False colour Taurus Tunable Filter (TTF) image of the centre of the Orion Nebula. The data were taken with the highest possible resolution of the TTF (6Å). Red is [SII], green [NII] and blue Halpha. Each individual image is a 10 second exposure. The composite image was made by Steve Lee.
The early chemical enrichment of the Galaxy, Sean Ryan et al
Fast CCD spectroscopy of black hole transients, Emilios Harlaftis et al
Circular polarimetry of OMC-1, Antonio Chrysostomou
Observational cosmology with the TTF, J. Bland-Hawthorn
Schmidt news, Local news
by Brian BoyleThe last three months have seen a number of significant developments for the AAO. The AAT Board had a successful meeting in March, where the Board decided that IRIS-2 should proceed as the next major instrumentation project for the AAO. It is currently planned to commission IRIS-2 by the second half of 1999. The initial stage in the IRIS-2 project will be a Phase A design study, which will be reviewed in August. The AAT Board once again affirmed its strong support for the vigorous scientific and technical programmes carried out at the AAO. This support is borne out by the continued demand for AAO facilities by the user communities. For the forthcoming semester, the AAT has received the greatest ever number of proposals from both PATT and ATAC.
The operations and development programme at the AAT also received strong support from the UK Ground Based Facilities Committee (GBFC). Nevertheless, the UK is facing significant financial pressures on its budget for ground-based astronomy and the Board also received notification, via the GBFC, that there would effectively be a phased reduction in the UK's contribution to operational costs of the Schmidt telescope beginning in 98/99. However, realising the significant future potential of the Schmidt, the Board have requested that the Schmidt Telescope panel (augmented by senior representatives of the UK and Australian communities) prepare a paper looking at Options for the UK Schmidt over the period 1999-2004. This is currently in progress, and the AAO has endeavoured to solicit community input, largely via a questionnaire which was distributed on the WWW. The response to this questionnaire has been very encouraging. It is clear that there is strong community support for new developments at the Schmidt, such as a new fibre-optic spectrograph with a robot positioner, and a large-format CCD mosaic camera. Both these projects would allow the UKST to tackle major new projects, such as the spectroscopic follow-up to all-sky surveys (DENIS, 2MASS) or to embark on major new imaging surveys of the Southern sky, at the depths required by the next generation of 8m-class telescopes. The AAO has also had very positive discussions with groups looking to provide additional funding for such projects.
As for the current operation of the AAO, 2dF continues to occupy most of the available resources. In this newsletter Karl Glazebrook and Ian Lewis report on the encouraging progress with 2dF commissioning. The major items on the critical path to a fully-working system are the spectrographs, and so it has been decided to cancel the May and June 2dF runs. This will give the 2dF team a significant period of time to work on 2dF off the telescope, prior to September, when it is hoped to have a system with 2 spectrographs and 400 fibres. Although dependent on progress made with 2dF over the next few months, the AAO plans to invite experienced observers to assist with the observations of their scheduled 2dF programs towards the latter half of the forthcoming semester.
Another positive aspect of the change-over from commissioning to shared-risks observing with 2dF, is that it has allowed the service scheme to be re-introduced at the AAT. Jessica Chapman reports on the 'new-look' service scheme to be introduced for the forthcoming semester.
The SPIRAL spectrograph also had a successful first commissioning run in March, despite less-than-ideal conditions. Although the current system is far from a common-user instrument, the AAO aims to make it available to the community in service mode, pending a successful second commissioning run in September. More details on the SPIRAL commissioning can be found in the article by Matthew Kenworthy and Ian Parry in this Newsletter.
As reported by Fred Watson in this newsletter, the new Halpha filter for the Schmidt telescope has now arrived and has already been used to obtain excellent images. A very successful workshop on the proposed Halpha survey was held at Epping in April, with an international audience providing extremely useful input to this survey.
Moving onto staff issues, I am pleased to announce that Dr Paul Butler from Berkeley will take up a post as a staff astronomer from 1 September. With Geoff Marcy, Paul Butler has been responsible for the discovery of 5 out of the 6 known extra-solar planets. His arrival continues the AAO tradition of providing excellent scientific support for instrumentation.
Finally, our readership will note that we have changed the design of the AAO Newsletter. After 20 years, it was felt that the Newsletter was due for a face-lift. Technology has moved on since the first Newsletter was produced, and now desk-top publishing packages exist which make the task of producing a well-designed Newsletter (which is also easily converted to the WWW) much more straightforward. I hope that the new design endures for at least as long as the original format.
THE EARLY CHEMICAL ENRICHMENT OF THE GALAXY
by Sean Ryan (AAO/RGO), John Norris (MSSSO) and Timothy Beers (Michigan State University)Sean Ryan worked at the AAO for four and a half years. He started as the AAO Research Fellow, moved on to an ARC position here, and then on to the staff as a fixed term Research Astronomer. He left this April to join the RGO. In this article he describes the research he carried out whilst working here.
Extremely-metal-poor stars exhibit the products of nucleosynthesis from the first high-mass, zero-metallicity objects to evolve and pollute the proto-Galaxy. Element abundance ratios observed in those low-mass stars that survive to the present allow us to probe the ejecta of these earliest supernovae (SN), and determine the nature of the stars and sites of nucleosynthesis during the first epochs of star formation in the Galaxy.
The element abundance ratios can be compared with the yields computed for SN of different masses to determine which ones have contributed to Galactic chemical enrichment and when. The observations feed back into nucleosynthesis calculations when unexpected element ratios are found, constraining efforts to produce more realistic SN models.
During the late 1980s, observations of very-metal-poor stars had revealed a number of unexpected new trends:
· the neutron-capture elements showed a range of behaviours, but broadly speaking Sr tracked Fe whereas the heavier element Ba became underabundant at [Fe/H]<2. Strontium also exhibited huge star to star differences (by a factor of 100 or more) in otherwise similar stars;
· conspicuous numbers of C-rich stars were being discovered, with [C/Fe]>~+1.0.
