arXiv subjects

Mark Fortune

Publications and source records attributed to Mark Fortune.

11 recordsLinked to original sources

arXiv2026

Hot Rocks Survey V: Secondary Eclipse Photometry of GJ 3473 b with JWST/MIRI

JWST is transforming our ability to characterise small exoplanets, from sub-Neptunes to rocky worlds. A key open question is whether highly irradiated rocky planets can retain atmospheres or are stripped bare by stellar irradiation -- a boundary that remains to be mapped observationally. Here we present the first JWST secondary eclipse observations of the rocky exoplanet GJ 3473 b, obtained with MIRI F1500W photometry. Using four visits, we confidently detect the eclipse at an average depth of 186$\pm$45 ppm, somewhat lower than expected for a blackbody. We test a wide range of data reduction and analysis assumptions and provide new insights into MIRI detector settling behaviour that will benefit future observations. We model a suite of airless surfaces with varied compositions, textures, and degrees of space weathering, as well as idealised atmospheric scenarios including the possibility of atmospheric collapse. Both atmospheric and bare-rock interpretations remain consistent with the data, but we exclude thick CO$_2$ atmospheres, placing a 95 % credible upper limit of 1.2-6.5 bar on the surface pressure. We also find tentative evidence for visit-to-visit variability in eclipse depth (33-371 ppm), though additional data are required to confirm this. Our results highlight the challenges and intrinsic degeneracies in interpreting MIRI F1500W eclipse measurements of rocky exoplanets, indicating that such observations alone may not uniquely distinguish between bare-rock and atmospheric scenarios. Future spectroscopic or phase-curve observations will be required to determine whether or not GJ 3473 b hosts a substantial atmosphere

astro-ph.EP↗

arXiv2025

JWST NIRSpec finds no clear signs of an atmosphere on TOI-1685 b

Determining the prevalence of atmospheres on terrestrial planets is a core objective in exoplanetary science. While M dwarf systems offer a promising opportunity, conclusive observations of terrestrial atmospheres have remained elusive, with many yielding flat transmission spectra. We observe four transits of the hot terrestrial planet TOI-1685 b using JWST's NIRSpec G395H instrument. Combining this with the transit from the previously-observed phase curve of the planet with the same instrument, we perform a detailed analysis to determine the possibility of an atmosphere on TOI-1685 b. From our retrievals, the Bayesian evidence favours a simple flat line model, indicating no evidence for an atmosphere on TOI-1685 b, in line with results from the phase curve analysis. Our results show that hydrogen-dominated atmospheres can be confidently ruled out. For heavier, secondary atmospheres we find a lower limit on the mean molecular weight of ~10, at a significance of ~5 sigma. Pure CO2, SO2, H2O, and CH4 atmospheres, or a mixed secondary atmosphere (CO+CO2+SO2) could explain the data (Delta lnZ < 3). However, pure CH4 atmospheres may be physically unlikely, and the pure H2O and CO2 cases require a high-altitude cloud, which could also be interpreted as a thin cloud-free atmosphere. We discuss the theoretical possibility for different types of atmosphere on this planet, and consider the effects of atmospheric escape and stellar activity on the system. Though we find that TOI-1685 b is likely a bare rock, this study also highlights the challenges of detecting secondary atmospheres on rocky planets with JWST.

astro-ph.EP↗

arXiv2025

Hot Rocks Survey IV: Emission from LTT 3780 b is consistent with a bare rock

It is an open question whether small planets around M dwarfs are able to maintain atmospheres. The Hot Rocks Survey aims to address this question by observing 9 rocky exoplanets orbiting M dwarfs with MIRI emission photometry to constrain the onset of atmospheres. In this paper, we present two MIRI F1500W (15$\mu$m) eclipses of LTT 3780 b, an ultra-short period super-Earth ($P=0.768$ d, $R=1.325 \,R_\oplus$, $M = 2.46\,M_\oplus$) that receives 111x Earth's instellation, the highest in the survey. We find a combined eclipse depth of $312\pm38$ ppm, which is consistent between different data reduction and analysis assumptions, bolstering our confidence in the eclipse detection. This eclipse depth is consistent with the thermal emission from a bare rock surface, with a dayside temperature of $T_d=1143^{+104}_{-99}$ K, $98\pm9$ % of the maximum temperature predicted for a zero albedo, zero heat redistribution blackbody. We are able to confidently rule out CO$_2$-based atmospheres down to 0.01 bar surface pressure to greater than 3$\sigma$ (ruling out an approximately Mars-like atmosphere). We are unable to rule out a pure H$_2$O 1 bar atmosphere, though we argue that this composition is unlikely on such a highly irradiated planet, nor O$_2$ atmospheres due to the lack of features in the bandpass, though we can put constraints on CO$_2$-mixture atmospheres. As a potential bare rock, we consider a variety of surface composition models, but are unable to distinguish between them. However, LTT 3780 b is an excellent target for follow-up JWST observations to determine its surface composition and rule out additional atmospheric compositions.

