# Dust evolution in the transition towards the denser ISM: impact on dust temperature, opacity, and spectral index

**Authors:** Melanie Köhler, Nathalie Ysard, Anthony P. Jones  
**arXiv:** [1506.01533](https://arxiv.org/abs/1506.01533)  
**Date:** 2015-06-04  
**Summary page:** https://www.summarizepaper.com/en/paper/1506.01533v1/

## Key points

- Study focuses on the evolution of dust properties in the transition from diffuse interstellar medium (ISM) to denser molecular clouds
- Changes in dust properties (SED, opacity, temperature) explained by evolutionary processes like coagulation and accretion
- Core-mantle grain model used to describe diffuse ISM-type grains
- Discrete dipole approximation (DDA) used to calculate how accretion and coagulation affect optical properties of grains
- DustEM and CRT used to calculate dust SED and extinction
- Accretion of aliphatic carbon mantle onto diffuse ISM-type dust increases FIR opacity by a factor of 2, decreases temperature by approximately 2 K, and increases FIR/submm spectral index from 1.5 to 1.8
- Coagulation of grains into aggregates further decreases temperature by 3 K and increases spectral index up to around 2
- Aggregates with additional ice-mantle exhibit significant increase in FIR opacity by factors of 3 and 7 compared to diffuse ISM-dust
- Dust evolution resulting from coagulation and accretion explains observed variations between diffuse ISM and denser regions in terms of grain optical properties
- Study provides insights into how evolutionary processes impact dust properties across different regions of interstellar space

## Summary

This study focuses on the evolution of dust properties in the transition from the diffuse interstellar medium (ISM) to denser molecular clouds. The authors investigate how changes in dust properties, such as variations in the spectral energy distribution (SED), opacity, and temperature, can be explained by evolutionary processes like coagulation and accretion. To understand this evolution, the authors use a core-mantle grain model to describe diffuse ISM-type grains. They employ the discrete dipole approximation (DDA) to calculate how the accretion of mantles and coagulation into aggregates affect the optical properties of these grains. Additionally, they utilize DustEM and the radiative transfer code CRT to calculate the dust SED and extinction. The results show that when an aliphatic carbon mantle is accreted onto diffuse ISM-type dust, there is an increase in FIR opacity by a factor of about 2. Furthermore, there is an increase in the FIR/submm spectral index from 1.5 to 1.8 and a decrease in temperature by approximately 2 K. The coagulation of these grains into aggregates further decreases the temperature by 3 K and increases the spectral index up to a value of around 2. Moreover, compared to diffuse ISM-dust, these aggregates (with an additional ice-mantle) exhibit a significant increase in FIR opacity by factors of 3 and 7. These findings suggest that dust evolution resulting from coagulation and accretion leads to substantial changes in grain optical properties that can explain observed variations between diffuse ISM and denser regions. In summary, this study provides insights into how evolutionary processes impact dust properties across different regions of interstellar space. By considering coagulation and accretion mechanisms it demonstrates that changes in grain composition can account for observed variations in dust emission and extinction characteristics between diffuse ISM and denser molecular clouds.