3,394 research outputs found
Triggered massive-star formation on the borders of Galactic HII regions. II. Evidence for the collect and collapse process around RCW 79
We present SEST-SIMBA 1.2-mm continuum maps and ESO-NTT SOFI JHK images of
the Galactic HII region RCW 79. The millimetre continuum data reveal the
presence of massive fragments located in a dust emission ring surrounding the
ionized gas. The two most massive fragments are diametrically opposite each
other in the ring. The near-IR data, centred on the compact HII region located
at the south-eastern border of RCW 79, show the presence of an IR-bright
cluster containing massive stars along with young stellar objects with near-IR
excesses. A bright near- and mid-IR source is detected towards maser emissions,
1.2 pc north-east of the compact HII region centre. Additional information,
extracted from the Spitzer GLIMPSE survey, are used to discuss the nature of
the bright IR sources observed towards RCW 79. Twelve luminous Class I sources
are identified towards the most massive millimetre fragments. All these facts
strongly indicate that the massive-star formation observed at the border of the
HII region RCW 79 has been triggered by its expansion, most probably by the
collect and collapse process.Comment: 14 pages, 15 figures. Accepted for publication in A&A. The images
have been highly compressed for astro-ph. A version of this paper with
higher-resolution figures is available at
http://www.oamp.fr/matiere/rcw79.pd
Triggered massive-star formation on the borders of Galactic HII regions. III. Star formation at the periphery of Sh2-219
Context. Massive-star formation triggered by the expansion of HII regions.
Aims. To understand if sequential star formation is taking place at the
periphery of the HII region Sh2-219. Methods. We present 12CO(2-1) line
observations of this region, obtained at the IRAM 30-m telescope (Pico Veleta,
Spain). Results. In the optical, Sh2-219 is spherically symmetric around its
exciting star; furthermore it is surrounded along three quarters of its
periphery by a ring of atomic hydrogen. This spherical symmetry breaks down at
infrared and millimetre wavelengths. A molecular cloud of about 2000\msol lies
at the southwestern border of Sh2-219, in the HI gap. Two molecular
condensations, elongated along the ionization front, probably result from the
interaction between the expanding HII region and the molecular cloud. In this
region of interaction there lies a cluster containing many highly reddened
stars, as well as a massive star exciting an ultracompact HII region. More
surprisingly, the brightest parts of the molecular cloud form a `chimney',
perpendicular to the ionization front. This chimney is closed at its south-west
extremity by H-alpha walls, thus forming a cavity. The whole structure is 7.5
pc long. A luminous H-alpha emission-line star, lying at one end of the chimney
near the ionization front, may be responsible for this structure. Confrontation
of the observations with models of HII region evolution shows that Sh2-219 is
probably 10^5 yr old. The age and origin of the near-IR cluster observed on the
border of Sh2-219 remain unknown.Comment: 11 pages, 10 figures. To be published in A&
The Nature of Radio Continuum Emission in the Dwarf Starburst Galaxy NGC 625
We present new multi-frequency radio continuum imaging of the dwarf starburst
galaxy NGC 625 obtained with the Very Large Array. Data at 20, 6, and 3.6 cm
reveal global continuum emission dominated by free-free emission, with only
mild synchrotron components. Each of the major HII regions is detected; the
individual spectral indices are thermal for the youngest regions (showing
strongest H Alpha emission) and nonthermal for the oldest. We do not detect any
sources that appear to be associated with deeply embedded, dense, young
clusters, though we have discovered one low-luminosity, obscured source that
has no luminous optical counterpart and which resides in the region of highest
optical extinction. Since NGC 625 is a Wolf-Rayet galaxy with strong recent
star formation, these radio properties suggest that the youngest star formation
complexes have not yet evolved to the point where their thermal spectra are
significantly contaminated by synchrotron emission. The nonthermal components
are associated with regions of older star formation that have smaller ionized
gas components. These results imply a range of ages of the HII regions and
radio components that agrees with our previous resolved stellar population
analysis, where an extended burst of star formation has pervaded the disk of
NGC 625 over the last ~ 50 Myr. We compare the nature of radio continuum
emission in selected nearby dwarf starburst and Wolf-Rayet galaxies,
demonstrating that thermal radio continuum emission appears to be more common
in these systems than in typical HII galaxies with less recent star formation
and more evolved stellar clusters.Comment: ApJ, in press; 27 pages, 5 figures. Full-resolution version may be
obtained at http://www.astro.umn.edu/~cannon/n625.vla.p
Star Formation and Selective Dust Extinction in Luminous Starburst Galaxies
We investigate the star formation and dust extinction properties of very
luminous infrared galaxies whose spectra display a strong Hdelta line in
absorption and a moderate [OII] emission (e[a] spectrum). This spectral
combination has been suggested to be a useful method to identify dusty
starburst galaxies at any redshift on the basis of optical data alone. We
compare the average e(a) optical spectrum with synthetic spectra that include
both the stellar and the nebular contribution, allowing dust extinction to
affect differentially the stellar populations of different ages. We find that
reproducing the e(a) spectrum requires the youngest stellar generations to be
significantly more extinguished by dust than older stellar populations, and
implies a strong ongoing star formation activity at a level higher than in
quiescent spirals. A model fitting the optical spectrum does not necessarily
produce the observed FIR luminosity and this can be explained by the existence
of stellar populations which are practically obscured at optical wavelengths.
