Wolf-Rayet (WR) stars are luminous, massive blue stars thought to be the immediate precursors to some supernovae. The existence of dust shells around such stars has been enigmatic since their ...discovery about 30 years ago, as the intense ultraviolet radiation from the star should be inimical to dust survival. Although dust creation models, including those involving interacting stellar winds, have been put forward to explain these dust shells, the high-resolution observations needed to distinguish between the models have hitherto been lacking. Here we present images of the dust outflow around WR104, obtained using a technique that allows us to resolve detail on scales of about 40 auat the distance of the star. Our images-taken at two epochs-show that the dust forms a spatially confined stream that follows precisely a linear (or archimedian) spiral trajectory with a rotation period of 220 ± 30 days. These results prove that, in this case, a binary companion is responsible for the creation of the circumstellar dust. Moreover, the spiral plume makes WR104 the prototype of a new class of circumstellar nebulae, which are unique to systems with interacting winds.
Abstract
While the importance of dusty asymptotic giant branch (AGB) stars to galactic chemical enrichment is widely recognized, a sophisticated understanding of the dust formation and wind-driving ...mechanisms has proven elusive due in part to the difficulty in spatially resolving the dust-formation regions themselves. We have observed 20 dust-enshrouded AGB stars as part of the Keck Aperture Masking Experiment, resolving all of them in multiple near-infrared bands between 1.5 and 3.1 μm. We find 45 per cent of the targets to show measurable elongations that, when correcting for the greater distances of the targets, would correspond to significantly asymmetric dust shells at par with the well-known cases of IRC + 10216 or CIT 6. Using radiative transfer models, we find the sublimation temperature of T
sub (silicates) = 1130 ± 90 K and T
sub (amorphous carbon) = 1170 ± 60 K, both somewhat lower than expected from laboratory measurements and vastly below temperatures inferred from the inner edge of young stellar objects discs. The fact that O-rich and C-rich dust types showed the same sublimation temperature was surprising as well. For the most optically thick shells (τ2.2 μm>2), the temperature profile of the inner dust shell is observed to change substantially, an effect we suggest could arise when individual dust clumps become optically thick at the highest mass-loss rates.
We present the first N-band nulling plus K- and L-band V super(2) observations of a young stellar object, MWC 325, taken with the 85 m baseline Keck Interferometer. The Keck nuller was designed for ...the study of faint dust signatures associated with debris disks, but it also has a unique capability for studying the temperature and density distribution of denser disks found around young stellar objects. Interferometric observations of MWC 325 at K, L, and N encompass a factor of five in spectral range and thus, especially when spectrally dispersed within each band, enable characterization of the structure of the inner disk regions where planets form. Fitting our observations with geometric models such as a uniform disk or a Gaussian disk show that the apparent size increases monotonically with wavelength in the 2-12 mum wavelength region, confirming the widely held assumption based on radiative transfer models, now with spatially resolved measurements over a broad wavelength range, that disks are extended with a temperature gradient. The effective size is a factor of about 1.4 and 2.2 larger in the L band and N band, respectively, compared to that in the K band. The existing interferometric measurements and the spectral energy distribution can be reproduced by a flat disk or a weakly shadowed nearly flat disk model, with only slight flaring in the outer regions of the disk, consisting of representative "sub-micron" (0.1 mum) and "micron" (2 mum) grains of a 50:50 ratio of silicate and graphite. This is in marked contrast to the disks previously found in other Herbig Ae/Be stars, suggesting a wide variety in the disk properties among Herbig Ae/Be stars.
We have measured nonzero closure phases for about 29% of our sample of 56 nearby asymptotic giant branch (AGB) stars, using the three-telescope Infrared Optical Telescope Array (IOTA) interferometer ...at near-infrared wavelengths (H band) and with angular resolutions in the range 5-10 mas. These nonzero closure phases can only be generated by asymmetric brightness distributions of the target stars or their surroundings. We discuss how these results were obtained and how they might be interpreted in terms of structures on or near the target stars. We also report measured angular sizes and hypothesize that most Mira stars would show detectable asymmetry if observed with adequate angular resolution.
Context.
Rocky planets form by the concentration of solid particles in the inner few au regions of planet-forming disks. Their chemical composition reflects the materials in the disk available in the ...solid phase at the time the planets were forming. Studying the dust before it gets incorporated in planets provides a valuable diagnostic for the material composition.
Aims.
We aim to constrain the structure and dust composition of the inner disk of the young Herbig Ae star HD 144432, using an extensive set of infrared interferometric data taken by the Very Large Telescope Interferometer (VLTI), combining PIONIER, GRAVITY, and MATISSE observations.
Methods.
We introduced a new physical disk model,
TGMdust
, to image the interferometric data, and to fit the disk structure and dust composition. We also performed equilibrium condensation calculations with GGchem to assess the hidden diversity of minerals occurring in a planet-forming disk such as HD 144432.
Results.
