（有删减）扩充适合进行详细天体测量和光谱研究的邻近年轻恒星直接成像伴星样本，对于其演化与大气过程理论的持续发展和验证至关重要。近期发布的 Gaia 天体测量星表使我们能够高效地搜寻这些难以捉摸的伴星，方法是通过锁定那些表现出由轨道伴星引起的天体测量反射运动的恒星。附近的（27 pc）、年轻的（24 Myr）恒星 AF Leporis (AF Lep) 因其天体测量加速度而被选为观测目标，这与可通过高对比度成像探测到的宽轨道行星伴星相一致。我们使用 VLT 上的 SPHERE 仪器在 AF Lep 附近搜寻暗淡的亚恒星伴星。我们使用与 AF Lep 交错的邻近恒星观测，来高效地扣除残余点扩散函数。这提供了对恒星 1 角秒（约 30 au）范围内暗淡行星质量伴星的探测灵敏度。我们在 339 mas（9 au）的间距处探测到了伴星 AF Lep b，位于其未分辨出的碎屑盘的内边缘。测得的 K 波段对比度和恒星年龄得出其依赖于模型的 4 至 6 倍木星质量（M_Jup），这与轨道拟合推导出的 4.3 加 2.9 减 1.2 倍 M_Jup 质量相一致。该行星的近红外光谱能量分布（SED）与处于 L-T 光谱型过渡的物体一致，但相较于场星重力天体而言光度偏低。AF Lep b 加入了在年轻 β Pic 移动星群恒星周围成像发现的越来越多的亚恒星伴星之列。其质量在 3 至 7 倍 M_Jup 之间，填补了该等时序列上的一个空白，介于较热、质量较大的伴星（如 PZ Tel B 和 β Pic b）与较冷的 51 Eri b（其冷到足以在其光球层内形成甲烷）之间。
英文摘要：
(abridged) Expanding the sample of directly imaged companions to nearby, young stars that are amenable to detailed astrometric and spectroscopic studies is critical for the continued development and validation of theories of their evolution and atmospheric processes. The recent release of the {\it Gaia} astrometric catalogue allows us to efficiently search for these elusive companions by targeting those stars that exhibit the astrometric reflex motion induced by an orbiting companion. The nearby (27 pc), young (24 Myr) star AF Leporis (AF Lep) was targeted because of its astrometric acceleration, consistent with a wide-orbit planetary companion detectable with high-contrast imaging.
We used the SPHERE instrument on the VLT to search for faint substellar companions in the immediate vicinity of AF Lep. We used observations of a nearby star interleaved with those of AF Lep to efficiently subtract the residual point spread function. This provided sensitivity to faint planetary-mass companions within 1 arcsec ($\sim$30 au) of the star. We detected the companion AF Lep b at a separation of 339 mas (9 au), within the inner edge of its unresolved debris disk. The measured $K$-band contrast and the age of the star yield a model-dependent mass of 4 and 6 $M_{\rm Jup}$, consistent with the mass derived from an orbital fit of $4.3_{-1.2}^{+2.9}$ $M_{\rm Jup}$. The near-infrared SED of the planet is consistent with an object at the L--T spectral type transition, but under-luminous with respect to field-gravity objects.
AF Lep b joins a growing number of substellar companions imaged around stars in the young $β$ Pic moving group. With a mass of between 3 and 7 $M_{\rm Jup}$, it occupies a gap in this isochronal sequence between the hotter, more massive companions like PZ~Tel~B and $β$~Pic~b, and the cooler 51~Eri~b, which is sufficiently cool for methane to form within its photosphere.
