午餐會報 (2023)

The epoch of matter-radiation equality, a fascinating era in the evolution of the Universe, leaves a unique imprint on the matter power spectrum, manifesting as a characteristic turnover. This intriguing feature can serve as an alternative standard ruler, the longest in the cosmos. I will present our latest findings derived from the quasar sample of the extended Baryon Oscillation Spectroscopic Survey (eBOSS). Additionally, I will outline our ongoing endeavors to accurately measure this turnover using the cutting-edge Dark Energy Spectroscopic Instrument (DESI).

Nanosecond precision in FRB time delay measurements opens up a qualitative new regime for lensing observations. I will outline the many novel applications that may arise in this new lensing paradigm, as well as discuss the necessary theoretical and observational developments needed to take full advantage of lensed FRBs.

We often talk about the star formation history of the Galaxy, but what about the planet formation history of the Milkyway? When did the first planets form in the Galaxy? Did rocky planets form first and gas giants later? Answering these questions is essential for understanding the formation and evolution of exoplanetary systems in the galactic context. Understanding the age distribution of exoplanet host stars and how it relates to the evolution of the Galaxy requires a large statistical study of the exoplanet host stars with uniformly determined ages. However, measuring the ages of the individual main-sequence-stars is challenging. One way to overcome this is to measure the ensemble ages of stars using the dispersion in their peculiar velocities. Stars in the solar neighborhood are known to show a strong correlation between stellar age and velocity dispersion. With Gaia DR3, we have the largest, most accurate measurements of proper motions, parallaxes, and radial velocities of stars, and a large statistical study is now possible. The [Fe/H] in the solar neighborhood also is a strong function of velocity dispersion. Hence studying the velocity dispersion as a function of age and [Fe/H] can help determine the [Fe/H] enrichment history of the Milkyway. From our analysis, we show that as the planet mass increases, the velocity dispersion (age) of the host stars decreases, suggesting that Jupiter-like planets only formed in the last 5-6 Gyr after significant enrichment of the galactic ISM with metals, while rocky planets have been forming for the last 10 Gyr. We further find that debris disks are younger (having smaller velocity dispersion) yet metal-poorer than Jupiter-hosting stars. In this talk, I will discuss our results and, in combination with the velocity dispersion-stellar metallicity relation, examine the implications of our results for planet formation in the context of galactic evolution.

Understanding orbital evolution and planet formation through the lenses of N-body simulations is crucial for chaotic dynamical systems. On the other hand, atmospheric and climate modeling can provide us with unique insights into atmospheric evolution, habitability, and the observation prospects of rocky extrasolar planets. In this talk, I will present results from two separate studies combining techniques of N-body simulations in conjunction with atmospheric and climate modeling. In the first study, we used a three-dimensional N-Rigid-Body integrator and an intermediately-complex general circulation model to simulate the evolving climates of TRAPPIST-1 e and f with different orbital and spin evolution pathways. We find that sporadic libration and rotation induced by planetary interactions, such as that due to mean motion resonances (MMRs) in compact planetary systems may destabilize attendant exoplanets away from synchronized states (or 1:1 spin-orbit ratio). Planet f perturbed by MMR effects with sporadic spin-variations are colder and dryer compared to their synchronized counterparts due to the zonal drift of the substellar point away from open ocean basins of their initial eyeball states. On the other hand, the differences between perturbed and synchronized planet e are minor due to higher instellation, warmer surfaces, and reduced climate hysteresis. This is the first study to incorporate the time-dependent outcomes of direct gravitational N-Rigid-Body simulations into 3D climate modeling of extrasolar planets and our results show that planets at the outer edge of the habitable zones in compact multi-planet systems are vulnerable to rapid global glaciations. In the absence of external mechanisms such as orbital forcing or tidal heating, these planets could be trapped in permanent snowball states. In a second study, we examine the building blocks of pre-planetary materials using the outputs of an N-body planet formation paradigm and a Python-based volatile accretion model. For Earth-like protoplanets, our results suggest that volatile elemental ratios (e.g., C/N, C/H) are driven by the complex interplay between delivery, atmospheric ablation, and mantle degassing.

