arXiv subjects

Federica Chiti

Publications and source records attributed to Federica Chiti.

5 recordsLinked to original sources

arXiv2026

Open Clusters as Laboratories for Cool Star Evolution: Highlights from the Cool Stars 23 Splinter Session

Open clusters remain among the most powerful laboratories for calibrating how fundamental stellar properties such as rotation, magnetic activity, surface chemistry, and internal structure evolve over the pre-main-sequence to main-sequence and post-main-sequence stellar lifetime. Because cluster members share a common age, galactic environment, and initial composition, they provide empirical anchors upon which age-dating techniques such as gyrochronology, activity-age relations, and chemical clocks are built. This paper summarizes the Cool Stars 23 splinter session - Open Clusters as Laboratories for Cool Star Evolution, held on 15 June 2026 in Tokyo, Japan. The session comprised one invited review, ten contributed talks, and ten poster pop-up presentations, organized around three themes: the evolution of rotation, magnetic activity, and chemical abundances (Li-depletion, [C/N], [Y/Mg], and neutron-capture elements). We close with a summary of the open questions identified in the panel discussion and the observational and theoretical work that new surveys (e.g. Gaia DR4, 4MOST, WEAVE) and missions (e.g. PLATO, Roman) will enable over the coming years. Together, these efforts will help assess the current state of the field and shape its future directions.

astro-ph.SR↗

arXiv2026

Revisiting the Exo-Mercury Candidate GJ 367 b with ESPRESSO and a Self-Consistent Tidal Distortion Model

We report revised mass and radius measurements for GJ 367 b, an ultra-short-period (7.7 hr) sub-Earth in a multi-planet system orbiting a nearby (~9 pc) M dwarf host. Previous mass and radius measurements have suggested GJ 367 b has an anomalously high bulk density, close to that of solid iron. The existence of such an iron-rich planet is in tension with established planet formation scenarios. We utilized newly available TESS short-cadence photometry to constrain the radius of GJ 367 b to 0.736 +/- 0.035 R_Earth. We consider observational and modeling effects such as photometric dilution, stellar activity, and tidal distortion to account for possible inaccuracies in the star and planet radius measurements. From our radial velocity (RV) analysis using VLT/ESPRESSO data covering nearly the full orbit in a single night, we find a mass of 0.503 +/- 0.078 M_Earth, corresponding to a bulk density of 6.9 +1.6/-1.4 g cm-1. We present a new tidal distortion and interior composition modeling framework to assess the iron mass fraction of GJ 367 b. Considering several different interior composition assumptions and radial aspect ratios, we find an iron fraction of ~50-70%, which is broadly consistent with that of Mercury and not as iron rich as previously suggested.

astro-ph.EP↗

arXiv2026

Spectropolarimetric Constraints on the Maunder Minimum Analog HD 166620: Evidence for Weakened Magnetic Braking

We present the first spectropolarimetric time-series analysis of the Maunder Minimum analog HD 166620, using 12 nights of data from CFHT/SPIRou and a single epoch from CFHT/ESPaDOnS. While individual Stokes $V$ profiles exhibit no significant polarization signatures, we leverage the rotational coverage of the SPIRou dataset to compute a grand average LSD profile. Forward modeling of the cumulative Stokes $V$ signal, assuming a purely axisymmetric dipole, yields a best-fit dipole field strength of $B_{\rm dip} = 1.10^{+0.95}_{-0.90}$G ($3σ$). This field strength matches simulations of the solar dipole during the Maunder Minimum phase. Our results are consistent with independent constraints on the dipole field strength from an LBT/PEPSI snapshot and exclude the presence of strong non-axisymmetric fields potentially missed by this single-epoch observation. These findings 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.

astro-ph.SR↗

arXiv2025

Zeeman Doppler Imaging of $τ$Ceti: The Weakest Magnetic Field Detected in a Sun-like Star

For nearly a decade, observations have shown that many older Sun-like stars spin faster than predicted, a phenomenon known as weakened magnetic braking (WMB). The leading hypothesis for WMB is a weakening of the large-scale dipole field, which leads to a less efficient angular momentum loss. To test this hypothesis on a star known to be in the WMB regime, we present the first Zeeman Doppler Imaging (ZDI) map of the Sun-like star $τ$Ceti, reconstructed using spectropolarimetric data from the Canada-France-Hawai'i Telescope (CFHT). Our ZDI analysis reveals a remarkably simple, stable and weak ($\langle B\rangle =0.17 \mathrm{G}$) magnetic field, characterized by a predominantly dipolar ($\sim92\%$ magnetic energy contained in $l=1$ modes), and highly axisymmetric ($\sim88\%$ magnetic energy contained in $m<l/2$ modes) morphology. We infer a dipole field strength of $B_{\mathrm{dip}}=0.31 \mathrm{G}$, nearly an order of magnitude weaker than standard braking model predictions, providing direct confirmation of the weakened large-scale dipole predicted by the WMB hypothesis. This work establishes a new benchmark for ZDI, demonstrating that even extremely quiet stars in the WMB regime are accessible to this technique.

astro-ph.SR↗

arXiv2024

Rotation at the Fully Convective Boundary: Insights from Wide WD + MS Binary Systems

Gyrochronology, a valuable tool for determining ages of low-mass stars where other techniques fail, relies on accurate calibration. We present a sample of 185 wide ($>$$100$ au) white dwarf + main sequence (WD + MS) binaries. Total ages of WDs are computed using all-sky survey photometry, Gaia parallaxes, and WD atmosphere models. Using a magnetic braking law calibrated against open clusters, along with assumptions about initial conditions and angular momentum transport, we construct gyrochrones to predict the rotation periods of MS stars. Both data and models show that, at the fully convective boundary (FCB), MS stars with WD ages up to 7.5 Gyr and within a $<50$ K effective temperature range experience up to a threefold increase in rotation period relative to stars slightly cooler than the FCB. We suggest that rapid braking at this boundary is driven by a sharp rise in the convective overturn timescale ($τ_{\mathrm{cz}}$) caused by structural changes between partially and fully convective stars and the $^3 \textrm{He}$ instability occurring at this boundary. While the specific location in mass (or temperature) of this feature varies with model physics, we argue that its existence remains consistent. Stars along this feature exhibit rotation periods that can be mapped, within 1$σ$, to a range of gyrochrones spanning $\approx 6$ Gyr. Due to current temperature errors ($\simeq$$50$ K), this implies that a measured rotation period cannot be uniquely associated to a single gyrochrone, implying that gyrochronology may not be feasible for M dwarfs very close to the FCB.

astro-ph.SR↗