A&A 535, A3 (2011)
DOI: 10.1051/0004-6361/201117230
c ESO 2011
Astronomy
&
Astrophysics
SuperWASP observations of pulsating Am stars
B. Smalley1, D. W. Kurtz2, A. M. S. Smith1, L. Fossati3, D. R. Anderson1, S. C. C. Barros4, O. W. Butters5,
A. Collier Cameron6, D. J. Christian7, B. Enoch6, F. Faedi4, C. A. Haswell3, C. Hellier1, S. Holmes3, K. Horne6,
S. R. Kane8, T. A. Lister9, P. F. L. Maxted1, A. J. Norton3, N. Parley6, D. Pollacco4, E. K. Simpson4, I. Skillen10,
J. Southworth1, R. A. Street9, R. G. West5, P. J. Wheatley11, and P. L. Wood1
1
Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK
e-mail: bs@astro.keele.ac.uk
2
Jeremiah Horrocks Institute of Astrophysics, University of Central Lancashire, Preston PR1 2HE, UK
3
Department of Physics & Astronomy, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
4
Astrophysics Research Centre, Main Physics Building, School of Mathematics & Physics, Queen’s University, University Road,
Belfast, BT7 1NN, UK
5
Department of Physics & Astronomy, University of Leicester, Leicester, LE1 7RH, UK
6
SUPA, School of Physics & Astronomy, University of St. Andrews, North Haugh, Fife, KY16 9SS, UK
7
Department of Physics & Astronomy, California State University, Northridge, CA, 91330, USA
8
NASA Exoplanet Science Institute, Caltech, MS 100-22, 770 South Wilson Avenue, Pasadena, CA, 91125, USA
9
Las Cumbres Observatory Global Telescope Network, 6740 Cortona Drive, Suite 102, Goleta, CA, 93117, USA
10
Isaac Newton Group of Telescopes, Apartado de Correos 321, 38700 Santa Cruz de la Palma, Tenerife, Spain
11
Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
Received 10 May 2011 / Accepted 30 June 2011
ABSTRACT
We have studied over 1600 Am stars at a photometric precision of 1 mmag with SuperWASP photometric data. Contrary to previous
belief, we find that around 200 Am stars are pulsating δ Sct and γ Dor stars, with low amplitudes that have been missed in previous,
less extensive studies. While the amplitudes are generally low, the presence of pulsation in Am stars places a strong constraint on
atmospheric convection, and may require the pulsation to be laminar. While some pulsating Am stars have been previously found to
be δ Sct stars, the vast majority of Am stars known to pulsate are presented in this paper. They will form the basis of future statistical
studies of pulsation in the presence of atomic diffusion.
Key words. asteroseismology – stars: chemically peculiar – stars: oscillations – stars: variables: delta Scuti – techniques: photometric
1. Introduction
In the region of the Hertzsprung-Russell (HR) diagram where
the Cepheid instability strip extends across the main sequence,
there is a complex relationship between stellar pulsation and
atmospheric abundance anomalies that is not fully understood.
This region ranges from the early A stars to mid-F stars in spectral
type, and from the zero age main sequence to the terminal
age main sequence in luminosity. Found here are the strongly
magnetic chemically peculiar Ap and Fp stars, the non-magnetic
metallic-lined Am stars, the rarer metal-deficient λ Boo stars,
the pulsating δ Sct stars, γ Dor stars and rapidly oscillating
Ap (roAp) stars. Much has been written about these stars and
their physics, which we briefly summarise here. For more detailed
discussions see Joshi et al. (2006), Kurtz & Martinez
(2000) and Kurtz (1989, 1978, 1976).
Most stars in the main-sequence region of the instability strip
are normal abundance δ Sct stars with relatively high rotational
velocities – usually v sin i ≥ 100 kms−1
. A large fraction of
A stars are Am stars, peaking at around 50 per cent at A8, but
Am stars are believed either not to pulsate as δ Sct stars, or may
do so with much smaller amplitudes than the normal abundance
δ Sct stars. Am stars are mostly found in short period binary
An extended version of Table 1 containing all the detected frequencies
and amplitudes is only available at the CDS via anonymous ftp to
cdsarc.u-strasbg.fr (130.79.128.5) or via
http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/535/A3
systems with orbital periods between 1−10 d, causing synchronous
rotation with v sin i ≤ 120 km s−1
(Abt 2009); a few
single Am stars with similar slow rotation are known.
