992 research outputs found
Are HI Supershells the Remnants of Gamma-Ray Bursts?
Gamma-Ray Bursts (GRBs) are thought to originate at cosmological distances
from the most powerful explosions in the Universe. If GRBs are not beamed then
the distribution of their number as a function of Gamma-ray flux implies that
they occur once per (0.3-40) million years per bright galaxy and that they
deposit >10^{53} ergs into their surrounding interstellar medium. The blast
wave generated by a GRB explosion would be washed out by interstellar
turbulence only after tens of millions of years when it finally slows down to a
velocity of 10 km/s. This rather long lifetime implies that there could be up
to several tens of active GRB remnants in each galaxy at any given time. For
many years, radio observations have revealed the enigmatic presence of
expanding neutral-hydrogen (HI) supershells of kpc radius in the Milky Way and
in other nearby galaxies. The properties of some supershells cannot be easily
explained in terms of conventional sources such as stellar winds or supernova
explosions. However, the inferred energy and frequency of the explosions
required to produce most of the observed supershells agree with the above GRB
parameters. More careful observations and analysis might reveal which fraction
of these supershells are GRB remnants. We show that if this link is
established, the data on HI supershells can be used to constrain the energy
output, the rate per galaxy, the beaming factor, and the environment of GRB
sources in the Universe.Comment: 8 pages, final version, ApJ Letters, in pres
Minimum local distance density estimation
We present a local density estimator based on first-order statistics. To estimate the density at a point, x, the original sample is divided into subsets and the average minimum sample distance to x over all such subsets is used to define the density estimate at x. The tuning parameter is thus the number of subsets instead of the typical bandwidth of kernel or histogram-based density estimators. The proposed method is similar to nearest-neighbor density estimators but it provides smoother estimates. We derive the asymptotic distribution of this minimum sample distance statistic to study globally optimal values for the number and size of the subsets. Simulations are used to illustrate and compare the convergence properties of the estimator. The results show that the method provides good estimates of a wide variety of densities without changes of the tuning parameter, and that it offers competitive convergence performance.United States. Department of Energy. Applied Mathematical Sciences Program (Award DE-FG02-08ER2585)United States. Department of Energy. Applied Mathematical Sciences Program (Award de-sc0009297
Rh-Catalyzed Hydroformylation of 1,3-Butadiene and Pent-4-enal to Adipaldehyde in CO2-Expanded Media
The homogeneous hydroformylation of pent-4-enal, the preferred aldehyde intermediate from 1,3-butadiene hydroformylation, was systematically investigated with Rh catalyst complexes in neat and CO2-expanded toluene media at 40â80 °C, syngas partial pressures ranging from 5â50 bar, and different ligand/Rh ratios. At similar operating conditions, the TOFs are generally greater with Rh/DIOP relative to a Rh/TPP catalyst. On both catalyst complexes, the chemoselectivity toward the dialdehydes ranges from 75%â100%, with the maximum adipaldehyde selectivity reaching approximately 75% (n/i ⌠3) at 60 °C, 10 bar syngas, and molar DIOP/Rh ratio of 2.5. By using CO2-expanded toluene, the regioselectivity toward the adipaldehyde (desired product), and therefore its yield, is significantly enhanced. Interestingly, even with the simple Rh/TPP catalyst complex, adipaldehyde selectivity of up to 85% (n/i ⌠5.6) is achieved at 60 °C, 10 bar syngas, and 50 bar CO2. The beneficial effects of CO2-expanded media are attributed to the facile tunability of the H2/CO ratio in such a phase with a fixed syngas feed composition. This approach to accelerate pent-4-enal hydroformylation to form adipaldehyde could also help in overcoming equilibrium limitations typically associated with the catalytic isomerization of pent-3-enal (the dominant product from 1,3-butadiene hydroformylation) to pent-4-enal (the preferred isomer)
Is the interstellar gas of starburst galaxies well mixed?
The extent to which the ISM in galaxies is well mixed is not yet settled.
Measured metal abundances in the diffuse neutral gas of star--forming gas--rich
dwarf galaxies are deficient with respect to that of the ionized gas. The
reasons, if real, are not clear and need to be based on firm grounds. Far-UV
spectroscopy of giant HII regions such as NGC604 in the spiral galaxy M33 using
FUSE allows us to investigate possible systematic errors in the metallicity
derivation. We still find underabundances of nitrogen, oxygen, argon, and iron
in the neutral phase by a factor of~6. This could either be explained by the
presence of less chemically evolved gas pockets in the sightlines or by dense
clouds out of which HIIregions form. Those could be more metallic than the
diffuse medium.Comment: 4 pages, 2 figures;contribution to Starbursts: from 30 Dor to Lyman
Break Galaxies, 6 -10 September 2004, Institute of Astronomy, University of
Cambridge, U
3D Spectroscopy of Blue Compact Galaxies. Diagnostic Diagrams
Here we present the analysis of 3D spectroscopic data of three Blue Compact
Galaxies (Mrk324, Mrk370, and IIIZw102). Each of the more than 22500 spectra
obtained for each galaxy has been fitted by a single gaussian from which we
have inferred the velocity dispersion (sigma), the peak intensity (Ipeak), and
the central wavelength (lambda_c). The analysis shows that the sigma vs Ipeak
diagrams look remarkably similar to those obtained for giant extragalactic HII
regions. They all present a supersonic narrow horizontal band that extends
across all the range of intensities and that result from the massive nuclear
star-forming regions of every galaxy. The sigma vs Ipeak diagrams present also
several inclined bands of lower intensity and an even larger sigma, arising
from the large galactic volumes that surround the main central emitting knots.
