Unusual Infrared Emission toward Sgr B2: Possible Planar C$_{24}$
  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 $\sim$
2% of the total interstellar carbon could have been locked up in graphene
without violating the observational constraints. While the possible detection
of planar C$_{24}$, 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 C$_{24}$ at $\sim$ 6.6, 9.8, 20 $\mu m$ toward
Sgr B2, a high-mass star formation region, and find a candidate target toward
R.A. (J2000) = $267^{\circ}.05855$ and Decl. (J2000) = $-28^{\circ}.01479$ in
Sgr B2 whose Spitzer/IRS spectra exhibit three bands peaking at $\sim$ 6.637,
9.853 and 20.050 $\mu m$ which appear to be coincident with that of C$_{24}$.
Possible features of C$_{60}$ 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 $\sim$ 5$^{\prime}$, suggesting that the creation
and/or excitation of C$_{24}$ could be related to star formation activities.
