arXiv subjects

S. Schäfer

Publications and source records attributed to S. Schäfer.

At least 19 recordsLinked to original sources

arXiv2026

MOSES -- The MONET Star and Exoplanet Spectrograph

We introduce MOSES, the new High-Resolution Echelle Spectrograph designated for the 1.2m MONET telescope at McDonald Observatory, Texas, USA. The science drivers are radial velocity experiments and activity monitoring in Sun-like stars. Set for installation in the final quarter of 2026, MOSES features a white pupil design and aims for a spectral resolution greater than 80,000 over the 380-680 nm wavelength range. It incorporates a pixel sampling rate of 3.5 and uses two fibers to facilitate a simultaneous calibration mode. Encased within a vacuum vessel and operating in a temperature-stabilized environment, MOSES is expected to achieve a radial velocity precision below 2 m/s, aided by a Fabry-Pérot etalon calibration system. This paper outlines the implementation of the fiber injection unit, the optical layout of the spectrograph, and the present status of the various subsystems under development.

astro-ph.IM↗

arXiv2026

The MICADO first light imager for the ELT: eliminating vibrations and excessive earthquake-loads on specific subsystems

Ensuring earthquake resilience is paramount for the upcoming Extremely Large Telescope (ELT). Instruments positioned on the ELT's Nasmyth platform are anticipated to endure substantial seismic accelerations, peaking at 3.6g in specific scenarios. These instruments have to be designed not only to survive such events but also to only require minimal repairs for sustained optimal functionality. This paper presents our approach to mitigate the dynamic forces impacting the rotary platform. This platform hosts various critical components, including cryogenic control units and mosaic detector control systems, essential for the operation of MICADO in its mechanical de-rotating motion. Furthermore, this paper tackles the challenge of minimizing instrument-induced vibrations affecting the telescope's structure. Given that the ELT's optical system, composed of 798 segments, operates very similar to an interferometer, maintaining an extremely stable wavefront is crucial - a key insight learned from the Very Large Telescope Interferometer (VLTI). Our focus is on identifying potential sources of vibration within specific frequency ranges where the telescope demonstrates increased sensitivity. We will explore strategies for mitigating these vibrations to maintain a wavefront error below 50nm.

astro-ph.IM↗

arXiv2026

Observing the integrated and spatially resolved Sun with ultra-high spectral resolution

The Institute for Astrophysics Göttingen operates a solar observatory that combines a 50\,cm siderostat with (1) a vacuum vertical telescope, (2) a very high resolution Fourier Transform Spectrograph ($R > 900,000$ at 600\,nm), and (3) a Laser Frequency Comb for extremely precise and accurate frequency calibration ($<10\,cm/s$). We introduce our setup that feeds the spectrograph with either a 32.5" field of view of the solar surface, or with disk-integrated sunlight for Sun-as-a-star observations and explain the necessary computational steps to guide specific positions on the solar surface into the fiber. Our instrument suite can deliver spectroscopic measurements with extremely accurate frequency calibration, which is valid across very large frequency regions (approx. 400-800\,nm in wavelength). This allows precision spectroscopy of individual lines in order to study the variability of spectral lines in Sun-as-a-star observations as well as determining the convective blueshift across the solar surface from many spectral lines.

astro-ph.IM↗

arXiv2026

Fiber-coupling of Fourier Transform Spectrographs

Fourier Transform Spectrographs (FTS) are versatile tools for measuring accurate, high resolution spectra. They are internally calibrated by a reference laser that runs in parallel to the science light. Therefore it is crucial to properly align these two beams with respect to each other. We show how this can be achieved by feeding a part of the reference light into the optical path of the science beam. For astronomical applications it's often useful to use optical fibers. We present a coupling setup for our Bruker Optics IFS 125 FTS, consisting of (1) two hexagonal input fibers, (2) dichroic beam-combining for measuring two light sources simultaneously and (3) optimized optics to match the original Bruker design. The hexagonal shape of the fiber cores secures sufficient mode scrambling inside the fibers, resulting in constant beam parameters and a more homogeneous illumination of the entrance aperture of the FTS.

