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ING Annual Report 1998
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Scientific Highlights
The following presents a selection of highlights, intended to be representative of the scientific quality and range of research being undertaken.
THE UNIVERSE WILL EXPAND FOREVER
WHT+ISIS, INT+PFC
New studies of supernovae in the farthest reaches of deep space indicate that the universe will expand forever because there isn't enough mass in the universe for its gravity to slow the expansion, which started with the Big Bang.
This result rests on analysis of 42 of the roughly 78 type Ia supernovae so far discovered by the Supernova Cosmology Project(1). By the time the light of the most distant supernova explosions so far discovered by the team reached telescopes on Earth, some seven billion years had passed since the stars exploded. After such a journey the starlight is feeble, and its wavelength has been stretched by the expansion of the universe, i.e. red-shifting its wavelength. By comparing the faint light of distant supernovae to that of bright nearby supernovae, one could tell how far the light had travelled. Distances combined with redshifts of the supernovae give the rate of expansion of the universe over its history, allowing a determination of how much the expansion rate is slowing. Although not all type Ia supernova have the same brightness, their intrinsic brightness can be determined by examining how quickly each supernova fades.
Since the most distant supernova explosions appear so faint from Earth, last for such a short time, and occur at unpredictable intervals, the Supernova Cosmology Project team had to develop a tightly choreographed sequence of observations to be performed at telescopes around the world, among them, the Isaac Newton and the William Herschel telescopes. While some team members are surveying distant galaxies using the largest telescopes in Chile and La Palma, others in Berkeley are retrieving that data over the Internet and analysing it to find supernovae. Once they detect a potential supernova they rush out to Hawaii to confirm its supernova status and measure the redshifts using the Keck telescope. Meanwhile, team members at telescopes outside Tucson and on La Palma are standing by to measure the supernovae as they fade away. The Hubble Space Telescope is called into action to study the most distant of the supernovae, since they are too hard to accurately measure from the ground.
Reaching out to these most distant supernovae teaches us about the cosmological constant. If the newly discovered supernovae confirm the story told by the previous 42, astrophysicists may have to invoke Einstein's cosmological constant to explain the observed accelerated expansion of the universe. This cosmological constant has nowadays an interpretation in terms of vacuum energy density which works against gravity to produce the observed accelerated rate of expansion.
(1)The Supernova Cosmology Project is a collaboration between the following institutions: Lawrence Berkeley National Laboratory (USA), Institute of Astrophysics, Cambridge and Royal Observatory of Edinburgh (UK), LPNHE, Paris and College de France, Paris (France), University of Barcelona (Spain), and Isaac Newton Group, La Palma (UK and The Netherlands), Stockholm University (Sweden), ESO (Chile), Yale University (USA) and STscI (USA).
References:
- S Perlmutter et al, 1997, "Measurements of the Cosmological Parameters Omega and Lambda from the First Seven Supernovae at z >= 0.35", Astrophys J, 483, 565.
- S Perlmutter et al, 1998, "Discovery of a supernova explosion at half the age of the universe", Nature, 391, 51.
- S Perlmutter et al, 1999, "Measurements of Omega and Lambda from 42 High-Redshift Supernovae", Astrophys J, 517, 565.
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Top: The observing strategy allows the team to find sets of high-redshift supernovae on the rising part of their light curves and guarantees the date of discovery, thus allowing follow-up photometry and spectroscopy of the transient supernovae to be scheduled. The supernova light curves are then followed with scheduled R-, I- and some B-band photometry at the INT and other telescopes. Top left: INT image of a high-redshift type Ia supernova thousands of millions of light years away. When a star explodes as a type Ia supernova its brightness is similar to the host galaxy. This latter feature along with the possibility of calibrating their maximum brightness, make type Ia supernovae the best known standard candles to investigate the geometry and the dynamics of our universe.
Middle left: Best-fit confidence regions in the OmegaMass – OmegaLambda plane. The 68%, 90%, 95%, and 99% statistical confidence regions are shown. Note that the spatial curvature of the universe — open, flat, or closed — is not determinative of the future of the universe's expansion, indicated by the near-horizontal solid line. In cosmologies above this near-horizontal line the universe will expand forever, while below this line the expansion of the universe will eventually come to a halt and recollapse. The upper-left shaded region, labelled 'no big bang', represents 'bouncing universe' cosmologies with no big bang in the past. The lower right shaded region corresponds to a universe that is younger than the oldest heavy elements for any value of H0>=50 kms-1Mpc-1.
