arXiv subjects

N. Piskunov

Publications and source records attributed to N. Piskunov.

At least 19 recordsLinked to original sources

arXiv2026

51 Peg b revisited with VLT/CRIRES+. Constraints on atmospheric thermal structure, chemical composition, and an alternative orbital solution

So far, the atmospheres of non-transiting exoplanets remain poorly explored, and the potential of high-resolution spectroscopy for determining key planetary parameters beyond their atmospheric properties has not been fully exploited. We obtained high-resolution emission spectra of the non-transiting hot Jupiter 51 Peg b with VLT/CRIRES+ to study its atmospheric thermochemical and dynamical structure and derive additional planetary parameters from the Doppler shift of its spectral lines. Using the cross-correlation technique, we confirmed the spectral signature of H$_2$O in the planetary emission spectrum and a non-inverted atmospheric temperature profile. An indication was also found for the presence of atmospheric CO. The atmospheric chemical and thermal conditions were quantitatively constrained by use of a Bayesian retrieval framework, which yielded a high metallicity value ($2.63_{-0.93}^{+1.00}$ dex) suggestive of chemical quenching, a solar C/O ratio ($0.54_{-0.23}^{+0.18}$), and a temperature profile in the upper atmosphere in line with the expected planetary equilibrium temperature. Moreover, we measured an excess of spectral line broadening. The planetary signal was recovered at an orbital semi-amplitude of $102.8_{-9.1}^{+8.3}$ km s$^{-1}$, differing from previous high-resolution measurements. From the obtained orbital semi-amplitude, we further derived the surface gravity ($3.15 \pm 0.12$ log cgs), mass ($0.61_{-0.05}^{+0.06}$ $M_\mathrm{Jup}$), and orbital inclination ($49.8_{-5.7}^{+5.8}$ deg) of 51 Peg b. The mass and inclination values differ from those reported in earlier works. Overall, this study provides new constraints on the thermochemical and dynamical properties of 51 Peg b's atmosphere and demonstrates the potential of high-resolution emission spectroscopy for measuring parameters related to the planetary orbital motion.

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arXiv2026

Cloudy with a chance of metals: Indications of CO$_2$ in the atmosphere of GJ 1214 b from high-resolution K-band spectroscopy

Sub-Neptune exoplanets frequently exhibit muted transmission spectra, with GJ 1214 b being the most prominent example. Following years of intense observing campaigns yielding featureless planetary spectra, recent observations with JWST revealed the first possible atmospheric signatures. We present high-resolution transmission spectroscopy of GJ 1214 b based on eight transits obtained with the CRIRES$^+$ spectrograph in the K band. We used SYSREM to remove telluric and stellar signals and searched for signatures of H2O, CO, CH4, H2S, NH3, and CO2 using the cross-correlation technique. We obtained non-detections for the first five molecules and used injection recovery tests to derive upper limits on the atmosphere. For CO$_2$ we measure a CCF signal at S/N ~ 3.6, with a detailed investigation showing no obvious indication that it is caused by correlated noise. A Welch t-test confirmed the in-trail and out-of-trail distributions to be different at $3.4 σ$ confidence. A Bayesian retrieval framework with free chemistry, resulted in volume mixing ratios corresponding to a metallicity of $[\mathrm{M/H}]=0.48^{+0.89}_{-1.70}$, an opacity deck pressure of $\log_{10}(P_\mathrm{c}) = -3.04^{+2.52}_{-1.53}$ and a planet temperature of $T_\mathrm{iso}=398^{+283}_{-197}$ K, consistent with a value intermediate between the day- and night-side T-p's derived from JWST data. While these values correspond to relatively large signal amplitudes predicted for CO2 features in the mid-infrared, they are compatible with JWST NIRSpec observations within the models' $1.5σ$ uncertainties. Further modelling and additional data are required to confirm the atmospheric signatures and obtain a comprehensive interpretation of low- and high-resolution data. Overall, our results support previous findings that CO2 is likely to be a significant component of the atmosphere of GJ 1214 b.

