Direct imaging searches for substellar companions to nearby stars [Elektronische Ressource] / presented by Markus Janson

Sujets

• Astronomie

Informations

• Publié par: ruprecht-karls-universitat_heidelberg
• Publié le: 01 janvier 2008
• Nombre de lectures: 15
• Langue: English
• Poids de l'ouvrage: 2 Mo

Extrait

Dissertation
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
M.Sc.
Markus Janson
born in S¨avedalen, Sweden
Oral examination: April 30, 2008Direct imaging searches for
substellar companions to nearby stars
Referees: Prof. Dr. Thomas Henning
Prof. Dr. Joachim WambsganssAbstract
Ever since the first detections of extrasolar planet candidates in the early 1990s, our knowl-
edgeofsuchobjectshasdrasticallyincreased,andexoplanetsciencetodayconstitutesamajor
branchofastrophysics, withafewhundredindividualdetections. Ourphysicalunderstanding
ishoweverlimitedbythefactthattheplanetsaregenerallyonlydetectedindirectly, withjust
a few constraints on its orbital and physical parameters. Direct imaging of exoplanets, where
theplanetcanbespatiallyresolvedfromthestar, opensupforamuchbroaderunderstanding
of these objects, with the opportunity to study their spectral characteristics. Alternatively,
eclipse spectroscopy, where the planetary signal can be temporally resolved in systems where
the orbital plane of the planet happens to align with the line of sight of an observer, can be
used for the same purpose. In this thesis, we study various approaches for direct imaging
and spectroscopy of exoplanets from the ground, using combinations of adaptive optics and
differential methods, in particular spectral and angular differential imaging. The possibility
of using an external occulter for the purpose of decreasing the star-planet contrast is stud-
ied. We also investigate the possibility to calibrate theoretical mass-luminosity relationships
in order to improve detection predictions and the interpretations of null-detection surveys.
Scientific results include an improved age range of the AB Dor system thanks to the spatial
distinguishing of AB Dor B as a close binary, and the best constrained upper limit to the
H-band brightness of any known exoplanet ever achieved, from a deep imaging search for
Eri b.
Zusammenfassung
Ausgehend von den ersten Entdeckungen von Kandidaten fur¨ extrasolare Planeten zu Beginn
der 1990er Jahre wuchs unser Wissen u¨ber derartige Objekte rasant an. Mit mehreren 100
bekanntenExoplanetenstelltihreUntersuchungheuteeinenHauptzweigderAstrophysikdar.
Allerdings beschr¨ankt der im allgemeinen indirekte Nachweis der Exoplaneten unser Wissen
ub¨ er Umlaufbahnen und physikalische Eigenschaften. Erst die direkte Abbildung der Exo-
planeten, d.h. die raumlic¨ he aufgel¨oste Trennung von Stern und Exoplanet erm¨oglicht eine
umfassendere Untersuchung ihrer spektralen Eigenschaften. Eine weitere M¨oglichkeit ist Be-
deckungsspektroskopie. Hierbei wird in Systemen, in denen die Sichtlinie des Beobachters
in der Umlaufebene des Planeten liegt, das Planetensignal zeitlich aufgel¨ost. In der vor-
liegenden Doktorarbeit werden unterschiedliche Ans¨atze zur direkten Abbildung und Spek-
troskopie der Exoplaneten vom Erdboden aus unter Verwendung von adaptiver Optik und
differentiellen Methoden, die auf spektralen Eigenschaften oder Drehungen beruhen, unter-
sucht. Es wird die Machbarkeit eines externen, freifliegenden Bedeckers mit dem Ziel, den
Kontrast zwischen Planet und Stern zu minimieren, studiert. Weiterhin wird mit dem Ziel,
Vorhersagen zur Entdeckung von Exoplaneten und die Interpretation von Nullergebnissen in
breitangelegenten Suchprogrammen zu verbesseren untersucht, wie sich theoretische Masse-
Leuchtkraft-Beziehungen eichen lassen. Zu den wissenschaftlichen Ergebnissen z¨ahlen eine
verbesserte Altersbestimmung des AB Dor-Systems, aufbauend auf den Eigenschaften des
engen visuellen Doppelsterns AB Dor B, und im Rahmen von tiefen Aufnahmen die beste
bisher erreichte Obergrenze fur¨ die H-Bandhelligkeit eines bekannten Exoplaneten im Falle
von Eps Eri b.
iiiPreface
This thesis work was performed at the Max-Planck-Institut fur¨ Astronomie in the period be-
tween August 2005 and early 2008 under the supervision of Dr. Wolfgang Brandner and Prof.
Dr.ThomasHenning. Themaincontentofthethesishasbeenpublishedinfivepeer-reviewed
journals. An article reference is given in the beginning of each chapter, showing which article
the chapter corresponds to. One chapter, ’A double-differential scheme for spectroscopy of