Here we also show that the sigma vs lambda_c and lambda_c vs Ipeak diagrams,
are powerful tools able to unveil the presence of high and low mass stellar
clusters, and thus allow for the possibility of inferring the star formation
activity of distant galaxies, even if these are not spatially resolved.Comment: 15 pages, 3 figures, accepted for publication in The Astronomical
Journa
On the Spatial Distribution of Stellar Populations in the Large Magellanic Cloud
We measure the angular correlation function of stars in a region of the Large
Magellanic Cloud (LMC) that spans 2 degrees by 1.5 degrees. We find that the
correlation functions of stellar populations are represented well by
exponential functions of the angular separation for separations between 2 and
40 arcmin (corresponding to ~ 30 pc and 550 pc for an LMC distance of 50 kpc).
The inner boundary is set by the presence of distinct, highly correlated
structures, which are the more familiar stellar clusters, and the outer
boundary is set by the observed region's size and the presence of two principal
centers of star formation within the region. We also find that the
normalization and scale length of the correlation function changes
systematically with the mean age of the stellar population. The existence of
positive correlation at large separations (~300 pc), even in the youngest
population, argues for large-scale hierarchical structure in current star
formation. The evolution of the angular correlation toward lower normalizations
and longer scale lengths with stellar age argues for the dispersion of stars
with time. We show that a simple, stochastic, self-propagating star formation
model is qualitatively consistent with this behavior of the correlation
function.Comment: 30 pages, 13 Figures. Scheduled for publication in AJ in June 199
Superbubble evolution including the star-forming clouds: Is it possible to reconcile LMC observations with model predictions?
Here we present a possible solution to the apparent discrepancy between the
observed properties of LMC bubbles and the standard, constant density bubble
model. A two-dimensional model of a wind-driven bubble expanding from a
flattened giant molecular cloud is examined. We conclude that the expansion
velocities derived from spherically symmetric models are not always applicable
to elongated young bubbles seen almost face-on due to the LMC orientation. In
addition, an observational test to differentiate between spherical and
elongated bubbles seen face-on is discussed.Comment: 25 pages, 7 figures, accepted to ApJ (September, 1999 issue
Retailers' knowledge of tobacco harm reduction following the introduction of a new brand of smokeless tobacco
<p>Abstract</p> <p>Background</p> <p>Tobacco retailers are potential public health partners for tobacco harm reduction (THR). THR is the substitution of highly reduced-risk nicotine products, such as smokeless tobacco (ST) or pharmaceutical nicotine, for cigarettes. The introduction of a Swedish-style ST product, du Maurier snus (dMS) (Imperial Tobacco Canada Limited), which was marketed as a THR product, provided a unique opportunity to assess retailers' knowledge. This study examined retailers' knowledge of THR and compliance with recommendations regarding tobacco sales to young adults.</p> <p>Methods</p> <p>Male researchers, who may have looked younger than 18 years old, visited 60 stores in Edmonton that sold dMS. The researchers asked the retailers questions about dMS and its health risks relative to those from other tobacco products. They also attempted to purchase dMS to ascertain whether retailers would ask for identification to verify that they were at least 18 years old.</p> <p>Results</p> <p>Overall, the retailers were only moderately knowledgeable about THR and the differences between dMS and other tobacco products. About half of the retailers correctly indicated that snus is safer than cigarettes; half of whom knew it is safer because it is smoke-free. Fifty percent incorrectly believed that snus causes oral cancer. Less than fifty percent indicated that dMS differs from chewing tobacco because it is in pouches and is used without spitting or chewing (making it more promising for THR). Most (90%) of the retailers asked the researchers for identification when selling dMS.</p> <p>Conclusion</p> <p>Tobacco retailers are potentially important sources of information about THR, particularly since there are restrictions on the promotion of all tobacco products (regardless of the actual health risks) in Canada. This study found that many retailers in Edmonton do not know the relative health risks of different tobacco products and are therefore unable to pass on accurate information to smokers.</p
Supernova 1996cr: SN 1987A's Wild Cousin?
We report on new VLT optical spectroscopic and multi-wavelength archival
observations of SN1996cr, a previously identified ULX known as Circinus Galaxy
X-2. Our optical spectrum confirms SN1996cr as a bona fide type IIn SN, while
archival imaging isolates its explosion date to between 1995-02-28 and
1996-03-16. SN1996cr is one of the closest SNe (~3.8 Mpc) in the last several
decades and in terms of flux ranks among the brightest radio and X-ray SNe ever
detected. The wealth of optical, X-ray, and radio observations that exist for
this source provide relatively detailed constraints on its post-explosion
expansion and progenitor history, including an preliminary angular size
constaint from VLBI. The archival X-ray and radio data imply that the
progenitor of SN1996cr evacuated a large cavity just prior to exploding: the
blast wave likely expanded for ~1-2 yrs before eventually striking the dense
circumstellar material which surrounds SN1996cr. The X-ray and radio emission,
which trace the progenitor mass-loss rate, have respectively risen by a factor
of ~2 and remained roughly constant over the past 7 yr. This behavior is
reminiscent of the late rise of SN1987A, but 1000 times more luminous and much
more rapid to onset. Complex Oxygen line emission in the optical spectrum
further hints at a possible concentric shell or ring-like structure. The
discovery of SN1996cr suggests that a substantial fraction of the closest SNe
observed in the last several decades have occurred in wind-blown bubbles. An
Interplanetary Network position allows us to reject a tentative GRB association
with BATSE 4B960202. [Abridged]Comment: 25 pages with tables, 12 figures (color), accepted to ApJ, comments
welcome; v2 - updated to reflect the subsequent rejection of our tentative
GRB association based on a revised error region from the Interplanetary
Network (thanks to Kevin Hurley) and include a few additional references; v3
- corrected some errors in Tables 7 and
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