In order to study these and other aspects of Galactic chemical evolution in more detail, we sought to discover and analyze larger numbers of extremely-metal-poor stars, with particular interest in those having [Fe/H]<3.0.
It is important to note that the yield predicted from a 25MO· SN, if mixed into a 106MO·primordial cloud, would result in metallicities comparable with those of the most metal-poor stars known. The implication is that by observing stars with [Fe/H]<3.5, we can see material which has been processed through only a single SN.
The lack of consistent behaviour among very metal-poor stars ([Fe/H]<3) emphasized that the star-forming environment during formation of the Galactic halo was not well mixed.
The program to identify new extremely-metal-poor stars followed in the footsteps of Beers, Preston & Shectman (1985), utilising Curtis Schmidt objective-prism exposures of the spectral region around the CaII H and K lines. Plates were visually scanned to find weak-lined candidates, and UBV photometry of these was obtained on the ANU 2.3m telescope to identify those showing an ultraviolet excess. Spectra at 1Å resolution were then obtained with the 2.3m telescope of the more promising cases to measure the metallicity, and high resolution spectra were sought of the most metal poor ones using UCLES on the AAT. Ten of the stars for which we have recently published new results have [Fe/H]<3.00 (Norris et al. 1996, 1997a,b; Ryan et al. 1996). Several other extremely-metal-poor stars remain to be observed with the AAT.
Abundances for a range of elements are shown in the figure. AAT results (including those from previous studies) and a few stars for which we have combined the measurements of several observers to form a composite data set are shown as filled circles, while other results from the literature are shown as small crosses.
Although the alpha-elements generally conform to the patterns seen in more-metal-rich halo stars, the [Mg/Fe] overabundance increases slightly towards lower metallicity, in contrast with the flat relations for [Ca/Fe] and [Ti/Fe]. However, the trend is weak, and could be due to systematic errors in the stellar atmospheres and/or analysis. Ideally the abundances would be verified by observing weaker lines and trying different model atmospheres.
If the trend is shown to be real, it may have implications for models of low metallicity SN. Although elements lighter than calcium are expelled in SN, they are chiefly produced prior to the explosion. (In contrast, heavier elements in the iron group are produced by explosive burning and are sensitive to the details of the explosion mechanism and its inherent uncertainties.) Woosley & Weaver (1995) experienced much difficulty getting model SN of metal-poor massive stars, which are more compact than higher-metallicity counterparts, to eject heavy elements at all, rather than having the envelope collapse back on the stellar remnant. If only the outermost layers are ejected, higher [Mg/Fe] ratios would obtain since Mg occupies the outermost layer in onion-skin models.
Woosley & Weaver's result also raises a 'tortoise and hare' question: were the most massive zero-metallicity stars the first to enrich the interstellar medium, or were they too massive to actually eject their enriched outer layers,
The present data show that the Ti overabundance continues down to the
limit of observation at [Fe/H]=-4, and it is difficult to avoid concluding
that Ti is over-produced in the SN of massive metal-poor stars. Nucleosynthesis
calculations, on the other hand, suggest it should be under-produced relative
to iron, not be over-abundant. Because Ti is produced deep in the SN, near
the mass cut, Ti yields from SN models depend sensitively on the details
of the explosion, with variations of ~0.35dex possible (Timmes et al. 1995).
Titanium thus provides an important constraint which the models cannot
The new data extend the plateau of [Al/Fe] seen in earlier AAT work down to [Fe/H]=-4.0. Two of the most metal poor stars in the figure have high [Al/Fe] abundances but "normal" values of [Al/Mg]. This is consistent with Al and Mg being produced under similar circumstances, but by a different mechanism from Fe, as nucleosynthesis computations predict. Since Mg and Al reside much further out in a SN progenitor than the region where Fe is formed, it is reasonable that the expelled fractions of Mg and Al on the one hand, and Fe on the other, should differ in different SN. Since models of massive, low-metallicity SN experience greater difficulty in ejecting the inner
layers, one might expect to encounter cases where less Fe has been ejected, leading to higher [Al/Fe] and [Mg/Fe] values than normal. Furthermore, such "weak" SN would not only occur in a low metallicity environment but would also produce less enrichment of the ISM by iron, so divergent [Al/Fe] ratios should be associated with the lowest [Fe/H] values, consistent with these observations.
The iron-group elements
When this work was begun, observations showed that the iron-group elements track iron depletion in metal-poor stars, with the exception of Mn. A highlight in recent studies of extremely-metal-poor stars was the discovery of a change in abundance ratios of several iron-group elements at [Fe/H]<=2.5 (McWilliam et al. 1995), which was also apparent in our AAT study. The figure shows that the two elements immediately lighter than iron, Cr and Mn, drop to greater overdeficiencies at [Fe/H]<2.5, whereas the next element heavier than iron, Co, is seen to rise to higher abundances. The average [Ni/Fe] also increases at lower metallicities, though not as steeply as [Co/Fe]. It is possible that the Ni result should be described as a dichotomy rather than a trend, with a different than expected Ni yield (relative to iron) in some low metallicity SN. Although a nucleosynthetic process is known that might explain the shift in iron-peak yield towards higher mass numbers - the alpha-rich freeze-out - its impact on Galactic chemical evolution had not been predicted. This provides an additional example of observational results feeding back into nucleosynthesis calculations.
The neutron-capture elements
The huge star to star spread in relative abundance of strontium has been confirmed for both dwarfs and giants, and is unlikely to be a result of an event during the stars' lifetimes. In contrast, barium is relatively well-behaved. Whereas the total range in [Ba/Fe] is almost the same as for [Sr/Fe] at [Fe/H]=2.8, the distribution in [Ba/Fe] is bimodal, comprising a major trend (comprising 3/4 of the stars) with [Ba/Fe]<0, and a few scattered points (1/4) at [Ba/Fe]>0. The bimodality of [Ba/Fe] has been confirmed by several different robust statistical tests.
