sign in sign up
<<
Home , Tau Scorpii, Ultraviolet astronomy

Sp.-V/AQuan/1999/10/07:19:25 Page 169

Chapter 8

Ultraviolet

Terry J. Teays

8.1 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 ’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 Adownto100û A.û (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,û and the extreme ultraviolet as being the region between 912 and 100 A.)û Note that wavelengths given in this chapter will always be vacuum ones. In the past ultraviolet wavelengths shorter than 2 000 Aû 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 (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.û

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 was obtained in 1946 using a captured V2 rocket, while the first stellar ultraviolet observations took place during 1955Ð1957. 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 , hot , 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- mission, but it continued to operate for over 18 . 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 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)û (A)û Reference

OAO-2 12/07/68Ð2/13/73 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 200Ð4 000 30 Vidicon 30 Vidicon 30 Vidicon 30 Vidicon Spectrometer 1 160Ð1 850 12 Spectrometer 1 850Ð3 600 22

Copernicus 8/21/72Ð12/31/80 80 Spectrometer 912Ð1 500 0.05 [2] Spectrometer 912Ð1 645 0.2 Spectrometer 1 640Ð3 185 0.01 Spectrometer 1 480Ð3 275 0.04

TD-1 3/12/72Ð1/9/80 27.5 Photometer 2 740 [3] Spectrophotometer 1 350Ð2 550

ANS 8/30/74Ð6/14/77 22 Photometer 1 550 [4] Photometer 1 800 Photometer 2 200 Photometer 2 500 Photometer 3 300

IUE 1/26/78Ð9/30/96 45 Echelle spectrograph 1 145Ð3 230 0.2 [5, 6] Spectrograph 1 150Ð3 300 6

HST 4/24/90Ð 240 GHRS 1 110Ð3 200 0.01-3.5 [7] FOS 1 150Ð7 000 1.2-7 WF/PC 1 200Ð10 000 FOC 1 200Ð6 500 HSP 1 150Ð8 000 STIS 1 150Ð10 000

ROSAT 6/1/90Ð aa Wide field camera 60Ð140 [8] Wide field camera 112Ð200 Wide field camera 150Ð220 Wide field camera 530Ð720

EUVE 6/7/92Ð Scanning photometer 44Ð360 [9] aa Scanning photometer 44Ð360 aa Scanning photometer 400Ð750 aa Deep survey 40Ð385 aa Spectrometer 70Ð190 0.5 aa Spectrometer 140Ð380 1 aa Spectrometer 280Ð760 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- 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 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 Encyclopedia, edited by S.P. Maran (Van Nostrand, New York), pp. 912Ð 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. Column 4 indicates the type of instrument, and column 5 gives the experiment’s wavelength range (for spectrographic and spectrophotometric instruments) or the effective and/or central wavelength (for photometric instruments). Column 6 gives the approximate average spectral resolution (in A)û for spectrographic instruments. (This will, of course, vary with wavelength in each instrument, so the entries in column 6 are intended to be representative only.) Finally, column 7 lists a representative reference which gives information about the mission.

8.3 SIGNIFICANT ATLASES AND CATALOGS

Table 8.2 gives titles and references for some of the more important catalogs and atlases of ultraviolet astronomical data.

Table 8.2. Important atlases and catalogs of ultraviolet data.

The Variation of Galactic Interstellar Extinction in the Ultraviolet [1] Atlas of the Wavelength Dependence of Ultraviolet Extinction in the [2] IUE-ULDA Access Guide No. 2: Comets [3] ANS Ultraviolet , Catalogue of Point Sources [4] An Atlas of Extreme Ultraviolet Explorer (EUVE) Sources [5] IUE Low-Dispersion Spectra Reference Atlas. Part 1. Normal Stars [6] IUE Ultraviolet Spectral Atlas of Selected Astronomical Objects [7] Ultraviolet Bright-Star Spectrophotometric Catalogue [8] Supplement to the Ultraviolet Bright-Star Spectrophotometric Catalogue [9] Catalogue of Stellar Ultraviolet Fluxes [10] Ultraviolet Photometry from the Orbiting Astronomical Observatory. XXXII. An Atlas of Ultraviolet Stellar Spectra [11] IUE Ultraviolet Spectral Atlas [12] IUE Ultraviolet Spectral Atlas [13] The Extreme Ultraviolet Explorer Stellar Spectral Atlas [14] Spectral Synthesis in the Ultraviolet. I. Far-Ultraviolet Stellar Library [15] An Atlas of High Resolution IUE Ultraviolet Spectra of 14 WolfÐRayet Stars [16] The Hopkins Ultraviolet Telescope Far-Ultraviolet Spectral Atlas of WolfÐRayet Stars [17] International Ultraviolet Explorer Atlas of O Type Spectra from 1200 to 1900 Aû [18] Ultraviolet Spectral Morphology of the O Stars. II. The [19] P Cygni and Related Profiles in the Ultraviolet Spectra of O-Stars [20] Sp.-V/AQuan/1999/10/07:19:25 Page 173

8.3 SIGNIFICANT ATLASES AND CATALOGS / 173

Table 8.2. (Continued.)

