The LuckyCam Survey for Very Low Mass Binaries II: 13 new M4.5-M6.0
  Binaries
Mon. Not. R. Astron. Soc. 000, 1–11 () Printed 2 November 2021 (MN LATEX style file v2.2)
The LuckyCam Survey for Very Low Mass Binaries II: 13 new
M4.5-M6.0 Binaries⋆
N.M. Law1,2†, S.T. Hodgkin2 and C.D. Mackay2
1Department of Astronomy, Mail Code 105-24, California Institute of Technology, 1200 East California Blvd., Pasadena, CA 91125, USA
2Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
ABSTRACT
We present results from a high-angular-resolution survey of 78 very low mass (VLM) binary
systems with 6.0 6 V-K colour 6 7.5 and proper motion > 0.15 arcsec/yr. Twenty-one VLM
binaries were detected, 13 of them new discoveries. The new binary systems range in separa-
tion between 0.18 arcsec and 1.3 arcsec. The distance-corrected binary fraction is 13.5+6.5
−4 %,
in agreement with previous results. Nine of the new binary systems have orbital radii > 10 AU,
including a new wide VLM binary with 27 AU projected orbital separation. One of the new
systems forms two components of a 2300 AU separation triple system. We find that the orbital
radius distribution of the binaries with V-K < 6.5 in this survey appears to be different from
that of redder (lower-mass) objects, suggesting a possible rapid change in the orbital radius
distribution at around the M5 spectral type. The target sample was also selected to investigate
X-ray activity among VLM binaries. There is no detectable correlation between excess X-Ray
emission and the frequency and binary properties of the VLM systems.
Key words: Binaries: close - Stars: low-mass, brown dwarfs - Instrumentation: high angular
resolution - Methods: observational - Techniques: high angular resolution
1 INTRODUCTION
Multiple star systems offer a powerful way to constrain the pro-
cesses of star formation. The distributions of companion masses,
orbital radii and thus binding energies provide important clues to
the systems’ formation processes. In addition, binaries provide us
with a method of directly determining the masses of the stars in
the systems. This is fundamental to the calibration of the mass-
luminosity relation (Henry & McCarthy 1993; Henry et al. 1999;
Ségransan et al. 2000).
A number of recent studies have tested the stellar multiplic-
ity fraction of low-mass and very-low-mass (VLM) stars. The
fraction of known directly-imaged companions to very-low-mass
stars is much lower than that of early M-dwarfs and solar type
stars. Around 57% of solar-type stars (F7–G9) have known stellar
companions (Abt & Levy 1976; Duquennoy & Mayor 1991), while
imaging and radial velocity surveys of early M dwarfs suggest that
between 25% & 42% have companions (Henry & McCarthy 1990;
Fischer & Marcy 1992; Leinert et al. 1997; Reid & Gizis 1997).
For M6–L1 primary spectral types direct imaging studies find bi-
nary fractions of only 10–20% (Close et al. 2003; Siegler et al.
⋆ Based on observations made with the Nordic Optical Telescope, operated
on the island of La Palma jointly by Denmark, Finland, Iceland, Norway,
and Sweden, in the Spanish Observatorio del Roque de los Muchachos of
the Instituto de Astrofisica de Canarias.
† E-mail: nlaw@astro.caltech.edu
2005; Law et al. 2006; Montagnier et al. 2006), and similar binary
fractions have been found for still later spectral types (Bouy et al.
2003; Gizis et al. 2003; Burgasser et al. 2003). Recent radial-
velocity work has, however, suggested that a large fraction of ap-
parently single VLM stars are actually very close doubles, and the
VLM multiplicity fraction may thus be comparable to higher mass
stars (Jeffries & Maxted 2005; Basri & Reiners 2006).
Very low mass M, L and T systems appear to have a tighter
and closer distribution of orbital separations, peaking at around
4 AU compared to 30 AU for G dwarfs (Close et al. 2003). How-
ever, the relatively few known field VLM binaries limit the sta-
tistical analysis of the distribution, in particular for studying the
frequency of the rare large-orbital-radii systems which offer strong
constraints on some formation theories (eg. Bate & Bonnell 2005;
Phan-Bao et al. 2005; Law et al. 2006; Close et al. 2006; Caballero
2007; Artigau et al. 2007).
We have been engaged in a programme to image a large
and carefully selected sample of VLM stars, targeting separations
greater than 1 AU (Law et al. 2005, 2006). The programme has
yielded a total of 18 new VLM binary systems, where VLM is de-
fined as a primary mass <0.11 M⊙. This paper presents the second
of the surveys, targeting field stars in the range M4.5–M6.0. The
spectral type range of this survey is designed to probe the transition
between the properties of the 30 AU median-radius binaries of the
early M-dwarfs and the 4 AU median-radius late M-dwarf binaries.
We observed 78 field M-dwarf targets with estimated spec-
tral types between M4.5 and M6.0, searching for companions with
c© RAS
http://arxiv.org/abs/0704.1812v1
2 N.M. Law et al.
separations between 0.1 and 2.0 arcsec. The surveyed primary stel-
lar masses range from 0.089 M⊙ to 0.11 M⊙ using the models in
Baraffe et al. (1998).
It has been suggested in Makarov (2002) that F & G field
stars detected in the ROSAT Bright Source Catalogue are 2.4 times
more likely to be members of wide (> 0.3 arcsec) multiple sys-
tems than those not detected in X-Rays. There is also a well-known
correlation between activity and stellar rotation rates (eg. Simon
1990; Soderblom et al. 1993; Terndrup et al. 2002). A correlation
between binarity and rotation rate would thus be detectable as a
correlation between activity and binarity. To test these ideas, we di-
vided our targets into two approximately equal numbered samples
on the basis of X-ray activity.
All observations used LuckyCam, the Cambridge Lucky
Imaging system. The system has been demonstrated to reliably
achieve diffraction-limited images in I-band on 2.5m telescopes
(Law 2007; Law et al. 2006; Mackay et al. 2004; Tubbs et al. 2002;
Baldwin et al. 2001). A Lucky Imaging system takes many rapid
short-exposure images, typically at 20-30 frames per second. The
turbulence statistics are such that a high-quality, near-diffraction-
limited frame is recorded a few percent of the time; in Lucky Imag-
ing only those frames are aligned and co-added to produce a final
high-resolution image. Lucky Imaging is an entirely passive pro-
cess, and thus introduces no extra time overheads beyond those re-
quired for standard CCD camera observations. The system is thus
very well suited to rapid high-angular-resolution surveys of large
numbers of targets.
