ORCID Profile
0000-0001-7841-3452
Current Organisation
Christian-Albrechts-Universität zu Kiel
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Publisher: EDP Sciences
Date: 2012
Publisher: EDP Sciences
Date: 12-2011
Publisher: EDP Sciences
Date: 07-2021
DOI: 10.1051/0004-6361/202140626
Abstract: Context. The nature of circumstellar envelopes (CSEs) around Cepheids is a matter of ongoing debate. The physical origin of their infrared (IR) excess could be shown to either be made up of a shell of ionized gas, a dust envelope, or a combination of both. Aims. This study is aimed at constraining the geometry and the IR excess of the environment of the bright long-period Cepheid ℓ Car ( P = 35.5 days) at mid-IR wavelengths in order to understand its physical nature. Methods. We first used photometric observations in various bands (from the visible domain to the infrared) and Spitzer Space Telescope spectroscopy to constrain the IR excess of ℓ Car. Then we analyzed the VLTI/MATISSE measurements at a specific phase of observation in order to determine the flux contribution as well as the size and shape of the environment of the star in the L band. Finally, we tested the hypothesis of a shell of ionized gas in order to model the IR excess. Results. We report the first detection in the L band of a centro-symmetric extended emission around ℓ Car, of about 1.7 R ⋆ in full width at half maximum, producing an excess of about 7.0% in this band.This latter value is used to calibrate the IR excess found when comparing the photometric observations in various bands and quasi-static atmosphere models. In the N band, there is no clear evidence for dust emission from VLTI/MATISSE correlated flux and Spitzer data. On the other side, the modeled shell of ionized gas implies a more compact CSE (1.13 ± 0.02 R ⋆ ) that is also fainter (IR excess of 1% in the L band). Conclusions. We provide new evidence supporting a compact CSE for ℓ Car and we demonstrate the capabilities of VLTI/MATISSE for determining common properties of CSEs. While the compact CSE of ℓ Car is likely to be of a gaseous nature, the tested model of a shell of ionized gas is not able to simultaneously reproduce the IR excess and the interferometric observations. Further Galactic Cepheid observations with VLTI/MATISSE are necessary for determining the properties of CSEs, which may also depend on both the pulsation period and the evolutionary state of the stars.
Publisher: American Astronomical Society
Date: 06-2022
Abstract: We present H -band (1.65 μ m) and SOFIA HAWC+ 154 μ m polarization observations of the low-mass core L483. Our H -band observations reveal a magnetic field that is overwhelmingly in the E–W direction, which is approximately parallel to the bipolar outflow that is observed in scattered IR light and in single-dish 12 CO observations. From our 154 μ m data, we infer a ∼45° twist in the magnetic field within the inner 5″ (1000 au) of L483. We compare these new observations with published single-dish 350 μ m polarimetry and find that the 10,000 au scale H -band data match the smaller-scale 350 μ m data, indicating that the collapse of L483 is magnetically regulated on these larger scales. We also present high-resolution 1.3 mm Atacama Large Millimeter/submillimeter Array data of L483 that reveals it is a close binary star with a separation of 34 au. The plane of the binary of L483 is observed to be approximately parallel to the twisted field in the inner 1000 au. Comparing this result to the ∼1000 au protostellar envelope, we find that the envelope is roughly perpendicular to the 1000 au HAWC+ field. Using the data presented, we speculate that L483 initially formed as a wide binary and the companion star migrated to its current position, causing an extreme shift in angular momentum thereby producing the twisted magnetic field morphology observed. More observations are needed to further test this scenario.
