Unexplained Spectral Phenomena in the Interstellar Medium: an introduction Sun Kwok1 Astrophysics and Space Science (in press) the demise of the Aristotelian concept of celestial ob- jects being composed of ether. Abstract In 1868, Norman Lockyer discovered a bright yellow- There exists a number of astronomical spectral phe- orange line in the solar spectrum, which he suggested to nomena that have remained unidentified after decades be due to a new element and named it “helium”. The of extensive observations. The diffuse interstellar existence of this new element was confirmed by the dis- bands, the 220 nm feature, unidentified infrared emis- covery of its terrestrial counterpart in 1895. In 1869, sion bands, extended red emissions, and 21 and 30 µm a green line was found in the spectrum of the corona emission features are seen in a wide variety of astro- of the Sun during the total solar eclipse, and this line physical environments. The strengths of these features was also thought to be a new element and was named suggest that they originate from chemical compounds “coronium”. In 1864, William Huggins found a bright made of common elements, possibly organic in nature. green emission line in the spectrum of the planetary The quest to understand how such organic materials nebula NGC 6543. Since this line did not match any are synthesized and distributed across the Galaxy rep- lines from known chemical elements, it was suggested resents a major challenge to our understanding of the to be due to a new element “nebulium”. It was not chemical content of the Universe. until the early 20th century that “coronium” was iden- 13+ Keywords astrobiology; astrochemistry; ISM: lines tified as electronic transitions from ionized iron (Fe ) and bands; ISM: molecules; planetary nebulae: general; due to the high temperature in the solar corona, and stars: AGB and post-AGB “nebulium” was identified as forbidden lines of ionized oxygen (O++) arising under low-density nebular condi- tions. These applications of quantum theory of atoms 1 Historical background and laboratory atomic spectroscopy to explain astro- nomical observations represented the beginning of the In 1814, Joseph Fraunhofer observed and tabulated 473 arXiv:2107.07571v1 [astro-ph.GA] 15 Jul 2021 modern discipline of astrophysics. dark lines in the spectrum of the Sun. By compar- In the early 21st century, we are facing similar chal- ing the Fraunhofer lines with the bright color lines ob- lenges in the form of a number of unexplained spectral served in heated chemical elements, Gustav Kirchhoff phenomena in the interstellar medium. The diffuse in- and Robert Bunsen identified the solar lines as originat- terstellar bands (DIBs) were discovered in 1922, when ing from elements sodium, calcium, magnesium, iron, two optical absorption lines of interstellar origin were chromium, nickel, barium, copper, and zinc. This led seen in the spectra of stars (Heger 1922). As of 2021, to the realization that the Sun is made of the same over 500 DIBs in the ultraviolet, visible, and infrared chemical elements as those of the Earth and marked wavelength regions have been cataloged along the line of sights of over 100 stars. Sun Kwok The 220 nm ultraviolet feature was discovered in 1 Department of Earth, Ocean, and Atmospheric Sciences, Uni- 1965 (Stecher 1965). This feature is seen in the ex- versity of British Columbia, Vancouver, Canada; corresponding author:[email protected] tinction curves of many stars, with characteristically consistent profiles and peak wavelengths. Its wide pres- 2 ence suggests that the carrier is a common constituent asymptotic-giant-branch stars. From the spectral en- of the diffuse interstellar medium. ergy distribution of 21 µm sources, it is found that The extended red emission (ERE) is a broad (∆λ ∼ the 21 and 30 µm features can carry respectively up 80 nm) emission band peaking between 650 and 800 to 8 and 20% of the total energy output of the objects nm, first discovered in the reflection nebula HD44179 (Hrivnak, Volk, and Kwok 2000). (Cohen et al. 1975). ERE has been detected in re- flection nebulae, dark nebulae, cirrus clouds, planetary nebulae, H ii regions, diffuse interstellar medium, and 2 Distribution in the Universe haloes of galaxies. It is commonly attributed to pho- toluminescence powered by far UV photons. It is esti- Although these unexplained spectral phenomena were mated that ∼4% of the energy absorbed by interstellar first discovered in the spectra of stars, they are not iso- dust at λ<0.55 µm is emitted in the form of the ERE. lated phenomena as they are observed in a wide range A family of unidentified infrared emission (UIE) fea- of celestial objects throughout the Universe. DIBs have tures at 3.3, 6.2, 7.7, 8.6, and 11.3 µm was discovered been detected in external galaxies with redshifts up to in the spectrum of the planetary nebula NGC 7027 0.5 (Sarre 2006). The 220 nm feature has been