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Chapter 8
Ultraviolet Astronomy
Terry J. Teays
8.1
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
ULTRAVIOLET WAVELENGTHS
The Earth’s atmosphere is an efficient absorber of ultraviolet radiation, and so astronomical observations 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 Å down to 100 Å. (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 Å, and the extreme ultraviolet as being the region between 912 and 100 Å.) Note
that wavelengths given in this chapter will always be vacuum ones. In the past ultraviolet wavelengths
shorter than 2 000 Å 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 Explorer (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 wavelengths 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 refrac169
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U LTRAVIOLET A STRONOMY
tion (n) of standard air as a function of vacuum wavelength. This algorithm was derived by Edlén [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:
n = 1 + 6.4328 × 10−5 +
2.949 81 × 10−2
2 554.0 × 10−4
+
,
146 × 108 − σ 2
41 × 108 − σ 2
(8.1)
where σ represents the wave number in vacuum, expressed in reciprocal Å.
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. TD1’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. Röntgensatellit (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).
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8.2 U LTRAVIOLET A STRONOMY S ATELLITE M ISSIONS / 171
Table 8.1. Major long-term ultraviolet astronomy missions.
Mission
Operational
dates
OAO-2
12/07/68–2/13/73
Tel.
apert.
(cm)
20
20
20
20
20
20
40
30
30
30
30
Spect.
resol.
(Å)
Instrument
λ (Å)
Photometer
Photometer
Photometer
Photometer
Photometer
Photometer
Nebular photometer
Vidicon
Vidicon
Vidicon
Vidicon
Spectrometer
Spectrometer
1 430
1 550
1 910
2 460
2 980
3 320
1 200–4 000
1 160–1 850
1 850–3 600
12
22
0.05
0.2
0.01
0.04
Reference
[1]
Copernicus
8/21/72–12/31/80
80
Spectrometer
Spectrometer
Spectrometer
Spectrometer
912–1 500
912–1 645
1 640–3 185
1 480–3 275
TD-1
3/12/72–1/9/80
27.5
Photometer
Spectrophotometer
2 740
1 350–2 550
[3]
ANS
8/30/74–6/14/77
22
Photometer
Photometer
Photometer
Photometer
Photometer
1 550
1 800
2 200
2 500
3 300
[4]
IUE
1/26/78–9/30/96
45
Echelle spectrograph
Spectrograph
1 145–3 230
1 150–3 300
0.2
6
[5, 6]
HST
4/24/90–
240
GHRS
FOS
WF/PC
FOC
HSP
STIS
1 110–3 200
1 150–7 000
1 200–10 000
1 200–6 500
1 150–8 000
1 150–10 000
0.01-3.5
1.2-7
[7]
ROSAT
6/1/90–
aa
Wide field camera
Wide field camera
Wide field camera
Wide field camera
60–140
112–200
150–220
530–720
[8]
EUVE
6/7/92–
Scanning photometer
Scanning photometer
Scanning photometer
Deep survey
Spectrometer
Spectrometer
Spectrometer
44–360
44–360
400–750
40–385
70–190
140–380
280–760
[9]
aa
aa
aa
aa
aa
aa
[2]
0.5
1
2
Note
a See 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
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Page 172
U LTRAVIOLET A STRONOMY
4.
5.
6.
7.
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
Kondo, Y., editor, 1987, Exploring the Universe with the IUE Satellite (Reidel, Dordrecht).
Newmark, J.S., Holm, A.V., Imhoff, C.I., Oliversen, N.A., Pitts, R.E., & Sonneborn, G. 1992, NASA IUE Newslett., 47, 1
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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 Å) 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 Galaxy [2]
IUE-ULDA Access Guide No. 2: Comets [3]
ANS Ultraviolet Photometry, 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 Wolf–Rayet Stars [16]
The Hopkins Ultraviolet Telescope Far-Ultraviolet Spectral Atlas of Wolf–Rayet Stars [17]
International Ultraviolet Explorer Atlas of O Type Spectra from 1200 to 1900 Å [18]
Ultraviolet Spectral Morphology of the O Stars. II. The Main Sequence [19]
P Cygni and Related Profiles in the Ultraviolet Spectra of O-Stars [20]
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8.3 S IGNIFICANT ATLASES AND C ATALOGS / 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 (125–198 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 Å 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, 2500–3230 Ř[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 Galaxies [49]
An Atlas of Hubble Space Telescope 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]
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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
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17.
18.
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25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
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45.
46.
47.
48.
49.
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51.
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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 Å.
