75 research outputs found
The Star Formation History of NGC 6822
Images of five fields in the Local Group dwarf irregular galaxy NGC 6822
obtained with the {\it Hubble Space Telescope} in the F555W and F814W filters
are presented. Photometry for the stars in these images was extracted using the
Point-Spread-Function fitting program HSTPHOT/MULTIPHOT. The resulting
color-magnitude diagrams reach down to , a level well below the red
clump, and were used to solve quantitatively for the star formation history of
NGC 6822. Assuming that stars began forming in this galaxy from low-metallicity
gas and that there is little variation in the metallicity at each age, the
distribution of stars along the red giant branch is best fit with star
formation beginning in NGC 6822 12-15 Gyr ago. The best-fitting star formation
histories for the old and intermediate age stars are similar among the five
fields and show a constant or somewhat increasing star formation rate from 15
Gyr ago to the present except for a possible dip in the star formation rate
from 3 to 5 Gyr ago. The main differences among the five fields are in the
higher overall star formation rate per area in the bar fields as well as in the
ratio of the recent star formation rate to the average past rate. These
variations in the recent star formation rate imply that stars formed within the
past 0.6 Gyr are not spatially very well mixed throughout the galaxy.Comment: 47 pages, 28 Figures, accepted for publication in the Astronomical
Journa
The Recent Star Formation in NGC 6822: an Ultraviolet Study
We characterize the star formation in the low-metallicity galaxy NGC 6822
over the past few hundred million years, using GALEX far-UV (FUV, 1344-1786 A)
and near-UV (NUV, 1771-2831 A) imaging, and ground-based Ha imaging. From GALEX
FUV image, we define 77 star-forming (SF) regions with area >860 pc^2, and
surface brightness <=26.8 mag(AB)arcsec^-2, within 0.2deg (1.7kpc) of the
center of the galaxy. We estimate the extinction by interstellar dust in each
SF region from resolved photometry of the hot stars it contains: E(B-V) ranges
from the minimum foreground value of 0.22mag up to 0.66+-0.21mag. The
integrated FUV and NUV photometry, compared with stellar population models,
yields ages of the SF complexes up to a few hundred Myr, and masses from 2x10^2
Msun to 1.5x10^6 Msun. The derived ages and masses strongly depend on the
assumed type of interstellar selective extinction, which we find to vary across
the galaxy. The total mass of the FUV-defined SF regions translates into an
average star formation rate (SFR) of 1.4x10^-2 Msun/yr over the past 100 Myr,
and SFR=1.0x10^-2 Msun/yr in the most recent 10 Myr. The latter is in agreement
with the value that we derive from the Ha luminosity, SFR=0.008 Msun/yr. The
SFR in the most recent epoch becomes higher if we add the SFR=0.02 Msun/yr
inferred from far-IR measurements, which trace star formation still embedded in
dust (age <= a few Myr).Comment: Accepted for publication in ApJ, 21 pages, 6 figures, 3 table
Stellar spectroscopy far beyond the Local Group
Multi-object spectroscopic observations of blue supergiants in NGC 3621, a
spiral galaxy at a distance of 6.7 Mpc, carried out with the ESO VLT and FORS
are presented. We demonstrate the feasibility of quantitative stellar
spectroscopy at distances approaching a ten-fold increase over previous
investigations by determining chemical composition, stellar parameters,
reddening, extinction and wind properties of one of our targets, a supergiant
of spectral type A1 Ia located in the outskirts of NGC 3621. The metallicity
(determined from iron group elements) is reduced by a factor of two relative to
the sun in qualitative agreement with results from previous abundance studies
based on H II region oxygen emission lines. Reddening and extinction are E(B-V)
= 0.12 and Av = 0.37, respectively, mostly caused by the galactic foreground.
Comparing stellar wind momentum and absolute V magnitude with galactic and M31
counterparts we confirm the potential of the wind momentum-luminosity
relationship as an alternative tool to estimate extragalactic distances.Comment: 9 pages, 4 figures, 1 table, accepted for publication in The
Astrophysical Journal Letter
The ARAUCARIA project: Grid-Based Quantitative Spectroscopic Study of Massive Blue Stars in NGC55
The quantitative study of the physical properties and chemical abundances of
large samples of massive blue stars at different metallicities is a powerful
tool to understand the nature and evolution of these objects. Their analysis
beyond the Milky Way is challenging, nonetheless it is doable and the best way
to investigate their behavior in different environments. Fulfilling this task
in an objective way requires the implementation of automatic analysis
techniques that can perform the analyses systematically, minimizing at the same
time any possible bias.
