On the dust properties of high-redshift molecular clouds and the connection to the 2175 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, AV, we also observe a correlation with the total-to-selective reddening, RV. All three quantities are also found to be connected to the presence and strength of the 2175 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 CI 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 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 highz absorbers.