Measuring and characterizing the line profile of HARPS with a laser frequency comb

High precision spectroscopy is one of the most successful methods to detect extra-solar planets. To enable the detection of Earth-like planets in the habitable zone, extremely precise instruments are required. Our lack of knowledge of the instrument line profile, non-linearity and charge transfer efficiency effects in the detector limits the achievable precision of an instrument. We report our studies on the HARPS (High Accuracy Radial- velocity Planet Searcher) line profiles, measured using the unresolved lines of a Laser Frequency Comb (LFC). We show how the line profile changes as a function of position and signal, and estimate the errors made in the line centroid measurement due to the variation of the line profile.

Stellar activity and planetary effects induce radial velocity (RV) offsets and cause temporal distortions in the shape of the stellar line profile. Hence, accurately probing the stellar line profile offers a wealth of information on both the star itself and any orbiting planets. Typically, cross-correlation functions (CCFs) are used as a proxy for the stellar line profile. The shape of CCFs, however, can be distorted by line blending and aliasing limiting the stellar and planetary physics that can be probed from them. Least-squares deconvolution (LSD) offers an alternative that directly fits the mean line profile of the spectrum to produce a high-precision profile. In this paper, we introduce our novel method ACID (Accurate Continuum fItting and Deconvolution) that builds on LSD techniques by simultaneously fitting the spectral continuum and line profile as well as performing LSD in effective optical depth. Tests on model data revealed ACID can accurately identify and correct the spectral continuum to retrieve an injected line profile. ACID was also applied to archival High Accuracy Radial-velocity Planet Searcher (HARPS) data obtained during the transit of HD189733b. The application of the Reloaded Rossiter–McLaughlin technique to both ACID profiles and HARPS CCFs shows ACID residual profiles improved the out-of-line root mean square (RMS) by over 5 per cent compared to CCFs. Furthermore, ACID profiles are shown to exhibit a Voigt profile shape that better describes the expected profile shape of the stellar line profile. This improved representation shows that ACID better preserves the stellar and planetary physics encoded in the stellar line profile shape for slow rotating stars.

Exoplanets of a few Earth masses can be now detected around nearby low-mass stars using Doppler spectroscopy. In this Letter, we investigate the radial velocity variations of Kapteyn's star, which is both a sub-dwarf M-star and the nearest halo object to the Sun. The observations comprise archival and new HARPS (High Accuracy Radial velocity Planet Searcher), High Resolution Echelle Spectrometer (HIRES) and Planet Finder Spectrograph (PFS) Doppler measurements. Two Doppler signals are detected at periods of 48 and 120 d using likelihood periodograms and a Bayesian analysis of the data. Using the same techniques, the activity indices and archival All Sky Automated Survey (ASAS-3) photometry show evidence for low-level activity periodicities of the order of several hundred days. However, there are no significant correlations with the radial velocity variations on the same time-scales. The inclusion of planetary Keplerian signals in the model results in levels of correlated and excess white noise that are remarkably low compared to younger G, K and M dwarfs. We conclude that Kapteyn's star is most probably orbited by two super-Earth mass planets, one of which is orbiting in its circumstellar habitable zone, becoming the oldest potentially habitable planet known to date. The presence and long-term survival of a planetary system seem a remarkable feat given the peculiar origin and kinematic history of Kapteyn's star. The detection of super-Earth mass planets around halo stars provides important insights into planet-formation processes in the early days of the Milky Way.

We present the discovery and confirmation of a transiting hot bloated super-Neptune using photometry from the Transiting Exoplanet Survey Satellite (TESS) and the Las Cumbres Observatory Global Telescope (LCOGT) and radial velocity measurements from the High Accuracy Radial velocity Planet Searcher (HARPS). The host star TOI-2498 is a V = 11.2, G-type (Teff = 5905 ± 12 K) solar-like star with a mass of 1.12 ± 0.02 M⊙ and a radius of 1.26 ± 0.04 R⊙. The planet, TOI-2498 b, orbits the star with a period of 3.7 d, has a radius of 6.1 ± 0.3 R⊕, and a mass of 35 ± 4 M⊕. This results in a density of 0.86 ± 0.25 g cm−3. TOI-2498 b resides on the edge of the Neptune desert; a region of mass–period parameter space in which there appears to be a dearth of planets. Therefore TOI-2498 b is an interesting case to study to further understand the origins and boundaries of the Neptune desert. Through modelling the evaporation history, we determine that over its ∼3.6 Gyr lifespan, TOI-2498 b has likely reduced from a Saturn-sized planet to its current radius through photoevaporation. Moreover, TOI-2498 b is a potential candidate for future atmospheric studies searching for species like water or sodium in the optical using high resolution spectroscopy, and for carbon-based molecules in the infrared using JWST.