In section 2 we describe the survey sample and the X-Ray
activity selection. Section 3 describes the observations and their
sensitivity. Section 4 describes the properties of the 13 new VLM
binaries, and section 5 discusses the results.
2 THE SAMPLE
We selected a magnitude and colour limited sample of nearby late
M-dwarfs from the LSPM-North High Proper motion catalogue
(Lépine & Shara 2005). The LSPM-North catalogue is a survey of
the Northern sky for stars with annual proper motions greater than
0.15”/year. Most stars in the catalogue are listed with both 2MASS
IR photometry and V-band magnitudes estimated from the photo-
graphic BJ and RF bands.
The LSPM-North high proper motion cut ensures that all stars
are relatively nearby, and thus removes contaminating distant giant
stars from the sample. We cut the LSPM catalogue to include only
stars with V-K colour >6 and 67.5, and K-magnitude brighter than
10. The colour cut selects approximately M4.5 to M6.0 stars; its
effectiveness is confirmed in Law et al. (2006).
2.1 X-ray selection
After the colour and magnitude cuts the sample contained 231 late
M-dwarfs. We then divide the stars into two target lists on the basis
of X-ray activity. We mark a star as X-ray active if the target star
has a ROSAT All-Sky Survey detection from the Faint Source Cat-
alogue (Voges 2000) or the Bright Source catalogue (Voges 1999)
within 1.5× the 1σ uncertainty in the X-ray position. Known or
high-probability non-stellar X-Ray associations noted in the QORG
catalogue of radio/X-ray sources (Flesch & Hardcastle 2004) are
removed. Finally, we manually checked the Digitized Sky Survey
(DSS) field around each star to remove those stars which did not
show an unambiguous association with the position of the X-ray
 4  5  6  7  8  9  10  11  12
2MASS K magnitude
Non-activeN
X-ray-active
 5.5  6  6.5  7  7.5  8
LSPM V-K colour
Non-active
 16 X-ray-active
 0  5  10  15  20  25  30
Photometric Estimated Distance / pc
Non-activeN
X-ray-active
Figure 1. The 2MASS K-magnitude, V-K colour and distance distributions
of the X-ray-active and non-X-ray-active samples. Distances are estimated
from the LSPM V-K colours of the samples and the V-K photometric ab-
solute magnitude relations detailed in Leggett (1992). The distances shown
in this figure have a precision of approximately 30%, and assume that all
targets are single stars.
detection. The completeness and biases of the X-Ray selection are
discussed in section 5.2.
It should be noted that the fraction of stars which show mag-
netic activity (as measured in Hα) reaches nearly 100% at a spectral
type of M7, and so the X-ray selection here picks only especially
active stars (Gizis et al. 2000; Schmitt & Liefke 2004). However,
for convenience, we here denote the stars without ROSAT evidence
for X-Ray activity as “non-X-ray active”.
One star in the remaining sample, LSPM J0336+3118, is
listed as a T-Tauri in the SIMBAD database, and was therefore re-
moved from the sample. We note that in the case of the newly de-
tected binary LSPM J0610+2234, which is ∼0.7σ away from the
ROSAT X-Ray source we associate with it, there is another bright
star at 1.5σ distance which may be the source of the X-Ray emis-
sion. GJ 376B is known to be a common-proper-motion companion
to the G star GJ 376, located at a distance of 134 arcsec (Gizis et al.
2000). Since the separation is very much greater than can detected
in the LuckyCam survey, we treat it as a single star in the following
analysis.
2.2 Target distributions
These cuts left 51 X-ray active stars and 179 stars without evidence
for X-Ray activity. We drew roughly equal numbers of stars at ran-
dom from these both these lists to form the final observing target set
c© RAS, MNRAS 000, 1–11
13 New VLM Binaries 3
LSPM ID Other Name K V-K Est. SpT PM/”/yr LSPM ID Other Name K V-K Est. SpT PM/”/yr
LSPM J0023+7711 LHS 1066 9.11 6.06 M4.5 0.839 LSPM J0722+7305 9.44 6.20 M4.5 0.178
LSPM J0035+0233 9.54 6.82 M5.0 0.299 LSPM J0736+0704 G 89-32 7.28 6.01 M4.5 0.383
LSPM J0259+3855 G 134-63 9.52 6.21 M4.5 0.252 LSPM J0738+4925 LHS 5126 9.70 6.34 M4.5 0.497
LSPM J0330+5413 9.28 6.92 M5.0 0.151 LSPM J0738+1829 9.81 6.58 M5.0 0.186
LSPM J0406+7916 G 248-12 9.19 6.43 M4.5 0.485 LSPM J0810+0109 9.74 6.10 M4.5 0.194
LSPM J0408+6910 G 247-12 9.40 6.08 M4.5 0.290 LSPM J0824+2555 9.70 6.10 M4.5 0.233
LSPM J0409+0546 9.74 6.34 M4.5 0.255 LSPM J0825+6902 LHS 246 9.16 6.47 M4.5 1.425
LSPM J0412+3529 9.79 6.25 M4.5 0.184 LSPM J0829+2646 V* DX Cnc 7.26 7.48 M5.5 1.272
LSPM J0414+8215 G 222-2 9.36 6.13 M4.5 0.633 LSPM J0841+5929 LHS 252 8.67 6.51 M5.0 1.311
LSPM J0417+0849 8.18 6.36 M4.5 0.405 LSPM J0849+3936 9.64 6.25 M4.5 0.513
LSPM J0420+8454 9.46 6.10 M4.5 0.279 LSPM J0858+1945 V* EI Cnc 6.89 7.04 M5.5 0.864
LSPM J0422+3900 9.67 6.10 M4.5 0.840 LSPM J0859+2918 LP 312-51 9.84 6.26 M4.5 0.434
LSPM J0439+1615 9.19 7.05 M5.5 0.797 LSPM J0900+2150 8.44 7.76 M6.5 0.782
LSPM J0501+2237 9.23 6.21 M4.5 0.248 LSPM J0929+2558 LHS 269 9.96 6.67 M5.0 1.084
LSPM J0503+2122 NLTT 14406 8.89 6.28 M4.5 0.177 LSPM J0932+2659 GJ 354.1 B 9.47 6.33 M4.5 0.277
LSPM J0546+0025 EM* RJHA 15 9.63 6.50 M4.5 0.309 LSPM J0956+2239 8.72 6.06 M4.5 0.533
LSPM J0602+4951 LHS 1809 8.44 6.20 M4.5 0.863 LSPM J1848+0741 7.91 6.72 M5.0 0.447
LSPM J0604+0741 9.78 6.15 M4.5 0.211 LSPM J2215+6613 7.89 6.02 M4.5 0.208
LSPM J0657+6219 GJ 3417 7.69 6.05 M4.5 0.611 LSPM J2227+5741 NSV 14168 4.78 6.62 M5.0 0.899
LSPM J0706+2624 9.95 6.26 M4.5 0.161 LSPM J2308+0335 9.86 6.18 M4.5 0.281
LSPM J0711+4329 LHS 1901 9.13 6.74 M5.0 0.676
Table 1. The observed non-X-ray-emitting sample. The quoted V & K magnitudes are taken from the LSPM catalogue. K magnitudes are based on 2MASS
photometry; the LSPM-North V-band photometry is estimated from photographic BJ and RF magnitudes and is thus approximate only, but is sufficient for
spectral type estimation – see section 4.2. Spectral types and distances are estimated from the V & K photometry (compared to SIMBAD spectral types) and
the young-disk photometric parallax relations described in Leggett (1992). Spectral types have a precision of approximately 0.5 spectral classes and distances
have a precision of ∼30%.
