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Liu, M.C. and Magnier, E.A. and Deacon, N.R. and Allers, K.N. and Dupuy, T.J. and Kotson, M.C. and
Aller, K.M. and Burgett, W.S. and Chambers, K.C. and Draper, P.W. and Hodapp, K.W. and Jedicke, R. and
Kaiser, N. and Kudritzki, R.-P. and Metcalfe, N. and Morgan, J.S. and Price, P.A. and Tonry, J.L. and
Wainscoat, R.J. (2013) 'The extremely red, young L Dwarf PSO J318.5338-22.8603 : a free-oating
planetary-mass analog to directly imaged young gas-giant planets.', Astrophysical journal letters., 777 (2). L20.
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The Astrophysical Journal Letters, 777:L20 (7pp), 2013 November 10 doi:10.1088/2041-8205/777/2/L20
C
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THE EXTREMELY RED, YOUNG L DWARF PSO J318.5338−22.8603: A FREE-FLOATING
PLANETARY-MASS ANALOG TO DIRECTLY IMAGED YOUNG GAS-GIANT PLANETS
Michael C. Liu
1,7
, Eugene A. Magnier
1
, Niall R. Deacon
2
, Katelyn N. Allers
3
, Trent J. Dupuy
4,8
,
Michael C. Kotson
1
, Kimberly M. Aller
1
, W. S. Burgett
1
, K. C. Chambers
1
, P. W. Draper
5
, K. W. Hodapp
1
,
R. Jedicke
1
,N.Kaiser
1
, R.-P. Kudritzki
1
, N. Metcalfe
6
, J. S. Morgan
1
, P. A. Price
5
,J.L.Tonry
1
, and R. J. Wainscoat
1
1
Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
2
Max Planck Institute for Astronomy, Konigstuhl 17, D-69117 Heidelberg, Germany
3
Department of Physics and Astronomy, Bucknell University, Lewisburg, PA 17837, USA
4
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
5
Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
6
Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
Received 2013 September 15; accepted 2013 September 25; published 2013 October 22
ABSTRACT
We have discovered using Pan-STARRS1 an extremely red late-L dwarf, which has (J − K)
MKO
= 2.78 and
(J − K)
2MASS
= 2.84, making it the r eddest known field dwarf and second only to 2MASS J1207−39b among
substellar companions. Near-IR spectroscopy shows a spectral type of L7 ± 1 and reveals a triangular H-band
continuum and weak alkali (K i and Na i) lines, hallmarks of low surface gravity. Near-IR astrometry from the Hawaii
Infrared Parallax Program gives a distance of 24.6 ± 1.4 pc and indicates a much fainter J-band absolute magnitude
than field L dwarfs. The position and kinematics of PSO J318.5−22 point to membership in the β Pic moving group.
Evolutionary models give a temperature of 1160
+30
−40
K and a mass of 6.5
+1.3
−1.0
M
Jup
, making PSO J318.5−22 one of
the lowest mass free-floating objects in the solar neighborhood. This object adds to the growing list of low-gravity
field L dwarfs and is the first to be strongly deficient in methane relative to its estimated temperature. Comparing
their spectra suggests that young L dwarfs with similar ages and temperatures can have different spectral signatures
of youth. For the two objects with well constrained ages (PSO J318.5−22 and 2MASS J0355+11), we find their
temperatures are ≈400 K cooler than field objects of similar spectral type but their luminosities are similar, i.e., these
young L dwarfs are very red and unusually cool but not “underluminous.” Altogether, PSO J318.5−22 is the first
free-floating object with the colors, magnitudes, spectrum, luminosity, and mass that overlap the young dusty
planets around HR 8799 and 2MASS J1207−39.
Key words: brown dwarfs – parallaxes – planets and satellites: atmospheres – proper motions – solar neighborhood
– surveys
Online-only material: color figures
1. INTRODUCTION
A major surprise arising from direct detection of gas-giant
planets around young stars is that the spectral properties of
these objects differ from those of field L and T dwarfs (e.g.,
Chauvin et al. 2005; Marois et al. 2008; Bowler et al. 2010, 2013;
Patience et al. 2010;Barmanetal.2011a). These young planets
have redder near-IR colors, fainter near-IR absolute magnitudes,
and peculiar spectra compared to their field analogs. Over the
last several years, this development has fostered closer scrutiny
of the long-standing paradigm that a simple physical sequence
connects the lowest mass stars to brown dwarfs to gas-giant
planets.