As part of the ARAUCARIA project we carry out the first quantitative
spectroscopic analysis of a sample of 12 B-type supergiants in the galaxy NGC55
at 1.94 Mpc away. By applying the methodology developed in this work, we derive
their stellar parameters, chemical abundances and provide a characterization of
the present-day metallicity of their host galaxy.
Based on the characteristics of the stellar atmosphere/line formation code
FASTWIND, we designed and created a grid of models for the analysis of massive
blue supergiant stars. Along with this new grid, we implemented a spectral
analysis algorithm. Both tools were specially developed to perform fully
consistent quantitative spectroscopic analyses of low spectral resolution of
B-type supergiants in a fast and objective way.
We present the main characteristics of our FASTWIND model grid and perform a
number of tests to investigate the reliability of our methodology. The
automatic tool is applied afterward to a sample of 12 B-type supergiant stars
in NGC55, deriving the stellar parameters and abundances. The results indicate
that our stars are part of a young population evolving towards a red supergiant
phase. The derived chemical composition hints to an average metallicity similar
to the one of the Large Magellanic Cloud, with no indication of a spatial trend
across the galaxy.Comment: 19 pages, 12 figures and 9 tables. Accpeted for publication in A&
A Survey of Local Group Galaxies Currently Forming Stars: II. UBVRI Photometry of Stars in Seven Dwarfs and a Comparison of the Entire Sample
We have obtained UBVRI images with the Kitt Peak and Cerro Tololo 4-m
telescopes and Mosaic cameras of seven dwarfs in (or near) the Local Group, all
of which have known evidence of recent star formation: IC10, NGC 6822, WLM,
Sextans B, Sextans A, Pegasus,and Phoenix. We construct color-magnitude
diagrams (CMDs) of these systems, as well as neighboring regions that can be
used to evaluate the degree of foreground contamination by stars in the Milky
Way. Inter-comparison of these CMDs with those of M31, M33, the LMC, and the
SMC permits us to determine improved reddening values for a typical OB star
found within these galaxies. All of the CMDs reveal a strong or modest number
of blue supergiants. All but Pegasus and Phoenix also show the clear presence
of red supergiants in the CMD, although IC10 appears to be deficient in these
objects given its large WR population. The bright stars of intermediate color
in the CMD are badly contaminated by foreground stars (30-100%), and
considerable spectroscopy is needed before statistics on the yellow supergiants
in these systems will be known. This study is intended to serve both as the
impetus and "finding charts" for further space-based imaging, and for many
spectroscopic programs at large aperture.Comment: Added Erratum. Revised versions of Tables 12 and 13 can be found at
http://www.lowell.edu/users/massey/Table12Revised.txt and
http://www.lowell.edu/users/massey/Table13Revised.tx
The Spatial Homogeneity of Nebular and Stellar Oxygen Abundances in the Local Group Dwarf Irregular Galaxy NGC 6822
To test the existence of a possible radial gradient in oxygen abundances
within the Local Group dwarf irregular galaxy NGC 6822, we have obtained
optical spectra of 19 nebulae with the EFOSC2 spectrograph on the 3.6-m
telescope at ESO La Silla. The extent of the measured nebulae spans
galactocentric radii in the range between 0.05 kpc and 2 kpc (over four
exponential scale lengths). In five H II regions (Hubble I, Hubble V, Kalpha,
Kbeta, KD28e), the temperature-sensitive [O III] 4363 emission line was
detected, and direct oxygen abundances were derived. Oxygen abundances for the
remaining H II regions were derived using bright-line methods. The oxygen
abundances for three A-type supergiant stars are slightly higher than nebular
values at comparable radii. Linear least-square fits to various subsets of
abundance data were obtained. When all of the measured nebulae are included, no
clear signature is found for an abundance gradient. A fit to only newly
observed H II regions with [O III] 4363 detections yields an oxygen abundance
gradient of -0.14 +/- 0.07 dex/kpc. The gradient becomes slightly more
significant (-0.16 +/- 0.05 dex/kpc) when three additional H II regions with [O
III] 4363 measurements from the literature are added. Assuming no abundance
gradient, we derive a mean nebular oxygen abundance 12+log(O/H) = 8.11 +/- 0.10
from [O III] 4363 detections in the five H II regions from our present data;
this mean value corresponds to [O/H] = -0.55.Comment: Accepted, Ap.J.; 25 pages (AASTeX 5.2; emulateapj) with 14 figures.
