Andreas W. Neitzel
CV
ESO/M. Zamani, CC BY 4.0

Publications

Oscillations of the dwarf ε Indi

The ESPRESSO spectrograph detected three-minute oscillations in ε Indi, a nearby star cooler than any other dwarf whose oscillations had been measured.

Expanding the frontiers of cool-dwarf asteroseismology with ESPRESSO

Scatter plot of stellar radius against surface temperature, with hotter stars on the left. Blue circles mark stars with oscillations detected in brightness measurements, orange diamonds those detected in radial velocity, and grey dots are other stars. Epsilon Indi, an orange diamond near 4650 K, lies to the right of every other detection.
Stars with measured oscillations; ε Indi, on the right, is cooler than all the others. From Campante et al. 2024, A&A 683, L16, Fig. 1, CC BY 4.0.

The question

Stars like the Sun vibrate with sound waves, and the tones reveal their mass, radius and age. K dwarfs, smaller and cooler than the Sun, vibrate so weakly that no dwarf cooler than about 5000 K had shown oscillations before this letter (Sect. 1, Fig. 1). These stars are a focus of searches for potentially habitable planets, and because they are common and long-lived they also record the Milky Way's chemical history (Sect. 1).

What the team did

The team watched the surface of ε Indi move towards and away from Earth. ESPRESSO on ESO's Very Large Telescope observed this K dwarf, 3.6 parsecs (12 light years) away, for six half nights in September 2022, taking 2084 exposures of 25 seconds each (Sect. 2). The team also re-analysed 2018 ESPRESSO data of HD 40307, a fainter K dwarf with a tentative earlier claim of oscillations (Sect. 1, Appendix D).

What came out

ε Indi oscillates with periods of about three minutes. The team measured 19 oscillation modes (individual tones), centred near 5.3 mHz (Sects. 3.1 and 3.2, Fig. 3). At 4649 K, the star was cooler than any other dwarf with measured oscillations at the time (Sect. 2, Fig. 1).

Its oscillations are weak. The peak amplitude, 2.6 ± 0.5 cm/s, is about 14 per cent of the Sun's and about half what the usual scaling predicts. Below about 5500 K the data favour a steeper scaling, close to (L/M)1.5, with L/M the ratio of luminosity to mass (Sects. 3.3 and 4, Fig. 5).

My part

I ran the statistical test of whether the HD 40307 data contain oscillations, the Bayes-factor test in Appendix D. A Bayes factor compares how well two models explain the same data: here, a power spectrum with a hump of oscillation power against one without. The test gave ln B < −2 for every assumed hump position from 3 to 6 mHz, strong evidence against a detection, so the letter plots HD 40307 as an upper limit of 4.93 cm/s (Fig. 5).