48 research outputs found
Are Those Who Bring Work Home Really Working Longer Hours? Implications for BLS Productivity Measures
An ongoing debate surrounding BLS productivity data is that official labor productivity measures may be overstating productivity growth because of an increase in unmeasured hours worked outside the traditional workplace. This paper uses both the ATUS and May CPS Work Schedules and Work at Home Supplements to determine whether the number of hours worked by nonfarm business employees are underestimated and increasing over time due to unmeasured hours worked at home. We find that 8 - 9 percent of nonfarm business employees bring some work home from the workplace. In addition, those who bring work home report working longer hours than those who work exclusively in a workplace, resulting in a 0.8 – 1.1 percent understatement of measured hours worked. However, we find no conclusive evidence that productivity trends were biased over the 1997-2005 period due to work brought home from the workplace.Work at Home, Productivity, Time Use
Predictions for mass-loss rates and terminal wind velocities of massive O-type stars
Mass loss forms an important aspect of the evolution of massive stars, as
well as for the enrichment of the surrounding ISM. Our goal is to predict
accurate mass-loss rates and terminal wind velocities. These quantities can be
compared to empirical values, thereby testing radiation-driven wind models. One
specific issue is that of the "weak-wind problem", where empirically derived
mass-loss rates fall orders of magnitude short of predicted values. We employ
an established Monte Carlo model and a recently suggested new line acceleration
formalism to solve the wind dynamics consistently. We provide a new grid of
mass-loss rates and terminal wind velocities of O stars, and compare the values
to empirical results. Our models fail to provide mass-loss rates for
main-sequence stars below a luminosity of log(L/Lsun) = 5.2, where we run into
a fundamental limit. At luminosities below this critical value there is
insufficient momentum transferred in the region below the sonic point to
kick-start the acceleration. This problem occurs at the location of the onset
of the weak-wind problem. For O dwarfs, the boundary between being able to
start a wind, and failing to do so, is at spectral type O6/O6.5. The direct
cause of this failure is a combination of the lower luminosity and a lack of Fe
V lines at the wind base. This might indicate that another mechanism is
required to provide the necessary driving to initiate the wind. For stars more
luminous than log(L/Lsun) = 5.2, our new mass-loss rates are in excellent
agreement with the mass-loss prescription by Vink et al. 2000. This implies
that the main assumption entering the method of the Vink et al. prescriptions -
i.e. that the momentum equation is not explicitly solved for - does not
compromise the reliability of the Vink et al. results for this part of
parameter space (Abridged).Comment: 10 pages, 10 figures, Astronomy & Astrophysics (in press
Testing the predicted mass-loss bi-stability jump at radio wavelengths
In this study, we test the theoretically predicted mass-loss behaviour as a
function of stellar effective temperature across the so-called `bi-stability'
jump (BSJ). We gathered radio observations of 30 OB supergiants (O8-B3). We
derived the radio mass-loss rates and wind efficiencies, and compared our
results with Halpha mass-loss rates and predictions based on radiation-driven
wind models. he wind efficiency shows the possible presence of a local maximum
around an effective temperature of 21~000 K -- in qualitative agreement with
predictions. We also find that the radio mass-loss rates show good agreement
with empirical Halpha rates. However, the empirical mass-loss rates are larger
than the predicted rates from radiation-driven wind theory for objects above
the BSJ temperature, whilst they are smaller for the rest. A new wind
momenta-luminosity relation for O8-B0 stars has been derived.Comment: 13 pages, 13 figures, A&
Mass-loss rates of Very Massive Stars
We discuss the basic physics of hot-star winds and we provide mass-loss rates
for (very) massive stars. Whilst the emphasis is on theoretical concepts and
line-force modelling, we also discuss the current state of observations and
empirical modelling, and address the issue of wind clumping.Comment: 36 pages, 15 figures, Book Chapter in "Very Massive Stars in the
Local Universe", Springer, Ed. Jorick S. Vin
Massive stars exploding in a He-rich circumstellar medium III. SN 2006jc: IR echoes from new and old dust in the progenitor CSM
We present near- and mid-infrared (IR) photometric data of the Type Ibn
supernova (SN) 2006jc obtained with the United Kingdom Infrared Telescope
(UKIRT), the Gemini North Telescope, and the Spitzer Space Telescope between
days 86 and 493 post-explosion. We find that the IR behaviour of SN 2006jc can
be explained as a combination of IR echoes from two manifestations of
circumstellar material. The bulk of the near-IR emission arises from an IR echo
from newly-condensed dust in a cool dense shell (CDS) produced by the
interaction of the ejecta outward shock with a dense shell of circumstellar
material ejected by the progenitor in a luminous blue variable (LBV) like
outburst about two years prior to the SN explosion. The CDS dust mass reaches a
modest 3.0 x 10^(-4) M(solar) by day 230. While dust condensation within a CDS
formed behind the ejecta inward shock has been proposed before for one event
(SN 1998S), SN 2006jc is the first one showing evidence for dust condensation
in a CDS formed behind the ejecta outward shock in the circumstellar material.