If the differences in [Ba/Fe] were due to some continuous range of parameters, a spread resembling that for [Sr/Fe] would be expected. That [Ba/Fe] has a much smaller scatter about its major trend plus a handful of separated high points leads us to conclude that the high [Ba/Fe] data points have a different origin from the majority of the barium data. What has yet to be determined is whether that bimodality is astrophysical in origin or results from non-Gaussian systematic errors in some of the data, but closer examination of the stars with [Ba/Fe]>0 is clearly warranted.
The difference between Sr and Ba may indicate that light neutron-capture elements have been produced more abundantly than heavier ones, but the site and process are currently unclear. One means by which this can occur is the "weak" s-process during core He burning of M>15MO·stars, which produces neutron-capture elements only between Cu and Zr. However, one customarily associates neutron-capture elements in the halo with the r-process, though s-processing with a primary neutron source in 17MO· stars, or r-processing in low mass (78MO·) stars could be responsible.
If r-process-rich stars can be identified so far only one is known it is possible to obtain an abundance for the radioactive species thorium, whose half-life is long enough to constrain the age of the universe. This element then provides a measurement of the minimum age of the Galaxy that is independent of cosmological arguments and globular cluster evolutionary tracks.
Pollution by the first supernovae
One deficiency of current Galactic chemical evolution models is that although nucleosynthetic yields are used to provide relative abundances for the elements, the details on how they are mixed into the environment are generally overlooked. It is often assumed that one cloud having the mass of the halo is enriched by numerous SN conforming to an adopted initial mass function, and the products of these are completely mixed into the cloud.
While such models should serve well in the Galactic disk and perhaps
at late times during formation of the halo, it is unlikely to be applicable
earlier in the formation of the halo when the gas clouds were poorly mixed,
independently evolving, and have been contaminated by only very small numbers
The well-defined abundance trends contrast with Searle & Zinn's (1978) view of a chaotic halo-formation mechanism, with independently-evolving fragments coalescing into the modern Galactic halo over a period of the order of 109yrs, which exceeds the lifetimes of the enriching SN progenitors by several orders of magnitude. That two SN on opposite sides of the Galaxy which produce the same [Cr/Fe] abundance also enrich their local ISM to the same [Fe/H] seems to require that the ejecta combine with identical gas masses. It is possible that the mass of ISM with which the ejecta mix depends primarily on the kinetic energy of the ejected SN layers, independent of the density or mass of the cloud. This scenario sees the enrichment outcomes dependent upon a universal property, namely SN physics, rather than on the details of the environment itself, and is consistent with both the well-defined abundance patterns and the lack of kinematic correlations with abundance which prompted Searle & Zinn's model. If this scenario is correct, it may also explain why the minimum metallicity for globular clusters coincides with the metallicity at which halo abundance ratios change, with greater diversity at [Fe/H]<2.5.
Beers, T. C., Preston, G. W., & Shectman, S. A. 1985, AJ, 90, 2089 McWilliam, A., Preston, G. W., Sneden, C., & Searle, L. 1995, AJ, 109, 2757 Norris, J. E., Ryan, S. G., & Beers, T. C. 1996, ApJS, 107, 391 Norris, J. E., Ryan, S. G., & Beers, T. C. 1997a, ApJ, in press (Paper III) Norris, J. E., Ryan, S. G., & Beers, T. C. 1997b, ApJ, submitted (Paper IV) Ryan, S. G., Norris, J. E., & Beers, T. C. 1996, ApJ, 471, 254 Searle, L. & Zinn, R. 1978, ApJ, 225, 357 Timmes, F. X., Woosley, S. E., & Weaver, T. A. 1995, ApJS, 98, 617 Woosley, S. E. & Weaver, T. A. 1995, ApJS, 101, 181FAST CCD SPECTROPHY OF BLACK HOLE TRANSIENTS
by Emilios Harlaftis, Keith Horne, St.Andrews, Raylee Stathakis, AAO, Ron Remillard, MITA new time domain can be explored with the time-series mode available on the CCD camera and the RGO spectrograph. Dynamical timescales in accretion discs of interacting binaries (white dwarf, neutron star or black hole being the primary star) are minutes to seconds as the gas spirals inwards. The shortest dynamical timescale is ~0.1 ms for gas near a 10 km neutron star. The sub-second timing regime is currently explored in X-rays by the RXTE satellite (see discovery of the ~1100 Hz quasi-periodic oscillation in Sco X-1 by Van der Klis et al. 1996). In X-ray binaries, most of the optical light is considered to arise through reprocessing of the X-rays in the accretion disc and the companion star. Such reprocessing will give rise to time delays (echoes) between the optical and X-ray photons due to the light-time travel between the X-ray source and the illuminated spot. In particular, the echo on the companion star contains information on the binary inclination and separation which can lead to estimates of the compact-star mass.
We designed an experiment to search for such echoes in an X-ray nova using the AAT and RXTE simultaneously. X-ray novae are appealing because at maximum they can be as bright as 13 mag, their outbursts last for months making it easy to monitor the accretion disc's evolution, and finally they usually harbour a black-hole primary star. The black-hole transient GRO J1655-40, which shows relativistic jets and has a 7MO·black hole, exhibited one of a series of outbursts in May 1996. On 10-12 May, we used the time-series mode at the AAT to match simultaneous RXTE observations of GRO J1655-40. Figure 1 shows the resulting AAT spectrum with absorption lines from the F spectral-type companion star and the accretion disc (Halpha, Hbeta, Nal) and the HeII/Bowen emission lines which are characteristic of high X-ray luminosity.
