24 research outputs found
A Survey For Infall Motions Toward Starless Cores. III. CS (3-2) and DCO+(2-1) Observations
We present CS(3-2) and DCO+(2-1) observations of 94 starless cores and
compare the results with previous CS(2-1) and N2H+(1-0) observations to study
inward motions in starless cores. The velocity shifts of the CS(3-2) and (2-1)
lines with respect to N2H+ correlate well with each other and have similar
distributions. This implies that, in many cores, systematic inward motions of
gaseous material may occur over a range of density of at least a factor 4. We
identify 18 infall candidates based on observations of CS(3-2), CS(2-1),
DCO+(2-1) and N2H+(1-0). The eight best candidates, L1355, L1498, L1521F,
L1544, L158, L492, L694-2, and L1155C-1, each show at least four indications of
infall asymmetry and no counter-indications. Fits of the spectra to a 2-layer
radiative transfer model in ten infall candidates suggest that the median
effective line-of-sight speed of the inward-moving gas is ~0.07 km/s for CS
(3-2) and ~0.04 km/s for CS(2-1). Considering that the optical depth obtained
from the fits is usually smaller in CS(3-2) than in (2-1) line, this may imply
that CS(3-2) usually traces inner denser gas in higher inward motions than
CS(2-1). However, it is also possible that this conclusion is not
representative of all starless core infall candidates, due to the statistically
small number analyzed here. Further line observations will be useful to test
this conclusion.Comment: 2 PS files for the manuscript and tables, and 17 gif files for the
figure
SWAS and Arecibo observations of H2O and OH in a diffuse cloud along the line-of-sight to W51
Observations of W51 with the Submillimeter Wave Astronomy Satellite (SWAS)
have yielded the first detection of water vapor in a diffuse molecular cloud.
The water vapor lies in a foreground cloud that gives rise to an absorption
feature at an LSR velocity of 6 km/s. The inferred H2O column density is
2.5E+13 cm-2. Observations with the Arecibo radio telescope of hydroxyl
molecules at ten positions in W51 imply an OH column density of 8E+13 cm-2 in
the same diffuse cloud. The observed H2O/OH ratio of ~ 0.3 is significantly
larger than an upper limit derived previously from ultraviolet observations of
the similar diffuse molecular cloud lying in front of HD 154368. The observed
variation in H2O/OH likely points to the presence in one or both of these
clouds of a warm (T > 400) gas component in which neutral-neutral reactions are
important sources of OH and/or H2O.Comment: 15 pages (AASTeX) including 4 (eps) figures. To appear in the
Astrophysical Journa
The Ionization Fraction in Dense Molecular Gas II: Massive Cores
We present an observational and theoretical study of the ionization fraction
in several massive cores located in regions that are currently forming stellar
clusters. Maps of the emission from the J = 1-> O transitions of C18O, DCO+,
N2H+, and H13CO+, as well as the J = 2 -> 1 and J = 3 -> 2 transitions of CS,
were obtained for each core. Core densities are determined via a large velocity
gradient analysis with values typically 10^5 cm^-3. With the use of
observations to constrain variables in the chemical calculations we derive
electron fractions for our overall sample of 5 cores directly associated with
star formation and 2 apparently starless cores. The electron abundances are
found to lie within a small range, -6.9 < log10(x_e) < -7.3, and are consistent
with previous work. We find no difference in the amount of ionization fraction
between cores with and without associated star formation activity, nor is any
difference found in electron abundances between the edge and center of the
emission region. Thus our models are in agreement with the standard picture of
cosmic rays as the primary source of ionization for molecular ions. With the
addition of previously determined electron abundances for low mass cores, and
even more massive cores associated with O and B clusters, we systematically
examine the ionization fraction as a function of star formation activity. This
analysis demonstrates that the most massive sources stand out as having the
lowest electron abundances (x_e < 10^-8).Comment: 35 pages (8 figures), using aaspp4.sty, to be published in
Astrophysical Journa
Herschel Observations of Extraordinary Sources: Analysis of the HIFI 1.2 THz Wide Spectral Survey toward Orion KL. I. Methods
We present a comprehensive analysis of a broadband spectral line survey of the Orion Kleinmann-Low nebula (Orion KL), one of the most chemically rich regions in the Galaxy, using the HIFI instrument on board the Herschel Space Observatory. This survey spans a frequency range from 480 to 1907 GHz at a resolution of 1.1 MHz. These observations thus encompass the largest spectral coverage ever obtained toward this high-mass star-forming region in the submillimeter with high spectral resolution and include frequencies >1 THz, where the Earth's atmosphere prevents observations from the ground. In all, we detect emission from 39 molecules (79 isotopologues). Combining this data set with ground-based millimeter spectroscopy obtained with the IRAM 30 m telescope, we model the molecular emission from the millimeter to the far-IR using the XCLASS program, which assumes local thermodynamic equilibrium (LTE). Several molecules are also modeled with the MADEX non-LTE code. Because of the wide frequency coverage, our models are constrained by transitions over an unprecedented range in excitation energy. A reduced Ï^2 analysis indicates that models for most species reproduce the observed emission well. In particular, most complex organics are well fit by LTE implying gas densities are high (>10^6 cm^(â3)) and excitation temperatures and column densities are well constrained. Molecular abundances are computed using H_2 column densities also derived from the HIFI survey. The distribution of rotation temperatures, T_(rot), for molecules detected toward the hot core is significantly wider than the compact ridge, plateau, and extended ridge T_(rot) distributions, indicating the hot core has the most complex thermal structure