We now know that most field brown dwarfs are not good
analogs to young exoplanets. In contrast to field T dwarfs of
similar temperature, the young planets around HR 8799 and
2MASS J1207−39 have no methane absorption and very red
colors. These properties are thought to arise from extreme
atmospheric conditions tied to their young (≈10–30 Myr)
ages and low gravities, e.g., enhanced vertical mixing,
7
Visiting Astronomer at the Infrared Telescope Facility, which is operated by
the University of Hawaii under Cooperative Agreement no. NNX-08AE38A
with the National Aeronautics and Space Administration, Science Mission
Directorate, Planetary Astronomy Program.
8
Hubble Fellow.
non-equilibrium chemistry, and unusual clouds (e.g., Barman
et al. 2011b; Madhusudhan et al. 2011; Marley et al. 2012). This
is corroborated by recent studies of the youngest (∼10–100 Myr)
field brown dwarfs, which find that low-gravity L dwarfs also
show very red colors and spectral peculiarities (e.g., McLean
et al. 2003; Kirkpatrick et al. 2008). Complicating this interpre-
tation, however, is the existence of very red L dwarfs that do
not show spectral signatures of youth (e.g., Kirkpatrick et al.
2010; Allers & Liu 2013). Thus, similarities between the colors
and spectra of field brown dwarfs and young planets can have
ambiguous interpretations.
There are two prominent shortfalls in our observational
knowledge. (1) There are only a handful of very red young
L dwarfs currently known (see compilation in Gizis et al. 2012),
and only one of them has a parallax measurement (Faherty
et al. 2013; Liu et al. 2013). (2) Most L dwarfs do not have as
red near-IR colors as young exoplanets, and none are as faint
in their near-IR absolute magnitudes. Therefore, the utility of
young field objects as exoplanet analogs may be limited, since
the existing samples of these two types of objects are small and
do not really overlap.
We have found an extraordinary young L dwarf that will help
shed light on these topics. It is the reddest field object found
to date and the first to have absolute magnitudes comparable to
directly imaged young dusty exoplanets.
1
The Astrophysical Journal Letters, 777:L20 (7pp), 2013 November 10 Liu et al.
2. OBSERVATIONS
We have been undertaking a search for T dwarfs using the
Pan-STARRS1 (PS1) 3π Survey (Deacon et al. 2011; Liu et al.
2011). We select objects using Pan-STARRS1 and Two Micron
All Sky Survey (2MASS) based on their colors and proper
motions and then obtain near-infrared photometry for further
screening. We observed PSO J318.5338−22.8603 (hereinafter
PSO J318.5−22) using WFCAM on the UK Infrared Telescope
on the 2010 September 15 UT. Conditions were photometric
with 0.9–1.
0 seeing. We found that the (J − K)
MKO
color for
PSO J318.5−22 was 2.74 ± 0.04 mag (Table 1 ), significantly
redder than any previously known field dwarf.
We obtained R ≈ 100 near-IR (0.8–2.5 μm) spectra on
2011 July 21 UT from NASA’s Infrared Telescope Facility.
(By coincidence, we observed PSO J318.5−22 immediately
before obtaining the spectrum of the similarly red L dwarf
WISE J0047+68 published in Gizis et al. 2012.) Conditions were
photometric with 1
seeing. We used the near-IR spectrograph
SpeX (Rayner et al. 1998) in prism mode with the 0.
8 slit. The
total on-source integration time was 16 minutes. All spectra were
reduced using version 3.4 of the SpeXtool software (Vacca et al.
2003; Cushing et al. 2004). We also used the final spectrum to
synthesize near-IR colors for PSO J318.5−22 (Table 1).
To assess the gravity of PSO J318.5−22, we obtained R ≈
1700 near-IR spectra using the GNIRS spectrograph (Elias et al.
2006) on the Gemini-North 8.1 m Telescope. Cross-dispersed
spectra were obtained using the 0.
