Table Of ContentMon.Not.R.Astron.Soc.000,1–12(2002) Printed28January2015 (MNLaTEXstylefilev2.2)
Spots on Am stars
L. A. Balona1, G. Catanzaro2, O. P. Abedigamba3, V. Ripepi4, B. Smalley5
1South African Astronomical Observatory, P.O. Box 9, Observatory 7935, Cape Town, South Africa
2INAF-Osservatorio Astrofisico di Catania, Via S.Sofia 78, I-95123, Catania, Italy
3Department of Physics, North-West University,Private Bag X2046, Mmabatho, 2735, South Africa
4INAF-Osservatorio Astronomico di Capodimonte, Via Moiariello 16, I-80131, Napoli, Italy
5Astrophysics Group, Keele University,Keele, Staffordshire, ST5 5BG, UK
5
1
0
2
n
ABSTRACT
a
J We investigate the light variations of 15 Am stars using four years of high-precision
7 photometry from the Kepler spacecraft and an additional 14 Am stars from the K2
2 Campaign 0 field. We find that most of the Am stars in the Kepler field have light
curves characteristic of rotational modulation due to star spots. Of the 29 Am stars
] observed,12areδ Scutivariablesandoneisaγ Doradusstar.Onestarisaneclipsing
R
binaryandanotherwasfoundtobeabinaryfromtime-delaymeasurements.TwoAm
S stars show evidence for flares which are unlikely to be due to a cool companion. The
h. fact that 10 out of 29 Am stars are rotationalvariables and that some may even flare
p strongly suggests that Am stars possess significant magnetic fields. This is contrary
- to the current understanding that the enhanced metallicity in these stars is due to
o
diffusion in the absence of a magnetic field. The fact that so many stars are δ Scuti
r
t variables is also at odds with the prediction of diffusion theory. We suggest that a
s
viable alternative is that the metal enhancement could arise from accretion.
a
[ Key words: stars: chemically peculiar – stars: oscillations – stars:variable
1
v
6
4
1 INTRODUCTION with the geometry of the field and is responsible for the
7
abundancepatchesinApstars.Ontheotherhand,theweak
6
0 TheAmstars(metallic-linedAstars)areagroupofA-and (or perhaps tangled) magnetic fields in Am stars leads to a
. earlyF-typestarsinwhichtheCaIIKlineisrelativelyweak, relatively homogeneous abundance distribution across the
1
and/or metallic lines relatively strong, compared with the stellar surface.
0
5 spectral type indicated by the Balmer lines. More detailed Early calculations predict a much larger over-
1 analysisshowsthatAmstarsarecharacterizedbyanunder- abundanceofheavyelementsinAmstarsthanactually ob-
: abundance of Ca (and/or Sc) and/or an over-abundance of served. Richeret al. (2000) found that it is important to
v FeandtheFe-groupelements.Theanomalousabundancesin consider the diffusion of heavy elements as well as helium.
i
X AmandApstarsarethoughttobearesultoftheinterplay Astheheavyelements settle,enrichmentof thedeeperiron
r between gravitational settling and radiative acceleration of bump opacity region of the iron-group causes a convective
a different atomic species. As a result, some elements diffuse zonetodevelopinthatregionwhichmixeswiththeconvec-
upwardsandotherssettledownwardsprovidedthereislittle tiveenvelopebyovershooting.Theresultisadilutionofthe
mixing. Mixing due to convection or meridional circulation abundance anomalies, leading to abundances more in line
duetorotationdestroysthenaturalsegregation ofelements withwhatisobserved.Morerecentworkshowsthatitisalso
due to diffusion that would otherwise occur. Convection in important to include the destabilizing effect (thermohaline
A stars is confined to a very thin sub-surface layer which instability)causedbythecompositiongradientproducedby
meansthatthediffusionprocessshouldproceedunhindered diffusion (µ gradient) (Vauclair & Th´eado 2012).
in slowly-rotating A stars.
It is thought that stars in binaries with orbital peri-
ThedifferencebetweenApandAmstarsisattributedto ods of 2–10d become Am stars because their rotational ve-
the fact that Ap stars have strong magnetic fields, whereas locities, v, have been reduced by tidal interactions below
Am stars do not. Auri`ereet al. (2010) have confirmed that v≈120kms−1,arequirementfordiffusiontoact.BothAm
there is at least an order of magnitude difference in mag- and normal stars occur in binaries with orbital periods be-
neticfieldstrengthbetweenAp/BpstarsandAmandHgMn tween10–100d,butwhythisisthecaseisnotunderstood.A
stars. The presence of a large-scale strong dipole magnetic recentsearchforeclipsingbinariesamongAmstarssuggests
field in Ap stars modifies the diffusion rate in accordance that around 60–70percent of Am stars are spectroscopic
(cid:13)c 2002RAS
2 L. A. Balona, G. Catanzaro, O. P. Abedigamba, V. Ripepi, B. Smalley
binaries (Smalley et al. 2014), which is consistent with ra- answermightexplainwhytherearemanyslowly-rotatingA
dialvelocitystudies(Abt& Levy1985;Carquillat & Prieur stars which are not Am stars and provide further clues as
2007).Clearly,asubstantialnumberofAmstarsareproba- to thenatureof theAm phenomenon.
blysingleorinwidebinariesanditisnolongerclearwhether
binarity is a requirement for theAm phenomenon.
