, some of the photometric improvements expected for the next data release were described.

2018
Gaia transients in galactic nuclei

Z. Kostrzewa-Rutkowska; P. G. Jonker; S. T. Hodgkin; Ł. Wyrzykowski; M. Fraser; D. L. Harrison; G. Rixon; A. Yoldaş; F. van Leeuwen; A. Delgado; M. van Leeuwen; S. E. Koposov

Monthly Notices of the Royal Astronomical Society · DOI ↗

The high spatial resolution and precise astrometry and photometry of the Gaia mission should make it particularly apt at discovering and resolving transients occurring in, or near, the centres of galaxies. Indeed, some nuclear transients are reported by the Gaia Science Alerts (GSA) team, but not a single confirmed Tidal Disruption Event has been published. In order to explore the sensitivity of GSA, we performed an independent and systematic search for nuclear transients using Gaia observations. Our transient search is driven from an input galaxy catalogue (derived from the Sloan Digital Sky Survey Release 12). We present a candidate detection metric which is independent from the existing GSA methodology, to see if Gaia Alerts are biased against the discovery of nuclear transients, and in particular which steps may have an impact. Our technique does require significant manual vetting of candidates, making implementation in the GSA system impractical for daily operations, although it could be run weekly, which for month-to-year long transients would make a scientifically valuable addition. Our search yielded ~480 nuclear transients, 5 of which were alerted and published by GSA. The list of (in some cases ongoing) transients includes candidates for events related to enhanced accretion onto a super-massive black hole and TDEs. An implementation of the detection methodology and criteria used in this paper as an extension of GSA could open up the possibility for Gaia to fulfil the role as a main tool to find transient nuclear activity as predicted in the literature.

2018
Galactic Archeology with 4MOST

S. Feltzing; T. Bensby; M. Bergemann; C. Chiappini; N. Christlieb; Cioni, M.; A. Helmi; M. J. Irwin; Ivan Minchev; Else Starkenburg; Roelof de Jong

MPG.PuRe (Max Planck Society)

4MOST is a new wide-field, high-multiplex spectroscopic survey facility for the VISTA telescope of ESO. Starting in 2022, 4MOST will deploy 2400 fibres in a 4.1 square degree field-of-view using a positioner based on the tilting spine principle. In this contribution we give an outline of the major science goals we wish to achieve with 4MOST in the area of Galactic Archeology. The 4MOST Galactic Archeology surveys have been designed to address long-standing and far-reaching problems in Galactic science. They are focused on four major themes: 1) Near-field cosmology tests, 2) Chemo-dynamical characterisation of the major Milky Way stellar components, 3) The Galactic Halo and beyond, and 4) Discovery and characterisation of extremely metal-poor stars. In addition to a top-level description of the Galactic surveys we provide information about how the community will be able to join 4MOST via a call for Public Spectroscopic Surveys that ESO will launch.

2018
GalMod: A Galactic Synthesis Population Model

S. Pasetto; E. K. Grebel; C. Chiosi; Denija Crnojević; Peter Zeidler; G. Busso; L. P. Cassará; L. Piovan; Rosaria Tantalo; Claudio Brogliato

The Astrophysical Journal · DOI ↗

Abstract We present a new Galaxy population synthesis Model, GalMod. GalMod is a star-count model featuring an asymmetric bar/bulge as well as spiral arms and related extinction. The model, initially introduced in Pasetto et al., has been here completed with a central bar, a new bulge description, new disk vertical profiles, and several new bolometric corrections. The model can generate synthetic mock catalogs of visible portions of the Milky Way, external galaxies like M31, or N -body simulation initial conditions. At any given time, e.g., at a chosen age of the Galaxy, the model contains a sum of discrete stellar populations, namely the bulge/bar, disk, and halo. These populations are in turn the sum of different components: the disk is the sum of the spiral arms, thin disks, a thick disk, and various gas components, while the halo is the sum of a stellar component, a hot coronal gas, and a dark-matter component. The Galactic potential is computed from these population density profiles and used to generate detailed kinematics by considering up to the first four moments of the collisionless Boltzmann equation. The same density profiles are then used to define the observed color–magnitude diagrams in a user-defined field of view (FoV) from an arbitrary solar location. Several photometric systems have been included and made available online, and no limits on the size of the FoV are imposed thus allowing full-sky simulations, too. Finally, we model the extinction by adopting a dust model with advanced ray-tracing solutions. The model's Web page (and tutorial) can be accessed at www.GalMod.org and support is provided at Galaxy.Model@yahoo.com.

2018
Low-mass eclipsing binaries in the WFCAM Transit Survey: the persistence of the M-dwarf radius inflation problem

P. Cruz; Marcos P. Diaz; Jayne Birkby; D. Barrado; Brigitta Sipőcz; S. T. Hodgkin

Monthly Notices of the Royal Astronomical Society · DOI ↗

We present the characterization of 5 new short-period low-mass eclipsing binaries from the WFCAM Transit Survey. The analysis was performed by using the photometric WFCAM J-mag data and additional low-and intermediate-resolution spectroscopic data to obtain both orbital and physical properties of the studied sample. The light curves and the measured radial velocity curves were modeled simultaneously with the JKTEBOP code, with Markov chain Monte Carlo simulations for the error estimates. The best-model fit have revealed that the investigated detached binaries are in very close orbits, with orbital separations of 2.9 a 6.7 R and short periods of 0.59 P orb 1.72 d, approximately. We have derived stellar masses between 0.24 and 0.72 M and radii ranging from 0.42 to 0.67 R . The great majority of the LMEBs in our sample has an estimated radius far from the predicted values according to evolutionary models. The components with derived masses of M < 0.6 M present a radius inflation of 9% or more. This general behavior follows the trend of inflation for partiallyradiative stars proposed previously. These systems add to the increasing sample of low-mass stellar radii that are not well-reproduced by stellar models. They further highlight the need to understand the magnetic activity and physical state of small stars. Missions like TESS will provide many such systems to perform high-precision radius measurements to tightly constrain low-mass stellar evolution models.