3 slit and the 32 lines mm
−1
grating on the nights of 2013 June 26, June 30, and July 1 UT.
The total integration time was 5400 s. We reduced the data using
a version of SpeXtool modified for GNIRS cross-dispersed data.
We combined the telluric-corrected spectra from the three nights
using a robust weighted mean to produce the final 0.95–2.5 μm
spectrum.
We conducted astrometric monitoring of PSO J318.5−22
with the facility near-IR camera WIRCam at the
Canada–France–Hawaii Telescope, obtaining nine epochs over
2.0 yr, starting on 2011 July 26 UT. Our methods are de-
scribed in Dupuy & Liu (2012). Using 116 reference stars in
the field of PSO J318.5−22, the resulting median astrometric
precision per epoch was 4.0 mas, and the best-fit proper motion
and parallax solution had χ
2
= 13.2 with 13 degrees of free-
dom. We applied a relative-to-absolute parallax correction of
0.74 ± 0.13 mas derived from the Besan¸con model of the Galaxy
(Robin et al. 2003). Table 1 gives our astrometry results. We did
not find any objects co-moving with PSO J318.5−22 in our
10.
4 × 10.
4 field of view within a range between 0.6 mag
fainter and 3.3 mag brighter at J band.
3. RESULTS
3.1. Spectrophotometric Properties
The colors of PSO J318.5−22 are extreme, with
(J − K)
MKO
= 2.78 mag, (J − K)
2MASS
= 2.84 mag, and
(W 1 − W 2) = 0.76 ± 0.04 mag, all being the reddest among
field L dwarfs (Figure 1 and also see Gizis et al. 2012). Such
colors are thought to arise from an unusually dusty atmosphere
that results from a low surface gravity (young age). The position
of PSO J318.5−22 on the near-IR color–magnitude diagram is
similarly extreme, being significantly fainter in J-band absolute
magnitude than field L dwarfs (Figure 1). It coincides with the
colors and magnitudes of the directly imaged planets around
HR 8799 and 2MASS J1207−39b.
Table 1
Measurements of PSO J318.5338−22.8603
Property Measurement
Nomenclature
2MASS 2MASS J21140802−2251358
Pan-STARRS1 PSO J318.5338−22.8603
WISE WISE J211408.13−225137.3
Astrometry (Equinox J2000)
2MASS R.A., decl. (ep 1999.34) 318.53344, −22.85996
Pan-STARRS1 R.A., decl. (ep 2010.0) 318.53380, −22.86032
WISE R.A., decl. (ep 2010.34) 318.53388, −22.86037
Proper motion μ
α
,μ
δ
(mas yr
−1
) 137.3 ± 1.3, −138.7 ± 1.4
Proper motion amplitude μ (
yr
−1
) 195.0 ± 1.3
Proper motion P.A. (
◦
) 135.3 ± 0.4
Parallax π (mas) 40.7 ± 2.4
Distance d (pc) 24.6 ± 1.4
v
tan
(km s
−1
) 22.7 ± 1.3
Photometry
PS1 g
P1
(AB mag) >23.6 (3σ )
a
PS1 r
P1
(AB mag) >23.1 (3σ )
a
PS1 i
P1
(AB mag) >22.9 (3σ )
a
PS1 z
P1
(AB mag) 20.80 ± 0.09
a
PS1 y
P1
(AB mag) 19.51 ± 0.07
a
2MASS J (mag) 16.71 ± 0.20
2MASS H (mag) 15.72 ± 0.17
2MASS K
S
(mag) 14.74 ± 0.12
MKO Y (mag) 18.81 ± 0.10
MKO J (mag) 17.15 ± 0.04
MKO H (mag) 15.68 ± 0.02
MKO K (mag) 14.41 ± 0.02
WISE W 1 (mag) 13.22 ± 0.03
WISE W 2 (mag) 12.46 ± 0.03
WISE W 3 (mag) 11.8 ± 0.4
WISE W 4(mag) >8.6 (2σ )
Synthetic photometry
b
2MASS J − H (mag) 1.678 ± 0.007 (0.06)
2MASS H − K
S
(mag) 1.159 ± 0.004 (0.03)
2MASS J − K
S
(mag) 2.837 ± 0.006 (0.07)