LSPM ID Other Name K V-K ST PM/as/yr ROSAT BSC/FSC ID ROSAT CPS
LSPM J0045+3347 9.31 6.50 M4.5 0.263 1RXS J004556.3+334718 2.522E-02
LSPM J0115+4702S 9.31 6.04 M4.5 0.186 1RXS J011549.5+470159 4.323E-02
LSPM J0200+1303 6.65 6.06 M4.5 2.088 1RXS J020012.5+130317 1.674E-01
LSPM J0207+6417 8.99 6.25 M4.5 0.283 1RXS J020711.8+641711 8.783E-02
LSPM J0227+5432 9.33 6.05 M4.5 0.167 1RXS J022716.4+543258 2.059E-02
LSPM J0432+0006 9.43 6.37 M4.5 0.183 1RXS J043256.1+000650 1.557E-02
LSPM J0433+2044 8.96 6.47 M4.5 0.589 1RXS J043334.8+204437 9.016E-02
LSPM J0610+2234 9.75 6.68 M5.0 0.166 1RXS J061022.8+223403 8.490E-02
LSPM J0631+4129 8.81 6.34 M4.5 0.212 1RXS J063150.6+412948 4.275E-02
LSPM J0813+7918 LHS 1993 9.13 6.07 M4.5 0.539 1RXS J081346.5+791822 1.404E-02
LSPM J0921+4330 GJ 3554 8.49 6.21 M4.5 0.319 1RXS J092149.3+433019 3.240E-02
LSPM J0953+2056 GJ 3571 8.33 6.15 M4.5 0.535 1RXS J095354.6+205636 2.356E-02
LSPM J0958+0558 9.04 6.17 M4.5 0.197 1RXS J095856.7+055802 2.484E-02
LSPM J1000+3155 GJ 376B 9.27 6.86 M5.0 0.523 1RXS J100050.9+315555 2.383E-01
LSPM J1001+8109 9.41 6.20 M4.5 0.363 1RXS J100121.0+810931 3.321E-02
LSPM J1002+4827 9.01 6.57 M5.0 0.426 1RXS J100249.7+482739 6.655E-02
LSPM J1125+4319 9.47 6.16 M4.5 0.579 1RXS J112502.7+431941 5.058E-02
LSPM J1214+0037 7.54 6.33 M4.5 0.994 1RXS J121417.5+003730 9.834E-02
LSPM J1240+1955 9.69 6.08 M4.5 0.307 1RXS J124041.4+195509 2.895E-02
LSPM J1300+0541 7.66 6.02 M4.5 0.959 1RXS J130034.2+054111 1.400E-01
LSPM J1417+3142 LP 325-15 7.61 6.19 M4.5 0.606 1RXS J141703.1+314249 1.145E-01
LSPM J1419+0254 9.07 6.29 M4.5 0.233 1RXS J141930.4+025430 2.689E-02
LSPM J1422+2352 LP 381-49 9.65 6.38 M4.5 0.248 1RXS J142220.3+235241 2.999E-02
LSPM J1549+7939 G 256-25 8.86 6.11 M4.5 0.251 1RXS J154954.7+793949 2.033E-02
LSPM J1555+3512 8.04 6.02 M4.5 0.277 1RXS J155532.2+351207 1.555E-01
LSPM J1640+6736 GJ 3971 8.95 6.91 M5.0 0.446 1RXS J164020.0+673612 7.059E-02
LSPM J1650+2227 8.31 6.38 M4.5 0.396 1RXS J165057.5+222653 6.277E-02
LSPM J1832+2030 9.76 6.28 M4.5 0.212 1RXS J183203.0+203050 1.634E-01
LSPM J1842+1354 7.55 6.28 M4.5 0.347 1RXS J184244.9+135407 1.315E-01
LSPM J1926+2426 8.73 6.37 M4.5 0.197 1RXS J192601.4+242618 1.938E-02
LSPM J1953+4424 6.85 6.63 M5.0 0.624 1RXS J195354.7+442454 1.982E-01
LSPM J2023+6710 9.17 6.60 M5.0 0.296 1RXS J202318.5+671012 2.561E-02
LSPM J2059+5303 GSC 03952-01062 9.12 6.34 M4.5 0.170 1RXS J205921.6+530330 4.892E-02
LSPM J2117+6402 9.18 6.62 M5.0 0.348 1RXS J211721.8+640241 3.628E-02
LSPM J2322+7847 9.52 6.97 M5.0 0.227 1RXS J232250.1+784749 2.631E-02
LSPM J2327+2710 9.42 6.07 M4.5 0.149 1RXS J232702.1+271039 4.356E-02
LSPM J2341+4410 5.93 6.48 M4.5 1.588 1RXS J234155.0+441047 1.772E-01
Table 2. The observed X-ray emitting sample. The star properties are estimated as described in the caption to table 1. ST is the estimated spectral type; the
ROSAT flux is given in units of counts per second.
c© RAS, MNRAS 000, 1–11
4 N.M. Law et al.
 0  2  4  6  8  10
X-ray active
Non-X-ray active
Figure 2. The observed samples, plotted in a V/V-K colour-magnitude dia-
gram. The background distribution shows all stars in the LSPM-North cat-
alogue.
