1,344 research outputs found
Non-contact temperature measurement of a falling drop
The 105 meter drop tube at NASA-Marshall has been used in a number of experiments to determine the effects of containerless, microgravity processing on the undercooling and solidification behavior of metals and alloys. These experiments have been limited, however, because direct temperature measurement of the falling drops has not been available. Undercooling and nucleation temperatures are calculated from thermophysical properties based on droplet cooling models. In most cases these properties are not well known, particularly in the undercooled state. This results in a large amount of uncertainty in the determination of nucleation temperatures. If temperature measurement can be accomplished then the thermal history of the drops could be well documented. This would lead to a better understanding of the thermophysical and thermal radiative properties of undercooled melts. An effort to measure the temperature of a falling drop is under way. The technique uses two color pyrometry and high speed data acquisition. The approach is presented along with some preliminary data from drop tube experiments. The results from droplet cooling models is compared with noncontact temperature measurements
A Model for Spectral States and Their Transition in Cyg X-1
A new accretion picture based on a small disk surrounding a black hole is
developed for the wind-fed source Cyg X-1. The hard and soft spectral states of
Cyg X-1 are interpreted in terms of co-spatial two component flows for the
innermost region of an accretion disk. The state transitions result from the
outward expansion and inward recession of this inner disk for the hard to soft
and soft to hard transition respectively. The theoretical framework for state
transitions in black hole X-ray binaries with high mass companions involving a
change in the inner disk size, thus, differs from systems with low mass
companions involving the change in the outer disk size. This fundamental
difference stems from the fact that matter captured and supplied to the black
hole in wind-fed systems has low specific angular momentum and is hot
essentially heated in the bow and spiral shocks, whereas it has high specific
angular momentum and is cool in Roche lobe overflow systems. The existence of a
weak cool disk around the ISCO region in the hard state allows for the presence
of a relativistically broadened Fe K line. The small disk fed by gas
condensation forms without an extensive outer disk, precluding thermal
instabilities and large outbursts, resulting in the lack of large amplitude
outbursts and hysteresis effects in the light curve of high mass black hole
X-ray binaries. Their relatively persistent X-ray emission is attributed to
their wind-fed nature.Comment: 13 pages, 2 figures. Accepted for publication in Ap
A cool disk in the Galactic Center?
We study the possibility of a cool disk existing in the Galactic Center in
the framework of the disk-corona evaporation/condensation model. Assuming an
inactive disk, a hot corona should form above the disk since there is a
continuous supply of hot gas from stellar winds of the close-by massive stars.
Whether the cool disk can survive depends on the mass exchange between the disk
and corona. If the disk-corona interaction is dominated by evaporation and the
rate is larger than the Bondi accretion rate in the Galactic Center, the disk
will be depleted within a certain time period and no persistent disk will
exist. On the other hand, if the interaction results in hot gas steadily
condensing into the disk, an inactive cool disk might survive. For this case we
further investigate the Bremsstrahlung radiation from the hot corona and
compare it with the observed X-ray luminosity. Our model shows that, for
standard viscosity in the corona (alpha=0.3), the mass evaporation rate is much
higher than the Bondi accretion rate and the coronal density is much larger
than that inferred from Chandra observations. An inactive disk can not survive
such strong evaporation. For small viscosity (alpha<0.07) we find condensation
solutions. But detailed computations show that in this case there is too much
X-ray radiation from the corona to be in agreement with the observations.
Therefore, we conclude that there should be no thin/inactive disk presently in
the Galactic Center. However, we do not exclude that the alternative
non-radiative model of Nayakshin (2004) might instead be realized in nature.Comment: 8 pages, including 3 figures, accepted for publication in A&
Optical properties of silicon carbide for astrophysical applications I. New laboratory infrared reflectance spectra and optical constants
Silicon Carbide (SiC) optical constants are fundamental inputs for radiative
transfer models of astrophysical dust environments. However, previously
published values contain errors and do not adequately represent the bulk
physical properties of the cubic (beta) SiC polytype usually found around
carbon stars. We provide new, uncompromised optical constants for beta- and
alpha-SiC derived from single-crystal reflectance spectra and investigate
quantitatively whether there is any difference between alpha- and beta-SiC that
can be seen in infrared spectra and optical functions.
