15 research outputs found
General Polytropic Magnetofluid under Self-Gravity: Voids and Shocks
We study the self-similar magnetohydrodynamics (MHD) of a quasi-spherical
expanding void (viz. cavity or bubble) in the centre of a self-gravitating gas
sphere with a general polytropic equation of state. We show various analytic
asymptotic solutions near the void boundary in different parameter regimes and
obtain the corresponding void solutions by extensive numerical explorations. We
find novel void solutions of zero density on the void boundary. These new void
solutions exist only in a general polytropic gas and feature shell-type density
profiles. These void solutions, if not encountering the magnetosonic critical
curve (MCC), generally approach the asymptotic expansion solution far from the
central void with a velocity proportional to radial distance. We identify and
examine free-expansion solutions, Einstein-de Sitter expansion solutions, and
thermal-expansion solutions in three different parameter regimes. Under certain
conditions, void solutions may cross the MCC either smoothly or by MHD shocks,
and then merge into asymptotic solutions with finite velocity and density far
from the centre. Our general polytropic MHD void solutions provide physical
insight for void evolution, and may have astrophysical applications such as
massive star collapses and explosions, shell-type supernova remnants and hot
bubbles in the interstellar and intergalactic media, and planetary nebulae.Comment: 21 pages, 15 figures, accepted for publication on New Astronom
GS305+04-26:Revisiting the ISM around the CenOB1 stellar association
Massive stars deeply modify their surrounding ISM via their high throughput
of ionizing photons and their strong stellar winds. In this way they may create
large expanding structures of neutral gas. We study a new large HI shell,
labelled GS305+04-26, and its relationship with the OB association CenOB1. To
carry out this study we have used a multi-wavelenght approach. We analyze
neutral hydrogen (HI) line data retrieved from the Leiden-Argentina-Bonn (LAB)
survey, new spectroscopic optical observations obtained at CASLEO, and make use
of proper motion databases available via Internet. The analysis of the HI data
reveals a large expanding structure GS305+04-26 centered at
(l,b)=(305^{\degr}, +4^{\degr}) in the velocity range from -33 to -17 km/s.
Based on its central velocity, -26 km/s, and using standard galactic rotation
models, a distance of 2.5(+-)0.9 kpc is inferred. This structure, elliptical in
shape, has major and minor axis of 440 and 270 pc, respectively. Its expansion
velocity, total gaseous mass, and kinetic energy are ~8 km/s, (2.4(+-)0.5)x10^5
Mo, and (1.6(+-)0.4)x10^{50} erg, respectively. Several stars of the
OB-association CenOB1 are seen projected onto, and within, the boundaries of
GS305+04-26. Based on an analysis of proper motions, new members of CenOB1 are
identified. The mechanical energy injected by these stars could have been the
origin of this HI structure.Comment: 14 pages, 6 figures, A&A (in press
HI Narrow Line Absorption in Dark Clouds
We have used the Arecibo telescope to carry out an survey of 31 dark clouds
in the Taurus/Perseus region for narrow absorption features in HI (
21cm) and OH (1667 and 1665 MHz) emission. We detected HI narrow
self--absorption (HINSA) in 77% of the clouds that we observed. HINSA and OH
emission, observed simultaneously are remarkably well correlated. Spectrally,
they have the same nonthermal line width and the same line centroid velocity.
Spatially, they both peak at the optically--selected central position of each
cloud, and both fall off toward the cloud edges. Sources with clear HINSA
feature have also been observed in transitions of CO, \13co, \c18o, and CI.
HINSA exhibits better correlation with molecular tracers than with CI.
The line width of the absorption feature, together with analyses of the
relevant radiative transfer provide upper limits to the kinetic temperature of
the gas producing the HINSA. Some sources must have a temperature close to or
lower than 10 K. The correlation of column densities and line widths of HINSA
with those characteristics of molecular tracers suggest that a significant
fraction of the atomic hydrogen is located in the cold, well--shielded portions
of molecular clouds, and is mixed with the molecular gas.
