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Brown Dwarfs
IAU Symposium, Vol. 211, 2003
E. L. Martn, ed.
Disks in Brown Dwarf Systems
Leonardo Testi, Antonella Natta
Osservatorio Astrosico di Arcetri, INAF, Firenze, Italy
Fernando Comeron
European Southern Observatory, Garching, Germany
Ernesto Oliva
Telescopio Nazionale Galileo and Centro Galileo Galilei, INAF
Francesca D'Antona
Osservatorio Astronomico di Roma, INAF, Roma, Italy
Abstract. We discuss evidence for and properties of disks associated
with brown dwarfs in the star-forming region  Oph. We derived pho-
tospheric parameters from low resolution near infrared spectroscopy and
modeled the mid-infrared excess of nine substellar object candidates in
the  OphISOCAM survey of Bontemps et al. (2001). In all cases, the
mid-infrared excess is consistent with the SED expected frm irradiated
disks. These results suggest that circumstellar disks are commonly as-
sociated to young brown dwarfs and planetary-mass objects. Finally, we
discuss the possibility of using these data to discriminate between various
formation scenarios for substellar objects.
1. Introduction
The discovery of large numbers of sub-stellar mass objects, down to planetary
masses, in regions of star formation has provoked an intense debate on the
formation mechanism of such objects. Do they form, as solar mass stars do, from
the collapse of a molecular core (Shu et al. 1987)? Are they stellar embryos,
whose further growth is prevented by dynamical ejections from small stellar
systems (Reipurth & Clarke 2001; Bate et al. 2002; see also the contributions
of Reipurth and Bate, this volume)? Or are they \planets" i.e., objects that
form within circumstellar disks (Papaloizou & Terquem 2001; Lin et al. 1998)?
Is there a single formation process for all substellar objects? What is the lowest
mass for the gravitational collapse mechanism?
A crucial contribution to this debate is expected from studies of the circum-
stellar disks (if any) associated with sub-stellar objects, since dierent theories
make very dierent predictions. Disks are a necessary step in any formation
mechanism that involves accretion from a parental core. If BDs form from core
1

2 Testi et al.
collapse, they should be associated to disks similar in properties to those found
around low mass pre-main{sequence stars (T Tauri stars; TTS). A prediction of
the stellar embryo theory is that the disks should be truncated by the ejection
mechanism, so that they should be small and short-lived. In the \planetary"
hypothesis, any circumstellar disk should be even less substantial.
In some young BDs, emission in excess of that due to the photosphere has
been detected in the near (Muench et al. 2001; Wilking et al. 1999) and mid-
infrared (Comeron et al. 1998; 2000), and has been interpreted, by analogy with
TTS, as evidence for circumstellar disks.
With the aim of investigating in more detail the disk hypothesis, we selected
a small but well dened sample of nine objects in the  Oph region. All objects
were detected by ISOCAM at both 6.7 and 14.3 m (Bontemps et al. 2001). We
obtained low-resolution near-infrared spectra for all of them, which we used to
derive the basic parameters of the central objects, namely eective temperature,
luminosity and mass. Because of the adopted selection criteria, all of these
objects have excess emission in the mid-IR. We then attempted to t the spectral
energy distributions by means of standard disk models.
2. Sample selection and observations
The nine objects in our sample are all those with visual extinction less than
8.5 mag and luminosity less than 0.04 L according to the Bontemps et
al. (2001) survey. The rst criterion ensures the possibility of obtaining high
signal to noise specta across the entire near infrared range. The low luminosity
was required to increase the chance of selecting objects in the range of masses
we are interested in. Some of the selected sources were known from previous
studies to be very low-mass objects. Hereafter we will refer to the objects using
their number in the Bontemps et al. (2001) ISOCAM list. Near-infrared spec-
tra for the objects in our sample were acquired at the TNG using the NICS
instrument and the Amici disperser. the resulting eective resolution is approx-
imately
= 100, approximately constant across the entire spectral range
(0.85{2.45 m). An identical instrumental conguration was used for the obser-
vations of eld dwarfs of known spectral type (Testi et al. 2001; see also Testi et
al. this volume). The spectroscopic observations were complemented by Gunn-i
photometry obtained at the Danish telescope on the ESO La Silla Observa-
tory, and by J, H, and K s photometry from 2MASS. Additional L' and R-band
photometry for some of the sources were available from Comeron et al. (1998).