Bottom left: Isochrones of constant H0t0, the age of the universe relative to the Hubble time, H0-1, with the best-fit 68% and 90% confidence regions in the OmegaMass – OmegaLambda plane. The isochrones are labelled for the case of H0=63 kms-1Mpc-1. If H0 were taken to be 10% larger, the age labels would be 10% smaller. The diagonal line labelled accelerating/decelerating is drawn for q0=OmegaMass/2 – OmegaLambda =0 and divides the cosmological models with an accelerating or decelerating expansion at present time. A value of OmegaLambda non-equal to zero is favored from the data of all the observed supernovae.
Bottom: Hubble diagram (effective B-magnitude at maximum versus redshift) containing 42 high-redshift supernovae (red dots) that could be width-luminosity corrected, and 18 from the lower-redshift Calán/Tololo Supernova Survey. Magnitudes have been K-corrected, and also corrected for the width-luminosity relation. The inner error bar corresponds to the photometry error alone, while the outer error bar includes the intrinsic dispersion of type Ia supernovae after stretch correction. The solid curves indicate theoretical model predictions based on different cosmological parameters.
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THE FAINTEST KUIPER BELT OBJECTSINT+WFC
Starting in 1992, astronomers have become aware of a vast population of small bodies orbiting the Sun beyond Neptune. There are at least 70,000 "Trans-Neptunians" with diameters larger than 100 km in the radial zone extending outwards from the orbit of Neptune (at 30 AU) to 50 AU. There may be many more similar bodies beyond 50 AU, but these are presently beyond the limits of detection. This population is generally referred to as the Kuiper Belt.
The Kuiper Belt holds significance for the study of the planetary system on at least two levels. First, it is likely that the Kuiper Belt objects are extremely primitive remnants from the early accretion phases of the Solar System. The inner, dense parts of the pre-planetary disk condensed into the major planets, probably within a few millions to tens of millions of years. The outer parts were less dense, and accretion progressed slowly. Evidently, a great many small objects were formed. Second, it is widely believed that the Kuiper Belt is the source of the short-period comets. It acts as a reservoir for these bodies in the same way that the Oort Cloud acts as a reservoir for the long-period comets.Recently, two new Kuiper Belt objects have been discovered, named 1997 UG25 and 1997 UF25, and they are some of the faintest objects ever seen orbiting our sun. One is estimated to be 150 km across and the other 110 km. Both are about 45 times farther from the Sun than Earth (4,200 million miles or 6,750 million kilometres), and more remote than the planet Pluto.
Based on present ideas about how Kuiper Belt objects formed, astronomers expected to be finding these faint objects at even greater distances. Since they did not, those ideas may need to be revised. It may be that the average size of the Kuiper Belt objects is smaller the farther away they are, so the most distant ones were too faint even for this survey. Or it might be that the objects actually discovered mark the outer edge of the Kuiper Belt.
The discovery team used the prime focus Wide-Field Camera at the Isaac Newton Telescope to image the sky for 7 nights, searching a total area slightly smaller than that covered by the full Moon. During each night they stared continuously at different patches of sky for up to four hours at a time. In each patch of sky several thousand distant stars and galaxies could be seen. However even these images were not sensitive enough to record the Solar System objects the team were seeking, so they combined the images by computer in a way that eliminated all stars, galaxies and nearby asteroids and revealed only faint solar-system objects at large distances from the Sun.
References:
- E Fletcher, M Irwin, and A Fitzsimmons, 1998, "1997 UF25", MPEC 1998-G08.
- E Fletcher, M Irwin, and A Fitzsimmons, 1998, "1997 UG25", MPEC 1998-G09.
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Left: Discovery image of 1997 UG25. The trans-neptunian object is the stellar-like object in the centre of the image. 1997 UF25 was discovered in images obtained on the 25th/26th October 1997. At a red magnitude of 25.0, it is so faint that it was only discovered by co-adding roughly 20 images of the same field. From observations over two nights, a distance of 44.9 AU was calculated. Right: Discovery image of 1997 UF25. Again, the trans-neptunian object is the stellar-like object in the centre of the image. At a red magnitude of 24.5, it was found in a similar manner to 1997 UF25. It may never be seen again, but was at a distance of around 43.3 AU at discovery.
