On the dust properties of high-redshift molecular clouds and the connection to the 2175 Å extinction bump

We present a study of the extinction and depletion-derived dust properties of gamma-ray burst (GRB) absorbers at 1 < z < 3 showing the presence of neutral atomic-carbon (C I). By modelling their parametric extinction laws, we discover a broad range of dust models characterizing the GRB C I absorption systems. In addition to the already well-established correlation between the amount of C I and visual extinction, A_V, we also observe a correlation with the total-to-selective reddening, R_V. All three quantities are also found to be connected to the presence and strength of the 2175 Å dust extinction feature. While the amount of C I is found to be correlated with the SED-derived dust properties, we do not find any evidence for a connection with the depletion-derived dust content as measured from [Zn/Fe] and N(Fe)_dust. To reconcile this, we discuss a scenario where the observed extinction is dominated by the composition of dust particles confined in the molecular gas-phase of the interstellar medium (ISM). We argue that since the depletion level trace non-carbonaceous dust in the ISM, the observed extinction in GRB C I absorbers is primarily produced by carbon-rich dust in the molecular cloud and is therefore only observable in the extinction curves and not in the depletion patterns. This also indicates that the 2175 Å dust extinction feature is produced by dust and molecules in the cold and molecular gas-phase. This scenario provides a possible resolution to the discrepancy between the depletion- and SED-derived amounts of dust in high- z absorbers.