Scholarly Works - Institute of Astronomy


Recent Submissions

Now showing 1 - 20 of 446
  • ItemPublished versionOpen Access
    Observations of solar system objects
    (EDP Sciences, 2018) Spoto, F; Tanga, P; Mignard, F; Berthier, J; Carry, B; Cellino, A; Dell'Oro, A; Hestroffer, D; Muinonen, K; Pauwels, T; Petit, JM; David, P; De Angeli, F; Delbo, M; Frézouls, B; Galluccio, L; Granvik, M; Guiraud, J; Hernández, J; Ordénovic, C; Portell, J; Poujoulet, E; Thuillot, W; Walmsley, G; Brown, AGA; Vallenari, A; Prusti, T; De Bruijne, JHJ; Babusiaux, C; Bailer-Jones, CAL; Biermann, M; Evans, DW; Eyer, L; Jansen, F; Jordi, C; Klioner, SA; Lammers, U; Lindegren, L; Luri, X; Panem, C; Pourbaix, D; Randich, S; Sartoretti, P; Siddiqui, HI; Soubiran, C; Van Leeuwen, F; Walton, NA; Arenou, F; Bastian, U; Cropper, M; Drimmel, R; Katz, D; Lattanzi, MG; Bakker, J; Cacciari, C; Castañeda, J; Chaoul, L; Cheek, N; Fabricius, C; Guerra, R; Holl, B; Masana, E; Messineo, R; Mowlavi, N; Nienartowicz, K; Panuzzo, P; Riello, M; Seabroke, GM; Thévenin, F; Gracia-Abril, G; Comoretto, G; Garcia-Reinaldos, M; Teyssier, D; Altmann, M; Andrae, R; Audard, M; Bellas-Velidis, I; Benson, K; Blomme, R; Burgess, P; Busso, G; Clementini, G; Clotet, M; Creevey, O; Davidson, M; De Ridder, J; Delchambre, L; Ducourant, C; Fernández-Hernández, J; Fouesneau, M; Frémat, Y; García-Torres, M; Evans, Dafydd [0000-0002-6685-5998]; van Leeuwen, Floor [0000-0003-1781-4441]; Walton, Nicholas [0000-0003-3983-8778]; Riello, Marco [0000-0002-3134-0935]
    Context. The Gaia spacecraft of the European Space Agency (ESA) has been securing observations of solar system objects (SSOs) since the beginning of its operations. Data Release 2 (DR2) contains the observations of a selected sample of 14,099 SSOs. These asteroids have been already identified and have been numbered by the Minor Planet Center repository. Positions are provided for each Gaia observation at CCD level. As additional information, complementary to astrometry, the apparent brightness of SSOs in the unfiltered G band is also provided for selected observations. Aims. We explain the processing of SSO data, and describe the criteria we used to select the sample published in Gaia DR2. We then explore the data set to assess its quality. Methods. To exploit the main data product for the solar system in Gaia DR2, which is the epoch astrometry of asteroids, it is necessary to take into account the unusual properties of the uncertainty, as the position information is nearly one-dimensional. When this aspect is handled appropriately, an orbit fit can be obtained with post-fit residuals that are overall consistent with the a-priori error model that was used to define individual values of the astrometric uncertainty. The role of both random and systematic errors is described. The distribution of residuals allowed us to identify possible contaminants in the data set (such as stars). Photometry in the G band was compared to computed values from reference asteroid shapes and to the flux registered at the corresponding epochs by the red and blue photometers (RP and BP). Results. The overall astrometric performance is close to the expectations, with an optimal range of brightness G ∼ 12 - 17. In this range, the typical transit-level accuracy is well below 1 mas. For fainter asteroids, the growing photon noise deteriorates the performance. Asteroids brighter than G ∼ 12 are affected by a lower performance of the processing of their signals. The dramatic improvement brought by Gaia DR2 astrometry of SSOs is demonstrated by comparisons to the archive data and by preliminary tests on the detection of subtle non-gravitational effects.
