Nulling at the Keck interferometer

Cotton

Millan-Gabet

et al. 2015

A&A

Aims. We present the results of observations with interferometers of a sample of pulsating asymptotic giant branch (AGB) stars in the infrared and at radio wavelengths. The goal of these observations is to explore the extended stellar atmospheres and to establish links between the spatial scales of molecular envelopes and of the dust shell. This is the key to better understand the process of dust formation and therefore of mass loss. Methods. We used the ESO VLTI/MIDI interferometer in the N band, the Keck Interferometer in the K band, and NRAO VLBA observations of SiO masers at 7 mm wavelength of a sample of AGB stars: U Ari, W Cnc, RX Tau, RT Tau, RT Aql, S Ser, and V Mon. The various instruments probe different altitudes of the atmosphere of the AGB stars. They are sensitive to regions below the silicate dust condensation distance and provide the opportunity of finding hints about how dust and its precursors form in the extended atmosphere of an AGB star. The K-band observations are sensitive to water and carbon-monoxyde vapors. Unfortunately, we were only able to observe S Ser in this wavelength range. Results. We find a ratio of 2.2 between the molecular envelope radius and the photospheric size, which is consistent with previous results. The N-band observations are mostly sensitive to vapors of SiO and water and to dust (alumina and silicate). The silicate dust shell is fully resolved, and no precise parameters can be deduced from the N-band observations other than a spatial extension of at least 12-16 R for our sample. The sizes found for the SiO region are consistent with the radii of the SiO maser rings provided by the VLBA observations. The sizes of the alumina and water vapor regions are systematically found to be larger. There is clear evidence that SiO is absent from regions farther from the star where silicate dust condenses. Conclusions. These observations support a possible scenario in which SiO is adsorbed by species such as corundum. An alternative explanation could be that SiO has chemically disappeared at this range of distances.

Section: Near-ir Observations and Data Reductionmentioning

confidence: 99%

High-resolution IR and radio observations of AGB stars

Cotton

Millan-Gabet

et al. 2015

A&A

“…By stabilizing the interferometric fringes to a fraction of the wavelength, such a system indeed enables exposure times of several minutes without the visibility loss inherent in the fringe motion, and therefore gives access to new sensitivity limits. Fringe trackers have since proven themselves to be essential, not only to observe faint targets (Müller et al 2010;Colavita et al 2013), but also to perform astrometry with a submilliarcsecond (mas) PhD Fellow of the Research Foundation -Flanders (FWO). accuracy (Lane & Muterspaugh 2004).…”

Section: Introductionmentioning

confidence: 99%

“…An efficient example of these efforts have been demonstrated at VLTI these past years, bringing the optical path length variations on the 8.2 m unit telescopes (UTs) from typical values of 280 nm rms to more than 1 μm rms in 2008 down to 145 to 380 nm rms in 2012, by damping pumps of some instruments, changing the close cyclo coolers of some others, and modifying the mechanical design of some mounts (Poupar et al 2010;Haguenauer et al 2010Haguenauer et al , 2012. Second, vibrations excited by the first mirrors of the system can be measured with accelerometers and independently compensated by optical delay lines (Colavita et al 2013;Haguenauer et al 2012). However, these solutions are not enough to completely suppress vibrations down the entire optical stream, and active compensation is necessary at the beam combination level.…”

Section: Introductionmentioning

confidence: 99%

Comparison of fringe-tracking algorithms for single-mode near-infrared long-baseline interferometers

Choquet

et al. 2014

A&A

To enable optical long baseline interferometry toward faint objects, long integrations are necessary despite atmospheric turbulence. Fringe trackers are needed to stabilize the fringes and thus increase the fringe visibility and phase signal-to-noise ratio (S/N), with efficient controllers robust to instrumental vibrations and to subsequent path fluctuations and flux drop-outs. We report on simulations, analysis, and comparison of the performances of a classical integrator controller and of a Kalman controller, both optimized to track fringes under realistic observing conditions for different source magnitudes, disturbance conditions, and sampling frequencies. The key parameters of our simulations (instrument photometric performance, detection noise, turbulence, and vibrations statistics) are based on typical observing conditions at the Very Large Telescope observatory and on the design of the GRAVITY instrument, a 4-telescope single-mode long-baseline interferometer in the near-infrared, next in line to be installed at VLT Interferometer. We find that both controller performances follow a two-regime law with the star magnitude, a constant disturbance limited regime, and a diverging regime limited by both the detector and photon-noise. Moreover, we find that the Kalman controller is optimal in the high and medium S/N regime owing to its predictive commands based on an accurate disturbance model. In the low S/N regime, the model is not accurate enough to be more robust than an integrator controller. Identifying the disturbances from high S/N measurements improves the Kalman performances in the case of strong optical path difference disturbances.

“…Coupling between AO systems and single-mode fibres in the J, H, and K bands was tested on the Canada-France-Hawaii (CFH) telescope and the Keck II telescope in 2002, and on the Gemini telescope in 2003 . The development and characterisation of a pair of 300 m long, single-mode fluoride glass fibres for the K band made the bypass of the Keck Interferometer (KI, Colavita et al 2013) beamtrains possible. The first onsky fringes were demonstrated in 2005, with the coherent light transport achieved in single-mode fibres between the two AO Nasmyth foci and the interferometric laboratory (Perrin et al 2006b).…”

Section: On-sky Demonstration Of the Reconfigurable Optical Interferomentioning

confidence: 99%

AGILIS: Agile Guided Interferometer for Longbaseline Imaging Synthesis

Woillez

Lai

et al. 2017

A&A

Context. In comparison to the radio and sub-millimetric domains, imaging with optical interferometry is still in its infancy. Due to the limited number of telescopes in existing arrays, image generation is a demanding process that relies on time-consuming reconfiguration of the interferometer array and super-synthesis. Aims. Using single mode optical fibres for the coherent transport of light from the collecting telescopes to the focal plane, a new generation of interferometers optimized for imaging can be designed. Methods. To support this claim, we report on the successful completion of the 'OHANA Iki project: an end-to-end, on-sky demonstration of a two-telescope interferometer, built around near-infrared single mode fibres, carried out as part of the 'OHANA project. Results. Having demonstrated that coherent transport by single-mode fibres is feasible, we explore the concepts, performances, and limitations of a new imaging facility with single mode fibres at its heart: Agile Guided Interferometer for Longbaseline Imaging Synthesis (AGILIS). Conclusions. AGILIS has the potential of becoming a next generation facility or a precursor to a much larger project like the Planet Formation Imager (PFI).