A Sample of Very Young Field L Dwarfs and Implications for the Brown Dwarf “Lithium Test” at Early Ages*

TLDR: In this paper, a large sample of optical spectra of late-type dwarfs was used to identify a subset of late M through L field dwarfs that, because of the presence of low-gravity features in their spectra, are believed to be unusually young.

Abstract: Using a large sample of optical spectra of late-type dwarfs, we identify a subset of late-M through L field dwarfs that, because of the presence of low-gravity features in their spectra, are believed to be unusually young. From a combined sample of 303 field L dwarfs, we find observationally that 7.6% ± 1.6% are younger than 100 Myr. This percentage is in agreement with theoretical predictions once observing biases are taken into account. We find that these young L dwarfs tend to fall in the southern hemisphere (decl: < 0°) and may be previously unrecognized, low-mass members of nearby, young associations like Tucana-Horologium, TW Hydrae, β Pictoris, and AB Doradus. We use a homogeneously observed sample of ~150 optical spectra to examine lithium strength as a function of L/T spectral type and further corroborate the trends noted by Kirkpatrick and coworkers. We use our low-gravity spectra to investigate lithium strength as a function of age. The data weakly suggest that for early- to mid-L dwarfs the line strength reaches a maximum for a few x 100 Myr, whereas for much older (few Gyr) and much younger (<100 Myr) L dwarfs the line is weaker or undetectable. We show that a weakening of lithium at lower gravities is predicted by model atmosphere calculations, an effect partially corroborated by existing observational data. Larger samples containing L dwarfs of well-determined ages are needed to further test this empirically. If verified, this result would reinforce the caveat first cited by Kirkpatrick and coworkers that the lithium test should be used with caution when attempting to confirm the substellar nature of the youngest brown dwarfs.

Figures

Fig. 3.—Same data shown in Fig. 2, except that the y-axis is scaled logarithmically to emphasize features at shorter wavelengths.

Fig. 16.—Zooms of the Keck /LRIS spectra in the 6300–7100 8 region for those objects from the southern L dwarf sample and literature sample exhibiting Li i absorption.

Fig. 15.—Hammer (equal-area) projection of the sky in J2000.0 equatorial coordinates showing members of the AB Doradus moving group (blue plus signs), the Tucana-Horologium association (green crosses), Pictoris moving group (red asterisks), and the TWHydrae association (magenta snowflakes) as listed in Zuckerman & Song (2004). Locations of the 20 low-gravity field dwarfs of Table 6 are shown by filled stars for objects with age estimates 100 Myr and by open stars for objects with age estimates 10 Myr.

Fig. 17.—Update of Fig. 7 from Kirkpatrick et al. (2000). (a) Li i equivalent widths as a function of spectral subclass for all L dwarfs for which we have ever obtained Keck-LRIS spectra. Filled circles denote objects having a detected Li i absorption line. Open circles with downward arrows denote upper limits to the Li i EW for those objects for which no line was detected. To avoid the overlapping of data caused by the quantization of spectral types, some of the points have been given slight offsets along the x-axis. (b) Percentage of L dwarfs showing Li i absorption as a function of spectral subclass. The only objects used in this computation are those for which a Li i equivalent width of 3 8 or more would be detectable. Points have been binned into integer subtypes in which L0 and L0.5 dwarfs have been combined in the L0 bin, L1 and L1.5 dwarfs have been combined in the L1 bin, etc. The total number of objects in each bin is given above each data point. (c) Median Li strength as a function of spectral class for those objects for which lithium absorption was detected.

Fig. 10.—Peculiar L1 dwarf 2MASS J1022+5828 compared to the standard L1 dwarf 2MASSW J1439+1929 and an L0 dwarf (Roque 25) in the Pleiades. Prominent features are marked. Spectra have been normalized to unity at 82508 and have been offset along the y-axis to separate the spectra vertically.

Fig. 22.—Synthetic spectral models at effective temperatures of 2400 K (left), 2100 K (middle), and 1800 K (right) for objects of solar abundance. Each panel illustrates the behavior of the spectra over a range of gravities, decreasing in half-dex increments from log g ¼ 6:0 at the top of each panel to log g ¼ 2:5 at the bottom. Note the universal weakening of the Li i line as gravity is lowered. The spectra displayed here have a resolution of 10 8, identical to that of our empirical spectra.