Radiative Transfer and Starless Cores
We develop a method of analyzing radio frequency spectral line observations
to derive data on the temperature, density, velocity, and molecular abundance
of the emitting gas. The method incorporates a radiative transfer code with a
new technique for handling overlapping hyperfine emission lines within the
accelerated lambda iteration algorithm and a heuristic search algorithm based
on simulated annnealing. We apply this method to new observations of N_2H^+ in
three Lynds clouds thought to be starless cores in the first stages of star
formation and determine their density structure. A comparison of the gas
densities derived from the molecular line emission and the millimeter dust
emission suggests that the required dust mass opacity is about
kappa_{1.3mm}=0.04 cm^2/g, consistent with models of dust grains that have
opacities enhanced by ice mantles and fluffy aggregrates.Comment: 42 pages, 17 figures, to appear in Ap
Water Absorption From Line-of-Sight Clouds Toward W49A
We have observed 6 clouds along the line-of-sight toward W49A using the
Submillimeter Wave Astronomy Satellite (SWAS) and several ground-based
observatories. The ortho-H2O 1-0 and OH (1665 and 1667 MHz) transitions are
observed in absorption, whereas the low-J CO, 13CO, and C18O lines, as well as
the [CI] 1-0 transition, are seen in emission. By using both the o-H218O and
o-H2O absorption lines, we are able to constrain the column-averaged o-H_2O
abundances in each line-of-sight cloud to within about an order of magnitude.
Assuming the standard N(H2)/N(CO) ratio of 10^4, we find N(o-H2O)/N(H2) = 8.1 x
10^-8 - 4 x 10^-7 for three clouds with optically thin water lines. In three
additional clouds, the HO lines are saturated so we have used observations
of the H218O ground-state transition to find upper limits to the water
abundance of 8.2x 10^-8 - 1.5x10^-6. We measure the OH abundance from the
average of the 1665 and 1667 MHz observations and find N(OH)/N(H2) = 2.3x10^-7
- 1.1x10^-6. The o-H2O and OH abundances are similar to those determined for
line-of-sight water absorption features towards W51 and Sgr B2 but are higher
than those seen from water emission lines in molecular clouds. However, the
clouds towards W49 have lower ratios of OH relative to H2O column densities
than are predicted by simple models which assume that dissociative
recombination is the primary formation pathway for OH and H2O. Building on the
work of Neufeld et al. (2002), we present photo-chemistry models including
additional chemical effects, which can also explain the observed OH and H2O
column densities as well as the observed H2O/CO abundance ratios.Comment: 32 pages, 7 figures, To appear in ApJ April 10 issu
Dense Gas and Star Formation: Characteristics of Cloud Cores Associated with Water Masers
We have observed 150 regions of massive star formation, selected originally
by the presence of a water maser, in the J = 5-4, 3-2, and 2-1 transitions of
CS, and 49 regions in the same transitions of CS. Over 90% of the 150
regions were detected in the J = 2-1 and 3-2 transitions of CS and 75% were
detected in the J=5-4 transition. We have combined the data with the J = 7-6
data from our original survey (Plume et al. 1992) to determine the density by
analyzing the excitation of the rotational levels. Using Large Velocity
Gradient (LVG) models, we have determined densities and column densities for 71
of these regions. The gas densities are very high (the mean log of the density
is 5.9), but much less than the critical density of the J=7-6 line. Small maps
of 25 of the sources in the J = 5-4 line yield a mean diameter of 1.0 pc. The
mean virial mass is 3800 solar masses. The mean ratio of bolometric luminosity
to virial mass (L/M) is 190, about 50 times higher than estimates using CO
emission, suggesting that star formation is much more efficient in the dense
gas probed in this study. The gas depletion time for the dense gas is roughly
1.3 x 10^7 yr. We find no statistically significant linewidth--size or
density--size relationships in our data. Instead, both linewidth and density
are larger for a given size than would be predicted by the usual relationships.
We find that the linewidth increases with density, the opposite of what would
be predicted by the usual arguments. We estimate that the luminosity of our
Galaxy (excluding the inner 400 pc) in the CS J = 5-4 transition is 15 to 23
L_sun, considerably less than the luminosity in this line within the central
100 pc of NGC 253 and M82. In addition, the ratio of far-infrared luminosity to
CS luminosity is higher in M82 than in any cloud in our sample.Comment: 26 pages, 6 postscript figures, 3 postscript tables. Uses AAS Latex
macros, accepted for Astrophysical Journa