Pleiades low-mass brown dwarfs
G. Bihain1,2
1
2
Instituto de Astrofı́sica de Canarias, c/ Vı́a Láctea, s/n, 38205 La Laguna,
Tenerife, Spain
gbihain@ll.iac.es
Consejo Superior de Investigaciones Cientı́ficas, Spain
Summary. We present results of a search for ∼ 0.040 − 0.020 MSol brown dwarfs
in the Pleiades open cluster. From an 1.8 deg2 photometric survey, combining nearinfrared J-band data obtained at the 3.5 m Calar Alto Telescope and previously
available optical I-band data, we identified 18 faint L-type candidates, with magnitudes J > 17.4 and colours I − J > 3.2. By follow-up HKs -band imaging we
found that out of 11 IJ candidates with proper motion measurements, six are cluster members and two are non-members. The three remaining candidates are possible
members depending on the intrinsic velocity dispersion of Pleiades brown dwarfs,
which is at least four times that of cluster stars more massive than solar. Five of
the seven IJ candidates without proper motion measurements are probable nonmembers because their J − Ks colours are bluer that confirmed members. In the
J vs. I − J colour–magnitude diagram, the sequence of L-type candidates is not
as red as predicted by theoretical models, whereas in the J vs. J − H and J − Ks
diagrams there is agreement with the predictions. In all three colour–magnitude
diagrams, the Pleiades and field L dwarf sequences overlap, suggesting that these
objects may share similar spectral energy distributions and luminosities, and thus
possibly similar radii. Finally we find a spectral index α = 0.5 ± 0.2 for the mass
spectrum dN/dM ∝ M −α (0.5–0.026 MSol ) in the cluster survey area. This value,
similar to that of much younger clusters, supports the possibility of finding objects
of planetary mass in the Pleiades open cluster.
1 Introduction
Brown dwarfs are defined as self-gravitating objects that do not have enough
mass to reach sufficient core temperature for stable hydrogen fusion (M <
0.075 MSol at solar metallicity, [9]). They lower mass limit can be defined as
that for deuterium fusion (M = 0.013 MSol ; [30]) and as the boundary with the
planetary mass domain. Because of insufficient production of thermonuclear
energy, brown dwarfs are expected to contract and cool down, progressively
during several Gyr. This implies they are brighter when young, as for example
in open clusters and star forming regions, and they may present the same
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G. Bihain
brightness for different masses and ages (luminosity degeneracy), which makes
their identification difficult among isolated field objects. In fact, models which
predict the temperature, radius and luminosity evolution of substellar objects
as a function of their mass (e.g. [7, 9]) need still to be calibrated with numerous
independent observations.
Since the discoveries of Teide 1 and Gl 229b [27, 24], hundreds of brown
dwarfs were found: in open clusters (Pleiades, α Per, Taurus, σ Orionis,
Trapezium, Chameleon, etc.), in the field (e.g. Kelu 1 and LP 944-20; 2MASS, DENIS- and SDSS objects, etc.) and in binaries or small multiple systems
(G 196-3, HD 29587, Epsilon Indi, HD 3651, etc.). These objects present spectral types M6–M9, L and T, corresponding to effective temperatures ∼ 3000 K
> Teff > 700 K. L-type spectra relative to M-type spectra are characterized by
the weakening of metal-oxide TiO and VO bands in the optical (formation of
dust grains; cf. the DUSTY models from [10]), the intensification of the bands
of metal-hydrides CrH, FeH and CaH and the bands of water, especially in
the near-infrared, and the intensification of the lines of neutral alkali metals
Na, K, Rb, Cs and Li in the optical [2]. Whereas T-type spectra relative to
L-type spectra are characterized by the apparition of methane bands in the
near-infrared. In the case of L dwarfs, typical colours are 3.3 < I − J < 3.8
(early L), 3.8 < I −J < 4.2 (late L), 1.4 < J −Ks < 2.0 and 0.8 < J −H < 1.3
[8, 18, 6].
The Pleiades open cluster (∼130 pc, ∼120 Myr), with its notable proper
motion [29] and applicability for the lithium test [26, 28, 20], allowed to confirm numerous brown dwarfs of M-type [27, 28, 19, 32, 33, 34, 5, 16, 31, 21,
22, 23, 13]. For a power-law approximation dN/dM ∝ M −α (0.5–0.03 MSol )
of the cluster mass spectrum, [15, 23] found a spectral index of α = 0.6 − 08.
We decided to extend the study to the fainter and redder objects – possibly of
L-type – that still needed to be confirmed as Pleiades low-mass brown dwarfs
(M < 0.04 MSol ). We performed a search as described in the following sections
(a more detailed description can be found in [4]).
2 Observations and data reduction
With the Omega-Prime instrument at the 3.5 m Calar Alto Telescope (Spain),
our group obtained J-band images in 1998 covering an area of ∼1.8 deg2 in the
Pleiades open cluster. This near-infrared survey overlapped 75% of an optical
RI-band survey from [5], obtained in 1996 with the 3.6 m Canada-FranceHawaii Telescope (CFHT)/UH8K instrument. Specifically it overlapped the
fields CFHT 2, 5 (north half), 6, 7, 8, 10, 11 and 13 (as designed in [5]).
