Near- and Mid-Infrared Photometry of the Pleiades and a New List of Substellar Candidate Members
ABSTRACT We make use of new near- and mid-IR photometry of the Pleiades cluster in order to help identify proposed cluster members. We also use the new photometry with previously published photometry to define the single-star main-sequence locus at the age of the Pleiades in a variety of color-magnitude planes. The new near- and mid-IR photometry extend effectively 2 mag deeper than the 2MASS All-Sky Point Source catalog, and hence allow us to select a new set of candidate very low-mass and substellar mass members of the Pleiades in the central square degree of the cluster. We identify 42 new candidate members fainter than Ks = 14 (corresponding to 0.1 M☉). These candidate members should eventually allow a better estimate of the cluster mass function to be made down to of order 0.04 M☉. We also use new IRAC data, in particular the images obtained at 8 μm, in order to comment briefly on interstellar dust in and near the Pleiades. We confirm, as expected, that—with one exception—a sample of low-mass stars recently identified as having 24 μm excesses due to debris disks do not have significant excesses at IRAC wavelengths. However, evidence is also presented that several of the Pleiades high-mass stars are found to be impacting with local condensations of the molecular cloud that is passing through the Pleiades at the current epoch.
NEAR- AND MID-INFRARED PHOTOMETRY OF THE PLEIADES
AND A NEW LIST OF SUBSTELLAR CANDIDATE MEMBERS1,2
John R. Stauffer
Spitzer Science Center, Caltech 314-6, Pasadena, CA 91125; firstname.lastname@example.org
Lee W. Hartmann
Astronomy Department, University of Michigan
Giovanni G. Fazio, Lori E. Allen, and Brian M. Patten
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138
Patrick J. Lowrance, Robert L. Hurt, and Luisa M. Rebull
Spitzer Science Center, Caltech, Pasadena, CA 91125
Roc M. Cutri and Solange V. Ramirez
Infrared Processing and Analysis Center, Caltech 220-6, Pasadena, CA 91125
Erick T. Young, George H. Rieke, Nadya I. Gorlova,3and James C. Muzerolle
Steward Observatory, University of Arizona, Tucson, AZ 85726
Cathy L. Slesnick
Astronomy Department, Caltech, Pasadena, CA 91125
Michael F. Skrutskie
Astronomy Department, University of Virginia, Charlottesville, VA 22903
Received 2007 February 23; accepted 2007 April 6
We make use of new near-and mid-IR photometry of thePleiades cluster inorder to helpidentifyproposed cluster
members. We also use the new photometry with previously published photometry to define the single-star main-
sequence locus at the age of the Pleiades in a variety of color-magnitude planes. The new near- and mid-IR pho-
new set of candidate very low-mass and substellar mass members of the Pleiades in the central square degree of the
cluster. We identify 42 new candidate members fainter than Ks¼ 14 (corresponding to 0.1 M?). These candidate
members shouldeventually allow abetterestimate ofthecluster mass function tobemade downtooforder 0.04 M?.
in and near the Pleiades. We confirm, as expected, that—with one exception—a sample of low-mass stars recently
identified as having 24 ?m excesses due to debris disks do not have significant excesses at IRAC wavelengths.
However, evidence is also presented that several of the Pleiades high-mass stars are found to be impacting with local
condensations of the molecular cloud that is passing through the Pleiades at the current epoch.
Subject headingg s: open clusters and associations: individual (Pleiades) — stars: low-mass, brown dwarfs
Online material: color figure, machine-readable tables
Because of its proximity, youth, richness, and location in the
northern hemisphere, the Pleiades has long been a favorite target
members identified via their common proper motion (Trumpler
a dozen proper-motion studies. Some of the earliest photoelectric
photometry was for members of the Pleiades (Cummings 1921),
and the cluster has beenthe subject ofdozensofpapersproviding
tifying its substellar population, and it was the first open cluster
1989). More than 20 papers have been subsequently published,
or studying their properties.
We have three primary goals for this paper. First, while exten-
sive optical photometry for Pleiades members is available in the
literature, photometry in the near- and mid-IR is relatively spotty.
We will remedy this situation by using new 2MASS JHKsand
Spitzer Infrared Array Camera (IRAC) photometry for a large
numberofPleiades members.We willuse these datatohelpiden-
1This work is based (in part) on observations made with the Spitzer Space
Telescope, which is operated by the Jet Propulsion Laboratory, California Institute
of Technology, under NASA contract 1407.
