One major goal in astrophysics is to extend previous successes in the
characterization and modeling of stellar atmospheres to the much cooler
atmospheres of extrasolar planets. A key pathway is the identification of
free-floating objects that not only share common temperatures with
exoplanets but that also share common masses and thus surface gravities.
In recent years, searches for ever colder free-floating brown dwarfs-
objects with masses below the hydrogen-fusing mass limit-have steadily
pushed the census of the solar neighborhood to ever lower masses and
finally perhaps into the planetary-mass regime ( 13 Jupiter masses).<
The detection of large samples of brown dwarfs at the beginning of
the last decade ushered in two, now widely accepted, spectral types de-
noted by the letters “L” and “T” that extend the canonical OBAFGKM
scheme for classifying stars that had stood untouched for nearly a centu-
ry. Over the last two years, candidates for a “Y” spectral class have been
uncovered in binary surveys (1, 2) and in all-sky imaging data from
WISE, the Wide-field Infrared Survey Explorer (3). The primary criterion
adopted to trigger this class has been the appearance of ammonia (NH3)
absorption in near-infrared (1-2.5 μm) spectra.
Y dwarfs probe colder atmospheric physics than before, with puta-
tive effective temperatures as low as Teff 300 K and masses of ≈ 5-20
Jupiter masses (3). If found orbiting a star, a Y dwarf would likely be
considered a gas-giant planet. However, these estimated properties of Y
dwarfs are speculative given the uncertainty in their temperatures, ages,
and luminosities. Temperatures have only been estimated from model
atmospheres that use incomplete molecular line lists and simple prescrip-
tions for complex processes like nonequilibrium chemistry and conden-
An independent approach for determining temperatures is to com-
bine bolometric luminosities (Lbol) with evolutionary model-predicted
radii (R ) and apply the Stefan-Boltzmann Law, Teff ≡ (4πσR 2
Recent observations of transiting substellar objects generally support
evolutionary model radius predictions over a wide range of masses (4–
6). Although many may not be ideal test cases, since they may have
formed via core accretion or have been intensely irradiated, variations in
radii are expected to be relatively small and not strongly influence our
resulting temperatures given the weak
dependence on radius (Teff R −1/2
Therefore, the key measurements need-
ed to determine temperatures via lumi-
nosity are accurate distances to Y
dwarfs, along with a method for com-
puting Lbol from multi-wavelength pho-
Trigonometric parallaxes provide
the only direct means of measuring
distances to stars. A star’s distance is
inversely proportional to the amplitude
of its apparent periodic motion on the
sky relative to more distant background
stars, which is due to the Earth’s orbital
motion around the Sun. The amplitude
of this effect is small, 0.1 arcseconds
for a star at 10 parsec, and thus measur-
ing parallaxes requires long-term, pre-
cise position measurements. We have
been using the Infrared Array Camera
(IRAC) on board the Spitzer Space
Telescope to obtain such astrometry of
late-T and Y dwarfs from 2011-2012.
Spitzer currently trails the Earth by ≈ 2
months in its solar orbit, and keeping its
solar shield directed at the Sun forces
the telescope to observe stars near par-
allax maximum. By maintaining a cold temperature Spitzer can obtain
sensitive images in the thermal mid-infrared, where Y dwarfs emit most
of their flux, giving it an advantage over ground-based near-infrared
observations of Y dwarfs. We also use an improved correction for the
nonlinear optical distortion of Spitzer/IRAC that enables 10× smaller
residual errors than the correction used by the standard data pipeline,
allowing us to unlock the precision astrometric capabilities of Spitzer.
Distances, Luminosities, and
Temperatures of the Coldest Known
Trent J. Dupuy1
and Adam L. Kraus1,2
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA.
epartment, University of Texas at Austin, 1 University Station C1400, Austin, TX 78712, USA.D
Corresponding author. E-mail: email@example.com
The coolest known brown dwarfs are our best analogs to extrasolar gas-giant
planets. The prolific detections of such cold substellar objects in the past two years
has spurred intensive followup, but the lack of accurate distances is a key gap in
our understanding. We present a large sample of precise distances based on
homogeneous mid-infrared astrometry that robustly establish absolute fluxes,
luminosities, and temperatures. The coolest brown dwarfs have temperatures of 400
to 450 kelvin and masses ≈5 to 20× that of Jupiter, showing they bridge the gap
between hotter brown dwarfs and gas-giant planets. At these extremes, spectral
energy distributions no longer follow a simple correspondence with temperature,
suggesting an increasing role of other physical parameters such as surface gravity,
vertical mixing, clouds, and metallicity.
