648 research outputs found
High-resolution Spectra of Very Low-Mass Stars
We present the results of high-resolution (1-0.4A) optical spectroscopy of a
sample of very low-mass stars. These data are used to examine the kinematics of
the stars at the bottom of the hydrogen-burning main sequence. No evidence is
found for a significant difference between the kinematics of the stars in our
sample with I-K > 3.5 (MBol > 12.8) and those of more massive M-dwarfs (MBol =
7-10). A spectral atlas at high (0.4A) resolution for M8-M9+ stars is provided,
and the equivalent widths of CsI, RbI and Halpha lines present in our spectra
are examined. We analyse our data to search for the presence of rapid rotation,
and find that the brown dwarf LP 944-20 is a member of the class of ``inactive,
rapid rotators''. Such objects seem to be common at and below the hydrogen
burning main sequence. It seems that in low-mass/low-temperature dwarf objects
either the mechanism which heats the chromosphere, or the mechanism which
generates magnetic fields, is greatly suppressed.Comment: 19 pages, 12 figure files. MNRAS style file. Accepted for publication
in MNRAS, August 199
The Intermediate Age Brown Dwarf LP 944-20
Observations are presented which show that LiI 6708A is detected with
equivalent width of 0.53+-0.05A in the proper-motion object LP 944-20 (which is
also known as BRI 0337-3535). This Li detection implies that LP 944-20 is a
brown dwarf with mass between 0.057 and 0.063Mo and age between 475 and 650Myr,
making it the first brown dwarf to have its mass and age precisely determined.Comment: 3 pages, 2 figure
Periodic photometric variability of the brown dwarf Kelu-1
We have detected a strong periodicity of 1.80+/-0.05 hours in photometric
observations of the brown dwarf Kelu-1. The peak-to-peak amplitude of the
variation is ~1.1% (11.9+/-0.8 mmag) in a 41nm wide filter centred on 857nm and
including the dust/temperature sensitive TiO & CrH bands. We have identified
two plausible causes of variability: surface features rotating into- and
out-of-view and so modulating the light curve at the rotation period; or,
elliposidal variability caused by an orbiting companion. In the first scenario,
we combine the observed vsin(i) of Kelu-1 and standard model radius to
determine that the axis of rotation is inclined at 65+/-12 degrees to the line
of sight.Comment: 7 pages, 9 figures. Accepted for publication in MNRA
The First Direct Distance and Luminosity Determination for a Self-Luminous Giant Exoplanet: The Trigonometric Parallax to 2MASS1207334-393254Ab
We present the first trigonometric parallax and distance for a young
planetary mass object. A likely TW Hya cluster member, 2MASSW J1207334-393254Ab
(hereafter 2M1207Ab) is an M8 brown dwarf with a mid to late L type planetary
mass companion. Recent observations of spectral variability have uncovered
clear signs of disk accretion and outflow, constraining the age of the system
to <10 Myr. Because of its late spectral type and the clearly youthful nature
of the system, 2M1207b is very likely a planetary mass object. We have measured
the first accurate distance and luminosity for a self-luminous planetary mass
object. Our parallax measurements are accurate to <2 mas (1sigma) for 2M1207Ab.
With 11 total epochs of data taken from January 2006 through April 2 007 (475
images for 2M1207Ab), we determine a distance of 58.8+-7.0 pc (17.0{+2.3}{-1.8}
mas, 1.28sigma) to 2M1207Ab and a calculated luminosity of 0.68-2.2x10^-5 Lsun
for 2M1207b. Hence 2M1207Ab is a clear member of the TW Hya cluster in terms of
its distance, proper motions, and youthful nature. However, as previously noted
by Mohanty and co-workers, 2M1207b's luminosity appears low compared to its
temperature according to evolutionary models.Comment: 12 pages, 3 figures, accepted to ApJ Letter
The 2MASS Wide-Field T Dwarf Search. IV Unting out T dwarfs with Methane Imaging
We present first results from a major program of methane filter photometry
for low-mass stars and brown dwarfs. The definition of a new methane filter
photometric system is described. A recipe is provided for the differential
calibration of methane imaging data using existing 2MASS photometry. We show
that these filters are effective in discriminating T dwarfs from other types of
stars, and demonstrate this with Anglo-Australian Telescope observations using
the IRIS2 imager. Methane imaging data and proper motions are presented for ten
T dwarfs identified as part of the 2MASS "Wide Field T Dwarf Search" -- seven
of them initially identified as T dwarfs using methane imaging.
We also present near-infrared moderate resolution spectra for five T dwarfs,
newly discovered by this technique. Spectral types obtained from these spectra
are compared to those derived from both our methane filter observations, and
spectral types derived by other observers. Finally, we suggest a range of
future programs to which these filters are clearly well suited: the winnowing
of T dwarf and Y dwarf candidate objects coming from the next generation of
near-infrared sky surveys; the robust detection of candidate planetary-mass
brown dwarfs in clusters; the detection of T dwarf companions to known L and T
dwarfs via deep methane imaging; and the search for rotationally-modulated
time-variable surface features on cool brown dwarfs.Comment: 20 pages. To appear in The Astronomical Journal, Nov. 200
A Spin-Orbit Alignment for the Hot Jupiter HATS-3b
We have measured the alignment between the orbit of HATS-3b (a recently
discovered, slightly inflated Hot Jupiter) and the spin-axis of its host star.