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arXiv2026

Mass determination of the ultra-short-period planet LHS 3844 b. First K-band radial velocity measurements with CRIRES+

We present the first planet mass measurement obtained with CRIRES+ radial velocity (RV) observations using the K-band gas cell. Our target, LHS 3844 b (TOI-136), is a transiting super-Earth with radius $R_b=1.286^{+0.043}_{-0.044}R_\oplus$ and an orbital period of $P_b = 0.462929709^{+0.000000044}_{-0.000000042}$d, placing it in the class of ultra-short-period (USP) planets. The host star LHS 3844 is an old ($7.8\pm1.6$Gyr), slowly rotating ($P_{rot} = 130.0^{+16.9}_{-13.4}$d) M5.0 dwarf with $M_\star = 0.151\pm0.014M_\odot$ at a distance of 15pc (V=15.2mag, K=9.2mag). Combining our CRIRES+ RVs with archival ESPRESSO spectra, and confirming the signal in each dataset independently, we detected periodic RV variations with a semi-amplitude $K_b=6.95^{+0.55}_{-0.60}$m/s, implying a planetary mass of $m_b = 2.37\pm0.25M_\oplus$ and a bulk density of $ρ_b = 6.15^{+0.60}_{-0.61}$gcm$^{-3}$, consistent with a predominantly rocky composition. We further found excess RV variability that may be attributed to stellar jitter or to an additional planetary signal, for which we identified a tentative super-Earth candidate with a period of $\approx6.88$d. Owing to its proximity to its M-dwarf host, LHS 3844 b experiences intense irradiation and is unlikely to retain a substantial H/He envelope. Interior modeling places an upper limit on the iron-core mass fraction, which is consistent with an Earth-like rocky composition. With an emission spectroscopy metric of 28, LHS 3844 b is a prime JWST target for atmospheric and surface characterization and the most promising surface-characterization target known. Phase-curve spectroscopy may reveal its surface mineralogy and enable the first robust detection of exoplanet surface spectral features. Our results demonstrate that near-infrared RVs obtained with CRIRES+ enable robust mass measurements of super-Earths orbiting late M dwarfs.

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arXiv2025

viper: High-precision radial velocities from the optical to the infrared (Reaching 3 m/s in the K band of CRIRES+ with telluric modelling)

In recent years, a number of new instruments and data reduction pipelines have been developed to obtain high-precision radial velocities (RVs). In particular in the optical, considerable progress has been made and RV precision below 50 cm/s has been reached. Yet, the RV precision in the near-infrared (NIR) is trailing behind. This is due to a number of factors, such as imprinted atmospheric absorption lines, lower stellar information content, different types of detectors, and usable calibration lamps. However, observations in the NIR are important for the search and study of exoplanets around cool low-mass stars that are faint at optical wavelengths. Not only are M dwarfs brightest in the NIR, the signal of stellar activity is also reduced at longer wavelengths. In this paper we introduce the RV pipeline viper (Velocity and IP EstimatoR). The philosophy of viper is to offer a publicly available and user-friendly code that is able to process data from various spectrographs. Originally designed to handle data from optical instruments, the code now has been extended to enable the processing of NIR data. viper uses a least-square fitting to model the stellar RV as well as the temporal and spatial variable IP. We have improved upon this method by adding a term for the telluric spectrum that enables the forward modelling of molecules present in the Earth's atmosphere. In this paper we use CRIRES+ observations in the K band to demonstrate viper's ability to handle data in the NIR. We show that it is possible to achieve an RV accuracy of 3 m/s over a period of 2.5 years with the use of a gas cell. Additionally, we present a study of the stability of atmospheric lines in the NIR. With viper it is possible to handle data taken with or without a gas cell, and we show that a long-term RV precision of around 10 m/s can be achieved when using only telluric lines for the wavelength calibration.

