Projects | Huanyu Teng

Stellar Obliquity and Young Systems

Mon, 01 Jul 2024 00:00:00 +0000

Short Introduction

The stellar obliquity, a probe to planet migration history, is the angle between the rotation axis of the host star and the normal of the orbital plane of the planet. Various migration theories with different timescales, e.g., primordial disk misalignment (< 3 Myr), Kozai-Lidov mechanism (10 kyr - 100 Myr), secular chaos (10 Myr - 100 Myr), differ strongly in their predictions on the obliquity. These mechanisms all functions during the youth of the planetary systems (< 1 Gyr). Young planetary systems are the promising targets to constrain the models and timescales of planetary formation and migration.

“The ∼50Myr Old TOI-942c is Likely on an Aligned, Coplanar Orbit and Losing Mass”

Asteroseismic masses of giant stars

Fri, 28 Oct 2022 00:00:00 +0000

Background

The mass of a planet is determined by the mass of its host star by using Kepler’s law. Accurate mass measurements on stars recover a true planet population. Among all types of stars, planets around intermediate-mass stars (1.5-3.0 M_⊙), which are ideal candidates to study the time scale of giant planet formation observationally, are less well-studied compared to solar-mass and low-mass stars. The East Asian Planet Search Network (EAPS-Net; Izumiura 2005) surveys totally more than 600 hundred giant stars initially considered to have intermediate masses and contributed about 40 planetary systems around these stars. Nonetheless, the mass determination of these stars was of low accuracy, especially in studies before 2018.

Traditionally, stellar masses are determined using spectroscopy and grid-based isochrones. However, evolutionary tracks of stars of different masses and metallicities strongly overlapped in the red-giant-branch (RGB) and Helium burning (HeB) phase. Thus, a large bias in mass measurements can be introduced, and consequently caused a biased planet population. In particular for the Okayama Planet Search Program (OPSP; Sato et al. 2005; the main branch of EAPS-Net), the mass measurements were judged to be probably overestimated by a factor of 2 (Takeda et al. 2015). Although the studies after that (e.g., Takarada et al. 2018; Teng et al. 2022a; Teng et al. 2022b) re-estimated the stellar masses with isochrones of fine grids, the error of the mass measurements was still as high as over 10 percent…

Asteroseismology can provide us with precise model-independent stellar masses. Currently, various high-quality data sets can be used for such study, e.g., photometry with Kepler and TESS, as well as high cadence radial velocities, e.g., SONG. Stello et al. (2017) found an overestimation of 15–20 percent in spectroscopic masses compared to the corresponding seismic masses. Malla2020 et al. (2022) found that stars above a mass threshold of 3 M_⊙ had a significant mass offset, while those below the threshold did not. Thanks to the full-sky survey by TESS, we now have high-cadence time-resolved data for many more stars with spectroscopically-derived masses in EAPS-Net. So far, several studies were carried out to study the asteroseismic mass of evolved stars (subgiant and giant stars) with TESS (e.g., Malla et al. submitted). And thus, we propose to use TESS light curves to measure the asteroseismic-based masses of EAPS-Net stars.

Asteroseismic mass

The asteroseismic mass can be estimated by the equation given in Sharma et al. 2016:

$$ \frac{M}{\rm {\it M}_\odot} \approx \left(\frac{\nu_{\rm max}}{f_{\nu_{\rm max}} \nu_{\rm max, \odot}}\right)^{3}\left(\frac{\Delta \nu}{f_{\Delta \nu} \Delta \nu_{\odot}}\right)^{-4}\left(\frac{T_{\rm eff}}{T_{\rm eff, \odot}}\right)^{1.5} $$ $$ \frac{R}{\it R_\odot} \approx \left(\frac{\nu_{\rm max}}{f_{\nu_{\rm max}} \nu_{\rm max, \odot}}\right)\left(\frac{\Delta \nu}{f_{\Delta \nu} \Delta \nu_{\odot}}\right)^{-2}\left(\frac{T_{\rm eff}}{T_{\rm eff, \odot}}\right)^{0.5} $$

where the global asteroseismic parameters $\nu_{\rm max}$ and $\Delta \nu$ are calculated from the power spectrum density fitting of TESS light curves (e.g. the figure), and $f_{\nu_{\rm max}}$ and $f_{\Delta \nu}$ are the correction factors obtained from asfgrid code.

In the following paper, we characterized two planet hosting giant stars using the asteroseismic methodology above:

“Two long-period giant planets around two giant stars: HD 112570 and HD 154391.”

Extended studies

We will try to perform precise asteroseismology for stars in TESS continuous viewing zone. We will also continue our planet discoveries based on EAPS-Net (OSPS and other branch programs), as well as refine planet masses and vet fake planets.

