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Dusty Globules and Globulettes
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.ORCID iD: 0000-0002-4932-7851
2018 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Stoft i globuler och globuletter (Swedish)
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: urn:nbn:se:ltu:diva-68252ISBN: 978-91-7790-092-4 (print)ISBN: 978-91-7790-093-1 (electronic)OAI: oai:DiVA.org:ltu-68252DiVA, id: diva2:1196362
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
List of papers
1. Globulettes as seeds of brown dwarfs and free-floating planetary-mass objects
Open this publication in new window or tab >>Globulettes as seeds of brown dwarfs and free-floating planetary-mass objects
2007 (English)In: Astronomical Journal, ISSN 0004-6256, E-ISSN 1538-3881, Vol. 133, no 4, p. 1795-1809Article in journal (Refereed) Published
Abstract [en]

Some H II regions surrounding young stellar clusters contain tiny dusty clouds, which on photos look like dark spots or teardrops against a background of nebular emission. From our collection of Hα images of 10 H II regions gathered at the Nordic Optical Telescope, we found 173 such clouds, which we call "globulettes," since they are much smaller than normal globules and form a distinct class of objects. Many globulettes are quite isolated and located far from the molecular shells and elephant trunks associated with the regions. Others are attached to the trunks (or shells), suggesting that globulettes may form as a consequence of erosion of these larger structures. None of our objects appear to contain stellar objects. The globulettes were measured for position, dimension, and orientation, and we find that most objects are smaller than 10 kAU. The Rosette Nebula and IC 1805 are particularly rich in globulettes, for which the size distributions peak at mean radii of ∼2.5 kAU, similar to what was found by Reipurth and coworkers and De Marco and coworkers for similar objects in other regions. We estimate total mass and density distributions for each object from extinction measures and conclude that a majority contain <13 MJ, corresponding to planetary-mass objects. We then estimate the internal thermal and potential energies and find, when also including the effects from the outer pressure, that a large fraction of the globulettes could be unstable and would contract on short timescales, < 106 yr. In addition, the radiation pressure and ram pressure exerted on the side facing the clusters would stimulate contraction. Since the globulettes are not screened from stellar light by dust clouds farther in, one would expect photoevaporation to dissolve the objects. However, surprisingly few objects show bright rims or teardrop forms. We calculate the expected lifetimes against photoevaporation. These lifetimes scatter around 4 × 10 6 yr, much longer than estimated in previous studies and also much longer than the free-fall time. We conclude that a large number of our globulettes have time to form central low-mass objects long before the ionization front, driven by the impinging Lyman photons, has penetrated far into the globulette. Hence, the globulettes may be one source in the formation of brown dwarfs and free-floating planetary-mass objects in the galaxy.

National Category
Other Physics Topics
Research subject
Fysik
Identifiers
urn:nbn:se:ltu:diva-16089 (URN)10.1086/512036 (DOI)facaeea0-6b3e-11dc-9e58-000ea68e967b (Local ID)facaeea0-6b3e-11dc-9e58-000ea68e967b (Archive number)facaeea0-6b3e-11dc-9e58-000ea68e967b (OAI)
Note
Validerad; 2007; 20070925 (bajo)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2018-04-09Bibliographically approved
2. The tiny globulettes in the Carina nebula
Open this publication in new window or tab >>The tiny globulettes in the Carina nebula
2014 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 565, article id A107Article in journal (Refereed) Published
Abstract [en]