  • ItemPublished versionOpen Access
    Spatial fluctuations of the intergalactic temperature-density relation after hydrogen reionization
    (Oxford University Press, 2018-07-11) Keating, Laura C; Puchwein, Ewald; Haehnelt, Martin G; Haehnelt, Martin [0000-0001-8443-2393]
    The thermal state of the post-reionization IGM is sensitive to the timing of reionization and the nature of the ionizing sources. We have modelled here the thermal state of the IGM in cosmological radiative transfer simulations of a realistic, extended, spatially inhomogeneous hydrogen reionization process, carefully calibrated with Ly α forest data. We compare these with cosmological simulations run using a spatially homogeneous ionizing background. The simulations with a realistic growth of ionized regions and a realistic spread in reionization redshifts show, as expected, significant spatial fluctuations in the temperature–density relation (TDR) of the post-reionization IGM. The most recently ionized regions are hottest and exhibit a flatter TDR. In simulations consistent with the average TDR inferred from Ly α forest data, these spatial fluctuations have a moderate but noticeable effect on the statistical properties of the Ly α opacity of the IGM at z ∼ 4–6. This should be taken into account in accurate measurements of the thermal properties of the IGM and the free-streaming of dark matter from Ly α forest data in this redshift range. The spatial variations of the TDR predicted by our simulations are, however, smaller by about a factor of 2 than would be necessary to explain the observed large spatial opacity fluctuations on large (≥50 h−1 comoving Mpc) scales atz ≳ 5.5.
  • ItemAccepted versionOpen Access
    Gaia Data Release 2: The astrometric solution
    (EDP Sciences, 2018) Lindegren, L; Hernandez, J; Bombrun, A; Klioner, S; Bastian, U; Ramos-Lerate, M; Torres, A de; Steidelmuller, H; Stephenson, C; Hobbs, D; Lammers, U; Biermann, M; Geyer, R; Hilger, T; Michalik, D; Stampa, U; McMillan, PJ; Castaneda, J; Clotet, M; Comoretto, G; Davidson, M; Fabricius, C; Gracia, G; Hambly, NC; Hutton, A; Mora, A; Portell, J; Leeuwen, F van; Abbas, U; Abreu, A; Altmann, M; Andrei, A; Anglada, E; Balaguer-Nunez, L; Barache, C; Becciani, U; Bertone, S; Bianchi, L; Bouquillon, S; Bourda, G; Brusemeister, T; Bucciarelli, B; Busonero, D; Buzzi, R; Cancelliere, R; Carlucci, T; Charlot, P; Cheek, N; Crosta, M; Crowley, C; Bruijne, J de; Felice, F de; Drimmel, R; Esquej, P; Fienga, A; Fraile, E; Gai, M; Garralda, N; Gonzalez-Vidal, JJ; Guerra, R; Hauser, M; Hofmann, W; Holl, B; Jordan, S; Lattanzi, MG; Lenhardt, H; Liao, S; Licata, E; Lister, T; Loffler, W; Marchant, J; Martin-Fleitas, J-M; Messineo, R; Mignard, F; Morbidelli, R; Poggio, E; Riva, A; Rowell, N; Salguero, E; Sarasso, M; Sciacca, E; Siddiqui, H; Smart, RL; Spagna, A; Steele, I; Taris, F; Torra, J; Elteren, A van; Reeven, W van; Vecchiato, A
    Gaia Data Release 2 (Gaia DR2) contains results for 1693 million sources in the magnitude range 3 to 21 based on observations collected by the European Space Agency Gaia satellite during the first 22 months of its operational phase. We describe the input data, models, and processing used for the astrometric content of Gaia DR2, and the validation of these results performed within the astrometry task. Some 320 billion centroid positions from the pre-processed astrometric CCD observations were used to estimate the five astrometric parameters (positions, parallaxes, and proper motions) for 1332 million sources, and approximate positions at the reference epoch J2015.5 for an additional 361 million mostly faint sources. Special validation solutions were used to characterise the random and systematic errors in parallax and proper motion. For the sources with five-parameter astrometric solutions, the median uncertainty in parallax and position at the reference epoch J2015.5 is about 0.04 mas for bright (G<14 mag) sources, 0.1 mas at G=17 mag, and 0.7 mas at G=20 mag. In the proper motion components the corresponding uncertainties are 0.05, 0.2, and 1.2 mas/yr, respectively. The optical reference frame defined by Gaia DR2 is aligned with ICRS and is non-rotating with respect to the quasars to within 0.15 mas/yr. From the quasars and validation solutions we estimate that systematics in the parallaxes depending on position, magnitude, and colour are generally below 0.1 mas, but the parallaxes are on the whole too small by about 0.03 mas. Significant spatial correlations of up to 0.04 mas in parallax and 0.07 mas/yr in proper motion are seen on small (<1 deg) and intermediate (20 deg) angular scales. Important statistics and information for the users of the Gaia DR2 astrometry are given in the appendices.
