773 research outputs found
Hubble Space Telescope Observations of Field Ultracool Dwarfs at High Galactic Latitude
We present a sample of 17 newly discovered ultracool dwarf candidates later
than ~M8, drawn from 231.90 arcmin2 of {\it Hubble Space Telescope} Wide Field
Camera 3 infrared imaging. By comparing the observed number counts for
17.5<J_125<25.5 AB mag to an exponential disk model, we estimate a vertical
scale height of z_scl=290 +- 25 (random) +- 30 (systematic) pc for a binarity
fraction of f_b=0. While our estimate is roughly consistent with published
results, we suggest that the differences can be attributed to sample
properties, with the present sample containing far more substellar objects than
previous work. We predict the object counts should peak at J_{125}~24 AB mag
due to the exponentially-declining number density at the edge of the disc. We
conclude by arguing that trend in scale height with spectral type may breakdown
for brown dwarfs since they do not settle onto the main sequence.Comment: 9 pages, 6 figures, 3 tables, accepted to ApJ (v2 is consistent with
Referee changes
Discovery of a Second L Subdwarf in the Two Micron All Sky Survey
I report the discovery of the second L subdwarf identified in the Two Micron
All Sky Survey, 2MASS J16262034+3925190. This high proper motion object (mu =
1.27+/-0.03 "/yr) exhibits near-infrared spectral features indicative of a
subsolar metallicity L dwarf, including strong metal hydride and H2O absorption
bands, pressure-broadened alkali lines, and blue near-infrared colors caused by
enhanced collision-induced H2 absorption. This object is of later type than any
of the known M subdwarfs, but does not appear to be as cool as the apparently
late-type sdL 2MASS 0532+8246. The radial velocity (Vrad = -260+/-35 km/s) and
estimated tangential velocity (Vtan ~ 90-210 km/s) of 2MASS 1626+3925 indicate
membership in the Galactic halo, and this source is likely near or below the
hydrogen burning minimum mass for a metal-poor star. L subdwarfs such as 2MASS
1626+3925 are useful probes of gas and condensate chemistry in low-temperature
stellar and brown dwarf atmospheres, but more examples are needed to study
these objects as a population as well as to define a rigorous classification
scheme.Comment: 11 pages, 3 figures, accepted for publication ApJ Letters, v. 614
October 200
SDSS J125637-022452: a high proper motion L subdwarf
We report the discovery of a high proper motion L subdwarf (
=0.617arcsec/yr) in the Sloan Digital Sky Survey spectral database. The optical
spectrum from the star SDSS J125637-022452 has mixed spectral features of both
late-M spectral subtype (strong TiO and CaH at 7000A) and mid-L spectral
subtype (strong wings of KI at 7700A, CrH and FeH), which is interpreted as the
signature of a very low-mass, metal-poor star (ultra-cool subdwarf) of spectral
type sdL. The near infrared (NIR) (J-Ks) colors from 2MASS shows the object to
be significantly bluer compared to normal L dwarfs, which is probably due a
strong collision induced absorption (CIA) due to H2 molecule. This is
consistent with the idea that CIA from H2 is more pronounced at low
metallicities. Proper motion and radial velocity measurements also indicate
that the star is kinematically "hot" and probably associated with the Galactic
halo population.Comment: 13 pages, 2 figures. Accepted for ApJ
The discovery of a T6.5 subdwarf
We report the discovery of ULAS J131610.28+075553.0, an sdT6.5 dwarf in the UKIDSS Large Area Survey 2 epoch proper motion catalogue. This object displays significant spectral peculiarity, with the largest yet seen deviations from T6 and T7 templates in the Y and K bands for this subtype. Its large, similar to 1 arcsec yr(-1), proper motion suggests a large tangential velocity of V-tan approximate to 240-340 km s(-1), if we assume its M-J lies within the typical range for T6.5 dwarfs. This makes it a candidate for membership of the Galactic halo population. However, other metal-poor T dwarfs exhibit significant under luminosity both in specific bands and bolometrically. As a result, it is likely that its velocity is somewhat smaller, and we conclude it is a likely thick disc or halo member. This object represents the only T dwarf earlier than T8 to be classified as a subdwarf, and is a significant addition to the currently small number of known unambiguously substellar subdwarfs.Peer reviewe
A search for lithium in metal-poor L dwarfs
The aim of the project is to search for lithium in absorption at 6707.8
Angstroms to constrain the nature and the mass of the brightest low-metallicity
L-type dwarfs (refered to as L subdwarfs) identified in large-scale surveys.
We obtained low- to intermediate-resolution (R~2500-9000) optical (~560-770
nm) spectra of two mid-L subdwarfs, SDSSJ125637.13-022452.4 (SDSS1256; sdL3.5)
and 2MASSJ162620.14+392519.5 (2MASS1626; sdL4) with spectrographs on the
European Southern Observatory Very Large Telescope and the Gran Telescopio de
Canarias.
