1,830 research outputs found
Detection of the old stellar component of the major Galactic bar
We present near-IR colour--magnitude diagrams and star counts for a number of
regions along the Galactic plane. It is shown that along the l=27 b=0 line of
sight there is a feature at 5.7 +-0.7kpc with a density of stars at least a
factor two and probably more than a factor five times that of the disc at the
same position. This feature forms a distinct clump on an H vs. J-H diagram and
is seen at all longitudes from the bulge to about l=28, but at no longitude
greater than this. The distance to the feature at l=20 is about 0.5kpc further
than at l=27 and by l=10 it has merged with, or has become, the bulge. Given
that at l=27 and l=21 there is also a clustering of very young stars, the only
component that can reasonably explain what is seen is a bar with half length of
around 4kpc and a position angle of about 43+-7.Comment: 5 pages, 5 figures accepted as a letter in MNRA
Inversion of stellar statistics equation for the Galactic Bulge
A method based on Lucy (1974, AJ 79, 745) iterative algorithm is developed to
invert the equation of stellar statistics for the Galactic bulge and is then
applied to the K-band star counts from the Two-Micron Galactic Survey in a
number of off-plane regions (10 deg.>|b|>2 deg., |l|<15 deg.).
The top end of the K-band luminosity function is derived and the morphology
of the stellar density function is fitted to triaxial ellipsoids, assuming a
non-variable luminosity function within the bulge. The results, which have
already been outlined by Lopez-Corredoira et al.(1997, MNRAS 292, L15), are
shown in this paper with a full explanation of the steps of the inversion: the
luminosity function shows a sharp decrease brighter than M_K=-8.0 mag when
compared with the disc population; the bulge fits triaxial ellipsoids with the
major axis in the Galactic plane at an angle with the line of sight to the
Galactic centre of 12 deg. in the first quadrant; the axial ratios are
1:0.54:0.33, and the distance of the Sun from the centre of the triaxial
ellipsoid is 7860 pc. The major-minor axial ratio of the ellipsoids is found
not to be constant. However, the interpretation of this is controversial. An
eccentricity of the true density-ellipsoid gradient and a population gradient
are two possible explanations.
The best fit for the stellar density, for 1300 pc<t<3000 pc, are calculated
for both cases, assuming an ellipsoidal distribution with constant axial
ratios, and when K_z is allowed to vary. From these, the total number of bulge
stars is ~ 3 10^{10} or ~ 4 10^{10}, respectively.Comment: 19 pages, 23 figures, accepted in MNRA
Old stellar Galactic disc in near-plane regions according to 2MASS: scales, cut-off, flare and warp
We have pursued two different methods to analyze the old stellar population
near the Galactic plane, using data from the 2MASS survey. The first method is
based on the isolation of the red clump giant population in the color-magnitude
diagrams and the inversion of its star counts to obtain directly the density
distribution along the line of sight. The second method fits the parameters of
a disc model to the star counts in 820 regions. Results from both independent
methods are consistent with each other. The qualitative conclusions are that
the disc is well fitted by an exponential distribution in both the
galactocentric distance and height. There is not an abrupt cut-off in the
stellar disc (at least within R<15 kpc). There is a strong flare (i.e. an
increase of scale-height towards the outer Galaxy) which begins well inside the
solar circle, and hence there is a decrease of the scale-height towards the
inner Galaxy. Another notable feature is the existence of a warp in the old
stellar population whose amplitude is coincident with the amplitude of the gas
warp.
