Solar system

Leech and Altieri, in a team led by Crovisier (Observatoire de Paris-Meudon) and in collaboration with Bockelee-Morvan and Lellouch, observed the new comet Hale-Bopp with the PHOT-S, Short Wavelength (SWS) and Long Wavelength (LWS) spectrometers on ISO (Crovisier et al., 1996). Data were obtained for three epochs. The first observation on 27 April 1996 was when Hale-Bopp was 4.6 AU from the Sun. ISOPHOT obtained the 2.5-12 µm spectrum at low resolution. This shows emission in the CO₂v3 band. The CO₂ production rate is about 1.3x10²⁸/s. The 6-12 µm spectrum shows thermal emission at a colour temperature of 162 K (6-8 µm range) and a strong silicate band around 10 µm, with a narrow feature at 11.2 µm indicative of crystalline silicates. Further observations with PHOT-S, SWS and LWS were taken in late September and early October 1996, indicating that the comet had brightened considerably. The SWS spectra in Fig. 4.3.6/1 show spectacular evidence for silicate bands and emission from water lines. The silicate features, strikingly similar to those seen in the dust envelopes of stars, strengthen the link between comets and the interstellar material. Spectra of the coma were also obtained in early September 1996. The spectra of this new comet will be compared with that of an older comet, P/Kopff.

Figure 4.3.6/1: An initial reduction of an ISO Short Wavelength Spectrometer observation of Comet Hale-Bopp. The broad features are probably due to silicates, and are similar to features seen in dust envelopes of stars.

The programme to establish far-IR standards for ISO-PHOT by using asteroids was continued by Schulz, in collaboration with Telesco, during an observing run on IRTF, Hawaii, in July 1995, where more observational data of candidate asteroids were gathered. Early ISO data on asteroids, as well as parallel JCMT observations at 300-1200 µm, already indicate some need for refinement of the standard thermal model, at least at the longest wavelengths (in collaboration with Müller, Osip and Telesco).

Together with Peschke and Grün, Schulz took advantage of the unique opportunity to observe 2060 Chiron with ISO before its distance to the Earth became larger than 8 AU. This special object, with characteristics between an asteroid and a comet, was observed for the first time in the ISOPHOT filters at 25, 60, 100 and 160 µm. Data analysis is ongoing.

Reference
Crovisier, J. et al. (1996). A&A 315, L385.

Stellar and circumstellar

A collaboration headed by Habing (University of Leiden), and including Jourdain de Muizon (Laeff, Spain), Heske, Kessler, Laureijs, Leech, Metcalfe, Salama, Sieben-morgen and Trams, is involved in a search for Vega-like discs around nearby main sequence stars by making photometric measurements with ISOPHOT at four wave-lengths between 25 µm and 180 µm. The goal of this survey is to establish how common Vega-like discs are, and how their presence depends on the fundamental properties of main sequence stars (mass, age, rotation, etc). The first results that concern Vega itself and g Lup were published in A&A (Habing et al., 1996). The results obtained by IRAS are confirmed and new fluxes at 135 µm and 180 µm are obtained. A first table of measurements on a larger sample has also been published. The preliminary conclusion is that Vega- like discs are common, but are not a general feature of all main sequence stars with masses above 0.9 M_Sun. At the time of writing, recent measurements of another programme star (with an IRAS-detected excess) appear to show a clear excess at 135 µm and 180 µm, which would indicate the presence of rather cold dust.

Salama, together with other members of the ISO-SOC (Barr, Leech and Kessler) and in collaboration with Evans, Eyres (Univ. of Keele, UK), has led a programme of ISO observations of a number of novae. Nova V1974 Cyg has been observed with the Short and Long Wavelength Spectrometers (SWS and LWS) and the spectrometer section of ISOPHOT in March 1996. The following lines were detected: [NeIII] at 15.5 µm, [NeV] at 14.3 and 24.3 µm, [NeVI] at 7.6 µm, [OIII] at 51.8 µm and [OIV] at 25.9 µm. On the other hand, [NeII] at 12.8 µm, which was strong during the early development of this nova after its recent outburst in 1992, was not securely detected. The lines in the SWS range, as shown in Figs. 4.3.6/2 and /3, are not only resolved, but also show structures, indicative of the breaking up of the ejecta into polar caps and tropical and equatorial rings, with velocities of 100-500 km/s. The Ne and O line profiles suggest that these species - and possibly even different ionisation states of Ne - arise in different regions of the ejecta. Electron temperatures of 10^5.35K and 10^5.05K in the Ne- and O-bearing regions, respectively, were derived. Ne was confirmed to be overabundant relative to O, in the ejecta (Salama et al., 1996).