The effect of diffusion on pulsation of Am stars is to
2 THE DATA
reducethewidth of theinstability strip, with theblueedge
shifting towardstherededge, eventuallyleadingtothedis- There are 15 known Am stars in the Kepler field
appearanceofinstabilitywhenheliumissufficientlydepleted (Balona et al.2011c).Wealsoinclude14Amstarsobserved
from the HeII ionization zone (Turcotte et al. 2000). The by the Kepler K2 Campaign 0 mission. These data are far
amountofdrivingisrelatedtotheamountofHestillpresent less favourable to detecting rotational modulation because
inthedrivingzone.Thecoolerthestar,thedeeperthemix- only about 45d of data are available and the photometric
ing and the greater the He abundance in the driving zone. precision is severely compromised by the telescope motion.
Models therefore predict that Am stars should not pulsate However, The K2 data are useful in detecting δ Sct pulsa-
unless they are sufficiently cool. More recent calculations tions in these stars.
suggest that rotational mixing is important in maintaining For the vast majority of stars, Kepler photometry is
theHeabundancein thedrivingregion (Talon et al. 2006). available only in long-cadence (LC) mode which consist of
These ideas can be tested if we can obtain sufficiently practically uninterrupted 30-min exposures covering a time
accurate physical parameters to place pulsating and non- span of over three years. For a few thousand stars, includ-
pulsating Am stars in the H-R diagram. Smalley et al. ing all the Am stars discussed here, short-cadence (SC) 1-
(2011) studied over 1600 Am stars and found that around min exposures are also available, but these usually cover
200 Am stars are pulsating δ Sct and γ Dor stars. It seems only one or two months. Characteristics of LC data are de-
thatAmstars,ingeneral,seemtopulsateatsomewhatlower scribedinJenkinset al.(2010b)whileGilliland et al.(2010)
amplitudesthannormalδSctstars.However,Smalley et al. describe the characteristics of SC data. The science pro-
(2011) could find no real difference between the location of cessing pipeline is described by Jenkinset al. (2010a). The
pulsatingAmstarsandδSctstarsintheinstabilitystrip,ex- Kepler data are divided into quarters of approximately 90-
ceptforaslighttendencyofpulsatingAmstarstobecooler d duration which are numbered Q0, Q1, Q2, etc., Q0 be-
than normal δ Sct stars. Catanzaro & Balona (2012) also ing the initial 10-d commissioning run. In this study we
concluded that there is a tendency for pulsating Am stars use all available LC data (Q0 – Q17) from JD 2454953
tobecoolerthannormalδSctstarsandthatthepercentage to JD 2456424 (1471 d). These data are publicly available
ofpulsatingAmstarsisaboutthesameasthepercentageof on the Barbara A. Mikulski Archive for Space Telescopes
δSct stars in the instability strip. Similar conclusions were (MAST, archive.stsci.edu).
obtained by Balona et al. (2011c). It is clear that the pre- The light curve files contain simple aperture photome-
diction of diffusion theory regarding pulsation needs to be try(SAP)valuesandamoreprocessedversionofSAPwith
re-examined. The diffusion process, in fact, seems to play artifact mitigation included called Pre-search Data Con-
onlyaminorroleindeterminingthepulsationalstability of ditioning (PDC) flux. The SAP photometry contains dis-
AandearlyFstars.EvenApstarspulsateasδ Sctstarsat continuities, outliers, systematic trends and other instru-
low amplitudes (Balona et al. 2011a). mental signatures. The PDC tries to remove these errors
Recent high-precision photometric observations of A whilepreservingastrophysically interestingsignals byusing
starsusingKeplerhavefurtherunderminedthegeneralcon- a subset of highly correlated and quiet stars to generate a
sensusthatthesestarshavestableenvelopeswherediffusion cotrending basis vector set. The PDC flux uses the best fit
has a strong competitive advantage. It seems that about thatsimultaneouslyremovessystematiceffectswhilereduc-
2percentofAstars observedbyKeplershowflares(Balona ing the signal distortion and noise injection. Details of how
2012, 2014c). It can be easily demonstrated that the flares this is accomplished are described in Stumpeet al. (2012)
are associated with the A star and not with a presumed and Smith et al. (2012). Because we are interested in low-
cool companion.Furthermore,theKeplerdatasuggest that frequency variations, we have used the PDC fluxes in this
atleast40percentofAstarsarerotationalvariables(Balona analysis.
2013),presumablyduetostarspots.Sinceitisgenerallybe- TheKeplerSAPdatacontainsdriftsandjumps.There
lieved that Am stars do not have magnetic fields, the inci- is a zero-point difference between data in different quarters
denceofrotationalmodulationshouldbeabsentorsmallin andtheinstrumentaldriftvariesfromquartertoquarter.Al-
Am stars compared tonormal A stars. though the PDC data are designed to remove these effects,
The previous study of Kepler Am stars (Balona et al. itispossiblethatresiduallong-terminstrumentaleffectsare
2011c) used only data for approximately the first 50d and stillpresentat alowlevel.Tounderstandhowcorrection of
was mainly aimed at detecting δ Sct pulsations in these data may effect the low-frequency end of the periodogram,
stars. The low-amplitude light variations due to rotational weperformedasimplesimulationwhereweappliedourown
modulation weresuspectedbutcouldnotbestudied.There correction to the SAP data of a few stars which appeared
arenowfouryearsofalmost continuoushigh-precisionpho- to be the least variable among 20000 Kepler stars. To cor-
tometryforthesestars,makingiteasiertodetectrotational recttheSAPdatawefittedandremovedpolynomialsoflow
modulation.Inthispaperweinvestigaterotationalmodula- degree (typically degree 3 or 5) to individual quarters. The
tion in Am stars using Kepler data. Ouraim is toestablish polynomials do not join smoothly between any two quar-
whethertheincidenceofrotational modulation orotherac- ters, but we ignored these small jumps. To these data we
tivity is lower in Am stars than in normal A stars. The added sinusoidal variations comprising of 100 frequencies
(cid:13)c 2002RAS,MNRAS000,1–12
Spots on Am stars 3
of equal amplitudes and random phases. These frequencies
start at zero frequency and are equally spaced by 0.1d−1. 3429637
Theperiodogramrecoversallfrequencies,includingthelow-
1000
est frequency of 0.1d−1. The same simulation using a sig-
nalof0.02d−1recoversalowestfrequencyof0.08d−1.From
thissimulation,wemaydeducethatfrequencieshigherthan
0
about0.08d−1 arenotaffectedbythecrudecorrection pro-
5219533
cedure. The corrections used to obtain the PDC data are,
ofcourse,moresophisticatedandwecansafelyassumethat 200
frequencies higher than 0.08d−1 are easily recovered.