The removal of noise typically correlated in time and wavelength is one of the main challenges for using the radial-velocity (RV) method to detect Earth analogues. We analyze τ Ceti RV data and find robust evidence for wavelength-dependent noise. We find that this noise can be modeled by a combination of moving average models and the so-called “differential radial velocities.” We apply this noise model to various RV data sets for τ Ceti, and find four periodic signals at 20.0, 49.3, 160, and 642 days, which we interpret as planets. We identify two new signals with orbital periods of 20.0 and 49.3 days while the other two previously suspected signals around 160 and 600 days are quantified to a higher precision. The 20.0 days candidate is independently detected in Keck data. All planets detected in this work have minimum masses less than with the two long-period ones located around the inner and outer edges of the habitable zone, respectively. We find that the instrumental noise gives rise to a precision limit of the High Accuracy Radial Velocity Planet Searcher (HARPS) around 0.2 m s−1. We also find correlation between the HARPS data and the central moments of the spectral line profile at around 0.5 m s−1 level, although these central moments may contain both noise and signals. The signals detected in this work have semi-amplitudes as low as 0.3 m s−1, demonstrating the ability of the RV technique to detect relatively weak signals.

\n Context. The solar spectrum is a primary reference for the study of physical processes in stars and their variation during activity cycles. High resolution spectra of the Sun are easily obtained from spatially selected regions of the solar disk, while those taken over the integrated disk are more problematic. However, a proxy can be obtained by using solar light reflected by small bodies of the solar system. \n Aims. In November 2010 an experiment with a prototype of a laser frequency comb (LFC) calibration system was performed with the HARPS spectrograph of the 3.6m ESO telescope at La Silla during which high signal-to-noise spectra of the Moon were obtained. We exploit those Echelle spectra to study a portion of the optical integrated solar spectrum and in particular to determine the solar photospheric line positions. \n Methods. The DAOSPEC program is used to measure solar line positions through Gaussian fitting in an automatic way. The solar spectra are calibrated both with an LFC and a Th-Ar.\n Results. We first apply the LFC solar spectrum to characterize the CCDs of the HARPS spectrograph. The comparison of the LFC and Th-Ar calibrated spectra reveals S-type distortions on each order along the whole spectral range with an amplitude of ±40 m s-1 . This confirms the pattern found in the first LFC experiment on a single order and extends the detection of the distortions to the whole analyzed region revealing that the precise shape varies with wavelength. A new data reduction is implemented to deal with CCD pixel inequalities to obtain a wavelength corrected solar spectrum. By using this spectrum we provide a new LFC calibrated solar atlas with 400 line positions in the range of 476–530, and 175 lines in the 534–585 nm range corresponding to the LFC bandwidth. The new LFC atlas is consistent on average with that based on FTS solar spectra, but it improves the accuracy of individual lines by a significant factor reaching a mean value of ≈10 m s-1 .\n Conclusions. The LFC–based solar line wavelengths are essentially free of major instrumental effects and provide a reference for absolute solar line positions at the date of Nov. 2010, i.e. an epoch of low solar activity. We suggest that future LFC observations could be used to trace small radial velocity changes of the whole solar photospheric spectrum in connection with the solar cycle and for direct comparison with the predicted line positions of 3D radiative hydrodynamical models of the solar photosphere. The LFC calibrated solar atlas can be also used to verify the accuracy of ground or space spectrographs by means of the solar spectrum. \n