  • ItemPublished versionOpen Access
    The Gaia-ESO Survey: a kinematical and dynamical study of four young open clusters
    (EDP Sciences, 2018-07-06) Bravi, L; Zari, E; Sacco, GG; Randich, S; Jeffries, RD; Jackson, RJ; Franciosini, E; Moraux, E; Lopez-Santiago, J; Pancino, E; Spina, L; Wright, N; Jimenez-Esteban, FM; Klutsch, A; Roccatagliata, V; Gilmore, G; Bragaglia, A; Flaccomio, E; Francois, P; Koposov, SE; Bayo, A; Carraro, G; Costado, MT; Damiani, F; Frasca, A; Hourihane, A; Jofre, P; Lardo, C; Lewis, J; Magrini, L; Morbidelli, L; Prisinzano, L; Sousa, SG; Worley, CC; Zaggia, S; Gilmore, Gerard [0000-0003-4632-0213]; Worley, Clare [0000-0001-9310-2898]
    Context. The origin and dynamical evolution of star clusters is an important topic in stellar astrophysics. Several models have been proposed to understand the formation of bound and unbound clusters and their evolution, and these can be tested by examining the kinematical and dynamical properties of clusters over a wide range of ages and masses. Aims. We use the Gaia -ESO Survey products to study four open clusters (IC 2602, IC 2391, IC 4665, and NGC 2547) that lie in the age range between 20 and 50 Myr. Methods. We employ the gravity index γ and the equivalent width of the lithium line at 6708 Å, together with e ff ective temperature T e ff , and the metallicity of the stars in order to discard observed contaminant stars. Then, we derive the cluster radial velocity disper- sions σ c , the total cluster mass M tot , and the half mass radius r hm . Using the Gaia -DR1 TGAS catalogue, we independently derive the intrinsic velocity dispersion of the clusters from the astrometric parameters of cluster members. Results. The intrinsic radial velocity dispersions derived by the spectroscopic data are larger than those derived from the TGAS data, possibly due to the di ff erent masses of the considered stars. Using M tot and r hm we derive the virial velocity dispersion σ v ir and we find that three out of four clusters are supervirial. This result is in agreement with the hypothesis that these clusters are dispersing, as predicted by the "residual gas expulsion" scenario. However, recent simulations show that the virial ratio of young star clusters may be overestimated if it is determined using the global velocity dispersion, since the clusters are not fully relaxed.
  • ItemPublished versionOpen Access
    The Milky Way Halo in Action Space
    (IoP Publishing, 2018-04-01) Myeong, GC; Evans, NW; Belokurov, V; Sanders, JL; Koposov, SE; Myeong, Gyuchul [0000-0002-5629-8876]; Evans, Wyn [0000-0002-5981-7360]; Belokurov, Vasily [0000-0002-0038-9584]; Sanders, Jason [0000-0003-4593-6788]; Koposov, Sergey [0000-0003-2644-135X]
    We analyze the structure of the local stellar halo of the Milky Way using ~60000 stars with full phase space coordinates extracted from the SDSS–Gaia catalog. We display stars in action space as a function of metallicity in a realistic axisymmetric potential for the Milky Way Galaxy. The metal-rich population is more distended toward high radial action J R as compared to azimuthal or vertical action, J phgr or J z . It has a mild prograde rotation $(\langle {v}_{\phi }\rangle \approx 25\,\mathrm{km}\,{{\rm{s}}}^{-1}$), is radially anisotropic and highly flattened, with axis ratio q ≈ 0.6–0.7. The metal-poor population is more evenly distributed in all three actions. It has larger prograde rotation $(\langle {v}_{\phi }\rangle \approx 50\,\mathrm{km}\,{{\rm{s}}}^{-1}$), a mild radial anisotropy, and a roundish morphology (q ≈ 0.9). We identify two further components of the halo in action space. There is a high-energy, retrograde component that is only present in the metal-rich stars. This is suggestive of an origin in a retrograde encounter, possibly the one that created the stripped dwarf galaxy nucleus, ωCentauri. Also visible as a distinct entity in action space is a resonant component, which is flattened and prograde. It extends over a range of metallicities down to [Fe/H] ≈ −3. It has a net outward radial velocity $\langle {v}_{R}\rangle \approx 12\,\mathrm{km}\,{{\rm{s}}}^{-1}$ within the solar circle at $| z| \lt 3.5\,\mathrm{kpc}$. The existence of resonant stars at such extremely low metallicities has not been seen before.
