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The formation of brown dwarfs as revealed by the mass function of IC 2391 [Elektronische Ressource] / presented by Steve Boudreault

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The formation of brown dwarfsas revealed by the massfunction of IC 2391Steve BoudreaultMax-Planck-Institut fu¨r AstronomieHeidelberg 2008Dissertation in Astronomysubmitted to theCombined Faculties for the Natural Sciences and for Mathematicsof the Ruperto-Carola University of Heidelberg, Germany.for the degree ofDoctor of Natural Sciencespresented byDipl.-Astron. Steve Boudreaultˆborn in Sept-Iles, CanadaOral examination: 20.11.2008, 14:00pmThe formation of brown dwarfsas revealed by the massfunction of IC 2391Referees: Prof. Dr. Ralf S. KlessenDr. Coryn A.L. Bailer-JonesiAbstractThis PhD thesis deals with the formation and evolution of brown dwarfs. Here, we presentthe stellar and substellar mass function of the open cluster IC 2391, plus its radial de-pendence, and use this to put constraints on the formation mechanism of brown dwarfs.Our multiband optical and infrared photometric survey with spectroscopic follow up cov-ers 11 square degrees, making it the largest survey of this cluster to date. We observethat there is no variation in the mass function over the stellar/substellar boundary at allthree cluster radius intervals analyzed. From this lack of discontinuity, we conclude thatthe stellar embryo ejection mechanism cannot be the unique brown dwarf formation pathif this formation mechanism produces a higher velocity dispersion for brown dwarfs thanthe stars obtain.

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Published 01 January 2008
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The formation of brown dwarfs
as revealed by the mass
function of IC 2391
Steve Boudreault
Max-Planck-Institut fu¨r Astronomie
Heidelberg 2008Dissertation in Astronomy
submitted to the
Combined Faculties for the Natural Sciences and for Mathematics
of the Ruperto-Carola University of Heidelberg, Germany.
for the degree of
Doctor of Natural Sciences
presented by
Dipl.-Astron. Steve Boudreault
ˆborn in Sept-Iles, Canada
Oral examination: 20.11.2008, 14:00pmThe formation of brown dwarfs
as revealed by the mass
function of IC 2391
Referees: Prof. Dr. Ralf S. Klessen
Dr. Coryn A.L. Bailer-Jonesi
Abstract
This PhD thesis deals with the formation and evolution of brown dwarfs. Here, we present
the stellar and substellar mass function of the open cluster IC 2391, plus its radial de-
pendence, and use this to put constraints on the formation mechanism of brown dwarfs.
Our multiband optical and infrared photometric survey with spectroscopic follow up cov-
ers 11 square degrees, making it the largest survey of this cluster to date. We observe
that there is no variation in the mass function over the stellar/substellar boundary at all
three cluster radius intervals analyzed. From this lack of discontinuity, we conclude that
the stellar embryo ejection mechanism cannot be the unique brown dwarf formation path
if this formation mechanism produces a higher velocity dispersion for brown dwarfs than
the stars obtain. Alternatively, the ejection mechanism could be a dominant brown dwarf
formation path only if it produces the same velocity dispersion for brown dwarfs as exists
for stars in the cluster. In addition, we observe a radial variation in the mass function
over the range 0.15 to 0.5M . We conclude that this is a signature of mass segregation⊙
via dynamical evolution. Analysis of mass functions of other open clusters from different
ages and environment, at each side of the stellar/substellar boundary, also indicates that
dynamical evolution influences the shape of the mass function.
Zusammenfassung
Diese Dissertation behandelt die Entstehung und Entwicklung von Braunen Zwergen. In
ihr wird die stellare und substellare Massenfunktion des offenen Sternhaufens IC 2391 ein-
schließlich ihrer radialen Abh¨angigkeit untersucht und die gefundenen Ergebnisse werden
dazu verwendet Randbedingungen fu¨rdie Entstehungsmechanismen von Braunen Zwergen
zu ermitteln. Unsere photometrische Vielfarbenuntersuchung im optischen und infraroten
Bereich u¨berdecktelfQuadratgrad. Diesistdiebisherumfassendste Studiedieses Haufens.
