Using the PIONIER visitor instrument that combines the light of the four Auxiliary Telescopes of ESOs Very Large Telescope Interferometer, we measure precisely the diameters of several symbiotic and related stars: HD 352, HD 190658, V1261 Ori, ER Del
, FG Ser, and AG Peg. These diameters - in the range of 0.6 to 2.3 milli-arcseconds - are used to assess the filling factor of the Roche lobe of the mass-losing giants and provide indications on the nature of the ongoing mass transfer. We also provide the first spectroscopic orbit of ER Del, based on CORAVEL and HERMES/Mercator observations. The system is found to have an eccentric orbit with a period of 5.7 years. In the case of the symbiotic star FG Ser, we find that the diameter is changing by 13% over the course of 41 days, while the observations of HD 352 are indicative of an elongation. Both these stars are found to have a Roche filling factor close to 1, as is most likely the case for HD 190658 as well, while the three other stars have factors below 0.5-0.6. Our observations reveal the power of interferometry for the study of interacting binary stars - the main limitation in our conclusions being the poorly known distances of the objects.
The VLTI Spectro Imager project aims to perform imaging with a temporal resolution of 1 night and with a maximum angular resolution of 1 milliarcsecond, making best use of the Very Large Telescope Interferometer capabilities. To fulfill the scientifi
c goals (see Garcia et. al.), the system requirements are: a) combining 4 to 6 beams; b) working in spectral bands J, H and K; c) spectral resolution from R= 100 to 12000; and d) internal fringe tracking on-axis, or off-axis when associated to the PRIMA dual-beam facility. The concept of VSI consists on 6 sub-systems: a common path distributing the light between the fringe tracker and the scientific instrument, the fringe tracker ensuring the co-phasing of the array, the scientific instrument delivering the interferometric observables and a calibration tool providing sources for internal alignment and interferometric calibrations. The two remaining sub-systems are the control system and the observation support software dedicated to the reduction of the interferometric data. This paper presents the global concept of VSI science path including the common path, the scientific instrument and the calibration tool. The scientific combination using a set of integrated optics multi-way beam combiners to provide high-stability visibility and closure phase measurements are also described. Finally we will address the performance budget of the global VSI instrument. The fringe tracker and scientific spectrograph will be shortly described.
For young Herbig Ae/Be stars, near-infrared interferometric measurements have revealed a correlation between the luminosity of the central object and the position of the disk inner rim. This correlation breaks down for the cooler T Tauri stars, a fac
t often interpreted in terms of disks with larger inner radii. In most cases, the conversion between the observed interferometric visibility and the calculated disk inner radius was done with a crude disk emission model. Here, we examine how the use of models that neglect scattered light can lead to an overestimation of the disk sizes. To do so, synthetic disk images (and visibilities) are calculated with a full treatment of the radiative transfer. The relative contributions of thermal emission and scattered light are compared. We find that the latter can not be neglected for cool stars. For further comparison, the model visibilities are also converted into inner disk radii using the same simple disk models as found in the literature. We find that reliable inner radii can only be estimated for Herbig Ae/Be stars with these models. However, they lead to a systematic overestimation of the disk size, by a factor of 2 to 3, for T Tauri stars. We suggest that including scattered light in the models is a simple (and sufficient) explanation of the current interferometric measurements of T Tauri stars.
