Issue |
A&A
Volume 513, April 2010
|
|
---|---|---|
Article Number | L9 | |
Number of page(s) | 4 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361/201014206 | |
Published online | 27 April 2010 |
LETTER TO THE EDITOR
Discovery of a nearby young brown dwarf binary candidate
A. Reiners1, - A. Seifahrt2
- S. Dreizler1
1 - Universität Göttingen, Institut für Astrophysik,
Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
2 - Department of Physics, University of California, One Shields
Avenue, Davis, CA 95616, USA
Received 5 February 2010 / Accepted 5 March 2010
Abstract
In near-infrared NaCo observations of the young brown dwarf
R2MASS J0041353-562112, we discovered a companion a little less
than a magnitude fainter than the primary. The binary candidate has a
separation of 143 mas, and the spectral types of the two
components are M 6.5 and M 9.0. Colors and flux
ratios of the components are consistent with their locations being at
the same distance minimizing the probability of the secondary being a
background object. The brown dwarf is known to exhibit Li absorption
constraining the age to be younger than 200 Myr, and has been suspected of
experiencing ongoing accretion, which implies an age as young as
10 Myr.
We estimate distance and orbital parameters of the binary as a function
of age. For an age of 10 Myr, the distance to the system is
50 pc, the orbital period is 126 yr, and the masses
of the components are
30
and
15
.
The binary brown dwarf fills a so far unoccupied region in the
parameters mass and age; it is a valuable new benchmark object for
brown dwarf atmospheric and evolutionary models.
Key words: stars: pre-main-sequence - stars: formation - stars: individual: R2MASS J0041353-562112 - stars: low-mass - brown dwarfs
1 Introduction
Very low mass binaries are of particular interest for a number of reasons. First, at very low masses, the binary fraction and the orbital properties of the binaries carry important information about the way binaries form (e.g., Burgasser et al. 2007; Close et al. 2003). Second, all components of a multiple system share the same evolutionary history so that a comparison between binary components is free of a number of degeneracies. Finally, binaries offer a model-independent way of determining the mass by means of measuring their orbital period. This third point is most important for our understanding of the evolution of low-mass stars and brown dwarfs in particular at young ages, where models still have relatively large uncertainties.
The number of very low mass binaries that have been detected
is now
substantial. An updated list of binaries with total masses below
0.2
can be found at the Very-Low-Mass Binary Archive
,
and contains 99 entries as of Jan. 2010. Of
particular importance are brown dwarf
binaries with independent age constraints because they provide
empirical constraints on brown dwarf evolution models. Furthermore,
to determine the mass from orbital motion on reasonable timescales,
the orbital period should be short enough, and the binaries should not
be too far away so that spectroscopic investigation is
possible. Usually, young binaries are members of star-forming regions
that are located at a distance of 100 pc or more, which makes
a
detailed investigation of low-mass members very difficult. Therefore,
young, nearby, low-mass systems are of very great value for our
understanding of low-mass star and brown dwarf evolution.
In this paper, we present the discovery of a new very low mass
binary,
R2MASS 0041353-562112 (or DENIS-P J0041353-562112; Phan Bao et al. 2001,
hereafter
2M0041), which is nearby and probably very young. The age
of 2M0041 is constrained to be lower than 200 Myr by the
detection of Li in an optical spectrum
(Reiners & Basri 2009).
Reiners (2009) presents
evidence of accretion
deduced from the intensity and shape of emission lines, in particular
H
.
Ongoing accretion would suggest that 2M0041 may even be as
young as 10 Myr. Space motion of 2M0041 is
consistent with it
being a member of the
20 Myr
old Tuc-Hor association or the
12 Myr
old
Pic
association. Unfortunately, the
distance to 2M0041 is not yet known because no parallax measurement
has so far been available. The distance from spectrophotometry would
be 17 pc if the object was an old, single field star, but the
real
distance is of course larger because the young object is more
luminous. As long as no parallax is measured for 2M0041, age, mass,
and distance are free parameters that can be constrained by measuring
the orbital period of the system.
2 Data and analysis
Data were obtained with NaCo, the Nasmyth Adaptive Optics System
(NAOS) and Near-Infrared Imager and Spectrograph (CONICA) at ESO's
Very Large Telescope (Lenzen
et al. 2003; Rousset et al. 2003).
Four images were
obtained in service mode on August 14, 2009: one image was taken in
J, one in H, and two images were
obtained in .
We obtained
ten individual frames, each of them being the average of 3
observations with an individual exposure time of 30 s each.
