67 research outputs found
Access to jobs : transportation barriers faced by low-skilled autoless workers in U.S. metropolitan areas
Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Urban Studies and Planning, 2002.Includes bibliographical references (leaves 240-249).One of the major concerns in today's urban labor market is spatial mismatch, the geographic separation between jobs and workers. Although numerous studies examine spatial mismatch, most of them focus on inner-city minorities, and the spatial mismatch problem for all autoless workers in a metropolitan area as a whole has not been well explored. Focusing on low-skilled workers and welfare recipients, this dissertation explores and quantifies the importance of job accessibility in employment outcomes for disadvantaged workers without autos in U.S. metropolitan areas. Metropolitan areas studied are Boston, San Francisco, and Los Angeles for low-skilled workers and Los Angeles for welfare recipients. An essential component of the analysis is the calculation of improved job-access measures that take into account supply and demand sides of the labor market and travel modes. The resulting measures indicate that, contrary to the perception of many spatial mismatch studies, central-city areas still offer more of a geographical advantage in accessing employment opportunities than suburban areas, despite the substantial suburbanization of employment. In other words, spatial mismatch is greater in suburban areas than in central-city areas. The measures also indicate that the levels of spatially accessible job opportunities are considerably lower for transit users than for auto users. In other words, spatial mismatch is much greater for transit users than for auto users. This transit/auto disparity is much greater than the central-city/suburb disparity, suggesting that the mode of travel has greater importance in determining job accessibility than location.(cont.) These findings suggest that spatial mismatch may pose a serious problem for autoless workers, particularly for those who live in suburban areas, although it may not be a problem for workers with autos. By incorporating the improved job-access measures into multinomial logit (MNL) models and regression models with Heckman correction, I find that improving job accessibility for transit users significantly augments the employment probability and the probability of working full-time for low-skilled autoless workers in San Francisco and Los Angeles. Further, in all three areas the job-access effect is greater for low-skilled autoless workers than for low-skilled auto-owning workers. Applying the same analytical framework for welfare recipients in Los Angeles, I find consistent results. I also find that job accessibility for transit users plays a more important role in employment outcomes in San Francisco and Los Angeles, more highly auto-dependent areas, than in Boston, a more compact area with relatively well-developed transit systems. The empirical findings together suggest that spatial mismatch is in fact the problem for autoless workers in suburban areas where jobs are dispersed and public transportation is poorly developed. The findings also suggest that spatial mismatch is more likely to be an employment barrier for those who live in suburban areas than for those who live in central-city areas, which contradicts the dominant view among spatial mismatch researchers. The empirical findings hold important policy implications...by Mizuki Kawabata.Ph.D
Correlation between Interstellar Polarization and Dust Temperature: Alignment of Grains by Radiative Torques is Ubiquitous?
We investigate the efficiency of interstellar polarization
where is the fractional linear polarization
and is extinction, in 16 lines of sight as a function of wavelength
. We have used the data obtained with the low-dispersion
spectropolarimeter HBS as well as those in literature. It is found that the
polarization efficiency is proportional to
in wavelength \lambda \approx 0.4-0.8 \micron, where
is a parameter which varies from 0.5 to 1.2 \micron. We find that
is negatively correlated with the dust temperature deduced from
infrared data by Schlegel et al., suggesting that the polarization efficiency
is higher in short wavelength for higher temperature. According to the
alignment theory by radiative torques (RATs), if the radiation is stronger and
bluer, RATs will make small grains align better, and the polarization
efficiency will increase in short wavelength. Our finding of the correlation
between and the temperature is consistent with what is expected with
the alignment mechanism by RATs.Comment: 5 pages, 3 figures. Accepted for publication in PASJ Letters, Vol.63,
October 2011 issu
Spectropolarimetric Study on Circumstellar Structure of Microquasar LS I +61deg 303
We present optical linear spectropolarimetry of the microquasar LS I
+61 303. The continuum emission is mildly polarized (up to 1.3 %) and
shows almost no temporal change. We find a distinct change of polarization
across the H emission line, indicating the existence of polarization
component intrinsic to the microquasar. We estimate the interstellar
polarization (ISP) component from polarization of the H line and derive
the intrinsic polarization component. The wavelength dependence of the
intrinsic component is well explained by Thomson scattering in equatorial disk
of the Be-type mass donor. The position angle (PA) of the intrinsic
polarization represents the rotational axis of the Be disk.
