Asteroseismic Identification and Characterisation of a
Rapidly Rotating Engulfment Candidate, or: TESS Asteroseismic Rotational Measurements, Today and Tomorrow

TESS Sci Con III | Slides at hyad.es/talks

Joel Ong
Hubble Fellow, Univ. of Hawaiʻi at Mānoa

July 30, 2024

(sun-like star)

Photometric \(P_\text{rot}\):
\(\ll 1\ \mu\text{Hz}\)
\(\downarrow\)

Spectroscopic \(V \sin i\):
\(\sim \text{PHz}\) regime \(\to\)

Power spectra of MDI dopplergrams

Cool main-sequence stars exhibit p-modes:
pure pressure waves.

Mode frequency measurements constrain
internal structure and rotation.

Zvrk:
A Rapidly Rotating Red Giant
Ong et al. 2024, ApJ, 966, 42

Very high \(\mathrm{A(Li)}\),
but otherwise innocuous

Asteroseismology happens…

Rotational splittings from seismology:

  • \(\ell = 1\): \(\delta\nu_{\text{rot},1} \sim 0.09\ \mu\mathrm{Hz}\)
  • \(\ell = 2\): \(\delta\nu_{\text{rot},2} \sim 0.10\ \mu\mathrm{Hz}\)
time evolution \to

\(\implies P_\text{rot} \sim 115\ \mathrm{d}?\)

Rotational signal probably got
detrended away
by systematic corrections…

TESS w/ Pixel-Level Decorrelation + Stitching

Very suggestive…
but is this real?

Probably yes!

Smoking Guns

Observational Overview

Asteroseismology:

  • \(M \sim 1.14 \pm 0.04\ M_\odot\)
  • \(R \sim 23.5 \pm 0.03\ R_\odot\)
  • \(\boxed{P_\text{rot,bulk} \sim 115 \pm 10\ \text{d}}\)
  • (Maybe rotational shear?)

Photometry:

  • \(\boxed{P_\text{rot,surf} \sim 99 \pm 3\ \text{d}}\)

Spectroscopy:

  • \(\boxed{V\text{sin } i \implies P_\text{rot} \sim 110 \pm 8\ \text{d}}\)
  • \(\mathrm{A(Li)} = 3.16 \pm 0.08\ \mathrm{dex}\)
  • \(^{14}\mathrm{N}\)-deficient relative to APOGEE sample

Gaia DR3:

  • RUWE of 1.06
  • \(\sigma_V = 0.16\ \mathrm{km/s}\)
    over 2.2 years

Gaia RV scatter rules out
large RV semiamplitudes…

\(P_\text{orb} \gg 99\ \mathrm{d}\) cannot spin star
up to 99-day rotational period…

Remaining permissible orbits are
unstable to tidal dissipation!

\(\implies\) ENGULFMENT?

So What?

Spin-Orbit Interactions?

Saunders et al. 2024: Thursday, after lunch!

With Misalignment

Core

Envelope

With enough patience,
seismology will get us
internal differential rotation.

Kepler-56: Huber et al. 2013

Kepler-56: Ong, in prep.

TESS Outlook

Engulfment Candidates: TESS is better with friends

  • Asteroseismology: astrophysical characterisation
  • ASAS-SN: Long-term stability

Seismic rotational measurements are pretty robust!

Qualitatively longer all-sky data sets will unlock
new kinds of internal rotational characterisation

What can TESS teach us about spin-orbit interactions?

Asteroseismic Identification and Characterisation
of an Engulfment Candidate

Zvrk is a lithium-rich, rapidly rotating,
and likely standalone red giant, with rotation
confirmed independently using ASAS-SN, seismology, \(V \sin i\).

Other constraints from spectroscopy and Gaia
strongly suggest an engulfment scenario.

Constraints on engulfment age (\(\lesssim 5\ \mathrm{Mya}\)) and mass (\(6\text{-}10\ M_J\)):
Ong et al. 2024, ApJ, 966, 42

\[\mathrm{j}\mathrm{o}\mathrm{e}\mathrm{l}\mathrm{o}\mathrm{n}\mathrm{g}\ \text{@}\ \text{hawaii}.\text{edu}\]

Supplementary Slides

Mode Identification

\[\ell = 0,2?\]

But Kepler says \(\ell = 0\) have to live here!
(and theory says so too…)

Magnetorotational Braking

Could it be a T Tauri Star?

V501 Aur: a red giant accidentally misclassified as a WTT!
(Vaňko et al. 2017: spectroscopic binary with \(K = 27\ \mathrm{km/s}, P \sim 69\ \mathrm{d}\))

Orbits of Engulfed Companions