High-resolution Near-infrared Spectroscopic Study of Galactic Supernova Remnants. I. Kinematic Distances

TL;DR

This study uses high-resolution near-infrared spectroscopy to measure the velocities and distances of 16 Galactic supernova remnants through shock-excited H₂ emission, providing new kinematic distance estimates and insights into their physical conditions.

Contribution

It presents the first kinematic distance measurements for four SNRs using H₂ lines and compares these with previous radio data, revealing velocity discrepancies and discussing their origins.

Findings

H₂ emission lines are mostly from shock-excited gas in SNRs.

Kinematic distances for 15 SNRs are derived, including four new estimates.

Discrepancies between H₂ and radio velocities are identified and analyzed.

Abstract

We have carried out high-resolution near-infrared spectroscopic observations toward 16 Galactic supernova remnants (SNRs) showing strong H$_{2}$ emission features. A dozen bright H$_{2}$ emission lines are clearly detected for individual SNRs, and we have measured their central velocities, line widths, and fluxes. For all SNRs except one (G9.9$-$0.8), the H$_{2}$ line ratios are well consistent with that of thermal excitation at $T\sim2000$ K, indicating that the H$_{2}$ emission lines are most likely from shock-excited gas and therefore that they are physically associated with the remnants. The kinematic distances to the 15 SNRs are derived from the central velocities of the H$_{2}$ lines using a Galactic rotation model. We derive for the first time the kinematic distances to four SNRs: G13.5$+$0.2, G16.0$-$0.5, G32.1$-$0.9, and G33.2$-$0.6. Among the remaining 11 SNRs, the central velocities of the H$_{2}$ emission lines for six SNRs are well consistent ($\pm5$ km s$^{-1}$) with those obtained in previous radio observations, while for the other five SNRs (G18.1$-$0.1, G18.9$-$1.1, Kes 69, 3C 396, W49B) they are significantly different. We discuss the velocity discrepancies in these five SNRs. In G9.9$-$0.8, the H$_{2}$ emission shows nonthermal line ratios and narrow line width ($\sim 4$ km s$^{-1}$), and we discuss its origin.