Survey of profile parameters of the 6196 Å diffuse interstellar band

From uniform profiles to Doppler splitting and blueshifts

¹ University of Vienna, Department of Astrophysics, Türkenschanzstrasse 17, 1180 Vienna, Austria
² University of Vienna, Research Network Data Science at Uni Vienna, Kolingasse 14–16, 1090 Vienna, Austria

^★ Corresponding author; martin.piecka@univie.ac.at

Received: 16 December 2024
Accepted: 19 February 2025

Abstract

The diffuse interstellar band (DIB) at 6196 Å exhibits notable profile variations across the Milky Way. This study addresses three open issues: the unusual broadening of the DIB profile towards Upper Sco, the lack of profile variations towards stars near η Car, and the origin of the blueshift observed in Sco OB1. Using archival spectra of 453 early-type stars across the Galactic disk and in its proximity, we created a catalogue of the DIB's profile parameters. Our analysis identified Doppler-split components within the DIB profiles across most regions with no evidence for these splits being able to account for the observed broadening (∼23 km s⁻¹) in Upper Sco or other regions such as Orion, Vela OB2, and Melotte 20 (α Per cluster). We propose that neither the ages of the studied stellar populations nor the distances between clusters and nearby clouds significantly contribute to the broadening. However, we detect a gradient in the full width at half maximum within the Sco-Cen and Orion regions, where broadening decreases with distance from the star-forming centres. This result points to a possible connection between the DIB broadening and star formation (likely via the impact of recent supernovae). Regarding the Carina Nebula, we confirm the lack of DIB profile variations in a small region near η Car, although an adjacent southern area exhibits significant variations, comparable to those in Upper Sco. In addition to the Carina Nebula, we find that the Rosette Nebula and NGC 6405 also show consistently narrow profiles (< 20 km s⁻¹) with minimal deviations from the median over spatial scales of a few parsecs. Finally, regarding the origin of the blueshift observed in Sco OB1, we used a comparison with the Lagoon Nebula and argue that the most natural explanation is the presence of an unresolved kinematic component in the profile of the DIB, shifting the measured centre of the band.