Models in which dust and stars are uniformly mixed yield a reddening of the
emerging emission lines which is too low compared to observations: additional
foreground reddening is required.Comment: 17 pages, 4 Postscript figures, ApJ in pres
Optical Spectral Signatures of Dusty Starburst Galaxies
We analyse the optical spectral properties of the complete sample of Very
Luminous Infrared Galaxies presented by Wu et al. (1998a,b) and we find a high
fraction (~50 %) of spectra showing both a strong H_delta line in absorption
and relatively modest [OII] emission (e(a) spectra). The e(a) signature has
been proposed as an efficient method to identify dusty starburst galaxies and
we study the star formation activity and the nature of these galaxies, as well
as the effects of dust on their observed properties. We examine their emission
line characteristics, in particular their [OII]/H_alpha ratio, and we find this
to be greatly affected by reddening. A search for AGN spectral signatures
reveals that the e(a)'s are typically HII/LINER galaxies. We compare the star
formation rates derived from the FIR luminosities with the estimates based on
the H_alpha line and find that the values obtained from the optical emission
lines are a factor of 10-70 (H_alpha) and 20-140 ([OII]) lower than the FIR
estimates (50-300 M_sun yr^-1). We then study the morphological properties of
the e(a) galaxies, looking for a near companion or signs of a
merger/interaction. In order to explore the evolution of the e(a) population,
we present an overview of the available observations of e(a)'s in different
environments both at low and high redshift. Finally, we discuss the role of
dust in determining the e(a) spectral properties and we propose a scenario of
selective obscuration in which the extinction decreases with the stellar age.Comment: 26 pages, Latex, including 7 postscript figures, accepted for
publication in the Astrophysical Journa
Validation of an electrogoniometry system as a measure of knee kinematics during activities of daily living
Purpose: The increasing use of electrogoniometry (ELG) in clinical research requires the validation of different instrumentation. The purpose of this investigation was to examine the concurrent validity of an ELG system during activities of daily living.
Methods: Ten asymptomatic participants gave informed consent to participate. A Biometrics SG150 electrogoniometer was directly compared to a 12 camera three dimensional motion analysis system during walking, stair ascent, stair descent, sit to stand, and stand to sit activities for the measurement of the right knee angle. Analysis of validity was undertaken by linear regression. Standard error of estimate (SEE), standardised SEE (SSEE), and Pearson’s correlation coefficient r were computed for paired trials between systems for each functional activity.
Results: The 95% confidence interval of SEE was reasonable between systems across walking (LCI = 2.43 °; UCI = 2.91 °), stair ascent (LCI = 2.09 °; UCI = 2.42 °), stair descent (LCI = 1.79 °; UCI = 2.10 °), sit to stand (LCI = 1.22 °; UCI = 1.41 °), and stand to sit (LCI = 1.17 °; UCI = 1.34 °). Pearson’s correlation coefficient r across walking (LCI = 0.983; UCI = 0.990), stair ascent (LCI = 0.995; UCI = 0.997), stair descent (LCI = 0.995; UCI = 0.997), sit to stand (LCI = 0.998; UCI = 0.999), and stand to sit (LCI = 0.996; UCI = 0.997) was indicative of a strong linear relationship between systems.
Conclusion: ELG is a valid method of measuring the knee angle during activities representative of daily living. The range is within that suggested to be acceptable for the clinical evaluation of patients with musculoskeletal conditions
Star formation around RCW 120, the perfect bubble
We take advantage of the very simple morphology of RCW 120 -- a perfect
bubble -- to understand the mechanisms triggering star formation around an HII
region and to establish what kind of stars are formed there. We present 870
microns observations of RCW 120, obtained with the APEX-LABOCA camera. These
show the distribution of cold dust, and thus of neutral material. We use
Spitzer-MIPS observations at 24 and 70 microns to detect the young stellar
objects (YSOs) present in this region and to estimate their evolutionary
stages.
A layer of dense neutral material surrounds the HII region, having been swept
up during the region's expansion. This layer has a mass greater than 2000 solar
masses and is fragmented, with massive fragments elongated along the ionization
front (IF). We measured the 24 microns flux of 138 sources. Of these, 39 are
Class I or flat-spectrum YSOs observed in the direction of the collected layer.
We show that several triggering mechanisms are acting simultaneously in the
swept-up shell, where they form a second generation of stars. No massive YSOs
are detected. However, a massive, compact 870 microns core lies adjacent to the
IF. A 70 microns source with no 24 microns counterpart is detected at the same
position. This source is a likely candidate for a Class 0 YSO. Also at 24
microns, we detect a chain of about ten regularly spaced Class I or flat
spectrum sources, parallel to the IF, in the direction of the most massive
fragment. We suggest that the formation of these YSOs is the result of Jeans
gravitational instabilities in the collected layer. Finally, the 870 microns
emission, the 24 microns emission, and the Halpha emission show the existence
of an extended and partially ionized photodissociation region around RCW 120.Comment: 14 pages, 17 figure
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