Full text

A SAMPLE OF VERY YOUNG FIELD L DWARFS AND IMPLICATIONS
FOR THE BROWN DWARF ‘‘LITHIUM TEST’’ AT EARLY AGES
1
J. Davy Kirkpatrick,
2
Kelle L. Cruz,
3
Travis S. Barman,
4
Adam J. Burgasser,
5
Dagny L. Looper,
6
C. G. Tinney,
7
Christopher R. Gelino,
8
Patrick J. Lowrance,
8
James Liebert,
9
John M. Carpenter,
3
Lynne A. Hillenbrand,
3
and John R. Stauffer
10
Received 2008 May 22; accepted 2008 August 21
ABSTRACT
Using a large sample of optical spectra of late-type dwarfs, we identify a subset of late-M through L field dwarfs
that, because of the presence of low-gravity features in their spectra, are believed to be unusually young. From a com-
bined sample of 303 field L dwarfs, we find observationally that 7:6%  1:6% are younger than 100 Myr. This per-
centage is in agreement with theoretical predictions once observing biases are taken into account. We find that these
young L dwarfs tend to fall in the southern hemisphere (decl:<0

) and may be previously unrecognized, low-mass
members of nearby, young associations like Tucana-Horologium, TW Hydrae,  Pictoris, and AB Doradus. We use a
homogeneously observed sample of 150 optical spectra to examine lithium strength as a function of L/T spectral
type and further corroborate the trends noted by Kirkpatrick and coworkers. We use our low-gravity spectra to inves-
tigate lithium strength as a function of age. The data weakly suggest that for early- to mid-L dwarfs the line strength
reaches a maximum for a few ; 100 Myr, whereas for much older (few Gyr) and much younger (<100 Myr) L dwarfs
the line is weaker or undetectable. We show that a weakening of lithium at lower gravities is predicted by model atmo-
sphere calculations, an effect partially corroborated by existing observational data. Larger samples containing L dwarfs
of well-determined ages are needed to further test this empirically. If verified, this result would reinforce the caveat
first cited by Kirkpatrick and coworkers that the lithium test should be used with caution when attempting to confirm
the substellar nature of the youngest brown dwarfs.
Subject headinggs: binaries: general — stars: fundamental parameters — stars: late-typ e —
stars: low-mass, brown dwarfs
1. INTRODUCTION
Determining the difference between an object stably burning
hydrogen (a main-sequence star) and an object of lower mass
incapable of such stable fusion (a brown dwarf ) is not always
straightforward. Although objects with main-sequence spectral
types of O, B, A, F, G, or K are always stars and objects with a
spectral type of T are always brown dwarfs, dwarfs of type M or
L comprise a mixture of stellar and substellar objects.
By definition, distinguishing a brown dwarf from a star re-
quires knowledge that the object is not burning hydrogen in its
core. There is no direct way to probe the interiors of these ob-
jects, so other techniques are required. The most common indi-
rect method used is the ‘‘lithium test’’ ( Rebolo et al. 1992). This
test recognizes the fact that the temperature (T > 3 ; 10
6
K;
Burrows et al. 1997; Nelson et al. 1993) needed to sustain core