An Atlas of Ultraviolet P Cygni Profiles [21] Identification of Lines in the Satellite Ultraviolet: The Spectrum of Tau Scorpii [22] Spectral Classification with the International Ultraviolet Explorer: An Atlas of B-Type Spectra [23] The IUE Spectral Atlas of Two Normal B Stars: π Ceti and ν Capricorni (125Ð198 nm) [24] Identification Lists of the Far UV Spectra of 7 Solar Chemical Composition Main Sequence Stars in the Spectral Range B2-B9.5 [25] A Catalog of 0.2 Aû Resolution Far-Ultraviolet Stellar Spectra Measured with Copernicus [26] The Copernicus Ultraviolet Spectral Atlas of Vega [27] The Copernicus Ultraviolet Spectral Atlas of Sirius [28] Early Type Strong Emission-Line Supergiants of the Magellanic Clouds: A Spectroscopic Zoology [29] Chromospheric Mg II Emission in A5 to K5 Main Sequence Stars from High Resolution IUE Spectra [30] Atlas of High Resolution IUE Spectra of Late-Type Stars, 2500Ð3230 Aû [31]˜ The Spectra of Late-Type Dwarfs and Sub-Dwarfs in the Near Ultraviolet. I. Line Identifications [32] Outer Atmospheres of Cool Stars. VII. High Resolution Absolute Flux Profiles of the Mg II h and k Lines in Stars of Spectral Types F8 to M5 [33] UV Fluxes of Pop II Stars [34] IUE Low Dispersion Observations of Symbiotic Objects [35] A Far-Ultraviolet Atlas of Symbiotic Stars Observed with IUE. I. The SWP Range [36] A Spectrophotometric Atlas of White Dwarfs Compiled from the IUE Archives [37] Ultraviolet Observations of Cataclysmic Variables: The IUE Archive [38] A Catalogue of Low-Resolution IUE Spectra of Dwarf Novae and Nova-Like Stars [39] An Atlas of UV Spectra of Supernovae [40] UV Observations of SN 1987a [41] International Ultraviolet Explorer Atlas of Planetary Nebulae, Central Stars, and Related Objects [42] UV Spectra of the Central Stars of Large Planetary Nebulae [43] A Survey of Ultraviolet Interstellar Absorption Lines [44] Galactic Interstellar Abundance Surveys with IUE. II. The Equivalent Widths & Column Densities [45] An Ultraviolet Spectral Atlas of Interstellar Lines toward SN 1987a [46] IUE UV Spectra of Extra Galactic H II Regions. I. The Catalogue & the Atlas [47] UV Observations by IUE of 31 Clusters of the LMC [48] IUE-ULDA Access Guide No. 3: Normal [49] An Atlas of Ultraviolet Images of Nearby Galaxies [50] An Atlas of Ultraviolet Spectra of Star-Forming Galaxies [51] IUE-ULDA Access Guide No. 4: Active Galactic Nuclei [52] The Ultraviolet Variability of Seyfert I Galaxies [53] An Ultraviolet Atlas of Quasar and Blazar Spectra [54]

References 1. Witt, A.N., Bohlin, R.C., & Stecher, T.P. 1984, ApJ, 279, 698 2. Aiello, S., Barsella, B., Chlewicki, G., Greenberg, J.M., Patriarchi, P., & Perinotto, M. 1988, A&AS, 73, 195 3. Festou, M.C. 1990, IUE-ULDA Access Guide No. 2: Comets (ESA SP-1134) 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. Shara, M.M., Bergeron, I.E., Christian, C.A., Craig, N., & Bowyer, S. 1997, PASP, 109, 998 6. Heck, A. 1987, in Exploring the Universe with the IUE Satellite, edited by Y. Kondo (Reidel, Dordrecht) p. 121 7. Wu, C.-C. et al. 1992, IUE Ultraviolet Spectral Atlas of Selected Astronomical Objects, NASA Tech. Memo. No. 1285 8. Jamar, C., Macau-Hercot, D., Monfils, A., Thompson, G.I., Houziaux, L., & Wilson, R. 1976, Ultraviolet Bright-Star Spectrophotometric Catalogue (ESA, Paris) 9. Macau-Hercot, D., Jamar, C., Monfils, A., Thompson, G.I., Houziaux, L., & Wilson, R. 1978, Supplement to the Ultraviolet Bright-Star Spectrophotometric Catalogue (ESA, Paris) 10. Thompson, G.I., Nandy, K., Jamar, D., Monfils, A., Houziaux, L., Carnochan, D.J., & Wilson, R. 1978, Catalogue of Stellar Ultraviolet Fluxes (Science Research Council, London) 11. Code, A.D., & Meade, M.R. 1979, ApJS, 39, 195 12. Wu, C.-C. et al. 1983, NASA IUE Newslett., 22,1 13. Wu, C.-C. et al. 1991, NASA IUE Newslett., 43,1 14. Craig, N., Abbott M., Finley, D., Jessop, H., Howell, S.B., Mathioudakis, M., Sommers, J., Vallerga, J.V., & Malina, R.F. 1997, ApJS, 113, 131 Sp.-V/AQuan/1999/10/07:19:25 Page 174

