Dream

Abstract

Context. The population of close-in exoplanets features a desert of hot Neptunes whose origin remains uncertain. These planets may have lost their atmosphere, eroding into mini-Neptunes and rocky super-Earths below the desert. Direct observations of evaporating atmospheres are essential to derive mass-loss estimates and constrain this scenario. The metastable He I triplet at 1083.3 nm represents a powerful diagnostic of atmospheric evaporation because it traces the hot gas in extended exoplanet atmospheres while being observed from the ground. In addition, it is located at the bright near-infrared stellar continuum and is very weakly affected by interstellar medium (ISM) absorption. Aims. We carried out a homogeneous He I transmission spectroscopy survey, targeting a selected sample of nine planets along the different edges of the desert, to interpret the absorption line profile with evaporation models and to better understand the role of photoevaporation in the desert formation. Methods. We observed one transit per planet using the high-resolution, near-infrared spectrograph GIANO-B mounted on the Telescopio Nazionale Galileo telescope. We focused our analysis on the He I triplet, based on a comparison of the in-transit and out-of-transit observations, and we computed high-resolution transmission spectra. We then employed the 1D p-winds model to calculate the planetary thermospheric structures and to interpret the observed transmission spectra. Results. We found no signatures of planetary absorption in the He I triplet in any of the investigated targets. We thus provided 3 σ upper-limit estimations on the thermosphere absorption, temperature and mass loss, and combined them with past measurements to search for correlations with parameters such as the stellar mass and XUV flux, which are thought to be key drivers in the formation of the He I triplet. Conclusions. These results strengthen the importance of performing homogeneous surveys and analyses in bringing clarity to He I detections and (thereby) to plausible Neptunian desert origins. Our findings corroborate literature expectations that state the He I absorption signal is correlated with the stellar mass and the received XUV flux. However, when translated in terms of mass-loss rates, these trends seem to disappear. Thus, further studies are essential to shed light on this aspect and to better understand the photoevaporation process.

A High-resolution Non-detection of Escaping Helium in the Ultrahot Neptune LTT 9779b: Evidence for Weakened Evaporation

“…These correspond to 14 and 22 lower-atmospheric scale heights, respectively (δR p /H < 14 at 2σ and δR p /H < 22 at 3σ), calculated as = m H k T g B eq with μ = 2.3 amu. Our reporting is slightly different than Allart et al (2023) and Guilluy et al (2023), who report 3σ for upper limits without the contribution of the mean absorption and calculate H assuming μ = 1.3 amu. Under their assumptions, our upper limits are 0.24% and δR p /H < 14 (the larger δR p is balanced out by the larger H, resulting in nearly the same δR p /H as our 2σ upper limit).…”

“…To set an upper limit on the planetary absorption, we first calculate the mean absorption in a 0.75 Å bin centered on the helium triplet (−0.04%), and we take σ to be the rms scatter of the transmission spectrum binned to 0.75 Å (0.08%). Estimating σ in this way accounts for correlated noise (Allart et al 2023;Guilluy et al 2023). Our resulting 2σ and 3σ upper limits in a 0.75 Å bin centered on the helium triplet are 0.12% and 0.20%, respectively.…”

“…Before continuing the analysis, we masked the cores of the deep Mg 10811 Å, Si 10827 Å, and Si 10844 Å lines, as we noticed weak (1%) time-correlated variations at these wavelengths. Such residuals are commonly seen and masked in helium observations (e.g., Guilluy et al 2023;Zhang et al 2023) and likely originate from the inadequacy of typical spectral extraction approaches for sharp lines in high-S/N data (Zhang et al 2021).…”

“…LTT 9779b is therefore an excellent target for transmission spectroscopy in the metastable He 10830 Å triplet, which probes atmospheric escape (Oklopčić & Hirata 2018;Spake et al 2018). Planets orbiting stars cooler than the Sun (Oklopčić 2019) are now routinely studied in He 10830 Å, allowing for population-level constraints on planetary massloss rates (Vissapragada et al 2022;Allart et al 2023;Guilluy et al 2023). Models suggest that planets orbiting late G stars like LTT 9779 should be favorable targets for helium outflow observations (Oklopčić 2019;Wang & Dai 2021;Biassoni et al 2023), but there have been relatively few He 10830 Å studies for planets orbiting G-type stars (Allart et al 2023;Bennett et al 2023;Guilluy et al 2023).…”

“…Planets orbiting stars cooler than the Sun (Oklopčić 2019) are now routinely studied in He 10830 Å, allowing for population-level constraints on planetary massloss rates (Vissapragada et al 2022;Allart et al 2023;Guilluy et al 2023). Models suggest that planets orbiting late G stars like LTT 9779 should be favorable targets for helium outflow observations (Oklopčić 2019;Wang & Dai 2021;Biassoni et al 2023), but there have been relatively few He 10830 Å studies for planets orbiting G-type stars (Allart et al 2023;Bennett et al 2023;Guilluy et al 2023). Recently, Edwards et al (2023) attempted to constrain helium absorption in the upper atmosphere of LTT 9779b with the HST Wide-Field Camera 3 (WFC3) in the G102 grism, but resolving power was ultimately a limiting factor.…”