We tested the time series mode (0.8s exposure and 0.26s dead-time) for 42 and 35 minutes on 10 and 11 May (total of 4500 spectra) obtaining a typical S/N of 11 (60007500 Å) (V=15.4 at maximum and high reddening, E(B-V)=1.3). The dead time of 0.26s was achieved with the CCD time-series mode (FAST readout; 7.2 e/pix) and with a CCD window of 1024x50 binned by 4x10. The absolute time accuracy necessary to correlate the X-ray and optical data is 0.1 seconds (AAT). The light curve of a representative segment of the time series is displayed in Figure 2 (under photometric conditions and 1 arcsecond seeing). Optical flickering is at a level of 3-4% on the 1-600s timescale. The
simultaneous XTE observations also show a similar level of flickering (photon statistics account only for 1%). Lack of any clear X-ray/optical correlation is puzzling. It may be related to the stage of the outburst at which our observations took place (outburst maximum). According to recent reprocessing models, the disc at maximum is so thick that the outer disc and companion star are not irradiated efficiently. Therefore, high flickering and possible correlation between optical/X-ray due to reprocessing may be damped by the thick outer disc. Alternatively, the generally-accepted notion of the optical emission being mainly due to reprocessing may be in error.
Our OVERRIDE programme now awaits the next bright X-ray nova. In view of our May observations, we plan to aim for for the plateau stage of the outburst when the disc is thinner and thus the companion star may be irradiated sufficiently by X-rays to show an optical echo from its inner face. Stay tuned!
by Antonio Chrysostomou (UKIRT), Tim Gledhill, Jim Hough (University of Hertfordshire), Francois Menard (Observatoire de Grenoble), Motohide Tamura (National Astronomical Observatory)Introduction
Polarisation mechanisms are now, in the main, well understood; unpolarised photons which scatter are linearly polarised with a position angle dependent on the scattering plane. However, the general Mie scattering equations do not allow the production of circular polarisation in this way. The incident radiation needs to be linearly polarised before it is scattered. In this way, circular polarisation relays some of the history of these photons, permitting a unique view of the complex morphology and physical conditions that is the circumstellar environment.
Through the study of linear polarisation, it is possible to discern information about the region where the incident light is being scattered, e.g. in the case of a YSO, this would be the reflection nebula or outflow cavity (Chrysostomou et al. 1996; Clark & McCall 1997). When the circular polarisation of the radiation is measured we are, in effect, travelling backwards along the light path, probing those regions before the final scatter, the regions very close to the YSO, regions for which we have previously had little geometrical information. For examples of some of our recent work which utilise this technique see Gledhill et al. 1996 and Chrysostomou et al. 1997.
The clearly unique and powerful combination of linear and circular polarimetry is still a very new technique in this field. The data that we present in this article, of the first imaging circular polarimetry of OMC-1, illustrates this point beautifully. These observations have helped to shed new light on our understanding of this important region, as well as exposing new questions and challenging our knowledge of polarisation mechanisms.
Observations of OMC-1
Our observations of OMC-1 were taken on the night of 20 November, 1996 in the Kn and H band filters. In Figure 1, we show the Kn band circular polarisation image only. The overall distribution of circular polarisation across the whole region is surprising. The average background polarisation to the north and south of the region is
~ 2%. This order of magnitude matches the highest circular polarisation
measurement in any of our studies thus far. In saying this, it was thus
surprising to find patches of intense circular polarisation to the SE and
SW of the region. Degrees of circular polarisation here fall in the region
between +10 and +
Comparison with Linear Polarimetry
Linear polarimetry measurements of OMC-1 have been relatively numerous at various wavelengths from the optical to the sub-millimeter. Perhaps the best example in the near-infrared is found in the study by Minchin et al. (1991), with which we compare. It was noted that there are specific regions whose linear polarisation is higher than the average. At the time, no satisfying explanation was seriously sought. However, what is now clear is that the regions of high circular polarisation that are seen in our data are correlated with those regions of enhanced linear polarisation.
Questions we are left to answer
Here we discuss some of the obvious outstanding questions which our circular polarimetry data presented here brings forward.
What mechanism can produce such high levels of circular polarisation?
We have already mentioned that it is not possible to produce circular polarisation by scattering unpolarised light off simple spherical grains. Indeed, even if the radiation is pre-polarised, spherical grains which are within the Rayleigh limit (alpha/lambda<<1, where alpha is the grain size) also cannot circularly polarise light. We can therefore confidently discount the possibility of single scattering off Rayleigh or MRN grains as an answer to this question.
Naturally, a multiple scattering solution was searched for. In this scenario the light source would have to be pre-polarised, possibly through interaction with a circumstellar and/or accretion disc. This particular model has gained favour from our studies of embedded sources thus far, and has enabled us to successfully model and derive physical parameters for these objects.
Indeed, we have used our Monte Carlo scattering code to determine that this mechanism is consistent with the large scale background circular polarisation pattern in OMC-1.
However, it may be difficult to reconcile this mechanism with the presence of isolated regions of high circular polarisation. Models have shown that it is possible to achieve such high degrees if the absorptive part of the grains' refractive index is large e.g. graphitic or mantled grains (and this property is used to great effect in our models). To produce localised areas of extremely high degrees of circular polarisation, this then requires the somewhat unappealing picture of isolated areas of OMC-1 with drastically different grain compositions in relation to their immediate environs.
One fact that is known about the grains in OMC-1 is that there is a
population of grains which are not only aspherical, but also generally
aligned with the ambient magnetic field. This evidence is borne to us by
near-infrared, mid-infrared and sub-millimeter polarimetric studies (Chrysostomou
et al. 1994; Aitken et al. 1997; Schleuning et al. 1996). This drove us
to derive a solution for the problem of scattering off aligned grains.
Importantly, our initial results have shown that it is possible to obtain
high degrees of circular polarisation by scattering unpolarised light
off elongated, aligned grains.
We can therefore assume, that in those regions where the linear and circular polarisations are enhanced, we are seeing photons being scattered to us off aligned grains.
Why aren't there high degrees of circular polarisation across all of OMC - 1?
If the bulk of the OMC-1 cloud receives its photons via multiply scattered routes, then these multiple scatters should be occurring off aligned grains, so shouldn't the degrees of polarisation still be higher than 2? An answer to this particular question is currently being sought. Nevertheless, the dependence on aligned grains is vital. Evidence of their existence relates, in the main, to the ambient and foreground molecular cloud. The prevalence of shocks in the region may serve to destroy this alignment in the interface between the outflow and the ambient cloud. These are also the surfaces where light is scattered to us. It is, therefore, only the portions of the cloud which have not suffered the passage of a shock that are seen with high degrees of circular polarisation.
