arXiv subjects

Hui-Gen Liu

Publications and source records attributed to Hui-Gen Liu.

31 records · Page 2Linked to original sources

arXiv2018

Planet Formation in Highly Inclined Binary Systems. II. Orbital Alignment or Anti-alignment and Planet Growth Boost in Intermediate Separation Binaries

Stars are commonly formed in binary systems, which provide a natural laboratory for studying planet formation in extreme conditions. In our first paper (Paper I) of a series Xie et al. (2011), we have shown that the intermediate stage - from planetesimals to planetary embryos/cores - of planet formation can proceed even in highly inclined binaries. Following Paper I, here we numerically study the late stage of terrestrial planet formation, i.e., from embryos to full planets, in binary systems of various orbital configurations. We identify an orbital alignment or anti-alignment effect; namely, although an inclined binary generally misaligns the planetary orbits with respect to the spin axis of the primary host star (i.e., causing large obliquity), it could align or anti-align the planetary orbits with respect to the binary orbit. Such an orbital (anti-)alignment effect is caused by the combination of orbital differential precession and self-damping, and it is mostly significant in cases of intermediate binary separations, i.e., a_B~40-200 AU for terrestrial planet formation around 1 AU from the primary stars. In such intermediate separation binaries, somewhat contrary to intuition, the binary companion can aid planet growth by having increased the rate of collisions, forming significantly more massive but fewer planets. In the other two ends, the companion is ether too close thus plays a violently disruptive role or too wide to have significant effect on planet formation. Future observations, which can discover more planet-bearing binary star systems and constrain their masses and 3-D orbital motions will test our numerical findings.

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arXiv2018

Transiting Exoplanet Monitoring Project (TEMP). IV. Refined System Parameters, Transit Timing Variations and Orbital Stability of the Transiting Planetary System HAT-P-25

We present eight new light curves of the transiting extra-solar planet HAT-P-25b obtained from 2013 to 2016 with three telescopes at two observatories. We use the new light curves, along with recent literature material, to estimate the physical and orbital parameters of the transiting planet. Specifically, we determine the mid-transit times (T$_{C}$) and update the linear ephemeris, T$_{C[0]}$=2456418.80996$\pm$0.00025 [$\mathrm{BJD}_\mathrm{TDB}$] and P=3.65281572$\pm$0.00000095 days. We carry out a search for transit timing variations (TTVs), and find no significant TTV signal at the $ΔT=$80 s-level, placing a limit on the possible strength of planet-planet interactions ($\mathrm{TTV_{G}}$). In the course of our analysis, we calculate the upper mass-limits of the potential nearby perturbers. Near the 1:2, 2:1, and 3:1 resonances with HAT-P-25b, perturbers with masses greater than 0.5, 0.3, and 0.5 $\mathrm{M_{\oplus}}$ respectively, can be excluded. Furthermore, based on the analysis of TTVs caused by light travel time effect (LTTE) we also eliminate the possibility that a long-period perturber exists with $M_{\rm p}> 3000 \,\mathrm{M_{J}}$ within $a=11.2\,{\rm AU}$ of the parent star.

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arXiv2017

Transiting Exoplanet Monitoring Project (TEMP). III. On the Relocation of the Kepler-9~b Transit

The Kepler-9 system harbors three known transiting planets. The system holds significant interest for several reasons. First, the outer two planets exhibit a period ratio that is close to a 2:1 orbital commensurability, with attendant dynamical consequences. Second, both planets lie in the planetary mass "desert" that is generally associated with the rapid gas agglomeration phase of the core accretion process. Third, there exist attractive prospects for accurately measuring both the sky-projected stellar spin-orbit angles as well as the mutual orbital inclination between the planets in the system. Following the original \textit{Kepler} detection announcement in 2010, the initially reported orbital ephemerides for Kepler-9~b and c have degraded significantly, due to the limited time base-line of observations on which the discovery of the system rested. Here, we report new ground-based photometric observations and extensive dynamical modeling of the system. These efforts allow us to photometrically recover the transit of Kepler-9~b, and thereby greatly improve the predictions for upcoming transit mid-times. Accurate ephemerides of this system are important in order to confidently schedule follow-up observations of this system, for both in-transit Doppler measurements as well as for atmospheric transmission spectra taken during transit.

