UV Extinction Towards a Quiescent Molecular Cloud in the SMC

TL;DR

This study measures the UV extinction law in a quiescent molecular cloud in the SMC, revealing significant variation among nearby stars and challenging the assumption of a universal extinction law in low-metallicity galaxies.

Contribution

It provides new UV extinction measurements for multiple stars in SMC B1-1, showing variability and refining understanding of dust properties in low-metallicity environments.

Findings

UV extinction law varies significantly among nearby stars.

The 2175 Å bump shows different strengths or absence.

Results challenge the universality of the SMC extinction law.

Abstract

Context: The mean UV extinction law for the Small Magellanic Cloud (SMC) is usually taken as a template for low-metallicity galaxies. However, its current derivation is based on only five stars, thus placing doubts on its universality. An increase in the number of targets with measured extinction laws in the SMC is necessary to determine its possible dependence on parameters such as metallicity and star-forming activity. Aims: To measure the UV extinction law for several stars in the quiescent molecular cloud SMC B1-1. Methods: We obtained HST/STIS slitless UV spectroscopy of a 25"x25" field of view and we combined it with ground-based NIR and visible photometry of the stars in the field. The results were processed using the Bayesian photometric package CHORIZOS to derive the visible-NIR extinction values for each star. The unextinguished Spectral Energy Distributions (SEDs) obtained in this way were then used to derive the UV extinction law for the four most extinguished stars. We also recalculated the visible-NIR extinction for the five SMC stars with preexisting UV extinction laws. Results: The UV extinction law for four SMC B1-1 stars within several pc of each other differs significantly from star to star. The 2175 {\AA} bump is moderately strong in one, weak in two, and absent in the fourth.