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February 1996

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Volume 13, number 1

S PACE TEL ESCOPE SCIENC E IN STITUTE
Highlights of this issue:

Newsletter
New HST Key Projects for Cycles 7-9
by Bob Williams
We have just finished a very successful year of scientific accomplishments with the HST, as evidenced by the number and impact of HSTrelated articles in the astronomical journals. The Institute is currently working with the HST Project at Goddard Space Flight Center, the STIS and NICMOS Principal Investigators and their teams to prepare for the Second Servicing Mission, scheduled for the early months of 1997. The selection process for the observing programs to be executed in Cycle 6 has now been completed, and we have already begun our preparations for the Call for Proposals for Cycle 7, which begins in July 1997. This early start is necessary so that we may inform the community of the unique new capabilities that STIS and NICMOS will provide. As part of these preparations, we intend to initiate a new round of Key Projects to take full advantage of the new scientific opportunities provided by STIS and NICMOS, as well as those still available through the use of the instruments that will remain on board after the Second Servicing Mission (WFPC 2, FOC, FGS). Key Projects were recommended to the Institute by the 1983 Space Telescope Advisory Committee (STAC) as a way to ensure that large-scale projects directed to answering fundamental questions in astronomy would be carried-out with HST. The concern was that the inevitably high oversubscription rate on HST would lead the Telescope Allocation Committees to limit the amount of observing time allocated per proposal so as to accomodate as many programs as possible. Some of the most important scientific objectives, those for which the HST itself was built, require extensive and uniform studies to reach definitive conclusions. Throughout 1983-1985 the STAC surveyed the astronomical opportunities for research with HST, with the specific task of identifying those programs which were of outstanding scientific importance, which could only be carried out by HST, and which required a large amount of observing time. Another important consideration was the creation of valuable and uniform databases for broad archival studies. The STAC obtained community input and made their recommendations to the Institute to implement three Key Projects for which "proposals to carry-out these projects would be particularly welcome". These projects were the Determination of the Extragalactic Distance Scale, the Quasar Absorption Lines Survey, and the HST Medium Deep Survey. Proposals for each of these three topics were reviewed and selected by the Cycle 1 TAC, and their progress and allocations have been reviewed in each of the following cycles. The Quasar Absorption Line Survey was completed in Cycle 3, and the remaining two Key Projects will be formally completed at the end of Cycle 6. The purpose of this article is to solicit your ideas for new and fundamental astronomical problems that should be tackled with HST and which would require large amounts of

· New Key Projects
-- page 1

· Hubble Deep Field
-- page 3

· Cycle 6 HST Programs
-- pages 12-22

observing time (100-150 orbits/year) over a 3-year period. Our plan is to collect these ideas and discuss them with a newly convened Advisory Committee during the month of March 1996, and to use this Committee's recommendations as topics for new Key Projects which will be solicited in the Call for Proposals for Cycle 7, to be issued in the late Spring of 1996. The success of this activity will depend largely on the scientific input we receive from the astronomical community, and therefore we strongly encourage you to send us your views. To facilitate your response to this request for suggestions for those topics to which Key Projects should be devoted, we have established a page on the World Wide Web : http:// www.stsci.edu/ftp/proposer/cycle7key/ keyprojects.html. We look forward to receiving your thoughts and ideas on this matter.

Hourglass Nebula
The planetary nebula MyCn 18 as seen with HST's WFPC2 camera.


ST ScI Telescope · Director's Perspective Spac e Newsletter Science Institute · N ewsletter

Director's Perspective
by Bob Williams & Mike Hauser
At the moment, the HST project and the Institute are passing through a period of stability and productivity with the telescope and instruments, a period in which we have not had any notable incidents or safe modes in almost a year. The data coming out of the telescope are proving to be of significance to a wide variety of problems in astronomy, and the steadily increasing data in the HST archive are clearly being valued and used by the community as an important resource. It is also pleasing to see the very positive view of the HST being presented by the press. We are now well into Cycle 5 observations, and have allocated most of the GO data reduction funds for these programs. The Cycle 6 TAC recently met and made its recommendations for telescope time, in which roughly 400 GO of the 1025 proposals submitted

