2MASS wide field extinction maps: V. Corona Australis

TL;DR

This paper presents a detailed near-infrared extinction map of the Corona Australis molecular cloud complex, revealing its large size, filamentary structure, and complex column density distribution, with implications for understanding cloud structure.

Contribution

It provides the first high-resolution, large-area extinction map of Corona Australis, analyzing its column density PDF and revealing limitations of previous models at low densities.

Findings

Corona Australis is three times larger than previous surveys indicated.

The cloud's column density PDF cannot be well described by a single log-normal function.

At high column densities, the PDF follows a power-law with index -3.

Abstract

We present a near-infrared extinction map of a large region ($\sim$870 deg$^2$) covering the isolated Corona Australis complex of molecular clouds. We reach a 1-$\sigma$ error of 0.02 mag 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 45 pc long complex of filamentary structure from the well known star forming Western-end (the head, $N \geq10^{23}$ cm$^{-2}$) to the diffuse Eastern-end the tail, ($N \leq10^{21}$ cm$^{-2}$). Remarkably, about two thirds of the complex both in size and mass lie beneath A$_V\sim1$ mag. We find that the 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$_K <$ 0.15 mag. Above A$_K \sim 0.15$ mag, 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.