He I λ10830 Resonant Scattering in M17: Toward a Diagnostic of Optical Depth and Gas Kinematics
Abstract
The lower level of the He I λ10830 transition is metastable and therefore well populated. In H II regions, the line can become optically thick, and its photons then undergo resonant scattering. Draine (2026) predicted that scattering splits the profile into several peaks. We present Magellan/WINERED spectra (R ≈ 70,000) at six pointings along a line from the ionizing source CEN 1 across the southwestern bar of M17, with He I λ10830 and Paγ in the same exposures. Every He I λ10830 profile is multi-peaked, while the Paγ profiles are single-peaked. Because Paγ does not undergo resonant scattering, it traces gas kinematics; its single peak argues against a purely kinematic origin of the He I peaks, and the shared exposures exclude an instrumental origin. We therefore attribute the He I peaks to resonant scattering, as predicted. We compare the profiles quantitatively with the static, spherically symmetric models of Draine (2026), using width and asymmetry measures independent of the systemic velocity. At two pointings far from CEN 1, no static model of any optical depth reproduces width and asymmetry together: both profiles are more red-weighted than static models of comparable width. Such profiles cannot arise from uniform motion but are expected from scattering in expanding gas. We argue that the profile is shaped by both the kinematics and the optical depth of the He+ zone, and resolved He I λ10830 profiles could therefore provide a new diagnostic of these properties in H II regions.
Related
- SN1987A constraints on the neutrino-dark-fermion interaction from resonant scattering with CνB
- 1D PIC Simulations of Resonant Scattering-Driven Pair Cascades in Magnetar Magnetospheres
- XRISM observations of the Perseus cluster along two arms: Chaotic ICM motions probed by resonant scattering
- CME Kinematics with Optical Flow: Multicoronagraph Insights into Internal Velocity Dispersion
- CME Kinematics with Optical Flow: Multi-Coronagraph Insights into Internal Velocity Dispersion
BibTeX
@misc{sameshima2026λ10830,
title = {{He I $λ$10830 Resonant Scattering in M17: Toward a Diagnostic of Optical Depth and Gas Kinematics}},
author = {Hiroaki Sameshima and Daisuke Taniguchi and Noriyuki Matsunaga and Shogo Otsubo and Tomomi Takeuchi},
howpublished = {arXiv},
year = {2026},
month = nov,
doi = {10.48550/arxiv.2611.03821},
eprint = {2611.03821},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2611.03821},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - He I λ10830 Resonant Scattering in M17: Toward a Diagnostic of Optical Depth and Gas Kinematics
AU - Hiroaki Sameshima
AU - Daisuke Taniguchi
AU - Noriyuki Matsunaga
AU - Shogo Otsubo
AU - Tomomi Takeuchi
PY - 2026
DA - 2026/11/02
PB - arXiv
DO - 10.48550/arxiv.2611.03821
UR - https://arxiv.org/abs/2611.03821
AB - The lower level of the He I λ10830 transition is metastable and therefore well populated. In H II regions, the line can become optically thick, and its photons then undergo resonant scattering. Draine (2026) predicted that scattering splits the profile into several peaks. We present Magellan/WINERED spectra (R ≈ 70,000) at six pointings along a line from the ionizing source CEN 1 across the southwestern bar of M17, with He I λ10830 and Paγ in the same exposures. Every He I λ10830 profile is multi-peaked, while the Paγ profiles are single-peaked. Because Paγ does not undergo resonant scattering, it traces gas kinematics; its single peak argues against a purely kinematic origin of the He I peaks, and the shared exposures exclude an instrumental origin. We therefore attribute the He I peaks to resonant scattering, as predicted. We compare the profiles quantitatively with the static, spherically symmetric models of Draine (2026), using width and asymmetry measures independent of the systemic velocity. At two pointings far from CEN 1, no static model of any optical depth reproduces width and asymmetry together: both profiles are more red-weighted than static models of comparable width. Such profiles cannot arise from uniform motion but are expected from scattering in expanding gas. We argue that the profile is shaped by both the kinematics and the optical depth of the He^+ zone, and resolved He I λ10830 profiles could therefore provide a new diagnostic of these properties in H II regions.
ER -