174 / 8 ULTRAVIOLET ASTRONOMY

15. Fanelli, M.N., O’Connell, R.W., & Thuan, T.X. 1987, ApJ, 221, 768 16. Willis, A.J., van der Hucht, K.A., Conti, P.S., & Garmany, D. 1986, A&AS, 43, 417 17. Schulte-Ladbeck, R.E., Hillier, D.J., & Herald, J.E. 1995, ApJ, 454, L51 18. Walborn, N.R., Nichols-Bohlin, J., & Panek, R.J. 1985, IUE Atlas of O-Type Spectra from 1200 to 1900 A,û NASA RP-1155 19. Walborn, N.R., & Panek, R.J. 1984, ApJ, 286, 718 20. Costero, R., & Stalio, R. 1984, A&AS, 58,95 21. Snow, T.P., Lamers, H.J.G.L.M., Lindholm, D.M., Odell, A.P. 1994, ApJS, 95, 163 22. Cowley, C.R., & Merritt, D.R. 1987, ApJ, 321, 553 23. Rountree, J. & Sonneborn, G. 1993, NASA Reference Publication No. 1312 (NASA, Washington) 24. Artru, M.-C., Borsenberger, J., & Lanz, T. 1989, A&AS, 80,17 25. Ramella, M., Castelli, F., Malagnini, M.L., Morossi, C., & Pasian, F. 1987, A&AS, 69,1 26. Snow, Jr., T.P., & Jenkins, E.B. 1977, ApJS, 33, 269 27. Rogerson, J.B. 1989, ApJS, 71, 1011 28. Rogerson, J.B. 1987, ApJS, 63, 369 29. Shore, S.N., & Sanduleak, N. 1984, ApJS, 55,1 30. Blanco, C., Bruca, L., Catalano, S., & Marilli, E. 1982, A&A, 115, 280 31. Wing, R.F., Carpenter, K.G., & Wahlgren, G.M. 1983, Perkins Obs. Special Pub. No. 1 32. Beckman, J.E., Crivellari, L., & Selvelli, P.L. 1982, A&AS, 47, 295 33. Stencel, R.E., Mullan, D.J., Linsky, J.L., Basri, G.S., & Worden, S.P. 1980, ApJS, 44, 383 34. Cacciari, C. 1985, A&AS, 61, 407 35. Sahade, J., Brandi, E., & Fountenla, J.M. 1984, A&AS, 56,17 36. Meier, S.R., Kafatos, M., Fahey, R.P., Michalitsianos, A.G. 1994 ApJS, 94, 183 37. Wegner, G., & Swanson, S.R. 1991, ApJS, 75, 507 38. Verbunt, F. 1987, A&AS, 71, 339 39. La Dous, C. 1990, Space Sci. Rev., 52, 203 40. Benvenuti, P., Sanz Fernandez de Cordoba, L., Wamsteker, W., Macchetto, F., Palumbo, G.C., & Panagia, N. 1982, ESA Special Pub. No. 1046 41. Kirschner, R.P., Sonneborn, G., Crenshaw, D.M., & Nassiopoulos, G.E. 1987, ApJ, 320, 602 42. Feibelman, W.A., Oliversen, N.A., Nichols-Bohlin, J., & Garhart, M.P. 1988, NASA Ref. Pub. No. 1203 43. Kaler, J.B., & Feibelman, W.A. 1985, ApJ, 297, 724 44. Bohlin, R.C., Hill, J.K., Jenkins, E.B., Savage, B.D., Snow, Jr., T.P., Spitzer, Jr., L.S., & York, D.G. 1983, ApJS, 51, 277 45. Van Steenberg, M.E., & Shull, J.M. 1988, ApJS, 67, 225 46. Blades, J.C., Wheatley, J.M., Panagia, N., Grewing, M., Pettini, M., & Wamstecker, W. 1988, ApJ, 334, 308 47. Rosa, M., Joubert, M., & Benvenuti, P. 1984, A&AS, 57, 361 48. Cassatella, A., Barbero, J., & Geyer, E.H. 1987, ApJS, 64,83 49. Longo, G., & Capaccioli, M. 1992, IUE-ULDA Access Guide No. 3: Normal Galaxies, ESA SP-1152 50. Maoz, D., Filippenko, A.V., Ho, L.C., Macchetto, F.D., Rix, H.-W. & Schneider, D.P. 1996, ApJS, 107, 215 51. Kinney, A.L., Bohlin, R.C., Calzetti, D., Panagia, N., & Wyse, R.F.G. 1993, ApJS, 86,5 52. Courvoisier, T.J.-L., & Paltani, S. 1992, IUE-ULDA Access Guide No. 4: Active Galactic Nuclei, ESA SP-1153 53. Chapman, G.N.F., Geller, M.J., & Huchra, J.P. 1985, ApJ, 297, 151 54. Kinney, A.L., Bohlin, R.C., Blades, J.C., & York, D.G. 1991, ApJS, 75, 645

8.4 INTERSTELLAR EXTINCTION IN THE ULTRAVIOLET

Since interstellar extinction is significantly stronger in the ultraviolet than at visual wavelengths, correcting for its effects is very important. The most prominent feature in the ultraviolet extinction curves is a broad peak centered at ≈ 2 175 A.û Equation (8.2) [6] gives some useful analytic functions which can be used to determine Aλ in the ultraviolet. Equation (8.2) is broken into three wavelength domains, and is parametrized in terms of σ , the wave number expressed in microns: 1.01 2.70 ≤ σ ≤ 3.65, Aλ/EB−V = 1.56 + 1.048σ + , (8.2a) [(σ − 4.60)2 + 0.280] Sp.-V/AQuan/1999/10/07:19:25 Page 175

8.5 COMMONLY OBSERVED ULTRAVIOLET EMISSION LINES / 175

1.01 3.65 ≤ σ ≤ 7.14, Aλ/EB−V = 2.29 + 0.848σ + , (8.2b) [(σ − 4.60)2 + 0.280] 2 7.14 ≤ σ ≤ 10, Aλ/EB−V = 16.17 − 3.20σ + 0.2975σ . (8.2c)

Savage and Mathis [7] adopt 3.1 for the value of AV /E(R−V ), while Seaton [6] uses 3.2. More detailed information is available in the review by Savage and Mathis [7], and additional references concerning ultraviolet extinction as a function of location in the sky are cited in Table 8.2.

8.5 COMMONLY OBSERVED ULTRAVIOLET EMISSION LINES

Table 8.3 (which is an expanded version of one given in Wu et al. [8]) gives a list of some of the more prominent ultraviolet emission lines observed in astronomical objects. The organization of Table 8.3 is as follows. Column 1 gives the wavelength (in A)û of the line, using the convention that a reasonably precise value (to 0.01 A)û is given for single lines, while an approximate value is given for lines formed of closely spaced individual lines of a given element. This value corresponds to the approximate location of the (blended) line which would be seen in low-resolution spectra, such as those taken in IUE’s low-dispersion mode. In cases where there is a spectral region which contains a large number of lines due to a single element, then the range of wavelengths is given in column 1. In the cases of multiple lines, column 4 gives more accurate wavelengths for the individual components that may be present. Column 2 specifies the ion which is the source of the emission line, while column 3 lists the type of objects in which this emission line is generally observed. The abbreviations used in column 3 to specify object type are given at the bottom of Table 8.3.

