3,618 research outputs found
Deep Hubble Space Telescope/ACS Observations of I Zw 18: a Young Galaxy in Formation
We present V and I photometry of the resolved stars in the most
metal-deficient blue compact dwarf galaxy known, I Zw 18 (Zsun/50), using
Hubble Space Telescope/Advanced Camera for Surveys (ACS) images, the deepest
ones ever obtained for this galaxy. The resulting I vs. V-I color-magnitude
diagram (CMD) reaches limiting magnitudes V=I=29 mag. It reveals a young
stellar population of blue main-sequence (MS) stars (age <30 Myr) and blue and
red supergiants (10 Myr<age<100 Myr), but also an older evolved population of
asymptotic giant branch (AGB) stars (100 Myr<age<500 Myr). We derive a distance
to I Zw 18 in the range 12.6 Mpc - 15 Mpc from the brightness of its AGB stars,
with preferred values in the higher range. The red giant branch (RGB) stars are
conspicuous by their absence, although, for a distance of I Zw 18 <15 Mpc, our
imaging data go ~ 1-2 mag below the tip of the RGB. Thus, the most evolved
stars in the galaxy are not older than 500 Myr and I Zw 18 is a bona fide young
galaxy. Several star formation episodes can be inferred from the CMDs of the
main body and the C component. There have been respectively three and two
episodes in these two parts, separated by periods of ~ 100-200 Myr. In the main
body, the younger MS and massive post-MS stars are distributed over a larger
area than the older AGB stars, suggesting that I Zw 18 is still forming from
the inside out. In the C component, different star formation episodes are
spatially distinct, with stellar population ages decreasing from the northwest
to the southeast, also suggesting the ongoing build-up of a young galaxy.Comment: 29 pages, 13 Postscript figures, accepted for publication in the
Astrophysical Journa
Sulfate and MSA in the air and snow on the Greenland Ice Sheet
Sulfate and methanesulfonic acid (MSA) concentrations in aerosol, surface snow, and snowpit samples have been measured at two sites on the Greenland Ice Sheet. Seasonal variations of the concentrations observed for these chemical species in the atmosphere are reproduced in the surface snow and preserved in the snowpit sequence. The amplitude of the variations over a year are smaller in the snow than in the air, but the ratios of the concentrations are comparable. The seasonal variations for sulfate are different at the altitude of the Ice Sheet compared to those observed at sea level, with low concentrations in winter and short episodes of elevated concentrations in spring. In contrast, the variations in concentrations of MSA are similar to those measured at sea level, with a first sequence of elevated concentrations in spring and another one during summer, and a winter low resulting from low biogenic production. The ratio MSA/sulfate clearly indicates the influence of high-latitude sources for the summer maximum of MSA, but the large impact of anthropogenic sulfate precludes any conclusion for the spring maximum. The seasonal pattern observed for these species in a snowpit sampled according to stratigraphy indicates a deficit in the accumulation of winter snow at the summit of the Greenland Ice Sheet, in agreement with some direct observations. A deeper snowpit covering the years 1985–1992 indicates the consistency of the seasonal pattern for MSA over the years, which may be linked to transport and deposition processes
Statistics of the electromagnetic response of a chaotic reverberation chamber
This article presents a study of the electromagnetic response of a chaotic
reverberation chamber (RC) in the presence of losses. By means of simulations
and of experiments, the fluctuations in the maxima of the field obtained in a
conventional mode-stirred RC are compared with those in a chaotic RC in the
neighborhood of the Lowest Useable Frequency (LUF). The present work
illustrates that the universal spectral and spatial statistical properties of
chaotic RCs allow to meet more adequately the criteria required by the Standard
IEC 61000-4-21 to perform tests of electromagnetic compatibility.Comment: 6 pages, 9 figure
The Carrington event not observed in most ice core nitrate records
The Carrington Event of 1859 is considered to be among the largest space weather events of the last 150 years. We show that only one out of 14 well-resolved ice core records from Greenland and Antarctica has a nitrate spike dated to 1859. No sharp spikes are observed in the Antarctic cores studied here. In Greenland numerous spikes are observed in the 40 years surrounding 1859, but where other chemistry was measured, all large spikes have the unequivocal signal, including co-located spikes in ammonium, formate, black carbon and vanillic acid, of biomass burning plumes. It seems certain that most spikes in an earlier core, including that claimed for 1859, are also due to biomass burning plumes, and not to solar energetic particle (SEP) events. We conclude that an event as large as the Carrington Event did not leave an observable, widespread imprint in nitrate in polar ice. Nitrate spikes cannot be used to derive the statistics of SEPs
Comment on “Low time resolution analysis of ice cores cannot detect impulsive nitrate events” by D. F. Smart et al.