The magnetic Ap stars are rarer, constituting less than
10 per cent of the A stars. They have very strong global magnetic
fields and are often roAp stars with high overtone p mode
pulsations with much shorter periods than the δ Sct stars. No
Ap star is known to be a δ Sct star. Our physical understanding
is that atomic diffusion – radiative levitation and gravitational
settling – stabilises the slowly rotating Am and Ap stars
so that low overtone p modes are not excited; particularly important
in this context is the gravitational settling of helium
from the He ii ionisation zone where the κ-mechanism drives
the pulsation of δ Sct stars (see Aerts et al. 2010). Otherwise,
the more rapidly rotating stars remain mixed because of turbulence
induced by meridional circulation and are excited by the
κ-mechanism (Turcotte et al. 2000).
The understanding of the relationship of the long-established
δ Sct stars to the more recently discovered γ Dor stars is currently
in flux. Previously, the δ Sct stars were known as p mode
pulsators, while the γ Dor stars were known as g mode pulsators.
The instability strips for these classes of stars partially overlap,
and some “hybrid” stars were discovered with pulsation in
both p modes and g modes. A striking case is that of HD 8801,
which is an Am star that shows both δ Sct and γ Dor p-mode and
g-mode pulsation (Henry & Fekel 2005).
Article published by EDP Sciences A3, page 1 of 32
A&A 535, A3 (2011)
Hybrid stars that show both p modes and g modes are of particular
interest asteroseismically because the p modes characterise
the conditions primarily in the outer part of the star, while
the g modes test the core conditions. Now with data from the
Kepler Mission, which is obtaining nearly continuous data for
over 150 000 stars for 3.5 y, mostly with 30-min cadence, but for
512 stars with 1-min cadence (Gilliland et al. 2010), the Kepler
Asteroseismic Science Consortium (KASC) is studying numbers
of δ Sct stars and γ Dor stars at μmag precision. It is becoming
clear that hybrid stars are common and may be the norm, so that
the classes of δ Sct and γ Dor stars are merging (Grigahcène et al.
2010). Interestingly, the latter authors find a possible correlation
among the hybrid stars and Am spectral classification.
The Kepler Mission through KASC will model individual
Am stars that are δ Sct pulsators with data of such high precision
that new insight into the physics of the relationship between
atomic diffusion and p mode pulsation will be obtained.
But Kepler has a limited number of Am stars in its 105 deg2
field-of-view. Another complementary source of information is
to look at the statistics of pulsation in Am stars over the entire
sky. That is now possible with the highly successful SuperWASP
planetary transit-finding programme (Pollacco et al. 2006) that
has surveyed a large fraction of both the northern and southern
skies. There now exists in the SuperWASP archive over 290 billion
photometric measurements for more than 30 million stars.
These light curves encompass many types of stars, including the
A stars in general, and Am stars in particular.
In this paper we have selected Am stars from the Renson &
Manfroid (2009) catalogue of peculiar stars for which we have
at least 1000 data points in SuperWASP light curves. While we
do not detect pulsation in all of our programme stars, for around
200 metallic-lined stars out of over 1600 tested we find δ Sct pulsation.
This is contrary to previous understanding that Am stars
are constant in brightness. The reason we have gained this new
understanding is that there has been no previous survey of so
many Am stars, and previous studies have not all reached the
SuperWASP detection threshold of only 1 mmag.
Many Am stars therefore do pulsate, generally with lower
amplitude than normal abundance δ Sct stars. This amplitude difference
is still to be understood in terms of atomic diffusion reducing
pulsation driving for the slowly rotating Am stars, but
there is not a complete lack of pulsation. That, has implications
for turbulence in the diffusive layers and may require that the
pulsation be laminar. Some striking examples of metallic-lined
stars with relatively high pulsation amplitude (these are rare)
address this question further, such as HD 188136 (Kurtz 1980;
Wegner 1981) and HD 40765 (Kurtz et al. 1995). More constraints
on the physics of the interaction of pulsation and atomic
diffusion may also be found in stars that show no δ Sct p modes
or γ Dor g modes at precisions of μmag. Some such A stars are
known in the CoRoT and Kepler data sets, but in-depth studies
have not yet been made, hence discussions of these have yet to
be published.
