arXiv subjects

Dieter Schertl

Publications and source records attributed to Dieter Schertl.

25 records · Page 2Linked to original sources

arXiv2010

A hot compact dust disk around a massive young stellar object

Circumstellar disks are an essential ingredient of the formation of low-mass stars. It is unclear, however, whether the accretion-disk paradigm can also account for the formation of stars more massive than about 10 solar masses, in which strong radiation pressure might halt mass infall. Massive stars may form by stellar merging, although more recent theoretical investigations suggest that the radiative-pressure limit may be overcome by considering more complex, nonspherical infall geometries. Clear observational evidence, such as the detection of compact dusty disks around massive young stellar objects, is needed to identify unambiguously the formation mode of the most massive stars. Here we report near-infrared interferometric observations that spatially resolve the astronomical unit-scale distribution of hot material around a high-mass (approx. 20 solar masses) young stellar object. The image shows an elongated structure with a size of about 13 x 19 astronomical units, consistent with a disk seen at an inclination angle of 45 degree. Using geometric and detailed physical models, we found a radial temperature gradient in the disk, with a dust-free region less than 9.5 astronomical units from the star, qualitatively and quantitatively similar to the disks observed in low-mass star formation. Perpendicular to the disk plane we observed a molecular outflow and two bow shocks, indicating that a bipolar outflow emanates from the inner regions of the system.

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arXiv2010

Detection of high-velocity material from the wind-wind collision zone of Eta Carinae across the 2009.0 periastron passage

We report near-IR spectroscopic observations of the Eta Carinae massive binary system during 2008-2009 using VLT/CRIRES. We detect a strong, broad absorption wing in He I 10833 extending up to -1900 km/s across the 2009.0 spectroscopic event. Archival HST/STIS ultraviolet and optical data shows a similar high-velocity absorption (up to -2100 km/s) in the UV resonance lines of Si IV 1394, 1403 across the 2003.5 event. UV lines from low-ionization species, such as Si II 1527, 1533 and C II 1334, 1335, show absorption up to -1200 km/s, indicating that the absorption with v from -1200 to -2100 km/s originates in a region markedly faster and more ionized than the nominal wind of the primary star. Observations obtained at the OPD/LNA during the last 4 spectroscopic cycles (1989-2009) also display high-velocity absorption in He I 10833 during periastron. Based on the OPD/LNA dataset, we determine that material with v < -900 km/s is present in the phase range 0.976 < phi < 1.023 of the spectroscopic cycle, but absent in spectra taken at phi < 0.947 and phi > 1.049. Therefore, we constrain the duration of the high-velocity absorption to be 95 to 206 days (or 0.047 to 0.102 in phase). We suggest that the high-velocity absorption originates from shocked gas in the wind-wind collision zone, at distances of 15 to 45 AU in the line-of-sight to the primary star. Using 3-D hydrodynamical simulations of the wind-wind collision zone, we find that the dense high-velocity gas is in the line-of-sight to the primary star only if the binary system is oriented in the sky so that the companion is behind the primary star during periastron, corresponding to a longitude of periastron of omega ~ 240 to 270 degrees. We study a possible tilt of the orbital plane relative to the Homunculus equatorial plane and conclude that our data are broadly consistent with orbital inclinations in the range i=40 to 60 degrees.

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arXiv2009

VLTI/AMBER unveils a possible dusty pinwheel nebula in WR118

Most Wolf-Rayet stars (WR) of WC9 sub-type exhibit a dusty circumstellar envelope, but it is still a matter of debate how dust can form in their harsh environment. In a few cases, a pinwheel-like structure of the dusty envelope has been detected. Therefore, it has been suggested that dust formation in all dusty WR stars might be linked to colliding winds in a binary system. We probed the innermost region of the circumstellar dust shell of the deeply embedded WR star WR 118. We carried out spectro-interferometric observations using the AMBER instrument of ESO's Very Large Telescope Interferometer in low-spectral resolution mode (R = 35). The K-band observations were obtained with three 1.8 m telescopes spanning projected baselines between 9.2 and 40.1 m. At high spatial frequencies, the AMBER visibilities exhibit a prominent lobe, indicating that the envelope contains one or several zones with a large local intensity gradient. The strong closure phase signal clearly shows that the circumstellar envelope of WR 118 can only be described by an asymmetric intensity distribution. We show that a pinwheel nebula seen at low inclination is consistent with the AMBER data. Its size was determined to be 13.9+-1.1 mas. WR 118 possibly harbors a pinwheel nebula, which suggests a binary nature of the system. According to our best model, the period of the system would be ~60 days (for d=3 kpc), making WR 118 the shortest-period pinwheel nebula known so far.

