598 research outputs found
A Universal Stellar Initial Mass Function? A Critical Look at Variations
Few topics in astronomy initiate such vigorous discussion as whether or not
the initial mass function (IMF) of stars is universal, or instead sensitive to
the initial conditions of star formation. The distinction is of critical
importance: the IMF influences most of the observable properties of stellar
populations and galaxies, and detecting variations in the IMF could provide
deep insights into the process by which stars form. In this review, we take a
critical look at the case for IMF variations, with a view towards whether other
explanations are sufficient given the evidence. Studies of the field, local
young clusters and associations, and old globular clusters suggest that the
vast majority were drawn from a "universal" IMF: a power-law of Salpeter index
() above a few solar masses, and a log normal or shallower
power-law () between a few tenths and a few solar masses
(ignoring the effects of unresolved binaries). The shape and universality of
the IMF at the stellar-substellar boundary is still under investigation and
uncertainties remain large, but most observations are consistent with a IMF
that declines () well below the hydrogen burning limit.
Observations of resolved stellar populations and the integrated properties of
most galaxies are also consistent with a "universal IMF", suggesting no gross
variations in the IMF over much of cosmic time. There are indications of
"non-standard" IMFs in specific local and extragalactic environments, which
clearly warrant further study. Nonetheless, there is no clear evidence that the
IMF varies strongly and systematically as a function of initial conditions
after the first few generations of stars.Comment: 49 pages, 5 figures, to appear in Annual Reviews of Astronomy and
Astrophysics (2010, volume 48
Colloidal Gels: Equilibrium and Non-Equilibrium Routes
We attempt a classification of different colloidal gels based on
colloid-colloid interactions. We discriminate primarily between non-equilibrium
and equilibrium routes to gelation, the former case being slaved to
thermodynamic phase separation while the latter is individuated in the
framework of competing interactions and of patchy colloids. Emphasis is put on
recent numerical simulations of colloidal gelation and their connection to
experiments. Finally we underline typical signatures of different gel types, to
be looked in more details in experiments.Comment: topical review, accepted in J. Phys. Condens. Matte
The intermediate-mass star-forming region Lynds 1340. An optical view
We have performed an optical spectroscopic and photometric search for young
stellar objects associated with the molecular cloud Lynds 1340, and examined
the structure of the cloud by constructing an extinction map, based on SDSS
data. The new extinction map suggests a shallow, strongly fragmented cloud,
having a mass of some 3700~Msun. Longslit spectroscopic observations of the
brightest stars over the area of L1340 revealed that the most massive star
associated with L1340 is a B4 type, about 5 solar mass star. The new
spectroscopic and photometric data of the intermediate mass members led to a
revised distance of 825 (+110 /-80) pc, and revealed seven members of the young
stellar population with M > 2 solar masses. Our search for H alpha emission
line stars, conducted with the Wide Field Grism Spectrograph 2 on the 2.2-meter
telescope of the University of Hawaii and covering a 30 arcmin x 40 arcmin
area, resulted in the detection of 75 candidate low-mass pre-main sequence
stars, 58 of which are new. We constructed spectral energy distributions of our
target stars, based on SDSS, 2MASS, Spitzer, and WISE photometric data, derived
their spectral types, extinctions, and luminosities from BVRIJ fluxes,
estimated masses by means of pre-main sequence evolutionary models, and
examined the disk properties utilizing the 2-24 micron interval of the spectral
energy distribution. We measured the equivalent width of the H alpha lines and
derived accretion rates. The optically selected sample of pre-main sequence
stars has a median effective temperature of 3970 K, stellar mass 0.7 Msun, and
accretion rate of 7.6 10^{-9} Msun/yr.Comment: 47 pages, 15 figures, 10 tables; accepted for publication in ApJ,
typos correcte
Proper Motions of the HH 47 Jet Observed with the Hubble Space Telescope
We present a proper motion study of the shock waves within the classic
stellar jet HH 47 based on Hubble Space Telescope H-alpha and [S II] images of
the region taken over two epochs. Individual knots within the jet and in the
bow shock/Mach disk working surface of HH 47A move significantly in the five
years that separate the images, and the excellent spatial resolution of HST
makes it possible to measure the proper motions with enough precision to easily
observe differential motions throughout the flow. The bright portion of the jet
emerges at 37.5 +/- 2.5 degrees from the plane of the sky with an average
velocity of 300 km/s. Dynamical ages of the shock waves in the jet range from a
few decades for knots recently ejected by the source to ~ 1300 years for the
faint extended bow shock HH 47D. The jet curves, but motions of knots in the
jet are directed radially away from the exciting source, and velocity
variability in the flow drives the shock waves that heat the jet internally.
The jet orientation angle varies with time by about 15 degrees, and currently
points to the northwestern portion of a cavity outlined by a reflection nebula,
where a quasi-stationary shock deflects the jet. The major working surface HH
47A is more complex than a simple bow shock/Mach disk, and contains numerous
clumps that move relative to one another with velocities of ~ +/- 40 km/s.
