arXiv subjects
Publications and source records attributed to Timothy R. Bedding.
Interferometric techniques are at the forefront of modern astronomical instrumentation. A new generation of instruments are either operating or nearing completion, including arrays of small telescopes as well as the ``big guns'' (VLTI and Keck). A number of space interferometers for the detection of extra-solar planets are also being planned. I review the current state of play and describe the latest developments in the field.
Oscillation frequencies are the most accurate properties one can measure for a star, potentially allowing detailed tests of stellar models and evolution theories. We briefly review asteroseismology for two classes of stars. In delta Scuti variables, the main problems are the identification of the observed modes and the theoretical treatment of rotation. In solar-like stars the main difficulty is the tiny amplitudes, but credible detections are now being made. These confirm that stars are oscillating at the approximately the expected levels, but suggest that amplitudes scale as 1/g rather than L/M. We also stress the importance of multi-site campaigns. Several space missions will be launched over the coming years, promising an exciting future for asteroseismology.
Asteroseismology involves using the resonant frequencies of a star to infer details about its internal structure and evolutionary state. Oscillation frequencies are most useful when accompanied by accurate measurements of the more traditional stellar parameters such as luminosity and effective temperature. The Hipparcos catalogue provides luminosities with precisions of a few percent or better for many oscillating stars. I briefly discuss the importance of Hipparcos measurements for interpreting asteroseismic data on three types of oscillating stars: delta Scuti variables, rapidly oscillating Ap stars and solar-like stars. I also retract the endorsement I made during my talk of Trimble's (1995) suggestion to change the spelling of "asteroseismology".
The MONS satellite will have two Star Trackers to sense the spacecraft attitude, and we plan to use them as scientific instruments to perform high-precision photometry of thousands of stars. We briefly describe the current plans for the Star Trackers and their expected capabilities.
K and M giants show variability on timescales from years down to days and possibly even hours. I discuss the contribution that can be made with high-precision photometry that will be obtained by the MONS Star Trackers. These include observations of flare-like events on Mira variables, and oscillation spectra for K giants and short-period M giants.
We have obtained time-series spectra for the pulsating hot subdwarf (sdB) PG1605+072. Previous time-series photometry of this star has shown maximum amplitude variations of about 0.1 mag and at least 50 periods. The pulsator has the largest amplitude and longest periods of all the pulsating sdBs (or EC14026 stars) discovered so far, and appears to be unusual in its class. Preliminary results of a search for velocity variations are presented here. With these variations, along with equivalent width measurements, oscillation mode identification should be possible.
Diffraction-limited images can be obtained with a large optical telescope using interferometry. One such method for objects of sufficient brightness is non-redundant masking (NRM), in which observations are made through a pupil mask that contains an array of small holes. However, NRM only uses a small fraction of the available light. Here I describe a method for Extended NRM in which a cylindrical lens allows interferograms from many one-dimensional arrays to be recorded side-by-side on a two-dimensional detector. For fainter objects, the holes in the aperture mask should be replaced by slits. In this case, the mask can be removed entirely, with the cylindrical lens effectively creating a continuous series of one-dimensional interferograms. This modified form of speckle interferometry, which I call MODS (Multiplexed One-Dimensional Speckle), is intermediate between NRM and conventional full-aperture speckle. An existing speckle camera can easily be converted to MODS observations by inserting a cylindrical lens. The feasibility of both MODS and Extended NRM are demonstrated using observations with MAPPIT at the Anglo-Australian Telescope.
We have been using a new method to search for solar-like oscillations that involves measuring temperature changes via their effect on the equivalent widths of the Balmer hydrogen lines. We observed alpha Cen A over six nights in 1995 with the 3.9-metre Anglo-Australian Telescope and the European Southern Observatory's 3.6-metre telescope in Chile. Oscillations were not detected, with an upper limit only slightly higher than the expected signal.
Despite numerous attempts and many hours of telescope time, there has so far been no confirmed detection of solar-like oscillations in any star except the Sun. We review recent efforts, with particular emphasis on the technique of monitoring equivalent widths of Balmer lines, including a description of the steps in data reduction.
The release of the Hipparcos catalogue provides an opportunity to check results from asteroseismology. Here we show that Hipparcos parallaxes for two stars are in good agreement with oscillation results. For eta Boo, a probable detection of solar-like oscillations by Kjeldsen at al. (1994; astro-ph/9411016) gave a frequency splitting in good agreement with the Hipparcos luminosity. For kappa_2 Boo, a delta Scuti variable, the Hipparcos parallax agrees well with the distance derived by Frandsen et al. (1995).
We review techniques for measuring stellar oscillations in solar-type stars. Despite great efforts, no unambiguous detections have been made. A new method, based on monitoring the equivalent widths of strong lines, shows promise but is yet to be confirmed. We also discuss several subtleties, such as the need to correct for CCD non-linearities and the importance of data weighting.
This tutorial gives a general introduction to optical and infrared interferometry, specifically addressing two questions: `Can I use VLTI to observe my favourite object?' and `What will it tell me?' The observables measured by an interferometer are fringe visibility and closure phase, which can then be fitted by a model or used to reconstruct an image. In order to measure fringes, the path lengths from the source to the detector via the different individual apertures must be equalized. This requires some form of fringe tracking, which constrains the type of observations that are feasible and has important implications for the complexity and size of the resulting image.
We have observed evidence for $p$-mode oscillations in the G0 IV star etaBoo (V = 2.68). This represents the first clear evidence of solar-like oscillations in a star other than the Sun. We used a new technique which measures fluctuations in the temperature of the star via their effect on the equivalent widths of the Balmer lines. The observations were obtained over six nights with the 2.5 m Nordic Optical Telescope on La Palma and consist of 12684 low-dispersion spectra. In the power spectrum of the equivalent-width measurements, we find an excess of power at frequencies around 850 microHz (period 20 minutes) which consists of a regular series of peaks with a spacing of $Δν=40.3$ microHz. We identify thirteen oscillation modes, with frequency separations in agreement with theoretical expectations. Similar observations of the daytime sky show the five-minute solar oscillations at the expected frequencies.
We have obtained H$α$ CCD images of 21 compact planetary nebulae in the galactic bulge. All objects are resolved and, after deconvolving the seeing disk, we find diameters of 1--2$''$ with typical uncertainties of 20\%. The values are generally in good agreement with radio measurements but are smaller than older optical measurements. Based on the new values, we find that the Shklovsky method for distance determination greatly overestimates the actual distance for all of our objects.
Although delta Scorpii is a bright and well-studied star, the details of its multiplicity have remained unclear. Here we present the first diffraction-limited image of this 0.12 arcsec binary star, made using optical interferometry, and resolve the confusion that has existed in the literature over its multiplicity. Examining published speckle measurements, together with the present result, reveals a periodicity of 10.5 yr and allows calculation of the orbital parameters. The orbit has a high eccentricity (e=0.82) and large inclination (i=70 degrees), making it a favourable target for radial velocity measurements during the next periastron (in 2000).