USco1606-1935: An Unusually Wide Low-Mass Triple System?
Draft version October 29, 2018
Preprint typeset using LATEX style emulateapj v. 08/22/09
USCO1606-1935: AN UNUSUALLY WIDE LOW-MASS TRIPLE SYSTEM?
Adam L. Kraus (alk@astro.caltech.edu), Lynne A. Hillenbrand (lah@astro.caltech.edu)
California Institute of Technology, Department of Astrophysics, MC 105-24, Pasadena, CA 91125
Draft version October 29, 2018
ABSTRACT
We present photometric, astrometric, and spectroscopic observations of USco160611.9-193532 AB,
a candidate ultrawide (∼1600 AU), low-mass (Mtot ∼0.4 M⊙) multiple system in the nearby OB
association Upper Scorpius. We conclude that both components are young, comoving members of
the association; we also present high-resolution observations which show that the primary is itself a
close binary system. If the Aab and B components are gravitationally bound, the system would fall
into the small class of young multiple systems which have unusually wide separations as compared to
field systems of similar mass. However, we demonstrate that physical association can not be assumed
purely on probabilistic grounds for any individual candidate system in this separation range. Analysis
of the association’s two-point correlation function shows that there is a significant probability (25%)
that at least one pair of low-mass association members will be separated in projection by .15′′, so
analysis of the wide binary population in Upper Sco will require a systematic search for all wide
systems; the detection of another such pair would represent an excess at the 98% confidence level.
Subject headings: stars:binaries:general; stars:low-mass,brown dwarfs;stars:pre-main se-
quence;stars:individual([PBB2002] USco160611.9-193532)
1. INTRODUCTION
The frequency and properties of multiple star systems
are important diagnostics for placing constraints on star
formation processes. This has prompted numerous at-
tempts to characterize the properties of nearby binary
systems in the field. These surveys (e.g. Duquennoy &
Mayor 1991; Fischer & Marcy 1992; Close et al. 2003;
Bouy et al. 2003; Burgasser et al. 2003) have found
that binary frequencies and properties are very strongly
dependent on mass. Solar-mass stars have high binary
frequencies (&60%) and maximum separations of up to
∼104 AU. By contrast, M dwarfs have moderately high
binary frequencies (30-40%) and few binary companions
with separations of more than ∼500 AU, while brown
dwarfs have low binary frequencies (∼15%) and few com-
panions with separations >20 AU.
The mass-dependent decline in the maximum observed
binary separation has been described by Reid et al.
(2001) and Burgasser et al. (2003) with an empirical
function which is exponential at high masses (amax ∝
103.3Mtot) and quadratic at low masses (amax ∝M
tot).
The mechanism that produces the mass dependence is
currently unknown; N-body simulations show that the
empirical limit is not a result of dynamical evolution in
the field (e.g. Burgasser et al. 2003; Weinberg et al.
1987) since the rate of disruptive stellar encounters is far
too low. This suggests that the limit must be set early in
stellar lifetimes, either as a result of the binary formation
process or during early dynamical evolution in relatively
crowded natal environments. Surveys of nearby young
stellar associations have identified several unusually wide
systems (Chauvin et al. 2004; Caballero et al. 2006; ;
Jayawardhana & Ivanov 2006; Luhman et al. 2006, 2007;
Close et al. 2007), but not in sufficient numbers to study
their properties in a statistically meaningful manner.
We have addressed this problem by using archival
2MASS data to systematically search for candidate wide
binary systems among all of the known members of three
nearby young associations (Upper Sco, Taurus-Auriga,
and Chamaeleon-I; Kraus & Hillenbrand 2007). Our re-
sults broadly agree with the standard paradigm; there
is a significant deficit of wide systems among very low-
mass stars and brown dwarfs as compared to their more
massive brethren. However, we did identify a small num-
ber of candidate wide systems. One of these candidates
is [PBB2002] USco160611.9-193532 (hereafter USco1606-
1935), a wide (10.87′′; 1600 AU) pair of stars with similar
fluxes and colors. The brighter member of the pair was
spectroscopically confirmed by Preibisch et al. (2002) to
be a young M5 star. The fainter member fell just below
the flux limit of their survey.
In this paper, we describe our photometric, as-
trometric, and spectroscopic followup observations for
USco1606-1935 and evaluate the probability that the sys-
tem is an unusually wide, low-mass binary. In Section 2,
we describe our observations and data analysis methods.
In Section 3, we use these results to establish that both
members of the pair are young and co-moving, and that
the primary is itself a close binary. Finally, in Section
4 we address the possibility that the pair is not bound,
but a chance alignment of young stars, by analyzing the
clustering of pre-main-sequence stars in Upper Sco.