By combining our parallaxes (Table S1, Fig. S1) with photometry
from the literature (7–9), we have determined absolute magnitudes in the
near-infrared YJHK bands (≈ 1.0-2.4 μm) and Spitzer’s mid-infrared
bands at 3.6 μm and 4.5 μm (Table S2, Fig. 1, Fig. 2). For each spectral
type bin, we computed the weighted mean absolute magnitude as well as
upper/lower limits on the amount of intrinsic scatter in the magnitudes
Objects classified as normal Y0 dwarfs are ≈2 magnitudes (≈6×)
fainter in the near-infrared compared to the latest type T dwarfs, yet they
generally share very similar colors. The most notable exception is that
the Y−J colors become much bluer for Y dwarfs (9), which we find is
due to flux at ≈1.25 μm dropping by 5× while flux at ≈ 1.05 μm only
drops by 2.5×. This behavior is consistent with prior speculation that Y
dwarfs may be so cool that the alkali atoms that dominate absorption at
blue wavelengths for warmer brown dwarfs finally become locked into
molecules like Na2S and KCl, thereby reducing the opacity at 1.05 μm
relative to 1.25 μm (10). The appearance of such molecules could result
in the return of substantial condensate clouds (11) and corresponding
In contrast to their near-infrared behavior, Y dwarfs show remarka-
ble diversity in their mid-infrared colors. Even though they are only ≈2×
fainter than the latest T dwarfs at these wavelengths, they range from the
same color as late-T dwarfs to much redder (≈0.8 magnitudes). One of
the reddest objects is WISEP J1405+5534, which has been typed as “Y0
peculiar?” because its H-band spectral peak is shifted 60 Å redder than
the Y0 standard WISEP J1738+2732 (3). We find that WISEP
J1405+5534 in fact has a very similar temperature to other Y0 dwarfs
(Table S5) indicating that its unusual spectrum is due to another physical
/ http://www.sciencemag.org/content/early/recent / 5 September 2013 / Page 1/ 10.1126/science.1241917
property. Both the mid-infrared color and peculiar spectrum may be
explained by a reduced level of nonequilibrium chemistry in the photo-
sphere, perhaps due to reduced vertical mixing. This would produce
enhanced NH3 absorption at H-band as compared to other Y0 dwarfs and
enhanced CH4 absorption relative to CO driving WISEP J1405+5534 to
redder [3.6]−[4.5] colors.
The coldest brown dwarfs also demonstrate unusual behavior in their
absolute fluxes as a function of spectral type. Despite the plummeting
near-infrared flux-normal Y0 dwarfs are ≈6× fainter than the latest T
dwarfs-Y0 dwarfs have indistinguishable fluxes compared to each other
to within 15%-25%. This is very unusual compared to warmer brown
dwarfs, which do not show such step-function behavior at any spectral
type transition and also show much larger intrinsic scatter (≈30%-50%)
in absolute fluxes for a given spectral type (12). This homogeneity
among the Y0 dwarfs is further unexpected because it reverses the trend
observed for the late-T dwarfs that the scatter increases substantially
with later type, cooler objects (Fig. 3). For example, here we double the
sample of T9 dwarfs with accurate distances and find that their near-
infrared fluxes typically have a scatter of 130%-210%.
Another unexpected result is that T9.5 dwarfs appear to be brighter
at all bandpasses than the mean for T9 dwarfs and the T9 standard UGPS
J0722−0540. Given the smaller sample of T9.5 dwarfs (three objects)
and their more uncertain distances, this brightening is currently a 2σ
result, i.e., the weighted means in Table S3 are consistent with being
equal at a p-value of 0.05. Such a brightening is reminiscent of the
change in near-infrared fluxes from late-L to early-T dwarfs (13, 14),
however we note that the brightening at the L/T transition only occurs at
blue near-infrared wavelengths whereas we see brightening at all bands
for the T9.5 dwarfs.