THE BRIGHTEST OBJECT EVER OBSERVED
INT+IDS, JKT+JAG CCD
APM 08279+5255 is an extremely bright quasar four to five million, billion times brighter than the Sun and about 100 times brighter than the next brightest object that has been observed. The light from the quasar has been travelling to us for roughly 11 billion years, nearly 90% of the age of the universe and set out on its long journey when the universe was only about 10% of its present age.
The only way such a huge amount of energy could be generated is from accretion of dust and gas particles onto a super massive black hole, located at the centre of the quasar. The object's apparent brightness actually comes from two different regions around the black hole. Light in the ultraviolet and optical range comes directly from an accretion disk surrounding the super massive black hole. Gas and dust and even entire stars are attracted by the black hole's gravitation and generate energy, including light, from friction as they are torn apart and fall toward the black hole.
The second source of brightness, in the infrared portion of the spectrum, comes from dust further away from the central engine, which is heated by radiation from the centre of the quasar and which re-radiates this radiation at much longer wavelengths in the infrared.
Quasars are generally the most energetic objects observed in the universe. Each quasar generates more energy than the rest of a galaxy's stars combined. Yet a quasar, its accretion disk and the glowing dust surrounding it occupy a relatively small amount of space, not much larger than the size of the Solar System.
Spectral energy distribution of APM 08279+5255 (z=3.87) compared to the ultra-luminous galaxies IRAS FSC 10214+4724 (z=2.29), SMM 02399-0136 (z=2.8), and H 1413+117 (z=2.56). [ JPEG | TIFF ] Most quasars are not bright enough to reveal this strong infrared signature. However a few, much closer, ultra-luminous galaxies have similar properties. By comparing the newly discovered object with these fainter nearby well studied examples, it is possible to weigh the amount of dust in the object and find a staggering value of almost a billion solar masses. This is more than the entire dust mass in the Milky Way, yet has been created and accreted in a small fraction of the time and is contained in a volume the size of the Solar System.
Since this quasar is such a powerful beacon of light and has travelled 11 billion light years, it can also be used to investigate intervening objects that leave an imprint on the light from the quasar. By studying these imprints we can learn what conditions in the early universe were like and measure how primordial gas was converted into the stars and galaxies that we see around us today.
It is possible that some of these intervening absorbing systems may have acted as giant gravitational lenses and magnified the light from the quasar. Gravitational lenses are often seen to be the cause of apparently extremely bright objects. Typically, such a lens might exaggerate the real light level by a factor of 30 or 40, which however in this case, would still make APM 0827+5255 an order of magnitude brighter than its nearest competitor.
References:
- A Blain, 1998, "Through a glass brightly", Nature News and Views, 393, 520.
- M J Irwin et al, 1998, "APM 08279+5255: an ultraluminous broad absorption line quasar at a redshift z=3.87", Astrophys J, 505, 529.
DISCOVERY OF A LOW-MASS BROWN DWARF COMPANION OF A YOUNG NEARBY STARWHT+ISIS, INT+IDS
Direct imaging searches for brown dwarfs and giant planets around stars explore a range of physical separations complementary to that of radial velocity measurements and provide key information on how substellar-mass companions are formed. Any companion uncovered by an imaging technique can be further investigated by spectroscopy, which allows information about its atmospheric conditions and evolutionary status to be obtained. Young, nearby, cool dwarf stars are ideal targets of searches for substellar-mass companions (brown dwarfs and giant planets) using direct imaging techniques, because (i) young substellar objects are considerably more luminous when undergoing the initial phases of gravitational contraction than at later stages; (ii) stars in the solar neighborhood (that is, within 50 pc of the Sun) allow the detection of faint companions at physical separations of several tens of astronomical units; and (iii) cool stars are among the least luminous stars, which favors full optimization of the dynamic range of current detectors to achieve detection of extremely faint companions by means of narrow-band imaging techniques at red wavelengths.
