2,910 research outputs found
A Universal Stellar Initial Mass Function? A Critical Look at Variations
Few topics in astronomy initiate such vigorous discussion as whether or not
the initial mass function (IMF) of stars is universal, or instead sensitive to
the initial conditions of star formation. The distinction is of critical
importance: the IMF influences most of the observable properties of stellar
populations and galaxies, and detecting variations in the IMF could provide
deep insights into the process by which stars form. In this review, we take a
critical look at the case for IMF variations, with a view towards whether other
explanations are sufficient given the evidence. Studies of the field, local
young clusters and associations, and old globular clusters suggest that the
vast majority were drawn from a "universal" IMF: a power-law of Salpeter index
() above a few solar masses, and a log normal or shallower
power-law () between a few tenths and a few solar masses
(ignoring the effects of unresolved binaries). The shape and universality of
the IMF at the stellar-substellar boundary is still under investigation and
uncertainties remain large, but most observations are consistent with a IMF
that declines () well below the hydrogen burning limit.
Observations of resolved stellar populations and the integrated properties of
most galaxies are also consistent with a "universal IMF", suggesting no gross
variations in the IMF over much of cosmic time. There are indications of
"non-standard" IMFs in specific local and extragalactic environments, which
clearly warrant further study. Nonetheless, there is no clear evidence that the
IMF varies strongly and systematically as a function of initial conditions
after the first few generations of stars.Comment: 49 pages, 5 figures, to appear in Annual Reviews of Astronomy and
Astrophysics (2010, volume 48
IN-SYNC. VIII. Primordial Disk Frequencies in NGC 1333, IC 348, and the Orion A Molecular Cloud
In this paper, we address two issues related to primordial disk evolution in
three clusters (NGC 1333, IC 348, and Orion A) observed by the INfrared Spectra
of Young Nebulous Clusters (IN-SYNC) project. First, in each cluster, averaged
over the spread of age, we investigate how disk lifetime is dependent on
stellar mass. The general relation in IC 348 and Orion A is that primordial
disks around intermediate mass stars (2--5) evolve faster than those
around loss mass stars (0.1--1), which is consistent with previous
results. However, considering only low mass stars, we do not find a significant
dependence of disk frequency on stellar mass. These results can help to better
constrain theories on gas giant planet formation timescales. Secondly, in the
Orion A molecular cloud, in the mass range of 0.35--0.7, we provide
the most robust evidence to date for disk evolution within a single cluster
exhibiting modest age spread. By using surface gravity as an age indicator and
employing 4.5 excess as a primordial disk diagnostic, we observe a
trend of decreasing disk frequency for older stars. The detection of
intra-cluster disk evolution in NGC 1333 and IC 348 is tentative, since the
slight decrease of disk frequency for older stars is a less than 1-
effect.Comment: 25 pages, 26 figures; submitted for publication (ApJ
Comparing student perceptions of the classroom climate created by U.S. American and International teaching assistants
Previous studies have revealed that American undergraduate students complain about International Teaching Assistants’ (ITAs) lack of English proficiency and rate ITAs lower than American Teaching Assistants (ATAs) on teaching evaluations. This study investigates student perceptions of classroom climate to discover how ITAs might overcome students’ ethnocentric preconceptions. Survey results from 485 undergraduate students found that student perceptions of classroom climate differed significantly with ITAs as compared to ATAs. Student perceptions of classroom climate also differed significantly by the biological sex of the student. The biological sex of the TA did not have a significant impact on student perceptions of classroom climate.DOI: 10.18870/hlrc.v4i3.20
Rethinking Evaluation Strategies for Student Participation
Basic communication course instructors encourage student participation in the classroom by employing a variety of strategies, including graded participation. The present study examined the methods that basic course instructors use to facilitate and assess student participation in the classroom through focus groups interviews exploring how students perceive graded participation in the basic course. The findings suggest that while there are conditions in which the focus group students enjoy participation, there are also conditions in which they perceive such strategies as a power issue for instructors and reject the notion that participation accurately measures their level of involvement and learning in the classroom. Moreover, results indicate that students perceive instructor immediacy to be a significant factor in their willingness to participate. Finally, the focus group members offered several suggestions for instructors to better facilitate student participation in the classroom
Exploring the Relationship Between Students with Accommodations and Instructor Self-Efficacy in Complying with Accommodations
The willingness and flexibility of university instructors to comply with and provide accommodations for students with disabilities is critical to academic success. The authors examine how communication between students needing accommodations and university instructors impacts instructor self-efficacy, or instructors’ perception that they can meet the accommodation. Specifically, the authors’ explored the relationship between student self-disclosure of a disability and instructor empathy, flexibility, and self-efficacy in meeting student accommodation needs. Results revealed that the more a student self-discloses about a needed accommodation, the more self-efficacy an instructor has in making that accommodation. For the low-disclosure condition, empathy and flexibility were both significant predictors of self-efficacy, whereas, for the high-disclosure condition, only flexibility was a significant predictor of self-efficacy. Finally, instructors’ levels of empathy and flexibility both decreased after reading both the high and low self-disclosure scenarios
Precision Spectroscopy of Polarized Molecules in an Ion Trap
Polar molecules are desirable systems for quantum simulations and cold
chemistry. Molecular ions are easily trapped, but a bias electric field applied
to polarize them tends to accelerate them out of the trap. We present a general
solution to this issue by rotating the bias field slowly enough for the
molecular polarization axis to follow but rapidly enough for the ions to stay
trapped. We demonstrate Ramsey spectroscopy between Stark-Zeeman sublevels in
180Hf19F+ with a coherence time of 100 ms. Frequency shifts arising from
well-controlled topological (Berry) phases are used to determine magnetic
g-factors. The rotating-bias-field technique may enable using trapped polar
molecules for precision measurement and quantum information science, including
the search for an electron electric dipole moment.Comment: Accepted to Scienc
Synthesizing the First 15 Years of the Basic Communication Course Annual: What Research Tells Us about Effective Pedagogy
Despite the popularity of the both the basic course in communication and the Basic Communication Course Annual, questions still remain about the empirical support for the ways in which we teach the basic course. This essay categorizes and synthesizes 61 empirical studies published from 1989 to 2004 in the Basic Communication Course Annual. The studies are classified into five categories: teaching strategies, teacher and student characteristics, status of the basic course, analyses of texts for the basic course, and assessment of the basic course. Several salient themes are developed and suggestions for future research are advanced
Observation of genuine three-photon interference
Multiparticle quantum interference is critical for our understanding and
exploitation of quantum information, and for fundamental tests of quantum
mechanics. A remarkable example of multi-partite correlations is exhibited by
the Greenberger-Horne-Zeilinger (GHZ) state. In a GHZ state, three particles
are correlated while no pairwise correlation is found. The manifestation of
these strong correlations in an interferometric setting has been studied
theoretically since 1990 but no three-photon GHZ interferometer has been
realized experimentally. Here we demonstrate three-photon interference that
does not originate from two-photon or single photon interference. We observe
phase-dependent variation of three-photon coincidences with 90.5 \pm 5.0 %
visibility in a generalized Franson interferometer using energy-time entangled
photon triplets. The demonstration of these strong correlations in an
interferometric setting provides new avenues for multiphoton interferometry,
fundamental tests of quantum mechanics and quantum information applications in
higher dimensions.Comment: 7 pages, 7 figure
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