2This publication makes use of data products from the Two Micron All Sky
Survey, which is a joint project of the University of Massachusetts and the In-
by the National Aeronautics and Space Administration and the National Science
The Astrophysical Journal Supplement Series, 172:663Y685, 2007 October
# 2007. The American Astronomical Society. All rights reserved. Printed in U.S.A.
our new IR imaging photometry of the center of the Pleiades to
identify a new set of candidate substellar members of the clus-
ter, extending down to stars expected to have masses of order
0.04 M?. Third, we will use the IRAC data to briefly comment
the interaction of the Pleiades stars with the molecular cloud
that is currently passing through the cluster.
with a necessary digression. As noted above, more than a dozen
proper-motion surveys ofthe Pleiades havebeenmadeinorder to
identify cluster members. However, no single catalog of the clus-
ter has been published that attempts to collect all of those candi-
date members in a single table and cross-identify those stars.
voted to providing optical photometry of cluster members, that
the number of photometric systems used. In x 3 and in the Ap-
pendix, we describe our efforts to create a reasonably complete
catalog of candidate Pleiades members and to provide optical
photometry transformed to the best of our ability onto a single
2. NEW OBSERVATIONAL DATA
2.1. 2MASS ‘‘6x’’ Imaging of the Pleiades
During the final months of Two Micron All Sky Survey
(2MASS; Skrutskie et al. 2006) operations, a series of special
observations were carried out that employed exposures 6 times
longer than used for the primary survey. These so-called ‘‘6x’’
observations targeted 30 regions of scientific interest including
a 3?; 2?area centered on the Pleiades cluster. The 2MASS 6x
data were reduced using anautomatedprocessingpipeline sim-
ilar to that used for the main survey data, and a calibrated 6x
and XSC have been released as part of the 2MASS Extended Mis-
Figure 1 shows the area on the sky imaged by the 2MASS 6x
observationsin the Pleiades field. The regionwas covered bytwo
rows of scans, each scan being 1?long (in declination) and 8.50
proximately 10in right ascension. There are small gaps in cov-
erage in the declination boundary between the rows, and one
complete scan in the southern row is missing because the data in
that scandid not meetthe minimum requiredphotometric quality.
The total area covered by the 6x Pleiades observations is approx-
imately 5.3 deg2.
bersexpected inthisregion, only?2%ofthe 6x-PSC sources are
cluster members, and the rest are field stars and background gal-
axies. The 6x-XSC objects are virtually all resolved background
in the 6x-XSC sources from the Pleiades region are shown in
Figures 2 and 3, respectively. The extragalactic sources tend to
numerous toward fainter magnitudes. Unresolved galaxies dom-
inate the point sources that are fainter than Ks>15:5 and redder
than J ? Ks>1:2 mag.
Fig. 1.—Spatialcoverageofthe6times deeper‘‘2MASS6x’’observationsof
the Pleiades. The 2MASS survey region is approximately centered on Alcyone,
region coveredwith the shallow IRAC surveyofthe cluster core. The star symbols
correspond to the brightest B star members of the cluster. The gray points are the
location of objects in the 2MASS 6x Point Source Catalog. [See the electronic edi-
tion of the Supplement for a color version of this figure.]
Fig. 2.—Color-magnitude diagram for the Pleiades derived from the 2MASS
6x observations. The red dots correspond to objects identified as unresolved,
whereas the green dots correspond to extended sources (primarily background
galaxies). The lack of green dots fainter than K ¼ 16 is indicative that too few
photons are available to identify sources as extended—the extragalactic popula-
tion presumably increases to fainter magnitudes.
STAUFFER ET AL.664Vol. 172
The 2MASS 6x observations were conducted using the same
freeze-frame scanning technique used for the primary survey
by increasing the ‘‘READ2-READ1’’ integration to 7.8 s from
the 1.3 s usedfor primarysurvey.However, the 51ms‘‘READ1’’
exposure time was not changed for the 6x observations. As a re-
sult,thereisaneffective‘‘sensitivitygap’’ inthe 8Y11magregion
where objects may be saturated in the 7.8 s READ2-READ1 6x
exposures, but too faint to be detected in the 51 ms READ1 ex-
posures. Because the sensitivity gap can result in incompleteness
and/or flux bias in the photometric overlap regime, the near-
infrared photometry for sources brighter than J ¼ 11 mag in
Table 2 (see x 3).
2.2. Shallow IRAC Imaging
team and the Multiband Imaging Photometer for Spitzer (MIPS)
instrumentteam.Initialresults ofthe MIPSsurveyofthe Pleiades
have already been reported in Gorlova et al. (2006). The IRAC
observations were obtained as two astronomical observing re-
at R:A: ¼ 03h47m00:0sand decl: ¼ 24?070(J2000.0), and con-
sisted of a 12 row by 12 column map, with ‘‘frame times’’ of 0.6
and 12.0 s and two dithers at each map position. The map steps
were 29000in both the column and row direction. The resultant
tion time per position of 24 s over most of the map. The second
AOR used the same basic mapping parameters, except it was
smaller (9 rows by 9 columns) and was instead centered north-
westfromtheclustercenteratR:A: ¼ 03h44m36:0sanddecl: ¼
25?240. A two-band color image of the AOR covering the cen-
ter of the Pleiades is shown in Figure 4. A pictorial guide to the
IRAC image providing Greek names for a few of the brightest
stars, and Hertzsprung (1947) numbers for several stars men-
tioned in x 6 is provided in Figure 5.
