245 research outputs found
The Origin of the Bifurcation in the Sagittarius Stream
The latest Sloan Digital Sky Survey data reveal a prominent bifurcation in
the distribution of debris of the Sagittarius dwarf spheroidal (Sgr) beginning
at a right ascension of roughly 190 degrees. Two branches of the stream (A and
B) persist at roughly the same heliocentric distance over at least 50 degrees
of arc. There is also evidence for a more distant structure (C) well behind the
A branch. This paper provides the first explanation for the bifurcation. It is
caused by the projection of the young leading (A) and old trailing (B) tidal
arms of the Sgr, whilst the old leading arm (C) lies well behind A. This
explanation is only possible if the halo is close to spherical, as the angular
difference between the branches is a measure of the precession of the orbital
plane.Comment: ApJ, in pres
New UltraCool and Halo White Dwarf Candidates in SDSS Stripe 82
A 2.5 x 100 degree region along the celestial equator (Stripe 82) has been
imaged repeatedly from 1998 to 2005 by the Sloan Digital Sky Survey. A new
catalogue of ~4 million light-motion curves, together with over 200 derived
statistical quantities, for objects in Stripe 82 brighter than r~21.5 has been
constructed by combining these data by Bramich et al. (2007). This catalogue is
at present the deepest catalogue of its kind. Extracting the ~130000 objects
with highest signal-to-noise ratio proper motions, we build a reduced proper
motion diagram to illustrate the scientific promise of the catalogue. In this
diagram disk and halo subdwarfs are well-separated from the cool white dwarf
sequence. Our sample of 1049 cool white dwarf candidates includes at least 8
and possibly 21 new ultracool H-rich white dwarfs (T_eff < 4000K) and one new
ultracool He-rich white dwarf candidate identified from their SDSS optical and
UKIDSS infrared photometry. At least 10 new halo white dwarfs are also
identified from their kinematics.Comment: 10 pages, 5 figures, published in MNRAS, minor text changes, final
versio
Qatar-1b: a hot Jupiter orbiting a metal-rich K dwarf star
We report the discovery and initial characterisation of Qatar-1b, a hot
Jupiter orbiting a metal-rich K dwarf star, the first planet discovered by the
Alsubai Project exoplanet transit survey. We describe the strategy used to
select candidate transiting planets from photometry generated by the Alsubai
Project instrument. We examine the rate of astrophysical and other false
positives found during the spectroscopic reconnaissance of the initial batch of
candidates. A simultaneous fit to the follow-up radial velocities and
photometry of Qatar-1b yield a planetary mass of 1.09+/-0.08 Mjup and a radius
of 1.16+/-0.05 Rjup. The orbital period and separation are 1.420033 days and
0.0234 AU for an orbit assumed to be circular. The stellar density, effective
temperature and rotation rate indicate an age greater than 4 Gyr for the
system.Comment: 8 pages, 5 figures, submitted to Monthly Notices of the Royal
Astronomical Societ
Kinematics of SDSS subdwarfs: Structure and substructure of the Milky Way halo
We construct a new sample of ~1700 solar neighbourhood halo subdwarfs from
the Sloan Digital Sky Survey, selected using a reduced proper motion diagram.
Radial velocities come from the SDSS spectra and proper motions from the
light-motion curve catalogue of Bramich et al. (2008). Using a photometric
parallax relation to estimate distances gives us the full phase-space
coordinates. Typical velocity errors are in the range 30-50 km/s. This halo
sample is one of the largest constructed to-date and the disc contamination is
at a level of < 1 per cent. This enables us to calculate the halo velocity
dispersion to excellent accuracy. We find that the velocity dispersion tensor
is aligned in spherical polar coordinates and that (sigma_r, sigma_phi,
sigma_theta) = (143 \pm 2, 82 \pm 2, 77 \pm 2) km/s. The stellar halo exhibits
no net rotation, although the distribution of v_phi shows tentative evidence
for asymmetry. The kinematics are consistent with a mildly flattened stellar
density falling with distance like r^{-3.75}. Using the full phase-space
coordinates, we look for signs of kinematic substructure in the stellar halo.