The light curves from the extended Kepler K2 Cam-
paign 0 mission were obtained from theraw pixel data files 0
using a suitable aperture. Our own software, keplc, was 7548479
used for this purpose. For fields where there were few or
1000
faint stars in close proximity to the target, we used the
largest possible aperture in order to minimize the instru-
mental light variation due to image motion. The telescope
0
motion can be found by tracking the centroid of the star. )
m
Thereisa4.08d−1 periodiccomponentintheimagemotion p 8881697
p
wthhisicfhreiqsureenflceyctaenddinallthiteslhigahrtmcounrivces.frWome tthheerelfiogrhetrceumrvoevesdo de ( 1000
astoenhancetheactualstellarsignal.Long-termdriftsand u
jumpswerealsoremoved.Theperiodogramoftheprocessed mplit 0
light curve was then calculated. Due to image motion and A 9204718
long-termdrifts,theperiodogramnoiselevelfortheK2data 50
increases strongly towards zero frequency. As a result, it is
generally not possible to detect low frequencies in the star
unless these are of high amplitude.
0
1000
9764965
3 KEPLER LIGHT CURVES
500
A list of known Am stars in the Kepler and K2 Cam-
paign 0 fields is shown in Table1. The identifications are 0
fromtheKeplerInputCatalogue(KIC,Brown et al.(2011)) 400
11402951
and from the K2 Ecliptic Plane Input Catalog (EPIC,
https://archive.stsci.edu/k2/KSCI-19082-002.pdf). The spectral classi- 200
fication for Am stars in the Kepler field are mostly quite
recentanddetailedmetalabundancesareavailableformost
of the stars. The Am nature of these stars is hardly in 0
doubt. The spectral classification for Am stars in the K2 11445913
500
field are far less secure. All of them are listed as Am stars
by Renson & Manfroid (2009), but the original reference is
oftenmissing.InTable1wehavereferencedwhatwebelieve
to be the original source, but sometimes the Am nature is
0
notevenmentioned.Partoftheproblemmaybeintheway
0 10 20 30 40 50
therecognition of theAm class has evolved with time.
-1
Examination of the light curves shows that of the 29 Frequency (d )
Am stars, there are 12 δ Sct variables, 1 γ Dor variable
and 2 detached eclipsing binaries. Most of the others have Figure1.Periodogramsofshort-cadencelightcurvesofAmstars
light curves similar to those generally attributed to rota- intheKeplerfieldwhichareδ Scutivariables.
tionalmodulation causedbystarspots.Periodograms ofthe
δ SctAmstars in theKepler fieldare shown in Fig.1.Peri-
odogramsoftheotherstarsintheKeplerfieldareshownin f(m) = 0.112 and a1 = 1.88×106km = 2.68R⊙. Kepler
Fig.2. observations confirm that this is an eclipsing binary with
It is evident from Fig.2 that highly significant, coher- P =1.540407dinadetachedAlgol-typesystemwithsini=
ent, low-frequency variations and harmonics are present in 0.96 (Slawson et al. 2011). The ratio of effective tempera-
allsixnon-δSctstars.InKIC3836439thiscanbeattributed tures T2/T1 = 0.758 and the sum of radii R1+R2 =0.35a
to proximity effects in a binary. Luyten (1936) found the whereaisthesemi-major axis.Slightlydifferentresultsare
star to be a spectroscopic binary with Porb = 1.54039d, given by Prˇsa et al. (2011): sini = 0.95, T2/T1 = 0.48 and
semi-amplitude K = 88.7 ±2.8kms−1 and a circular or- R1+R2 =0.37a. Itis possible that theAm classification is
bit. Batten et al. (1978) further lists the mass function a result of a composite spectrum.
(cid:13)c 2002RAS,MNRAS000,1–12
4 L. A. Balona, G. Catanzaro, O. P. Abedigamba, V. Ripepi, B. Smalley
Table1.ListofAmstarsintheKeplerfield(top)andtheK2Campaign0field(bottom).ThestarsareidentifiedbytheirKICnumber
andtheirEPICnumberrespectivelywhilethesecondcolumnistheHDnumber.Thespectralclassificationisgiveninthethirdcolumn.