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:
2.70 ≤ σ ≤ 3.65,
Aλ /E B−V = 1.56 + 1.048σ +
1.01
,
[(σ − 4.60)2 + 0.280]
(8.2a)
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8.5 C OMMONLY O BSERVED U LTRAVIOLET E MISSION L INES / 175
1.01
,
[(σ − 4.60)2 + 0.280]
3.65 ≤ σ ≤ 7.14,
Aλ /E B−V = 2.29 + 0.848σ +
7.14 ≤ σ ≤ 10,
Aλ /E B−V = 16.17 − 3.20σ + 0.2975σ 2 .
(8.2b)
(8.2c)
Savage and Mathis [7] adopt 3.1 for the value of A V /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 Å) of the line, using the convention that a reasonably
precise value (to 0.01 Å) 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 (Å)a
Ion
Type of object where observed b
Individual components in multiplets
538
584.33
834
916
O II
He I
O III
N II
C
SSO
C
C
537.83, 538.26, 538.32, 539.13
933.4
977.02
1 033
1 066.66
1 085
S VI
C III
O IV
Ar I
N II
SNR
SNR
SNR
SSO, C
C
1 175
C III
WR, PN, CS, SS
1 199
S III
SSO
1 215.67
1 240
HI
NV
1 247.38
1 256
1 279
C III
S II
CI
(all sources)
PN, SS, WR, CV, XRB, SN, IB,
N, SQ, SNR
SS, WR
SSO
TT, LTS
1 299
1 304
1 309
Si III
OI
Si II
SS, IB, TT
RS, LTS, N, SQ, C
PN
832.93, 833.74, 835.29
915.61, 915.96, 916.02, 916.10, 916.35,
916.70, 916.71
1 031.93, 1 033.82, 1 037.62
1 083.99, 1 084.56, 1 084.58, 1 085.53,
1 085.55, 1 085.70, 1 085.12
1 174.93, 1 175.26, 1 175.59, 1 175.71,
1 175.99, 1 176.37
1 190.21, 1 194.06, 1 194.46, 1 197.56,
1 200.97, 1 201.73, 1 202.13
1 238.82, 1 240.15, 1 242.80
1 250.58, 1 253.81, 1 256.12, 1 259.52
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
1 298.89, 1 298.96
1 302.17, 1 303.49, 1 304.86, 1 306.03
1 304.37, 1 307.64, 1 309.28
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176 / 8
U LTRAVIOLET A STRONOMY
Table 8.3. (Continued.)
λ (Å)a
Ion
Type of object where observed b
Individual components in multiplets
1 335
C II
1 334.53, 1 335.31, 1 335.66, 1 335.71
1 342
1 371.29
1 394
O IV
OV
Si IV
1 397–1 407
O IV
TT, PN, LTS, RS, WR, CV, N,
SNR, C
CS, SS, WR, XRB
PN, CS, SS, XRB, SNR
PN, LTS, RS, TT, XRB, CV, IV,
N, SQ
PN, SS, N
1 402.77
Si IV
1 460
CI
PN, LTS, RS, TT, XRB, CV, IV,
N, SQ
TT
1 473
SI
RS, LTS
1 483.32
1 486
1 487
1 550
N IV
SI
N IV
C IV
1 561
CI
PN, SS, WR, N
RS, LTS
PN, SS, WR, N, SNR
TT, PN, LTS, SS, N, WR, CV, IB,
XRB, SQ, SNR
C
1 574.77
1 577
1 602
1 640
Ne V
C III
Ne IV
He II
1 641.31
1 657
OI
CI
1 663
O III
1 670.79
1 710
1 718.55
1 728.94
1 750
Al II
Si II
N IV
S III
N III
1 760
1 815
1 814.63
1 860
1 882.71
1 892.03
C II
Si II
Ne III
Al III
Si III
Si III
1 900.29
1 908.73
SI
C III
1 914.70
1 993.62
2 321.67
2 326
SI
CI
O III
C II
PN, N
SS
PN, N, SS
TT, PN, LTS, RS, WR, XRB, SQ,
SNR
RS, LTS, SS, N
C, RS, LTS, TT
PN, WR, SQ, N, LTS, HII, SS,
SNR
IB, LTS
PN, WR
PN, WR, XRB, CV, N
SSO
WR, TT, HII, N, SN, SNR
PN
TT, PN, RS, LTS
PN, N
LTS, IB
PN, LTS, HII, SN, N, SQ, SNR
TT, PN, LTS, HII, SN, N, SQ,
SNR
RS, LTS, SN, HII
TT, PN, LTS, WR, HII, N, SN,
SQ, ELG
RS, LTS
RS, LTS
PN
RS, LTS, SQ
1 342.99, 1 343.51
1 393.76, 1 396.75, 1 398.13
1 397.23, 1 399.78, 1 401.16, 1 404.81,
1 407.38
1 459.03, 1 463.34, 1 467.40, 1 467.88,
1 468.41
1 472.97, 1 473.02, 1 473.07, 1 473.99,
1 474.38, 1 474.57, 1 478.50
1 485.62, 1 487.15
1 486.50, 1 487.89
1 548.20, 1 550.77
1 560.31, 1 560.68, 1 560.71, 1 561.05,
1 561.34, 1 561.37, 1 561.44
1 576.48, 1 577.30, 1 577.89
1 601.50, 1 601.68
1 640.47, 1 640.49
1 656.28, 1 656.93, 1 657.01, 1 657.38,
