525 research outputs found
The "True" Column Density Distribution in Star-Forming Molecular Clouds
We use the COMPLETE Survey's observations of the Perseus star-forming region
to assess and intercompare three methods for measuring column density in
molecular clouds: extinction mapping (NIR); thermal emission mapping (FIR); and
mapping the intensity of CO isotopologues. The structures shown by all three
tracers are morphologically similar, but important differences exist.
Dust-based measures give similar, log-normal, distributions for the full
Perseus region, once careful calibration corrections are made. We also compare
dust- and gas-based column density distributions for physically-meaningful
sub-regions of Perseus, and we find significant variations in the distributions
for those regions. Even though we have used 12CO data to estimate excitation
temperatures, and we have corrected for opacity, the 13CO maps seem unable to
give column distributions that consistently resemble those from dust measures.
We have edited out the effects of the shell around the B-star HD 278942. In
that shell's interior and in the parts where it overlaps the molecular cloud,
there appears to be a dearth of 13CO, likely due either to 13CO not yet having
had time to form in this young structure, and/or destruction of 13CO in the
molecular cloud. We conclude that the use of either dust or gas measures of
column density without extreme attention to calibration and artifacts is more
perilous than even experts might normally admit. And, the use of 13CO to trace
total column density in detail, even after proper calibration, is unavoidably
limited in utility due to threshold, depletion, and opacity effects. If one's
main aim is to map column density, then dust extinction seems the best probe.
Linear fits amongst column density tracers are given, quantifying the inherent
uncertainties in using one tracer (when compared with others). [abridged]Comment: Accepted in ApJ. 13 pages, 6 color figures. It includes small changes
to improve clarity. For a version with high-resolution figures see
http://www.cfa.harvard.edu/COMPLETE/papers/Goodman_ColumnDensity.pd
Abundant cyanopolyynes as a probe of infall in the Serpens South cluster-forming region
We have detected bright HC7N J = 21-20 emission toward multiple locations in
the Serpens South cluster-forming region using the K-Band Focal Plane Array at
the Robert C. Byrd Green Bank Telescope. HC7N is seen primarily toward cold
filamentary structures that have yet to form stars, largely avoiding the dense
gas associated with small protostellar groups and the main central cluster of
Serpens South. Where detected, the HC7N abundances are similar to those found
in other nearby star forming regions. Toward some HC7N `clumps', we find
consistent variations in the line centroids relative to NH3 (1,1) emission, as
well as systematic increases in the HC7N non-thermal line widths, which we
argue reveal infall motions onto dense filaments within Serpens South with
minimum mass accretion rates of M ~ 2-5 M_sun Myr^-1. The relative abundance of
NH3 to HC7N suggests that the HC7N is tracing gas that has been at densities n
~ 10^4 cm^-3, for timescales t < 1-2 x 10^5 yr. Since HC7N emission peaks are
rarely co-located with those of either NH3 or continuum, it is likely that
Serpens South is not particularly remarkable in its abundance of HC7N, but
instead the serendipitous mapping of HC7N simultaneously with NH3 has allowed
us to detect HC7N at low abundances in regions where it otherwise may not have
been looked for. This result extends the known star-forming regions containing
significant HC7N emission from typically quiescent regions, like the Taurus
molecular cloud, to more complex, active environments.Comment: 19 pages, 13 figures, accepted to MNRAS. Version with full resolution
figures available at http://www.dunlap.utoronto.ca/~friesen/Friesen_HC7N.pd
Determining the Mass of Dark Matter Particles with Direct Detection Experiments
In this article I review two data analysis methods for determining the mass
(and eventually the spin-independent cross section on nucleons) of Weakly
Interacting Massive Particles with positive signals from direct Dark Matter
detection experiments: a maximum likelihood analysis with only one experiment
and a model-independent method requiring at least two experiments.