MKO Y − J (mag) 1.37 ± 0.02 (0.05)
MKO J − H (mag) 1.495 ± 0.007 (0.06)
MKO H − K (mag) 1.279 ± 0.004 (0.03)
MKO J − K (mag) 2.775 ± 0.007 (0.07)
(J
2MASS
− J
MKO
) (mag) 0.111 ± 0.002
(H
2MASS
− H
MKO
)(mag) −0.072 ± 0.001
(K
S
,2MASS
− K
MKO
) (mag) 0.049 ± 0.001
log(L
bol
/L
) (dex) −4.42 ± 0.06
Spectral classification
c
J-band type (SpeX, GNIRS) L8 ± 1, L9 ± 1
H-band type (SpeX, GNIRS) L6 ± 1, L6 ± 1
H
2
OD type (SpeX, GNIRS) L6.0 ± 0.9, L6.0 ± 0.8
Gravity score (SpeX, GNIRS) XXX2, 2X21
Final near-IR spectral type L7 ± 1
Near-IR gravity class vl-g
Physical Properties (age = 12
+8
−4
Myr)
Mass (M
Jup
) 6.5 (−1.0, +1.3)
T
eff ,evol
(K) 1160 (−40, +30)
log(g
evol
) (cgs) 3.86 (−0.08, +0.10)
Radius (R
Jup
) 1.53 (−0.03, +0.02)
Physical Properties (age = 10–100 Myr)
Mass (M
Jup
)12± 3
T
eff ,evol
(K) 1210 (−50, +40)
log(g
,evol
) (cgs) 4.21 (−0.16, +0.11)
Radius (R
Jup
) 1.40 (−0.04, +0.06)
Notes.
a
Average of multi-epoch photometry. The optical non-detections are consistent
with the colors of late-L dwarfs (i
P1
− z
P1
≈ 2.0–2.5 mag; Best et al. 2013).
b
Colors were synthesized from our SpeX spectrum, with formal errors derived
from the spectrum’s measurement errors. The values in parentheses are the
estimated systematic errors, calculated from comparing the measured JHK
colors for ∼100 objects to the colors synthesized from the SpeX Prism Library.
This additional uncertainty is needed to reconcile the two sets of results such
that P (χ
2
) ≈ 0.5. No systematic errors (0.02 mag) were measurable for the
2MASS-to-MKO conversion within a given bandpass (Dupuy & Liu 2012).
c
Based on the classification system of Allers & Liu (2013).
2
The Astrophysical Journal Letters, 777:L20 (7pp), 2013 November 10 Liu et al.
−1 0 1 2 3
(J−K)
MKO
[mag]
17
16
15
14
13
12
11
M(J
MKO
) [mag]
2M0355+11
PSO
318.5−22
2M0122 B
HR 8799 bcd
2M1207 b
β Pic b
CD−35 2722 B
κ And b
HN Peg B
Ross 458 C
AB Pic b
LP 261−75 B
2M0103−55 ABb
>15 M
Jup
11−15 M
Jup
<11 M
Jup
M dwarf
L dwarf
T dwarf
Figure 1. Left: color–color diagrams using optical+IR (top left) and IR-only (bottom left) photometry. Field objects are plotted based on our PS1 photmetry (y
P1
)
and the MKO photometry (JHK) compilation by Leggett et al. (2010). The extreme colors of PSO J318.5 −22 compared to the field population are evident. Also
shown are the young planetary-mass object 2MASS 1207−39b (Chauvin et al 2005) and the very red field L dwarfs 2MASS J2148+40, 2MASS J2244+20, 2MASS
J0355+11, and WISE J0047+68. Right: PSO J318.5−22 compared to known substellar objects based on the compilations of Dupuy & Liu (2012), Bowler et al. (2013),
and references therein. Young substellar companions are highlighted, with the AB Pic b data from Biller et al. (2013). PSO J318.5−22 is very red and faint compared
to field L dwarfs, with magnitudes and colors comparable to the planets around HR 8799 and 2MASS J1207−39. (Its measurements uncertainties are smaller than the
symbol size.)
(A color version of this figure is available in the online journal.)