astro-ph.EP↗

arXiv2025

Hot Rocks Survey III: A deep eclipse for LHS 1140c and a new Gaussian process method to account for correlated noise in individual pixels

Time-series photometry at mid-infrared wavelengths is becoming a common technique to search for atmospheres around rocky exoplanets. This method constrains the brightness temperature of the planet to determine whether heat redistribution is taking place - indicative of an atmosphere - or whether the heat is reradiated from a low albedo bare rock. By observing at 15$\mu$m we are also highly sensitive to CO$_2$ absorption. We observed three eclipses of the rocky super-Earth LHS 1140c using MIRI/Imaging with the F1500W filter. We found significant variation in the initial settling ramp for these observations and identify a potential trend between detector settling and the previous filter used by MIRI. We analysed our data using aperture photometry but also developed a novel approach which joint-fits pixel light curves directly using a shared eclipse model and a flexible multi-dimensional Gaussian process which models changes in the PSF over time. We demonstrate using simulated data that our method has the ability to weight away from particular pixels which show increased systematics, allowing for the recovery of eclipse depths in a more robust and precise way. Both methods and an independent analysis detect the eclipse at $>5\sigma$ and are highly consistent with a low albedo bare rock. We recover a dayside brightness temperature of $T_\mathrm{day} = 561\pm44$ K, close to the theoretical maximum of $T_\text{day; max} = 537\pm9$ K. We rule out a wide range of atmospheric forward models to $>3\sigma$ including pure CO$_2$ atmospheres with surface pressure $\ge10$ mbar and pure H$_2$O atmospheres with surface pressure $\ge1$ bar. Our strict constraints on potential atmospheric composition, in combination with future observations of the exciting outer planet LHS 1140b, could provide a powerful benchmark to understand atmospheric escape around M dwarfs.

astro-ph.EP↗

arXiv2025

New perspectives on MASCARA-1b: A combined analysis of pre- and post-eclipse emission data using CRIRES+

We present high-resolution emission spectroscopy observations of the ultra-hot Jupiter MASCARA-1b with CRIRES+ in the K-band, covering the post-eclipse phases of its orbit. These observations complement previously published pre-eclipse data. The stellar and telluric features were removed using SysRem, and the planetary signal was analysed with the cross-correlation technique. After confirming the presence of chemical species in our atmospheric model, we combined the pre- and post-eclipse datasets for a joint analysis. By employing a Bayesian retrieval framework, this joint retrieval enabled us to constrain the spatially varying temperature-pressure (T-P) profile and atmospheric carbon-to-oxygen (C/O) ratio. We detected strong emission signatures of CO and H$_2$O in the post-eclipse and combined datasets. While a well-mixed retrieval model results in a super-solar C/O, allowing for vertically varying chemistry yields C/O values consistent with solar. The retrieved parameters are not only consistent across the datasets but also across different chemical regimes. We did not identify any significant velocity shifts between the detected species or across the datasets, which could otherwise serve as proxies for possible atmospheric dynamics. We also explored phase dependence through the model scaling factor and found no substantial changes in atmospheric properties throughout the observed phases. Due to strong degeneracies between the temperature gradient and chemical abundances, our retrieved temperatures are broadly consistent with either a full redistribution of heat or strong day-night contrasts. While this complicates direct comparisons with recent Spitzer phase curve analyses suggesting inefficient recirculation, we find no clear evidence of spatial variation in the chemical or temperature structure of MASCARA-1b from pre- to post-eclipse, nor temporal variation over $\approx$2 years.

astro-ph.EP↗

arXiv2024

Hot Rocks Survey I : A possible shallow eclipse for LHS 1478 b

M dwarf systems offer an opportunity to study terrestrial exoplanetary atmospheres due to their small size and cool temperatures. However, the extreme conditions imposed by these host stars raise question about whether their close-in rocky planets are able to retain any atmosphere at all. The Hot Rocks Survey aims to answer this question by targeting nine different M dwarf rocky planets. Of these, LHS 1478 b orbits an M3-type star, has an equilibrium temperature of T$_{eq}$ = 585 K and receives 21 times Earth's instellation. We observe two secondary eclipses using photometric imaging at 15$\mu$m using the Mid-Infrared Instrument on the James Webb Space Telescope (JWST MIRI) to measure thermal emission from the dayside of the planet. We compare these values to atmospheric models to evaluate potential heat transport and CO$_2$ absorption signatures. We find that a secondary eclipse depth of $138\pm 53$ppm at the expected time for a circular orbit is preferred over a null model at $2.8\sigma$, a moderate detection, though dynamical models do favour a non-eccentric orbit for this planet. The second observation results in a non-detection due to significantly larger unexplained systematics. Based on the first observation alone, we can reject the null hypothesis of the dark (zero Bond albedo) no atmosphere bare rock model with a confidence level of 3.3$\sigma$, though for $A_B=0.2$ the significance decreases to $2.1\sigma$. The tentative secondary eclipse depth is consistent with the majority of atmospheric scenarios we considered, spanning CO$_2$-rich atmospheres with surface pressures from 0.1 to 10 bar. However, we stress that the two observations from our program do not yield consistent results, and more observations are needed to verify our findings. The Hot Rocks Survey serves as a relevant primer for the Director's Discretionary Time (DDT) Rocky Worlds program.

