1,128 research outputs found
COVID-19 and the Demand for Online Grocery Shopping:Empirical Evidence from the Netherlands
There has been a pronounced increase in online shopping since the start of the COVID-19 pandemic. We study the effect of the pandemic on demand for online grocery shopping specifically, using municipality-level data from a Dutch online supermarket. We find that an additional hospital admission increased app traffic by 7.3 percent and sales per order by 0.31 percent. Local hospital admissions do not correlate with the variety of groceries ordered, but online search behavior does, suggesting that hoarding behavior is driven by the general perception and impact of the virus rather than local conditions. Local COVID-19 conditions also have different effects in urban versus non-urban municipalities, with local hospital admissions increasing app traffic in urban areas but lowering sales per order as compared to non-urban areas. It remains to be seen whether the demand for online grocery shopping will permanently increase as a result of the COVID-19 pandemic.</p
Modelling the atmosphere of the carbon-rich Mira RU Vir
Context. We study the atmosphere of the carbon-rich Mira RU Vir using the
mid-infrared high spatial resolution interferometric observations from
VLTI/MIDI. Aims. The aim of this work is to analyse the atmosphere of the
carbon-rich Mira RU Vir, with state of the art models, in this way deepening
the knowledge of the dynamic processes at work in carbon-rich Miras. Methods.
We compare spectro-photometric and interferometric measurements of this
carbon-rich Mira AGB star, with the predictions of different kinds of modelling
approaches (hydrostatic model atmospheres plus MOD-More Of Dusty,
self-consistent dynamic model atmospheres). A geometric model fitting tool is
used for a first interpretation of the interferometric data. Results. The
results show that a joint use of different kind of observations (photometry,
spectroscopy, interferometry) is essential to shed light on the structure of
the atmosphere of a carbon-rich Mira. The dynamic model atmospheres fit well
the ISO spectrum in the wavelength range {\lambda} = [2.9, 25.0] {\mu}m.
Nevertheless, a discrepancy is noticeable both in the SED (visible), and in the
visibilities (shape and level). A possible explanation are intra-/inter-cycle
variations in the dynamic model atmospheres as well as in the observations. The
presence of a companion star and/or a disk or a decrease of mass loss within
the last few hundred years cannot be excluded but are considered unlikely.Comment: 15 pages. Accepted in A&
The mass-loss return from evolved stars to the Large Magellanic Cloud III. Dust properties for carbon-rich asymptotic giant branch stars
We present a 2Dust model for the dust shell around a LMC long-period variable
(LPV) previously studied as part of the OGLE survey. OGLE LMC LPV 28579 (SAGE
J051306.40-690946.3) is a carbon-rich asymptotic giant branch (AGB) star for
which we have photometry and spectra from the Spitzer SAGE and SAGE-Spec
programs along with UBVIJHK_s photometry. By modeling this source, we obtain a
baseline set of dust properties to be used in the construction of a grid of
models for carbon stars. We reproduce its spectral energy distribution using a
mixture of AmC and SiC (15% by mass). The grain sizes are distributed according
to the KMH model. The best-fit model has an optical depth of 0.28 for the shell
at the peak of the SiC feature, with R_in~1430 R_sun or 4.4 R_star. The
temperature at this inner radius is 1310 K. Assuming an expansion velocity of
10 km s^-1, we obtain a dust mass-loss rate of 2.5x10^-9 M_sun yr-1. We
calculate a 15% variation in this rate by testing the fit sensitivity against
variation in input parameters. We also present a simple model for the molecular
gas in the extended atmosphere that could give rise to the 13.7 \mu m feature
seen in the spectrum. We find that a combination of CO and C_2H_2 gas at an
excitation temperature of about 1000 K and column densities of 3x10^21 cm^-2
and 10^19 cm^-2 respectively are able to reproduce the observations. Given that
the excitation temperature is close to T_dust(R_in), most of the molecular
contribution probably arises from the inner shell region. The luminosity
corresponding to the first epoch of SAGE observations is 6580 L_sun. For an
effective temperature of about 3000 K, this implies a stellar mass of 1.5-2
M_sun and an age of 1-2.5 Gyr. For a gas:dust ratio of 200, we obtain a gas
mass-loss rate of 5.0x10^-7 M_sun yr^-1, consistent with the gas mass-loss
rates estimated from the period, color and 8 \mu m flux of the source.Comment: 14 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
Fast thermistor string observations at the slope of Great Meteor Seamount
International audienceA very fast thermistor string has been built to accommodate the scientific need to accurately monitor fast and vigorous internal wave and overturning processes above sloping bottoms in the ocean. The thermistors and their custom designed electronics can register temperature at an estimated precision of about 1mK with a response time faster than 0.25 s down to depths of 6000 m. The present string holds 128 synoptically measuring sensors at 0.5 m intervals, which are all read-out within 0.5 s. When sampling at 1Hz, the batteries and the memory capacity of the recorder allow for deployments of up to 2 weeks. Detailed examples of the first field observations are presented, which show overturning and very high-frequency (Doppler-shifted) internal waves besides occasionally large turbulent bores moving up the sloping side of Great Meteor Seamount, Canary Basin, North-Atlantic Ocean
High sampling rate thermistor string observations at the slope of Great Meteor Seamount
