National Astronomical Observatory of China

Chaoyang, China

National Astronomical Observatory of China

Chaoyang, China
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Zheng X.,Peking University | Zheng X.,University of California at Santa Cruz | Zheng X.,Tsinghua University | Lin D.N.C.,Peking University | And 5 more authors.
Astrophysical Journal | Year: 2017

The distribution of heavy elements is anomalously low in the asteroid main belt region compared with elsewhere in the solar system. Observational surveys also indicate a deficit in the number of small (≲50 km size) asteroids, which is two orders of magnitude lower than what is expected from the single power-law distribution that results from a collisional coagulation and fragmentation equilibrium. Here, we consider the possibility that a major fraction of the original asteroid population may have been cleared out by Jupiter's secular resonance, as it swept through the main asteroid belt during the depletion of the solar nebula. This effect leads to the excitation of the asteroids' orbital eccentricities. Concurrently, hydrodynamic drag and planet-disk tidal interaction effectively damp the eccentricities of sub-100 km-size and of super-lunar-size planetesimals, respectively. These combined effects lead to the asteroids' orbital decay and clearing from the present-day main belt region (∼2.1-3.3 au). Eccentricity damping for the intermediate-size (50 to several hundreds of kilometers) planetesimals is less efficient than for small or large planetesimals. These objects therefore preferentially remain as main belt asteroids near their birthplaces, with modest asymptotic eccentricities. The smaller asteroids are the fragments of subsequent disruptive collisions at later times as suggested by the present-day asteroid families. This scenario provides a natural explanation for both the observed low surface density and the size distribution of asteroids in the main belt, without the need to invoke special planetesimal formation mechanisms. It also offers an explanation for the confined spatial extent of the terrestrial planet building blocks without the requirement of extensive migration of Jupiter, which is required in the grand-tack scenario. © 2017. The American Astronomical Society. All rights reserved.

Liu B.,University of Amsterdam | Ormel C.W.,University of Amsterdam | Lin D.N.C.,University of California at Santa Cruz | Lin D.N.C.,Tsinghua University | And 2 more authors.
Astronomy and Astrophysics | Year: 2017

Context. The Kepler mission has discovered that close-in super-Earth planets are common around solar-type stars. They are often seen together in multiplanetary systems, but their period ratios do not show strong pile-ups near mean motion resonances (MMRs). One scenario is that super-Earths form early, in the presence of a gas-rich disk. These planets interact gravitationally with the disk gas, inducing their orbital migration. However, for this scenario disk migration theory predicts that planets will end up at resonant orbits due to their differential migration speed. Aims. Motivated by the discrepancy between observation and theory, we seek a mechanism that moves planets out of resonances. We examine the orbital evolution of planet pairs near the magnetospheric cavity during the gas disk dispersal phase. Our study determines the conditions under which planets can escape resonances. Methods. We extend Type I migration theory by calculating the torque a planet experiences at the interface of the empty magnetospheric cavity and the disk, namely the one-sided torque. We perform two-planet N-body simulations with the new Type I expressions, varying the planet masses, stellar magnetic field strengths, disk accretion rates, and gas disk depletion timescales. Results. As planets migrate outwards with the expanding magnetospheric cavity, their dynamical configurations can be rearranged. Migration of planets is substantial (minor) in a massive (light) disk. When the outer planet is more massive than the inner planet, the period ratio of two planets increases through outward migration. On the other hand, when the inner planet is more massive, the final period ratio tends to remain similar to the initial one. Larger stellar magnetic field strengths result in planets stopping their migration at longer periods. We apply this model to two systems, Kepler-170 and Kepler-180. By fitting their present dynamical architectures, the disk and stellar B-field parameters at the time of disk dispersal can be retrieved. Conclusions. We highlight "magnetospheric" rebound as an important ingredient able to reconcile disk migration theory with observations. Even when planets are trapped into MMRs during the early gas-rich stage, subsequent cavity expansion induces substantial changes to their orbits that move them out of resonance. © ESO 2017.

Ahn K.,Chosun University | Mesinger A.,Normal School of Pisa | Alvarez M.A.,Canadian Institute for Theoretical Astrophysics | Chen X.,National Astronomical Observatory of China
Proceedings of Science | Year: 2014

We present an overview of the theory of high-redshift star and X-ray source formation, and how they affect the 21-cm background. Primary focus is given to Lyα pumping and X-ray heating mechanisms at cosmic dawn, opening a new observational window for high-redshift astrophysics by generating sizable fluctuations in the 21-cm background. We describe observational prospects for power spectrum analysis and 3D tomography (imaging) of the signature of these early astrophysical sources by SKA1-LOW and SKA2. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence.