To derive bolometric luminosities from the absolute fluxes, we com-
puted “super-magnitudes” by summing the fluxes in near- and mid-
infrared bandpasses. This is an approximation to the standard method of
integrating the observed spectral energy distribution as a function of
wavelength, which is not possible for Y dwarfs given the current lack of
sensitive mid-infrared spectrographs. We derive a multiplicative correc-
tion to account for the remaining flux not captured in these bands from a
large grid of model atmospheres (11, 15). The weak dependence on these
models is highlighted by the 8% fractional uncertainty in this correction
factor (Fig. S4).
We used the Cond evolutionary models (16) to estimate radii and
thereby temperatures, masses, and surface gravities from the bolometric
luminosities of our sample (Fig. 4, Table S5). We assumed fiducial ages
of 1 Gyr and 5 Gyr as expected for the field population (17, 18). The
tangential velocities for our sample are consistent with having such typi-
cal ages. We find that the fractional change in temperature over this
narrow range of spectral types is remarkably large: the mean temperature
of T8 dwarfs is 685-745 K (for 1-5 Gyr), and this drops to 410-440 K for
Y0 dwarfs (Table S6). Thus, these two subtypes alone span the same
fractional range in temperature as the entire sequence of FGK stars
(7300-4400 K) that are 10× hotter.
Although much cooler than their late-T counterparts, Y0 dwarfs turn
out to be significantly warmer than previously suggested from model
atmosphere fitting (Fig. S5). The most common best-fit models of Y0
dwarfs in previous work have Teff = 350 K, with plausible model fits of
400 K in some cases (3). Thus, model fits are typically 60-90 K (≈15%-
25%) cooler than we find from our distances combined with evolution-
ary model radii. If the fault lies with our assumed radii, they would need
to be 30%-50% larger than expected because Teff R −1/2
. This would
require very young ages ( 100 Myr) or very large systematic errors in
the evolutionary models that are not likely given the aforementioned
empirical validation from transiting brown dwarfs. Rather, we suggest
that parameters derived from fitting model atmospheres to near-infrared
spectra, where < 5% of the flux emerges, are less likely to be accurate
because current atmospheres imperfectly reproduce observed spectra.
Using our luminosity measurements, we find that the coldest brown
dwarfs would be 6-10 Jupiter masses given an age of 1 Gyr. An older
age of 5 Gyr implies 16-25 Jupiter masses. These masses therefore
straddle the current demarcation of “planetary mass” set by the deuteri-
um-fusing mass limit of ≈ 13 Jupiter masses (19–21). Thus, it is possible
that the atmospheres of our objects harbor deuterated molecules such as
HDO or CH3D that have not yet been detected because of the observa-
tional challenges (22).
Given the interest in both identifying the coldest atmospheric
benchmarks and searching for the bottom of the initial mass function we
briefly consider the most extreme objects in our sample in terms of tem-
perature and mass. WISEP J1828+2650 has been dubbed the archetypal
Y dwarf with a model-atmosphere temperature of < 300 K, i.e., room
temperature, based on extremely red colors implying that the Wien tail
of its underlying blackbody distribution has moved into the near-infrared
(3). Our luminosity for this object is inconsistent with such a low tem-
perature, and we find it must be at least 420 K at 2σ; our calculations
give K at an age of 1 Gyr. (If WISEP J1828+2650 is a binary as
proposed by (9) the 2σ limit at 1 Gyr only drops to 360 K and 340 K for
the hypothetical two components.) Its atypical properties compared to
other Y dwarfs may simply be due to a slightly lower surface gravity,
i.e., slightly younger age, which qualitatively agrees with model predic-
tions that the collapse in near-infrared flux happens at warmer tempera-
tures for lower surface gravity (23). Ross 458C is a contender for the
lowest mass object, at 7 Jupiter masses, if its age is near the 150 Myr
lower limit of its proposed age range (24). However, WD 0806−661B is
the most secure case for both lowest temperature (330-375 K) and lowest
mass (6-10 Jupiter masses) object known, given that it has a precise age
of 2.0 ± 0.5 Gyr (25).