With an emphasis on improving the fidelity even in super-resolution regimes, new imaging techniques have been intensively developed over the last several years, which may provide substantial improvements to the interferometric observation (such as ALMA) of protoplanetary disks (PPDs). In this talk, I will introduce a novel super-resolution imaging technique utilizing sparse modeling to enhance the fidelity and spatial resolution of the ALMA images, as well as its principle. Next, I will turn to our recent results of high-resolution images of 43 Taurus-Auriga protoplanetary disks with spatial resolutions ranging from 0.01 to 0.1 arcseconds (1-14 au at a distance of 140 pc), 2-3 times higher than those obtained from conventional imaging technique. The resolved dust disks have radii that widely range from 8 to 238 au but exhibit a roughly similar average surface brightness temperature of 8 K. The analysis revealed various substructures, such as gap and ring, regardless of disk size. The locations of the gaps show bimodal distribution peaking at 10-20 au and 30-100 au. The locations of gaps and rings show a linear relationship, while several systems do not lie in this correlation. Such outlier’s gap widths are 2-3 times larger than typical sizes on the corresponding locations. Finally, I will discuss two types of these disks with different gap sizes that indicate the presence of two distinct planet-forming mechanisms by assuming that the gaps are carved by planets and that their sizes are proportional to planetary masses.

Planet formation begins with the coagulation of dust grains in protoplanetary disks. Therefore, constraints on the dust size distribution in the disks can provide a clue to understanding planet formation. Previous studies have estimated the dust size distribution using the spectral index derived from multi-wavelength observations or dust polarization observations. However, these studies give different results depending on their methods and model assumptions and do not reach a consensus. In this work, we propose another indirect method to constrain the dust size distribution by using the wavelength dependency of the dust ring widths. In many disks, there are dust and gas rings, and larger dust grains are more effectively trapped in the gas rings, forming narrower dust rings. As a result, the dust rings are expected to appear narrower at longer wavelengths since the observations are sensitive to the dust grains whose size is comparable to the observed wavelength. We analyze high-resolution Band 4 (2.1 mm) and Band 6 (1.3 mm) images of the HD 163296 disk. We find for the first time that the width of the outer dust ring (100 au) appears 1.2 times narrower at the longer wavelength, while the width of the inner ring (67 au) is similar between the two bands. The difference in the ring width depending on the observed wavelength is consistent with the dust trapping scenario in a gas ring. We constrain the maximum dust size a_max and the exponent of the dust size distribution p from the difference in dust ring width between the two bands, together with the spectral index, and found that 1 mm < a_max < 20 mm and p < 3.5 in the inner ring, and a_max > 30 mm and 3.4 < p < 3.6 in the outer ring. These results suggest that the degree of dust growth is spatially dependent, which could affect the formation of planetesimals.

Starless cores are the potential sites for star and planet formation. Although we know gravity plays the main role during evolution, the details, in particular the timescale, are not yet well understood. The lifetime suggested by different scenarios could vary by more than a factor of ten. With time-dependent chemical analysis, measuring the chemical timescale of the cores allows us to infer possible evolutionary scenarios. We determine the density, temperature, and molecular abundance profiles of two nearby low-mass starless cores, L1512 and L1498, with dust extinction measurements from near-infrared observations and non-local thermal equilibrium radiative transfer with single-dish radio observations (H2D+ as well as deuterated N-bearing tracers with IRAM30m, JCMT, and GBT). Then we perform chemical modeling of the two targets to measure their chemical timescales (t) using deuterium fractionation as a chemical clock. We find that L1512 is chemically evolved while L1498 is chemically young. This might imply that the magnetic field is stronger in L1512 than in L1498. Consequently, ambipolar diffusion may have slowed the contraction of L1512 or even halted it to the present state.

Protostellar jets are the most intriguing characteristics in the magnetized, accreting young stellar objects. The ejection of jets and accretion outbursts are closely related to the formation of complex organic molecules (COMs) in the class 0/I protostars. However, the launching mechanism of the protostellar jet and its correlation with COMs formation are not constrained observationally. In this presentation, I shall briefly introduce the current theory of jet launching. Then I shall present our recent results on jet/outflow from the high spatial resolution and high sensitivity observations with Atacama Large Millimeter/submillimeter Array (ALMA), which is currently the largest radio interferometry array on Earth. Finally, I shall describe the possible connection of the jet with the accretion burst and formation of complex organic molecules within the disk, which would be the chemical composition of future planets.