astro-ph.EP↗

arXiv2024

High-resolution atmospheric retrievals of WASP-76b transmission spectroscopy with ESPRESSO: Monitoring limb asymmetries across multiple transits

Direct atmospheric retrievals of exoplanets at high-resolution have recently allowed for a more detailed characterisation of their chemistry and dynamics from the ground. By monitoring the longitudinal distribution of species, as well as the varying vertical temperature structure and dynamics between the limbs of WASP-76b, across multiple transits, we aim to enhance our understanding of the 3D nature and chemical/dynamical evolution of such objects over the timescales of months/years. We present retrievals of three VLT/ESPRESSO observations of the ultra-hot Jupiter WASP-76b, including one not yet reported in the literature, from which we constrain the atmospheric abundances, vertical temperature structure, and atmospheric dynamics for both the leading and trailing limbs of the atmosphere separately, via novel rotational broadening kernels. We confirm the presence of VO recently reported in the atmosphere of WASP-76b. We find a uniform longitudinal distribution of Fe and Mg across the limbs of the atmosphere, for each of our transits, consistent with previous works as well as with stellar values. We constrain substellar Na/Fe and Cr/Fe ratios across each of our transits, consistent with previous studies of WASP-76b. Where constrained, V/Fe and VO/Fe ratios were also found to be broadly consistent between the limbs of the atmosphere for each of the transits, as well as with previous studies. However, for two of the transits, both V and VO were unconstrained in the leading limb, suggesting possible depletion due to recombination and condensation. The consistency of our constraints across multiple high-resolution observations, as well as with previous studies using varying modelling/retrieval frameworks and/or instruments, affirms the efficacy of high-resolution ground-based retrievals of exoplanetary atmospheres.

astro-ph.EP↗

arXiv2024

How do wavelength correlations affect transmission spectra? Application of a new fast and flexible 2D Gaussian process framework to transiting exoplanet spectroscopy

The use of Gaussian processes (GPs) is a common approach to account for correlated noise in exoplanet time series, particularly for transmission and emission spectroscopy. This analysis has typically been performed for each wavelength channel separately, with the retrieved uncertainties in the transmission spectrum assumed to be independent. However, the presence of noise correlated in wavelength could cause these uncertainties to be correlated, which could significantly affect the results of atmospheric retrievals. We present a method that uses a GP to model noise correlated in both wavelength and time simultaneously for the full spectroscopic dataset while avoiding the use of a 'common-mode' correction. To make this analysis computationally tractable, we introduce a new fast and flexible GP method that can analyse 2D datasets when the input points lie on a (potentially non-uniform) 2D grid - in our case a time by wavelength grid - and the kernel function has a Kronecker product structure. This simultaneously fits all light curves and enables the retrieval of the covariance matrix of the transmission spectrum. By testing on synthetic datasets, we demonstrate that our new approach can reliably recover atmospheric features contaminated by noise correlated in time and wavelength. In contrast, fitting each spectroscopic light curve separately performed poorly when wavelength-correlated noise was present. It frequently underestimated the uncertainty of the scattering slope and overestimated the uncertainty in the strength of sharp absorption peaks in transmission spectra. Two archival VLT/FORS2 transit observations of WASP-31b were re-analysed, with our method strongly constraining the presence of wavelength-correlated noise in both datasets and recovering significantly different constraints on atmospheric features such as the scattering slope and strength of sodium and potassium features.

astro-ph.EP↗

arXiv2023

High-resolution emission spectroscopy retrievals of MASCARA-1b with CRIRES+: Strong detections of CO, H$_2$O and Fe emission lines and a C$/$O consistent with solar