Name Ref.
GJ 3417 Henry et al. (1999)
G 89-32B Henry et al. (1997)
V* EI Cnc Gliese & Jahreiß (1991)
LP 595-21 Luyten (1997)
GJ 1245 McCarthy et al. (1988)
GJ 3928 McCarthy et al. (2001)
GJ 3839 Delfosse et al. (1999)
LHS 1901 Montagnier et al. (2006)
Table 3. The previously known binaries which were re-detected by Lucky-
Cam in this survey.
of 37 X-Ray active stars and 41 non-X-ray active stars (described
in tables 1 and 2). Four of the X-Ray active stars and 4 of the non-
X-ray stars were previously known to be binary systems (detailed
in table 3), but were reimaged with LuckyCam to ensure a uniform
survey sensitivity in both angular resolution and detectable com-
panion contrast ratio.
Figure 1 shows the survey targets’ distributions in K magni-
tude, V-K colour and photometrically estimated distance. Figure 2
compares the targets to the rest of the stars in the LSPM catalogue.
The X-ray and non-X-ray samples are very similar, although the
non-X-ray sample has a slightly higher median distance, at 15.4pc
rather than 12.2pc (the errors on the distance determination are
about 30%).
3 OBSERVATIONS
We imaged all 78 targets in a total of 11 hours of on-sky time in
June and November 2005, using LuckyCam on the 2.56m Nordic
Optical Telescope. Each target was observed for 100 seconds in
both i’ and the z’ filters. Most of the observations were performed
through varying cloud cover with a median extinction on the order
of three magnitudes. This did not significantly affect the imaging
performance, as all these stars are 3-4 magnitudes brighter than
the LuckyCam guide star requirements, but the sensitivity to faint
objects was reduced and no calibrated photometry was attempted.
3.1 Binary detection and photometry
Companions were detected according to the criteria described in
detail in Law et al. (2006). We required 10σ detections above both
photon and speckle noise; the detections must appear in both i’ and
z’ images. Detection is confirmed by comparison with point spread
function (PSF) reference stars imaged before and after each target.
In this case, because the observed binary fraction is only ∼30%,
other survey sample stars serve as PSF references. We measured
resolved photometry of each binary system by the fitting and sub-
traction of two identical PSFs to each image, modelled as Moffat
functions with an additional diffraction-limited core.
3.2 Sensitivity
The sensitivity of the survey was limited by the cloud cover. Be-
cause of the difficulty of flux calibration under very variable extinc-
tion conditions we do not give an overall survey sensitivity. How-
ever, a minimum sensitivity can be estimated. LuckyCam requires
an i’=+15.5m guide star to provide good correction; all stars in
this survey must appear to be at least that bright during the obser-
vations1. The sensitivity of the survey around a i=+15.5m star is
calculated in Law et al. (2006) and the sensitivity as a function of
companion separation is discussed in section 5.4.
The survey is also sensitive to white dwarf companions around
all stars in the sample. However, until calibrated resolved photom-
etry or spectroscopy is obtained for the systems it is not possi-
ble to distinguish between M-dwarf and white-dwarf companions.
Since a large sample of very close M-dwarf companions to white
dwarf primaries have been found spectroscopically (for example,
Delfosse et al. 1999; Raymond et al. 2003), but very few have been
resolved, it is unlikely that the companions are white dwarfs. It
will, however, be of interest to further constrain the frequency of
white-dwarf M-dwarf systems.
4 RESULTS & ANALYSIS
We found 13 new very low mass binaries. The binaries are shown
in figure 3 and the observed properties of the systems are detailed
in table 4. In addition to the new discoveries, we also confirmed
eight previously known binaries, detailed in tables 3 and 4.
4.1 Confirmation of physical association
Seven of the newly discovered binaries have moved more than one
DSS PSF-radius between the acquisition of DSS images and these
observations (table 5). With a limiting magnitude of iN ∼ +20.3m
(Gal et al. 2004), the DSS images are deep enough for clear de-
tection of all the companions found here, should they actually be
stationary background objects. None of the DSS images show an
object at the present position of the detected proposed companion,
confirming the common proper motions of these companions with
their primaries.
The other binaries require a probabilistic assessment. In the
entire LuckyCam VLM binary survey, covering a total area of
(22′′ × 14.4′′) × 122 fields, there are 10 objects which would have
1 LSPM J2023+6710 was observed though ∼5 magnitudes of cloud, much
more than any other target in the survey, and was too therefore faint for
good performance Lucky Imaging. However, its bright companion is at 0.9
arcsec separation and so was easily detected.
c© RAS, MNRAS 000, 1–11
13 New VLM Binaries 5
LSPM J0035+0233
0.25"
LSPM J0409+0546
NLTT 14406
0.26"
LSPM J0610+2234
0.18"
LHS 1901
0.26"
LHS 5126
LP 312-51
0.26"
LSPM J0045+3347
0.27"
LSPM J0115+4702
0.68"
LSPM J0227+5432
0.90"
G 134-63
GJ 3554
0.90"
LSPM J2023+6710
1.30"
LSPM J1832+2030
Figure 3. The newly discovered binaries. All images are orientated with North up and East to the left. The images are the results of a Lucky Imaging selection
of the best 10% of the frames taken in i’, with the following exceptions: LSPM J0409+0546, LSPM J0610+2234 and LP 312-51 are presented in the z’
band, as the cloud extinction was very large during their i’ observations. The image of LSPM LHS 5126 uses the best 2% of the frames taken and LSPM
J0115+4702S uses the best 1%, to improve the light concentration of the secondary. LSPM J2023+6710 was observed through more than 5 magnitudes of
cloud extinction, and was thus too faint for Lucky Imaging; a summed image with each frame centroid-centred is presented here, showing clear binarity. LHS
1901 was independently found by Montagnier et al. (2006) during a similar M-dwarf survey. We present our image here to confirm its binarity.
been detected as companions if they had happened to be close to the
target star. One of the detected objects is a known wide common
proper motion companion; others are due to random alignments.
For the purposes of this calculation we assume that all detected
widely separated objects are not physically associated with the tar-
get stars.
