Exploring the Wolf Approach to Constraining NIR Extinction Laws in the Corona Australis Molecular Cloud

TL;DR

This study evaluates the star count (Wolf) method for constraining near-infrared extinction laws in the Corona Australis cloud, finding consistent extinction ratios and a power-law index around 2.0, with some spatial variation hints.

Contribution

It demonstrates the viability of the Wolf method with deep photometry to measure NIR extinction laws and explores potential spatial variations within the cloud.

Findings

Extinction ratios align with Galactic averages.

Power-law index of extinction law is approximately 2.0.

Possible spatial variations in extinction properties.

Abstract

The viability of the star count (Wolf) method is assessed as a means of constraining the near-infrared (NIR) extinction law toward the Corona Australis molecular cloud. Using deep $JHK_S$ photometry from the VISIONS survey, extinction maps with 1 arcmin spatial resolution are constructed. The derived extinction ratios are $A_J/A_H=1.73\pm0.07$, $A_H/A_{K_S}=1.70\pm0.11$, and $A_J/A_{K_S}=3.02\pm0.22$, which are consistent with Galactic literature means. Assuming a power-law form ($A_\lambda \propto \lambda^{-\alpha}$) for the NIR extinction law, we derive indices of $\alpha\approx 2.0$ across all wavelength combinations, with no statistically significant wavelength dependence throughout the NIR wavelength range. While spatial variations in extinction properties are tentatively observed across the cloud, concerns persist regarding the impact of photometric completeness, and the role of reference field selection. Continued research is required to refine the approach, and scrutinize the veracity of potential extinction law variations over a more expansive region of sky.