ING 2002 Scientific Highlights
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Scientific Highlights

Astronomical discoveries following from observations
carried out with the ING telescopes

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The adaptive optics system NAOMI on the WHT was used to take a remarkable image of a Near Earth Asteroid (NEA). On the night of August 17 to 18 NAOMI imaged the NEA 2002 NY40 just before its closest approach to the Earth. These are the first images of a NEA obtained with an Adaptive Optics system.

The asteroid was observed when it was only 750,000 kilometres away, twice the distance to the Moon, and moving rapidly across the sky at 65,000 kilometres per hour. Despite the technical difficulties this caused, the astronomers using the WHT obtained very high quality images in the near-infrared with a resolution of 0.11 arcseconds. This resolution is close to the theoretical limit of the telescope, and sets an upper limit to the size of the asteroid, which is only 400 metres across at the time of the observations.

  NEA 2002 NY40
  H-band (1.63 microns) NAOMI image of asteroid 2002 NY40 taken on the night of August 17 to 18, 2002. [JPEG | TIFF]

Near Earth asteroids are those that periodically approach or cross the orbit of our planet, and there is a very small probability that one could collide with the Earth. Measuring the size of asteroids helps astronomers understand their nature and how they formed, as well as the potential threat they pose. Variations in the brightness of 2002 NY40 suggest that it is highly elongated and is tumbling. Further monitoring of these variations will tell the astronomers whether the asteroid was viewed end-on or side-on, thus allowing them to determine the size and shape more precisely.

NAOMI is the first Adaptive Optics system on a UK telescope, and was built by a team from the University of Durham and the UK Astronomy Technology Centre. In good conditions, it can deliver images as sharp as those from the Hubble Space Telescope.

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The lithium content of halo stars near the main-sequence turnoff is of great importance for several reasons. The near constancy of the Li abundances, which are broadly independent of metallicity or effective temperature, means that these were hardly altered from the primordial value. That discovery prompted numerous studies over the ensuing two decades, with the aim of using the inferred primordial abundance as a constraint on the baryon density of the universe, Ω B.

Lithium is also important because it is a sensitive probe of mixing below the stellar surfaces. Since Li is destroyed in stars at relatively low temperatures, it survives in halo main-sequence turnoff stars only in a thin surface layer making up a few percent of the stellar mass.

Although the majority of halo main-sequence turnoff stars have almost identical Li abundances, about 7% have very low (thus far undetected) Li abundances. It has been unclear why these small numbers of stars should differ so significantly from the Li-normal stars. Their evolutionary states and the presence (or lack) of abundance anomalies for other elements had thus far failed to provide unambiguous evidence of their origin.

Astronomers analysed the Li abundances of 18 halo main-sequence turnoff stars. They found that four of them were ultra-Li-deficient objects. During detailed spectral analysis of other elements using UES on the WHT, they recognised that three of the Li-depleted stars, but none of the Li-normal stars, exhibited unusually broad absorption lines. They believe that this is due to rotational broadening.

In principle, most 14-billion-year-old stars do not spin very fast at all but these ones had up to 16 times as much spin energy as the Sun. The extra energy could come from only one source; another star.

When these stars formed out of the gas clouds, not just one but two stars formed very near one another. As they grew older, the smaller one captured the outer layers of the larger one. Very little now remains of what was the larger star; it has been cannibalised by its companion. The material captured by the companion carried orbital energy that was converted into spin energy. The scientists believe that the lithium was destroyed in nuclear reactions shortly before the star-eating episode occurred.

It was the discovery of the excessive spin energy that revealed the history of the objects and it explains one of the mysteries surrounding the Big Bang. Astronomers conclude that such objects must be avoided in studies of the primordial Li abundance and in investigations into the way normal single stars process their initial Li.

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Quiescent black hole X-ray transients provide the best evidence we have for the existence of stellar mass black holes within our own Galaxy. These X-ray binaries contain a relatively low-mass star accreting onto a likely black hole via Roche lobe overflow and an accretion disc. The gas becomes so hot that it glows with X-rays.

In their quiescent state the accretion flow becomes extremely faint and so the companion star can be directly observed. It is the companion, and in particular its radial velocity variations, that provides the key to measuring the black hole mass. It is believed that less gas is falling onto the black hole or neutron star at these times, but quiescent systems with black holes appear even fainter than the ones with neutron stars. This might be because energy is disappearing past the black hole's event horizon - the point of no return beyond which energy is irretrievably lost. But to be sure,
astronomers need to know more about how the dribble of gas flows onto the
black hole during the quiescent period.

To investigate this, a team of astronomers from UK and Spain
used the William Herschel Telescope and the Jacobus Kapteyn Telescope to look at the visible light from the gas disc of a quiescent black-hole X-ray binary star (V404 Cygni). The glow from the disc varied by a large amount - during flares lasting for a few hours, gas all around the black hole was lit up, most likely by X-rays shining on it.

The strongest flares involved development of asymmetry in the line profile, with the red wing usually strongest independent of orbital phase. Based on the line profile changes during the flares, the researchers conclude that the most likely origin for the variability is variable photoionization by the central source, although local flares within the disc cannot be ruled out.

So astronomers probing the intimate details of apparently quiescent stellar black holes
have discovered that in reality they are dynamic, lively places, subject to flares that briefly illuminate the whole of the gas disc around the black hole. These observations are helping to build up a picture of precisely where X-rays are generated in the gas as it heats up to extreme temperatures and swirls around under incredible gravitational forces before cascading into the black hole itself.