Theapparentmagnitudeandthederivedeffectivetemperature, Teff,aremostlyfromtheKICexcept wherenoted. Thesurfacegravity,
logg and projected rotation velocity, vsini, are from Uytterhoeven etal. (2011) except where noted. The variable star classification is
estimated from the light curve and periodogram. The rotation period of the star (Prot in days) and the rotational light amplitude (A
in ppm) is estimated from the periodogram. References are as follows: 1 - Abt (1981), 2 - Abt (1984), 3 - Ammonsetal. (2006), 4 -
Antocietal. (2011), 5 - Bertaud (1960), 6 - Catanzaroetal. (2011), 7 - Catanzaro&Ripepi (2014), 8 - Catanzaroetal. (2015), 9 -
Floquet(1970),10-Floquet(1975),11-(Halbedel1985),12-McCuskey(1967),13-McCuskey(1959),14-Renson&Manfroid(2009).
KIC/EPIC HD Sp.Type V Teff logg vsini Type Prot A
(K) (dex) (kms−1) (d) (ppm)
3429637 178875 kF2hA9mF32 7.71 71006 4.00 50 DSCT
3836439 178661 kA2.5hF0mF01 7.57 7166 3.90 EA
5219533 226766 kA2hA8mA82 9.20 7409 3.90 115 DSCT
7548479 187547 A4Vm4 8.40 75004 3.90 10 DSCT/ROT 0.6530 150
8323104 188911 A2mF010 9.66 7587 3.90 ROT 6.0447 7
8703413 187254 A2mF010 8.71 79607 3.807 157 ROT 6.5292 215
8881697 A5m5 10.57 7732 3.90 DSCT/ROT 0.4319 200
9117875 190165 A2mF210 7.51 74607 3.607 587 ROT 1.4949 42
9204718 176843 A3mF010 8.79 76107 3.807 277 DSCT/ROT 8.7308 90
9272082 179458 A5m9 9.00 81507 3.907 757 ROT 0.9703 4
9349245 185658 Am:8 8.15 83008 4.008 808 ROT 0.9420 7
9764965 181206 Am:8 8.85 78008 3.888 878 DSCT/ROT 0.4872 135
11402951 183489 kF3hA9mF52 8.13 71506 3.50 100 DSCT
11445913 178327 kA7hA9mF52 8.49 72006 3.50 51 DSCT
12253106 180347 Am8 8.41 79008 3.858 128 ROT 4.1043 82
202059291 40788 A212,A2m14 9.04 75763 SB2 2.984 188
202059336 43509 Am8 8.90 79008 3.978 288
202061329 250408 A212,A2m14 10.36 83293
202061333 250786 A712,A5m14 10.40 74313 DSCT
202061336 251071 F0V12,A3m14 10.34 73163
202061338 251150 A212,A0m14 10.57 79593 DSCT
202061349 251947 A3p:12,A2m14 10.95 65613
202061353 252154 F2V12,A5m14 9.85 69253 DSCT
202061363 A0m..13 11.00 45693
202062436 A2-F012 11.64 6500 DSCT
202062447 251717 A3-F212 11.20 73963 GDOR
202062450 256320B kA5hA8mF511 10.80
202062454 251812 A5-F2:12 11.61 67503
202062455 251463 A5-F212 10.70 62203
In theother stars, thelow-frequency variations are un- tude and the presence of an harmonic (Balona 2013). The
likely to be due to proximity effects in a binary because presumedrotationperioddecreaseswithvsini,asmightbe
the amplitudes often vary and the periodogram lines at- expected.
tributed to rotation are broadened in some stars. This is
KIC8703413 was first reported as an Am star by
tobeexpectedinarotationalvariablebecausespotschange
Mendoza (1974) and subsequently classified as kA2mF0
in size and intensity with time. Differential rotation, which
by Floquet (1975). The star was confirmed as Am by
appears to be a general feature in A stars (Balona 2013),
Catanzaro & Ripepi (2014). As can be seen in Fig.4, the
will also cause such broadening. As an example of rota-
Kepler light curve shows distinct beating and traveling
tional modulation, Fig.3 shows part of the long-cadence
wave characteristic of star spots (Uytterhoevenet al. 2011;
PDC light curve of KIC8323104. Quasi-regular light vari-
Balona 2013). The periodogram shown in the middle panel
ability is present with a peak-to-peak amplitude of about
40ppm and a timescale of around 6d. This cannot be a has a main peak at f1 = 0.1532d−1 and its harmonics.
result of proximity effects in a binary because of the irreg- In addition, there is another peak f2 = 0.1404d−1 which
looks to be a bit broader. It is not clear how this pattern
ularity of the variations. The most plausible explanation is
maybeinterpreted,butitseemsreasonabletosupposethat
that it is a result of rotational modulation due to stellar
activity. either f1 or f2 could be the rotation frequency. Perhaps
both frequencies could be ascribed to starspots if the star
The second-last column of Table1 shows the rotation is in differential rotation. No other frequencies are present.
periodestimatedfromtheperiodogram.Inthecaseofδ Sct The projected rotational velocity, vsini = 15±2kms−1,
starsthereareoften manyotherlow-frequencypeakswhich is compatible with the rotation frequency, giving a radius
are always present in any δ Sct star. In this case we identi- Rsini≈1.96R⊙.Therearesevenradialvelocitiesinthelit-
fied the rotation peak because of its relatively high ampli- erature(Fehrenbachet al.1997),whichisnotenoughforan
(cid:13)c 2002RAS,MNRAS000,1–12
Spots on Am stars 5
3836439
0.5
2000 pt)
p
p ( 0
K
0 ∆
8323104 -0.5
5
0 50 100 150 200
Time
0
200
8703413 )
pt 0.2
p
(
e
d
u
m) 0 plit 0.1
m
pp 40 9117875 A
(
e 0
ud 20 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
plit Frequency (d-1)
m
0
A
9272082
5
)
pt 0.5
p
(
p
K
0 ∆
9349245 0
5
-2 -1 0 1 2 3 4
Time (hr)
0
12253106 Figure4.Toppanel:partoftheKeplerlong-cadencelightcurve
50 of KIC8703413 showing traveling features typical of starspots.