At later epochs, a substantial and growing contribution to the IR fluxes arises
from an IR echo from pre-existing dust in the progenitor wind. The mass of the
pre-existing CSM dust is at least ~8 x 10^(-3) M(solar). This work therefore
adds to the evidence that mass-loss from the progenitors of core-collapse
supernovae could be a major source of dust in the universe. However, yet again,
we see no direct evidence that the explosion of a supernova produces anything
other than a very modest amount of dust.Comment: 18 pages, 10 figures, accepted to MNRA
The Tarantula Massive Binary Monitoring: I. Observational campaign and OB-type spectroscopic binaries
© ESO, 2017.Context. Massive binaries play a crucial role in the Universe. Knowing the distributions of their orbital parameters is important for a wide range of topics from stellar feedback to binary evolution channels and from the distribution of supernova types to gravitational wave progenitors, yet no direct measurements exist outside the Milky Way. Aims. The Tarantula Massive Binary Monitoring project was designed to help fill this gap by obtaining multi-epoch radial velocity (RV) monitoring of 102 massive binaries in the 30 Doradus region. Methods. In this paper we analyze 32 FLAMES/GIRAFFE observations of 93 O- and 7 B-type binaries. We performed a Fourier analysis and obtained orbital solutions for 82 systems: 51 single-lined (SB1) and 31 double-lined (SB2) spectroscopic binaries. Results. Overall, the binary fraction and orbital properties across the 30 Doradus region are found to be similar to existing Galactic samples. This indicates that within these domains environmental effects are of second order in shaping the properties of massive binary systems. A small difference is found in the distribution of orbital periods, which is slightly flatter (in log space) in 30 Doradus than in the Galaxy, although this may be compatible within error estimates and differences in the fitting methodology. Also, orbital periods in 30 Doradus can be as short as 1.1 d, somewhat shorter than seen in Galactic samples. Equal mass binaries (q> 0.95) in 30 Doradus are all found outside NGC 2070, the central association that surrounds R136a, the very young and massive cluster at 30 Doradus's core. Most of the differences, albeit small, are compatible with expectations from binary evolution. One outstanding exception, however, is the fact that earlier spectral types (O2-O7) tend to have shorter orbital periods than later spectral types (O9.2-O9.7). Conclusions. Our results point to a relative universality of the incidence rate of massive binaries and their orbital properties in the metallicity range from solar (Z) to about half solar. This provides the first direct constraints on massive binary properties in massive star-forming galaxies at the Universe's peak of star formation at redshifts z ~ 1 to 2 which are estimated to have Z 0.5 Z
Controlling crystallization and its absence: Proteins, colloids and patchy models
The ability to control the crystallization behaviour (including its absence)
of particles, be they biomolecules such as globular proteins, inorganic
colloids, nanoparticles, or metal atoms in an alloy, is of both fundamental and
technological importance. Much can be learnt from the exquisite control that
biological systems exert over the behaviour of proteins, where protein
crystallization and aggregation are generally suppressed, but where in
particular instances complex crystalline assemblies can be formed that have a
functional purpose. We also explore the insights that can be obtained from
computational modelling, focussing on the subtle interplay between the
interparticle interactions, the preferred local order and the resulting
crystallization kinetics. In particular, we highlight the role played by
``frustration'', where there is an incompatibility between the preferred local
order and the global crystalline order, using examples from atomic glass
formers and model anisotropic particles.Comment: 11 pages, 7 figure
How much H and He is "hidden" in SNe Ib/c? I. - low-mass objects
H and He features in photospheric spectra have seldom been used to infer
quantitatively the properties of Type IIb, Ib and Ic supernovae (SNe IIb, Ib
and Ic) and their progenitor stars. Most radiative transfer models ignored NLTE
effects, which are extremely strong especially in the He-dominated zones. In
this paper, a comprehensive set of model atmospheres for low-mass SNe IIb/Ib/Ic
is presented. Long-standing questions such as how much He can be contained in
SNe Ic, where He lines are not seen, can thus be addressed. The state of H and
He is computed in full NLTE, including the effect of heating by fast electrons.
The models are constructed to represent iso-energetic explosions of the same
stellar core with differently massive H/He envelopes on top. The synthetic
spectra suggest that 0.06 - 0.14 M_sun of He and even smaller amounts of H
suffice for optical lines to be present, unless ejecta asymmetries play a major
role. This strongly supports the conjecture that low-mass SNe Ic originate from
binaries where progenitor mass loss can be extremely efficient.Comment: 19 pages, 12 figures. Published in MNRAS. V2 with corrections and
additional reference
SN 2008S: an electron capture SN from a super-AGB progenitor?
We present comprehensive photometric and spectroscopic observations of the
faint transient SN 2008S discovered in NGC 6946. SN 2008S exhibited slow
photometric evolution and almost no spectral variability during the first nine
months, implying a high density CS medium. The light curve is similar in shape
to that of SN 1998S and SN 1979C, although significantly fainter at maximum
light. Our quasi-bolometric lightcurve extends to 300 days and shows a tail
phase decay rate consistent with that of ^{56}Co. We propose that this is
evidence for an explosion and formation of ^{56}Ni (0.0015 +/- 0.0004 M_Sun).
The large MIR flux detected shortly after explosion can be explained by a light
echo from pre-exisiting dust. The late NIR flux excess is plausibly due to a
combination of warm newly-formed ejecta dust together with shock-heated dust in
the CS environment. We reassess the progenitor object detected previously in
Spitzer archive images, supplementing this discussion with a model of the MIR
spectral energy distribution. This supports the idea of a dusty, optically
thick shell around SN 2008S with an inner radius of nearly 90AU and outer
radius of 450AU, and an inferred heating source of 3000 K and luminosity of L ~
10^{4.6} L_Sun. The combination of our monitoring data and the evidence from
the progenitor analysis leads us to support the scenario of a weak electron
capture supernova explosion in a super-AGB progenitor star (of initial mass 6-8
M_sun) embedded within a thick CS gaseous envelope. We suggest that all of main
properties of the electron capture SN phenomenon are observed in SN 2008S and
future observations may allow a definitive answer.Comment: accepted for publication in MNRAS (2009 May 7
SN 2016coi/ASASSN-16fp: an example of residual helium in a type Ic supernova?
The optical observations of Ic-4 supernova (SN) 2016coi/ASASSN-16fp, from ∼2 to ∼450 d after explosion, are presented along with analysis of its physical properties. The SN shows the broad lines associated with SNe Ic-3/4 but with a key difference. The early spectra display a strong absorption feature at ∼5400 Å which is not seen in other SNe Ic-3/4 at this epoch. This feature has been attributed to He I in the literature. Spectral modelling of the SN in the early photospheric phase suggests the presence of residual He in a C/O dominated shell. However, the behaviour of the He I lines is unusual when compared with He-rich SNe, showing relatively low velocities and weakening rather than strengthening over time. The SN is found to rise to peak ∼16 d after core-collapse reaching a bolometric luminosity of Lp∼3 × 1042 erg s−1. Spectral models, including the nebular epoch, show that the SN ejected 2.5–4 M⊙ of material, with ∼1.5 M⊙ below 5000 km s−1, and with a kinetic energy of (4.5–7) × 1051 erg. The explosion synthesized ∼0.14 M⊙ of 56Ni. There are significant uncertainties in E(B − V)host and the distance, however, which will affect Lp and MNi. SN 2016coi exploded in a host similar to the Large Magellanic Cloud (LMC) and away from star-forming regions. The properties of the SN and the host-galaxy suggest that the progenitor had MZAMS of 23–28 M⊙ and was stripped almost entirely down to its C/O core at explosion