have been granted time on HST. Because of the decrease in GTO time and increasing efficiency of telescope operations, GO proposers in Cycle 6 have more available time than in any previous cycle. The development of the two new instruments, STIS and NICMOS, to be installed in HST in the 1997 Servicing Mission is proceeding, as reported elsewhere in this issue. Commensurate with the new capabilities provided by these instruments, the Institute plans to initiate a process by which new Key Projects will be defined and selected beginning in Cycle 7. Information on this process can be found elsewhere in this issue and also on the Institute home page on the WWW. The community should be aware of an important new initiative in which the Institute will help shape follow-up missions to HST. In anticipation of the recommendations of the `HST and Beyond' Committee

chaired by Alan Dressler. NASA headquarters has asked Goddard Space Flight Center (GSFC) and the Institute to lead a study of large telescope technologies for space. The intent is to identify ways in which a 4m class telescope, IR optimized and passively cooled, might be launched into orbit at a project cost well below that of HST. Dr. John Campbell, the HST Project Manager, is leading the study, and he will be assisted by John Mather of GSFC and Peter Stockman of the Institute. Technical experts from GSFC, Marshall Space Flight Center, Jet Propulsion Laboratory, and other organizations will participate in this study, and a panel of senior scientists from the astronomical community will be convened to provide scientific oversight. The study should be completed within two years. The results will provide the basis for development of the next generation large optical/IR telescope in orbit.

Deep Images

On the left we can see the image of the Hubble Deep Field (HDF) from one of the Wide Field chips. For comparison on the right is a 4000 sec V image of the cluster of galaxies CL 0939+4713 in the V band. In this black and white printed image of the HDF you can't appreciate its incredible detail. We invite you to see the full color version or to retrieve the actual files from http://www.stsci.edu/ftp/observer/hdf/hdf.html

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Flux Density [MJy/sr]

The Hubble Deep Field project
H.C. Ferguson While many were home feasting, HST spent the holidays staring at a single non-descript patch of sky, now known as the Hubble Deep Field (HDF). The project arose out of the desire to use a large portion of director 's discretionary time effectively to further studies of galaxy evolution. While several deep images at high galactic latitude had been obtained

8 adjacent "flanking fields" shortly before and after the main observations. The data were released January 15, after three weeks of intensive effort to process, register, and co-add the images. The processed data are available via ftp from stdatu.stsci.edu in the directory /pub/hdf, and details of the observations and data reduction can be found on the observer page of the STScI web site (http://www.stsci.edu/). Version 1 of the data reduction used about 80% of the available data. By the

Figure 1. Example of a simple shift and combine (left) and the result of the "drizzling" technique (right) to obtain the final image. already with the telescope, it was clear that significantly more information on the faintest galaxies could be obtained by (1) targeting a field in the continuous viewing zone and (2) observing through a variety of filters. It was also clear that such images would have a wide variety of uses, and that immediate release of the data to the astronomical community would stimulate a great deal of research. Following the recommendation of a special Institute Advisory committee that met in March 1995, a working group was constituted within STScI, with help from members of the ST-ECF, to plan and carry out the observations. The details of planning issues were highlighted in articles in the previous issues of the STScI and ST-ECF Newsletters. The bulk of the observations were carried out from 18-29 December, 1995, with the addition of 10 orbits on the time this newsletter appears, version 2 of the reduced data should be available. This version will use nearly all of the available data, combined with more nearly optimal weighting. One of the innovations developed for the HDF was the use of "drizzling" to combine frames taken at different positions. For each HDF filter, observations were made at nine different pointing positions in an irregular pattern covering a space of + - 1.3 arcsec from the central position. The drizzling technique was developed by Andy Fruchter and Richard Hook to correct for geometric distortion during the image combination phase, while simultaneously improving the resolution and doing as little injustice as possible to the noise. A comparison of images combined with integer pixel shifts and images combined and subsampled using drizzling is shown in

Fig. 1. The drizzling software is still being modified and improved, but will make its way into STSDAS over the next several months. As anticipated, study of the HDF images and followup observations are now proceeding at a vigorous pace. The images are being used by astronomers at STScI and elsewhere to study the stellar luminosity function in the galactic halo, to measure the optical extragalactic background, to search for weak lensing due to large scale structure, to quantify the morphology of faint blue galaxies and of high-redshift protogalaxies, and to study the clustering of galaxies at the faintest observable levels. Redshifts of several galaxies have already been obtained at the Keck observatory. Infrared Imaging and further spectroscopy will be obtained by various groups this spring from Keck, KPNO, and Calar Alto. We invite all groups and individuals participating in HDF research to make their plans and results known through "HDF Clearinghouse" on the World-Wide Web. This service is simply a set of links to web locations at other institutions, where HDF research can be described. To participate, simply create a web page and send its address to ferguson@stsci.edu. Observers interested in making their data available through the HST archive are also encouraged to contact us.

