MIDI Visibilities Research Articles

A multi-scale exploration of a massive young stellar object

Context.The rarity of young massive stars combined with the fact that they are often deeply embedded has limited the understanding of the formation of stars larger than 8M⊙. Ground based mid-infrared (IR) interferometry is one way of securing the spatial resolution required to probe the circumstellar environments of massive young stellar objects (MYSOs). Given that the spatial-frequency coverage of such observations is often incomplete, direct-imaging can be supplementary to such a dataset. By consolidating these observations with modelling, the features of a massive protostellar environment can be constrained.Aims.This paper aims to detail the physical characteristics of the protostellar environment of the MYSO G305.20+0.21 at three size-scales by fitting one 2.5D radiative transfer model to three different types of observations simultaneously, providing an extensive view of the accreting regions of the MYSO.Methods.Interferometry, imaging and a multi-wavelength spectral energy distribution (SED) are combined to study G305.20+0.21. The high-resolution observations were obtained using the Very Large Telescope’s MIDI and VISIR instruments, producing visibilities in theN-band and near-diffraction-limited imaging in theQ-band respectively. By fitting simulated observables, derived from the radiative transfer model, to our observations the properties of the MYSO are constrained.Results.The VISIR image shows elongation at 100 mas scales and also displays a degree of asymmetry. From the simulated observables derived from the radiative transfer model output we find that a central protostar with a luminosity of ~5 × 104L⊙surrounded by a low-density bipolar cavity, a flared 1M⊙disk and an envelope is sufficient to fit all three types of observational data for G305.20+0.21. The weak silicate absorption feature within the SED requires low-density envelope cavities to be successfully fit and is an atypical characteristic in comparison to previously studied MYSOs.Conclusions.The fact that the presence of a dusty disk provides the best fit to the MIDI visibilities implies that this MYSO is following a scaled-up version of the low-mass star formation process. The low density, low extinction environment implies the object is a more evolved MYSO and this combined with large inner radius of the disk suggests that it could be an example of a transitional disk around an MYSO.

Study of the inner disk of the Herbig star MWC480

The inner structure and properties (temperature, mass) of the circumstellar disk of Herbig star MWC480 are studied by stellar interferometry method used in the infrared and are interpreted using semi-analytical models. From these models, the SED (Spectral Energy Distribution) was fitted and multi- wavelength intensity map of the source were calculated. The intensity map provides the input for modeling the Keck Interferometer (KI) data in the near-infrared (near-IR) and the data of the Very Large Telescope Interferometer (VLTI) with the mid-infrared instrument MIDI. We conclude that with our limited set of data, we can fit the SED, the Keck visibilities and the MIDI visibilities using a two-components disk model. Furthermore, we suspect that MWC480 has a transitional dusty disk. However, we need more MIDI observations with different baseline orientations to confirm our modeling.

Revealing the inner circumstellar disk of the T Tauri star S Coronae Australis N using the VLTI

Aims: We investigate the structure of the circumstellar disk of the T Tauri star S CrA N and test whether the observations agree with the standard picture proposed for Herbig Ae stars. Methods: Our observations were carried out with the VLTI/AMBER instrument in the H and K bands with the low spectral resolution mode. For the interpretation of our near-infrared AMBER and archival mid-infrared MIDI visibilities, we employed both geometric and temperature-gradient models. Results: To characterize the disk size, we first fitted geometric models consisting of a stellar point source, a ring-shaped disk, and a halo structure to the visibilities. In the H and K bands, we measured ring-fit radii of 0.73 +- 0.03 mas (corresponding to 0.095 +- 0.018 AU for a distance of 130 pc) and 0.85 +- 0.07 mas (0.111 +- 0.026 AU), respectively. This K-band radius is approximately two times larger than the dust sublimation radius of ~0.05 AU expected for a dust sublimation temperature of 1500 K and gray dust opacities, but approximately agrees with the prediction of models including backwarming (namely a radius of ~0.12 AU). The derived temperature-gradient models suggest that the disk is approximately face-on consisting of two disk components with a gap between star and disk. The inner disk component has a temperature close to the dust sublimation temperature and a quite narrow intensity distribution with a radial extension from 0.11 AU to 0.14 AU. Conclusions: Both our geometric and temperature-gradient models suggest that the T Tauri star S CrA N is surrounded by a circumstellar disk that is truncated at an inner radius of ~0.11 AU. The narrow extension of the inner temperature-gradient disk component implies that there is a hot inner rim.

A Compact Dust Shell in the Symbiotic System HM Sagittae

We present high spatial resolution observations of the mid-infrared core of the dusty symbiotic system HM Sge. The MIDI interferometer was used with the VLT Unit Telescopes and Auxiliary Telescopes providing baselines oriented from PA=42° to 105°. The MIDI visibilities are compared with the ones predicted in the frame of various spherical dust shells published in the literature involving single or double dusty shells intended to account for the influence of the hot White Dwarf. The mid-IR environment is unresolved by a 8m telescope and the MIDI spectrum exhibits a level similar to the ISO spectra recorded 10 yr ago. The discrepancies between the HWHM at different angle orientations suggest an increasing level of asymmetry from 13 to 8 μ m. The observations are surprisingly well fitted by the densest (optically thick in the N band) and smallest spherical model published in the literature based on the ISO data, although such a model does not account for the variations of near-IR photometry due to the Mira pulsation cycle suggesting a much smaller optical thickness. These observations also discard the two shells models, developed in an attempt to take into account the effect of the White Dwarf illumination onto the dusty wind of the Mira. These models are too extended, and lead to a level of asymmetry of the dusty environment tightly constrained by the MIDI visibilities. These observations show that a high rate of dust formation is occurring in the vicinity of the Mira which seems to be not highly perturbed by the hot companion.

A compact dusty disk around the Herbig Ae star HR 5999 resolved with VLTI / MIDI

Aims. We have used mid-infrared long-baseline interferometry to resolve the circumstellar material around the Herbig Ae star HR 5999, providing the first direct measurement of its angular size, and t o derive constraints on the spatial distribution of the dust . Methods. MIDI at the VLTI was used to obtain a set of ten spectrally dispersed (8− 13� m) interferometric measurements of HR 5999 at different projected baseline lengths and position angles. To derive constraints on the geometrical distribution of the dus t, we compared our interferometric measurements to 2D, frequency-dependent radiation transfer simulations of circumstellar disks and envelopes. Results. The derived visibility values between ∼0.5 and∼0.9 show that the mid-infrared emission from HR 5999 is clearly resolved. The characteristic size of the emission region depends on the projected baseline length and position angle, and it ranges between∼ 5− 15 milliarcseconds (Gauss FWHM), corresponding to remarkably small physical sizes of∼ 1− 3 AU. For disk models with radial power-law density distributions, the relatively weak but very extended emission from outer disk regions (> 3 AU) leads to model visibilities that are significantly lower than the observed visibilities, making these models inconsistent with the MIDI data. Disk models in which the density is truncated at outer radii of∼ 2− 3 AU, on the other hand, provide good agreement with the data. Conclusions. A satisfactory fit to the observed MIDI visibilities of HR 599 9 is found with a model of a geometrically thin disk that is truncated at 2.6 AU and seen under an inclination angle of 58 ◦ (i.e. closer to an edge-on view than to a face-on view). Neither models of a geometrically thin disk seen nearly edge-on, nor models of spherical dust shells can achieve agreement between the observed and predicted visibilities. The reason why the disk is so compact remains unclear; we speculate that it has been truncated by a close binary companion.