Limiting the detection radius to 2 arcsec around the target star
(we confirm wider binaries by testing for common proper motion
against DSS images) 0.026 random alignments would be expected
in our dataset. This corresponds to a probability of only 2.5 per
cent that one or more of the apparent binaries detected here is a
chance alignment of the stars. We thus conclude that all the detected
binaries are physically associated systems.
4.2 Constraints on the nature of the target stars
Clouds unfortunately prevented calibrated resolved photometry for
the VLM systems. However, unresolved V & K-band photometry
listed in the LSPM survey gives useful constraints on the spectral
types of the targets. About one third of the sample has a listed spec-
tral type in the SIMBAD database (from Jaschek 1978). To obtain
estimated spectral types for the VLM binary systems, we fit the
LSPM V-K colours to those spectral types. The fit has a 1σ un-
certainty of ∼0.5 spectral types. The colour-magnitude relations in
Leggett (1992) show the unresolved system colour is dominated
by the primary for all M2–M9 combinations of primary and sec-
ondary. We then estimate the secondaries’ spectral types by: 1/ as-
suming the estimated primary spectral type to be the true value and
2/ using the spectral type vs. i’ and z’ absolute magnitude relations
in Hawley et al. (2002) to estimate the difference in spectral types
between the primary and secondary. This procedure gives useful
constraints on the nature of the systems, although resolved spec-
troscopy is required for definitive determinations.
c© RAS, MNRAS 000, 1–11
6 N.M. Law et al.
Name ∆i′ ∆z′ Sep. (arcsec) P.A. (deg) Epoch X-ray emitter?
LSPM J0035+0233 1.30 ± 0.30 · · · 0.446 ± 0.01 14.3 ± 1.4 2005.9
LSPM J0409+0546 < 1.5 < 1.5 0.247 ± 0.01 40.0 ± 3.2 2005.9
NLTT 14406 1.30 ± 0.30 0.77 ± 0.30 0.310 ± 0.01 351.6 ± 1.1 2005.9
LSPM J0610+2234 < 1.0 < 1.0 0.255 ± 0.01 268.4 ± 2.7 2005.9 *
LHS 5126 0.50 ± 0.20 0.50 ± 0.30 0.256 ± 0.02 235.1 ± 3.4 2005.9
LP 312-51 0.74 ± 0.10 0.51 ± 0.10 0.716 ± 0.01 120.5 ± 1.1 2005.9
LSPM J0045+3347 0.80 ± 0.35 0.77 ± 0.35 0.262 ± 0.01 37.6 ± 1.9 2005.9 *
LSPM J0115+4702S 0.55 ± 0.25 0.73 ± 0.25 0.272 ± 0.01 249.8 ± 1.3 2005.9 *
LSPM J0227+5432 0.60 ± 0.10 0.59 ± 0.10 0.677 ± 0.01 275.8 ± 1.1 2005.9 *
G 134-63 1.55 ± 0.10 1.35 ± 0.10 0.897 ± 0.01 13.6 ± 1.1 2005.9
GJ 3554 0.51 ± 0.20 0.57 ± 0.20 0.579 ± 0.01 44.0 ± 1.1 2005.9 *
LSPM J2023+6710 0.55 ± 0.20 · · · 0.900 ± 0.15 232.5 ± 3.2 2005.9 *
LSPM J1832+2030 0.48 ± 0.10 0.45 ± 0.10 1.303 ± 0.01 20.6 ± 1.1 2005.4 *
GJ 3417 1.66 ± 0.10 1.42 ± 0.10 1.526 ± 0.01 −39.8 ± 1.0 2005.9
LHS 1901 1.30 ± 0.70 1.30 ± 0.70 0.177 ± 0.01 51.4 ± 1.6 2005.9
G 89-32 0.43 ± 0.10 0.38 ± 0.10 0.898 ± 0.01 61.3 ± 1.0 2005.9
V* EI Cnc 0.62 ± 0.10 0.49 ± 0.10 1.391 ± 0.01 76.6 ± 1.0 2005.9
LP 595-21 0.74 ± 0.10 0.60 ± 0.10 4.664 ± 0.01 80.9 ± 1.0 2005.9 *
GJ 1245AC 2.95 ± 0.20 2.16 ± 0.20 1.010 ± 0.01 −11.3 ± 1.0 2005.4 *
GJ 3928 2.32 ± 0.20 2.21 ± 0.20 1.556 ± 0.01 −10.7 ± 1.0 2005.4 *
LP 325-15 0.36 ± 0.10 0.33 ± 0.10 0.694 ± 0.01 −21.5 ± 1.0 2005.4 *
Table 4. The observed properties of the detected binaries. The top group are stars with newly detected companions; the bottom group are the previously known
systems. LSPM J0409+0546 and LSPM J0610+2234 were observed though thick cloud and in poor seeing, and so only upper limits on the contrast ratio are
given. LSPM J2023+6710 was not observed in z’, and cloud prevented useful z’ observations of LSPM J0035+0233.
LSPM ID Years since DSS obs. Dist. moved
1RXS J004556.3+334718 16.2 4.3”
G 134-63 16.2 4.1”
NLTT 14406 19.1 3.4”
LHS 5126 6.8 3.4”
LP 312-51 7.6 3.3”
GJ 3554 15.8 5.0”
LSPM J2023+6710 14.2 4.2”
Table 5. The newly discovered binaries which have moved far enough since
DSS observations to allow confirmation of the common proper motion of
their companions.
4.3 Distances
The measurement of the distances to the detected binaries is a vital
step in the determination of the orbital radii of the systems. None of
the newly discovered binaries presented here has a measured par-
allax (although four2 of the previously known systems do) and cal-
2 G 132-25 (NLTT 2511) is listed in Reid & Cruz (2002) and the SIM-
BAD database as having a trigonometric parallax of 14.7 ± 4.0 mas,
based on the Yale General Catalogue of Trigonometric Stellar Parallaxes
(van Altena et al. 2001). However, this appears to be a misidentification, as
the star is not listed in the Yale catalogue. The closest star listed, which
does have the parallax stated for G 132-25 in Reid & Cruz (2002), is LP
294-2 (NLTT 2532). This star has a very different proper motion speed and
direction to G 132-25 (0.886 arcsec/yr vs. 0.258 arcsec/yr in the LSPM cat-
alogue & SIMBAD). In addition, the G 132-25 LSPM V and K photometry
is inconsistent with that of an M-dwarf at a distance of 68pc. We thus do
not use the stated parallax for G 132-25.
ibrated resolved photometry is not available for almost all the sys-
tems. We therefore calculate distances to the newly discovered sys-
tems using the V-K colour-absolute magnitude relations described
in Leggett (1992). Calculation of the distances in this manner re-
quires care, as the V and K-band photometry is unresolved, and so
two luminous bodies contribute to the observed colours and mag-
nitudes.