6 4
Flux Density [MJy/beam]

NGC 891 Radio Thin Disk

2 0 2 0 2 0 125

Halo NW Halo SE

4000

FIR

III

100
3000

III Vx I x III x III III x x V x x x II V x x x V I 1000 O3 x I V III O4 O5 O6 O7 O8 O9 B0 x I x I x I x

75
2000

x V I V I

50 25 0

Spectral Type

B1

4

2

0

-2

-4

Relative Distance Along Major Axis [ ']

Probing Globular Clusters' Cores with the FOC
by Guido De Marchi (ESO)
The FOC has had a strong impact in the field of dynamics and stellar evolution in dense cores of galactic globular clusters (GCs). Thanks to its high spatial resolution, less than 0.05" FWHM) and UV sensitivity (115-- 300nm), it can effectively probe the core population of even the densest clusters in search of dynamically induced abnormalities. The presence of a central cusp in the surface brightness or density radial profile of a GC has usually been interpreted as the signature of a

Science News
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"collapsed" state of the core, i.e. of a very advanced evolutionary state. With its unprecedented resolving power the FOC makes it possible to measure the distributions of stars in GC cores with a much greater accuracy than could be done from the ground. We should then expect to be able to understand the cluster dynamical history much better. Yet, FOC's higher resolution has revealed that the most concentrated cores are indeed so small that there are

Figure 1. Near UV Objective Prism image of a region of ~ 11" в 11" in the core of 47 Tuc obtained with the FOC before the servicing mission. The prism generates, at the position of each star, a dispersion figure (the oblique features seen on the frame), whose length depends on the spectral energy distribution of the object: the bluer the longer. The NUV prism is mostly efficient in the range 190-400 nm. Longward of 400 nm the low dispersion causes the light from the objects to concentrate in a bright spot (red-end). Short, stubby images are the spectra of red objects in the core (particularly red giants with temperature ~4,000K. The prominent feature crossing the frame from side to side is the spectrum of the dwarf nova V2 at outburst, the bluest object in the field. The spectra obtained with the NUV prism have medium resolution (~0.5nm at 250nm), and are particularly useful at estimating the UV continuum of blue objects.

not enough stars to characterize their structure. The concept of density becomes then meaningless, as the distribution of masses can not be approximated by a fluid. The cluster M15 is one of these elusive objects: its

projected density distribution of turn· off mass stars (0.8 Mo, those contributing the most to the total light) is fitted by several different models going from a power-law function all the way to the center, to a King-type model with a small core radius (~ 1.8"). Similarly, 47 Tuc has seen its core radius drop by more than a factor 2 to ~ 10" as a result of high resolution observations. Intuitively, one would think that going to fainter magnitudes would increase the sample by adding stars less massive than those above the turnoff, and therefore improve in a statistical sense the determination of the dynamical state. Because of their lower masses, however, these fainter objects have a radial distribution which differs from that of the brighter stars, and their number decreases rather than increasing at fainter magnitudes, as a result of mass segregation (see below). These difficulties although discouraging, had long been predicted by theoretical studies. Collapse may also involve only certain types of heavier stars that decouple from the system leaving the profile of other objects unchanged. Understanding the dynamical state of the cluster on the basis of these profiles therefore becomes tremendously difficult, if at all possible. With the FOC, one can look for stellar populations whose characteristics deviate from those expected from a normal evolutionary sequence of single, coeval, and isolated stars. The specifics of the collapse process immediately suggest that such populations would preferentially inhabit the central core regions where they are formed in greatest abundance. The very high densities reached even for a short time in the core coupled to the relatively low stellar velocities automatically ensure a copious production of binaries by 3-body dynamical or 2-body tidal capture and their evolutionary products such as blue straggler stars (BSS), stripped red giant cores, contact binaries, etc. These populations would tend to cluster physically in the core and parametrically in the blue-UV region of the

