Examinando por Autor "Van Holstein, R. G."

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    Characterizing the morphology of the debris disk around the low-mass star GSC 07396-00759
    (European Southern Observatory (ESO), 2021) Adam, C.; Olofsson, Johan; Van Holstein, R. G.; Bayo, Amelia; Milli, J.; Boccaletti, A.; Kral, Q.; Ginski, C.; Henning, Th.; Montesinos, M.; Pawellek, N.; Zurlo, A.; Langlois, M.; Delboulbé, A.; Pavlov, A.; Ramos, J.; Weber, L.; Wildi, F.; Rigal, F.; Sauvage, J.-F.
    Context. Debris disks have commonly been studied around intermediate-mass stars. Their intense radiation fields are believed to efficiently remove the small dust grains that are constantly replenished by collisions. For lower-mass central objects, in particular M stars, the dust removal mechanism needs to be further investigated given the much weaker radiation field produced by these objects. Aims. We present new observations of the nearly edge-on disk around the pre-main-sequence M-type star GSC 07396-00759, taken with VLT/SPHERE IRDIS in dual-beam polarimetric imaging mode, with the aim to better understand the morphology of the disk, its dust properties, and the star-disk interaction via the stellar mass-loss rate. Methods. We model the polarimetric observations to characterize the location and properties of the dust grains using the Henyey–Greenstein approximation of the polarized phase function. We use the estimated phase function to evaluate the strength of the stellar winds. Results. We find that the polarized light observations are best described by an extended and highly inclined disk (i ≈ 84.3 ° ± 0.3) with a dust distribution centered at a radius r0 ≈ 107 ± 2 au. Our modeling suggests an anisotropic scattering factor g ≈ 0.6 to best reproduce the polarized phase function S12. We also find that the phase function is reasonably well reproduced by small micron-sized dust grains with sizes s > 0.3μm. We discuss some of the caveats of the approach, mainly that our model probably does not fully recover the semimajor axis of the disk and that we cannot readily determine all dust properties due to a degeneracy between the grain size and the porosity. Conclusions. Even though the radius of the disk may be overestimated, our best-fit model not only reproduces the observations well but is also consistent with previous published data obtained in total intensity. Similarly to previous studies of debris disks, we suggest that using a given scattering theory might not be sufficient to fully explain key aspects, such as the shape of the phase function or the dust grain size. Taking into consideration the aforementioned caveats, we find that the average mass-loss rate of GSC 07396-00759 can be up to 500 times stronger than that of the Sun, supporting the idea that stellar winds from low-mass stars can evacuate small dust grains in an efficient way.
  • Miniatura
    Ítem
    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
    (European Southern Observatory (ESO), 2021) Ginski, C.; Mugrauer, M.; Adam, C.; Vogt, N.; Van Holstein, R. G.
    Aims. We are studying the influence of stellar multiplicity on exoplanet systems and, in particular, systems that have been detected via radial-velocity searches. We are specifically 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 that have been detected during our ongoing survey of exoplanet hosts with VLT/SPHERE (Spectro-Polarimetric High-Contrast Exoplanet Research). Methods. 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. Results. We detected new stellar companions to the exoplanet host stars HD 1666, HIP 68468, HIP 107773, and HD 109271. With an angular separation of only 0.38′′ (40 au of projected separation), HIP 107773 is among the closest companions found for 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 a significant influence on the evolution of the planet-forming disk around the primary star. We find that the companion in the HD 1666 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 shows, furthermore, that the near infrared faint companion around HD 109271 was detected in the optical and it is significantly brighter than in the near infrared, making it a white dwarf companion.
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    Ítem
    Revealing asymmetrical dust distribution in the inner regions of HD141569
    (European Southern Observatory (ESO), 2021) Singh, G.; Bhowmik, T.; Boccaletti, A.; Thébault, P.; Kral, Q.; Milli, J.; Mazoyer, J.; Pantin, E.; Van Holstein, R. G.; Olofsson, Johan; Boukrouche, R.; Di Folco, E.; Janson, M.; Langlois, M.; Maire, A.-L.; Vigan, A.; Benisty, M.; Augereau, J.-C.; Perrot, C.; Gratton, R.; Henning, T.; Ménard, F.; Rickman, E.; Wahhaj, Z.; Zurlo, A.; Biller, B.; Bonnefoy, M.; Chauvin, G.; Delorme, P.; Desidera, S.; D’Orazi, V.; Feldt, M.; Hagelberg, J.; Keppler, M.; Kopytova, T.; Lagadec, E.; Lagrange, A.-M.; Mesa, D.; Meyer, M.; Rouan, D.; Sissa, E.; Schmidt, T. O. B.; Jaquet, M.; Fusco, T.; Pavlov, A.; Rabou, P.
    Context. The combination of high-contrast imaging with spectroscopy and polarimetry offers a pathway to studying the grain distribution and properties of debris disks in exquisite detail. Here, we focus on the case of a gas-rich debris disk around HD 141569A, which features a multiple-ring morphology first identified with SPHERE in the near-infrared. Aims. We obtained polarimetric differential imaging with SPHERE in the H-band to compare the scattering properties of the innermost ring at 44 au with former observations in total intensity with the same instrument. In polarimetric imaging, we observed that the intensity of the ring peaks in the south-east, mostly in the forward direction, whereas in total intensity imaging, the ring is detected only at the south. This noticeable characteristic suggests a non-uniform dust density in the ring. With these two sets of images, we aim to study the distribution of the dust to solve for the actual dust distribution. Methods. We implemented a density function varying azimuthally along the ring and generated synthetic images both in polarimetry and in total intensity, which are then compared to the actual data. The search for the best-fit model was performed both with a grid-based and an MCMC approach. Using the outcome of this modelization, we further measured the polarized scattering phase function for the observed scattering angle between 33° and 147° as well as the spectral reflectance of the southern part of the ring between 0.98 and 2.1 μm. We tentatively derived the grain properties by comparing these quantities with MCFOST models and assuming Mie scattering. Results. We find that the dust density peaks in the south-west at an azimuthal angle of 220°~238° with a rather broad width of 61°~127°. The difference in the intensity distributions observed in polarimetry and total intensity is the result of this particular morphology. Although there are still uncertainties that remain in the determination of the anisotropic scattering factor, the implementation of an azimuthal density variation to fit the data proved to be robust. Upon elaborating on the origin of this dust density distribution, we conclude that it could be the result of a massive collision when we account for the effect of the high gas mass that is present in the system on the dynamics of grains. In terms of grain composition, our preliminary interpretation indicates a mixture of porous sub-micron sized astro-silicate and carbonaceous grains. Conclusions. The SPHERE observations have allowed, for the first time, for meaningful constraints to be placed on the dust distribution beyond the standard picture of a uniform ring-like debris disk. However, future studies with a multiwavelength approach and additional detailed modeling would be required to better characterize the grain properties in the HD 141569 system.