5,568 research outputs found
Public Transportation Ridership Levels
This article uses linear regression analysis to examine the determinants of public transportation ridership in over 100 U. S. cities in 2007. The primary determinant of ridership appears to be availability of public transportation service. In fact, the relationship is nearly one to one: a 1% increase in availability is associated with a 1% increase in ridership. The relative unimportance of price may be an indicator of the heavy subsidization of fares in most cities, leaving availability as the more effective policy tool to encourage use of public transport.identification, public transportation, ridership
Towards precision distances and 3D dust maps using broadband Period--Magnitude relations of RR Lyrae stars
We determine the period-magnitude relations of RR Lyrae stars in 13
photometric bandpasses from 0.4 to 12 {\mu}m using timeseries observations of
134 stars. The Bayesian formalism, extended from our previous work to include
the effects of line-of-sight dust extinction, allows for the simultaneous
inference of the posterior distribution of the mean absolute magnitude, slope
of the period-magnitude power-law, and intrinsic scatter about a perfect
power-law for each bandpass. In addition, the distance modulus and
line-of-sight dust extinction to each RR Lyrae star in the calibration sample
is determined, yielding a sample median fractional distance error of 0.66%. The
intrinsic scatter in all bands appears to be larger than the photometric
errors, except in WISE W1 (3.4 {\mu}m) and W2 (4.6 {\mu}m) where the
photometric error ( mag) is to be comparable or larger
than the intrinsic scatter. Additional observations at these wavelengths could
improve the inferred distances to these sources further. As an application of
the methodology, we infer the distance to the RRc-type star RZCep at low
Galactic latitude () to be mag
( pc) with colour excess mag. This
distance, equivalent to a parallax of microarcsec, is consistent
with the published HST parallax measurement but with an uncertainty that is 13
times smaller than the HST measurement. If our measurements (and methodology)
hold up to scrutiny, the distances to these stars have been determined to an
accuracy comparable to those expected with Gaia. As RR Lyrae are one of the
primary components of the cosmic distance ladder, the achievement of sub-1%
distance errors within a formalism that accounts for dust extinction may be
considered a strong buttressing of the path to eventual 1% uncertainties in
Hubble's constant.Comment: 21 pages, 29 figures, 2 tables, abstract abridged for arXiv. Comments
solicited on referee report (received June 9, 2014) linked:
https://gist.github.com/profjsb/c6c4e2f3a20ea02f1762 . Public archive of code
used to generate results and figures:
https://github.com/ckleinastro/period_luminosity_relation_fittin
Radiation Pressure in Massive Star Formation
Stars with masses of >~ 20 solar masses have short Kelvin times that enable
them to reach the main sequence while still accreting from their natal clouds.
The resulting nuclear burning produces a huge luminosity and a correspondingly
large radiation pressure force on dust grains in the accreting gas. This effect
may limit the upper mass of stars that can form by accretion. Indeed,
simulations and analytic calculations to date have been unable to resolve the
mystery of how stars of 50 solar masses and up form. We present two new ideas
to solve the radiation pressure problem. First, we use three-dimensional
radiation hydrodynamic adaptive mesh refinement simulations to study the
collapse of massive cores. We find that in three dimensions a configuration in
which radiation holds up an infalling envelope is Rayleigh-Taylor unstable,
leading radiation driven bubbles to collapse and accretion to continue. We also
present Monte Carlo radiative transfer calculations showing that the cavities
created by protostellar winds provides a valve that allow radiation to escape
the accreting envelope, further reducing the ability of radiation pressure to
inhibit accretion.Comment: To be appear in "IAU 227: Massive Star Birth: A Crossroads of
Astrophysics"; 6 pages, 1 figur
Public Transportation Ridership Levels
This article uses linear regression analysis to examine the determinants of public transportation ridership in over 100 U. S. cities in 2007. The primary determinant of ridership appears to be availability of public transportation service. In fact, the relationship is nearly one to one: a 1% increase in availability is associated with a 1% increase in ridership. The relative unimportance of price may be an indicator of the heavy subsidization of fares in most cities, leaving availability as the more effective policy tool to encourage use of public transport
The Kinematics of Molecular Cloud Cores in the Presence of Driven and Decaying Turbulence: Comparisons with Observations
In this study we investigate the formation and properties of prestellar and
protostellar cores using hydrodynamic, self-gravitating Adaptive Mesh
Refinement simulations, comparing the cases where turbulence is continually
driven and where it is allowed to decay. We model observations of these cores
in the CO, NH, and NH lines, and from
the simulated observations we measure the linewidths of individual cores, the
linewidths of the surrounding gas, and the motions of the cores relative to one
another. Some of these distributions are significantly different in the driven
and decaying runs, making them potential diagnostics for determining whether
the turbulence in observed star-forming clouds is driven or decaying. Comparing
our simulations with observed cores in the Perseus and Ophiuchus clouds
shows reasonably good agreement between the observed and simulated core-to-core
velocity dispersions for both the driven and decaying cases. However, we find
that the linewidths through protostellar cores in both simulations are too
large compared to the observations. The disagreement is noticably worse for the
decaying simulation, in which cores show highly supersonic infall signatures in
their centers that decrease toward their edges, a pattern not seen in the
observed regions. This result gives some support to the use of driven
turbulence for modeling regions of star formation, but reaching a firm
conclusion on the relative merits of driven or decaying turbulence will require
more complete data on a larger sample of clouds as well as simulations that
include magnetic fields, outflows, and thermal feedback from the protostars.Comment: 18 pages, 12 figures, accepted to A
Molecular Line Emission from Massive Protostellar Disks: Predictions for ALMA and the EVLA
We compute the molecular line emission of massive protostellar disks by
solving the equation of radiative transfer through the cores and disks produced
by the recent radiation-hydrodynamic simulations of Krumholz, Klein, & McKee.
We find that in several representative lines the disks show brightness
temperatures of hundreds of Kelvin over velocity channels ~10 km s^-1 wide,
extending over regions hundreds of AU in size. We process the computed
intensities to model the performance of next-generation radio and submillimeter
telescopes. Our calculations show that observations using facilities such as
the EVLA and ALMA should be able to detect massive protostellar disks and
measure their rotation curves, at least in the nearest massive star-forming
regions. They should also detect significant sub-structure and non-axisymmetry
in the disks, and in some cases may be able to detect star-disk velocity
offsets of a few km s^-1, both of which are the result of strong gravitational
instability in massive disks. We use our simulations to explore the strengths
and weaknesses of different observational techniques, and we also discuss how
observations of massive protostellar disks may be used to distinguish between
alternative models of massive star formation.Comment: 15 pages, 9 figures, emulateapj format, accepted for publication in
ApJ. Resolution of figures severely degraded to fit within size limits.
Download the full paper from
http://www.astro.princeton.edu/~krumholz/recent.htm
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