arXiv:1606.00052v1 [astro-ph.SR] 31 May 2016
Version from June 2, 2016
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ROTATION IN THE PLEIADES WITH K2: I. DATA AND FIRST RESULTS
L. M. Rebull
1,2
, J. R. Stauffer
2
, J. Bouvier
3
, A. M. Cody
4
, L. A. Hillenbrand
5
, D. R. Soderblom
6
, J. Valenti
6
,
D. Barrado
7
, H. Bouy
7
, D. Ciardi
8
, M. Pinsonneault
9
, K. Stassun
10
, G. Micela
11
, S. Aigrain
12
, F. Vrba
13
,
G. Somers
9
, J. Christiansen
8
, E. Gillen
12,14
, A. Collier Cameron
15
1
Infrared Science Archive (IRSA), Infrared Processing and Analysis Center (IPAC), 1200 E. California Blvd., California Institute of Tech-
nology, Pasadena, CA 91125, USA; rebull@ipac.caltech.edu
2
Spitzer Science Center (SSC), Infrared Processing and Analysis Center (IPAC), 1200 E. California Blvd., California Institute of Technology,
Pasadena, CA 9112, USA5
3
Universit´e de Grenoble, Institut de Plan´etologie et d’Astrophysique de Grenoble (IPAG), F-38000 Grenoble, France; CNRS, IPAG, F-38000
Grenoble, France
4
NASA Ames Research Center, Kepler Science Office, Mountain View, CA 94035, USA
5
Astronomy Department, Cali fornia Institute of Technology, Pasadena, CA 91125, USA
6
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA; Center for Astrophysical Sciences, Johns Hopkins
University, 3400 North Charles St., Baltimore, MD 21218, USA
7
Centro de Astrobiolog´ıa, Dpto. de Astrof´ısica, INTA-CSIC, E-28692, ESAC Campus, Villanueva de la Ca˜nada, Madrid, Spain
8
NASA Exoplanet Science Institute (NExScI), Infrared Processing and Analysis Center (IPAC), 1200 E. California Blvd., California Institute
of Technology, Pasadena, CA 91125, USA
9
Department of Astronomy, The Ohio State University, Columbus, OH 43210, USA; Center for Cosmology and Astroparticle Physics, The
Ohio State University, Columbus, OH 43210, USA
10
Department of Physi cs and Astronomy, Vanderbilt University, Nashville, TN 37235, USA; Department of Physics, Fisk University, Nashvill e,
TN 37208, USA
11
INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134, Palermo, Italy
12
Department of Physics, University of Oxford, Keble Road, Oxford OX3 9UU, UK
13
US Naval Observatory, Flagstaff Station, P.O. Box 1149, Flagstaff, AZ 86002, USA
14
Astrophysics Group, Cavendish Laboratory, J.J. Thomson Avenue, Cambridge CB3 0HE, UK
15
School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK
ABSTRACT
Young (125 Myr ), populous (>1000 members), and relatively nearby, the Pleiades has provided an
anchor for s tellar angular momentum models for both younger and older star s. We used K2 to explore
the distribution of rotation periods in the Pleiades. With more than 500 new periods for Pleiades
members, we are vastly ex panding the number of Pleiads with periods, particularly at the low mass
end. Abo ut 92% of the members in our sample have at least one measured spot-modulated rotation
period. For the ∼8% of the members without periods, non-astrophysical effects often dominate (sa t-
uration, etc.), such that periodic signals might have been detectable, all other things being equal.
We now have an unusually complete view of the rotation distr ibutio n in the Pleiades. The relation-
ship between P and (V − K
s
)
0
follows the overa ll trends found in other Pleiades studies. There is
a slowly rotating se quence for 1.1 .(V − K
s
)
0
. 3.7, and a primarily rapidly ro tating population for
(V − K
s
)
0
& 5.0. There is a region in which there seems to be a dis organized relationship between P
and (V − K
s
)
0
for 3.7 .(V − K
s
)
0
. 5.0. Paper II continues the discussion, focusing on multi-period
structures, and Paper III sp e culates about the origin and evolution of the period distribution in the
Pleiades.