Photospheric parameters for all the objects were derived by comparing spectra
and broad band magnitudes with comparison eld dwarfs and theoretical models
(from Allard et al. 2001). The procedure used and the accuracy of the results
are described in Testi et al. (2002) and Natta et al. (2002). All objects are found
to be sub-stellar, one of them (#33, also known as GY11) is probably below the
deuterium burning limit.
3. Disk models
For each system we computed the SED predicted by disk+photophere models,
assuming that the disk is heated by the radiation of the central object. To com-

Disks in Brown Dwarf Systems 3
pute the disk emission we follow the formalism of Chiang & Goldreich (1997),
with some improvements and modications. The disk is in hydrostatic equilib-
rium in the vertical direction ( ared), and, at each radius, the vertical temper-
ature structure of the disk is described in terms of two components: the disk
surface, optically thin to the stellar radiation, and the disk midplane. Such disk
is a scaled-down version of TTS typical disks. It extends inward to the stellar
radius, and outward to RD=110 15 cm (67 AU). The total mass is MD0.03 M ? ,
and the surface density varies as R 1 . The dust in the disk midplane has opacity
 = 0:01(=1:3mm) 1 cm 2 g 1 . As pointed out in Natta & Testi (2001), most
of the disk parameters are irrelevant for the calculation of the mid-infrared disk
emission, or appear in combinations, and cannot be determined individually.
As long as the disk midplane remains optically thick to mid-infrared radiation,
the only parameters that aect the SED in the near and mid-infrared are the
geometrical shape of the disk (i.e., the aring angle), the inclination to the line
of sight and, to some degree, the disk inner radius R i
. There is also some de-
pendence of the shape of the SED on the surface dust model; however, since
the luminosity intercepted and re-radiated by the optically thin surface layers
is xed, variations due to (reasonable) changes of the grain properties are well
within the uncertainty of the existing observations.
All ared disk models have strong strong silicate emission at 10 m and a
rather at spectral slope between the two ISO bands at 6.7 and 14.3 m. If,
rather than extending all the way to the stellar surface the disk is truncated
further out, as predicted by magnetospheric accretion models in TTS, at each
radius the surface of a ared disk intercepts and reprocesses a larger fraction of
the stellar radiation. The disk emission increases correspondingly at all wave-
lengths but in the near-infrared, where one is sensitive to the lack of the hottest
disk dust. A model with R i  3R ?
may account for the large observed mid-
infrared excess of #033, as discussed in Testi et al (2002). Large variations of
the predicted SED occur if the disk shape changes. Geometrically thin, \ at"
disks, i.e., disks where the grains are not well mixed with the gas, but have col-
lapsed onto the disk midplane predict a much lower mid-infrared emission and
a SED close to a power law F  /  4=3 (see also Apai et al. 2002).
4. Implications
The comparison of the ISO observations to the model predictions shows that
irradiated disk models can account for the observed mid-infrared excess. More
precisely, and in spite of the large uncertainties of the ISO data, the computa-
tions of Natta et al. (2002) show that there are ve systems out of nine (#030,
#032, #102, #160, #176) are extremely well t by at disk models. Two objects
(#023 and #033) seem to require ared, face-on disks, while two others have a
lower mid-infrared excess, consistent with disks seen rather edge-on. However,
given the large error bars and the model uncertainties, most objects with at
disks are also consistent with ared disk models with large inclination.
