Amplitude modulation of pulsation modes in Delta Scuti stars

216 Pages·2017·19.667 MB·English

by  Bowman, Dominic

Springer Theses
Recognizing Outstanding Ph.D. Research
Dominic M. Bowman

Amplitude
Modulation of
Pulsation Modes
in Delta Scuti Stars
Springer Theses
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Dominic M. Bowman
Amplitude Modulation
of Pulsation Modes in
Delta Scuti Stars
Doctoral Thesis accepted by
University of Central Lancashire, Preston, UK
123
Author Supervisor
Dr. Dominic M.Bowman Prof. DonKurtz
Institute of Astronomy University of Central Lancashire
KULeuven Preston
Leuven UK
Belgium
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-3-319-66648-8 ISBN978-3-319-66649-5 (eBook)
DOI 10.1007/978-3-319-66649-5
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’
Supervisor s Foreword
The fundamental data of asteroseismology are the frequencies of stellar pulsation
modes, which are modelled to give direct measurement of stellar interior physics.
To obtain these frequencies, a principal method is analysis of time series data of
photometric variations in stellar brightness. For decades, we were limited to
observingjustone,oratbestafew,starsatatimeduringnight-timehoursunderthe
best weather conditions at observatories placed in the remote driest regions of
Earth. The photometric variations of solar-like oscillators were generally too small
tobedetectedfromtheground,andthelargeramplitudepulsatorsneededmulti-site
campaignstoovercometheconfusionthatresultsfromgapsinthetimeseriesdata.
Theprecisioninpulsationamplitudethatwasobtainedwasoftheorderof100parts
per million (ppm).
Then, camethespace photometry revolution,with itsculmination inthe Kepler
mission. Kepler observed nearly 200 000 stars over its four-year lifetime, with
precision in the best cases of 1 ppm, and routinely of 10 ppm. Because of its
heliocentric orbit, it obtained nearly continuous data. Just as the CCD has made
photographicfilmobsoleteasanastronomicaldetector, theKeplerdatahavemade
ground-based photometric time series observations for asteroseismology a thing
of the past. The Kepler mission data are truly revolutionary for asteroseismology,
with hundreds of solar-like pulsators and thousands of red giants yielding
unprecedented understanding of stellar interiors.
Theparameterspaceforunderstandingstellarastrophysicsisfarwiderthanjust
thecoolstars.Inparticular,thestarsoftheuppermainsequencehaverichpotential
for asteroseismic inference, yet the Kepler mission was designed primarily for the
study ofcool starsinthequest for eta-Earth.Asteroseismicstudies havenow been
carriedoutonmanyhundredsofmainsequencesolar-likepulsatorsandmorethan
10000redgiants.Itiseasytounderstandwhythosestochasticallydrivenstarshave
highly variable amplitudes. Over much of the rest of the HR diagram, stellar
pulsations are driven by heat mechanisms, which are much more regular than the
stochastic driving in solar-like stars. Yet for decades, amplitude and frequency
modulation of pulsation modes have been observed in almost all types of
heat-drivenpulsatingstars.Isthismodulationastrophysical,andcanwelearnmore
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vi Supervisor’sForeword
about the pulsating stars from it? Or is it simply beating of modes with stable
amplitudes, unrecognised because of a lack offrequency? This outstanding thesis
by Dominic Bowman provides the most thorough investigation of these
long-standing questions. He has studied a uniform sample of 983 d Sct stars—the
most common type of main sequence heat engine pulsators—that were observed
nearly continuously for 4 years at stunning photometric precision of only a few
partspermillionbytheKeplerSpaceMission.Hehasshownthattheamplitudeand
frequencymodulationareastrophysical,andhehasinvestigateditsimplicationsand
prospects, and provided new insight into this, for the d Sct stars, and for the many
other types of heat engine pulsators across the HR diagram. With no mission
planned to supersede the Kepler four-year data set, this thesis will stand as the
definitive study of these questions for many years.
The recognition of the impact of this thesis led to Dominic taking up a presti-
giouspostdoctoral fellowship withProf. ConnyAertsattheKULeuven,Belgium,
in one of the top asteroseismology groups in the world. He is continuing and
expandinghisasteroseismologystudiestotheas-yet-not-understood-low-frequency
variations of the most massive stars known, the O-type stars.
ThisthesisisanoutstandingintroductiontothestudyofpulsationindSctstars,
and to the Kepler data and their analysis. It gives an excellent introduction to, and
discussion of, pulsational non-linearity and mode coupling, particularly from the
observational viewpoint.
Preston, UK Prof. Don Kurtz
July 2017
Preface
Context
Astronomy covers numerous aspects of physics and mathematics across many
orders of magnitude in size, from the quantum mechanical laws that determine
nuclear fusion reactions in the centre of a star, to the dynamics of galaxy clusters
over cosmological distances. Astronomy studies the smallest and largest known
structures in the universe and the physical laws that govern them. Over the last
century,ageneraldescriptionofstellarstructureandevolutionhasbeendeveloped.
However,theinclusionofphysicssuchasrotation,binarity,pulsationandmagnetic
fields causes stellar structure and evolution to become more complicated.
