Federica Chiti

PhD Candidate in Astronomy · Amelia Earhart Fellow · Institute for Astronomy, University of Hawaiʻi at Mānoa

I study how stars spin down as they age, and how their magnetic fields drive that evolution.

Just like us, stars slow down as they age! Most do it in a predictable way over billions of years, which makes stellar rotation one of the best clocks we have for measuring stellar ages. By knowing the ages of a large population of stars, we can study how our galaxy formed and evolved over time. And by knowing the ages of stars that host planets, we can understand how long those planets have had to build, or lose, an atmosphere, and how that affects their habitability.

However, stellar rotation is not a perfect clock! I work in the regimes where this clock currently fails, combining stellar evolution models with direct measurements of stellar magnetic fields to understand why, and how to fix it.

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My Favorite Research Tools

Stellar evolution: rotation and magnetism

A star's rotation and its magnetic field are fundamentally coupled. Rotation and convection drive the internal dynamo that generates the field, while that same field dictates how efficiently the stellar wind carries away angular momentum over billions of years, causing the star to spin down. To understand this coupled evolution, I bridge two complementary approaches: theoretical rotation modeling and direct observational magnetic mapping.

Rotation · modeling

Spin-down as a stellar clock

Cool stars with deep convective envelopes generate magnetized stellar winds that carry away angular momentum as they age—a process known as magnetic braking. Because spin-down is rapid early on and slows over time, stars of a given mass eventually converge onto a predictable relationship between rotation period and age, shedding memory of their initial birth spins.

This convergence underpins gyrochronology: inferring a star's age from its measured rotation period and mass (or temperature). For cool, low-mass stars—which make up the vast majority of our Galaxy—rotation is often the only reliable clock available when other age indicators fail.

How I model stellar rotational evolution

Animation of rotation period versus effective temperature for six open clusters of increasing age, from the Pleiades to M67, with a red rotational-evolution model curve tracking the data at each age.

Rotation periods of stars in six open clusters, from the 120-Myr-old Pleiades to the 4-Gyr-old M67 (grey points), compared with rotational-evolution models from my Chiti et al. (2024) paper. As the clusters age, stars spin-down and fall onto tight sequences, which we can model to infer stellar ages.

Magnetism · observation

Mapping stellar magnetic fields

Because stellar magnetic fields dictate the efficiency of magnetic braking, accurately modeling spin-down requires knowing the field's strength and geometry. Since these global fields are notoriously difficult to detect, standard rotational models (like those shown to the left) rely on scaling relations to estimate them. My work puts these theoretical proxies to the test. I use high-resolution spectropolarimetry and Zeeman-Doppler Imaging (ZDI) to map large-scale surface fields directly. By computing braking torques from these magnetic field measurements, and comparing them against the proxy-driven models, I can pinpoint exactly where standard spin-down theory succeeds—and where it breaks down.

How I map magnetic fields: polarization from a rotating magnetic star

A star with a dipole field, colored by the field component pointing toward us (red) or away (blue). The solid grey line is the rotation axis; the dashed amber line is the magnetic axis; the amber dot marks the visible magnetic pole.

magnetic tilt 60°

viewing angle 60°

phase0.00

Magnetic fields imprint a circular polarization (Stokes V) on starlight through the Zeeman effect, while rotation Doppler-shifts each part of the surface to a different position in the line profile. The curve shows the resulting signature. Zeeman Doppler Imaging (ZDI) inverts a full rotation's worth of these signatures into a map of the surface field. This animation runs the physics forward (field → signal); the maps elsewhere on this page come from solving the inverse problem.

My Research Program

Rotation is a clock — where does it break?

Over the past decade, observations have revealed three distinct regimes where the standard picture of spin-down fails—putting strict limits on the reliability of the rotation clock. My research tackles all three. By combining rotational-evolution modeling with direct magnetic-field measurements, I am working to uncover the physical mechanisms driving these breakdowns. Explore the three puzzles below to see what I have found, and how these results challenge our current understanding of stellar evolution.

1 Spin-Down Stalling: Stars that appear to stop spinning down Click to explore 2 The Fully Convective Boundary: Where one rotation period can mean many different ages Click to explore 3 Weakened Magnetic Braking: Old Sun-like stars spinning faster than expected Click to explore

Spin-down Stalling: Stars that appear to stop spinning down

Observations The first ZDI maps from PEPSI/LBT spectropolarimetry of stars during spin-down stalling

Models YREC + rotevol rotational models at the Hyades' age

Paper Chiti et al. (in prep)

Explore the Magnetic Field Maps of Hyades stars: Click any gold star in the chart below to view the magnetic field map that I reconstructed with ZDI. For stars with multiple observations, use the arrows to switch between observing seasons.