We find evidence for four discrete overdensities localised in angular momentum
and suggest that they may be possible accretion remnants. The most prominent is
the solar neighbourhood stream previously identified by Helmi et al. (1999),
but the remaining three are new. One of these overdensities is potentially
associated with a group of four globular clusters (NGC5466, NGC6934, M2 and
M13) and raises the possibility that these could have been accreted as part of
a much larger progenitor.Comment: 16 pages, 13 figures, MNRAS (in press). Revised following referee's
comments; using new and improved parallax relation. Results and conclusions
unchange
A systematic fitting scheme for caustic-crossing microlensing events
We outline a method for fitting binary-lens caustic-crossing microlensing
events based on the alternative model parameterisation proposed and detailed in
Cassan (2008). As an illustration of our methodology, we present an analysis of
OGLE-2007-BLG-472, a double-peaked Galactic microlensing event with a source
crossing the whole caustic structure in less than three days. In order to
identify all possible models we conduct an extensive search of the parameter
space, followed by a refinement of the parameters with a Markov Chain-Monte
Carlo algorithm. We find a number of low-chi2 regions in the parameter space,
which lead to several distinct competitive best models. We examine the
parameters for each of them, and estimate their physical properties. We find
that our fitting strategy locates several minima that are difficult to find
with other modelling strategies and is therefore a more appropriate method to
fit this type of events.Comment: 12 pages, 11 figure
Substructure revealed by RR Lyraes in SDSS Stripe 82
We present an analysis of the substructure revealed by 407 RR Lyraes in Sloan
Digital Sky Survey (SDSS) Stripe 82. Period estimates are determined to high
accuracy using a string-length method. A subset of 178 RR Lyraes with
spectrally derived metallicities are employed to derive
metallicity-period-amplitude relations, which are then used to find
metallicities and distances for the entire sample. The RR Lyraes lie between 5
and 115 kpc from the Galactic center. They are divided into subsets of 316 RRab
types and 91 RRc types based on their period, colour and metallicity. The
density distribution is not smooth, but dominated by clumps and substructure.
Samples of 55 and 237 RR Lyraes associated with the Sagittarius Stream and the
Hercules-Aquila Cloud respectively are identified. Hence, ~ 70 % of the RR
Lyraes in Stripe 82 belong to known substructure. There is a sharp break in the
density distribution at Galactocentric radii of 40 kpc, reflecting the fact
that the dominant substructure in Stripe 82 - the Hercules-Aquila Cloud and the
Sagittarius Stream - lies within 40 kpc. In fact, almost 60 % of all the RR
Lyraes in Stripe 82 are associated with the Hercules-Aquila Cloud alone, which
emphasises its pre-eminence. Additionally, evidence of a new and distant
substructure - the Pisces Overdensity - is found, consisting of 28 faint RR
Lyraes centered on Galactic coordinates (80 deg, -55 deg) and with distances of
~ 80 kpc. The total stellar mass in the Pisces Overdensity is ~10000 solar
masses and its metallicity is [Fe/H] ~ -1.5.Comment: 15 pages, submitted to MNRA
The Hercules-Aquila Cloud
We present evidence for a substantial overdensity of stars in the direction
of the constellations of Hercules and Aquila. The Cloud is centered at a
Galactic longitude of about 40 degrees and extends above and below the Galactic
plane by at least 50 degrees. Given its off-centeredness and height, it is
unlikely that the Hercules-Aquila Cloud is related to the bulge or thick disk.
More likely, this is a new structural component of the Galaxy that passes
through the disk. The Cloud stretches about 80 degrees in longitude. Its
heliocentric distance lies between 10 and 20 kpc so that the extent of the
Cloud in projection is roughly 20 kpc by 15 kpc. It has an absolute magnitude
of -13 and its stellar population appears to be comparable to, but somewhat
more metal-rich than, M92.Comment: ApJ (Letters), in pres
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