Thus, the
total exposure time is 15 min per image. Individual images
were
jittered using a 5
jitter box, this
allowed efficient
reduction of the sky background. The adaptive optics system was used
with the N90C10 dichroic using 90% of the light for AO and
only 10%
for the science camera. We chose this option because no bright
AO source was available nearby. All observations were carried
out at low
airmass (<1.3) and good seeing conditions (<
).
We used
the S13 camera with a field of view of
and
a pixel scale of 13.2 mas/pix. For our analysis, we use the
standard pipeline products that are provided for service mode
observations. Data reduction includes dark subtraction, flat fielding,
and sky subtraction using the jittered images. A close-up of one of
the two
-band
images is shown in Fig. 1.
We
clearly resolve 2M0041 as a binary with two components that differ
somewhat in brightness; the component to the SW appears brighter than
the NE component.
![]() |
Figure 1: NACO K-band image of 2M0041, North is up and East is left. Axis labels denote coordinates in mas relative to the center of the image. |
Open with DEXTER |
To measure the separation and the flux ratio of the two components, we estimate the PSF from the science images in an iterative procedure. We start with an image of a standard star. We use the so-called zeropoint images that are routinely taken for NaCo service observations. With this first guess of the PSF, we search for stars in our science images and determine their position and flux using the public domain IDL package StarFinder (Diolaiti et al. 2000). Next, we extract the PSF from the science image obtaining information about the flux and position using the routine psf_extract from the StarFinder package. In the next step, we use this PSF to iteratively search for the two components and redetermine the shape of the PSF.
The deduction of the PSF from the science image has the advantage that we do not have to rely on a PSF that was taken at a different time, direction, and object brightness. The shape of the PSF depends sensitively on the adaptive optics performance, which in turn depends on the brightness of the reference target. On the other hand, the separation of the two components that we wish to distinguish is not much larger than the width of the PSF, particularly in the J-band. When determining the PSF, this may cause the algorithm to construct a PSF consisting of both components. To avoid this problem, we assume that the PSF has an axisymmetric shape. After each PSF determination, we construct a rotationally averaged version of the PSF that we use in the next iteration of StarFinder.
We successfully identified the two components in all four images using the procedure described above. As a result, we obtain the positions and individual fluxes of the two components, and the PSF of each image.
Table 1: Separation and flux measurements in the four images.
We show the separation of the two components, their flux
ratio, and
the width of the PSF from our four images in
Table 1.
The results of the separation are
consistent with each other. The flux ratios of the two -band
images agree very well, and the flux ratio seems to be a function of
color, which is consistent with two components of different
temperature. The width of the PSF also varies with wavelength, as
expected because the Strehl ratio is lower at shorter wavelengths
(Strehl ratios are 0.50 in
,
0.33 in H, and 0.18 in J). In
the
-band,
we note that the width of the PSF is on the order of
the diffraction limit. Going to even longer wavelengths would not lead
to higher image quality because the diffraction limit increases with
wavelength (for example, it is
100 mas at L'). In
the
J-band, the two components are not separated well.
We tried to use a
PSF consisting of different elliptical components, but did not succeed
in reproducing our results better than when using our procedure
described above. The J-band position angle differs
from the H- and
-band
results by about 1.5 degree, which may be caused by the
slightly irregular shape of the J-band PSF. In
particular,
irregularities in the wings of the J-band PSF may
lead to a small
shift in the suspected stars' positions (here,
0.2 pix or
2.5% of the J-band
FWHM). This probably leads to a slight
dependence of the photocenter position on the stars' intensity in the
J-band.
To quantify the uncertainty in our measurements, we performed
Monte
Carlo simulations for all three bands. In each band, we constructed a
series of 1000 images of a binary consisting of two objects
with
different flux ratios, separations, and position angles. We uniformly
varied these values about those found in the real data, and used
asymmetric PSFs consisting of two elliptical Gaussian components that
scatter about a description optimized to fit the rotationally
symmetric PSF from the procedure described above. Noise was added to
the artificial data to match the quality of the real images. In the
H- and K-bands, the fitting
process turned out to be very
stable. The clear separation of the two components because of the
narrow PSF allows a robust fitting process even for an asymmetric
PSF. We derived the uncertainties in the flux ratio from the scatter
about the mean of our simulations. The 2-scatter in the flux
ratio is 3% for H and 2% for
.
