18,762 research outputs found
A dynamical and kinematical model of the Galactic stellar halo and possible implications for galaxy formation scenarios
We re-analyse the kinematics of the system of blue horizontal branch field
(BHBF) stars in the Galactic halo (in particular the outer halo), fitting the
kinematics with the model of radial and tangential velocity dispersions in the
halo as a function of galactocentric distance r proposed by Sommer-Larsen,
Flynn & Christensen (1994), using a much larger sample (almost 700) of BHBF
stars. The basic result is that the character of the stellar halo velocity
ellipsoid changes markedly from radial anisotropy at the sun to tangential
anisotropy in the outer parts of the Galactic halo (r greater than approx 20
kpc). Specifically, the radial component of the stellar halo's velocity
ellipsoid decreases fairly rapidly beyond the solar circle, from approx 140 +/-
10 km/s at the sun, to an asymptotic value of 89 +/- 19 km/s at large r. The
rapid decrease in the radial velocity dispersion is matched by an increase in
the tangential velocity dispersion, with increasing r.
Our results may indicate that the Galaxy formed hierarchically (partly or
fully) through merging of smaller subsystems - the 'bottom-up' galaxy formation
scenario, which for quite a while has been favoured by most theorists and
recently also has been given some observational credibility by HST observations
of a potential group of small galaxies, at high redshift, possibly in the
process of merging to a larger galaxy (Pascarelle et al 1996).Comment: Latex, 16 pages. 2 postscript figures. Submitted to the Astrophysical
Journal. also available at http://astro.utu.fi/~cflynn/outerhalo.htm
Computer and photogrammetric general land use study of central north Alabama
The object of this report is to acquaint potential users with two computer programs, developed at NASA, Marshall Space Flight Center. They were used in producing a land use survey and maps of central north Alabama from Earth Resources Technology Satellite (ERTS) digital data. The report describes in detail the thought processes and analysis procedures used from the initiation of the land use study to its completion, as well as a photogrammetric study that was used in conjunction with the computer analysis to produce similar land use maps. The results of the land use demonstration indicate that, with respect to computer time and cost, such a study may be economically and realistically feasible on a statewide basis
Observations of atmospheric water vapor with the SAGE 2 instrument
The Stratospheric Aerosol and Gas Experiment 2 (SAGE 2) is discussed. The SAGE 2 instrument was a multichannel spectrometer that inferred the vertical distribution of water vapor, aerosols, nitrogen dioxide, and ozone by measuring the extinction of solar radiation at spacecraft sunrise/sunset. At altitudes above 20 km, the SAGE 2 and LIMS (Limb Infrared Monitor of the Stratosphere) data are in close agreement. The discrepancies below this altitude may be attributed to differences in the instruments' field of view and time of data acquisition
Low-background temperature sensors fabricated on parylene substrates
Temperature sensors fabricated from ultra-low radioactivity materials have
been developed for low-background experiments searching for neutrinoless
double-beta decay and the interactions of WIMP dark matter. The sensors consist
of electrical traces photolithographically-patterned onto substrates of
vapor-deposited parylene. They are demonstrated to function as expected, to do
so reliably and robustly, and to be highly radio-pure. This work is a
proof-of-concept study of a technology that can be applied to broad class of
electronic circuits used in low-background experiments
Faint Fuzzy Star Clusters in NGC1023 as Remnants of Merged Star Cluster Complexes
In the lenticular galaxy NGC1023 a third population of globular clusters
(GCs), called faint fuzzies (FFs), was discovered next to the blue and red GC
populations by Larsen & Brodie. While these FFs have colors comparable to the
red population, the new population is fainter, larger (R_eff > 7 pc) and, most
importantly, shows clear signs of co-rotation with the galactic disk of
NGC1023. We present N-body simulations verifying the hypothesis that these
disk-associated FFs are related to the young massive cluster complexes (CCs)
observed by Bastian et. al in M51, who discovered a mass-radius relation for
these CCs. Our models have an initial configuration based on the observations
from M51 and are placed on various orbits in a galactic potential derived for
NGC1023. All computations end up with a stable object containing 10 to 60% of
the initial CC mass after an integration time of 5 Gyr. A conversion to visual
magnitudes demonstrates that the resulting objects cover exactly the observed
range for FFs. Moreover, the simulated objects show projected half-mass radii
between 3.6 and 13.4 pc, in good agreement with the observed FF sizes. We
conclude that objects like the young massive CCs in M51 are likely progenitors
of the FFs observed in NGC1023.Comment: Accepted for publication in Ap
Hole Spin Coherence in a Ge/Si Heterostructure Nanowire
Relaxation and dephasing of hole spins are measured in a gate-defined Ge/Si
nanowire double quantum dot using a fast pulsed-gate method and dispersive
readout. An inhomogeneous dephasing time
exceeds corresponding measurements in III-V semiconductors by more than an
order of magnitude, as expected for predominately nuclear-spin-free materials.
Dephasing is observed to be exponential in time, indicating the presence of a
broadband noise source, rather than Gaussian, previously seen in systems with
nuclear-spin-dominated dephasing.Comment: 15 pages, 4 figure
Young and intermediate-age massive star clusters
An overview of our current understanding of the formation and evolution of
star clusters is given, with main emphasis on high-mass clusters. Clusters form
deeply embedded within dense clouds of molecular gas. Left-over gas is cleared
within a few million years and, depending on the efficiency of star formation,
the clusters may disperse almost immediately or remain gravitationally bound.
