266 research outputs found
Kelvin-Helmholtz Instability in a Weakly Ionized Medium
Ambient interstellar material may become entrained in outflows from massive
stars as a result of shear flow instabilities. We study the linear theory of
the Kelvin - Helmholtz instability, the simplest example of shear flow
instability, in a partially ionized medium. We model the interaction as a two
fluid system (charged and neutral) in a planar geometry. Our principal result
is that for much of the relevant parameter space, neutrals and ions are
sufficiently decoupled that the neutrals are unstable while the ions are held
in place by the magnetic field. Thus, we predict that there should be a
detectably narrower line profile in ionized species tracing the outflow
compared with neutral species since ionized species are not participating in
the turbulent interface with the ambient ISM. Since the magnetic field is
frozen to the plasma, it is not tangled by the turbulence in the boundary
layer.Comment: 21 pages, 4 figure
Cooling, Gravity and Geometry: Flow-driven Massive Core Formation
We study numerically the formation of molecular clouds in large-scale
colliding flows including self-gravity. The models emphasize the competition
between the effects of gravity on global and local scales in an isolated cloud.
Global gravity builds up large-scale filaments, while local gravity --
triggered by a combination of strong thermal and dynamical instabilities --
causes cores to form. The dynamical instabilities give rise to a local focusing
of the colliding flows, facilitating the rapid formation of massive
protostellar cores of a few 100 M. The forming clouds do not reach an
equilibrium state, though the motions within the clouds appear comparable to
``virial''. The self-similar core mass distributions derived from models with
and without self-gravity indicate that the core mass distribution is set very
early on during the cloud formation process, predominantly by a combination of
thermal and dynamical instabilities rather than by self-gravity.Comment: 13 pages, 12 figures, accepted by Ap
Space Velocities of Southern Globular Clusters. IV. First Results for Inner-Galaxy Clusters
We have measured the absolute proper motions of four low-latitude,
inner-Galaxy globular clusters. These clusters are: NGC 6266 (M62), NGC 6304,
NGC 6316 and NGC 6723. The proper motions are on the Hipparcos system, as no
background extragalactic objects are found in these high-extinction regions.
The proper-motion uncertainties range between 0.3 and 0.6 mas/yr.
We discuss the kinematics of these clusters and of three additional bulge
clusters -- NGC 6522, NGC 6528 and NFC 6553 -- whose proper motions with
respect to bulge stars had been determined previously. We find that all of the
clusters have velocities that confine them to the bulge region. Of the three
metal poor clusters ([Fe/H] < -1.0), NGC 6522, and NGC 6723 have kinematics
consistent with halo membership. The third cluster, NGC 6266 however, appears
to belong to a rotationally-supported system. Of the four metal rich clusters
([Fe/H] >= -1.0), NGC 6304 and NGC 6553 also have kinematics consistent with
membership to a rotationally-supported system. NGC 6528 has kinematics,
metallicity and mass that argue in favor of a genuine Milky-Way bar cluster.
NGC 6316's kinematics indicate membership to a hotter system than the bar.Comment: 4 figures, 5 tables; accepted for publication in A
Age Constraints for an M31 Globular Cluster from Main Sequence Photometry
We present a color-magnitude diagram (CMD) of the globular cluster SKHB-312
in the Andromeda galaxy (M31), obtained with the Advanced Camera for Surveys on
the Hubble Space Telescope. The cluster was included in deep observations taken
to measure the star formation history of the M31 halo. Overcoming a very
crowded field, our photometry of SKHB-312 reaches V ~ 30.5 mag, more than 1 mag
below the main sequence turnoff. These are the first observations to allow a
direct age estimate from the turnoff in an old M31 cluster. We analyze its CMD
and luminosity function using a finely-spaced grid of isochrones that have been
calibrated using observations of Galactic clusters taken with the same camera
and filters. The luminosity difference between the subgiant and horizontal
branches is ~0.2 mag smaller in SKHB-312 than in the Galactic clusters 47 Tuc
and NGC 5927, implying SKHB-312 is 2-3 Gyr younger. A quantitative comparison
to isochrones yields an age of 10 +2.5/-1 Gyr
Effects of Non-Circular Motions on Azimuthal Color Gradients
Assuming that density waves trigger star formation, and that young stars
preserve the velocity components of the molecular gas where they are born, we
analyze the effects that non-circular gas orbits have on color gradients across
spiral arms. We try two approaches, one involving semi-analytical solutions for
spiral shocks, and another with magnetohydrodynamic (MHD) numerical simulation
data. We find that, if non-circular motions are ignored, the comparison between
observed color gradients and stellar population synthesis models would in
principle yield pattern speed values that are systematically too high for
regions inside corotation, with the difference between the real and the
measured pattern speeds increasing with decreasing radius. On the other hand,
image processing and pixel averaging result in systematically lower measured
spiral pattern speed values, regardless of the kinematics of stellar orbits.