The reduction of the J-band data, including aperture and PSF photometry,
was done using standard scripts and routines within the IRAF environment.
The PSF photometry was calibrated with objects in common with the Two
Micron All Sky Survey (2MASS) Point Source Catalog [12], providing errors
of calibrated magnitudes smaller than 0.1 mag. The completeness and limiting
Pleiades low-mass brown dwarfs
3
magnitudes of the data were Jcpl ∼ 19.0 and Jlim ∼ 19.5, respectively. The
reduction of the optical data, on the other hand, was performed by [3], and
the photometry was calibrated with objects from [5]. Errors in the magnitudes
were typically of 0.1 mag. The completeness and limiting magnitudes were
Rcpl ∼ 23.5, Icpl ∼ 22.5 and Rlim ∼ 25.0, Ilim ∼ 23.5, respectively.
We correlated the J- and I-band sources and found 34 Pleiades very lowmass star and brown dwarf candidates, for their redder I − J colours than
the bluest cluster members confirmed by proper motion or lithium. Of these
candidates, 18 were fainter than J = 17.4 and redder than I − J = 3.3, thus
possibly Pleiades L dwarfs, with masses smaller than 0.040 MSol .
We then performed follow-up imaging for the 18 low-mass brown dwarf
candidates in the near-infrared HKs -bands, with 1.55 m Telescopio Carlos
Sánchez (TCS)/CAIN-II (Teide Observatory, Spain) only for photometry,
and with 4.2 m William Hershel Telescope/LIRIS (La Palma Observatory,
Spain) and 3.5 m Calar Alto Telescope/Ω2000, for photometry and proper
motion measurements. Those TCS targets that presented red J − Ks colours
were observed at subarcsecond precision with the larger telescopes and their
instruments, in the H-band mostly. The proper motion measurements implied a more careful data reduction than that of the JKs -band data, taking
into account bad pixels, distortions and accurate aligning. We included the
Pleiades brown dwarf Teide 1 in the target list of LIRIS and Ω2000, to improve its proper motion measurement and to check if there are any systematic errors when using different instruments. A first-epoch reduced I-band
image from 2.5 m Isaac Newton Telescope/TEK3 (La Palma Observatory)
was used for Teide 1. We obtained the transformation between first-epoch
images (CFHT/UH8K, INT/TEK3) and second-epoch images (WHT/LIRIS,
3.5 m/Ω2000) using reference objects that minimized the transformation error. Comparing the positions of our candidates with those predicted by the
transformation provided their displacements in pixels, that we converted into
proper motions in equatorial coordinates.
3 Pleiades low-mass brown dwarfs
We obtained proper motion measurements for 11 Pleiades low-mass brown
dwarf candidates, that we represented in a vector point diagram of proper
motion. As a criterion of cluster membership, we considered a circle centred
on the average motion of the open cluster [29] and of radius three times our average proper motion uncertainty. This uncertainty is as low as ∼3.7 mas yr−1 ,
thanks to the large baseline of more than eight years between our first- and
second epoch images. Those objects with proper motions within the circle were
considered as cluster members (six candidates) and those much further away
were discarded as non-members (two candidates). If we measure the dispersion
of the objects within the circle (including Teide 1) and subtract quadratically
by the average proper motion uncertainty, we obtain (4.3, 4.1) mas yr−1 . This
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G. Bihain
estimate of the intrinsic velocity dispersion of Pleiades brown dwarfs is at least
four times that of Pleiades stars more massive than the Sun [17, 25], and appears consistent with a nearly relaxed cluster [25]. The availability in our data
of the massive lithium brown dwarf ”twins”, Teide 1 and Calar 3 [27, 28, 19],
allowed us to better consider the membership status of the remaining three
low-mass candidates, which were outside the circle but close to the region surrounding it. Teide 1 lies very near to the cluster average motion (with LIRIS
and Ω2000 measurements agreeing within 1 σ), whereas Calar 3 lies slightly
outside of the circle. This suggests that our low-mass brown dwarfs could have
a velocity dispersion even greater than those within the circle. We thus need
to confirm further (e.g. by lithium observations) the membership of the three
peculiar low-mass candidates.
Comparing all of our 18 fainter IJ-band candidates in a J vs. I −J colour–
magnitude diagram with the 120 Myr DUSTY isochrone from [10] and shifted
to the cluster distance, we observe a disagreement at J > 18, where the predicted colours are too red. Comparing also with nearby field dwarfs compiled
by [8] and shifted to the cluster distance, we observe that the field L dwarf
sequence overlaps the sequence of Pleiades low-mass brown dwarfs that we
confirmed by proper motion.