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arXiv2017

Searching for the Transit of the Earth--mass exoplanet Proxima~Centauri~b in Antarctica: Preliminary Result

Proxima Centauri is known as the closest star from the Sun. Recently, radial velocity observations revealed the existence of an Earth-mass planet around it. With an orbital period of ~11 days, the surface of Proxima Centauri b is temperate and might be habitable. We took a photometric monitoring campaign to search for its transit, using the Bright Star Survey Telescope at the Zhongshan Station in Antarctica. A transit-like signal appearing on 2016 September 8th, is identified tentatively. Its midtime, $T_{C}=2,457,640.1990\pm0.0017$ HJD, is consistent with the predicted ephemeris based on RV orbit in a 1$σ$ confidence interval. Time-correlated noise is pronounced in the light curve of Proxima Centauri, affecting detection of transits. We develop a technique, in a Gaussian process framework, to gauge the statistical significance of potential transit detection. The tentative transit signal reported here, has a confidence level of $2.5σ$. Further detection of its periodic signals is necessary to confirm the planetary transit of Proxima Centauri b. We plan to monitor Proxima Centauri in next Polar night at Dome A in Antarctica, taking the advantage of continuous darkness. \citet{Kipping17} reported two tentative transit-like signals of Proxima Centauri b, observed by the Microvariability and Oscillation of Stars space Telescope in 2014 and 2015, respectively. The midtransit time of our detection is 138 minutes later than that predicted by their transit ephemeris. If all the signals are real transits, the misalignment of the epochs plausibly suggests transit timing variations of Proxima Centauri b induced by an outer planet in this system.

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arXiv2016

Detecting planet pairs in mean motion resonances via astrometry method

GAIA leads us to step into a new era with a high astrometry precision of 10 uas. Under such a precision, astrometry will play important roles in detecting and characterizing exoplanets. Specially, we can identify planet pairs in mean motion resonances(MMRs) via astrometry, which constrains the formation and evolution of planetary systems. In accordance with observations, we consider two Jupiters or two super-Earths systems in 1:2, 2:3 and 3:4 MMRs. Our simulations show the false alarm probabilities(FAPs) of a third planet are extremely small while the real two planets can be good fitted with signal-to-noise ratio(SNR)> 3. The probability of reconstructing a resonant system is related with the eccentricities and resonance intensity. Generally, when SNR >= 10, if eccentricities of both planets are larger than 0.01 and the resonance is quite strong, the probabilities to reconstruct the planet pair in MMRs >= 80%. Jupiter pairs in MMRs are reconstructed more easily than super-Earth pairs with similar SNR when we consider the dynamical stability. FAPs are also calculated when we detect planet pairs in or near MMRs. FAPs for 1:2 MMR are largest, i.e., FAPs > 15% when SNR <= 10. Extrapolating from the Kepler planet pairs near MMRs and assuming SNR to be 3, we will discover and reconstruct a few tens of Jupiter pairs and hundreds of super-Earth pairs in 2:3 and 1:2 MMRs within 30 pc. We also compare the differences between even and uneven data cadence and find that planets are better measured with more uniform phase coverage.

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arXiv2016

Exoplanet Orbital Eccentricities Derived From LAMOST-Kepler Analysis

The nearly circular (mean eccentricity ~0.06) and coplanar (mean mutual inclination ~3 deg) orbits of the Solar System planets motivated Kant and Laplace to put forth the hypothesis that planets are formed in disks, which has developed into the widely accepted theory of planet formation. Surprisingly, the first several hundred extrasolar planets (mostly Jovian) discovered using the Radial Velocity (RV) technique are commonly on eccentric orbits ( ~ 0.3). This raises a fundamental question: Are the Solar System and its formation special? The Kepler mission has found thousands of transiting planets dominated by sub-Neptunes, but most of their orbital eccentricities remain unknown. By using the precise spectroscopic host star parameters from the LAMOST observations, we measure the eccentricity distributions for a large (698) and homogeneous Kepler planet sample with transit duration statistics. Nearly half of the planets are in systems with single transiting planets (singles), while the other half are multiple-transiting planets (multiples). We find an eccentricity dichotomy: on average, Kepler singles are on eccentric orbits with ~0.3, while the multiples are on nearly circular ( = 0.04^{+0.03}_{-0.04}) and coplanar ( = 1.4^{+0.8}_{-1.1} deg) orbits similar to the Solar System planets. Our results are consistent with previous studies of smaller samples and individual systems. We also show that Kepler multiples and solar system objects follow a common relation ~(1-2)x between mean eccentricities and mutual inclinations. The prevalence of circular orbits and the common relation may imply that the solar system is not so atypical in the galaxy after all.