Where are the other similar objects?
The consensus is slowly developing that it is possible for grains to be aligned under almost any physical condition, implying that for most of the embedded objects which are studied, the grains in their environment must be aligned. If so, then why is OMC-1 the only object where we have seen such high degrees of circular polarisation?
The answer to this question may already have been provided above. All of the objects we have studied have outflows associated with them. The momentum provided by these outflows could be destroying any grain alignment, and thus when radiation is scattered to us off the cavity walls we do not see the signatures of aligned grains in the circular polarimetry. One presumes, therefore, that it is only the unique nature of the OMC-1 region which has allowed us a privileged insight into this aspect of nature.
One can begin to answer this question by making a similar study of reflection nebulae, where the scattering surfaces remain relatively undisturbed and thus may still preserve their grain alignment.
Finally, we should, at this point, introduce a caveat. The statistics
in our study thus far are poor. Fewer than 5 objects have been studied
to date, and OMC-1 is the only high mass region. Our understanding of the
scattering process in relation to aligned grains is still far from complete,
and surely nobody doubts that future observations will unlock yet further
secrets of the OMC-1 cloud. The questions we have raised need to be addressed,
and it is certainly the intention of this group to attempt to do so through
modelling and observing
progammes using facilities such as those provided by the AAT.
We hope that once the statistics have become more significant we will be much closer to understanding not only the OMC-1 region itself, but also the formation and evolution of circumstellar regions in general.
Aitken et al. 1997, MNRAS, 286, 85 Chrysostomou et al. 1994, MNRAS, 268, 325 Chrysostomou et al. 1996, MNRAS, 278, 449 Chrysostomou et al. 1997, MNRAS, 285, 750 Clark & McCall 1997, MNRAS, 284, 513 Gledhill et al. 1996, MNRAS, 282, 1418 Schleuning et al. 1996, ASP Conference Series 97, "Polarimetry of the Interstellar Medium", p 2852DF COMMISSIONING
Karl GlazebrookIn the last newsletter there was a very detailed report on the successes and progress of the 2dF commissioning during the previous 6 months. Since then we have had two more commissioning runs with the 200-fibre system in March and April with generally clear weather and here offer this briefer update on our progress and future plans.
A number of tests were done during the March and April runs. A better focus was achieved (<2 pixels) over the CCD making the automatic pipeline data reduction much easier. We also tested the new Fibre Autoguider software, techniques for offsetting standard stars into already configured fibres and did numerous exposures to get test data for fibre throughput variations and sky-subtraction.
Some time was lost to computer hardware and software glitches, primarily network related. The 2dF hardware itself behaved generally very well, though there was a persistent problem with the closed-cycle cooler for the CCD. There appears to be some contamination in the helium lines which are now being cleaned out. The fibre positioner operated quite reliably, though due to effort diverted to the cooling problem there was none to spare for testing the new, faster, version of the positioner control system.
As in the December and January runs most of the time was spent doing a number of science observations for a variety of projects. Some of them, such as the Centaurus-A planetary nebulae observations for Ken Freeman, can even be classed as complete! We observed another 6 test fields for the 2dF Redshift Survey project, which completes their initial allocation of test data, and as a bonus many of these fields included high-
The 2dF Team now feel we have learned as much as we need to from commissioning
observations with the current 200-fibre system. We now see the priority
as the upgrade to the final 400-fibre/dual spectrograph full system. We
have determined that this is best achieved by a longer period of down-time
off the telescope. To this end we have abandoned the May and June 2dF runs
to give us 12 clear weeks for engineering work. In late July and August
we will be going back on the telescope to test the new spectrograph control
system, positioner system and other ancillary systems. By then both spectrographs
should have a complete control system. After that we have to fit the final
complement of 200 fibres to each plate, a job that will take several weeks
in itself, and we plan to take the first 400-fibre observations in September.
It is not expected that many science observations will be taken during
the May-September period due to the number of new systems to be commissioned.
Any that are obtained will be a bonus. The engineering work is tightly
scheduled and so it is always possible that unforeseen problems may lose
us a month or so, but we are determined to have the 400-fibre system available
in some form for the forthcoming SGP season.
ADAPTIVE OPTICS TESTS AT THE AAO
by John O'Byrne (Sydney University) and Kristen Larson (Rensselaer Polytechnic)Adaptive Optics (AO) activity at the AAT in early 1997 has been primarily aimed at evaluating the atmosphere at Siding Spring. In January, Nick Wooder and Miles Adcock from Imperial College London came out for ten nights' observing on the AAT and the MSO 40-inch telescope. They brought the Imperial College
SCIDAR camera system which is used to determine the altitude of turbulent layers in the atmosphere above a site. The poor weather in late January resulted in only one night of data out of ten! The data are being processed, but a quick initial look shows the presence of at least three turbulent layers on that night.
In late March, a total of two nights of Director's time provided a good
range of seeing conditions to exercise the tip-tilt component of the AO
system. Seeing ranged from below 1 arcsecond to 'really bad'. Again, the
data are still being processed, but it is quite clear that the tip-tilt
correction dramatically reduces low frequency image motion. This is apparent
from the IRIS image display during observations and from calculated power
spectra of the tip-tilt sensor. As an example, the panels reproduced above
illustrate radial motion of the image centroid of a star (V~6) with the
servo on and off, as seen by IRIS early on the night of 25 March 1997.
These data sets were taken in quick succession using the IRIS intermediate
field mode (0.27 arcseconds per pixel) in median seeing. Each panel represents
500 0.2 second integrations at 1.64 microns for a total duration of 100s.