We report the presence of a feature at the nominal position of the lithium
absorption doublet at 6707.8 Angstroms in the spectrum of SDSS1256, with an
equivalent width of 66+/-27 Angstroms at 2.4 sigma, which we identify as
arising from a CaH molecular transition based on atmosphere models. We do not
see any feature at the position of the lithium feature in the spectrum of
2MASS1626. The existence of overlapping molecular absorption sets a confusion
detection limit of [Li/H]=-3 for equivalently-typed L subdwarfs. We provided
improved radial velocity measurements of -126+/-10 km/s and -239+/-12 km/s for
SDSS1256 and 2MASS1626, respectively, as well as revised Galactic orbits. We
implemented adjusting factors for the CaH molecule in combination with the
NextGen atmosphere models to fit the optical spectrum of SDSS1256 in the
6200-7300 Angstroms range. We also estimate the expected Li abundance from
interstellar accretion ([Li/H]=-5), place limits on circumstellar accretion
(10^9 g/yr), and discuss the prospects of Li searches in cooler L and T
subdwarfs.Comment: 9 pages, 6 figures, 4 tables, accepted to A&
Spectroscopic Detection of Carbon Monoxide in Two Late-type T Dwarfs
M band spectra of two late-type T dwarfs, 2MASS J09373487+2931409, and Gliese
570D, confirm evidence from photometry that photospheric CO is present at
abundance levels far in excess of those predicted from chemical equilibrium.
These new and unambiguous detections of CO, together with an earlier
spectroscopic detection of CO in Gliese 229B and existing M band photometry of
a large selection of T dwarfs, suggest that vertical mixing in the photosphere
drives the CO abundance out of chemical equilibrium and is a common, and likely
universal feature of mid-to-late type T dwarfs. The M band spectra allow
determinations of the time scale of vertical mixing in the atmosphere of each
object, the first such measurements of this important parameter in late T
dwarfs. A detailed analysis of the spectral energy distribution of 2MASS
J09373487+2931409 results in the following values for metallicity, temperature,
surface gravity, and luminosity: [M/H]~-0.3, T_eff=925-975K, log g=5.20-5.47,
log L/L_sun=-5.308 +/- 0.027. The age is 3-10 Gyr and the mass is in the range
45-69 M_Jup.Comment: 36 pages incl. 12 figures and 3 tables, accepted by Ap
Discovery of An Unusually Blue L Dwarf Within 10 pc of the Sun
We report the discovery of an unusually blue L5 dwarf within 10 pc of the Sun
from a search of Sloan Digital Sky Survey (SDSS) spectra. A spectrophotometric
distance estimate of 8.0+/-1.6 pc places SDSS J141624.08+134826.7 among the six
closest known L dwarfs. SDSS 1416+13 was overlooked in infrared color-based
searches because of its unusually blue J-K_S color, which also identifies it as
the nearest member of the blue L dwarf subclass. We present additional infrared
and optical spectroscopy from the IRTF/SpeX and Magellan/MagE spectrographs and
determine UVW motions that indicate thin disk kinematics. The inclusion of SDSS
1416+13 in the 20 pc sample of L dwarfs increases the number of L5 dwarfs by
20% suggesting that the L dwarf luminosity function may be far from complete.Comment: 15 pages, 3 figures, accepted for publication in AJ; updated version
includes corrected radial velocit
Clouds in the Coldest Brown Dwarfs: FIRE Spectroscopy of Ross 458C
Condensate clouds are a salient feature of L dwarf atmospheres, but have been
assumed to play little role in shaping the spectra of the coldest T-type brown
dwarfs. Here we report evidence of condensate opacity in the near-infrared
spectrum of the brown dwarf candidate Ross 458C, obtained with the Folded-Port
Infrared Echellette (FIRE) spectrograph at the Magellan Telescopes. These data
verify the low-temperature nature of this source, indicating a T8 spectral
classification, log Lbol/Lsun = -5.62+/-0.03, Teff = 650+/-25 K, and a mass at
or below the deuterium burning limit. The data also reveal enhanced emission at
K-band associated with youth (low surface gravity) and supersolar metallicity,
reflecting the properties of the Ross 458 system (age = 150-800 Myr, [Fe/H] =
+0.2 to +0.3). We present fits of FIRE data for Ross 458C, the T9 dwarf ULAS
J133553.45+113005.2, and the blue T7.5 dwarf SDSS J141624.08+134826.7B, to
cloudless and cloudy spectral models from Saumon & Marley. For Ross 458C we
confirm a low surface gravity and supersolar metallicity, while the temperature
differs depending on the presence (635 [+25,-35] K) or absence (760 [+70,-45]
K) of cloud extinction. ULAS J1335+1130 and SDSS J1416+1348B have similar
temperatures (595 [+25,-45] K), but distinct surface gravities (log g = 4.0-4.5
cgs versus 5.0-5.5 cgs) and metallicities ([M/H] ~ +0.2 versus -0.2). In all
three cases, cloudy models provide better fits to the spectral data,
significantly so for Ross 458C. These results indicate that clouds are an
important opacity source in the spectra of young cold T dwarfs, and should be
considered when characterizing the spectra of planetary-mass objects in young
clusters and directly-imaged exoplanets. The characteristics of Ross 458C
suggest it could itself be regarded as a planet, albeit one whose cosmogony
does not conform with current planet formation theories.Comment: Accepted for publication to ApJ: 18 pages, 11 figures in emulateapj
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