hydrogen fusion in the lowest mass stars is only slightly higher
than that needed to burn lithium (T  2 ; 10
6
K; Pozio 1991). If
lithium does not burn, that means hydrogen is also not being
fused. Fortuitously, lithium, once destroyed, is not easily manu-
factured again in stellar interiors, so stars and brown dwarfs will
never have more than their natal abundance of this element. In
principle, the presence or absence of lithium in the spectrum can
thus provide knowledge of inner temperatures.
In practice there are several drawbacks when applying the
lithium test to M and L dwarfs:
1. The ground-state resonance line of Li i is located at 6708 8,
a portion of the spectrum where the flux is quite low for late-M
and L dwarfs. The low flux levels means that large telescopes are
needed for observation of the line. Even then, very few late-type
dwarfs are bright enough that they can be acquired at high resolu-
tion, so to obtain a sizable sample of spectra moderate resolution
(k  5
10 8) is required. Fortunately, this is sufficient for
measuring Li i equivalent widths of a few angstroms or greater
provided that the signal-to-noise ratio is high.
2. As stated above, there is a 1 ; 10
6
K difference between
the temperatures for lithium ignition and normal hydrogen igni-
tion. In other words, some brown dwarfs are capable of burning
lithium and for those the lithium test will fail. These higher mass
brown dwarfs are unrecognizable as substellar based on this test
alone.
1
Most of the spectroscopic data presented here in were obtained at the W. M.
Keck Observatory, which is operated as a scientific partnership among th e Cali-
fornia Institute of Technology, the University of California, and the National
Aeronautics and Space Administration. The Observatory was made possible by
the generous financial support of the W. M. Keck Founda tion. Other spectro -
scopic data were collected at the Subar u Tele scope, the twin telescopes of the
Gemini Observatory, the Mage llan-Clay Telescope, the Kitt Peak National Ob-
servatory Mayall Telescope , a nd the Cerro Tololo Interamerican Observatory
Blanco Telesc ope.
2
Infrared Processing and Analysis Center, MS 100-22, California Institute
of Technol ogy, Pasadena, CA 91125; davy@ip ac.caltech.edu.
3
Department of Astronomy, MS 105-24, California Institute of Technology,
Pasadena, CA 91125.
4
Lowell Observatory, Planetary Research Centre, 1400 West Mars Hill Road,
Flagstaff, AZ 86001.
5
Massachusetts Institute of Technology, 77 Massachusetts Avenue,
Building 37, Cambridge, MA 02139.
6
Institute for Astronomy, University of Hawai’i, 2680 Woodlawn Drive,
Honolulu, HI 96822.
7
Department of Astrophysics, School of Physics, University of New South
Wale s, NSW 2052, Australia.
8
Spitzer Science Center, MS 220-6, California Institute of Technology,
Pasadena, CA 91125.
9
Steward Observatory, 933 North Cherry Avenue, University of Arizona,
Tucson, AZ 85721.
10
Spitzer Science Center, MS 314-6, California Inst itute of Techno logy,
Pasadena, CA 91125.
1295
The Astrophysical Journal, 689:1295–1326, 2008 December 20
# 2008. The American Astronomical Society. All rights reserved. Printed in U.S.A.