Using X-ray emission as an indicator of youth, a number of late-type stars (K and M spectral classes) was selected in the solar neighbourhood, of which deep images were obtained. After several targets of the programme were observed, a very red companion to the high-proper-motion M-class dwarf star G 196-3 was discovered 16.2 arcsec away from the star. This red companion was called G 196-B. Further photometry and spectroscopy allowed the astronomers to constrain spectral classification and proper motions of both stars, coming to the conclusion that G 196-3 is a M2.5 star and G 196-3B is a L brown dwarf. From the comparison with other known brown dwarfs they derived a temperature of 1800±200 K.
The observed optical and infrared colors present no strange anomaly that might be attributed to an unresolved less massive companion to G 196-3, and no indication of changes in the radial velocity is found beyond the uncertainties of the measurements determined with high-resolution spectra taken at the Isaac Newton Telescope over a time interval of several hours to days. This makes it very unlikely that the star is actually a close-contact binary. The spectral type combined with the observed fluxes indicate that the star is at a minimum distance of 15.4 pc.
An upper limit to the age of G 196-3 can be imposed from comparison to the Hyades cluster (600 My), where the average chromospheric and coronal emission of M2-M3 stars is considerably lower than in G 196-3. This star appears to be substantially younger than the Hyades, and hence 300 My is adopted, an age intermediate between that of the Pleiades and Hyades, as a reasonable upper age limit. The lower age limit can be derived from observations of Li I at 670.8 nm. Lithium is a fragile element that burns efficiently in the interiors of fully convective stars over short time scales (a few tens of millions of years). Convection drains material from the stellar atmosphere into the innermost layers, where the temperature is high enough for Li burning to take place. There are several models in the literature that predict the Li depletion rate as function of mass for low-mass stars and give consistent results. A search was made for the Li I line in G 196-3, and an optical spectrum was obtained with the Intermediate Dispersion Spectrograph. An upper limit on the equivalent width of 0.005 nm was imposed, which gives a Li depletion factor larger than 1,000 with respect to its original abundance. This constrains the age of the star to be older than 20 My. All these considerations provide a most likely age for G 196-3 that locates the star in the pre-main sequence evolutionary phase and thus at a more luminous stage than expected for its main-sequence lifetime. According to the age range derived, the most probable distance from Earth to the system is 21±6 pc, the minimum value corresponding to the case of the primary star already on the main sequence and the maximum distance taking into account the youngest possible age.
Assuming this distance interval, the luminosity of the companion G 196-3B can be estimated from the measured I and K magnitudes and the K bolometric correction as a function of the colour (I through K). The values obtained are log L/Lo = 4.1 when the oldest age (main sequence) is assumed and log L/Lo = 3.6 for the youngest age (Lo, Sun luminosity). The comparison of the optical and infrared magnitudes with the recent evolutionary tracks, which include dust condensation, allows the astronomers to conclude that the mass of G 196-3B is 25–10+15 Jupiter masses (MJup), where the upper and lower values result from the age limits discussed above.
An independent confirmation of the substellar nature of this faint companion was achieved with the detection of the Li I resonance doublet at 670.8 nm. An intermediate-resolution optical spectrum was obtained at the William Herschel Telescope using the ISIS double-arm spectrograph. The equivalent width of the doublet is 0.5±0.1 nm that, using model atmospheres, gives an atmospheric abundance consistent with no depletion at all of Li. The presence of Li, combined with the low atmospheric temperature, rules out the possibility that the object is a star. Any brown dwarf with a mass below 65 MJup should preserve its initial Li content for its entire lifetime, and an object with such a small mass as that of G 196-3B should necessarily show a high Li content. Although in more massive substellar objects the presence of Li would help to determine its evolutionary stage more precisely through the time dependence of Li burning, for our object this detection provides a necessary check of consistency.
Intermediate-resolution spectrum obtained at the WHT telescope showing the Li detection at 670.8 nm in G 196-3B. [ TIFF ] The distance to the system implies a physical separation between the two components of more than 250 AU, being 350 AU at 21 pc. It could be even larger if the system were younger and therefore more distant from the Sun. This large distance and the high mass ratio of 16:1 between the two components favor the fragmentation of a collapsing cloud as the most plausible explanation for the formation of the system. The possibility cannot be excluded, however, that the accretion of matter in a protoplanetary disc may produce an object more massive than 15 MJup at such large distances. Accretion discs extending up to several hundred astronomical units are known to exist around several stars. Surveys similar to that conducted here will provide a statistically significant number of substellar-mass companions that can be used to test the proposed formation mechanisms and may well promote the development of new ideas, as occurred because of the recent findings of giant planets with highly eccentric orbits around solar-type stars.