Our best-fit model has three disk zones with ring-like structures at 0.15, 1.3, and 4.1 au. Assuming that the dark regions in the disk at ~0.9 au and at ~3 au are gaps opened by planets, we estimate the masses of the putative gap-opening planets to be around a Jupiter mass. We find evidence for an optically thin emission (τ < 0.4) from the inner two disk zones
(r <
4 au) at
λ >
3 µm. Our silicate compositional fits confirm radial mineralogy gradients, as for the mass fraction of crystalline silicates we get around 61% in the innermost zone (
r
< 1.3 au), mostly from enstatite, while only ~20% in the outer two zones. To identify the dust component responsible for the infrared continuum emission, we explore two cases for the dust composition, one with a silicate+iron mixture and the other with a silicate+carbon one. We find that the iron-rich model provides a better fit to the spectral energy distribution. Our GGchem calculations also support an iron-rich and carbon-poor dust composition in the warm disk regions (
r
< 5 au,
T >
300 K).
Conclusions.
We propose that in the warm inner regions (
r
< 5 au) of typical planet-forming disks, most if not all carbon is in the gas phase, while iron and iron sulfide grains are major constituents of the solid mixture along with forsterite and enstatite. Our analysis demonstrates the need for detailed studies of the dust in inner disks with new mid-infrared instruments such as MATISSE and JWST/MIRI.
We report the results of a high angular resolution near-infrared survey of dusty Wolf-Rayet stars using the Keck I Telescope, including new multiwavelength images of the pinwheel nebulae WR 98a, WR ...104, and WR 112. Angular sizes were measured for an additional eight dusty Wolf-Rayet stars using aperture-masking interferometry, allowing us to probe characteristic sizes down to 620 mas (640 AU for typical sources). With angular sizes and specific fluxes, we can directly measure the wavelength-dependent surface brightness and size relations for our sample. We discovered tight correlations of these properties within our sample that could not be explained by simple spherically symmetric dust shells or even the more realistic "pinwheel nebula" (three-dimensional) radiative transfer model, when using Zubko's optical constants. While the tightly correlated surface brightness relations we uncovered offer compelling indirect evidence of a shared and distinctive dust shell geometry among our sample, long-baseline interferometers should target the marginally resolved objects in our sample in order to conclusively establish the presence or absence of the putative underlying colliding-wind binaries thought to produce the dust shells around WC Wolf-Rayet stars.
ALMA observations of α Centauri Liseau, R; Vlemmings, W; Bayo, A ...
Astronomy and astrophysics (Berlin),
1/2015, Volume:
573
Journal Article
Peer reviewed
Open access
The precise mechanisms that provide the non-radiative energy for heating the chromosphere and the corona of the Sun and those of other stars constitute an active field of research. By studying ...stellar chromospheres one aims at identifying the relevant physical processes. Earlier observations with Herschel and APEX have revealed the temperature minimum of alpha Cen, but these were unable to spatially resolve the binary into individual components. With the data reported in this Letter, we aim at remedying this shortcoming. Furthermore, these earlier data were limited to the wavelength region between 100 and 870 mu m. The previously obtained flux ratio of 0.44+ or -0.18, which was based on measurements in the optical and at 70 mu m, is consistent with the present ALMA results, albeit with a large error bar. The observed 3.1 mm emission greatly exceeds what is predicted from the stellar photospheres, and undoubtedly arises predominantly as free-free emission in the ionized chromospheric plasmas of both stars.
Parasitic Interference in Nulling Interferometry Matter, A.; Defrere, D.; Danchi, W. C. ...
Monthly notices of the Royal Astronomical Society,
05/2013, Volume:
431, Issue:
2
Journal Article, Web Resource
Peer reviewed
Open access
Nulling interferometry aims to detect faint objects close to bright stars. Its principle is to produce a destructive interference along the line of sight so that the stellar flux is rejected, while ...the flux of the off-axis source can be transmitted. In practice, various instrumental perturbations can degrade the nulling performance. Any imperfection in phase, amplitude or polarization produces a spurious flux that leaks to the interferometer output and corrupts the transmitted off-axis flux. One of these instrumental perturbations is the crosstalk phenomenon, which occurs because of multiple parasitic reflections inside transmitting optics, and/or diffraction effects related to beam propagation along finite size optics. It can include a crosstalk of a beam with itself, and a mutual crosstalk between different beams. This can create a parasitic interference pattern, which degrades the intrinsic transmission map - or intensity response - of the interferometer. In this context, we describe how this instrumental effect impairs the performance of a Bracewell interferometer. A simple formalism is developed to derive the corresponding modified intensity response of the interferometer, as a function of the two parameters of interest: the crosstalk level (or contamination rate) and the phase shift between the primary and secondary - parasitic - beams. We then apply our mathematical approach to a few scientific cases, both analytically and using the GENIESIM simulation software, adapted to handle coherent crosstalk. Our results show that a coherent crosstalk level of about 1 per cent implies a 20 per cent drop of the signal-to-noise ratio at most. Careful attention should thus be paid to reduce the crosstalk level inside an interferometric instrument and ensure an instrumental stability that provides the necessary sensitivity through calibration procedures.
We present high spatial and frequency resolution images of the SiO masers in Stokes I, Q, U, and V around the asymptotic giant branch star IK Tau and describe and exploit a new technique for making ...accurate calibration of Stokes V. This technique also resulted in improved images of Stokes I. An evaluation of the results suggests that the circular polarization is neither the result of Zeeman splitting nor an alternate propagation effect. The pattern of circular and linear polarization across the maser lines shows no tendency toward that expected for simple Zeeman splitting. The fractional circular polarization greatly exceeds that expected from the alternate mechanism. The overall shape of the masing ring has changed from the elliptical form repeatedly observed over the last decade and a half.
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