The combination of the all-sky mmag precision of
SuperWASP with the μmag precision of CoRoT and Kepler on
selected stars, calls for new attempts to model the physics of
the interaction of pulsation, rotation and atomic diffusion in the
A stars.
2. Observations
The WASP project is surveying the sky for transiting extrasolar
planets (Pollacco et al. 2006) using two robotic telescopes, one
at the Observatorio del Roque de los Muchachos on the island of
La Palma in the Canary Islands, and the other at the Sutherland
Station, South African Astronomical Observatory (SAAO). Both
telescopes consist of an array of eight 200-mm, f/1.8 Canon
telephoto lenses and Andor CCDs, giving a field of view of
7.8◦
× 7.8◦
and pixel size of around 14 . The observing strategy
is such that each field is observed with a typical cadence
of the order of 10 min. WASP provides good quality photometry
with a precision exceeding 1 per cent per observation in the
approximate magnitude range 9 ≤ V ≤ 12.
The SuperWASP data reduction pipeline is described in detail
in Pollacco et al. (2006). The aperture-extracted photometry
from each camera on each night are corrected for primary
and secondary extinction, instrumental colour response and system
zero-point relative to a network of local secondary standards.
The resultant pseudo-V magnitudes are comparable to
Tycho V magnitudes. Additional systematic errors affecting all
the stars are identified and removed using the SysRem algorithm
of Tamuz et al. (2005). The final light curves are stored in the
WASP project’s searchable archive (Butters et al. 2010).
3. Am star selection and analysis
We have selected Am stars from the Renson & Manfroid (2009)
catalogue of peculiar stars for which we have data in the WASP
archive and when individual light curves have at least 1000 data
points (i.e. for a single camera and during a single season). Any
stars known, or found, to be eclipsing binary systems were excluded
from the analysis. Stars were also rejected when two
approximately equal brightness stars were within the 3.5-pixel
(∼50 ) SuperWASP photometry aperture. However, unresolved
close pairs in DSS images (separation <∼2 ) and systems with
fainter companions (>∼2 mag) were retained.
For each individual light curve, periodograms were calculated
using the fast computation of the Lomb periodogram
method of Press & Rybicki (1989) as implemented in the
Numerical Recipes fasper routine (Press et al. 1992). Spectral
window functions were also calculated, in order to identify peaks
which had arisen due to the gaps in the observations. The periodograms
were examined for any evidence of variability. Stars
were rejected if the false alarm probability of the strongest peaks
exceeded 0.1 (Horne & Baliunas 1986). The remaining stars
were examined in more detail using the Period04 program (Lenz
& Breger 2005). For stars in which variability was confirmed,
frequencies continued to be selected so long as their amplitude
was >4 times the average background of the pre-whitened residuals
(Breger et al. 1993). Formal uncertainties on frequencies
and amplitudes were obtained from the least-squares fitting using
the method of Montgomery & O’Donoghue (1999).
Of the 1620 Am stars initially selected, a total of 227 (14%
of the total) have been found to pulsate. The remaining 1393
stars were deemed as “not found to pulsate”, since low-level pulsation
could be present below the SuperWASP detection limits.
Table 1 provides a summary of the pulsating Am stars. The individual
periodograms and phase-folded lightcurves are presented
in Fig. 1.
4. Stellar parameters
To place stars on the HR diagram we require values of Teff
and log L. For stars with uvbyβ photometry in the Hauck &
Mermilliod (1998) catalogue, we used the uvbybeta code of
Moon (1985) to obtain de-reddened indices, and the (b − y, c0)
grids of Smalley & Kupka (1997) to determine Teff and log g.
For stars with only uvby photometry the above procedure was
used but without the de-reddening step. For stars without uvby
photometry, Geveva photometry from Rufener (1988) was used
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B. Smalley et al.: SuperWASP observations of pulsating Am stars
Table 1. Pulsating Am stars.
Ren ID Name Sp. Type log Teff log L Method Freq. Amp. nFreq. ΔT Class
(K) (L ) (d−1
) (mmag) (d)
10 HD 154A A9mF2 3.862 0.89 a 4.7672 22.4 2 1135 γ Dor
110 HD 719 A3mF0 3.855 1.01 a 14.1368 3.2 10 473 δ Sct
113 HD 728 Am δ Del 14.9268 2.8 2 119 δ Sct
140 HD 923 A6mF2 3.908 1.39 a 18.6684 2.2 1 50 δ Sct
210 HD 1097 A4mF4 Sr 3.840 0.11 a 15.5491 2.0 2 50 δ Sct
Notes. The full version of this table can be found at the CDS.