Figure 4.3.6/2: Structure in the [NeV] 14.3 µm line of the nova V1974 Cyg, as observed with the ISO-SWS in March 1996. The dashed line is the fit to the underlying rebinned spectrum. Gaussian fitted components are shown below, shifted for clarity. Dotted lines indicate the range in which the baseline has been determined.

Figure 4.3.6/3: Structure in the [NeV] 24.3 µm line of the nova V1974 Cyg, as observed with the ISO-SWS in March 1996.

Trams, in collaboration with Waters (University of Amsterdam) and Voors (University of Utrecht), has obtained IR images, at eight wavelengths using ISOCAM with the Circular Variable Filters and a pixel size of 6 arcsec, of AG Car and HR Car, two Luminous Blue Variables (LBVs) known to have nebulae from imaging in the optical. The ISOCAM images were taken at the following wavelengths: 8.689, 8.993 ([ArIII]), 11.480 (continuum and PAH), 12.410 (Humphreys alfa), 12.820 ([NeII]), 13.530, 15.580 ([NeIII]) and 15.960 µm. The images show that the nebula around these stars is especially bright in the [NeII] line, with a virtual absence of [NeIII]. Also, the nebulae are brighter at 11.480 µm, suggesting the presence of PAHs in these circumstellar shells. The images of AG Car allow the independent determination of the spectral energy distribution of the star and the nebula at 9-16 µm. From this it is clear that most (>85%) of the IR flux seen in this object originates in the nebula, and is due to thermal emission from dust in the nebula (Trams et al., 1996). Images of a third object (G79.29+0.46), shown in Fig. 4.3.6/4, were obtained in November 1996. This LBV candidate is known to have a shell visible at radio wavelengths.

Figure 4.3.6/4: The ISOCAM images (ISOCAM Off Line Processing data products) of G79.29+0.46. The wavelengths are (from top left to bottom right): 8.689, 8.993 ([ArIII]), 9.986, 11.480 (continuum and PAH), 12.410 (Humphreys alfa), 12.820 ([NeII]), 13.530, 15.580 ([NeIII]), 15.960 and 16.980 µm.

New, absolutely calibrated, continuous spectra at 3-35 µm for three southern standard stars were constructed (Cohen et al., 1996) from observations from the NASA Kuiper Airborne Observatory (KAO) by Salama and Metcalfe, participating within the group led by Witteborn. The 15-30 µm range was covered by Si:P BIB detectors, provided by the Astrophysics Division.

References
Habing, H.J. et al. (1996). A&A 315, L233.
Salama, A. (1996). A&A
315, L209.
Trams, N.R., Waters, L.B.F.M. & Voors, R.H.M. (1996). A&A 315, L213.
Cohen M. et al. (1996). AJ 112, 241.

Interstellar medium

Laureijs, Schulz and Gabriel, together with co-workers from the ISOPHOT consortium (Acosta-Pulido, Abráham, Kinkel, Klaas, Castaineda, Cornwall, Heinrichsen, Lemke, Pelz and Walker), observed the reflection nebula NGC 7023 at various positions using wide and narrow band ISOPHOT photometry centred on the 7.7 µm and 11.3 µm PAH emission features and wide band photometry at 20 µm and 25 µm. No significant variations were detected over the nebula in the 7.3 µm and 20 µm continuum relative to the total emission in the 6-15 µm range (hereafter Gamma). This result supports the hypothesis that most of the continuum emission in both bands comes from very small grains. In contrast, the 7.7 µm and 11.3 µm PAH emission features show clear variations. It is found that, in weaker radiation fields of the nebula, the 11.3 µm and 7.7 µm features are not the dominant emission components in the 6-15 µm range. The 11.5 µm band increases whereas the 11.3 µm emission feature decreases relative to Gamma in the outer parts of the nebula. The effect can be explained by increased hydrogenation of PAH molecules.

Using ISOPHOT, Laureijs, Burgdorf and Prusti, in collaboration with Haikala, Clark, Liljeström and Mattila, detected very cold dust in a small and isolated dust cloud. Observations at 60, 90, 135 and 200 µm show a constant surface brightness ratio I₁₃₅/I₂₀₀ and a flattening of I₉₀/I₂₀₀ towards the cloud centre. The dust temperature derived from the far-IR colours can be confined to the range of 12-15 K assuming a lambda^-2 dust emissivity. The 60 µm emission comes mainly from outer dust layers surrounding the core. It is inferred that a fraction of the power in the 90 µm band comes from the warmer grain component also causing I₆₀. This fraction is approximately equal to the total power in the 60 µm band. The cloud is also detected on ground in CO, ¹³CO, C¹⁸O, HCO⁺ and CS. The detections indicate that the dust emission is associated with a molecular cloud core where n(H₂ )>3x10⁴/cm³. Using the column density derived from the molecular observations, an extinction cross section of 4.1-9.5 cm²/H-atom is derived.