Table 8.3. Emission lines commonly found in ultraviolet spectra. λ (Aû )a Ion Type of object where observed b Individual components in multiplets

538 O II C 537.83, 538.26, 538.32, 539.13 584.33 He I SSO 834 O III C 832.93, 833.74, 835.29 916 N II C 915.61, 915.96, 916.02, 916.10, 916.35, 916.70, 916.71 933.4 S VI SNR 977.02 C III SNR 1 033 O IV SNR 1 031.93, 1 033.82, 1 037.62 1 066.66 Ar I SSO, C 1 085 N II C 1 083.99, 1 084.56, 1 084.58, 1 085.53, 1 085.55, 1 085.70, 1 085.12 1 175 C III WR, PN, CS, SS 1 174.93, 1 175.26, 1 175.59, 1 175.71, 1 175.99, 1 176.37 1 199 S III SSO 1 190.21, 1 194.06, 1 194.46, 1 197.56, 1 200.97, 1 201.73, 1 202.13 1 215.67 H I (all sources) 1 240 N V PN, SS, WR, CV, XRB, SN, IB, 1 238.82, 1 240.15, 1 242.80 N, SQ, SNR 1 247.38 C III SS, WR 1 256 S II SSO 1 250.58, 1 253.81, 1 256.12, 1 259.52 1 279 C I TT, LTS 1 276.48, 1 276.75, 1 277.19, 1 277.25, 1 277.28, 1 277.46, 1 277.51, 1 277.55, 1 277.72, 1 277.95, 1 279.06, 1 279.23, 1 279.50, 1 279.89, 1 280.14, 1 280.33, 1 280.36, 1 280.40, 1 280.60, 1 280.85 1 299 Si III SS, IB, TT 1 298.89, 1 298.96 1 304 O I RS, LTS, N, SQ, C 1 302.17, 1 303.49, 1 304.86, 1 306.03 1 309 Si II PN 1 304.37, 1 307.64, 1 309.28 Sp.-V/AQuan/1999/10/07:19:25 Page 176

176 / 8 ULTRAVIOLET ASTRONOMY

Table 8.3. (Continued.)

λ (Aû )a Ion Type of object where observed b Individual components in multiplets

1 335 C II TT, PN, LTS, RS, WR, CV, N, 1 334.53, 1 335.31, 1 335.66, 1 335.71 SNR, C 1 342 O IV CS, SS, WR, XRB 1 342.99, 1 343.51 1 371.29 O V PN, CS, SS, XRB, SNR 1 394 Si IV PN, LTS, RS, TT, XRB, CV, IV, 1 393.76, 1 396.75, 1 398.13 N, SQ 1 397Ð1 407 O IV PN, SS, N 1 397.23, 1 399.78, 1 401.16, 1 404.81, 1 407.38 1 402.77 Si IV PN, LTS, RS, TT, XRB, CV, IV, N, SQ 1 460 C I TT 1 459.03, 1 463.34, 1 467.40, 1 467.88, 1 468.41 1 473 S I RS, LTS 1 472.97, 1 473.02, 1 473.07, 1 473.99, 1 474.38, 1 474.57, 1 478.50 1 483.32 N IV PN, SS, WR, N 1 486 S I RS, LTS 1 485.62, 1 487.15 1 487 N IV PN, SS, WR, N, SNR 1 486.50, 1 487.89 1 550 C IV TT, PN, LTS, SS, N, WR, CV, IB, 1 548.20, 1 550.77 XRB, SQ, SNR 1 561 C I C 1 560.31, 1 560.68, 1 560.71, 1 561.05, 1 561.34, 1 561.37, 1 561.44 1 574.77 Ne V PN, N 1 577 C III SS 1 576.48, 1 577.30, 1 577.89 1 602 Ne IV PN, N, SS 1 601.50, 1 601.68 1 640 He II TT, PN, LTS, RS, WR, XRB, SQ, 1 640.47, 1 640.49 SNR 1 641.31 O I RS, LTS, SS, N 1 657 C I C, RS, LTS, TT 1 656.28, 1 656.93, 1 657.01, 1 657.38, 1 657.59, 1 657.91, 1 658.12 1 663 O III PN, WR, SQ, N, LTS, HII, SS, 1 660.81, 1 666.15 SNR 1 670.79 Al II IB, LTS 1 710 Si II PN, WR 1 710.83, 1 711.30 1 718.55 N IV PN, WR, XRB, CV, N 1 728.94 S III SSO 1 750 N III WR, TT, HII, N, SN, SNR 1 746.82, 1 748.65, 1 749.67, 1 752.16, 1 754.00 1 760 C II PN 1 760.47, 1 760.82 1 815 Si II TT, PN, RS, LTS 1 808.01, 1 816.93, 1 817.45 1 814.63 Ne III PN, N 1 860 Al III LTS, IB 1 854.72, 1 862.79 1 882.71 Si III PN, LTS, HII, SN, N, SQ, SNR 1 892.03 Si III TT, PN, LTS, HII, SN, N, SQ, SNR 1 900.29 S I RS, LTS, SN, HII 1 908.73 C III TT, PN, LTS, WR, HII, N, SN, 1 906.68, 1 908.73, 1 909.60 SQ, ELG 1 914.70 S I RS, LTS 1 993.62 C I RS, LTS 2 321.67 O III PN 2 326 C II RS, LTS, SQ 2 324.21, 2 325.40, 2 326.11, 2 327.65, 2 328.84 Sp.-V/AQuan/1999/10/07:19:25 Page 177

8.5 COMMONLY OBSERVED ULTRAVIOLET EMISSION LINES / 177

Table 8.3. (Continued.)