astro-ph.IM↗

arXiv2023

Convective characteristics of Fe I lines across the solar disc

Solar convection is visible as a net blueshift of absorption lines, which becomes apparent when observing quiet Sun granulation. This blueshift exhibits variations from the disc centre to the solar limb due to differing projection angles onto the solar atmosphere. Our goal is to investigate convective Doppler velocities based on observations from the disc centre to the solar limb. Consequently, we aim to improve our understanding of atmospheric hydrodynamics and contribute to the improvement of solar and stellar atmospheric models. We used resolved quiet-Sun spectra to investigate the convective velocity shifts of more than 1000\,\ion{Fe}{I} lines across multiple centre-to-limb positions on the solar disc. We determined the Doppler velocities with respect to the line depth. Additionally, we calculated the formation temperature and investigated its correlation with Doppler velocities. The general behaviour of convective line shifts shows a decreasing blueshift as the lines become deeper for all observing positions from the centre to limb. For spectra obtained at the solar limb, even deeper lines exhibit redshifts. We observe a velocity trend for the different observation angles, with a less pronounced convective blueshift towards the solar limb. Convective velocities show a wavelength dependence for each observing angle when analysing on the basis of line depths. We observe a decreasing convective blueshift as the formation temperatures of the lines decrease. The velocity change over temperature ranges proceeds slower towards the solar limb. When investigating Doppler velocities with respect to formation temperature, the disc centre does not exhibit the strongest blueshift.

astro-ph.SR↗

arXiv2023

The IAG spectral atlas of the spatially resolved Sun: Centre-to-limb observations

Solar surface magneto-convection appears as granulation pattern that impacts spectral lines in terms of both shape and wavelength. Such induced effects also tend to vary over the observed solar disc because of the changing observation angle and, thus, the changing observation height as well. Centre-to-limb observations of the resolved Sun offer an insight into the variable spectral behaviour across different heliocentric observing positions, providing crucial information about limb darkening, convective velocities, and line profile variability relevant to radial velocity (RV) calculations. Thus, RV measurements and exoplanet transit spectroscopy depend on precise reference templates. We want to provide a spectroscopic centre-to-limb solar atlas at high spectral resolution and high-frequency accuracy. The atlas shall help improve the understanding of the solar atmosphere and convection processes. We performed high-resolution observations of the resolved quiet Sun with a Fourier transform spectrograph at the Institut für Astrophysik und Geophysik in Göttingen. Our dataset contains a wavelength range from 4200Å to 8000Å. We obtained 165 spectra in total, with a spectral resolution of $Δν$ = 0.024cm$^{-1}$, corresponding to a resolving power $R$ of 700,000 at $\sim$6000Å. We present a centre-to-limb solar atlas containing 14 heliocentric positions. To check for consistency, we investigated the FeI ~6175Å absorption line and compared our line profiles with previous centre-to-limb observations and also with simulations. The line profile and also the bisector profiles are generally consistent with previous observations, but we have identified differences to model line profiles, especially close to the solar limb.

astro-ph.SR↗

arXiv2023

The CARMENES search for exoplanets around M dwarfs. Guaranteed time observations Data Release 1 (2016-2020)

The CARMENES instrument was conceived to deliver high-accuracy radial velocity (RV) measurements with long-term stability to search for temperate rocky planets around a sample of nearby cool stars. The broad wavelength coverage was designed to provide a range of stellar activity indicators to assess the nature of potential RV signals and to provide valuable spectral information to help characterise the stellar targets. The CARMENES Data Release 1 (DR1) makes public all observations obtained during the CARMENES guaranteed time observations, which ran from 2016 to 2020 and collected 19,633 spectra for a sample of 362 targets. The CARMENES survey target selection was aimed at minimising biases, and about 70% of all known M dwarfs within 10 pc and accessible from Calar Alto were included. The data were pipeline-processed, and high-level data products, including 18,642 precise RVs for 345 targets, were derived. Time series data of spectroscopic activity indicators were also obtained. We discuss the characteristics of the CARMENES data, the statistical properties of the stellar sample, and the spectroscopic measurements. We show examples of the use of CARMENES data and provide a contextual view of the exoplanet population revealed by the survey, including 33 new planets, 17 re-analysed planets, and 26 confirmed planets from transiting candidate follow-up. A subsample of 238 targets was used to derive updated planet occurrence rates, yielding an overall average of 1.44+/-0.20 planets with 1 M_Earth < M sin i < 1000 M_Earth and 1 d < P_orb < 1000 d per star, and indicating that nearly every M dwarf hosts at least one planet. CARMENES data have proven very useful for identifying and measuring planetary companions as well as for additional applications, such as the determination of stellar properties, the characterisation of stellar activity, and the study of exoplanet atmospheres.