1 657.59, 1 657.91, 1 658.12
1 660.81, 1 666.15
1 710.83, 1 711.30
1 746.82, 1 748.65, 1 749.67, 1 752.16,
1 754.00
1 760.47, 1 760.82
1 808.01, 1 816.93, 1 817.45
1 854.72, 1 862.79
1 906.68, 1 908.73, 1 909.60
2 324.21, 2 325.40, 2 326.11, 2 327.65,
2 328.84
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8.5 C OMMONLY O BSERVED U LTRAVIOLET E MISSION L INES / 177
Table 8.3. (Continued.)
λ (Å)a
Ion
Type of object where observed b
Individual components in multiplets
2 328–2 414
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
2 335
2 381.13
2 424
2 471.04
2 511.96
2 586–2 632
Si II
Si II
He II
Ne IV
O II
He II
Fe II
RS, LTS
PN
PN
SQ
PN, SN, HII
PN
LTS, TT, N, MSG
2 664.06
2 696.92
2 724.00
2 734.14
2 764.62
2 783.03
2 786.81
2 794
2 800
He I
He I
He I
He I
He I
Mg V
Ar V
Mg II
Mg II
2 829.91
2 838
2 852.96
2 854.48
2 869.00
2 928.34
2 933
2 945.97
2 950.07
2 973.15
2 978
3 005.36
3 024.33
3 046
3 068
3 109
3 133.77
3 188.67
3 204.03
He I
C II
Mg I
Ar IV
Ar IV
Mg V
Mg II
He I
Mn II
OI
N III
Ar III
O III
O III
N II
Ar III
O III
He I
He II
PN
PN
PN
PN
PN, HII
PN
PN
PN
PN, LTS, RS, TT, IB, N, SQ,
ELG
PN, HII
PN
PN, HII
PN
PN
PN
PN
PN
PN, TT
C
PN
PN
PN
PN
PN
PN
PN, TT, N, LTS
PN
PN
2 335.12, 2 335.32, 2 344.92, 2 350.89
2 422.51, 2 425.15
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
2 798.81, 2 791.59
2 796.35, 2 803.53
2 837.54, 2 838.44
2 929.49, 2 937.36
2 973.43, 2 979.70
3 043.91, 3 048.02
3 063.72, 3 071.44
3 109.16, 3 110.06
Notes
a Wavelengths (in vacuum) are taken from: Aller, L.H. 1984, Physics 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 Å: 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); Köppen, 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.
b The 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, solar
system objects; TT, T Tau stars; WR, Wolf–Rayet stars; XRB, low-mass X-ray binaries.
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8.6
U LTRAVIOLET A STRONOMY
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 NicholsBohlin [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 mÅ to effective temperature.
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 luminosity 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 (Å)
Ion
Comments
1 175
C III
1 216
HI
1 240
NV
1 247
C III
1 255
1 264
Fe V
Si II
1 300
Si III
1 310
Si II
1 336
C II
1 339
O IV
1 371
OV
In low dispersion this blend of six lines (λλ 1174.933–1176.370) is seen to increase from O4 to a
maximum at B1, and disappears at B6 into the L y α wing. In high dispersion one can see dramatic P
Cygni profiles for all supergiants from O4 I–B0.5 Ia, for bright giants as late as O9.5, and for giants
as late as O8.
When not affected by interstellar or circumstellar components has a half-width at half-maximum
which increases from 10 Å at O9 to 100 Å at B8.
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.
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 (B0–B1), 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).
Decreases from O3 to O7.
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.
Probably the most sensitive diagnostic of O star temperatures. Increases sharply from O3 to B2, then
levels out in strength from B2 to B5.
Useful diagnostic in B stars. It is weaker than λ1300 at B2, greater than or equal to 1300 at B3–B4,
and dominates the spectrum at B5–B8.
