2,737 research outputs found
Spectroscopy of Giant Stars in the Pyxis Globular Cluster
The Pyxis globular cluster is a recently discovered globular cluster that
lies in the outer halo (R_{gc} ~ 40 kpc) of the Milky Way. Pyxis lies along one
of the proposed orbital planes of the Large Magellanic Cloud (LMC), and it has
been proposed to be a detached LMC globular cluster captured by the Milky Way.
We present the first measurement of the radial velocity of the Pyxis globular
cluster based on spectra of six Pyxis giant stars. The mean heliocentric radial
velocity is ~ 36 km/sec, and the corresponding velocity of Pyxis with respect
to a stationary observer at the position of the Sun is ~ -191 km/sec. This
radial velocity is a large enough fraction of the cluster's expected total
space velocity, assuming that it is bound to the Milky Way, that it allows
strict limits to be placed on the range of permissible transverse velocities
that Pyxis could have in the case that it still shares or nearly shares an
orbital pole with the LMC. We can rule out that Pyxis is on a near circular
orbit if it is Magellanic debris, but we cannot rule out an eccentric orbit
associated with the LMC. We have calculated the range of allowed proper motions
for the Pyxis globular cluster that result in the cluster having an orbital
pole within 15 degrees of the present orbital pole of the LMC and that are
consistent with our measured radial velocity, but verification of the tidal
capture hypothesis must await proper motion measurement from the Space
Interferometry Mission or HST. A spectroscopic metallicity estimate of [Fe/H] =
-1.4 +/- 0.1 is determined for Pyxis from several spectra of its brightest
giant; this is consistent with photometric determinations of the cluster
metallicity from isochrone fitting.Comment: 22 pages, 5 figures, aaspp4 style, accepted for publication in
October, 2000 issue of the PAS
The extent of NGC 6822 revealed by its C stars population
Using the CFH12K camera, we apply the four band photometric technique to
identify 904 carbon stars in an area 28' x 42' centered on NGC 6822. A few C
stars, outside of this area were also discovered with the Las Campanas Swope
Telescope. The NGC 6822 C star population has an average I of 19.26 mag leading
to an average absolute I magnitude of
-4.70 mag, a value essentially identical to the mean magnitude obtained for
the C stars in IC 1613. Contrary to stars highlighting the optical image of NGC
6822, C stars are seen at large radial distances and trace a huge slightly
elliptical halo which do not coincide with the huge HI cloud surrounding
NGC6822. The previously unknown stellar component of NGC 6822 has a exponential
scale length of 3.0' +/- 0.1' and can be traced to five scale lengths. The C/M
ratio of NGC 6822 is evaluated to br 1.0 +/- 0.2.Comment: accepted, to be published in A
Photometric Survey of the Polar ring galaxy NGC 6822
We have previously established, from a carbon star survey, that the Local
Group dwarf irregular galaxy NGC 6822 is much larger than its central bright
core.
Four MegaCam fields are acquired to survey a 2 2 area
centred on NGC 6822 to fully determine its extent and map its stellar
populations. Photometry of over one million stars is obtained in the SDSS g,
r, i to three magnitudes below the TRGB. RGB stars, selected from their
magnitudes and colours, are used to map the NGC 6822 stellar distribution up to
a distance of 60 arcmin. We map the reddening over the whole area. We establish
that the stellar outer structure of NGC 6822 is elliptical in shape, with
and a major-axis PA = 65, contrasting with the
orientation of the HI disk. The density enhancement can be seen up to a
semi-major axis of 36 making NGC 6822 as big as the Small Magellanic Cloud.
We fit two exponentials to the surface density profile of the spheroid, and
identify a bulge with a scale length of 3.85 and an outer spheroid with a
scale length of 10.0. We find intermediate-age C stars up to 40
while demonstrating that the SDSS filters are unsuitable to identify
extragalactic C stars. NGC 6822 is a unique Local Group galaxy with shape and
structure suggesting a polar ring configuration. Radial velocities of carbon
stars have indeed demonstrated that there are two kinematical systems in NGC
6822.Comment: A&A in pres
Constraining the History of the Sagittarius Dwarf Galaxy Using Observations of its Tidal Debris
We present a comparison of semi-analytic models of the phase-space structure
of tidal debris with observations of stars associated with the Sagittarius
dwarf galaxy (Sgr). We find that many features in the data can be explained by
these models. The properties of stars 10-15 degrees away from the center of Sgr
--- in particular, the orientation of material perpendicular to Sgr's orbit
(c.f. Alard 1996) and the kink in the velocity gradient (Ibata et al 1997) ---
are consistent with those expected for unbound material stripped during the
most recent pericentric passage ~50 Myrs ago. The break in the slope of the
surface density seen by Mateo, Olszewski & Morrison (1998) at ~ b=-35 can be
understood as marking the end of this material. However, the detections beyond
this point are unlikely to represent debris in a trailing streamer, torn from
Sgr during the immediately preceding passage ~0.7 Gyrs ago, but are more
plausibly explained by a leading streamer of material that was lost more that 1
Gyr ago and has wrapped all the way around the Galaxy. The observations
reported in Majewski et al (1999) also support this hypothesis. We determine
debris models with these properties on orbits that are consistent with the
currently known positions and velocities of Sgr in Galactic potentials with
halo components that have circular velocities v_circ=140-200 km/s. The best
match to the data is obtained in models where Sgr currently has a mass of ~10^9
M_sun and has orbited the Galaxy for at least the last 1 Gyr, during which time
it has reduced its mass by a factor of 2-3, or luminosity by an amount
equivalent to ~10% of the total luminosity of the Galactic halo. These numbers
suggest that Sgr is rapidly disrupting and unlikely to survive beyond a few
more pericentric passages.Comment: 19 pages, 5 figures, accepted to Astronomical Journa