Table 2. Am stars with both SuperWASP and Kepler data.
KIC Ren ID Max Amp Ref
(mmag)
9204718 49340 0.13 Bal
11445913 49650 2.5 Cat, Bal
9272082 49840 <0.01 Bal
12253106 50070 <0.01
9764965 50230 1.0
8881697 50420 1.9
11402951 50670 1.2 Cat,Bal
9349245 51233 <0.1
8703413 51640 <0.1 Bal
8323104 52260 <0.1 Bal
Notes. The second column gives the identification number (Ren ID)
from the Renson & Manfroid (2009) catalogue. Column 3 gives the
amplitude (Max Amp) of the highest peak in the Kepler periodogram.
Column 4 gives reference to published Kepler data: Cat: Catanzaro et al.
(2011), Bal: Balona et al. (2011).
and the calibration of Künzli et al. (1997) used to determine Teff
and log g, assuming zero reddening. In all of the above cases, the
Torres et al. (2010) relations were used to determine log L. For
stars without suitable intermediate-band photometry, but with
Hipparcos parallaxes (van Leeuwen 2007), spectral energy distributions
(SEDs) were constructed using literature broad-band
photometry. Values of Teff were determined by fitting Kurucz
flux distributions to the SEDs and log L determined from the
bolometric flux at the earth (f⊕) and the Hipparcos parallax. The
typical uncertainties are estimated to be ±200 K in Teff (±0.01 in
log Teff) and ±0.25 in log L. The stellar parameters are given in
Table 1. In total around a third of the Am stars investigated have
stellar parameters determined.
5. Am stars in Kepler field
The sky coverage of the SuperWASP survey overlaps with a
large fraction of the Kepler field. For Am stars with light curves
in both the Kepler Public archive and the SuperWASP database
we have compared the frequencies and amplitudes. This allows
us to evaluate the detection limits of SuperWASP. Of the 10 stars
with both Kepler and SuperWASP data, four have clear pulsations
with amplitudes >∼1 mmag (Table 2), while the other six
stars have amplitudes below the SuperWASP detectability limit.
The period04 analysis (Table 3) shows good agreement
above the nominal SuperWASP 1 mmag amplitude limit. There
is a suggestion that the amplitudes found using SuperWASP
lightcurves are slightly higher than those from Kepler. In addition,
the SuperWASP frequency can differ from the “true”
frequency by a small integer number of 1 d−1
aliases. The
comparison also shows that it is possible with SuperWASP data
to detect frequencies slightly below the 1 mmag level (Fig. 2).
Naturally, the variable data quality of ground-based photometry
means that not all stars with suitable variability will be detected.
Table 3. Comparison between frequencies and amplitudes found in the
Kepler and SuperWASP data for the four Am stars common to both.
Kepler SuperWASP
Freq. Amp.a
Freq. Amp.