The net effect is that roughly the correct pattern speeds are recovered,
although the trend of higher measured at lower radii (as expected
when non-circular motions exist but are neglected) should still be observed. We
examine the Martinez-Garcia et al. (2009) photometric data and confirm that
this is indeed the case. The comparison of the size of the systematic pattern
speed offset in the data with the predictions of the semi-analytical and MHD
models corroborates that spirals are more likely to end at Outer Lindblad
Resonance, as these authors had already found.Comment: 32 pages, 15 figures, accepted to Ap
An ammonia spectral map of the L1495-B218 filaments in the Taurus molecular cloud. I. Physical properties of filaments and dense cores
We present deep NH3 observations of the L1495-B218 filaments in the Taurus molecular cloud covering over a 3° angular range using the K-band focal plane array on the 100 m Green Bank Telescope. The L1495-B218 filaments form an interconnected, nearby, large complex extending over 8 pc. We observed NH3 (1, 1) and (2, 2) with a spectral resolution of 0.038 km s−1 and a spatial resolution of 31''. Most of the ammonia peaks coincide with intensity peaks in dust continuum maps at 350 and 500 μm. We deduced physical properties by fitting a model to the observed spectra. We find gas kinetic temperatures of 8–15 K, velocity dispersions of 0.05–0.25 km s−1, and NH3 column densities of 5 × 1012 to 1 × 1014 cm−2. The CSAR algorithm, which is a hybrid of seeded-watershed and binary dendrogram algorithms, identifies a total of 55 NH3 structures, including 39 leaves and 16 branches. The masses of the NH3 sources range from 0.05 to 9.5 . The masses of NH3 leaves are mostly smaller than their corresponding virial mass estimated from their internal and gravitational energies, which suggests that these leaves are gravitationally unbound structures. Nine out of 39 NH3 leaves are gravitationally bound, and seven out of nine gravitationally bound NH3 leaves are associated with star formation. We also found that 12 out of 30 gravitationally unbound leaves are pressure confined. Our data suggest that a dense core may form as a pressure-confined structure, evolve to a gravitationally bound core, and undergo collapse to form a protostar
Giant Molecular clouds: what are they made from, and how do they get there?
