4,331 research outputs found
The effect of collisional enhancement of Balmer lines on the determination of the primordial helium abundance
This paper describes a new determination of the primordial helium abundance
(Y_P), based on the abundance analysis of five metal-poor extragalactic HII
regions. For three regions of the sample (SBS 0335-052, I Zw 18, and H29) we
present tailored photoionization models based on improved calculations with
respect to previous models. In particular, we use the photoionization models to
study quantitatively the effect of collisional excitation of Balmer lines on
the determination of the helium abundance (Y) in the individual regions. This
effect is twofold: first, the intensities of the Balmer lines are enhanced with
respect to the pure recombination value, mimicking a higher hydrogen abundance;
second, the observed reddening is larger than the true extinction, due to the
differential effect of collisions on different Balmer lines. In addition to
these effects, our analysis takes into account the following features of HII
regions: (i) the temperature structure, (ii) the density structure, (iii) the
presence of neutral helium, (iv) the collisional excitation of the HeI lines,
(v) the underlying absorption of the HeI lines, and (vi) the optical thickness
of the HeI lines. The object that shows the highest increase in Y after the
inclusion of collisional effects in the analysis is SBS 0335-052, whose helium
abundance has been revised by Delta Y = +0.0107. The revised Y values for the
five objects in our sample yield an increase of +0.0035 in Y_P, giving Y_P =
0.2391 +/- 0.0020.Comment: 59 pages, 8 figures. AAS Latex. Accepted for publication in the
Astrophysical Journa
The young stellar population of NGC 4214 as observed with HST. II. Results
We present the results of a detailed UV-optical study of the nearby dwarf
starburst galaxy NGC 4214 using multifilter HST/WFPC2+STIS photometry. The
stellar extinction is found to be quite patchy, with some areas having values
of E(4405-5495)< 0.1 mag and others, associated with star forming regions, much
more heavily obscured, a result which is consistent with previous studies of
the nebular extinction. We determined the ratio of blue-to-red supergiants and
found it to be consistent with theoretical models for the metallicity of the
SMC. The stellar IMF of the field in the range 20-100 solar masses is found to
be steeper than Salpeter. A number of massive clusters and associations with
ages between a few and 200 million years are detected and their properties are
discussed.Comment: 49 pages, 12 figures, 6 table
Model-based Aeroservoelastic Design and Load Alleviation of Large Wind Turbine Blades
This paper presents an aeroservoelastic modeling approach for dynamic load alleviation
in large wind turbines with trailing-edge aerodynamic surfaces. The tower, potentially on a
moving base, and the rotating blades are modeled using geometrically non-linear composite
beams, which are linearized around reference conditions with arbitrarily-large structural
displacements. Time-domain aerodynamics are given by a linearized 3-D unsteady vortexlattice
method and the resulting dynamic aeroelastic model is written in a state-space
formulation suitable for model reductions and control synthesis. A linear model of a single
blade is used to design a Linear-Quadratic-Gaussian regulator on its root-bending moments,
which is finally shown to provide load reductions of about 20% in closed-loop on the full
wind turbine non-linear aeroelastic model
Ultraviolet Properties of Primeval Galaxies: Theoretical Models from Stellar Population Synthesis
The ultraviolet luminosity evolution of star-forming galaxies is explored
from the theoretical point of view, especially focusing on the theory of UV
energetics in simple and composite stellar populations and its relationship to
the star formation rate and other main evolutionary parameters.
Galaxy emission below 3000 Angstroms directly correlates with actual star
formation, not depending on the total mass of the system. A straightforward
calibration is obtained, in this sense, from the theoretical models at 1600,
2000 and 2800 Angstroms, and a full comparison is carried out with IUE data and
other balloon-borne observations for local galaxies.
The claimed role of late-type systems as prevailing contributors to the
cosmic UV background is reinforced by our results; at 2000 Angstroms Im
irregulars are found in fact nearly four orders of magnitude brighter than
ellipticals, per unit luminous mass.
