48 research outputs found
Logarithmically Slow Expansion of Hot Bubbles in Gases
We report logarithmically slow expansion of hot bubbles in gases in the
process of cooling. A model problem first solved, when the temperature has
compact support. Then temperature profile decaying exponentially at large
distances is considered. The periphery of the bubble is shown to remain
essentially static ("glassy") in the process of cooling until it is taken over
by a logarithmically slowly expanding "core". An analytical solution to the
problem is obtained by matched asymptotic expansion. This problem gives an
example of how logarithmic corrections enter dynamic scaling.Comment: 4 pages, 1 figur
Thermal Instability-Induced Interstellar Turbulence
We study the dynamics of phase transitions in the interstellar medium by
means of three-dimensional hydrodynamic numerical simulations. We use a
realistic cooling function and generic nonequilibrium initial conditions to
follow the formation history of a multiphase medium in detail in the absence of
gravity. We outline a number of qualitatively distinct stages of this process,
including a linear isobaric evolution, transition to an isochoric regime,
formation of filaments and voids (also known as "thermal" pancakes), the
development and decay of supersonic turbulence, an approach to pressure
equilibrium, and final relaxation of the multiphase medium. We find that 1%-2%
of the initial thermal energy is converted into gas motions in one cooling
time. The velocity field then randomizes into turbulence that decays on a
dynamical timescale E_k ~ t^-n, 1 < n < 2. While not all initial conditions
yield a stable two-phase medium, we examine such a case in detail. We find that
the two phases are well mixed with the cold clouds possessing a fine-grained
structure near our numerical resolution limit. The amount of gas in the
intermediate unstable phase roughly tracks the rms turbulent Mach number,
peaking at 25% when M_rms ~ 8, decreasing to 11% when M_rms ~ 0.4.Comment: To appear in the ApJ Letters, April 2002; 5 pages, 3 color figures,
mpeg animations available at http://akpc.ucsd.edu/ThermalLetter/thermal.htm
Superbubble evolution including the star-forming clouds: Is it possible to reconcile LMC observations with model predictions?
Here we present a possible solution to the apparent discrepancy between the
observed properties of LMC bubbles and the standard, constant density bubble
model. A two-dimensional model of a wind-driven bubble expanding from a
flattened giant molecular cloud is examined. We conclude that the expansion
velocities derived from spherically symmetric models are not always applicable
to elongated young bubbles seen almost face-on due to the LMC orientation. In
addition, an observational test to differentiate between spherical and
elongated bubbles seen face-on is discussed.Comment: 25 pages, 7 figures, accepted to ApJ (September, 1999 issue
Ten Million Degree Gas in M 17 and the Rosette Nebula: X-ray Flows in Galactic H II Regions
We present the first high-spatial-resolution X-ray images of two high-mass
star forming regions, the Omega Nebula (M 17) and the Rosette Nebula (NGC
2237--2246), obtained with the Chandra X-ray Observatory Advanced CCD Imaging
Spectrometer (ACIS) instrument. The massive clusters powering these H II
regions are resolved at the arcsecond level into >900 (M 17) and >300 (Rosette)
stellar sources similar to those seen in closer young stellar clusters.
However, we also detect soft diffuse X-ray emission on parsec scales that is
spatially and spectrally distinct from the point source population. The diffuse
emission has luminosity L_x ~ 3.4e33 ergs/s in M~17 with plasma energy
components at kT ~0.13 and ~0.6 keV (1.5 and 7 MK), while in Rosette it has L_x
\~6e32 ergs/s with plasma energy components at kT ~0.06 and ~0.8 keV (0.7 and 9
MK). This extended emission most likely arises from the fast O-star winds
thermalized either by wind-wind collisions or by a termination shock against
the surrounding media. We establish that only a small portion of the wind
energy and mass appears in the observed diffuse X-ray plasma; in these blister
H II regions, we suspect that most of it flows without cooling into the
low-density interstellar medium. These data provide compelling observational
evidence that strong wind shocks are present in H II regions.Comment: 35 pages, including 11 figures; to appear in ApJ, August 20, 2003. A
version with high-resolution figures is available at
ftp://ftp.astro.psu.edu/pub/townsley/diffuse.ps.g
Dynamic Evolution Model of Isothermal Voids and Shocks
We explore self-similar hydrodynamic evolution of central voids embedded in
an isothermal gas of spherical symmetry under the self-gravity. More
specifically, we study voids expanding at constant radial speeds in an
isothermal gas and construct all types of possible void solutions without or
with shocks in surrounding envelopes. We examine properties of void boundaries
and outer envelopes. Voids without shocks are all bounded by overdense shells
and either inflows or outflows in the outer envelope may occur. These
solutions, referred to as type void solutions, are further
divided into subtypes and
according to their characteristic behaviours across the sonic critical line
(SCL). Void solutions with shocks in envelopes are referred to as type
voids and can have both dense and quasi-smooth edges.
