460 research outputs found
Cosmic background explorer (COBE) navigation with TDRSS one-way return-link Doppler in the post-helium-venting phase
A navigation experiment was performed which establishes Ultra-Stable Oscillator (USO) frequency stabilized one way return link Doppler TDRSS tracking data as a feasible option for mission orbit determination support at the Goddard Space Center Flight Dynamics Facility. The study was conducted using both one way and two way Tracking and Data Relay Satellite System (TDRSS) tracking measurements for the Cosmic Background Explorer (COBE) spacecraft. Tracking data for a 4 week period immediately follow the depletion of the helium supply was used. The study showed that, for both definitive orbit solution and short term orbit prediction (up to 4 weeks), orbit determination results based on one way return link Doppler tracking measurements are comparable to orbit determination results based on two way range and two way Doppler tracking measurements
Knowledge-based monitoring of the pointing control system on the Hubble space telescope
A knowledge-based system for the real time monitoring of telemetry data from the Pointing and Control System (PCS) of the Hubble Space Telescope (HST) that enables the retention of design expertise throughout the three decade project lifespan by means other than personnel and documentation is described. The system will monitor performance, vehicle status, success or failure of various maneuvers, and in some cases diagnose problems and recommend corrective actions using a knowledge base built using mission scenarios and the more than 4,500 telemetry monitors from the HST
A 95 GHz Class I Methanol Maser Survey Toward A Sample of GLIMPSE Point Sources Associated with BGPS Clumps
We report a survey with the Purple Mountain Observatory (PMO) 13.7-m radio
telescope for class I methanol masers from the 95 GHz (8_0 - 7_1 A^+)
transition. The 214 target sources were selected by combining information from
both the Spitzer GLIMPSE and 1.1 mm BGPS survey catalogs. The observed sources
satisfy both the GLIMPSE mid-IR criteria of [3.6]-[4.5]>1.3, [3.6]-[5.8]>2.5,
[3.6]-[8.0]>2.5 and 8.0 um magnitude less than 10, and also have an associated
1.1 mm BGPS source. Class I methanol maser emission was detected in 63 sources,
corresponding to a detection rate of 29% for this survey. For the majority of
detections (43), this is the first identification of a class I methanol maser
associated with these sources. We show that the intensity of the class I
methanol maser emission is not closely related to mid-IR intensity or the
colors of the GLIMPSE point sources, however, it is closely correlated with
properties (mass and beam-averaged column density) of the BGPS sources.
Comparison of measures of star formation activity for the BGPS sources with and
without class I methanol masers indicate that the sources with class I methanol
masers usually have higher column density and larger flux density than those
without them. Our results predict that the criteria
log(S_{int})22.1,
which utilizes both the integrated flux density (S_{int}) and beam-averaged
column density (N_{H_{2}}^{beam}) of the BGPS sources, are very efficient for
selecting sources likely to have an associated class I methanol maser. Our
expectation is that searches using these criteria will detect 90% of the
predicted number of class I methanol masers from the full BGPS catalog (~
1000), and do so with a high detection efficiency (~75%).Comment: Accepted for publication in ApJ Supplement. 58 pages, 12 figures, 7
table
Warm Saturns: On the Nature of Rings around Extrasolar Planets that Reside Inside the Ice Line
We discuss the nature of rings that may exist around extrasolar planets.
Taking the general properties of rings around the gas giants in the Solar
System, we infer the likely properties of rings around exoplanets that reside
inside the ice line. Due to their proximity to their host star, rings around
such exoplanets must primarily consist of rocky materials. However, we find
that despite the higher densities of rock compared to ice, most of the observed
extrasolar planets with reliable radii measurements have sufficiently large
Roche radii to support rings. For the currently known transiting extrasolar
planets, Poynting-Robertson drag is not effective in significantly altering the
dynamics of individual ring particles over a time span of years provided
that they exceed about 1 m in size. In addition, we show that significantly
smaller ring particles can exist in optically thick rings, for which we find
typical ring lifetimes ranging from a few times to a few times
years. Most interestingly, we find that many of the rings could have nontrivial
Laplacian planes due to the increased effects of the orbital quadrupole caused
by the exoplanets' proximity to their host star, allowing a constraint on the
of extrasolar planets from ring observations. This is particular
exciting, since a planet's reveals information about its interior
structure. Furthermore, measurements of an exoplanet's oblateness and of its
, from warped rings, would together place limits on its spin period. Based
on the constraints that we have derived for extrasolar rings, we anticipate
that the best candidates for ring detections will come from transit
observations by the Kepler spacecraft of extrasolar planets with semi-major
axes AU and larger.Comment: Accepted for publication in Ap
Galactic H2CO Densitometry I: Pilot survey of Ultracompact HII regions and methodology
We present a pilot survey of 21 lines of sight towards UCHII regions in the
H2CO 1-1 (6cm) and 2-2 (2cm) transitions, using the H2CO centimeter lines as a
molecular gas densitometer. Using Arecibo and Green Bank beam-matched
observations, we measure the density of 51 detected H2CO line pairs and present
upper limits on density for an additional 24 detected 1-1 lines. We analyze the
systematic uncertainties in the H2CO densitometer, achieving H2 density
measurements with accuracies ~ 0.1-0.3 dex. The densities measured are not
correlated with distance, implying that it is possible to make accurate density
measurements throughout the galaxy without a distance bias. We confirm that
ultracompact HII regions are associated with gas at densities n(H2)~10^5cm^-3.
