[Paper Review] 2MASS wide field extinction maps: V. Corona Australis

J. Alves, M. Lombardi+1 more|arXiv (Cornell University)|Jan 13, 2014

Astrophysics and Star Formation Studies56 references20 citations

TL;DR

This study presents a wide-field near-infrared extinction map of the Corona Australis molecular complex using the 2MASS survey and the Nicer/Nicest algorithms, achieving 3 arcmin resolution and 0.02 mag sensitivity. It reveals a 45 pc filamentary cloud with two-thirds of its mass below A_K ≈ 0.15 mag, where the PDF is dominated by noise; the molecular component above this threshold best fits a power-law with index –3, challenging standard log-normal models.

ABSTRACT

We present a near-infrared extinction map of a large region ($\sim$870 deg$^2$) covering the isolated Corona Australis complex of molecular clouds. We reach a 1-$σ$ 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

Motivation & Objective

• To produce a high dynamic range, wide-field extinction map of the Corona Australis molecular complex using near-infrared stellar extinction.
• To characterize the large-scale structure and column density distribution of the cloud, including its diffuse outer regions.
• To assess the intrinsic probability density function (PDF) of column densities and determine whether it follows a log-normal or power-law distribution.
• To investigate the limitations imposed by measurement noise on deriving the true PDF at low column densities.
• To evaluate the role of diffuse, low-column-density envelopes in cloud mass and structure budgets, particularly in isolated systems.

Proposed method

• Applied the Nicer (Near-Infrared Color Excess Revisited) and Nicest (improved Nicer) algorithms to 10.7 million 2MASS stars to derive K-band extinction across ~870 deg².
• Used a multi-band stellar color excess technique to estimate extinction, correcting for foreground stellar contamination and background inhomogeneities.
• Employed a pixel-based approach with 3 arcmin resolution to map extinction, minimizing bias from sparse stellar sampling in high-column-density regions.
• Modeled the column density PDF using both log-normal and power-law functions, and tested hybrid models (log-normal + power-law) to assess fit quality.
• Quantified noise contributions in the PDF at low column densities by comparing observed PDF shape and amplitude to expected statistical uncertainty (1-σ ≈ 0.02 mag).
• Used the Nicest method to correct for sub-pixel inhomogeneities and extinction bias in dense regions, improving accuracy in high-column-density regimes.

Experimental results

Research questions

• RQ1 What is the true extent and mass distribution of the Corona Australis molecular cloud, including its diffuse outer regions?
• RQ2 How does the column density PDF of the Corona Australis cloud deviate from a single log-normal distribution, and what does this imply about its formation and evolution?
• RQ3 To what extent is the observed PDF at low column densities (A_K < 0.15 mag) shaped by measurement noise rather than intrinsic cloud structure?
• RQ4 Can the molecular component of the cloud (A_K > 0.15 mag) be better described by a power-law or a broad log-normal PDF?
• RQ5 What is the significance of the diffuse, low-column-density envelope in the cloud’s mass and structural budget, and how might it form?

Key findings

• The Corona Australis complex spans ~870 deg² and extends over 45 pc in a curved, filamentary structure, with the star-forming Western-end (head) reaching A_K > 5 mag and the Eastern-end (tail) extending down to A_K ~ 0.05 mag.
• Approximately 65% of the cloud’s mass lies below A_V ≈ 1 mag, indicating a significant reservoir of low-column-density material previously undetected in CO or dust emission surveys.
• The observed PDF of column densities is not well described by a single log-normal function; instead, it shows a significant excess at high column densities.
• Below A_K ≈ 0.15 mag, the amplitude and shape of the PDF are dominated by measurement noise (1-σ ≈ 0.02 mag), making it impractical to infer the intrinsic cloud PDF in this regime.
• Above A_K ≈ 0.15 mag, the molecular component’s PDF is best described by a power-law with index –3, though it could also be interpreted as the tail of a broad, low-amplitude log-normal PDF peaking at very low column densities.
• The diffuse envelope in the Eastern-end is likely atomic or transition-phase material, suggesting Corona Australis may be a unique laboratory for studying atomic-to-molecular transitions and feedback processes in isolated clouds.