786 research outputs found
A CS J = 2 1 survey of the galactic center region
A CS map of the galactic center region is presented consisting of 15,000 spectra covering -1 deg. less than 3. deg. 6 min., -0 deg.4 min. less than b less than 0 deg. 4 min., each having an rms noise of 0.15 K in 1 MHz filters. CS is a high-excitation molecule, meaning that it is excited into emission only when the ambient density is less than n much greater than or approx. 2 x 10 to the 4th power/cu cm CS emission in the inner 2 deg. of the galaxy is nearly as pervasive as CO emission, in stark contrast to the outer galaxy where CS emission is confined to cloud cores. Galactic center clouds are on average much more dense than outer Galaxy clouds. This can be understood as a necessary consequence of the strong tidal stresses in the inner galaxy
Mass return to the interstellar medium from highly-evolved carbon stars
Data produced by the Infrared Astronomy Satellite (IRAS) was surveyed at the mid- and far-infrared wavelengths. Visually-identified carbon stars in the 12/25/60 micron color-color diagram were plotted, along with the location of a number of mass-losing stars that lie near the location of the carbon stars, but are not carbon rich. The final sample consisted of 619 objects, which were estimated to be contaminated by 7 % noncarbon-rich objects. The mass return rate was estimated for all evolved circumstellar envelopes. The IRAS Point Source Catalog (PSC) was also searched for the entire class of stars with excess emission. Mass-loss rates, lifetimes, and birthrates for evolved stars were also estimated
Large Area Mapping at 850 Microns. V. Analysis of the Clump Distribution in the Orion A South Molecular Cloud
We present results from a 2300 arcmin^2 survey of the Orion A molecular cloud
at 450 and 850 micron using the Submillimetre Common-User Bolometer Array
(SCUBA) on the James Clerk Maxwell Telescope. The region mapped lies directly
south of the OMC1 cloud core and includes OMC4, OMC5, HH1/2, HH34, and L1641N.
We identify 71 independent clumps in the 850 micron map and compute size, flux,
and degree of central concentration in each. Comparison with isothermal,
pressure-confined, self-gravitating Bonnor-Ebert spheres implies that the
clumps have internal temperatures T_d ~ 22 +/- K and surface pressures log
(k^-1 P cm^-3 K) = 6.0 +/- 0.2. The clump masses span the range 0.3 - 22 Msun
assuming a dust temperature T_d ~ 20 K and a dust emissivity kappa_850 = 0.02
cm^2 g^-1. The distribution of clump masses is well characterized by a
power-law N(M) propto M^-alpha with alpha = 2.0 +/- 0.5 for M > 3.0 Msun,
indicating a clump mass function steeper than the stellar Initial Mass
Function. Significant incompleteness makes determination of the slope at lower
masses difficult. A comparison of the submillimeter emission map with an H_2
2.122 micron survey of the same region is performed. Several new Class 0
sources are revealed and a correlation is found between both the column density
and degree of concentration of the submillimeter sources and the likelihood of
coincident H_2 shock emission.Comment: 44 pages, 17 figures, accepted by Ap
A Disk Shadow Around the Young Star ASR 41 in NGC 1333
We present images of the young stellar object ASR 41 in the NGC 1333 star
forming region at the wavelengths of H_alpha and [SII] and in the I, J, H, and
K-bands.
ASR 41 has the near-infrared morphology of an edge-on disk object, but
appears an order of magnitude larger than typical systems of this kind.
We also present detailed models of the scattering and radiative transfer in
systems consisting of a young star surrounded by a proto-planetary disk, and
the whole system being embedded in either an infalling envelope or a uniform
molecular cloud. The best fit to the observed morphology can be achieved with a
disk of approx. 200 AU diameter, immersed in a low density cloud. The low cloud
density is necessary to stay below the sub-mm flux upper limits and to preserve
the shadow cast by the disk via single scattering.