References:
- R Rebolo et al, 1998, "Discovery of a Low-Mass Brown Dwarf Companion of the Young Nearby Star G 196-3", Science, 282, 1309.
AN ARC OF EXTENDED EMISSION IN THE GRAVITATIONAL LENS SYSTEM Q2237+0305WHT+INTEGRAL
The quadruple system of images Q2237+0305 at z=1.695 is one of the most interesting gravitational lens system owing to the proximity of the lens galaxy and to the high degree of symmetry for which it is also named the 'Einstein Cross'.
Two-dimensional spectroscopy of this system was obtained with the INTEGRAL fiber system in subarcsecond seeing conditions. The four components of the system, compact QSO images, appeared clearly separated in the continuum intensity maps. However, the intensity map of the C III] 1909 Å line exhibited an arc of extended emission connecting three of the four refracted components. This result can be explained if, as is usually assumed, the continuum arises from a compact source <0.05 pc in extent in the nucleus of the object while the line emission comes from a much larger region. A lens model fitted to the positions of the four compact images also accounts for the arc morphology. In the framework of this model, the region generating the C III] 1909 Å emission would have dimensions of about 400 h-1 pc across. The astronomers interpret the observed arc as a gravitational lens image of the extended narrow line region of the source.
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Left: Image of the Einstein Cross obtained with INTEGRAL. The four components of the lens system can be seen, and superimposed on these, the spectra obtained with INTEGRAL at the William Herschel Telescope. Right: Intensity map of the emission in the C III] 1909 Å line. Orientation is as in the figure on the left. Isophotes are linearly scaled between 0.02 and 0.2 with steps of 0.02 (units are arbitrary). These results add to the observational domain a new type of gravitational lens system for integral field spectroscopy, where the lens galaxy images the extended narrow line region of the lensed QSO host.
INTEGRAL is an optical fiber system for two-dimensional spectroscopy which links the Nasmyth focus of the WHT with the WYFFOS spectrograph. INTEGRAL was designed and built by the Instituto de Astrofísica de Canarias in collaboration with the Royal Greenwich Observatory and the ING.
References:
- E Mediavilla et al, 1998, "Two-dimensional spectroscopy reveals an arc of extended emission in the gravitational lens system Q2237+0305", Astrophys J, 503, L27.
- S Arribas et al, 1998, "INTEGRAL: a matrix optical fiber system for WYFFOS", Proc SPIE, 3355, 821.
A STRIPPED-DOWN STELLAR COREWHT+ISIS
In most binary systems, the component stars evolve essentially independently. But in others, the stars are so closely interacting that regular stellar evolution is disrupted. AL Comae Berenices, a 21st-magnitude object, consists of a 20,000 K white dwarf and a ruddy, low-mass companion star. That companion has been steadily losing material to an accretion disk around the white dwarf. The two stars are closer to each other than Earth is to its Moon; they orbit one another every 82 minutes.
What sets AL Comae apart from other cataclysmic binaries is the amount of mass shed by the white dwarf's companion. Although it seems to have started out as a type-G or K dwarf with a mass approaching our Sun's, the astronomers place the companion's present-day mass at a mere 0.04 to 0.09 solar masses (40 to 90 Jupiters). This mass value is similar to those of brown dwarfs and of some massive planets in other solar systems.
But AL Comae's lightweight constituent is neither of these. Rather, it appears to be an exposed stellar core whose interior was mixed up as it lost mass to its white-dwarf companion. This notion is buttressed by the system's hydrogen-poor spectrum and by the companion's inferred density: a thousand times smaller than a white dwarf's, yet a hundred times larger than that of a regular main-sequence star.
References:
- S B Howell et al, 1998, "Time-resolved Spectroscopy of AL Comae", Astrophys J, 494, L223.
PALOMAR 1: A YOUNG GALACTIC HALO GLOBULAR CLUSTERINT+PFC, IDS
Globular clusters are well known as the oldest conglomerations of stars in the Milky Way. Once thought all to have formed at roughly the same time, a small number of these clusters were recently found to have ages at least 3 Gyr younger that their siblings.
