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publications
The HARPS search for southern extra-solar planets - XLV. Two Neptune mass planets orbiting HD 13808: a study of stellar activity modelling’s impact on planet detection
Published in MNRAS, 2021
We present a comprehensive analysis of 10 yr of HARPS radial velocities (RVs) of the K2V dwarf star HD 13808, which has previously been reported to host two unconfirmed planet candidates. We use the state-of-the-art nested sampling algorithm POLYCHORD to compare a wide variety of stellar activity models, including simple models exploiting linear correlations between RVs and stellar activity indicators, harmonic models for the activity signals, and a more sophisticated Gaussian process regression model. We show that the use of overly simplistic stellar activity models that are not well-motivated physically can lead to spurious ‘detections’ of planetary signals that are almost certainly not real. We also reveal some difficulties inherent in parameter and model inference in cases where multiple planetary signals may be present. Our study thus underlines the importance both of exploring a variety of competing models and of understanding the limitations and precision settings of one’s sampling algorithm. We also show that at least in the case of HD 13808, we always arrive at consistent conclusions about two particular signals present in the RV, regardless of the stellar activity model we adopt; these two signals correspond to the previously reported though unconfirmed planet candidate signals. Given the robustness and precision with which we can characterize these two signals, we deem them secure planet detections. In particular, we find two planets orbiting HD 13808 at distances of 0.11, 0.26 au with periods of 14.2, 53.8 d, and minimum masses of 11, 10 Earth masses.
Recommended citation: Ahrer, E. et al. (2021), MNRAS, 503, 1248 http://evamariaa.github.io/files/ahrer-harps-search-for-exoplanets-xlv-hd13808bc.pdf
LRG-BEASTS: Sodium absorption and Rayleigh scattering in the atmosphere of WASP-94A b using NTT/EFOSC2
Published in MNRAS, 2022
We present an optical transmission spectrum for WASP-94A b, the first atmospheric characterization of this highly-inflated hot Jupiter. The planet has a reported radius of 1.72 Jupiter radii, a mass of only 0.456 Jupiter masses, and an equilibrium temperature of 1508 K. We observed the planet transit spectroscopically with the EFOSC2 instrument on the ESO New Technology Telescope (NTT) at La Silla, Chile: the first use of NTT/EFOSC2 for transmission spectroscopy. We achieved an average transit-depth precision of 128 ppm for bin widths of ~200 Å. This high precision was achieved in part by linking Gaussian Process hyperparameters across all wavelength bins. The resulting transmission spectrum, spanning a wavelength range of 3800-7140 Å, exhibits a sodium absorption with a significance of 4.9σ, suggesting a relatively cloud-free atmosphere. The sodium signal may be broadened, with a best-fitting width of 78 Å in contrast to the instrumental resolution of 27.2 ± 0.2 Å. We also detect a steep slope in the blue end of the transmission spectrum, indicating the presence of Rayleigh scattering in the atmosphere of WASP-94A b. Retrieval models show evidence for the observed slope to be super-Rayleigh and potential causes are discussed. Finally, we find narrow absorption cores in the CaII H&K lines of WASP-94A, suggesting the star is enshrouded in gas escaping the hot Jupiter.
