Unusual infrared emission toward Sgr B2: possible planar C24

Interstellar graphene could be present in the interstellar medium (ISM), resulting from the photochemical processing of polycyclic aromatic hydrocarbon (PAH) molecules and/or collisional fragmentation of graphitic particles. Indeed, by comparing the observed ultraviolet (UV) extinction and infrared (IR) emission of the diffuse ISM with that predicted for graphene, as much as ∼2% of total interstellar carbon could be locked up in graphene without violating the observational constraints. While the possible detection of planar C₂₄, a small piece of a graphene sheet, has been reported towards several Galactic and extragalactic planetary nebulae, graphene has not yet been detected in interstellar environments. In this work, we search for the characteristic IR features of C₂₄ at ∼6.6, 9.8 and 20 μm toward Sgr B2, a high-mass star formation region, and find a candidate target toward R.A. (J2000) = 267.05855° and Decl. (J2000) = -28.01479° in Sgr B2 whose Spitzer/IRS spectra exhibit three bands peaking at ∼6.637, 9.853 and 20.050 μm which appear to be coincident with those of C₂₄. Possible features of C₆₀ are also seen in this region. The candidate region is a warm dust environment heated by massive stars or star clusters, associated with a WISE spot (a tracer of star formation activity), close to the HII region candidate IRAS 17450-2759, and is surrounded by seven young stellar object candidates within ∼5‧, suggesting that the creation and/or excitation of C₂₄ could be related to star formation activities.