We determine the near-IR spectral type of PSO J318.5−22
using the Allers & Liu (2013) system, which provides
gravity-insensitive types consistent with optical spectral types.
For late-L dwarfs, visual classification in the J and K bands and
index-based classifications with the H
2
OD index are applicable.
For the GNIRS spectrum of PSO J318.5−22, we visually as-
sign a J-band type of L9 ± 1 and a K-band type of L6 ± 1. The
H
2
OD index corresponds to L6.0 ± 0.8. The weighted mean
of these three determinations leads to a final type of L7 ± 1.
Spectral typing of our low-resolution SpeX spectrum gives the
same classification (Table 1).
PSO J318.5−22 shows a triangular H-band continuum, which
is considered a hallmark of youth (e.g., Lucas et al. 2001).
However, Allers & Liu (2013) caution that very red L-dwarfs
having no signatures of youth (low gravity) can display a
triangular H-band shape (e.g., 2MASS J2148+40 in Figure 2).
At moderate resolution, there are other indicators of youth for
late-L dwarfs. Our GNIRS spectrum displays a weak 1.20 μm
FeH band as well as weak Na i (1.14 μm) and K i (1.17
and 1.25 μm) lines, which indicate a low gravity. Using the
gravity-sensitive indices of Allers & Liu (2013), we classify
PSO J318.5−22 as vl-g, which Allers & Liu suggest correspond
to ages of ∼10–30 Myr based on the (small) sample of young
late-M/early-L dwarfs with good age constraints. Altogether,
PSO J318.5−22 visually appears most similar to the red
L dwarfs WISE J0047+68 (Gizis et al. 2012) and 2MASS
J2244+20 (McLean et al. 2003), in accord with the similar
spectral types and gravity classifications for these three objects.
Allers & Liu (2013) note that objects of the same age
and spectral type (temperature) can display different spectral
signatures of youth, based on two young L dwarfs in the AB Dor
moving group. Our new discovery affirms this idea. The spectra
of 2MASS J1207−39b and PSO J318.5−22 are quite different
(Figure 2), despite their similar colors and absolute magnitudes
and the fact they may be coeval (Section 3.2). PSO J318.5−22
shows a negative continuum slope from 2.12–2.28 μm, whereas
2MASS J1207−39b has a positive slope. The H-band continuum
of PSO J318.5−22 displays a “shoulder” at 1.58 μm, whereas
2MASS J1207−39b has a very peaked continuum. Overall,
the spectrum of 2MASS J1207−39b appears most similar to
that of 2MASS J0355+11 (both objects have near-IR types of
L3 vl-g), despite their large differences in ages, colors, and
absolute magnitudes. Altogether, these comparisons hint that
determining relative ages and temperatures from NIR spectra
may be unexpectedly complex.
To assess the physical parameters, we fit the low-resolution
near-IR spectra of PSO J318.5−22 and 2MASS J0355+11 with
the Ames/DUSTY model atmospheres (Allard et al. 2001)
and the BT-Settl model atmospheres (Allard et al. 2011) with
two different assumed solar abundances (Asplund et al. 2009
[AGSS] and Caffau et al. 2011 [CIFIST]). Since both objects
have parallaxes, the scaling factors (R
2
/d
2
) from the fits provide
3
The Astrophysical Journal Letters, 777:L20 (7pp), 2013 November 10 Liu et al.