1. INTRODUCTION
The three most fundamental parameters of a star are its mass, its composition, and its angular momentum. Together,
they determine how the star evolves from birth through the pre-main sequence phase to main sequence hydroge n bur n-
ing, and beyond, and further, whether and how planets form and migrate. Angular momentum evolution is tied during
star formation to cloud core fragmentation pr ocesses and stellar multiplicity, a nd during pre-main sequence evolution
2
to star-disk interactions coupled with simple radial contraction and internal structural changes. Main sequence angular
momentum evolution is dominated by spin-down due to mass loss and core-envelope coupling efficiencie s. Although
theoretical guidance addressing these matters for stars from Myr to Gyr ages has been significant (see,e.g., Bouvier
et al. 2014 and references therein), the problems to be addressed are still lacking in e mpirical guida nc e in critical areas.
Because the Pleiades is populous (over 1000 members; e.g., Bouy et al. 2015), relatively young (125 Myr; Stauffer
et al. 1998a), and nearby (136 pc; Melis et al. 2014), it has provided an anchor for stellar angular momentum models
for both younger and older stars. As such, we need a thorough understanding of the rotational distribution of stars
in the Pleiades. There is ample evidence that angular momentum evolution depends on stellar mass, so obtaining a
reliable rotation distribution for stars of a wide range of masses is critically important. The NASA K2 mission (Howell
et al. 2014), using the repurposed 1-m Kepler spac ecraft, observed the Pleiades cluster nearly continuously for 72 days,
enabling us to probe rotation rates to lower masses and to higher precision than ever before.
The Pleiades has been ex tensively studied for decades (e.g., Trumpler 192 1, Hertzsprung 1947, Johnso n & Mitchell
1958), and more recent surveys (e.g., Lodieu et al. 2012, Sarro et al. 2014, Bouy et al. 2015) have identified candidate
members down to at le ast ∼0.03 M
⊙
(K
s
∼18, or R >22), past where K2 can obtain a viable light curve in the Pleiades
(K
s
∼14.5, or K
p
∼18). More than 1000 candidate members for the Pleiades were included in K2’s Campaign 4 , down
to mass ∼0.09 M
⊙
.
The rotation of stars in the Pleiades has been the subject of study for quite some time, both spectroscopically (e.g.,
Anderson, Kra ft, & Stoe ckly 1966, Stauffer & Hartman 1987, Soderblom et a l. 1993a,b, Terndrup et al. 2000 , Queloz
et al. 1998) a nd photometrically (e.g., van Leeuwen et al. 1987, Stauffer & Hartmann 1 987, Stauffer et al. 1987, Prosser
et al. 1993a,b, 1995). There have been two recent extensive photometric surveys of Pleiades rotation periods. Hartman
et al. (2010) used the Hungarian Automa ted Telesco pe Network (HATNet) to obtain rotation periods for nearly 400
Pleiades members down to M ∼0.4 M
⊙
, with estimated completeness to M ∼0.7 M
⊙
. More recently, Covey et al.
(2016) present new rotation period observations for more than 100 Pleiads from the Palomar Transient Facility (PTF),
which greatly expanded the known periods for lower mass Pleia ds down to M ∼0.18 M
⊙
. These ground- based surveys,
however, necessarily were biased towards larger amplitude variability, and against periods near ∼1d.
Because K2 provides precision, sensitivity, and continuous (as opposed to diurnal) time coverage, in the present
paper we push the known per iods down to lower mass and lower amplitude tha n has ever been done before in the
Pleiades. In the process of doing this, we have found other repeated patterns in the light curves (LCs). We have
already scoured the K2 data for eclipsing binaries (David et al. 2015, 2016). Other periods that do not app e ar to be
spot-modulated ro tation periods are included in the Appendix. The re st of the periods are nearly a ll consistent with
spot-modulated rotation periods (though a few are likely pulsation; see Paper II).