Exomoon detections might be feasible with NASA's Kepler or ESA's upcoming PLATO mission or the ground-based E-ELT. To use observational resources most efficiently we need to know where the largest, most easily detected moons can form. We explore the possibility of large exomoons by following the movement of water (H2O) ice lines in the accretion disks around young super-Jovian planets. We want to know how different heating sources in those disks affect the H2O ice lines. We simulate 2D rotationally symmetric accretion disks in hydrostatic equilibrium around super-Jovian exoplanets. The energy …
Acoustic oscillations in stars can be driven by gravitational waves. However, at present it is not feasible to use the helioseismology data for their detection, since it is impossible to disentangle the uncertain driving contributions originating in the Sun itself. We here point out that any such wave will affect also Jupiter and Saturn in a similar $and$ $coordinated$ $way$; after all, they are mostly spheres of gas like the Sun, only one order of magnitude smaller. Hence, akin to the concept of coincidence detection in particle physics experiments, evaluation of the time-correlation function…
Several exoplanets have recently been imaged at wide separations of >10 AU from their parent stars. These span a limited range of ages (<50 Myr) and atmospheric properties, with temperatures of 800--1800 K and very red colors (J - H > 0.5 mag), implying thick cloud covers. Furthermore, substantial model uncertainties exist at these young ages due to the unknown initial conditions at formation, which can lead to an order of magnitude of uncertainty in the modeled planet mass. Here, we report the direct imaging discovery of a Jovian exoplanet around the Sun-like star GJ 504, detected as part of …
We present an updated dynamical and statistical analysis of outer Oort cloud cometary evidence suggesting the sun has a wide-binary Jovian mass companion. The results support a conjecture that there exists a companion of mass ~ 1-4 M_Jup orbiting in the innermost region of the outer Oort cloud. Our most restrictive prediction is that the orientation angles of the orbit normal in galactic coordinates are centered on the galactic longitude of the ascending node Omega = 319 degree and the galactic inclination i = 103 degree (or the opposite direction) with an uncertainty in the normal direction s…
Studies have shown that remnants of destroyed planets and debris-disk planetesimals can survive the volatile evolution of their host stars into white dwarfs, but detection of intact planetary bodies around white dwarfs are few. Simulations predict that planets in Jupiter-like orbits around stars of $\lt 8 M_\odot$ avoid being destroyed by the strong tidal forces of their stellar host, but as yet there has been no observational confirmation of such a survivor. Here we report on the non-detection of a main-sequence lens star in the microlensing event MOA-2010-BLG-477Lb using near-infrared observ…
Most of the exoplanets with known masses at Earth-like distances to Sun-like stars are heavier than Jupiter, which raises the question of whether such planets are accompanied by detectable, possibly habitable moons. Here we simulate the accretion disks around super-Jovian planets and find that giant moons with masses similar to Mars can form. Our results suggest that the Galilean moons formed during the final stages of accretion onto Jupiter, when the circumjovian disk was sufficiently cool. But in contrast to other studies, with our assumptions, we show that Jupiter was still feeding from the…
We report the discovery of planetary companions orbiting four low-luminosity giant stars with M$_\star$ between 1.04 and 1.39 M$_\odot$. All four host stars have been independently observed by the EXoPlanets aRound Evolved StarS (EXPRESS) program and the Pan-Pacific Planet Search (PPPS). The companion signals were revealed by multi-epoch precision radial velocities obtained during nearly a decade. The planetary companions exhibit orbital periods between $\sim$ 1.2 and 7.1 years, minimum masses of m$_{\rm p}$sini $\sim$ 1.8-3.7 M$_{jup}$ and eccentricities between 0.08 and 0.42. Including these…
Kepler-454 (KOI-273) is a relatively bright (V = 11.69 mag), Sun-like starthat hosts a transiting planet candidate in a 10.6 d orbit. From spectroscopy, we estimate the stellar temperature to be 5687 +/- 50 K, its metallicity to be [m/H] = 0.32 +/- 0.08, and the projected rotational velocity to be v sin i <2.4 km s-1. We combine these values with a study of the asteroseismic frequencies from short cadence Kepler data to estimate the stellar mass to be 1.028+0:04-0:03 M_Sun, the radius to be 1.066 +/- 0.012 R_Sun and the age to be 5.25+1:41-1:39 Gyr. We estimate the radius of the 10.6 d planet …
We report the discovery of a super-Jovian planet in the microlensing event KMT-2016-BLG-1836, which was found by the Korea Microlensing Telescope Network's high-cadence observations (Γ~ 4~{hr}^{-1}). The planet-host mass ratio q ~ 0.004. A Bayesian analysis indicates that the planetary system is composed of a super-Jovian M_{planet} = 2.2_{-1.1}^{+1.9} M_{J} planet orbiting an M or K dwarf M_{\rm host} = 0.49_{-0.25}^{+0.38} M_{Sun}, at a distance of D_{L} = 7.1_{-2.4}^{+0.8} kpc. The projected planet-host separation is 3.5^{+1.1}_{-0.9} AU, implying that the planet is located beyond the snowl…
