arXiv subjects

Christian Adam

Publications and source records attributed to Christian Adam.

5 recordsLinked to original sources

arXiv2026

Diffuse and specular brightness models applied to LEO satellites. Case study: The ONEWEB constellation

Context. To better understand the observed brightness of low Earth orbit satellites, we must characterize their reflectivity, which in turn depends importantly on their bus designs. The reflectivity of a body can be described by Lambert's law, in terms of its albedo, cross-sectional area, range (distance), phase angles, and the mixing coefficient between diffuse and specular reflection components. Aims. We aim to analyze the reflectivity of more than 300 ONEWEB satellites using the diffuse Lambertian sphere, diffuse and specular Lambertian sphere, and the relative reflectance brightness models. Methods. Astrometric and photometric measurements, plus two-line elements (TLE) orbital information were used to compute the apparent and range-magnitude, as well as the relevant angles related to the orientation of the Sun, the satellites, and the observer. A differential evolution Monte Carlo algorithm was used to obtain each model's parameters that best fit the data. Results. All models can fit the mean observed brightness of the satellites but cannot describe the observed phase-angle-dependent brightness modulations. The residuals in all cases have a standard deviation of $\sim$0.6 magnitudes, while the observational photometric errors are on average $\sim$0.2 magnitudes. Conclusions. The studied brightness models, which depend on the relative Sun-body-observer position but are independent of the specific orientation of the reflecting body surface(s) with respect to the observer, cannot entirely explain the observed brightness of the ONEWEB constellation satellites. Accounting for the real shape and the changing attitude of the satellite, as well as the effect of Earth's albedo is needed to better explain satellite photometric observations

astro-ph.IM↗

arXiv2022

ALMA's view of the M-dwarf GSC 07396-00759's edge-on debris disc: AU Mic's coeval twin

We present new ALMA Band 7 observations of the edge-on debris disc around the M1V star GSC 07396-00759. At ~20 Myr old and in the beta Pictoris Moving Group along with AU Mic, GSC 07396-00759 joins it in the handful of low mass M-dwarf discs to be resolved in the sub-mm. With previous VLT/SPHERE scattered light observations we present a multi-wavelength view of the dust distribution within the system under the effects of stellar wind forces. We find the mm dust grains to be well described by a Gaussian torus at 70 au with a FWHM of 48 au and we do not detect the presence of CO in the system. Our ALMA model radius is significantly smaller than the radius derived from polarimetric scattered light observations, implying complex behaviour in the scattering phase function. The brightness asymmetry in the disc observed in scattered light is not recovered in the ALMA observations, implying that the physical mechanism only affects smaller grain sizes. High resolution follow-up observations of the system would allow investigation into its unique dust features as well as provide a true coeval comparison for its smaller sibling AU Mic, singularly well observed amongst M-dwarfs systems.

astro-ph.EP↗

arXiv2021

Discovery of a directly imaged planet to the young solar analog YSES 2

Abbreviated. By selecting stars with similar ages and masses, the Young Suns Exoplanet Survey (YSES) aims to detect and characterize planetary-mass companions to solar-type host stars in the Scorpius-Centaurus association. Our survey is carried out with VLT/SPHERE with short exposure sequences on the order of 5 min per star per filter. The subtraction of the stellar point spread function (PSF) is based on reference star differential imaging (RDI) using the other targets in the survey in combination with principal component analysis. We report the discovery of YSES 2b, a planetary-mass companion to the K1 star YSES 2 (TYC 8984-2245-1). The primary has a Gaia EDR3 distance of 110 pc, and we derive a revised mass of $1.1\,M_\odot$ and an age of approximately 14 Myr. We detect the companion in two observing epochs southwest of the star at a position angle of 205$^\circ$ and with a separation of $\sim1.05''$, which translates to a minimum physical separation of 115 au at the distance of the system. We derive a photometric planet mass of $6.3^{+1.6}_{-0.9}\,M_\mathrm{Jup}$ using AMES-COND and AMES-dusty evolutionary models; this mass corresponds to a mass ratio of $q=(0.5\pm0.1)$% with the primary. This is the lowest mass ratio of a direct imaging planet around a solar-type star to date. We discuss potential formation mechanisms and find that the current position of the planet is compatible with formation by disk gravitational instability, but its mass is lower than expected from numerical simulations. Formation via core accretion must have occurred closer to the star, yet we do not find evidence that supports the required outward migration, such as via scattering off another undiscovered companion in the system. YSES 2b is an ideal target for follow-up observations to further the understanding of the physical and chemical formation mechanisms of wide-orbit Jovian planets.