Usingtraditionalobservationaltechniquessuchasphotometryandspectroscopy,
onebarelyscratchesbeneaththevisiblesurfaceofastar,becausethesemethodsare
only sensitive to the stellar atmosphere. Until the advent of asteroseismology,
astronomy lacked the techniques to validate or invalidate theoretical predictions
oftheinteriorconditionsofastarthroughobservationandexperimentation.Thisis
famously encapsulatedby thetext TheInternalConstitutionoftheStars published
in 1926 by Sir Arthur Stanley Eddington, who asked the following questions:
“AtfirstsightitwouldseemthatthedeepinterioroftheSunandstarsislessaccessibleto
scientific investigation than any other region in the universe. Our telescopes may probe
fartherandfartherintothedepthsofspace;buthowcanweeverobtaincertainknowledge
ofthatwhichishiddenbehindsubstantialbarriers?Whatappliancecanpiercethroughthe
outerlayersofastarandtesttheconditionswithin?”
Asteroseismology is the tool that Eddington sought, which provides direct
insight into the interior of a star by studying its oscillations. The pulsations within
an oscillating star are governed by the physics of stellar structure, so from obser-
vations and analysis of these pulsations the interior properties of a star can be
measured. Many different types of pulsator have been discovered, including our
nearest stellar neighbour—the Sun. Its proximity has allowed the Sun to be
extensively studied since the discovery of its oscillations in the 1960s.
Asteroseismologyhasrapidlyexpandedinrecentyearsbecauseofimprovementsin
vii
viii Preface
instrumentationandhasbeenappliedtomanytypesofstarsacrossawiderangein
stellar mass and for different stages of stellar evolution.
Motivation
ThismonographiscentredonagroupofvariablestarscalleddSctstars,whichare
themostcommontypeofpulsatoramongtheintermediate-massAandFstars,and
havebeenstudiedsincetheirdiscoveryintheearlytwentiethcentury.Historically,
using several hours of ground-based observations, these variable stars were
straightforward to classify because they have similar effective temperatures and
pulsate with periods of order hours and minutes.
Prior to the twenty-first century, only a handful of d Sct stars had been exten-
sively studied using ground-based campaigns, which provided intermittent obser-
vations with duty cycles typically less than 50 per cent. These investigations were
limitedbythedatacoverageandprecisionoftheinstrumentationused,butrevealed
that we lack a complete physical description of these pulsators, primarily because
ofthediversityof observed pulsationsinthese stars. Afew dSctstarswerefound
to exhibit a phenomenon called amplitude modulation—in the time domain this
appears as a change in the maxima and minima of the light excursions over time,
and in the Fourier domain this appears as variable pulsation mode amplitudes.
Naturally, this causes problems for intermittent observations over a few days if
thepulsationmodeamplitudesofavariablestarchangeontimescalesoforderyears
todecades.Fromonlyafewin-depthstudies,itwasnotclearwhatdeterminesthis
effect in d Sctstars, asnotall stars exhibited such behaviour. This provides strong
motivation to study amplitude modulation in d Sct stars further. The space-based
telescopes CoRoT and Kepler, which were launched in 2006 and 2009, respec-
tively,haveledtoaphotometryrevolution.TheKeplerSpaceTelescopeprovideda
large increase in high-quality observations of hundreds of thousands of stars,
including many d Sct stars, which form the basis of this research text.
Overview
This monograph addresses Eddington’s questions by using asteroseismology to
expand our understanding of d Sct stars and studies the incidence of amplitude
modulation in approximately 1000 d Sct stars observed by the Kepler Space
Telescope.Intheoriginaleditionofthisthesis,thepeer-reviewedjournalpapersby
the author were included as they were published, but in this monograph they have
beendiscussedaspartoftheliterature.Asasecondedition,muchofthecontentis
thesame,althoughithasbeenupdatedandexpandeduponsincetheoriginalwork
waspublished.Thegeneralthemeofthistextisthejourneythroughthediscoveries
that were made throughout the work.
Preface ix
The pulsations in d Sct stars are excited by a heat engine driving mechanism
caused by increased opacity in their surface layers and have pulsation periods of
order a few hours. Observations from the Kepler Space Telescope have revealed a
diverse range of pulsational behaviour in these stars, which has been investigated
usinganensembleof983dSctstarsthatwereobservedcontinuouslyforfouryears
at an unprecedented level of photometric precision. A search for amplitude mod-
ulation in these stars showed that the majority exhibit amplitude modulation in at
leastasinglepulsationmode.Thus,timescalesofyearsandlongeraresignificantin
these stars both observationally and theoretically. The various causes of amplitude
modulation include beating, non-linear mode coupling and stellar evolution, each
having a characteristic pattern that must be recognised and disentangled to truly
understandthephysicalprocessesoccurringwithinastar.Theobservationalstudies
of individual stars in this monograph provide strong evidence that non-linear
processes are clearly at work in the many d Sct stars and provide valuable con-
straints for future asteroseismic modelling.
Leuven, Belgium Dr. Dominic M. Bowman