International audienceA high sampling rate (1 Hz) thermistor string has been built to accommodate the scientific need to accurately monitor high-frequency and vigorous internal wave and overturning processes in the ocean. The thermistors and their custom designed electronics can register temperature at an estimated precision of about 0.001° C with a response time faster than 0.25 s down to depths of 6000 m. With a quick in situ calibration using SBE 911 CTD an absolute accuracy of 0.005° C is obtained. The present string holds 128 sensors at 0.5 m intervals, which are all read-out within 0.5 s. When sampling at 1 Hz, the batteries and the memory capacity of the recorder allow for deployments of up to 2 weeks. In this paper, the instrument is described in some detail. Its performance is illustrated with examples from the first moored observations, which show Kelvin-Helmholtz overturning and very high-frequency (Doppler-shifted) internal waves besides occasionally large turbulent bores moving up the sloping side of Great Meteor Seamount, Canary Basin, North-Atlantic Ocean
The envelope of IRC+10216 reflecting the galactic light: UBV surface brightness photometry and interpretation
We present and analyse new optical images of the dust envelope surrounding
the high mass-loss carbon star IRC+10216. This envelope is seen due to external
illumination by galactic light. Intensity profiles and colors of the nebula
were obtained in the UBV bandpasses. The data are compared with the results of
a radiative transfer model calculating multiple scattering of interstellar
field photons by dust grains with a single radius. The data show that the
observed radial shape of the nebula, especially its half maximum radius, does
not depend on wavelength (within experimental errors), suggesting that grains
scatter in the grey regime, etc, etc (this abstract has been shortened)Comment: accepted by A
A Submillimeter HCN Laser in IRC+10216
We report the detection of a strong submillimeter wavelength HCN laser line
at a frequency near 805 GHz toward the carbon star IRC+10216. This line, the
J=9-8 rotational transition within the (04(0)0) vibrationally excited state, is
one of a series of HCN laser lines that were first detected in the laboratory
in the early days of laser spectroscopy. Since its lower energy level is 4200 K
above the ground state, the laser emission must arise from the inner part of
IRC+10216's circumstellar envelope. To better characterize this environment, we
observed other, thermally emitting, vibrationally excited HCN lines and find
that they, like the laser line, arise in a region of temperature approximately
1000 K that is located within the dust formation radius; this conclusion is
supported by the linewidth of the laser. The (04(0)0), J=9-8 laser might be
chemically pumped and may be the only known laser (or maser) that is excited
both in the laboratory and in space by a similar mechanism.Comment: 11 pages, 3 figure
Evolution and Nucleosynthesis of Zero Metal Intermediate Mass Stars
New stellar models with mass ranging between 4 and 8 Mo, Z=0 and Y=0.23 are
presented. The models have been evolved from the pre Main Sequence up to the
Asymptotic Giant Branch (AGB). At variance with previous claims, we find that
these updated stellar models do experience thermal pulses in the AGB phase. In
particular we show that: a) in models with mass larger than 6 Mo, the second
dredge up is able to raise the CNO abundance in the envelope enough to allow a
"normal" AGB evolution, in the sense that the thermal pulses and the third
dredge up settle on; b) in models of lower mass, the efficiency of the CNO
cycle in the H-burning shell is controlled by the carbon produced locally via
the 3alpha reactions. Nevertheless the He-burning shell becomes thermally
unstable after the early AGB. The expansion of the overlying layers induced by
these weak He-shell flashes is not sufficient by itself to allow a deep
penetration of the convective envelope. However, immediately after that, the
maximum luminosity of the He flash is attained and a convective shell
systematically forms at the base of the H-rich envelope. The innermost part of
this convective shell probably overlaps the underlying C-rich region left by
the inter-shell convection during the thermal pulse, so that fresh carbon is
dredged up in a "hot" H-rich environment and a H flash occurs. This flash
favours the expansion of the outermost layers already started by the weak
thermal pulse and a deeper penetration of the convective envelope takes place.
Then, the carbon abundance in the envelope rises to a level high enough that
the further evolution of these models closely resembles that of more metal rich
AGB stars. These stars provide an important source of primary carbon and
nitrogen.Comment: 28 pages, 5 tables and 17 figures. Accepted for publication in Ap
An independent distance estimate to CW Leo
CW Leo has been observed six times between October 2009 and June 2012 with
the SPIRE instrument on board the Herschel satellite. Variability has been
detected in the flux emitted by the central star with a period of 639 \pm 4
days, in good agreement with determinations in the literature. Variability is
also detected in the bow shock around CW Leo that had previously been detected
in the ultraviolet and Herschel PACS/SPIRE data. Although difficult to prove
directly, our working hypothesis is that this variability is directly related
to that of the central star. In this case, fitting a sine curve with the period
fixed to 639 days results in a time-lag in the variability between bow shock
and the central star of 402 \pm 37 days. The orientation of the bow shock
relative to the plane of the sky is unknown (but see below). For an inclination
angle of zero degrees, the observed time-lag translates into a distance to CW
Leo of 130 \pm 13 pc, and for non-zero inclination angles the distance is
smaller. Fitting the shape of the bow shock with an analytical model (Wilkin
1996), the effect of the inclination angle on the distance may be estimated.
Making the additional assumption that the relative peculiar velocity between
the interstellar medium (ISM) and CW Leo is determined entirely by the star
space velocity with respect to the local standard of rest (i.e. a stationary
ISM), the inclination angle is found to be (-33.3 \pm 0.8) degrees based on the
observed proper motion and radial velocity. Using the Wilkin model, our current
best estimate of the distance to CW Leo is 123 \pm 14 pc. For a distance of 123
pc, we derive a mean luminosity of 7790 \pm 150 Lsol (internal error).Comment: Accepted A&A Letter
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