3. A sufficient amount of time needs to elapse for the natal
supply of lithium to be extinguished. The Li i line may therefore
still be present not because the object is substellar, but because it
is a star too young to have fused its lithium store. In a very young,
low-mass M star, lithium may still be present in the spectrum
either (1) because the core has not yet reached lithium-burning
temperatures; (2) because the object has not yet cooled to the
temperatures at which it is fully convective, and is thus unable to
recirculate the surface lithium to the core where it can be de-
stroyed; or (3) because there has been insufficient time for con-
vective recirculation to have destroyed all the lithium in the star
and in particular all of the lithium in the photosphere (but this
recirculation is expected to be rapid compared to the other two
timescales considered here; Basri 1998). Despite these potential
pitfalls, Basri (1998) has demonstrated that any object with
lithium absorption and T
eA
< 2670 K is unambiguously a brown
dwarf. Thus, if we confine our focus to objects with temperatures
cooler than this, the lithium test can be applied regardless of these
considerations. Using the relation of optical dwarf spectral type to
temperature from Kirkpatrick (2008b)—T
eA
(K ) ¼ 3759  135x,
where, e.g., x ¼ 0 for M0, 9 for M9, 10 for L0, and 18 for L8—
we find that an effective temperature of 2670 K corresponds to a
dwarf of optical type M8. For the remainder of this paper, we
therefore restrict our discussion of the lithium test to dwarfs with
optical types of M8 and later.
4. It is expected that the 6708 8 line will disappear at very
cool temperatures as lithium forms molecular species. Lodders
(1999) has shown that below T
eA
 1500 K Li-bearing mole-
cules like LiCl and LiOH begin to form. For brown dwarfs of
solar age, this temperature corresponds to optical spectral types
of late-L through mid-T (see Fig. 7 of Kirkpatrick 2005). Stars of
solar abundance do not exist at temperatures this low (see Fig. 10
of Kirkpatrick 2005), so we can safely assume that objects in this
range of spectral types are all brown dwarfs. For these the lithium
test is not needed. This theoretical prediction of lithium disappear-
ance at these types should nonetheless be tested with empirical
data.
5. Young (<100 Myr) brown dwarfs have radii larger than
older stars and brown dwarfs of the same spectral type because
they are still contracting to their final structural configuration
(Burrows et al. 1997). These young brown dwarfs are also lower
in mass than stars and older brown dwarfs of the same spectral
type. Both the larger radius and the lower mass mean that these
objects will have lower surface gravities, which should be detect-
able even with low-resolution spectroscopy. (See x 4.1.) Lower
gravity will weaken the Li i line due to the lower atmospheric
pressure, making it much harder to detect. This has been observed
by Kirkpatrick et al. (2006) in th e low-gravity, early-L dwarf
2MASS J014158234633574, which shows no Li i line down
to an equivalent width of 1 8. Weak Li i is seen despite the fact
that this object is believed to be a young (1–50 Myr), low-mass
(6–25 M
Jup
) brown dwarf and despite the fact that other brown
dwarfs of similar spectral type are known that show prominent
Li i lines—such as the L0 dwarf 2MASS J115442233400390
with a Li i equivalent width of 3 8.
We can test points (4) and (5) using a unique set of spectro-
scopic data. Over the past several years we have amassed a
library of optical spectra of L and T dwarfs primarily using the
W. M. Keck Observatory. Data from Kirkpatrick et al. (1999b,
2000, 2001, 2006; Burgasser et al. 2000, 2003a, 2003b; Wilson
et al. 2001; Thorstensen & Kirkpatrick 2003; McGovern et al.
2004) combined with new data presented in this paper now gives
us a library of  15 0 spectra sp anni ng the wavele ngth regime
6300–10000 8 and covering spectral types L0 through T8. With
such a large set of homogeneous spectra, we can better charac-
terize the behavior of the Li i line as a function of spectral type.
Moreover, this large set of spectra enables us to identify peculiar
objects, particularly those with unusual spectra indicative of low
gravity. As stated above, low gravity in L and T dwarfs is an in-
dicator of youth, so these spectra also enable us to characterize
the Li i strength as a function of age.
In this paper we present the newest spectra in our optical
spectral library (x 2), analyze objects with detected H (x 3), and
identify objects with low-gravity ( young) signatures (x 4). We
perform further analysis of the young objects (x 5) and build sta-
tistics on the behavior of Li i line strengths as a function of both
spectral type and age (x 6). Conclusions are given in x 7.
2. OBJECT SELECTION, OBSERVATION,
AND CLASSIFICATION
2.1. The Two Samples
Over the past several years we have conducted Keck Low
Resolution Imaging Spectrometer ( LRIS; Oke et al. 1995) ob-
servations of two different samples. The first is a color-selected
sample of southern hemisphere L dwarf candidates chosen from
2MASS. The secon d sam ple is a collec tion o f late -L dwarf s,
early-T dwarfs, and a few other interesting cases drawn from the
literature. We present details on each sample below.
2.1.1. The Southern L Dwarf Sample from 2MASS
In 2000 July, one of us (C. G. T.) wished to begin a parallax pro-
gram for ultracool dwarfs using facilities at the Anglo Australian
Observatory (AAO). At this time, very few L dwarfs were recog-
nized south of decl: ¼ 0

