Radial-velocity Searches for Planets Around Active Stars

152 Pages·2016·6.15 MB·English

by  Raphaëlle D. Haywood (auth.)

Springer Theses
Recognizing Outstanding Ph.D. Research
Raphaëlle D. Haywood

Radial-velocity
Searches for
Planets Around
Active Stars
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Rapha lle D. Haywood
Radial-velocity Searches
for Planets Around
Active Stars
Doctoral Thesis accepted by
the University of St Andrews, UK
123
Author Supervisor
Dr. Raphaëlle D.Haywood Prof. AndrewCollier Cameron
Harvard CollegeObservatory Schoolof Physics andAstronomy
Cambridge, MA University of St Andrews
USA St Andrews
UK
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-3-319-41272-6 ISBN978-3-319-41273-3 (eBook)
DOI 10.1007/978-3-319-41273-3
LibraryofCongressControlNumber:2016943804
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A mon père,
modèle de force, courage,
patience et détermination
’
Supervisor s Foreword
Raphaëlle Haywood’s thesis develops powerful new methods for detecting reflex
orbital motions of solar-type stars hosting extra-solar planets, in the presence of
stellar magnetic activity.
During the first two decades of exoplanet research, the sensitivity of
radial-velocity spectrometers improved exponentially, at the rate of one order
of magnitude per decade. After 2011, however, the detection threshold stalled at
orbital velocity amplitudes close to 1 m s−2. Although the new generation of
high-accuracy radial-velocity spectrometers is capable of detecting velocity shifts
an order of magnitude smaller than this, the photospheric physics of the host stars
themselves is now the limiting factor.
High-resolutionimagesofourneareststar,theSun,showthatitssurfacechurns
with convective flows on a wide range of length scales. The solar granulation in
particularissuppressedintheregionsofhighmagneticfieldstrengththatsurround
sunspotgroupsandmakeupthewidersolarmagneticnetwork.AstheSunrotates,
sunspotsandactiveregionspassinandoutofview,modulatingtheSun’sapparent
radial velocity by several m s−2. Space-borne observations of other Sun-like stars
show similar patterns of modulation in brightness, which are closely related to the
velocityvariationsthatplagueeffortstodeterminethemassesoftheirsmallplanets.
The need to overcome this barrier has become more acute since the advent of
space-basedphotometrymissionssuchasCoRoTandKepler,whichhavedetected
thetransits ofplanetsdown to thesize ofthe Earth and even smaller.To have any
hopeofdistinguishingrockyEarthanaloguesfrommini-Neptuneswithlow-density
ice mantles, their masses must be found by measuring the orbital reflex motions
oftheirhoststarsanddisentanglingthemfromthehigheramplitudestellaractivity
signal.
Using the High-Accuracy Radial-velocity Planet Searcher (HARPS) on the
EuropeanSouthernObservatory’s3.6-mtelescopeatLaSilla,RaphaëlleHaywood
conducted observations of the compact planetary system orbiting the magnetically
active star CoRoT-7, simultaneously with photometry from the CoRoT spacecraft.
Heranalysisofthesedatasetsdemonstratesthattheradial-velocityvariationsarise
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viii Supervisor’sForeword
mainly from suppression of photospheric convection by magnetic fields. A key
resultofHaywood’sworkonCoRoT-7wastherecognitionthatwhilestellaractive
regions come and go, a true planetary signal remains constant in phase and
amplitude. Her work provides the first practical demonstration that Gaussian pro-
cess regression is adept at teasing them apart, given a sufficiently long and
well-sampled data train.
Haywood’s second major achievement was to carry out the first systematic
campaign of radial-velocity observations of the Sun using the HARPS instrument,
using integrated sunlight scattered from the surface of a bright asteroid. She used
data from the Solar Dynamics Observatory to identify the types of solar surface
activity that drive the full-disc velocity variations. She demonstrated that the sup-
pressionofconvectiveblueshiftinsolaractiveregions,andthevelocitymodulation
caused by dark spots and bright faculae rotating across the face of the Sun, was
directly measurable from the SDO images. She found them to be an excellent
predictor of the Sun-as-a-star radial-velocity fluctuations measured over two solar
rotations with the HARPS instrument.
The clarity of Haywood’s writing makes her thesis popular with researchers in
thefieldseekingtomasterandadoptthestate-of-the-artstatisticalmethodsthatshe
employed. These include Gaussian-process regression for modelling the correlated
signals arising from evolving active regions on a rotating star and Bayesian model
selection methods for distinguishing genuine planetary reflex motion from false
positives arising from stellar magnetic activity.
Her study represents a significant step towards measuring the masses of
potentially habitable planets orbiting Sun-like stars with solar-like magnetic