The estimated distances to the systems, and the resulting or-
bital separations, are given in table 6. The stated 1σ distance ranges
include the following contributions:
• A 0.6 magnitude Gaussian-distributed uncertainty in the V-K
colour of the system (a combination of the colour uncertainty noted
in the LSPM catalogue and the maximum change in the V-K colour
of the primary induced by a close companion).
• A 0.3 magnitude Gaussian-distributed uncertainty in the abso-
lute K-band magnitude of the system, from the uncertainty in the
colour-absolute magnitude relations from Leggett 1992.
• A 0.75 magnitude flat-distributed uncertainty in the absolute
K-band magnitude of the system, to account for the unknown K-
band contrast ratio of the binary system.
The resulting distances have 1σ errors of approximately 35%,
with a tail towards larger distances due to the K-band contrast ratio
uncertainties.
4.4 NLTT 14406 – A Newly Discovered Triple System
We found NLTT 14406 to have a 0.31 arcsec separation companion.
NLTT 14406 is identified with LP 359-186 in the NLTT catalogue
(Luyten 1995), although it is not listed in the revised NLTT cat-
alogue (Salim & Gould 2003). LP 359-186 is a component of the
common-proper-motion (CPM) binary LDS 6160 (Luyten 1997),
with the primary being LP 359-216 (NLTT 14412), 167 arcsec dis-
tant and listed in the SIMBAD database as a M2.5 dwarf.
c© RAS, MNRAS 000, 1–11
13 New VLM Binaries 7
Name Parallax / mas Distance / pc Orbital rad. / AU Prim. ST (est.) Sec. ST (est.)
LSPM J0035+0233 · · · 14.5+6.3
−2.4 6.8
−1.0 M5.0 M6.0
LSPM J0409+0546 · · · 19.9+9.1
−3.8 4.9
−0.7 M4.5 6M6.0
NLTT 14406 · · · 13.7+6.5
4.4+2.3
−0.7 M4.5 M5.5
LSPM J0610+2234 · · · 17.0+7.5
−2.9 4.6
−0.8 M5.0 6M6.0
LHS 5126 · · · 19.5+8.9
−3.7 4.9
−0.6 M4.5 M5.0
LP 312-51 · · · 21.5+10.1
−4.0 16.1
−2.7 M4.5 M5.0
LSPM J0045+3347 · · · 14.9+7.0
−2.6 4.0
−0.6 M4.5 M5.5
LSPM J0115+4702S · · · 18.7+9.3
−3.6 5.2
−0.9 M4.5 M5.0
LSPM J0227+5432 · · · 18.6+9.5
−3.4 13.2
−2.2 M4.5 M5.0
G 134-63 · · · 18.8+9.3
−3.4 17.6
−2.8 M4.5 M5.5
GJ 3554 · · · 11.8+5.6
−2.2 7.1
−1.2 M4.5 M4.5
LSPM J2023+6710 · · · 13.6+5.9
−2.5 12.8
−2.6 M5.0 M5.0
LSPM J1832+2030 · · · 20.6+9.6
−3.9 27.0
+14.6
−4.0 M4.5 M5.0
GJ 3417 87.4 ± 2.3 11.4+0.3
−0.3 17.5
−0.5 M4.5 M6.5
G 89-32 · · · 7.3+3.9
−1.3 6.5
−1.1 M4.5 M5.0
LHS 1901 · · · 12.3+5.6
−2.0 2.3
−0.4 M4.5 M6.0
V* EI Cnc 191.2 ± 2.5 5.23+0.07
−0.07 7.27
+0.11
−0.11 M5.5 M6.0
LP 595-21 · · · 16.5+8.2
−2.7 76.2
+38.7
−11.8 M4.5 M5.5
GJ 1245AC 220.2 ± 1.0 4.54+0.02
−0.02 4.6
+0.05
−0.05
M5.0 M8.5
GJ 3928 · · · 10.2+5.6
−1.7 15.7
−2.5 M4.5 M6.5
LP 325-15 62.2 ± 13.0 16.1+3.4
−3.4 11.2
−2.4 M4.5 M4.5
Table 6. The derived properties of the binary systems. The top group are stars with newly detected companions; the bottom group are the previously known
binaries. All parallaxes are from the Yale General Catalogue of Trigonometric Stellar Parallaxes (van Altena et al. 2001). Distances and orbital radii are
estimated as noted in the text; the stated errors are 1σ. The primaries’ spectral types have a 1σ uncertainty of ∼0.5 subtypes (section 4.2); the difference in
spectral types is accurate to ∼0.5 spectral subtypes.
The identification of these high proper motion stars can be
occasionally problematic when working over long time baselines.
As a confirmatory check, the LSPM-listed proper motion speeds
and directions of these candidate CPM stars agree to within 1σ
(using the stated LSPM proper motion errors).
In the LSPM catalogue, the two stars are separated by 166.3
arcsec at the J2000.0 epoch. We thus identify our newly discovered
4.4 AU separation companion to NLTT 14406 as a member of a
triple system with an M2.5 primary located at 22801080
−420 AU separa-
tion.
5 DISCUSSION
5.1 The binary frequency of stars in this survey
We detected 13 new binaries in a sample of 78 VLM stars, as well
as a further 8 previously known binaries. This corresponds to a bi-
nary fraction of 26.9+5.5
−4.4%, assuming Poisson errors. However, the
binaries in our sample are brighter on average than single stars of
the same colour and so were selected from a larger volume. Cor-
recting for this, assuming a range of contrast ratio distributions be-
tween all binaries being equal magnitude and all constrast ratios
being equally likely (Burgasser et al. 2003), we find a distance-
corrected binary fraction of 13.5+6.5
−4 %.