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arXiv2025

Abundance analysis of benchmark M dwarfs

Abundances of M dwarfs, being the most numerous stellar type in the Galaxy, can enhance our understanding of planet formation processes. They can also be used to study the chemical evolution of the Galaxy, where in particular alpha-capture elements play an important role. We aim to obtain abundances for Fe, Ti, and Ca for a small sample of well-known M dwarfs for which interferometric measurements are available. These stars and their abundances are intended to serve as a benchmark for future large-scale spectroscopic studies. We analysed spectra obtained with the GIANO-B spectrograph. Turbospectrum and the wrapper TSFitPy were used with MARCS atmospheric models in order to fit synthetic spectra to the observed spectra. We performed a differential abundance analysis in which we also analysed a solar spectrum with the same method and then subtracted the derived abundances line-by-line. The median was taken as the final abundance for each element and each star. Our abundances of Fe, Ti, and Ca agree mostly within uncertainties when comparing with other values from the literature. However, there are few studies to compare with.

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arXiv2025

Retrieving day- and nightside atmospheric properties of the ultra-hot Jupiter TOI-2109b. Detection of Fe and CO emission lines and evidence for inefficient heat transport

The ultra-hot Jupiter (UHJ) TOI-2109b marks the lower edge of the equilibrium temperature gap between 3500 K and 4500 K, an unexplored thermal regime that separates KELT-9b, the hottest planet yet discovered, from all other currently known gas giants. To study the structure of TOI-2109b's atmosphere, we obtained high-resolution emission spectra of both the planetary day- and nightsides with CARMENES and CRIRES$^+$. By applying the cross-correlation technique, we identified the emission signatures of Fe I and CO, as well as a thermal inversion layer in the dayside atmosphere; no significant H$_2$O signal was detected from the dayside. None of the analyzed species were detectable from the nightside atmosphere. We applied a Bayesian retrieval framework that combines high-resolution spectroscopy with photometric measurements to constrain the dayside atmospheric parameters and derive upper limits for the nightside hemisphere. The dayside thermal inversion extends from 3200 K to 4600 K, with an atmospheric metallicity consistent with that of the host star (0.36 dex). Only weak constraints could be placed on the C/O ratio ($>$ 0.15). The retrieved spectral line broadening is consistent with tidally locked rotation, indicating the absence of strong dynamical processes. An upper temperature limit of 2400 K and a maximum atmospheric temperature gradient of 700 K/log bar could be derived for the nightside. Comparison of the retrieved dayside T-p profile with theoretical models, the absence of strong atmospheric dynamics, and significant differences in the thermal constraints between the day- and nightside hemispheres suggest a limited heat transport efficiency across the planetary atmosphere. Overall, our results place TOI-2109b in a transitional regime between the UHJs below the thermal gap, which show both CO and H$_2$O emission lines, and KELT-9b, where molecular features are largely absent.

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arXiv2025

Comparative study of small-scale magnetic fields on $ξ$ Boo A using optical and near-infrared spectroscopy

Magnetic field investigations of Sun-like stars, using Zeeman splitting of non-polarised spectra, in the optical and H-band have found significantly different magnetic field strengths for the same stars, the cause of which is currently unknown. We aim to further investigate this issue by systematically analysing the magnetic field of $ξ$ Boo A, a magnetically active G7 dwarf, using spectral lines at different wavelengths. We used polarised radiative transfer accounting for the departures from local thermodynamic equilibrium to generate synthetic spectra. To find the magnetic field strengths in the optical, H-band, and K-band, we employed MCMC sampling analysis of high-resolution spectra observed with the spectrographs CRIRES$^+$, ESPaDOnS, NARVAL, and UVES. We also determine the formation depth of different lines by calculating the contribution functions for each line employed in the analysis. We find that the magnetic field strength discrepancy between lines in the optical and H-band persists even when treating the different wavelength regions consistently. In addition, the magnetic measurements derived from the K-band appear to more closely align with the optical. The H-band appears to yield magnetic field strengths $\sim$ 0.4 kG with a statistically significant variation while the optical and K-band is stable at $\sim$ 0.6 kG for observations spanning about two decades. The contribution functions reveal that the optical lines form at a significantly higher altitude in the photosphere compared to those in the H- and K-band. While we find that the discrepancy remains, the variation of formation depths could indicate that the disagreement between magnetic field measurements obtained at different wavelengths is linked to the variation of the magnetic field along the line of sight and between different structures, such as star spots and faculae, in the stellar photosphere.

