Bandwidth smearing in optical interferometry: Analytic model of the transition to the double fringe packet

Bandwidth smearing is a chromatic aberration due to the finite frequency bandwidth. In long-baseline optical interferometry terms, it is when the angular extension of the source is greater than the coherence length of the interferogram. As a consequence, separated parts of the source will contribute to fringe packets that are not fully overlapping; it is a transition from the classical interferometric regime to a double or multiple fringe packet. While studied in radio interferometry, there has been little work on the matter in the optical, where observables are measured and derived in a different manner, and are more strongly impacted by the turbulent atmosphere. We provide here the formalism and a set of usable equations to model and correct for the impact of smearing on the fringe contrast and phase, with the case of multiple stellar systems in mind. The atmosphere is briefly modeled and discussed.Comment: To appear in the proceedings of the 2012 SPIE Conference "Astronomical Telescopes and Instrumentation" (9 pages, 3 figures

Inner disk regions revealed by infrared interferometry

I review the results obtained by long-baseline interferometry at infrared wavelengths on the innermost regions around young stars. These observations directly probe the location of the dust and gas in the disks. The characteristic sizes of these regions found are larger than previously thought. These results have motivated in part a new class of models of the inner disk structure. However the precise understanding of the origin of these low visibilities is still in debate. Mid-infrared observations have probed disk emission over a larger range of scales revealing mineralogy gradients in the disk. Recent spectrally resolved observations allow the dust and gas to be studied separately. The few results shows that the Brackett gamma emission can find its origin either in a wind or in a magnetosphere but there are no definitive answers yet. In a certain number of cases, the very high spatial resolution seems to reveal very close companions. In any case, these results provide crucial information on the structure and physical properties of disks surrounding young stars especially as initial conditions for planet formation.Comment: 11 page

On marginally resolved objects in optical interferometry

With the present and soon-to-be breakthrough of optical interferometry, countless objects shall be within reach of interferometers; yet, most of them are expected to remain only marginally resolved with hectometric baselines. In this paper, we tackle the problem of deriving the properties of a marginally resolved object from its optical visibilities. We show that they depend on the moments of flux distribution of the object: centre, mean angular size, asymmetry, and curtosis. We also point out that the visibility amplitude is a second-order phenomenon, whereas the phase is a combination of a first-order term, giving the location of the photocentre, and a third-order term, more difficult to detect than the visibility amplitude, giving an asymmetry coefficient of the object. We then demonstrate that optical visibilities are not a good model constraint while the object stays marginally resolved, unless observations are carried out at different wavelengths. Finally, we show an application of this formalism to circumstellar discs.Comment: 10 pages 8 figures (layout slightly different from that of A&A

Gavotte Badine

https://digitalcommons.library.umaine.edu/mmb-ps/1532/thumbnail.jp

Quiescent X-ray emission from an evolved brown dwarf ?

I report on the X-ray detection of Gl569Bab. During a 25ksec Chandra observation the binary brown dwarf is for the first time spatially separated in X-rays from the flare star primary Gl569A. Companionship to Gl569A constrains the age of the brown dwarf pair to ~300-800 Myr. The observation presented here is only the second X-ray detection of an evolved brown dwarf. About half of the observing time is dominated by a large flare on Gl569Bab, the remainder is characterized by weak and non-variable emission just above the detection limit. This emission -- if not related to the afterglow of the flare -- represents the first detection of a quiescent corona on a brown dwarf, representing an important piece in the puzzle of dynamos in the sub-stellar regime.Comment: to appear in ApJ

The vertical structure of T Tauri accretion discs IV. Self-irradiation of the disc in the FU Orionis outburst phase

I investigate the self-irradiation of intensively accreting circumstellar discs (backwarmed discs). It is modelled using the two-layer disc approach by Lachaume et al. (2003) that includes heating by viscous dissipation and by an external source of radiation. The disc is made of a surface layer directly heated by the viscous luminosity of the central parts of the disc, and of an interior heated by viscosity as well as by reprocessed radiation from the surface. This model convincingly accounts for the infrared excess of some FU Orionis objects in the range 1-200 microns and supports the backwarmed disc hypothesis sometimes invoked to explain the mid- and far-infrared excesses whose origins are still under debate. Detailed simulation of the vertical radiative transfert in the presence of backwarming is still needed to corroborate these results and spectroscopically constrain the properties of intensively accreting discs.Comment: 6 pages, 2 figures, 2 tables, accepted by Astronomy & Astrophysic

First scattered light images of debris disks around HD 53143 and HD 139664

We present the first scattered light images of debris disks around a K star (HD 53143) and an F star (HD 139664) using the coronagraphic mode of the Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope (HST). With ages 0.3 - 1 Gyr, these are among the oldest optically detected debris disks. HD 53143, viewed ~45 degrees from edge-on, does not show radial variation in disk structure and has width >55 AU. HD 139664 is seen close to edge-on and has belt-like morphology with a dust peak 83 AU from the star and a distinct outer boundary at 109 AU. We discuss evidence for significant diversity in the radial architecture of debris disks that appears unconnected to stellar spectral type or age. HD 139664 and possibly the solar system belong in a category of narrow belts 20-30 AU wide. HD 53143 represents a class of wide disk architecture with characteristic width >50 AU.Comment: 7 pages, 3 figure

The vertical structure of T Tauri accretion discs III. Consistent interpretation of spectra and visibilities with a two-layer model

We present a two-layer accretion disc model developed to simultaneously fit optical long baseline visibilities and spectral energy distributions of T Tauri accretion discs. Our model includes viscous heating and absorption of stellar radiation. It analytically expresses the vertical structure of the disc using a two-layer approach: the outer layer is heated by the star and by the inner layer, and the inner layer by visous dissipation and by the outer layer. We compare three prescriptions for the flaring of the irradiated surface, and conclude that a smooth profile with r ~ r^1/8->1/7 has predictions close to those of comparable numerical models. The observables (spectra, images and visibilities) are numerically determined. We study the influence of disc parameters on the structure and observables. Then we apply it to three stars observed in IR interferometry. For T Tau and SU Aur we find a model fit consistent with both visibilities and spectrum but we could not in the case of T Tau North, which might come from caveats in the flux correction from the close South companion. We find that even a single interferometric measurement at one wavelength can bring a very strong constraint on disc models. We predict that future massive interferometric observations will provide a breakthrough in the understanding of accretion disc physics.Comment: 19 pages 10 figures Important conceptual changes concerning the flaring of the irradiated surfac