  • ItemPublished versionOpen Access
    Observational hertzsprung-russell diagrams
    (EDP Sciences, 2018) Babusiaux, C; Van Leeuwen, F; Barstow, MA; Jordi, C; Vallenari, A; Bossini, D; Bressan, A; Cantat-Gaudin, T; Van Leeuwen, M; Brown, AGA; Prusti, T; De Bruijne, JHJ; Bailer-Jones, CAL; Biermann, M; Evans, DW; Eyer, L; Jansen, F; Klioner, SA; Lammers, U; Lindegren, L; Luri, X; Mignard, F; Panem, C; Pourbaix, D; Randich, S; Sartoretti, P; Siddiqui, HI; Soubiran, C; Walton, NA; Arenou, F; Bastian, U; Cropper, M; Drimmel, R; Katz, D; Lattanzi, MG; Bakker, J; Cacciari, C; Castañeda, J; Chaoul, L; Cheek, N; De Angeli, F; Fabricius, C; Guerra, R; Holl, B; Masana, E; Messineo, R; Mowlavi, N; Nienartowicz, K; Panuzzo, P; Portell, J; Riello, M; Seabroke, GM; Tanga, P; Thévenin, F; Gracia-Abril, G; Comoretto, G; Garcia-Reinaldos, M; Teyssier, D; Altmann, M; Andrae, R; Audard, M; Bellas-Velidis, I; Benson, K; Berthier, J; Blomme, R; Burgess, P; Busso, G; Carry, B; Cellino, A; Clementini, G; Clotet, M; Creevey, O; Davidson, M; De Ridder, J; Delchambre, L; Dell'Oro, A; Ducourant, C; Fernández-Hernández, J; Fouesneau, M; Frémat, Y; Galluccio, L; García-Torres, M; González-Núñez, J; González-Vidal, JJ; Gosset, E; Guy, LP; Halbwachs, JL; Hambly, NC; Harrison, DL; Hernández, J; Hestroffer, D; Hodgkin, ST; Hutton, A; Jasniewicz, G; Jean-Antoine-Piccolo, A; Jordan, S; Korn, AJ; Krone-Martins, A; Lanzafame, AC; Lebzelter, T; van Leeuwen, Floor [0000-0003-1781-4441]; Evans, Dafydd [0000-0002-6685-5998]; Walton, Nicholas [0000-0003-3983-8778]; Riello, Marco [0000-0002-3134-0935]; Harrison, Diana [0000-0001-8687-6588]; Hodgkin, Simon [0000-0002-5470-3962]
    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.
  • ItemAccepted versionOpen Access
    Homogeneous Analysis of the Dust Morphology of Transition Disks Observed with ALMA: Investigating Dust Trapping and the Origin of the Cavities
    (American Astronomical Society, 2018) Pinilla, P; Tazzari, M; Pascucci, I; Youdin, AN; Garufi, A; Manara, CF; Testi, L; Van Der Plas, G; Barenfeld, SA; Canovas, H; Cox, EG; Hendler, NP; Pérez, LM; Van Der Marel, N; Pinilla, P [0000-0001-8764-1780]; Pascucci, I [0000-0001-7962-1683]; Manara, CF [0000-0003-3562-262X]; Testi, L [0000-0003-1859-3070]; Van Der Plas, G [0000-0001-5688-187X]; Barenfeld, SA [0000-0001-5222-6851]; Canovas, H [0000-0001-7668-8022]; Cox, EG [0000-0002-5216-8062]; Hendler, NP [0000-0002-3164-0428]; Pérez, LM [0000-0002-1199-9564]; Van Der Marel, N [0000-0003-2458-9756]
    We analyze the dust morphology of 29 transition disks (TDs) observed with ALMA at (sub-) millimeter-emission. We perform the analysis in the visibility plane to characterize the total flux, cavity size, and shape of the ring-like structure. First, we found that the $M_{\rm{dust}}-M_\star$ relation is much flatter for TDs than the observed trends from samples of class II sources in different star forming regions. This relation demonstrates that cavities open in high (dust) mass disks, independent of the stellar mass. The flatness of this relation contradicts the idea that TDs are a more evolved set of disks. Two potential reasons (not mutually exclusive) may explain this flat relation: the emission is optically thick or/and millimeter-sized particles are trapped in a pressure bump. Second, we discuss our results of the cavity size and ring width in the context of different physical processes for cavity formation. Photoevaporation is an unlikely leading mechanism for the origin of the cavity of any of the targets in the sample. Embedded giant planets or dead zones remain as potential explanations. Although both models predict correlations between the cavity size and the ring shape for different stellar and disk properties, we demonstrate that with the current resolution of the observations, it is difficult to obtain these correlations. Future observations with higher angular resolution observations of TDs with ALMA will help to discern between different potential origins of cavities in TDs.