astro-ph.EP↗

arXiv2020

Orbital angular momentum superposition states in transmission electron microscopy and bichromatic multiphoton ionization

The coherent control of electron beams and ultrafast electron wave packets dynamics have attracted significant attention in electron microscopy as well as in atomic physics. In order to unify the conceptual pictures developed in both fields, we demonstrate the generation and manipulation of tailored electron orbital angular momentum (OAM) superposition states either by employing customized holographic diffraction masks in a transmission electron microscope or by atomic multiphoton ionization utilizing pulse-shaper generated carrier-envelope phase stable bichromatic ultrashort laser pulses. Both techniques follow similar physical mechanisms based on Fourier synthesis of quantum mechanical superposition states allowing the preparation of a broad set of electron states with uncommon symmetries. We describe both approaches in a unified picture based on an advanced spatial and spectral double slit and point out important analogies. In addition, we analyze the topological charge and discuss the control mechanisms of the free-electron OAM superposition states. Their generation and manipulation by phase tailoring in transmission electron microscopy and atomic multiphoton ionization is illustrated on a 7-fold rotationally symmetric electron density distribution.

quant-ph↗

arXiv2020

Phase-ordering of charge density waves traced by ultrafast low-energy electron diffraction

We introduce ultrafast low-energy electron diffraction (ULEED) in backscattering for the study of structural dynamics at surfaces. Using a tip-based source of ultrashort electron pulses, we investigate the optically-driven transition between charge-density wave phases at the surface of 1T-TaS2. The large transfer width of the instrument allows us to employ spot-profile analysis, resolving the phase-ordering kinetics in the nascent incommensurate charge-density wave phase. We observe a coarsening that follows a power-law scaling of the correlation length, driven by the annihilation of dislocation-type topological defects of the charge-ordered lattice. Our work opens up the study of a wide class of structural transitions and ordering phenomena at surfaces and in low-dimensional systems.

cond-mat.mes-hall↗

arXiv2020

The CARMENES search for exoplanets around M dwarfs. Measuring precise radial velocities in the near infrared: the example of the super-Earth CD Cet b

The high-resolution, dual channel, visible and near-infrared spectrograph CARMENES offers exciting opportunities for stellar and exoplanetary research on M dwarfs. In this work we address the challenge of reaching the highest radial velocity precision possible with a complex, actively cooled, cryogenic instrument, such as the near-infrared channel. We describe the performance of the instrument and the work flow used to derive precise Doppler measurements from the spectra. The capability of both CARMENES channels to detect small exoplanets is demonstrated with the example of the nearby M5.0 V star CD Cet (GJ 1057), around which we announce a super-Earth ($4.0\pm0.4\,M_\oplus$) companion on a 2.29 d orbit.

astro-ph.EP↗

arXiv2016

MiNDSTEp differential photometry of the gravitationally lensed quasars WFI2033-4723 and HE0047-1756: Microlensing and a new time delay