In Se ction
2, we present the observations and data reduction, as well as assembly of Pleiades members out of the
1020 K 2 LCs of candidate Pleiads . The overall distribution of K2-derived rotation rates is discussed in Section
3.
Section
4 summar iz es our main results.
This is the first of three papers focus e d on rotation pe riods in the Pleiades. Paper II, Rebull et al. (20 16), discus ses
the several types of LCs and periodogram structures that we found in the K2 data, and some of the properties of
these multi-period sta rs. Stauffer et al. (2016), Paper III, s peculates about the origin a nd evolution of the period
distribution in the Ple iades.
2. OBSERVATIONS AND MET HODS
2.1. K2 Data
Members of the Pleiades wer e obse rved in K2 c ampaign 4, which lasted for 72d. Note that the field of view is not
centered on the cluster; see Fig.
1. All of the stars shown were observed in the long-cadence (∼30 min exposure) mode.
Thirty-four of these stars were additionally observed in fast cadence (∼1 min exposure), but those data are beyond
the scope of the present paper. There are 1020 unique K2 long-cadence light curves.
Kepler pix e l sizes ar e relatively larg e , 3.98
′′
× 3.98
′′
, and the 95% encircle d energy diameter ranges from 3.1 to 7.5
pixels with a median value of 4.2 pixels. During the K2 portion of the mission, becaus e only two reaction wheels can
be used, the whole spacecraft slowly drifts and then repositions regularly every 0.245 d.
We have used several different sets of LCs employing different reductions. (1) The pre-s earch data conditioning
(PDC) version generated by the Kepler project and o btained from MAST, the Mikulski Archive for Space Telescopes.
(2) A version with moving apertures o btained following Cody et al. in prep. (3) A version using a semiparametric
Gaussian proces s model used by Aigrain et al. (2015, 2016). (4) The ‘self-flat-fielding’ approach us ed by Vanderburg &
Johnson (2014) as obtained from MAST. We removed any data points corresponding to thruster firings and any others
3
62 60 58 56 54 52
RA (deg; J2000)
18
20
22
24
26
28
Dec (deg; J2000)
1
2
3
4
5
6
7
Figure 1. All 1020 candidate Pleiades members with K2 LCs projected onto the sk y. Red numbers correspond as follows:
1-Electra=HII468; 2-Taygeta=HII 563; 3-Maia=HII785; 4-Merope=HII980; 5-Alcyone=etaTau=HII1432; 6-Atlas=HII2168; 7-
Pleione=HII2181. Note that the entire Pleiades cluster, centered roughly on Alcyone, is not included in the K2 fields; the tidal
radius of the Pleiades is ∼6
◦
. Note also the gaps between K2 detectors.
with bad data flags set in the corresponding data product. The times (as shown in figures in this and our subsequent
papers) are Kepler baric entric Julian day.
We inspected LCs from each reduction approa ch, and we selected the visually ‘best’ LC from among the four, such
as the LC with the least discontinuities, or the one with the leas t overall trend, or the one leas t subject to saturation
effects, etc. Out of our entire sample of 1020 LCs, the PDC LC was the best for ∼58% of the sample, 11% of the LCs
had the best version from Aigrain et al., ∼8% had the best version from Cody et al., and ∼5% of the LCs were b est
in the Vanderburg & Johnson approach. It is important to note that in most cases, the period appear s as a significant
peak in the periodograms fo r all four LC versions, but the subtleties of the processing mea n that one version is slightly
better than another and is the one that we used to obtain the periods reported here. In general, the PDC LC was
best for .3 d; both the Aigrain and Vander burg approaches were on average best for the longer periods. For ∼18%
of the 1020, it was not clear which was the best LC, either because the LC was s aturated (too bright) or too faint,
or adversely affected by nearby bright stars, or all the LC versions were different enough that no one LC could be
selected as the best a nd most reliable. None of these latter confusing LCs were found to be periodic.