This comparison between models and observations proves that the mid-
infrared excess associated to many young BDs can be accounted for by the
emission of circumstellar disks heated by the radiation of the central object. Few
disk properties, however, are convincingly constrained by existing observations,

4 Testi et al.
and we do not want to overinterpret our results, given the large uncertainties of
the observed uxes, and the simplicity of the adopted models. However, in our
limited sample of nine stars we nd disks of dierent avours, and, in particular,
an indication that many BDs may have at disks. If we consider also the three
objects in Cha I studied in Natta & Testi (2001) we have three objects with
clear evidence of ared disks, and nine where at disks seem more appropriate,
although we cannot rule out almost edge-on ared disks for some of them. This
is potentially an interesting result, since it seems natural to associate at disks
with dust sedimentation toward the midplane. In our selection of ISO sources,
we have an obvious strong bias against objects with at disks, since we required
that the sources were detected by ISO in both bands. So, the fact that our
objects with the lowest 6.7 m uxes (Cha H1 and #033) have ared disks is
not surprising. However, there is no bias against selecting ared disk objects
of higher luminosity, and we nd only one (#023). The possibility of dust
settling in these very young low-mass objects is intriguing. However, it needs to
be conrmed by high-quality photometric observations at longer wavelengths,
before entering into further speculations.
The ejected embryos hypothesis does not exclude that BDs may have a
small, and therefore short-lived, circumstellar disk. Estimates by Bate et al. (2002)
give disk radii of about 20 AU or less. The existing infrared data do not allow
us to rule out such possibility, since the SED of a model with RD=20 AU will
dier from the SED of a disk with RD=75 AU only at wavelengths 40m.
The mass of the disk is not predicted by existing calculation, nor constrained
by the observations, since the only constraint we can set is that the disk has
to be optically thick in the mid-infrared. This, however, only requires a disk
mass of 10 5 {10 6 M (or MD /Mstar 10 4 ), which is still consistent with a
typical disk (having MD /Mstar 0.03, RD=75 AU), truncated at RD=20 AU.
Until far-infrared and millimeter data become available, the only way to validate
these models is to determine the fraction of disks in unbiased samples of BDs of
known age.
In all objects, the mid-infrared excess is consistent with the predictions of
disks irradiated by the central object. We nd no evidence of strong accretion
occurring in these systems, based on the fact the observed near-infrared uxes
are dominated by the emission of the photospheres, and there is very little
contribution (if any) from hot dust. However, it is not clear to which degree
the near-infrared excess in very low-luminosity objects is a sensitive indicator
of accretion (see, for an example of an actively accreting object with no near-
infrared excess, Fernandez & Comeron 2001; Comeron this volume), and this
issue should be explored more quantitatively in the future.
We want to emphasize that our results do not discriminate yet between
dierent formation mechanisms, namely between the possibility that BDs form
from the gravitational collapse of individual, very low-mass cores, and the ejected
embryo theory. We t the observed mid-infrared excess with a scaled-down ver-
sion of disks around the more massive TTS. This, however, just implies that
\normal" disks can account for the existing observations, since few parameters
are actually constrained. Only observations at longer wavelengths can measure
the disk radius and mass, since the lower limits that we can derive from the con-
ditions that the disk is optically thick in the mid-infrared are hardly signicant.

Disks in Brown Dwarf Systems 5
References
Apai I., Pascucci I., Henning Th., et al. 2002, ApJ, in press
Bate M.R., Bonnell I.A., Bromm V. 2002, MNRAS, 332, L65
Bontemps S., Andre P., Kaas A.A., et al. 2001, A&A, 372, 173
Chiang E.I. , Goldreich P. 1997, ApJ, 490, 368
Comeron F., Rieke G.H., Claes P., Torra J., Laureijs R.J. 1998, A&A, 335, 522
Comeron F., Neuhauser R., Kaas A.A. 2000, A&A, 359, 269
Fernandez M., Comeron F. 2001, A&A, 380, 264
Lin D.N.C., Laughlin G., Bodenheimer P., Rozyczka M. 1998, Science, 281,2025
Muench A.A., Alves J.A., Lada C.J., Lada E.A., 2001, ApJ, 558, L51
Natta A., Testi L. 2001, A&A, 367, L22
Natta A., Testi L., Comeron F. et al. 2002, A&A in press (astro-ph/0207463)
Papaloizou J.C.B., Terquem C. 2001, MNRAS, 325, 221
Reipurth B., Clarke C.J. 2001, AJ, 122, 432
Shu F.H., Adams F.C., Lizano S. 1987, ARA&A, 25,23
Testi L., D'Antona F., Ghinassi F., et al. 2001, ApJ, 552, L147
Testi L., Natta A., Oliva E., et al. 2002, ApJ, 571, L155