2. OBSERVATIONS AND DATA ANALYSIS
Most binary surveys, including our discovery survey,
identify companions based on their proximity to the pri-
mary star and argue for physical association based on
the (usually very low) probability that an unbound star
would have been observed in chance alignment. How-
ever, the probability of contamination is much higher
for very wide systems like USco1606-1935, so we de-
cided to pursue additional information in order to con-
firm its multiplicity and further characterize its system
components. In this section, we describe our followup
efforts: a search of publicly available databases to ob-
http://arxiv.org/abs/0704.0455v1
tain additional photometry and astrometry, acquisition
of intermediate-resolution spectra to measure the sec-
ondary spectral type and test for signatures of youth,
and acquisition of high-resolution images to determine if
either component is itself a tighter binary and to test for
common proper motion.
2.1. Archival Data
We identified USco1606-1935 AB as a candidate bi-
nary system using archival data from 2MASS (Skrut-
skie et al. 2006). The binary components are bright
and clearly resolved, so we were able to retrieve ad-
ditional photometry and astrometry from several other
wide-field imaging surveys. We collated results for the
binary components themselves and for nearby field stars
from 2MASS, the Deep Near Infrared Survey (DENIS;
Epchtein et al. 1999), United States Naval Observatory
B1.0 survey (USNO-B; Monet et al. 2003), and the Su-
perCOSMOS Sky Survey (SSS; Hambly et al. 2001).
The DENIS and 2MASS source catalogues are based on
wide-field imaging surveys conducted in the optical/NIR
(IJK and JHK, respectively) using infrared array de-
tectors, while the USNO-B and SSS source catalogues
are based on independent digitizations of photographic
plates from the First Palomar Observatory Sky Survey
and the ESO Southern-Sky Survey.
2.1.1. Photometry
After evaluating the data, we decided to base our anal-
ysis on the JHK magnitudes measured by 2MASS and
the photographic I magnitude of USNO-B (hereafter de-
noted I2, following the nomenclature of the USNO-B
catalog, to distinguish it from Cousins IC). We chose
these observations because their accuracy can be directly
tested using the independent IJK magnitudes measured
by DENIS; this comparison shows that the fluxes are con-
sistent within the uncertainties. We do not directly use
the DENIS observations because they are not as deep as
the other surveys. We adopted the photometric uncer-
tainties suggested in each survey’s technical reference.
2.1.2. Astrometry
As we describe in Section 3.3, there appear to be large
systematic differences in the astrometry reported by the
USNO-B and SSS source catalogs. These surveys rep-
resent digitizations of the same photographic plates, so
these systematic discrepancies suggest that at least one
survey introduces systematic biases in the digitization
and calibration process. Given the uncertainty in which
measurements to trust, we have chosen to disregard all
available photographic astrometry and only use results
from 2MASS and DENIS.
Our discovery survey already measured 2MASS rel-
ative astrometry for each filter directly from the pro-
cessed atlas images, so we have adopted those values.
We extracted DENIS astrometry from the source catalog,
which contains the average positions for all three filters.
Both surveys quote astrometric uncertainties of 70-100
mas for stars in the brightness range of our targets, but
that value includes a significant systematic term result-
ing from the transformation to an all-sky reference frame.
We have conducted tests with standard binary systems
of known separation which suggest that relative astrom-
etry on angular scales of <1′ is accurate to ∼40 mas,
so we adopt this value as the astrometric uncertainty for
each survey.
2.2. Optical Spectroscopy
We obtained an intermediate-resolution spectrum of
USco1606-1935 B with the Double Spectrograph (Oke &
Gunn 1982) on the Hale 5m telescope at Palomar Obser-
vatory. The spectrum presented here was obtained with
the red channel using a 316 l/mm grating and a 2.0′′ slit,
yielding a spectral resolution of R ∼1250 over a wave-
length range of 6400-8800 angstroms. Wavelength cali-
bration was achieved by observing a standard lamp after
the science target, and flux normalization was achieved
by observation of the spectrophotometric standard star
Feige 34 (Massey et al. 1988). The spectrum was pro-
cessed using standard IRAF1 tasks.
Our field and young spectral type standards were
drawn from membership surveys of Upper Sco and Tau-
rus by Slesnick et al. (2006a, 2006b) which used identical
instrument settings for the spectroscopic confirmation of
photometrically selected candidate members.
2.3. High-Resolution Imaging
We observed USco1606-1935 A and B on February 7,
2006 (JD=2453773) using laser guide star adaptive op-
tics (LGSAO; Wizinowich et al. 2006) on the Keck-II
telescope with NIRC2 (K. Matthews, in prep), a high
spatial resolution near-infrared camera. The seeing was
average to poor (&1′′) for most of the observing run, but
the system delivered nearly diffraction-limited correction
in K ′ (60 mas FWHM) during the period of these ob-
servations. The system performance was above average
given the low elevation (34 degrees; 1.8 airmasses), most
likely due to the proximity and brightness of the tip-tilt
reference star (R = 14.2, d = 14′′).