The RMS image motion of the centroid is reduced from 0.59 pixels (0.16
arcseconds) to 0.22 pixels (0.06 arcseconds) by the servo. However the
size of the final image in a large telescope will typically be dominated
by image blurring rather than image motion. Averaging the 500 images in
each data set yields the long exposure image size in each case. For this
AAO SERVICE PROGRAM RESUMES
Jessica ChapmanWith the commissioning of 2dF now reaching its final stages, it has been decided that the AAT service observing programme will be relaunched. There have been several changes to the programme. From now on deadlines will be set for applications. Limited exceptions will be made for targets-of-opportunity, but other projects requiring service time must be received by the relevant deadline. It is expected that these deadlines will be set every two months. The maximum length of time given to a service project will be increased to 3 hours. All applications will be refereed, by a panel comprised of an ATAC member, a PATT AAT TAG member and an in-house staff member. From
SPIRAL PHASE A COMMISSIONING
Matthew Kenworthy and Ian Parry (Institute of Astronomy)The phase A SPIRAL prototype was successfully commissioned during the middle of Feburary. SPIRAL is a prototype fibre-fed spectrograph optimised for work on the AAT re-imaging optics take light from the f/8 Cassegrain focus and cast an enlarged image scale onto the macro-lens integral field unit (IFU). Optical fibres then take the light from the 37 hexagonal lenses to the Littrow configuration spectrograph sitting on the dome floor, just below the telescope. As this is a prototype, the field of view of the IFU is about three arcseconds, with each lenslet matching to 0.5 arcseconds on the sky. The advantage of SPIRAL is that there are no slit losses, unlike a conventional spectrograph, and by using a 1200R grating we attained 0.125Å/pixel on the Tek 1024 CCD. Another mode of SPIRAL called the pupil imaging mode allows us to do single object spectroscopy and allows extra fibres in the IFU to simultaneously record sky background, again without slit losses.
The commissioning run took place over three nights just before re-aluminisation of the primary mirror. The weather and seeing gave us about one and a half usable nights in which we measured the throughput of the whole system to be just under 12% for a resolution of ~10,000 and a wavelength of 6750Å at the zenith. This allowed us to demonstrate the integral field capability by looking at Circinus and NGC 1068, two galaxies with large internal velocity fields. Even though this is just a prototype, the system already offers higher throughput at a resolution of 10,000 than the RGO spectrograph.
Due to bad weather on the last night, we did not test the pupil imaging
mode, but initial tests looked promising. The next SPIRAL phase A run,
in September, will have a scientific rather than a technical basis and
will further test the pupil imaging mode. We are now planning the construction
of SPIRAL phase B, which will have 500 lenses in a macro-array and have
a scale of 0.7 arcseconds per lenslet.
AAT AND UKST OBSERVERS REPORTS
Jessica ChapmanSince November 1996, observers at the Anglo-Australian Observatory and UK Schmidt have been asked to complete a questionnaire on the Web at the end of their observations. The information collected is used to monitor the level of user satisfaction with observing and computing facilities, staff support and the general working environments at the observatories.
The responses received so far have provided extremely useful feedback from observers. While the level of user satisfaction is generally high, the comments and statistical information obtained assist with making improvements where needed.
We thank observers who have completed the questionnaire and request that the 'AAT and UKST Observer's Report Form' be completed at the end of each observing run. The URL address of the form is http://www.aao.gov.au/local/www/jmc/surveyform.html.
OBSERVATIONAL COSMOLOGY WITH THE TTF
by J. Bland-HawthornThe TAURUS Tunable Filter (TTF) has begun to produce impressive data for a diverse range of astrophysical studies. This instrument is set to revolutionize the way in which intermediate to narrowband imaging is carried out at observatories. The instrument allows for wide-field (4.59arcmin) narrowband imaging over 63009600Å where the bandpass can be set anywhere in the range 660Å FWHM. The instrument is simple to operate from the observer's perspective and even simpler to apply for time with! The TTF provides the capability, for the first time, to synchronize frequency switching with movement of charge on a CCD. This has important ramifications for many astrophysical experiments, not least for averaging out temporal variations due to the atmosphere or measurement apparatus. We provide more details after a brief summary of recent TTF projects in observational cosmology (in order of increasing cosmic time).
B.J. Boyle, K. Glazebrook and JBH have begun a search for Lyalpha emitting companions associated with z~4.5 quasars. We have already observed one radio loud QSO (PKS 1251407, z=4.46) as part of this program. The reduction and analysis is only preliminary, but a comparison in Fig. 1 of a deep B image with a stacked TTF image at 6650Å (corresponding to Lyalpha in the rest frame of the QSO) shows a clear excess of objects around the QSO in the TTF image, consistent with galaxies at z = 4.5. A large amount of work still needs to be done to confirm this tentative, but potentially important, result. R. Hunstead (U. Sydney), J.C. Baker (MRAO) and M. Bremer (IAP) are undertaking a related program to look for [OII]3727 emission around radio-loud quasars at z~1. An example of TTF data obtained for one quasar, showing both emission-line galaxies and extended line emission at z=0.9, is shown in Fig. 2. Several more emission-line galaxies (brightening in one or two bands) were easily identifiable by eye over the whole field, and more are found at lower significance levels (down to star formation rates of a few solar masses per year). The faint magnitudes of these galaxies, typically R = 2223, and their broadband colours, make it highly likely that they are the quasar redshift rather than intervening Halpha or [OIII]5007 emitters. Their projected separations from the quasar range from 200700 kpc (HO = 50 km/s/Mpc, Omega =1).
Figure 1: The central 30arcsec region of the TAURUS 4.5arcmin field of view imaged in April 1997. The image on the left is a 4 hour B exposure in a combined seeing of 1.5arcsec FWHM. The righthand image is a composite of 10 15Å wide TTF images taken around redshifted Lyalpha (total exposure 5 hours) with a combined seeing of 0.7arcsec FWHM. Apart from the QSO, which itself peaks up strongly in the TTF image, a number of other objects strongly emitting in the redshifted Lyalpha band are clearly visible, including the galaxy closest to the QSO.
Figure 2: Zoomed TTF images of emission-line objects (G1 and G2) in the field of the z=0.898 radio-loud quasar MRC 0450221 (Q) as imaged by TTF in February 1997 (Hunstead, Baker, Bremer). The field was scanned in a sequence of seven adjoining 10Å bands centred on [OII]3727 at the quasar redshift (sub-image 4). Extended line emission is clearly seen around the quasar reaching up to 1000 km/s redward of the nuclear emission.
