arXiv subjects

Markus Wittkowski

Publications and source records attributed to Markus Wittkowski.

25 records · Page 2Linked to original sources

arXiv2008

Asymmetric silicate dust distribution toward the silicate carbon star BM Gem

Silicate carbon stars show the 10 micron silicate emission, despite their carbon-rich photospheres. They are considered to have circumbinary or circum-companion disks, which serve as a reservoir of oxygen-rich material shed by mass loss in the past. We present N-band spectro-interferometric observations of the silicate carbon star BM Gem using MIDI at the Very Large Telescope Interferometer (VLTI). Our aim is to probe the spatial distribution of oxygen-rich dust with high spatial resolution. BM Gem was observed with VLTI/MIDI at 44--62 m baselines using the UT2-UT3 and UT3-UT4 baseline configurations. The N-band visibilities observed for BM Gem show a steep decrease from 8 to ~10 micron and a gradual increase longward of ~10 micron, reflecting the optically thin silicate emission feature emanating from sub-micron-sized amorphous silicate grains. The differential phases obtained at baselines of ~44--46 m show significant non-zero values (~ -70 degrees) in the central part of the silicate emission feature between ~9 and 11 micron, revealing a photocenter shift and the asymmetric nature of the silicate emitting region. The observed N-band visibilities and differential phases can be fairly explained by a simple geometrical model in which the unresolved star is surrounded by a ring with azimuthal brightness modulation. The best-fit model is characterized by a broad ring (~70 mas across at 10 micron) with a bright region which is offset from the unresolved star by ~20 mas at a position angle of ~280 degrees. This model can be interpreted as a system with a circum-companion disk and is consistent with the spectroscopic signatures of an accretion disk around an unseen companion recently discovered in the violet spectrum of BM Gem.

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arXiv2008

Spatially resolved dusty torus toward the red supergiant WOH G64 in the Large Magellanic Cloud

We present N-band spectro-interferometric observations of the red supergiant WOH G64 in the Large Magellanic Cloud (LMC) using MIDI at the Very Large Telescope Interferometer (VLTI). The location of WOH G64 on the H-R diagram based on the previously estimated luminosities is in serious disagreement with the current stellar evolution theory. The dust envelope around WOH G64 has been spatially resolved with a baseline of ~60 m--the first MIDI observations to resolve an individual stellar source in an extragalactic system. The observed N-band visibilities show a slight decrease from 8 to 10 micron and a gradual increase longward of 10 micron, reflecting the 10 micron silicate feature in self-absorption. The visibilities measured at four position angles differing by ~60 degrees but at approximately the same baseline length (~60 m) do not show a noticeable difference, suggesting that the object appears nearly centrosymmetric. The observed N-band visibilities and spectral energy distribution can be reproduced by an optically and geometrically thick silicate torus model viewed close to pole-on. The luminosity of the central star is derived to be 2.8 x 10^5 Lsun, which is by a factor of 2 lower than the previous estimates based on spherical models. The lower luminosity newly derived from our MIDI observations and two-dimensional modeling brings the location of WOH G64 on the H-R diagram in much better agreement with theoretical evolutionary tracks for a 25 Msun star. We also identify the H2O absorption features at 2.7 and 6 micron in the spectra obtained with the Infrared Space Observatory and the Spitzer Space Telescope. The 2.7 micron feature originates in the photosphere and/or the extended molecular layers, while the 6 micron feature is likely to be of circumstellar origin.

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arXiv2005

Observing the Seyfert 2 nucleus of NGC 1068 with the VLT Interferometer

Dusty tori have been suggested to play a crucial role in determining the physical characteristics of active galactic nuclei (AGN), but investigation of their properties has stalled for lack of high resolution mid-IR imaging. Recently, a long-awaited breakthrough in this field was achieved: NGC 1068, a nearby AGN, was the first extragalactic object to be observed with a mid-IR interferometer, thereby obtaining the needed angular resolution to study the alleged torus. In this proceeding, first the field of AGN research is briefly reviewed, with an emphasis on models of dusty tori. Second, the general properties of the key object NGC 1068 are discussed. Third, the MIDI mid-IR interferometric data set is presented together with a first attempt to interpret this data in the context of tori models. Fourth, preliminary MIDI interferometric spectra of the nucleus of the nearby starbursting galaxy Circinus are presented. Finally, we briefly discuss the prospects of ESA's Darwin mission for observing nearby and distant AGN. This mission will allow detailed mapping of tori of low luminosity AGN such as NGC 1068 up to redshifts of 1 - 2 and more luminous AGN up to redshift of 10 and beyond (abridged).