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arXiv2016

Global Instability of Exo-Moon System Triggered by Photo-Evaporation

Many exoplanets have been found in orbits close to their host stars and thus they are subject to the effects of photo-evaporation. Previous studies have shown that a large portion of exoplanets detected by the Kepler mission have been significantly eroded by photo-evaporation. In this paper, we numerically study the effects of photo-evaporation on the orbital evolution of a hypothesized moon system around a planet. We find that photo-evaporation is crucial to the stability of the moon system. Photo-evaporation can erode the atmosphere of the planet thus leading to significant mass loss. As the planet loses mass, its Hill radius shrinks and its moons increase their orbital semi-major axes and eccentricities. When some moons approach their critical semi-major axes, global instability of the moon system would be triggered, which usually ends up with two, one or even zero surviving moons. Some lost moons could escape from the moon system to become a new planet orbiting the star or run away further to become a free-floating object in the Galaxy. Given the destructive role of photo-evaporation, we speculate that exo-moons are less common for close-in planets ($<0.1$ AU), especially those around M-type stars, because they are more X-ray luminous and thus enhancing photo-evaporation. The lessons we learn in this study may be helpful for the target selection of on-going/future exomoon searching programs.

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arXiv2015

Eclipsing Binaries From the CSTAR Project at Dome A, Antarctica

The Chinese Small Telescope ARray (CSTAR) has observed an area around the Celestial South Pole at Dome A since 2008. About $20,000$ light curves in the i band were obtained lasting from March to July, 2008. The photometric precision achieves about 4 mmag at i = 7.5 and 20 mmag at i = 12 within a 30 s exposure time. These light curves are analyzed using Lomb--Scargle, Phase Dispersion Minimization, and Box Least Squares methods to search for periodic signals. False positives may appear as a variable signature caused by contaminating stars and the observation mode of CSTAR. Therefore the period and position of each variable candidate are checked to eliminate false positives. Eclipsing binaries are removed by visual inspection, frequency spectrum analysis and locally linear embedding technique. We identify 53 eclipsing binaries in the field of view of CSTAR, containing 24 detached binaries, 8 semi-detached binaries, 18 contact binaries, and 3 ellipsoidal variables. To derive the parameters of these binaries, we use the Eclipsing Binaries via Artificial Intelligence (EBAI) method. The primary and the secondary eclipse timing variations (ETVs) for semi-detached and contact systems are analyzed. Correlated primary and secondary ETVs confirmed by false alarm tests may indicate an unseen perturbing companion. Through ETV analysis, we identify two triple systems (CSTAR J084612.64-883342.9 and CSTAR J220502.55-895206.7). The orbital parameters of the third body in CSTAR J220502.55-895206.7 are derived using a simple dynamical model.

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arXiv2015

Eight Planets in Four Multi-planet Systems via Transit Timing Variations in 1350 Days

Analysis of the transit timing variations (TTVs) of candidate pairs near mean-motion resonances (MMRs) is an effective method to confirm planets. Hitherto, 68 planets in 34 multi-planet systems have been confirmed via TTVs. We analyze the TTVs of all candidates from the most recent Kepler data with a time span as long as about 1350 days (Q0-Q15). The anti-correlations of TTV signals and the mass upper limits of candidate pairs in the same system are calculated, using an improved method suitable for long-period TTVs. If the false alarm probability (FAP) of a candidate pair is less than 10^{-3} and the mass upper limit for each candidate is less than 13 M_J, we confirm them as planets in the same system. Finally, 8 planets in 4 multi-planet systems are confirmed via analysis of their TTVs. All of the 4 planet pairs are near first-order MMRs, including KOI-2672 near 2:1 MMR, KOI-1236, KOI-1563 and KOI-2038 near 3:2 MMR. Four planets have relatively long orbital periods (> 35 day). KOI-2672.01 has a period of 88.51658 days and a fit mass of 17 M_\oplus. It is the longest-period planet confirmed, to date, near first-order MMR via TTVs.

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arXiv2014

Transits of Planets with Small Intervals in Circumbinary Systems

Transit times around single stars can be described well by a linear ephemeris. However, transit times in circumbinary systems are influenced both by the gravitational perturbations and the orbital phase variations of the central binary star. Adopting a coplanar analog of Kepler-16 as an example, we find that circumbinary planets can transit the same star more than once during a single planetary orbit, a phenomenon we call "tight transits". In certain geometric, the projected orbital velocity of the planet and the secondary star can approach zero and change sign, resulting in very long transits and/or 2-3 transits during a single binary orbit. Whether tight transits are possible for a particular system depends primarily on the binary mass ratio and the orbital architecture of both the binary and the planet. We derive a time-dependent criterion to judge when tight transits are possible for any circumbinary system. These results are verified with full dynamical integrations that also reveal other tight transit characteristics, i.e., the transit durations and the intervals between tight transits. For the seven currently known circumbinary systems, we estimate these critical parameters both analytically and numerically. Due to the mutual inclination between the planet and the binary, tight transits can only occur across the less massive star B in Kepler-16, -34, -35, and -47 (for both planets). The long-term average frequency of tight transits (compared to typical transits) for Kepler-16, -34, and -35 are estimated to be several percent. Using full numerical integrations, the next tight transit for each system is predicted and the soonest example appears to be Kepler-47b and -47c, which are likely to have tight transits before 2025. These unique and valuable events often deserve special observational scrutiny.