The average number density ratio [HI]/[\h2] is . The
inferred HI density appears consistent with but is slightly higher than the
value expected in steady state equilibrium between formation of HI via cosmic
ray destruction of H and destruction via formation of H on grain
surfaces. The distribution and abundance of atomic hydrogen in molecular clouds
is a critical test of dark cloud chemistry and structure, including the issues
of grain surface reaction rates, PDRs, circulation, and turbulent diffusion.Comment: 40 pages, 10 figures, accepted by Ap
A formation scenario of young stellar groups in the region of the Scorpio Centaurus OB association
The main objective of this work is to investigate the role played by Lower
Centaurus Crux (LCC) and Upper Centaurus Lupus (UCL), both subcomponents of the
Scorpio Centaurus OB association (Sco-Cen), in the formation of the groups beta
Pictoris, TW Hydrae and the eta Chamaeleontis cluster. The dynamical evolution
of all the stellar groups involved and of the bubbles and shells blown by LCC
and UCL are calculated and followed from the past to the present. This leads to
a formation scenario in which (1) the groups beta Pictoris, TW Hydrae were
formed in the wake of the shells created by LCC and UCL, (2) the young cluster
eta Chamaeleontis was born as a consequence of the collision of the shells of
LCC and UCL, and (3) the formation of Upper Scorpius (US), the other main
subcomponent of the Sco-Cen association, may have been started by the same
process that created eta Chamaeleontis
The Spitzer Survey of Interstellar Clouds in the Gould Belt. III. A Multi-Wavelength View of Corona Australis
We present Spitzer Space Telescope IRAC and MIPS observations of a 0.85 deg^2
field including the Corona Australis (CrA) star-forming region. At a distance
of 130 pc, CrA is one of the closest regions known to be actively forming
stars, particularly within its embedded association, the Coronet. Using the
Spitzer data, we identify 51 young stellar objects (YSOs) in CrA which include
sources in the well-studied Coronet cluster as well as distributed throughout
the molecular cloud. Twelve of the YSOs discussed are new candidates, one of
which is located in the Coronet. Known YSOs retrieved from the literature are
also added to the list, and a total of 116 candidate YSOs in CrA are compiled.
Based on these YSO candidates, the star formation rate is computed to be 12 M_o
Myr^-1, similar to that of the Lupus clouds. A clustering analysis was also
performed, finding that the main cluster core, consisting of 68 members, is
elongated (having an aspect ratio of 2.36), with a circular radius of 0.59 pc
and mean surface density of 150 pc^-2.
In addition, we analyze outflows and jets in CrA by means of new CO and H_2
data. We present 1.3 mm interferometric continuum observations made with the
Submillimeter Array (SMA) covering R CrA, IRS 5, IRS 7, and IRAS 18595-3712
(IRAS 32). We also present multi-epoch H_2 maps and detect jets and outflows,
study their proper motions, and identify exciting sources. The Spitzer and
ISAAC/VLT observations of IRAS 32 show a bipolar precessing jet, which drives a
CO (2-1) outflow detected in the SMA observations. There is also clear evidence
for a parsec-scale precessing outflow, E-W oriented, and originating in the SMA
2 region, likely driven by SMA 2 or IRS 7A.Comment: Accepted for publication in ApJS. 112 pages, 42 figures (quality
reduced), 13 tables. Full resolution version can be found at
http://www.cfa.harvard.edu/~dpeterson/CrA/CrA_highres.pd
Physical Conditions in the Foreground Gas of Reflection Nebulae: NGC 2023, vdB 102, and NGC 7023
High resolution optical spectra of HD 37903 and HD 147009, which illuminate
the reflection nebulae, NGC 2023 and vdB 102, were obtained for comparison with
our results for HD 200775 and NGC 7023. Ground-based measurements of the
molecules, CH, C, and CN, and the atoms, Na I and K I, were analyzed to
extract physical conditions in the foreground cloud. Estimates of the gas
density, gas temperature and flux of ultraviolet radiation were derived and
were compared with the results from infrared and radio studies of the main
molecular cloud. The conditions are similar to those found in studies of
diffuse clouds. The foreground material is less dense than the gas in the
molecular cloud behind the star(s). The gas temperature was set at 40 K, the
temperature determined for the foreground gas in NGC 7023. The flux of
ultraviolet radiation was found to be less intense than in the molecular
material behind the star(s). The column densities of Na I and K I were
reproduced reasonably well when the extinction curve for the specific line of
sight was adopted. We obtained NEWSIPS data from the IUE archive for HD 37903
and HD 200775. The ultraviolet data on C I and CO allow extraction of the
physical conditions by alternate methods. General agreement among the various
diagnostics was found, leading to self-consistent pictures of the foreground
photodissociation regions. An Appendix describes checks on the usefulness of
IUE NEWSIPS data for interstellar studies. (Abridged)Comment: 65 pages, 18 tables, 14 figures, Accepted for publication in ApJ