core's color magnitude diagram. With the FOC for the first time, these strange populations can be studied in detail right where they are produced. The first example of this application was the discovery made soon after launch by the FOC of a centrally concentrated population of BSS in 47 Tuc (Paresce et al. 1991), followed by similar findings in all the clusters obeserved thereafter (M15, NGC6254, NGC6397, NGC6752, M3, Cen). These results consistently suggest a BSS source rate which is actually enhanced in crowded environments due, most likely, to mergers of main sequence stars as a consequence of direct collisions involving single and binary stars. The latter are particularly important in the dynamical evolution of a cluster, in that their presence may retard or prevent collapse from occurring at all, but they can also power the re-expansion of the core after collapse by energy transfer to other stars. The sensitivity of the FOC to the UV light makes this instrument particularly efficient at revealing interacting binary systems in GC cores. Besides the two dozen BSS, which are supposed to have a binary origin, two cataclysmic variables have been found in the core of 47 Tuc. The first, V1 (Paresce, De Marchi & Ferraro 1992), is likely the UV counterpart to the variable X-ray source X0021.8-7221. If it is, then it is almost certainly an asynchronously rotating DQ Her type binary with a period of around 6 hours. The second one, V2 (Paresce & De Marchi 1994), is the first dwarf nova detected at outburst right in the core of a GC. The astounding low-resolution spectrum of this object obtained with the FOC's near-UV Objective Prism (Figure 1) clearly shows the signature of an eruption typical of dwarf novae (F -2.3). Four blue variable objects have been found in the core of NGC6397 (De Marchi & Paresce 1994b), most likely the UV counterpart to multiple X-ray sources discovered with ROSAT (Cool et al. 1993), with fluxes typical of cataclysmic variables. Their broad-

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band UV spectra are compatible with temperatures in the range 15 20,000K, as expected for accretion disks around interacting binaries. Finally, the FOC has shown that the core of M15 harbors a tightly concentrated group of about 15 very blue objects (bluer than the BSS), which could be the result of catastrophic encounters between red giants and compact stars, probably in binaries, that would strip the extended gaseous envelope of the former revealing a hot, blue core (De Marchi & Paresce 1994a). Similar but less numerous objects are also seen at the center of NGC6752 (Shara et al. 1995). With this cornucopia of exotic objects in their cores, it is no longer necessary to rely on the ambiguous surface brightness profiles to define these clusters' dynamical state: a postcollapse phase is legitimate for all of them, wherein the core stellar population is strongly and directly modified by dynamical effects. But GC cores undergo also much quieter and smoother population changes which are, nevertheless, fundamental in their dynamical life. Because of the relaxation process, these systems evolve towards energy equipartition through repeated stellar encounters, in which more massive stars transfer kinetic energy to lighter objects (which then move outward onto larger orbits) and sink into the potential well at the cluster center. The net effect of this differential migration, called mass segregation, had long been predicted theoretically, yet had so far proved tremendously difficult to confirm observationally, particularly in dense cores. The first deep investigations with the COSTAR-corrected FOC of the inner few arcseconds of 47 Tuc (Paresce, De Marchi & Jedrzejewski 1995, see Figure 2), NGC6397 (King, Sosin & Cool 1995), and M15 (De Marchi & Paresce 1996) have revealed a dramatic drop in the luminosity function (LF) of main sequence stars that begins right from the turn-off and continues all the way down to the detection limit. Low mass stars are strongly depleted in the core

with respect to the outer regions, as is evident in Figure 3. The drop observed in the core LF of these clusters further confirms their advanced evolutionary state. In the near future, with STIS, radial velocity measurements will become possible also for main sequence stars in dense cores. Coupled to proper motion determinations, they will allow a firmer discrimination between pre- and post-collapse phases. Studying the properties of the local stellar population in GCs, however, will remain one of the most powerful ways of investigating the dynamical history of these fascinating objects. References Cool, A., et al. 1993, ApJ, 410, L103 De Marchi, G., & Paresce, F. 1994a, ApJ, 422, 597 De Marchi, G., & Paresce, F. 1994b, A&A, 281, L13 De Marchi, G., & Paresce, F. 1996, ApJ, in the press King, I.R., Sosin, C., & Cool, A. 1995, ApJ, 452, L33 Paresce, F., et al. 1991, Nature, 352, 297 Paresce, F., & De Marchi, G. 1994, ApJ, 427, L33 Paresce, F., De Marchi, G., & Ferraro, F. 1992, Nature, 360, 46 Paresce, F., De Marchi, G., & Jedrzejewski, R. 1995, ApJ, 442, L57 Shara, M., Drissen, L., Bergeron, L., & Paresce, F. 1995, ApJ, 441, 617