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arXiv2004

VINCI-VLTI measurements of HR 4049: the physical size of the circumbinary envelope

We present the first detection of the envelope which surrounds the post-AGB binary source HR 4049. VINCI-VLTI K-band interferometric observations of this source imply the existence of a large structure with a Gaussian angular FWHM 22.4 +/- 1.4 mas or uniform disk diameter of 34.9 +/- 1.9 mas. With the Hipparcos parallax of 1.50 +/- 0.64 mas these values correspond to a physical size of 14.9 (+11.1,-4.4) AU and 23.3 (+17.3,-7.0) AU, respectively. Our measurements, covering an azimuth range of about 60 degrees, for the sky-projected baseline, provide information on the geometry of the emitting region and show that there is only a slight variation of the measured angular values along the different directions sampled. Thus, our results are consistent with a spherical geometry of the envelope. However, we cannot completely rule out the existence of an asymmetric envelope (like the circumbinary disk envisaged by some recent models) because of the limited spatial frequency and azimuth range covered by the observations.

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arXiv2003

Joint VLTI/VLBA observations of Mira stars

We present preliminary results on a recently started project to perform coordinated observations of Mira stars using near-infrared and radio long-baseline interferometry. We concentrate on recent observations of the Mira star S Ori. Observations with the ESO Very Large Telescope Interferometer (VLTI) were performed to measure the near-infrared diameter of the stellar surface. Concurrent VLBA observations of SiO maser emission towards this Mira star were performed to probe the structure and dynamics of the circumstellar atmosphere. Our near-infrared measurements suggest a diameter change consistent with the photometric period. The SiO maser emission appears to be at a distance of $\sim$ 2 stellar radii ($Θ_{\rm UD} \sim$ 10.7 mas, $Θ_{\rm SiO} \sim$ 20 mas, both at stellar phase $\sim$ 0.7), and to show a clumpy distribution within a ring-like structure.

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arXiv2003

Round Table Summary: Stellar interferometry as a tool to investigate atmospheres and to compare observations with models

Long-baseline interferometry at optical and near-infrared wavelengths is an emerging technology which is quickly becoming a useful tool to investigate stellar atmospheres and to compare observations with models. Stellar atmosphere models have so far mainly been constrained by comparisons with stellar spectra which are integrated over the stellar disks. Interferometric observations provide spatially and spectrally resolved information and can thus provide important complementary observational information which can be compared to model predictions. Here, I summarize the different aspects on this topic which were discussed at a round table on Thursday, June 20, 2002, during IAU Symposium 210. This summary gives an overview on discussed interferometric facilities and techniques, concepts to study atmospheres by optical interferometry, and particular classes of objects. We conclude that more frequent interactions between the efforts of atmosphere modelling and interferometric observations promise to lead to increased confidence in stellar model atmospheres and to further advancement of the field in the next years.

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arXiv2001

The potential of near-infrared high-resolution studies on the field of galaxies in the young universe

So far, high resolution techniques on the one hand provide morphological information on bright nearby objects. On the other hand, telescopes with large collecting areas allow us to detect very faint and distant objects, but not to obtain a spatial resolution which is sufficient for detailed morphological studies. Currently, the construction of large optical and infrared interferometers like the Keck Interferometer, the Very Large Telescope Interferometer (VLTI) and the Large Binocular Telescope Interferometer (LBTI) is in progress. These instruments will simultaneously provide larger collecting areas and higher spatial resolutions than current instruments. Thus, they might enable for the first time near-infrared studies of galaxies in the young universe with an absolute spatial resolution as available today only for the closest galaxies. Using recent results in the field of high resolution studies of nearby galaxies, a rough idea of what might be expected to be observed is given. The concepts of the forthcoming interferometers is reviewed and technical aspects that are essential for observations of distant galactic centers are discussed. An outlook is given on which observational tasks can be addressed to these instruments and how the results will increase our knowledge in the field of the evolution of galaxies and the universe.

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