217 research outputs found
A two micron polarization survey toward dark clouds
A near infrared (2.2 micron) polarization survey of about 190 sources was conducted toward nearby dark clouds. The sample includes both background field stars and embedded young stellar objects. The aim is to determine the magnetic field structure in the densest regions of the dark clouds and study the role of magnetic fields in various phases of star formation processes, and to study the grain alignment efficiency in the dark cloud cores. From the polarization of background field stars and intrinsically unpolarized embedded sources, the magnetic field structure was determined in these clouds. From the intrinsic polarization of young stellar objects, the spatial distribution was determined of circumstellar dust around young stars. Combining the perpendicularity between the disks and magnetic fields with perpendicularity between the cloud elongation and magnetic fields, it is concluded that the magnetic fields might have dominated nearly all aspects of cloud dynamics, from the initial collapse of the clouds right through to the formation of disks/tori around young stars in these low to intermediate mass star forming clouds of the Taurus, Ophiuchus, and Perseus
Methods For Evaluating The Performance Of Small Acoustic Filters
Experimental methods are needed for determining the characteristics of small acoustic filters used in systems with pulsating gas flows. These characteristics could then be used to predict the performance of proposed filter designs in a particular system, according to plane wave acoustic theory. Dependence on trial-and-error experimentation in solving noise control problems would thus be minimized. A literature survey revealed only a few earlier attempts to evaluate the acoustic performance of small filters and filter elements. Three methods for determining reflection and transmission factors are described, evaluated, and compared. A method employing a standing wave tube of unique design is recommended for determination of reflection factors. Transmission factors are obtained from reflection factors and pressure measurements at the filter inlet and outlet. Use of an anechoic termination simplifies the calculations and increases accuracy; design and evaluation of such a termination is described. © 1969, Acoustical Society of America. All rights reserved
Extragalactic infrared spectroscopy
The spectra of galaxies in the near infrared atmospheric transmission windows are explored. Emission lines were detected due to molecular hydrogen, atomic hydrogen recombination lines, a line attributed to FEII, and a broad CO absorption feature. Lines due to H2 and FEII are especially strong in interacting and merging galaxies, but they were also detected in Seyferts and normal spirals. These lines appear to be shock excited. Multi-aperture measurements show that they emanate from regions as large as 15 kpc. It is argued that starbursts provide the most plausible and consistent model for the excitation of these lines, but the changes of relative line intensity of various species with aperture suggest that other excitation mechanisms are also operating in the outer regions of these galaxies
NH3 in the Central 10 pc of the Galaxy I: General Morphology and Kinematic Connections Between the CND and GMCs
New VLA images of NH3 (1,1), (2,2), and (3,3) emission in the central 10
parsecs of the Galaxy trace filamentary streams of gas, several of which appear
to feed the circumnuclear disk (CND). The NH3 images have a spatial resolution
of 16.5''x14.5'' and have better spatial sampling than previous NH3
observations. The images show the ``southern streamer,'' ``50 km/s cloud,'' and
new features including a ``western streamer'', 6 parsecs in length, and a
``northern ridge'' which connects to the CND. NH3(3,3) emission is very similar
to 1.2 mm dust emission indicating that NH3 traces column density well. Ratios
of the NH3(2,2) to (1,1) line intensities give an estimate of the temperature
of the gas and indicate high temperatures close to the nucleus and CND. The new
data cover a velocity range of 270 km/s, including all velocities observed in
the CND, with a resolution of 9.8 km/s. Previous NH3 observations with higher
resolution did not cover the entire range of velocities seen in the CND. The
large-scale kinematics of the CND do not resemble a coherent ring or disk. We
see evidence for a high velocity cloud within a projected distance of 50'' (2
pc) which is only seen in NH3(3,3) and is likely to be hot. Comparison to 6 cm
continuum emission reveals that much of the NH3 emission traces the outer edges
of Sgr A East and was probably pushed outward by this expanding shell. The
connection between the northern ridge (which appears to be swept up by Sgr A
East) and the CND indicates that Sgr A East and the CND are in close proximity
to each other. Kinematic evidence for these connections is presented in this
paper, while the full kinematic analysis of the central 10 pc will be presented
in Paper II.Comment: 16 pages (containing 6 figures), 8 additional JPEG figures. Accepted
for publication in ApJ. For full resolution images, see
http://cfa-www.harvard.edu/~rmcgary/SGRA/nh3_figures.htm
Spitzer IRAC Observations of Star Formation in N159 in the LMC
We present observations of the giant HII region complex N159 in the LMC using
IRAC on the {\it Spitzer Space Telescope}. One of the two objects previously
identified as protostars in N159 has an SED consistent with classification as a
Class I young stellar object (YSO) and the other is probably a Class I YSO as
well, making these two stars the youngest stars known outside the Milky Way. We
identify two other sources that may also be Class I YSOs. One component,
N159AN, is completely hidden at optical wavelengths, but is very prominent in
the infrared. The integrated luminosity of the entire complex is L L, consistent with the observed radio emission assuming a
normal Galactic initial mass function (IMF). There is no evidence for a red
supergiant population indicative of an older burst of star formation. The N159
complex is 50 pc in diameter, larger in physical size than typical HII regions
in the Milky Way with comparable luminosity. We argue that all of the
individual components are related in their star formation history. The
morphology of the region is consistent with a wind blown bubble $\approx
1-2Myr-old that has initiated star formation now taking place at the rim. Other