K. Glazebrook, R. Abrahams (Cambridge) and JBH took the TTF to the WHT to observe the Hubble Deep Field. In four clear nights and 0.7arcsec seeing, we covered 3 redshift slices in our search for [OII] emission. These were z=0.8940.908 (0.2h/slice), z=1.1731.191 (1h/slice) and z=1.4261.448 (1.5h/slice), for a total of 26 hours exposure and 68 data frames (221 total runs). The bandpass was tuned to 16 Å which corresponds to 300 km/s steps (rest frame) at these redshifts. Our primary motivation is to look for the evolution in the star formation rate as this is expected to turn over in the interval z = 12.
D.H. Jones (MSO) and JBH have begun to construct Halpha luminosity functions in clusters and in the field at redshift intervals near z=0.2 and z=0.4. For the rich cluster ACO 3665 (AC 106), we have identified more than 30 Halpha emitting candidates above a 3sigma threshold with many more candidates at lower thresholds. Our experimental "efficiency" in finding star-forming objects will be measured with the LDSS by observing both TTF-selected objects and a control sample of objects. Fig. 3 shows a small subset of objects found, and Fig. 4 illustrates the excess of objects which show up in the on band (cluster redshift z = 0.232) compared with off bands which straddle the on band.
The total TTF throughput (TTF+CCD+telescope) is 30% (600-700nm), 25% (800nm) and 13% (900nm). The prospective applicant need only determine the sky contribution within a chosen bandpass (6-60Å FWHM) and that's it. The Tek 1Kx1K CCD camera gives a pixel scale of 0.59arcsec/pix (f/8) or 0.32arcsec/pix (f/15), compared with 0.37arcsec/pix (f/8) or 0.20arcsec/pix (f/15) with the soon-to-be commissioned MIT-LL 2Kx4K CCD. The latter will allow for the full field to be shuffled in concert with frequency switching. For most projects, the data analysis is straightforward since the TTF produces a stream of discrete images. A suite of IRAF procedures developed by D.H. Jones now exists. Standard object-finding packages (e.g. Sextractor) can be run in real time at the telescope, and are sufficiently reliable to track variations in atmospheric conditions during the night.
The TTF has largely removed the need for buying arbitrary narrow and intermediate interference filters, as one can tune the bandpass and the centroid of the bandpass with the TTF. At low resolution (R ~ 300), conventional R and I band filters are needed for order separation. At high resolution (R ~ 1000), six intermediate band filters are used to divide up the R and I bands. These filters are illustrated on the TTF web page which can be reached from the AAO home page http://www.aao.gov.au/
Figure 4: (a) Narrow bands used in our search for Halpha emission
in ACO 3665. (b) Colour-colour plot showing an excess of emission-line
objects in the lower right quadrant. Error bars are shown for all objects
with an excess signal in the on band which exceeds 3s. (c) Histograms showing
the number of objects as a function of "colour" radius from the origin
in (b). Shown are the distribution of objects in the bottom right quadrant
(dashed), top right quadrant (shaded), and the difference (solid).
Fred WatsonEarly in April, the Schmidt Telescope took delivery of its new Halpha filter for the proposed Galactic Plane/Magellanic Clouds Halpha survey. This survey will take advantage of the wide field of the telescope and the high resolution of Tech Pan film to explore the Galactic Halpha distribution in unprecedented detail. With a useful diameter of 254 mm - almost 5 degrees - the narrow-band filter is possibly the largest ever fabricated for astronomy.
The delivery of the filter from Barr Associates in the USA was very timely, allowing a number of test exposures to be obtained before the commencement of an international workshop on the exploitation of the filter. The workshop, Prospects for the AAO/UKST Galactic Plane Halpha Survey, was held at the CSIRO Radiophysics Laboratory at Epping from 16th to 18th April.
Some 57 astronomers discussed a wide variety of research topics, all of which stand to benefit from the new survey. It also emerged that there is much to be gained by extending the survey to greater galactic latitudes. The filter has been designed so that its ~70Å passband is centred to the redward of Halpha, allowing investigation of the nearest galaxy clusters; prospects for such work were also aired.
Strategy for the new survey was discussed in an informal workshop session by several members of the Consortium that proposed it. The Consortium is led by Quentin Parker (AAO) and Steve Phillipps (Bristol), and will now formally propose the survey to the Schmidt Telescope Panel with the expectation that it will begin almost immediately. Eventually, it is anticipated that the survey will be issued in digital form as well as on film copies.
While the workshop was taking place, the filter itelf was being subjected to a detailed evaluation by the CSIRO National Measurement Laboratory at West Lindfield. Preliminary results from these tests indicate that it meets its specification.
EVENTS AT EPPING
Helen WoodsThe AAO is pleased to welcome Ofer Lahav who is taking sabbatical leave from the Institute of Astronomy in Cambridge and will be in Sydney until September. Ofer's research here will be associated with the 2dF Galaxy redshift survey.
Also joining the AAO for six weeks is Mark Whittle, on sabbatical from the University of Virginia. He is keen to interact with a number of AAO astronomers in such areas as active galaxies and star formation in galaxies.
After more than 4 years with the AAO, Sean Ryan has accepted an appointment with the Royal Greenwich Observatory in Cambridge. Sean joined the AAO as an ARC/SERC Research Fellow in October 1992 and gained a contract as Research Scientist in July 1994. His contribution to the Observatory has been both tremendous and varied. Sean has been the support astronomer for UCLES and UHRF which form the high resolution optical spectroscopic facility at the AAT. He has been the driving force behind the upgrades to these systems, including the UCLES short camera project and the use of the UCLES slit with UHRF. During his time with the AAO Sean also put a considerable amount of effort into the running and maintenance of the AAT schedule. Of course we all hope very much to be seeing Sean at regular intervals in the future, and he takes with him our very best wishes for his future career.
