We report the discovery of two binary M dwarf systems in the immediate solar neighbourhood using the Two Micron All Sky Survey (2MASS). The first is an M6.5 companion to the nearby G star HD 86728 (Gl 376). The known properties of HD 86728 indicate that the M dwarf (Gl 376B) is old, metal-rich and only 14.9 parsec away. The M dwarf is highly active, with both Hα and X-ray emission. Thus, Gl 376B offers the opportunity to study an old, bright, active M dwarf with known metallicity, age and luminosity. We show that it is probable that Gl 376B is itself an unresolved pair. The other system consists of an M6.5 and an M8 dwarf with 14.5 arcsec separation. We estimate a distance of ∼16 parsec for this very low-mass pair. Stronger activity is observed in the M6.5 dwarf, supporting evidence that chromospheric activity is weakening near the hydrogen-burning limit

Burgasser, Adam J.

Gizis, John E.

Kirkpatrick, J. Davy

Monet, David G.

Reid, I. Neill

Caltech Authors - Main

Mon. Not. R. Astron. Soc. 311, 385±388 (2000)
Two nearby M dwarf binaries from the Two Micron All Sky Survey
John E. Gizis,1 David G. Monet,2 I. Neill Reid,3 J. Davy Kirkpatrick4
and Adam J. Burgasser5
1Department of Physics and Astronomy, University of Massachusetts, Amherst, MA 01003, USA
2US Naval Observatory, PO Box 1149, Flagstaff, AZ 86002, USA
3Department of Physics and Astronomy, University of Pennsylvania, 209 South 33rd Street, Philadelphia, PA 19104-6396, USA
4Infrared Processing and Analysis Center (IPAC), 100-22, California Institute of Technology, Pasadena, California 91125, USA
5Department of Physics, 103-33, California Institute of Technology, Pasadena, California 91125, USA
Accepted 1999 August 24. Received 1999 July 28
A B S T R A C T
We report the discovery of two binary M dwarf systems in the immediate solar
neighbourhood using the Two Micron All Sky Survey (2MASS). The first is an M6.5
companion to the nearby G star HD 86728 (Gl 376). The known properties of HD 86728
indicate that the M dwarf (Gl 376B) is old, metal-rich and only 14.9 parsec away. The M
dwarf is highly active, with both Ha and X-ray emission. Thus, Gl 376B offers the
opportunity to study an old, bright, active M dwarf with known metallicity, age and
luminosity. We show that it is probable that Gl 376B is itself an unresolved pair. The other
system consists of an M6.5 and an M8 dwarf with 14.5 arcsec separation. We estimate a
distance of ,16 parsec for this very low-mass pair. Stronger activity is observed in the M6.5
dwarf, supporting evidence that chromospheric activity is weakening near the hydrogen-
burning limit.
Key words: binaries: general ± stars: low-mass, brown dwarfs ± solar neighbourhood ±
infrared: stars.
1 I N T R O D U C T I O N
Nearby binary stars have considerable importance for astronomi-
cal studies. The nearby star sample is the basis for determining the
properties of the field disc population, such as the luminosity and
mass functions, kinematics, binary fraction, and chromospheric
and coronal activity. Furthermore, binary systems offer special
opportunities to astronomers. Since the components of a system
can be assumed to have the same age and composition, studies of
one component can be used to constrain the properties of the other
component. This is especially important for the coolest M dwarfs,
whose complicated atmospheres are difficult to model. G dwarf
primaries allow many otherwise unmeasurable properties of their
M dwarf secondaries to be constrained, but surprisingly few wide
systems suitable for ground-based follow-up are known.
We have recently begun a search using the Two Micron All Sky
Survey (2MASS) aimed at identifying the nearest isolated M and
L dwarfs in the solar neighbourhood. Here we report the discovery
of two nearby wide M dwarf secondaries of special interest in the
course of that search.
2 DATA
Candidate nearby M dwarfs were identified using the pairing of
2MASS detections with USNO PMM scans (Monet, private
communication) of the Second Palomar Sky Survey (POSSII)
plates (Reid et al. 1991). On the basis of the resulting BRIJHK
magnitudes, nearby M dwarfs can be identified with high
confidence. More details are given in Gizis et al. (in preparation).
The selection criteria are designed to produce a complete sample
of M8 and cooler dwarfs, but necessarily include many M6.5 and
M7 dwarfs. Objects with Ks , 12; J 2 Ks . 1:0 were selected for
initial follow-up.
In the course of compiling the nearby star candidate list, we
noted that two of the M dwarfs appeared to be companions to
brighter stars, and marked them for priority in follow-up
spectroscopy. The 2MASS J2000 positions and magnitudes are
given in Table 1. The first, 2MASSW J10005031315545, is close
(133 arcsec west, 20 arcsec north) to the nearby G star HD 86728,
