Least-squares Fourier phase estimation from the modulo 2Pi bispectrum phase

The recovery of Fourier phases from measurements of the bispectrum occupies a vital role in many astronomical speckle imaging schemes. In arecent paper [J. Opt. Soc. Am. A 7, 14 (1990)] it was suggested that a least-squares solution to this problem must fail if the bispectrum phase is known only modulo 2π. Here an alternative nonlinear least-squares algorithm is presented that differs from the linear method discussed in the aforementioned paper and that permits the fitting of Fourier phases directly to modulo 2π measurements of the bispectrum phase, thus eliminating any need for phase unwrapping. Numerical simulations of this method confirm that it is reliable and robust in the presence of noise and verify its enhanced performance when compared with a linear least-squares method that includes the unwrapping of the bispectral phase before Fourier phase retrieval

Model Bisnis Pengembangan Kawasan Tertinggal (Studi Kasus : Kabupaten Nias Selatan)

ABSTRACT  South Nias Regency is currently still in the category of lagging areas based on composite index in North Sumatera province. However, South Nias Regency has a variety of potential, especially maritime tourism potential, fisheries, and agriculture. Focus Group Discussion with the stakeholders in South Nias Regency was done to formulate the right business model in South Nias regency. Based on the results of FGD and the analysis of business model conducted, business activities that can be done by South Nias Regency was to focus on the development of maritime tourism activities through surfing and agroindustry activities. The business Model would be able to succeed if the cooperation between stakeholders, such as the central government, local government, tourism business, and local communities with various models of operational cooperation. Keywords: business model, disadvantaged area, South Nias, maritime tourism.   ABSTRAK Kabupaten Nias Selatan saat ini masih masuk dalam kategori daerah tertinggal berdasarkan Indeks Komposit Ketertinggalan di Provinsi Sumatera Utara. Akan tetapi, Kabupaten Nias Selatan memiliki berbagai potensi, terutama potensi wisata bahari, perikanan, dan pertanian. Focus Group Discussion bersama para stakeholder yang ada di Kabupaten Nias Selatan pun dilakukan guna merumuskan model bisnis yang tepat di Kabupaten Nias Selatan. Berdasarkan hasil FGD dan analisis model bisnis yang dilakukan, kegiatan bisnis yang dapat dilakukan oleh Kabupaten Nias Selatan adalah dengan memfokuskan pengembangan aktivitas pariwisata bahari melalui kegiatan berselancar dan agroindustry. Model bisnis tersebut akan dapat berhasil jika adanya kerjasama antar stakeholder, seperti Pemerintah Pusat, Pemerintah Daerah, Pelaku Usaha Wisata, dan Masyarakat Lokal dengan berbagai model kerjasama operasional Kata kunci: model bisnis, kawasan tertinggal, Nias Selatan, pariwisata bahari

Imaging the Radio Photospheres of Mira Variables

We have used the VLA at 43 GHz to image the radio continuum emission from o Ceti, R Leo, and W Hya and to precisely locate their SiO maser emission with respect to the star. The radio continuum emission region for all three stars has a diameter close to 5.6 AU. These diameters are similar to those measured at infrared wavelengths in bands containing strong molecular opacity and about twice those measured in line-free regions of the infrared spectrum. Thus, the radio photosphere and the infrared molecular layer appear to be coextensive. The 43 GHz continuum emission is consistent with temperatures near 1600 K and opacity from H-minus free-free interactions. While the continuum image of o Ceti appears nearly circular, both R Leo and W Hya display significant elongations. The SiO masers for all three stars show partial rings with diameters close to 8 AU.Comment: 14 pages; 3 figure

Multi-wavelength visibility measurements of the red giant R Doradus

We present visibility measurements of the nearby Mira-like star R Doradus taken over a wide range of wavelengths (650--990 nm). The observations were made using MAPPIT (Masked APerture-Plane Interference Telescope), an interferometer operating at the 3.9-m Anglo-Australian Telescope. We used a slit to mask the telescope aperture and prism to disperse the interference pattern in wavelength. We observed in R Dor strong decreases in visibility within the TiO absorption bands. The results are in general agreement with theory but differ in detail, suggesting that further work is needed to refine the theoretical models.Comment: 8 pages; SPIE Conf. 4006 "Interferometry in Optical Astronomy

