ING Scientific Highlights in 1986-1987
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ING Scientific Highlights
in 1986-1987*

*Astronomical discoveries following from observations carried out with the ING telescopes

[ 1984-1985 Scientific Highlights | 1988 Scientific Highlights


A model of AC211Alone among the dozens of globular clusters visible in the northern sky, M15 in Pegasus harbors a bright, compact X-ray source. Images obtained several years ago by the Einstein X-ray satellite narrowed the location of the high-energy emitter - known as 4U 2127 + 12 - to within a circle some 7 arc seconds across in the cluster core. And despite the extraordinary concentration of stars there, a visible counterpart, AC 211,  was identified. AC 211 appears bright and variable at ultraviolet wavelengths. It bears little resemblance to ordinary cluster variables like pulsating Cepheids or RR Lyrae stars. Such objects are typically cool and red. In contrast, AC 211 is clearly very hot.

Is that enough to guarantee that AC 211 is the X-ray star? No, but a team of British astronomers led by Phillip A. Charles (Oxford University) discovered ionized-helium emission from the star, indicating the presence of gas heated by X-rays. As in X-ray binaries seen elsewhere in our galaxy, the intesely energetic emission presumably arises when gas from a normal star falls toward a neutron-star companion and heats up to temperatures as high as 10 million degrees Kelvin.

Comparison of the ultraviolet and X-ray light curves indicates that an accretion disk around the neutron star contributes a substantial fraction of the blue light from AC 211. Other properties of this remarkable system have recently come to light. T. Naylor (Oxford University) and co-workers, using sensitive optical spectroscopy, have found an absorption line due to neutral helium. It probably arises in the disk around the compact object. The observed wavelength of the feature shifts slightly back and forth once every 9 hours or so, consistent with the binary's orbital motion derived from ultraviolet and X-ray observations.

Astonishingly, however, the average line-of-sight velocity derived from optical the optical spectra is 150 kilometres per second smaller than that of M15 itself. This suggests that AC 211 is hurtling out of the cluster core. If it maintains its present velocity, it will leave the cluster altogether about 100,000 years from now.

More information

ING facilities involved: 

  • Isaac Newton Telescope using the Intermediate Dispersion Spectrograph with the 235-mm camera and the IPCS detector
Pictures: Some references: 
  • Charles, P.A., Jones, D.C. & Naylor, T., 1986, "An emission-line object in the core of M15", Nature, 323, 417.
  • Naylor, T., Charles, P.A., Drew, J.E. and Hassall, B.J.M., 1988, "Spectroscopy of the M15 X-ray source: discovery of binary motion and an unusual systemic velocity", MNRAS, 233, 285.
  • "M15's X-ray Binary", Sky & Telescope, November 1988, 459.



Plot of Z versus
Log M totalThe importance of finding regions of interstellar matter of relatively low metal content has been emphasized for diverse problems such as the evolution of galaxies and primordial nucleosynthesis. Also, the difficulty of finding young, unevolved galaxies showing oxygen abundances less than 0.1 of solar led some authors to hypothesize that bursts of star formation contaminate their surroundings with heavy elements, leading to a minimum observable metal content of apparently 0.1 solar.

It remains possible, however, that searches for low abundance galaxies have not been optimized for finding extremely low abundance regions. Recent searches focus on selecting the highest excitation (generally most metal poor) regions from surveys capable of detecting intense bursts of star formation. The chosen samples of objects failed to produce candidates with oxygen abundance less than 0.05 of solar.

In retrospect, it may be that the previous programs have been conducted backwards, i.e. it makes more sense to look for star formation regions in galaxies suspected of low abundance rather than vice versa. This above statement may appear fatuous by reason of the fact that one does not know, a priori, where to look for low abundance regions. However, as early as 1968, van der Bergh suggested that the average metal content was a function of total galaxian mass. Lequeux and collaborators were the first to show a good correlation between galaxian mass and oxygen abundance for Dwarf Irregular galaxies. Most subsequent studies show a good relation between the total galaxy mass and observed oxygen abundance for irregular galaxies. Wh this relationship has been challenged by some, a good test is to measure the oxygen abundance in extremely low mass gas rich galaxies.

