arXiv subjects

O. Kochukhov

Publications and source records attributed to O. Kochukhov.

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

Testing the reliability of magnetic field strength measurements for M dwarfs

M dwarfs host strong magnetic fields that can be measured using several complementary techniques. However, the impact of key methodological choices on Zeeman broadening diagnostics has not been systematically quantified. We assess the reliability of different approaches for inferring magnetic fields in M dwarfs using synthetic Stokes $I$ spectra of Ti I lines generated from three-dimensional magnetohydrodynamic simulations combined with MARCS model atmospheres. Synthetic observations were produced for different surface magnetic field strengths, projected rotational velocities, and inclination angles. Zeeman broadening and intensification were analysed using polarised radiative transfer calculations coupled with Markov chain Monte Carlo inference. We evaluated several statistical criteria (BIC, AIC, and WAIC) for selecting the number of magnetic filling factors and compared alternative strategies for treating line strengths. In most cases, BIC, AIC, and WAIC favoured the same number of components. For some active and rapidly rotating models, BIC and AIC selected fewer components and yielded lower field estimates, while WAIC generally produced closer agreement with the input field strengths. Treating individual line strengths as free parameters underestimated the field strength by 30--50%, whereas fitting a joint element abundance together with continuum scaling recovered the input field more reliably. Our results show that Zeeman broadening diagnostics can robustly recover magnetic fields in M dwarfs, but their accuracy depends strongly on methodological choices.

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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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arXiv2026

Magnetic field measurements in a sample of Class I and flat-spectrum protostars observed with SPIRou

Magnetic fields play a crucial role throughout stellar evolution, regulating angular momentum, channelling accretion, and launching jets and outflows. While the magnetic properties of Classical T Tauri Stars (CTTS) are well characterised, those of their progenitors, Class I and Flat-Spectrum (FS) protostars, remain poorly constrained due to observational challenges linked to their embedded nature. We aim to detect and characterise large-scale magnetic fields in a sample of Class I and FS protostars, which are expected to host strong dynamo-generated fields. Using SPIRou, a high-resolution near-infrared spectropolarimeter, we analysed polarised spectra and applied the Least Squares Deconvolution (LSD) technique to extract magnetic signatures and measure longitudinal fields from Stokes V profiles. We report new detections of large-scale magnetic fields in 5 FS protostars. Including the previously known magnetic FS protostar V347 Aur, 40% of our sample (15 objects) is confirmed to be magnetic. These stars exhibit clear Zeeman signatures, with longitudinal field strengths ranging from ~80 to ~200 G. The remaining targets show no detectable Stokes V signature, with upper limits on dipolar fields between 500 G and >5 kG. These results indicate that Class I and FS protostars can host large-scale magnetic fields, possibly weaker than in CTTS, supporting the idea that magnetic processes are already active during the main accretion phase and may influence star-disk interactions from the earliest stages.

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arXiv2025

Magnetic braking and dynamo evolution of $β$ Hydri

The evolution of magnetic braking and dynamo processes in subgiant stars is essential for understanding how these stars lose angular momentum. We investigate the magnetic braking and dynamo evolution of $β$ Hydri, a G-type subgiant, to test the hypothesis of weakened magnetic braking and the potential rejuvenation of large-scale magnetic fields. We analyze spectropolarimetric observations from HARPS (HARPSpol; polarimetric mode of High Accuracy Radial velocity Planet Searcher), and combine them with archival X-ray data and asteroseismic properties from TESS (Transiting Exoplanet Survey Satellite) to estimate the current wind braking torque of $β$ Hydri. Despite experiencing weakened magnetic braking during the second half of its main-sequence lifetime, our results indicate that $β$ Hydri has regained significant magnetic activity and a large-scale magnetic field. This observation aligns with the "born-again" dynamo hypothesis. Furthermore, our estimated wind braking torque is considerably stronger than what would be expected for a star in the weakened magnetic braking regime. This suggests that subgiants with extended convective zones can temporarily re-establish large-scale dynamo action. These results provide critical constraints on stellar rotation models and improve our understanding of the interplay between magnetic field structure, stellar activity cycles, and angular momentum evolution in old solar-type stars.

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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

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

HD 34736: An intensely magnetised double-lined spectroscopic binary with rapidly-rotating chemically peculiar B-type components

We report the results of a comprehensive study of the spectroscopic binary (SB2) system HD 34736 hosting two chemically peculiar (CP) late B-type stars. Using new and archival observational data, we characterise the system and its components, including their rotation and magnetic fields. Fitting of the radial velocities yields $P_\mathrm{orb}=83.\!^\mathrm{d}219(3)$ and $e=0.8103(3)$. The primary component is a CP He-wk star with $T_{\mathrm{eff}A}=13000\pm500$ K and $\upsilon_\mathrm{e}\sin i\;=75\pm3$ km/s, while the secondary exhibits variability of Mg and Si lines, and has $T_{\mathrm{eff}B}=11500\pm1000$ K and $\upsilon_\mathrm{e}\sin i=110$-180 km/s. TESS and KELT photometry reveal clear variability of the primary component with a rotational period $P_{\mathrm{rot}A}=1.\!^\mathrm{d}279\,988\,5(11)$, which is lengthening at a rate of $1.26(6)$ s/yr. For the secondary, $P_{\mathrm{rot}B}=0.\!^\mathrm{d}522\,693\,8(5)$, reducing at a rate of $-0.14(3)$ s/yr. The longitudinal component $\langle B_\mathrm{z}\rangle$ of the primary's strongly asymmetric global magnetic field varies from $-6$ to +5 kG. Weak spectropolarimetric evidence of a magnetic field is found for the secondary star. The observed X-ray and radio emission of HD 34736 may equally be linked to a suspected T Tau-like companion or magnetospheric emission from the principal components. Given the presence of a possible third magnetically active body, one can propose that the magnetic characteristics of the protostellar environment may be connected to the formation of such systems.