Small clumps or instabilities affect the Mach disk, and dense clumps may move
all the way through the working surface to cause the bumpy morphology seen at
the bow shock. A localized area between the bow shock and Mach disk varies
significantly between the two sets of images.Comment: full resolution figures available at
http://sparky.rice.edu/~hartigan/pub.html; Accepted for publication in the
Astronomical Journa
The Stability of Radiatively Cooled Jets in Three Dimensions
The effect of optically thin radiative cooling on the Kelvin-Helmholtz
instability of three dimensional jets is investigated via linear stability
theory and nonlinear hydrodynamical simulation. Two different cooling functions
are considered: radiative cooling is found to have a significant effect on the
stability of the jet in each case. The wavelengths and growth rates of unstable
modes in the numerical simulations are found to be in good agreement with
theoretical predictions. Disruption of the jet is found to be sensitive to the
precessional frequency at the origin with lower frequencies leading to more
rapid disruption. Strong nonlinear effects are observed as the result of the
large number of normal modes in three dimensions which provide rich mode-mode
interactions. These mode-mode interactions provide new mechanisms for the
formation of knots in the flows. Significant structural features found in the
numerical simulations appear similar to structures observed on protostellar
jets.Comment: 32 pages, 13 figures, figures included in page tota
Recipes for stellar jets: results of combined optical/infrared diagnostics
We examine the conditions of the plasma along a sample of 'classical'
Herbig-Haro jets located in the Orion and Vela star forming regions, through
combined optical-infrared spectral diagnostics. Our sample includes HH 111, HH
34, HH 83, HH 73, HH 24 C/E, HH 24 J, observed at moderate spatial/spectral
resolution. The obtained spectra cover a wide wavelength range from 0.6-2.5 um,
including many transitions from regions of different excitation conditions.
This allows us to probe the density and temperature stratification which
characterises the cooling zones behind the shock fronts along the jet. The
derived physical parameters (such as the extinction, the electron density and
temperature, the ionisation fraction, and the total density) are used to
estimate the depletion onto dust grains of Calcium and Iron with respect to
solar abundances. This turns out to be between 70% and 0% for Ca and ~90% for
Fe, suggesting that the weak shocks present in the beams are not capable of
completely destroying the dust grains. We then derive the mass flux rates
(Mdot_jet is on average 5 10^-8 M_solar yr^-1) and the associated linear
momentum fluxes. The latter are higher than, or of the same order as, those
measured in the coaxial molecular flows, suggesting that the flows are jet
driven. Finally, we discuss differences between jets in our sample.Comment: 19 pages, 15 figures, accepted by A&
A pair of gigantic bipolar dust jets close to the solar system
We report the discovery of two adjacent jet candidates with a length of about
9 degrees each -- 10 times longer than the largest known jets -- detected by us
on 60 and 100 micron IRAS maps, but not observed at any other wavelength. They
are extremely collimated (length-to-width ratios 20--50), curved, knotty, and
end in prominent bubbles. Their dust temperatures are 25 K and 30 K,
respectively. Both harbour faint stars, one having a high proper motion (0.23
arcsec/yr) and being very red, suggesting a distance of about 60 pc. At this
distance, the total mass of both jet candidates is about about 1 solar mass. We
suspect that these gigantic (9 pc length respectively) jets are of fossil type
and have a common origin, due to the decay of a system of evolved stars. They
are the first examples of jets radiating in the far IR and might, because of
their closeness, be of interest for further studies of the acceleration and
collimation processes of astrophysical jets.Comment: 4 pages, 4 figures in reduced quality, accepted by Astronomy &
Astrophysics (Letter) february 10, 2004. See
http://astro.uibk.ac.at/dustjets/ for the full resolution and color version
of the image
Near infrared imaging of the cometary globule CG12
Cometary globule 12 is a relatively little investigated medium- and low mass
star forming region 210 pc above the Galactic plane. NIR J, H, and Ks imaging
and stellar photometry is used to analyse the stellar content and the structure
of CG 12. Several new members and member candidates of the CG 12 stellar
cluster were found. The new members include in particular a highly embedded
source with a circumstellar disk or shell and a variable star with a
circumstellar disk which forms a binary with a previously known A spectral type
cluster member. The central source of the known collimated molecular outflow in
CG 12 and an associated "hourglass"-shaped object due to reflected light from
the source were also detected. HIRES-enhanced IRAS images are used together
with SOFI J,H,Ks imaging to study the two associated IRAS point sources,
13546-3941 and 13547-3944. Two new 12 micrometer sources coinciding with NIR
excess stars were detected in the direction of IRAS 13546-3941. The IRAS
13547-3944 emission at 12 and 25 micrometers originates in the Herbig AeBe star
h4636n and the 60 and 100 micrometer emission from an adjacent cold source.Comment: Accepted Astronomy and Astrophysics Oct. 15 200
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