1,082 research outputs found
Star formation in Chamaeleon I and III: a molecular line study of the starless core population
The Chamaeleon clouds are excellent targets for low-mass star formation
studies. Cha I and II are actively forming stars while Cha III shows no sign of
ongoing star formation. We aim to determine the driving factors that have led
to the very different levels of star formation activity in Cha I and III and
examine the dynamical state and possible evolution of the starless cores within
them. Observations were performed in various molecular transitions with APEX
and Mopra. Five cores are gravitationally bound in Cha I and one in Cha III.
The infall signature is seen toward 8-17 cores in Cha I and 2-5 cores in Cha
III, which leads to a range of 13-28% of the cores in Cha I and 10-25% of the
cores in Cha III that are contracting and may become prestellar. Future
dynamical interactions between the cores will not be dynamically significant in
either Cha I or III, but the subregion Cha I North may experience collisions
between cores within ~0.7 Myr. Turbulence dissipation in the cores of both
clouds is seen in the high-density tracers N2H+ 1-0 and HC3N 10-9. Evidence of
depletion in the Cha I core interiors is seen in the abundance distributions of
C17O, C18O, and C34S. Both contraction and static chemical models indicate that
the HC3N to N2H+ abundance ratio is a good evolutionary indicator in the
prestellar phase for both gravitationally bound and unbound cores. In the
framework of these models, we find that the cores in Cha III and the southern
part of Cha I are in a similar evolutionary stage and are less chemically
evolved than the central region of Cha I. The measured HC3N/N2H+ abundance
ratio and the evidence for contraction motions seen towards the Cha III
starless cores suggest that Cha III is younger than Cha I Centre and that some
of its cores may form stars in the future. The cores in Cha I South may on the
other hand be transient structures. (abridged)Comment: Accepted for publication in A&A. The resolution of Figure 2 has been
degraded and the abstract in the metadata has been shortened to fit within
the limits set by arXi
Student Engagement: A Study of the Relationship between Teacher Credibility and Student Self-Efficacy
Given the current economic and subsequent employment uncertainty, people are returning to college to update their skills and acquire the credentials they need to be competitive in today\u27s workforce. As a result, faculty must be prepared to facilitate the learning process for an ever-changing and more diverse student body.
The purpose of this research is to further the extant body of research in the area of effective student engagement. The variables being used to represent credibility are competence, goodwill and trustworthiness. They were measured using McCroskey and Teven\u27s (1999) Source Credibility Questionnaire. The data for this study were provided by students attending a community college located in the metropolitan area of a Midwestern city. Using competence, trustworthiness and goodwill as the antecedents to represent instructor credibility, this research tests the relationship between instructor credibility and student self-efficacy.
Conducting a study of this type will provide faculty and administrators looking for new approaches to teacher leadership with data to help them enhance student engagement and increase their rate of persistence to graduation
The evolutionary state of the southern dense core Cha-MMS1
Aims: Our goal is to set constraints on the evolutionary state of the dense
core Cha-MMS1 in the Chamaeleon I molecular cloud. Methods: We analyze
molecular line observations carried out with the new submillimeter telescope
APEX. We look for outflow signatures around the dense core and probe its
chemical structure, which we compare to predictions of models of gas-phase
chemistry. We also use the public database of the Spitzer Space Telescope (SST)
to compare Cha-MMS1 with the two Class 0 protostars IRAM 04191 and L1521F,
which are at the same distance. Results: We measure a large deuterium
fractionation for N2H+ (11 +/- 3 %), intermediate between the prestellar core
L1544 and the very young Class 0 protostar L1521F. It is larger than for HCO+
(2.5 +/- 0.9 %), which is probably the result of depletion removing HCO+ from
the high-density inner region. Our CO(3-2) map reveals the presence of a
bipolar outflow driven by the Class I protostar Ced 110 IRS 4 but we do not
find evidence for an outflow powered by Cha-MMS1. We also report the detection
of Cha-MMS1 at 24, 70 and 160 microns by the instrument MIPS of the SST, at a
level nearly an order of magnitude lower than IRAM 04191 and L1521F.
Conclusions: Cha-MMS1 appears to have already formed a compact object, either
the first hydrostatic core at the very end of the prestellar phase, or an
extremely young protostar that has not yet powered any outflow, at the very
beginning of the Class 0 accretion phase.Comment: Accepted by Astronomy & Astrophysics as a letter, to appear in the
special issue on the APEX first result
Detection of OD towards the low-mass protostar IRAS16293-2422
Although water is an essential and widespread molecule in star-forming
regions, its chemical formation pathways are still not very well constrained.
Observing the level of deuterium fractionation of OH, a radical involved in the
water chemical network, is a promising way to infer its chemical origin. We aim
at understanding the formation mechanisms of water by investigating the origin
of its deuterium fractionation. This can be achieved by observing the abundance
of OD towards the low-mass protostar IRAS16293-2422, where the HDO distribution
is already known. Using the GREAT receiver on board SOFIA, we observed the
ground-state OD transition at 1391.5 GHz towards the low-mass protostar
IRAS16293-2422. We also present the detection of the HDO 111-000 line using the
APEX telescope. We compare the OD/HDO abundance ratio inferred from these
observations with the predictions of chemical models. The OD line is detected
in absorption towards the source continuum. This is the first detection of OD
outside the solar system. The SOFIA observation, coupled to the observation of
the HDO 111-000 line, provides an estimate of the abundance ratio OD/HDO ~
17-90 in the gas where the absorption takes place. This value is fairly high
compared with model predictions. This may be reconciled if reprocessing in the
gas by means of the dissociative recombination of H2DO+ further fractionates OH
with respect to water. The present observation demonstrates the capability of
the SOFIA/GREAT instrument to detect the ground transition of OD towards
star-forming regions in a frequency range that was not accessible before.