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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

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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arXiv2024

Statistical relations between spectropolarimetric observables and the polar strength of the stellar dipolar magnetic field

Global magnetic fields of early-type stars are commonly characterised by the mean longitudinal magnetic field $\langle B_{\rm z} \rangle$ and the mean field modulus $\langle B \rangle$, derived from the circular polarisation and intensity spectra, respectively. Observational studies often report a root mean square (rms) of $\langle B_{\rm z} \rangle$ and an average value of $\langle B \rangle$. In this work, I used numerical simulations to establish statistical relationships between these cumulative magnetic observables and the polar strength, $B_{\rm d}$, of a dipolar magnetic field. I show that in the limit of many measurements randomly distributed in rotational phase, $\langle B_{\rm z} \rangle_{\rm rms}$=$0.179^{+0.031}_{-0.043}$$B_{\rm d}$ and $\langle B \rangle_{\rm avg}$=$0.691^{+0.020}_{-0.023}$$B_{\rm d}$. The same values can be recovered with only three measurements, provided that the observations are distributed uniformly in the rotational phase. These conversion factors are suitable for ensemble analyses of large stellar samples, where each target is covered by a small number of magnetic measurements.

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arXiv2024

Multi-scale magnetic field investigation of the M-dwarf eclipsing binary CU Cancri

We aim to characterise the magnetic field of the eclipsing binary CU Cnc. The determination of magnetic field parameters of this target enables comparisons with both observations of similar stars and theoretical predictions of the magnetic field strength for CU Cnc. The target is therefore providing an excellent opportunity to test our understanding of the generation of magnetic fields in low-mass stars and its impact on stellar structure. We use spectropolarimetric observations obtained with ESPaDOnS to investigate the magnetic properties of CU Cnc. We generate average line profiles with LSD, which are used to extract information about the radial velocities of the components, expanding the number of radial velocity measurements available and allowing for a determination of orbital parameters. Stokes V LSD profiles are used with ZDI to obtain large-scale magnetic field structures on both components. We also use polarised radiative transfer modelling to investigate the small-scale fields by utilising Zeeman splitting of magnetically sensitive Ti I lines in non-polarised spectra. The large-scale fields are dominantly poloidal and have an average strength of ~100 G on both components. This analysis of the large-scale fields likely suffers from some amount of hemisphere degeneracy due to the high inclination of the target. Both components also show unusual magnetic field configurations compared to stars with similar parameters, the primary is weakly axisymmetric (~10%) and the secondary has a strong torroidal contribution (~20%). The small-scale fields are significantly stronger, at 3.1 and 3.6 kG for the primary and secondary respectively. This measurement is in excellent agreement with surface field strength predictions for CU Cnc from magnetoconvective stellar evolution models. These results indicates that magnetic fields play a significant role in radius inflation of active stars.

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arXiv2023

Surface distribution of small-scale magnetic field on the active cool star LQ Hydrae

It is well known that small-scale magnetism dominates the surface magnetic field topologies of active late-type stars. However, little information is available on the spatial distribution of this key magnetic field component. Here, we take advantage of the recently developed magnetic field diagnostic procedure relying on the magnetic intensification of iron atomic lines in the optical. We extend this methodology from measuring a single average field strength value to simultaneous Doppler imaging reconstruction of the two-dimensional maps of temperature and magnetic field strength. We applied this novel surface mapping approach to two spectroscopic data sets of the young active Sun-like star LQ Hya. For both epochs, we found a fairly uniform field strength distribution, apart from a latitudinal trend of the field strength increasing from 1.5-2.0 kG at low latitudes to 3.0-3.5 kG, close to the rotational poles. This distribution of the small-scale field does not display a clear correlation with the locations of temperature spots or the global magnetic field structure reconstructed for the same epochs.

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arXiv2023

TESS Cycle 2 observations of roAp stars with 2-min cadence data

We present the results of a systematic search of the Transiting Exoplanet Survey Satellite (TESS) 2-min cadence data for new rapidly oscillating Ap (roAp) stars observed during the Cycle 2 phase of its mission. We find seven new roAp stars previously unreported as such and present the analysis of a further 25 roAp stars that are already known. Three of the new stars show multiperiodic pulsations, while all new members are rotationally variable stars, leading to almost 70 per cent (22) of the roAp stars presented being $α^2$ CVn-type variable stars. We show that targeted observations of known chemically peculiar stars are likely to overlook many new roAp stars, and demonstrate that multi-epoch observations are necessary to see pulsational behaviour changes. We find a lack of roAp stars close to the blue edge of the theoretical roAp instability strip, and reaffirm that mode instability is observed more frequently with precise, space-based observations. In addition to the Cycle 2 observations, we analyse TESS data for all known roAp stars. This amounts to 18 further roAp stars observed by TESS. Finally, we list six known roAp stars that TESS is yet to observe. We deduce that the incidence of roAp stars amongst the Ap star population is just 5.5 per cent, raising fundamental questions about the conditions required to excite pulsations in Ap stars. This work, coupled with our previous work on roAp stars in Cycle 1 observations, presents the most comprehensive, homogeneous study of the roAp stars in the TESS nominal mission, with a collection of 112 confirmed roAp stars in total.

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