which appears in the nearby star catalogue as Gl 376 (Gliese &
Jahreiû 1991). Measurement of the POSSI and POSSII plates
reveals that the proper motion of the 2MASS source is identical to
that of Gl 376. Indeed, it is a little surprising that this source has
not been noticed before. It is easily visible on the Palomar Sky
Survey plates, and its proper motion is sufficient to be included in
the LHS Catalogue (Luyten 1979), although it is within the halo of
the bright star. We estimate R < 14:5 ^ 0:5 from the uncalibrated
PMM scans. The common proper motion indicates that it is
associated, and we therefore denote this object Gl 376B. We
henceforth refer to the primary (HD 86728) as Gl 376A.
A spectrum of Gl 376B was obtained on 1999 March 4 with the
LRIS spectrograph (Oke et al. 1995) on the Keck II telescope
q 2000 RAS
386 J. E. Gizis et al.
using the setup described in Kirkpatrick et al. (1999). A second
spectrum was obtained at the Hale 200-inch telescope at Palomar
Mountain on 1999 May 24. The Keck spectrum is shown in Fig. 1.
The spectral type of Gl 376B is M6.5 V with an Ha equivalent
width of 16 AÊ in the Keck spectrum and 13 AÊ in the lower-
resolution Palomar spectrum. Spectral types are on the Kirk-
patrick, Henry & McCarthy (1991) system, and were determined
by visually comparing our spectrum to stars of known spectral
type taken with the same setup.
Our second system consists of the M dwarfs 2MASSW
J10471261402643 and 2MASSW J10471381402649. The two
sources show common proper motion of ma  20:30 and md 
20:03 arcsec yr21 relative to other stars in the field, as measured
from the POSSI and 2MASS images, and have a separation of
14.5 arcsec. Both the position and the proper motion agree with the
two Luyten (1979-1980) NLTT sources LP 213-67 and LP 213-68,
and we adopt those names henceforth. The two M dwarfs were
observed at Keck II on 1999 March 5. The primary, LP 213-67,
has a spectral type of M6.5 V and an Ha equivalent with of 6.9 AÊ .
The secondary, LP 213-68, is an M8 V with an Ha equivalent
width of 3.7 AÊ .
3 D I S C U S S I O N
Gl 376A has been extensively studied, and, as a result, its
properties are well determined. The Hipparcos parallax places it at
14.9 parsec (Perryman et al. 1997). Taylor's (1995) analysis of all
metallicity determinations of the star indicates that Fe=H 
10:170 ^ 0:052: Using the Hipparcos distance and the photo-
metry of Edvardsson et al. (1993); Ng & Bertelli (1998) find
M  1:02 M( and logage  9:854 ^ 0:083. This age is in good
agreement with Barry's (1988) estimate of logage  9:93 Gyr
based on the Ca ii chromospheric emission lines. Duquennoy,
Mayor & Halbwachs (1991) find a constant radial velocity of
155:83 km s21, and both the Hipparcos observations (Perryman
et al. 1997) and ground-based astrometry (Heintz 1988) indicate
that there is no close companion. Although there is an IDS and
CCDM entry for a bright, nearby companion to Gl 376A in
Simbad, this entry has been deleted from the more recent
Washington Visual Double Star Catalog (Worley & Douglass
1997) and there is clearly no such double on the Palomar plates.
We note that Gl 376B should have been included in the
Duquennoy & Mayor (1991) analysis of the frequency of binary
companions, though it appears in a separation range in which they
assumed 50 per cent incompleteness and therefore, in a sense, was
accounted for by their analysis.
The Hipparcos parallax and 2MASS photometry imply MK 
8:19 for Gl 376B. However, the typical absolute magnitude of an
M6.5 dwarf is MK  9:60 (Kirkpatrick & McCarthy 1994).
Furthermore, our estimate of R < 14:5 implies MR < 13:4, which
is brighter than the typical M6.5 value of MR  15:13 (Kirk-
patrick & McCarthy 1994), but consistent with the Ks discrepancy.
The apparent overluminosity of Gl 376B may be partially
explained if it is actually a near-equal luminosity binary (or
triple!). The high metallicity of the system may also partially
account for the apparent overluminosity of the M dwarf with
respect to the typical field main sequence.
We may compare the Ha emission strength of Gl 376B to
similar known M dwarfs observed by Hawley, Gizis & Reid
(1996). Despite its great age, Gl 376B is one of the most active
nearby field M dwarfs. Since strong emission was observed
twice, it is unlikely that a flare is responsible. We note that the
ROSAT All-Sky Survey (Voges et al. 1999) source 1RXSJ
100050.91315555 is coincident with Gl 376B. Adopting the
calibration given by Fleming et al. (1993), we find f X  1:4 
10212 erg s21 cm22 if the M dwarf is the X-ray source. With an
implied luminosity of log LX  27:5 (or 27.2 if it is an equal-
luminosity binary), Gl 376B is very active compared to the limited
sample of late M dwarfs observed by Fleming et al. While we
cannot rule out the possibility that some of the flux is caused by