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arXiv2013

Configurations of Bounded and Free-floating Planets in Very Young Open Clusters

Open clusters(OCs) are usually young and suitable for studying the formation and evolution of planetary systems. Hitherto, only four planets have been found with radial velocity measurements in OCs. Meanwhile, a lot of free-floating planets(FFPs) have been detected. We utilize N-body simulations to investigate the evolution and final configurations of multi-planetary systems in very young open clusters with an age <10 Myr. After an evolution of 10 Myr, 61%-72% of the planets remain bounded and more than 55% of the planetary systems will maintain their initial orbital configurations. For systems with one planet ejected, more than 25% of them have the surviving planets in misaligned orbits. In the clusters, the fraction of planetary systems with misalignment is > 6%, and only 1% have planets in retrograde orbits. We also obtain a positive correlation between the survival planet number and the distance from the cluster center $r$: planetary systems with a larger $r$ tend to be more stable. Moreover, stars with a mass >2.5 $M_\odot$ are likely unstable and lose their planets. These results are roughly consistent with current observations. Planetary systems in binaries are less stable and we achieve a rough criterion: most of the binary systems with $a_{\rm b}(1-e_{\rm b}^2)$>100 AU can keep all the initial planets survived. Finally, 80% of the FFPs are ejected out of the clusters, while the rest (~20%) still stay in host clusters and most of them are concentrated in the center (<2 pc).

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arXiv2013

Where to Find Habitable "Earths" in Circumbinary Systems

Hitherto, six P-type planets are found around five binary systems, i.e. Kepler-16 b, 34 b, 35 b, 38 b, 47 b, c, which are all Neptune or Jupiter-like planets. The stability of planets and the habitable zones are influenced by the gravitational and radiative perturbations of binary companions. In this Letter, we check the stability of an additional habitable Earth-mass planet in each system. Based on our simulations in 10 Myr, a habitable "Earth" is hardly stable in Kepler-16 while a stable "Earth" in Kepler-47 close to the boundaries of the habitable zone is possible. In contrast, Kepler-34, 35 and 38 seem to have high probabilities of being able to tolerate a stable "Earth" in their habitable zones. The affects of transit time variations are quite small due to the small mass of an undetected "Earth", except that of Kepler-16 b. With a time precision of 0.001 day(~ 88 s), an "Earth" in the corotational resonance with Kepler-16 b can be detected in 3 years, while habitable "Earths" in Kepler-34 and 38 systems can be detected in 10 years. Habitable "Earths" in Kepler-35 and 47 are not likely to be detected in 10 years under this precision.

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arXiv2013

A Mechanism of Exciting Planetary Inclination and Eccentricity through a Residual Gas Disk

Accordling to the theory of Kozai resonance, the initial mutual inclination between a small body and a massive planet in an outer circular orbit is as high as $\sim39.2^{\circ}$ for pumping the eccentricity of the inner small body. Here we show that, with the presence of a residual gas disk outside two planetary orbits, the inclination can be reduced as low as a few degrees. The presence of disk changes the nodal precession rates and directions of the planet orbits. At the place where the two planets achieve the same nodal processing rate, vertical secular resonance would occur so that mutual inclination of the two planets will be excited, which might trigger the Kozai resonance between the two planets further. However, in order to pump an inner Jupiter-like planet, the conditions required for the disk and the outer planet are relatively strict. We develop a set of evolution equations, which can fit the N-body simulation quite well but be integrated within a much shorter time. By scanning the parameter spaces using the evolution equations, we find that, a massive planet ($10M_J$) at 30AU with $6^o$ inclined to a massive disk ($50M_J$) can finally enter the Kozai resonance with an inner Jupiter around the snowline. And a $20^{\circ}$ inclination of the outer planet is required for flipping the inner one to a retrograde orbit. In multiple planet systems, the mechanism can happen between two nonadjacent planets, or inspire a chain reaction among more than two planets. This mechanism could be the source of the observed giant planets in moderate eccentric and inclined orbits, or hot-Jupiters in close-in, retrograde orbits after tidal damping.

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