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arXiv2024

Retrieving wind properties from the ultra-hot dayside of WASP-189b with CRIRES$^+$

The extreme temperature gradients from day- to nightside in the atmospheres of hot Jupiters generate fast winds in the form of equatorial jets or day-to-night flows. Observations of blue-shifted and red-shifted signals in the transmission and dayside spectra of WASP-189b have sparked discussions about the nature of winds on this planet. To investigate the structure of winds in the atmosphere of the ultra-hot Jupiter WASP-189b, we studied its dayside emission spectrum with CRIRES$^+$ in the spectral K band. We used the cross-correlation method to detect emission signals of CO and Fe, and employed a Bayesian framework to retrieve the atmospheric parameters relating to the temperature-pressure structure and chemistry. The retrieval incorporated a numerical model of the line profile influenced by various dynamic effects to determine the wind structure. The cross-correlation signals of CO and Fe showed a velocity offset of ~6km/s, which could be caused by a fast day-to-night wind in the atmosphere of WASP-189b. The atmospheric retrieval showed that the line profile of the observed spectra is best fitted by the presence of a day-to-night wind of 4.4km/s, while the retrieved equatorial jet velocity of 1.0km/s is consistent with the absence of such a jet. Such a wind pattern is consistent with the observed line broadening and can explain the majority of the velocity offset, while uncertainties in the ephemerides and the effects of a hot spot could also contribute to this offset. We further retrieved an inverted temperature-pressure profile and determined the C/O ratio and metallicity. We showed that red-shifts of a few km/s in the dayside spectra could be explained by day-to-night winds. Further studies combining transmission and dayside observations could advance our understanding of WASP-189b's atmospheric circulation by improving the uncertainties in the velocity offset and wind parameters.

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arXiv2024

CRIRES$^+$ transmission spectroscopy of WASP-127b. Detection of the resolved signatures of a supersonic equatorial jet and cool poles in a hot planet

General circulation models of gas giant exoplanets predict equatorial jets that drive inhomogeneities in the atmospheric physical parameters across the planetary surface. We studied the transmission spectrum of the hot Jupiter WASP-127\,b during one transit in the K band with CRIRES$^+$. Telluric and stellar signals were removed from the data using SYSREM. The planetary signal was investigated using the cross-correlation technique. After detecting a spectral signal indicative of atmospheric inhomogeneities, we employed a Bayesian retrieval framework with a 2D modelling approach tailored to address this scenario. We detected strong signals of H$_2$O and CO, which exhibited not one but two distinct cross-correlation peaks. The double-peaked signal can be explained by a supersonic equatorial jet and muted signals at the planetary poles, with the two peaks representing the signals from the planet's morning and evening terminators. We calculated a jet velocity of $7.7\pm0.2$ km~s$^{-1}$ and derive distinct atmospheric properties for the two terminators as well as the polar region. Our retrieval yields a solar C/O ratio and metallicity and challenges previous studies of WASP-127b's atmosphere. It provides tentative evidence for the morning terminator to be cooler than the evening terminator by $-175^{+133}_{-117}$K and shows that the muted signals from the poles can be explained by either significantly lower temperatures or a high cloud deck. The presence of a clear double-peaked signal highlights the importance of taking planetary 3D structure into account during interpretation of atmospheric signals. The supersonic jet velocity and lack of signal from the polar regions, representing a detection of latitudinal inhomogeneity in a spatially unresolved target, showcases the power of high-resolution transmission spectroscopy for the characterisation of global circulation in exoplanet atmospheres.

