Solar system studies

Andernach, in collaboration with the Target of Oppor-tunity Teams for Comets under the leadership of Festou (Observatoire du Midi-Pyrenees) and Feldman (Johns Hopkins University), has made an extensive series of observations of comets Hale-Bopp and Hyakutake. The observations of Hale-Bopp were especially of great interest, since they allowed the study of the onset of water sublimation. The spectra taken at different epochs allow a very detailed study of the start of the H ₂O evaporation process taking over from CO and thus controlling the temperature of the nucleus. This is the first time that this phenomenon has been monitored at wavelengths other than in the radio domain, and suggests that water can sublimate at much larger heliocentric distances than pre-viously thought. This is probably due to sublimation near the subsolar point and suggests that the temperature in that region is higher than expected. The difficult observations, in that the cometary motion reached 1°/h and the new 1-gyro control system had only just been implemented on IUE, have allowed the simultaneous determination of the H₂O, CO and CO₂ production rates at the nucleus of Comet Hyakutake to be made. The H₂O production rate showed a significant but brief increase during the 'break-up' event of the comet, suggesting that the comet lost only small pieces and did not really fragment. The difference between the nuclear CO production rate found in the UV and that derived from the sub-mm observations, suggests that most of the CO in the coma originates from a distributed source, as was found for Comet Halley.

Stellar studies

Fernley, Wamsteker and Solano, in collaboration with Neckel (Hamburger Sternwarte), have completed their comparison of Solar Analogue stars with the Sun (Fernley et al., 1996). For the closest analogues, it was found that, after normalisation of the fluxes in the optical, a significant discrepancy remained of some 16% in the sense that the solar UV fluxes are too high or the UV flux scale is too low. Further work on the high resolution IUE spectra of Solar Analogues in collaboration with Greve (IRAM) is ongoing and an UV Atlas of Solar Analogues is in preparation.

González-Riestra has made an extensive study of the large database of novae spectra built up during IUE's lifetime. This has clarified the apparent differences seen in nova light curves and led to the creation of a schematic description of the evolution of a classical nova in the UV (González-Riestra, 1995). The general evolution of a classical nova can be described by four stages, as shown in Fig. 4.3.3/1.

Figure 4.3.3/1: A sketch of the UV evolution of a nova during the first 2 years after the outburst. The light curve in the central panel corresponds to the UV integrated flux of Nova Cygni 1992. Each of the spectra shown belongs to a different object, as specified, in a different stage of evolution, which corresponds to the part of the light curve indicated.

• the 'fireball' phase occurs well before the optical maximum, and is characterised by a rather strong UV continuum. Only Nova Cygni 1992 has been observed in this stage;
• the 'optically thick' phase is very close to optical maximum. It is characterised by very low UV flux as a consequence of the recombination of the iron peak elements and the corresponding opacity increase, which blocks essentially all the radiation shortward 1700 Å. During the first days to weeks after the outburst, the UV flux gradually increases as the optical depth of the UV lines decreases. This is referred to as the 'lifting of the iron curtain';
• the 'UV maximum' when the usual nebular lines start to appear and the resonance lines develop strong P-Cygni profiles. This phase can last up to several weeks;
• the 'nebular' phase where the P-Cygni profiles disappear and the UV spectrum shows only a very faint continuum with the typical nebular emission lines. Most of the novae have narrow emission lines (i.e. not resolved in the IUE low dispersion mode) during this stage, but novae with broad lines during the outburst still have them in the nebular phase. During this phase some novae can show strong dimming in their UV light curve, associated with the condensation of dust in the envelope.

In collaboration with Viotti (IAS, Frascati) and Greiner (Max- Planck, Munich), González-Riestra has for the first time been able to study the behaviour of a symbiotic star during two consecutive outbursts, in June 1994 and July 1995. These two outbursts of the star AG Draconis have been fully covered by co- ordinated ground and IUE and ROSAT observations (Viotti et al., 1996). The X-ray flux had remained constant during the 3 years prior to the outburst. The quiescent data are consistent with a black-body temperature of 15.4 eV. The high luminosity suggests that the hot component is burning hydrogen steadily during the quiescence periods. During the optical outbursts, the UV continuum flux increased by a factor of 10, while the X-ray intensity dropped by at least a factor of 100. The X-ray decrease is consistent with a temperature decrease, which is also supported by the changes in the UV spectrum, as shown in Fig. 4.3.3/2). Such a temperature decrease could be produced by an increased mass transfer to the hot star, causing it to expand slowly to about twice its original size.