Doublet, which increases from B0 to a maximum at B8. The wind profiles achieve maximum strength
at B1–B2 Ia. There is a very strong interstellar contribution to this line.
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).
This line declines from O3 until it disappears at O7.
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8.6 U LTRAVIOLET S PECTRAL C LASSIFICATION / 179
Table 8.4. (Continued.)
λ (Å)
Ion
Comments
1 400
Si IV
1 428
C III
1 430
1 453
1 485
1 527
1 533
1 550
Fe V
Blend
Si II
Si II
Si II
C IV
1 608
Fe II
1 640
He II
1 655
1 670
CI
Al II
1 718
N IV
1 723
Al II
1 750
N III
1 859
Al III
1 862
Al II
1 891
Fe III
1 926
1 967
Fe III
Fe III
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 lowdispersion spectra at O7, and gets stronger as surface gravity 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 V–III, 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 O6.5–O9.7.
(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 O7–B1 region,
where it can be compared to λ1430.
The ratio λ1429/λ1430 = 1 for this Fe V doublet between O3 and O4, and declines at O5 and later.
In low dispersion, has a maximum at O4 and disappears at B0.
Blend of three lines. First present at B2 and becomes stronger through B9.
Absorption feature becomes prominent in late B.
Absorption feature becomes prominent in late B.
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 O3–O6, 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.
A large collection of Fe II lines exist in the λλ 1600–1610 region. These blends increase in strength
with increasing luminosity, while showing little temperature effect. In O stars there is a noticeable
interstellar component.
Present throughout the O star regime, is still strong at B0, still noticeable, but declining in B0.5–B1,
weak at B1.5, and weak to absent at B2.
Increases in strength as spectral type gets later. It is a prominent line in B5–B9.
Becomes prominent in late B (N.B.: there is frequently a strong interstellar line seen in O stars, due
to this ion).
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.
Blend. The components are at λλ 1719.44, 1721.24, 1721.27, 1724.95, 1724.98. Line strength
increase with luminosity in B stars.
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.
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.
Strong in O stars. Blended with λ1855 in low-resolution spectra. Shows an increased strength with
increased luminosity class.
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.
Similar to λ1891.
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].
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8.7
U LTRAVIOLET A STRONOMY
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
HD 2151
HD 3360
BPM 16274
Feige 11
HD 10144
HD 11636
HD 15318
GD 50
HZ 4
LB 227
HZ 2
G191B2B
HD 32630
HD 34816
HD 35468
HD 35580
HD 38666
PG 0549 + 158
HD 45557
HD 49798
HD 60753
CD −31◦ 4800
HD 61421
HD 66811
BD +75◦ 325
HD 80007
AGK +81◦ 266
BD +48◦ 1777
HD 87901
Feige 34
β Hyi
ζ Cas
α Eri
β Ari
ξ 2 Cet
η Aur
λ Lep
γ Ori
κ Pic
µ Col
GD 71
α CMi
ζ Pup
β Car
α Leo
α(2000)
δ(2000)
Sp. Type
V
00:25:45.4
00:36:58.3
00:50:03.2
01:04:21.6
01:37:42.9
01:54:38.3
02:28:09.5
03:48:50.1
03:55:21.7
04:09:28.8
04:12:43.5
05:01:31.0
05:06:30.8
05:19:34.4
05:25:07.8
05:22:22:2
05:44:08.4
05:52:27.5
06:24:13.7
06:48:04.6
07:33:27.3
07:36:30.2
07:39:18.1
08:03:35.1
08:10:49.3
09:13:12.1
09:21:19.1
09:30:46.6
10:08:22.3
10:39:36.7
−77:15:16
+53:53:49
−52:08:17
+04:13:38
−57:14:12
+20:48:29
+08:27:36
−00:58:30
+09:47:19
+17:07:54.4
+11:51:50
+52:45:48
+41:14:04
−13:10:37
+06:20:59
−56:08:04
−32:19:27
+15:53:17
−60:16:52
−44:18:59
−50:35:04
−32:12:45
+05:13:30
−40:00:12
+74:57:58
−69:43:02
+81:43:29
+48:16:26
+11:58:02
+43:06:10
G2 IV
B2 IV
DA
B0 VI
B3 Vpe
A5 V
B9 III
DA
DA
DA
DA
DA
B3 V
B0.5 IV
B2 III
B8–9 V
O9.5 IV
DA
A0 V
sdO6
B3 IV
O8 AI
F5 IV–V
O5f
O5p
A2 IV
sdO
O VI
B7 V
DO
2.80
3.68
14.2
12.06
0.46
2.64
4.29
14.06
14.52
15.34
13.86
11.78
3.17
4.29
1.64
6.11
5.17
13.04
5.80
8.30
6.69
10.50
0.38
2.26
9.54
1.68
11.92
10.37
1.35
11.18
Observed bya
H
OTAVI
H
OIH
OCTI
OTI
H
H
H
H
H
VIHE
OTAI
OTAI
OTI
TI
VIH
VIE
TI
VIH
TIH
AI
OCTAI
OCTVIH
OTAVIH
OTI
AIH
AI
OCTAVIH
VIH
Sp.-V/AQuan/1999/10/07:19:25
Page 181
8.7 U LTRAVIOLET S PECTROPHOTOMETRIC S TANDARDS / 181
Table 8.5. (Continued.)