Uncertainties and caveats of these methods will also be discussed.Comment: 24 pages, 10 figures, 1 reference added, typos fixed, published
version, to appear in the NJP Focus Issue on "Dark Matter and Particle
Physics
Infall/Expansion Velocities in the Low-Mass Dense Cores L492, L694-2, and L1521F: Dependence on Position and Molecular Tracer
Although surveys of infall motions in dense cores have been carried out for
years, few surveys have focused on mapping infall across cores using multiple
spectral line observations. To fill this gap, we present IRAM 30-m Telescope
maps of N2H+(1-0), DCO+(2-1), DCO+(3-2), and HCO+(3-2) emission towards two
prestellar cores (L492 and L694-2) and one protostellar core (L1521F). We find
that the measured infall velocity varies with position across each core and
choice of molecular line, likely as a result of radial variations in core
chemistry and dynamics. Line-of-sight infall speeds estimated from DCO+(2-1)
line profiles can decrease by 40-50 m/s when observing at a radial offset >=
0.04 pc from the core's dust continuum emission peak. Median infall speeds
calculated from all observed positions across a core can also vary by as much
as 65 m/s depending on the transition. These results show that while
single-pointing, single-transition surveys of core infall velocities may be
good indicators of whether a core is either contracting or expanding, the
magnitude of the velocities they measure are significantly impacted by the
choice of molecular line, proximity to the core center, and core evolutionary
state.Comment: Accepted for publication in Ap
Evidence for dust evolution within the Taurus Complex from Spitzer images
We present Spitzer images of the Taurus Complex (TC) and take advantage of
the sensitivity and spatial resolution of the observations to characterize the
diffuse IR emission across the cloud. This work highlights evidence of dust
evolution within the translucent sections of the archetype reference for
studies of quiescent molecular clouds. We combine Spitzer 160 um and IRAS 100
um observations to produce a dust temperature map and a far-IR dust opacity map
at 5' resolution. The average dust temperature is about 14.5K with a dispersion
of +/-1K across the cloud. The far-IR dust opacity is a factor 2 larger than
the average value for the diffuse ISM. This opacity increase and the
attenuation of the radiation field (RF) both contribute to account for the
lower emission temperature of the large grains. The structure of the TC
significantly changes in the mid-IR images that trace emission from PAHs and
VSGs. We focus our analysis of the mid-IR emission to a range of ecliptic
latitudes where the zodiacal light residuals are small. Within this cloud area,
there are no 8 and 24 um counterparts to the brightest 160 um emission
features. Conversely, the 8 and 24 um images reveal filamentary structure that
is strikingly inconspicuous in the 160 um and extinction maps. The IR colors
vary over sub-parsec distances across this filamentary structure. We compare
the observed colors with model calculations quantifying the impact of the RF
intensity and the abundance of stochastically heated particles on the dust SED.
To match the range of observed colors, we have to invoke variations by a factor
of a few of both the interstellar RF and the abundance of PAHs and VSGs. We
conclude that within this filamentary structure a significant fraction of the
dust mass cycles in and out the small size end of the dust size distribution.Comment: 43 pages, 13 figures, accepted for publication in Ap
A (sub)millimetre study of dense cores in Orion B9
We aim to further constrain the properties and evolutionary stages of dense
cores in Orion B9. The central part of Orion B9 was mapped at 350 micron with
APEX/SABOCA. A sample of nine cores in the region were observed in C17O(2-1),
H13CO+(4-3) (towards 3 sources), DCO+(4-3), N2H+(3-2), and N2D+(3-2) with
APEX/SHFI. These data are used in conjunction with our previous APEX/LABOCA
870-micron dust continuum data. Many of the LABOCA cores show evidence of
substructure in the higher-resolution SABOCA image. In particular, we report on
the discovery of multiple very low-mass condensations in the prestellar core
SMM 6. Based on the 350-to-870 micron flux density ratios, we determine dust
temperatures of ~7.9-10.8 K, and dust emissivity indices of ~0.5-1.8. The CO
depletion factors are in the range ~1.6-10.8. The degree of deuteration in N2H+
is ~0.04-0.99, where the highest value (seen towards the prestellar core SMM 1)
is, to our knowledge, the most extreme level of N2H+ deuteration reported so
far. The level of HCO+ deuteration is about 1-2%. We also detected D2CO towards
two sources. The detection of subcondensations within SMM 6 shows that core
fragmentation can already take place during the prestellar phase. The origin of