Figure 2. Deep UV color-magnitude diagram of the population in the core of 47 Tuc. The main sequence becomes sparser towards fainter magnitude, as a result of mass segregation. The triangle marks the quiescent state of V2.

Figure 3. Solid line: LF of the core population of M15 in the F346M band. Dashed line: LF of a region located ~5' away from the center obtained with the WFPC2 in the F814W band, converted to F346M and normalized to the other . at the turn-off. At m346~22 (~ 0.65 Mo) the number densities differ by a factor 10, in fair agreement with theoretical expectations.

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Paris Meeting
by Mario Livio
The second conference on science using HST was held in Paris, December 4-8 1995. In spite of the difficulties introduced by the general transportation strike in Paris, the meeting was attended by more than 250 participants. The most striking thing about the data presented at the meeting was the extent to which HST has impacted ALL of the areas of current astronomical research. Observations of the solar system provided both spectacular images and abundance determinations in objects ranging from Venus to Pluto. Observations of star-formation regions and of young stellar objects revealed proto-planetary disks, jets emanating from the centers of accretion disks and unprecedented details in the structure of jets and Herbig-Haro objects. The study of stars of all masses now permits the construction of theoretical models for winds from massive stars, for supernova explosions, and for the shaping of planetary nebulae and structure formation in supernova remnants. The ability of HST to resolve stars in extremely crowded

fields and to determine their properties has facilitated amazing progress in the study of globular clusters and in the understanding of stellar populations. In extragalactic astronomy, the HST resolution was again instrumental in revealing the properties of a variety of Active Galactic Nuclei, the characteristics of their host galaxies and the detailed structure of the jets emanating from them. HST provided convincing evidence for the presence of supermassive black holes in M87 and NGC 4261. Observations of the intergalactic medium, of QSO absorption line systems and of galaxies and clusters at redshifts ranging from zero to 3.4 begin to place very meaningful constraints on cosmological models and on galaxy formation. HST is the key tool used for recent determinations of the Hubble Constant, and for the calibration of a variety of methods of distance scale determinations. A special session was devoted to future HST science instrumentation. In this session, the servicing missions of 1997, 1999 and 2002 were discussed, as well as the recommendations of the "HST And Beyond" study chaired by

Alan Dressler. The capabilities and expected performance of the future HST instruments were also presented. Finally, an education session was held, in which, among other things, innovative ways of using HST data through the Internet were discussed. A hands-on activity on object classification was demonstrated, utilizing many of the concepts which the HST data can help elucidate.

H0 Key Project
To diseminate the results to a wider audience the H0 Key project has set up a WWW page at: http://www.ipac.caltech.edu/ H0kp/ In that page you can currently find the abstracts of published papers, postscript files of accepted papers, and addresses of authors to contact for preprint information. Eventually, an archive of H0 Key Project data will be available.

News
HST & Beyond Committee Report
by Goetz Oertel, AURA President
The Oertel Report of the "HST & Beyond" Committee was released in January 1996. This report makes specific recommendations toward the scientific goals of: (1) direct study of the birth and evolution of galaxies like the Milky Way; and (2) the detection of Earth-like planets around other stars and the search for evidence of life on them. Draft copies of the Report were available at the January 1996 AAS meeting in San Antonio. The HST & Beyond Committee was established by AURA in the spring of 1994. The Committee is chaired by Alan Dressler (Carnegie Observatories) and comprised of members from a cross-section of the U.S. astronomy community and Richard Ellis for the ESA community. Thanks to the committee for its visionary and thoughtful report!

Explore HST's greatest hits at http://www.stsci.edu/public.html.

