![]() | Annu. Rev. Astron. Astrophys. 1994. 32:
531-590 Copyright © 1994 by Annual Reviews. All rights reserved |
9.5. Infrared Searches for Brown Dwarfs
Even though brown dwarfs do not burn hydrogen, they still generate some luminosity in the infrared. They radiate first by gravitational contraction (for about 107 y) and then by degenerate cooling. If the disk or halo dark matter is in the form of brown dwarfs, it is therefore important to consider whether they can be detected via this infrared emission. Current constraints on BDs are rather weak (Low 1986, van der Kruit 1987, Beichmann et al 1990, Nelson et al 1993) but the prospects of detection will be much better with impending space satellites such as ISO and SIRTF.
The problem has been addressed in various contexts by several authors.
Karimabadi & Blitz (1984)
have calculated the expected intensity from BDs
with a discrete IMF comprising an
= 1 cosmological
background.
Adams & Walker (1990)
have discussed the possibility of detecting the collective emission
of the brown dwarfs in our own Galactic halo for both a discrete and
power-law IMF.
Daly & McLaughlin (1992)
have considered the prospects of detecting the
emission of individual halo brown dwarfs of a given mass and age in the
Solar
vicinity, as well as the collective emission of brown dwarfs in other
galaxy halos.
Kerins & Carr (1994)
have considered the possibility that the BDs are assembled
into dark clusters and also discuss how infrared observations at different
wavelengths could be used to probe the mass spectrum of the brown dwarfs.
As an illustration of the feasibility of detecting radiation from BDs,
let us
consider the prospects of detecting the nearest one in our halo. If the
BDs all have the same mass m, then the local halo density
(0 =
0.01 M
pc-3) implies
that the expected distance to the nearest one is 0.55(m / 0.01
M
)1/3pc. The
expected spectra are shown in Figure 9 and
compared to the sensitivities of IRAS
and ISO. This assumes the temperature and luminosity of
Stevenson (1986)
where the BD age and opacity are taken to be 1010y and 0.01
cm2g-1
(corresponding to electron-scattering). Although IRAS gives no
useful constraints (it
is too weak by a factor of 2 even for the optimal mass of 0.07
M
), the
ISOCAM instrument on ISO could detect 0.08
M
BDs in
a few hours, 0.04
M
BDs in
a few days, and 0.02
M
BDs in
a few months. Note that disk BDs, would be
younger, locally more numerous, and more opaque than halo BDs, increasing
the peak flux by 6 and decreasing the peak wavelength by
0.6. IRAS results already imply that BDs with a discrete IMF
could provide the disk dark matter only if their mass is below 0.01
M
.
One might expect the BDs to be easier to detect if they are in
clusters. This
is because, although the distance to the nearest source is increased by
a factor
(Mc / m)1/3, the luminosity is
increased by (Mc / m), giving an increase in flux
of (Mc / m)1/3.
Rix & Lake (1993)
have already used this to exclude the cluster
scenario. However, they assume that the clusters are point sources and, as
illustrated in Figure 4, the dynamical
constraints discussed in Section 6.4 imply
that the clusters will always be extended sources. In fact, the
IRAS extended
source sensitivity (EES) at 12 µ is too low to permit the
detection of clusters.
The ISO extended source sensitivity at 6.75 µ will
suffice, but the time required
to find these clusters is very sensitive to their mass and radius. Note
that the
halo clusters will cover the sky if they are large enough, corresponding
to the
line Kh > 1 in Figure 4,
in which case detecting the clusters is equivalent to
detecting the halo background. ISO would take several months to
detect the
Galactic background, even in the optimal case with m = 0.08
M
(Kerins & Carr 1994).
The background spectrum in this case is also indicated in
Figure 6.