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I am actually a theorist working on the fundamental building blocks of astrophysics: stars. In a more detailed context, I specialize in modeling stellar and binary evolution, astroseismology, and tidal physics in binaries. I have made key contributions to our understanding of stars as a core developer of two widely used public codes: GYRE (an open-source stellar oscillations code) and POSYDON (a single and binary-star population synthesis code), as well as a long-term user in modelling stars with MESA (an open-source stellar evolution code). I have published papers on various projects spanning diverse topics, from exoplanets to massive stars, and from young stars to evolved stars and compact objects.

GYRE-tides: a new open-source code to model stellar tides

The regular GYRE code calculates wave frequencies and functions for the (unforced) natural modes from a giving stellar profile. In Sun et al. (2023), we show the code capabilities have been extended to encompass forced oscillations, known as GYRE-tides, such that the interior perturbations of the primary star caused by the gravitational field of a companion can be calculated. Again, in our code, we do not use the tidal Q approach to simulate tides that involves parameterized equations, as this method usually underestimates the circularization rate and overestimates the orbital shrinking rate. GYRE-tides undertakes a full numerical solution of the tidal equations without approximations.

The code is a couple of orders of magnitude faster than the other publicly available forced oscillation code, and can be widely applied to modeling the orbital evolution of close binaries hosting intermediate- and high-mass stars. Just as MESA and GYRE allowed a much larger community to carry out stellar structure calculations, we expect that GYRE-tides will allow non-specialists to carry out state-of-the-art calculations of tidal friction in binary and exoplanet systems. 

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The timescale for the orbital circularization rate is shown as a function of orbital period for a 5 solar mass main-sequence star hosting a 1.4 solar mass neutron star. Resonances are labeled beneath with the mode classification, and in parentheses, the harmonic index k and the sense of propagation in the corotating frame (p = prograde, r = retrograde). If the resonance is with an overstable mode, then the label border is shown in bold.

Core Developer of the POSYDON Code

 

Two types of simulation models help us predict the population of binary compact objects we may observe: rapid binary population synthesis (BPS) codes and detailed stellar and binary evolution codes (e.g. MESA, which is the predominantly used in star community, for evolving various types of stars and binaries). The latter code calculates stellar structures at each time step during the evolution, and the effects of binary evolution is included. However, the simulation is computationally expensive. Due to computational power limitations, fast BPS codes do not self-consistently evolve each star's structure with the orbit; instead, they rely on fitting formulas that connect important stellar parameters and orbital configurations. Therefore, fast BPS codes could provide inaccurate information about the population of compact objects, as well as incorrect formation histories.

 

POSYDON is a next-generation and publicly available BPS code that utilizes detailed stellar structure and binary simulations generated by MESA. The POSYDON code balances both speed and accuracy, providing the best of both worlds. The second version of POSYDON expands the metallicity coverage range from metal-poor to metal-rich stars. We successfully created more than 300,000 MESA binary tracks to comprehend the broader scope of detailed binary evolution and to train with machine learning algorithms based on these detailed simulations.

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The above Figure shows the complicated mapping from initial binary parameters and metallicities to different evolutionary outcomes and formation histories of compact objects.

Extremely Low-mass White Dwarfs in Binaries and Astroseismology

In recent developments, extremely low-mass (ELM, its mass is usually less than 0.2 solar mass) helium white dwarfs have been observed pulsating in close binaries. Notably, the ELM white dwarf binary system J1112+1117 exhibits pressure-driven pulsations (p-modes), making it the only white dwarf with detected p-modes. These pulsations provide insights into the star's internal structure, giving us a look into the star itself: gravity-driven modes (g-modes) reveal core details, while p-modes indicate envelope size.

 

In contrast to the garden-variety, isolated white dwarfs, these ELM white dwarfs form exclusively through binary interactions. In Sun & Arras (2018), we explored the possible formation channels of ELM WDs. Using my code and GYRE, I conducted a detailed asteroseismic analysis, providing a better understanding of both core and envelope structures for ELM WDs.

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This plot shows the MESA binary modeling evolutionary tracks in the effective temperature and gravity plane of those discovered ELM white dwarfs in black dots surrounded by boxes, which indicate the error bars. The color indicates the helium-core mass.

Tides in WASP-12 System

WASP-12b is a massive hot Jupiter with an orbital period of only 1.1 day, making it one of the shortest-period giant planets known. Transit timing observations by Maciejewski et al. (2016) and Patra et al. (2017) measured a decreasing orbital period over a timescale of 3.2 Myr. This is the only exoplanet system that has been detected with unambiguous orbital decay. In Weinberg et al. (2017) , I computed the orbital decay rate caused by the resonant excitation of the star's internal gravity waves (also known as "dynamical tides"), which is of the same order of magnitude as the observed decay rate. This study also found that the host star of WASP-12b is a subgiant.

Stellar Tides in APOGEE Binaries Containing Evolved Stars


Motivated by the discovery of hundreds of close binary systems containing red giant branch stars (where tidal effects can be strong) covering a wide range of parameters such as primary and secondary masses and orbital period from the APOGEE-1 survey (Alam et al. 2015, Troup et al. 2016, Majewski et al. 2017), in Sun et al (2018), we computed the tidal dissipation rate from first principles. This approach is distinct from the old method of treating tidal decay (causing orbital shrinking) with parameterized equations that rely on important tidal Q quantities calibrated by observations. We conducted a parameter survey that studied orbital decay with low- to intermediate-mass stars and companions ranging from low-mass WDs to hot Jupiters. The main finding is that observations align well with our theoretical predictions. For low-mass stars, the system can survive (avoiding tidal decay leading to merger) outside 0.05 to 0.15 AU.

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The critical semimajor axis (within this distance, the secondary will merge with the primary due to tides) as a function of gravity for a 1 solar mass primary with three companion masses: 1 Jupiter mass (black), 10 Jupiter masses (green), and 0.1 solar mass (cyan). The black dots represent the APOGEE data, with the primary mass between 0.5 to 1.5 solar masses and the secondary mass between 1 Jupiter mass to 0.1 solar mass.

The First Detailed Mass Transfer Binary Evolution Calculations for Blue Stragglers

 

The WIYN Open Cluster Survey, conducted over 20 years using the WIYN 3.5m telescope, has identified numerous blue stragglers in binaries using the radial velocity method. The orbital period distribution of the blue straggler binaries in the open cluster favors the stable mass transfer formation channel (Geller and Mathieu 2011).

 

Two blue straggler and white dwarf pairs, WOCS 5379 and 4540, represent the shortest and longest orbital periods within the old open cluster NGC 188. I modeled these two extreme case to explain how blue stragglers form and evolve in open clusters. In contrast to field binaries, studying binaries in stellar clusters offers an advantage in better constraining our theoretical models, as we have the age and metallicity information from the cluster. We presented our detailed modeling, discussed different physical scenario, and showed the main results for close and wide blue straggler binaries in two publications Sun et al. (2021) and Sun and Mathieu (2023).

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