Images were obtained using the K ′ filter in both the
narrow and wide camera modes. The pixel scales in
these modes are 9.942 mas pix−1 (FOV=10.18′′) and
39.686 mas pix−1 (FOV=40.64′′). All wide-camera ob-
servations were centered on the close Aab binary. The A
and B components were too wide to fit reasonably into
a single narrow-camera exposure, so we took separate
exposure sequences centered on each. We obtained four
wide-camera exposures of the AB system, seven narrow-
camera exposures of A, and four narrow-camera expo-
sures of B; the total integration times for each image set
are 80s, 175s, and 100s, respectively. Each set was pro-
duced with a 3-point box dither pattern that omitted the
bottom-left position due to higher read-noise for the de-
tector in that quadrant. Single exposures were also taken
at the central position.
Our science targets are relatively bright, so all obser-
vations were taken in correlated double-sampling mode,
for which the array read noise is 38 electrons/read. The
read noise is the dominant noise term for identifying
faint sources, yielding 10σ detection limits of K ∼ 19.2
for the wide camera observations, K ∼ 18.8 for the
narrow-camera observations centered on component A,
1 IRAF is distributed by the National Optical Astronomy Ob-
servatories, which are operated by the Association of Universities
for Research in Astronomy, Inc., under cooperative agreement with
the National Science Foundation.
and K ∼ 18.3 for the narrow-camera observations cen-
tered on component B; the detection limits for B are
slightly shallower due to the shorter total integration
time. The data were flat-fielded and dark- and bias-
subtracted using standard IRAF procedures. The images
were distortion-corrected using new high-order distortion
solutions (P. Cameron, in prep) that deliver a significant
performance increase as compared to the solutions pre-
sented in the NIRC2 pre-ship manual2; the typical resid-
uals are ∼4 mas in wide camera mode and ∼0.6 mas in
narrow camera mode. We adopt these systematic limits
as the uncertainty in astrometry for bright objects; all
faint objects (K ∼16-18) have larger uncertainties (∼10
mas) due to photon statistics.
We measured PSF-fitting photometry and astrome-
try for our sources using the IRAF package DAOPHOT
(Stetson 1987), and specifically with the ALLSTAR rou-
tine. We analyzed each frame separately in order to esti-
mate the uncertainty in individual measurements and to
allow for the potential rejection of frames with inferior
AO correction; our final results represent the mean value
for all observations in a filter.
In the wide-camera observations, we produced a tem-
plate PSF based on the B component and the field star
F1 (see Section 3.1 and Figure 1), both of which appear
to be single sources. In the narrow-camera observations
centered on A or B, the science target was the only bright
object detected in our observations, so there was not a
separate source from which to adopt a template PSF. We
could have adopted a template PSF from another set of
observations, but the AO correction usually varies sig-
nificantly between targets since it is very sensitive to the
seeing, elevation, laser return, and tip-tilt separation and
brightness. We found that no other target in our survey
provided a good PSF match.
We addressed this issue for the Aab binary pair by
developing a procedure to reconstruct the single-source
PSF directly from the observations of the binary system.
Our algorithm begins with a preliminary estimate of the
single-source PSF, then iteratively fits both components
of the binary system with the estimated PSF and uses
the synthetic PSF to subtract the best-fit estimate of the
secondary flux. This residual image (which is dominated
by the primary flux distribution) is then used to fit an
improved estimate of the single-source PSF.
DAOPHOT characterizes an empirical PSF in terms
of an analytical function and a lookup table of residuals,
so we first iterated the procedure using a purely ana-
lytical function until it converged, then added a lookup
table to the estimated PSF and iterated until its con-
tents also converged. Observations of single stars sug-
gested that the penny2 function (a gaussian core with
lorentzian wings) would provide the best analytic fit, so
we chose it as our analytic function. Four iterations of
the fitting process were required for the analytic func-
tion to converge and 3 iterations were required for the
lookup table to converge. Our algorithm does not work
for the B component because it appears to be single, so
we adopted the average synthetic single-source PSF from
analysis of the Aab system to perform PSF fitting and
verify that it is single.
We calibrated our photometry using 2MASS K mag-
2 http://www2.keck.hawaii.edu/realpublic/inst/nirc2/
nitudes for the A and B components and the nearby field
star F1 (Section 3). The 2MASS observations were con-
ducted using the Ks filter rather than K
′, but the the-
oretical isochrones computed by Kim et al. (2005) for
the Ks and K
′ systems differ by .0.01 magnitudes for
objects in this color range; this is much smaller than
other uncertainties in the calibration. Carpenter (2001)
found typical zero point shifts of .0.03 magnitudes be-
tween 2MASS Ks and several standard K bandpasses,
all of which are more distinctly different from Ks than
K ′, which also demonstrates that the zero point shift
between Ks and K
′ should be negligible.
The calibration process could introduce systematic un-
certainties if any of the three calibration sources are vari-
able, but based on the small deviation in the individual
calibration offsets for each source (0.03 mag), variability
does not appear to be a significant factor. We tested the
calibration using DENIS K magnitudes and found that
the two methods agree to within 0.01 mag, albeit with a
higher standard deviation (0.12 mag) for DENIS.
