分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The orientation between a star's spin axis and a planet's orbital plane provides valuable information about the system's formation and dynamical history. For non-transiting planets at wide separations, true stellar obliquities are challenging to measure, but lower limits on spin-orbit orientations can be determined from the difference between the inclination of the star's rotational axis and the companion's orbital plane ($\Delta i$). We present results of a uniform analysis of rotation periods, stellar inclinations, and obliquities of cool stars (SpT $\gtrsim$ F5) hosting directly imaged planets and brown dwarf companions. As part of this effort, we have acquired new $v \sin i_*$ values for 22 host stars with the high-resolution Tull spectrograph at the Harlan J. Smith telescope. Altogether our sample contains 62 host stars with rotation periods, most of which are newly measured using light curves from the Transiting Exoplanet Survey Satellite. Among these, 53 stars have inclinations determined from projected rotational and equatorial velocities, and 21 stars predominantly hosting brown dwarfs have constraints on $\Delta i$. Eleven of these (52$^{+10}_{-11}$% of the sample) are likely misaligned, while the remaining ten host stars are consistent with spin-orbit alignment. As an ensemble, the minimum obliquity distribution between 10-250 AU is more consistent with a mixture of isotropic and aligned systems than either extreme scenario alone--pointing to direct cloud collapse, formation within disks bearing primordial alignments and misalignments, or architectures processed by dynamical evolution. This contrasts with stars hosting directly imaged planets, which show a preference for low obliquities. These results reinforce an emerging distinction between the orbits of long-period brown dwarfs and giant planets in terms of their stellar obliquities and orbital eccentricities.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We present the discovery of Kepler-129 d ($P_{d}=7.2^{+0.4}_{-0.3}$ yr, $m\sin i_{d}=8.3^{+1.1}_{-0.7}\ \rm M_{Jup}$, $ e_{d}=0.15^{+0.07}_{-0.05} $) based on six years of radial velocity (RV) observations from Keck/HIRES. Kepler-129 also hosts two transiting sub-Neptunes: Kepler-129 b ($P_{b}=15.79$ days, $r_{b}=2.40\pm{0.04}\ \rm{R_{\oplus}}$) and Kepler-129 c ($P_{c}=82.20$ days, $r_{c}=2.52\pm{0.07}\ \rm{R_{\oplus}}$) for which we measure masses of $m_{b}<20\ \rm{M_{\oplus}}$ and $m_{c}=43^{+13}_{-12}\ \rm{M_{\oplus}}$. Kepler-129 is an hierarchical system consisting of two tightly-packed inner planets and an external companion whose mass is close to the deuterium burning limit. In such a system, two inner planets precess around the orbital normal of the outer companion, causing their inclinations to oscillate with time. Based on an asteroseismic analysis of Kepler data, we find tentative evidence that Kepler-129 b and c are misaligned with stellar spin axis by $\gtrsim 38$ deg, which could be torqued by Kepler-129 d if it is inclined by $\gtrsim 19$ deg relative to inner planets. Using N-body simulations, we provide additional constraints on the mutual inclination between Kepler-129 d and inner planets by estimating the fraction of time during which two inner planets both transit. The probability that two planets both transit decreases as their misalignment with Kepler-129 d increases. We also find a more massive Kepler-129 c enables the two inner planets to become strongly coupled and more resistant to perturbations from Kepler-129 d. The unusually high mass of Kepler-129 c provides a valuable benchmark for both planetary dynamics and interior structure, since the best-fit mass is consistent with this $\rm{2.5R_{\oplus}}$ planet having a rocky surface.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Inference is crucial in modern astronomical research, where hidden astrophysical features and patterns are often estimated from indirect and noisy measurements. Inferring the posterior of hidden features, conditioned on the observed measurements, is essential for understanding the uncertainty of results and downstream scientific interpretations. Traditional approaches for posterior estimation include sampling-based methods and variational inference. However, sampling-based methods are typically slow for high-dimensional inverse problems, while variational inference often lacks estimation accuracy. In this paper, we propose alpha-DPI, a deep learning framework that first learns an approximate posterior using alpha-divergence variational inference paired with a generative neural network, and then produces more accurate posterior samples through importance re-weighting of the network samples. It inherits strengths from both sampling and variational inference methods: it is fast, accurate, and scalable to high-dimensional problems. We apply our approach to two high-impact astronomical inference problems using real data: exoplanet astrometry and black hole feature extraction.