Zhu M.,National Astronomical Observatory of China | Davis C.J.,Joint Astronomy Center | Wu Y.,Peking University | Whitney B.A.,Space Science Institute | And 2 more authors.
Astrophysical Journal | Year: 2011

We report the discovery of an extremely red object embedded in the massive SCUBA core JCMT 18354-0649S. This object is not associated with any known radio or far-IR source, though it appears in Spitzer IRAC data obtained as part of the GLIMPSE survey. At shorter wavelengths, this embedded source exhibits an extreme color, K - L′ = 6.7. At an assumed distance of 5.7 kpc, this source has a near-IR luminosity of ∼1000 L⊙. Its spectral energy distribution (SED) rises sharply from 2.1 μm to 8 μm, similar to that of a Class0 young stellar object. Theoretical modeling of the SED indicates that the central star has a mass of 6-12 M⊙, with an optical extinction of more than 30. As both inflow and outflow motions are present in JCMT 18354-0649S, we suggest that this deeply embedded source is (1) a massive protostar in the early stages of accretion, and (2) the driving source of a massive molecular outflow evident in HCN J = 3-2 profiles observed toward this region. © 2011. The American Astronomical Society. All rights reserved..

Liu B.,University of Amsterdam | Liu B.,Peking University | Zhang X.,University of California at Santa Cruz | Lin D.N.C.,Peking University | And 3 more authors.
Astrophysical Journal | Year: 2016

Radial velocity and transit surveys have found that the fraction of FGKM stars with close-in super-Earth(s) (η ⊕) is around 30%-50%, independent of the stellar mass M ∗ and metallicity Z ∗. In contrast, the fraction of solar-Type stars harboring one or more gas giants (η J) with masses M p > 100 M ⊕ is nearly 10%-15%, and it appears to increase with both M ∗ and Z ∗. Regardless of the properties of their host stars, the total mass of some multiple super-Earths systems exceeds the core mass of Jupiter and Saturn. We suggest that both super-Earths and supercritical cores of gas giants were assembled from a population of embryos that underwent convergent type I migration from their birthplaces to a transition location between viscously heated and irradiation-heated disk regions. We attribute the cause for the η ⊕-η J dichotomy to conditions required for embryos to merge and to acquire supercritical core mass () for the onset of efficient gaseous envelope accretion. We translate this condition into a critical disk accretion rate, and our analysis and simulation results show that it weakly depends on M ∗ and decreases with metallicity of disk gas Z d. We find that embryos are more likely to merge into supercritical cores around relatively massive and metal-rich stars. This dependence accounts for the observed η J-M ∗. We also consider the dispersed relationship and reproduce the observed η J-Z ∗ correlation. © 2016. The American Astronomical Society. All rights reserved.

Ren Z.,Peking University | Wu Y.,Peking University | Zhu M.,National Astronomical Observatory of China | Liu T.,Peking University | And 3 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

We present a multi-wavelength observational study towards the high-mass young stellar object G8.68-0.37. A single massive gas-and-dust core is observed in the (sub)millimetre continuum and molecular line emissions. We fitted the spectral energy distribution (SED) from the dust continuum emission. The best-fitting SED suggests the presence of two components with temperature of T d= 20 and 120K, respectively. The core has a total mass of up to 1.5 × 10 3M ⊙ and a bolometric luminosity of 2.3 × 10 4L ⊙. Both the mass and luminosity are dominated by the cold component (T d= 20K). The molecular lines of C 18O, C 34S, DCN and thermally excited CH 3OH are detected in this core. Prominent infall signatures are observed in the 12CO (1 - 0) and (2 - 1). We estimated an infall velocity of 0.45kms -1 and a mass infall rate of 7 × 10 -4M ⊙ yr -1. From the molecular lines, we have found a high DCN-to-HCN abundance ratio of 0.07. The overabundant DCN may originate from a significant deuteration in the previous cold pre-protostellar phase. And the DCN should now be rapidly sublimated from the grain mantles to maintain the overabundance in the gas phase. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Pattle K.,University of Central Lancashire | Ward-Thompson D.,University of Central Lancashire | Kirk J.M.,University of Central Lancashire | White G.J.,Open University Milton Keynes | And 74 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2015

In this paper, we present the first observations of the Ophiuchus molecular cloud performed as part of the James Clerk Maxwell Telescope (JCMT) Gould Belt Survey (GBS) with the SCUBA-2 instrument.We demonstratemethods for combining these datawith previous HARP CO, Herschel, and IRAM N2H+ observations in order to accurately quantify the properties of the SCUBA-2 sources in Ophiuchus. We produce a catalogue of all of the sources found by SCUBA-2. We separate these into protostars and starless cores. We list all of the starless cores and perform a full virial analysis, including external pressure. This is the first time that external pressure has been included in this level of detail. We find that the majority of our cores are either bound or virialized. Gravitational energy and external pressure are on average of a similar order of magnitude, but with some variation from region to region. We find that cores in the Oph A region are gravitationally bound prestellar cores, while cores in the Oph C and E regions are pressure-confined. We determine that N2H+ is a good tracer of the bound material of prestellar cores, although we find some evidence for N2H+ freeze-out at the very highest core densities.We find that non-thermal linewidths decrease substantially between the gas traced by C18O and that traced by N2H+, indicating the dissipation of turbulence at higher densities. We find that the critical Bonnor-Ebert stability criterion is not a good indicator of the boundedness of our cores. We detect the pre-brown dwarf candidate Oph B-11 and find a flux density and mass consistent with previous work. We discuss regional variations in the nature of the cores and find further support for our previous hypothesis of a global evolutionary gradient across the cloud from south-west to north-east, indicating sequential star formation across the region. © 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Yu Q.,Peking University | Lu Y.,National Astronomical Observatory of China | Mohayaee R.,CNRS Paris Institute of Astrophysics | Colin J.,CNRS Paris Institute of Astrophysics
Astrophysical Journal | Year: 2011