Spin-down Stalling in Clusters and My Work on the Hyades

Cluster data from Curtis et al. (2019, 2020) and Douglas et al. (2019). The dashed line represents a standard braking model at the age of the Hyades. While Sun-like stars in the older NGC 6811 cluster spin down as expected, cooler K dwarfs overlap with the younger Hyades and Praesepe sequences—demonstrating that their rotation has temporarily stalled. In the grey band, Praesepe stars exhibit anomalously high starspot coverage for their rotation, a secondary signature of this stalled state.

Selected star

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The Problem: K-dwarfs (stars slightly cooler than the Sun) appear to temporarily stop spinning down at ages between 600 Myr and 4 Gyr. This phenomenon, known as spin-down stalling, breaks our stellar clock in exactly the mass range where it should be most reliable. The field has debated two competing explanations: either the magnetic braking torque has physically weakened, or a faster-spinning interior is transferring angular momentum to the surface, counteracting the loss at the surface and therefore masking the spin-down.

The Method & Result: To solve this puzzle, I directly measured the stellar magnetic field. Using PEPSI/LBT spectropolarimetry and Zeeman-Doppler Imaging, I reconstructed the first surface magnetic field maps of Hyades K dwarfs caught precisely in this stalled state. The maps revealed strong, dipole-dominated magnetic fields that produce normal, expected braking torques. We also see the same patter of overactivity in the stalled Hyades stars as seen in the Praesepe cluster (Cao et al. 2023). To explain the stall without changing the physics of our models, the torque would need to drop by two to three orders of magnitude—a scenario my direct measurements of magnetic fields disfavored.

Implications: The magnetic brake isn't broken. By proving that the magnetic wind continues to drain angular momentum at a normal rate for the first time, my work provides evidence supporting the scenario that spin-down stalling is due to redistribution of internal angular momentum.

Paper coming up soon, stay tuned!

Watch the talk: Video of my talk on the Hyades spin-down stalling work →

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The Fully Convective Boundary: Where one rotation period can mean many different ages

Observations White dwarfs in 185 wide binaries, used as independent stellar clocks

Models YREC + rotevol gyrochrones, calibrated on six open clusters

Paper Chiti et al. (2024) →

The spike in spin-down at the fully convective boundary

Rotation period versus effective temperature for the full 0-10 Gyr sample, with model gyrochrones shaded around a sharp spike right at the fully convective boundary (green band), and an inset zooming in on two wide-binary points with error bars right at the spike.

The green band marks the fully convective boundary; the shaded curves are model gyrochrones. Rotation spikes sharply right at the boundary. The inset zooms in on the spike, highlighting two wide-binary stars (yellow) whose measured periods sit consistent with ages spanning the full width of the spike.

Why it happens: an oscillating convective envelope

$Three panels showing convective zone mass fraction, convective zone radius fraction, and convective overturn timescale as a function of age, for stellar masses spanning the fully convective boundary, with masses near the transition showing repeated oscillations between partially and fully convective states.$

For stellar masses right at the fully convective transition (0.31–0.34 M☉), non-equilibrium helium-3 burning makes the convective envelope's depth cycle repeatedly between partially convective (PC) and fully convective (FC) states before settling down. Each cycle drives a sharp swing in the convective overturn timescale (bottom panel) that controls magnetic braking efficiency — the underlying cause of the spike shown to the left.

The Problem: As stars become less massive (around 0.33 M☉), they undergo a major structural shift, losing their radiative cores to become fully convective. It was previously unknown how this abrupt transition (i.e. the fully convective boundary) affected magnetic braking and the stellar rotation clock. Testing this was incredibly difficult because open clusters—our standard age calibrators—rarely live long enough or contain enough of these faint stars to probe this boundary deep into the main sequence.

The Method & Result: To bypass the limits of star clusters, I used a different independent clock: 185 wide binaries pairing a low-mass star with a white dwarf. The white dwarf's cooling rate provided a precise age for the system. When I used these to calibrate my rotational models, I found a dramatic breakdown right at the fully convective boundary. Rotation periods suddenly triple across a temperature range of just ~50 K. Within this narrow strip, a single measured rotation period becomes completely degenerate, corresponding to ages spanning up to six billion years.

Implications: This defines a well-characterized blind spot in the gyrochronology toolkit. I traced the root cause to non-equilibrium helium-3 burning, which forces the convective envelope to oscillate wildly between partially and fully convective states. By precisely mapping the physics of where and why this happens, we now know that gyrochronology is fundamentally unreliable at this boundary (until we get better, more precise temperature measurements!).