Middle pane: periodogram of all LC data. Bottom panel: pre-
sumedflareatJD2455164.88seeninSCdata.
0
0 1 2 3 analysis. The velocity range is quitelarge and it is possible
Frequency (d-1) that thestar may be a binary.
KIC8703413 is a flare star (Balona 2012) as can be
seen in the bottom panel of Fig.4. The flare intensity is
Figure2.Periodogramsoflong-cadencelightcurvesofAmstars
about 0.7ppt. The A star has a luminosity of about 100
intheKeplerfieldwhicharenotδ Sctvariables.
times larger than a K dwarf, which means that if the flare
is attributed to an unseen K dwarf companion, its rela-
40 tive intensity would have to be about 70ppt. The mean
flare intensities in Kepler field K-dwarfs are about 3.3ppt
) 20 (Walkowicz et al. 2011), which means that this particular
m
flarewouldhavetobeofunusualintensity.Ingeneral,flares
p
p 0 in A stars cannot be attributed to a cool companion since
(
k p theaverageflareintensityisabouttwoordersofmagnitude
∆ -20 larger than that in a typical K or M dwarf.
For KIC3429637, Catanzaro et al. (2011) obtained
-40 Teff = 7300±200K, logg = 3.16±0.25 and vsini = 50±
0 20 40 60 80 100 5kms−1, values later confirmed by Murphyet al. (2012).
Time (d) The remarkable amplitude increase in the δ Sct pulsations
of this star has been discussed by Murphyet al. (2012).
Since that time, additional Kepler data show a more com-
Figure 3. Portion of the corrected Kepler light curve of
plex amplitude variation for the frequencies of largest am-
KIC8323104showingregularvariations.
plitude (Fig.5, top panel). While f1 = 10.337d−1 and
(cid:13)c 2002RAS,MNRAS000,1–12
6 L. A. Balona, G. Catanzaro, O. P. Abedigamba, V. Ripepi, B. Smalley
f3 =10.936d−1 showsteadyamplitudechanges,thesteady
f1 amplitude increase for f2 = 12.472d−1 reverses and de-
ppt) 2 ff23 cinredaicsaetseshthaarptltyhaeraomunpdlitJuDde24o5f5f9250is.,Tohnecelaatgeastino,binsecrrveaatsiionngs.
e ( Thetimescale seemstobetooshort foranevolutionary ex-
d planation, as suggested by Murphyet al. (2012).
u
plit 1 KIC5219533 has avery rich δ Sct frequency spectrum,
m including a large number of modes in the γ Dor range.
A The projected rotational velocity is vsini = 115kms−1.
0 The principal peak at f1 = 10.2853d−1 is clearly variable
in frequency. The phase, φ, varies smoothly in the range
0 500 1000 1500
2.0 < φ < 4.0radians over 400d which corresponds to ap-
JD 2454950+ proximately 0.0008d−1.
3.15 A recent abundance analysis of KIC9117875 by
f1 Catanzaro & Ripepi (2014) indicates Teff = 7400±150K,
logg = 3.6±0.1dex, and vsini = 58±6kms−1. Both Ca
) and Sc show underabundances of about 1 dex, while the
s 3.1
n heavy elements are all overabundant: 0.5 dex for iron-peak
a
di elementsandabout2dexforBa.TheAmnatureofthisstar
ra isthusconfirmed.TheKeplerlightcurveshowsbeatingand
(
φ 3.05 low frequencies resembling a γ Dor star and it was classi-
fiedassuchbyUytterhoevenet al.(2011).However,amore
detailedexaminationoftheperiodogramshowsabroadfea-
tureataboutf1 =0.672d−1withaverysharpneighbouring
0 500 1000 1500 peakatf2 =0.7157d−1.Severalharmonicsoff2 arevisible,
JD 2454950+ which is quite unlike the structure found in typical γ Dor
1 stars.Infact,thismysteriousstructureisfoundinaconsid-
f erablefractionofAstars,butisunexplained(Balona2013).
2
Balona(2014c)suggeststhatthesharpfeaturemightbeex-
0.5
plained by a synchronously orbiting planet, not necessarily
)
s
n transiting. Assuming that f1 is the rotation frequency and
a
di 0 using the above projected rotational velocity leads to a ra-
ra diusRsini≈1.69R⊙, which is reasonable for an A star.
(
φ There is a further mystery regarding this star in that
-0.5
it appears to flare. In Fig.7 we show examples of possible
flaresinthisstarin theKeplerLCdata.Itshouldbenoted
-1 thatLCdataisnot idealtodetectflaressincetheexposure
0 500 1000 1500 time of 30min is a considerable fraction of the duration of
theflare.Short-cadencedata areavailable for thisstar, but
JD 2454950+
only for about 10d during Q0.