de- (Russell, Soifer, and Willner 1977). The 3.3 µm fea- tected in interplanetary dust particles in the Solar Sys- ture was first identified as the C−H stretching mode tem (Bradley et al. 2005) as well as in distant galaxies of aromatic compounds by Knacke (1977). The or- with redshift >2 (El´ıasd´ottir et al. 2009). A survey of ganic origin of the UIE bands was extensively discussed 150 galaxies by the AKARI satellite found that ∼0.1% by Duley & Williams (1981), who assigned the 3.3 and of the total energy of the parent galaxies is emitted 11.3 µm features to graphitic (aromatic) materials. through the 3.3 µm UIE band (Imanishi et al. 2010). In Also present in astronomical spectra are emission some active galaxies, up to 20% of the total luminosity features around 3.4 µm, which arise from symmetric of the galaxy is emitted in the UIE bands (Smith et al. and anti-symmetric C−H stretching modes of methyl 2007). The 3.4 µm aliphatic feature has been de- and methylene groups (Jourdain de Muizon, D’Hendecourttected and Geba in absorptionlle in ultraluminous infrared galaxies 1990). The bending modes of these groups also man- (Mason et al. 2004; Risaliti et al. 2006). The detection ifest themselves at 6.9 and 7.3 µm. In addition, there of UIE bands in high-redshift galaxies (Teplitz et al. are unidentified emission features at 15.8, 16.4, 17.4, 2007) and quasars (Lutz et al. 2007) implies that or- 17.8, and 18.9 µm. The emission bands themselves are ganic compounds were widely present as early as 10 often accompanied by strong, broad emission plateaus billion years ago. This suggests that abiological syn- features at 6−9, 10−15, and 15−20 µm. The first two thesis of complex organics was active through most of plateau features have been identified as superpositions the history of the Universe. of in-plane and out-of-plane bending modes emitted by a mixture of aliphatic side groups attached to aromatic rings (Kwok, Volk, and Bernath 2001). This collection 3 Chemical nature of the carriers of features in the UIE family has been observed in plan- etary nebulae, reflection nebulae, novae, Hii regions, Because of the strengths and ubiquitous nature of and galaxies. the features, the carrier must be made of common, The unidentified infrared emission feature around 30 abundant elements, with the element carbon proba- µm was discovered from Kuiper Airborne Observatory bly playing a major role. While the DIBs are com- observations (Forrest, Houck, and McCarthy 1981). It monly believed to be due to electronic transitions was first seen in carbon-rich asymptotic giant branch of gas-phase carbon-based molecules, the carrier of stars, planetary nebulae, and proto-planetary neb- the 220 nm feature is more likely to be a carbona- ulae. Among planetary nebulae in the Magellanic ceous solid such as amorphous carbon (Mennella et al. Clouds, about half of them possess the 30 µm feature 1998), carbon onions (Iglesias-Groth 2004), hydro- (Bernard-Salas et al. 2009). genated fullerences (Cataldo & Iglesias-Groth 2009), or The 21 µm emission feature was first discovered in polycrystalline graphite (Papoular & Papoular 2009). Infrared Astronomical Satellite Low Resolution Spec- Among the hundreds of DIBs, only two (963.2 and troscopic survey (Kwok, Volk, and Hrivnak 1989). The 957.7 nm) have been positively identified as originat- + feature peaks around 20.1 µm and shows a broad (∼ 2 ing from ionized fullerene (C60, Foing & Ehrenfreund + µm) and smooth profile. The 21 µm feature is almost 1994; Campbell et al. 2015). Two weaker lines of C60 always accompanied by the 30 µm feature. The 21 at 942.8 and 936.6 nm have also been suggested to have µm feature is primarily observed in carbon-rich post- counterparts in DIBs (Walker et al. 2015). 3 A variety of chemical structures have been sug- time scales of ∼103 years in the proto-planetary nebu- gested as the carriers of the UIE bands. These include lae phase (Kwok, Volk, & Hrivnak 1999), and over time polycyclic aromatic hydrocarbon (PAH) molecules scales of weeks in novae (Helton et al. 2011). This sug- (L´eger and Puget 1984; Allamandola, Tielens, and Barker gests that the synthesis of the UIE carriers is extremely 1989), small carbonaceous molecules (Bernstein and Lynch efficient. How such synthesis can occur so rapidly under 2009), hydrogenated amorphous carbon (HAC), soot low-density conditions is not understood by our current and carbon nanoparticles (Hu and Duley 2008), quenched chemical models. carbonaceous composite particles (QCC, Sakata et al. Could the carriers of these unexplained spectral phe- 1987), coal and kerogen (Papoular et al. 1989; Papoular nomena be new chemical compounds unobserved on 2001), petroleum fractions (Cataldo, Feheyan, and HeymannEarth, as in the case of the discovery of helium in 2002), and mixed aromatic/aliphatic organic nanopar- the Sun, or could they be the result of unusual phys- ticles (MAON, Kwok & Zhang 2013).