λ (Aû )a Ion Type of object where observed b Individual components in multiplets

2 328Ð2 414 Fe II LTS, TT 2 328.11, 2 333.52, 2 338.73, 2 344.21, 2 344.70, 2 345.00, 2 349.02, 2 359.83, 2 365.55, 2 367.59, 2 374.46, 2 381.49, 2 382.77, 2 383.79, 2 389.36, 2 394.98, 2 396.15, 2 396.36, 2 399.97, 2 405.16, 2 405.62, 2 407.39, 2 411.25, 2 411.80, 2 414.05 2 329.23 Si II RS, LTS 2 335 Si II PN 2 335.12, 2 335.32, 2 344.92, 2 350.89 2 381.13 He II PN 2 424 Ne IV SQ 2 422.51, 2 425.15 2 471.04 O II PN, SN, HII 2 511.96 He II PN 2 586Ð2 632 Fe II LTS, TT, N, MSG 2 586.65, 2 599.15, 2 600.17, 2 607.87, 2 611.41, 2 612.65, 2 614.61, 2 618.40, 2 621.19, 2 622.45, 2 626.45, 2 629.08, 2 631.83, 2 632.11 2 664.06 He I PN 2 696.92 He I PN 2 724.00 He I PN 2 734.14 He I PN 2 764.62 He I PN, HII 2 783.03 Mg V PN 2 786.81 Ar V PN 2 794 Mg II PN 2 798.81, 2 791.59 2 800 Mg II PN, LTS, RS, TT, IB, N, SQ, 2 796.35, 2 803.53 ELG 2 829.91 He I PN, HII 2 838 C II PN 2 837.54, 2 838.44 2 852.96 Mg I PN, HII 2 854.48 Ar IV PN 2 869.00 Ar IV PN 2 928.34 Mg V PN 2 933 Mg II PN 2 929.49, 2 937.36 2 945.97 He I PN 2 950.07 Mn II PN, TT 2 973.15 O I C 2 978 N III PN 2 973.43, 2 979.70 3 005.36 Ar III PN 3 024.33 O III PN 3 046 O III PN 3 043.91, 3 048.02 3 068 N II PN 3 063.72, 3 071.44 3 109 Ar III PN 3 109.16, 3 110.06 3 133.77 O III PN, TT, N, LTS 3 188.67 He I PN 3 204.03 He II PN

Notes aWavelengths (in vacuum) are taken from: Aller, L.H. 1984, of Thermal Gaseous Nebula (Reidel, Dordrecht); Kelly, R.L. 1979, Atomic Emission Lines in the Near Ultraviolet; Hydrogen through Krypton, NASA Tech. Memo. No. 80268; Kelly, R.L. 1987, Atomic and Ionic Spectrum Lines Below 2000 A:û Hydrogen through Krypton (American Chemical Society, New York); Kelly, R.L. & Palumbo, L.J. 1973, Atomic and Ionic Emission Lines Below 2000 Angstroms (Naval Research Lab., Washington, DC); Koppen,¬ J., & Aller, L.H. 1987, in Exploring the Universe with the IUE Satellite, edited by Y. Kondo (Reidel, Dordrecht), p. 589; and Morton, D.C. 1991, ApJS, 77, 119. bThe astronomical objects where these lines are frequently seen in emission are noted by the abbreviated code in column 3. They are: C, comets; CS, carbon stars; CV, cataclysmic variables (N.B. novae have a separate listing); ELG, emission line galaxies; HII, H II regions; IB, interacting binaries; LTS, late-type stars; MSG, massive supergiants; N, novae; PN, planetary nebulae; RS, RS CVn stars; SQ, Seyfert galaxies and QSOs; SN, supernovae; SS, symbiotic stars; SSO, objects; TT, T Tau stars; WR, WolfÐRayet stars; XRB, low-mass X-ray binaries. Sp.-V/AQuan/1999/10/07:19:25 Page 178

178 / 8 ULTRAVIOLET ASTRONOMY

8.6 ULTRAVIOLET SPECTRAL CLASSIFICATION

Studies of spectral classification of O and B stars based on ultraviolet spectra have been made using Copernicus data and the extensive IUE archive. Low-dispersion spectra were used by Heck et al. [9], Heck [10], and Jaschek and Jaschek [11]. High-dispersion studies have been conducted by Snow and Morton [12], Walborn and Panek [13], Walborn et al. [14], Walborn and Nichols- Bohlin [15], Massa [16], Bates and Gilheany [17], Prinja [18], and Rountree and Sonneborn [19]. For detailed quantitative comparisons, the papers by Massa and Prinja are convenient, because they give tables and/or figures which show the equivalent widths as a function of spectral type or temperature. Prinja [18] gives two useful formulas relating equivalent widths (Wa) in mAû to . The most sensitive diagnostic for O stars temperatures is Si III λ1299:

log(Wa) = 17.89 − 3.43 log Teff.(3)

For B stars, the Si II λ1265 is the most sensitive temperature indicator [16]:

log(Wa) = 20.57 − 4.21 log Teff.(4)

The information in Table 8.4 is taken from these studies. Table 8.4 gives the approximate wavelength and identification for classification lines in its first two columns, and summarizes their changing characteristics as a function of spectral type and in the final column. (More accurate wavelengths can be found in Table 8.3.)

Table 8.4. Lines useful for spectral classification of O and B stars. λ (A)û Ion Comments

