Sp.-V/AQuan/1999/10/07:19:25 Page 169 Chapter 8 Ultraviolet Astronomy Terry J. Teays 8.1 Ultraviolet Wavelengths ................. 169 8.2 Ultraviolet Astronomy Satellite Missions ....... 170 8.3 Significant Atlases and Catalogs ............ 172 8.4 Interstellar Extinction in the Ultraviolet ........ 174 8.5 Commonly Observed Ultraviolet Emission Lines .. 175 8.6 Ultraviolet Spectral Classification ............ 178 8.7 Ultraviolet Spectrophotometric Standards ....... 180 8.1 ULTRAVIOLET WAVELENGTHS The Earth’s atmosphere is an efficient absorber of ultraviolet radiation, and so astronomical observa- tions in this wavelength regime are pretty well limited to space-based instruments. As such, I adopt the nomenclature that “ultraviolet” refers to the wavelengths in the region from the atmospheric cutoff at ≈ 3 200 Adownto100˚ A.˚ (The terms “far ultraviolet” and “extreme ultraviolet” are frequently used to refer to the shorter end of the ultraviolet wavelength range, but the usage has not been consistent in the literature. Generally one thinks of the far ultraviolet as referring to wavelengths shorter than that of the Lyman limit at 912 A,˚ and the extreme ultraviolet as being the region between 912 and 100 A.)˚ Note that wavelengths given in this chapter will always be vacuum ones. In the past ultraviolet wavelengths shorter than 2 000 A˚ were expressed as vacuum values, while those longward of this were given with regard to wavelengths in air. This convention has been continued in the International Ultraviolet Ex- plorer (IUE) Project, but is currently being changed in their newest pipeline processing system, and eventually the entire archive will make use of only vacuum wavelengths. Newer missions such as the Hubble Space Telescope (HST) and Extreme Ultraviolet Explorer (EUVE) are using vacuum wave- lengths exclusively. This practice conforms to Resolution C15 of the 21st General Assembly of the International Astronomical Union. Equation (8.1) is the algorithm for calculating the index of refrac- 169 Sp.-V/AQuan/1999/10/07:19:25 Page 170 170 / 8 ULTRAVIOLET ASTRONOMY tion (n) of standard air as a function of vacuum wavelength. This algorithm was derived by Edlen´ [1], and was the one officially adopted by the International Astronomical Union (IAU) [2]. The wavelength in air is the vacuum wavelength divided by the index of refraction: −2 −4 − 2.949 81 × 10 2 554.0 × 10 n = 1 + 6.4328 × 10 5 + + , (8.1) 146 × 108 − σ 2 41 × 108 − σ 2 where σ represents the wave number in vacuum, expressed in reciprocal A.˚ 8.2 ULTRAVIOLET ASTRONOMY SATELLITE MISSIONS There have been numerous balloon and rocket flights devoted to ultraviolet astronomy, as well as various short-term studies, such as those conducted from manned space missions. The first ultraviolet spectrum of the Sun was obtained in 1946 using a captured V2 rocket, while the first stellar ultraviolet observations took place during 1955–1957. The first stellar ultraviolet spectrophotometry, by Stecher and Milligan [3], was accomplished by a rocket-borne instrument, while the first ultraviolet stellar spectroscopy (i.e., wavelength resolution sufficient to resolve individual spectral lines) was achieved in a 1965 rocket flight [4]. A balloon-borne stellar spectrograph first examined the very important Mg II resonance doublet in 1971 [5]. The principal long-term ultraviolet astronomy missions are summarized in Table 8.1. Note that the extensive number of missions that have been devoted to ultraviolet solar studies have not been included in the table. The first column in Table 8.1 gives the mission’s name or acronym. OAO-2 stands for the second satellite in the Orbiting Astronomical Observatory series (the first having failed). It was the first instrument to carry out an extensive survey of the ultraviolet sky. The fourth satellite in this series was named Copernicus. It made substantial contributions to our understanding of the interstellar medium, hot stars, and stellar chromospheres. The TD-1 mission (named after the launch vehicle—a Thor Delta) was a European Space Agency (ESA) mission which had two ultraviolet experiments on board, including the S2/68 Ultraviolet Sky Survey Telescope. TD- 1’s primary legacy is the catalog of ultraviolet fluxes, which is cited in Table 8.2. ANS, the Astronomy Netherlands Satellite, had one ultraviolet experiment. Though well known for their spectacular success in planetary encounter missions, each of the two Voyager spacecraft have an ultraviolet spectrometer (UVS) that has been used for stellar spectroscopy, now that the primary mission objectives are completed. IUE, the International Ultraviolet Explorer, was a joint project of