Young stars and their planetary systems form from the collapse of their embryo, a cold core. The stellar growth is thought to occur primarily when the star is still deeply embedded in this natal envelope and visible only in the far-IR and sub-mm wavelengths. The accretion processes that cause the star to grow also drive powerful outflows that disrupt the stellar envelope. Meanwhile, much of the mass in the envelope has settled into a disk, which will be the birthplace of planets.
As the envelope dissipates the star becomes visible in the UV/optical/infrared wavelengths, allowing for the accretion and disk evolution processes to be directly studied. The disk is truncated by the stellar magnetosphere at several stellar radii, from where gas flows along the magnetic field lines until it strikes the stellar surface. At larger radii in the disk, giant planets are thought to sculpt the disk as they form, producing dust traps that may trigger formation of additional planetary cores. Eventually the primordial disk must dissipate by a combination of accretion, photoevaporation, and planet formation. The resulting planetary systems are diverse, including hot Jupiters, super-Earths, systems with 6 known planets, and systems of multiple planets located at tens of AU from the central star.
Each of these stages last for 10^5-10^6 years, so observations of the evolution of young stellar systems requires the identification and study of objects in each stage. My research observationally investigates the physical processes that determine the evolution from cold cores to planetary systems. At the youngest stages, Herschel/PACS spectroscopy of young stars reveal the heating and cooling in outflows as they interact with the dense stellar envelopes. At older stages, far-ultraviolet spectroscopy shows unique diagnostics of the accretion flow and outflows, and directly demonstrates the effect of strong irradiation of the disk by light produced by accretion onto the central star. With optical spectroscopy we measure the accretion rate and its variability due to stellar rotation and disk instabilities.
(1) Contribution to a Science article, "A Major Asymmetric Dust Trap in a Transition Disk', by van der Marel et al., 2013, Science, 340, 1199. A press release describing the work is here:
A similar English-language press release is here:
(2) A student working with me, Yifan Zhou, is about to submit a Letter describing measurements of accretion rate onto young planetary mass objects. Seeing these planets during their formation helps to improve our understanding of formation mechanisms. We will likely have a press release when accepted.
(3) A paper by Herczeg & Hillenbrand will soon be accepted deriving new stellar properties of stars in one of the nearest and best studied star forming regions, Taurus. Our approach for measuring spectral type, extinction, and veiling simultaneously significantly improves age estimates and our understanding of the observational uncertainties when using pre-main sequence tracks.
INVOLVEMENT IN LARGE PROGRAMS AND PROGRAMS RELATED TO THE CHINESE ASTRONOMY COMMUNITY:
P.I. of a large HST/COS+STIS program, "Disks, Accretion, and Outflows from T Tauri Stars", 111 orbits to survey T Tauri stars in the far-ultraviolet. 9 published papers to date
WISH and DIGIT Herschel Key Projects (P.I. van Dishoeck and Evans): Co-I in large Herschel Key Programs to survey young stellar objects in the far-IR with PACS and HIFI spectroscopy.
Beijing Exoplanet Meetings: Participation in monthly meetings of astronomers in Beijing who work on exoplanets, led by Tian Feng.
ISSI-BJ: Co-convener of an ISSI-Beijing workshop on exoplanets and planet formation, to be hosted by NSSC in Beijing in late August 2014