(d−1
) (mmag) (d−1
) (mmag)
Ren ID 49650 (KIC 11445913, 1SWASP J190540.61+491820.7)
f1 31.5577 ± 0.0003 2.8 31.5577 ± 0.0001 3.2 ± 0.1
f2 25.3799 ± 0.0007 1.1 25.3769 ± 0.0001 1.2 ± 0.1
f3 22.1307 ± 0.0009 0.8 22.1306 ± 0.0002 1.0 ± 0.1
f4 37.8182 ± 0.0011 0.6
f5 29.7394 ± 0.0012 0.6
Ren ID 50230 (KIC 9764965, 1SWASP J191724.91+463535.2)
f1 27.1777 ± 0.0001 1.1 27.1778 ± 0.0001 1.2 ± 0.1
f2 21.3819 ± 0.0002 0.6 22.3891 ± 0.0001 0.9 ± 0.1
f3 31.9895 ± 0.0002 0.4 31.9902 ± 0.0001 0.9 ± 0.1
f4 19.9579 ± 0.0004 0.2
Ren ID 50420 (KIC 8881697, 1SWASP J192136.03+450706.8)
f1 16.5567 ± 0.0003 1.9 16.5565 ± 0.0005 2.1 ± 0.1
f2 32.0477 ± 0.0004 1.5 32.0481 ± 0.0006 1.6 ± 0.1
f3 25.2105 ± 0.0005 1.2 25.2064 ± 0.0008 1.2 ± 0.1
f4 30.0120 ± 0.0006 1.1 30.0111 ± 0.0009 1.1 ± 0.1
f5 34.3647 ± 0.0007 0.9 34.3661 ± 0.0009 1.0 ± 0.1
f6 30.6537 ± 0.0008 0.9 30.6569 ± 0.0010 0.9 ± 0.1
f7 28.8044 ± 0.0009 0.7 27.8049 ± 0.0011 0.8 ± 0.1
f8 34.0106 ± 0.0009 0.7
f9 27.4073 ± 0.0010 0.7
f10 16.0119 ± 0.0013 0.5
Ren ID 50670 (KIC 11402951, 1SWASP J192732.81+491523.5)
f1 23.8493 ± 0.0004 1.3 23.8464 ± 0.0008 1.4 ± 0.2
f2 23.2770 ± 0.0004 1.1 23.2790 ± 0.0008 1.4 ± 0.2
f3 27.4616 ± 0.0007 0.7 27.4643 ± 0.0012 1.0 ± 0.2
f4 15.1001 ± 0.0007 0.7
f4 14.4967 ± 0.0009 0.5
Notes. (a)
Uncertainties on Kepler Amplitudes are all <0.05 mmag.
6. Discussion
The pulsating Am stars (see Fig. 3) are concentrated within the
fundamental radial mode red and blue edges of Dupret et al.
(2005). This is in agreement with that found by Balona et al.
(2011) for Am stars within the Kepler field. These studies show
that pulsating Am stars are concentrated in the cooler region of
the instability strip. Hot Am stars do not appear to pulsate at the
precision of the Kepler data.
The standard interpretation of the Am phenomenon is that
atomic diffusion – radiative levitation and gravitational settling –
in the outer stellar envelope gives rise to the observed atmospheric
abundance anomalies. For a typical mid-A star, Teff ∼
8000 K, there are two thin convection zones in the outer envelope.
The atmosphere itself is a convection zone a few thousand
km thick where ionisation of H drives the convection. Deeper in
the atmosphere, at T ∼ 50 000 K, the ionisation of He ii also creates
a thin convection zone, where the κ-mechanism drives δ Sct
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A&A 535, A3 (2011)
Fig. 2. Comparison between the period04 periodograms from Kepler (left) and SuperWASP (right) for four Am stars with pulsations detected by
SuperWASP (see Table 3 for details of frequencies identified).
pulsation. It has long been clear that some Am stars and related
types do pulsate, particularly the marginal Am stars (labelled
spectroscopically as Am: stars), the evolved Am stars (δ Del or
ρ Pup stars), and some more extreme cases, such as HD 188136
(Kurtz 1980; Wegner 1981) and HD 40765 (Kurtz et al. 1995).
The pulsation modes that we observe in Am stars are low
radial order, low spherical degree p modes. The surface of the
star is an anti-node. With the low radial order, the vertical wavelength
is long compared to the depth of the envelope above the
He ii ionisation zone. With the decrease in density with height
in the atmosphere, conservation of kinetic energy density means
that the pulsation amplitude increases with height in the atmosphere,
or conversely, decreases with depth.
In Am stars, the microturbulence velocity is also peculiar, as
it is generally much higher than that of chemically normal stars.
This high microturbulence arises from large velocity fields in the
stellar atmosphere (Landstreet 1998), which are even supersonic
for some Am stars. We do not really know what causes these
large velocity fields to develop exclusively in Am stars and how
chemical peculiarities and velocity fields coexist. The results
shown by Landstreet et al. (2009) suggest that there is a connection
between Teff and the velocity fields, peaking at around
Teff ∼ 8000 K, although we do not know what happens for cooler
Am stars.