60
KIC9204718 was confirmed as an Am star by
40 Catanzaro & Ripepi (2014) who find Teff = 7600±150K,
logg = 3.8 ± 0.1dex and vsini = 27 ± 3kms−1. There
20
are clearly several low-frequency peaks in theperiodogram,
s) 0 as shown in Fig.8. The largest peak is at 0.1145d−1 and
(
τ its harmonic. These peaks are both rather broad suggest-
-20 ing amplitude or frequency modulation; in fact, the light
variation with this frequency is clearly visible in the light
-40
curve (Fig.8). This variation is suggestive of a starspot, in
-60 whichcase0.1145d−1 wouldbetherotationfrequency.This
0 0.2 0.4 0.6 0.8 1 1.2 1.4 is reasonable since we measured vsini=27±3kms−1.
Even more interesting is the presence of several har-
Phase
monics of 0.5474d−1. If we remove the 0.1145d−1 and the
δSctfrequenciesfromthedata,whatisleftisthe0.5474d−1
Figure 5. Top panel: amplitude variation of the three highest
variation anditsharmonics.By binningthelight curveand
amplitudepulsationmodesinKIC3429637fromKeplerphotom-
averagingthedataineachbin,weobtainedthephasedlight
etry.Thenextpanelsfromthetopshowsthephasevariationoff1 curveshown in thebottom panelof Fig.8.Thereis nosim-
andf2.Inthe bottom panel thetimedelay variation,τ,derived
pleexplanationforthisvariation.Itcannotbetherotational
fromf1andf2arephasedwiththeorbitalperiodof704.7d.The
solid line is from the orbital solution. The time of phase zero is frequency, since this has already been identified. The light
JD2454950.0. curve resembles that of a contact binary, but if so it im-
pliesthattherotationalperiodisnotsynchronizedwiththe
orbital period.
(cid:13)c 2002RAS,MNRAS000,1–12
Spots on Am stars 7
4
400
) 200 3.5 5112.43
m
p
p 0
p ( 3 5347.58
K
∆-200
2.5
-400
)
pt
p
0 20 40 60 80 100 ( 2
y
Days sit 5762.89
n
50 e 1.5
nt
I
) 40 5768.88
m
p 1
p
( 30
e 5779.03
d
plitu 20 0.5
m 6011.86
A 10 0
0
0 0.5 1 1.5 2 2.5 3 -2 -1 0 1 2 3
Frequency (d-1) Time (hrs)
50
Figure 7.PossibleflaresinLCdataofKIC9117875. Thelabels
givetheJDofpeakflareintensityrelativetoJD2450000.00.
)
m
p 0
(p panel). There is a constant frequency f1 =1.03058d−1 and
p its harmonic, suggestive of rotational modulation. The cor-
K
∆ rectedLCdataphasedwith thisfrequencyshowsaroughly
-50 sinusoidal variation (Fig.9), which we assume to be rota-
tional modulation.
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Phase 4 BINARY MOTION FROM TIME DELAY
ItisgenerallythoughtthatmostAmstarsarebinarieswith
Figure6.Toppanel:partoftheKeplerlong-cadencelightcurve
orbitalperiodsintherange1–10d(Abt1967).Itisofinter-
ofKIC9117875.Themiddlepanelistheperiodogramshowingthe
est todeterminewhetherany ofthestarsin oursample are
broadfeature,f1 andthesharppeakf2anditsmanyharmonics.
spectroscopic binaries. A pulsating star in a binary system
Thebottom panelshowsthelightcurvewhenallfrequenciesex-
ceptforf2anditsharmonicshavebeenremoved.Thedatapoints acts as a clock. If the star is a member of a binary sys-
havebeenaveragedforclarity. tem, its distance will vary as it orbits the barycentre. The
changingdistanceleadstoavariablephaseofpulsation.The
method has been discussed by Shibahashi & Kurtz (2012)
Thereisagreatdealofuncertaintyregardingthenature using data transformed to the frequency domain. More re-
of KIC9272082. Macrae (1952) noted its possible peculiar cently,Murphy et al. (2014) has used themore direct anal-
spectrum,butdidnotgiveanydetails.Bertaud(1960)gives ysis of phase variation. Application to δ Sct stars is diffi-
aclassification ofA5m,butFloquet (1970) classified it asa cultbecauseofthemanyclosefrequencieswhichdistortthe
normal A7 star. The star is included as an Am star in the phase variation. By direct minimization of the photometric
Renson & Manfroid (2009) catalogue. Catanzaro & Ripepi error,itispossibletoconstructadiagramsimilartothepe-
(2014) found it to bean A4main sequencestar with Teff = riodogram where the orbital semi-major axis is shown as a
8500±200K,logg=3.9±0.1dexandvsini=75±7kms−1. function of the orbital period (the binarogram). This tech-
Mostoftheelementsinthisstararesomewhatoverabundant niqueallows relatively quick detection of binary candidates
by about 0.5–1.8 dex. However, Si, Cr, Ni, and Ba have (Balona 2014a).