On the Formation of Multiple-Shells Around Asymptotic Giant Branch Stars

Two types of models for the formation of semi-periodic concentric multiple shells (M-shells) around asymptotic giant branch (AGB) stars and in planetary nebulae are compared against observations. Models that attribute the M-shells to processes in an extended wind acceleration zone around AGB stars result in an optically thick acceleration zone, which reduces the acceleration efficiency in outer parts of the extended acceleration zone. This makes such models an unlikely explanation for the formation of M-shells. Models which attribute the M-shell to semi-periodic variation in one or more stellar properties are most compatible with observations. The only stellar variation models on time scales of 50-1500 years that have been suggested are based on an assumed solar-like magnetic cycle. Although ad-hoc, the magnetic cycle assumption fits naturally into the increasingly popular view that magnetic activity plays a role in shaping the wind from upper AGB stars.Comment: 8 pages, Submitted to Ap

Resolving the hot dust around HD69830 and eta Corvi with MIDI and VISIR

Most of the known debris discs exhibit cool dust in regions analogous to the Edgeworth-Kuiper Belt. However, a rare subset show hot excess from within a few AU, which is often inferred to be transient. We examine 2 such sources to place limits on their location to help distinguish between different interpretations for their origin. We use MIDI on the VLTI to observe the debris discs around eta Corvi and HD69830 using baseline lengths from 44-130m. New VISIR observations of HD69830 at 18.7um are also presented. These observations are compared with disc models to place limits on disc size. The visibility functions measured with MIDI for both sources show significant variation with wavelength across 8-13um in a manner consistent with the disc flux being well resolved, notably with a dip at 10-11.5um due to the silicate emission feature. The average ratio of visibilities measured between 10-11.5um and 8-9um is 0.934+/-0.015 for HD69830 and 0.880+/-0.013 for eta Corvi over the 4 baselines for each source, a departure of 4 and 9sigma from that expected if the discs were unresolved. HD69830 is unresolved by VISIR at 18.7um. The combined limits from MIDI and 8m imaging constrain the warm dust to lie within 0.05-2.4AU for HD69830 and 0.16-2.98AU for eta Corvi. These results represent the first resolution in the mid-IR of dust around main sequence stars. The constraints placed on the location of the dust are consistent with radii predicted by SED modelling. Tentative evidence for a common position angle for the dust at 1.7AU with that at 150AU around eta Corvi, which might be expected if the hot dust is fed from the outer disc, demonstrates the potential of this technique for constraining the origin of the dust and more generally for the study of dust in the terrestrial regions of main sequence stars.Comment: Accepted for publication in Astronomy and Astrophysic

Mid-infrared spectra of late-type stars: Long-term evolution

Recent ground-based mid-infrared spectra of 29 late-type stars, most with substantial dust shells, are compared to ground-based spectra of these stars from the 1960s and 1970s and to IRAS-LRS spectra obtained in 1983. The spectra of about half the stars show no detectable changes, implying that their distributions of circumstellar material and associated dust grain properties have changed little over this time interval. However, many of the stars with strong silicate features showed marked changes. In nearly all cases the silicate peak has strengthened with respect to the underlying continuum, although there is one case (VY~CMa) in which the silicate feature has almost completely disappeared. This suggests that, in general, an oxygen-rich star experiences long periods of gradual silicate feature strengthening, punctuated by relatively rare periods when the feature weakens. We discuss various mechanisms for producing the changes, favoring the slow evolution of the intrinsic dust properties (i.e., the chemical composition or grain structure). Although most IRAS spectra agree well with ground-based spectra, there are a number of cases where they fall well outside the expected range of uncertainty. In almost all such cases the slopes of the red and blue LRS spectra do not match in their region of overlap.Comment: Accepted in ApJ, 20 pages, 5 figures, 1 tabl

Photon counting strategies with low light level CCDs

Low light level charge coupled devices (L3CCDs) have recently been developed, incorporating on-chip gain. They may be operated to give an effective readout noise much less than one electron by implementing an on-chip gain process allowing the detection of individual photons. However, the gain mechanism is stochastic and so introduces significant extra noise into the system. In this paper we examine how best to process the output signal from an L3CCD so as to minimize the contribution of stochastic noise, while still maintaining photometric accuracy. We achieve this by optimising a transfer function which translates the digitised output signal levels from the L3CCD into a value approximating the photon input as closely as possible by applying thresholding techniques. We identify several thresholding strategies and quantify their impact on photon counting accuracy and effective signal-to-noise. We find that it is possible to eliminate the noise introduced by the gain process at the lowest light levels. Reduced improvements are achieved as the light level increases up to about twenty photons per pixel and above this there is negligible improvement. Operating L3CCDs at very high speeds will keep the photon flux low, giving the best improvements in signal-to-noise ratio.Comment: 7 pages, accepted by MNRA