Overall, our results strengthen the connection between the coolest
brown dwarfs and gas-giant exoplanets. We validate that they probe an
extreme physical regime that bridges the gap between previously known,
hotter brown dwarfs and Jupiter-like planets. We find that objects of
very similar temperatures can have widely varying spectral energy dis-
tributions and absorption features, e.g., a range of 0.8 magnitudes in
mid-IR colors for the same Teff. Along with the fact that the ≥Y2 dwarf is
warmer than the Y0 dwarfs, this implies that temperature is not the prin-
cipal determinant in shaping spectra but rather seems to be on compara-
ble footing with other physical properties such as surface gravity,
vertical mixing, clouds, and perhaps metallicity. Consequently, the cur-
rent spectral classification scheme used to identify Y dwarfs may not
strongly correlate with temperature as it generally does for L and T
dwarfs. This could explain the unusually homogeneous fluxes for Y0
dwarfs, unusually heterogeneous fluxes for T9 dwarfs, and plateau or
brightening of flux from T9 to T9.5.
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Acknowledgments: We thank M. C. Liu for many helpful suggestions for
improving this article; C. V. Morley for fruitful discussions regarding her and
collaborators’ model atmospheres, particularly the Vega zero points; M. C.
Cushing for making available published spectra of our targets; and K. N.
Allers for assistance in computing photometry from our IRAC data. This work
is based on observations made with the Spitzer Space Telescope, which is
operated by the Jet Propulsion Laboratory, California Institute of Technology
under a contract with NASA. T.J.D. acknowledges support from Hubble
Fellowship grant HST-HF-51271.01-A awarded by the Space Telescope
Science Institute, which is operated by AURA for NASA, under contract NAS
5-26555. A.L.K. is supported by a Clay Fellowship. Our imaging data are
available in the Spitzer Heritage Archive at
http://sha.ipac.caltech.edu/applications/Spitzer/SHA, and the astrometric
measurements we derived are given in Table S1.
Materials and Methods
Figs. S1 to S5
Tables S1 to S6
13 June 2013; accepted 23 August 2013
Published online 05 September 2013
Fig. 1. Color-magnitude diagrams for
all objects with spectral types T8 and
later that have direct distance
measurements. Data points are color
coded according to spectral type, with
open/white points indicating that no
spectra are available. Small gray points
are earlier type field brown dwarfs.
Near-infrared photometry is on the
Mauna Kea Observatory (MKO)
Fig. 2. Absolute magnitude as a function of spectral type for near-infrared and mid-infrared bandpasses.
Objects typed as peculiar are shown as open white symbols. Objects with very uncertain distances are plotted
with smaller gray symbols. Error bars for spectral types are not plotted, and small x-axis offsets have been
added to the spectral types for clarity.
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Fig. 4. Bolometric luminosities (Lbol) and effective temperatures
for objects of spectral type T8 and later; spectrally peculiar
objects are denoted by white symbols. Objects with Lbol
uncertainties larger than 0.2 dex are shown as smaller, gray
symbols. These are either objects with very uncertain distances
or the components of tight binaries where the lack of resolved
mid-infrared photometry results in a very uncertain bolometric
flux. Error bars for spectral types are not plotted, and small x-
axis offsets have been added to the spectral types for clarity.
Effective temperatures are derived from our Lbol measurements
and Cond evolutionary model radii. Upward and downward
pointing triangles correspond to the median Lbol and lower and
upper age limits used (see Table S5). Error bars show the range
of temperatures corresponding to the ±1σ range of Lbol over the
same age range.
Fig. 3. Intrinsic photometric scatter among objects at each spectral type; unfilled symbols are
upper limits where no scatter is detected. Y0 dwarfs show remarkably low intrinsic scatter in
the near-infrared, reversing the trend observed at the end of the T dwarf sequence that later
type T dwarfs show increasing dispersion in their near-infrared absolute magnitudes. The
much smaller sample of T9.5 dwarfs are also suggestive of this reversal at J and H bands.
There is no evidence for such a reversal in the mid-infrared where both late-T and Y dwarfs
show much less intrinsic scatter and distance uncertainties for Y dwarfs do not permit strong
enough upper limits.
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