  • ItemAccepted versionOpen Access
    Discovery of fe Kα X-Ray reverberation around the black holes in MCG-5-23-16 and NGC 7314
    (American Astronomical Society, 2013) Zoghbi, A; Reynolds, C; Cackett, EM; Miniutti, G; Kara, E; Fabian, AC; Reynolds, Christopher [0000-0002-1510-4860]; Fabian, Andrew [0000-0002-9378-4072]
    Several X-ray observations have recently revealed the presence of reverberation time delays between spectral components in AGN. Most of the observed lags are between the power-law Comptonization component, seen directly, and the soft excess produced by reflection in the vicinity of the black hole. NGC 4151 was the first object to show these lags in the iron K band. Here, we report the discovery of reverberation lags in the Fe K band in two other sources: MCG-5-23-16 and NGC 7314. In both objects, the 6-7 keV band, where the Fe K line peaks, lags the bands at lower and higher energies with a time delay of ~ 1 kilo-seconds. These lags are unlikely to be due to the narrow Fe K line. They are fully consistent with reverberation of the relativistically-broadened iron K line. The measured lags, their time-scale and spectral modeling, indicate that most of the radiation is emitted at ~ 5 and 24 gravitational radii for MCG-5-23-16 and NGC 7314 respectively.
  • ItemAccepted versionOpen Access
    X-ray dips in the seyfert galaxy fairall 9: Compton-thick "cOMETS" or a failed radio galaxy?
    (American Astronomical Society, 2012) Lohfink, AM; Reynolds, CS; Mushotzky, RF; Wilms, J; Reynolds, Christopher [0000-0002-1510-4860]
    We investigate the spectral variability of the Seyfert galaxy Fairall 9 using almost 6 years of monitoring with the Rossi X-ray Timing Explorer (RXTE) with an approximate time resolution of 4 days. We discover the existence of pronounced and sharp dips in the X-ray flux, with a rapid decline of the 2--20 keV flux of a factor 2 or more followed by a recovery to pre-dip fluxes after ~10 days . These dips skew the flux distribution away from the commonly observed log-normal distribution. Dips may result from the eclipse of the central X-ray source by broad line region (BLR) clouds, as has recently been found in NGC 1365 and Mrk 766. Unlike these other examples, however, the clouds in Fairall 9 would need to be Compton-thick, and the non-dip state is remarkably free of any absorption features. A particularly intriguing alternative is that the accretion disk is undergoing the same cycle of disruption/ejection as seen in the accretion disks of broad line radio galaxies (BLRGs) such as 3C120 but, for some reason, fails to create a relativistic jet. This suggests that a detailed comparison of Fairall 9 and 3C120 with future high-quality data may hold the key to understanding the formation of relativistic jets in AGN.
  • ItemAccepted versionOpen Access
    NuSTAR and XMM-Newton observations of NGC 1365: Extreme absorption variability and a constant inner accretion disk
    (American Astronomical Society, 2014) Walton, DJ; Risaliti, G; Harrison, FA; Fabian, AC; Miller, JM; Arevalo, P; Ballantyne, DR; Boggs, SE; Brenneman, LW; Christensen, FE; Craig, WW; Elvis, M; Fuerst, F; Gandhi, P; Grefenstette, BW; Hailey, CJ; Kara, E; Luo, B; Madsen, KK; Marinucci, A; Matt, G; Parker, ML; Reynolds, CS; Rivers, E; Ross, RR; Stern, D; Zhang, WW; Walton, Dominic [0000-0001-5819-3552]; Fabian, Andrew [0000-0002-9378-4072]; Parker, Michael [0000-0002-8466-7317]; Reynolds, Christopher [0000-0002-1510-4860]
    We present a spectral analysis of four coordinated NuSTAR+XMM-Newton observations of the Seyfert galaxy NGC 1365. These exhibit an extreme level of spectral variability, which is primarily due to variable line-of-sight absorption, revealing relatively unobscured states in this source for the first time. Despite the diverse range of absorption states, each of the observations displays the same characteristic signatures of relativistic reflection from the inner accretion disk. Through time-resolved spectroscopy we find that the strength of the relativistic iron line and the Compton reflection hump relative to the intrinsic continuum are well correlated, as expected if they are two aspects of the same broadband reflection spectrum. We apply self-consistent disk reflection models to these time-resolved spectra in order to constrain the inner disk parameters, allowing for variable, partially covering absorption to account for the vastly different absorption states observed. Each of the four observations is treated independently to test the consistency of the results obtained for the black hole spin and the disk inclination, which should not vary on observable timescales. We find both the spin and the inclination determined from the reflection spectrum to be consistent, confirming NGC 1365 hosts a rapidly rotating black hole; in all cases the dimensionless spin parameter is constrained to be a* > 0.97 (at 90% statistical confidence or better).