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Scientific Instruments Status
by Ron Gilliland
All of the instruments on HST have operated smoothly and productively throughout the fall and early winter. Even the long idle f/48 relay of the Faint Object Camera is now returning excellent science observations. For all of the instruments efforts continue to maintain and extend the calibrations and definition of data characteristics. More information on the science instruments may be obtained from the World Wide Web which is a repository for frequently asked questions, calibration details, and late-breaking instrument news.

FOC
The f/96 relay has continued to provide excellent science observations. As detailed in recent Newsletter articles the FOC f/48 relay has been making a comeback from operational problems with the high-voltage system first encountered in September 1992. Development of more robust operational procedures, and extensive characterization of the instrument has resulted in bringing the f/48 relay back to limited science observing status in order to allow use of its unique longslit spectroscopic capability. The first science program using the f/48 spectrograph -- Long-Slit Spectroscopy of the Center of M31, Dr. Ivan King executed successfully on 5 December 1995. The target acquisition strategy appears to have worked perfectly. Use of an 8-hour warmup period allowed the detector background noise level to stabilize somewhat, avoiding the high levels that had been seen shortly after the high voltage switch-on in earlier f/48 tests.

performing FOS target acquisitions, the FOS has been particularly sensitive to the new FGS behavior. When guide star lock is not established at the time the FOS prepares to begin an exposure, the FOS is shut down for the duration of the observational sequence as a safety precaution. Five FOS observational sequences have been lost since 1 October '95 due to the new FGS1 lossof-lock problem. Four of the occurrences were during ACQ/PEAK target acquisition sequences and one during an initial moving target tracking slew. Such lost observations are automatically repeated, but of course result in lowered observing efficiency overall. New operational procedures are in place and are resulting in a much lower rate of lost observations. During this quarter the FOS was used to obtain the highest time-resolution data of the post-COSTAR era with 0.25 second readout times for the November 1995 stellar occultation by Saturn and its rings (A. Bosh, Lowell Observatory). Also during November a near-record volume of RAPID mode data were obtained during nearly 10 continuous orbits (CVZ) of high time-resolution observations (3- and 6-sec readouts) to facilitate tomographic analysis of the cataclysmic variable HT Cas (J. Wood, Keele University).

was below the background rate, and consequently the FLYLIM option was used. FLYLIM is an on-board software algorithm to increase the S/N by suppressing bursts of background noise. The PI of this program (C. Hogan, U. of Washington) hopes to derive more stringent limits on the He II Gunn-Peterson effect. NGC 1741 is a starburst galaxy whose optical spectrum indicates the presence of hundreds of Wolf-Rayet stars. A G140L spectrum was obtained, whose quality rivals any other ultraviolet spectrum in existence for this class of object. The PI Of the program (P. Conti, U. of Colorado) will use the numerous stellar and interstellar lines in the GHRS spectrum to study the stellar population and the interstellar medium of NGC 1741.

WFPC2
The WFPC2 continues to work extremely well. As WFPC2 is the youngest science instrument on HST, the level of data characterization issues under investigation remains significant. The most troublesome issue to surface in recent months concerns a possible photometric zeropoint difference for long versus short exposures. This effect was first reported by GOs performing careful calibrations in support of the extragalactic distance scale key project. Further investigation shows that "long" vs. "short" is probably a misnomer. The level of background appears to be the main parameter rather than the exposure time. The magnitude difference measured between short and long exposures is more pronounced for faint stars in large apertures, where it can reach 0.05 mag, and is essentially absent for stars with more than 1000 total counts. The dependence on aperture and magnitude appears consistent with a charge transfer efficiency problem. The offset of faint star magnitudes can be explained by a loss of 0.3 DN (2 e-) in each pixel used in the aperture. We are actively pursuing this problem with further testing and analysis and encourage those who require accurate absolute photometry for their observations to

GHRS
As with the FOS, the GHRS has continued to provide a steady stream of high quality science observations, although the GHRS also experienced a few lost observation sets due to the FGS1 loss-of-lock problem. The GHRS in combination with grating G140L on Side-1 is becoming more and more popular among observers of faint (by GHRS standards) extragalactic targets. One of the reasons is the capability of the GHRS to make use of the FOS acquisitions of faint targets, which has been made available in Cycle 5. One of the faintest objects ever observed with the GHRS was the QSO Q0302-003, which is famous as a test case to study the He II GunnPeterson effect. A series of successful spectra was obtained during several v