this substructure is likely caused by thermal Jeans fragmentation of the
elongated parent core. A low depletion factor and the presence of gas-phase
D2CO in SMM 1 suggest that the core chemistry is affected by the nearby
outflow. The very high N2H+ deuteration in SMM 1 is likely to be remnant of the
earlier CO-depleted phase.Comment: 20 pages, 10 figures, 10 tables. Accepted for publication in
Astronomy and Astrophysic
Redesign of Schneider electric rack layout into a centralized warehouse layout
This capstone design project sponsored by Schneider Electric was to create a centralized warehouse layout based on provided part usages, forklift practices, ergonomics, and warehouse procedures. Previously, storage areas were separated based on assembly lines, resulting in long times to pick parts for orders. The objective was to optimize current part placement in a centralized storage area and develop a tool to determine where to place new parts after the layout was set. Through task analyses, employee interviews, and time studies, customer needs and product specifications were determined. A root cause analysis was done to determine the main causes to be addressed in the concept generation phase. Tools including VBA, FLAP, and ergonomic standards were considered in concept generation. Moving forward, each concept will go through several iterations of testing and refining before the most beneficial solution will be chosen and implemented at the plant
A Sub-arcsecond Survey Toward Class 0 Protostars in Perseus: Searching for Signatures of Protostellar Disks
We present a CARMA 1.3 mm continuum survey toward 9 Class 0 protostars in the
Perseus molecular cloud at 0.3 (70 AU) resolution. This
study approximately doubles the number of Class 0 protostars observed with
spatial resolutions 100 AU at millimeter wavelengths, enabling the presence
of protostellar disks and proto-binary systems to be probed. We detect
flattened structures with radii 100 AU around 2 sources (L1448 IRS2 and
Per-emb-14) and these sources may be strong disk candidates.
Marginally-resolved structures with position angles within 30 of
perpendicular to the outflow are found toward 3 protostars (L1448 IRS3C, IRAS
03282+3035, and L1448C) and are considered disk candidates. Two others (L1448
IRS3B and IRAS 03292+3039) have resolved structure, possibly indicative of
massive inner envelopes or disks; L1448 IRS3B also has a companion separated by
0.9 (210 AU). IC348-MMS does not have well-resolved
structure and the candidate first hydrostatic core L1451-MMS is marginally
resolved on 1 scales. The strong disk candidate sources were
followed-up with CO () observations, detecting velocity
gradients consistent with rotation, but it is unclear if the rotation is
Keplerian. We compare the observed visibility amplitudes to radiative transfer
models, finding that visibility amplitude ratios suggest a compact component
(possibly a disk) is necessary for 5 of 9 Class 0 sources; envelopes alone may
explain the other 4 systems. We conclude that there is evidence for the
formation of large disks in the Class 0 phase with a range of radii and masses
dependent upon their initial formation conditions.Comment: Accepted to ApJ, 58 pages, 19 Figures, 5 Table
The COMPLETE Nature of the Warm Dust Ring in Perseus
The Perseus molecular cloud complex is a ~30pc long chain of molecular clouds
most well-known for the two star-forming clusters NGC1333 and IC348 and the
well-studied outflow source in B5. However, when studied at mid- to
far-infrared wavelengths the region is dominated by a ~10pc diameter shell of
warm dust, likely generated by an HII region caused by the early B-star
HD278942. Using a revised calibration technique the COMPLETE team has produced
high-sensitivity temperature and column-density maps of the Perseus region from
IRAS Sky Survey Atlas (ISSA) 60 and 100um data. In this paper, we combine the
ISSA based dust-emission maps with other observations collected as part of the
COMPLETE Survey, along with archival H-alpha and MSX observations. Molecular
line observations from FCRAO and extinction maps constructed by applying the
NICER method to the 2MASS catalog provide independent estimates of the ``true''
column-density of the shell. H-alpha emission in the region of the shell
confirms that it is most likely an HII region located behind the cloud complex,
and 8um data from MSX indicates that the shell may be interacting with the
cloud. Finally, the two polarisation components previously seen towards
background stars in the region can be explained by the association of the
stronger component with the shell. If confirmed, this would be the first
observation of a parsec-scale swept-up magnetic field.Comment: Accepted by ApJ. Figures have been compressed - full resolution
version available at http://cfa-www.harvard.edu/COMPLETE/results.htm
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