The J-band uncertainties are
larger, not only because of the larger FWHM of the
PSF but also
because of the asymmetry in the PSF mentioned above. Thus, the fitting
process in the J-band image depends sensitively on
assumptions about
the shape of the PSF. Because deconvolving that shape is less reliable
in the J-band image, we decided not to simply adopt
the J-band
uncertainties inferred by our Monte-Carlo approach, but conservatively
estimate them to be roughly a factor of two larger because of
PSF-dependent systematics. We adopt a J-band flux
ratio uncertainty
of 5%, which agrees with the scatter that we found during our
attempts using different PSF assumptions.
3 Photometry of the components
Photometric measurements of 2M0041 in J, H,
and
are
available from the Two Micron All Sky Survey (R2MASS, Skrutskie et al. 2006).
The R2MASS magnitude of 2M0041 reflects the combined flux of both
components. With the NaCo observations, we can now separate the
combined flux into two components using the R2MASS magnitudes and the
flux ratios f1/f2
in the three filters, for the J-band obtaining
![]() |
(1) |
The results for individual J, H, and


Table 2: Measured parameters of the system.
The linear relation between spectral type and absolute
magnitude
enables us to use the combined spectral type (M 7.5) to
determine the
spectral type range of 2M0041 A and B. Using flux ratio and
individual
magnitudes, we can construct an artificial average magnitude of the
combined system 2M0041AB (
,
),
which in this system must coincide
with spectral type M 7.
.
From this, we derive individual
spectral types for 2M0041 A and B of M 6.
and M 9.
,
respectively.
An independent check of our assumption that both components are located at the same distance can be make by examining the colors of 2M0041 A and B. The typical J-K colors of late-M objects are given, e.g., by Hawley et al. (2002), Dahn et al. (2002), and West et al. (2008). The colors of 2M0041 A are indicative of a spectral type in the range M 5-M 9, while 2M0041 B falls in the range M 8.5-early L. Thus, the colors and flux ratios of 2M0041 A and B are consistent with the assumption that both components belong to a binary system. Given a dispersion of 0.2 mag in the J-K relation and much smaller uncertainties in our measurements of flux ratios and colors, the difference in distance modulus between the two components is unlikely to be greater than 6 pc.
Table 2
also indicates the mean position angle and
separation. For the position angle, the individual measurements are
weighted according to the inverse PSF widths of the images to obtain
.
The mean separation calculated from the four
exposures is 142.8 mas, we use the scatter of 0.5 mas
as a
conservative estimate of the uncertainty in this value.
The proper motion of 2M0041 is 140 mas yr-1(Phan Bao & Bessel 2006),
i.e., the primary travels roughly by one full
observed separation per year in SE direction. The secondary is
sufficiently bright to be visible in archive nIR measurements, if the
separation from the primary is large enough. We can use nIR or red
observations of the region to search for a second object. Assuming
that the secondary would exhibit negligible proper motion, the objects
should be separated by
1.5
on
the R2MASS images taken in
1999, which would be difficult to detect given the
3
FWHM of the R2MASS PSF. On the other hand, ESO.R-MAMA
plates taken in
1988 should show two objects separated by about one FWHM,
which is
2.5-3
.
We found a second object at the position of 2M0041 in
neither the R2MASS images from 1999 nor the ESO.R-MAMA images
from 1988, and we see no signs of an elongated PSF, which
would be
indicative of a barely resolved secondary object close to the primary
(
1
). We conclude that the second
object in our images is
probably physically bound to the primary. The confirmation of a binary
status, however, can only be accomplished by verifying a common proper
motion in an exposure taken at a second epoch.
4 System parameters
From the new photometry, we can determine the parameters of the two components, and estimate the mass and the orbital period for a given age. The distance to 2M0041 can be estimated from the difference between absolute and apparent magnitude. For field objects, the absolute magnitude as a function of spectral type was given by Dahn et al. (2002) and Cruz et al. (2003). We can use that estimate to determine a lower limit to the distance that would apply if the objects were field stars. To calculate the distance that would apply if the objects were very young, we scale the absolute magnitude by the radius difference between young and old objects assuming that the temperature does not change significantly with age for a given spectral type. We use the radius-age dependence from Baraffe et al. (1998,2002).
All parameters and their uncertainties are given in Table 3. Uncertainties are related to the spectral type uncertainty, which has an important effect on the absolute magnitude, and to this we add the uncertainty in J in quadrature. Based on the assumption that 2M0041 A is a field star, the distance to the system is 24 pc. As expected, the new distance is larger than the 17 pc calculated earlier (Faherty et al. 2009), because in that calculation, all the flux measured in J was assumed originate in a single (and cooler) object. We can now use the radius-age relation to estimate the distance for different ages of 2M0041 A (see Reiners 2009). The distances to 2M0041 for ages of 5 and 10 Myr are 71 and 50 pc, respectively. To estimate the semi-major axis, we apply a correction factor of 1.26 to account for projection effects (Fischer & Marcy 1992). The separations between 2M0041 A and B are inferred to be 8.9 and 12.8 AU for a 10 Myr and a 5 Myr binary, respectively.
