33 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
Riskometer—Voting with Your Feet
As Extension professionals, we are faced with the need to educate youth and adults about risk and to assess their preference for risk; however, many find the concepts difficult to illustrate. This article lays out a simple technique that can be used to illustrate how personal attitudes toward risk change with changes in situations. Youth and adults who participate in the exercise will have a better understanding of risk and their attitudes about risk, which will lead them to make personal choices that are right for them
Search for Nanosecond Near-infrared Transients around 1280 Celestial Objects
Stars and planetary system
A Young Super Star Cluster in the Nuclear Region of NGC 253
We present observations of a massive star cluster near the nuclear region of
the nearby starburst galaxy NGC 253. The peak of near-infrared emission, which
is spatially separated by 4" from the kinematic center of the galaxy, is
coincident with a super star cluster whose properties we examine with
low-resolution (R ~ 1,200) infrared CTIO spectroscopy and optical/near-infrared
HST imaging. Extinction, measured from [FeII] lines, is estimated at Av = 17.7
+/- 2.6. The age of the cluster is estimated at 5.7 Myr, based on Bry
equivalent width for an instantaneous burst using Starburst99 modeling.
However, a complex star formation history is inferred from the presence of both
recombination emission and photospheric CO absorption. The ionizing photon flux
has a lower limit of 7.3 +/- 2.5 x 10^53 inverse seconds, corrected for
extinction. Assuming a Kroupa IMF, we estimate a cluster mass of 1.4 +/- 0.4 x
10^7 solar masses. We observe a strong Wolf-Rayet signature at 2.06 microns and
report a weak feature at 2.19 microns which may be due to a massive stellar
population, consistent with the derived mass and age of this cluster.Comment: 8 pages, 5 figures, accepted for publication in Ap
The Crowdsourced Replication Initiative: Investigating Immigration and Social Policy Preferences. Executive Report.
In an era of mass migration, social scientists, populist parties and social movements raise concerns over the future of immigration-destination societies. What impacts does this have on policy and social solidarity? Comparative cross-national research, relying mostly on secondary data, has findings in different directions. There is a threat of selective model reporting and lack of replicability. The heterogeneity of countries obscures attempts to clearly define data-generating models. P-hacking and HARKing lurk among standard research practices in this area.This project employs crowdsourcing to address these issues. It draws on replication, deliberation, meta-analysis and harnessing the power of many minds at once. The Crowdsourced Replication Initiative carries two main goals, (a) to better investigate the linkage between immigration and social policy preferences across countries, and (b) to develop crowdsourcing as a social science method. The Executive Report provides short reviews of the area of social policy preferences and immigration, and the methods and impetus behind crowdsourcing plus a description of the entire project. Three main areas of findings will appear in three papers, that are registered as PAPs or in process
Qualitative Analysis of Bluff Erosion and Changes in the Le Sueur River
Over time, the Le Sueur River has been undergoing channel changes and creating bluff erosion in Southern Minnesota. The bluff erosion has become a threat to homes near the river causing them to be in danger of destruction. While other researchers have mapped river changes using two years of imagery, we analyzed five sets of aerial photos, ranging from 1939 to the present, of a study area southeast of Mankato to determine the extent of bluff erosion, channel changes and threats to residents. This series of images allowed us to see whether the stream changes have been steady or increasing at a more rapid rate than in the past. We interpreted the photos using Stereo Analyst in ERDAS Imagine and created anaglyphs to interpret erosion processes in stereo. We also digitized the river’s course in ArcGIS to map the changes in course and erosion over time. Spatial Analyst allowed us to produce a detailed 3-D shaded relief model of the river channel to illustrate the geomorphic changes occurring in the area over time. Our conclusion determines whether bluff erosion on the Le Sueur River is increasing as time goes on. This research could affect future development and zoning restrictions in the area