Full paper with color figures may be retrieved from
http://www.astro.umn.edu/~hlee
Fluorescence strategies for high-throughput quantification of protein interactions
Advances in high-throughput characterization of protein networks in vivo have resulted in large databases of unexplored protein interactions that occur during normal cell function. Their further characterization requires quantitative experimental strategies that are easy to implement in laboratories without specialized equipment. We have overcome many of the previous limitations to thermodynamic quantification of protein interactions, by developing a series of in-solution fluorescence-based strategies. These methods have high sensitivity, a broad dynamic range, and can be performed in a high-throughput manner. In three case studies we demonstrate how fluorescence (de)quenching and fluorescence resonance energy transfer can be used to quantitatively probe various high-affinity protein–DNA and protein–protein interactions. We applied these methods to describe the preference of linker histone H1 for nucleosomes over DNA, the ionic dependence of the DNA repair enzyme PARP1 in DNA binding, and the interaction between the histone chaperone Nap1 and the histone H2A–H2B heterodimer
Science and Adaptive Optics Requirements of MICADO, the E-ELT adaptive optics imaging camera
MICADO is the adaptive optics imaging camera being studied for the E-ELT. Its
design has been optimised for use with MCAO, but will have its own SCAO module
for the initial operational phase; and in principle could also be used with
GLAO or LTAO. In this contribution, we outline a few of the science drivers for
MICADO and show how these have shaped its design. The science drivers have led
to a number of requirements on the AO system related to astrometry, photometry,
and PSF uniformity. We discuss why these requirements have arisen and what
might be done about them.Comment: 6 pages, to appear in the proceedings of the AO4ELT conference, held
in Paris, 22-26 June 200
MOONS: a Multi-Object Optical and Near-infrared Spectrograph for the VLT
MOONS is a new conceptual design for a Multi-Object Optical and Near-infrared
Spectrograph for the Very Large Telescope (VLT), selected by ESO for a Phase A
study. The baseline design consists of 1000 fibers deployable over a field of
view of 500 square arcmin, the largest patrol field offered by the Nasmyth
focus at the VLT. The total wavelength coverage is 0.8um-1.8um and two
resolution modes: medium resolution and high resolution. In the medium
resolution mode (R=4,000-6,000) the entire wavelength range 0.8um-1.8um is
observed simultaneously, while the high resolution mode covers simultaneously
three selected spectral regions: one around the CaII triplet (at R=8,000) to
measure radial velocities, and two regions at R=20,000 one in the J-band and
one in the H-band, for detailed measurements of chemical abundances. The grasp
of the 8.2m Very Large Telescope (VLT) combined with the large multiplex and
wavelength coverage of MOONS - extending into the near-IR - will provide the
observational power necessary to study galaxy formation and evolution over the
entire history of the Universe, from our Milky Way, through the redshift desert
and up to the epoch of re-ionization at z>8-9. At the same time, the high
spectral resolution mode will allow astronomers to study chemical abundances of
stars in our Galaxy, in particular in the highly obscured regions of the Bulge,
and provide the necessary follow-up of the Gaia mission. Such characteristics
and versatility make MOONS the long-awaited workhorse near-IR MOS for the VLT,
which will perfectly complement optical spectroscopy performed by FLAMES and
VIMOS.Comment: 9 pages, 5 figures. To appear in the proceedings of the SPIE
Astronomical Instrumentation + Telescopes conference, Amsterdam, 201
Simulation vs. Reality: A Comparison of In Silico Distance Predictions with DEER and FRET Measurements
Site specific incorporation of molecular probes such as fluorescent- and nitroxide spin-labels into biomolecules, and subsequent analysis by Förster resonance energy transfer (FRET) and double electron-electron resonance (DEER) can elucidate the distance and distance-changes between the probes. However, the probes have an intrinsic conformational flexibility due to the linker by which they are conjugated to the biomolecule. This property minimizes the influence of the label side chain on the structure of the target molecule, but complicates the direct correlation of the experimental inter-label distances with the macromolecular structure or changes thereof. Simulation methods that account for the conformational flexibility and orientation of the probe(s) can be helpful in overcoming this problem. We performed distance measurements using FRET and DEER and explored different simulation techniques to predict inter-label distances using the Rpo4/7 stalk module of the M. jannaschii RNA polymerase. This is a suitable model system because it is rigid and a high-resolution X-ray structure is available. The conformations of the fluorescent labels and nitroxide spin labels on Rpo4/7 were modeled using in vacuo molecular dynamics simulations (MD) and a stochastic Monte Carlo sampling approach. For the nitroxide probes we also performed MD simulations with explicit water and carried out a rotamer library analysis. Our results show that the Monte Carlo simulations are in better agreement with experiments than the MD simulations and the rotamer library approach results in plausible distance predictions. Because the latter is the least computationally demanding of the methods we have explored, and is readily available to many researchers, it prevails as the method of choice for the interpretation of DEER distance distributions
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