  • ItemAccepted versionOpen Access
    Hard X-ray lags in active galactic nuclei: Testing the distant reverberation hypothesis with NGC 6814
    (American Astronomical Society, 2013) Walton, DJ; Zoghbi, A; Cackett, EM; Uttley, P; Harrison, FA; Fabian, AC; Kara, E; Miller, JM; Reis, RC; Reynolds, CS; Walton, Dominic [0000-0001-5819-3552]; Fabian, Andrew [0000-0002-9378-4072]; Reynolds, Christopher [0000-0002-1510-4860]
    We present an X-ray spectral and temporal analysis of the variable active galaxy NGC 6814, observed with Suzaku during November 2011. Remarkably, the X-ray spectrum shows no evidence for the soft excess commonly observed amongst other active galaxies, despite its relatively low level of obscuration, and is dominated across the whole Suzaku bandpass by the intrinsic powerlaw-like continuum. Despite this, we clearly detect the presence of a low frequency hard lag of ~1600s between the 0.5-2.0 and 2.0-5.0 keV energy bands at greater than 6-sigma significance, similar to those reported in the literature for a variety of other AGN. At these energies, any additional emission from e.g. a very weak, undetected soft excess, or from distant reflection must contribute less than 3% of the observed countrates (at 90% confidence). Given the lack of any significant continuum emission component other than the powerlaw, we can rule out models that invoke distant reprocessing for the observed lag behavior, which must instead be associated with this continuum emission. These results are fully consistent with a propagating fluctuation origin for the low frequency hard lags, and with the interpretation of the high frequency soft lags - a common feature seen in the highest quality AGN data with strong soft excesses - as reverberation from the inner accretion disk.
  • ItemPublished versionOpen Access
    Protoplanetary disc truncation mechanisms in stellar clusters: Comparing external photoevaporation and tidal encounters
    (Oxford University Press (OUP), 2018) Winter, AJ; Clarke, CJ; Rosotti, G; Ih, J; Facchini, S; Haworth, TJ; Winter, Andrew [0000-0002-7501-9801]; Clarke, Catherine [0000-0003-4288-0248]; Rosotti, Giovanni [0000-0003-4853-5736]; Haworth, Thomas [0000-0002-9593-7618]
    Most stars form and spend their early life in regions of enhanced stellar density. Therefore the evolution of protoplanetary discs (PPDs) hosted by such stars are subject to the influence of other members of the cluster. Physically, PPDs might be truncated either by photoevaporation due to ultraviolet flux from massive stars, or tidal truncation due to close stellar encounters. Here we aim to compare the two effects in real cluster environments. In this vein we first review the properties of well studied stellar clusters with a focus on stellar number density, which largely dictates the degree of tidal truncation, and far ultraviolet (FUV) flux, which is indicative of the rate of external photoevaporation. We then review the theoretical PPD truncation radius due to an arbitrary encounter, additionally taking into account the role of eccentric encounters that play a role in hot clusters with a 1D velocity dispersion $\sigma_v > 2$ km/s. Our treatment is then applied statistically to varying local environments to establish a canonical threshold for the local stellar density ($n_{c} > 10^4$ pc$^{-3}$) for which encounters can play a significant role in shaping the distribution of PPD radii over a timescale $\sim 3$ Myr. By combining theoretical mass loss rates due to FUV flux with viscous spreading in a PPD we establish a similar threshold for which a massive disc is completely destroyed by external photoevaporation. Comparing these thresholds in local clusters we find that if either mechanism has a significant impact on the PPD population then photoevaporation is always the dominating influence.
