The Interstellar Extinction Toward the Milky Way Bulge with Planetary Nebulae, Red Clump, and RR Lyrae stars

TL;DR

This review summarizes recent progress in mapping interstellar extinction toward the Milky Way bulge using various stellar tracers, highlighting advances, remaining challenges, and the implications for understanding the extinction curve.

Contribution

It provides a comprehensive overview of extinction studies focusing on planetary nebulae, RR Lyrae, and red clump stars, emphasizing recent improvements and ongoing issues.

Findings

Extinction maps have increased in spatial coverage by ~100 times in 20 years.

Total-to-selective extinction ratios have shifted by 20-25%, indicating a steeper extinction curve.

Progress has been made in modeling differential extinction and understanding extinction coefficients.

Abstract

I review the literature covering the issue of interstellar extinction toward the Milky Way bulge, with emphasis placed on findings from planetary nebulae, RR Lyrae, and red clump stars. I also report on observations from HI gas and globular clusters. I show that there has been substantial progress in this field in recent decades, most particularly from red clump stars. The spatial coverage of extinction maps has increased by a factor $\sim 100 \times$ in the past twenty years, and the total-to-selective extinction ratios reported have shifted by $\sim$20-25\%, indicative of the improved accuracy and separately, of a steeper-than-standard extinction curve. Problems remain in modelling differential extinction, explaining anomalies involving the planetary nebulae, and understanding the difference between bulge extinction coefficients and "standard" literature values.