The densities measured in line-of-sight molecular clouds suggest that they
consist of low volume filling factor (f~10^-2) gas at high (n(H2)>10^4cm^-3)
density, which is inconsistent with purely supersonic turbulence and requires
high-density clumping greater than typically observed in gravoturbulent
simulations. We observe complex line morphologies that indicate density
variations with velocity around UCHII regions, and we classify a subset of the
UCHII molecular envelopes as collapsing or expanding. We compare these
measurements to Bolocam Galactic Plane Survey 1.1mm observations, and note that
most UCHII regions have 1.1mm emission consisting of significant (5-70%)
free-free emission and are therefore not necessarily dominated by optically
thin dust emission. A comparison of our data with the Mangum et al. starburst
sample shows that the area filling factor of dense (n(H2)~10^5cm^-3) molecular
gas in typical starburst galaxies is <~0.01, but in extreme starburst galaxies
like Arp 220, is ~0.1, suggesting that Arp 220 is physically similar to an
oversized UCHII region.Comment: 39 pages, 46 figures, 8 tables, EmulateApJ 2-column format. Accepted
for publication in ApJ. Full resolution version:
http://casa.colorado.edu/~ginsbura/papers/h2co_pilot_draft0607.pdf, published
at http://stacks.iop.org/0004-637X/736/14
An Infrared through Radio Study of the Properties and Evolution of IRDC Clumps
We examine the physical properties and evolutionary stages of a sample of 17
clumps within 8 Infrared Dark Clouds (IRDCs) by combining existing infrared,
millimeter, and radio data with new Bolocam Galactic Plane Survey (BGPS) 1.1 mm
data, VLA radio continuum data, and HHT dense gas (HCO+ and N2H+) spectroscopic
data. We combine literature studies of star formation tracers and dust
temperatures within IRDCs with our search for ultra-compact (UC) HII regions to
discuss a possible evolutionary sequence for IRDC clumps. In addition, we
perform an analysis of mass tracers in IRDCs and find that 8 micron extinction
masses and 1.1 mm Bolocam Galactic Plane Survey (BGPS) masses are complementary
mass tracers in IRDCs except for the most active clumps (notably those
containing UCHII regions), for which both mass tracers suffer biases. We find
that the measured virial masses in IRDC clumps are uniformly higher than the
measured dust continuum masses on the scale of ~1 pc. We use 13CO, HCO+, and
N2H+ to study the molecular gas properties of IRDCs and do not see any evidence
of chemical differentiation between hot and cold clumps on the scale of ~1 pc.
However, both HCO+ and N2H+ are brighter in active clumps, due to an increase
in temperature and/or density. We report the identification of four UCHII
regions embedded within IRDC clumps and find that UCHII regions are associated
with bright (>1 Jy) 24 micron point sources, and that the brightest UCHII
regions are associated with "diffuse red clumps" (an extended enhancement at 8
micron). The broad stages of the discussed evolutionary sequence (from a
quiescent clump to an embedded HII region) are supported by literature dust
temperature estimates; however, no sequential nature can be inferred between
the individual star formation tracers.Comment: 33 pages, 26 figures, 6 tables, accepted for publication in ApJ. Full
resolution version available here:
http://casa.colorado.edu/~battersb/Publications.htm
A Study of Direct Author Subvention for Publishing Humanities Books at Two Universities: A Report to the Andrew W. Mellon Foundation by Indiana University and University of Michigan
This report was produced as the main deliverable from the Andrew W. Mellon Foundation Grant 41400692, “A Study of Direct Author Subvention for Publishing Humanities Books at Two Universities.” The Indiana University team led by PI Carolyn Walters, consisted of Jason Baird Jackson, Scott Smart, Nick Fitzgerald, Gary Dunham and Shayna Pekala. The University of Michigan team led by PI James Hilton consisted of Paul Courant, Sidonie Smith, Meredith Kahn, Charles Watkinson, Jim Ottaviani, and Aaron McCollough. Lead authorship of the different sections in this report is indicated in the opening paragraphs. Supplemental data to this report is available at http://hdl.handle.net/2022/20358.This white paper presents recommendations about how a system of monographic publication fully funded by subventions from authors’ parent institutions might function, based on research activities supported by the Andrew W. Mellon Foundation at Indiana University and the University of Michigan. While the contributors present a strong argument for implementing such an “author subvention” system, they describe a number of challenges and potential unintended consequences. Particular issues discussed include how to determine which publishers would be eligible for support, how best to support untenured faculty, and how to avoid disenfranchising scholars at less well-funded institutions.Andrew W. Mellon Foundatio
Star formation towards the Scutum tangent region and the effects of Galactic environment
By positional matching to the catalogue of Galactic Ring Survey molecular
clouds, we have derived distances to 793 Bolocam Galactic Plane Survey (BGPS)
sources out of a possible 806 located within the region defined by Galactic
longitudes l = 28.5 degr to 31.5 degr and latitudes |b| < 1 degr. This section
of the Galactic Plane contains several major features of Galactic structure at
different distances, mainly mid-arm sections of the Perseus and Sagittarius
spiral arms and the tangent of the Scutum-Centarus arm, which is coincident
with the end of the Galactic Long Bar. By utilising the catalogued cloud
distances plus new kinematic distance determinations, we are able to separate
the dense BGPS clumps into these three main line-of-sight components to look
for variations in star-formation properties that might be related to the
different Galactic environments. We find no evidence of any difference in
either the clump mass function or the average clump formation efficiency (CFE)
between these components that might be attributed to environmental effects on
scales comparable to Galactic-structure features.