We analyse the results of four simulations of isolated galaxies: two with a
rigid spiral potential of fixed pattern speed, but with different degrees of
star-formation induced feedback, one with an axisymmetric galactic potential
and one with a `live' self-gravitating stellar component. Since we use a
Lagrangian method we are able to select gas that lies within giant molecular
clouds (GMCs) at a particular timeframe, and to then study the properties of
this gas at earlier and later times. We find that gas which forms GMCs is not
typical of the interstellar medium at least 50 Myr before the clouds form and
reaches mean densities within an order of magnitude of mean cloud densities by
around 10 Myr before. The gas in GMCs takes at least 50 Myr to return to
typical ISM gas after dispersal by stellar feedback, and in some cases the gas
is never fully recycled. We also present a study of the two-dimensional,
vertically-averaged velocity fields within the ISM. We show that the velocity
fields corresponding to the shortest timescales (that is, those timescales
closest to the immediate formation and dissipation of the clouds) can be
readily understood in terms of the various cloud formation and dissipation
mechanisms. Properties of the flow patterns can be used to distinguish the
processes which drive converging flows (e.g.\ spiral shocks, supernovae) and
thus molecular cloud formation, and we note that such properties may be
detectable with future observations of nearby galaxies.Comment: 13 pages, 8 figures, accepted for publication in MNRA
Turbulent Control of the Star Formation Efficiency
Supersonic turbulence plays a dual role in molecular clouds: On one hand, it
contributes to the global support of the clouds, while on the other it promotes
the formation of small-scale density fluctuations, identifiable with clumps and
cores. Within these, the local Jeans length \Ljc is reduced, and collapse
ensues if \Ljc becomes smaller than the clump size and the magnetic support
is insufficient (i.e., the core is ``magnetically supercritical''); otherwise,
the clumps do not collapse and are expected to re-expand and disperse on a few
free-fall times. This case may correspond to a fraction of the observed
starless cores. The star formation efficiency (SFE, the fraction of the cloud's
mass that ends up in collapsed objects) is smaller than unity because the mass
contained in collapsing clumps is smaller than the total cloud mass. However,
in non-magnetic numerical simulations with realistic Mach numbers and
turbulence driving scales, the SFE is still larger than observational
estimates. The presence of a magnetic field, even if magnetically
supercritical, appears to further reduce the SFE, but by reducing the
probability of core formation rather than by delaying the collapse of
individual cores, as was formerly thought. Precise quantification of these
effects as a function of global cloud parameters is still needed.Comment: Invited review for the conference "IMF@50: the Initial Mass Function
50 Years Later", to be published by Kluwer Academic Publishers, eds. E.
Corbelli, F. Palla, and H. Zinnecke
The Frequency of Mid-Infrared Excess Sources in Galactic Surveys
We have identified 230 Tycho-2 Spectral Catalog stars that exhibit 8 micron
mid-infrared extraphotospheric excesses in the MidCourse Space Experiment (MSX)
and Spitzer Space Telescope Galactic Legacy MidPlane Survey Extraordinaire
(GLIMPSE) surveys. Of these, 183 are either OB stars earlier than B8 in which
the excess plausibly arises from a thermal bremsstrahlung component or evolved
stars in which the excess may be explained by an atmospheric dust component.
The remaining 47 stars have spectral classifications B8 or later and appear to
be main sequence or late pre-main-sequence objects harboring circumstellar
disks. Six of the 47 stars exhibit multiple signatures characteristic of
pre-main-sequence circumstellar disks, including emission lines, near-infrared
K-band excesses, and X-ray emission. Approximately one-third of the remaining
41 sources have emission lines suggesting relative youth. Of the 25 GLIMPSE
stars with SST data at >24 microns, 20 also show an excess at 24 microns. Three
additional objects have 24 micron upper limits consistent with possible
excesses, and two objects have photospheric measurements at 24 microns. Six MSX
sources had a measurement at wavelengths >8 microns. We modeled the excesses in
26 stars having two or more measurements in excess of the expected photospheres
as single-component blackbodies. We determine probable disk temperatures and
fractional infrared luminosities in the range 191 < T < 787 and 3.9x10^-4 <
L_IR/L_* < 2.7x10^-1. We estimate a lower limit on the fraction of Tycho-2
Spectral Catalog main-sequence stars having mid-IR, but not near-IR, excesses
to be 1.0+-0.3%.Comment: Accepted to Ap
Diffusion in supersonic, turbulent, compressible flows
We investigate diffusion in supersonic, turbulent, compressible flows.
Supersonic turbulence can be characterized as network of interacting shocks. We
consider flows with different rms Mach numbers and where energy necessary to
maintain dynamical equilibrium is inserted at different spatial scales. We find
that turbulent transport exhibits super-diffusive behavior due to induced bulk
motions. In a comoving reference frame, however, diffusion behaves normal and
can be described by mixing length theory extended into the supersonic regime.Comment: 11 pages, incl. 5 figures, accepted for publication in Physical
Review E (a high-resolution version is available at
http://www.aip.de./~ralf/Publications/p21.abstract.html
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