We present V and R photometry of the gravitationally lensed quasars WFI2033-4723 and HE0047-1756. The data were taken by the MiNDSTEp collaboration with the 1.54 m Danish telescope at the ESO La Silla observatory from 2008 to 2012. Differential photometry has been carried out using the image subtraction method as implemented in the HOTPAnTS package, additionally using GALFIT for quasar photometry. The quasar WFI2033-4723 showed brightness variations of order 0.5 mag in V and R during the campaign. The two lensed components of quasar HE0047-1756 varied by 0.2-0.3 mag within five years. We provide, for the first time, an estimate of the time delay of component B with respect to A of $Δt= 7.6\pm1.8$ days for this object. We also find evidence for a secular evolution of the magnitude difference between components A and B in both filters, which we explain as due to a long-duration microlensing event. Finally we find that both quasars WFI2033-4723 and HE0047-1756 become bluer when brighter, which is consistent with previous studies.

astro-ph.GA↗

arXiv2015

Transits and starspots in the WASP-6 planetary system

We present updates to \textsc{prism}, a photometric transit-starspot model, and \textsc{gemc}, a hybrid optimisation code combining MCMC and a genetic algorithm. We then present high-precision photometry of four transits in the WASP-6 planetary system, two of which contain a starspot anomaly. All four transits were modelled using \textsc{prism} and \textsc{gemc}, and the physical properties of the system calculated. We find the mass and radius of the host star to be $0.836\pm 0.063\,{\rm M}_\odot$ and $0.864\pm0.024\,{\rm R}_\odot$, respectively. For the planet we find a mass of $0.485\pm 0.027\,{\rm M}_{\rm Jup}$, a radius of $1.230\pm0.035\,{\rm R}_{\rm Jup}$ and a density of $0.244\pm0.014\,ρ_{\rm Jup}$. These values are consistent with those found in the literature. In the likely hypothesis that the two spot anomalies are caused by the same starspot or starspot complex, we measure the stars rotation period and velocity to be $23.80 \pm 0.15$\,d and $1.78 \pm 0.20$\,km\,s$^{-1}$, respectively, at a co-latitude of 75.8$^\circ$. We find that the sky-projected angle between the stellar spin axis and the planetary orbital axis is $λ= 7.2^{\circ} \pm 3.7^{\circ}$, indicating axial alignment. Our results are consistent with and more precise than published spectroscopic measurements of the Rossiter-McLaughlin effect. These results suggest that WASP-6\,b formed at a much greater distance from its host star and suffered orbital decay through tidal interactions with the protoplanetary disc.

astro-ph.EP↗

arXiv2013

Flux and color variations of the doubly imaged quasar UM673

With the aim of characterizing the flux and color variations of the multiple components of the gravitationally lensed quasar UM673 as a function of time, we have performed multi-epoch and multi-band photometric observations with the Danish 1.54m telescope at the La Silla Observatory. The observations were carried out in the VRi spectral bands during four seasons (2008--2011). We reduced the data using the PSF (Point Spread Function) photometric technique as well as aperture photometry. Our results show for the brightest lensed component some significant decrease in flux between the first two seasons (+0.09/+0.11/+0.05 mag) and a subsequent increase during the following ones (-0.11/-0.11/-0.10 mag) in the V/R/i spectral bands, respectively. Comparing our results with previous studies, we find smaller color variations between these seasons as compared with previous ones. We also separate the contribution of the lensing galaxy from that of the fainter and close lensed component.

astro-ph.CO↗

arXiv2012

Mode conversion by symmetry breaking of propagating spin waves

We study spin-wave transport in a microstructured Ni81Fe19 waveguide exhibiting broken translational symmetry. We observe the conversion of a beam profile composed of symmetric spin-wave width modes with odd numbers of antinodes n=1,3,... into a mixed set of symmetric and asymmetric modes. Due to the spatial homogeneity of the exciting field along the used microstrip antenna, quantized spin-wave modes with an even number n of antinodes across the stripe's width cannot be directly excited. We show that a break in translational symmetry may result in a partial conversion of even spin-wave waveguide modes

cond-mat.mes-hall↗

arXiv2012

The Transiting System GJ1214: High-Precision Defocused Transit Observations and a Search for Evidence of Transit Timing Variation

