Welcome

Hello, I’m Feng Long, an SMA Fellow at the Harvard-Smithsonian Center for Astrophysics. I obtained my PhD in 2019 at Kavli Institute for Astronomy and Astrophysics of Peking University (北京大学), working with Prof. Gregory Herczeg. Before that, I received my BS degree also from Peking University.

I have broad interests in star and planet formation. In particually, my research focuses on the formation and evolution of protoplanetary disks - the cradle of young planets, using observations from ALMA, SMA, and VLA etc, to understand the initial conditions of planet formation.

Research

  • Disk Small-scale Substructures

    The striking image of HL Tau from the ALMA Science Verification observation in 2014 has opened up the window of observational planet formation at its earliest stage. The revealed features with dust gaps and rings can naturally explain the long-standing radial drift barrier in planet formation and facilitate the build-up of young planets. Questions like in what forms, how prevelence, how diverse, disk substructure are, remain to be answered with surveys towards disk samples spanning sufficient parameter space. Works towards their origins and detailed consequences for planet formation has just started.

    Our Taurus survey serves as one of the earliest explorations towards a sample view of disk substructures. We have performed a high-resolution (~0.1”) ALMA survey (PI. Herczeg) at 1.3 mm for a sample of 32 disks with spectral type earlier than M3 in Taurus Clouds. Follow-up observations for a sub-sample has then conducted at the longer 3 mm with comparable angular resolution (PI. Long). Key results include:

    1) Dust substructures are prevalent, observed in 1/3 of our sample, mostly seen as axisymmetric rings and gaps; Long, F., Pinilla, P., Herczeg, G. J, et al. ApJ 2018 ; Liu, Y., Dipierro, G., Ragusa, E. et al. A&A 2019

    2) The correlation between gap locations and widths, the intensity contrast between rings and gaps, and the separations of rings and gaps could all be explained if most gaps are opened by low-mass planets (super-Earths and Neptunes) with low viscosity; see also Lodato, G., Dipierro, G., Ragusa, E., Long, F. et al. ApJ 2019

    3) The gap locations are not well correlated with the expected locations of CO and N2 ice lines, so condensation fronts are unlikely to be a universal mechanism to create gaps and rings.

    4) The similar dust emission morphology at 1.3 and 3 mm for three ring disks imply the presence of pressure bumps at outer disk, which could be created by planet-disk interactions. Dust rings, with high density and enhanced grain growth, however, serve as the promising sites for the formation of new planets; Long, F., Pinilla, P., Herczeg, G. J, et al. ApJ 2020


  • Diversity in Disk Dust Evolution

    The observed rich diversity in planetary architecture, at least in part, originates from their natal protoplanetary disk. From the aformentioned ALMA Taurus survey, 2/3 of our sample do not present detectable substructures, whose properties reveal interesting features:

    1) The sub-sample of disks without detetabel disk features are compact (with disk radius < 50au), while they share similar distributions in disk brightness and stellar mass to the disks with substructures; Long, F., Herczeg, G. J, Harsono, D. et al. ApJ 2019

    2) Among the compact disks, the dust disk radii for disks in multiple systems are smaller than these in single stellar systems, implying the effect of tidal truncation; Manara, C., Tazzari, M., Long, F. et al. ApJ 2019


  • Disk Mass Budget

    The mass of a protoplanetary disk limits the formation and future evolution of any planet, and is thus one of the most important input quantities for planet formation models. ALMA has allowed us to take snapshot observations for the millimeter brightness of hundreds to thousands of disks in the young (1-10 Myr) nearby star-forming regions in the past few years. These observations enable a statistic understanding of the mass budget in disks.

    Our Chamaeleon I survey is one of the many early experienments. We have conducted the 0.89 mm continuum and CO gas observations (PI. Pascucci) for 93 protoplanetary disks in the nearby and young (2-3 Myr) Chamaeleon I star-forming region, with key results:

    1) Disk dust masses are well correlated with the stellar masses with a steeper-than-linear scaling relation; Pascucci, I., Testi, L., Herczeg, G. J, Long, F. et al. ApJ 2016

    2) The stellar-disk mass correlation bears with considerable scatters, which is in part due to stellar multiplicity; Long, F., Herczeg, G. J,Pascucci, I. et al. ApJ 2018

    3) Assuming typical ISM CO-to-H2 abundance ratio, resulting gas masses are implausibly low, implying an early giant planet formation process. Alternatively, the gas masses may be severely underestimated if CO-to-H2 abundance ratio is lower than the ISM value, which may be caused by C and/or O depletion and lock-up, or if CO freeze-out is underestimated Long, F., Herczeg, G. J,Pascucci, I. et al. ApJ 2017


  • Gas Disk Size

    The disk extension of the gas component provides crucial information on disk formation and subsequent viscous evolution. Their comparison to the dust disk radii (Rgas/Rdust) also reveals the differential evolution between dust grains and gas particles, e.g., efficiency of dust radial drift. Stay tuned for gas disk measurement for a sample of 44 disks :)


  • The Stellar and Disk Architecture of V892 Tau System

    V892 Tau, a young system in the Taurus cloud, contains two A stars, separated by 50 mas (~7au), surroundded by a gas and dust-rich circumbinary disk. We will study the binary-disk interaction by constraining the binary orbit and the disk geometry, as well as interesting gas kinematic features introduced by binary-disk interaction in the inner disk and tidal features from the pontenial 4" stellar companion. Stay tuned!


  • Modeling HNC and HCN line emission in disks

    Long et al. 2020 A&A in revision

Publications

Resources

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