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Moor A.,Hungarian Academy of Sciences | Szabo G.M.,Hungarian Academy of Sciences | Szabo G.M.,ELTE Gothard Lendulet Research Group | Kiss L.L.,Hungarian Academy of Sciences | And 6 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

In the past decade many kinematic groups of young stars (<100 Myr) were discovered in the solar neighbourhood. Since the most interesting period of planet formation overlaps with the age of these groups, their well dated members are attractive targets for exoplanet searches by direct imaging. We combined astrometric, photometric and X-ray data, and applied strict selection criteria to explore the stellar content of five nearby moving groups. We identified more than 100 potential new candidate members in the β Pic moving group, and in the Tucana-Horologium, Columba, Carina and Argus associations. In order to further assess and confirm their membership status, we analysed radial velocity data and lithium equivalent widths extracted from high-resolution spectra of 54 candidate stars. We identified 35 new probable/possible young moving group members: four in the β Pic moving group, 11 in the Columba association, 16 in the Carina association and four in the Argus association.We found serendipitously a new AB Dor moving group member as well. For four Columba systems Hipparcos-based parallaxes have already been available and as they are consistent with the predicted kinematic parallaxes, they can be considered as secure new members. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.


Regaly Zs.,Hungarian Academy of Sciences | Regaly Zs.,ELTE Gothard Lendulet Research Group | Sandor Zs.,University of Vienna | Csomos P.,University of Innsbruck | Ataiee S.,University of Heidelberg
Monthly Notices of the Royal Astronomical Society | Year: 2013

In this paper the migration of a 10 M⊕ planetary core is investigated at the outer boundary of the 'dead zone' of a protoplanetary disc by means of 2D hydrodynamic simulations done with the graphics processor unit version of the FARGO code. In the dead zone, the effective viscosity is greatly reduced due to the disc self-shielding against stellar UV radiation, X-rays from the stellar magnetosphere and interstellar cosmic rays. As a consequence, mass accumulation occurs near the outer dead zone edge, which is assumed to trap planetary cores enhancing the efficiency of the core-accretion scenario to form giant planets. Contrary to the perfect trapping of planetary cores in 1D models, our 2D numerical simulations show that the trapping effect is greatly dependent on the width of the region where viscosity reduction is taking place. Planet trapping happens exclusively if the viscosity reduction is sharp enough to allow the development of large-scale vortices due to the Rossby wave instability. The trapping is only temporarily, and its duration is inversely proportional to the width of the viscosity transition. However, if the Rossby wave instability is not excited, a ring-like axisymmetric density jump forms, which cannot trap the 10 M⊕ planetary cores. We revealed that the stellar torque exerted on the planet plays an important role in the migration history as the barycentre of the system significantly shifts away from the star due to highly non-axisymmetric density distribution of the disc. Our results still support the idea of planet formation at density/pressure maximum, since the migration of cores is considerably slowed down enabling them further growth and runaway gas accretion in the vicinity of an overdense region.© 2013 The Authors.


Freeman K.,Australian National University | Ness M.,Australian National University | Wylie-de-Boer E.,Australian National University | Athanassoula E.,Aix - Marseille University | And 9 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

We describe the motivation, field locations and stellar selection for the Abundances and Radial velocity Galactic Origins Survey (ARGOS) spectroscopic survey of 28 000 stars in the bulge and inner disc of the Milky Way galaxy across latitudes of b = -5° to -10°. The primary goal of this survey is to constrain the formation processes of the bulge and establish whether it is predominantly a merger or instability remnant. From the spectra (R = 11 000), we have measured radial velocities and determined stellar parameters, including metallicities and [α/Fe] ratios. Distances were estimated from the derived stellar parameters and about 14 000 stars are red giants within 3.5 kpc of the Galactic Centre. In this paper, we present the observations and analysis methods. Subsequent papers (III and IV)will discuss the stellarmetallicity distribution and kinematics of the Galactic bulge and inner disc, and the implications for the formation of the bulge. © 2012 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Ness M.,Australian National University | Freeman K.,Australian National University | Athanassoula E.,Aix - Marseille University | Wylie-de-Boer E.,Australian National University | And 9 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