, so we performed a photometric search
for L dwarf candidates using data from the Two Micron All Sky
Survey (2MASS; Skrutskie et al. 2006). This search was performed
in two steps. First, the 2MASS Second Incremental Data Release
(IDR2) Point Source Catalog
11
was searched for objects meeting
the following criteria: 50

< decl:<þ5

,8:0 < K
s
< 13:6,
J  K
s
> 1:30, R  K
s
> 6:5 (where optical magnitudes are
available from the USNO-A), and jbj > 25

. In order to weed
out fainter objects at early-L types, a further cut was employed
to drop objects with K
s
> 13:0ifJ  K
s
< 1:6. To supplement
this data set, we also performed on the 2MASS Point Source
Working Database a search for southern hemisphere data taken
after 1999 February 20 (the cutoff date for the 2MASS IDR2) but
before 2000 July. Selection criteria on this set stipulated that the
candidates have 50

< decl:<þ5

, jbj > 25

, J  K
s
> 1:30,
R  K
s
> 5:5 (where optical magnitudes are available from the
USNO-A), and 9:5 < K
s
< 13:6. Visual inspection of the DSS,
XDSS, and 2MASS images was used to drop objects contami-
nated by bright stars or galaxies, lying in nebular regions such as
the Lynds 134 cloud complexes, or having bright R-band counter-
parts on the optical plates. The latter objects are not always elim-
inated using the R  K
s
selection criterion because the USNO-A
Catalog has as one of its constraints that any real object must be
detected on both the blue and red plates; as some late-type giants
will be R-band only sources on the optical sky plates, these will
not be included in the USNO-A (Monet et al. 2003).
Our final candidate list of 68 objects is given in Table 1. Ob-
jects verified spectroscopically to be late-M or L dwarfs are given
in the upper portion of the table; those objects known not to be
late-M or L dwarfs are given in the lower portion. 2MASS source
11
Available at http://irsa.ipac.caltech.edu.
KIRKPATRICK ET AL.1296