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Since the discovery of brown dwarfs (i.e., objects unable to burn hydrogen stably in their interiors and with masses below 72 MJup) both in the field and in young open clusters, many questions remain unanswered. A very important one is the minimum mass for the formation of very low mass objects in isolation, which would represent the bottom end of the Initial Mass Function (IMF) for free-floating objects. Very recent photometric and spectroscopic searches suggest that the IMF extends further below the deuterium burning mass threshold at around 13 MJup. This is usually referred as the "planetary-mass" domain. The least massive objects so far identified in young stellar clusters of Orion have masses around 5-10 MJup and cover the full range of the spectral type L.

Now a group of researchers have discovered a free-floating methane dwarf toward the direction of Orion. They present evidence for its membership in the σ Ori star cluster, which implies that this object is likely the least massive planetary-mass body imaged to date outside the solar system.

INGRID image of S Ori 70
An image of the σ Ori region. The multiple star σ Ori, which is visible with the naked eye, is at the center. A box indicates the position of the planet candidate, which is only 8.7 arcminutes from the star. The image was taken from the Digital Sky Survey and has a size of 23 x 22 square arcminutes. The inset shows the infrared image obtained using INGRID at the William Herschel Telescope. [ JPEG | TIFF ]

The candidate was selected from a JH near-infrared survey, in which the south western region of the young σ Ori cluster was targeted down to 3σ detection limit of JH approximate 21 mag. An area of 55.4 arcmin2 was covered with the near-infrared camera INGRID mounted at the Cassegrain focus of the William Herschel Telescope. This camera is equipped with a 1024 ×1024 Hawaii detector, which provides a pixel size of 0.242" projected onto the sky. The total integration time was 3240s in each of the J and H filters. S Ori 70, as it is called by the discovery team,  showed a rather blue J-H colour of about -0.1 mag and J=20.28 mag.

Based on the object's far-red, optical, and near-infrared photometry and spectroscopy, the astronomers conclude that it is a possible member of the σ Ori association. and if it is a true member of σ Ori, the comparison of the photometric and spectroscopic properties of S Ori 70 with state-of-the-art evolutionary models yields a mass of 3+5-1 Jupiter mass for ages between 1 and 8 Myr. The presence of such a low-mass object in the small search area would indicate a rising substellar initial mass function in the σ Ori cluster, even for planetary masses.

This discovery indicates that objects only slightly heavier than Jupiter may exist free-floating in σ Ori. Their formation process is not yet established. Theory predicts that opacity-limited fragmentation of cool gravitationally collapsing gas clouds is capable of producing 7-10 MJup Population I objects in isolation. Moreover, this minimum Jeans mass seems to be insensitive to changes in the opacity of protostellar clouds (amount of dust, size of grains, cosmic-ray flux). These models, however, do not include rotation, magnetic fields, and further external accretion onto the cloud fragment, which might alter the final mass of the nascent object. S Ori 70 is probably less massive than the minimum Jeans mass of 7-10 MJup and thus prompts us to refine the collapse-and-fragmentation models and/or to rethink possible formation mechanisms for such low-mass objects. Recently, several formation scenarios have been suggested that include tidal interactions and ejection of low-mass objects from multiple systems before brown dwarfs and planetary-mass objects can accrete enough gas to become stars. Others suggest that brown dwarfs are formed in the same way as more massive hydrogen burning stars, that is, by the process of supersonic turbulent fragmentation.

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Large ground and space telescopes combined with solid state detectors have revolutionized optical astronomy over the past two decades, yet deriving physical diagnostics of stars and galaxies still requires the somewhat indirect methods of filter photometry or dispersive spectroscopy to measure spectral features, energy distributions, and redshifts. The recent development of high-efficiency superconducting detectors has introduced the possibility of measuring individual optical photon energies directly. Many extensive observational programmes which aim at determining the large-scale structure of the Universe, and galaxy formation and evolution, demand high-efficiency extragalactic spectroscopy.

For the first time astronomers obtained optical measurements of spectral energy distributions of quasars using an imaging detector with intrinsic energy resolution. They also showed that they can determine their redshifts directly with excellent precision.

They observed 11 quasars in the redshift range z=2.2-4.1, the sample comprising relatively bright high-redshift Lyman-limit quasars from the published literature, supplemented by three lower redshift objects, two of which were discovered in objective prism-type surveys.

Observations used the ESA superconducting tunnel junction (STJ) camera, S-CAM 2, on the William Herschel Telescope. The camera is a 6×6 array of 25×25 µm2 (0.6×0.6 arcsec2) tantalum junctions, providing individual photon arrival time accuracies to about 5 µs, a resolving power of R approximate 8 at λ=500 nm, and high sensitivity from 310 nm (the atmospheric cutoff) to about 720 nm (currently set by long-wavelength filters to reduce the thermal noise photons).

They determined each quasar redshift by comparing the calibrated energy distribution with a single rest-frame composite quasar spectrum. Only one differed significantly from the literature value. It was discovered in a thin prism survey, classified as a possible quasar, and tentatively assigned a redshift of z approximate 2.30, but with an uncertain line identification. Although the quality of the S-CAM 2 fit was acceptable, the investigators obtained z=2.976. Subsequently they determined a spectroscopic redshift z=3.04, which agrees with the S-CAM 2 estimate to about 2%, and confirming that the literature value was incorrect.

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Last modified: 24 November 2011