The results demonstrate that ASR 41 is probably not inherently different from
typical edge-on disk objects, and that its large apparent size is due to the
shadow of a much smaller disk being projected into the surrounding dusty
molecular material
The lower mass function of the young open cluster Blanco 1: from 30 Mjup to 3 Mo
We performed a deep wide field optical survey of the young (~100-150 Myr)
open cluster Blanco1 to study its low mass population well down into the brown
dwarf regime and estimate its mass function over the whole cluster mass
range.The survey covers 2.3 square degrees in the I and z-bands down to I ~ z ~
24 with the CFH12K camera. Considering two different cluster ages (100 and 150
Myr), we selected cluster member candidates on the basis of their location in
the (I,I-z) CMD relative to the isochrones, and estimated the contamination by
foreground late-type field dwarfs using statistical arguments, infrared
photometry and low-resolution optical spectroscopy. We find that our survey
should contain about 57% of the cluster members in the 0.03-0.6 Mo mass range,
including 30-40 brown dwarfs. The candidate's radial distribution presents
evidence that mass segregation has already occured in the cluster. We took it
into account to estimate the cluster mass function across the
stellar/substellar boundary. We find that, between 0.03Mo and 0.6Mo, the
cluster mass distribution does not depend much on its exact age, and is well
represented by a single power-law, with an index alpha=0.69 +/- 0.15. Over the
whole mass domain, from 0.03Mo to 3Mo, the mass function is better fitted by a
log-normal function with m0=0.36 +/- 0.07Mo and sigma=0.58 +/- 0.06. Comparison
between the Blanco1 mass function, other young open clusters' MF, and the
galactic disc MF suggests that the IMF, from the substellar domain to the
higher mass part, does not depend much on initial conditions. We discuss the
implications of this result on theories developed to date to explain the origin
of the mass distribution.Comment: 18 pages, 15 figures and 5 tables accepted in A&
N-Methylimidazole Promotes The Reaction Of Homophthalic Anhydride With Imines
The addition of N-methylimidazole (NMI) to the reaction of homophthalic anhydride with imines such as pyridine-3-carboxaldehyde-N-trifluoroethylimine (9) reduces the amount of elimination byproduct and improves the yield of the formal cycloadduct, tetrahydroisoquinolonic carboxylate 10. Carboxanilides of such compounds are of interest as potential antimalarial agents. A mechanism that rationalizes the role of NMI is proposed, and a gram-scale procedure for the synthesis and resolution of 10 is also described
Dust-penetrated morphology in the high-redshift universe: clues from NGC 922
Results from the Hubble Deep Field (HDF) North and South show a large
percentage of high-redshift galaxies whose appearance falls outside traditional
classification systems. The nature of these objects is poorly understood, but
sub-mm observations indicate that at least some of these systems are heavily
obscured (Sanders 2000). This raises the intriguing possibility that a
physically meaningful classification system for high-redshift galaxies might be
more easily devised at rest-frame infrared wavelengths, rather than in the
optical regime. Practical realization of this idea will become possible with
the advent of the Next Generation Space Telescope (NGST). In order to explore
the capability of NGST for undertaking such science, we present NASA-IRTF and
SCUBA observations of NGC 922, a chaotic system in our local Universe which
bears a striking resemblance to objects such as HDF 2-86 (z=0.749) in the HDF
North. If objects such as NGC 922 are common at high-redshifts, then this
galaxy may serve as a local morphological `Rosetta stone' bridging low and
high-redshift populations. In this paper we demonstrate that quantitative
measures of galactic structure are recoverable in the rest-frame infrared for
NGC 922 seen at high redshifts using NGST, by simulating the appearance of this
galaxy at redshifts z=0.7 and z=1.2 in rest-frame K'. Our results suggest that
the capability of efficiently exploring the rest-wavelength IR morphology of
high-z galaxies should probably be a key factor in deciding the final choice of
instruments for the NGST.Comment: 7 pages, 12 Figures. Accepted for publication in A&A. Better version
of the figures can be found at http://www.inaoep.mx/~puerari/ngs
Herschel Observations of the W43 "mini-starburst"