We explore atmospheric escape from close-in exoplanets with the highest mass loss rates. First, we locate the transition from stellar X-ray and UV-driven escape to rapid Roche lobe overflow, which occurs once the 10-100 nbar pressure level in the atmosphere reaches the Roche lobe. Planets enter this regime when the ratio of the substellar radius to the polar radius along the visible surface pressure level, that aligns with a surface of constant Roche potential, is X/Z~$\gtrsim$~1.2 for Jovian planets (Mp~$\gtrsim$~100 M$_{\Earth}$) and X/Z~$\gtrsim$~1.02 for sub-Jovian planets ($M_p \approx$~1…
We present radial velocity measurements of two stars observed as part of the Lick Subgiants Planet Search and the Keck N2K survey. Variations in the radial velocities of both stars reveal the presence of Jupiter-mass exoplanets in highly eccentric orbits. HD 16175 is a G0 subgiant from the Lick Subgiants Planet Search, orbited by a planet having a minimum mass of 4.4 M_Jup, in an eccentric (e = 0.59), 2.71 yr orbit. HD 96167 is a G5 subgiant from the N2K ("Next 2000") program at Keck Observatory, orbited by a planet having a minimum mass of 0.68 M_Jup, in an eccentric (e = 0.71), 1.366 yr orbi…
Jupiter's enhancement in nitrogen relative to hydrogen when compared to the Sun has been interpreted as evidence that its early formation occurred beyond the N$_{2}$ snowline ($\sim$ 20-40 AU). However, the rapid growth necessary to form Jupiter before the dissipation of the solar nebula would lead to the forming planet's core reaching very high temperatures ($>$1000 K), which would lead to it warming its surroundings. Here, we explore the effects of a luminous planetary core on the solids that it ultimately accretes. We find that a critical transition occurs where very hot (rapidly accreting)…
Stellar flybys in star clusters may perturb the evolution of young planetary systems in terms of disk truncation, planetary migration and planetary mass accretion. We investigate the feedback of a young planetary system during a single close stellar encounter in a typical open young stellar cluster. We consider 5 masses for the stellar perturbers: 0.5, 0.8, 1, 3 and 8 M$_{Sun}$, in coplanar, prograde and retrograde orbits respect to the planetary disk, varying the pertruber-host star orbital periastron from 100 AU to 500 AU. We have made 3D modelizations with the smooth particle hydrodynamics …
We present an analysis of ~5 years of Lick Observatory radial velocity measurements targeting a uniform sample of 31 intermediate-mass subgiants (1.5 < M*/Msun < 2.0) with the goal of measuring the occurrence rate of Jovian planets around (evolved) A-type stars and comparing the distributions of their orbital and physical characteristics to those of planets around Sun-like stars. We provide updated orbital solutions incorporating new radial velocity measurements for five known planet-hosting stars in our sample; uncertainties in the fitted parameters are assessed using a Markov Chain Monte Car…
Exoplanet systems with multiple transiting planets are natural laboratories for testing planetary astrophysics. One such system is HD 191939 (TOI-1339), a bright (V=9) and Sun-like (G9V) star, which TESS found to host three transiting planets (b, c, and d). The planets have periods of 9, 29, and 38 days each with similar sizes from 3 to 3.4 $R_{\oplus}$. To further characterize the system, we measured the radial velocity (RV) of HD 191939 over 415 days with Keck/HIRES and APF/Levy. We find that $M_b = 10.4 \pm 0.9 M_{\oplus}$ and $M_c = 7.2 \pm 1.4 M_{\oplus}$, which are low compared to most k…
We report the discovery of a planet in the microlensing event OGLE-2018-BLG-1269, with planet-host mass ratio $q \sim 6\times10^{-4}$, i.e., $0.6$ times smaller than the Jupiter/Sun mass ratio. Combined with the $Gaia$ parallax and proper motion, a strong one-dimensional constraint on the microlens parallax vector allows us to significantly reduce the uncertainties of lens physical parameters. A Bayesian analysis that ignores any information about light from the host yields that the planet is a cold giant $(M_{2} = 0.69_{-0.22}^{+0.44}\,M_{\rm J})$ orbiting a Sun-like star $(M_{1} = 1.13_{-0.3…
We report the discovery of a planetary system from observation of the high-magnification microlensing event OGLE-2012-BLG-0026. The lensing light curve exhibits a complex central perturbation with multiple features. We find that the perturbation was produced by two planets located near the Einstein ring of the planet host star. We identify 4 possible solutions resulting from the well-known close/wide degeneracy. By measuring both the lens parallax and the Einstein radius, we estimate the physical parameters of the planetary system. According to the best-fit model, the two planet masses are ~0.…
The detection of Earth-size exoplanets around low-mass stars -- in stars such as Proxima Centauri and TRAPPIST-1 -- provide an exceptional chance to improve our understanding of the formation of planets around M stars and brown dwarfs. We explore the formation of such planets with a population synthesis code based on a planetesimal-driven model previously used to study the formation of the Jovian satellites. Because the discs have low mass and the stars are cool, the formation is an inefficient process that happens at short periods, generating compact planetary systems. Planets can be trapped …