Dual active galactic nuclei (AGNs) are natural byproducts of hierarchical mergers of galaxies in the ΛCDM cosmogony. Recent observations have shown that only a small fraction (0.1%-2.5%) of AGNs at redshift z ≲ 0.3 are dual with kpc-scale separations, which is rather low compared to the high merger rate of galaxies. Here we construct a phenomenological model to estimate the number density of dual AGNs and its evolution according to the observationally estimated major merger rates of galaxies and various scaling relations on the properties of galaxies and their central massive black holes. We show that our model reproduces the observed frequency and separation distribution of dual AGNs provided that significant nuclear activities are triggered only in gas-rich progenitor galaxies with central massive black holes and only when the nuclei of these galaxies are roughly within the half-light radii of their companion galaxies. Under these constraints, the observed low dual AGN frequency is consistent with the relatively high merger rate of galaxies and supports the hypothesis that major mergers lead to AGN/QSO activities. We also predict that the number of kpc-scale dual AGNs decreases with increasing redshift and only about 0.02%-0.06% of AGNs are dual AGNs with double-peaked narrow line features at redshifts of z 0.5-1.2. Future observations of high-redshift dual AGNs would provide a solid test for this prediction. © 2011. The American Astronomical Society. All rights reserved.

Liu J.,National Astronomical Observatory of China | Liu J.,Harvard - Smithsonian Center for Astrophysics | Di Stefano R.,Harvard - Smithsonian Center for Astrophysics | Wang T.,Harvard - Smithsonian Center for Astrophysics | And 2 more authors.
Astrophysical Journal | Year: 2012

The explosion of a Type Ia supernova, SN2011fe, in the nearby Pinwheel galaxy (M101 at 6.4 Mpc) provides an opportunity to study pre-explosion images and search for the progenitor, which should consist of a white dwarf (WD), possibly surrounded by an accretion disk, in orbit with another star. We report on our use of deep Chandra observations and Hubble Space Telescope observations to limit the luminosity and temperature of the pre-explosion WD. It is found that if the spectrum was a blackbody, then pre-SN WDs with steady nuclear burning of the highest possible temperatures and luminosities are excluded assuming moderate n H values, but values of kT between roughly 10eV and 60eV are permitted even if the WD was emitting at the Eddington luminosity. This allows the progenitor to be an accreting nuclear-burning WD with an expanded photosphere 4-100 times the WD itself, or a super-critically accreting WD blowing off an optically thick strong wind, or possibly a recurrent nova with luminosities an order of magnitude lower than Eddington. The observations are also consistent with a double degenerate scenario, or a spinning down WD that has been spun up by accretion from the donor. © 2012. The American Astronomical Society. All rights reserved.

Liu J.,National Astronomical Observatory of China | Liu J.,Harvard - Smithsonian Center for Astrophysics | Orosz J.,San Diego State University | Bregman J.N.,University of Michigan
Astrophysical Journal | Year: 2012

Dynamical mass measurements hold the key to answering whether ultraluminous X-ray sources (ULXs) are intermediate-mass black holes (IMBHs) or stellar-mass black holes with special radiation mechanisms. NGC 1313 X-2 is so far the only ULX with Hubble Space Telescope light curves, the orbital period, and the black hole's radial velocity amplitude based on the He II λ4686 disk emission line shift of ∼ 200kms-1. We constrain its black hole mass and other parameters by fitting observations to a binary light curve code with accommodations for X-ray heating of the accretion disk and the secondary. Given the dynamical constraints from the observed light curves and the black hole radial motion and the observed stellar environment age, the only acceptable models are those with 40-50Myr old intermediate-mass secondaries in their helium core and hydrogen shell burning phase filling 40%-80% of their Roche lobes. The black hole can be a massive black hole of a few tens of M⊙ that can be produced from stellar evolution of low-metallicity stars, or an IMBH of a few hundred to above 1000 M⊙ if its true radial velocity 2K′ < 40kms-1. Further observations are required to better measure the black hole radial motion and the light curves in order to determine whether NGC 1313 X-2 is a stellar-mass black hole or an IMBH. © 2012. The American Astronomical Society. All rights reserved.

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