Aims. We present a new list of thorium and argon emission lines in the visible obtained by analyzing high-resolution (R = 110 000) spectra of a ThAr hollow cathode lamp. The aim of this new line list is to allow significant improvements in the quality of wavelength calibration for medium- to high-resolution astronomical spectrographs. Methods. We use a series of ThAr lamp exposures obtained with the HARPS instrument (High Accuracy Radial-velocity Planet Searcher) to detect previously unknown lines, perform a systematic search for blended lines and correct individual wavelengths by determining the systematic offset of each line relative to the average wavelength solution. Results. We give updated wavelengths for more than 8400 lines over the spectral range 3785–6915 A. The typical internal uncertainty on the line positions is estimated to be ∼10 m s −1 (3.3 parts in 10 8 or 0.18 mA), which is a factor of 2–10 better than the widely used Los Alamos Atlas of the Thorium Spectrum (Palmer & Engleman 1983). The absolute accuracy of the global wavelength scale is the same as in the Los Alamos Atlas. Using this new line list on HARPS ThAr spectra, we are able to obtain a global wavelength calibration which is precise at the 20 cm s −1 level (6.7 parts in 10 10 or 0.0037 mA). Conclusions. Several research fields in astronomy requiring high-precision wavelength calibration in the visible (e.g. radial velocity planet searches, variability of fundamental constants) should benefit from using the new line list.

Thorium-Argon hollow cathode lamps are commonly used as wavelength calibration lamps for high-resolution astronomical spectrographs (e.g. the European Southern Observatory’s (ESO) High Accuracy Radial velocity Planet Searcher (HARPS) and Ultraviolet and Visual Echelle Spectrograph (UVES)). They have been instrumental in supporting high precision work such as the search for extra-solar planets using the radial-velocity method. However, several years ago astronomers found that the quality of commercial Th-Ar lamps had deteriorated, when new lamps showed a “forest” of lines at low intensity levels obscuring faint atomic thorium lines rendering them useless for wavelength calibration in some regions. Based on information provided by the manufacturers the presence of molecular emission from thorium oxides has been suspected as the likely cause of this problem. We have now conclusively identified the observed emission bands as being due to strong molecular bands of ThO, confirming the source of the contamination of the hollow cathode lamps. We have recorded spectra of new Th-Ar lamps showing contamination using the high-resolution echelle spectrograph at the University of Wisconsin and ESO’s new Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO) spectrograph in order to determine the positions of the ThO lines. We shall present our initial analyses of these spectra, describing the wavelength regions most affected, their dependence on the lamp current, and measurements of the ThO line positions.

New analysis of the ν5 and 2 ν9 bands of HNO 3 by infrared and millimeter wave techniques: line positions and intensities

Laser frequency combs are an ideal calibration source for precision astronomical spectrographs. We report on the demonstrated long term operation of a laser frequency comb that we designed and built as the primary calibrator for the Habitable Zone Planet Finder (HPF). The core technology of the comb is based on robust, polarization maintaining fiber coupled electro-optic modulators and broadband supercontinuum generation spanning 700-1600 nm in an efficient silicon nitride waveguide. The comb is continuously maintained on and ready to use, and since May 2018 the laser frequency comb has had a total uptime of 97%.

Context. Physical and chemical properties, such as kinetic temperature, volume density, and molecular composition of interstellar clouds are inherent in their line spectra at submillimeter wavelengths. Therefore, the spectral line profiles could be used to estimate the physical conditions of a given source. Aims. We present a new bottom-up approach, based on machine learning (ML) algorithms, to extract the physical conditions in a straightforward way from the line profiles without using radiative transfer equations. Methods. We simulated, for the typical physical conditions of dense molecular clouds and star-forming regions, the emission in spectral lines of the two isomers HCN and HNC, from J = 1–0 to J = 5–4 between 30 and 500 GHz, which are commonly observed in dense molecular clouds and star forming regions. The generated data cloud distribution has been parametrised using the line intensities and widths to enable a new way to analyse the spectral line profiles and to infer the physical conditions of the region. The line profile parameters have been charted to the HNC/HCN ratio and the excitation temperature of the molecule(s). Three ML algorithms have been trained, tested, and compared aiming to unravel the excitation conditions of HCN and HNC and their abundance ratio. Results. Machine learning results obtained with two spectral lines, one for each isomer HCN and HNC, have been compared with the local thermodynamic equilibrium (LTE) analysis for the cold source R CrA IRS 7B. The estimate of the excitation temperature and of the abundance ratio, in this case considering the two spectral lines, is in agreement with our LTE analysis. The complete optimisation procedure of the algorithms (training, testing, and prediction of the target quantities) have the potential to predict interstellar cloud properties from line profile inputs at lower computational cost than before. Conclusions. It is the first time that the spectral line profiles are mapped according to the physical conditions charting the ratio of two isomers and the excitation temperature of the molecules. In addition, a bottom-up approach starting from a set of simulated spectral data at different physical conditions is proposed to interpret line observations of interstellar regions and to estimate their physical conditions. This new approach presents the potential relevance to unravel hidden interstellar conditions with the use of ML methods.