3. RESULTS
3.1. Images
In Figure 1, we show a NIRC2 wide-camera image of
the field surrounding USco1606-1935. The A and B com-
ponents are labeled, as are 6 apparent field stars (named
F1 through F6) which we use as astrometric comparison
stars. We found counterparts for the first three field stars
in existing survey catalogues: F1 was detected by all four
sky surveys, F2 was detected by DENIS, USNO-B, and
SSS, and F3 was detected only by USNO-B and SSS.
In Figure 2, we show individual contour plots drawn
from NIRC2 narrow-camera images of the A and B
components. These high-resolution images show that
USco1606-1935 A is itself composed of two sources; we
designate these two components Aa and Ab. We do not
possess any direct diagnostic information to determine if
Aa and Ab are physically associated, but there are only
two other bright sources in the field of view. If the source
count is representative of the surface density of bright
(K < 15) sources along the line of sight, the probability
of finding an unbound bright source within <100 mas of
the A component is only ∼ 10−5. Thus, we consider Aa
and Ab to comprise a physically bound binary system.
3.2. Photometry
Photometric data are generally sufficient to reject most
nonmember interlopers because association members fol-
low a bright, well-defined cluster sequence in color-
magnitude diagrams and most field stars will fall be-
low or bluer than the association sequence. In Table
2, we summarize the observed and archival photometry
for each source in the NIRC2 wide-camera images. In
Figure 3, we show three color-magnitude diagrams (K
versus J − K, H − K, and I2 − K) for our observed
sources and for all spectroscopically-confirmed members
of Upper Sco (as summarized in Kraus & Hillenbrand
2007).
The colors and magnitudes for USco1606-1935 B are
consistent with the known members of Upper Sco, which
supports the assertion that it is an association member.
B is located marginally above and redward of the mean
cluster sequence in the (K,J −K) and (K,H −K) dia-
grams; if this result is genuine and not a consequence of
Fig. 1.— The field surrounding USco1606-1935. The A and B components are labeled, as are 6 apparent field stars. The separation
between the Aa and Ab components is too small to be apparent in this image.
Fig. 2.— Contour plots showing our LGSAO observations of USco1606-1935. The first panel shows an original exposure for the Aab
pair, the second and third panels show Aa and Ab after subtracting best-fit values for the other component, and the last panel shows an
original exposure for B. The contours are drawn at 5% to 95% of the peak pixel values.
Fig. 3.— Color-magnitude diagrams showing all spectroscopically-confirmed members of Upper Sco (black crosses), the A and B binary
components (red), and the other six objects detected in our LGSAO images (blue). The NIR CMDs (top) demonstrate that F1 lies
significantly below the association sequence, and therefore is an unrelated field star. The optical-NIR CMD (bottom) supports this
identification and demonstrates that F2 and F3 are also field stars that lie below the association sequence. We measure formal upper limits
only for stars F4-F6, but marginal R band detections in the POSS plates suggest that F4 and F6 are also field stars. Typical uncertainties
are plotted on the left edge of each plot.
TABLE 1
Coordinates and Photometry
Name RAa DECa KLGS
b K2MASS
b Hb Jb I2b
A 16 06 11.99 -19 35 33.1 11.04 11.02 11.35 12.01 14.1
Aa - - 11.71 - - - -
Ab - - 11.88 - - - -
B 16 06 11.44 -19 35 40.5 11.74 11.78 12.32 13.00 14.9
F1 16 06 12.09 -19 35 18.3 11.51 11.50 11.62 12.27 13.5
F2 16 06 12.90 -19 35 36.1 16.32 - - - 17.8
F3 16 06 13.23 -19 35 23.7 16.66 - - - 18.7
F4 16 06 11.75 -19 35 32.0 17.43 - - - -
F5 16 06 12.40 -19 35 40.3 17.28 - - - -
F6 16 06 12.94 -19 35 44.6 16.97 - - - -
Note. — Photometry is drawn from our observations (KLGS), 2MASS (JHK2MASS),
and the USNO-B1.0 catalogue (I2).
a Coordinates are derived from the 2MASS position for USco1606-1935 A and the rel-
ative separations we measure using LGSAO. The absolute uncertainty in the 2MASS
position with respect to the International Coordinate Reference System (ICRS) is .0.1′′.
b Photometric uncertainties are ∼0.03 mag for LGSAO and 2MASS photometry and
∼0.25 mag for USNO-B1.0 photometry.
TABLE 2
Relative Astrometry
LGSAO K 2MASS K 2MASS H 2MASS J DENIS IJK
(JD=2453773) (JD=2451297) (JD=2451297) (JD=2451297) (JD=2451332)
∆α ∆δ ∆α ∆δ ∆α ∆δ ∆α ∆δ ∆α ∆δ
Aa -0.0132 -0.0149 - - - - - - - -
Ab +0.0201 +0.0266 - - - - - - - -
B -7.825 -7.460 -7.757 -7.455 -7.749 -7.395 -7.834 -7.382 -7.865 -7.448
F1 +1.453 +14.844 +1.401 +14.762 +1.446 +14.732 +1.479 +14.735 +1.418 +14.728
F2 +12.839 -3.017 - - - - - - -a -a
F3 +17.571 +9.370 - - - - - - - -
F4 -3.438 +1.056 - - - - - - - -
F5 +5.805 -7.224 - - - - - - - -
F6 +13.385 -11.540 - - - - - - - -
Note. — The zero-point for all coordinate offsets is the photocenter of the unresolved Aab system. The relative
astrometric uncertainties for 2MASS and DENIS results are ∼40 mas; uncertainties for the LGSAO results are ∼5
mas for bright objects and ∼10 mas for faint objects.
