TESS Observes
a Rapidly Rotating
Engulfment Candidate  

TESS Mission Update | Slides at http://hyad.es/talks

Joel Ong, Marc Hon, Melinda Soares-Furtado,
Alexander Stephan, Jamie Tayar, et al., in review

June 23, 2023

and Rotation

(sun-like star)

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

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

Power spectra of MDI dopplergrams

Main-sequence stars exhibit p-modes:
pure pressure waves.

Mode frequencies satisfy \(\nu_{n\ell} \sim \Delta\nu\left(n + {\ell \over 2} + \epsilon_\ell(\nu)\right) + \mathcal{O}(1/\nu)\)

Key Application: Global Parameter Estimation

Hare “Zebedee”, Cunha+ (incl. Ong, 2021)

Precise measurements of field stars: \[ {\sigma_R \over R} \lesssim 2 \%; {\sigma_M \over M} \lesssim 5 \% \]

Key Application: Rotational Measurement

Eigenfunctions indexed by
three quantum numbers \(n, \ell, m\): \[ \begin{aligned} \ell &= 0, 1, 2, \ldots \\ m &= -\ell, -\ell+1, \ldots, \ell-1, \ell \end{aligned} \]

Zonal (\(m = 0\))

Prograde sectoral
(\(m = +\ell\))

Retrograde sectoral
(\(m = -\ell\))

A Rapidly Rotating Red Giant

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

Mode Identification

\[\ell = 0,2?\]

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

Asteroseismology Happens…

Rotational splittings from seismology:

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

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

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

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?


TESS Takeaways

  • TESS is better with friends
    • Asteroseismology: various properties etc.
    • ASAS-SN: Stability
  • It’s easier to smell gunpowder than catch fireworks
  • Qualitatively longer is better


TESS finds 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.

If engulfment, \(< 5\ \mathrm{Mya}\), with mass about \(6\)-\(10\ M_J\).

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

Supplementary Slides

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 \(K = 27\ \mathrm{km/s}, P \sim 69\ \mathrm{d}\))

Consolidated Constraints

Observational Overview


  • \(M \sim 1.14 \pm 0.04\ M_\odot\)
  • \(R \sim 23.5 \pm 0.03\ R_\odot\)
  • \(P_\text{rot,bulk} \sim 115 \pm 10\ \mathrm{d}\)
  • (Weak evidence for rotational shear)


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


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

Gaia DR3:

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

Orbits of Engulfed Companions