The role of dust absorption in the observation of high-redshift galaxies is
assessed, comparing model output and observed spectral energy distribution of
local galaxy samples. Similar to what we observe in our own galaxy, a quick
evolution in the dust environment might be envisaged in primeval galaxies, with
an increasing fraction of luminous matter that would escape the regions of
harder and "clumpy" dust absorption on a timescale of some 10^7 yr, comparable
with the lifetime of stars of 5-10 solar masses.Comment: 20 pages and 14 PS figures - To appear in the March issue of the
Astronomical Journal. See http://www.merate.mi.astro.it/~eps/home.html for
more info and model retrieva
TYC 2675-663-1: A newly discovered W UMa system in an active state
The recently discovered eclipsing binary system TYC 2675-663-1 is a X-ray
source, and shows properties in the optical that are similar to the W UMa
systems, but are somewhat unusual compared to what is seen in other contact
binary systems. The goal of this work is to characterize its properties and
investigate its nature by means of detailed photometric and spectroscopic
observations. We have performed extensive V-band photometric measurements with
the INTEGRAL satellite along with ground-based multi-band photometric
observations, as well as high-resolution spectroscopic monitoring from which we
have measured the radial velocities of the components. These data have been
analysed to determine the stellar properties, including the absolute masses and
radii. Additional low-resolution spectroscopy was obtained to investigate
spectral features. From the measured eclipse timings we determine an orbital
period for the binary of P=0.4223576+-0.0000009 days. The light-curve and
spectroscopic analyses reveal the observations to be well represented by a
model of an overcontact system composed of main-sequence F5 and G7 stars
(temperature difference of nearly 1000 K), with the possible presence of a
third star. Low-resolution optical spectroscopy reveals a complex H alpha
emission, and other features that are not yet understood. The unusually large
mass ratio of q=0.81+-0.05 places it in the rare "H" (high mass ratio) subclass
of the W UMa systems, which are presumably on their way to coalescence.Comment: 12 pages in double column format. Accepted for publication in
Astronomy and Astrophysic
Patchy Reconnection in a Y-Type Current Sheet
We study the evolution of the magnetic field in a Y-type current sheet
subject to a brief, localized magnetic reconnection event. The reconnection
produces up- and down-flowing reconnected flux tubes which rapidly decelerate
when they hit the Y-lines and underlying magnetic arcade loops at the ends of
the current sheet. This localized reconnection outflow followed by a rapid
deceleration reproduces the observed behavior of post-CME downflowing coronal
voids. These simulations support the hypothesis that these observed coronal
downflows are the retraction of magnetic fields reconnected in localized
patches in the high corona.Comment: 4 pages, 3 figure
The Luminous Starburst Ring in NGC 7771: Sequential Star Formation?
Only two of the twenty highly luminous starburst galaxies analyzed by Smith
et al. exhibit circumnuclear rings of star formation. These galaxies provide a
link between 10^11 L_sun systems and classical, less-luminous ringed systems.
We report the discovery of a near-infrared counterpart to the nuclear ring of
radio emission in NGC 7771. A displacement between the ~10 radio bright clumps
and the ~10 near-infrared bright clumps indicates the presence of multiple
generations of star formation. The estimated thermal emission from each radio
source is equivalent to that of ~35000 O6 stars. Each near-infrared bright knot
contains ~5000 red supergiants, on average. The stellar mass of each knot is
estimated to be ~10^7 M_sun. The implied time-averaged star formation rate is
\~40 M_sun per yr. Several similarities are found between the properties of
this system and other ringed and non-ringed starbursts. Morphological
differences between NGC 7771 and the starburst + Seyfert 1 galaxy NGC 7469
suggest that NGC 7771 may not be old enough to fuel an AGN, or may not be
capable of fueling an AGN. Alternatively, the differences may be unrelated to
the presence or absence of an AGN and may simply reflect the possibility that
star formation in rings is episodic.Comment: accepted for publication in The Astrophysical Journal (10 January
1999); 48 pages including 13 figures; AAS LaTe
Statistical properties of giant pulses from the Crab pulsar
We have studied the statistics of giant pulses from the Crab pulsar for the
first time with particular reference to their widths. We have analyzed data
collected during 3.5 hours of observations conducted with the Westerbork
Synthesis Radio Telescope operated in a tied-array mode at a frequency of 1200
MHz. The PuMa pulsar backend provided voltage recording of X and Y linear
polarization states in two conjugate 10 MHz bands. We restricted the time
resolution to 4 microseconds to match the scattering on the interstellar
inhomogeneities. In total about 18000 giant pulses (GP) were detected in full
intensity with a threshold level of 6 sigma. Cumulative probability
distributions (CPD) of giant pulse energies were analyzed for groups of GPs
with different effective widths in the range 4 to 65 microseconds. The CPDs
were found to manifest notable differences for the different GP width groups.
The slope of a power-law fit to the high-energy portion of the CPDs evolves
from -1.7 to -3.2 when going from the shortest to the longest GPs. There are
breaks in the CPD power-law fits indicating flattening at low energies with
indices varying from -1.0 to -1.9 for the short and long GPs respectively. The
GPs with a stronger peak flux density were found to be of shorter duration. We
compare our results with previously published data and discuss the importance
of these peculiarities in the statistical properties of GPs for the heoretical
understanding of the emission mechanism responsible for GP generation.Comment: 5 pages, 2 figures. Accepted by Astronomy and Astrophysic
Detection of an optical transient following the 13 March 2000 short/hard gamma-ray burst
We imaged the error box of a gamma-ray burst of the short (0.5 s), hard type
(GRB 000313), with the BOOTES-1 experiment in southern Spain, starting 4 min
after the gamma-ray event, in the I-band. A bright optical transient (OT
000313) with I = 9.4 +/- 0.1 was found in the BOOTES-1 image, close to the
error box (3-sigma) provided by BATSE. Late time VRIK'-band deep observations
failed to reveal an underlying host galaxy. If the OT 000313 is related to the
short, hard GRB 000313, this would be the first optical counterpart ever found
for this kind of events (all counterparts to date have been found for bursts of
the long, soft type). The fact that only prompt optical emission has been
detected (but no afterglow emission at all, as supported by theoretical models)
might explain why no optical counterparts have ever been found for short, hard
GRBs.This fact suggests that most short bursts might occur in a low-density
medium and favours the models that relate them to binary mergers in very
low-density enviroments.Comment: Revised version. Accepted for publication in Astronomy and
Astrophysics Letters, 5 pages, 3 figure
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