265 research outputs found
Erupting Dwarf Novae in the Large Magellanic Cloud
We report the first likely detections of erupting Dwarf Novae (DN) in an
external galaxy: the Large Magellanic Cloud. Six candidates were isolated from
approximately a million stars observed every second night over 11 nights with
the CTIO 8K x 8K Mosaic2 CCD imager. Artificial dwarf nova and completeness
tests suggest that we are seeing only the brightest of the LMC DN, probably SS
Cygni-like CVs, but possibly SU UMa-type cataclysmics undergoing
superoutbursts. We derive crude but useful limits on the LMC DN surface
density, and on the number of DN in the LMC. Many thousands of cataclysmic
variables in the Magellanic Clouds can be discovered and characterized with 8
meter class telescopes.Comment: Accepted for publication in AJ, 28 pages, 9 figures total, Figures 1
and 8 are supplied separately in jpeg forma
An Age Difference of 2 Gyr between a Metal-Rich and a Metal-Poor Globular Cluster
Globular clusters trace the formation history of the spheroidal components of
both our Galaxy and others, which represent the bulk of star formation over the
history of the universe. They also exhibit a range of metallicities, with
metal-poor clusters dominating the stellar halo of the Galaxy, and higher
metallicity clusters found within the inner Galaxy, associated with the stellar
bulge, or the thick disk. Age differences between these clusters can indicate
the sequence in which the components of the Galaxy formed, and in particular
which clusters were formed outside the Galaxy and later swallowed along with
their original host galaxies, and which were formed in situ. Here we present an
age determination of the metal-rich globular cluster 47 Tucanae by fitting the
properties of the cluster white dwarf population, which implies an absolute age
of 9.9 (0.7) Gyr at 95% confidence. This is about 2.0 Gyr younger than inferred
for the metal-poor cluster NGC 6397 from the same models, and provides
quantitative evidence that metal-rich clusters like 47 Tucanae formed later
than the metal-poor halo clusters like NGC 6397.Comment: Main Article: 10 pages, 4 figures; Supplementary Info 15 pages, 5
figures. Nature, Aug 1, 201
A New All-Sky Catalog of Stars with Large Proper Motions
A new all-sky catalog of stars with proper motions pm>0.15"/yr is presented.
The catalog is largely a product of the SUPERBLINK survey, a data-mining
initiative in which the entire Digitized Sky Surveys are searched for moving
stellar sources. Findings from earlier proper motions surveys are also
incorporated. The new all-sky catalog supersedes the great historic proper
motion catalogs assembled by W. J. Luyten (LHS, NLTT), and provides a virtually
complete (>98%) census of high proper motion stars down to magnitude R=19.Comment: To appear in proceedings of IAU Symposium No. 248 - A Giant Step:
from Milli- to Micro-arcsecond Astrometry, Shangai : China (2007
LSR0602+3910 - Discovery of a Bright Nearby L-type Brown Dwarf
We report the discovery of LSR0602+3910, an L dwarf of class L1. The object
was initially identified by Lepine et al. (2002) as a new high proper motion
star lying close to the Galactic plane. Its 2MASS J-K_s=1.43 is consistent with
an L dwarf, which we now confirm spectroscopically. In addition, we see a
signature of Li I absorption, making LSR0602+3910 a brown dwarf, one of the
brightest known (K_s=10.86). Among L dwarfs it is second in brightness to the
combined light of 2MASS 0746+20, a close binary system. We see no indication
that LSR0602+3910 is a binary, although high-resolution imaging will be
required to confirm this. Spectroscopic and photometric distance estimates
agree very well, placing LSR0602+3910 at d=10.6+-0.8 pc. LSR0602+3910 was most
likely missed in previous searches because of its proximity to the plane, the
region that most searches avoided. We estimate that some 40% of bright L dwarfs
are missed because of this selection effect.Comment: Accepted to ApJL. Revised version discusses LP 944-20 and has an
updated L-dwarf tabl
The Space Motion of the Globular Cluster NGC 6397
As a by-product of high-precision, ultra-deep stellar photometry in the
Galactic globular cluster NGC 6397 with the Hubble Space Telescope, we are able
to measure a large population of background galaxies whose images are nearly
