Dusty Globules and Globulettes

212 Pages·2017·29.97 MB·English

DOCTORA L T H E S IS
Department of Engineering Sciences and Mathematics
Division of Material Science
T Dusty Globules and Globulettes
i
i
ISSN 1402-1544 a
G
ISBN 978-91-7790-092-4 (print)
r
e
ISBN 978-91-7790-093-1 (pdf) n
m
a
Luleå University of Technology 2018 n

D
u
s
t
y

G
lo
b
u
le
s

a
n
d

G
lo
b
u
le
t
t
e
s

Tiia Grenman
Applied Physics

Doctoral Thesis
Dusty Globules and Globulettes
Tiia Grenman
Division of Applied Physics
Lule˚a University of Technology
SE-971 87 Lule˚a
Sweden
E-mail: [email protected]
Lule˚a, May 2018
© 2018 Tiia Grenman
Division of Applied Physics
Department of Engineering Sciences and Mathematics
Luleå University of Technology
SE-971 87 Luleå
Sweden
The cover image shows the northern part of the Rosette Nebula. In the lower
right part of the image the Wrench, an elephant trunk is shown. This trunk
is mainly made up of thin threads that are twisted and connected to two jaws
at the lower massive head. In the upper part of this Wrench a string of dark
dots, globulettes, are seen in silhouette against the bright background. This
false-color image shows emission from lines of Sulfur, Hydrogen and Oxygen
(shaded red, green and blue). Credit: Astronomy Picture of the Day, March
2014.
Printed by Luleå University of Technology, Graphic Production 2018
ISSN 1402-1544
ISBN 978-91-7790-092-4 (print)
ISBN 978-91-7790-093-1 (pdf)
Luleå 2018
www.ltu.se
i
We are just an advanced breed of monkeys on a minor planet of a very
average star. But we can understand the Universe. That makes us something
very special. Stephen Hawking
To my family

ABSTRACT
Interstellargasanddustcancondenseintocloudsofverydifferentsize,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 of e.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, <13M (Jupiter masses). Very few objects have masses
J
above 100M ≈ 0.1M (Solar masses). Hence, there is no smooth overlap
J (cid:12)
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
iii
iv
found globulettes are all either expanding or disrupting. When the ram and
the radiation pressure were included, we found that about half of our objects
aregravitationallyboundorunstabletocontractionandcouldcollapsetoform
brown dwarfs or free floating planets.
Wealsoestimatedtheamountofglobulettesandthenumberoffreefloating
planetary mass objects, originating from globulettes, during the history of the
Milky Way. We found that a conservative value of the number of globulettes
formedis5.7×1010. Alessconservativeestimategave2×1011 globulettesand
if10%oftheseformsfreefloatingplanetsthentheglobuletteshavecontributed
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
wehavefoundinHIIregions. Thetotalmassofdustinglobuleswasestimated
to be 4.5×10−4M , which corresponds to (cid:46) 2% of the total dust content of
(cid:12)
the nebula. These globules move outward from the center with transversal
velocities of 60–1600kms−1. Using the extinction law for globules, we found
thatthedustgrainsaresimilartotheinterstellardustgrains. Thismeansthat
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
globulesmaybeproductsofcell-likeblobsorgranulesintheatmosphereofthe
progenitor star. Theses blobs collapse and form globules during the passage of
the blast wave during the explosion.
Preface
The research behind this doctoral thesis has been carried out at the Division
of Applied Physics, Lule˚a University of Technology. It had not been possible
withoutthehelpandsupportofmanyindividuals, who, inonewayoranother,
have helped me to complete my work.
First of all, I would like to express my gratitude to my supervisors, Hans
Weber and Erik Elfgren for invaluable advice, guidance, comments, and for
theirencouragement. Ialsowantexpressmythankstomysupervisor,professor
Go¨sta Gahm at the AlbaNova Centre of Stockholm University. He suggested
my research topic, provided the observational data, and has been advising me
through the research work. His expertise in astronomy has been necessary for
my daily work and he is also a great inspiration. I also wish to express my
gratitude to my late supervisor, Sverker Fredriksson for his support during the
first part of this work.
Special thanks to my friends, Britt-Mari, Armi and Rose-Marie who kept
me smiling through even the hardest of times.
I am grateful also for support from the National Graduate School of Space
Technology. Finally, andmostofall, IwouldliketothankmypartnerLeifand
my children Liina, Jane, and Lucia for their moral support and unconditional
love over the years. I also thank my grandchildren Athena, Leah, Nathalie and
Ameliah for they always make me smile.
Astronomers are one of those lucky people who can make a living out of
their pure interest. I feel lucky.
Tiia Grenman
v