So ISO has finally run out of Liquid Helium and, after a one month Technology Test Phase (TTP), was switched off. Information on these events can be found on the front cover and in the `Project Status' article.

Even though ISO ran out of Helium, observations with SWS continued through the TTP! The band 1 detectors (2.36 - 4.08 microns) could still operate at the enhanced temperatures encountered, and a special observing programme was designed prior to Helium depletion to take advantage of any spare time in the TTP schedule. Information on how SWS coped with the new temperature regime and of the observing programme can be found in the articles `SWS operations post-Helium' and `The SWS stellar classification programme'.

This issue also contains observing statistics for the routine phase and the final list of successful discretionary time proposals.

Figures 1 to 6 give details of the number, type and length of observations carried out by ISO. This list is close to final; however the quality control process is continuing for the last few revolutions. Note that the given numbers do not include observations made during the Performance Verification phase, or calibration observations (which also includes a few polarimetry observations).

Shown in figures 1 to 4 are the distribution of observing time per instrument and per observing mode, expressed both in terms of number of observations, and also by observing time.

By the time ISO's Helium had been depleted, 26200 successful science observations had been performed, along with approximately 4000 calibration observations performed using the normal observing modes. Each of these 30000 observations lasted on average just over 23 minutes. The facility to concatenate two or more observations was used rather extensively however, and therefore each observation chain lasted on average 40 minutes. Due to the skewness of the observation duration distribution, the median length of an observing block lies between 10 and 20 minutes. Distributions are shown in figures 5 and 6.

Figure 5: Log distribution of the number of observations against time for all observations

Figure 6: Linear distribution of number of observations against time for observations of less than 1 hour

ISO's supply of liquid helium ran out on 8 April, thus bringing to an end the highly successful routine operations phase of the world's first true orbiting infrared observatory. This phase lasted nearly 1 year longer than specified. Activities then turned to a 1-month period of calibrations and technology tests prior to the de-commissioning of the satellite on 16 May. During the next 3.5 years, astronomers all over the globe will collaborate to re-analyse and re-calibrate all of ISO's 30000 individual observations to extract the maximum scientific return from this unique mission and to leave behind a legacy archive to serve the community for the next decades.

Some of the main activities during the technology test programme were testing the operation of the star trackers at low altitudes, i.e. in the radiation belts, use of the on-board redundant units that were not needed during the routine operations due to the superb performance of the satellite, and evaluation of the software intended to overcome multiple gyro failures. The results of these test will be of great benefit to other ESA spacecraft, e.g. XMM and Integral, which use some of the same components. During gaps in this programme, the shortest-wavelength detectors in the Short Wavelength Spectrometer were used to measure the 2.36 - 4.08 micron spectrum of nearly 300 stars to extend their spectral classification to the infrared.

The `last light' of ISO occurred just before midnight on 10 May and was an SWS spectrum of Brackett alpha emission in a hot supergiant star. Preliminary results show that this star may well have a disk!

Two `de-orbiting' manoeuvres were carried out on 11 and 14 May to lower the perigee from 1380km to 715km. In this new orbit, it is expected that ISO\ will re-enter the Earth's atmosphere in some 20-30 years time. At 14.00 CET on 16 May, the final ground command to turn off the transmitter was executed, thus ending the operational life of ISO.

Just prior to exhaustion of liquid helium, some of ISO's latest results were announced to the Press at a briefing in London. Among these discoveries were the finding of water vapour on Saturn's moon Titan, complementing ISO's many detections of water throughout the cosmos, detections of young stars in the Orion region of the sky and measurements of infrared galaxies at immense distances.

The detailed analysis and interpretation of the ISO results is only just beginning. All ISO data will be re-processed with the latest software over the summer and an archive will be opened to the astronomical community in the autumn. Thereafter, there will be an international effort, co-ordinated by the ESA ISO Data Centre at Villafranca in Spain, to maximise the exploitation of the ISO data and to prepare the best possible final archive to leave as a legacy from ISO to future astronomers. Many more discoveries still await us!

A small pool of ISO observing time, called ``Discretionary Time'', was kept available for observations which could not have been foreseen at the time of the proposal process. A list of discretionary time proposals for which observing time was recommended since the last newsletter follows. Note however, that observations were not made for all of these programmes due to the date at which ISO's liquid Helium became depleted.