From the evolutionary tracks of Baraffe et al., we can also estimate the mass of the two components at a given age. With knowledge of both the masses and the separation, we can then estimate the orbital period of the binary assuming that the distance we observe is the true semimajor-axis of the system.
Table 3: System parameters for different ages1.
The orbital period that would be derived if the binary were
old is yr.
This is the lower boundary to the period; the true
period must be longer because an age of above a few hundred Myr is
excluded by the detection of Li. If 2M0041 had an age of
10 Myr, the
period would be on the order of 126 yr, i.e., a factor
of 5 longer. If the system were as young as
5 Myr, the period would be
about 228 yr, i.e., another factor of two longer. We show the
estimated period as a function of age in
Fig. 2.
Using the photometric information gathered
about this object, the possible orbital period is found to be a steep
function of age.
![]() |
Figure 2:
Top panel: estimated orbital period as a
function of age for 2M0041A+B (solid line). The grey region indicates 1 |
Open with DEXTER |
5 Summary
In a NaCo image of the nearby young brown dwarf 2M0041, we have
discovered the binarity of this object. The system consists of an
M 6.5
primary and a secondary of spectral type M 9.0. We
found a separation
of 143 mas and derived individual J, H,
and
magnitudes for
both components. Flux ratio and colors of both components are
consistent, which means that the chances of the secondary being a
background object are very small.
The distance, age, mass, and orbital period of the system
remain
unknown, but we have presented possible solutions as a function of
age. The object contains lithium, which means that it is a brown dwarf
younger than a few hundred Myr. The detection of H
and other
optical emission lines indicate that the object may be accreting, and
that its age may be as low as 10 Myr. For this age,
the period is
predicted to be on the order of 125 yr at a
separation of
9 AU
and a distance of 50 pc. Age, semi-major axis, and
distance are steep
functions of the orbital period. So far, no parallax measurement is
available so that the distance to the binary is unknown given that we
do not know its age with any great certainty.
Objects of spectral type M 9.0 are much fainter at
optical wavelengths
than M 6.5, so the secondary contributes negligible
background
continuum to the H
and Li measurement. On the other hand, the
H
emission seen in the combined spectrum may originate in
either of the two components, or both, which would affect the measured
accretion rate: if both objects were accreting, the accretion rate per
star would be lower, but the shape and strength of H
would
still be indicative of accretion. Alternatively, if the accretion
timescale were longer for the secondary, it may be the only accreting
object in the system. In that case, the accretion rate would be higher
than the value originally derived. An image taken in H
or even
spatially resolved spectroscopy would help resolve this issue.
The discovery of binarity in this young brown dwarf provides an opportunity to directly determine the mass of two young nearby brown dwarfs that may be accreting. Without knowing the distance, the age of the system can be determined from the orbital period. In the next few years, the first estimate of the orbital period should become available. If it were to be as young as a few ten million years, it would be the first object in this mass/age regime for which a direct mass estimate would be available. A measured parallax would give us independent information about the distance and place strong constraints on the system's parameters. This system will be an important benchmark for brown dwarf evolutionary models at young ages.
AcknowledgementsWe thank an anonymous referee for a very helpful report. Based on observations made with the European Southern Observatory, PID 383.C0708. This publication has made use of the Very-Low-Mass Binaries Archive housed at http://www.vlmbinaries.org and maintained by Nick Siegler, Chris Gelino, and Adam Burgasser. A.R. acknowledges research funding from the DFG as an Emmy Noether fellow under RE 1664/4-1, A.S. acknowledges financial support from NSF grant AST07-08074.
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Footnotes
- ...
- Emmy Noether Fellow.
- ... Archive
- http://www.vlmbinaries.org
All Tables
Table 1: Separation and flux measurements in the four images.
Table 2: Measured parameters of the system.
Table 3: System parameters for different ages1.
All Figures
![]() |
Figure 1: NACO K-band image of 2M0041, North is up and East is left. Axis labels denote coordinates in mas relative to the center of the image. |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Top panel: estimated orbital period as a
function of age for 2M0041A+B (solid line). The grey region indicates 1 |
Open with DEXTER | |
In the text |
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