Radial velocities from far red spectra of Carina Arm O and early B stars

TL;DR

This study demonstrates that far-red spectra can reliably measure radial velocities of OB stars, enabling detailed kinematic analysis of distant, dust-obscured regions in the Milky Way's Carina Arm.

Contribution

It introduces a method using far-red spectra (8400-8800 Å) for radial velocity measurements of OB stars, expanding the potential for Galactic kinematic studies beyond previous wavelength limitations.

Findings

Radial velocities have errors of 3-10 km/s with systematic uncertainties of 2-3 km/s.

Up to 22 stars are identified as likely runaways, with some possibly being undetected binaries.

The mean azimuthal motion aligns with Galactic rotation, and there is evidence of modest infall at ~10 km/s.

Abstract

Massive O and early B stars are important markers of recent star formation and exert a significant influence on their environments during their short lives via photoionization and winds and when they explode as supernovae. In the Milky Way they can be detected at great distances but often lie behind large dust columns, making detection at short wavelengths difficult. In this study the use of the less extinguished far-red spectrum (8400 -- 8800 \AA ) for radial velocity measurement is examined. Results are reported for a sample of 164 confirmed OB stars within a 2-degree field positioned on the Carina Arm. Most stars are at distances between 3 and 6 kpc, and Westerlund 2 is at the field edge. The measured radial velocities have errors concentrated in the 3--10 km s$^{-1}$ range, with a systematic uncertainty of 2--3 km s$^{-1}$. These are combined with Gaia-mission astrometry to allow full space motions to be constructed. Up to 22 stars are likely to be runaways although 8 of them are as likely to be interloping (so far undetected) binaries. The mean azimuthal motion of the sample fits in with recent measurements of Galactic disk rotation. In the Galactocentric radial direction the mean motion indicates modest infall at a speed of $\sim$ 10 km s$^{-1}$. This experiment shows that weak Paschen lines in the far-red can yield credible radial velocity determination, offering the prospect of exploring OB-star kinematics over much more of the Galactic disk than has hitherto been possible.