However, because the binaries are more distant on average
than the single stars in this survey, they also have a lower aver-
age proper motion. The LSPM proper motion cut will thus pref-
erentially remove binaries from a sample which is purely selected
on magnitude and colour. The above correction factor for the in-
creased magnitude of binary systems does not include this effect,
and so will underestimate the true binary fraction of the survey.
5.2 Biases in the X-ray sample
Before testing for correlations between X-ray emission and binary
parameters, it is important to assess the biases introduced in the
selection of the X-ray sample. The X-ray flux assignment criteria
described in section 2.1 are conservative. To reduce false associa-
tions, the X-ray source must appear within 1.5σ of the candidate
star, which implies that ∼13% of true associations are rejected. The
requirement for an unambiguous association will also reject some
fraction of actual X-ray emitters (10% of the candidate emitting
systems were rejected on this basis). The non-X-ray emitting sam-
ple will thus contain some systems that do actually meet the X-ray
flux-emitting limit.
The X-ray source detection itself, which cuts only on the de-
tection limit in the ROSAT Faint Source catalogue, is biased both
towards some sky regions (the ROSAT All-Sky Survey does not
have uniform exposure time (Voges 1999)) and towards closer stars.
However, these biases have only a small effect: all but three of the
target stars fall within the relatively constant-exposure area of the
ROSAT survey, where the brightness-cutoff is constant to within
c© RAS, MNRAS 000, 1–11
8 N.M. Law et al.
 6  6.1  6.2  6.3  6.4  6.5  6.6  6.7  6.8  6.9  7
Figure 4. The fraction of stellar luminosity which appears as X-Ray emis-
sion. Empty circles denote single stars; filled circles denote the binaries
detected in this survey. No binarity correction is made to either the X-Ray
flux or K-magnitude. The two high points are likely to be due to flaring.
about 50%. The samples also do not show a large bias in distance
– the X-ray stars’ median distance is only about 10% smaller than
that of the non-X-ray sample (figure 1).
Finally, the X-Ray active stars also represent a different stel-
lar population from the non-active sample. In particular, the X-ray
active stars are more likely to be young (eg. Jeffries (1999) and ref-
erences therein). It may thus be difficult to disentangle the biases
introduced by selecting young stars from those intrinsic to the pop-
ulation of X-ray emitting older stars. As the results below show,
there are no detectable correlations between binarity and X-ray
emission. If correlations are detected in larger samples, constraints
on the ages of the targets would have to be found to investigate the
causes of the correlations.
5.3 Is X-ray activity an indicator of binarity?
11 of the 21 detected binaries are X-ray active. The non-distance-
corrected binary fraction of X-Ray active targets in our survey is
thus 30+8
−6%, and that of non-X-ray-active targets is 24
−5%. X-Ray
activity therefore does not appear to be an indicator of binarity.
The fraction of the X-ray target’s bolometric luminosity which
is in the X-Ray emission (Lx/Lbol) is shown in figure 4, and again
no correlation with binarity is found. The two targets with very
large Lx/Lbol (GJ 376B and LSPM J1832+2030) are listed as flar-
ing sources in Fuhrmeister & Schmitt (2003) and thus were prob-
ably observed during flare events (although Gizis et al. (2000) ar-
gues that GJ 376B is simply very active).
This contrasts with the 2.4 times higher binarity among the
similarly-selected sample of F & G type X-ray active stars in
Makarov (2002). However, the binary fractions themselves are very
similar, with a 31% binary fraction among X-ray active F & G stars,
compared with 13% for X-ray mute F & G stars. Since the fraction
of stars showing X-Ray activity increases towards later types, it
is possible that the Makarov sample preferentially selects systems
containing an X-ray emitting late M-dwarf. However, most of the
stellar components detected in Makarov (2002) are F & G types.
The much longer spin-down timescales of late M-dwarfs,
in combination with the rotation-activity paradigm, may ex-
plain the lack of activity-binarity correlation in late M-dwarfs.
Delfosse et al. (1998) show that young disk M dwarfs with spectral
types later than around M3 are still relatively rapidly rotating (with
vsini’s up to 40 km/s and even 60 km/s in one case), while earlier
spectral types do not have detectable rotation periods to the limit of
their sensitivity (around 2 km/s). Indeed solar type stars spin down
 0  10  20  30  40  50
Separation / AU
Figure 5. The i-band contrast ratios of the detected binaries, plotted as a
function of binary separation in AU. For reasons of clarity, the 76AU binary
and the contrast ratio errorbars (table 4) have been omitted. Filled circles
are X-ray emitters.
 0  10  20  30  40  50  60  70  80  90  100
Separation / AU
Figure 6. The histogram distribution of the orbital radii of the detected
binaries in the sample.
on relatively short timescales, for example in the 200 Myr old open
cluster M34 Irwin et al. (2006) find that the majority of solar type
stars have spun down to periods of around 7 days (Vrot ∼ 7 km/sec).
The M-dwarfs thus have a high probability of fast rotation and thus
activity even when single, which could wash-out any obvious bina-
rity correlation with X-ray activity.
5.4 Contrast ratios
In common with previous surveys, the new systems have low con-
trast ratios. All but two of the detected systems have contrast ratios
<1.7 mags. This is well above the survey sensitivity limits, as illus-
trated by the two binaries detected at much larger contrast ratios.
Although those two systems are at larger radii, they would have
been detected around most targets in the survey at as close as 0.2-
0.3 arcsec.
It is difficult to obtain good constraints on the mass contrast
ratio for these systems because of the lack of calibrated photome-
try, and so we leave the determination of the individual component
masses for future work. However, we note that an interesting fea-
ture of the sample is that no binaries with contrast ratios consistent
with equal mass stars are detected.
There is no obvious correlation between the orbital radius and
the i-band contrast ratios, nor between X-ray emission and the con-
trast ratios (figure 5).
5.5 The distribution of orbital radii
Early M-dwarfs and G-dwarfs binaries have a broad or-
bital radius peak of around 30 AU (Fischer & Marcy 1992;
Duquennoy & Mayor 1991), while late M-dwarfs have a peak at
around 4 AU (eg. Close et al. 2005). Our survey covers a narrow
c© RAS, MNRAS 000, 1–11
13 New VLM Binaries 9
5.8 6.0 6.2 6.4 6.6 6.8 7.0 7.2
V-K colour
Figure 7. Orbital radius in the detected binaries as a function of colour. V-
K=6 corresponds approximately to M4.5, and V-K=7 to M5.5. Filled circles
are X-ray emitters. For clarity, the ∼0.3 mags horizontal error bars have
been omitted. There is no obvious correlation between X-ray emission and
orbital radius.