Context. Galactic Cepheids in the vicinity of the Sun have a residual line-of-sight velocity, or γ -velocity, which shows a systematic blueshift of about 2 km s-1 compared to an axisymmetric rotation model of the Milky Way. This term is either related to the space motion of the star and, consequently, to the kinematic structure of our Galaxy, or it is the result of the dynamical structure of the Cepheids' atmosphere.Aims. We aim to show that these residual γ -velocities are an intrinsic property of Cepheids.Methods. We observed eight galactic Cepheids with the HARPS (High Accuracy Radial velocity Planetary Search project developed by the European Southern Observatory.) spectroscope, focusing specifically on 17 spectral lines. For each spectral line of each star, we computed the γ -velocity (resp. γ -asymmetry) as an average value of the interpolated radial velocity (resp. line asymmetry) curve.Results. For each Cepheid in our sample, a linear relation is found between the γ -velocities of the various spectral lines and their corresponding γ -asymmetries, showing that residual γ -velocities stem from the intrinsic properties of Cepheids. We also provide a physical reference to the stellar γ -velocity: it should be zero when the γ -asymmetry is zero. Following this definition, we provide very precise and physically calibrated estimates of the γ -velocities for all stars of our sample [ in km s-1 ] : -11.3 ± 0.3 [R TrA], -3.5 ± 0.4 [S Cru], -1.5 ± 0.2 [Y Sgr], 9.8 ± 0.1 [ β Dor] , 7.1 ± 0.1 [ ζ Gem] , 24.6 ± 0.4 [RZ Vel], 4.4 ± 0.1 [ Car] , 25.7 ± 0.2 [RS Pup]. Finally, we investigated several physical explanations for these γ -asymmetries like velocity gradients or the relative motion of the line-forming region compared to the corresponding mass elements. However, none of these hypotheses seems to be entirely satisfactory to explain the observations.Conclusions. To understand this very subtle γ -asymmetry effect, further numerical studies are needed. Cepheids' atmosphere are strongly affected by pulsational dynamics, convective flows, nonlinear physics, and complex radiative transport. Hence, all of these effects have to be incorporated simultaneously and consistently into the numerical models to reproduce the observed line profiles in detail.

Aims. We have studied the emission of CO ro-vibrational lines in the disc around the Herbig Be star HD 100546 to determine physical properties, disc asymmetries, the CO excitation mechanism, and the spatial extent of the emission, with the final goal of using the CO ro-vibrational lines as a diagnostic to understand inner disc structure in the context of planet formation. Methods. High-spectral-resolution infrared spectra of CO ro-vibrational emission at eight different position angles were taken with the CRyogenic high-resolution InfraRed Echelle Spectrograph (CRIRES) at the Very Large Telescope (VLT). From these spectra flux tables, line profiles for individual CO ro-vibrational transitions, co-added line profiles, and population diagrams were produced. We have investigated variations in the line profile shapes and line strengths as a function of slit position angle. We used the thermochemical disc modelling code ProDiMo based on the chemistry, radiation field, and temperature structure of a previously published model for HD 100546. We calculated line fluxes and profiles for the whole set of observed CO ro-vibrational transitions using a large CO model molecule that includes the lowest two electronic states, each with 7 vibrational levels and within them 60 rotational levels. Comparing observations and the model, we investigated the possibility of disc asymmetries, the excitation mechanism (UV fluorescence), the geometry, and physical conditions of the inner disc. Results. The observed CO ro-vibrational lines are largely emitted from the inner rim of the outer disc at 10–13 AU. The line shapes are similar for all v levels and line fluxes from all vibrational levels vary only within one order of magnitude. All line profile asymmetries and variations can be explained with a symmetric disc model to which a slit correction and pointing offset is applied. Because the angular size of the CO emitting region (10–13 AU) and the slit width are comparable the line profiles are very sensitive to the placing of the slit. The model reproduces the line shapes and the fluxes of the v = 1–0 lines as well as the spatial extent of the CO rovibrational emission. It does not reproduce the observed band ratios of 0.5–0.2 with higher vibrational bands. We find that lower gas volume densities at the surface of the inner rim of the outer disc can make the fluorescence pumping more efficient and reproduce the observed band ratios.