transiting planets from the ground’, is based on as-of-yet unpublished research. The research
of that chapter was performed in collaboration with Mark Swain (JPL, Pasadena), Christina
AfonsoandThomasHenning(MPIA,Heidelberg), andDanielAngerhausen, ChristofIserlohe
and Alfred Krabbe (Univ. of Cologne).
Acknowledgements
Iamparticularlygratefultomythesissupervisors, WolfgangBrandnerandThomasHenning,
for their help and the useful discussions. During my PhD work, I have received financial sup-
port via IMPRS Heidelberg, for which I am naturally also very thankful. Kudos to Christian
Fendt for a job well done in administrating this program.
Several free online services exist that hugely simplify life for an astrophysicist. I have made
extensive use of VIZIER, SIMBAD, NASA ADS, and obviously the Extrasolar planets ency-
clopediaaspartofmyscientificwork. Astronomicalcataloguessuchas2MASSandHipparcos
have been particularly useful for my purposes.
Thanks to the computer department at MPIA for keeping a system up that hardly crashed at
allduringmywholetimeattheinstitute–afeatfarfromaccomplishedatpreviousworkplaces
I’ve encountered. General thanks also to all the administrative and servicing personnel at
MPIA, as well as Paranal and Calar Alto, for keeping things running without noticeable
problems.
To all collaborators I’ve had during my PhD studies, and my colleagues at MPIA for the
interesting scientific discussions – many thanks. The positive and creative spirit of the work-
place, and the scientific environment in general, is a joy to experience. I wish I could list
you all individually, but it would make a long list, and inevitably I would forget someone. A
specific sentence of thanks, however, is well spent indeed on the other members of Wolfgang’s
research group – Felix, Mica, Boyke, Kerstin, and Sebastian. Thanks also to the people I
have already thanked in the acknowledgments of my individual articles for various reasons,
iiibuthavesofarnotbeenspecificallymentionedhere–BenZuckerman, StanMetchev, Andrea
Stolte, Alessandro Berton, Webster Cash, Stefan Hippler, and all the anonymous referees.
ThankstoAndersJohansenforthethesismacro–ifyou,thereader,findthegeneralstructure
of this thesis to be aesthetically pleasing, the cred goes his way. Whenever I’ve had questions
about everyday problems with tax returns, insurance issues, or Linux shortcuts, my office
mates have always been eager to help. Many thanks to Michael, Marie-Helene, Jan, and
Sebastian for that.
Finally, great thanks to my family and friends for all the obvious reasons.
ivContents
Introduction 1
1 Extrasolar planets 3
1.1 Planet formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Brown dwarf formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Planet and brown dwarf definitions . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4 Planet detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.4.1 Dynamical methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.4.2 Eclipse methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.4.3 Imaging methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.4.4 Other methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2 Early ComeOn+ adaptive optics observation of GQ Lup 21
2.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.3 Observations and data analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.3.1 Analysis of the suspected companion . . . . . . . . . . . . . . . . . . . 23
2.3.2 Examination of the background hypothesis . . . . . . . . . . . . . . . 25
2.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.4.1 The data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.4.2 The GQ Lup system . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
v2.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3 Improved age constraints for the AB Dor quadruple system 35
3.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.3 Observations and Data Analysis . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.4 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.4.1 NACO data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.4.2 Isochronal age . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.4.3 Kinematical group membership . . . . . . . . . . . . . . . . . . . . . . 41
3.4.4 Consequences for AB Dor C . . . . . . . . . . . . . . . . . . . . . . . . 43
3.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . .