334 research outputs found
The impact of alcohol and drug use on employment: A labor market study using the National Longitudinal Survey of Youth
The purpose of this study was, first, to estimate of the impact of alcohol and drug use on the employment status of men and women, and second, to examine whether a history of past use, as opposed to current use, adversely affects the propensity to be employed. Using data from the National Longitudinal Survey of Youth we conducted a cross-sectional and a longitudinal analysis with logistic regression estimation to model the probability that a person was employed in 1992. In addition to usual regressors, interactions between substance use measures, between substance use measures and human capital variables, and between substance use measures and race dummies were included in the equation. The longitudinal analysis utilized a conditional likelihood method based on employment data in 1992 and 1988 and included the difference between 1992 regressors and their 1988 counterparts. A comparison was made between the prediction accuracy of the logit choice model, linear discriminant analysis, k-nearest neighbor analysis, and three modern classification methods that are used extensively in the area of machine learning. Results showed that the logit model performs relatively well in classifying individuals into employed and unemployed categories based on individual attributes. Results of the cross-sectional and longitudinal analysis were mixed, but not inconsistent with our prior expectations that use of alcohol or drug has a negative impact on a person's propensity to be employed. Cross-sectional results show a clear negative impact of past substance use on a person's employment probability among all demographic groups examined (by gender: all persons, blacks, Hispanics, families with income below the poverty line, and high users of alcohol or drugs). However, when current and past use are considered together, only women seem to experience negative impacts. The results of the longitudinal analysis are less clear, although they do indicate that negative impacts are associated with the interaction between substance use measures and human capital variables. Limitations of the study are pointed out and suggestions are made for future research.
Emission Line Variability of the Accreting Young Brown Dwarf 2MASSW J1207334-393254: From Hours to Years
We have obtained a series of high-resolution optical spectra for the brown
dwarf 2MASSW J1207334-393254 (2M1207) using the ESO Very Large Telescope with
the UVES spectrograph during two consecutive observing nights (time resolution
of ~12 min) and the Magellan Clay telescope with the MIKE spectrograph.
Combined with previously published results, these data allow us to investigate
changes in the emission line spectrum of 2M1207 on timescales of hours to
years. Most of the emission line profiles of 2M1207 are broad, in particular
that of Halpha, indicating that the dominant fraction of the emission must be
attributed to disk accretion rather than to magnetic activity. From the Halpha
10% width we deduce a relatively stable accretion rate between
10^(-10.1...-9.8) Msun/yr for two nights of consecutive observations.
Therefore, either the accretion stream is nearly homogeneous over (sub-)stellar
longitude or the system is seen face-on. Small but significant variations are
evident throughout our near-continuous observation, and they reach a maximum
after ~8 h, roughly the timescale on which maximum variability is expected
across the rotation cycle. Together with past measurements, we confirm that the
accretion rate of 2M1207 varies by more than one order of magnitude on
timescales of months to years. Such variable mass accretion yields a plausible
explanation for the observed spread in the accretion rate vs. mass diagram. The
magnetic field required to drive the funnel flow is on the order of a few
hundred G. Despite the obvious presence of a magnetic field, no radio nor X-ray
emission has been reported for 2M1207. Possibly strong accretion suppresses
magnetic activity in brown dwarfs, similar to the findings for higher mass T
Tauri stars.Comment: accepted for publication in Ap
The SONYC survey: Towards a complete census of brown dwarfs in star forming regions
SONYC, short for "Substellar Objects in Nearby Young Clusters", is a survey
program to provide a census of the substellar population in nearby star forming
regions. We have conducted deep optical and near-infrared photometry in five
young regions (NGC1333, rho Ophiuchi, Chamaeleon-I, Upper Sco, and Lupus-3),
combined with proper motions, and followed by extensive spectroscopic campaigns
with Subaru and VLT, in which we have obtained more than 700 spectra of
candidate low-mass objects. We have identified and characterized more than 60
new substellar objects, among them a handful of objects with masses close to,
or below the Deuterium burning limit. Through SONYC and surveys by other
groups, the substellar IMF is now well characterized down to ~ 5 - 10 MJup, and
we find that the ratio of the number of stars with respect to brown dwarfs lies
between 2 and 6. A comprehensive survey of NGC 1333 reveals that, down to
~5MJup, free-floating objects with planetary masses are 20-50 times less
numerous than stars, i.e. their total contribution to the mass budget of the
clusters can be neglected.Comment: to appear in the proceedings of the conference 'Brown dwarfs come of
age', May 20-24 2013, Memorie della Societa Astronomica Italian
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