1,302 research outputs found
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
Natural Cycles, Gases
The major gaseous components of the exhaust of stratospheric aircraft are expected to be the products of combustion (CO2 and H2O), odd nitrogen (NO, NO2 HNO3), and products indicating combustion inefficiencies (CO and total unburned hydrocarbons). The species distributions are produced by a balance of photochemical and transport processes. A necessary element in evaluating the impact of aircraft exhaust on the lower stratospheric composition is to place the aircraft emissions in perspective within the natural cycles of stratospheric species. Following are a description of mass transport in the lower stratosphere and a discussion of the natural behavior of the major gaseous components of the stratospheric aircraft exhaust
A Panchromatic Study of the Globular Cluster NGC 1904. I: The Blue Straggler Population
By combining high-resolution (HST-WFPC2) and wide-field ground based (2.2m
ESO-WFI) and space (GALEX) observations, we have collected a multi-wavelength
photometric data base (ranging from the far UV to the near infrared) of the
galactic globular cluster NGC1904 (M79). The sample covers the entire cluster
extension, from the very central regions up to the tidal radius. In the present
paper such a data set is used to study the BSS population and its radial
distribution. A total number of 39 bright () BSS has been
detected, and they have been found to be highly segregated in the cluster core.
No significant upturn in the BSS frequency has been observed in the outskirts
of NGC 1904, in contrast to other clusters (M 3, 47 Tuc, NGC 6752, M 5) studied
with the same technique. Such evidences, coupled with the large radius of
avoidance estimated for NGC 1904 ( core radii), indicate that
the vast majority of the cluster heavy stars (binaries) has already sunk to the
core. Accordingly, extensive dynamical simulations suggest that BSS formed by
mass transfer activity in primordial binaries evolving in isolation in the
cluster outskirts represent only a negligible (0--10%) fraction of the overall
population.Comment: ApJ accepte
The Tropical Upper Troposphere and Lower Stratosphere in the GEOS-2 GCM
The structure of the tropical upper troposphere and lower stratosphere in the GEOS-2 General Circulation Model (GCM) is discussed. The emphasis of this study is on the reality of monthly-mean temperature and water vapor distributions in the model, compared to reasonable observational estimates. It is shown that although the zonal-mean temperature is in good agreement with observations, the GCM supports an excessive zonal asymmetry near the tropopause compared to the ECMWF Reanalyses. In reality there is a QBO-related variability in the zonally averaged lower stratospheric temperature which is not captured by the model. The observed upper tropospheric temperature and humidity fields show variations related to those in the sea surface temperature, which are not incorporated in the GCM; nevertheless, there is some interannual variability in the GCM, indicating a component arising from internal processes. The model is too moist in the middle troposphere (500 hPa) but too dry in the upper troposphere, suggesting that there is too little vertical transport or too much drying in the GCM. Transport into the stratosphere shows a pronounced annual cycle, with drier air entering the tropical stratosphere when the tropopause is coldest in northern winter; while the alternating dry and moist air masses can be traced ascending through the tropical lower stratosphere, the progression of the anomalies is too rapid
The surprising external upturn of the Blue Straggler radial distribution in M55
By combining high-resolution HST and wide-field ground based observations, in
ultraviolet and optical bands, we study the Blue Straggler Star (BSS)
population of the low density galactic globular cluster M55 (NGC 6809) over its
entire radial extent. The BSS projected radial distribution is found to be
bimodal, with a central peak, a broad minimum at intermediate radii, and an
upturn at large radii. Similar bimodal distributions have been found in other
globular clusters (M3, 47 Tucanae, NGC 6752, M5), but the external upturn in
M55 is the largest found to date. This might indicate a large fraction of
primordial binaries in the outer regions of M55, which seems somehow in
contrast with the relatively low (\sim 10%) binary fraction recently measured
in the core of this cluster.Comment: in press on Ap
Recommended from our members
OA02.01. Effect of macronutrient composition of weight loss diets on reduction of the inflammatory marker hsCRP
Discovery of another peculiar radial distribution of Blue Stragglers in Globular Clusters: The case of 47 Tuc
We have used high resolution WFPC2-HST and wide field ground-based
observations to construct a catalog of blue straggler stars (BSS) in the
globular cluster 47 Tuc spanning the entire radial extent of the cluster.
The BSS distribution is highly peaked in the cluster center, rapidly
decreases at intermediate radii, and finally rises again at larger radii. The
observed distribution closely resembles that discovered in M3 by Ferraro et al
(1993,1997). To date, complete BSS surveys covering the full radial extent (HST
in the center and wide field CCD ground based observations of the exterior)
have been performed for only these two clusters. Both show a bimodal radial
distribution, despite their different dynamical properties. BSS surveys
covering the full spatial extent of more globular clusters are clearly required
to determine how common bimodality is and what its consequence is for theories
of BSS formation and cluster dynamics.Comment: 21 pages, 6 figures, accepted for pubblication in Ap
Horizontal-Branch Models and the Second-Parameter Effect. IV. The Case of M3 and Palomar 3
We present a detailed analysis of the "second-parameter pair" of globular
clusters M3 (NGC 5272) and Palomar 3. Our main results can be summarized as
follows: i) The horizontal-branch (HB) morphology of M3 is significantly bluer
in its inner regions (observed with the Hubble Space Telescope) than in the
cluster outskirts (observed from the ground), i.e., M3 has an internal second
parameter. Most plausibly the mass loss on the red giant branch (RGB) has been
more efficient in the inner than in the outer regions of the cluster. ii) The
dispersion in mass of the Pal 3 HB is found to be very small -- consistent with
zero -- and we argue that this is unlikely to be due to a statistical
fluctuation. It is this small mass dispersion that leads to the most apparent
difference in the HB morphologies of M3 and Pal 3. iii) The relative HB types
of M3 and Pal 3, as measured by mean colors or parameters involving the number
of blue, variable, and red HB stars, can easily be accounted for by a fairly
small difference in age between these clusters, of order 0.5-1 Gyr -- which is
in good agreement with the relative age measurement, based on the clusters'
turnoffs, by VandenBerg (2000).Comment: 20 pages, 12 figures, emulateapj5 style. The Astrophysical Journal,
in press. Figs. 1, 6, 9, 10 are in png format. The preprint (postscript
format) with full resolution (embedded) figures is available from
http://www.astro.virginia.edu/~mc6v
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