2014A&A...565A..18A

We present a near-infrared extinction map of a large region (∼870deg²) covering the isolated Corona Australis complex of molecular clouds. We reach a 1-σ error of 0.02mag in the K-band extinction with a resolution of 3-arcmin over the entire map. We find that the Corona Australis cloud is about three times as large as revealed by previous CO and dust emission surveys. The cloud consists of a 45pc long complex of filamentary structure from the well known star forming Western-end (the head, N ≥10²³cm^–2) to the diffuse Eastern-end (the tail, N ≤10²¹cm^–2). Remarkably, about two thirds of the complex both in size and mass lie beneath A_V∼1mag. We find that the probability density function (PDF) of the cloud cannot be described by a single log-normal function. Similar to prior studies, we found a significant excess at high column densities, but a log-normal + power-law tail fit does not work well at low column densities. We show that at low column densities near the peak of the observed PDF, both the amplitude and shape of the PDF are dominated by noise in the extinction measurements making it impractical to derive the intrinsic cloud PDF below A_K<0.15mag. Above A_K∼0.15mag, essentially the molecular component of the cloud, the PDF appears to be best described by a power-law with index -3, but could also described as the tail of a broad and relatively low amplitude, log-normal PDF that peaks at very low column densities.