888 research outputs found
New surveys of UBV photometry and absolute proper motions at intermediate latitude
A photometric and proper motion survey has been obtained in 2 directions at
intermediate latitude: (, ;
,) and
(, ; ,
). The survey covers 7.13 and 20.84 square
degrees, respectively. The limiting magnitude is about 18.5 in V for both
directions. We have derived the density laws for stars (M 3.5) as a
function of distance from the galactic plane. The density laws for stars follow
a sum of two exponentials with scale heights of 240 pc (thin disk) and 790 pc
(thick disk), respectively. The local density of thick disk is found to be
6.13 % relative to the thin disk. The kinematical distribution of stars
has been probed to distances up to 3.5 kpc above the galactic plane. New
estimates of the parameters of velocity ellipsoid have been derived for the
thick disk of the Galaxy. A comparison of our data sets with the Besan\c con
model star count predictions has been performed, giving a good agreement in the
magnitude range V = 13 to 18.Comment: 13 pages, 8 PS figures, To appear in A&
Dimensionality reduction, and function approximation of poly (lactic-co-glycolic acid) micro-and nanoparticle dissolution rate
Prediction of poly(lactic-co-glycolic acid) (PLGA) micro- and nanoparticles’ dissolution rates plays a significant role in pharmaceutical and medical industries. The prediction of PLGA dissolution rate is crucial for drug manufacturing. Therefore, a model that predicts the PLGA dissolution rate could be beneficial. PLGA dissolution is influenced by numerous factors (features), and counting the known features leads to a dataset with 300 features. This large number of features and high redundancy within the dataset makes the prediction task very difficult and inaccurate. In this study, dimensionality reduction techniques were applied in order to simplify the task and eliminate irrelevant and redundant features. A heterogeneous pool of several regression algorithms were independently tested and evaluated. In addition, several ensemble methods were tested in order to improve the accuracy of prediction. The empirical results revealed that the proposed evolutionary weighted ensemble method offered the lowest margin of error and significantly outperformed the individual algorithms and the other ensemble techniques
Evolution of microstructure in spray formed Al–18%Si alloy
Spray atomization and deposition process has emerged as an alternative to ingot and powder metallurgy routes. In the present investigation, we study the microstructural control during spray deposition of hypereutectic Al–Si alloy, employing different nozzle to substrate distances. Spray deposition is carried out using convergent–divergent close-coupled nozzle design at deposition distances of 200, 300, 450, and 550 mm. Microstructural characterization of oversprayed powders as well as spray formed deposits was performed. Microstructural features obtained at smaller deposition distance consist of co-existing primary Si phase and needle like eutectic Si. Dendrites of α-Al phase are observed indicating a large undercooling of the liquid pool prior to solidification. A large number of pre-solidified particles with very fine microstructure, embedded in a relatively coarse region, typically characterize those evolved at large deposition distances. However, at intermediate deposition distances, uniform and refined primary Si phases, 3–8 μm size, are observed. An undercooling effect is manifested in both the spray deposits as well as powder particles. These microstructural features have been discussed in light of a proposed model, which describes the presence of two layers (1) solidification layer and (2) interaction layer in the liquid pool
Star Formation Activity in the Galactic HII Complex S255-S257
We present results on the star-formation activity of an optically obscured
region containing an embedded cluster (S255-IR) and molecular gas between two
evolved HII regions S255 and S257. We have studied the complex using optical,
near-infrared (NIR) imaging, optical spectroscopy and radio continnum mapping
at 15 GHz, along with Spitzer-IRAC results. It is found that the main exciting
sources of the evolved HII regions S255 and S257 and the compact HII regions
associated with S255-IR are of O9.5 - B3 V nature, consistent with previous
observations. Our NIR observations reveal 109 likely young stellar object (YSO)
candidates in an area of ~ 4'.9 x 4'.9 centered on S255-IR, which include 69
new YSO candidates. Our observations increased the number of previously
identified YSOs in this region by 32%. To see the global star formation, we
constructed the V-I/V diagram for 51 optically identified IRAC YSOs in an area
of ~ 13' x 13' centered on S255-IR. We suggest that these YSOs have an
approximate age between 0.1 - 4 Myr, indicating a non-coeval star formation.
Using spectral energy distribution models, we constrained physical properties
and evolutionary status of 31 and 16 YSO candidates outside and inside the gas
ridge, respectively. The models suggest that the sources associated within the
gas ridge are of younger population (mean age ~ 1.2 Myr) than the sources
outside the gas ridge (mean age ~ 2.5 Myr). The positions of the young sources
inside the gas ridge at the interface of the HII regions S255 and S257, favor a
site of induced star formation.Comment: 46 pages, 14 figures, 5 tables. Accepted for publication in The
Astrophysical Journa
A Multicolor Survey of Absolute Proper Motions : Galactic Structure and Kinematics in the Direction of Galactic Center at Intermediate Latitude
We have derived a new photographic photometry and proper motions for 20000
stars with completeness to V = 18 in the direction of galactic center at the
intermediate latitude (l = 3 deg,b = 47 deg; alpha(1950)=15h 18m, delta(1950) =
+02deg 16') for a 15.5 sq deg field. The combination of four glass copies of
the Palomar Observatory Sky Survey (i.e. POSS 1402 E&O and POSS 1429 E&O) has
been used as a first epoch for proper motion determination. The random error of
the proper motions is approximately 0.''3/cen to V=17. The photometric accuracy
ranges between 0.07 to 0.10 in the V, B and U bands. We stress the importance
of the magnitude and color effects in astrometric surveys of field stars. Using
color-magnitude diagrams of a few cluster member stars, a new distance of
6.9+-0.5 kpc is derived for M5 and 20.3+-0.8 kpc for Pal 5 globular clusters.
This is in good agreement with other determinations. We have analyzed the
components of U+W and V galactic space motions resulting from the accurate
proper motions survey. No dependence with z distance is found in the asymmetric
drift of the thick disk population. New estimates of the parameters of the
velocity ellipsoid have been derived for the thin disk, thick disk and halo
populations of the Galaxy.Comment: PS file, 15 pages, 13 figures (available on request), paper accepted
for publication in Astron. & Astrophy
Efficient absolute aspect determination of a balloon borne far infrared telescope using a solid state optical photometer
The observational and operational efficiency of the TIFR 1 meter balloon
borne far infrared telescope has been improved by incorporating a multielement
solid state optical photometer (SSOP) at the Cassegrain focus of the telescope.
The SSOP is based on a 1-D linear photo diode array (PDA). The online and
offline processing schemes of the PDA signals which have been developed, lead
to improvement in the determination of absolute telescope aspect (
0\farcm8), which is very crucial for carrying out the observations as well as
offline analysis. The SSOP and its performance during a recent balloon flight
are presented here.Comment: To appear in the February 2000 issue of the PAS
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