¨ ¨Wir haben in der Massenfunktion keine Anderung beim Ubergang von der stellaren zur
substellaren Grenze gefunden; dies gilt fu¨r alle drei untersuchten ringf¨ormigen Haufen-
regionen. Aus dieser Stetigkeit schließen wir, dass der Auswurfsmechanismus von stel-
laren Embryos nicht der einzige Entstehungsmechanismus von Brauner Zwergen sein kann,
falls dieser Entstehungsprozess eine h¨ohere Geschwindigkeitsverteilung fu¨r Braune Zwerge
als fu¨r Sterne zur Folge hat. Der Auswurfmechanismus k¨onnte nur dann ein dominanter
Entstehungsmechanismus BraunerZwergesein,fallserdieselbeGeschwindigkeitsverteilung
fu¨r Braune Zwerge und Sterne in dem Haufen erzeugt. Zus¨atzlich beobachten wir eine ra-
¨diale Anderung in der Massenfunktion im Bereich von 0.15 bis 0.5M . Wir schließen⊙
daraus, dass es sich um ein Anzeichen fu¨r Massensegregation durch dynamische Entwick-
lunghandelt. DieAnalyse derMassenfunktion beidseitig der stellaren/substellaren Grenze
in anderer offener Sternhaufen unterschiedlichen Alters und in unterschiedlichen Umge-
bungen deutet auch darauf hin, dass dynamische Evolution die Form der Massenfunktion
beeinflusst.Contents
1 Introduction 1
1.1 Brown dwarfs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Possible formation mechanisms of brown dwarfs . . . . . . . . . . . . . . . 2
1.2.1 Star-like formation . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2.2 Formation by gravitational instability of a disk . . . . . . . . . . . 4
1.2.3 Ejection from accretion envelope . . . . . . . . . . . . . . . . . . . 5
1.2.4 Photoevaporation of the accretion envelope . . . . . . . . . . . . . 7
1.3 Constraints on the formation mechanisms: survey of brown dwarfs in open
clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.4 Surveys of IC 2391 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.5 Outline of this work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2 Physical concepts used in the analysis 15
2.1 Mass function determination of a population . . . . . . . . . . . . . . . . . 16
iiiiv Contents
2.2 Dynamical evolution of stellar clusters . . . . . . . . . . . . . . . . . . . . 17
2.3 Atmosphere model and evolutionary track used . . . . . . . . . . . . . . . 20
3 Observations, data reduction and mass estimations 25
3.1 Photometric and spectroscopic data available . . . . . . . . . . . . . . . . . 26
3.1.1 Choice of fields, instruments and filters for photometric observations
and data obtained . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.1.2 Choice of instrument for spectroscopic follow-up and data obtained 28
3.2 Photometric data reduction procedure . . . . . . . . . . . . . . . . . . . . 29
3.2.1 Overscan, bias, darks and trimming . . . . . . . . . . . . . . . . . . 29
3.2.2 Flat-fielding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.2.3 Sky and fringe subtraction . . . . . . . . . . . . . . . . . . . . . . . 30
3.2.4 Registration and combination of images . . . . . . . . . . . . . . . . 30
3.2.5 Detection and aperture photometry of detected sources . . . . . . . 31
3.2.6 Coordinate determination and flux calibration . . . . . . . . . . . . 31
3.3 Spectroscopic data reduction procedure . . . . . . . . . . . . . . . . . . . . 33
3.4 Effective temperature and mass determination . . . . . . . . . . . . . . . . 33
4 Candidate selection procedure 49
4.1 Photometric selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
4.1.1 First candidate selection: colour-magnitude diagrams . . . . . . . . 50
4.1.2 Second candidate selection: colour-colour diagrams . . . . . . . . . 50
4.1.3 Rejection of contaminants based on proper motion. . . . . . . . . . 51
4.1.4 Rejectionofobjectsbasedonobserved magnitudevs.predictedmag-
nitude discrepancy . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.2 Spectroscopic selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52