a F2 was marginally detected in i by DENIS, but the astrometry is not sufficiently precise to be useful in calculating
its proper motion.
the photometric uncertainties, it could be a consequence
of differential reddening, aK band excess associated with
a hot disk, or the presence of an unresolved tight binary
companion. However, B does not appear to be as red
in DENIS data (J −K = 0.98), which suggests that the
2MASS result may not be genuine.
The three sources for which we have colors (F1, F2,
and F3) all sit below the Upper Sco member sequence in
the (K,I2 − K) color-magnitude diagram. Some USco
members also fall marginally blueward of the associa-
tion sequence in (K,I2−K); we can find no correlation
with location, multiplicity, or other systematic factors,
so this feature may be a result of intrinsic variability be-
tween the epochs of K and I2. This result suggests that
the (K,I2−K) CMD is not sufficient for ruling out the
membership of F1. However, F1 also sits at the extreme
blueward edge of the association sequence in (K,J −K)
and is clearly distinct from the association sequence in
(K,H − K). We therefore judge that all three sources
are unassociated field star interlopers.
We do not possess sufficient information to determine
whether these three stars are field dwarfs in the Milky
Way disk or background giants in the Milky Way bulge;
the unknown nature of these sources could complicate fu-
ture efforts to calculate absolute proper motions because
comparison to nonmoving background giants is the best
way to establish a nonmoving astrometric frame of refer-
ence. As we will show in Section 3.3, F1 possesses a small
total proper motion (<10 mas yr−1), so it may be a dis-
tant background star. Its 2MASS colors (J −H = 0.65,
H −K = 0.12) place it on the giant sequence in a color-
color diagram, but reddened early-type stars with spec-
tral type <M0 can also reproduce these colors.
We are unable to measure colors for the stars F4, F5,
and F6 because they were detected only in our LGSAO
observations. However, visual inspection of the digitized
POSS plates via Aladdin (Bonnarel et al. 2000) found
possible R band counterparts to F4 and F6 that were
not identified by USNO-B. If these detections are genuine
and these two sources fall near the USNO-B survey limit
(R ∼ 20 − 21), their colors (R − K ∼ 3 − 4 or I2 −
K ∼ 2−3) are too blue to be consistent with association
membership.
3.3. Astrometry
Fig. 4.— Relative separations from the A component to the B component (left) and the field star F1 (right) for our LGSAO data and
archival 2MASS/DENIS data. The blue circles denote LGSAO data, the red circles denote 2MASS data for each filter (J , H, and K), and
the green circles denote the average DENIS values for all three filters (IJK). The black line shows the expected relative astrometry as a
function of time for a stationary object, and the predicted archival astrometry values for the non-moving (background) case are shown on
these curves with red asterisks. The results for component B are consistent with common proper motion; the results for F1 are inconsistent
with common proper motion and suggest that the total proper motion is small, denoting a probable background star.
Fig. 5.— The spectrum of USco1606-1935 B (red) as compared
to a set of standard stars drawn from the field and from the young
Taurus and Upper Sco associations. The overall continuum shape
is best fit by a field standard with spectral type M5; the spec-
trum around the Na doublet at 8189 angstroms is better fit by an
intermediate-age (5 Myr) M5 than a young (1-2 Myr) or field M5,
suggesting that the B component is also intermediate-aged.
The standard method for confirming physical associa-
tion of candidate binary companions is to test for com-
mon proper motion. This test is not as useful for young
stars in associations because other (gravitationally un-
bound) association members have similar proper motions
to within .2-3 mas yr−1. However, proper motion anal-
ysis can still be used to eliminate nearby late-type field
stars and background giants that coincidentally fall along
the association color-magnitude sequence but possess dis-
tinct kinematics.
In Table 2, we summarize the relative astrometry for
the three system components and for the field stars F1-F6
as measured with our LGSAO observations and archival
data from 2MASS and DENIS. All offsets are given with
respect to the photocenter of the unresolved Aab sys-
tem; Aa and Ab have similar fluxes and do not appear
to be variable in any of these measurements (Section 2.3),
so this zero point should be consistent between different
epochs. We evaluated the possibility of including astro-
metric data from older photographic surveys like USNO-
B and SSS, but rejected this idea after finding that the
two surveys reported very large (up to 1′′) differences
in the separation of the A-B system from digitization
of the same photographic plates. We calculated relative
proper motions in each dimension by averaging the four
first-epoch values (2MASS and DENIS; Table 2), then
comparing the result to our second-epoch observation ob-
tained with LGSAO. We did not attempt a least-squares
fit because the 2MASS values are coeval and the DENIS
results were measured only 35 days after the 2MASS re-
sults.