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arXiv2024

End-to-End simulation framework for astronomical spectrographs: SOXS, CUBES and ANDES

We present our numerical simulation approach for the End-to-End (E2E) model applied to various astronomical spectrographs, such as SOXS (ESO-NTT), CUBES (ESO-VLT), and ANDES (ESO-ELT), covering multiple wavelength regions. The E2E model aim at simulating the expected astronomical observations starting from the radiation of the scientific sources (or calibration sources) up to the raw-frame data produced by the detectors. The comprehensive description includes E2E architecture, computational models, and tools for rendering the simulated frames. Collaboration with Data Reduction Software (DRS) teams is discussed, along with efforts to meet instrument requirements. The contribution to the cross-correlation algorithm for the Active Flexure Compensation (AFC) system of CUBES is detailed.

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arXiv2024

Exploring the ultra-hot Jupiter WASP-178b. Constraints on atmospheric chemistry and dynamics from a joint retrieval of VLT/CRIRES$^+$ and space photometric data

Despite recent progress in the spectroscopic characterization of individual exoplanets, the atmospheres of key ultra-hot Jupiters (UHJs) still lack comprehensive investigations. These include WASP-178b, one of the most irradiated UHJs known to date. We observed the dayside emission signal of this planet with CRIRES$^+$ in the spectral K-band. By applying the cross-correlation technique and a Bayesian retrieval framework to the high-resolution spectra, we identified the emission signature of $^{12}$CO (S/N = 8.9) and H$_2$O (S/N = 4.9), and a strong atmospheric thermal inversion. A joint retrieval with space-based secondary eclipse measurements from TESS and CHEOPS allows us to refine our results on the thermal profile and thus to constrain the atmospheric chemistry, yielding a solar to super-solar metallicity (1.4$\pm$1.6 dex) and a solar C/O ratio (0.6$\pm$0.2). We infer a significant excess of spectral line broadening and identify a slight Doppler-shift between the $^{12}$CO and H$_2$O signals. These findings provide strong evidence for a super-rotating atmospheric flow pattern and suggest the possible existence of chemical inhomogeneities across the planetary dayside hemisphere. In addition, the inclusion of photometric data in our retrieval allows us to account for stellar light reflected by the planetary atmosphere, resulting in an upper limit on the geometric albedo (0.23). The successful characterization of WASP-178b's atmosphere through a joint analysis of CRIRES$^+$, TESS, and CHEOPS observations highlights the potential of combined studies with space- and ground-based instruments and represents a promising avenue for advancing our understanding of exoplanet atmospheres.

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arXiv2024

Are the surface abundance structures stable in rapidly rotating Ap star 56 Ari?

The surface magnetic and abundance inhomogeneities in chemically peculiar Ap/Bp stars are coupled and responsible for their rotationally modulated variability. Within the framework of fossil field hypothesis these inhomogeneities are considered to be essentially stable over the Main Sequence (MS) timescale. However, a small group of Ap/Bp stars show rotational period changes, which are currently not well understood. We present results of Doppler Imaging (DI) of rapidly rotating Ap star 56 Ari for which changes in period were previously detected. Reconstruction of the surface distribution of silicon in 56 Ari reveals its complex spot pattern, which is responsible for the rotationally light variability and correlated with magnetic field modulation. Comparison of abundance maps obtained over the unprecedentedly long for such studies interval from 1986 to 2014 confirms stability and rigid rotation of the spot pattern. Thus, the period change in 56 Ari is not caused by rearrangement of the surface magnetic structures and/or atomic diffusion operating on short time scale. It is also unlikely to be explained by the visibility changes of the spots due to free-body precession of stellar rotational axis. In the end of the paper we briefly discuss possible alternative explanations of period variability.

