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Aligned, Misaligned and Polar Orbits of Hot Jupiters: Measuring Spin-Orbit Angles via Doppler Tomography with HARPS-N

Z. Balkóová, J. Žák, M. Skarka, E. Knudstrup, P. Gajdoš, A. Bignamini, P. Kabáth

TL;DR

This study uses Doppler tomography with HARPS‑N data to measure the projected spin–orbit angles λ for three hot Jupiters, exploring how different CCF processing methods influence the detected Doppler shadows. It finds a polar orbit for HAT‑P‑49 b (λ ≈ -85.3°), a well‑aligned orbit for HAT‑P‑57 A (λ ≈ -0.4°), and a moderately misaligned orbit for XO‑3 A (λ ≈ 38°), while highlighting how template choice and pulsations can affect λ in Doppler tomographic analyses. The work also situates these configurations within dynamical timescales such as Kozai–Lidov cycles, tidal circularization, and radiative realignment, arguing that the observed diversity likely reflects multiple migration histories and long‑term evolution. Overall, the paper emphasizes both the scientific value of spin–orbit measurements and the methodological caveats related to CCF construction, advocating for larger, diverse samples to robustly map spin‑orbit architectures and migration pathways in exoplanet systems.

Abstract

Although the migration of hot Jupiters is not yet fully understood, measurements of the projected spin-orbit angle λ help shed light on the processes involved. Here we present Doppler tomography of three known hot Jupiters to determine their λ orientation: HAT-P-49 b, HAT-P-57A b, and XO-3A b. Our analysis explores the impact of cross-correlation processing methods on the detectability and characterization of the planet's Doppler shadow using up to three independent routines for cross-correlation functions extraction; those being: Yabi, iSpec and IRAF. After accounting for differences among the results obtained with the various routines, we report: first, the HAT-P-49 system is a case of a hot Jupiter on a polar orbit with λ=-85.3{\pm}1.7°, second, HAT-P-57A indicates practically no deviation of the planet's projected orbit from the host's equatorial plane with λ=-0.4^{+1.4}_{-1.9}°, and third, the XO-3A system with the measured value of λ=38{+3}_{-4}° lies in between an aligned and a perpendicular orientation, which is a less populated region of the spin-orbit distribution. Our findings highlight both the diversity of spin-orbit angles among close-in giant planets and the potential discrepancies in their measurement that can arise from different approaches to constructing the cross-correlation functions.

Aligned, Misaligned and Polar Orbits of Hot Jupiters: Measuring Spin-Orbit Angles via Doppler Tomography with HARPS-N

TL;DR

This study uses Doppler tomography with HARPS‑N data to measure the projected spin–orbit angles λ for three hot Jupiters, exploring how different CCF processing methods influence the detected Doppler shadows. It finds a polar orbit for HAT‑P‑49 b (λ ≈ -85.3°), a well‑aligned orbit for HAT‑P‑57 A (λ ≈ -0.4°), and a moderately misaligned orbit for XO‑3 A (λ ≈ 38°), while highlighting how template choice and pulsations can affect λ in Doppler tomographic analyses. The work also situates these configurations within dynamical timescales such as Kozai–Lidov cycles, tidal circularization, and radiative realignment, arguing that the observed diversity likely reflects multiple migration histories and long‑term evolution. Overall, the paper emphasizes both the scientific value of spin–orbit measurements and the methodological caveats related to CCF construction, advocating for larger, diverse samples to robustly map spin‑orbit architectures and migration pathways in exoplanet systems.

Abstract

Although the migration of hot Jupiters is not yet fully understood, measurements of the projected spin-orbit angle λ help shed light on the processes involved. Here we present Doppler tomography of three known hot Jupiters to determine their λ orientation: HAT-P-49 b, HAT-P-57A b, and XO-3A b. Our analysis explores the impact of cross-correlation processing methods on the detectability and characterization of the planet's Doppler shadow using up to three independent routines for cross-correlation functions extraction; those being: Yabi, iSpec and IRAF. After accounting for differences among the results obtained with the various routines, we report: first, the HAT-P-49 system is a case of a hot Jupiter on a polar orbit with λ=-85.3{\pm}1.7°, second, HAT-P-57A indicates practically no deviation of the planet's projected orbit from the host's equatorial plane with λ=-0.4^{+1.4}_{-1.9}°, and third, the XO-3A system with the measured value of λ=38{+3}_{-4}° lies in between an aligned and a perpendicular orientation, which is a less populated region of the spin-orbit distribution. Our findings highlight both the diversity of spin-orbit angles among close-in giant planets and the potential discrepancies in their measurement that can arise from different approaches to constructing the cross-correlation functions.
Paper Structure (10 sections, 4 equations, 10 figures, 5 tables)

This paper contains 10 sections, 4 equations, 10 figures, 5 tables.

Table of Contents

  1. Introduction
  2. Data samples
  3. Methodology
  4. Data Analysis
  5. HAT-P-49
  6. HAT-P-57 A
  7. The Pulsations of HAT-P-57 A
  8. XO-3 A
  9. Timescales
  10. Discussion

Figures (10)

  • Figure 1: From left to right: data - best-fit model - residuals from fitting the Doppler shadow of HAT-P-49 using the Yabi CCFs on a grid of -60 to +60 km/s. Solid blue horizontal lines depict the first (at the bottom) and last (at the top) planet's contact with the stellar disk, dashed blue lines border the total transit. Solid red lines denote the borders behind which no influence of the planet on the line profiles is observed, i.e., borders for normalization baseline. The zero-velocity represents the center of the stellar disc, and the combination of a high projected obliquity and impact parameter - reflected by inclination - implies that the planet’s trajectory does not intersect the redshifted hemisphere of the star.
  • Figure 2: Comparison of fitting three different CCF datasets made out of the same spectral dataset of HAT-P-49; green: Yabi pipeline, blue: IRAF, red: iSpec. Contours denote 1, 2 and 3 $\sigma$ confidences.
  • Figure 3: Top row: Doppler tomography of HAT-P-57 A from three observing nights created from CCFs on sharp-lined synthetic template ($v\sin i_*=2$ km/s). The dashed horizontal lines mark the orbital phases of the first and fourth transit contacts. Bottom row: Third of the selected nights (2019-06-28) plotted as created from CCFs on synthetic templates with $v\sin i_*$ of 2 and 20 km/s, and a dataset template for the last case. Besides the Doppler shadow, we can see additional diagonal distortions with different slope, which extend beyond the transit boundaries. These can be attributed to stellar oscillations.
  • Figure 4: From left to right: data - best-fit model - residuals from fitting the Doppler shadow of HAT-P-57 A, depicted for 2nd night on a grid of -140 to +140 km/s.
  • Figure 5: Top panel: Sector 80 TESS data with 20-seconds (red) and 120-seconds cadences (blue) around one of the observed transits. The frequency spectrum of the 20-seconds data once the transits have been masked out is shown in the bottom panel.
  • ...and 5 more figures