Catalog ID
HD 93521
HD 100889
HD 103287
HZ 21
PG 1254 + 223
HZ 44
Grw +70◦ 5824
HD 120315
HD 121263
HD 122451
HD 125924
HD 128801
HD 137389
HD 137744
BD +33◦ 2642
HD 142669
HD 145454
G153−41
HD 149438
HD 149757
HD 155763
HD 164058
HD 172167
HD 172883
HD 177724
HD 186427
HD 196519
HD 197637
HD 201908
LDS 749B
BD +28◦ 4211
G93−48
HD 209952
NGC 7293
HD 214680
HD 214923
PG 2309 + 105
Feige 110
Common name
θ Crt
γ UMa
GD 153
η UMa
ζ Cen
β Cen
ι Dra
ρ Sco
τ Sco
ζ Oph
ζ Dra
γ Dra
α Lyr
ζ Aql
16 Cyg B
υ Pav
α Gru
10 Lac
ζ Peg
GD 246
α(2000)
10:48:23.5
11:36:40.8
11.53:49.8
12:13:56.4
12:57:04.5
13:23:35.4
13:38:51.8
13:47:32.4
13:55:32.3
14:03:49.5
14:22:43.0
14:38:48.1
15:22:37.1
15:24:55.7
15:51:59.9
15:56:53.0
16:06:19.5
16:17:55.4
16:35:52.9
16:37:09.5
17:08:47.1
17:56:30.4
18:36:56.3
18:39:52.7
19:05:24.5
19:41:52.0
20:41:57.1
20:36:00.6
21:05:29.2
21:32:15.8
21:51:11.1
21:52:25.3
22:08:13.9
22:29:38.5
22:39:15.6
22:41:27.7
23:12:35.3
23:19:58.4
δ(2000)
Sp. Type
V
+37:34:13
−09:48:08
+53:41:41
+32:56:31
+22:12:45
+36:08:00
+70:17:09
+49:18:48
−47:17:18
−60:22:23
−08:14:54
+07:54:44
+62:02:50
+58:57:58
+32:56:55
−29:12:50
+67:48:36
−15:35:49
−28:12:58
−10:34:02
+65:42:53
+51:29:20
+38:47:01
+52:11:46
+13:51:48
+50:31:03
−66:45:39
+79:25:49
+78:07:35
+00:15:14
+28:51:52
+02:23:24
−46:57:40
−20:50:13
+39:03:01
+10:49:53
+10:50:27
−05:09:56
O9 Vp
B9.5 Vn
A0 Ve
DO
DA
sdO
DA
B3 V
B2.5 IV
B1 III
B2 IV
B9
A0pSi
K2 III
B2 IV
B2 IV–V
A0 Vn
DA
B0 V
O9.5 Vn
B6 III
K5 III
A0 V
A0pHg
A0 Vn
G1.5 V
B9 III
B3
B8 Vn
DB
sdOp
DA
B7 IV
PNN
O9 V
B8 V
DA
DOp
7.04
4.70
2.44
14.68
13.4
11.66
12.77
1.86
2.55
0.61
9.70
8.80
5.98
3.29
10.81
3.88
5.44
13.42
2.82
2.56
3.17
2.22
0.03
6.00
2.99
6.20
5.15
6.78
5.91
14.67
10.51
12.74
1.74
13.51
4.88
3.40
13.10
11.82
Observed bya
TAVIH
IH
IH
H
VIE
VH
H
OCTVIH
OCTAI
H
TAI
TAI
TAI
H
OTAIH
OTAI
TI
VIH
OCTAVI
OCTVIH
OCTAI
H
OCTAVIH
TI
OTAI
IH
TAI
TI
OTI
H
OTAVIH
H
OCTI
VIH
OCTAI
H
IHE
H
Note
a Observations 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.
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