Astrophysics Group

Cavendish Laboratory

COAST Astronomical Results

This page highlights some astronomical results obtained with COAST over the years. These are listed in chronological order (most recent first). Some details of each result are given lower down this page.

Multi-waveband imaging of evolved supergiants

Following on from our earlier work on Betelgeuse, we have imaged both Betelgeuse and Alpha Herculis (another evolved supergiant) in a number of visible wavebands, during 2002, 2003 and 2004. The observations in 2004 were obtained at the same time as infrared data from the IOTA interferometer in Arizona. Hence we hope to constrain the wavelength-dependence of the asymmetry over both small and large wavelength ranges simultaneously.

The images of Betelgeuse below were reconstructed from COAST data taken in early 2004. From left to right, the wavelengths of observation were 750, 782, and 905nm.




More details can be found in the conference papers Young et al. (2002) and Young et al. (2004 – poster presented at UK National Astronomy Meeting).

Cyclic variations in the angular diameter of Chi Cygni

Measurements made between July 1997 and September 1999, using COAST and the William Herschel Telescope, indicate periodic changes in the 905-nm apparent diameter at of the Mira variable Chi Cygni. The amplitude is significantly larger than that seen for R Leonis, being 45% of the smallest value.

See Young et al. (2000).

Surface imaging of Betelgeuse

Contemporaneous parametric images of the red supergiant Betelgeuse at three widely-spaced wavelengths were secured in November 1997, using COAST and the William Herschel Telescope. These reveal a dramatic change in the apparent symmetry of the stellar disk with wavelength. More information on these Betelgeuse results is available on a separate page.

The First Infrared Image

The binary star Capella was observed with COAST using the infrared correlator at a wavelength of 1.3 microns on 25 October 1997. We have produced the image below from these data. The bottom contours are 4% of the peak flux. The noise level in the map is consistent with the uv coverage, also shown below.

This is the first infrared image from an aperture synthesis array.



The first direct detection of long term periodic diameter variations in a long-period variable

As part of a continuing programme to monitor the angular sizes of nearby long-period variable stars, COAST has, for the first time, directly detected the stellar pulsation that underlies their brightness variations. Measurements of the apparent diameter of R Leonis, a nearby and well-studied long-period variable, obtained with COAST over a period of two years have revealed a periodic modulation of the stellar size of 35%. Contemporaneous interferometric observations secured at the William Herschel Telescope confirm this result, and suggest that the regularity of the variations can be sustained for many years.

See Burns et al. (1998).

Surface structure and limb-darkening profile of Betelgeuse

The M supergiant Betelgeuse (alpha Orionis) was observed on four nights in October 1995. These observations yielded the first image of a resolved stellar disk from a separated-element interferometer. Our data indicate strong limb-darkening, but there was no evidence for “hotspots”, which are often detected on Betelgeuse.

See Burns et al. (1997).

The First Images

The spectroscopic binary star Capella (alpha Aurigae) was observed at 830nm at two epochs during September 1995, and the first images were produced during the same month. The position angle and separation of the components are in excellent agreement with predictions from the best currently available set of orbital elements (Hummel C.A. et al, 1994, AJ, 107, 1859). The maps show no noise above the 5% level, as expected from the uv coverage. These maps are in fact the first from any optical aperture synthesis array.

See Baldwin et al. (1996).

Stellar Diameter Measurements

The angular diameters of a number of stars were determined at various wavelengths and several epochs during 1995. These measurements are all in very good agreement with values in the literature.