We began our analysis with the basic calibrated data (BCDs)
from the Spitzer pipeline, using the S13 version of the Spitzer
Science Center pipeline software. Artifact mitigation and mask-
ing was done using the IDL tools provided on the Spitzer con-
tributed software Web site. For each AOR, the artifact-corrected
the post-BCD ‘‘MOPEX’’ package (Makovoz & Marleau 2005).
The mosaic images were constructed with 1:2200; 1:2200pixels
(i.e.,approximatelythe same pixel size as the native IRAC arrays).
and the ‘‘phot’’ routine in DAOPHOT. In both cases, we used a
3 pixel radius aperture and a sky annulus from 3 to 7 pixels (ex-
cept that for channel 4, for the phot package we used a 2 pixel
Fig. 4.—Two-color (4.5 and 8.0 ?m) mosaic of the central square degree of
the Pleiades from the IRAC survey. North is approximately vertical, and east is
approximately to the left. The bright star nearest the center is Alcyone; the bright
Fig. 3.—Same as Fig. 2, except in this case the axes are J ? H and H ? Ks.
The extragalactic objects are very red in both colors.
Fig. 5.—Finding chart corresponding approximately to the region imaged with
IRAC. The large, five-pointed stars are all of the Pleiades members brighter than
V ¼ 5:5.Thesmallopencirclescorrespondtootherclustermembers.Severalstars
with 8 ?m excesses are labeled by their HII numbers and are discussed further in
the residual optical polarization (after subtraction of a constant foreground compo-
nent), as provided in Fig. 6 of Breger (1986).
INFRARED OBSERVATIONS OF THE PLEIADES
665No. 2, 2007
radius aperture and a 2Y6 pixel annulus because that provided
magnitude calibrations provided in the IRAC data handbook
and the aperture corrections provided in the same handbook
(multiplicative flux correction factors of 1.124, 1.127, 1.143, and
etry from APEX to that from phot and shows that the two pack-
ference between the derived 3.6 and 4.5 ?m magnitudes for
Pleiades members. Based on previous studies (e.g., Allen et al.
2004),weexpectedthisdifference tobe essentiallyzeroformost
stars, and the Pleiades data corroborate that expectation. For
½3:6? < 10:5,thermsdispersionofthemagnitudedifferencebe-
of order 0.017 mag. The absolute calibration uncertainty for the
IRAC fluxes is currently estimated at of order 0.02 mag. Figure 7
also shows that fainter than ½3:6? ¼ 10:5 (spectral type later than
about M0), the ½3:6? ? ½4:5? color for M dwarfs departs slightly
from zero, becoming increasingly redder to the limit of the data
3. A CATALOG OF PLEIADES CANDIDATE MEMBERS
If one limits oneself to only stars visible with the naked eye, it
is easy to identify which stars are members of the Pleiades—all
of the stars within a degree of the cluster center that have V < 6
are indeed members. However, if one were to try to identify the
M dwarf stellar members of the cluster (roughly 14 < V < 23),
only of order 1% of the stars toward the cluster center are likely
to be members, and it is much harder to construct an uncontam-
inated catalog. The problem is exacerbated by the fact that the
Pleiades is old enough that mass segregation through dynamical
processes has occurred, and therefore one has to survey a much
larger region of the sky in order to include all of the M dwarf
The other primary difficulty in constructing a comprehensive
member catalog for the Pleiades is that the pedigree of the can-
didates varies greatly. For the best-studied stars, astrometric po-
sitions can be measured over temporal baselines ranging up to a
century ormore, and the separationofcluster members fromfield
stars in a vector point diagram (VPD) can be extremely good. In
addition, accurate radial velocities and other spectral indicators
are available for essentially all of the bright cluster members, and
itive. Conversely, at the faint end (for stars near the hydrogen-
limit of the existing wide-field photographic plates, and the errors
on the proper motions become correspondingly large, causing the
separation of cluster members from field stars in the VPD to be-
come poor. These stars are also sufficiently faint that spectra
of the faint candidates have had such spectra obtained. There-
fore, any comprehensive catalog created for the Pleiades will
necessarily have stars ranging from certain members to candi-
dates for which very little is known and where the fraction of
spurious candidate members increases to lower masses.