Figure 4.3.3/2: IR to X-ray energy distribution of the symbiotic system AG Draconis during quiescence and outburst. Full line: quiescent IUE spectrum (April 1993) and blackbody of 14.5 eV (best-fit to ROSAT PSPC data). Dashed line: outburst IUE spectrum (July 1995) with blackbody of 11 eV. Dotted line: outburst IUE spectrum (September 1995) with blackbody of 9.5 eV. The temperature of the outburst blackbodies is estimated from the ROSAT HRI count rate. Optical and IR photometry were taken during quiescence.

References
Fernley, J., Neckel, H., Solano, E. & Wamsteker, W. (1996). A&A 311, 245.
Viotti, R., González-Riestra, R., Montagni, F., Mattei, J., Maesano, M., Greiner, J., Friedjung M. & Altamore, A. (1996). In Supersoft X-Ray Sources, (Ed. J. Greiner), Springer, p.259.
González-Riestra, R. (1995). In Cataclysmic Variables, (Eds. Bianchini, A. et al.), Kluwer, p.21.

Extragalactic studies

In collaboration with Mas-Hesse (LAEFF), Rodriguez-Pascual has completed the study of the spectral energy distribution from radio to X-rays of various sub-classes of active galaxies (Mas- Hesse et al., 1995). Strong evidence was found for the existence of important star formation in the Seyfert II class, giving support to the current unification models. For NGC 4619, evidence for broad lines was found in the UV, while for IRAS15564+6359 rapid X-ray variability was indicated, suggesting that star formation alone cannot account for all the properties of these objects.

Schartel and Andernach, in collaboration with Greiner (Max- Planck, Munich) have made a study of the positional coincidences of gamma-ray bursts (GRBs) and quasar/AGN from the Veron-Cetty & Veron Catalogue. For most classes of active galaxies and BL Lacs, no correlations were found. However, the sample of nearest and brightest radio-quiet quasars (M<-24.2 and z<1.0) showed a larger than 99.7% probability of being associated with the GRB sample.

Rodriguez-Pascual and Wamsteker continued their collaboration with Reimers (Hamburger Sternwarte) to search for unobscured sightlines in the Universe, and discovered two more high redshift systems at z=2.19 and z=2.13 (Reimers et al., 1995). Optical-UV spectroscopy of these confirms the result obtained earlier from HS1700+6416 (z=2.72), i.e. that the turnover in the EUV, to connect with the X-rays, occurs at energies higher than 27 eV (2 Rydberg). Over the area of the sky surveyed, the three unabsorbed QSOs discovered are still consistent with the expected numbers at the sensitivity of the survey.

The extensive collaboration of Rodriguez-Pascual and Wamsteker with Peterson (Ohio State University) in the context of the International AGN Watch has continued at its normal pace of one size determination experiment per year (Crenshaw et al., 1996). With the high density sampling of Fairall-9 (a high luminosity Seyfert I galaxy), the full range of luminosities of Seyfert galaxies has been covered and a serious attempt can be made to evaluate the existence of a size-luminosity relation for these objects. The emission line lags found for Fairall-9 are remarkably short and support the general picture of a stratified broad-line region. During the final IUE observing programme, the best reverberation light curve ever of an AGN was obtained on NGC 7469, as shown in Fig. 4.3.3/3. This campaign was carried out simultaneously with XTE in collaboration with Nandra (GSFC).

Figure 4.3.3/3: Continuum light curve of the Seyfert I galaxy NGC 7469 taken as part of the International AGN Watch.

The first radio-loud AGN (3C390.3) has been studied through a campaign of 9-day sampling intervals. Wamsteker and Schartel have, in collaboration with Vio (University of Padova) and Wang (University of Science and Technology of China), found the first evidence for differences in the behaviour between radio-loud and radio-quiet AGN. Contrary to what has been found for all Seyfert galaxies studied until now, a strong correlation between the line ratio CIV/Ly alfa and the continuum brightness was found for 3C390.3.

References
Mas-Hesse, J.M., Rodriguez-Pascual, P.M., Sanz Fernández de C., L., Mirabel, F.I., Wamsteker, W., Makino, F. & Otani, C. (1995). A&A 298, 22.
Reimers, D., Clavel, J., Chini, R., Dahlem, M., Engels, D., Fink, H., Hagen, H.-J., Heber, U., Heras, A., Hopp, U., Wamsteker, W. & Wisotzki, L. (1995). A&A 303, 449.
Crenshaw, D.M., Rodriguez-Pascual, P.M., Penton, S.V., Edelson, R.A., Alloin, D., Ayres, T.R., Clavel, J., Horne, K., Johnson, W.N., Kaspi, S., Korista, K.T., Kriss, G.A., Krolik, J.H., Malkan, M.A., Maoz, D., Netzer, H., O'Brien, P.T., Peterson, B.M., Reichert, G.A., Shull, J.M., Ulrich, M. H., Wamsteker and 64 co-Authors. (1996). ApJ 470, 322.