This PA is nearly perpendicular to the PA of the radio jet found during
quiescent phases. Assuming an orthogonal disk-jet geometry around the compact
star, the rotational axis of the accretion disk is almost perpendicular to that
of the Be disk. Moreover, according to the orbital parameters of the
microquasar, the compact star is likely to get across the Be disk around their
periastron passage. We discuss the peculiar circumstellar structure of this
microquasar inferred from our observation and possible connection with its
high-energy activities.Comment: 17pages, 7figures; accepted for Publications of the Astronomical
Society of Japa
Detection of Polarimetric Variations Associated with the Shortest Time-Scale Variability in S5 0716714
We present the result of near-infrared and optical observations of the BL Lac
object S5 0716714 carried out by the KANATA telescope. S5 0716714 has
both a long term high-amplitude variability and a short-term variability within
a night. The shortest variability (microvariability) time-scale is important
for understanding the geometry of jets and magnetic field, because it provides
a possible minimum size of variation sources. Here, we report the detection of
15-min variability in S5 0716714, which is one of the shortest time-scales
in optical and near-infrared variations observed in blazars. The detected
microvariation had an amplitude of mag in band and a blue
color of . Furthermore, we successfully detected
an unprecedented, short time-scale polarimetric variation which correlated with
the brightness change. We revealed that the microvariation had a specific
polarization component. The polarization degree of the variation component was
higher than that of the total flux. These results suggest that the
microvariability originated from a small and local region where the magnetic
field is aligned.Comment: 5 pages, 4 figures; Publications of the Astronomical Society of Japan
letter accepte
Dwarf Novae in the Shortest Orbital Period Regime: I. A New Short Period Dwarf Nova, OT J055717+683226
We report the observation of a new dwarf nova, OT J055717+683226, during its
first-ever recorded superoutburst in December 2006. Our observation shows that
this object is an SU UMa-type dwarf nova having a very short superhump period
of 76.67+/- 0.03 min (0.05324+/-0.00002 d). The next superoutburst was observed
in March 2008. The recurrence time of superoutbursts (supercycle) is, hence,
estimated to be ~480 d. The supercycle is much shorter than those of WZ
Sge-type dwarf novae having supercycles of >~ 10 yr, which are a major
population of dwarf novae in the shortest orbital period regime (<~85 min).
Using a hierarchical cluster analysis, we identified seven groups of dwarf
novae in the shortest orbital period regime. We identified a small group of
objects that have short supercycles, small outburst amplitudes, and large
superhump period excesses, compared with those of WZ Sge stars. OT
J055717+683226 probably belongs to this group.Comment: 14 pages, 11 figures, accepted for publication in PAS
Optical and Near-Infrared Photometry of Nova V2362 Cyg : Rebrightening Event and Dust Formation
We present optical and near-infrared (NIR) photometry of a classical nova,
V2362 Cyg (= Nova Cygni 2006). V2362 Cyg experienced a peculiar rebrightening
with a long duration from 100 to 240 d after the maximum of the nova. Our
multicolor observation indicates an emergence of a pseudophotosphere with an
effective temperature of 9000 K at the rebrightening maximum. After the
rebrightening maximum, the object showed a slow fading homogeneously in all of
the used bands for one week. This implies that the fading just after the
rebrightening maximum ( less or equal 1 week ) was caused by a slowly shrinking
pseudophotosphere. Then, the NIR flux drastically increased, while the optical
flux steeply declined. The optical and NIR flux was consistent with blackbody
radiation with a temperature of 1500 K during this NIR rising phase. These
facts are likely to be explained by dust formation in the nova ejecta. Assuming
an optically thin case, we estimate the dust mass of 10^(-8) -- 10^(-10)
M_solar, which is less than those in typical dust-forming novae. These results
support the senario that a second, long-lasting outflow, which caused the
rebrightening, interacted with a fraction of the initial outflow and formed
dust grains.Comment: 6 pages, 4 figures, 2010, PASJ, 62, 1103--1108, in pres
Low- and Medium-Dispersion Spectropolarimetry of Nova V475 Sct (Nova Scuti 2003): Discovery of an Asymmetric High-Velocity Wind in a Moderately Fast Nova
We present low-resolution () and medium-resolution ()
spectropolarimetry of Nova V475 Sct with the HBS instrument, mounted on the
0.91-m telescope at the Okayama Astrophysical Observatory, and with FOCAS,
mounted on the 8.2-m Subaru telescope. We estimated the interstellar
polarization toward the nova from the steady continuum polarization components
and H line emission components. After subtracting the interstellar
polarization component from the observations, we found that the H
emission seen on 2003 October 7 was clearly polarized. In the polarized flux
spectrum, the H emission had a distinct red wing extending to km s and a shoulder around km s, showing a
constant position angle of linear polarization \theta_{\rm *}\simeq
155\arcdeg\pm 15\arcdeg. This suggests that the nova had an asymmetric outflow
with a velocity of km s or more, which is six
times higher than the expansion velocity of the ionized shell at the same
epoch. Such a high-velocity component has not previously been reported for a
nova in the `moderately fast' speed class. Our observations suggest the
occurrence of violent mass-loss activity in the nova binary system even during
the common-envelope phase. The position angle of the polarization in the
H wing is in good agreement with that of the continuum polarization
found on 2003 September 26 (--0.6 %), which disappeared
within the following 2 d. The uniformity of the PA between the continuum
polarization and the wing polarization on October 7 suggests that the axis of
the circumstellar asymmetry remained nearly constant during the period of our
observations.Comment: 27 pages, 7 figures, accepted for publication in A
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