Atomic diffusion occurs in the radiative zone below the turbulent
outer convective layer, which is far below the observable
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B. Smalley et al.: SuperWASP observations of pulsating Am stars
Fig. 3. HR diagram showing the location of Am stars. The filled circles
are the Am stars which were found to pulsate, while the open circles are
the Am stars which were not found to pulsate. The solid lines indicate
the location of the ZAMS and the fundamental radial mode red and
blue edges of the instability strip (Dupret et al. 2005). The large cross
indicates the typical uncertainties in log Teff and log L. The dots are the
δ Sct stars from the catalogue of Rodríguez et al. (2000).
atmosphere. In this radiative layer there must be no turbulence at
the diffusion velocity, which is of the order of 10−4
— 1 cm s−1
.
The photometric amplitudes found in Am stars are consistent
with atmospheric pulsation radial velocity amplitudes of a few
km s−1
. Taking into account the decrease in pulsation amplitude
with depth – largely because of the increase in density, but also
because of the radial wave function – the pulsation velocity in
the radiative layer where atomic diffusion is most important in
Am stars is still of the order of a km s−1
. With such pulsations
in a layer where atomic diffusion is operating at sub-cms−1
velocities,
it must be that the pulsation is laminar; i.e., producing
no turbulence at the sub-cms−1
level.
With the results from the Kepler mission (Balona et al. 2011)
and now our results from SuperWASP we conclude that the loss
of helium by gravitational settling from the He ii ionisation zone
reduces driving, but does not suppress it entirely. Thus Am stars
can pulsate as δ Sct stars, but typically with relatively low amplitudes
compared to normal abundance δ Sct stars. Some Am stars
show no pulsation whatsoever at Kepler μmag precision. It has
yet to be shown whether this lack of pulsation can also occur in
the more rapidly rotating normal abundance stars in the δ Sct instability
strip. Study of this question is in progress with Kepler
data. As was concluded for the individual cases of HD 188136
and HD 40765, we may now state in general: in Am stars the pulsation
must be laminar, not generating turbulence to mix away
the observed effects of atomic diffusion in the outer atmosphere.
The Fm δ Del subclass are evolved Am stars above the mainsequence,
many of which have been found to show variability
(Kurtz 1976). Not unexpectedly, many stars classed as Fm δ Del
are found to be pulsating in the WASP data, but clearly not all. Of
the 227 Am stars that we found to be pulsating 55 are classed as
Fm δ Del: 24% of the Am stars found to pulsate. This compares
to a total of 186 Fm δ Del stars out of the 1620 Am stars investigated
using WASP data, around 11% of the sample. Therefore,
30% of the Fm δ Del stars have been found to pulsate, compared
to just 12% of other Am stars. Thus pulsation amplitude either
grows in Am stars as they evolve, or some non-pulsating Am
stars begin pulsating as they move off the main sequence. This is
likely to be a consequence of the driving region moving deeper
Fig. 4. Location of the pulsating Am stars in the HR diagram. The circles
are pulsating Am stars, with the filled circles indicating those with
spectral classification noted as δ Del. The crosses are the Fm δ Del stars
which were not found to pulsate. The solid lines indicate the location
of the ZAMS and the fundamental radial mode red and blue edges of
the instability strip (Dupret et al. 2005). The large cross indicates the
typical uncertainties in log Teff and log L.
Fig. 5. Frequency-amplitude diagram for pulsating Am stars shown as
circles, with filled circles indicating those with spectral classification
noted as δ Del. Note that in multi-periodic systems only the frequency
of the highest amplitude is shown, as given in Table 1. The dots are the
δ Sct stars from the catalogue of Rodríguez et al. (2000).
into the star where the helium abundance is higher than in the
main sequence He ii ionisation zone (see Turcotte et al. 2000 for
theoretical discussion).
The location of the pulsating Fm δ Del stars in the HR diagram
is shown in Fig. 4. There is a tendency for the pulsating
Fm δ Del stars to be located toward the cooler (and/or)
slightly more evolved parts of the instability strip, whereas
the non-pulsating Fm δ Del stars are distributed more uniformly.
The frequency-amplitudediagram (Fig. 5) shows that the
Fm δ Del stars occupy the same regions as the other Am stars, but
with an absence of high-frequency (>∼20 d−1
) pulsations; this is
not surprising, given that they are cooler and more evolved than
average δ Sct stars.
Several factors are thought to play a role in the development
of pulsating Am stars, but stellar rotation is probably one of the
most important. Charbonneau & Michaud (1991) showed that
Am chemical peculiarity develops in stars that rotate slower than
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A&A 535, A3 (2011)
Table 4. The number of pulsating Am stars and percentage in each of
the four pulsation classes as defined by Grigahcène et al. (2010).
