4,391 research outputs found
Employment Changes Play Major Role in Access to Employer Health Coverage
Highlights findings on the factors that drive short-term changes in employer-sponsored health insurance coverage, including the rising cost of health insurance and changes in employment rates and availability of better jobs during macroeconomic cycles
A Comparison of the Merger and Acquisition Provisions of Present Law with the Provisions in the Senate Finance Committee\u27s Draft Bill
Mr. Thompson presents a detailed comparison of the corporate merger and acquisition provisions of present law with the changes proposed by the Senate Finance Committee Staff Report and the Draft Bill, prepared by the Senate Finance Committee staff. Professor Ginsburg attempts to illustrate how the changes proposed by the SFC Report do not resolve many of the more sophisticated problems generated by use of multiple corporations and selective acquisitions of some of the target\u27s assets or stocks. Professor Ginsburg argues that the rules work well in the unreal world of one corporation operating one business, but not in the real world of multiple commonly controlled entities. He provides several examples of corporations that operate through affiliates that may end up with results not necessarily by the proposed changes
Subclasses of Presburger Arithmetic and the Weak EXP Hierarchy
It is shown that for any fixed , the -fragment of
Presburger arithmetic, i.e., its restriction to quantifier alternations
beginning with an existential quantifier, is complete for
, the -th level of the weak EXP
hierarchy, an analogue to the polynomial-time hierarchy residing between
and . This result completes the
computational complexity landscape for Presburger arithmetic, a line of
research which dates back to the seminal work by Fischer & Rabin in 1974.
Moreover, we apply some of the techniques developed in the proof of the lower
bound in order to establish bounds on sets of naturals definable in the
-fragment of Presburger arithmetic: given a -formula
, it is shown that the set of non-negative solutions is an ultimately
periodic set whose period is at most doubly-exponential and that this bound is
tight.Comment: 10 pages, 2 figure
On the pumping of the CS() masers in W51 e2e
We present the results of numerically solving the rate equations for the
first 31 rotational states of CS in the ground vibrational state to determine
the conditions under which the J=1-0, J=2-1 and J=3-2 transitions are inverted
to produce maser emission. The essence of our results is that the CS()
masers are collisionally pumped and that, depending on the spectral energy
distribution, dust emission can suppress the masers. Apart from the J=1-0 and
J=2-1 masers the calculations also show that the J=3-2 transition can be
inverted to produce maser emission. It is found that beaming is necessary to
explain the observed brightness temperatures of the recently discovered CS
masers in W51 e2e. The model calculations suggest that a CS abundance of a few
times and CS() column densities of the order
are required for these masers. The rarity of the CS
masers in high mass star forming regions might be the result of a required high
CS abundance as well as due to attenuation of the maser emission inside as well
as outside of the hot core.Comment: 13 pages, 15 figure
VLASSICK: The VLA Sky Survey in the Central Kiloparsec
At a distance of 8 kpc, the center of our Galaxy is the nearest galactic
nucleus, and has been the subject of numerous key projects undertaken by great
observatories such as Chandra, Spitzer, and Herschel. However, there are still
no surveys of molecular gas properties in the Galactic center with less than
30" (1 pc) resolution. There is also no sensitive polarization survey of this
region, despite numerous nonthermal magnetic features apparently unique to the
central 300 parsecs. In this paper, we outline the potential the VLASS has to
fill this gap. We assess multiple considerations in observing the Galactic
center, and recommend a C-band survey with 10 micro-Jy continuum RMS and
sensitive to molecular gas with densities greater than 10^4 cm^{-3}, covering
17 square degrees in both DnC and CnB configurations ( resolution ~5"),
totaling 750 hours of observing time. Ultimately, we wish to note that the
upgraded VLA is not just optimized for fast continuum surveys, but has a
powerful correlator capable of simultaneously observing continuum emission and
dozens of molecular and recombination lines. This is an enormous strength that
should be fully exploited and highlighted by the VLASS, and which is ideally
suited for surveying the center of our Galaxy.Comment: 13 pages, 3 figures, a White Paper submitted to provide input in
planning the Very Large Array Sky Surve
Feedback from massive stars at low metallicities : MUSE observations of N44 and N180 in the Large Magellanic Cloud
Accepted for publication in MNRAS, 27 pages, 21 figuresWe present MUSE integral field data of two HII region complexes in the Large Magellanic Cloud (LMC), N44 and N180. Both regions consist of a main superbubble and a number of smaller, more compact HII regions that formed on the edge of the superbubble. For a total of 11 HII regions, we systematically analyse the radiative and mechanical feedback from the massive O-type stars on the surrounding gas. We exploit the integral field property of the data and the coverage of the HeII5412 line to identify and classify the feedback-driving massive stars, and from the estimated spectral types and luminosity classes we determine the stellar radiative output in terms of the ionising photon flux . We characterise the HII regions in terms of their sizes, morphologies, ionisation structure, luminosity and kinematics, and derive oxygen abundances via emission line ratios. We analyse the role of different stellar feedback mechanisms for each region by measuring the direct radiation pressure, the pressure of the ionised gas, and the pressure of the shock-heated winds. We find that stellar winds and ionised gas are the main drivers of HII region expansion in our sample, while the direct radiation pressure is up to three orders of magnitude lower than the other terms. We relate the total pressure to the star formation rate per unit area, , for each region and find that stellar feedback has a negative effect on star formation, and sets an upper limit to as a function of increasing pressure.Peer reviewe
Hypervelocity Stars: Predicting the Spectrum of Ejection Velocities
The disruption of binary stars by the tidal field of the black hole in the
Galactic Center can produce the hypervelocity stars observed in the halo. We
use numerical models to simulate the full spectrum of observable velocities of
stars ejected into the halo by this binary disruption process. Our model
includes a range of parameters for binaries with 3-4 M_Solar primaries,
consideration of radial orbits of the ejected stars through an approximate mass
distribution for the Galaxy, and the impact of stellar lifetimes. We calculate
the spectrum of ejection velocities and reproduce previous results for the mean
ejection velocity at the Galactic center. The model predicts that the full
population of ejected stars includes both the hypervelocity stars with
velocities large enough to escape from the Galaxy and a comparable number of
ejected, but bound, stars of the same stellar type. The predicted median speeds
of the population of ejected stars as a function of distance in the halo are
consistent with current observations. Combining the model with the data also
shows that interesting constraints on the properties of binaries in the
Galactic Center and on the mass distribution in the Galaxy can be obtained even
with modest samples of ejected stars.Comment: 26 pages, including 6 figures, accepted for publication in the
Astrophysical Journa
Slow group velocity and Cherenkov radiation
We theoretically study the effect of ultraslow group velocities on the
emission of Vavilov-Cherenkov radiation in a coherently driven medium. We show
that in this case the aperture of the group cone on which the intensity of the
radiation peaks is much smaller than that of the usual wave cone associated
with the Cherenkov coherence condition. We show that such a singular behaviour
may be observed in a coherently driven ultracold atomic gas.Comment: 4 pages, 4 figure
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