Thus a spectrophotometric study of the HII regions in GR 8 was undertaken. GR 8 was discovered in search for dwarf galaxies in the Virgo cluster. The best estimate of its distance gives 1.0 Mpc, making GR 8 a Local Group member. Using a distance of 1.1 Mpc, an absolute magnitude of -10.7 is derived, establishing GR 8 as one of the faintest known gas rich irregulars.

The first series of observations were obtained in February 1984 with the SIT vidicon attached to the RC-spectrograph of the CTIO 4-m telescope. GR 8 was observed for a total of 5,400 seconds. Unfortunately in these spectra the measurement of the critical [OIII] 4363 Å line was uncertain, due to confusion with the strong Hg I 4368 Å night sky line. However, the temperature sensitive [OIII] [OIII] 4363 Å line was clearly detected in spectra taken with the IPCS in the IDS spectrograph of the Isaac Newton Telescope. The abundance analysis gives an electron temperature of 18,500 K and a relative oxygen content of 0.024 of solar. This is, within the errors, equal to the oxygen abundance in I Zw 18, the lowest abundance HII region known.

Confidence in the abundance-mass relationship for irregular galaxies may speed up the process in determining the primordial helium abundance and lead to a better understanding of the evolution of dwarf irregular galaxies.

More information

ING facilities involved:

  • Isaac Newton Telescope, using IDS and IPCS
Pictures: Some references: 
  • Skillman, D., Melnick, J., Terlevich, R., and Moles, M., 1988, "The extremely low oxygen abundance of GR 8: a very low luminosity dwarf irregular galaxy", Astron & Astrophys, 196, 31.
  • Terlevich, R., and Skillman, E., 1987, "The extremely low metal content of GR 8", Gemini Newsletter, 16, 4. 



Diffraction-limited imagesAstronomical imaging with the full diffraction-limited resolution of large optical telescopes has been an attractive though largely unfulfilled prospect ever since Michelson's pioneering work in the 1920s with the Mt. Wilson stellar interferometer. The principle of interferometric imaging is well known and straightforward - take light from a star incident on 2 small apertures; form interference fringes by combining the two beams; and measure the position (phase) and modulation (visibility) of the fringes for a range of interferometer baselines. A Fourier transform of the results the gives the brightness distribution of the source. The experimental problem is that, whereas the visibility can be measured with sensitive modern detectors, the fringe phase is entirely corrupted due to the different atmospherically disturbed optical paths from the source to the two apertures. For Michelson this was unimportant since stellar discs could be assumed to be symmetric and the expected fringe phase was either 0° or 180°; for imaging of objects of arbitrary structure it is a crucial difficulty.

The revival of interest in optical Michelson interferometry stems from the remarkable success it has had in radio astronomy. Aperture synthesis has been providing diffraction-limited images of ever increasing resolution over the past 30 years, but only because the radio interferometer phases are relatively undisturbed by atmospheric fluctuations. In VLBI, however, where the telescopes are separated by as much as 10,000 km, large and varying phase uncertainties do become important. The solution adopted by radio astronomers has been not to consider the corrupted interferometer phases themselves rather their sums round closed loops of baselines - the so-called closure phases. Such sums of phases are completely independent of the atmospheric contributions above each telescope; they neither represent the whole of the phase information nor do they present it in a convenient form, but fortunately they almost always uniquely constrain the images that fit the visibility amplitudes. Radio images, with dynamic ranges of several hundred to one, of complex jets in galactic nuclei have been made with arrays of fewer than 10 antennae. The high quality of these images encourages one to believe that the closure phase technique may be equally succesful for optical imaging.