Data were obtained using the CYCLOPS2 optical-fiber bundle and its simultaneous
calibration system feeding the UCLES spectrograph on the Anglo-Australian
Telescope. The sky-projected spin-orbit angle of was
determined from spectroscopic measurements of Rossiter-McLaughlin effect. This
is the first exoplanet discovered through the HATSouth transit survey to have
its spin-orbit angle measured. Our results indicate that the orbital plane of
HATS-3b is consistent with being aligned to the spin axis of its host star. The
low obliquity of the HATS-3 system, which has a relatively hot mid F-type host
star, agrees with the general trend observed for Hot Jupiter host stars with
effective temperatures K to have randomly distributed spin-orbit angles.Comment: 13 pages. Accepted for publication in the Astrophysical Journa
Ks band secondary eclipses of WASP-19b and WASP-43b with the Anglo-Australian Telescope
We report new Ks band secondary eclipse observations for the hot-Jupiters
WASP-19b and WASP-43b. Using the IRIS2 infrared camera on the Anglo-Australian
Telescope (AAT), we measured significant secondary eclipses for both planets,
with depths of 0.287 -0.020/+0.020% and 0.181 -0.027/+0.027% for WASP-19b and
WASP-43b respectively. We compare the observations to atmosphere models from
the VSTAR line-by-line radiative transfer code, and examine the effect of C/O
abundance, top layer haze, and metallicities on the observed spectra. We
performed a series of signal injection and recovery exercises on the observed
light curves to explore the detection thresholds of the AAT+IRIS2 facility. We
find that the optimal photometric precision is achieved for targets brighter
than Kmag = 9, for which eclipses as shallow as 0.05% are detectable at >5
sigma significance.Comment: Accepted for publication in MNRAS, 13 pages, 10 figure
Infrared Spectra and Spectral Energy Distributions of Late-M- and L-Dwarfs
We have obtained 1.0-2.5um spectra at R~600 of 14 disk dwarfs with spectral
types M6 to L7. For four of the dwarfs we have also obtained infrared spectra
at R~3000 in narrow intervals. In addition, we present new L' photometry for
four of the dwarfs in the sample, which allows improved determinations of their
bolometric luminosities. We resolve the L-dwarf Denis-P J 0205-1159 into an
identical pair of objects separated by 0.35". The spectra, with the published
energy distribution for one other dwarf, are compared to synthetic spectra
generated by upgraded model atmospheres. Good matches are found for 2200> Teff
K>1900 (spectral types around M9 to L3), but discrepancies exist at Teff> 2300
K (M8) and for Teff<1800 K (L4-L7). At the higher temperatures the mismatches
are due to incompleteness in the water vapor linelist. At the lower
temperatures the disagreement is probably due to our treatment of dust: we
assume a photospheric distribution in equilibrium with the gas phase. We derive
effective temperatures for the sample from the comparison with synthetic
spectra and also by comparing our observed total intrinsic luminosities to
structural model calculations (which are mostly independent of the atmosphere
but are dependent on the unknown masses and ages of the targets). The two
derivations agree to ~200 K except for the faintest object in the sample where
the discrepancy is larger. Agreement with other temperature determinations is
also ~200 K, except for the L7 dwarf.Comment: 31 pages incl. 5 Tables and 12 Figures, accepted by ApJ for Feb 2001
issu
Planetary Formation Scenarios Revistied: Core-Accretion Versus Disk Instability
The core-accretion and disk instability models have so far been used to
explain planetary formation. These models have different conditions, such as
planet mass, disk mass, and metallicity for formation of gas giants. The
core-accretion model has a metallicity condition ([Fe/H] > −1.17 in the
case of G-type stars), and the mass of planets formed is less than 6 times that
of the Jupiter mass MJ. On the other hand, the disk instability model does not
have the metallicity condition, but requires the disk to be 15 times more
massive compared to the minimum mass solar nebulae model. The mass of planets
formed is more than 2MJ. These results are compared to the 161 detected planets
for each spectral type of the central stars. The results show that 90% of the
detected planets are consistent with the core-accretion model regardless of the
spectral type. The remaining 10% are not in the region explained by the
core-accretion model, but are explained by the disk instability model. We
derived the metallicity dependence of the formation probability of gas giants
for the core-accretion model. Comparing the result with the observed fraction
having gas giants, they are found to be consistent. On the other hand, the
observation cannot be explained by the disk instability model, because the
condition for gas giant formation is independent of the metallicity.
Consequently, most of planets detected so far are thought to have been formed
by the core-accretion process, and the rest by the disk instability process.Comment: accepted for publication in The Astrophysical Journa
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