Yapeng Zhang
Astronomer
I am a PhD student at Leiden Observatory, working on characterisations of exoplanet atmospheres under the supervision of Prof. Ignas Snellen. I obtained my Master's degree in Astronomy and Data Science at Leiden University. Prior to that, I studied astronomy at Nanjing University in China. My research interests include various aspects of extra-solar planets and formation of planetary systems, with a particular focus on exoplanet atmospheres. Here is my Curriculum Vitae.
ADS library of Yapeng Zhang's publications.
Sep 2019 - Present
Characterization of exoplanet atmospheres with high-resolution spectroscopy
With the rapidly growing number of extra-solar planets detected, we have entered an era of characterizing their physical properties. As exoplanets show immense diversity in their masses, sizes, compositions and orbital configurations, they pose great challenges to planet-formation theories. The characterization of exoplanet atmospheres is essential for investigating these questions because atmospheres provide fossil records of the formation condition and evolution history of planets. Moreover, the detection of biosignatures such as water, oxygen and ozone in atmospheres can indicate the habitability of exoplanets. Spectroscopy of exoplanets can reveal physical, chemical and dynamical properties of their atmospheres, however these observations tend to be overwhelmed by the light coming from the host stars. One way of overcoming this is to use high resolution spectroscopy, which resolves the atomic or molecular features into tens or hundreds of individual lines. The Doppler shift of the spectral lines due to the orbital motion of planets can help disentangle the planetary signals from the stationary stellar and telluric lines. This method is sensitive to the depth, shape and position of the lines, therefore can constrain various aspects of atmospheres including chemical composition, thermal structure, clouds and hazes, spin, high-altitude winds and atmospheric escape. This has been successfully implemented on giant planets, and the aim of future work is to push it to the limit that enables characterization of smaller Earth-like planets.
Sep 2017 - July 2019
Dust evolution in protoplanetray disks
Supervised by Prof. Michiel Hogerheijde and Dr. Leon Trapman, we studied the effect of global dust evolution and gap formation on multi-wavelength continuum observations of TW Hydrae. This project involves processing of radio data at multiple wavelengths and modeling of the physical disk, by which we investigated the dust distribution and evolution in the disk.
Probing exoplanet atmospheres with Integral-Field Spectroscopy
Supervised by Prof. Ignas Snellen and Dr. Paul Wilson, the project utilised Integral-Field Spectroscopic data for atmospheric characterisation of directly imaged giant exoplanets with large orbital separation, e.g. beta Pictoris b and HR 8799c.
Sep 2013 - July 2017
Simulations on planet formation in highly inclined binary systems
Under the supervision of Prof. Jiwei Xie, this project numerically studied the late stage of terrestrial planet formation, i.e., from embryos to full planets, in binary systems of various orbital configurations. An orbital alignment effect and enhancement of planet formation in cases of intermediate binary separations were suggested.
yzhang@strw.leidenuniv.nl
Leiden Observatory, Niels Bohrweg 2, 2333CA Leiden, The Netherlands
Oort 535
Dust evolution in protoplanetary disks plays an essential role in the formation of planets. Multi-wavelength thermal continuum emission of disks can provide constraints on the dust distribution and dust evolution processes such as radial drift, grain growth and fragmentation. We analyze archival ALMA observations of the TW Hydrae disk at three different wavelengths, Band 3 (3.1 mm), Band 6 (1.3 mm) and Band 9 (0.454 mm), and implement three approaches of disk modeling in order to infer the dust surface density profile. Firstly, the optical depth radial profile at each wavelength is parameterized as a broken power-law, which reproduces the observed visibilities. The steepening of continuum emissions at large radius is in agreement with the expectations for dust evolution. The shapes of the radial profiles at different wavelengths are distinct, indicating the radial dependence of the dust opacity. Taking that into account, a physical dust evolution model for TW Hya is implemented, but it turns out to provide insufficient dust opacity in comparison with the observations. As it is supposed that the presence of gaps can halt dust grains at large radius to maintain the dust content, we superimpose gaps on the parameterized radial profiles to investigate this effect. However, none of these models result in a consistent description of the emission at all wavelengths. This implies that the dynamical interaction between dust grains and gaps significantly shapes the dust distribution. In this case, a global dust evolution model is ineffectual in interpreting the dust distribution due to such locally coupled processes. Instead, statistical studies of populations of disks may unveil the origin of gaps and therefore put constraints on the co-evolving dust distribution.
Searching for Earth-like exoplanets has been one of the holy grails of astronomers. Unlike the planets within our solar system, the exoplanets are so far away that the chances of approching them using spacecrafts are rather dim. Thus, the feasible way of probing exoplanets is via telescopes. There are several the state-of-art detection methods, including transit, radial velocity, microlensing and direct imaging, among which the direct imaging technique is most straightforward.
High contrast imaging (HCI) techniques, which are assisted by adaptive optics (AO), coronagraphs and innovations on the observation and data reduction techniques, have been well established to enable the direct imaging of young Jovian exoplanets. However, HCI su↵ers from the quasi-stationary speckle noise when it comes to small angular separations. In parallel, the high dispersion spectroscopy (HDS) has shown potential on characterizing exoplanet atmospheres, although it is limited by the overwhelming starlight. The new concept of combining the HCI and HDS techniques is able to enhance the contrast limit of the detection, and directly detect the absorptions by molecules in exoplanet atmospheres. It takes advantage of the fact that the planetary spectrum is distinct from the stellar spectrum at high spectral resolution because of the molecular absorption features which are exclusively present in the planetary spectrum. In this way, the planetary signal can be distinguished from the stellar speckle noise utilizing the cross-correlation technique. The aim of this thesis is exploring the application of this new method to the integral-field spectroscopic (IFS) data obtained with OSIRIS on the Keck telescope and SINFONI on the Very Large Telescope. For OSIRIS data, we analyzed the archival observation of HR 8799 c and new data of GJ 504 b. We managed to recover HR 8799 c, but got no detection of GJ 504 b because of the high brightness contrast of the planet relative to its host star. For SINFONI data, the previous result of ! Pictoris b was successfully reproduced so as to validate our method, which was then applied to the recent observations of HD 100546 b. The analysis ended up with no detection of HD 100546 b due to integration time limitations, while we suggest that the detection is probable provided a reasonably longer integration time. Consequently, we draw the conclusion that our method performs properly, and the detailed analysis in this thesis demonstrates the potential of applying our method in detecting and characterizing exoplanet atmospheres.
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Stars are commonly formed in binary systems, which provide a natural laboratory for studying planet formation in extreme conditions. We numerically study the late stage of terrestrial planet formation, i.e., from embryos to full planets, in binary systems of various orbital configurations. We identify an orbital alignment effect; namely, although an inclined binary generally misaligns the planetary orbits with respect to the spin axis of the primary host star (i.e., causing large obliquity), it could align the planetary orbits with respect to the binary orbit. Such an orbital alignment effect is caused by the combination of orbital differential precession and self-damping, and it is mostly significant in cases of intermediate binary separations, i.e., 40–200 au for planet formation around 1 au from the primary stars. In such intermediate separation binaries, somewhat contrary to intuition, the binary companion can aid planet growth by having increased the rate of collisions, forming significantly more massive but fewer planets. On the other two ends, the companion is either too close, and thus plays a violently disruptive role, or too wide to have a significant effect on planet formation. Future observations that can discover more planet-bearing binary star systems and constrain their masses and 3D orbital motions will test our numerical findings.