In Figure 4, we plot the relative astrometry between A
and B and between A and F1 as measured by 2MASS,
DENIS, and our LGSAO survey. We also show the ex-
pected relative motion curve if B or F1 are nonmoving
background stars and A moves with the mean proper mo-
tion and parallax of Upper Sco, (µα,µδ)=(-9.3,-20.2) mas
yr−1 and π=7 mas (de Zeeuw et al. 1999; Kraus & Hil-
lenbrand 2007). The total relative motion of B over the
6.8 year observation interval is (+24±25,-40±25) mas;
the corresponding relative proper motion is (+3.5±3.7,-
5.9±3.7) mas yr−1, which is consistent with comovement
to within <2σ. This result is inconsistent with the hy-
pothesis that B is a nonmoving background star at the
8σ level.
The relative motion of F1 is (+17±25,+105±25) mas
or (+2.5±3.7,+15.4±3.7)mas yr−1, which is inconsistent
with comovement at the 4σ level. The absolute proper
motion of F1, assuming A moves with the mean proper
motion of Upper Sco, is (-7±4,-5±4) mas yr−1, which is
consistent with nonmovement to within <2σ. The impli-
cation is that F1 is probably a distant background star,
either a giant or a reddened early-type star.
3.4. Spectroscopy
The least ambiguous method for identifying young
stars is to observe spectroscopic signatures of youth like
lithium or various gravity-sensitive features. Spectro-
scopic confirmation is not strictly necessary in the case of
USco1606-1935 since we confirmed common proper mo-
tion for the A-B system, but a spectral type is also useful
in constraining the physical properties of the secondary,
so we decided to obtain an optical spectrum.
In the top panel of Figure 5, we plot our spectrum
for B in comparison to three standard field dwarfs with
spectral types of M4V-M6V. We qualitatively find that
the standard star which produces the best fit is GJ 866
(M5V). The M4V and M6V standards do not adequately
fit either the overall continuum shape or the depths of
the TiO features at 8000 and 8500 angstroms, so the
corresponding uncertainty in the spectral type is .0.5
subclasses.
In the bottom panel of Figure 5, we plot a restricted
range of the spectrum (8170-8210 angstroms) centered
on the Na-8189 absorption doublet. The depth of the
doublet is sensitive to surface gravity (e.g. Slesnick et
al. 2006a, 2006b); high-gravity dwarfs possess very deep
absorption lines, while low-gravity giants show almost
no absorption. We also plot standard stars of identi-
cal spectral type (M5) spanning a range of ages. The
depth of the B component’s Na 8189 doublet appears to
be consistent with the depth for a member of USco (5
Myr), deeper than that of a Taurus member (1-2 Myr),
and shallower than that of a field star, which confirms
that the B component is a pre-main sequence member of
Upper Sco.
We have quantified our analysis by calculating the
spectral indices TiO-7140, TiO-8465, and Na-8189,
which measure the depth of key temperature- and
gravity-sensitive features (Slesnick et al. 2006a). We
find that T iO7140 = 2.28, T iO8465 = 1.23, and Na8189 =
0.92; all three indices are consistent with our assessment
that B is a young M5 star which has not yet contracted
to the zero-age main sequence.
3.5. Stellar and Binary Properties
In Table 3, we list the inferred stellar and binary
properties for the Aa-Ab and A-B systems, which we
estimate using the methods described in Kraus & Hil-
lenbrand (2007). This procedure calculates component
masses by combining the 5 Myr isochrone of Baraffe et
al. (1998) and the M dwarf temperature scale of Luhman
TABLE 3
Binary Properties
Property Aa-Ab A-B
Measured
Sep (mas) 53.2±1.0 10874±5
PA (deg) 38.7±1.0 226.45±0.03
∆K (mag) 0.17±0.05 0.70±0.05
aproj (AU) 7.7±1.2 1600±200
Inferred
q 0.88±0.05 0.53±0.08
SpTPrim M5±0.5 M5+M5.2(±0.5)
SpTSec M5.2±0.5 M5±0.5
MPrim 0.14±0.02 0.26±0.04
MSec 0.12±0.02 0.14±0.02
Note. — The center of mass for the Aa-Ab
pair is unknown, so we calculate all A-B separa-
tions with respect to the K band photocenter.
et al. (2003) to directly convert observed spectral types
to masses. Relative properties (mass ratios q and relative
spectral types) are calculated by combining the Baraffe
isochrones and Luhman temperature scale with the em-
pirical NIR colors of Bessell & Brett (1998) and the K-
band bolometric corrections of Leggett et al. (1998) to
estimate q and ∆SpT from the observed flux ratio ∆K.
We have adopted the previously-measured spectral
type for A (M5; Preibisch et al. 2002) as the type for
component Aa, but the inferred spectral type for Ab is
only 0.2 subclasses later, so this assumption should be ro-
bust to within the uncertainties (∼0.5 subclasses). The
projected spatial separations are calculated for the mean
distance of Upper Sco, 145±2 pc (de Zeeuw et al. 1999).
