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History of Globulettes in the Milky Way
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.ORCID iD: 0000-0002-4932-7851
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.ORCID iD: 0000-0002-4400-3276
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.ORCID iD: 0000-0003-1564-870X
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.

##### Place, publisher, year, edition, pages
Springer, 2018. Vol. 363, no 2, article id 28
##### Keywords [en]
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
ISI: 000424039100002Scopus ID: 2-s2.0-85040453332OAI: oai:DiVA.org:ltu-67161DiVA, id: diva2:1170838
##### Note

Available from: 2018-01-04 Created: 2018-01-04 Last updated: 2018-09-27Bibliographically 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
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)
##### Supervisors
Available from: 2018-04-10 Created: 2018-04-09 Last updated: 2018-05-25Bibliographically approved

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