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arXiv2020

How many suns are in the sky? A SPHERE multiplicity survey of exoplanet host stars I -- Four new close stellar companions including a white dwarf

We are studying the influence of stellar multiplicity on exoplanet systems, in particular systems that have been detected via radial-velocity searches. We are in particular interested in the closest companions as they would have a strong influence on the evolution of the original planet forming disks. In this study we present new companions detected during our ongoing survey of exoplanet hosts with VLT/SPHERE. We are using the extreme adaptive optics imager SPHERE at the ESO/VLT to search for faint (sub)stellar companions. We utilized the classical coronagraphic imaging mode to perform a snapshot survey (3-6 min integration time) of exoplanet host stars in the Ks-band. We detected new stellar companions to the exoplanet host stars HD1666, HIP68468, HIP107773, and HD109271. With an angular separation of only 0.38" (40 au of projected separation) HIP107773 is among the closest companions found to exoplanet host stars. The presence of the stellar companion explains the linear radial-velocity trend seen in the system. At such a small separation the companion likely had significant influence on the evolution of the planet forming disk around the primary star. We find that the companion in the HD1666 system may well be responsible for the high orbit eccentricity (0.63) of the detected Jupiter class planet, making this system one of only a few where such a connection can be established. A cross-match with the Gaia DR2 catalog showed furthermore that the near infrared faint companion around HD109271 had been detected in the optical and is significantly brighter than in the near infrared making it a white dwarf companion.

astro-ph.EP↗

arXiv2019

Exploring the innermost dust formation region of the oxygen-rich AGB star IK~Tau with VLT/SPHERE-ZIMPOL and VLTI/AMBER

Low- and intermediate-mass stars at the asymptotic giant branch (AGB) are known to be prevalent dust providers to galaxies. However, the mechanisms responsible for the formation and acceleration of dust in the cool extended atmospheres of AGB stars are still open to debate. We present visible polarimetric imaging observations of the oxygen-rich AGB star IK Tau obtained with SPHERE-ZIMPOL (phase 0.27) as well as interferometric observations with AMBER. IK Tau was observed with ZIMPOL at three wavelengths in the pseudo-continuum (645, 748, and 820 nm), in the H$\alpha$ line at 656.3 nm, and in the TiO band at 717 nm. The AMBER observations were carried out in the wavelength region of the CO first overtone lines near 2.3 $\mu$m (R=12 000). The polarimetric imaging capabilities of SPHERE-ZIMPOL have allowed us to spatially resolve clumpy dust clouds at 20--50 mas from the central star, which corresponds to 2--5 $R_{\star}$ when combined with a central star's angular diameter of 20.7$\pm$1.53 mas measured with AMBER. The diffuse, asymmetric dust emission extends out to $\sim$73 $R_{\star}$. We find that the TiO emission extends to 150 mas (15 $R_{\star}$). The AMBER data in the CO lines also suggest a molecular outer atmosphere extending to $\sim$1.5 $R_{\star}$. The results of our 2-D Monte Carlo radiative transfer modelling of dust clumps suggest that the polarized intensity and degree of linear polarization can be reasonably explained by small-sized (0.1 $\mu$m) grains of Al$_{2}$O$_{3}$, MgSiO$_{3}$, or Mg$_{2}$SiO$_{4}$ in an optically thin shell ($\tau_{550\mathrm{nm}}$=0.5$\pm$0.1) with an inner boundary radius of 3.5 $R_{\star}$. IK Tau's mass-loss rate is 20 to 50 times higher than the previously studied AGB stars W Hya, R Dor, and $o$ Cet. Nevertheless, our observations of IK Tau revealed that clumpy dust formation occurs close to the star as seen in those low mass-rate AGB stars.

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