If the total radial depth of Upper Sco is equal to its angu-
lar extent (∼15o or ∼40 pc), then the unknown depth of
USco1606-1935 within Upper Sco implies an uncertainty
in the projected spatial separation of ±15%. The sys-
tematic uncertainty due to the uncertainty in the mean
distance of Upper Sco is negligible (.2%).
4. IS USCO1606-1935 AB A BINARY SYSTEM?
The unambiguous identification of pre-main sequence
binaries is complicated by the difficulty of distinguishing
gravitationally bound binary pairs from coeval, comov-
ing association members which are aligned in projection.
Most traditional methods used to confirm field binary
companions do not work in the case of young binaries in
clusters and associations because all association members
share common distances and kinematics (to within cur-
rent observational uncertainties), so the only remaining
option is to assess the probability of chance alignment.
We address this challenge by quantifying the clustering
of PMS stars via calculation of the two-point correlation
function (TPCF) across a wide range of angular scales
(1′′ to >1 degree). This type of analysis has been at-
tempted in the past (e.g. Gomez et al. 1993 for Taurus;
Simon 1997 for Ophiuchus, Taurus, and the Trapezium),
but these studies were conducted using samples that were
significantly incomplete relative to today.
The TPCF, w(θ), is defined as the number of excess
pairs of objects with a given separation θ over the ex-
pected number for a random distribution (Peebles 1980).
The TPCF is linearly proportional to the surface density
of companions per star, Σ(θ) = (N∗/A)[1 +w(θ)], where
A is the survey area and N∗ is the total number of stars.
Fig. 6.— The surface density of companions as a function of separation for young stars and brown dwarfs in Upper Sco. Red symbols
denote results from our wide-binary survey using 2MASS (Kraus & Hillenbrand 2007) and blue symbols denote data for all spectroscopically-
confirmed members in two fields surveyed by Preibisch et al. (2002). The data appear to be well-fit by two power laws (dashed lines) which
most likely correspond to gravitationally bound binaries and unbound clusters of stars that have not yet completely dispersed from their
formation environments. The data points which were used to fit these power laws are denoted with circles; other points are denoted with
crosses.
However, it is often easier to evaluate the TPCF via a
Monte Carlo-based definition, w(θ) = Np(θ)/Nr(θ) − 1,
where Np(θ) is the number of pairs in the survey area
with separations in a bin centered on θ and Nr(θ) is the
expected number of pairs for a random distribution of ob-
jects over the same area (Hewett 1982). The advantage
of this method is that it does not require edge correc-
tions, unlike direct measurement of Σ(θ). We adopted
this method due to its ease of implementation, but we
report our subsequent results in terms of Σ(θ) since it is
a more intuitive quantity.
The current census of Upper Sco members across the
full association is very incomplete, so we implemented
our analysis for intermediate and large separations (θ >
6.4′′) using only members located in two heavily-studied
fields originally observed by Preibisch et al. (2001, 2002;
the 2df-East and 2df-West fields). The census of mem-
bers in these fields may not be complete, but we ex-
pect that it is the least incomplete. The census of com-
panions at smaller separations (1.5-6.4′′) has been uni-
formly studied for all spectroscopically-confirmed mem-
bers (Kraus & Hillenbrand 2007), so we have maximized
the sample size in this separation range by considering
the immediate area around all known members, not just
those within the Preibisch fields. Our survey was only
complete for mass ratios q >0.25, so we do not include
companions with mass ratios q < 0.25.
These choices might lead to systematic biases if the
Preibisch fields are still significantly incomplete or if
the frequency and properties of binary systems show
intra-association variations, but any such incompleteness
would probably change the result by no more than a fac-
tor of 2-3. As we will subsequently show, Σ(θ) varies by 4
orders of magnitude across the full range of θ. The well-
established mass dependence of multiplicity should not
affect our results since the mass function for the Preibisch
fields is similar to that seen for the rest of the association.
In Figure 6, we plot Σ(θ) for Upper Sco, spanning
the separation range −3.5 < log(θ) < 0.25 (1.14′′ to
1.78 deg ). We have fit this relation with two power
laws, one which dominates at small separations (.15-
30′′) and one at larger separations. We interpret the
two segments, following Simon (1997), to be the result
of gravitationally-bound binarity and gravitationally un-
bound intra-association clustering, respectively. We fit
the binary power law to the three lowest-separation bins
(log(θ) < −2.75) because this is the separation range
over which we possess uniform multiplicity data. The
cluster power law was fit to the six highest-separation
bins (log(θ) > −1.25) because those bins have the small-
est uncertainties. Bins corresponding to intermediate
separations seem to follow the two power laws.