The precision of radial velocity (RV) measurements depends on the precision attained on the wavelength calibration. One of the available options is to use atmospheric lines as a natural, freely available wavelength reference. Figueira et al. measured the RV of O2 lines using High Accuracy Radial velocity Planet Searcher (HARPS) and showed that the scatter was only of ∼10 m s−1 over a time-scale of 6 yr. Using a simple but physically motivated empirical model, they demonstrated a precision of 2 m s−1, roughly twice the average photon noise contribution. In this paper, we take advantage of a unique opportunity to confirm the sensitivity of the telluric absorption lines’ RV to different atmospheric and observing conditions by means of contemporaneous in situ wind measurements. This opportunity is a result of the work done during site testing and characterization for the European Extremely Large Telescope (E-ELT). The HARPS spectrograph was used to monitor telluric standards while contemporaneous atmospheric data were collected using radiosondes. We quantitatively compare the information recovered by the two independent approaches. The RV model fitting yielded results similar to that of Figueira et al., with lower wind magnitude values and varied wind direction. The probes confirmed the average low wind magnitude and suggested that the average wind direction is a function of time as well. However, these results are affected by large uncertainty bars that probably result from a complex wind structure as a function of height. The two approaches deliver the same results in what concerns wind magnitude and agree on wind direction when fitting is done in segments of a couple of hours. Statistical tests show that the model provides a good description of the data on all time-scales, being always preferable to not fitting any atmospheric variation. The smaller the time-scale on which the fitting can be performed (down to a couple of hours), the better the description of the real physical parameters is. We then conclude that the two methods deliver compatible results, down to better than 5 m s−1 and less than twice the estimated photon noise contribution on O2 lines’ RV measurement. However, we cannot rule out that parameters α and γ (dependence on airmass and zero-point, respectively) have a dependence on time or exhibit some cross-talk with other parameters, an issue suggested by some of the results.

Variations of line profiles of eclipsing binary systems during the eclipse are discussed. The previous formula by KOPAL (1959) is extended so as to include the effect of the center-limb variation of line profile. The variations are characterized by the rotational effect and the parameters for asymmetry of a line, half width, and residual intensity at the deepest point in the line profile. The effects of tidal distortion and differential rotation upon the line profile are estimated for two eclipsing binaries outside the eclipse. The observed rotational effects are compared with the calculated ones for two eclipsing binaries R CMa and YZ Cas. The variation of residual intensity at the deepest point in the line profile of R CMa is compared with the observation by KITAMURA (1969). It is found that the observed residual intensity, R, of Fe I 4045-line does not show any peak at mid-eclipse, while the theoretical one, Id, shows a large peak at mid-eclipse. However, the curve of R may agree generally with that of the quantity h if we allow for errors in observation. The variation of R may be mostly due to the eclipse. Asymmetry of the Hγ-line profile in RZ Sct is examined in the observations by HANSEN and MCNAMARA (1959) and KITAMURA and SATO (1967). The sense of asymmetries appearing in the observed line profiles is opposite to that calculated, suggesting that the asymmetry must be affected by some additional absorption by gaseous matter which would exist in such close binary systems.