# 王加冕 Joel Ong

Hi! I’m Joel Ong, a theoretical astrophysicist interested in various aspects of stellar astrophysics, particularly as localised at stellar surfaces. My research combines new developments in analytic technique with the extensive use of high-performance computing and cutting-edge spectroscopic instrumentation.

I am presently working towards a PhD in Astronomy at Yale University, where I do research with Profs. Sarbani Basu and Debra Fischer. Before this, I received my BSc (Hons) from the National University of Singapore, in Physics (with a specialisation in Astrophysics), supervised by A/P Kuldip Singh.

# Research

## Theoretical Asteroseismology

Stars are made of fluids, and (small) perturbations to this fluid may propagate as sound waves. As with any wave phenomena, some waves may resonate at certain characteristic frequencies associated with the structure of the containing medium (in this case, the star); these resonant frequencies emerge as solutions to a quadratic eigenvalue problem. Near the surfaces of stars, these waves are pressure driven (and are hence called “p-modes”).

Observationally, some stars (like the sun) can be seen to grow and shrink over time at these characteristic frequencies. For small deviations from a spherically symmetric homogenous sphere of gas, this pulsation can be decomposed into spherical harmonics, indexed by quantum numbers $$l$$ and $$m$$. These frequency eigenvalues approximately satisfy a “comb” parameterisation: $\nu_{nlm} = \Delta\nu\left(n + {l \over 2} + \epsilon_{lm}(\nu)\right),$ where $$\Delta\nu$$ is called the “large frequency separation”. If you think of the star a resonating mode cavity, this is its approximate free spectral range. $$\epsilon(\nu)$$ is a function of frequency that specifies the eigenvalues for a given pair of $$l, m$$; to a first approximation, it doesn’t vary much, and the “spherical mode cavity” approximation holds fairly well up to a constant offset.

Accurate measurement of these eigenvalues requires quite long observational campaigns. Often, the only quantities that are directly accessible are average values of $$\Delta\nu$$ and $$\epsilon$$, as well as a characteristic excitation frequency called $$\nu_\text{max}$$. Moreover, many unconstrained aspects of stellar surface astrophysics (e.g. global magnetic activity, radiative turbulence) enter into the boundary problem via the choice of surface boundary conditions. These perturb theoretical results relative to the frequencies observed in real stars.

Much of my work revolves around the theory of linear adiabatic oscillations, including:

• Constructing a refined integral estimator of $$\Delta\nu$$ that accounts for secular variations in $$\epsilon$$ using the WKB approximation,
• Constructing evolutionary and structural diagnostic information for stars from isochrones on the $$\Delta\nu-\epsilon$$ plane, and
• Deriving a weak-interaction limit with corresponding scattering-matrix elements to describe the coupling of different mode cavities in evolved stars, via analogies with few-body quantum mechanics.

More generally, I am interested in transitions between different asymptotic regimes. I am currently working on the transition from p- to g-dominated mixed modes in first-ascent red giants, and on the breakdown of the asymptotic regime in very luminous red giants.

#### Selected Publications:

• Ong, J. M. J., & Basu, S. 2019, ApJ, 870, 41 — WKB estimator for $$\Delta\nu$$
• Ong, J. M. J., & Basu, S. 2019, ApJ, 885, 26 — Phenomenology of $$\epsilon_0(\nu)$$
• Ong, J. M. J., & Basu, S. 2020, ApJ, 898, 127 — Hybridisation of mode cavities
• Ong, J. M. J., Basu, S., & Roxburgh, I. W. 2021 — Avoided crossings and perturbation theory

## Stellar Modelling

Astronomers (as opposed to astrophysicists) are far more interested in the inverse problem: that is, in inferring properties of a star given a set of observed oscillation frequencies. My theoretical constructions are intended to aid the construction of stellar models which more closely replicate the interior structures implied by observed oscillation frequencies. I’ve also worked with constructing such models myself from time to time.

For the past few years, I’ve been working on results from the NASA Transiting Exoplanet Survey Satellite (TESS) mission through the TESS Asteroseismic Science Consortium (TASC) (in working groups 1 and 2), helping to constrain the global properties and interior structures of these stars in this manner. For this purpose, I am both a methodologist and a practitioner: I develop and investigate methods used in deriving these constraints, as well as actually perform the calculations for actual stars. I’m glad to be able to make contributions to such a large collaboration. Interesting targets I’ve worked on include:

• TOI-197, which formed the basis of the first TESS asteroseismology paper since other people found a “hot saturn” exoplanet around it
• ν Indi, a metal-poor but otherwise chemically enriched star with unusual kinematic properties, which was observed with TESS
• δ Eridani, which was observed simultaneously with TESS and the SONG radial-velocity spectrograph

At the moment, I am undertaking several projects involving the modelling of evolved stars in the Kepler, K2 and TESS samples. I am also leading a study using these stars to investigate the metallicity-helium enrichment relation in our local neighbourhood.

