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Dusty globules in the Crab Nebula
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.ORCID iD: 0000-0002-4932-7851
Stockholm Observatory, AlbaNova University Centre, Stockholm University.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.ORCID iD: 0000-0002-4400-3276
Number of Authors: 3
2017 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 599, article id A110Article in journal (Refereed) Published
Abstract [en]

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

Place, publisher, year, edition, pages
2017. Vol. 599, article id A110
National Category
Other Physics Topics Energy Engineering
Research subject
Energy Engineering; Applied Physics
Identifiers
URN: urn:nbn:se:ltu:diva-61736DOI: 10.1051/0004-6361/201629693ISI: 000395821900088Scopus ID: 2-s2.0-85015159073OAI: oai:DiVA.org:ltu-61736DiVA, id: diva2:1070234
Note

Validerad; 2017; Nivå 2; 2017-03-24 (andbra)

Available from: 2017-01-31 Created: 2017-01-31 Last updated: 2018-05-02Bibliographically approved
In thesis
1. Dusty Globules and Globulettes
Open this publication in new window or tab >>Dusty Globules and Globulettes
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Stoft i globuler och globuletter
Abstract [en]

Interstellar gas and dust can condense into clouds of very different size, ranging from giant molecular cloud complexes to massive, isolated, dark cloudlets, called globules with a few solar masses.

This thesis focuses on a new category of small globules, named globulettes.These have been found in the bright surroundings of H II regions of young, massive stellar clusters. The globulettes are much smaller and less massive than normal globules. The analysis is based on H-alpha images ofe.g., the Rosette Nebula and the Carina Nebula collected with the Nordic Optical Tele-scope and the Hubble Space Telescope.

Most globulettes found in different H II regions have distinct contours and are well isolated from the surrounding molecular shell structures. Masses and densities were derived from the extinction of light through the globulettes and the measured shape of the objects. A majority of the globulettes have planetary masses,<13MJ (Jupiter masses). Very few objects have masses above 100MJ≈0.1M(Solar masses). Hence, there is no smooth overlap between globulettes and globules, which makes us conclude that globulettes represent a distinct, new class of objects.

Globulettes might have been formed either by the fragmentation of larger filaments, or by the disintegration of large molecular clouds originally hosting compact and small cores. At a later stage, globulettes expand, disrupt or evaporate. However, preliminary calculations of their lifetimes show that some might survive for a relatively long time, in several cases even longer than their estimated contraction time.

The tiny high density globulettes in the Carina Nebula indicate that they are in a more evolved state than those in the Rosette Nebula, and hence they may have survived for a longer time. It is possible that the globulettes could host low mass brown dwarfs or planets.

Using the virial theorem on the Rosette Nebula globulettes and including only the thermal and gravitational potential energy indicated that the 133 found globulettes are all either expanding or disrupting. When the ram and the radiation pressure were included, we found that about half of our objectsare gravitationally bound or unstable to contraction and could collapse to form brown dwarfs or free floating planets.

We also estimated the amount of globulettes and the number of free floating planetary mass objects, originating from globulettes, during the history of the Milky Way. We found that a conservative value of the number of globulettes formed is 5.7×1010. A less conservative estimate gave 2×1011globulettes andif 10% of these forms free floating planets then the globulettes have contributed about 0.2 free floating planets per star.

In the Crab Nebula, which is a supernova remnant from the explosion of a massive old star, one can find dusty globules appearing as dark spots against the background nebulosity. These globules are very similar to the globulettes we have found in H II regions. The total mass of dust in globules was estimated to be 4.5×10−4M, which corresponds to.2% of the total dust content of the nebula. These globules move outward from the center with transversal velocities of 60–1600 km s−1. Using the extinction law for globules, we found that the dust grains are similar to the interstellar dust grains. This means that they contribute to the ISM dust population. We concluded that the majority of the globules are not located in bright filaments and we proposed that these globules may be products of cell-like blobs or granules in the atmosphere of the progenitor star. Theses blobs collapse and form globules during the passage of the blast wave during the explosion.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2018
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Physical Sciences Other Physics Topics
Research subject
Applied Physics
Identifiers
urn:nbn:se:ltu:diva-68252 (URN)978-91-7790-092-4 (ISBN)978-91-7790-093-1 (ISBN)
Public defence
2018-06-19, E 231, Luleå, 10:00 (English)
Opponent
Supervisors
Available from: 2018-04-10 Created: 2018-04-09 Last updated: 2018-05-25Bibliographically approved

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Grenman, TiiaElfgren, Erik

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