1 175 C III In low dispersion this blend of six lines (λλ 1174.933Ð1176.370) is seen to increase from O4 to a maximum at B1, and disappears at B6 into the Ly α wing. In high dispersion one can see dramatic P Cygni profiles for all supergiants from O4 IÐB0.5 Ia, for bright giants as late as O9.5, and for giants as late as O8. 1 216 H I When not affected by interstellar or circumstellar components has a half-width at half-maximum which increases from 10 AatO9to100û AatB8.û 1 240 N V λλ 1239, 1243 show wind profiles in most O stars. Shows a dependence on luminosity at O9.5, since the stellar wind effects have declined by then. 1 247 C III Blended with Fe II λλ 1246.8, 1247.8, and can be severely affected by emission component of NV λ1240 P Cygni lines in luminous stars. Generally increases in strength from early to late O. Strongest in early B (B0ÐB1), and then slowly declines. The ratio C III λ1247/O IV λ1339 depends on luminosity class, being higher for more luminous stars. This ratio can be as large as 4 between supergiants and main-sequence stars at a given temperature (Prinja, R.K. 1990, MNRAS, 246, 392). The comparison of this line with Si II λ1265 shows a slight dependence on luminosity class (N.B.: can be affected by a reseaux mark in high-dispersion IUE spectra). 1 255 Fe V Decreases from O3 to O7. 1 264 Si II Becomes visible at B1; at B1.5 it is clearly present but weaker than λ1247; at B2 it is as strong as λ1247; and by B4 it is much stronger. Continues to increase through B9. Does not show any luminosity effect. 1 300 Si III Probably the most sensitive diagnostic of O star temperatures. Increases sharply from O3 to B2, then levels out in strength from B2 to B5. 1 310 Si II Useful diagnostic in B stars. It is weaker than λ1300 at B2, greater than or equal to 1300 at B3ÐB4, and dominates the spectrum at B5ÐB8. 1 336 C II Doublet, which increases from B0 to a maximum at B8. The wind profiles achieve maximum strength at B1ÐB2 Ia. There is a very strong interstellar contribution to this line. 1 339 O IV Shows a well-defined temperature sequence for luminosity classes I and V in O stars, decreasing as temperature declines. Generally only the λ1339 line is used in this doublet, since the λ1343 line is blended with a nearby Si III line (as well as lying in an awkward location in IUE echelle spectra). 1 371 O V This line declines from O3 until it disappears at O7. Sp.-V/AQuan/1999/10/07:19:25 Page 179

8.6 ULTRAVIOLET SPECTRAL CLASSIFICATION / 179

Table 8.4. (Continued.)

λ (A)û Ion Comments

1 400 Si IV Blend of the λ1394 and λ1403 lines of Si IV. In low-dispersion spectra this blended pair is a useful luminosity indicator for late O, and a spectral type discriminator for B. First strongly visible in low- dispersion spectra at O7, and gets stronger as decreases. In high dispersion, at O6.5 lines display stellar wind effects which increase with luminosity, from none at V to a full P Cyg profile at Ia. At O9.5 the doublet shows no stellar-wind effect in luminosity classes VÐIII, but it develops gradually as a function of luminosity from classes II through Ia. In the B stars, Si IV is strong in B0 and B1 and decreases in strength until it disappears at about B6. The intensity ratio Si IV λ1400/C IV λ1550 is very sensitive to the O star spectral type, being ≈ 1 at O6, and greater than 1 for O6.5ÐO9.7. 1 428 C III (In low dispersion the λ1426 and λ1428 lines are blended, though they are never especially strong. They increase from O4 to a maximum at B1.) Especially fine discriminator in the O7ÐB1 region, where it can be compared to λ1430. 1 430 Fe V The ratio λ1429/λ1430 = 1 for this Fe V doublet between O3 and O4, and declines at O5 and later. 1 453 Blend In low dispersion, has a maximum at O4 and disappears at B0. 1 485 Si II Blend of three lines. First present at B2 and becomes stronger through B9. 1 527 Si II Absorption feature becomes prominent in late B. 1 533 Si II Absorption feature becomes prominent in late B. 1 550 C IV Resonance doublet is one of the most prominent UV lines. Strong in O stars, decreasing from O3 to B2 (in dwarfs) where it disappears. If seen in mid-B, indicates a supergiant. Saturated P Cyg profiles from O3ÐO6, declining at O7. Continues to show strong wind absorption through O9, becoming purely photospheric at B1. At the transition type O9.5 there is an increase in strength with luminosity class. 1 608 Fe II A large collection of Fe II lines exist in the λλ 1600Ð1610 region. These blends increase in strength with increasing luminosity, while showing little temperature effect. In O stars there is a noticeable interstellar component. 1 640 He II Present throughout the O star regime, is still strong at B0, still noticeable, but declining in B0.5ÐB1, weak at B1.5, and weak to absent at B2. 1 655 C I Increases in strength as spectral type gets later. It is a prominent line in B5ÐB9. 1 670 Al II Becomes prominent in late B (N.B.: there is frequently a strong interstellar line seen in O stars, due to this ion). 1 718 N IV Unsaturated subordinate line which shows P Cyg profiles through O6, then declines in strength with increasingly later spectral type. It is still strong at B0, much less prominent at B0.5, and weak to absent at B1. At B0 it is stronger in giants than dwarfs. 1 723 Al II Blend. The components are at λλ 1719.44, 1721.24, 1721.27, 1724.95, 1724.98. Line strength increase with luminosity in B stars. 1 750 N III Doublet at λλ 1748, 1752. The strength of both lines increases between O3 and O4, and the ratio λ1748/λ1752 increases dramatically between O3 and O4. The pair remains distinct through B0, but starts to weaken at B0.5, and disappears as B1. 1 859 Al III Doublet at λλ 1855, 1862. Purely interstellar in O stars. In B stars increases with increasing luminosity class. There is a strong wind maximum at B1-2 Ia. 1 862 Al II Strong in O stars. Blended with λ1855 in low-resolution spectra. Shows an increased strength with increased luminosity class. 1 891 Fe III Present in early B stars. Shows a positive luminosity effect. There are many Fe III lines in this wavelength region. The use of this line and others below is most generally useful in low-dispersion spectra. 1 926 Fe III Similar to λ1891. 1 967 Fe III Similar to λ1891.