Asymptotically, outflows, breezes, inflows, accretions and static outer
envelopes may all surround such type voids. Both cases of
constant and varying temperatures across isothermal shock fronts are analyzed;
they are referred to as types and
void shock solutions. We apply the `phase net matching procedure' to construct
various self-similar void solutions. We also present analysis on void
generation mechanisms and describe several astrophysical applications. By
including self-gravity, gas pressure and shocks, our isothermal self-similar
void (ISSV) model is adaptable to various astrophysical systems such as
planetary nebulae, hot bubbles and superbubbles in the interstellar medium as
well as supernova remnants.Comment: 24 pages, 13 figuers, accepted by ApS
Formation of Multiple Populations in Globular Clusters: Another Possible Scenario
While chemical composition spreads are now believed to be a universal
characteristic of globular clusters (GCs), not all of them present multiple
populations in their color-magnitude diagrams (CMDs). Here we present a new
scenario for the formation of GCs, in an attempt to qualitatively explain this
otherwise intriguing observational framework. Our scenario divides GCs into
three groups, depending on the initial mass (M_I) of the progenitor structure
(PS), as follows. i) Massive PSs can retain the gas ejected by massive stars,
including the ejecta of core-collapse SNe. ii) Intermediate-mass PSs can retain
at least a fraction of the fast winds of massive stars, but none of the
core-collapse SNe ejecta. iii) Low-mass PSs can only retain the slow winds of
intermediate-mass stars. Members of the first group would include omega
Centauri (NGC 5139), M54 (NGC 6715), M22 (NGC 6656), and Terzan 5, whereas NGC
2808 (and possibly NGC 2419) would be members of the second group. The
remaining GCs which only present a spread in light elements, such as O and Na,
would be members of the third group. According to our scenario, the different
components in omega Cen should not display a sizeable spread in age. We argue
that this is consistent with the available observations. We give other simple
arguments in favor of our scenario, which can be described in terms of two main
analytical relations: i) Between the actual observed ratio between first and
second generation stars (R_SG^FG) and the fraction of first generation stars
that have been lost by the GC (S_L); and ii) Between S_L and M_I. We also
suggest a series of future improvements and empirical tests that may help
decide whether the proposed scenario properly describes the chemical evolution
of GCs.Comment: Accepted for publication in Astronomy and Astrophysic
Physics of Solar Prominences: II - Magnetic Structure and Dynamics
Observations and models of solar prominences are reviewed. We focus on
non-eruptive prominences, and describe recent progress in four areas of
prominence research: (1) magnetic structure deduced from observations and
models, (2) the dynamics of prominence plasmas (formation and flows), (3)
Magneto-hydrodynamic (MHD) waves in prominences and (4) the formation and
large-scale patterns of the filament channels in which prominences are located.
Finally, several outstanding issues in prominence research are discussed, along
with observations and models required to resolve them.Comment: 75 pages, 31 pictures, review pape
Control of star formation by supersonic turbulence
Understanding the formation of stars in galaxies is central to much of modern
astrophysics. For several decades it has been thought that stellar birth is
primarily controlled by the interplay between gravity and magnetostatic
support, modulated by ambipolar diffusion. Recently, however, both
observational and numerical work has begun to suggest that support by
supersonic turbulence rather than magnetic fields controls star formation. In
this review we outline a new theory of star formation relying on the control by
turbulence. We demonstrate that although supersonic turbulence can provide
global support, it nevertheless produces density enhancements that allow local
collapse. Inefficient, isolated star formation is a hallmark of turbulent
support, while efficient, clustered star formation occurs in its absence. The
consequences of this theory are then explored for both local star formation and
galactic scale star formation. (ABSTRACT ABBREVIATED)Comment: Invited review for "Reviews of Modern Physics", 87 pages including 28
figures, in pres
Wind-Blown Bubbles around Evolved Stars
Most stars will experience episodes of substantial mass loss at some point in
their lives. For very massive stars, mass loss dominates their evolution,
although the mass loss rates are not known exactly, particularly once the star
has left the main sequence. Direct observations of the stellar winds of massive
stars can give information on the current mass-loss rates, while studies of the
ring nebulae and HI shells that surround many Wolf-Rayet (WR) and luminous blue
variable (LBV) stars provide information on the previous mass-loss history. The
evolution of the most massive stars, (M > 25 solar masses), essentially follows
the sequence O star to LBV or red supergiant (RSG) to WR star to supernova. For
stars of mass less than 25 solar masses there is no final WR stage. During the
main sequence and WR stages, the mass loss takes the form of highly supersonic
stellar winds, which blow bubbles in the interstellar and circumstellar medium.
In this way, the mechanical luminosity of the stellar wind is converted into
kinetic energy of the swept-up ambient material, which is important for the
dynamics of the interstellar medium. In this review article, analytic and
numerical models are used to describe the hydrodynamics and energetics of
wind-blown bubbles. A brief review of observations of bubbles is given, and the
degree to which theory is supported by observations is discussed.Comment: To be published as a chapter in 'Diffuse Matter from Star Forming
Regions to Active Galaxies' - A volume Honouring John Dyson. Eds. T. W.
Harquist, J. M. Pittard and S. A. E. G. Falle. 22 pages, 12 figure
Limb Spicules from the Ground and from Space
We amassed statistics for quiet-sun chromosphere spicules at the limb using
ground-based observations from the Swedish 1-m Solar Telescope on La Palma and
simultaneously from NASA's Transition Region and Coronal Explorer (TRACE)
spacecraft. The observations were obtained in July 2006. With the 0.2 arcsecond
resolution obtained after maximizing the ground-based resolution with the
Multi-Object Multi-Frame Blind Deconvolution (MOMFBD) program, we obtained
specific statistics for sizes and motions of over two dozen individual
spicules, based on movies compiled at 50-second cadence for the series of five
wavelengths observed in a very narrow band at H-alpha, on-band and in the red
and blue wings at 0.035 nm and 0.070 nm (10 s at each wavelength) using the
SOUP filter, and had simultaneous observations in the 160 nm EUV continuum from
TRACE. The MOMFBD restoration also automatically aligned the images,
facilitating the making of Dopplergrams at each off-band pair. We studied 40
H-alpha spicules, and 14 EUV spicules that overlapped H-alpha spicules; we
found that their dynamical and morphological properties fit into the framework
of several previous studies. From a preliminary comparison with spicule
theories, our observations are consistent with a reconnection mechanism for
spicule generation, and with UV spicules being a sheath region surrounding the
H-alpha spicules