TABLE 1
List of L Dwarf Candidates
2MASS Designation
a
(1)
J
(mag)
(2)
H
(mag)
(3)
K
s
(mag)
(4)
J  K
s
Color
(5)
Opt. Sp. Type
(6)
Type Ref.
(7)
Discovery Ref.
(8)
Other Designation
(9)
Confirmed Late-M and L Dwarfs
2MASS J001455754844171.......................... 14.0500.035 13.1070.036 12.7230.030 1.3270.046 L2.5 pec 1 1
2MASS J001659534056541.......................... 15.3160.061 14.2060.048 13.4320.038 1.8840.072 L3.5 1 1
2MASS J002424630158201.......................... 11.9920.035 11.0840.022 10.5390.023 1.4530.042 M9.5 2 3, 4 BRI 00210214
2MASS J003323861521309.......................... 15.2860.056 14.2080.051 13.4100.039 1.8760.068 L2 pec 1 5
2MASS J005110781544169.......................... 15.2770.050 14.1640.048 13.4660.039 1.8110.063 L3.5 6 6
2MASS J005318993631102.......................... 14.4450.026 13.4800.031 12.9370.029 1.5080.039 L3.5 1 1
2MASS J005842530651239.......................... 14.3110.026 13.4440.030 12.9040.033 1.4070.042 L0 6 6
2MASS J011747483403258.......................... 15.1780.036 14.2090.039 13.4890.037 1.6890.052 L2: 7 7
2MASS J014158234633574.......................... 14.8320.043 13.8750.026 13.0970.032 1.7350.054 L0 pec 8 8
2MASS J014435360716142.......................... 14.1910.026 13.0080.029 12.2680.023 1.9230.035 L5 1 9
2MASS J020529401159296.......................... 14.5870.030 13.5680.037 12.9980.030 1.5890.042 L7 2 10 DENIS-P J0205.41159
2MASS J025114900352459.......................... 13.0590.027 12.2540.024 11.6620.019 1.3970.033 L3 7 7
2MASS J025503574700509.......................... 13.2460.027 12.2040.024 11.5580.024 1.6880.036 L8 1 11 DENIS-P J02554700
2MASS J025725813105523.......................... 14.6720.039 13.5180.032 12.8760.032 1.7960.050 L8 1 1
2MASS J031854033421292.......................... 15.5690.055 14.3460.044 13.5070.039 2.0620.067 L7 1 1
2MASS J033703591758079.......................... 15.6210.058 14.4120.050 13.5810.041 2.0400.071 L4.5 6 6
2MASS J035726954417305.......................... 14.3670.032 13.5310.026 12.9070.027 1.4600.042 L0 pec 1 12 DENIS-P J035726.9441730
2MASS J040829051450334.......................... 14.2220.030 13.3370.030 12.8170.023 1.4050.038 L2 7 13
2MASS J042348580414035.......................... 14.4650.027 13.4630.035 12.9290.034 1.5360.043 L7.5 1 14, 15 SDSSp J042348.57041403.5
2MASS J042850962253227.......................... 13.5070.023 12.6680.027 12.1180.026 1.3890.035 L0.5 16 16
2MASS J043901012353083.......................... 14.4080.029 13.4090.029 12.8160.023 1.5920.037 L6.5 7 7
2MASS J04433761+0002051........................... 12.5070.026 11.8040.024 11.2160.021 1.2910.033 M9 pec 1 17 SDSS J044337.61+000205.1
2MASS J044553873048204.......................... 13.3930.026 12.5800.024 11.9750.021 1.4180.033 L2 7 7
2MASS J045326471751543.......................... 15.1420.035 14.0600.035 13.4660.035 1.6760.049 L3: 7 7
2MASS J051206362949540.......................... 15.4630.057 14.1560.048 13.2850.042 2.1780.071 L4.5 1 7
2MASS J052338221403022.......................... 13.0840.024 12.2200.021 11.6380.027 1.4460.036 L2.5 7 7
2MASS J052643484455455.......................... 14.0820.033 13.3070.028 12.7050.027 1.3770.043 M9.5 1 1
2MASS J090957490658186.......................... 13.8900.024 13.0900.021 12.5390.026 1.3510.035 L0 1 18 DENIS-P J09090658
2MASS J095321261014205.......................... 13.4690.028 12.6440.027 12.1420.022 1.3270.036 L0: 19 19
2MASS J101014800406499.......................... 15.5080.059 14.3850.037 13.6190.046 1.8890.075 L6 7 7
2MASS J104524000149576.......................... 13.1600.024 12.3520.025 11.7800.023 1.3800.033 L1 20 20
2MASS J105847871548172.......................... 14.1550.035 13.2260.025 12.5320.029 1.6230.045 L3 2 10 DENIS-P J1058.71548
2MASS J115442233400390.......................... 14.1950.033 13.3310.028 12.8510.033 1.3440.047 L0 1 1
2MASS J121303360432437.......................... 14.6830.035 13.6480.025 13.0140.030 1.6690.046 L5 7 7
2MASS J121859570550282.......................... 14.0500.027 13.3270.024 12.7800.030 1.2700.040 M8 1 7
2MASS J122815231547342.......................... 14.3780.030 13.3470.032 12.7670.030 1.6110.042 L5 2 10 DENIS-P J1228.21547
2MASS J130540192541059.......................... 13.4140.026 12.3920.025 11.7470.023 1.6670.035 L2 2 21 Kelu1
2MASS J140903103357565.......................... 14.2480.026 13.4240.033 12.8650.029 1.3830.039 L2 1 1