According to deep V and I CCD images of the loosely populated galactic globular cluster Palomar 1 and the surrounding field obtained with the Isaac Newton Telescope, an estimated age at 6.3 to 8 Gyr was derived. That makes Palomar 1 just over half as old as typical globulars and the youngest Galactic globular cluster identified so far. Also surprising are its comparatively low luminosity and uncrowded population of stars — unusual traits for a globular.
Central 4.3' × 4.3' image of Palomar 1, taken with the I filter and an exposure time of 600 s. [ TIFF ] The astronomers have discussed the possibility that Palomar 1 is in fact a very old open cluster. But that would be an even worse fit for its properties. Furthermore, Palomar 1's location in the outer halo, about 55,000 light-years from the Galaxy's centre, would be difficult to reconcile with an open-cluster classification. An alternative explanation, which may also account for the other young globular clusters, could be a different formation process. Most globulars are thought to have coalesced at the same time as the Galaxy itself. The younger ones, on the other hand, may have come in three other ways: as gas clouds that survived in the halo after the Milky Way's formation, later to form stars; as captured intergalactic star groups; or as cannibalised dwarf galaxies.
References:
- A Rosenberg et al, 1998, "Palomar 1: Another Young Galactic Halo Globular Cluster?", Astron J, 115, 648.
- A Rosenberg et al, 1998, "The Metallicity of Palomar 1", Astron J, 115, 658.
THE SEARCH FOR EXTRASOLAR PLANETSPlanetary Systems: Their Formation and Properties
WHT, INT, JKT
The team EXPORT (EXoPlanetary Observational Research Team) was awarded the 1998 International Time Programme 'Planetary Systems: their formation and properties'. The project focused on formation and evolution of planetary systems, the search for spectral signatures of extrasolar planet atmospheres, and planet searches.
Formation and evolution of planetary systems. An enormous data set including high and intermediate resolution spectroscopy, optical photopolarimetry and near-IR photometry of Herbig AeBe, T Tauri, UXORs and Beta Pic-like stars at different stellar evolutions stages was collected. The data were taken simultaneously, which is crucial since many of the phenomena that have to do with preliminary stages in planet formation are variable. Monitoring took place on time-scales of one night (hours), consecutive nights (days), different runs (months). The wide spectral coverage allowed the astronomers to study the behaviour of many transitions of different species and to establish correlations — if any — with broad-band photometry and polarimetry. A large number of the spectra show interesting and puzzling events: evidence of infalling solid bodies (possibly comets) by red-shifted Ca II and Na I components, rotating (possibly stable condensed structures) by flips in H-alpha components, and many other dynamical features. This data set seems to contain a key for the understanding of disk structures that might ultimately lead to planet formation.
Spectral signatures of extra-solar planet atmospheres. After the detection of massive planets in close orbits around their star the question of their nature is raised. Although it seems that these planets are large gas giants, similar to Jupiter, large solid planets cannot be ruled out. For these observations, the astronomers adopted the hypothesis that the planets around the stars 51 Peg and tau Boo are large gas giants. Due to interaction with the UV flux of the star, stellar wind, or thermal escape, atoms and molecules of their atmosphere may escape and fill a large volume around the planet. Such extended exosphere could be detected by obtaining spectra of the stars during transit of their planets through the Earth-star line. Spectra of 51 Peg and tau Boo were obtained at the WHT using UES covering several atomic and ionic transitions of potential constituents in a Jupiter-like planet. The stars were observed on two consecutive nights: during transit of the planet and when the planet was not in the line-of-sight. The analysis of the spectra includes a very careful comparison of spectra taken on and off-transit.
Search for planets. More than a dozen exo-planets haven been reported. These detections are based on the radial velocity method. Two competitive observing strategies are microlensing and planet transit searches in clusters. Both techniques can be carried out with small 1-m class telescopes. The JKT was hence used to obtain CCD images of two open clusters. The observing strategy consisted in taking R band images of the same cluster position, each image corresponding to a 10 min exposure. Several hundred images were obtained with roughly 1,000 stars within the field of view. This large amount of data provided very accurate light curves of the cluster stars. Many images have already been tested for transit events and several possible candidates have been found.