#### Selected Publications:

• Ong, J. M. J., Basu, S., & McKeever, J. M. 2021 (methodology)
• Ong, J. M. J., Basu, S., Lund, M. N., et al. 2021 (methodology)

## Extreme Precision Radial Velocity (EPRV) Instrumentation

The tale of exoplanet detection and discovery is long and storied, but the oldest and still most reliable way of detecting planets, and measuring their masses, is through the radial velocity method, which requires spectroscopic characterisation. As planets orbit their stars, they exert an opposite gravitational pull on the star that causes it to also orbit (much more slowly) their common centre of mass, moving towards and then away from us over time. From Earth, we are able to measure this time-varying reflex velocity by the Doppler shift it induces on stellar spectra.

These measurements are made on high-resolution spectrographs. One of them, the EXtreme PREcision Spectrograph, was built at Yale. I wrote parts of its calibration and analysis software, including its template cross-correlation-function (CCF) radial velocity code, and various infrastructural components of its automated optimal-extraction data reduction pipeline.

While the ultimate goal of EXPRES is, of course, to discover long-period, low-mass planets with the radial-velocity method that would otherwise not be accessible to short-baseline space photometry, the high-resolution spectra returned from this process are very useful for other purposes. One example of this is Doppler imaging of active stellar surface, which can be further refined by way of joint constraints with contemporaneous photometry.

#### Selected Publications:

• Blackman, Ong, et al. 2019 — wavelength-dependent barycentric corrections
• Petersburg, Ong, et al. 2020 — The EXPRES data reduction pipeline

# Publications

### First-Author

1. Ong, J. M. J., Basu, S., Lund, M. N., et al. 2021, accepted to ApJ (ADS Link)
“Mixed Modes and Asteroseismic Surface Effects: II. Subgiant Systematics”
2. Ong, J. M. J., Basu, S., & Roxburgh, I. W. 2021, accepted to ApJ (ADS Link)
“Mixed Modes and Asteroseismic Surface Effects: I. Analytic Treatment”
3. Ong, J. M. J., Basu, S., & McKeever, J. M. 2021, ApJ, 906, 54 (ADS Link)
“Differential Modelling Systematics across the HR Diagram from Asteroseismic Surface Corrections”
4. Ong, J. M. J., & Basu, S. 2020, ApJ, 898, 127 (ADS Link)
“Semianalytic Expressions for the Isolation and Coupling of Mixed Modes”
5. Ong, J. M. J., & Basu, S. 2019, 885, 26 (ADS Link)
“Structural and Evolutionary Diagnostics from Asteroseismic Phase Functions”
6. Ong, J. M. J., & Basu, S. 2019, ApJ, 870, 41 (ADS Link)
“Explaining Deviations from the Scaling Relationship of the Large Frequency Separation”

### Co-authored

1. Hill, M., Kane, S., Campante, T., et al. (including Ong, J. M. J.). accepted to AJ,
“Asteroseismology of ι Draconis and Discovery of an Additional Long-Period Companion”
2. Chontos, A., Huber, D., Kjeldsen, H., et al. (including Ong, J. M. J.). accepted to ApJ,
“TESS Asteroseismology of α Mensae: Benchmark Ages for a G7 Dwarf and its M-dwarf Companion”
3. Lillo-Box, J., Ribas, A., Montesinos, B., Santos, N. C., et al. (including Ong, J. M. J.). accepted to A&A, “An Eclipsing Brown Dwarf In A Hierarchical Triple With Two Evolved Stars”
4. Nielsen, M. B., Davies, G. R., Ball, W. H., et al. (including Ong, J. M. J.). 2021, AJ, 161, 62,
“PBjam: A Python Package for Automating Asteroseismology of Solar-like Oscillators”
5. Ball, W. H., Chaplin, W. J., Nielsen, M. B., et al. (including Ong, J. M. J.). 2020, MNRAS, 499, 6084,
“Robust asteroseismic properties of the bright planet host HD 38529”
6. Brewer, J. M., Fischer, D. A., Blackman, R. T., et al. (including Ong, J. M. J.). 2020, AJ, 160, 67, “EXPRES I. HD 3651 as an Ideal RV Benchmark”
7. Jiang, C., Bedding, T. R., Stassun, K. G., et al. (including Ong, J. M. J.). 2020, ApJ, 896, 65,
“TESS Asteroseismic Analysis of the Known Exoplanet Host Star HD 222076”
8. Blackman, R. T., Fischer, D. A., Jurgenson, C. A., et al. (including Ong, J. M. J.). 2020, AJ, 159, 238, “Performance Verification of the EXtreme PREcision Spectrograph”
9. Petersburg, R. R., Ong, J. M. J., Zhao, L. L., et al. 2020, AJ, 159, 187,