The Flattest Infrared Extinction Curve in Four Isolated Dense Molecular Cloud Cores

TL;DR

This study measures the infrared extinction law in four dense molecular cloud cores, revealing an exceptionally flat curve indicative of large dust grains and suggesting significant grain growth in dense environments.

Contribution

It provides the most detailed infrared extinction curves for dense cloud cores to date, demonstrating a high degree of flattening and aligning with the diffuse interstellar dust model.

Findings

Flat infrared extinction curves in all four cores

Evidence for large dust grain presence

Consistency with the Astrodust model

Abstract

The extinction curve of interstellar dust in the dense molecular cloud cores is crucial for understanding dust properties, particularly size distribution and composition. We investigate the infrared extinction law in four nearby isolated molecular cloud cores, L429, L483, L673, and L1165, across the 1.2 - 8.0 $\mu$m wavelength range, using deep near-infrared (NIR) and mid-infrared (MIR) photometric data from UKIDSS and Spitzer Space Telescope. These observations probe an unprecedented extinction depth, reaching $A_V\sim$ 40-60 mag in these dense cloud cores. We derive color-excess ratios $E(K-\lambda)/E(H-K)$ by fitting color-color diagrams of $(K-\lambda)$ versus $(H-K)$, which are subsequently used to calculate the extinction law $A_\lambda/A_K$. Our analysis reveals remarkably similar and exceptionally flat infrared extinction curves for all four cloud cores, exhibiting the most pronounced flattening reported in the literature to date. This flatness is consistent with the presence of large dust grains, suggesting significant grain growth in dense environments. Intriguingly, our findings align closely with the Astrodust model for a diffuse interstellar environment proposed by Hensley \& Draine. This agreement between dense core observations and a diffuse medium model highlights the complexity of dust evolution and the need for further investigation into the processes governing dust properties in different interstellar environments.