371 research outputs found
CCD Photometry of Twenty Open Clusters
Fundamental astrophysical parameters have been derived for 20 open clusters
(O\!Cs) using CCD~ photometric data observed with the 84~cm
telescope at the San Pedro M\'artir National Astronomical Observatory,
M\'exico.
The interstellar reddenings, metallicities, distances, and ages have been
compared to the literature values. Significant differences are usually due to
the usage of diverse empirical calibrations and differing assumptions, such as
concerning cluster metallicity, as well as distinct isochrones which correspond
to differing element-abundance ratios, internal stellar physics, and
photometric systems. Different interstellar reddenings, as well as varying
reduction and cluster-membership techniques, are also responsible for these
kinds of systematic differences and errors.
The morphological ages, which are derived from the morphological indices
( and ) in the CM diagrams, are in good agreement with the
isochrone ages of 12 O\!Cs, those with good red clump (RC) and red giant (RG)
star candidates. No metal abundance gradient is detected for the range kpc, nor any correlation between the cluster ages and
metal abundances for these 20 O\!Cs.
Young, metal-poor O\!Cs, observed here in the third Galactic quadrant, may be
associated with stellar over-densities, such as that in Canis Major (Martin et
al.) and the Monoceros Ring (Newberg et al.), or signatures of past accretion
events, as discussed by Yong et al. and Carraro et al.Comment: 68 pages, 33 figures, 15 tables. Accepted in New Astronom
The Galactic Disk-Halo transition - Evidence from Stellar Abundances
New information on the relations between the Galactic disks, the halo, and
satellite galaxies is being obtained from elemental abundances of stars having
metallicities in the range -1.5 < [Fe/H] < -0.5. The first results for a sample
of 26 halo stars and 13 thick-disk stars observed with the ESO VLT/UVES
spectrograph are presented. The halo stars fall in two distinct groups: One
group (9 stars) has [alpha/Fe]= 0.30 +-0.03 like the thick-disk stars. The
other group (17 stars) shows a clearly deviating trend ranging from [alpha/Fe]=
0.20 at [Fe/H]= -1.3 to [alpha/Fe]= 0.08 at [Fe/H]= -0.8. The kinematics of the
stars are discussed and the abundance ratios Na/Fe, Ni/Fe, Cu/Fe and Ba/Y are
applied to see if the low-alpha stars are connected to the thin disk or to
Milky Way satellite galaxies. Furthermore, we compare our data with simulations
of chemical abundance distributions in hierarchically formed stellar halos in a
LambdaCDM Universe.Comment: 6 pages, 6 figures. To appear in proceedings of the IAU Symposium No.
254 "The Galaxy Disk in Cosmological Context", Copenhagen 9-13 June 2008,
Eds. J. Andersen, J.Bland-Hawthorn & B. Nordstrom, Cambridge University Pres
Observational signatures for depletion in the Spite plateau: solving the cosmological Li discrepancy?
We present Li abundances for 73 stars in the metallicity range -3.5 < [Fe/H]
< -1.0 using improved IRFM temperatures (Casagrande et al. 2010) with precise
E(B-V) values obtained mostly from interstellar NaI D lines, and high-quality
equivalent widths (errors ~ 3%). At all metallicities we uncover a
fine-structure in the Li abundances of Spite plateau stars, which we trace to
Li depletion that depends on both metallicity and mass. Models including atomic
diffusion and turbulent mixing seem to reproduce the observed Li depletion
assuming a primordial Li abundance ALi = 2.64 dex (MARCS models) or 2.72
(Kurucz overshooting models), in good agreement with current predictions (ALi =
2.72) from standard BBN. We are currently expanding our sample to have a better
coverage of different evolutionary stages at the high and low metallicity ends,
in order to verify our findings.Comment: In press, Light elements in the Universe, Proceedings IAU Symposium
No. 268, 2010. C. Charbonnel, M. Tosi, F. Primas & C. Chiappini, ed
Infrared Observations of the Helix Planetary Nebula
We have mapped the Helix (NGC 7293) planetary nebula (PN) with the IRAC instrument on the Spitzer Space Telescope. The Helix is one of the closest bright PNs and therefore provides an opportunity to resolve the small-scale structure in the nebula. The emission from this PN in the 5.8 and 8 μm IRAC bands is dominated by the pure rotational lines of molecular hydrogen, with a smaller contribution from forbidden line emission such as [Ar III] in the ionized region. The IRAC images resolve the "cometary knots," which have been previously studied in this PN. The "tails" of the knots and the radial rays extending into the outer regions of the PN are seen in emission in the IRAC bands. IRS spectra on the main ring and the emission in the IRAC bands are consistent with shock-excited H_2 models, with a small (~10%) component from photodissociation regions. In the northeast arc, the H_2 emission is located in a shell outside the Hα emission
New intrinsic-colour calibration for -- photometry
A new intrinsic-colour calibration (----) is presented for
the -- photometric system, making use of re-calibrated Hipparcos
parallaxes and published reddening maps. This new calibration for
----, our Equation (1), has been based upon stars with
pc in the photometric catalogues of Schuster et al. (1988, 1993,
2006), provides a small dispersion, , and has a positive ``standard''
coefficient, which is not too different from the
coefficients of Crawford (+1.11; 1975a) and of Olsen (+1.34; 1988). For 61
stars with spectra from CASPEC, UVES/VLT, and FIES/NOT databases, without
detectable Na I lines, the average reddening value
shows that any zero-point correction to our intrinsic-colour equation must be
minuscule.Comment: 18 pages, 3 figures, accepted to New Astronom
Time-Accurate Unsteady Pressure Loads Simulated for the Space Launch System at Wind Tunnel Conditions
A transonic flow field about a Space Launch System (SLS) configuration was simulated with the Fully Unstructured Three-Dimensional (FUN3D) computational fluid dynamics (CFD) code at wind tunnel conditions. Unsteady, time-accurate computations were performed using second-order Delayed Detached Eddy Simulation (DDES) for up to 1.5 physical seconds. The surface pressure time history was collected at 619 locations, 169 of which matched locations on a 2.5 percent wind tunnel model that was tested in the 11 ft. x 11 ft. test section of the NASA Ames Research Center's Unitary Plan Wind Tunnel. Comparisons between computation and experiment showed that the peak surface pressure RMS level occurs behind the forward attach hardware, and good agreement for frequency and power was obtained in this region. Computational domain, grid resolution, and time step sensitivity studies were performed. These included an investigation of pseudo-time sub-iteration convergence. Using these sensitivity studies and experimental data comparisons, a set of best practices to date have been established for FUN3D simulations for SLS launch vehicle analysis. To the author's knowledge, this is the first time DDES has been used in a systematic approach and establish simulation time needed, to analyze unsteady pressure loads on a space launch vehicle such as the NASA SLS
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