Dissociative recombination of H2DO+ may play an important role in setting a
high OD abundance. Measuring the branching ratios of this reaction in the
laboratory will be of great value for chemical models.Comment: 6 pages, 6 figures, 3 tables, accepted for publication in A&A
SOFIA/GREAT special issu
CO and CH3OH observations of the BHR71 outflows with APEX
Context : Highly-collimated outflows are believed to be the earliest stage in
outflow evolution, so their study is essential for understanding the processes
driving outflows. The BHR71 Bok globule is known to harbour such a
highly-collimated outflow, which is powered by a protostar belonging to a
protobinary system. Aims : We aimed at investigating the interaction of
collimated outflows with the ambient molecular cloud by using molecular
tracers. Methods : We mapped the BHR71 highly-collimated outflow in CO(3-2)
with the APEX telescope, and observed several bright points of the outflow in
the molecular transitions CO(4-3), 13CO(3-2), C18O(3-2), and CH3OH(7-6). We use
an LVG code to characterise the temperature enhancements in these regions.
Results : In our CO(3-2) map, the second outflow driven by IRS2, which is the
second source of the binary system, is completely revealed and shown to be
bipolar. We also measure temperature enhancements in the lobes. The CO and
methanol LVG modelling points to temperatures between 30 and 50K in the IRS1
outflow, while the IRS2 outflow seems to be warmer (up to 300K).Comment: 4 pages, 5 Figures, accepted by A&A Letters, to appear in the APEX
First results special issu
EXPERIMENTAL ANALYSIS OF A HEAT PUMP ASSISTED RECUPERATIVE AIR DEHUMIDIFIER
This paper describes the experimental analysis of a heat pump assisted recuperative air dehumidifier. The system consisted of an air-to-air vapor compression heat pump, coupled to the air ducting. Dehumidification was generated by reduction of the air temperature through the evaporator below the dew point, and thus promoting the condensation of the water vapor. Moist air was then warmed up in the condenser, resulting in a lowtemperature low-humidity air stream. Low energy consumption values are achieved in such systems as the latent heat of the water vapor acts as the heat pump own heat source. Occasionally, the compressor heat is also recoverable. The innovative feature of the present analysis was the introduction of an air-to-air plate recuperator, to further promote dehumidification, yet at the expense of greater compressor energy consumption. An experimental apparatus was constructed to perform comparative tests of the dehumidifier operating with and without the recuperator. A closed air circuit was employed, with the air mass flow rate as the controlling parameter of the experiment. Tests were also carried out with an open circuit
Lattice Boltzmann method for warm fluid simulations of plasma wakefield acceleration
A comprehensive characterization of lattice Boltzmann (LB) schemes to perform warm fluid numerical simulations of particle wakefield
acceleration (PWFA) processes is discussed in this paper. The LB schemes we develop hinge on the moment matching procedure, allowing
the fluid description of a warm relativistic plasma wake generated by a driver pulse propagating in a neutral plasma. We focus on fluid
models equations resulting from two popular closure assumptions of the relativistic kinetic equations, i.e., the local equilibrium and the warm
plasma closure assumptions. The developed LB schemes can, thus, be used to disclose insights on the quantitative differences between the
two closure approaches in the dynamics of PWFA processes. Comparisons between the proposed schemes and available analytical results are extensively addressed
SIMULATION AND EXPERIMENTAL EVALUATION OF AN INNOVATIVE ROTARY COMPRESSOR WITH VARIABLE SPEED DISPLACERS
This paper describes preliminary studies of a new rotary compressor with variable speed displacers. Two displacers rotate concentrically, in an annular space, at variable and phased angular velocities, thus creating two variable-volume compression spaces between them. The displacers are individually driven by two concentric shafts. An innovative driving mechanism imposes phased variable angular speeds to the shafts and, consequently, to the displacers, thus providing a volume variation in the gas compression spaces. The driving mechanism also offers a convenient way of capacity control, from zero to 100%, at constant electric motor speed. A mathematical model simulating the performance was developed. A traditional simulation model for positive displacement compressors was employed, where mass and energy conservation equations, in differential form, were applied to the control volumes (two compression spaces). Uniformly distributed thermodynamic properties of gas, varying with time, were assumed for each control volume. Equations describing the volume variation with time, the gas to cylinder wall heat transfer and gas flow through valve ports and leakage passages were also employed. The resulting mathematical model was a system of ordinary differential equations, the numerical integration of which provides the time-history of pressure and temperature of the gas inside the compression chambers. A prototype was also constructed and tested. First performance results are presented, showing the compressor behaviour under different operational conditions
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Ultraviolet Spectrum of the Crab Nebula
Data from 65 hours of observation of the Crab Nebula with the International Ultraviolet Explorer are reported, together with new ground-based spectrophotometry. We have measured the important C IV λ1549, He II λ1640, and C III] λ1908 emission line intensities and placed upper limits on other ultraviolet features for the brightest filamentary region in the nebula. We have also measured some ultraviolet continuum surface brightnesses at two places in the Crab. The emission lines imply an average ionic abundance ratio n(C^+2)/n(O^+2 ) in the range from 0.4 to 1.5 in the observed gaseous condensations. The elemental abundance ratio of carbon to oxygen is probably in the same range. This range may be narrowed through the use of ionization calculations, but even our simple analysis shows that there is no perceptible excess of carbon, due to presupernova nucleosynthesis, in the observed region. The large helium abundance, small carbon and oxygen abundances, and presence of a neutron star in the Crab Nebula suggest that the presupernova star had a mass close to 8 M0 when it was on the main sequence
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