In order to address the membership uncertainties and biases,
we have chosen a sliding scale for inclusion in our catalog. For
all stars, werequire that the available photometry yields location
ter membership. For the stars with well-calibrated photoelectric
photometry, this means the star should not fall below the Pleiades
single-star locus by more than about 0.2 mag or above that locus
by more than about 1.0 mag (the expected displacement for a
Fig. 6.—Comparison of aperture photometry for Pleiades members derived
from the IRAC 3.6 ?m mosaic using the Spitzer APEX package and the IRAF
implementation of DAOPHOT.
Fig. 7.—Difference between aperture photometry for Pleiades members for
IRAC channels 1 and 2. The ½3:6? ? ½4:5? color begins to depart from essentially
STAUFFER ET AL.666Vol. 172
hierarchical triple with three nearly equal mass components). For
stars with only photographic optical photometry, where the 1 ?
uncertainties are of order 0.1Y0.2 mag, we still require the star’s
photometry to be consistent with membership, but the allowed
displacements from the single-star locus are considerably larger.
Where accurate radial velocities are known, we require that the
star be considered a radial velocity member based on the paper
where the radialvelocitieswerepresented.Where stars havebeen
troscopic indices, we adopt those conclusions.
Twoother relevantpieces ofinformation aresometimes avail-
abilities are provided by the various membership surveys. If no
for a given candidate is less than 0.1, we exclude that star from
our final catalog. However, often a star appears in several cata-
logs; if it appears in two or more proper-motion membership
lists, we include it in the final catalog even if P < 0:1 in one of
those catalogs. Second, an entirely different means to identify
candidate Pleiades members is via flare star surveys toward the
membership probability in one catalog but whose photometry is
is retained in our catalog.
Further details of the catalog construction are provided in the
tometry have been homogenized. A full discussion and listing of
all of the papers from which we have extracted astrometric and
photometric information is also provided in the Appendix. Here
we simply provide a very brief description of the inputs to the
We include candidate cluster members from the following
proper-motion surveys: Trumpler (1921), Hertzsprung (1947),
Jones (1981), Pels & Lub (as reported in van Leeuwen et al.
1986), Stauffer et al. (1991), Artyukhina (1969), Hambly et al.
Hambly (2004). Another important compilation that provides
the initial identification of a significant number of low-mass clus-
ter members is the flare star catalog of Haro et al. (1982). Table 1
provides a brief synopsis of the characteristics of the candidate
member catalogs from these papers. The Trumpler paper is listed
twice in Table 1 because there are two membership surveys in-
cluded in that paper, with differing spatial coverages and different
In our final catalog, we have attempted to follow the standard
naming convention whereby the primary name is derived from
the paper where it was first identified as a cluster member. An
exception to this arises for stars with both Trumpler (1921) and
Hertzsprung (1947) names, where we use the Hertzsprung num-
bers as the standard name because that is the most commonly
identifications to some of the other catalogs, particularly where
alternate names. For the brightest cluster members, we provide
additional cross-references (e.g., Greek names, Flamsteed num-
bers, HD numbers).
For each star, we attempt to include an estimate for Johnson B
and V, and for Cousins I (IC). Only a very small fraction of the
cluster members have photoelectric photometry in these systems,
tained in other systems, including Walraven, Geneva, Kron, and
In other cases, photometry is available in a natural I-band system,
conversionusingstarsforwhichwe alreadyhavean ICestimateas
well as the natural I measurement. Details of these issues are pro-
vided in the Appendix.
Finally, we have cross-correlated the cluster candidates cata-
log with the 2MASS All-Sky PSC and also with the 6x-PSC for
the Pleiades. For every star in the catalog, we obtain JHKspho-
2MASS data and data from the 6x catalog, we adopt the 6x data
otherwise. We verified that the two catalogs do not have any
obvious photometric or astrometric offsetsrelative to each other.
The coordinates we list in our catalog are entirely from these
2MASS sources, and hence they inherit the very good and homo-
geneous 2MASS positional accuracies of order 0.100rms.
We have then plotted the candidate Pleiades members in a va-
riety of color-magnitude diagrams and color-color diagrams and
required that a star must have photometry that is consistent with
cluster membership. Figure8 illustrates thisprocess and indicates
Pleiades Membership Surveys Used as Sources
Artyukhina (1969) .......................
Haro et al. (1982) ........................
van Leeuwen et al. (1986)..........
Stauffer et al. (1991)....................
Hambly et al. (1993) ...................
Pinfield et al. (2000)....................
Adams et al. (2001).....................
Deacon & Hambly (2004)...........
32.5 < B < 14.5
2.5 < B < 10
2.5 < V < 15.5
2.5 < B < 12.5
11 < V < 17.5
2.5 < B < 13
14 < V < 18
10 < I < 17.5
13.5 < I < 19.5
8 < Ks< 14.5
10 < R < 19
coverages and different limiting magnitudes.
INFRARED OBSERVATIONS OF THE PLEIADES
667No. 2, 2007