Smart et al. (2014) suggested that the detection of nitrate spikes in polar ice cores from solar energetic particle (SEP) events could be achieved if an analytical system with sufficiently high resolution was used. Here we show that the spikes they associate with SEP events are not reliably recorded in cores from the same location, even when the resolution is clearly adequate. We explain the processes that limit the effective resolution of ice cores. Liquid conductivity data suggest that the observed spikes are associated with sodium or another nonacidic cation, making it likely that they result from deposition of sea salt or similar aerosol that has scavenged nitrate, rather than from a primary input of nitrate in the troposphere. We consider that there is no evidence at present to support the identification of any spikes in nitrate as representing SEP events. Although such events undoubtedly create nitrate in the atmosphere, we see no plausible route to using nitrate spikes to document the statistics of such events
A spectroscopic study of component C and the extended emission around I Zw 18
Long-slit Keck II, 4m Kitt Peak, and 4.5m MMT spectrophotometric data are
used to investigate the stellar population and the evolutionary status of I Zw
18C, the faint C component of the nearby blue compact dwarf galaxy I Zw 18.
Hydrogen H and H emission lines are detected in the spectra of I
Zw 18C, implying that ionizing massive stars are present. High signal-to-noise
Keck II spectra of different regions in I Zw 18C reveal H, H
and higher order hydrogen lines in absorption. Several techniques are used to
constrain the age of the stellar population in I Zw 18C. Ages derived from two
different methods, one based on the equivalent widths of the H,
H emission lines and the other on H, H absorption lines
are consistent with a 15 Myr instantaneous burst model. We find that a small
extinction in the range = 0.20 -- 0.65 mag is needed to fit the observed
spectral energy distribution of I Zw 18C with that model. In the case of
constant star formation, all observed properties are consistent with stars
forming continuously between ~ 10 Myr and < 100 Myr ago. We use all available
observational constraints for I Zw 18C, including those obtained from Hubble
Space Telescope color-magnitude diagrams, to argue that the distance to I Zw 18
should be as high as ~ 15 Mpc. The deep spectra also reveal extended ionized
gas emission around I Zw 18. H emission is detected as far as 30" from
it. To a B surface brightness limit of ~ 27 mag arcsec we find no
observational evidence for extended stellar emission in the outermost regions,
at distances > 15" from I Zw 18.Comment: 38 pages, 11 Postscript figures, accepted for publication in the
Astrophysical Journa
A Serendipitous Search for High-Redshift Lyman alpha Emission: Two Primeval Galaxy Candidates at z~3
In the course of our ongoing search for serendipitous high-redshift Lyman
alpha (LyA) Emissionin deep archival Keck spectra, we discovered two very high
equivalent width (W_{obs} ~ 450A, 2-sigma) LyA emission line candidates at z ~3
in a moderate dispersion (R~1200) spectrogram. Both lines have low velocity
dispersions (sigma_v ~ 60 km/s) and deconvolved radii r ~ 1 kpc (h = 0.5). We
argue that the lines are LyA, and are powered by stellar ionization. The
surface density of robust, high equivalent width LyA candidates is estimated to
be ~3 \pm 2 per arcmin^2 per unit redshift at z ~ 3, consistent with the
estimate of Cowie etal (1998). The LyA emission line source characteristics are
consistent with the galaxies undergoing their first burst of star formation,
ie, with being primeval. Source sizes and velocity dispersions are comparable
to the theoretical primeval galaxy model of Lin and Murray (1992) based on the
inside-out, self-similar collapse of an isothermal sphere. In this model, star
formation among field galaxies is a protracted process. Galaxies are thought to
be able to display high equivalent widths for only the first few x 10 Myr. This
time is short in relation to the difference in look back times between z=3 and
z=4, and implies that a substantial fraction of strong line-emitting galaxies
at z=3 were formed at redshifts z < 4. We discuss the significance of
high-equivalent width LyA-emitting galaxies in terms of the emerging picture of
the environment, and the specific characteristics of primeval galaxy formation
at high redshift.Comment: 17 pages, 3 figures, one table. To appear in the Astrophysical
Journa
Stress fractures in 2011: practical approach
Stress fractures occur when excessive loads are applied to a bone whose mechanical strength is normal. Bone insufficiency fractures, in contrast, are due to physiological loads applied to bone of inadequate mechanical strength [1]. This contradistinction is obviously an oversimplifi cation. In practice, a continuum exists between these two clearly defined situations.