PI	Institute	Proposal
Ehrenfreund, P.	Sterrenwacht Leiden, NL	Spectroscopy of Dust and Ice in MonR2 IRS2
Eriksson, K.	Uppsala Astr. Observatory, S	A Spectrum of the Envelope around the Red Giant R Doradus
Forveille, T	Observatoire de Grenoble, F	ISOCAM CVF spectrum of the nearby Brown Dwarf
		DENIS-P J0205-1159
Gorny, S.	N. Copernicus Astronomical	Planetary Nebulae with [WR]-Type Central Stars:
	Center, Torun, Poland	Transition from O- to C-rich dust composition.
Hough, J.	University of Hertfordshire, UK	Polarimetry of the nucleus of active galaxies:
		AGN with hidden broadlines and rising IR polarization
Jourdain de	LAEFF-INTA, Madrid, E	The Remarkable Dust Shell of the B[e] star MWC922:
Muizon, M.		ISO discovers strongest olivine bands ever
Klaas, U.	ISO SOC, VILSPA, E	Far Infrared Polarisation of Beta Pictoris
Krugel, E.	MPI für Radioastr., Bonn, D	The shape of grains in the normal galaxy N1808
Lagage, P.O.	C.E.A., Saclay, F	Gas and Dust in the Kepler SNR
Lemke, D.	MPI für Astronomie, Heidelberg, D	Large-scale Map of the Orion Molecular Cloud/Star
		Formation Complex at 200 m
Lemke, D.	MPI für Astronomie, Heidelberg, D	Completion of 175 m map of the SMC
Malfait, K.	Instituut voor Sterrekunde,	Circumstellar Chemistry of the Main Sequence Star
	Leuven, B	Beta Pictoris
Martin-Pintado, J.	Observatorio Astronomico	The spatial distribution of the recently discovered
	Nacional, E	ionized and hot neutral gas in the Galactic Center
Mattila, K.	University of Helsinki	Large-scale Mapping of the Orion-B Molecular Cloud/
	Observatory, SF	Star Formation Complex (Lynds 1630) at 200 m
Metcalfe, L.	ISO SOC, VILSPA, E	FIR photometry and CAM CVF observations of a
		disk around BD+31 643
Morris, M.	UCLA, USA	Large-Scale Shocks in the Galactic centre and other Regions
Morris, P.	ISO SOC, VILSPA, E	The Molecular Content of the WR Nebula NGC2359
Mueller, T.	ISO SOC, VILSPA, E	Polarimetry of Asteroids
Pilbratt, G.	SA, ESTEC, NL	SWS spectroscopy of an unknown extended source
		in the M16 area
Prusti, T.	ISO SOC, VILSPA, E	Variable infrared emission from circumstellar
		environment of young intermediate mass stars
Prusti, T.	ISO SOC, VILSPA, E	Evolution of the circumstellar environment during
		an FU Ori outburst
Schutte, W.	Sterrenwacht Leiden, NL	The 6.2 m Feature of Carbonaceous Dust
		towards CYGOB2# 12
Siebenmorgen, R.	ISO SOC, VILSPA, E	Mid-IR polarimetric imaging of the protostellar
		candidate HH108MMS
Stark, R.	MPI für Radioastr., Bonn, D	Density structure and evolutionary state of TMC-1
Stickel, M	MPI für Astronomie, Heidelberg, D	ISOPHOT Follow-up of interesting Serendipity Sources
Stringfellow, G.	Center for Astrophysics and Space	Temporal Evolution and Dust Formation(?) in Nova
	Astr., Univ. of Colorado, USA	Sgr 1998: The ISO Mission Ends with a Bang!
Szczerba, R.	N. Copernicus Astronomical	Infrared carbon stars with OH emission:
	Center, Torun, Poland	Transition from O- to C-rich stars
Thompson. D.	MPI für Astronomie, Heidelberg, D	Far-Infrared Photometry of an Extremely Red Galaxy
Thornley, M.	MPE, Garching, D	The Arches Cluster: Probing the interaction between
		massive stars and the ISM in the Galactic Center
Thuan, T.	Univ. of Virginia, Dep.	Dust and infrared emission in the extremely
	of Astronomy, USA	metal-deficient blue compact galaxy SBS 0335-052
Trams, N.	ISO SOC, VILSPA, E	The LMC AGB star IRAS04496-6958: A carbon
		star with silicate dust?
Unger, S.	IPAC, CalTech, USA	Observations of the SNR 3C58 using the ISO Spectrometers
Andre, P.	C.E.A., Saclay, F	Spectral Energy Distribution of a Newly-Discovered
		Protostar in Taurus
Barvainis, R.	Haystack Observatory, USA	An Einstein Ring with Strong Submillimeter Emission
Bertoldi. F.	MPE, Garching, D	High H rotational lines in Orion IRc2 Pk1
Bregman, J.	University of Michigan, USA	The Origin of Mid-IR Emission from Elliptical Galaxies
Castro-Tirado, A.	LAEFF-INTA, Madrid, E	The nature of the superluminal galactic source GRS 1915+105:
		ISO monitoring of the most powerful source in the Galaxy
Cernicharo, J.	CSIC, Madrid, E	Discovery of a Dust Jet and a Circumstellar Disk in HH1-2
Cernicharo, J.	CSIC, Madrid, E	Warm Dust around Hot Stars : The Ionizing Source
		of the Trifid Nebula
van der Werf, P.	Sterrenwacht Leiden, NL	Stellar population in the z=3.5 radio galaxy
		6C1909+722: A genuine primaeval galaxy?
van Hoof, P.	University of Kentucky, USA	Accurate abundance determination of CNO
		in PN: An investigation into the t problem.
Waters, R.	Astronomical Inst. Amsterdam, NL	First Detection of Crystalline Silicates in the LMC:
		Unravelling the Mass Loss History of R71
Waters, R.	Astronomical Inst. Amsterdam, NL	A protoplanetary disk around the Red Rectangle?
Wesselius, P.	SRON, Groningen, NL	Follow-up SWS observations on
		intermediate-mass YSOs
Wright, C.	Sterrenwacht Leiden, NL	ISOPHOT Polarisation Observations of two
		Bipolar Outflow Sources
Zavagno, A.	Obs. Marseille, F	Imaging of a massive star forming region:
		Study of AFGL4029

