Near-infrared diffuse interstellar bands towards Her 36

Abstract

Discovered almost a century ago, the Diffuse Interstellar Bands (DIBs) still lack convincing and comprehensive identification. Hundreds of DIBs have now been observed in the near-ultraviolet (NUV), visible and near-infrared (NIR). They are widely held to be molecular in origin, and modelling of their band profiles offers powerful constraints on molecular constants. Herschel 36, the illuminating star of the Lagoon Nebula, has been shown to possess unusually broad and asymmetric DIB profiles in the visible, and is also bright enough for NIR observation. We present here high-resolution spectroscopic observations targeting the two best-known NIR DIBs at 11797.5 and 13175 A toward this object and a nearby comparison O-star, 9 Sgr, using the GNIRS instrument on Gemini North. We show a clear detection of the 13175Å DIB in both stars, and find (i) that it does not exhibit the unusual wing structure of some of the visual DIBs in Her 36 and (ii) that the depth of the band in the two objects is very similar, also contrary to the behaviour of the visual DIBs. We discuss the implications of these results for multiple DIB carrier candidates, and the location of their carriers along the observed lines of sight.

Absorption Spectroscopy of Solid-Phase Fullerene C60 between 1.65 and 2.78 μm

“…Recently, the presence of C 60 + was firmly established in diffuse clouds through an observed match between its electronic spectrum and a few DIBs. , The approach was based on the photo-fragmentation of C 60 + –He clusters confined in a cryogenic radiofrequency multipole ion trap. This work confirmed the hypothesis proposed by Foing and Ehrenfreund based on low-temperature Ne matrix isolation spectra obtained by Fulara et al DIBs are broad absorption features (with bandwidths varying between 2 and 80 cm –1 ) predominantly covering from 0.4 to 0.9 μm range in the spectra of reddened stars. − The original DIB spectral window has been progressively extended to the NIR range through several observational studies. − Primarily driven by the PAH-DIB hypothesis, , the matrix isolated NIR spectra of a few ionic PAHs were recorded and several electronic transitions between 0.7 and 2.5 μm spectral range were measured . Spectral shifts in the position of the rovibronic bands, induced by the interaction of the isolated PAHs with the matrix, however prevented decisive identification of the NIR DIBs observed by Cox et al Barring electronic transitions, the NIR spectral region is also associated with higher-order vibrational transitions, which have been less scrutinized in the context of laboratory astrophysics.…”

“…[35][36][37][38] The original DIB spectral window has been progressively extended to the NIR range through several observational studies. [39][40][41][42][43][44][45][46][47] Primarily driven by the PAH-DIB hypothesis, 48,49 the matrix isolated NIR spectra of a few ionic PAHs were recorded and several electronic transitions between 0.7 and 2.5 µm spectral range were measured. 50 Spectral shifts in the position of the rovibronic bands, induced by the interaction of the isolated PAHs with the matrix, however prevented decisive identification of the NIR DIBs observed by Cox et al 42 .…”

Low-temperature gas-phase formation of indene in the interstellar medium

“…Suprathermal reactions occur quickly (95,96), with rate coefficients incorporated based on the attempt frequency (94). Whenever experimental production rates in the ices were not available, we computed the rate coefficients using expression (3) where  is the branching fraction and  is the G value of formation, which gives the number of molecules produced per 100 eV deposited in the system.…”

“…Since Léger’s and Puget’s very first hypothesis of polycyclic aromatic hydrocarbons (PAHs)—organic molecules composed of fused benzene rings—as the missing link between small carbon molecules and carbonaceous nanoparticles (interstellar grains) ( 1 ), PAHs ( 2 ) along with their (de)hydrogenated ( 3 ), alkylated ( 4 , 5 ), protonated ( 6 ), and ionized ( 7 ) counterparts have been associated with the diffuse interstellar bands (DIBs) ( 8 )—discrete absorption features superimposed on the interstellar extinction curve ranging from the visible (400 nm) to the near-infrared (1.2 μm)—and the unidentified infrared (UIR) emission bands ( 9 , 10 ) in the 3- to14-μm range. Encompassing up to 20% of the carbon budget in our galaxy ( 11 , 12 ), the identification of PAHs in carbonaceous chondrites such as Murchison, Allende, and Orgueil ( 13 , 14 ) along with 13 C/ 12 C and D/H isotopic analyses ( 15 ) advocates a circumstellar origin of aromatics in carbon-rich asymptotic giant branch (AGB) stars and planetary nebulae as the descendants of AGB stars at elevated temperatures of a few 1000 K through molecular mass growth processes involving hydrogen abstraction–carbon addition (HACA) sequences ( 16 ).…”

The Role of Methylaryl Radicals in the Growth of Polycyclic Aromatic Hydrocarbons: The Formation of Five-Membered Rings

“…Polycyclic aromatic hydrocarbons (PAHs), organic molecules composed of fused benzene rings, together with their (de)­hydrogenated, alkylated, , protonated, and ionized , derivatives, are believed to be omnipresent in the interstellar space and in circumstellar envelopes, with up to 20% of the carbon budget in our galaxy , attributed to the molecules of this class. It has been also hypothesized that they represent the missing link between small carbon molecules and carbonaceous nanoparticles (interstellar grains) .…”