The objective of the third ODISSEE meetings [2,3] held under the aegis of the GRIO was to review current knowledge on stress fractures. The pathophysiology of stress fractures is still poorly understood. When loads are applied to a bone, particularly in a repetitive manner, an elastic deformity occurs, followed by a plastic deformity and, finally, by microfractures. Bone strength varies across individuals. It depends not only on the intrinsic qualities of the bone tissue, but also on the magnitude and repetitiveness of the loads applied to the bone. Bone tissue fatigue is an inability to repair the microdamage caused by mechanical loading. The number and length of the microfractures increase, resulting in a fracture with clinical symptoms and radiographic changes [4]. Stress fractures are a common reason for physician visits among athletes and military recruits. They account for 5% to 14% of all physician visits, depending on the study population [5,6].
Although stress fractures can arise at any site, the most common locations are the tibia, particularly in runners; the metatarsals (most notably the second and third metatarsals) in hikers, runners, dancers, and military recruits; the iliopubic and ischiopubic rami of the pelvis in military recruits, gymnasts, dancers, and soccer players; and the femur in cross-country runners. The calcaneus is also a common site of involvement in all populations. Stress fractures are rare at the upper limbs, except in high- level gymnasts country runners. The calcaneus is also a common site of involvement in all populations. Stress fractures are rare at the upper limbs,except in high-level gymnasts
An imaging and spectroscopic study of the very metal-deficient blue compact dwarf galaxy Tol 1214--277
We present a spectrophotometric study based on VLT/FORS I observations of one
of the most metal-deficient blue compact dwarf (BCD) galaxies known, Tol
1214-277 (Z ~ Zsun/25). The data show that roughly half of the total luminosity
of the BCD originates from a bright and compact starburst region located at the
northeastern tip of a faint dwarf galaxy with cometary appearance. The
starburst has ignited less than 4 Myr ago and its emission is powered by
several thousands O7V stars and ~ 170 late-type nitrogen Wolf-Rayet stars
located within a compact region with < 500 pc in diameter. For the first time
in a BCD, a relatively strong [Fe V] 4227 emission line is seen which together
with intense He II 4686 emission indicates the presence of a very hard
radiation field in Tol 1214-277. We argue that this extraordinarily hard
radiation originates from both Wolf--Rayet stars and radiative shocks in the
starburst region. The structural properties of the low-surface-brightness (LSB)
component underlying the starburst have been investigated by means of surface
photometry down to 28 B mag/sq.arcsec. We find that, for a surface brightness
level fainter than ~ 24.5 B mag/sq.arcsec, an exponential fitting law provides
an adequate approximation to its radial intensity distribution. The broad-band
colors in the outskirts of the LSB component of Tol 1214-277 are nearly
constant and are consistent with an age below one Gyr. This conclusion is
supported by the comparison of the observed spectral energy distribution (SED)
of the LSB host with theoretical SEDs.Comment: 17 pages, 11 Postscript figures, uses emulateapj.sty, to appear in
Astronomical Journa
Selective amplification of scars in a chaotic optical fiber
In this letter we propose an original mechanism to select scar modes through
coherent gain amplification in a multimode D-shaped fiber. More precisely, we
numerically demonstrate how scar modes can be amplified by positioning a gain
region in the vicinity of specific points of a short periodic orbit known to
give rise to scar modes
- …