TABLE 1—Continued
2MASS Designation
a
(1)
J
(mag)
(2)
H
(mag)
(3)
K
s
(mag)
(4)
J  K
s
Color
(5)
Opt. Sp. Type
(6)
Type Ref.
(7)
Discovery Ref.
(8)
Other Designation
(9)
Confirmed Late-M and L Dwarfs
2MASS J144137160945590.......................... 14.0200.029 13.1900.031 12.6610.030 1.3590.042 L0.5 1 11 DENIS-P J14410945; G 124-62B
b
2MASS J150747691627386.......................... 12.8300.027 11.8950.024 11.3120.026 1.5180.037 L5 22 22
2MASS J153941890520428.......................... 13.9220.029 13.0600.026 12.5750.029 1.3470.041 L4:
c
1 23 DENIS-P J153941.96052042.4
2MASS J161845031321297.......................... 14.2470.024 13.4020.026 12.9200.026 1.3270.035 L0: 1 1
2MASS J162026140416315.......................... 15.2830.049 14.3480.040 13.5980.038 1.6850.062 L2.5 24 24 Gl 618.1B
2MASS J205754090252302.......................... 13.1210.024 12.2680.024 11.7240.025 1.3970.035 L1.5 1 7
2MASS J210414911037369.......................... 13.8410.029 12.9750.025 12.3690.024 1.4720.038 L2.5 1 7
2MASS J210731690307337.......................... 14.2000.032 13.4430.031 12.8780.030 1.3220.044 sd:M9 1 7
2MASS J213044640845205.......................... 14.1370.032 13.3340.032 12.8150.033 1.3220.046 L1.5 1 1
2MASS J215804571550098.......................... 15.0400.040 13.8670.033 13.1850.036 1.8550.054 L4: 1 1
2MASS J220644984217208.......................... 15.5550.066 14.4470.061 13.6090.055 1.9460.086 L2 6 6
2MASS J222443810158521.......................... 14.0730.027 12.8180.026 12.0220.023 2.0510.035 L4.5 6 6
2MASS J234406240733282.......................... 14.8020.037 13.8460.035 13.2320.033 1.5700.050 L4.5 1 1
Objects Confirmed as Other Types
2MASS J012412364537057.......................... 13.4100.029 12.4930.026 12.0760.026 1.3340.039 M giant 1 ...
2MASS J013435660931030.......................... 16.1870.131 14.7930.074 13.5790.055 2.6080.142 Carbon star 1 ...
2MASS J044757500553241.......................... 13.7890.029 12.4410.032 11.7340.019 2.0550.035 Reddened 1 ... ( Early type, reddened star)
2MASS J101525920204318.......................... 14.0500.027 12.8660.025 11.9460.026 2.1040.037 Carbon star 1 ...
2MASS J122740040027506.......................... 12.7570.024 11.5130.021 10.5450.023 2.2120.033 Carbon star 25 ... FASTT 542
2MASS J125621450811144.......................... 11.3170.024 10.4260.024 9.9940.021 1.3230.032 M giant 1 ...
2MASS J134147370813470.......................... 12.8720.024 11.8130.021 11.4440.026 1.4280.035 M giant 1 ...
2MASS J134517890829573.......................... 15.6220.068 14.1650.039 12.8220.027 2.8000.073 QSO 1 1 ( Redshift z ¼ 0:57)
2MASS J135920633023395.......................... 14.6130.033 13.0810.028 11.8200.025 2.7930.041 Carbon star 20 ...
2MASS J143228740531178.......................... 13.9890.028 12.5310.023 11.2400.021 2.7490.035 Carbon star 25 ...
2MASS J150106930531388.......................... 13.5880.026 12.3910.021 11.4980.021 2.0900.033 Carbon star 7 ...
2MASS J151511061332278.......................... 12.5990.026 11.5110.023 10.7950.021 1.8040.033 Carbon star 20 ...
2MASS J155149210750489.......................... 16.8670.181 15.0800.072 13.5330.042 3.3340.186 Carbon star 1 ...
2MASS J160142651249447.......................... 14.4160.029 13.1050.029 12.3820.029 2.0340.041 Carbon star 1 ...
2MASS J201351522806020.......................... 14.2420.030 13.4610.028 12.9440.027 1.2980.040 M8–9 III 1 ...
2MASS J210018790606550.......................... 15.3060.045 13.5570.025 12.0730.021 3.2330.050 Carbon star 1 ...
2MASS J223513224835588.......................... 13.8090.029 12.9490.029 12.1240.026 1.6850.039 QSO ... 26
a
Source designations from the 2MASS All-Sky Point Source Catalog are given as ‘‘2MASS Jhhmmss[.]ssddmmss[.]s,’’ where the suffix is the sexigesimal right ascension and declination at J2000.0 equinox.
b
Common proper motion for DENIS-P J14410945 and G 124-62A confirmed by Seifahrt et al. (2005).
c
DENIS J15390520 is typed in the optical as L3.5 by Reid et al. (2008).
References.— (1) This paper; (2) Kirkpatrick et al. 1999b; (3) Luyten 1980; (4) Irwin et al. 1991; (5) Gizis et al. 2003; (6) Kirkpatrick et al. 2000; (7) Cruz et al. 2003; (8) Kirkpatrick et al. 2006; (9) Liebert et al. 2003;
(10) Delfosse et al. 1997 ; (11) Martı´n et al. 1999b; (12) Bouy et al. 2003; (13) Wilson et al. 2003; (14) Geball e et al. 2002; (15) Schneider et al. 2002; (16) Kendall et al. 2003; (17) Hawley et al. 2002; (18) Delfosse et al.
1999; (19) Cruz et al. 2007; (20) Gizis 2002 ; (21) Ruiz et al. 1997; (22) Reid et al. 2000; (2 3) Kendall et al. 2004; (24) Wilson et al. 2001; (25 ) Totten & Irwin 1998; (26 ) Savage et al. 1976.