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arXiv2008

Phase Closure Image Reconstruction for Future VLTI Instrumentation

Classically, optical and near-infrared interferometry have relied on closure phase techniques to produce images. Such techniques allow us to achieve modest dynamic ranges. In order to test the feasibility of next generation optical interferometers in the context of the VLTI-spectro-imager (VSI), we have embarked on a study of image reconstruction and analysis. Our main aim was to test the influence of the number of telescopes, observing nights and distribution of the visibility points on the quality of the reconstructed images. Our results show that observations using six Auxiliary Telescopes (ATs) during one complete night yield the best results in general and is critical in most science cases; the number of telescopes is the determining factor in the image reconstruction outcome. In terms of imaging capabilities, an optical, six telescope VLTI-type configuration and ~200 meter baseline will achieve 4 mas spatial resolution, which is comparable to ALMA and almost 50 times better than JWST will achieve at 2.2 microns. Our results show that such an instrument will be capable of imaging, with unprecedented detail, a plethora of sources, ranging from complex stellar surfaces to microlensing events.

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arXiv2008

Phase Referencing in Optical Interferometry

One of the aims of next generation optical interferometric instrumentation is to be able to make use of information contained in the visibility phase to construct high dynamic range images. Radio and optical interferometry are at the two extremes of phase corruption by the atmosphere. While in radio it is possible to obtain calibrated phases for the science objects, in the optical this is currently not possible. Instead, optical interferometry has relied on closure phase techniques to produce images. Such techniques allow only to achieve modest dynamic ranges. However, with high contrast objects, for faint targets or when structure detail is needed, phase referencing techniques as used in radio interferometry, should theoretically achieve higher dynamic ranges for the same number of telescopes. Our approach is not to provide evidence either for or against the hypothesis that phase referenced imaging gives better dynamic range than closure phase imaging. Instead we wish to explore the potential of this technique for future optical interferometry and also because image reconstruction in the optical using phase referencing techniques has only been performed with limited success. We have generated simulated, noisy, complex visibility data, analogous to the signal produced in radio interferometers, using the VLTI as a template. We proceeded with image reconstruction using the radio image reconstruction algorithms contained in AIPS IMAGR (CLEAN algorithm). Our results show that image reconstruction is successful in most of our science cases, yielding images with a 4 milliarcsecond resolution in K band. (abridged)

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arXiv2002

Properties of the close binary and circumbinary torus of the Red Rectangle

New diffraction-limited speckle images of the Red Rectangle in the wavelength range 2.1--3.3 microns with angular resolutions of 44--68 mas and previous speckle images at 0.7--2.2 microns revealed well-resolved bright bipolar outflow lobes and long X-shaped spikes originating deep inside the outflow cavities. This set of high-resolution images stimulated us to reanalyze all infrared observations of the Red Rectangle using our two-dimensional radiative transfer code. The new detailed modeling, together with estimates of the interstellar extinction in the direction of the Red Rectangle enabled us to more accurately determine one of the key parameters, the distance D=710 pc with model uncertainties of 70 pc, which is twice as far as the commonly used estimate of 330 pc. The central binary is surrounded by a compact, massive (M=1.2 Msun), very dense dusty torus with hydrogen densities reaching n_H=2.5x10^12 cm^-3 (dust-to-gas mass ratio rho_d/rho~0.01). The bright component of the spectroscopic binary HD 44179 is a post-AGB star with mass M*=0.57 Msun, luminosity L*=6000 Lsun, and effective temperature T*=7750 K. Based on the orbital elements of the binary, we identify its invisible component with a helium white dwarf with Mwd~0.35 Msun, Lwd~100 Lsun, and Twd~6x10^4 K. The hot white dwarf ionizes the low-density bipolar outflow cavities inside the dense torus, producing a small HII region observed at radio wavelengths. We propose an evolutionary scenario for the formation of the Red Rectangle nebula, in which the binary initially had 2.3 and 1.9 Msun components at a separation of 130 Rsun. The nebula was formed in the ejection of a common envelope after Roche lobe overflow by the present post-AGB star.

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arXiv1999

Bispectrum speckle interferometry of the Orion Trapezium stars: detection of a close (33 mas) companion of Theta1 Ori C

We present bispectrum speckle interferometry observations with the SAO 6 m telescope of the four brightest stars in the Orion Trapezium. Diffraction-limited images with an unprecedented resolution lambda/D of 57 mas and 76 mas were obtained in the H- and K-band, respectively. The The H and K images of Theta1 Ori C (the star responsible for the proplyds) show for the first time that Theta1 Ori C is a close binary with a separation of only 33 mas (H-band observation). The sub-arcsecond companions of Theta1 Ori A and Theta1 Ori B reported by Petr et al. (1998) are confirmed. We use the magnitudes and colors of the companions to derive information about their stellar properties from the HR-diagram. In addition we briefly discuss the multiplicity of the Trapezium stars. Considering both, the visual and the spectroscopic companions of the 4 Trapezium stars, there are at least 7 companions, i.e. at least 1.75 companions per primary on average. This number is clearly higher than that found for the low-mass stars in the Orion Nebula cluster as well as in the field population. This suggests that a different mechanism is at work in the formation of high-mass multiple systems in the dense Trapezium cluster than for low-mass stars.

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