Argon (Ar) is also one of the most abundant rare gases in the Universe and Ar ion lines are observed in solar and astrophysical plasmas as well as in the laboratory. It is observed that the spectra of early B-type stars contain a large number of unblended Ar ii lines (Keenan et al. 1990; Lanz et al. 2008). Lanz et al. analysed the blue spectra of Ar ii in the 10 B-type main-sequence stars in the Orion association and inferred about the mean abundance of Ar in the star HD 35299. Direct observation of the fine structure splittings in the ground states of Ne ii and Ar ii were carried out by Yamada, Kanamori & Hirota (1985). These two transitions were measured with frequencies 780.4240 ± 0.0011 cm−1 and 1431.5831 ± 0.0007 cm−1 for Ne ii and Ar ii, respectively. They are advantageous for infrared spectroscopy over optical, radio frequency and microwave regions to estimate the abundance of Ne and Ar. Because these infrared lines are relatively unaffected by interstellar dust, they can be easily identified by high-resolution telescopes. In fact, the considered infrared line of Ne ii is the only line that can be observed using a ground-based observatory among all other most abundant elements in the Universe at their various stages of ionization. Krypton (Kr) ion spectra were also detected in the interstellar medium (Cardelli & Mayer 1997), in the galactic disc (Cartledge, Meyer & Lauroesch 2003) and in planetary nebulae (Dinerstein 2001). With the help of the Goddard high-resolution spectrograph in the Hubble Space Telescope, many Kr ii lines have also been observed in the interstellar medium (Cardelli, Sarage & Ebbets 1991). More importantly, the materials responsible for the formation of the young early-type stars are enriched with Kr (Leckrone et al. 1993). Extensive analysis of the Kr ii spectra has been carried out by deBruin, Humphreys & Meggers (1933), who quoted uncertainties of the sharp lines of about 0.01 Å. A second analysis was carried out by Minnhagen, Strihed & Petersson (1969) for two sets of wavelength ranges, 800–1350 and 2350–2450 Å. Humphreys & Paul (1970) also reported wavelengths of the 43 lines in Kr ii most precisely from the interferometric observations, using isotropically pure 86Kr. Following this, Bredice et al. (1988) obtained 36 additional lines between 1134 and 8652 Å using an energetically excited source with an accuracy of 0.01 Å. There have been numerous attempts made in the last two decades to discover accurate values of the transition probabilities in Kr ii (Koozekanani & Trusty 1969; Levchenko 1971; Miller, Roig & Bengtson 1972) for various inter-combination transitions. Similarly, the lines of singly charged xenon (Xe ii) have been identified in many HgMn young stars, such as k Cnc, 112 Her, 46 Aql (HD 186122), 33 Gem, HR 7143, HR 7245, HR 7361, HD 175640, etc. (Whitford 1962; Bidelman 1966), which were analysed using the available spectroscopic data of Xe ii (Castelli & Hubrig 2004; Dworetsky, Persaud & Patel 2008). Yüce, Castelli & Hubrig (2011) also investigated the Xe ii lines lying in the ranges 3900–4521, 4769–7542 and 7660–8000 Å in HgMn stars. Observations of the optical lines from heavy elements, especially from radioactive atoms, in the atmosphere of Procyon have been reported (Yushchenko & Gopka 1996). It happens that the wavelength of the emission line of the 7p 2P3/2 → 7p 2P1/2 transition in singly ionized radon (Rn ii) is about 3216 Å and could be useful for estimating the abundance of Rn in astronomical objects.
Citation: Cardelli, Sarage & Ebbets 1991
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Citation: Keenan et al. 1990
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Citation: Yamada, Kanamori & Hirota (1985)
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Citation: Leckrone et al. 1993
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Citation: deBruin, Humphreys & Meggers (1933)
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Citation: Minnhagen, Strihed & Petersson (1969)
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Citation: Humphreys & Paul (1970)
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Citation: Bredice et al. (1988)
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Citation: Miller, Roig & Bengtson 1972
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