Recommended citation: Ahrer, E. et al. (2022), MNRAS, 510, 4857 http://evamariaa.github.io/files/ahrer-lrg-beasts-wasp-94ab.pdf
Early Release Science of the exoplanet WASP-39b with JWST NIRCam
Published in Nature, 2023
Measuring the metallicity and carbon-to-oxygen (C/O) ratio in exoplanet atmospheres is a fundamental step towards constraining the dominant chemical processes at work and, if in equilibrium, revealing planet formation histories. Transmission spectroscopy (for example, refs. 1,2) provides the necessary means by constraining the abundances of oxygen- and carbon-bearing species; however, this requires broad wavelength coverage, moderate spectral resolution and high precision, which, together, are not achievable with previous observatories. Now that JWST has commenced science operations, we are able to observe exoplanets at previously uncharted wavelengths and spectral resolutions. Here we report time-series observations of the transiting exoplanet WASP-39b using JWST’s Near InfraRed Camera (NIRCam). The long-wavelength spectroscopic and short-wavelength photometric light curves span 2.0-4.0 micrometres, exhibit minimal systematics and reveal well defined molecular absorption features in the planet’s spectrum. Specifically, we detect gaseous water in the atmosphere and place an upper limit on the abundance of methane. The otherwise prominent carbon dioxide feature at 2.8 micrometres is largely masked by water. The best-fit chemical equilibrium models favour an atmospheric metallicity of 1-100-times solar (that is, an enrichment of elements heavier than helium relative to the Sun) and a substellar C/O ratio. The inferred high metallicity and low C/O ratio may indicate significant accretion of solid materials during planet formation (for example, refs. 3,4,) or disequilibrium processes in the upper atmosphere (for example, refs. 5,6).
Recommended citation: Ahrer, E. et al. (2023), Nature, 614, 653 http://evamariaa.github.io/files/paper3.pdf
LRG-BEASTS: evidence for clouds in the transmission spectrum of HATS-46 b
Published in MNRAS, 2023
We have performed low-resolution ground-based spectroscopy of HATS-46 b in transmission, using the EFOSC2 instrument on the ESO New Technology Telescope (NTT). HATS-46 b is a highly inflated exoplanet that is a prime target for transmission spectroscopy, having a Jupiter-like radius (0.95 RJup) but a much lower mass (0.16 MJup). It orbits a G-type star with a 4.7 d period, giving an equilibrium temperature of 1100 K. We observed one transit of HATS-46 b with the NTT, with the time-series spectra covering a wavelength range of 3900-9000 Å at a resolution (R) of ~380. We achieved a remarkably precise transmission spectrum of 1.03 × photon noise, with a median uncertainty of 357 ppm for ~200 Å-wide bins, despite the relative faintness of the host star with Vmag = 13.6. The transmission spectrum does not show strong absorption features and retrievals favour a cloudy model, ruling out a clear atmosphere with 3.0σ confidence. We also place a conservative upper limit on the sodium abundance under the alternative scenario of a clear atmosphere. This is the eighth planet in the LRG-BEASTS (Low-Resolution Ground-Based Exoplanet Atmosphere Survey using Transmission Spectroscopy) survey, which uses 4 m-class telescopes such as the NTT to obtain low-resolution transmission spectra of hot Jupiters with precisions of around one atmospheric scale height.
Recommended citation: Ahrer, E. et al. (2023), MNRAS, 521, 5636 http://evamariaa.github.io/files/ahrer-lrg-beasts-hats-46b.pdf
Atmospheric characterization and tighter constraints on the orbital misalignment of WASP-94 A b with HARPS
Published in MNRAS, 2024
We present high spectral resolution observations of the hot Jupiter WASP-94 A b using the HARPS instrument on ESO’s 3.6-m telescope in La Silla, Chile. We probed for Na absorption in its atmosphere as well as constrained the previously reported misaligned retrograde orbit using the Rossiter-McLaughlin effect. Additionally, we undertook a combined atmospheric retrieval analysis with previously published low-resolution data. We confirm the retrograde orbit as well as constrain the orbital misalignment with our measurement of a projected spin-orbit obliquity of λ = 123.0 ± 3.0°. We find a tentative detection of Na absorption in the atmosphere of WASP-94 A b, independent of the treatment of the Rossiter-McLaughlin effect in our analysis (3.6σ and 4.4σ). We combine our HARPS high-resolution data with low-resolution data from the literature and find that while the posterior distribution of the Na abundance results in a tighter constraint than using a single data set, the detection significance does not improve (3.2σ), which we attribute to degeneracies between the low- and high-resolution data.