Read the full paper here: Chiti, van Saders, Heintz, Hermes, Ong, Hey, Ramirez-Weinhouse & Dugas 2024, ApJ, 977, 15 →

Watch the talk: Video of my talk on the fully convective boundary result →

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Weakened Magnetic Braking: Old Sun-like stars spinning faster than expected

Observations ESPaDOnS & SPIRou (CFHT) spectropolarimetric time series at part-per-million precision

Paper Chiti et al. (2025) · Chiti et al. (2026)

τ Ceti — first ZDI map, weakest field on a Sun-like star

Zeeman Doppler Imaging map of tau Ceti showing radial, meridional, and azimuthal magnetic field components as a function of latitude and longitude, alongside the observed and best-fit model Stokes I and Stokes V line profiles at each observed rotational phase.

Reconstructed radial, meridional, and azimuthal field components (left three panels) from ESPaDOnS/CFHT spectropolarimetry, with the observed (black) and best-fit model (red) Stokes I and Stokes V profiles used to derive the map (right panels).

The Problem: Past the middle of their main-sequence lives, Sun-like stars stop braking as expected, spinning faster than age models predict. The leading theory suggests their large-scale dipole fields—the primary lever arm for magnetic braking—dramatically weaken. Testing this, however, requires detecting ultra weak global fields (which imprint polarization signals of just a few parts per million), pushing observational techniques to their absolute limits.

The Method & Result: To determine whether these stars have weak dipole fields, I targeted two of the oldest, quietest stars known in this regime: τ Ceti and HD 166620. Using ESPaDOnS and SPIRou spectropolarimetry at CFH telescope, I reconstructed the first Zeeman-Doppler Imaging map of τ Ceti. It revealed a global field of just 0.17 gauss—nearly ten times weaker than standard models predict, making it the weakest large-scale field ever mapped on a Sun-like star. For HD 166620, the first confirmed Maunder Minimum analog, I found a 1.1 gauss dipole field right at the edge of instrumental sensitivity from combining 12 nights worth of data.

Implications: These measurements provide direct empirical evidence that the transition to weakened magnetic braking involves a weakening of the large-scale magnetic field and suggest that HD 166620 represents a state comparable to the Sun near the peak activity of a grand minimum. My work also establishes a new benchmark for ZDI, demonstrating that even extremely quiet stars, which are the prime targets for direct imaging of temperate rocky planets by future missions, are accessible to this technique.

Read the τ Ceti paper here: Chiti, Kochukhov, van Saders & Metcalfe 2025, ApJL, 991, L13

Read the HD 166620 paper here: Chiti, van Saders, Kochukhov & Metcalfe 2026, ApJ, 1000, 175

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About me

I'm a PhD candidate at the Institute for Astronomy in Honolulu, working with Jennifer van Saders. Before my dissertation work on stellar rotation and magnetism, I studied the chemistry of protoplanetary disks with high-resolution infrared spectroscopy at NASA IRTF (see or listen to my AAS poster here), and earned my BSc (Hons) in Physics with Astrophysics at the University of Dundee, Scotland, supervised by Aurora Sicilia-Aguilar. While in Dundee, I was a Cormarck Undergraduate Vacation Scholar and worked with David Pontin and Roger Scott to build a toolkit for locating magnetic null points in solar-wind models.

I am originally from Italy, so I firmly believe the universe is held together by gravity, magnetic fields, and a plate of spaghetti cooked al dente 🍝

Federica Chiti photographing from the summit of Haleakalā, observatory domes visible in the background above the clouds

Teaching, Mentoring & Service

HI STAR (Maui)

I mentor middle- and high-school students in the University of Hawaiʻi's HI STAR summer research program. In 2025 I guided Reina Mae Lana, Lyngel Paguel, and Anthony Phillips through a project using Gaia data and stellar-evolution models to age-date open clusters.

Maunakea Scholars

Through this program, Hawaiʻi public-school students win observing time on Maunakea telescopes. I advised Jahnea Ordona (Kealakehe High School) on a CFHT spectroscopy project searching for biosignatures on Europa — work that earned her the NASA Earth System Science Project Award.

Teaching

Observational Astronomy Projects (ASTR 301), UH Mānoa, 2022
Observational Astronomy Lab (ASTR 300L), UH Mānoa, 2021
Introduction to Programming (EG21007), University of Dundee, 2018–2020

Service & community

Referee for AAS Journals
Member of Graduate Women in Science at Hawaiʻi (GWISH) and Zonta International
Leading the Italian Scientists & Scholars In North America Foundation (ISSNAF) Chapter in Hawaiʻi
LOC for the TESS/Kepler Asteroseismic Science Consortium meeting (Honolulu, 2023)
Institutional outreach at the Institute for Astronomy

Off duty

Photography

Outside of science, I am a passionate photographer. I love taking pictures of nature, including landscapes, animals, flowers, and the night sky. Here is a small gallery.

Contact

Get in touch

fchiti@hawaii.edu
Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822