From Solar to Stellar:�Investigating Supergranulation Across Spectral Types

Niamh K. O’Sullivan

University of Oxford

Image credit: NASA

GPRV 2022 – Gran/SG Talks and Posters

1 talk….

GPRV 2026 – Gran/SG Talks and Posters

5 talks!!!

& 2 posters!

Supergranulation

• Seen in Doppler imaging of the Sun
• Horizontal flow of 200 – 300 m/s
• Horizontal scale of 30-35 Mm
• Convective origin
• Timescale of ~ 1 day
• 0.5-1 m/s RV scatter

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• Review by Rincon & Rieutord 2018

Image credit: SDO/HMI

Areas of Uncertainty for Supergranulation for EPRV

• What causes supergranulation?
• Interaction of supergranules and magnetic fields?
• How does supergranulation present itself in other stars?
• Does supergranulation actually scale like granulation?
• Does supergranulation have spectral footprints?
• A supergranulation proxy?

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Characterising Stellar Signals with a Power Spectral Density (PSD)

Supergranulation

Granulation

Rotational Modulation

Al Moulla et al (2023)

Characterising Stellar Signals with Gaussian Processes (GPs)

O’Sullivan & Aigrain (2024)

Define the data set as a multivariant Gaussian, and fit the correlation between data points

Frequency vs Time Domain – FFT vs GP

O’Sullivan & Aigrain 2024

Regular Time Sampling

Irregular Time Sampling

Collier Cameron et al. (2019), Dumusque et al. (2021), Dumusque et al. (in prep)

Lakeland et al (2024), Cretignier et al (2021)

Supergranulation Cycle?

O’Sullivan et al. (2025)

Interaction between Supergranulation and Magnetic Fields

Rincon & Rieutord (2018)

O’Sullivan et al. (2025)

Areas of Uncertainty for Supergranulation for EPRV

• What causes supergranulation?
• Interaction of supergranules and magnetic fields?
• How does supergranulation present itself in other stars?
• Does supergranulation actually scale like granulation?
• Does supergranulation have spectral footprints?
• A supergranulation proxy?

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If we take normal �RV time sampling…

Gran in HD 166620

𝛔 = 0.23 m/s

𝝉 = 43.53 mins

John, Al Moulla & O’Sullivan et al. 2025

SG in 18 Sco

Gran: 𝛔 = 0.56 m/s

𝝉 = 69 mins

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O’Sullivan et al. (in prep)

SG:

𝛔 = 1.2 m/s

𝝉 = 1.51 days

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The EPRV RCN�Standard Star Survey

The Targets:

• HD 10476
• HD 10700
• HD 32147

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• HD 4628
• HD 126053
• HD 157347

The Instruments:

• KPF
• MAROON-X
• NEID
• HARPS-N

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• EXPRES
• CARMENES
• HARPS/NIRPS
• PARVI
• ESPRESSO

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The EPRV RCN Standard Star Survey

PARVI

HARPS

EXPRESS

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First Round of Observations – Sept 2024

O’Sullivan, Luhn , and Gupta al. In Prep

126 data points

131 data points

201 data points

117 data points

First Round of Observations – Sept 2024

O’Sullivan, Luhn , and Gupta al. In Prep

101 data points

85 data points

198 data points

78 data points

Second Round of Observations – June 2025

Luhn, O’Sullivan, & Gupta et al. In Prep

531 data points

454 data points

Second Round of Observations – June 2025

Luhn, O’Sullivan, & Gupta et al. In Prep

181 data points

148 data points

Searching for Supergranulation Scaling Laws

O’Sullivan et al. (in prep)

Searching for Supergranulation Scaling Laws

O’Sullivan et al. (in prep)

Searching for Supergranulation Scaling Laws

O’Sullivan et al. (in prep)

Future Supergranulation Observations

Simulating �Supergranulation with GRANOLA�

O’Sullivan et al. (in prep)

(GRANulation and OsciLlation Analysis )

Impact of Exposure Time

O’Sullivan et al. (in prep)

Impact of Exposure Time and Spectral Type

O’Sullivan et al. (in prep)

EPRV RCN Standard Star Survey Simulations

How much do we need to observe?

Gupta + Bedell (2024)

A larger spread of observations is better

What we need to observe supergranulation

3 Key Ingredients:

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• Bright stars to get high SNR

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• Lots of exposures spread randomly across the whole night to sample the SG timescale

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• Stars that are visible for most of the night for a month+

Image credit: SDO/HMI

The Terra Hunting Experiment (and SHE)�on HARPS3 at the INT

Coming Soon!!!

The Supergranulation Hunting Experiment (SHE)

• Observe 8 stars every night
• Change between targets quickly
• ~ 6 weeks
• Observing standard stars and bright stars observable for 6+ hours

O’Sullivan et al. (2025)

RV Simulations for the SHE

Scheduling done by Jay Fitzpatrick, University of Exeter

Fisher Information for the SHE

Summary

• We need to mitigate supergranulation to detect Earth-twins.

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• Understanding how supergranulation changes via spectral type will help us develop mitigation techniques.

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• We need to observe supergranulation in more targets to test our scaling laws.

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• The SHE Survey will aim to detect supergranulation in the first dedicated single instrument survey.

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Email: niamh.osullivan@physics.ox.ac.uk

Observing SG across a Stellar Activity Cycle�18 Sco – Observations in Progress!