Extrasolar Planetary Transits
JKT+JAG CCD, INT+PFC
The TEP (Transits of Extrasolar Planets) network has been observing photometrically the eclipsing binary CM Dra since 1994. This is the first long-term observational application of the transit method for the detection of extrasolar planets.
The transit method is based on observing small drops in the brightness of a stellar system, resulting from the transit of a planet across the disk of its central star. Such transits would cause characteristic changes in the central star's brightness and, to a lesser extend, colour. The depth of a transit is proportional to the surface area of the planet, and the duration of a transit is indicative of the planet's velocity. If the central star's mass is known, the distance and period can be obtained with great precision.
Previous observational tests have been prevented by the required photometric precision (which is about 1 part in 105 in the case of an Earth-sized planet transiting a sun-like star), and by the generally low probability that a planetary plane is aligned correctly to produce transits. An observationally appealing application is available with close binary systems, where the probability is high that the planetary orbital plane is coplanar with the binary orbital plane, and thus in the line of sight. This makes the observational detection of planetary transits feasible in systems with an inclination very close to 90º.
The CM Dra system is the eclipsing binary system with the lowest mass known. The total surface area of the system's components is about 12% of the Sun's, and the transits of a planet with 3.2 RE (Earth radii), corresponding to 2.5% of the volume of Jupiter, would cause a brightness drop of about 0.01 magnitudes, which is within easy reach of current differential photometric techniques. The low temperature of CM Dra also implies that planets in the thermal regime of solar system terrestrial planets would circle the central binary with orbital periods on the order of weeks. This allows for a high detection probability of planetary transits by observational campaigns with coverages lasting more than one planetary period. Planets with orbital periods of 10-30 days around CM Dra are especially interesting, since they would lie within the habitable zone, which is the region around a star where planetary surface temperatures can support liquid water, and therefore the development of organic life. CM Dra is relatively close (17.6 pc) and has a near edge-on inclination of 89.82º. With this inclination, coplanar planets within a distance of CM Dra of 0.35 AU aproximately will cause a transit event. This maximum distance corresponds to a circular orbit with a period of about 125 days. There is also a low probability of observing orbits from planets inclined out of CM Dra's binary orbital plane, if the ascending or descending nodes of the planetary orbits are precessing across the line of sight.
To obtain sufficient observational coverage, the TEP network was formed with the participation of several observatories in 1994. The final lightcurve contains 17,176 points acquired over three years, and gives a complete phase coverage for CM Dra. Six suspicious events, one of them detected by the JKT, were found by planets with sizes between 1.5 and 2.5 RE. Such events are typified by being temporary faintenings of CM Dra's brightness by a few milimagnitudes, with normal durations of 45-90 minutes. However, none of these events has amplitudes compatible with planets larger than 2.5 RE. Planets smaller than 1.5 RE cannot be detected in the data without a sub-noise detection algorithm. A preliminary signal detection analysis shows that there is a 50% detection confidence for 2 RE planets with a period from 10 to 30 days with the current data.
References:
- H J Deeg et al, 1998, "Near-term detectability of terrestrial extrasolar planets: TEP network observations of CM Draconis", Astron Astrophys, 338, 479.
Planetary transit event candidate as observed by the JKT. The lightcurve is plotted against the phase of CM Dra. The data are shown as squares; the line indicates a smoothing fit to the data. [ TIFF ] OTHER HIGHLIGHTS
Observations carried out by the Isaac Newton Telescope greatly improved the orbit determination of two recently discovered distant irregular moons of Uranus (B J Gladman et al, 1998, "Discovery of two distant irregular moons of Uranus", Nature, 392, 897). Both moons, S/1997 U1 and S/1997 U2, are unusually red in colour, suggesting a link between these objects —which were presumably captured by Uranus early in the Solar System's history— and other recently discovered bodies orbiting in the outer Solar System.
The INT and the Prime Focus Camera participated in a new Whole Earth Telescope (WET) campaign to observe AM Canum Venaticorum. 143.2 hours of time-series photometry were collected over a 12-day period. Thanks to the detection of 5 harmonically related frequency modulations, a successful disco-seismological interpretation was achieved for the first time (J-E Solheim et al, 1998, "Whole Earth Telescope observations of AM Canum Venaticorum — discoseismology at last", Astron Astrophys, 332, 939).
