731 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
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
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
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
Planets in Spin-Orbit Misalignment and the Search for Stellar Companions
The discovery of giant planets orbiting close to their host stars was one of
the most unexpected results of early exoplanetary science. Astronomers have
since found that a significant fraction of these 'Hot Jupiters' move on orbits
substantially misaligned with the rotation axis of their host star. We recently
reported the measurement of the spin-orbit misalignment for WASP-79b by using
data from the 3.9 m Anglo-Australian Telescope. Contemporary models of
planetary formation produce planets on nearly coplanar orbits with respect to
their host star's equator. We discuss the mechanisms which could drive planets
into spin-orbit misalignment. The most commonly proposed being the Kozai
mechanism, which requires the presence of a distant, massive companion to the
star-planet system. We therefore describe a volume-limited direct-imaging
survey of Hot Jupiter systems with measured spin-orbit angles, to search for
the presence of stellar companions and test the Kozai hypothesis.Comment: Accepted for publication in the peer-reviewed proceedings of the 13th
annual Australian Space Science Conferenc
A Dynamical Analysis of the Proposed Circumbinary HW Virginis Planetary System
In 2009, the discovery of two planets orbiting the evolved binary star system
HW Virginis was announced, based on systematic variations in the timing of
eclipses between the two stars. The planets invoked in that work were
significantly more massive than Jupiter, and moved on orbits that were mutually
crossing - an architecture which suggests that mutual encounters and strong
gravitational interactions are almost guaranteed. In this work, we perform a
highly detailed analysis of the proposed HW Vir planetary system. First, we
consider the dynamical stability of the system as proposed in the discovery
work. Through a mapping process involving 91,125 individual simulations, we
find that the system is so unstable that the planets proposed simply cannot
exist, due to mean lifetimes of less than a thousand years across the whole
parameter space. We then present a detailed re-analysis of the observational
data on HW Vir, deriving a new orbital solution that provides a very good fit
to the observational data. Our new analysis yields a system with planets more
widely spaced, and of lower mass, than that proposed in the discovery work, and
yields a significantly greater (and more realistic) estimate of the uncertainty
in the orbit of the outermost body. Despite this, a detailed dynamical analysis
of this new solution similarly reveals that it also requires the planets to
move on orbits that are simply not dynamically feasible. Our results imply that
some mechanism other than the influence of planetary companions must be the
principal cause of the observed eclipse timing variations for HW Vir. If the
sys- tem does host exoplanets, they must move on orbits differing greatly from
those previously proposed. Our results illustrate the critical importance of
performing dynamical analyses as a part of the discovery process for
multiple-planet exoplanetary systems.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Societ
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