normalabundances.Althoughtheabundancepatternisnot KIC3429637hasbeendiscussedaboveinthecontextof
typical, it does not conform to that expected for Am stars. its δ Sct pulsations. Fig.5 also shows the phase variations
The light curve (top panel of Fig.9) shows distinct of f1 and f2. We used a window with semi-width of 20d
variations which areconfirmed bytheperiodogram (middle and a sampling interval of 20d. The dominant frequency,
(cid:13)c 2002RAS,MNRAS000,1–12
8 L. A. Balona, G. Catanzaro, O. P. Abedigamba, V. Ripepi, B. Smalley
40
)
m 30
p ) 20
p m
e ( 20 pp
d (
u p 0
plit K∆
m 10
A
-20
0
0 0.5 1 1.5 2 2.5 3 0 50 100 150 200 250
Frequency (d-1) Days
8
400
)
m 6
) 200 p
m p
(pp 0 de ( 4
p u
K∆-200 plit
m 2
A
-400
0
0 50 100 150 200 0 1 2 3
Days Frequency (d-1)
20
5
) )
m m
pp 0 pp 0
( (
p p
K K
∆ ∆
-5
-20
0 0.2 0.4 0.6 0.8 1 1.2 1.4 0 0.2 0.4 0.6 0.8 1 1.2 1.4
Phase Phase
Figure 8.Toppanel: periodogramofKIC9204718 showingsev- Figure9.Toppanel:portionofthecorrectedKeplerlightcurve
erallowfrequencies.Middlepanel:partofthelightcurveshowing of KIC9272082. The light curve has been smoothed by a Gaus-
the dominant 0.1145d−1 frequency. Bottom panel: phased and sianfilterwithFWHM=0.5dforbettervisibility.Middlepanel:
averagedlightcurveshowingthe0.5474d−1 lightvariation. periodogram of the LC data. Bottom panel: binned light curve
phasedwithfrequencyf1=1.03058d−1.
f1,showsaclearperiodicvariationwithP =704.7d.There
isarathersuddenchangeinphaseoff2 (correspondingtoa
changeinfrequency)atatimesoonafterthesharpdecrease we obtain the orbital solution shown in Table4 and shown
inamplitudeshowninthetoppanel.Closeinspectionreveals in the bottom panelof Fig.5.
thatthe704.7-dperiodisalsopresentinthephasevariation ForKIC11445913shortcadenceobservationsshowthat
off2.Short-cadenceobservations areavailablefor thisstar, thedominantpeaksareatf1 =31.5578, f2 =25.3771, f3 =
so there is no ambiguity regarding thefrequencies. 22.1330 and f4 = 37.8204d−1. As usual there are quite a
Weappliedthetime-delayalgorithmdescribedaboveto numberoflow-frequencypeaks,butnonethatcouldbeiden-
both f1 and f2. Because there is a strong sudden decrease tified as due to starspots. The value vsini=51±1kms−1
ofamplitudeandfrequencyaround JD2455950, wedecided (Balona et al. 2011c). Time delay analysis of f1, f2 and f3
to use only the data up to this date. There is still a long- shows that theyhavethesame phasevariation, but thepe-
term trend which was removed by fitting a Fourier series riod, if it exists, is considerably longer than the almost 4yr
combined with a polynomial of third degree. The derived duration of the Kepler data. Fig.10 shows the resulting ra-
time delay variation for f1 and f2 are in good agreement dial velocities derived from the first derivative of the time
and both are shown in the bottom panel of Fig.5 phased delay variation. This star might be a binary with a period
with the orbital period. By fitting the time delay variation of around 10yror longer.
(cid:13)c 2002RAS,MNRAS000,1–12
Spots on Am stars 9
Table 2. Binary elements of KIC3429637 from the time delay
of the Kepler light curve. The orbital period Porb, half-range of 1 202061333
primaryradial velocity variation K1, eccentricity e,longitude of
periastronω,JDofperiastronTper,projectedsemi-majoraxisof 0.5
theprimarya1sini,andmassfunctionf(M).
0
Parameter Value Units
202061338
Porb 704.7±4.4 days
K1 1.79±0.08 kms−1 pt) 0.5
e 0.61±0.04 p
(
ω 3.05±0.02 radians e
d
Tper 2455204.6±3.6 u 0
a1sini 0.100±0.005 AU plit 6 202061353
f(M) (2.69±0.03)×10−4 M⊙ m
A 4
2
0
f 0.2 202062436
2 1
f
2
f
) 3 0.1
1
-s 1
m
k 0
(
V r 0 0 5 10 15 20
-1
Frequency (d )
-1
Figure 11.Periodogramsof Am δ Sct stars observed inthe K2
500 1000 1500
Campaign0.
JD 2454950.00+
Figure 10. Radial velocity curves derived from frequencies f1,
f2 andf3 inKIC11445913.
peratureof EPIC202062436 was obtained through our own
analysis of themulticolour photometry.
5 K2 LIGHT CURVES
EPIC202061353 isaknownδ Sctstarfrom SuperWasp
Owing to the failure of two of the four reaction wheels, the photometry (Smalley et al. 2011). This is confirmed by the
Keplerspacecrafthaslostitspointingability.Thetelescope K2 data (Fig.11). The main feature is a triplet of low-
drift is corrected at regular intervals, with the result that frequencypeaksat1.452,0.980and2.000d−1 whicharenot
there is substantial image drift and a large increase in pho- quiteequally spaced.
tometric error from about 20 ppm for the original Kepler The periodogram of EPIC202059291 shows only two
field to about 300ppm for the K2 field. Here we present significant peaks. Considering the very low amplitude, the
results for Am stars observed in the first K2 scientific mis- variabilityisbestunderstoodasrotationalmodulationwith
sion, Campaign 0. The observations occur over a timespan period P = 2.984d, but it could also be interpreted as a
of about 77d, but with a large gap of 26.5d. The total on- contact binary. One of us (GC) obtained a single spectrum
target time is about 47d. This rather short time and the ofthisstarwhichappearstobeadouble-linedspectroscopic
much increased scatter severely compromises detection of binary. It can probably be ruled out as an Am star. The
significant frequencies less than about 3d−1. periodogram and phased light curveis shown in Fig.12.
Many of these stars were observed by the STEREO ThelightcurveofEPIC202062447(Fig.13)showsthat
spacecraft. No significant variability could be found it is a γ Dor of the ASYM type (Balona et al. 2011b). The
for EPIC202059291, 202059336, 202061329, 202061333, periodogramshowsdominantperiodsof0.912dand0.773d.