More evidence for a population of intracluster planetary nebulae in the Virgo cluster was found by observing a blank, 50-square-arcminute patch of the Virgo Cluster's core using the Prime Focus Camera on the WHT. A planetary nebula can be seen only for a few thousand years of a star's multi-billion-year life, and only the brightest planetaries can be detected at the cluster's distance (50 million light-years). Consequently, the planetaries found may indicate that a significant fraction of the Virgo Cluster's stars are intergalactic (R H Méndez et al, 1997, "More evidence for a population of intracluster planetary nebulae in the Virgo Cluster", Astrophys J, 491, L23).
From high resolution spectra obtained with the UES, astronomers have found that the spectral type of the post-red supergiant IRC+10420 changed from F8I+ in 1973 to early A type in 1994, and is probably en route to landing among the Wolf-Rayet stars. So far IRC+10420 is the only object known in this transition phase (R D Oudmaijer, 1998, "High resolution spectroscopy of the post-red supergiant IRC+10420", Astron Astrophys Suppl, 129, 541).
Pulsars spin from several dozen times to a second to once every few seconds. Their rotation is left over from their birth, when the core of a massive star collapsed in a supernova explosion. Millisecond pulsars however weren't born that way; a collapsing supernova core might not have that much angular momentum. Something probably spun them up later. The usual explanation has been that a close companion star transferred angular momentum to the pulsar by pouring mass onto it. A binary system called SAX J1808.4-3658 has finally been caught in the midst of this spin-up process. In April this object flared up for several weeks and astronomers examined it with the Rossi X-Ray Timing Explorer, finding that its X-ray output pulses 401 times per second. Announcements followed and quickly brought other teams into the chase. Finally the JKT identified a 16.6 magnitude star at the X-ray source's location as the optical counterpart of the X-ray transient (P Roche, 1998, "SAX J1808.4-3658 = XTE J1808-369", IAU Circular 6885), which allowed other astronomers to carry out optical observations confirming the binarity of the system. The entire picture of SAX J1808-3658 was assembled by independent teams in just 10 days and it's the first binary millisecond X-ray pulsar ever found.
Motivated by recent discoveries of nearby galaxies in the Zone of Avoidance (e.g. Dwingeloo 1 discovered by the INT in optical light in 1994) a new search for more new galaxies through the dusty, obscuring plane of the Milky Way was carried out. 10 out of 18 candidates were confirmed spectroscopically as galaxies by the INT, making for a better than 50 percent success rate (O Lahav, 1998, "Galaxy candidates in the Zone of Avoidance", MNRAS, 299, 24).
Thanks to ISIS observations of the cataclysmic variable BZ Camelopardalis, astronomers were able to measure the acceleration law of a cataclysmic variable wind for the first time. They found that the velocity increases linearly with time in 6 to 8 minutes after starting near rest. They also found a subsequent linear deceleration to nearly rest in 30-40 minutes (F A Ringwald and T Naylor, 1998, "High-speed optical spectroscopy of a cataclysmic variable wind: BZ Camelopardalis", Astron J, 115, 286).
The ING telescopes continued to search for optical counterparts of Gamma-Ray Bursts and follow them up photometrically. The wide field of view and sensitivity of the imaging instruments on the WHT, INT and JKT, along with a rapid response thanks to the override programme, allowed the astronomers to observe and make stronger restrictions on the possible models for GRBs (IAU Circulars 6806, 6848, 6855).
A spectroscopic study of seven galactic H II regions was performed using the INT, with the aim of determining and comparing the gaseous iron abundance in nebulae located at different galactocentric distances and characterised by different physical conditions. The resulting iron abundances relative to oxygen are found to be 3 to 30 times lower than the solar value, implying that most of the iron atoms are depleted on to the dust grains known to coexist with the ionised gas.
The fifth large MUSICOS (Multi-SIte COntinuous Spectroscopy) campaign took place at the end of 1998 and it involved 13 telescopes mostly equipped with cross-dispersed echelle spectrographs. The ESA-MUSICOS spectrograph, first commissioned in April 1996 and installed in the INT, participated again in this campaign. This spectrograph has also been offered to the general community and used for programmes of stellar variability, support to space observations, and as part of multi-site campaigns.
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