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Orbital eccentricities directly trace the formation mechanisms and dynamical histories of substellar companions. Here, we study the effect of hyperpriors on the population-level eccentricity distributions inferred for the sample of directly imaged substellar companions (brown dwarfs and cold Jupiters) from hierarchical Bayesian modeling (HBM). We find that the choice of hyperprior can have a significant impact on the population-level eccentricity distribution inferred for imaged companions, an effect that becomes more important as the sample size and orbital coverage decrease to values that mirror the existing sample. We reanalyse the current observational sample of imaged giant planets in the 5-100 AU range from Bowler et al. (2020) and find that the underlying eccentricity distribution implied by the imaged planet sample is broadly consistent with the eccentricity distribution for close-in exoplanets detected using radial velocities. Furthermore, our analysis supports the conclusion from that study that long-period giant planets and brown dwarf eccentricity distributions differ by showing that it is robust to the choice of hyperprior. We release our HBM and forward modeling code in an open-source Python package, ePop!, and make it freely available to the community.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We report the discovery of TOI-1444b, a 1.4-$R_\oplus$ super-Earth on a 0.47-day orbit around a Sun-like star discovered by {\it TESS}. Precise radial velocities from Keck/HIRES confirmed the planet and constrained the mass to be $3.87 \pm 0.71 M_\oplus$. The RV dataset also indicates a possible non-transiting, 16-day planet ($11.8\pm2.9M_\oplus$). We report a tentative detection of phase curve variation and secondary eclipse of TOI-1444b in the {\it TESS} bandpass. TOI-1444b joins the growing sample of 17 ultra-short-period planets with well-measured masses and sizes, most of which are compatible with an Earth-like composition. We take this opportunity to examine the expanding sample of ultra-short-period planets ($3R_\oplus$, $>2000F_\oplus$ TOI-849 b, LTT9779 b and K2-100). We find that 1) USPs have predominately Earth-like compositions with inferred iron core mass fractions of 0.32$\pm$0.04; and have masses below the threshold of runaway accretion ($\sim 10M_\oplus$), while ultra-hot Neptunes are above the threshold and have H/He or other volatile envelope. 2) USPs are almost always found in multi-planet system consistent with a secular interaction formation scenario; ultra-hot Neptunes ($P_{\rm orb} \lesssim$1 day) tend to be ``lonely' similar to longer-period hot Neptunes($P_{\rm orb}$1-10 days) and hot Jupiters. 3) USPs occur around solar-metallicity stars while hot Neptunes prefer higher metallicity hosts. 4) In all these respects, the ultra-hot Neptunes show more resemblance to hot Jupiters than the smaller USP planets, although ultra-hot Neptunes are rarer than both USP and hot Jupiters by 1-2 orders of magnitude.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: TOI-561 is a galactic thick disk star hosting an ultra-short period (0.45 day orbit) planet with a radius of 1.37 R$_{\oplus}$, making it one of the most metal-poor ([Fe/H] = -0.41) and oldest ($\sim$10 Gyr) sites where an Earth-sized planet has been found. We present new simultaneous radial velocity measurements (RVs) from Gemini-N/MAROON-X and Keck/HIRES, which we combined with literature RVs to derive a mass of M$_{b}$=2.24 $\pm$ 0.20 M$_{\oplus}$. We also used two new Sectors of TESS photometry to improve the radius determination, finding R$_{b}$=$1.37 \pm 0.04 R_\oplus$, and confirming that TOI-561 b is one of the lowest-density super-Earths measured to date ($\rho_b$= 4.8 $\pm$ 0.5 g/cm$^{3}$). This density is consistent with an iron-poor rocky composition reflective of the host star's iron and rock-building element abundances; however, it is also consistent with a low-density planet with a volatile envelope. The equilibrium temperature of the planet ($\sim$2300 K) suggests that this envelope would likely be composed of high mean molecular weight species, such as water vapor, carbon dioxide, or silicate vapor, and is likely not primordial. We also demonstrate that the composition determination is sensitive to the choice of stellar parameters, and that further measurements are needed to determine if TOI-561 b is a bare rocky planet, a rocky planet with an optically thin atmosphere, or a rare example of a non-primordial envelope on a planet with a radius smaller than 1.5 R$_{\oplus}$.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The detection of satellites around extrasolar planets, so called exomoons, remains a largely unexplored territory. In this work, we study the potential of detecting these elusive objects from radial velocity monitoring of self-luminous directly imaged planets. This technique is now possible thanks to the development of dedicated instruments combining the power of high-resolution spectroscopy and high-contrast imaging. First, we demonstrate a sensitivity to satellites with a mass ratio of 1-4% at separations similar to the Galilean moons from observations of a brown-dwarf companion (HR 7672 B; Kmag=13; 0.7" separation) with the Keck Planet Imager and Characterizer (KPIC; R~35,000 in K band) at the W. M. Keck Observatory. Current instrumentation is therefore already sensitive to large unresolved satellites that could be forming from gravitational instability akin to binary star formation. Using end-to-end simulations, we then estimate that future instruments such as MODHIS, planned for the Thirty Meter Telescope, should be sensitive to satellites with mass ratios of ~1e-4. Such small moons would likely form in a circumplanetary disk similar to the Jovian satellites in the solar system. Looking for the Rossiter-McLaughlin effect could also be an interesting pathway to detecting the smallest moons on short orbital periods. Future exomoon discoveries will allow precise mass measurements of the substellar companions that they orbit and provide key insight into the formation of exoplanets. They would also help constrain the population of habitable Earth-sized moons orbiting gas giants in the habitable zone of their stars.