668 research outputs found
Sediment dynamics of a nearshore sandbank: Results from TELEMAC-2D, TOMAWAC and SISYPHE modelling
Water Qualit
Long-Term Potentiation: One Kind or Many?
Do neurobiologists aim to discover natural kinds? I address this question in this chapter via a critical analysis of classification practices operative across the 43-year history of research on long-term potentiation (LTP). I argue that this 43-year history supports the idea that the structure of scientific practice surrounding LTP research has remained an obstacle to the discovery of natural kinds
NGC 346 in the Small Magellanic Cloud. III. Recent Star Formation and Stellar Clustering Properties in the Bright HII Region N 66
In the third part of our photometric study of the star-forming region NGC
346/N~66 and its surrounding field in the Small Magellanic Cloud (SMC), we
focus on the large number of low-mass pre-main sequence (PMS) stars revealed by
the Hubble Space Telescope Observations with the Advanced Camera for Surveys.
We investigate the origin of the observed broadening of the pre-main sequence
population in the , CMD. The most likely explanations are either the
presence of differential reddening or an age spread among the young stars.
Assuming the latter, simulations indicate that we cannot exclude the
possibility that stars in NGC 346 might have formed in two distinct events
occurring about 10 and 5 Myr ago, respectively. We find that the PMS stars are
not homogeneously distributed across NGC 346, but instead are grouped in at
least five different clusters. On spatial scales from 0.8 to 8 (0.24 to
2.4 pc at the distance of the SMC) the clustering of the PMS stars as computed
by a two-point angular correlation function is self-similar with a power law
slope . The clustering properties are quite similar to
Milky Way star forming regions like Orion OB or Oph. Thus molecular
cloud fragmentation in the SMC seems to proceed on the same spatial scales as
in the Milky Way. This is remarkable given the differences in metallicity and
hence dust content between SMC and Milky Way star forming regions.Comment: Accepted for publication in ApJ. 16 pages, 13 (low-resolution)
figures, emulateapj.cls LaTeX styl
Hubble Space Telescope Photometry of the Dwarf Spheroidal Galaxy ESO 410-G005
We present HST WFPC2 imaging of the nearby low-surface-brightness dwarf
spheroidal galaxy ESO 410-G005, which has been resolved into stars for the
first time. The resulting color-magnitude diagram for about 2500 stars shows a
red giant branch branch with a tip at I=(22.4+-0.15), which yields a distance
of D=(1.9+-0.2) Mpc. ESO 410-G005 is found to be metal-poor with a mean
metallicity of (-1.8+-0.4) dex estimated from its red giant branch. Upper
asymptotic giant branch stars appear to be present near the center of the
galaxy, indicative of a substantial, centrally concentrated intermediate-age
population, unless these objects are artifacts of crowding. Previous studies
did not detect ESO 410-G005 in H alpha or in HI. ESO 410-G005 is a probable
member of the Sculptor group. Its linear separation from the nearest spiral,
NGC 55, is 230 kpc on the sky. The deprojected separation ranges from 340 to
615 kpc depending on the assumed distance of NGC 55. ESO 410 G005 appears to be
a relatively isolated dSph within the Sculptor group. Its absolute magnitude,
Mv = (-12.1+-0.2) mag, its central surface brightness, mu_V = (22.7+-0.1)
mag/arcsec^2, and its mean metallicity, [Fe/H] = (-1.8+-0.4) dex, follow the
trend observed for dwarf galaxies in the Local Group. (abridged)Comment: Accepted for publication in the Astrophysical Journal, Vol. 542 (Oct
20). 23 pages in AASTEX style, 9 figures, partially in gif format to save
spac
Mapping the stellar structure of the Milky Way thick disk and halo using SEGUE photometry
We map the stellar structure of the Galactic thick disk and halo by applying
color-magnitude diagram (CMD) fitting to photometric data from the SEGUE
survey, allowing, for the first time, a comprehensive analysis of their
structure at both high and low latitudes using uniform SDSS photometry.
Incorporating photometry of all relevant stars simultaneously, CMD fitting
bypasses the need to choose single tracer populations. Using old stellar
populations of differing metallicities as templates we obtain a sparse 3D map
of the stellar mass distribution at |Z|>1 kpc. Fitting a smooth Milky Way model
comprising exponential thin and thick disks and an axisymmetric power-law halo
allows us to constrain the structural parameters of the thick disk and halo.
The thick-disk scale height and length are well constrained at 0.75+-0.07 kpc
and 4.1+-0.4 kpc, respectively. We find a stellar halo flattening within ~25
kpc of c/a=0.88+-0.03 and a power-law index of 2.75+-0.07 (for 7<R_{GC}<~30
kpc). The model fits yield thick-disk and stellar halo densities at the solar
location of rho_{thick,sun}=10^{-2.3+-0.1} M_sun pc^{-3} and
rho_{halo,sun}=10^{-4.20+-0.05} M_sun pc^{-3}, averaging over any
substructures. Our analysis provides the first clear in situ evidence for a
radial metallicity gradient in the Milky Way's stellar halo: within R<~15 kpc
the stellar halo has a mean metallicity of [Fe/H]=-1.6, which shifts to
[Fe/H]=-2.2 at larger radii. Subtraction of the best-fit smooth and symmetric
model from the overall density maps reveals a wealth of substructures at all
latitudes, some attributable to known streams and overdensities, and some new.