1.1 1.2 1.3
Wavelength (μm)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Normalized F
λ
+ Constant
FeH
Na
K
K
1.5 1.6 1.7
Wavelength (μm)
H
2
O
FeH
2.0 2.1 2.2 2.3
Wavelength (μm)
H
2
O
CO
2M 1207−39b
PSO J318.5−22
W 0047+68
2M 0355+11
2M 2244+20
2M 2148+40
2M 0103+19
1.0 1.5 2.0 2.5
Wavelength (μm)
0.0
0.5
1.0
1.5
2.0
2.5
Normalized F
λ
PSO 318.5−22
(L7 VL−G)
2M 1207−39b
(L3 VL−G)
W 0047+68
(L7 INT−G)
2M 0103+19
(L7 std/L6 INT−G)
2M 2244+20
(L6 VL−G)
2M 2148+40
(L6 FLD−G)
2M 0355+11
(L3 VL−G)
Figure 2. Spectrum of PSO J318.5−22 compared to the L7 near-IR standard 2MASS J0103+19 of Kirkpatrick et al. (2010); the dusty field object 2MASS J2148+40
(Looper et al. 2008, which we classify as L6 ± 1 fld-g); the L6 vl-g standard 2MASS J2244+20 from Allers & Liu (2013); the very red L dwarfs 2MASS J0355+11
(Reid et al. 2008; Faherty et al. 2013; Allers & Liu 2013) and WISE J0047+68 (Gizis et al. 2012; J. E. Gizis et al. in preparation); and the young planetary-mass object
2MASS 1207−39b (Patience et al. 2010). The labels use near-IR types and gravity classifications on the Allers & Liu system, except for the Kirkpatrick et al. (2010)
L7 standard (which Allers & Liu classify as L6 int-g in the near-IR). Top: low-resolution (R ≈ 100) spectra, normalized to the J-band peak (1.26–1.31 μm). The
2MASS J1207−39b spectrum has been lightly smoothed. Note that despite having similar colors, luminosities, and ages, 2MASS J1207−39b and PSO J318.5−22
have very different H-band continuum shapes. Bottom: moderate-resolution spectra for the same young and/or dusty objects, all smoothed to R = 750. The weaker
J-band Na i and K i lines for PSO J318.5−22 compared to 2MASS J2244+20 and WISE J0047+68 indicate a lower gravity.
(A color version of this figure is available in the online journal.)
an estimate of the objects’ radii. Using χ
2
minimization, all
three sets of models indicates that PSO J318.5−22 (T
eff
,atm
=
1400–1600 K) is 100–200 K cooler than 2MASS J0355+11
(T
eff
,atm
= 1600–1700 K). BT-Settl/AGSS models give the
best fits, with the resulting temperatures being comparable to
previous fitting of field L dwarfs but with both young objects
having lower surface gravities (log(g) = 4.0–4.5 dex) than
field objects (Cushing et al. 2008; Stephens et al. 2009;Testi
2009, though the number of fitted field objects is small). Most
strikingly, the fitted radii are implausibly small (≈0.8 R
Jup
),
indicating discord between the data and models even though the
quality of fit seems adequate by eye. A similar radius mismatch
occurs in model atmosphere fitting of young dusty planets (e.g.,
Bowler et al. 2010;Barmanetal.2011a; cf. Marley et al.
2012).
3.2. Group Membership
There is no radial velocity (RV) for PSO J318.5−22, but we
can place modest constraints on its space motion. Published
RVs (Blake et al. 2010; Seifahrt et al. 2010; Rice et al. 2010;
Shkolnik et al. 2012) find that low-gravity late-M and L dwarfs
reside in the range of [−20, +25] km s
−1
, with the smaller RV
range compared to field objects being expected given the young
ages. Figure 3 plots the spatial (XYZ) and kinematic (UVW)
location of PSO J318.5−22 compared to the young moving
groups (YMGs) from Torres et al. (2008).
9
Its XYZ position
is coincident with the β Pic, Tuc-Hor, and AB Dor moving
9
U and X are positive toward the Galactic center, V and Y are positive toward
the direction of galactic rotation, and W and Z are positive toward the north
Galactic pole.
4
![Figure 3. Kinematic and spatial position of PSO J318.5−22 (red circle) compared to known YMGs from Torres et al. (2008). For the UVW plots, we adopt an RV of [−20, +25] km s−1 (Section 3.2), with the larger red circles being more positive velocities and the red line being the full RV range. Uncertainties arising from errors in the distance and proper motion (but not the RV) are show at the three red circles. The members of the nearest YMGs to PSO J318.5−22 are shown in color and the others in gray, with the ellipses representing the rms of the known members. Consideration of both the XYZ and UVW data indicate the β Pic group is the only possibility. The middle (black-outlined) circle shows the UVW position of PSO J318.5−22 for an RV of −6 km s−1.](/figures/figure-3-kinematic-and-spatial-position-of-pso-j318-5-22-red-me8krps4.webp)
