Dusty globules in the Crab Nebula

Abstract

Context. Dust grains are widespread in the Crab Nebula. A number of small dust concentrations are visible as dark spots against the background of continuous synchrotron emission in optical images. Aims. Our aim is to catalogue such roundish, 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 −6 M , 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.Conclusions.

Modelling the ArH+ emission from the Crab nebula

“…Using an increased AV /NH ratio more appropriate for the Crab Nebula, and a total extinction within the range found by Grenman et al (2017) for Crab Nebula dusty globules, we find that models with nH = 1900 cm −3 reproduce the observed ArH + and OH + surface brightnesses measured by Barlow et al (2013), while underpredicting the H2 surface brightness compared to Loh et al (2010Loh et al ( , 2011Loh et al ( , 2012 Gomez et al (2012), although surface brightness ratios are comparable with observed values. We suggest that these dusty globules are the source of most of the observed molecular emission in the Crab Nebula, with denser gas accounting for the ∼ 10% of globules associated with H2 vibrational emission.…”

“…The globule sizes range from 6×10 15 cm to 3 × 10 16 cm, similar to the sizes of the H2 emitting knots observed by Loh et al (2011), although Grenman et al (2017) note that only ∼ 10% of their dusty globules are coincident with H2 knots. While the observed globule sizes are similar to our nH = 1900 cm −3 model cloud thicknesses, none of our preferred models match the extinctions found by Grenman et al (2017). These models used a standard interstellar AV /NH ratio of 6.289 × 10 −22 cm 2 mag, whereas with the increased dust-to-gas ratio in the Crab Nebula the true value is likely to be higher.…”

“…The dusty globules detected by Grenman et al (2017) have radii of ≤ 0.5 arcsec, far smaller than the beam sizes in Gomez et al (2012) (9.4 ′′ × 9.4 ′′ for each PACS spaxel). The ratio of globule angular area to PACS spaxel area is ∼ 3 × 10 −3 , which is low enough to explain the discrepancy between our models and the observations if each spaxel contains only a few globules, although unlike the moleuclar emission, there is no reason to assume that the far-infrared lines only originate in dense, shielded clumps.…”

“…Loh et al (2012) estimated the thickness of the H2-emitting regions they observed to be of order 10 14 cm, which is comparable to our nH = 2 × 10 4 cm −3 models. Grenman et al (2017) used Hubble Space Telescope optical images of the Crab Nebula to determine the extinctions of several dusty globules observed as dark spots against the synchrotron background, obtaining values of AV = 0.20−0.34. The globule sizes range from 6×10 15 cm to 3 × 10 16 cm, similar to the sizes of the H2 emitting knots observed by Loh et al (2011), although Grenman et al (2017) note that only ∼ 10% of their dusty globules are coincident with H2 knots.…”

“…Grenman et al (2017) used Hubble Space Telescope optical images of the Crab Nebula to determine the extinctions of several dusty globules observed as dark spots against the synchrotron background, obtaining values of AV = 0.20−0.34. The globule sizes range from 6×10 15 cm to 3 × 10 16 cm, similar to the sizes of the H2 emitting knots observed by Loh et al (2011), although Grenman et al (2017) note that only ∼ 10% of their dusty globules are coincident with H2 knots. While the observed globule sizes are similar to our nH = 1900 cm −3 model cloud thicknesses, none of our preferred models match the extinctions found by Grenman et al (2017).…”