It is shown for low latitude stars (mean height: |z|~300 pc) in the outer
disc (galactocentric radius R>6 kpc) that: the scale-height in the solar circle
is h_z(R_sun)=3.6e-2 R_sun, the scale-length of the surface density is h_R=0.42
R_sun and the scale-length of the space density in the plane (i.e. including
the effect of the flare) is H=0.25 R_sun. The variation of the scale-height due
to the flare follows roughly a law h_z(R) =~ h_z(R_sun) exp
[(R-R_\odot)/([12-0.6R(kpc)] kpc)] (for R<~15 kpc; R_sun=7.9 kpc). The warp
moves the mean position of the disc to a height z_w=1.2e-3 R(kpc)^5.25
sin(phi+(5 deg.)) pc (for R<~13 kpc; R_sun=7.9 kpc).Comment: LaTEX, 20 pages, 23 figures, accepted to be published in A&
An excess of very bright stars in the inner bulge
From an analysis of the stars remaining in central regions of the Galaxy
after subtracting those belonging to the disc and the bulge, we deduce that the
inner bulge must have an extra young population with respect the rest of the
bulge. It is shown that there is a higher ratio of very bright stars in the
central bulge than than there is in the outer bulge. This is interpreted as
being an additional young component due to the presence of star formation
regions near the Galactic Centre which is absent in the outer bulge.Comment: 6 pages, 12 figures, accepted to be published in MNRA
The Interpretation of Near-Infrared Star Counts at the South Galactic Pole
We present new deep counts of stars at the South Galactic Pole (SGP)
taken with the NAOJ PICNIC camera to . Star-galaxy separation to
was accomplished effectively using image profiles because the pixel
size we used is 0.509 arcsec. We interpret these counts using the SKY (Cohen
1994) model of the Galactic point source sky and determine the relative
normalization of halo-to-disk populations, and the location of the Sun relative
to the Galactic plane, within the context of this model. The observed star
counts constrain these parameters to be: halo/disk 1/900 and
z=16.52.5 pc. These values have been used to correct our SGP
galaxy counts for contamination by the point source Galactic foreground.Comment: accepted for publication in AJ, 15 pages with 2 figure
The bulge luminosity functions in the MSX infrared bands
We use an inversion technique to derive the luminosity functions of the
Galactic bulge from point source counts extracted from the Midcourse Space
Experiment's Point Source Catalog (version 1.2).Comment: 5 pages, 2 figures, to be published in A&
A major star formation region in the receding tip of the stellar Galactic bar. II. Supplementary information and evidence that the bar is not the same structure as the triaxial bulge previouly reported
This paper is the second part of Garzon et al. (1997: ApJ 491, L31) in which
we presented an outline of the analysis of 60 spectra from a follow-up program
to the Two Micron Galactic Survey (TMGS) project in the l=27 deg., b=0 deg.
area. In this second part, we present a more detailed explanation of the
analysis as well a library of the spectra for more complete information for
each of the 60 stars, and further discussions on the implications for the
structure of the Galaxy.
This region contains a prominent excess in the flux distribution and star
counts previously observed in several spectral ranges, notably in the TMGS.
More than 50% of the spectra of the stars detected with m_K<5.0 mag, within a
very high confidence level, correspond to stars of luminosity class I, and a
significant proportion of the remainder are very late giants which must also be
rapidly evolving. We make the case, using all the available evidence, that we
are observing a region at the nearer end of the Galactic bar, where the Scutum
spiral arm breaks away, and that this is powerful evidence for the presence of
the bar. Alternative explanations do not give nearly such a satisfactory
account of the observations.
The space localization of one and, a fortiori, of both ends of the bar allows
us to infer a position angle for the bar of around 75 deg. with respect to the
Sun-Galactic centre line. The angle is different from that given by other
authors for the bar and this, we think, is because they refer to the triaxial
bulge and not to the bar as detected here.Comment: 21 pages, 1 table, 9 figures, accepted in A
Search for star clustering: methodology and application to the Two Micron Galactic Survey
A new approach to the study of the large-scale stellar cluster distribution
in the Galaxy based on two-point correlation techniques is presented. The basic
formalism for this method is outlined and its applications are then
investigated by the use of a simple model of cluster distribution in the
Galaxy. This provides an estimate of the potentials of the two-point
correlation function for indicating clustering in the measured star positions,
which can be related to the presence of star clusters in the observed field.
This technique is then applied to several areas of the Two Micron Galactic
Survey catalogue, from which information is obtained on the distribution of
clusters according to position in the Galaxy, as well as about age, density of
stars, etc.Comment: 16 pages, 14 figures, accepted in MNRA
The local atomic quasicrystal structure of the icosahedral Mg25Y11Zn64 alloy
A local and medium range atomic structure model for the face centred
icosahedral (fci) Mg25Y11Zn64 alloy has been established in a sphere of r = 27
A. The model was refined by least squares techniques using the atomic pair
distribution (PDF) function obtained from synchrotron powder diffraction. Three
hierarchies of the atomic arrangement can be found: (i) five types of local
coordination polyhedra for the single atoms, four of which are of Frank-Kasper
type. In turn, they (ii) form a three-shell (Bergman) cluster containing 104
atoms, which is condensed sharing its outer shell with its neighbouring
clusters and (iii) a cluster connecting scheme corresponding to a
three-dimensional tiling leaving space for few glue atoms. Inside adjacent
clusters, Y8-cubes are tilted with respect to each other and thus allow for
overall icosahedral symmetry. It is shown that the title compound is
essentially isomorphic to its holmium analogue. Therefore fci-Mg-Y-Zn can be
seen as the representative structure type for the other rare earth analogues
fci-Mg-Zn-RE (RE = Dy, Er, Ho, Tb) reported in the literature.Comment: 12 pages, 8 figures, 2 table
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