202061336, 202061338 and 202061363 from STEREO ob- The field of EPIC202062450 contains two stars of al-
servations Paunzen et al. (2013). The remarkably low most equal brightness separated by 5arcsec. The brighter
effective temperature for EPIC202061363 reported in star,FIOri,isadetachedeclipsingbinarywithP =4.4478d
Ammonset al. (2006) is confirmed by our own analysis of and spectral type K0III/IV (Halbedel 1985). The eclipses
available multicolourphotometry.STEREOobservations of are very easily seen in the K2 combined light of the two
EPIC202061349 alsodonotshowanysignificantvariability stars. The fainter star, HD256320B has a spectral type
(Wraight et al.2012).Ofthesestars,wedetectedvariability kA5hA8mF5 (Halbedel 1985). We constructed an aperture
inEPIC202059291,202061333andEPIC202061338(seebe- which only included this star, but could not find any ev-
low).SomeofthestarsareδSctvariables.Theperiodograms idence of periodic variation with an amplitude exceeding
of the δ Sct stars are shown in Fig.11. The effective tem- 5ppt.
(cid:13)c 2002RAS,MNRAS000,1–12
10 L. A. Balona, G. Catanzaro, O. P. Abedigamba, V. Ripepi, B. Smalley
2 6 DISCUSSION AND CONCLUSION
Itisevidentfrom Table1thatmostAmstarsintheKepler
pt) 1.5 fieldexhibitrotational modulation.Anideaofthespotsize
p
( may be gained from the light amplitude of the modulation
de 1 shown in Table1.There areonly 10Am starsin theKepler
u
plit firaenldgekinsolawrnge.toThbeemroeatantaiomnpallitvuadreiaibsl9es3±a2n5dptphme.aTmhpelmitueadne
m
0.5
A rotational amplitude from 875 A stars in Balona (2013) is
541±86ppm.This seems to suggest that, in general, spots
0 on Am stars are smaller than on normal A stars, but it is
0 0.2 0.4 0.6 0.8 1 1.2 notpossibletodrawadefiniteconclusionowingtothesmall
Frequency (d-1) sample.
One or more flares appear to be seen in two confirmed
Am stars, KIC8703413 and KIC9117875. The relatively
4
large flare intensities imply that if the origin of the flare
is a cool companion, the flare must be about two orders of
2
pt) magnitudemoreintensethanonanisolated coolstarowing
(p 0 to the much larger luminosity of the Am star. While this
p is not impossible, it seems veryimprobable. It is clear from
K
∆ -2 a study of a much larger number of A stars that the flares
cannot allbeattributedtoa cool companion (Balona 2012,
-4 2013).
The presence of starspots and flares on Am stars sug-
0 0.2 0.4 0.6 0.8 1 1.2 1.4 gests that Am stars may have significant magnetic fields
similar in strength to normal A stars. The magnetic fields
Phase
in both Am and normal A stars are sufficiently intense to
form starspots and flares. Hence the idea that the superfi-
Figure 12. Top panel: periodogram of EPIC202059291. Light
cialmetalenrichmentinAmstarsisaresultoftheeffectsof
curvephasedwithperiodP =2.984d.
diffusionandgravitationalsettlingintheabsenceofamag-
neticfieldmayneedtoberevised.Furthermore,therelative
numberofpulsatingAmstarsissimilartotherelativenum-
40
ber of δ Sct stars among the A star population. Am stars
which are δ Sct variables occupy the same region of the in-
stabilitystripasnormalδSctstars(Smalley et al.2011).All
pt) 20 these observations are contrary to the view that Am stars
p
( areformedbydiffusionofelementsintheabsenceofamag-
p
K neticfield andthat δ Sct pulsation in Am stars onlyoccurs
∆ 0 near thered edge of the instability strip. Given these facts,
one has to conclude either that the diffusion model for Am
stars needs to berevised or that it is incorrect.
Thermohaline convection takes place when a layer of
720 740 760 780 800 enhancedmetalabundanceisformed abovelayersoflighter
Time(d) mean molecular weight. This happens in the case of accre-
tion of predominantly metal-rich material (Vauclair 2004),
and also when such a layer is formed due to atomic diffu-
6
sion. This important effect has only recently been studied.
)
pt Th´eado et al. (2009) finds that accumulation of heavy ele-
p
( ments due to atomic diffusion is attenuated when thermo-
e 4
d haline mixing is taken into account, though not completely
u
plit suppressed.Theinclusion of thethermohaline instability in
m 2 A-star models needs to be more fully investigated to see
A whether this can reconcile the diffusion model of Am stars
with observations.
0 ThepresenceofpulsationsinAmstarsisonlyonesymp-
0 1 2 3 4 tomofamoregeneralproblemwhich hasbeenencountered
Frequency (d-1) sincetheadventofKeplerphotometry.Forexample,itisnot
understood whyless thanhalf of stars in theδ Sct instabil-
itystrippulsate(Balona & Dziembowski2011),orwhylow-
Figure 13. Top panel: light curve of EPIC202062447 showing
frequency pulsations are a general feature of all δ Sct stars
typical asymmetric light curve of a γ Dor star of type ASYM.
(Balona 2014b). Further exploration of pulsational driving
Thebottom panelshowstheperiodogram.
needs to be undertaken. For example, coherent pulsations
(cid:13)c 2002RAS,MNRAS000,1–12