Pulsation Class Number Percentage
δ Sct 169 75
δ Sct/γ Dor 23 10
γ Dor 30 13
γ Dor/δ Sct 5 2
90 km s−1
and that the He ii ionisation zone deepens with decreasing
rotation. This was later confirmed by more advanced
diffusion model calculations by Talon et al. (2006) and observationally
by Fossati et al. (2008), who found a correlation between
Am chemical peculiarities and v sin i in Am stars belonging to
the Praesepe open cluster. The vast majority of the Am stars already
known to pulsate have a rather large v sin i, between 40
and 90 km s−1
, thus avoiding the He ii ionisation zone sinking
too deep into the star and therefore allowing the development of
pulsation driven by the κ-mechanism. On the other hand, for the
very slowly rotating pulsating Am stars, the pulsation could be
laminar. It is therefore likely there are two different mechanisms
driving pulsation in Am stars.
Our results show a wide variety of pulsations, from singly
periodic to complex multiperiodic, and also some examples of
what appear to be hybrid γ Dor/δSct pulsators. This is similar
to the range of behaviour seen in normal abundance δ Sct stars,
as can be seen in the study of Kepler data by Grigahcène et al.
(2010). Those authors reclassified pulsation types with the following
scheme:
δ Sct: frequencies above 5 d−1
;
δ Sct/γ Dor hybrid: most frequencies above 5 d−1
, but some low
frequencies present;
γ Dor: frequencies lower than 5 d−1
;
γ Dor/δSct hybrid: most frequencies lower than 5 d−1
, but some
high frequencies present.
Our results are summarized in Table 4 and the individual classes
for each star are given in Table 1. The majority of the pulsators
we found are δ Sct stars, with the remaining quarter split between
γ Dor stars and mostly δ Sct/γ Dor hybrids. Given that the
SuperWASP data are affected by daily aliases and systematics
at low frequencies, the true number of stars with γ Dor pulsations
may indeed be higher. However, given that Am stars
are thought to be members of binary systems and tidal effects
slow the stellar rotation rate, it is possible that some of the lowfrequency
signatures found in the SuperWASP data are due to
ellipsoidal effects in close binaries. Assuming a rotation limit
of v sin i <∼ 120 km s−1
for an Am star and a radius of 1.5 R ,
the shortest period for a binary system containing a tidallysynchronised
Am star is ∼0.6 d. Close binary systems with dissimilar
components have two maxima and minima per orbital
period, and this value dominates over the orbital value in periodograms.
Hence, frequencies <∼3.3 d−1
may have arisen due
to ellipsoidal variations in close binaries. Thus, we caution that
some of the stars presented in Table 1 could have erroneously
been classified as having γ Dor pulsations. In addition, it is possible
that long-period pulsations in close binaries could be tidally
excited (Handler et al. 2002).
It is clear from examination of the Kepler data set that
the δ Sct stars show frequencies ranging from nearly zero d−1
up to 100 d−1
; some stars even show the full range, including
frequencies between the g mode and p mode ranges seen
in models. These intermediate frequencies are unexplained at
present. It is clear that the δ Sct stars are complex pulsators that
show g modes, p modes, mixed modes and many nonlinear cross
terms. Whether there are differences between abnormal abundance,
slowly rotating Am stars that are δ Sct stars and the more
rapidly rotating, normal abundance δ Sct stars is yet to be determined.
The objects we present here from SuperWASP greatly
increases the number of pulsating Am stars for statistical study
of this question.
Acknowledgements. The WASP project is funded and operated by Queen’s
University Belfast, the Universities of Keele, St. Andrews and Leicester, the
Open University, the Isaac Newton Group, the Instituto de Astrofisica de
Canarias, the South African Astronomical Observatory and by STFC. This research
has made use of the SIMBAD database, operated at CDS, Strasbourg,
France. Some of the data presented in this paper were obtained from the
Multimission Archive at the Space Telescope Science Institute (MAST). STScI
is operated by the Association of Universities for Research in Astronomy, Inc.,
under NASA contract NAS5-26555. Support for MAST for non-HST data is
provided by the NASA Office of Space Science via grant NNX09AF08G and by
other grants and contracts.