The ultraviolet is particularly suitable for classifying O and B stars, due to the strong fluxes for these objects in that wavelength regime. Difficulties with classifying OB stars include the contamination of some lines by strong interstellar components, and the fact that ultraviolet resonance lines are frequently severely affected by stellar winds. Snow and Morton [12] found that all O and B supergiants exhibited mass loss, with P Cygni profiles being seen to as late as B1. For bright giants and giants, strong P Cygni profiles were noted as late as O9.5 and O9, respectively, and all main-noted sequence O stars showed evidence of mass loss. A further complication is that the wind profiles of some B supergiants have been found to be variable. Exactly how much of the dispersion in wind line strengths is due to variations in the intrinsic stellar properties, and how much is due to variability or abundance anomalies, is uncertain [17, 20]. Sp.-V/AQuan/1999/10/07:19:25 Page 180

180 / 8 ULTRAVIOLET ASTRONOMY

8.7 ULTRAVIOLET SPECTROPHOTOMETRIC STANDARDS

Spectrophotometric calibration has always been a thorny problem for long-term ultraviolet satellite missions. Early efforts tended to focus on using hot subdwarfs as reasonably line-free continuum sources, which were not generally variable, and had very small or negligible interstellar reddening. The current IUE absolute calibration is based on comparison with the earlier measurements of some baseline standard stars made by OAO-2 and TD-1, and normalized to the flux values for the fundamental calibration star, η UMa. The stars used were HD 60753, BD + 75◦ 325, HD 93521, BD + 33◦ 2642, and BD + 28◦ 4211 for the low-dispersion data, while ζ Cas, λ Lep, and τ Sco were used for the high-dispersion data. It should be noted that both ζ Cas and η UMa have shown some indications of microvariability [21]. A more complete list of IUE standards can be found in [22], while the HST standards are cited in [23]. More recently a shift has been made to using hot DA (i.e., essentially pure helium) white dwarfs as fundamental calibrators. The reasoning behind this is that the models for these stars are very simple and well understood, as well as being unaffected by spectral lines. The IUE Project’s Final Archive is making use of white dwarfs for their new absolute calibration. The EUVE used this approach from the very beginning. The fundamental calibrator that is being used is G191B2B. Table 8.5 lists some of the ultraviolet standard stars that have been used in common by many missions. Columns 1 and 2 give the star’s catalog number and common name, while columns 3 and 4 list the star’s coordinates. Columns 5 and 6 give the spectral type and visual magnitude, while column 7 indicates which missions have observed this star for calibration purposes.

Table 8.5. Selected ultraviolet spectrophotometric standard stars. Catalog ID Common name α(2000) δ(2000) Sp. Type V Observed bya HD 2151 β Hyi 00:25:45.4 −77:15:16 G2 IV 2.80 H HD 3360 ζ Cas 00:36:58.3 +53:53:49 B2 IV 3.68 OTAVI BPM 16274 00:50:03.2 −52:08:17 DA 14.2 H Feige 11 01:04:21.6 +04:13:38 B0 VI 12.06 OIH HD 10144 α Eri 01:37:42.9 −57:14:12 B3 Vpe 0.46 OCTI HD 11636 β Ari 01:54:38.3 +20:48:29 A5 V 2.64 OTI HD 15318 ξ2 Cet 02:28:09.5 +08:27:36 B9 III 4.29 H GD 50 03:48:50.1 −00:58:30 DA 14.06 H HZ 4 03:55:21.7 +09:47:19 DA 14.52 H LB 227 04:09:28.8 +17:07:54.4 DA 15.34 H HZ 2 04:12:43.5 +11:51:50 DA 13.86 H G191B2B 05:01:31.0 +52:45:48 DA 11.78 VIHE HD 32630 η Aur 05:06:30.8 +41:14:04 B3 V 3.17 OTAI HD 34816 λ Lep 05:19:34.4 −13:10:37 B0.5 IV 4.29 OTAI HD 35468 γ Ori 05:25:07.8 +06:20:59 B2 III 1.64 OTI HD 35580 κ Pic 05:22:22:2 −56:08:04 B8Ð9 V 6.11 TI HD 38666 µ Col 05:44:08.4 −32:19:27 O9.5 IV 5.17 VIH PG 0549 + 158 GD 71 05:52:27.5 +15:53:17 DA 13.04 VIE HD 45557 06:24:13.7 −60:16:52 A0 V 5.80 TI HD 49798 06:48:04.6 −44:18:59 sdO6 8.30 VIH HD 60753 07:33:27.3 −50:35:04 B3 IV 6.69 TIH ◦ CD −31 4800 07:36:30.2 −32:12:45 O8 AI 10.50 AI HD 61421 α CMi 07:39:18.1 +05:13:30 F5 IVÐV 0.38 OCTAI HD 66811 ζ Pup 08:03:35.1 −40:00:12 O5f 2.26 OCTVIH ◦ BD +75 325 08:10:49.3 +74:57:58 O5p 9.54 OTAVIH HD 80007 β Car 09:13:12.1 −69:43:02 A2 IV 1.68 OTI ◦ AGK +81 266 09:21:19.1 +81:43:29 sdO 11.92 AIH ◦ BD +48 1777 09:30:46.6 +48:16:26 O VI 10.37 AI HD 87901 α Leo 10:08:22.3 +11:58:02 B7 V 1.35 OCTAVIH Feige 34 10:39:36.7 +43:06:10 DO 11.18 VIH Sp.-V/AQuan/1999/10/07:19:25 Page 181

8.7 ULTRAVIOLET SPECTROPHOTOMETRIC STANDARDS / 181

Table 8.5. (Continued.)