A simple warp cannot account for the low latitude substructure, as
overdensities occur simultaneously above and below the Galactic plane.
(abridged)Comment: 13 pages, 10 figures, accepted for publication in Astrophysical
Journa
Improved distance determination to M51 from supernovae 2011dh and 2005cs
The appearance of two recent supernovae, SN 2011dh and 2005cs, both in M51,
provides an opportunity to derive an improved distance to their host galaxy by
combining the observations of both SNe. We apply the Expanding Photosphere
Method to get the distance to M51 by fitting the data of these two SNe
simultaneously. In order to correct for the effect of flux dilution, we use
correction factors (zeta) appropriate for standard type II-P SNe atmospheres
for 2005cs, but find zeta ~ 1 for the type IIb SN 2011dh, which may be due to
the reduced H-content of its ejecta. The EPM analysis resulted in D_M51 = 8.4
+/- 0.7 Mpc. Based on this improved distance, we also re-analyze the HST
observations of the proposed progenitor of SN 2011dh. We confirm that the
object detected on the pre-explosion HST-images is unlikely to be a compact
stellar cluster. In addition, its derived radius (~ 277$ R_sun) is too large
for being the real (exploded) progenitor of SN 2011dh. The supernova-based
distance, D = 8.4 Mpc, is in good agreement with other recent distance
estimates to M51.Comment: 6 pages, 5 figures, accepted for publication in A&
Cloning, primary sequence and chromosomal localization of human FMO2, a new member of the flavin-containing mono-oxygenase family
Deep Hubble Space Telescope Imaging of IC 1613 II. The Star Formation History
We present deep HST WFPC2 imaging of the Local Group dwarf irregular galaxy
IC 1613. The photometry is the deepest to date for an isolated dwarf irregular
galaxy. The resulting color-magnitude diagram (CMD) is analyzed using three
different methods to derive a star formation history (SFH). All three find an
enhanced star formation rate (SFR), from 3 to 6 Gyr ago, and similar
age-metallicity relationships (AMR). A comparison of the newly observed outer
field with an earlier studied central field of IC 1613 shows that the SFR in
the outer field has been significantly depressed during the last Gyr. This
implies that the optical scale length of the galaxy has been decreasing with
time and that comparison of galaxies at intermediate redshift with present day
galaxies should take this effect into account. We find strong similarities
between IC 1613 and the more distant Milky Way dSph companions in that all are
dominated by star formation at intermediate ages. In particular, the SFH and
AMR for IC 1613 and Leo I are indistinguishable. This implies that dIrr
galaxies cannot be distinguished from dSphs by their intermediate age stellar
populations. This type of a SFH may also be evidence for slower or suppressed
early star formation in dwarf galaxies due to photoionization after the
reionization of the universe by background radiation. Assuming that IC 1613 is
typical of a dIrr evolving in isolation, since most of the star formation
occurs at intermediate ages, these dwarf systems cannot be responsible for the
fast chemical enrichment of the IGM which is seen at high redshift. There is no
evidence for any large amplitude bursts of star formation in IC 1613, and we
find it highly unlikely that analogs of IC 1613 have contributed to the excess
of faint blue galaxies in existing galaxy redshift surveys.Comment: 32 pages, including 1 table and 17 figures, accepted for publication
in the Astrophysical Journal October 10, 2003 issu
The Panchromatic Hubble Andromeda Treasury
The Panchromatic Hubble Andromeda Treasury (PHAT) is an on-going HST
Multicycle Treasury program to image ~1/3 of M31's star forming disk in 6
filters, from the UV to the NIR. The full survey will resolve the galaxy into
more than 100 million stars with projected radii from 0-20 kpc over a
contiguous 0.5 square degree area in 828 orbits, producing imaging in the F275W
and F336W filters with WFC3/UVIS, F475W and F814W with ACS/WFC, and F110W and
F160W with WFC3/IR. The resulting wavelength coverage gives excellent
constraints on stellar temperature, bolometric luminosity, and extinction for
most spectral types. The photometry reaches SNR=4 at F275W=25.1, F336W=24.9,
F475W=27.9, F814W=27.1, F110W=25.5, and F160W=24.6 for single pointings in the
uncrowded outer disk; however, the optical and NIR data are crowding limited,
and the deepest reliable magnitudes are up to 5 magnitudes brighter in the
inner bulge. All pointings are dithered and produce Nyquist-sampled images in
F475W, F814W, and F160W. We describe the observing strategy, photometry,
astrometry, and data products, along with extensive tests of photometric
stability, crowding errors, spatially-dependent photometric biases, and
telescope pointing control. We report on initial fits to the structure of M31's
disk, derived from the density of RGB stars, in a way that is independent of
the assumed M/L and is robust to variations in dust extinction. These fits also
show that the 10 kpc ring is not just a region of enhanced recent star
formation, but is instead a dynamical structure containing a significant
overdensity of stars with ages >1 Gyr. (Abridged)Comment: 48 pages including 22 pages of figures. Accepted to the Astrophysical
Journal Supplements. Some figures slightly degraded to reduce submission siz
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