than its large physical size, star formation in N159 appears to be
indistinguishable from star formation in the Milky Way.Comment: 14 figure
A SCUBA survey of the NGC 2068/2071 protoclusters
We report the results of a submillimeter dust continuum survey of the
protoclusters NGC 2068 and NGC 2071 in Orion B carried out at 850 microns and
450 microns with SCUBA on JCMT. The mapped region is ~ 32' x 18' in size (~ 4
pc x 2 pc) and consists of filamentary dense cores which break up into
small-scale (~ 5000 AU) fragments, including 70 starless condensations and 5
circumstellar envelopes/disks. The starless condensations, seen on the same
spatial scales as protostellar envelopes, are likely to be gravitationally
bound and pre-stellar in nature. Their mass spectrum, ranging from ~ 0.3 Msun
to ~ 5 Msun, is reminiscent of the stellar initial mass function (IMF). Their
mass-size relation suggests that they originate from gravitationally-driven
fragmentation. We thus argue that pre-collapse cloud fragmentation plays a
major role in shaping the IMF.Comment: 6 pages, 4 figures, 1 table. Letter accepted by Astronomy &
Astrophysic
Infalling Gas Towards the Galactic Center
VLA maps of ammonia emission were made for the Galactic Center region. The
NH3(1,1) and NH3(2,2) transitions were observed in three 2' x 2' fields
covering Sgr A* and the region 3' immediately south of it. In the central 3
parsecs surrounding Sgr A* we find emission which appears to be associated with
the circumnuclear disk (CND), both morphologically and kinematically. This
central emission is connected to a long, narrow 2 pc x 10 pc streamer of clumpy
molecular gas located towards the south, which appears to be carrying gas from
the nearby 20 km/s giant molecular cloud (GMC) to the circumnuclear region. We
find a velocity gradient along the streamer, with progressively higher
velocities as the gas approaches Sgr A*. The streamer stops at the location of
the CND, where the line width of the NH3 emission increases dramatically. This
may be the kinematic signature of accretion onto the CND. The ratio of the
NH3(2,2)/NH3(1,1) emission indicates that the gas is heated at the northern tip
of the streamer, located inside the eastern edge of the CND. The morphology,
kinematics and temperature gradients of the gas all indicate that the southern
streamer is located at the Galactic Center and is interacting with the
circumnuclear region.Comment: 11 pages, 10 figures, accepted by The Astrophysical Journal (figure 1
contours have been corrected
Warm Molecular Gas Traced with CO J=7->6 in the Galaxy's Central 2 Parsecs: Dynamical Heating of the Circumnuclear Disk
We present an 11 arcsec resolution map of the central two parsecs of the
Galaxy in the CO J =7->6 rotational transition. The CO emission shows rotation
about Sgr A*, but also evidence for non-circular turbulent motion and a clumpy
morphology. We combine our dataset with available CO measurements to model the
physical conditions in the disk. We find that the molecular gas in the region
is both warm and dense, with T~200-300 K, n_H2~50,000-70,000 cm^-3. The mass of
warm molecular gas we measure in the central two parsecs is at least 2000
M_solar, about 20 times the UV-excited atomic gas mass, ruling out an UV
heating scenario for the molecular material. We compare the available spectral
tracers with theoretical models and conclude that molecular gas is heated with
magneto-hydrodynamic shocks with v~10-20 kms and B~0.3-0.5 mG. Using the
conditions derived with the CO analysis, we include the other important
coolants--neutral oxygen and molecular hydrogen--to estimate the total cooling
budget of the molecular material. We derive a mass to luminosity ratio of 2-3
M_solar/ L_solar, which is consistent with the total power dissipated via
turbulent decay in 0.1 pc cells with v_rms~15 kms. These size and velocity
scales are comparable to the observed clumping scale and the velocity
dispersion. At this rate, the material near Sgr A* its dissipating its orbital
energy on an orbital timescale, and cannot last for more than a few orbits. Our
conclusions support a scenario in which the features near Sgr A* such as the
CND and northern arm are generated by infalling clouds with low specific
angular momentum.Comment: 31 pages, including 5 figures, accepted for publication in Ap
Near-Infrared Molecular Hydrogen Emission from the Central Regions of Galaxies: Regulated Physical Conditions in the Interstellar Medium
The central regions of many interacting and early-type spiral galaxies are
actively forming stars. This process affects the physical and chemical
properties of the local interstellar medium as well as the evolution of the
galaxies. We observed near-infrared H2 emission lines: v=1-0 S(1), 3-2 S(3),
1-0 S(0), and 2-1 S(1) from the central ~1 kpc regions of the archetypical
starburst galaxies, M82 and NGC 253, and the less dramatic but still vigorously
star-forming galaxies, NGC 6946 and IC 342. Like the far-infrared continuum
luminosity, the near-infrared H2 emission luminosity can directly trace the
amount of star formation activity because the H2 emission lines arise from the
interaction between hot and young stars and nearby neutral clouds. The observed
H2 line ratios show that both thermal and non-thermal excitation are
responsible for the emission lines, but that the great majority of the
near-infrared H2 line emission in these galaxies arises from energy states
excited by ultraviolet fluorescence. The derived physical conditions, e.g.,
far-ultraviolet radiation field and gas density, from [C II] and [O I] lines
and far-infrared continuum observations when used as inputs to
photodissociation models, also explain the luminosity of the observed H2 v=1-0
S(1) line. The ratio of the H2 v=1-0 S(1) line to far-IR continuum luminosity
is remarkably constant over a broad range of galaxy luminosities; L_H2/L_FIR =
about 10^{-5}, in normal late-type galaxies (including the Galactic center), in
nearby starburst galaxies, and in luminous IR galaxies (LIRGs: L_FIR > 10^{11}
L_sun). Examining this constant ratio in the context of photodissociation
region models, we conclude that it implies that the strength of the incident UV
field on typical molecular clouds follows the gas density at the cloud surface.Comment: Accepted for ApJ, 24 pages, 17 figures, for complete PDF file, see
http://kao.re.kr/~soojong/mypaper/2004_pak_egh2.pd
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