NASA, ESA, and the British SERC. It was originally intended for a three-year mission, but it continued to operate for over 18 years. One of the first major international satellites, IUE was operated in real-time from NASA’s Goddard Space Flight Center for 16 hours per day, and from the ESA tracking station near Madrid for the remaining 8 hours. It is in an eccentric geosynchronous orbit. Rontgensatellit¨ (ROSAT) is primarily an X-ray mission, but it has a wide field camera which operates in the ultraviolet wavelength range and has been used to produce an all-sky survey. The Hubble Space Telescope contains a battery of instruments, most with a number of configurations, which operate at ultraviolet wavelengths. For example, the Goddard High Resolution Spectrograph (GHRS) had a number of gratings and echelle cross-dispersers, which have not been detailed specifically in the table, rather representative ranges have been listed. These instruments, referred to by their acronyms in Table 8.1, are the GHRS, Faint Object Spectrograph (FOS), Wide Field/Planetary Camera (WF/PC), Faint Object Camera (FOC), High Speed Photometer (HSP), and the Space Telescope Imaging Spectrograph (STIS). Sp.-V/AQuan/1999/10/07:19:25 Page 171 8.2 ULTRAVIOLET ASTRONOMY SATELLITE MISSIONS / 171 Table 8.1. Major long-term ultraviolet astronomy missions. Tel. Spect. Operational apert. resol. Mission dates (cm) Instrument λ (A)˚ (A)˚ Reference OAO-2 12/07/68–2/13/73 20 Photometer 1 430 [1] 20 Photometer 1 550 20 Photometer 1 910 20 Photometer 2 460 20 Photometer 2 980 20 Photometer 3 320 40 Nebular photometer 1 200–4 000 30 Vidicon 30 Vidicon 30 Vidicon 30 Vidicon Spectrometer 1 160–1 850 12 Spectrometer 1 850–3 600 22 Copernicus 8/21/72–12/31/80 80 Spectrometer 912–1 500 0.05 [2] Spectrometer 912–1 645 0.2 Spectrometer 1 640–3 185 0.01 Spectrometer 1 480–3 275 0.04 TD-1 3/12/72–1/9/80 27.5 Photometer 2 740 [3] Spectrophotometer 1 350–2 550 ANS 8/30/74–6/14/77 22 Photometer 1 550 [4] Photometer 1 800 Photometer 2 200 Photometer 2 500 Photometer 3 300 IUE 1/26/78–9/30/96 45 Echelle spectrograph 1 145–3 230 0.2 [5, 6] Spectrograph 1 150–3 300 6 HST 4/24/90– 240 GHRS 1 110–3 200 0.01-3.5 [7] FOS 1 150–7 000 1.2-7 WF/PC 1 200–10 000 FOC 1 200–6 500 HSP 1 150–8 000 STIS 1 150–10 000 ROSAT 6/1/90– aa Wide field camera 60–140 [8] Wide field camera 112–200 Wide field camera 150–220 Wide field camera 530–720 EUVE 6/7/92– Scanning photometer 44–360 [9] aa Scanning photometer 44–360 aa Scanning photometer 400–750 aa Deep survey 40–385 aa Spectrometer 70–190 0.5 aa Spectrometer 140–380 1 aa Spectrometer 280–760 2 Note aSee text for aperture discussion. References 1. Code, A.S., Houck, T.E., McNall, J.F., Bless, R.C., & Lillie, C.F. 1970, ApJ, 161, 377 2. Rogerson, J.B., Spitzer, L., Drake, J.F., Dressler, K., Jenkins, E.B., Morton, D.C., & York, D.G. 1973, ApJ, 181,97 3. Jamar, C., Macau-Hercot, D., Monfils, A., Thompson, G.I., Houziaux, L., & Wilson, R. 1976, Ultraviolet Bright-Star Spectrophotometric Catalogue (ESA, Paris) Sp.-V/AQuan/1999/10/07:19:25 Page 172 172 / 8 ULTRAVIOLET ASTRONOMY 4. Wesselius, P.R., van Duinen, R.J., de Jonge, A.R.W., Aalders, J.W.G., Luinge, W., & Wildeman, K.J. 1982, A&AS, 49, 427 5. Kondo, Y., editor, 1987, Exploring the Universe with the IUE Satellite (Reidel, Dordrecht). 6. Newmark, J.S., Holm, A.V., Imhoff, C.I., Oliversen, N.A., Pitts, R.E., & Sonneborn, G. 1992, NASA IUE Newslett., 47,1 7. Bless, R.C. 1992, in The Astronomy and Astrophysics Encyclopedia, edited by S.P. Maran (Van Nostrand, New York), pp. 912– 915 8. Pye, J.P., Watson, M.G., Pounds, K.A., & Wells, A. 1991, in Extreme Ultraviolet Astronomy, edited by R.F. Malina and S. Bowyer (Pergamon, New York), p. 409 9. EUVE Guest Observer Center 1992, EUVE Guest Observer Program Handbook (Appendix G of NASA NRA 92-OSS-5) This configuration will change as a result of servicing missions for HST. The Extreme Ultraviolet Explorer (EUVE) is still in operation at the time of writing. The ROSAT and EUVE missions provided the first extensive and detailed look at this wavelength regime. HST and EUVE are in low-Earth orbits. Column 2 of Table 8.1 gives the mission’s operational dates (the first date is the launch date, and so science operations will have begun somewhat later). Column 3 gives, when applicable, the size of the telescope objective (in cm) for the satellite or specific instrument. The notation “a” is used for the ROSAT and EUVE instruments to indicate that the matter of aperture is not as straightforward in the case of those instruments. They make use of various types of segmented filter masks which allow a given instrument to make use of a specific fraction of the aperture.