the G primary, the ROSAT position with its 30 arcsec uncertainty
does not match the G star, and Barry (1988) finds little
chromospheric activity, which suggests that much or all of the
observed X-ray emission is due to the M dwarf. HuÈnsch et al.
(1999) find nearby old G stars with log LX , 27, and they do not
attribute the ROSAT source to Gl 376A.
The high activity level cannot be due to youth, since the G
primary is known to be old from two independent techniques. This
strongly suggests that Gl 376B is a short-period binary that
maintains a rapid rotation rate via tidal interaction (Young et al.
1987). An unresolved companion would thus explain both the high
activity level and the apparent overluminosity of Gl 376B. The
postulated companion, Gl 376C, should be detectable via high-
resolution spectroscopy and radial velocity monitoring.
Both members of the LP 213-67/68 system show Ha emission.
We note that the M6.5 primary has stronger emission than the M8
secondary. This is consistent with the general observation that, in
the Pleiades and the field, the chromospheric activity levels are
becoming weaker as the hydrogen-burning limit is approached
(Tinney & Reid 1998), although the hotter component could be
more active by chance or as the result of the influence of an
unseen companion.
We have derived the relationship MK  7:59 1 2:25  J 2 K
using trigonometric parallax data for M7 and later dwarfs (Gizis
et al., in preparation). This implies a distance of 16 parsec for the
M8 secondary. Another estimate may be obtained using
Kirkpatrick & McCarthy's (1994) value of MK  9:6 for M6.5
dwarfs. In this case, the estimated distance for the primary is
14.5 parsec. The tangential velocity of ,23 km s21 does not a give
a strong clue to the age of this system, but an age of a few
gigayears is likely given the activity level of M6.5.
4 C O N C L U S I O N
We have discovered an M dwarf member of the Gl 376 system.
This offers the opportunity to study an old, metal-rich M dwarf
with known properties. Most active, metal-rich objects are young
(such as the Hyades), and therefore it is difficult to disentangle the
q 2000 RAS, MNRAS 311, 385±388
Table 1. The 2MASS data.
Name RA Dec. J H Ks Epoch
Gl 376B 10:00:50.30 131:55:45.9 10.30 9.68 9.27 1998 March 21
LP 213-67 10:47:12.65 140:26:43.7 11.42 10.78 10.40 1998 April 5
LP 213-68 10:47:13.82 140:26:49.3 12.45 11.71 11.28 1998 April 5
Two nearby M dwarf binaries from 2MASS 387
effects of youth and metallicity. It is perhaps unfortunate that Gl
376B is probably an unresolved double, because we cannot
presently use its position to measure the shift of the main sequence
with metallicity. On the other hand, detection of Gl 376C would
allow the mass ratio for the M dwarf to be determined, giving an
additional constraint for models, and would allow the luminosity
ratio of the system to be measured. Of course, if a direct orbital
mass determination proves to be possible, or if the system proves
to be eclipsing, this system would provide a unique constraint on
stellar structure, atmospheric and coronal models near the
hydrogen-burning limit.
We have also identified a nearby pair of cool M dwarfs. LP 213-
67 and LP 213-68 appear to be a normal pair of M dwarfs just
above the hydrogen-burning limit. We note that the two
components are easily resolved and would have simultaneously
measurable parallaxes. With a separation of ,230 au, an orbital
period is not likely in the near future, but the system should allow
comparative study of M6.5 and M8 dwarfs.
AC K N OW L E D G M E N T S
We thank Hartmut Jahreiû for discussion of the nature of Gl 376A
and James Liebert for comments. This work was funded in part by
NASA grant AST-9317456 and JPL contract 960847. JDK and
AJB acknowledge the support of JPL, CalTech, which is operated
under contract with NASA. INR and JDK acknowledge funding
through a NASA/JPL grant to 2MASS Core Project science; AJB
q 2000 RAS, MNRAS 311, 385±388
Figure 1. The Keck spectra of the three nearby M dwarfs.
388 J. E. Gizis et al.
acknowledges support through this grant. This publication makes
use of data products from 2MASS, which is a joint project of the
University of Massachusetts and IPAC, funded by NASA and
NSF. Some of the data presented herein were obtained at the Keck
Observatory, which is operated as a scientific partnership by
CalTech, the University of California and NASA. The Observatory
was made possible by the generous financial support of the W. M.
Keck Foundation. This research has made use of the Simbad data
base, operated at CDS, Strasbourg, France.
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q 2000 RAS, MNRAS 311, 385±388


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Two nearby M dwarf binaries from the Two Micron All Sky 

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