We present the kinematic results from our ARGOS spectroscopic survey of the Galactic bulge of the Milky Way. Our aim is to understand the formation of the Galactic bulge. We examine the kinematics of about 17 400 stars in the bulge located within 3.5 kpc of the Galactic Centre, identified from the 28 000 star ARGOS survey. We aim to determine if the formation of the bulge has been internally driven from disc instabilities as suggested by its boxy shape, or if mergers have played a significant role as expected from lambda cold dark matter simulations. From our velocity measurements across latitudes b=-5°,-7.5° and-10° we find the bulge to be a cylindrically rotating system that transitions smoothly out into the disc. From observations of 3 fields at b=+10, the kinematics of the bulge show North-South symmetry about the major axis.Within the bulge, we find a kinematically distinct metal-poor population ([Fe/H]>-1.0) that is not rotating cylindrically. The 5 per cent of our stars with [Fe/H] > -1.0 are a slowly rotating spheroidal population, which we believe are stars of the metal-weak thick disc and halo which presently lie in the inner Galaxy. The kinematics of the two bulge components that we identified in ARGOS Paper III (mean [Fe/H]≈-0.25 and [Fe/H]≈+0.15, respectively) demonstrate that they are likely to share a common formation origin and are distinct from the more metal-poor populations of the thick disc and halo which are co-located inside the bulge. We do not exclude an underlying merger generated bulge component but our results favour bulge formation from instabilities in the early thin disc. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Tran K.,Kavli Institute for Astrophysics and Space Research | Levine A.,37 575 Mit Kavli Institute For Astrophysics And Space Research | Rappaport S.,Kavli Institute for Astrophysics and Space Research | Borkovits T.,Baja Astronomical Observatory | And 5 more authors.
Astrophysical Journal | Year: 2013

We report a study of the eclipse timing variations in contact binary systems, using long-cadence lightcurves from the Kepler archive. As a first step, observed minus calculated (O-C) curves were produced for both the primary and secondary eclipses of some 2000 Kepler binaries. We find ∼390 short-period binaries with O-C curves that exhibit (1) random walk-like variations or quasi-periodicities, with typical amplitudes of ±200-300 s, and (2) anticorrelations between the primary and secondary eclipse timing variations. We present a detailed analysis and results for 32 of these binaries with orbital periods in the range of 0.35 ± 0.05 days. The anticorrelations observed in their O-C curves cannot be explained by a model involving mass transfer, which, among other things, requires implausibly high rates of ∼0.01 MȮ yr-1. We show that the anticorrelated behavior, the amplitude of the O-C delays, and the overall random walk-like behavior can be explained by the presence of a starspot that is continuously visible around the orbit and slowly changes its longitude on timescales of weeks to months. The quasi-periods of ∼50-200 days observed in the O-C curves suggest values for k, the coefficient of the latitude dependence of the stellar differential rotation, of ∼0.003-0.013. © 2013. The American Astronomical Society. All rights reserved.


Regaly Zs.,Konkoly Observatory | Regaly Zs.,ELTE Gothard Lendulet Research Group | Kiraly S.,Konkoly Observatory | Kiss L.L.,Konkoly Observatory | Kiss L.L.,University of Sydney
Astrophysical Journal Letters | Year: 2014

We investigate the formation of double-peaked asymmetric line profiles of CO in the fundamental band spectra emitted by young (1-5 Myr) protoplanetary disks hosted by a 0.5-2 M star. Distortions of the line profiles can be caused by the gravitational perturbation of an embedded giant planet with q = 4.7 × 10-3 stellar-to-planet mass ratio. Locally isothermal, two-dimensional hydrodynamic simulations show that the disk becomes globally eccentric inside the planetary orbit with stationary 0.2-0.25 average eccentricity after 2000 orbital periods. For orbital distances 1-10 AU, the disk eccentricity is peaked inside the region where the fundamental band of CO is thermally excited. Hence, these lines become sensitive indicators of the embedded planet via their asymmetries (both in flux and wavelength). We find that the line shape distortions (e.g., distance, central dip, asymmetry, and positions of peaks) of a given transition depend on the excitation energy (i.e., on the rotational quantum number J). The magnitude of line asymmetry is increasing/decreasing with J if the planet orbits inside/outside the CO excitation zone (R CO ≤ 3, 5, and 7 AU for a 0.5, 1, and 2 M star, respectively), thus one can constrain the orbital distance of a giant planet by determining the slope of the peak asymmetry-J profile. We conclude that the presented spectroscopic phenomenon can be used to test the predictions of planet formation theories by pushing the age limits for detecting the youngest planetary systems. © 2014. The American Astronomical Society. All rights reserved..


Fuller J.,Cornell University | Derekas A.,Hungarian Academy of Sciences | Derekas A.,University of Sydney | Borkovits T.,Hungarian Academy of Sciences | And 8 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

We investigate the nature of tidal effects in compact triple-star systems. The hierarchical structure of a triple system produces tidal forcing at high frequencies unobtainable in binary systems, allowing for the tidal excitation of high-frequency p-modes in the stellar components. The tidal forcing exists even for circular, aligned and synchronized systems. We calculate the magnitude and frequencies of three-body tidal forcing on the central primary star for circular and coplanar orbits, and we estimate the amplitude of the tidally excited oscillation modes.We also calculate the secular orbital changes induced by the tidally excited modes and show that they can cause significant orbital decay. During certain phases of stellar evolution, the tidal dissipation may be greatly enhanced by resonance locking. We then compare our theory to observations of HD 181068, which is a hierarchical triply eclipsing star system in the Kepler field of view. The observed oscillation frequencies in HD 181068 can be naturally explained by three-body tidal effects. We then compare the observed oscillation amplitudes and phases in HD 181068 to our predictions, finding mostly good agreement. Finally, we discuss the past and future evolution of compact triple systems like HD 181068. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Ness M.,Australian National University | Freeman K.,Australian National University | Athanassoula E.,Aix - Marseille University | Wylie-de-Boer E.,Australian National University | And 8 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