Inside cold and dense clouds, dust grains are modified in three ways: they acquire ice mantles, coagulate (thus growing) and they obtain a fluffy structure during coagulation. Krügel & Siebenmorgen (1996) have calculated how these modifications change the wave-length-dependent extinction coefficient of the dust. The result should be considered as an improved guess for the dust cross section in protostellar clouds. As an example, the transfer of radiation in the frosty nebulae HH100 IRS was discussed by Siebenmorgen (1996) and Siebenmorgen, Gredel & Starck (1996). Comprehensive model calculations are presented to interpret the spectral energy distribution of HH100 IRS as a function of dust parameters such as the grain size, the ice volume fraction and the fluffiness of the particles. The radiative transfer calculations treat in detail the spectroscopic signature of ice bands. The observed IR spectrum together with the strength of the water ice band of HH100 IRS is successfully reproduced if an upper size limit of the grains well below 1 µm is used. Contributions from scattering to the bands superimposed to the long wavelength wing of the 3.08 µm H₂O absorption are thus excluded. The strength of the H₂O ice absorption increases with increasing fluffiness of the particles. Comet-like grains, with sizes above 1 µm, result in a poor fit to the observations.

References
Laureijs, R.J. et al. (1996). A&A 315, L313.
Laureijs, R.J. et al. (1996). A&A 315, L317.
Krügel, E. & Siebenmorgen R. (1996). ESO conference on 'The role of dust in the formation of stars', (Eds. H.U. Käufl & R. Siebenmorgen), Springer, Berlin- Heidelberg- New York.
Siebenmorgen, R. (1996). From stardust to planetesimals, (Eds. M.E. Kress, A.G.G.M. Tielens & Y. Pendelton), NASA CP3343.
Siebenmorgen, R., Gredel, R. & Starck, J.-L. (1996). ApJ, submitted.

Normal and starburst galaxies

Work on dwarf starburst galaxies was continued by Metcalfe in collaboration with colleagues at University College Dublin (McBreen, Steel et al.) and the Regional Technical College Cork (Smith). Results of spectrophotometry on the dwarf starburst galaxy Haro 3 (NGC 3353) were published, reporting the detection of the characteristic spectral features of Wolf-Rayet stars in this galaxy (Steel et al., 1996). Results were indicative of a starburst less than 5 Myears old, containing about 84 WR stars, and superimposed upon an older stellar substrate. Intrinsic starburst triggering mechanisms, rather than a triggering interaction, were inferred. Further work on Haro 3 was carried out by Metcalfe, together with other members of the ISO-SOC (Barr, Clavel, Gallais, Laureijs and Leech) and the UCD and Cork RTC collaborators (listed above), with the execution of ISO observations using ISOCAM and ISOPHOT. The resulting paper (Metcalfe et al., 1996) reported strong Unidentified Infrared Band (UIB) and SIV emission in the PHT-S spectrum and, from CAM and visible images, a strong correspondence between the IR (UIB) and visible contours of the galaxy. An IR peak occurs at the position of the optical peak. The 7-15 µm colour peaks in the vicinity of the star formation knots. A relative trough in the colour map, indicative of a relative decline in 12-18 µm emission, and possible local destruction of the UIB emitters contributing longward of 12 µm, occurs at the location of the WR emission. Several other dwarf starburst galaxies have been observed with ISO and data reduction is underway.

Schulz, Laureijs and Gabriel, in collaboration with the ISOPHOT teams (Acosta-Pulido, Klaas, Kinkel, Abráham, Castaineda, Cornwall, Heinrichsen, Herbstmeier, Krüger and Pelz), reported on the interacting galaxy system NGC 6090. The data were gathered from commissioning measurements during the performance verification phase of ISOPHOT. Broadband photometry over the full wavelength range 3.6-240 µm, including a low resolution spectrum at 2.5-11.5 µm, show the spectral energy distribution (SED) in unprecedented detail. It was shown that the starburst component is dominating the spectrum. The radiation from PAHs below 20 µm was found to contribute significantly to the total emission.

References
Metcalfe, L. et al. (1996). A&A 315, L105.
Steel, S.J. et al. (1996). A&A 311, 721.
Acosta-Pulido, J.A. et al. (1996). A&A 315, L121.

AGN and QSOs

The effort to map the accretion flow in active galactic nuclei (AGN) was pursued by Clavel and a large number of collaborators from the international AGN Watch consortium. To this end, the nuclei of NGC 5548 and NGC 4151 have been monitored in the optical, UV and X-ray domains using a large number of ground- and space-based telescopes. In both galaxies, it is found that the optical and far-UV continuum vary essentially in phase with no delay larger than 0.1 days between the two wavebands. This result invalidates AGN models where the continuum originates from thermal emission in a geometrically thin accretion disc. In the case of NGC 4151, the X-ray emission also varies in phase with the optical and UV flux, to within 0.3 day. This is also at odds with the thin disc model. However, these results could be explained if the X-ray source is external to the disc (e.g. a hot corona) such that its emission is thermally reprocessed inside the disc in the form of UV and optical photons.

