Re: NCTS Astrophysics Student Lunch Seminars [July to December 2025]

Dear colleagues and students,

We will have two NCTS student seminars this week on Friday, August 22. The talks will take place in Cosmology Hall at 12:00 p.m. on the 4th floor. You can sign up for lunch boxes here:

https://forms.gle/8MXzPRfRN2qey7eb8

Cheers, Min-Kai

1. Model the streamer Structure in Collapsing Prestellar Cores.

Tsung-Han Chuang (NTNU)

Streamers have been observed with high-resolution ALMA observations around many protoplanetary disks undergoing formation. They have been suggested to dominate the mass accretion budget from the collapsing prestellar core, and have important consequences on the disk dynamics. Understanding the formation of streamers is therefore important for explaining how protoplanetary disks receive mass from the envelope. We propose a model to explain the formation of streamer structures by considering density enhancements due to gravitational instability. We test our model against sources where streamers have been detected (ex. Per-emb-2 and Per-emb-50 observed with NOEMA) and fit for model parameters. This allows us to gain deeper insights into the physical origin of streamers and their role in mass transport from the core to the disk. This model can be applied to analyze new observations as well as archival data that show signs of streamers.

2. A Rotation Dip in the Envelope-Disk Transition Region: Evidence of Magnetic Braking

Jyun-Heng Lin (NTHU/ASIAA)

The envelope-disk transition region in protostellar system is key to understanding how angular momentum is removed during star formation. Magnetic braking, predicted to operate in this region, can remove angular momentum and regulate disk growth, but direct observational evidence remains limited. Using ALMA C18O (J = 2–1) data of the HH 111 VLA1 system, we analyzed gas kinematics within 6000 au of the protostar. We find clear deviations from simple free-fall with conserved angular momentum across the transition region (~5200–160 au), which can be divided into three zones: (1) an outer region with reduced infall velocity, likely due to magnetic tension from pinched field lines; (2) a middle region showing a sharp drop in rotation velocity and angular momentum, consistent with strong magnetic braking; and (3) an inner region where rotation rises toward Keplerian velocity and infall ceases, likely due to weaker braking from ambipolar diffusion. The resulting dip in the rotation profile provides observational evidence for magnetic braking.