Exoplanets display incredible diversity, from planetary system architectures around Sun-like stars that are very different to our Solar System, to planets orbiting post-main sequence stars or stellar remnants. Recently the B-star Exoplanet Abundance STudy (BEAST) reported the discovery of at least two super-Jovian planets orbiting massive stars in the Sco Cen OB association. Whilst such massive stars do have Keplerian discs, it is hard to envisage gas giant planets being able to form in such hostile environments. We use N-body simulations of star-forming regions to show that these systems can …
This paper reports a blind search for magnetospheric emissions from planets around nearby stars. Young stars are likely to have much stronger stellar winds than the Sun, and because planetary magnetospheric emissions are powered by stellar winds, stronger stellar winds may enhance the radio luminosity of any orbiting planets. Using various stellar catalogs, we selected nearby stars (<~ 30 pc) with relatively young age estimates (< 3 Gyr). We constructed different samples from the stellar catalogs, finding between 100 and several hundred stars. We stacked images from the 74-MHz (4-m wavelength)…
We have found that Proxima Centauri, the star closest to our Sun, will pass close to a pair of faint background stars in the next few years. Using Hubble Space Telescope (HST) images obtained in 2012 October, we determine that the passage close to a mag 20 star will occur in 2014 October (impact parameter 1.6"), and to a mag 19.5 star in 2016 February (impact parameter 0.5"). As Proxima passes in front of these stars, the relativistic deflection of light will cause shifts in the positions of the background stars by ~0.5 and 1.5 mas, respectively, readily detectable by HST imaging, and possibly…
Recent studies have shown that large exomoons can form in the accretion disks around super-Jovian extrasolar planets. These planets are abundant at about 1 AU from Sun-like stars, which makes their putative moons interesting for studies of habitability. Technological advances could soon make an exomoon discovery with Kepler or the upcoming CHEOPS and PLATO space missions possible. Exomoon climates might be substantially different from exoplanet climates because the day-night cycles on moons are determined by the moon's synchronous rotation with its host planet. Moreover, planetary illumination…
An Outer Giant Planet or Brown Dwarf in the 51 Pegasi System?
University of California, Santa Barbara（加州大学圣塔芭芭拉分校）；University of California, Santa Cruz（加州大学圣克鲁兹分校）；University of Hawai’i at Mänoa（夏威夷大学马诺阿分校）；Yale University（耶鲁大学）；The University of Texas at Austin（德克萨斯大学奥斯汀分校）；University of California Los Angeles（加州大学洛杉矶分校）；Michigan State University（密歇根州立大学）
Context. Centaurs go around the Sun between the orbits of Jupiter and Neptune. Only a fraction of the known centaurs have been found to display comet-like features. Comet 29P/Schwassmann-Wachmann 1 is the most remarkable active centaur. It orbits the Sun just beyond Jupiter in a nearly circular path. Only a handful of known objects follow similar trajectories. Aims. We present photometric observations of 2020 MK4, a recently found centaur with an orbit not too different from that of 29P, and we perform a preliminary exploration of its dynamical evolution. Methods. We analyzed broadband Cousins…
Direct imaging is a powerful exoplanet discovery technique that is complementary to other techniques and offers great promise in the era of 30 meter class telescopes. Space-based transit surveys have revolutionized our understanding of the frequency of planets at small orbital radii around Sun-like stars. The next generation of extremely large ground-based telescopes will have the angular resolution and sensitivity to directly image planets with $R < 4R_\oplus$ around the very nearest stars. Here, we predict yields from a direct imaging survey of a volume-limited sample of Sun-like stars with …
The nearby M-dwarf AP Col was recently identified by Riedel et al. 2011 as a pre-main sequence star (age 12 - 50 Myr) situated only 8.4 pc from the Sun. The combination of its youth, distance, and intrinsically low luminosity make it an ideal target to search for extrasolar planets using direct imaging. We report deep adaptive optics observations of AP Col taken with VLT/NACO and Keck/NIRC2 in the L-band. Using aggressive speckle suppression and background subtraction techniques, we are able to rule out companions with mass m >= 0.5 - 1M_Jup for projected separations a>4.5 AU, and m >= 2 M_Jup…
Abbreviated. By selecting stars with similar ages and masses, the Young Suns Exoplanet Survey (YSES) aims to detect and characterize planetary-mass companions to solar-type host stars in the Scorpius-Centaurus association. Our survey is carried out with VLT/SPHERE with short exposure sequences on the order of 5 min per star per filter. The subtraction of the stellar point spread function (PSF) is based on reference star differential imaging (RDI) using the other targets in the survey in combination with principal component analysis. We report the discovery of YSES 2b, a planetary-mass companio…
The recent announcement of a Neptune-sized exomoon candidate around the transiting Jupiter-sized object Kepler-1625 b could indicate the presence of a hitherto unknown kind of gas giant moons, if confirmed. Three transits have been observed, allowing radius estimates of both objects. Here we investigate possible mass regimes of the transiting system that could produce the observed signatures and study them in the context of moon formation in the solar system, i.e. via impacts, capture, or in-situ accretion. The radius of Kepler-1625 b suggests it could be anything from a gas giant planet somew…