Publisher: SPIE
Date: 09-07-2018
DOI: 10.1117/12.2312683
Publisher: EDP Sciences
Date: 30-08-2013
Publisher: Springer Berlin Heidelberg
Date: 2007
Publisher: EDP Sciences
Date: 19-06-2013
Publisher: EDP Sciences
Date: 08-2021
DOI: 10.1051/0004-6361/202141175
Abstract: Context. Carbon is one of the most abundant components in the Universe. While silicates have been the main focus of solid phase studies in protoplanetary discs (PPDs), little is known about the solid carbon content especially in the planet-forming regions (~0.1–10 au). Fortunately, several refractory carbonaceous species present C-H bonds (such as hydrogenated nano-diamond and amorphous carbon as well as polycyclic aromatic hydrocarbons), which generate infrared (IR) features that can be used to trace the solid carbon reservoirs. The new mid-IR instrument MATISSE, installed at the Very Large Telescope Interferometer (VLTI), can spatially resolve the inner regions (~1–10 au) of PPDs and locate, down to the au-scale, the emission coming from carbon grains. Aims. Our aim is to provide a consistent view on the radial structure, down to the au-scale, as well as basic physical properties and the nature of the material responsible for the IR continuum emission in the inner disk region around HD 179218. Methods. We implemented a temperature-gradient model to interpret the disk IR continuum emission, based on a multiwavelength dataset comprising a broadband spectral energy distribution and VLTI H -, L -, and N -bands interferometric data obtained in low spectral resolution. Then, we added a ring-like component, representing the carbonaceous L -band features-emitting region, to assess its detectability in future higher spectral resolution observations employing mid-IR interferometry. Results. Our temperature-gradient model can consistently reproduce our dataset. We confirmed a spatially extended inner 10 au emission in H - and L -bands, with a homogeneously high temperature (~1700 K), which we associate with the presence of stochastically heated nano-grains. On the other hand, the N -band emitting region presents a ring-like geometry that starts at about 10 au with a temperature of 400 K. Moreover, the existing low resolution MATISSE data exclude the presence of aromatic carbon grains (i.e., producing the 3.3 μm feature) in close proximity tothe star (≲1 au). Future medium spectral resolution MATISSE data will confirm their presence at larger distances. Conclusions. Our best-fit model demonstrates the presence of two separated dust populations: nano-grains that dominate the near- to mid-IR emission in the inner 10 au region and larger grains that dominate the emission outward. The presence of such nano-grains in the highly irradiated inner 10 au region of HD 179218 requires a replenishment process. Considering the expected lifetime of carbon nano-grains from The Heterogeneous dust Evolution Model for Interstellar Solids (THEMIS model), the estimated disk accretion inflow of HD 179218 could significantly contribute to feed the inner 10 au region in nano-grains.Moreover, we also expect a local regeneration of those nano-grains by the photo-fragmentation of larger aggregates.
Publisher: Wiley
Date: 06-2012
Abstract: The far‐infrared excesses produced by debris disks are common features of stellar systems. These disks are thought to contain solids ranging from micron‐sized dust to planetesimals. Naturally, their formation and evolution are linked to those of potential planets. With this motivation, the Herschel open time key programme DUNES (DUst around NEarby Stars) aims at further characterising known debris disks and discovering new ones in the regime explored by the Herschel space observatory. On the one hand, in their survey of 133 nearby FGK stars, DUNES discovered a class of extremely cold and faint debris disks, different from well‐known disks such as the one around Vega in that their inferred typical grain sizes are rather large, indicating low dynamical excitation and low collision rates. On the other hand, for the more massive disk around the sun‐like star HD 207129, well‐resolved PACS images confirmed the ring‐liked structure seen in HST images and provided valuable information for an in‐depth study and benchmark for models. Employing both models for power‐law fitting and collisional evolution we found the disk around HD 207129 to feature low collision rates and large grains, as well. Transport by means of Poynting‐Robertson drag likely plays a role in replenishing the dust seen closer to the star, inside of the ring. The inner edge is therefore rather smooth and the contribution from the extended halo of barely bound grains is small. Both slowly self‐stirring and planetary perturbations could potentially have formed and shaped this disk (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Publisher: EDP Sciences
Date: 16-08-2006
Publisher: EDP Sciences
Date: 03-2014
Publisher: EDP Sciences
Date: 05-2012
Publisher: SPIE
Date: 04-08-2016
DOI: 10.1117/12.2231067
Publisher: EDP Sciences
Date: 14-04-2011
Publisher: EDP Sciences
Date: 02-2022
DOI: 10.1051/0004-6361/202142514
Abstract: Context. VX Sgr is a cool, evolved, and luminous red star whose stellar parameters are difficult to determine, which affects its classification. Aims. We aim to spatially resolve the photospheric extent as well as the circumstellar environment. Methods. We used interferometric observations obtained with the MATISSE instrument in the L (3–4 μm), M (4.5–5 μm), and N (8–13 μm) bands. We reconstructed monochromatic images using the MIRA software. We used 3D radiation-hydrodynamics simulations carried out with CO 5 BOLD and a uniform disc model to estimate the apparent diameter and interpret the stellar surface structures. Moreover, we employed the radiative transfer codes OPTIM3D and RADMC3D to compute the spectral energy distribution for the L , M , and N bands, respectively. Results. MATISSE observations unveil, for the first time, the morphology of VX Sgr across the L , M , and N bands. The reconstructed images show a complex morphology with brighter areas whose characteristics depend on the wavelength probed. We measured the angular diameter as a function of the wavelength and showed that the photospheric extent in the L and M bands depends on the opacity through the atmosphere. In addition to this, we also concluded that the observed photospheric inhomogeneities can be interpreted as convection-related surface structures. The comparison in the N band yielded a qualitative agreement between the N -band spectrum and simple dust radiative transfer simulations. However, it is not possible to firmly conclude on the interpretation of the current data because of the difficulty in constraing the model parameters using the limited accuracy of our absolute flux calibration. Conclusions. MATISSE observations and the derived reconstructed images unveil the appearance of VX Sgr’s stellar surface and circumstellar environment across a very large spectral domain for the first time.
