Unexplained Spectral Phenomena in the Interstellar Medium: An

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arXiv:2107.07571v1 [astro-ph.GA] 15 Jul 2021 hmcleeet stoeo h at n marked same and the Earth of the made of is those Sun as the led elements that This chemical realization zinc. the and iron, to copper, magnesium, barium, calcium, nickel, sodium, chromium, elements originat- as from lines solar ing Kirchhoff compar- the Gustav identified By Bunsen ob- elements, Robert lines and chemical color Sun. heated bright the the in with of served lines spectrum Fraunhofer the the ing in 473 lines tabulated and dark observed Fraunhofer Joseph 1814, In background Historical 1 post-AGB and general; AGB nebulae: stars: planetary molecules; ISM: bands; and Keywords the of Universe. understanding the our of to content chemical challenge rep- Galaxy major materials the a organic across resents such distributed and how synthesized understand nature. are to in organic quest possibly The compounds elements, chemical common from of astro- originate made of features they variety these that wide of suggest strengths a The in environments. 30 seen physical and are 21 interstellar and features emis- emissions, emission infrared diffuse red unidentified extended The bands, feature, sion nm 220 the observations. bands, decades extensive after unidentified of remained have that nomena Abstract Kwok Sun introduction Interstellar an the in Medium: Phenomena Spectral Unexplained est fBiihClmi,Vnovr aaa correspon Canada; Vancouver, author:[email protected] Columbia, British of versity 1 u Kwok Sun eateto at,Oen n topei cecs Uni- Sciences, Atmospheric and Ocean, Earth, of Department hr xssanme fatooia pcrlphe- spectral astronomical of number a exists There press) (in Science Space and Astrophysics 1 srbooy srceity S:lines ISM: astrochemistry; astrobiology; ding µ m 95(tce 95.Ti etr sse nteex- pres- wide the characteristically Its in with wavelengths. peak seen stars, and profiles is many consistent feature of This curves tinction 1965). (Stecher 1965 stars. line 100 over the infrared of along sights and 2021, cataloged of of visible, been As ultraviolet, have the regions 1922). wavelength in (Heger DIBs stars 500 of were over spectra origin the interstellar in of when seen lines 1922, absorption in optical discovered in- two were diffuse (DIBs) The spectral bands medium. unexplained terstellar interstellar of number the a in of phenomena form the in lenges astrophysics. the of of discipline astro- beginning modern the explain represented to observations atoms spectroscopy nomical of atomic theory laboratory quantum of and applications These tions. u otehg eprtr nteslrcrn,and ionized corona, of solar lines (O forbidden the oxygen as in identified temperature was “nebulium” high the to due ob u oanweeet“euim.I a not (Fe was iron It ionized iden- from was transitions “nebulium”. “coronium” electronic as that element tified century new 20th early a the suggested any to until was match due it not be elements, did planetary to chemical line known the this from of Since lines spectrum 6543. the NGC bright in nebula a named line found was emission Huggins and line William element green this 1864, new and In a corona eclipse, be “coronium”. the to solar of thought total spectrum also the was the during 1869, in Sun In the found of was 1895. line in green counterpart dis- The a terrestrial the its by “helium”. confirmed of it was named covery element and new this element of new existence a to suggested to he due which be spectrum, solar the in line orange ob- celestial ether. of of concept composed Aristotelian being jects the of demise the h 2 mutailtfauewsdsoee in discovered was feature ultraviolet nm 220 The chal- similar facing are we century, 21st early the In n16,Nra oke icvrdabih yellow- bright a discovered Lockyer Norman 1868, In ++ rsn ne o-est eua condi- nebular low-density under arising ) 13+ ) 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 reflection 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´ıasdóttir 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 planetary 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- (Léger 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). ical environment as in the case of “coronium” and Proposed carriers for the 21 µm feature include hy- “nebulium”? Further development in laboratory spec- drogenated fullerenes (Webster 1995), titanium carbide troscopy will hold the key to the identification of the (von Helden et al. 2000), silicon carbide (Speck and Hofmeistercarriers. 2004), and thiourea groups attached to aromatic/aliphatic Snow (2014) has suggested that DIBs alone represent structures (Papoular 2011). Since ERE is the result of the largest reservoir of organic material in the Galaxy, photoluminescience, the carrier is likely a semiconduc- so no matter what the exact nature of the carriers is, tor with a nonzero band gap. Other proposed carbon- they must be an important constituent of the Cosmos. based carriers include QCC (Sakata etal 1992), C60 The resolution of these spectral mysteries may bring (Webster 1993), and nanodiamonds (Chang, Chen, & Kwokabout a new view of our understanding of the chemical 2006). content of the Universe.

Acknowledgements 4 Future outlook This work is supported by a grant from the Natural and Engineering Research Council of Canada. In this topical collection, the recent developments in our understanding of the ERE and the 21/30 µm features are reviewed by Witt and Lai (2020), and Volk, Sloan, and Kraemer (2020) respectively. The possible ERE-DIB connection is also discussed by Witt and Lai (2020). The scenario of coal and petroleum related compounds as carriers of UIE is discussed by Cataldo, Garc´ıa-Hern´andez and Manchado (2020). The possibilities of hydrogenated fullerenes as carriers of DIB, ERE, and 220 nm features are reviewed by Zhang, Sadjadi, and Hsia (2020). Is it possible that these unexplained spectral phenomena be related to each other? The identiﬁcation + of two DIBs with C60 suggests the possibility that the DIB carrier molecules may be breakdown products of large, complex organic compounds, such as MAONs or other amorphous hydrocarbons. The presence of the 8 and 12 µm UIE plateau features in fullerene sources also suggests that fullerenes and UIE carriers may share common precursors. While we do not know when and where the carriers of DIBs and 220 nm features are synthesized, the observations of UIE bands, ERE, and 21/30 µm features in the circumstellar environment dictate that the chemical synthesis time scale is constrained by the evolutionary and dynamical time scale of the circumstellar envelopes. For example, UIE bands are observed to emerge over 4

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