In two cases, there are pairs of lightcurves that are indistinguishable. EPIC 211076026 and 211076042 are ADS2755A
and ADS2755B, which are sometimes jointly referred to as HII956 or HD 23479. These two stars are a visual binary
with a separation o f ∼0.7
′′
, so close that the K2 light curves are effectively identical. We dropped 211076026 and
kept 211076042; the LC is not periodic. EPIC 211066337 (HII298) and EPIC 211066412 (HII29 9) are functionally
indistinguishable light c urves. They are a visual binary separated by ∼6
′′
. We have kept EPIC 211066337 and dropped
EPIC 211066412. The net LC in EPIC 211066337 has two periods, 6.156, and 2.932 d, and we suspect that is one
period per binary component (see Paper II).
4
2.2. Finding Periods
We looked for periodic signals using primarily the NASA Exoplanet Archive Periodogram Service
1
(Akeson et al.
2013). This serv ice provides period calculations using Lomb-Scargle (LS; Sca rgle 1982), Box-fitting Least Squares
(BLS; Kov´acs et al. 2002), and Plavchan (Plavchan et al. 2008) algorithms. We also looked for per iods us ing CLEAN
(Rob e rts et al. 198 7).
In practice, though, the periodic signals ar e generally not a mbigous and any method yields very similar periods.
Different LC versions can make more of a difference in the derived period than different period-finding algorithms
because of the influence of artifacts. We used LS for the analysis discussed her e, because most of the periodic signals
are sinusoidal.
Some LCs, periodograms, and phased LCs can be found in Fig.
2. These are representatives from a range of
brightnesses and per iods. The power spectra indicate unambiguously pe riodic signa ls – the peak is so high that little
structure other than the peak can be seen in the power spectrum, and when there is substructure, it is a harmonic
of the main signal. (However, see Paper II for multi-periodic stars.) For signals like those in Fig. 2, the false alarm
probability (FAP) returned by the LS algorithm is 0; for ∼97% of the sample with p eriods, the FAP of the main peak
is very small, ∼0. For many stars, the FAP of the second or third peak is also ∼0, which gives rise to the multi-periodic
discoveries in Paper II. The only situations in which we took a star to be periodic when the FAP for the peak calculated
over the whole LC was not ∼0 were situations in which, e.g., half the LC was co rrupted by instrumental effects and
thus we took a P derived from the unaffected portion (which then meant that the FAP computed for that peak on
that portion of the LC was very low), or the three stars in Sec.
2.3.1 where there is a c lear pea k at the same loca tion
as others found for this star in an independent dataset, e ven if the formal FAP calculated for that peak from the K2
data was high.
For star s of the mass range considered here, the periods that we measure are, by and large, star spot-modulated
rotation periods. Spot modulation is the simplest explanation for sinusoidal (or sinusoidal-like) variations where there
are changes over an entire rotation phase.
To be conservative, we required a t least 2 complete cycles of a pattern to call it periodic, thus the maximum period
we searched for was 35 d. We do not expect Pleiades members to be rotating more slowly than 35d. Indeed, the
distribution of periods we fo und (se e Figure
3) is strongly p e aked at <1 day; only ∼3% of the periods over a ll 1020
LCs (not just members identified in Sec.
2.5 below) are longer than 10 d. Because the number of rotation periods falls
off so strongly, we suspect that few or no legitimate rotation periods of Pleiades members a re >35d, and our approach
is not unduly biasing our derived distribution of rotation periods in the Pleiades. The re may be some patterns that are
repeated on timescales longer than 35d, but they are not rotation pe riods – the shapes of the LCs are much different
than the rotation periods in the data.