The attainment of diffraction-limited images with large ground-based optical telescopes is an important objective for many astronomical programmes. The limited resolution set by atmospheric fluctuations in refractive index can be overcome by applying aperture symthesis and phase-closure techniques to short-exposure images taken through non-redundant aperture masks. 

Most previous attempts to obtain high-resolution astronomical images from the ground used short-exposure 'speckle' images obtained by using the whole telescope aperture. Some of these methods enable the amplitudes, but not the pases, of the spatial coherence function to be measured and hence does not in general permit unambiguous image reconstruction. Later developments have enabled some phase information to be retrieved and true images have been made in a few cases. The alternative approach is to mask the telescope mirror with an array of apertures each no larger than the scale size r0 of the atmospheric fluctuations. Short-exposure images can then be analysed to give both amplitudes and phases.

The discovery team had already demostrated that closure phases can be obtained at high light levels. They have made systematic observations to measure the spatial-coherence function with this technique at low photon rates and have derived high-resolution images from the data.

First observing run on the Isaac Newton Telescope, in November 1985, used the Image Photon Counting System (IPCS) together with the empty TAURUS box as a bench for the optical components. A 136-mm focal length lens behind the f/15 Cassegrain focus re-imaged the pupil at a diameter of 9.0 mm. The aperture mask consisted of four holes whose separations gave six uniformly space non-redundant baselines in one dimension: the hole diameter corresponded to 5.6 cm at the pupil and the unit baseline separation to 16 cm. After passing through the aperture mask the collimated beam was refocused onto the IPCS at an image scale of 1.24 arcsec/mm, giving 3.8 pixels per fringe for the finest fringes. An area of 512 pixels × 128 pixels was scanned every 16 ms and the coordinates of each photon recorded on magnetic tape. A 12-nm (FWHM) interference filter centred on 512 nm defined the bandwidth and no atmospheric dispersion corrector was used. Stellar profiles, observed with the whole of the telescope pupil, had widths of 1.2 arcsec, which corresponds to a value of  r0 of approximately 9 cm.

Two of the stars observed were Lambda Peg, a 3.95-m single star and Phi And, a binary system with a separation of 0.45 arcsec and magnitude difference of 1.2 m. Lambda is unresolved at a 1-m baseline, providing a good calibration source, whereas the wide double is well resolved at the maximum baseline.

Subsequent observing runs have taken advantage of the new RGO/RSRE imaging box. This contains two easily accessible optical benches for the reimaging and magnifying optics. Two more bunary stars were observed in July 1987. A linear 4-hole mask, similar to the one employed in the first experiments, was used in an almost identical instrumental configuration. Again both images are diffraction-limited and have good dynamic ranges. One of the major successes of the run is the image of Delta Equ. This is the first reconstruction from data obtained with a 2 m maximum baseline and has a resolution of 50 milliarcseconds.

The images produced show that good results can be obtained even at low photon rates. For the simplest image-plane interferometer, such as the one described here, a limiting magnitude as faint as +11 is expected to be achievable if larger apertures and wider bandwidths are used. By working at longer wavelengths, where both the seeing scale size and the atmospheric coherence time increase favourably, and by correcting for image motions in the focal plane, high-quality images should be obtained at much fainter limiting magnitudes .

More information

ING facilities involved:

  • Isaac Newton Telescope, using the IPCS detector and an own optical configuration 
Pictures: Some references: 
  • Baldwin, J.E., Buscher, D.F., Warner, P.J., Haniff, C.A., Mackay, C.D., 1988, "Optical Aperture Synthesis on the INT", Gemini Newsletter, 20, 2.
  • Haniff, C.A., Mackay, C.D., Titterington, D. J., Sivia, D., Baldwin, J.E. & Warner, P.J., 1987, "The first images from optical aperture synthesis", Nature, 328, 694.
  • Haniff, C.A., 1987, "Milliarcsecond ground-based imaging with single telescopes", Proc. ESA workshop: Optical interferometry in space, (ESA SP-273), 171

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Last modified: 13 December 2010