We found that the slope of the cluster power law (-
0.14±0.02) is very close to zero, which implies that there
is very little clustering on scales of .1 deg. This re-
sult is not unexpected for intermediate-age associations
like Upper Sco; given the typical intra-association veloc-
ity dispersion (∼1 km s−1) and the age (5 Myr), most
association members have dispersed ∼5 pc (2 deg) rel-
ative to their formation point, averaging out structure
on smaller spatial scales. Simon (1997) found that the
slopes for Taurus, Ophiuchus, and the ONC are steeper,
suggesting that more structure is present on these small
scales at young ages (∼1-2 Myr). The slope of the binary
power law (-3.03±0.24) is much steeper than the clus-
ter regime. The separation range represented is much
larger than the peak of the binary separation distribu-
tion (∼30 AU for field solar-mass stars; Duquennoy &
Mayor 1991), so the steep negative slope corresponds to
the large-separation tail of the separation distribution
function. The two power laws seem to cross at separa-
tions of ∼15-30′′ (aproj ∼ 2500− 5000 AU), though this
result depends on the sample completeness in the binary
and cluster regimes. We interpret this to be the maxi-
mum separation range at which binaries can be identified.
If we extrapolate the cluster power law into the separa-
tion regime of the binary power law, we find that the ex-
pected surface density of unbound coincidentally-aligned
companions is ∼60 deg−2. Given this surface density,
there should be ∼1 chance alignment within 15′′ among
the 366 spectroscopically confirmed members of Upper
Sco. Among the 173 known late-type stars and brown
dwarfs (SpT≥M4) for which this separation range is un-
usually wide, the expected number of chance alignments
with any other member is 0.5. If the mass function of
known members is similar to the total mass function,
approximately half (∼0.25 chance alignments) are ex-
pected to occur with another low-mass member. There-
fore, we expect ∼0.25 chance alignments which might be
mistaken for a low-mass binary pair.
The probability that one or more such chance align-
ments actually exists for a known low-mass USco member
is 25% (based on Poisson statistics), which suggests that
the nature of a single candidate wide pair like USco1606-
1935 AB can not be unambiguously determined. If any
more pairs can be confirmed, then they would represent a
statistically significant excess. The corresponding prob-
ability of finding 2 chance alignments of low-mass mem-
bers is only 2%. As we have described in our survey
of wide multiplicity with 2MASS (Kraus & Hillenbrand
2007), we have identified at least three additional can-
didate ultrawide systems in Upper Sco, so spectroscopic
and astrometric followup of these candidate systems is a
high priority.
5. SUMMARY
We have presented photometric, astrometric, and spec-
troscopic observations of USco1606-1935, a candidate ul-
trawide (∼1600 AU), low-mass (Mtot ∼0.4 M⊙) hierar-
chical triple system in the nearby OB association Upper
Scorpius. We conclude that the ultrawide B component
is a young, comoving member of the association, and
show that the primary is itself a close binary system.
If the Aab and B components are gravitationally
bound, the system would join the growing class of young
multiple systems which have unusually wide separations
as compared to field systems of similar mass. However,
we demonstrate that binarity can not be assumed purely
on probabilistic grounds. Analysis of the association’s
two-point correlation function shows that there is a sig-
nificant probability (25%) that at least one pair of low-
mass association members will be separated by .15′′, so
analysis of the wide binary population requires a system-
atic search for all wide binaries. The detection of another
pair of low-mass members within 15′′ would represent
an excess at the 98% confidence level. In principle, bina-
rity could also be demonstrated by measuring common
proper motion with precision higher than the internal
velocity scatter of the association; given the astromet-
ric precision currently attainable with LGSAO data (.1
mas), the test could be feasible within .5 years.
The authors thank C. Slesnick for providing guidance
in the analysis of young stellar spectra, P. Cameron for
sharing his NIRC2 astrometric calibration results prior
to publication, and the anonymous referee for returning
a helpful and very prompt review. The authors also wish
to thank the observatory staff, and particularly the Keck
LGSAO team, for their tireless efforts in commissioning
this valuable addition to the observatory. Finally, we
recognize and acknowledge the very significant cultural
role and reverence that the summit of Mauna Kea has
always had within the indigenous Hawaiian community.
We are most fortunate to have the opportunity to con-
duct observations from this mountain.
This work makes use of data products from the Two
Micron All-Sky Survey, which is a joint project of the
University of Massachusetts and the Infrared Process-
ing and Analysis Center/California Institute of Tech-
nology, funded by the National Aeronautics and Space
Administration and the National Science Foundation.
This work also makes use of data products from the
DENIS project, which has been partly funded by the
SCIENCE and the HCM plans of the European Com-
mission under grants CT920791 and CT940627. It is
supported by INSU, MEN and CNRS in France, by
the State of Baden-Wrttemberg in Germany, by DG-
ICYT in Spain, by CNR in Italy, by FFwFBWF in
Austria, by FAPESP in Brazil, by OTKA grants F-
4239 and F-013990 in Hungary, and by the ESO C&EE
grant A-04-046. Finally, our research has made use of
the USNOFS Image and Catalogue Archive operated by
the United States Naval Observatory, Flagstaff Station
(http://www.nofs.navy.mil/data/fchpix/).
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