(0.02M⊙) mass range in the region between the two populations
and so allows us to test the rapidity of the transition in binary prop-
erties.
The orbital radius distribution derived in this survey (figure
6) replicates the previously known VLM-star 4 AU orbital radius
peak. However, 9 of the 21 systems are at a projected separation of
more than 10 AU. These wide VLM binaries are known to be rare
– for example, in the 36 M6-M7.5 M-dwarf sample of Siegler et al.
(2005) 5 binaries are detected but none are found to be wider than
10 AU.
To test for a rapid transition between the low-mass and very-
low-mass binary properties in the mass range covered by our sur-
vey, we supplemented the V-K > 6.5 systems from the LuckyCam
sample with the known VLM binaries from the Very Low Mass Bi-
naries archive3 (which, due to a different mass cut, all have a lower
system mass than the LuckyCam sample). To reduce selection ef-
fects from the instrumental resolution cut-offs we only considered
VLM binaries with orbital radius > 3.0 AU.
The resulting cumulative probability distributions are shown
in figure 8. There is a deficit in wider systems in the redder sample
compared to the more massive, bluer systems. A K-S test between
the two orbital radius distributions gives an 8% probability that they
are derived from the same underlying distribution. This suggests a
possibly rapid change in the incidence of systems with orbital radii
> 10AU, at around the M5-M5.5 spectral type. However, confirma-
tion of the rapid change will require a larger number of binaries and
a more precise mass determination for each system.
5.6 The LuckyCam surveys in the context of formation
mechanisms
VLM star formation is currently usually modelled as fragmentation
of the initial molecular cloud core followed by ejection of the low
mass stellar embryos before mass accretion has completed – the
ejection hypothesis (Reipurth & Clarke 2001). Multiple systems
formed by fragmentation are limited to be no smaller than 10AU
by the opacity limit (eg. Boss 1988), although closer binaries can
3 collated by Nick Siegler; VLM there is defined at the slightly lower cutoff
of total system mass of < 0.2M⊙
 0  10  20  30  40  50
Orbital radius / AU
V-K < 6.5
V-K > 6.5
Figure 8. The cumulative distribution of orbital radii of the detected bina-
ries in the sample with V-K < 6.5 (dashed line). The solid line shows those
with V-K > 6.5, with the addition of the full sample of known VLM binaries
with total system masses < 0.2M⊙, collated by Siegler. Neither distribution
reaches a fraction of 1.0 because of a small number of binaries wider than
50 AU.
be formed by dynamical interactions and orbital decay (Bate et al.
2002).
The ejection hypothesis predicted binary frequency is about
8% (Bate & Bonnell 2005); few very close (< 3AU) binaries are
expected (Umbreit et al. 2005) without appealing to orbital decay.
Few wide binaries with low binding energies are expected to sur-
vive the ejection, although recent models produce some systems
wider than 20AU when two stars are ejected simultaneously in the
same direction (Bate & Bonnell 2005). The standard ejecton hy-
pothesis orbital radius distribution is thus rather tight and centered
at about 3-5 AU, although its width can be enlarged by appealing
to the above additional effects.
The LuckyCam VLM binary surveys (this work and Law et al.
2006) found several wide binary systems, with 11 of the 24 de-
tected systems at more than 10 AU orbital radius and 3 at more
than 20 AU. With the latest models, the ejection hypothesis can-
not be ruled out by these observations, and indeed (as suggested
in Bate & Bonnell 2005) the frequency of wider systems will be
very useful for constraining more sophisticated models capable
of predicting the frequency in detail. The observed distance-bias-
corrected binary frequency in the LuckyCam survey is consistent
with the ejection hypothesis models, but may be inconsistent when
the number of very close binaries undetected in the surveys is taken
into account (Maxted & Jeffries 2005; Jeffries & Maxted 2005).
For fragmentation to reproduce the observed orbital radius dis-
tribution, including the likely number of very close systems, dy-
namical interactions and orbital decay must be very important pro-
cesses. However, SPH models also predict very low numbers of
close binaries. An alternate mechanism for the production of the
closest binaries is thus required (Jeffries & Maxted 2005), as well
as modelling constraints to test against the observed numbers of
wider binaries. Radial velocity observations of the LuckyCam sam-
ples to test for much closer systems would offer a very useful in-
sight into the full orbital radius distribution that must be reproduced
by the models.
6 CONCLUSIONS
We found 21 very low mass binary systems in a 78 star sample,
including one close binary in a 2300 AU wide triple system and
c© RAS, MNRAS 000, 1–11
10 N.M. Law et al.
one VLM system with a 27 AU orbital radius. 13 of the binary sys-
tems are new discoveries. All of the new systems are significantly
fainter than the previously known close systems in the sample. The
distance-corrected binary fraction is 13.5+6.5
−4 %, in agreement with
previous results. There is no detectable correlation between X-Ray
emission and binarity. The orbital radius distribution of the binaries
appears to show characteristics of both the late and early M-dwarf
distributions, with 9 systems having an orbital radius of more than
10 AU. We find that the orbital radius distribution of the binaries
with V-K < 6.5 in this survey appears to be different from that of
lower-mass objects, suggesting a possible sharp cutoff in the num-
ber of binaries wider than 10 AU at about the M5 spectral type.
ACKNOWLEDGEMENTS
The authors would like to particularly thank the staff members at
the Nordic Optical Telescope. We would also like to thank John
Baldwin and Peter Warner for many helpful discussions. NML ac-
knowledges support from the UK Particle Physics and Astronomy
Research Council (PPARC). This research has made use of the
SIMBAD database, operated at CDS, Strasbourg, France. We also
made use of NASA’s Astrophysics Data System Bibliographic Ser-
vices.
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	Introduction
	The Sample
	X-ray selection
	Target distributions
	Observations
	Binary detection and photometry
	Sensitivity
	Results & Analysis
	Confirmation of physical association
	Constraints on the nature of the target stars
	Distances
	NLTT 14406 – A Newly Discovered Triple System
	Discussion
	The binary frequency of stars in this survey
	Biases in the X-ray sample
	Is X-ray activity an indicator of binarity?
	Contrast ratios
	The distribution of orbital radii
	The LuckyCam surveys in the context of formation mechanisms
	Conclusions