Despite having a very high star-formation rate, and containing at least one
cloud with a very high CFE, the star formation associated with the
Scutum-Centarus tangent does not appear to be in any way abnormal or different
to that in the other two spiral-arm sections. Large variations in the CFE are
found on the scale of individual clouds, however, which may be due to local
triggering agents as opposed to the large-scale Galactic structure.Comment: 11 pages, 8 figures. Accepted for publication in the Monthly Notices
of the Royal Astronomical Societ
The Bolocam Galactic Plane Survey: Survey Description and Data Reduction
We present the Bolocam Galactic Plane Survey (BGPS), a 1.1 mm continuum
survey at 33" effective resolution of 170 square degrees of the Galactic Plane
visible from the northern hemisphere. The survey is contiguous over the range
-10.5 < l < 90.5, |b| < 0.5 and encompasses 133 square degrees, including some
extended regions |b| < 1.5. In addition to the contiguous region, four targeted
regions in the outer Galaxy were observed: IC1396, a region towards the Perseus
Arm, W3/4/5, and Gem OB1. The BGPS has detected approximately 8400 clumps over
the entire area to a limiting non-uniform 1-sigma noise level in the range 11
to 53 mJy/beam in the inner Galaxy. The BGPS source catalog is presented in a
companion paper (Rosolowsky et al. 2010). This paper details the survey
observations and data reduction methods for the images. We discuss in detail
the determination of astrometric and flux density calibration uncertainties and
compare our results to the literature. Data processing algorithms that separate
astronomical signals from time-variable atmospheric fluctuations in the data
time-stream are presented. These algorithms reproduce the structure of the
astronomical sky over a limited range of angular scales and produce artifacts
in the vicinity of bright sources. Based on simulations, we find that extended
emission on scales larger than about 5.9' is nearly completely attenuated (>
90%) and the linear scale at which the attenuation reaches 50% is 3.8'.
Comparison with other millimeter-wave data sets implies a possible systematic
offset in flux calibration, for which no cause has been discovered. This
presentation serves as a companion and guide to the public data release through
NASA's Infrared Processing and Analysis Center (IPAC) Infrared Science Archive
(IRSA). New data releases will be provided through IPAC IRSA with any future
improvements in the reduction.Comment: Accepted for publication in Astrophysical Journal Supplemen
The Bolocam Galactic Plane Survey V: HCO+ and N2H+ Spectroscopy of 1.1 mm Dust Continuum Sources
We present the results of observations of 1882 sources in the Bolocam
Galactic Plane Survey (BGPS) at 1.1 mm with the 10m Heinrich Hertz Telescope
simultaneously in HCO+ J=3-2 and N2H+ J=3-2. We detect 77% of these sources in
HCO^+ and 51% in N2H+ at greater than 3. We find a strong correlation
between the integrated intensity of both dense gas tracers and the 1.1 mm dust
emission of BGPS sources. We determine kinematic distances for 529 sources (440
in the first quadrant breaking the distance ambiguity and 89 in the second
quadrant) We derive the size, mass, and average density for this subset of
clumps. The median size of BGPS clumps is 0.75 pc with a median mass of 330
M (assuming T_{Dust}=20 K). The median HCO+ linewidth is 2.9 km
s indicating that BGPS clumps are dominated by supersonic turbulence or
unresolved kinematic motions. We find no evidence for a size-linewidth
relationship for BGPS clumps. We analyze the effects of the assumed dust
temperature on the derived clump properties with a Monte Carlo simulation and
we find that changing the temperature distribution will change the median
source properties (mass, volume-averaged number density, surface density) by
factors of a few. The observed differential mass distribution has a power-law
slope that is intermediate between that observed for diffuse CO clouds and the
stellar IMF. BGPS clumps represent a wide range of objects (from dense cores to
more diffuse clumps) and are typically characterized by larger sizes and lower
densities than previously published surveys of high-mass star-forming regions.
This collection of objects is a less-biased sample of star-forming regions in
the Milky Way that likely span a wide range of evolutionary states.Comment: 48 pages, 25 figures, Accepted for publicatio
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