We present the metallicity results from the ARGOS spectroscopic survey of the Galactic bulge. Our aim is to understand the formation of the Galactic bulge: did it form via mergers, as expected from Λ cold dark matter theory, or from disc instabilities, as suggested by its boxy/peanut shape, or both? Our stars are mostly red clump giants, which have a well-defined absolute magnitude from which distances can be determined. We have obtained spectra for 28 000 stars at a spectral resolution of R = 11 000. From these spectra, we have determined stellar parameters and distances to an accuracy of <1.5 kpc. The stars in the inner Galaxy span a large range in [Fe/H], -2.8 ≤ [Fe/H] ≤ +0.6. From the spatial distribution of the red clump stars as a function of [Fe/H], we propose that the stars with [Fe/H] > -0.5 are part of the boxy/peanut bar/bulge. We associate the lower metallicity stars ([Fe/H] < -0.5) with the thick disc, which may be puffed up in the inner region, and with the inner regions of the metal-weak thick disc and inner halo. For the bulge stars with [Fe/H] > -0.5, we find two discrete populations: (i) stars with [Fe/H] ≈ -0.25 which provide a roughly constant fraction of the stars in the latitude interval b = -5° to -10°, and (ii) a kinematically colder, more metal-rich population with mean [Fe/H] ≈ +0.15 which is more prominent closer to the plane. The changing ratio of these components with latitude appears as a vertical abundance gradient of the bulge. We attribute both of these bulge components to instability-driven bar/bulge formation from the thin disc. We associate the thicker component with the stars of the early less metal-rich thin disc, and associate the more metal-rich population concentrated to the plane with the colder more metal-rich stars of the early thin disc, similar to the colder and younger more metal-rich stars seen in the thin disc in the solar neighbourhood today. We do not exclude a weak underlying classical merger-generated bulge component, but see no obvious kinematic association of any of our bulge stars with such a classical bulge component. The clear spatial and kinematic separation of the two bulge populations (i) and (ii) makes it unlikely that any significant merger event could have affected the inner regions of the Galaxy since the time when the bulge-forming instabilities occurred. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Rappaport S.,Kavli Institute for Astrophysics and Space Research | Deck K.,Kavli Institute for Astrophysics and Space Research | Levine A.,37 575 Mit Kavli Institute For Astrophysics And Space Research | Borkovits T.,Baja Astronomical Observatory | And 7 more authors.
Astrophysical Journal | Year: 2013

We present the results of a search through the photometric database of Kepler eclipsing binaries looking for evidence of hierarchical triple-star systems. The presence of a third star orbiting the binary can be inferred from eclipse timing variations. We apply a simple algorithm in an automated determination of the eclipse times for all 2157 binaries. The "calculated" eclipse times, based on a constant period model, are subtracted from those observed. The resulting O-C (observed minus calculated times) curves are then visually inspected for periodicities in order to find triple-star candidates. After eliminating false positives due to the beat frequency between the ∼1/2 hr Kepler cadence and the binary period, 39 candidate triple systems were identified. The periodic O-C curves for these candidates were then fit for contributions from both the classical Roemer delay and so-called physical delay, in an attempt to extract a number of the system parameters of the triple. We discuss the limitations of the information that can be inferred from these O-C curves without further supplemental input, e.g., ground-based spectroscopy. Based on the limited range of orbital periods for the triple-star systems to which this search is sensitive, we can extrapolate to estimate that at least 20% of all close binaries have tertiary companions. © 2013. The American Astronomical Society. All rights reserved..


Szabados L.,Hungarian Academy of Sciences | Anderson R.I.,Observatoire de Geneva | Derekas A.,Hungarian Academy of Sciences | Derekas A.,University of Sydney | And 6 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

We present an analysis of spectroscopic radial velocity and photometric data of three bright Galactic Cepheids: LR Trianguli Australis (LR TrA), RZ Velorum (RZ Vel) and BG Velorum (BG Vel). Based on new radial velocity data, these Cepheids have been found to be members of spectroscopic binary systems. The ratio of the peak-to-peak radial velocity amplitude to photometric amplitude indicates the presence of a companion for LR TrA and BG Vel. IUE spectra indicate that the companions of RZ Vel and BG Vel cannot be hot stars. The analysis of all available photometric data revealed that the pulsation period of RZ Vel and BG Vel varies monotonically, due to stellar evolution. Moreover, the longest period Cepheid in this sample, RZ Vel, shows period fluctuations superimposed on the monotonic period increase. The lighttime effect interpretation of the observed pattern needs long-term photometric monitoring of this Cepheid. The pulsation period of LR TrA has remained constant since the discovery of its brightness variation. Using statistical data, it is also shown that a large number of spectroscopic binaries still remain to be discovered among bright classical Cepheids. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

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