Publisher: Springer Science and Business Media LLC
Date: 16-02-2022
DOI: 10.1038/S41586-021-04311-7
Abstract: In the widely accepted 'unified model'
Publisher: EDP Sciences
Date: 03-2022
DOI: 10.1051/0004-6361/202141785
Abstract: Context. Optical interferometry is at a key development stage. The Very Large Telescope Interferometer (VLTI) has established a stable, robust infrastructure for long-baseline interferometry that is usable by general astronomical observers. The present second-generation instruments offer a wide wavelength coverage and improved performance. Their sensitivity and measurement accuracy lead to data and images of high reliability. Aims. We have developed the Multi AperTure mid-Infrared SpectroScopic Experiment (MATISSE) to access, for the first time, high resolution imaging in a wide spectral domain. Many front-line topics are explored with this new equipment, including: stellar activity and mass loss planet formation and evolution in the gas and dust disks around young stars and environment interaction and accretion processes around super massive black holes in active galactic nuclei. Methods. The instrument is a spectro-interferometric imager in the transmission windows called L , M , and N , from 2.8 to 13.0 microns, combining four optical beams from the VLTI’s unit or auxiliary telescopes. Its concept, related observing procedure, data reduction, and calibration approach, is the product of 30 years of instrumental research and has benefitted from the expertise developed in the frame of the VLTI’s first generation instruments. The instrument utilises a multi-axial beam combination that delivers spectrally dispersed fringes. The signal provides the following quantities at several spectral resolutions: photometric flux, coherent fluxes, visibilities, closure phases, wavelength differential visibilities and phases, and aperture-synthesis imaging. Results. This article provides an overview of the physical principle of the instrument and its functionalities. The motivation of the choice of the instrumental concept and the characteristics of the delivered signal are detailed with a description of the observing modes and of their performance limit. MATISSE offers four spectral resolutions in L & M bands, namely 30, 500, 1000 and 3400, and 30 and 220 in the N band, and it provides an angular resolution down to 3 mas for the shortest wavelengths. The MATISSE stand-alone sensitivity limits are 60 mJy in L and 300 mJy in N . The paper gives details of the sensitivity limits for the different measurables and their related precision criteria, considering telescope configurations and spectral resolutions. We also discuss the gain provided with the GRA4MAT fringe tracker. An ensemble of data and reconstructed images illustrate the first acquired key observations. Conclusions. The instrument has been in operation at Cerro Paranal, ESO, Chile, since 2018, and has been open for science use by the international community since April 2019. The first scientific results are being published now.
Publisher: EDP Sciences
Date: 10-2014
Publisher: EDP Sciences
Date: 03-2021
DOI: 10.1051/0004-6361/202039400
Abstract: Context. A complex environment exists in the inner few astronomical units of planet-forming disks. High-angular-resolution observations play a key role in our understanding of the disk structure and the dynamical processes at work. Aims. In this study we aim to characterize the mid-infrared brightness distribution of the inner disk of the young intermediate-mass star HD 163296 from early VLTI/MATISSE observations taken in the L - and N -bands. We put special emphasis on the detection of potential disk asymmetries. Methods. We use simple geometric models to fit the interferometric visibilities and closure phases. Our models include a smoothed ring, a flat disk with an inner cavity, and a 2D Gaussian. The models can account for disk inclination and for azimuthal asymmetries as well. We also perform numerical hydrodynamical simulations of the inner edge of the disk. Results. Our modeling reveals a significant brightness asymmetry in the L -band disk emission. The brightness maximum of the asymmetry is located at the NW part of the disk image, nearly at the position angle of the semimajor axis. The surface brightness ratio in the azimuthal variation is 3.5 ± 0.2. Comparing our result on the location of the asymmetry with other interferometric measurements, we confirm that the morphology of the r 0.3 au disk region is time-variable. We propose that this asymmetric structure, located in or near the inner rim of the dusty disk, orbits the star. To find the physical origin of the asymmetry, we tested a hypothesis where a vortex is created by Rossby wave instability, and we find that a unique large-scale vortex may be compatible with our data. The half-light radius of the L -band-emitting region is 0.33 ±0.01 au, the inclination is 52° −7° +5° , and the position angle is 143° ± 3°. Our models predict that a non-negligible fraction of the L -band disk emission originates inside the dust sublimation radius for μ m-sized grains. Refractory grains or large (≳10 μ m-sized) grains could be the origin of this emission. N -band observations may also support a lack of small silicate grains in the innermost disk ( r ≲ 0.6 au), in agreement with our findings from L -band data.
Publisher: EDP Sciences
Date: 03-2014
Publisher: Oxford University Press (OUP)
Date: 25-11-2014
Publisher: American Astronomical Society
Date: 02-07-2013
Publisher: EDP Sciences
Date: 2014
Publisher: EDP Sciences
Date: 2010
DOI: 10.1051/EAS/1044012
Publisher: AIP
Date: 2005
DOI: 10.1063/1.2146244
Location: United States of America
No related grants have been discovered for Sebastian Wolf.