The origin of the IR continuum in the Circinus galaxy is being addressed by Siebenmorgen in collaboration with Moorwood, Freudling and Käufl. New IR and mm-wave observations of the Circinus galaxy yielding FWHM continuum sizes of ~0.3 arcsec (6 pc) for the 3-5 µm emission and upper limits of 51.5 arcsec (30 pc) around 10 µm and 20 µm and <23 arcsec (460 pc) at 1.3 µm have been obtained. The central source appears to have been marginally resolved with a size of 1.3 arcsec (26 pc) in a narrow band image centred close to the 11.3 µm PAH feature. The 1.3 µm measurements also provide an estimate of the gas mass of about 1.6 10⁸M_Sun. Circinus is of particular interest for such studies as it is the closest (4 Mpc) example of a galaxy containing an Active Galactic Nucleus and circumnuclear starburst activity. It has been shown, using a radiative transfer code, that both the observed sizes and overall spectral energy distribution, including the 9.7 µm silicate absorption and 3.28-11.3 µm PAH emission features, are consistent with dust heating by a central power-law source. In contrast to earlier conclusions, it appears that the total IR luminosity is dominated by emission from dust heated by the AGN rather than in the starburst region.

Leech, together with Barr, Salama, Saxton, Long and Altieri, made ISOPHOT observations of three quasars where dust may be present. The z=4.69 radio-quiet quasar BR 1202-0725 has a 1.25 mm flux of 10.5±1.5 mJy, an 800 µm flux of 50±7 mJy and a 450 µm flux of 92±38 mJy. If its far-IR spectrum is similar to that of IRAS 10214+4724, the ultra high luminosity IRAS source, then BR 1202-0725 has an IR luminosity of the order of 10¹⁴L_Sun. This makes it one of the most luminous objects in the Universe. A dust model, with dust heated to about 50 K, can be fitted to the 450 µm - 1.25 mm data. From this, the fluxes at various IR wavelengths are predicted to be of the order of tens to hundreds of mJy, and the expected turnover in the spectrum to be within 150-300 µm. Based on this, we obtained PHOT-C data of this object to confirm or refute the presence of dust and to determine the dust temperature (assuming present). Only upper limits were obtained from the PHOT data, indicating either a lack of dust or that the dust is cooler than had been expected.

Reference
Siebenmorgen, R. (1996). A&A, submitted.

Cosmology

Metcalfe, in collaboration with ISO-SOC colleagues (Altieri, Leech and Schulz, lately joined by Okumura) and University College Dublin (McBreen), prepared a programme of ISO observations using cluster gravitational lenses to study high redshift galaxies in the IR. At the time of writing, a strong ISO detection at the location of the Abell 370 gravitational arc (z=0.7) is under study. Further work, including ISOCAM imaging of higher red-shift arcs, is ongoing.

ISOPHOT serendipity mode

As ISO slews from target to target, data are taken in one of the long wavelength channels of ISOPHOT. This will lead to a partial sky survey at an unexplored wavelength centred at 175 µm. Members of the SOC (Burgdorf, Kessler, Laureijs and Schulz) are involved in the data analysis together with MPIA Heidelberg, AIP Potsdam, IPAC Pasadena and ICSTM London. The first data from this survey (Bogun et al., 1996) led to an initial detection limit of about 1 Jy being obtained. This leads to an estimate of several thousand source detections during the whole mission of ISO. The value of the observing mode for learning more about extended emission, such as cirrus, was also shown.

Reference
Bogun, S. et al. (1996). A&A 315, L71.

ISOCAM parallel mode

Whenever another instrument is being operated on ISO, ISOCAM gathers data in a parallel mode at a position offset from that of the prime instrument by some 12-17 arcmin. Siebenmorgen, together with other members of the SOC (Altieri, Kessler, Metcalfe, Ott, Pollock and Prusti) and in collaboration with the ISOCAM teams (Abergel, Biviano, Blommaert, Boulade, Cesarsky, Gallais, Guest, Okumura, Perault, Sauvageon and Starck), published the first results of a survey being made in a broadband 6.75 µm filter in this mode at about 6 arcsec resolution (Siebenmorgen et al., 1996). So far, a sky area of ~1.375 deg² down to a limiting flux of 5 mJy has been analysed. A total of 287 objects has been detected. The final survey will cover a sky area of about 33 deg², most of which will be done in staring mode. The final catalogue should reach a typical sensitivity limit of 1 mJy and should include more than 10 000 sources. Siebenmorgen et al. estimate that, at the detection limit, 99% of the objects will have a Galactic origin.

Reference
Siebenmorgen, R. et al. (1996). A&A 315, L169.