designations are listed in column (1), and photometric data from
the final 2MASS All-Sky Release Point Source Catalog (Cutri et al.
2003), which postdates our selection, are given in columns (2–5).
Using this later catalog as the data source, some of the objects we
originally selected may no longer fall within our constraints (such
as the J  K
s
color of 1.29 for 2MASS J04433761+0002051).
Columns (6) and (7) give the optical spectral type and reference
for the classification. For objects discovered previously by other
surveys, alternate source designations are given in column (9).
As for the AAO L dwarf parallax program itself, observations
of our confirmed candidates continue. Results from the program
will be presented in a forthcoming paper.
2.1.2. Interesting Objects Pulled from the Literature
The goal of the second sample was to obtain optical spectra of
late-M, L, and T dwarfs that either had no previous spectroscopic
follow-up in the optical or that would benefit from an optical spec-
trum with higher signal-to-noise ratio or covering a new epoch.
These interesting objects
12
are listed in approximate order of right
ascension in the following:
2MASS J04150935.—This object is the latest optical T dwarf
standard from Burgasser et al. (2003a).
2MASS J051852828.—This object is a discovery from the
ongoing 2MASS L dwarf search of Cruz et al. (2003) and because
of a very unusual near-infrared spectrum was hypothesized to be
a late-L + T dwarf double (Cruz et al. 2004). Its binary nature has
been confirmed via HST imaging (Burgasser et al. 2006c).
SDSS J0830+4828, SDSS J08370000, SDSS J0857+5708,
SDSS J10210304, and SDSS J12540122.—These objects
are all classified in the near-infrared as late-L or early-T dwarfs
(Leggett et al. 2000; Geballe et al. 2002), and we obtained LRIS
spectra o f these to explore the L/T transition at far optical
wavelengths.
Gl 337CD.—This object adds to this sample near the L/T
transition and is relatively bright (K
s
¼ 14:0). Its optical type
is L8 (Wilson et al. 2001), whereas its near-infrared type is T0
(Burgasser et al. 2006b).
2MASS J12091004.—This object adds another data point to
the L / T transition. This object is the T3 near-infrared standard on
the Burgasser et al. (2006b) scheme.
2MASS J13152649.—This object is an L dwarf with ab-
normally strong and variable H emission ( Hall 2002a; Gizis
2002; Hall 2002b; Riaz & Gizis 2007) and we chose to observe it
again to check its H strength.
2MASS J2244+2043.—This object is an extremely red (J 
K
s
¼ 2:45  0:16) late-L dwarf uncovered during a hunt for red
QSOs in 2MASS. We chose to observe this object to see if the
optical spectrum revealed any clues regarding its unusually red
near-infrared photometry.
2.2. Spectroscopic Obser
vations
Optical spectra were obtained with LRIS on the 10 m Keck-I
Observatory atop Mauna Kea, Hawaii. A 400 lines mm
1
grating
blazed at 8500 8 was used with a 1
00
slit and 2048 ; 2048 CCD to
produce 10 8 resolution spectra covering the range 6300–10100 8.
The OG570 order-blocking filter was used to eliminate second-
order light. The data were reduced and calibrated using standard
IRAF routines. Flat-field exposures of the interior of the tele-
scope dome were used to normalize the response of the detector.
Individual stellar spectra were extracted using the apextract
routine in IRAF,
13
allowing for the slight curvature of a point-
source spectrum viewed through the LRIS optics and using a tem-
plate where necessary. Wavelength calibration was achieved using
neon+argon arc lamp exposures taken after each program object.
Finally, the spectra were flux-calibrated using observations of
standards from Hamuy et al. (1994). Most of the data have not
been corrected for telluric absorption, so the atmospheric O
2
bands
at 6867–7000, 7594–7685 8 and H
2
O bands at 7186–7273,
8161–8282, 8950–9300, and 9300–9650 8 are still present
in the spectra. In the sections that follow, a few cases are noted
where a correction for telluric absorption was applied by using
the spectrum of a nearby field late-F/early-G star taken just be-
fore or after the spectrum of the program object.
Table 2 lists the UT dates of observation, program principal
investigator, other observers assisting, and sky conditions for the
TABLE 2
Nights of Observation at Keck
Obs. Date ( UT)
(1)
Principal Investigator
(2)
Other Observer Assisting
(3)
Sky Conditions
(4)
2000 Aug 23.............. Kirkpatrick ( None) Cirrus throughout night
2000 Dec 26 .............. Kirkpatrick Liebert Clear
2000 Dec 27 .............. Carpenter Hillenbrand Clear
2000 Dec 28 .............. Carpenter Hillenbrand Clear, seeing poorer than average
2001 Feb 20 ............... Kirkpatrick Liebert Mostly clear, light clouds late in night
2001 Nov 13.............. Stauffer Kirkpatrick Spotty clouds
2002 Jan 1.................. Carpenter ( None) Clear
2002 Jan 2.................. Carpenter ( None) Clear
2002 Jan 3.................. Carpenter ( None) Clear first half, then fog forced closure
2002 Feb 19 ............... Kirkpatrick Lowrance Never opened (fog and snow)
2002 Feb 20 ............... Kirkpatrick Lowrance Never opened (fog)
2003 Jan 2.................. Kirkpatrick Lowrance Clear
2003 Jan 3.................. Kirkpatrick Lowrance Clear
2003 Dec 22 .............. Kirkpatrick Lowrance Clear
2003 Dec 23 .............. Kirkpatrick Lowrance Clear
2003 Dec 24 .............. Kirkpatrick Lowrance Clear
12
In the text we abbreviate sources n ames with a prefix such as 2MASS, DENIS,
or SDSS and a suffix of the form Jhhmmddmm, where hhmm is the truncated
J2000.0 right ascension in hours and minutes and ddmm is the truncated J2000.0
declination in degrees and minutes. Full designations are given in the tables.
13
IRAF is distributed by the National Optical Astronomy Observatory,
which is operated by the Association of Universities for Research in Astronomy,
Inc., under cooperative agreement with the National Science Foundation.
LITHIUM TEST 1299