Rotating Disks and Non-Kinematic Double Peaks

TL;DR

Double-peaked line profiles, often linked to rotating disks, can also result from radiative transfer effects in non-rotating sources, making it challenging to identify true rotation solely from line profiles.

Contribution

The paper provides exact solutions for line transfer in thin Keplerian disks and clarifies when double peaks indicate rotation versus opacity effects.

Findings

Double peaks can arise from opacity effects, not just rotation.

Disentangling kinematic and opacity effects is generally difficult.

Triple and quadruple profiles may better indicate rotation in certain cases.

Abstract

Double-peaked line profiles are commonly considered a hallmark of rotating disks, with the distance between the peaks a measure of the rotation velocity. However, double-peaks can arise also from radiative transfer effects in optically thick non-rotating sources. Utilizing exact solutions of the line transfer problem we present a detailed study of line emission from geometrically thin Keplerian disks. We derive the conditions for emergence of kinematic double peaks in optically thin and thick disks, and find that it is generally impossible to disentangle the effects of kinematics and line opacity in observed double-peaked profiles. Unless supplemented by additional information, a double-peaked profile alone is not a reliable indicator of a rotating disk. In certain circumstances, triple and quadruple profiles might be better indicators of rotation in optically thick disks.