Current evidence suggests that a few percent of star formation occurs in
clusters that remain bound, although it is not yet clear if this fraction is
truly universal. Internal two-body relaxation and external shocks will lead to
further, gradual dissolution on timescales of up to a few hundred million years
for low-mass open clusters in the Milky Way, while the most massive clusters (>
10^5 Msun) have lifetimes comparable to or exceeding the age of the Universe.
The low-mass end of the initial cluster mass function is well approximated by a
power-law distribution, dN/dM ~ M^{-2}, but there is mounting evidence that
quiescent spiral discs form relatively few clusters with masses M > 2 x 10^5
Msun. In starburst galaxies and old globular cluster systems, this limit
appears to be higher, at least several x 10^6 Msun. The difference is likely
related to the higher gas densities and pressures in starburst galaxies, which
allow denser, more massive giant molecular clouds to form. Low-mass clusters
may thus trace star formation quite universally, while the more long-lived,
massive clusters appear to form preferentially in the context of violent star
formation.Comment: 21 pages, 3 figures. To appear as invited review article in a special
issue of the Phil. Trans. Royal Soc. A: Ch. 9 "Star clusters as tracers of
galactic star-formation histories" (ed. R. de Grijs). Fully peer reviewed.
PDFLaTeX, requires rspublic.cls style fil
Inclusion of a Parental Component in a Sports-Based HIV Prevention Program for Dominican Youth
Background: Underprivileged youth in the Dominican Republic (DR) are at high risk of acquiring the human immunodeficiency virus (HIV). Protective parenting practices may inhibit sexual risk-taking. Objective: We investigated whether parental involvement in a sports-based HIV prevention program increased self-efficacy to prevent HIV and safe sex behavior among Dominican youth. Method: The study had a quasi-experimental design with repeated measures. N = 90 participants between 13 and 24 years of age participated in the program through two different trainings, UNICA and A Ganar, both of which had an experimental (i.e., program with parental component) and a control (i.e., program without parental component) condition. Results: Self-efficacy to prevent HIV significantly increased among participants in the experimental condition of UNICA. Self-efficacy for safe sex increased among sexually active participants in the experimental condition of A Ganar. Implications for Impact: These findings are important to meet the United Nations’ Sustainable Development Goal of good health and wellbeing, as they suggest that parental involvement in sports-based HIV prevention programs can enhance their positive effects for increasing youth’s self-efficacy to practice HIV-preventive behaviors. Randomized control trials and longitudinal studies are needed.</p
Bulge Globular Clusters in Spiral Galaxies
There is now strong evidence that the metal-rich globular clusters (GC) near
the center of our Galaxy are associated with the Galactic bulge rather than the
disk as previously thought. Here we extend the concept of bulge GCs to the GC
systems of nearby spiral galaxies. In particular, the kinematic and metallicity
properties of the GC systems favor a bulge rather than a disk origin. The
number of metal-rich GCs normalized by the bulge luminosity is roughly constant
(i.e. bulge S_N ~ 1) in nearby spirals, and this value is similar to that for
field ellipticals when only the red (metal--rich) GCs are considered. We argue
that the metallicity distributions of GCs in spiral and elliptical galaxies are
remarkably similar, and that they obey the same correlation of mean GC
metallicity with host galaxy mass. We further suggest that the metal-rich GCs
in spirals are the direct analogs of the red GCs seen in ellipticals. The
formation of a bulge/spheroidal stellar system is accompanied by the formation
of metal-rich GCs. The similarities between GC systems in spiral and elliptical
galaxies appear to be greater than the differences.Comment: 5 pages, Latex, 2 figures, 1 table, Accepted for publication in ApJ
Letter
The Chemical Properties of Milky Way and M31 Globular Clusters: I. A Comparative Study
A comparative analysis is performed between high-quality integrated spectra
of 30 globular clusters in M31, 20 Milky Way clusters, and a sample of field
and cluster elliptical galaxies. We find that the Lick CN indices in the M31
and Galactic clusters are enhanced relative to the bulges of the Milky Way,
M31, and elliptical spheroids. Although not seen in the Lick CN indices, the
near-UV cyanogen feature (3883 A) is strongly enhanced in M31 clustesr with
respect to the Galactic globulars at metallicities, --1.5<[Fe/H]<--0.3. Carbon
shows signs of varying amongst these two groups. For [Fe/H]>--0.8, we observe
no siginificant differences in the Hdelta, Hgamma, or Hbeta indices between the
M31 and Galactic globulars. The sample of ellipticals lies offset from the loci
of all the globulars in the Cyanogen--[MgFe], and Balmer--[MgFe] planes. Six of
the M31 cluster spectra appear young, and are projected onto the M31 disk.
Population synthesis models suggest that these are metal-rich clusters with
ages 100--800 Myr, metallicities --0.20 < [Fe/H] <0.35, and masses 0.7
-7.0x10^4 Msun. Two other young clusters are Hubble V in NGC 205, and an older
(~3 Gyr) cluster ~7 kpc away from the plane of the disk. The six clusters
projected onto the disk rotate in a similar fashion to the HI gas in M31, and
three clusters exhibit thin disk kinematics (Morrison et al.). Dynamical masses
and structural parameters are required for these objects to determine whether
they are massive open clusters or globular clusters. If the latter, our
findings suggest globular clusters may trace the build up of galaxy disks. In
either case, we conclude that these clusters are part of a young, metal-rich
disk cluster system in M31, possibly as young as 1 Gyr old.Comment: 52 pages, 14 figures, 8 tables, minor revisions in response to
referee, conclusions remain the same. Scheduled to appear in the October 2004
issue of The Astronomical Journa
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