activity. The techniques she developed are influential in the design of new
observing strategies that allow intrinsic stellar variability to be fully characterised
and separated from planetary motion,using thedataanalysis methodsdescribedin
the thesis. Although the first mass measurement of a true Earth analogue orbiting
inthehabitablezoneofaSun-likestarisstillsomewayoff,themethodspioneered
in this thesis represent an influential milestone along the journey.
St Andrews, UK Prof. Andrew Collier Cameron
February 2016
Preface
Since the discovery of the first planet orbiting another star than our Sun, just over
twenty years ago, hundreds of new extra-solar planets have been identified, and
thousands of more discoveries are awaiting confirmation. The first exoplanets that
weredetectedhadsizessimilartothoseofJupiterandSaturn,thegiantsinoursolar
system.Inrecentyears,instrumentprecisionandtelescopepowerhaveimprovedso
much that discovering and characterising planets as small as the Earth is now a
reality.Thesearchforworldssimilartoourownisoneofthefastestgrowingfields
in astronomy; it is a young and exciting field and captivates the interest of the
public like no other.
One of the most successful ways to find extra-solar planets is to look for stars
thatwobble.Asaplanetorbitsarounditsparentstar,itexertsatinypullonthestar.
This causes the starlight to periodically stretch and compress, making the star
appear redderandbluer.Thiseffect,knownastheDoppler shift, isthesame effect
that makes the siren of an ambulance sound high-pitched then low-pitched as it
drivespast.Theseminusculechangesinthecolourofthestar’slight,whichreflect
thevariationsofthestar'svelocityalongourlineofsight,canbedetectedbycurrent
state-of-the-art spectrographs.
There are still several challenges to be overcome in the quest for other Earths.
One major difficulty arises from the intrinsic magnetic activity of the host stars
themselves.Indeed,thecorrelatednoisethatarisesfromtheirnaturalradial-velocity
variability can easily mimic or conceal the orbital signals of super-Earth and
Earth-mass exoplanets, and there is currently no reliable method to untangle the
signal of a planet from this stellar “noise”.
TheworkIundertookaspartofmythesiswasintendedtotacklethisissueviaa
twofold approach. First, I developed an intuitive and robust data analysis frame-
workinwhichtheactivity-inducedvariationsaremodelledwithaGaussianprocess
that has the frequency structure of the photometric variations of the star, thus
allowing me to determine precise and reliable planetary masses (Chap. 2); and
second,Iexploredthephysicaloriginofstellar-inducedDopplervariationsthrough
the study of our best-known star, the Sun (Chap. 4).
ix
x Preface
Iappliedmynewdata-modellingtechniquetothreerecentlydiscoveredplanetary
systems: CoRoT-7, Kepler-78, and Kepler-10 (Chap. 3). I determined the masses
of the transiting super-Earth CoRoT-7b and the small Neptune CoRoT-7c to be
4:73(cid:1)0:95M(cid:3) and 13:56(cid:1)1:08M(cid:3), respectively. The density of CoRoT-7b is
6:61(cid:1)1:72 g cm−3, which is compatible with a rocky composition. I carried out
Bayesianmodelcomparisontoassessthenatureofapreviouslyidentifiedsignalat9
daysandfoundthatitisbestinterpretedasstellaractivity.Despitethehighlevelsof
activityofitshoststar,IdeterminedthemassoftheEarth-sizedplanetKepler-78bto
be 1:76(cid:1)0:18M(cid:3). With a density of 6:2(cid:4)þ11:4:8 g cm−3, it is also a rocky planet.
I found the masses of Kepler-10b and Kepler-10c to be 3:31(cid:1)0:32M(cid:3)
and 16:25(cid:1)3:66M(cid:3), respectively. Their densities, of 6:4(cid:4)þ01:7:1 g cm−3 and
8:1(cid:1)1:8 g cm−3, imply that they are both of rocky composition—even the 2
Earth-radius planet Kepler-10c!
In parallel, I deepened our understanding of the physical origin of stellar
radial-velocityvariabilitythroughthestudyoftheSun,whichistheonlystarwhose
surfacecanbeimagedathighresolution.Ifoundthatthefull-discmagneticfluxis
an excellent proxy for activity-induced radial-velocity variations; this result may
become key to breaking the activity barrier in coming years.
IalsofoundthatinthecaseofCoRoT-7,thesuppressionofconvectiveblueshift
leads toradial-velocityvariationswithan RMSof1.82ms−1,while themodulation
induced by the presence of dark spots on the rotating stellar disc has an RMS of
0.46 m s−1. For the Sun, I found these contributions to be 2.22 m s−1 and
0.14 m s−1, respectively. These results suggest that for slowly rotating stars, the
suppression of convective blueshift is the dominant contributor to the
activity-modulated radial-velocity signal, rather than the rotational Doppler shift
of the flux blocked by starspots.
Gainingadeeperunderstandingofthephysicsattheheartofactivity-drivenRV
variability will ultimately enable us to better model and remove this contribution
from RV observations, thus revealing the planetary signals.
Cambridge, MA, USA Dr. Raphaëlle D. Haywood