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arXiv2024

The Mean Longitudinal Magnetic Field and its Uses in Radial-Velocity Surveys

This work focuses on the analysis of the mean longitudinal magnetic field as a stellar activity tracer in the context of small exoplanet detection and characterisation in radial-velocity (RV) surveys. We use SDO/HMI filtergrams to derive Sun-as-a-star magnetic field measurements, and show that the mean longitudinal magnetic field is an excellent rotational period detector and a useful tracer of the solar magnetic cycle. To put these results into context, we compare the mean longitudinal magnetic field to three common activity proxies derived from HARPS-N Sun-as-a-star data: the full-width at half-maximum, the bisector span and the S-index. The mean longitudinal magnetic field does not correlate with the RVs and therefore cannot be used as a one-to-one proxy. However, with high cadence and a long baseline, the mean longitudinal magnetic field outperforms all other considered proxies as a solar rotational period detector, and can be used to inform our understanding of the physical processes happening on the surface of the Sun. We also test the mean longitudinal magnetic field as a "stellar proxy" on a reduced solar dataset to simulate stellar-like observational sampling. With a Gaussian Process regression analysis, we confirm that the solar mean longitudinal magnetic field is the most effective of the considered indicators, and is the most efficient rotational period indicator over different levels of stellar activity. This work highlights the need for polarimetric time series observations of stars.

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arXiv2024

A conclusive non-detection of magnetic field in the Am star o Peg with high-precision near-infrared spectroscopy

The A-type metallic-line (Am) stars are typically considered to be non-magnetic or possessing very weak sub-G magnetic fields. This view has been repeatedly challenged in the literature, most commonly for the bright hot Am star o Peg. Several studies claimed to detect 1-2 kG field of unknown topology in this object, possibly indicating a new process of magnetic field generation in intermediate-mass stars. In this study, we revisit the evidence of a strong magnetic field in o Peg using new high-resolution spectropolarimetric observations and advanced spectral fitting techniques. The mean magnetic field strength in o Peg is estimated from the high-precision CRIRES+ measurement of near-infrared sulphur lines. This observation is modelled with a polarised radiative transfer code, including treatment of the departures from local thermodynamic equilibrium. In addition, the least-squares deconvolution multi-line technique is employed to derive longitudinal field measurements from archival optical spectropolarimetric observations of this star. Our analysis of the near-infrared S I lines reveals no evidence of Zeeman broadening, ruling out magnetic field with a strength exceeding 260 G. This null result is compatible with the relative intensification of Fe II lines in the optical spectrum taking into account blending and uncertain atomic parameters of the relevant diagnostic transitions. Longitudinal field measurements at three different nights also yield null results with a precision of 2 G. This study refutes the claims of kG-strength dipolar or tangled magnetic field in o Peg. This star is effectively non-magnetic, with the surface magnetic field characteristics no different from those of other Am stars.

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arXiv2023

Doppler imaging of a southern ApSi star HD 152564

We present the results of the spectroscopic study of a chemically peculiar star HD 152564. Using medium-resolution ($R=37000$) observations obtained with the HRS spectrograph mounted on the South African Large Telescope we determined atmospheric parameters T$_{eff}$=11950$\pm200$ K and log g=3.6$\pm0.2$ dex. Abundance analysis revealed mild deficiency of the light elements and an overabundance of up to $\sim$2 dex of metals with greatest excess for the silicon. With these characteristics HD 152564 is a typical member of the silicon subgroup of Ap stars. Rotational modulation of the light curve and line profiles of HD 152564 are typical for inhomogeneous surface distribution of elements in its atmosphere. We performed multi-element Doppler imaging of the HD 152564 surface. Abundance maps constructed for He, O, Mg, Si, and Fe revealed concentration of these elements in a sequence of equatorial spots as well as in the circumpolar rings. The photometric maximum of the light curve coincided with the visibility of two most overabundant silicon spots. Abundances determined from the different ionisation stages of Fe and Si show clear evidence for vertical stratification of these elements in HD 152564 atmosphere. Meanwhile, the horizontal distribution of silicon reconstructed from the lines of different ionisation stages and excitation energies appeared to be identical with increasing average abundance deeper in atmosphere.