Context. Small molecular cloudlets are abundant in many H ii regions surrounding newborn stellar clusters. In optical images these so-called globulettes appear as dark silhouettes against the bright nebular background. Aims. We aim to make an inventory of the population of globulettes in the Carina nebula complex, and to derive sizes and masses for comparisons with similar objects found in other H ii regions. Methods. The globulettes were identified from Hα images collected at the Hubble Space Telescope. Results. We have located close to 300 globulettes in the Carina complex, more than in any other region surveyed so far. The objects appear as well-confined dense clumps and, as a rule, lack thinner envelopes and tails. Objects with bright rims are in the minority, but more abundant than in other regions surveyed. Some globulettes are slightly elongated with their major axes oriented in the direction of young clusters in the complex. Many objects are quite isolated and reside at projected distances >1.5 pc from other molecular structures in the neighbourhood. No globulette coincides in position with recognized pre-main-sequence objects in the area. The objects are systematically much smaller, less massive, and much denser than those surveyed in other H ii regions. Practically all globulettes are of planetary mass, and most have masses less than one Jupiter mass. The average number densities exceed 105 cm-3 in several objects. We have found a statistical relation between density and radius (mass) in the sense that the smallest objects are also the densest. Conclusions. The population of small globulettes in Carina appears to represent a more advanced evolutionary state than those investigated in other H ii regions. The objects are subject to erosion in the intense radiation field, which would lead to a removal of any thinner envelope and an unveiling of the core, which becomes more compact with time. We discuss the possibility that the core may become gravitationally unstable, in which case free-floating planetary mass objects can form.

National Category
Other Physics Topics
Research subject
Tillämpad fysik
Identifiers
urn:nbn:se:ltu:diva-5368 (URN)10.1051/0004-6361/201423532 (DOI)3743f3c7-054c-4937-920f-357bf0b4e17f (Local ID)3743f3c7-054c-4937-920f-357bf0b4e17f (Archive number)3743f3c7-054c-4937-920f-357bf0b4e17f (OAI)
Note
Validerad; 2014; 20140611 (andbra)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2018-04-09Bibliographically approved
3. Dusty globules in the Crab Nebula
Open this publication in new window or tab >>Dusty globules in the Crab Nebula
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

National Category
Other Physics Topics Energy Engineering
Research subject
Energy Engineering; Applied Physics
Identifiers
urn:nbn:se:ltu:diva-61736 (URN)10.1051/0004-6361/201629693 (DOI)000395821900088 ()2-s2.0-85015159073 (Scopus ID)
Note

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

Available from: 2017-01-31 Created: 2017-01-31 Last updated: 2018-05-02Bibliographically approved
4. History of Globulettes in the Milky Way
Open this publication in new window or tab >>History of Globulettes in the Milky Way
2018 (English)In: Astrophysics and Space Science, ISSN 0004-640X, E-ISSN 1572-946X, Vol. 363, no 2, article id 28Article in journal (Refereed) Published
Abstract [en]

Globulettes are small (radii $< 10$ kAU) dark dust clouds, seen against the background of bright nebulae.

A majority of the objects have planetary mass.

These objects may be a source of brown dwarfs and free floating planetary mass objects in the galaxy.

In this paper we investigate how many globulettes could have formed in the Milky Way and how they could contribute to the total population of free floating planets.

In order to do that we examine H-alpha images of 27 H~II regions. In these images, we find 778 globulettes.

We find that a conservative value of the number of globulettes formed is $5.7\times 10^{10}$.

If 10 \% of the globulettes form free floating planets then they have contributed with $5.7\times 10^{9}$ free floating planets in the Milky Way. A less conservative number of globulettes would mean that the globulettes could contribute $2.0\times 10^{10}$ free floating planets.

Thus the globulettes could represent a non-negligible source of free floating planets in the Milky Way.

Keywords
Free Floating Planets; Globulettes; H II Regions; ISM; Milky Way
National Category
Astronomy, Astrophysics and Cosmology Other Physics Topics Energy Engineering
Research subject
Applied Physics; Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-67161 (URN)10.1007/s10509-017-3233-6 (DOI)000424039100002 ()2-s2.0-85040453332 (Scopus ID)
Note

Validerad;2018;Nivå 2;2018-01-23 (andbra)

Available from: 2018-01-04 Created: 2018-01-04 Last updated: 2018-05-02Bibliographically approved

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