Dusty globules in the Crab Nebula^⋆

¹ Division of Physics, Luleå University of Technology, 97187 Luleå, Sweden
e-mail: tiia.grenman@gmail.com
² Stockholm Observatory, AlbaNova University Centre, Stockholm University, 106 91 Stockholm, Sweden

Received: 12 September 2016
Accepted: 19 October 2016

Abstract

Context. Dust grains are widespread in the Crab Nebula. A number of small, dusty globules, are visible as dark spots against the background of continuous synchrotron emission in optical images.

Aims. Our aim is to catalogue such dusty globules and investigate their properties.

Methods. From existing broad-band images obtained with the Hubble Space Telescope, we located 92 globules, for which we derived positions, dimensions, orientations, extinctions, masses, proper motions, and their distributions.

Results. The globules have mean radii ranging from 400 to 2000 AU and are not resolved in current infrared images of the nebula. The extinction law for dust grains in these globules matches a normal interstellar extinction law. Derived masses of dust range from 1 to 60 × 10^-6M_⊙, and the total mass contained in globules constitute a fraction of approximately 2% or less of the total dust content of the nebula. The globules are spread over the outer part of the nebula, and a fraction of them coincide in position with emission filaments, where we find elongated globules that are aligned with these filaments. Only 10% of the globules are coincident in position with the numerous H₂-emitting knots found in previous studies. All globules move outwards from the centre with transversal velocities of 60 to 1600 km s^-1, along with the general expansion of the remnant. We discuss various hypotheses for the formation of globules in the Crab Nebula.