In the J vs. J − H and J − Ks colour–magnitude diagrams, the predicted
colours of the DUSTY model however agree with the data points of the confirmed members, indicating therefore a possible underestimation of the I-band
flux relative to the near-infrared flux. In the second diagram, five candidates
present much bluer J − Ks than the objects confirmed by proper motion,
implying these five are probable contaminants. Finally, in these diagrams,
the field L dwarf sequence and the Pleiades low-mass brown dwarf sequence
overlap also.
The fact that many of the Pleiades L dwarf candidates share the same
colours and absolute magnitudes as the field L dwarfs (which is not observed
for the Pleiades and field M dwarfs) suggests they may have the same spectral
energy distributions and luminosities. If the effective temperature estimation
from spectral energy distributions depends weakly on gravity (as indicated
by spectral synthesis), then they may have the same effective temperatures
as well, and thus the same radii. However field L dwarfs could have ages of
3–4 Gyr and masses 0.06–0.07 MSol [1], which traduce in radii of ∼0.09 RSol
(DUSTY models, [10]), i.e. these objects have almost reached the end of contraction. Whereas Pleiades low-mass dwarfs should have radii of ∼0.125 RSol
(DUSTY models, [10]), inconsistent with the previous estimate. This inconsistency could be resolved by considering that Pleiades L dwarfs may contract
faster or field L dwarfs may be more massive or younger. An issue to this question could come in the future from precise spectroscopy, for the determination
of the radii from the luminosities and the effectives temperatures.
For the luminosity function (number of objects per bin of J-band magnitudes) and in particular the mass spectrum (number of objects per unit
of mass), we made an estimation of contaminants. First we subdivided the
Pleiades low-mass brown dwarfs
5
region of the J vs. I − J colour–magnitude diagram occupied by our candidates in small sections. Then we considered the colours and magnitudes of
field dwarfs (late M to early L) contaminating each of these sections, and
computed their distances and volumes towards the Pleiades survey area. We
considered the numerical densities of these field dwarfs in the solar neighbourhood and computed their densities at the distances above the Galactic
plane. Finally we multiplied the volumes by the corresponding densities and
obtained the number of contaminants in function of the J-band magnitude.
Predictions of luminosities and effective temperatures are mostly independent of atmospheric properties for the DUSTY and NextGen models [10], and
bolometric luminosities obtained using specific bolometric corrections agree
well with SPITZER/IRS spectra from M, L and T field dwarfs [11]. For these
reasons we derived bolometric luminosities applying a bolometric correction
(depending of the I −J colour, [8]) on the J-band magnitudes of all our candidates. These luminosities were then compared with the theoretical predictions
(for the age and distance of the Pleiades cluster) to the obtain the masses. Our
least massive Pleiades brown dwarfs confirmed by proper motion are BRB 28
and BRB 29 (∼25 MJup ). We buildt mass bins corrected for contaminants
and added also higher mass bins, containing the more massive cluster members confirmed by proper motion by [13] and present in our survey fields. This
allowed us to obtained a mass spectrum. A power-law approximation of the
mass spectrum dN/dM ∝ M −α from 0.5 MSol down to the completeness mass
of the survey, 0.026 MSol , provided a spectral index of α = 0.5±0.2. This value
is in agreement with that of [23] and very similar to estimates for younger open
clusters, as e.g. the 30–100 Myr IC 4665 [14], implying that the differential
evaporation of low-mass brown dwarfs relative to more massive members has
not been very significant in the case of the Pleiades cluster. Moreover it implies that very low-mass brown dwarfs (VLMBD) and planetary mass objects
(PMO) may be found. The extrapolation of the mass spectrum slope leads
to a prediction of 5 VLMBDs (14–27 MJup ) and 4 PMOs (5–14 MJup ) in the
Pleiades survey area.
4 Conclusions
From a near-infrared and optical survey in the Pleiades we obtained the following: (i) 18 L-type brown dwarf candidates with theoretical masses 0.040–
0.020 MSol , (ii) the proper motion measurements for 11 of these (six cluster
members, three possible members and two non-members), (iii) from the seven
other candidates, five are discarded because of their too blue J − Ks colours,
(iv) the L brown dwarf photometric sequence at ∼120 Myr is similar to the
field L dwarf sequence (in three colour–magnitude diagrams), with the possible implication that Pleiades low-mass brown dwarfs may contract faster or
the field L dwarfs may be more massive of younger, (v) the intrinsic velocity
dispersion of Pleiades brown dwarfs is at least four times larger than that of
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G. Bihain
Pleiades stars more massive than the Sun, and (vi) the spectral index of the
survey mass spectrum corrected for contaminants is α = 0.5 ± 0.2, suggesting the existence of as much as two planetary mass objects (5–14 MJup ) per
square degree.
Acknowledgements
G.B. acknowledges coauthors of [4], R. Rebolo, V. J. S. Béjar, J. A. Caballero,
C. A. L. Bailer-Jones, R. Mundt, J. A. Acosta-Pulido and A. Manchado Torres.
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