1,337 research outputs found
Ultraviolet Radiation from Evolved Stellar Populations: II. The Ultraviolet Upturn Phenomenon in Elliptical Galaxies
We present an analysis of the far-ultraviolet upturn phenomenon (UVX)
observed in elliptical galaxies and spiral galaxy bulges. Our premise is that
the UV radiation from these systems emanates primarily from extreme horizontal
branch (EHB) stars and their progeny. We re-derive the broad-band UV colors
and for globular clusters and elliptical galaxies from the
available satellite data and investigate color-color and color-line strength
correlations. We also provide the ingredients necessary for constructing models
with arbitrary HB morphologies.Comment: uuencoded compressed postscript file, 60pp. (revisions on pp.
7,8,22,33,37 & 57
Landscape response to Pleistocene-Holocene precipitation change in the Western Cordillera, Peru: 10Be concentrations in modern sediments and terrace fills
The landscape response to climate change is frequently investigated with models because natural experiments on geologic timescales are rare. In Quebrada Veladera, in the western Andes Mountains, the formation of alluvial terraces during periods of high precipitation presents opportunities for such an experiment. We compare drainage-average erosion rates during Pleistocene terrace deposition with Holocene rates, using cosmogenic 10Be samples for seven pairs of quartz sand taken from the trunk and tributaries of Quebrada Veladera and adjacent terraces. Each pair consists of sediment collected from the modern channel and excavated from an adjacent fill terrace. The terrace fill was deposited at ~16 ka and preserved an isotopic record of paleoerosion rates in the Late Pleistocene. Modern sands yield 10Be concentrations between 1.68 × 105 and 2.28 × 105 atoms/g, corresponding to Holocene erosion rates between 43 ± 3 and 58 ± 4 mm/kyr. The 10Be concentrations in terrace sands range from 9.46 × 104 to 3.73 × 105 atoms/g, corresponding to paleoerosion rates from 27 ± 2 to 103 ± 8 mm/kyr. Smaller, upstream tributaries show a substantial decline in erosion rate following the transition from a wet to dry climate, but larger drainage areas show no change. We interpret this trend to indicate that the wetter climate drove landscape dissection, which ceased with the return to dry conditions. As channel heads propagated upslope, erosion accelerated in low-order drainages before higher-order ones. This contrast disappeared when the drainage network ceased to expand; at that point, erosion rates became spatially uniform, consistent with the uniformity of modern hillslope gradients. Key Points Landscape response to climate change evaluated with 10Be erosion rates Wetter climate associated with rapid erosion in smaller, upstream drainages Drier, Holocene climate associated with spatially uniform erosion rates ©2013. American Geophysical Union. All Rights Reserved
The Electron Temperature Gradient in the Galactic Disk
We derive the electron temperature gradient in the Galactic disk using a
sample of HII regions that spans Galactocentric distances 0--17 kpc. The
electron temperature was calculated using high precision radio recombination
line and continuum observations for more than 100 HII regions. Nebular
Galactocentric distances were calculated in a consistent manner using the
radial velocities measured by our radio recombination line survey. The large
number of nebulae widely distributed over the Galactic disk together with the
uniformity of our data provide a secure estimate of the present electron
temperature gradient in the Milky Way. Because metals are the main coolants in
the photoionized gas, the electron temperature along the Galactic disk should
be directly related to the distribution of heavy elements in the Milky Way. Our
best estimate of the electron temperature gradient is derived from a sample of
76 sources for which we have the highest quality data. The present gradient in
electron temperature has a minimum at the Galactic Center and rises at a rate
of 287 +/- 46 K/kpc. There are no significant variations in the value of the
gradient as a function of Galactocentric radius or azimuth. The scatter we find
in the HII region electron temperatures at a given Galactocentric radius is not
due to observational error, but rather to intrinsic fluctuations in these
temperatures which are almost certainly due to fluctuations in the nebular
heavy element abundances. Comparing the HII region gradient with the much
steeper gradient found for planetary nebulae suggests that the electron
temperature gradient evolves with time, becoming flatter as a consequence of
the chemical evolution of the Milky Way's disk.Comment: 43 pages, 9 figures (accepted for publication in the ApJ
The pure non-collisional Blue Straggler population in the giant stellar system omega Centauri
We have used high spatial resolution data from the Hubble Space Telescope and
wide-field ground-based observations to search for blue straggler stars (BSS)
over the entire radial extent of the large stellar system omega Centauri. We
have detected the largest population of BSS ever observed in any stellar
system. Even though the sample is restricted to the brightest portion of the
BSS sequence, more than 300 candidates have been identified. BSS are thought to
be produced by the evolution of binary systems (either formed by stellar
collisions or mass exchange in binary stars). Since systems like Galactic
globular clusters (GGC) and omega Cen evolve dynamically on time-scales
significantly shorter than their ages, binaries should have settled toward the
center, showing a more concentrated radial distribution than the ordinary, less
massive single stars. Indeed, in all GGCs which have been surveyed for BSS, the
BSS distribution is peaked at the center. Conversely, in omega Cen we find that
the BSS share the same radial distribution as the adopted reference
populations. This is the cleanest evidence ever found that such a stellar
system is not fully relaxed even in the central region. We further argue that
the absence of central concentration in the BSS distribution rules out a
collisional origin. Thus, the omega Cen BSS are the purest and largest
population of non-collisional BSS ever observed. Our results allow the first
empirical quantitative estimate of the production rate of BSS via this channel.
BSS in omega Cen may represent the best local template for modeling the BSS
populations in distant galaxies where they cannot be individually observed.Comment: 21 pages, 6 figures, accepted for publication by Ap
A Superwind from Early Post-Red Giant Stars?
We suggest that the gap observed at 20,000 K in the horizontal branches of
several Galactic globular clusters is caused by a small amount of extra mass
loss which occurs when stars start to "peel off" the red giant branch (RGB),
i.e., when their effective temperature starts to increase, even though they may
still be on the RGB. We show that the envelope structure of RGB stars which
start to peel off is similar to that of late asymptotic giant branch stars
known to have a super-wind phase. An analogous super-wind in the RGB peel-off
stars could easily lead to the observed gap in the distribution of the hottest
HB stars.Comment: 9 pages; Accepted by ApJ Letters; Available also at
http://www.astro.puc.cl/~mcatelan
UV Properties of Galactic Globular Clusters with GALEX II. Integrated colors
We present ultraviolet (UV) integrated colors of 44 Galactic globular
clusters (GGCs) observed with the Galaxy Evolution Explorer (GALEX) in both FUV
and NUV bands. This data-base is the largest homogeneous catalog of UV colors
ever published for stellar systems in our Galaxy. The proximity of GGCs makes
it possible to resolve many individual stars even with the somewhat low spatial
resolution of GALEX. This allows us to determine how the integrated UV colors
are driven by hot stellar populations, primarily horizontal branch stars and
their progeny. The UV colors are found to be correlated with various parameters
commonly used to define the horizontal branch morphology. We also investigate
how the UV colors vary with parameters like metallicity, age, helium abundance
and concentration. We find for the first time that GCs associated with the
Sagittarius dwarf galaxy have (FUV-V) colors systematically redder than GGCs
with the same metallicity. Finally, we speculate about the presence of an
interesting trend, suggesting that the UV color of GCs may be correlated with
the mass of the host galaxy, in the sense that more massive galaxies possess
bluer clusters.Comment: Accepted for publication by The Astronomical Journal. 36 pages, 9
figures, 1 tabl
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