Additionally, by inspection of individual LCs, we deemed some periodic signals with periods shorter than 35 d to
not nec e ssarily be rotation periods. Two objects, EPIC 211082420 (HII1 431) and 210822691 (AKII4 65) are ec lipsing
binaries (see David et a l. 201 5, 2016). We have re moved these periods from our sample because they are not spot-
modulated rotation periods (AKII465 is a lso likely not a member of the Pleiades). There are other eclipsing binaries
in our data, but for those , there is also a periodic signal fro m the primary, which we retain here because it is likely
to be a rotation rate; see, for e xample, 211093684/HII2407 in David et al. (2015). There are 28 additional objects
that have features in their LS pe riodograms that suggest possible periods P <35 d, but that which we b elieve are not
unambiguously periodic. Those stars are listed Appe ndix
B for reference and those periods have been removed from
subsequent analysis.
We find per iods for 798 out of our sample of 1020 K2 LCs of candidate Pleiads. However, not all of those stars may
be members; see Sec. 2.5.
2.3. Comparison to Literature Values
2.3.1. Literature Periods
In order to verify our period-finding approa ch, it is useful to compare to prior Pleiades results. There are two
recent papers that obtain periods in the Pleiades from large -field pho tometric monitoring. Hartman et al. (2010) used
HATNet and rep orted periods for 383 Pleiads. We have 225 per iodic objects in common (given spatial and brightness
constraints), and we agree to within 10% of the derived P for 85% of the objects; see Figure
4. The median fractio nal
difference (|(P
Hartman
− P
Rebull
)|/P
Rebull
) is 0.7%. Covey et al. (2016) used PTF and report periods fo r 138 Pleiads.
1
http://exoplanetarchive.ipac.caltech.edu/cgi-bin/Periodogram/nph-simpleupload
5
0 20 40 60
times- 2228 (d)
1.3×10
4
1.4×10
4
1.5×10
4
1.6×10
4
flux
210872505/DH146
0.1 1.0 10.0
Period (d)
0
500
1000
1500
2000
Power
0.0 0.2 0.4 0.6 0.8 1.0
phase
0.90
0.95
1.00
1.05
1.10
relative flux
0.53d
0 10 20 30 40 50 60
times- 2231 (d)
0.96
0.98
1.00
1.02
1.04
flux
211026087/DH166
0.1 1.0 10.0
Period (d)
0
500
1000
1500
Power
0.0 0.2 0.4 0.6 0.8 1.0
phase
0.96
0.98
1.00
1.02
1.04
relative flux
10.02d
0 20 40 60
times- 2228 (d)
2500
3000
3500
flux
211053678/HHJ206
0.1 1.0 10.0
Period (d)
0
500
1000
1500
Power
0.0 0.2 0.4 0.6 0.8 1.0
phase
0.8
0.9
1.0
1.1
1.2
relative flux
0.77d
0 20 40 60
times- 2228 (d)
6.0×10
4
6.5×10
4
7.0×10
4
7.5×10
4
flux
211023687/HII915
0.1 1.0 10.0
Period (d)
0
500
1000
1500
2000
Power
0.0 0.2 0.4 0.6 0.8 1.0
phase
0.85
0.90
0.95
1.00
1.05
1.10
1.15
relative flux
3.93d
0 20 40 60
times- 2228 (d)
2.35×10
4
2.40×10
4
2.45×10
4
2.50×10
4
2.55×10
4
2.60×10
4
flux
210892390/s4868524
0.1 1.0 10.0
Period (d)
0
500
1000
1500
Power
0.0 0.2 0.4 0.6 0.8 1.0
phase
0.94
0.96
0.98
1.00
1.02
1.04
1.06
relative flux
1.41d
Figure 2. Five examples of finding periods in the K2 Pleiades data. Left column: full LC; middle column: LS periodogram;
right column: phased LC, with best period (in days) as indicated. Rows, in order: EPIC 210872505/DH146, 211026087/DH166,
211053678/HHJ206, 211023687/HII915, 210892390/s4868524. These are representatives from a range of brightnesses and pe-
riods. Note that in each case, the power spectrum indicates unambiguously periodic signals – the peak is so high that little
structure other than the peak can be seen in the power spectrum. These LCs are best interpreted as large spots or spot groups
rotating into and out of view.