Catalog ID Common name α(2000) δ(2000) Sp. Type V Observed bya

HD 93521 10:48:23.5 +37:34:13 O9 Vp 7.04 TAVIH HD 100889 θ Crt 11:36:40.8 −09:48:08 B9.5 Vn 4.70 IH HD 103287 γ UMa 11.53:49.8 +53:41:41 A0 Ve 2.44 IH HZ 21 12:13:56.4 +32:56:31 DO 14.68 H PG 1254 + 223 GD 153 12:57:04.5 +22:12:45 DA 13.4 VIE HZ 44 13:23:35.4 +36:08:00 sdO 11.66 VH ◦ Grw +70 5824 13:38:51.8 +70:17:09 DA 12.77 H HD 120315 η UMa 13:47:32.4 +49:18:48 B3 V 1.86 OCTVIH HD 121263 ζ Cen 13:55:32.3 −47:17:18 B2.5 IV 2.55 OCTAI HD 122451 β Cen 14:03:49.5 −60:22:23 B1 III 0.61 H HD 125924 14:22:43.0 −08:14:54 B2 IV 9.70 TAI HD 128801 14:38:48.1 +07:54:44 B9 8.80 TAI HD 137389 15:22:37.1 +62:02:50 A0pSi 5.98 TAI HD 137744 ι Dra 15:24:55.7 +58:57:58 K2 III 3.29 H ◦ BD +33 2642 15:51:59.9 +32:56:55 B2 IV 10.81 OTAIH HD 142669 ρ Sco 15:56:53.0 −29:12:50 B2 IVÐV 3.88 OTAI HD 145454 16:06:19.5 +67:48:36 A0 Vn 5.44 TI G153−41 16:17:55.4 −15:35:49 DA 13.42 VIH HD 149438 τ Sco 16:35:52.9 −28:12:58 B0 V 2.82 OCTAVI HD 149757 ζ Oph 16:37:09.5 −10:34:02 O9.5 Vn 2.56 OCTVIH HD 155763 ζ Dra 17:08:47.1 +65:42:53 B6 III 3.17 OCTAI HD 164058 γ Dra 17:56:30.4 +51:29:20 K5 III 2.22 H HD 172167 α Lyr 18:36:56.3 +38:47:01 A0 V 0.03 OCTAVIH HD 172883 18:39:52.7 +52:11:46 A0pHg 6.00 TI HD 177724 ζ Aql 19:05:24.5 +13:51:48 A0 Vn 2.99 OTAI HD 186427 16 Cyg B 19:41:52.0 +50:31:03 G1.5 V 6.20 IH HD 196519 υ Pav 20:41:57.1 −66:45:39 B9 III 5.15 TAI HD 197637 20:36:00.6 +79:25:49 B3 6.78 TI HD 201908 21:05:29.2 +78:07:35 B8 Vn 5.91 OTI LDS 749B 21:32:15.8 +00:15:14 DB 14.67 H ◦ BD +28 4211 21:51:11.1 +28:51:52 sdOp 10.51 OTAVIH G93−48 21:52:25.3 +02:23:24 DA 12.74 H HD 209952 α Gru 22:08:13.9 −46:57:40 B7 IV 1.74 OCTI NGC 7293 22:29:38.5 −20:50:13 PNN 13.51 VIH HD 214680 10 Lac 22:39:15.6 +39:03:01 O9 V 4.88 OCTAI HD 214923 ζ Peg 22:41:27.7 +10:49:53 B8 V 3.40 H PG 2309 + 105 GD 246 23:12:35.3 +10:50:27 DA 13.10 IHE Feige 110 23:19:58.4 −05:09:56 DOp 11.82 H

Note aObservations were made of these standards by many of the ultraviolet astronomy missions, and they are listed in column 7, where the letters refer to O = OAO-2, C = Copernicus, T = TD-1, A = ANS, V = Voyager UVS, I = IUE, H = HST, E = EUVE.

REFERENCES

1. Edlen,« B. 1953, JOSA, 43, 339 No. 1285 2. Oosterhoff, P.T. 1957, Trans. IAU, 9, 202 9. Heck, A., Egret, D., Jaschek, M., & Jaschek, C. 1984, 3. Stecher, T.P., & Milligan, J.E. 1962, ApJ, 136,1 IUE Low-Resolution Spectra Reference Atlas: Part 1. 4. Morton, D.C., & Spitzer, L. 1966, ApJ, 144,1 Normal Stars (ESA, Paris) 5. Kondo, Y., Giuli, T., Modisette, J.L., & Rydgren, A.E. 10. Heck, A. 1987, in Exploring the Universe with the IUE 1972, ApJ, 176, 153 Satellite, edited by Y. Kondo (Reidel, Dordrecht), p. 121 6. Seaton, M.J. 1979, MNRAS, 187,73 11. Jaschek, C., & Jaschek, M. 1987, The Classification of 7. Savage, B.D., & Mathis, J.S. 1979, ARA&A, 17,73 Stars (Cambridge University Press, Cambridge) 8. Wu, C.-C. et al. 1992, IUE Ultraviolet Spectral Atlas 12. Snow, Jr., T.P., & Morton, D.C. 1976, ApJS, 32, 429 of Selected Astronomical Objects, NASA Tech. Memo. 13. Walborn, N.R., & Panek, R.J. 1984, ApJ, 286, 718 Sp.-V/AQuan/1999/10/07:19:25 Page 182

182 / 8 ULTRAVIOLET ASTRONOMY

14. Walborn, N.R., Nichols-Bohlin, J., & Panek, R.J. 1985, 20. Massa, D., Altner, B., Wynne, D., & Lamers, IUE Atlas of O-Type Spectra from 1200 to 1900 A,û H.J.G.L.M. 1991, A&A, 242, 188 NASA RP-1155 21. Taylor, B.J. 1984, ApJS, 54, 259 15. Walborn, N.R., & Nichols-Bohlin, J. 1987, PASP, 99, 22. Perez,« M.R., Oliversen, N.A., Garhart, M.P., & Teays, 40 T.J. 1990, in Evolution in Astrophysics: IUE Astronomy 16. Massa, D. 1989, A&A, 224, 131 in the Era of New Space Missions, edited by E.J. Rolfe 17. Bates, B., & Gilheany, S. 1990, MNRAS, 243, 320 (ESA, Noordwijk), p. 349 18. Prinja, R.K. 1990, MNRAS, 246, 392 23. Turnshek, D.A., Bohlin, R.C., Williamson, R.L., Lupie, 19. Rountree, J., & Sonneborn, G. 1991, ApJ, 369, 515 O.L., & Koorneef, J. 1990, ApJ, 99, 1243