The characterization of exoplanet atmospheres has proven to be successful using high-resolution spectroscopy. Phase curve observations of hot/ultra-hot Jupiters can reveal their compositions and thermal structures, thereby allowing the detection of molecules and atoms in the planetary atmosphere using the cross-correlation technique. We present pre-eclipse observations of the ultra-hot Jupiter, MASCARA-1b, observed with the recently upgraded CRIRES+ high-resolution infrared spectrograph at the VLT. We report a detection of $\rm Fe$ ($\approx$8.3$\sigma$) in the K-band and confirm previous detections of $\rm CO$ (>15$\sigma$) and $\rm H_2O$ (>10$\sigma$) in the day-side atmosphere of MASCARA-1b. Using a Bayesian inference framework, we retrieve the abundances of the detected species and constrain planetary orbital velocities, $T$-$P$ profiles, and the carbon-to-oxygen ratio ($\rm C/O$). A free retrieval results in an elevated $\rm CO$ abundance ($\log_{10}$($\chi_{\rm{{}^{12}CO}}$) = $-2.85^{+0.57}_{-0.69}$), leading to a super-solar $\rm C/O$ ratio. More realistically, allowing for vertically-varying chemistry in the atmosphere by incorporating a chemical-equilibrium model results in a $\rm C/O$ of $0.68^{+0.12}_{-0.22}$ and a metallicity of $[\rm M/H] = 0.62^{+0.28}_{-0.55}$, both consistent with solar values. Finally, we also report a slight offset of the $\rm Fe$ feature in both K$_{\rm p}$ and v$_{\rm sys}$ that could be a signature of atmospheric dynamics. Due to the 3D structure of exoplanet atmospheres and the exclusion of time/phase dependence in our 1D forward models, further follow-up observations and analysis are required to confirm or refute this result.

astro-ph.EP↗

arXiv2022

High-resolution atmospheric retrievals of WASP-121b transmission spectroscopy with ESPRESSO: Consistent relative abundance constraints across multiple epochs and instruments

Recent progress in high-resolution transmission spectroscopy has offered new avenues in which to characterise the atmospheres of transiting exoplanets. High-resolution cross-correlation spectroscopy allows for the unambiguous detection of molecules/atoms. It has also been used to map both atmospheric dynamics and longitudinal variations in the abundance of species across the morning and evening limbs. We present multiple VLT/ESPRESSO observations of the ultra-hot Jupiter WASP-121b, from which we constrain relative abundances of various neutral metals consistently across all observations, whilst accounting for the distortion of the exoplanet's signal caused by traditional data processing techniques. We also constrain planetary orbital velocities and $T$-$P$ profiles. We compare our abundance constraints with previous constraints using VLT/UVES transmission spectroscopy of WASP-121b, and find our results to be consistent between observations, and also in agreement with stellar values for species previously detected in the atmosphere of WASP-121b. Our retrieval framework can also be used to identify potential exospheric species, resulting in extended absorption features beyond the transit equivalent Roche limit of WASP-121b ($R_{\rm eqRL}$ $\sim$ 1.3 $R_{\rm p}$). H$\alpha$, Fe II, and Ca II were found to extend to high altitudes ($1.54\pm0.04$ $R_{\rm p}$, $1.17\pm0.01$ $R_{\rm p}$, and $2.52\pm0.34$ $R_{\rm p}$, respectively), which are broadly consistent with literature values. The consistency of our constraints across multiple high-resolution observations is a strong validation of our model filtering and retrieval framework, as well as the stability of the atmosphere over the timescales of months/years, and could allow for planet formation processes to be inferred from future ground-based observations of exoplanetary atmospheres.

astro-ph.EP↗

arXiv2022

Toward Development of Machine Learned Techniques for Production of Compact Kinetic Models

Chemical kinetic models are an essential component in the development and optimisation of combustion devices through their coupling to multi-dimensional simulations such as computational fluid dynamics (CFD). Low-dimensional kinetic models which retain good fidelity to the reality are needed, the production of which requires considerable human-time cost and expert knowledge. Here, we present a novel automated compute intensification methodology to produce overly-reduced and optimised (compact) chemical kinetic models. This algorithm, termed Machine Learned Optimisation of Chemical Kinetics (MLOCK), systematically perturbs each of the four sub-models of a chemical kinetic model to discover what combinations of terms results in a good model. A virtual reaction network comprised of n species is first obtained using conventional mechanism reduction. To counteract the imposed decrease in model performance, the weights (virtual reaction rate constants) of important connections (virtual reactions) between each node (species) of the virtual reaction network are numerically optimised to replicate selected calculations across four sequential phases. The first version of MLOCK, (MLOCK1.0) simultaneously perturbs all three virtual Arrhenius reaction rate constant parameters for important connections and assesses the suitability of the new parameters through objective error functions, which quantify the error in each compact model candidate's calculation of the optimisation targets, which may be comprised of detailed model calculations and/or experimental data. MLOCK1.0 is demonstrated by creating compact models for the archetypal case of methane air combustion. It is shown that the NUGMECH1.0 detailed model comprised of 2,789 species is reliably compacted to 15 species (nodes), whilst retaining an overall fidelity of ~87% to the detailed model calculations, outperforming the prior state-of-art.

physics.chem-ph↗