Sandra Harrison, Public Relations Officer for the AAO and AAT Board Secretary for over seven years, has found a new position as Executive Officer at the Australian Museum in Sydney. Sandra's original three-year contract with the AAO began in September 1989 and was renewed twice, ample indication of how much she enjoyed her time here and how much we valued both her company and her enthusiastic contribution to the Observatory. On the positive side, we have no doubt that we will be seeing more of Sandra in a social capacity.
Victoria Thompson, AAO Receptionist at Epping for the last 18 months also left recently to join her family at the Gold Coast. We all wish her well with her move and her new job. The new AAO receptionist is in fact a well-known face. Belinda Steele, AAO receptionist for about four years from 1991 to 1994 has rejoined us; so the first point of contact will, for many of you, be a familiar face.
Sandra RickettsA group of astronomy librarians met at Siding Spring and Narrabri early in April. Jeanette Regan (MSSSO) and the AAO librarian travelled from Siding Spring to Narrabri and spent a most interesting afternoon being shown over the Australia Telescope and the Sydney University Stellar Interferometer, as well as visiting the library. We both found it fascinating to see how the other astronomical half operates, and greatly appreciated the time taken by astronomers to show us around. The following day Chris van der Leeuw from CSIRO travelled to Siding Spring where we all visited the AAT, the UKST and the ANU 40 inch telescope. Again we greatly appreciated the time taken to show a group of librarians how things work.
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LETTER FROM COONABARABRAN
Rhonda MartinA time for us to look smug, with more-or-less constant sub-arcsecond seeing and very contented looking astronomers using wheelbarrows to carry away their data .... it must be a dry spell!
There has been a great deal of interest about Comet Hale-Bopp from the general public, and subsequent disappointment that it will not be a good southern hemisphere object. But still, we had Comet Hyakutake last year and it was a winner so perhaps we should not be greedy. Astronomical events such as these always seem to exert a rather odd influence on certain types of people who then call us to tell us about it - these people are always treated with courtesy and attention and we learn lots of things we would never have known otherwise!
2dF is progressing well with No 2 spectrograph wired up and currently undergoing testing in preparation for software trials. No 1 spectrograph is off the top-end and its wiring is being upgraded to match No 2.
It is good to have Darren Stafford back working with the AAT again. His dry humour is always appreciated, even if we don't know just what he is talking about.
It was with regret that the AAT farewelled the Board Secretary, Sandra Harrison, with an autumn barbeque. Sandra has always been a pleasure to work with and has always been ready to listen to all and sundry. Her PR work for the observatory has been superlative and we will miss her. May she do well in her new position, and may her flamenco heels wake up the sleepy corridors of the Australian Museum. Happy dancing, Sandra!
A VIP who enjoyed himself immensely recently was the Federal Minister for Science and Technology, Mr Peter McGauran MP. On a lightning visit the Minister was shown the latest developments of the Observatory, including 2dF, by the Director. He was also treated to spectacular views over the Warrumbungle National Park from within the Prime Focus top end - a ride not for the faint-hearted!
Coonabarabran Rescue Squad also paid the AAT a visit with a view to working out how to effect a rescue should the need arise. This was a valuable experience for the members, most of whom had never been in the dome before and had never appreciated the difficulties of a rescue. It was instructive for both the Squad and the staff who attended.
During November some intrepid travellers from Mt Wilson made the mistake of flying to Site in some of the worst storm weather in living memory. Full credit to the young pilot who had to deal with ferocious storms that had not been foreseen but every passenger arrived with grey hair and a deep desire never to fly anywhere, ever again. I understand that the Mt Wilson party walked home across the Pacific after their fingers had been prised off the Australian coastline.
Then there was the little Scots-chicken, the Epping Administrator General, who had a bumpy flight to Site, and didn't want to go home, no way, no how, not in anything with wings!
THE AAOBRITISH COUNCIL 'NIGHTSKIES' PHOTOGRAPHIC EXHIBITION
The British Council is celebrating its 50 years in Australia with a number of high profile events with a strong Anglo-Australian flavour. Among these are a series of activities highlighting the AAO as a model of successful Anglo-Australian collaboration. The most conspicuous event is a photographic exhibition of colour photographs of astrononomical objects made by David Malin from plates taken with the Observatory's telescopes, an example of which is shown on the back page.
There are two identical photographic exhibitions currently circulating in the UK and in Australia, each consisting of 40 large (and some huge) mounted prints. The British exhibition opened at Australia House in London in January and is currently on the Isle of Wight. The Australian launch was at the Victorian Arts Centre in Melbourne in March and it will be at New Parliament House in Canberra from June 4 and at the Australian Museum in Sydney from July 6. At most Australian venues and some overseas there will be public lectures presented by David Malin.
The exhibition is accompanied by a very glossy illustrated catalogue, and copies are available from Coral Cooksley. The British Council have also involved us in the preparation of a series of twelve posters featuring Australian astronomy and a pamphlet about the southern sky and southern hemisphere astronomy to be circulated to schools and seen in British Council offices around the world. Coral also has copies of these.
This series of events, generously funded by the British Council, has greatly enhanced the profile of the Observatory and its work in the general, non-science community, both in Australia and in the UK. Much of the liaison with the British Council that made the exhibition, the catalogue, pamphlet and posters possible was the work of Sandra Harrison, the AAT Board secretary, who, completely exhausted, has now taken up the even more demanding role of Executive Officer of the Australian Museum Society, where we wish her every success.
For tour details see:
editor STUART LUMSDEN editorial assistant SANDRA RICKETTS ISSN 0728-5833 Published by ANGLO-AUSTRALIAN OBSERVATORY PO Box 296 Epping, NSW 2121 Australia Epping Lab Telephone +61 2 9372 4800 Fax +61 2 9372 4880 email < email@example.com >
AAT/Schmidt Telephone +61 68 42 6291 Fax +61 68 84 2298 email < firstname.lastname@example.org >
URL < http://www.aao.gov.au >
ANGLO-AUSTRALIAN OBSERVATORY NEWSLETTER APRIL 97page 16