Previous optical constants for SiC do not reflect the true bulk properties,
and they are only valid for a narrow grain size range. The new optical
constants presented here will allow narrow constraints to be placed on the
grain size and shape distribution that dominate in astrophysical environments.
In addition, our calculated absorption coefficients are much higher than
laboratory measurements, which has an impact on the use of previous data to
constrain abundances of these dust grains.Comment: 12 pages; 10 figures; laboratory optical constants available from
CDS. Accepted by Astronomy & Astrophysic
Characteristics of Low-Latitude Coronal Holes near the Maximum of Solar cycle 24
We investigate the statistics of 288 low-latitude coronal holes extracted
from SDO/AIA-193 filtergrams over the time range 2011/01/01 to 2013/12/31. We
analyse the distribution of characteristic coronal hole properties, such as the
areas, mean AIA-193 intensities, and mean magnetic field densities, the local
distribution of the SDO/AIA-193 intensity and the magnetic field within the
coronal holes, and the distribution of magnetic flux tubes in coronal holes. We
find that the mean magnetic field density of all coronal holes under study is
3.0 +- 1.6 G, and the percentage of unbalanced magnetic flux is 49 +- 16 %. The
mean magnetic field density, the mean unsigned magnetic field density, and the
percentage of unbalanced magnetic flux of coronal holes depend strongly
pairwise on each other, with correlation coefficients cc > 0.92. Furthermore,
we find that the unbalanced magnetic flux of the coronal holes is predominantly
concentrated in magnetic flux tubes: 38 % (81 %) of the unbalanced magnetic
flux of coronal holes arises from only 1 % (10 %) of the coronal hole area,
clustered in magnetic flux tubes with field strengths > 50 G (10 G). The
average magnetic field density and the unbalanced magnetic flux derived from
the magnetic flux tubes correlate with the mean magnetic field density and the
unbalanced magnetic flux of the overall coronal hole (cc > 0.93). These
findings give evidence that the overall magnetic characteristics of coronal
holes are governed by the characteristics of the magnetic flux tubes.Comment: 15 figure
Hysteresis in spectral state transitions - a challenge for theoretical modeling
Many low-mass X-ray binaries show both hard and soft spectral states. For
several sources the transitions between these states have been observed, mostly
from the soft to the hard state during a luminosity decrease. In a few cases
also the transition from the hard to the soft state was observed, coincident
with an increase of the luminosity. Surprisingly this luminosity was not the
same as the one during a following change back to the hard state. The values
differed by a factor of about 3 to 5. We present a model for this hysteresis in
the light curves of low-mass X-ray binaries (sources with neutron stars or
black holes). We show that the different amount of Compton cooling or heating
acting on the accretion disk corona at the time of the transition causes this
switch in the accretion mode at different mass accretion rates and therefore
different luminosities. The inner disk during the soft state provides a certain
amount of Compton cooling which is either not present or much less if the inner
region is filled with a hot advection-dominated accretion flow (ADAF) that
radiates a hard spectrum.Comment: 8 pages, 2 figures, accepted for publication in Astron. Astrophy
Environmentally conscious consumption patterns in Hungarian households
This article provides a comprehensive review of the literature on the theoretical aspects of sustainable consumption. The conditions for consumers’ social responsibility and the formation of environmentally conscious behavior patterns will also be discussed, along with possible methods for motivating behavioral changes. The authors have completed a primary research study with the purpose of surveying environmentally conscious consumption patterns in Hungary. They also examined how the provision of appropriate information and the raising of awareness might encourage sustainable consumption. According to their findings, the respondents’ knowledge on environmentally conscious behavior was rather limited, and reinforcement was needed in identifying appropriate activity alternatives. This paper provides a summary of the qualitative research phase which employed in-depth interviews, logging and focus groups. The consecutive application of these methods enabled the authors to keep track of the process and the consequences of raising awareness
- …