Continuous, Semi-discrete, and Fully Discretized Navier-Stokes Equations
The Navier--Stokes equations are commonly used to model and to simulate flow
phenomena. We introduce the basic equations and discuss the standard methods
for the spatial and temporal discretization. We analyse the semi-discrete
equations -- a semi-explicit nonlinear DAE -- in terms of the strangeness index
and quantify the numerical difficulties in the fully discrete schemes, that are
induced by the strangeness of the system. By analyzing the Kronecker index of
the difference-algebraic equations, that represent commonly and successfully
used time stepping schemes for the Navier--Stokes equations, we show that those
time-integration schemes factually remove the strangeness. The theoretical
considerations are backed and illustrated by numerical examples.Comment: 28 pages, 2 figure, code available under DOI: 10.5281/zenodo.998909,
https://doi.org/10.5281/zenodo.99890
Magellanic Cloud Periphery Carbon Stars IV: The SMC
The kinematics of 150 carbon stars observed at moderate dispersion on the
periphery of the Small Magellanic Cloud are compared with the motions of
neutral hydrogen and early type stars in the Inter-Cloud region. The
distribution of radial velocities implies a configuration of these stars as a
sheet inclined at 73+/-4 degrees to the plane of the sky. The near side, to the
South, is dominated by a stellar component; to the North, the far side contains
fewer carbon stars, and is dominated by the neutral gas. The upper velocity
envelope of the stars is closely the same as that of the gas. This
configuration is shown to be consistent with the known extension of the SMC
along the line of sight, and is attributed to a tidally induced disruption of
the SMC that originated in a close encounter with the LMC some 0.3 to 0.4 Gyr
ago. The dearth of gas on the near side of the sheet is attributed to ablation
processes akin to those inferred by Weiner & Williams (1996) to collisional
excitation of the leading edges of Magellanic Stream clouds. Comparison with
pre LMC/SMC encounter kinematic data of Hardy, Suntzeff, & Azzopardi (1989) of
carbon stars, with data of stars formed after the encounter, of Maurice et al.
(1989), and Mathewson et al. (a986, 1988) leaves little doubt that forces other
than gravity play a role in the dynamics of the H I.Comment: 30 pages; 7 figures, latex compiled, 1 table; to appear in AJ (June
2000
Absolute Proper Motion of the Fornax Dwarf Spheroidal Galaxy from Photographic and HST WFPC2 Data
We have measured the absolute proper motion of the Fornax dwarf spheroidal
galaxy from a combination of photographic plate material and HST WFPC2 data
that provide a time baseline of up to 50 years. The extragalactic reference
frame consists of 8 QSO images and 48 galaxies. The absolute proper motion is
mu_alpha cos(delta) = 0.59 +-0.16 mas/yr and mu_delta = -0.15 +- 0.16 mas/yr.
The corresponding orbit of Fornax is polar, with an eccentricity of 0.27, and a
radial period of 4.5 Gyr. Fornax's current location is near pericenter. The
direction of the motion of Fornax supports the notion that Fornax belongs to
the Fornax-LeoI-LeoII-Sculptor-Sextans stream as hypothesized by Lynden-Bell
(1976, 1982) and Majewski (1994).
According to our orbit determination, Fornax crossed the Magellanic plane
\~190 Myr ago, a time that coincides with the termination of the star-formation
process in Fornax. We propose that ram-pressure stripping due to the passage of
Fornax through a gaseous medium denser than the typical intragalactic medium
left behind from the LMC may have caused the end of star formation in Fornax.
The excess, anomalous clouds within the South Galactic pole region of the
Magellanic Stream whose origin has long been debated in the literature as
constituents of either the Magellanic Stream or of the extragalactic Sculptor
group, are found to lie along the orbit of Fornax. We speculate that these
clouds are stripped material from Fornax as the dwarf crossed the Magellanic
Clouds' orbit.Comment: Accepted for publication in Astronomical Journal. The version with
high resolution figures can be found at
ftp://pegasus.astro.yale.edu/pub/dana/paper
Cellâtoâcell and cellâtoâmatrix interactions mediate chemokine expression: an important component of the inflammatory lesion
Although many studies have characterized soluble factors that stimulate or inhibit chemokine secretion, in this review we focus on the event of cellular adhesion as a novel mechanism for stimulating chemokine expression. Recent work has demonstrated chemokine expression following cellâtoâcell and cellâtoâmatrix adhesion. The specificity of this finding was demonstrated utilizing various techniques that illustrate that adhesion, and not a soluble stimulus, is in some cases responsible for initiating or augmenting chemokine expression. For example, coâcultures of peripheral blood monocytes and endothelial cells secreted elevated levels of ILâ8 and MCPâ1 compared with either cell type alone. When coâcultured in transwells, this effect was significantly attenuated. In other experiments, neutralizing monoclonal antibodies to various adhesion molecules inhibited chemokine expression. The effects of adhesion were not limited to leukocytes. Both immune and nonâimmune cell types were evaluated as potential sources of adhesionâmediated chemokine expression. Not suprisingly, expression of some chemokines was associated with adhesion, whereas others were not, supporting the notion that adhesion differentially signals chemokine secretion during the inflammatory response. We hypothesize that as a recruited leukocyte encounters different adhesion substrates such as endothelial cells, basement membrane, extracellular matrix, and fibroblasts, the expression of chemokines from both the leukocyte and the substrate may be initiated, inhibited, or augmented. Careful characterization of the contribution of adhesion to regulation of chemokine expression will provide insight into the pathogenesis of many human diseases where chemokines have a central role. J. Leukoc. Biol.62: 612â619; 1997.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/142209/1/jlb0612.pd
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