Recommended citation: Ahrer, E. et al. (2024), MNRAS, 530, 2749 http://evamariaa.github.io/files/ahrer-harps-wasp-94ab.pdf
Escaping Helium and a Highly Muted Spectrum Suggest a Metal-enriched Atmosphere on Sub-Neptune GJ 3090 b from JWST Transit Spectroscopy
Published in ApJL, 2025
Sub-Neptunes, the most common planet type, remain poorly understood. Their atmospheres are expected to be diverse, but their compositions are challenging to determine, even with JWST. Here, we present the first JWST spectroscopic study of the warm sub-Neptune GJ 3090 b (2.13 R⊕, Teq,A = 0.3 ∼ 700 K), which orbits an M2V star, making it a favorable target for atmosphere characterization. We observed four transits of GJ 3090 b: two each using JWST NIRISS/SOSS and NIRSpec/G395H, yielding wavelength coverage from 0.6 to 5.2 μm. We detect the signature of the 10833 Å metastable helium triplet at a statistical significance of 5.5σ with an amplitude of 434 ± 79 ppm, marking the first such detection in a sub-Neptune with JWST. This amplitude is significantly smaller than predicted by solar-metallicity forward models, suggesting a metal-enriched atmosphere that decreases the mass-loss rate and attenuates the helium feature amplitude. Moreover, we find that stellar contamination, in the form of the transit light source effect, dominates the NIRISS transmission spectra, with unocculted spot and faculae properties varying across the two visits separated in time by approximately 6 months. Free retrieval analyses on the NIRSpec/G395H spectrum find tentative evidence for highly muted features and a lack of CH4. These findings are best explained by a high-metallicity atmosphere (>100× solar at 3σ confidence for clouds at ∼μbar pressures) using chemically consistent retrievals and self-consistent model grids. Further observations of GJ 3090 b are needed for tighter constraints on the atmospheric abundances and to gain a deeper understanding of the processes that led to its potential metal enrichment.
Recommended citation: Ahrer, E. et al. (2025a), ApJL, 985, L10 http://evamariaa.github.io/files/ahrer-jwst-gj3090b.pdf
Tracing the formation and migration history: molecular signatures in the atmosphere of misaligned hot Jupiter WASP-94Ab using JWST NIRSpec/G395H
Published in MNRAS, 2025
The discovery of hot Jupiters that orbit very close to their host stars has long challenged traditional models of planetary formation and migration. Characterising their atmospheric composition - mainly in the form of the carbon-to-oxygen (C/O) ratio and metallicity - can provide insights into their formation locations and evolution pathways. With JWST we can characterise the atmospheres of these types of planets more precisely than previously possible, primarily because it allows us to determine both their atmospheric oxygen and carbon composition. Here, we present a JWST NIRSpec/G395H transmission spectrum from 2.8 - 5.1 µm of WASP-94Ab, an inflated hot Jupiter with a retrograde misaligned orbit around its F-type host star. We find a relatively cloud-free atmosphere, with absorption features of H2O and CO2 at detection significances of ~4σ and ~11σ, respectively. In addition, we detect tentative evidence of CO absorption at ~3σ, as well as hints of sulphur with the detection of H2S at a ~2.5σ confidence level. Our favoured equilibrium chemistry model determines a C/O ratio of 0.49+0.08−0.13 for WASP-94Ab’s atmosphere, which is substellar compared to the star’s C/O ratio of 0.68 ± 0.10. The retrieved atmospheric metallicity is similar to the star’s metallicity as both are ~2 × solar. We find that this sub-stellar C/O ratio and stellar metallicity can be best explained by pebble accretion or planetesimal accretion in combination with large-distance migration of the planet.
Recommended citation: Ahrer, E. et al. (2025b), MNRAS, in press http://evamariaa.github.io/files/ahrer-jwst-wasp-94ab.pdf