point-like. These provide an extragalactic reference frame of unprecedented
accuracy, relative to which we measure the most accurate absolute proper motion
ever determined for a globular cluster. We find mu_alpha = 3.56 +/- 0.04 mas/yr
and mu_delta = -17.34 +/- 0.04 mas/yr. We note that the formal statistical
errors quoted for the proper motion of NGC 6397 do not include possible
unavoidable sources of systematic errors, such as cluster rotation. These are
very unlikely to exceed a few percent. We use this new proper motion to
calculate NGC 6397's UVW space velocity and its orbit around the Milky Way, and
find that the cluster has made frequent passages through the Galactic disk.Comment: 5 pages including 3 figures, accepted for publication in the
Astrophysical Journal Letters. Very minor changes in V2. typos fixe
The White Dwarf Cooling Sequence of NGC6397
We present the results of a deep Hubble Space Telescope (HST) exposure of the
nearby globular cluster NGC6397, focussing attention on the cluster's white
dwarf cooling sequence. This sequence is shown to extend over 5 magnitudes in
depth, with an apparent cutoff at magnitude F814W=27.6. We demonstrate, using
both artificial star tests and the detectability of background galaxies at
fainter magnitudes, that the cutoff is real and represents the truncation of
the white dwarf luminosity function in this cluster. We perform a detailed
comparison between cooling models and the observed distribution of white dwarfs
in colour and magnitude, taking into account uncertainties in distance,
extinction, white dwarf mass, progenitor lifetimes, binarity and cooling model
uncertainties. After marginalising over these variables, we obtain values for
the cluster distance modulus and age of \mu_0 = 12.02 \pm 0.06 and T_c = 11.47
\pm 0.47Gyr (95% confidence limits). Our inferred distance and white dwarf
initial-final mass relations are in good agreement with other independent
determinations, and the cluster age is consistent with, but more precise than,
prior determinations made using the main sequence turnoff method. In
particular, within the context of the currently accepted \Lambda CDM
cosmological model, this age places the formation of NGC6397 at a redshift z=3,
at a time when the cosmological star formation rate was approaching its peak.Comment: 56 pages, 30 figure
The Galactic Inner Halo: Searching for White Dwarfs and Measuring the Fundamental Galactic Constant, Vo/Ro
We establish an extragalactic, zero-motion frame of reference within the
deepest optical image of a globular star cluster, an HST 123-orbit exposure of
M4 (GO 8679, cycle 9). The line of sight beyond M4 (l,b (deg) = 351,16)
intersects the inner halo (spheroid) of our Galaxy at a tangent-point distance
of 7.6 kpc (for Ro = 8 kpc). We isolate these spheroid stars from the cluster
based on their proper motions over the 6-year baseline between these and
previous epoch HST data (GO 5461, cycle 4). Distant background galaxies are
also found on the same sight line using image-morphology techniques. This fixed
reference frame allows us to independently determine the fundamental Galactic
constant, Vo/Ro = 25.3 +/- 2.6 km/s/kpc, thus providing a velocity of the Local
Standard of Rest, v = 202.7 +/- 24.7 km/s for Ro = 8.0 +/- 0.5 kpc. Secondly,
the galaxies allow a direct measurement of M4's absolute proper motion,
mu_total = 22.57 +/- 0.76 mas/yr, in excellent agreement with recent studies.
The clear separation of galaxies from stars in these deep data also allow us to
search for inner-halo white dwarfs. We model the conventional Galactic
contributions of white dwarfs along our line of sight and predict 7.9 (thin
disk), 6.3 (thick disk) and 2.2 (spheroid) objects to the limiting magnitude at
which we can clearly delineate stars from galaxies (V = 29). An additional 2.5
objects are expected from a 20% white dwarf dark halo consisting of 0.5 Mo
objects, 70% of which are of the DA type. After considering the kinematics and
morphology of the objects in our data set, we find the number of white dwarfs
to be consistent with the predictions for each of the conventional populations.
However, we do not find any evidence for dark halo white dwarfs.Comment: 31 pages, including 6 diagrams and 2 tables. Accepted for publication
in Ap
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