After Helium loss, and prior to the start of the Technology Test Phase, a series of tests were carried out on SWS to verify the proper functioning of its shortest wavelength band 1 detectors (2.36 - 4.08 microns) at the elevated temperatures then being experienced in the cryostat. Despite the focal plane temperature being above nominal, and increasing at the slow rate of about 4 K per day, the band 1 detectors were seen to function properly with only minor adjustments to the instrument settings needed. The go-ahead was therefore given to continue observing up to the start of, and, thereafter, in any gaps in the Technology Test Phase.

The stellar classification programme started on 13 April. At that time, the instrument temperature sensors were out of range (with T ;SPMgt; 20 K), and the upper baffle temperature was around 25K. About 20 band 1 observations were scheduled and some quickly reduced, with the data quality looking very good. Compared to nominal mission conditions, detector dark current and dark noise had not significantly changed, while the responsivity had increased. On the second day, responsivity had increased by 25% to 45% across the 12-element detector array. The wavelength calibration had been slightly drifting, by up to 1 scanner step per day, depending on wavelength.

The high quality of the data was confirmed by spot checks of a few science observations. As an example, spectra of three carbon stars of the same spectral type were found to be practically identical. The data quality was further confirmed at temperatures as high as 40 K, the upper limit of the ISO upper baffle temperature sensor calibrated range. For this reason, pending observations in the ISO Mission Database, making explicit use of the Band 1 detector, were introduced into the post-Helium programme.

As the observation programme progressed, the detector response and the wavelength calibration continued to drift. In addition, changes were recorded in the Relative Spectral Response Function, in the form of low-resolution features. These trends will be fully characterised, using a number of detector and wavelength calibration measurements which were inserted into the programme, and it is not expected that they will cause a problem for data reduction (although they will have to be processed in a different manner to normal observations).

By the time ISO was switched off, almost 150 hours had been filled with observations of nearly 300 stars and the band 1 detectors of SWS had operated successfully at temperatures of up to 60K.

The post-helium stellar classification programme was aimed at obtaining deep spectra at the full SWS grating resolution in the 2.36 to 4.08

m wavelength range for a sample of bright stars covering the whole MK classification scheme. This will allow stars to be characterized at infrared wavelengths in the context of this scheme (effective temperature, gravity, luminosity, and chemical composition).

Since the MK classification is widely used to determine the stellar type, and from that the evolutionary state, the extension to the infrared will be very useful for cases where only the infrared part of the spectrum is available. A good example of such a case is the massive, evolved stars near the galactic centre, that are only observable at infrared wavelengths.

For late type stars, the peak of the energy distribution is at near-infrared wavelengths. This wavelength range contains many important molecular bands (e.g. CO, H

O, OH, SiO, HCN, C

) that will constrain the spectral type of stars, sometimes even better than the optical spectra. For hot stars, the 2.36 - 4.08

m spectral range contains HI lines of the Brackett, Pfund and Humphreys series, as well as strong HeI and HeII lines. All these lines are very sensitive to temperature, gravity, and the presence of a stellar outflow.

The spectra in the sample are also important for case studies of individual stars; indeed, many of the brightest stars serve as prototypes of classes of objects, and these observations will extended the characterisation to the infrared wavelengths.

In addition, the sample can be used for stellar population synthesis studies. Observations of distant stellar systems (globular clusters, galaxies) at infrared wavelengths contain the integrated light of all stars in such a system. In order to characterize the stellar population (age, star formation history, metallicity) a database of observed stars covering a wide range of spectral types at infrared wavelengths is very useful.

The data will become public as soon as data reduction is at a satisfactory level, and an atlas of stellar spectra will be published.

Observant readers of this issue of ISO Info will have noticed that the URL for the ISOWEB (printed on the front page) has changed from ESTEC, NL (http://www.iso.vilspa.esa.es/) to Vilspa, Spain (http://www.iso.vilspa.esa.es). This move was taken to centralise all ISO services at one site (Vilspa), and was carried out in mid-May. Currently all accesses to the old URL are being re-routed automatically, but users are requested to switch their bookmarks to the new URL.

ISO INFO is edited by:
Kieron Leech ISO Resident Astronomer
Villafranca del Castillo,
Satellite Tracking Station,
P.O. Box 50727, 28080 Madrid, Spain
Telephone: International +34-91-813-1254
Telefax: International +34-91-813-1308
e-mail: `kleech@iso.vilspa.esa.es'

To receive a copy of this newsletter please contact K. Leech at the above address. If you wish to be added to the ISO Info mailing list please supply your name, address, phone and fax numbers and e-mail address.

Liquid Helium depleted on April 8 1998: Satellite switched off on May 16