Aims: We present 11 high-precision photometric transit observations of the transiting super-Earth planet GJ1214b. Combining these data with observations from other authors, we investigate the ephemeris for possible signs of transit timing variations (TTVs) using a Bayesian approach. Methods: The observations were obtained using telescope-defocusing techniques, and achieve a high precision with random errors in the photometry as low as 1mmag per point. To investigate the possibility of TTVs in the light curve, we calculate the overall probability of a TTV signal using Bayesian methods. Results: The observations are used to determine the photometric parameters and the physical properties of the GJ1214 system. Our results are in good agreement with published values. Individual times of mid-transit are measured with uncertainties as low as 10s, allowing us to reduce the uncertainty in the orbital period by a factor of two. Conclusions: A Bayesian analysis reveals that it is highly improbable that the observed transit times is explained by TTV, when compared with the simpler alternative of a linear ephemeris.

astro-ph.EP↗

arXiv2012

Kondo physics and orbital degeneracy interact to boost thermoelectrics on the nanoscale

We investigate the transport through a nanoscale device consisting of a degenerate double-orbital Anderson dot coupled to two uncorrelated leads. We determine the thermoelectric transport properties close to the one-electron regime and compare them to a corresponding single-orbital dot. The linear and nonlinear regimes are addressed, the latter via a non-equilibrium generalization of the non-crossing approximation based on the Keldysh formalism. Power output and efficiency in the Kondo regime are shown to be strongly enhanced through the presence of a second orbital. We predict an experimentally relevant optimal operating point which benefits from the concomitant increase of the Kondo temperature in the two-orbital setup. An approximation based on the transport coefficients and fulfilling the thermodynamic balance is proven to remain appropriate even far beyond the expected range of validity of such approaches. Finally, the double-orbital Kondo regime reveals itself as a promising candidate to avoid, at least partially, the generic dilemma between optimal thermoelectric efficiency on one hand, and fair power output on the other.

cond-mat.mes-hall↗

arXiv2012

Characterization of low temperature metallic magnetic calorimeters having gold absorbers with implanted $^{163}$Ho ions

For the first time we have investigated the behavior of fully micro-fabricated low temperature metallic magnetic calorimeters (MMCs) after undergoing an ion-implantation process. This experiment had the aim to show the possibility to perform a high precision calorimetric measurement of the energy spectrum following the electron capture of $^{163}$Ho using MMCs having the radioactive $^{163}$Ho ions implanted in the absorber. The implantation of $^{163}$Ho ions was performed at ISOLDE-CERN. The performance of a detector that underwent an ion-implantation process is compared to the one of a detector without implanted ions. The results show that the implantation dose of ions used in this experiment does not compromise the properties of the detector. In addition an optimized detector design for future $^{163}$Ho experiments is presented.

physics.ins-det↗

arXiv2011

Carrier Confinement in GaN/AlGaN Nanowire Heterostructures for 0 < x <= 1

The three dimensional carrier confinement in GaN nanodiscs embedded in GaN/AlGaN nanowires and its effect on their photoluminescence properties is analyzed for Al concentrations between x = 0.08 and 1. Structural analysis by high resolution transmission electron microscopy reveals the presence of a lateral AlGaN shell due to a composition dependent lateral growth rate of the barrier material. The structural properties are used as input parameters for three dimensional numerical simulations of the confinement which show that the presence of the AlGaN shell has to be considered to explain the observed dependence of the emission energy on the Al concentration in the barrier. The simulations reveal that the maximum in the emission energy for x ~ 30% is assigned to the smallest lateral strain gradient and consequently the lowest radial internal electric fields in the nanodiscs. Higher Al-concentrations in the barrier cause high radial electric fields that can overcome the exciton binding energy and result in substantially reduced emission intensities. Effects of polarization-induced axial internal electric fields on the photoluminescence characteristics have been investigated using nanowire samples with nanodisc heights ranging between 1.2 nm and 3.5 nm at different Al concentrations. The influence of the quantum confined Stark effect is significantly reduced compared to GaN/AlGaN quantum well structures which is attributed to the formation of misfit dislocations at the heterointerfaces which weakens the internal electric polarization fields.

cond-mat.mes-hall↗