  • ItemAccepted versionOpen Access
    ALMA observations of the narrow HR 4796A debris ring
    (Oxford University Press (OUP), 2018) Kennedy, GM; Marino, S; Matrà, L; Panić, O; Wilner, D; Wyatt, MC; Yelverton, B; Kennedy, Grant [0000-0001-6831-7547]; Marino, Sebastian [0000-0002-5352-2924]; Wyatt, Mark [0000-0001-9064-5598]
  • ItemAccepted versionOpen Access
    Shaping HR8799's outer dust belt with an unseen planet
    (Oxford University Press (OUP), 2018) Read, MJ; Wyatt, MC; Marino, S; Kennedy, GM; Wyatt, Mark [0000-0001-9064-5598]; Marino, Sebastian [0000-0002-5352-2924]
  • ItemPublished versionOpen Access
    A soft x-ray reverberation lag in the agn ESO 113-G010
    (American Astronomical Society, 2013) Cackett, EM; Fabian, AC; Zogbhi, A; Kara, E; Reynolds, C; Uttley, P; Fabian, Andrew [0000-0002-9378-4072]; Reynolds, Christopher [0000-0002-1510-4860]
    Reverberation lags have recently been discovered in a handful of nearby, variable AGN. Here, we analyze a ~100 ksec archival XMM-Newton observation of the highly variable AGN, ESO 113-G010 in order to search for lags between hard, 1.5 - 4.5 keV, and soft, 0.3 - 0.9 keV, energy X-ray bands. At the lowest frequencies available in the lightcurve (<1.5E-4 Hz), we find hard lags where the power-law dominated hard band lags the soft band (where the reflection fraction is high). However, at higher frequencies in the range (2-3)E-4 Hz we find a soft lag of -325 +/- 89 s. The general evolution from hard to soft lags as the frequency increases is similar to other AGN where soft lags have been detected. We interpret this soft lag as due to reverberation from the accretion disk, with the reflection component responding to variability from the X-ray corona. For a black hole mass of 7E6 M(solar) this corresponds to a light-crossing time of ~9 R_g/c, however, dilution effects mean that the intrinsic lag is likely longer than this. Based on recent black hole mass-scaling for lag properties, the lag amplitude and frequency are more consistent with a black hole a few times more massive than the best estimates, though flux-dependent effects could easily add scatter this large.
  • ItemPublished versionOpen Access
    Kuiper belt analogues in nearby M-type planet-host systems
    (Oxford University Press, 2018-06-01) Kennedy, GM; Bryden, G; Ardila, D; Eiroa, C; Lestrade, J-F; Marshall, JP; Matthews, BC; Moro-Martin, A; Wyatt, MC; Kennedy, Grant [0000-0001-6831-7547]; Wyatt, Mark [0000-0001-9064-5598]
    We present the results of a Herschel survey of 21 late-type stars that host planets discovered by the radial velocity technique. The aims were to discover new discs in these systems and to search for any correlation between planet presence and disc properties. In addition to the known disc around GJ 581, we report the discovery of two new discs, in the GJ 433 and GJ 649 systems. Our sample therefore yields a disc detection rate of 14 per cent, higher than the detection rate of 1.2 per cent among our control sample of DEBRIS M-type stars with 98 per cent confidence. Further analysis however shows that the disc sensitivity in the control sample is about a factor of two lower in fractional luminosity than for our survey, lowering the significance of any correlation between planet presence and disc brightness below 98 per cent. In terms of their specific architectures, the disc around GJ 433 lies at a radius somewhere between 1 and 30 au. The disc around GJ 649 lies somewhere between 6 and 30 au, but is marginally resolved and appears more consistent with an edge-on inclination. In both cases the discs probably lie well beyond where the known planets reside (0.06–1.1 au), but the lack of radial velocity sensitivity at larger separations allows for unseen Saturn–mass planets to orbit out to ~5 au, and more massive planets beyond 5 au. The layout of these M-type systems appears similar to Sun-like star + disc systems with low-mass planets.
  • ItemPublished versionOpen Access
    Gaia Data Release 2: processing of the photometric data
    (EDP Sciences, 2018) Riello, M; Angeli, F De; Evans, DW; Busso, G; Hambly, NC; Davidson, M; Burgess, PW; Montegriffo, P; Osborne, PJ; Kewley, A; Carrasco, JM; Fabricius, C; Jordi, C; Cacciari, C; Leeuwen, F van; Holland, G; Riello, Marco [0000-0002-3134-0935]; Evans, Dafydd [0000-0002-6685-5998]
    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.
  • ItemPublished versionOpen Access
    GALARIO: A GPU accelerated library for analysing radio interferometer observations
    (OUP, 2018-06-01) Tazzari, M; Beaujean, F; Testi, L; Tazzari, Marco [0000-0003-3590-5814]
    © 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. We present GALARIO, a computational library that exploits the power of modern graphical processing units (GPUs) to accelerate the analysis of observations from radio interferometers like Atacama Large Millimeter and sub-millimeter Array or the Karl G. Jansky Very Large Array. GALARIO speeds up the computation of synthetic visibilities from a generic 2D model image or a radial brightness profile (for axisymmetric sources). On a GPU, GALARIO is 150 faster than standard PYTHON and 10 times faster than serial C++ code on a CPU. Highly modular, easy to use, and to adopt in existing code,GALARIOcomes as two compiled libraries, one for Nvidia GPUs and one for multicore CPUs, where both have the same functions with identical interfaces. GALARIO comes with PYTHON bindings but can also be directly used in C or C++. The versatility and the speed of GALARIO open new analysis pathways that otherwise would be prohibitively time consuming, e.g. fitting high-resolution observations of large number of objects, or entire spectral cubes of molecular gas emission. It is a general tool that can be applied to any field that uses radio interferometer observations. The source code is available online at under the open source GNU Lesser General Public License v3.