Aims: To explore the infrared and radio properties of one of the closest
Galactic starburst regions. Methods: Images obtained with the Herschel Space
Observatory at wavelengths of 70, 160, 250, 350, and 500 microns using the PACS
and SPIRE arrays are analyzed and compared with radio continuum VLA data and 8
micron images from the Spitzer Space Telescope. The morphology of the
far-infrared emission is combined with radial velocity measurements of
millimeter and centimeter wavelength transitions to identify features likely to
be associated with the W43 complex. Results: The W43 star-forming complex is
resolved into a dense cluster of protostars, infrared dark clouds, and ridges
of warm dust heated by massive stars. The 4 brightest compact sources with L >
1.5 x 10^4 Lsun embedded within the Z-shaped ridge of bright dust emission in
W43 remain single at 4" (0.1 pc) resolution. These objects, likely to be
massive protostars or compact clusters in early stages of evolution are
embedded in clumps with masses of 10^3 to 10^4 Msun, but contribute only 2% to
the 3.6 x 10^6 Lsun far-IR luminosity of W43 measured in a 16 by 16 pc box. The
total mass of gas derived from the far-IR dust emission inside this region is
~10^6 Msun. Cometary dust clouds, compact 6 cm radio sources, and warm dust
mark the locations of older populations of massive stars. Energy release has
created a cavity blowing-out below the Galactic plane. Compression of molecular
gas in the plane by the older HII region near G30.684-0.260 and the bipolar
structure of the resulting younger W43 HII region may have triggered the
current mini-star burst.Comment: 5 pages, 3 figures, accepted for A&A Special Issu
Chandra Orion Ultradeep Project: Observations and Source Lists
We present a description of the data reduction methods and the derived
catalog of more than 1600 X-ray point sources from the exceptionally deep
January 2003 Chandra X-ray Observatory observation of the Orion Nebula Cluster
and embedded populations around OMC-1. The observation was obtained with
Chandra's Advanced CCD Imaging Spectrometer (ACIS) and has been nicknamed the
Chandra Orion Ultradeep Project (COUP). With an 838 ks exposure made over a
continuous period of 13.2 days, the COUP observation provides the most uniform
and comprehensive dataset on the X-ray emission of normal stars ever obtained
in the history of X-ray astronomy.Comment: 52 pages, 11 figures, 12 tables. Accepted for publication in ApJS,
special issue dedicated to Chandra Orion Ultradeep Project. A version with
high quality figures can be found at
http://www.astro.psu.edu/users/gkosta/COUP_Methodology.pd
Transient evolution of C-type shocks in dusty regions of varying density
Outflows of young stars drive shocks into dusty, molecular regions. Most
models of such shocks assume that they are steady and propagating perpendicular
to the magnetic field. Real shocks often violate both of these assumptions and
the media through which they propagate are inhomogeneous. We use the code
employed previously to produce the first time-dependent simulations of
fast-mode, oblique C-type shocks interacting with density perturbations. We
include a self-consistent calculation of the thermal and ionisation balances
and a fluid treatment of grains. We identify features that develop when a
multifluid shock encounters a density inhomogeneity to investigate whether any
part of the precursor region ever behaves in a quasi-steady fashion. If it does
the shock may be modelled approximately without solving the time-dependent
hydromagnetic equations. Simulations were made for initially steady oblique
C-type shocks encountering density inhomogeneities. For a semi-finite
inhomogeneity with a density larger than the surrounding medium, a transmitted
shock evolves from being J-type to a steady C-type shock on a timescale
comparable to the ion-flow time through it. A sufficiently upstream part of the
precursor of an evolving J-type shock is quasi-steady. The ion-flow timescale
is also relevant for the evolution of a shock moving into a region of
decreasing density. The models for shocks propagating into regions in which the
density increases and then decreases to its initial value cannot be entirely
described in terms of the results obtained for monotonically increasing and
decreasing densities. For the latter model, the long-term evolution to a C-type
shock cannot be approximated by quasi-steady models.Comment: 11 pages, 9 figure
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