Exoplanet searching around evolved stars

Thu, 04 Aug 2022 00:00:00 +0000

Background

To date, more than 5000 extra-solar planets (exoplanets) have been detected around different types of stars. These exoplanets were detected by different methods, among which the radial velocity (RV) method (Doppler method) contributed to around 800 of them. For these planets, about 60% are detected around solar-mass (0.7–1.5 M_⊙) stars, while only about 1% are detected around intermediate-mass (1.5–5.0 M_⊙).

Intermediate-mass stars are B- or A- or F-type stars at main sequence, and G- or K- type at more evolved stages (subgiants and giants). Compared to their main-sequence counterparts, these stars are better for planet searching with RV method because they rotate slowly and have sharp absorption lines.

Since the intermediate-mass stars have shorter lifetime than less-massive stars, they are valuable objects to constrain the mechanism and timescale of planet formation.

There are two popular planet formation scenarios for gaseous giant planets. One is core accretion, which has a timescale of about a few million years. In this model, a rocky core forms through the coagulation of planetesimals until it is sufficiently massive (about 10 M_⊕) to accrete a gaseous envelope. Another one is disk instability, which has a timescale of about only a few thousand years. In this model, gas giant planet forms directly as a result of (local) gravitational instability in the protoplanetary disk.

To characterize how the giant planets are and to recover how they form, it is urgent to survey intermediate-mass stars at their evolved stages.

Issues in my research

Thanks to large surveys during the last 20 years, more interesting issues are realized about giant planet populations. Here I present some of them which are within my interest:

“Planet desert”

Close-in (a < 0.6 au), low-mass (Mp < 0.6M_J ) planets were seldom found around intermediate-mass planets. Especially for more evolved giant stars, most detected planets reside at more than 0.5 au from their hosts.

The desert could be attributed to a scaling of the proto-planetary disk mass with the mass of the central star, and it could also be a result of planet engulfment while a star is ascending the red giant branch.

Related publication:

“A Close-in Planet Orbiting Giant Star HD 167768”

The giant-planet metallicity correlation

Among all detected giant exoplanets, they are more likely to reside around metal-rich stars. For the evolved stars, the existence of giant-planet–metallicity correlation is a more popular. But some give no clear correlation. It is somewhat under-debating…

Related publication:

“Revisiting Planetary Systems in Okayama Planet Search Program: A new long-period planet, RV astrometry joint analysis, and multiplicity-metallicity trend around evolved stars.” “Regular radial velocity variations in nine G- and K-type giant stars: Eight planets and one planet candidate”

Multiplicity of giant planets

To date, there are around 30 multi-planet systems discovered around deeply evolved stars (logg<3.5). Most of them are in the pattern of massive planet pairs and having intermediate periods (100 ~ 1000 days), and many of pairs are considered to be in mean motion resonance. Why are they mostly in pairs? What have they experienced?

Related publication:

“A Trio of Giant Planets Orbiting Evolved Star HD 184010”

Data in my research

Observations

I use precise radial velocity data within the framework of Okayama Planet Search Program and its extended programs.

Statistics

Data are obtained from NASA Exoplanet Archive and Exoplanet.eu.

Exoplanet searching around cool stars

Tue, 01 Jan 2019 00:00:00 +0000

Short Introduction

Not like solar-like stars, which are hard to detect Earth analogs in the habitable zone (HZ) for their low possibility of transit (<1%, ~1/year) and weak RV signals (~0.1 m/s), cool stars such as mid-to-late M-dwarfs, are promising targets to search for habitable planets because they are much cooler and thus with closer HZ as well as stronger and detectable RV signals (~1-5m/s). In addition, studies on planet occurrence revealed that terrestrial planets are more likely to reside around M dwarfs.

Program

Subaru/IRD is one of the most powerful near-infrared spectrograph mounted on large-aperture telescopes with capability in detecting Earth analogs around cool stars. I am in the project of IRD-SSP (PI: Bun’ei Sato), an exoplanet survey in the framework of the Subaru Strategic Program (SSP), and IRD TESS Intensive Follow-up Project (PI: Norio Narita).

Other studies in exoplanets

Mon, 01 Jan 2018 00:00:00 +0000

A wide interest…

I have a wide interest in exoplanetary world, and am now working with people in diverse fields, including joint characterization using radial velocity and astrometry, planets around more evolved stars (AGB stars), orbital evolution of planetary systems, planet population, stellar jitter modeling…

Projects | Huanyu Teng

Stellar Obliquity and Young Systems

Short Introduction

Related works:

Asteroseismic masses of giant stars

Background

Asteroseismic mass

Extended studies

Exoplanet searching around evolved stars

Background

Issues in my research

“Planet desert”

The giant-planet metallicity correlation

Multiplicity of giant planets

Data in my research

Observations

Statistics

Exoplanet searching around cool stars

Short Introduction

Program

Other studies in exoplanets

A wide interest…