astro-ph.SR↗

arXiv2023

Retrieval of the dayside atmosphere of WASP-43b with CRIRES+

Accurately estimating the C/O ratio of hot Jupiter atmospheres is a promising pathway towards understanding planet formation and migration, as well as the formation of clouds and the overall atmospheric composition. The atmosphere of the hot Jupiter WASP-43b has been extensively analysed using low-resolution observations with HST and Spitzer, but these previous observations did not cover the K band, which hosts prominent spectral features of major carbon-bearing species such as CO and CH$_{4}$. As a result, the ability to establish precise constraints on the C/O ratio was limited. Moreover, the planet has not been studied at high spectral resolution, which can provide insights into the atmospheric dynamics. In this study, we present the first high-resolution dayside spectra of WASP-43b with the new CRIRES$^+$ spectrograph. By observing the planet in the K band, we successfully detected the presence of CO and provide evidence for the existence of H$_2$O using the cross-correlation method. This discovery represents the first direct detection of CO in the atmosphere of WASP-43b. Furthermore, we retrieved the temperature-pressure profile, abundances of CO and H$_2$O, and a super-solar C/O ratio of 0.78 by applying a Bayesian retrieval framework to the data. Our findings also shed light on the atmospheric characteristics of WASP-43b. We found no evidence for a cloud deck on the dayside, and recovered a line broadening indicative of an equatorial super-rotation corresponding to a jet with a wind speed of $\sim$ 5 km s$^{-1}$, matching the results of previous forward models and low-resolution atmospheric retrievals for this planet.

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arXiv2023

Determination of small-scale magnetic fields on Sun-like stars in the near-infrared using CRIRES$^+$

We aim to characterise the small-scale magnetic fields for a sample of 16 Sun-like stars and investigate the capabilities of the newly upgraded near-infrared (NIR) instrument CRIRES$^+$ at the VLT in the context of small-scale magnetic field studies. Our targets also had their magnetic fields studied in the optical, which allows us to compare magnetic field properties at different spatial scales on the stellar surface and to contrast small-scale magnetic field measurements at different wavelengths. We analyse the Zeeman broadening signature for six magnetically sensitive and insensitive \ion{Fe}{I} lines in the H-band to measure small-scale magnetic fields on the stellar surface. We use polarised radiative transfer modelling and NLTE departure coefficients in combination with MCMC to determine magnetic field characteristics together with non-magnetic stellar parameters. We use two different approaches to describe small-scale magnetic fields. The first is a two-component model with a single magnetic region and a free magnetic field strength. The second model contains multiple magnetic components with fixed magnetic field strengths. We find average magnetic field strengths ranging from $\sim 0.4$ kG down to $<0.1$ kG. The results align closely with other results from high resolution NIR spectrographs such as SPIRou. We find that the small-scale fields correlate with the large-scale fields and that the small-scale fields are at least 10 times stronger than the large-scale fields inferred with Zeeman Doppler imaging. The two- and multi-component models produce systematically different results as the strong fields from the multi-component model increase the obtained mean magnetic field strength. When comparing our results with the optical measurements of small-scale fields we find a systematic offset of 2--3 times stronger fields in the optical.

astro-ph.SR↗

arXiv2023

CRIRES+ detection of CO emissions lines and temperature inversions on the dayside of WASP-18b and WASP-76b

The dayside atmospheres of ultra-hot Jupiters (UHJs) are predicted to possess temperature inversion layers with extremely high temperatures at high altitudes. We observed the dayside thermal emission spectra of WASP-18b and WASP-76b with the new CRIRES+ high-resolution spectrograph at near-infrared wavelengths. Using the cross-correlation technique, we detected strong CO emission lines in both planets, which confirms the existence of temperature inversions on their dayside hemispheres. The two planets are the first UHJs orbiting F-type stars with CO emission lines detected; previous detections were mostly for UHJs orbiting A-type stars. Evidence of weak H2O emission signals is also found for both planets. We further applied forward-model retrievals on the detected CO lines and retrieved the temperature-pressure profiles along with the CO volume mixing ratios. The retrieved logarithmic CO mixing ratio of WASP-18b (-2.2) is slightly higher than the value predicted by the self-consistent model assuming solar abundance. For WASP-76b, the retrieved CO mixing ratio (-3.6) is broadly consistent with the value of solar abundance. In addition, we included the equatorial rotation velocity (Veq ) in the retrieval when analyzing the line profile broadening. The obtained Veq is 7.0 km/s for WASP-18b and 5.2 km/s for WASP-76b, which are consistent with the tidally locked rotational velocities.

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