References
Abt, H. A. 2009, AJ, 138, 28
Aerts, C., Christensen-Dalsgaard, J., & Kurtz, D. W. 2010, Asteroseismology
(Heidelberg: Springer)
Balona, L., Ripepi, V., Catanzaro, G., et al. 2011, MNRAS, 414, 792
Breger, M., Stich, J., Garrido, R., et al. 1993, A&A, 271, 482
Butters, O. W., West, R. G., Anderson, D. R., et al. 2010, A&A, 520, L10
Catanzaro, G., Ripepi, V., Bernabei, S., et al. 2011, MNRAS, 411, 1167
Charbonneau, P., & Michaud, G. 1991, ApJ, 370, 693
Dupret, M.-A., Grigahcène, A., Garrido, R., Gabriel, M., & Scuflaire, R. 2005,
A&A, 435, 927
Fossati, L., Bagnulo, S., Landstreet, J., et al. 2008, A&A, 483, 891
Gilliland, R. L., Brown, T. M., Christensen-Dalsgaard, J., et al. 2010, PASP, 122,
131
Grigahcène, A., Antoci, V., Balona, L., et al. 2010, ApJ, 713, L192
Handler, G., Balona, L. A., Shobbrook, R. R., et al. 2002, MNRAS, 333, 262
Hauck, B., & Mermilliod, M. 1998, A&AS, 129, 431
Henry, G. W., & Fekel, F. C. 2005, AJ, 129, 2026
Horne, J. H., & Baliunas, S. L. 1986, ApJ, 302, 757
Joshi, S., Mary, D. L., Martinez, P., et al. 2006, A&A, 455, 303
Joshi, S., Mary, D. L., Chakradhari, N. K., Tiwari, S. K., & Billaud, C. 2009,
A&A, 507, 1763
Künzli, M., North, P., Kurucz, R. L., & Nicolet, B. 1997, A&AS, 122, 51
Kurtz, D. W. 1976, ApJS, 32, 651
Kurtz, D. W. 1978, ApJ, 221, 869
Kurtz, D. W. 1980, MNRAS, 193, 29
Kurtz, D. W. 1989, MNRAS, 238, 1077
Kurtz, D. W., & Martinez, P. 2000, Baltic Astron., 9, 253
Kurtz, D. W., Garrison, R. F., Koen, C., Hofmann, G. F., & Viranna, N. B. 1995,
MNRAS, 276, 199
Landstreet, J. D. 1998, A&A, 338, 1041
Landstreet, J. D., Kupka, F., Ford, H. A., et al. 2009, A&A, 503, 973
Lenz, P., & Breger, M. 2005, CoAst, 146, 53
Montgomery, M. H., & O’Donoghue, D. 1999, DSSN, 13, 28
Moon, T. T. 1985, Commun. Univ. London Obs., 78
Rufener, F. 1998, Observations in the Geneva Photometric System 4 [CDS
Catalog II/169/]
Pollacco, D. L., Skillen, I., Collier Cameron, A., et al. 2006, PASP, 118, 1407
Press, W. H., & Rybicki, G. B. 1989, ApJ, 338, 277
Press, W. H., Teukolsky, S. A., Vetterling, W. T., & Flannery, B. P. 1992,
Numerical recipes in FORTRAN, 2nd ed. (Cambridge: Cambridge Univ.
Press)
Renson, P., & Manfroid, J. 2009, A&A, 498, 961
Rodríguez, E., López-González, M. J., & López de Coca, P. 2002, A&AS, 144,
469
Smalley, B., & Kupka, F. 1997, A&A, 328, 349
Talon, S., Richard, O., & Michaud, G. 2006, ApJ, 645, 634
Tamuz, O., Mazeh, T., & Zucker, S. 2005, MNRAS, 356, 1466
Torres, G., Andersen, J., & Giménez, A. 2010, A&AR, 18, 67
Turcotte, S., Richer, J., Michaud, G., & Christensen-Dalsgaard, J. 2000, A&A,
360, 603
van Leeuwen, F. 2007, A&A, 474, 653
Wegner, G. 1981, ApJ, 247, 969
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B. Smalley et al.: SuperWASP observations of pulsating Am stars
Fig. 1. SuperWASP periodograms for the pulsating Am stars (top) and corresponding lightcurves folded on the principal frequency (below). The
lightcurves have been binned in 0.01 phase steps and phase 0.0 corresponds to the time of maximum light.
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