  • ItemAccepted versionOpen Access
    Gaia: 3-dimensional census of the Milky Way Galaxy
    (Taylor & Francis, 2018-03-24) Gilmore, Gerard; Gilmore, Gerard [0000-0003-4632-0213]
    Astrometry from space has unique advantages over ground-based observations: the all-sky coverage, relatively stable, and temperature and gravity invariant, operating environment delivers precision, accuracy and sample volume several orders of magnitude greater than ground-based results. Even more importantly, absolute astrometry is possible. The European Space Agency Cornerstone mission Gaia is delivering that promise. Gaia provides 5-D phase space measurements, 3 spatial coordinates and two space motions in the plane of the sky, for a representative sample of the Milky Way’s stellar populations (over 2billion stars, being ~1% of the stars over 50% of the radius). Full 6-D phase space data is delivered from line-of-sight (radial) velocities for the 300million brightest stars. These data make substantial contributions to astrophysics and fundamental physics on scales from the Solar System to cosmology. Reliable parallax distances in astronomy were available for of order 10^4 stars to milliarcsec (mas) precision in the 1980s, for of order 10^5 stars to mas accuracy in the 2000s, and with Gaia for more than 10^9 stars to 10μas accuracy. A knowledge revolution is underway.
  • ItemPublished versionOpen Access
    The Gaia-ESO Survey: open clusters in Gaia-DR1 A way forward to stellar age calibration
    (EDP Sciences, 2018-04-01) Randich, S; Tognelli, E; Jackson, R; Jeffries, RD; Degl'Innocenti, S; Pancino, E; Fiorentin, P Re; Spagna, A; Sacco, G; Bragaglia, A; Magrini, L; Moroni, PG Prada; Alfaro, E; Franciosini, E; Morbidelli, L; Roccatagliata, V; Bouy, H; Bravi, L; Jimenez-Esteban, FM; Jordi, C; Zari, E; Tautvaisiene, G; Drazdauskas, A; Mikolaitis, S; Gilmore, G; Feltzing, S; Vallenari, A; Bensby, T; Koposov, S; Korn, A; Lanzafame, A; Smiljanic, R; Bayo, A; Carraro, G; Costado, MT; Heiter, U; Hourihane, A; Jofre, P; Lewis, J; Monaco, L; Prisinzano, L; Sbordone, L; Sousa, SG; Worley, CC; Zaggia, S; Gilmore, Gerard [0000-0003-4632-0213]; Worley, Clare [0000-0001-9310-2898]
    Context. Determination and calibration of the ages of stars, which heavily rely on stellar evolutionary models, are very challenging, while representing a crucial aspect in many astrophysical areas. Aims. We describe the methodologies that, taking advantage of Gaia-DR1 and the Gaia-ESO Survey data, enable the comparison of observed open star cluster sequences with stellar evolutionary models. The final, long-term goal is the exploitation of open clusters as age calibrators. Methods. We perform a homogeneous analysis of eight open clusters using the Gaia-DR1 TGAS catalogue for bright members and information from the Gaia-ESO Survey for fainter stars. Cluster membership probabilities for the Gaia-ESO Survey targets are derived based on several spectroscopic tracers. The Gaia-ESO Survey also provides the cluster chemical composition. We obtain cluster parallaxes using two methods. The first one relies on the astrometric selection of a sample of bona fide members, while the other one fits the parallax distribution of a larger sample of TGAS sources. Ages and reddening values are recovered through a Bayesian analysis using the 2MASS magnitudes and three sets of standard models. Lithium depletion boundary (LDB) ages are also determined using literature observations and the same models employed for the Bayesian analysis. Results. For all but one cluster, parallaxes derived by us agree with those presented in Gaia Collaboration (2017, A&A, 601, A19), while a discrepancy is found for NGC 2516; we provide evidence supporting our own determination. Inferred cluster ages are robust against models and are generally consistent with literature values. Conclusions. The systematic parallax errors inherent in the Gaia DR1 data presently limit the precision of our results. Nevertheless, we have been able to place these eight clusters onto the same age scale for the first time, with good agreement between isochronal and LDB ages where there is overlap. Our approach appears promising and demonstrates the potential of combining Gaia and ground-based spectroscopic datasets.