Research
In order to understand the origin of our solar system and the diverse properties of exo-planetary systems, it is essential to observationally study when and where planets form in protoplanetary disks. Although protoplanetary disks are often observed around young stellar objects, such as T Tauri or Herbig Ae/Be stars, it remains unclear as to how and when such disks come to existence, and whether they possess forming planets or not. To answer these questions, I have been studying two topics:
Formation and evolution of circumstellar disks around protostars: The goals are to understand when and how circumstellar disks form around protostars and evolve to be protoplanetary disks, and the underlying physics that regulates the disk formation and evolution.
Searching for forming planets in protoplanetary disks: The goals are to find the signposts of forming planets, that is the perturbation by forming planets on the structures and dynamics in protoplanetary disks, and thus, to understand the location and environment of planet formation.
Below are some of my research results.
Figure. (a) Schematic figure of our kinematic model. Red and blue arrows denote the directions of the outflow from the protostar. This protostar is surrounded by a Keplerian disk, and there two streams of infalling flow connecting the disk. The infalling flows transfer mass and angular momentum to the disk, allowing it to grow. (b) Image of the observed gas distribution around the protostar (contour) overlaid on the model distribution generated using our kinematic model (color). A cross shows the location of the protostar.
We have conducted ALMA observations toward one of protostars likely having forming disks to investigate detailed kinematics of its ambient gas. Our observational results show that most of the ambient gas is following Keplerian rotation around the protostar, suggesting a circumstellar disk is indeed formed. Moreover, we further find that the motional velocity of the outer ambient gas is deviated from the Keplerian rotation. Our analyses of its structures and velocity features show that the outer gas is free falling toward the disk. Such the gas motion can transfer mass and angular moment to the disk from its envelope materiel, allowing the disk to grow. Our ALMA observations capture this on-going disk formation. (Yen et al. 2014, ApJ, 793, 1).
Figure. Protostellar mass versus disk radius for a sample of protostars. Blue diamonds and red squares present the Class I and 0 protostars. Red squares with an arrow display the Class 0 protostars without directly observed Keplerian disks. Their inferred protostellar masses are lower limits. Their disk radii are upper limits. The results show that the Keplerian disk radius quickly grows from <10 AU to ~100 as the protostellar mass increases from <0.1 solar mass to ~0.2 solar mass. Dark and light green lines denote the scaling relations between protostellar mass and disk radius in the collapse models in Terebey et al. (1984) and Basu (1998), respectively.
In order to investigate the disk formation and evolution, I have observed Keplerian disks around a sample of young protostars (Yen et al. 2013, 2014) and constrained disk sizes at the early evolutionary stage (Yen et al. 2010, 2015b, 2017) by analyzing gas kinematics with SMA and ALMA observations. Combining my results, especially my three measurements of the protostars <0.1 solar mass, with additional results from the literature shows the sign of rapid disk growth from the Class 0 to I stages, and the disk growth rate declines at the Class I stage (Yen et al. 2017).
Figure. Averaged spectra of H2CO lines in the protoplanetary disk around HD 163296 obtained with the ALMA archival data. Black histograms present the conventional averaged spectra from the original image cubes, and red ones are from my method that aligns the centroid velocities of the spectra according to their Keplerian velocities at different positions within the disk area before averaging. With my method, the originally undetectable spectra of the H2CO lines are materialized at more than 3 sigma.
In order to find the signposts of forming planets, I have been developing methods to detect fine features in structures and kinematic patterns of protoplanetary disks, for example, revealing gas gaps in the disk around HL Tau and enhancing S/Ns of molecular-line detections in protoplanetary disks (Yen et al. 2016a,b). This method takes advantage of the Keplerian rotational velocity pattern. It aligns spectra according to their different centroid velocities at their different positions in a disk and stacks them. My method can be applied to search for faint molecular lines in protoplanetary disks, and to measure intensity profiles of molecular lines more accurately and at higher spatial resolutions