310 research outputs found
Stellar winds, dead zones, and coronal mass ejections
Axisymmetric stellar wind solutions are presented, obtained by numerically
solving the ideal magnetohydrodynamic (MHD) equations. Stationary solutions are
critically analysed using the knowledge of the flux functions. These flux
functions enter in the general variational principle governing all axisymmetric
stationary ideal MHD equilibria. The magnetized wind solutions for
(differentially) rotating stars contain both a `wind' and a `dead' zone. We
illustrate the influence of the magnetic field topology on the wind
acceleration pattern, by varying the coronal field strength and the extent of
the dead zone. This is evident from the resulting variations in the location
and appearance of the critical curves where the wind speed equals the slow,
Alfven, and fast speed. Larger dead zones cause effective, fairly isotropic
acceleration to super-Alfvenic velocities as the polar, open field lines are
forced to fan out rapidly with radial distance. A higher field strength moves
the Alfven transition outwards. In the ecliptic, the wind outflow is clearly
modulated by the extent of the dead zone. The combined effect of a fast stellar
rotation and an equatorial `dead' zone in a bipolar field configuration can
lead to efficient thermo-centrifugal equatorial winds. Such winds show both a
strong poleward collimation and some equatorward streamline bending due to
significant toroidal field pressure at mid-latitudes. We discuss how coronal
mass ejections are then simulated on top of the transonic outflows.Comment: scheduled for Astrophys. J. 530 #2, Febr.20 2000 issue. 9 figures (as
6 jpeg and 8 eps files
Convective magneto-rotational instabilities in accretion disks
We present a study of instabilities occuring in thick magnetized accretion
disks. We calculate the growth rates of these instabilities and characterise
precisely the contribution of the magneto-rotational and the convective
mechanism. All our calculations are performed in radially stratified disks in
the cylindrical limit. The numerical calculations are performed using the
appropriate local dispersion equation solver discussed in Blokland et al.
(2005). A comparison with recent results by Narayan et al. (2002) shows
excellent agreement with their approximate growth rates only if the disks are
weakly magnetized. However, for disks close to equipartition, the dispersion
equation from Narayan et al. (2002) loses its validity. Our calculations allow
for a quantitative determination of the increase of the growth rate due to the
magneto-rotational mechanism. We find that the increase of the growth rate for
long wavelength convective modes caused by this mechanism is almost neglible.
On the other hand, the growth rate of short wavelength instabilities can be
significantly increased by this mechanism, reaching values up to 60%.Comment: 10 pages, 9 figures, Accepted for publication in Astronomy &
Astrophysic
Stable isotopic composition of bivalve shell organic matrix: <i>Mytilus edulis</i> collected along the Scheldt estuary
Bivalve shells are biostructures composed of a mineral and an organic phase. For paleoclimatology applications, the mineral part (carbonates) is most widely studied. In contrast, understanding of the composition and the proxyfunction of the organic matrix is much less developed. The quantity of organic matrix in shells is relatively small compared to the mineral phase (a few wt %) and the biochemical composition is quite complex, consisting mainly of sugars and proteins. Lipids, which represent a small fraction of the organic matrix, are rather poorly known. We studied the potential of stable isotope composition (C, N, H) of bulk organic matrix and specific lipid compounds of Mytilis edulis shells, as environmental and climatic proxies, with special focus on the effects due to changing salinity. Mytilus specimens were collected along the salinity gradient of the Scheldt estuary (The Netherlands) and we analysed the isotopic composition of the organic matrix and associated specific lipid compounds and related these to averaged physico-chemical characteristics of the water, in particular salinity. We discuss these relationships in the light of their usefulness as proxies for reconstructing past environmental conditions
3D Structures on Relativistic Jets
The properties of wave-like helically twisted normal mode structures on
steady relativistic jets are summarized. Wave speeds are a function of the
wavelength and less than the jet speed. However, normal mode interference can
lead to both stationary and superluminal phase effects. A maximum pressure
fluctuation criterion suggested by numerical simulations of axisymmetric
relativistic jets is used to find the maximum asymmetric jet distortions and
velocity fluctuations. Cyclic transverse velocity fluctuation can lead to
variation in the flow direction on the order of the relativistic beaming angle.
Resulting variation in the Doppler boost factor can lead to significant
brightness asymmetries as helical structures twist around the jet beam. Growth
of these structures is reduced as the jet density, Lorentz factor or Mach
number are increased. Maximum jet distortion is reduced as the Lorentz factor
increases and this suggests a reduction in mass entrainment or other non-linear
disruptive processes that influence the morphological development of radio
sources.Comment: to appear in Life Cycles of Radio Galaxies, ed. J. Biretta et al.,
New Astronomy Reviews; 8 pages, including 3 figure
Paleoclimate in continental northwestern Europe during the Eemian and early Weichselian (125–97 ka): insights from a Belgian speleothem
The last interglacial serves as an excellent time interval for studying climate dynamics during past warm periods. Speleothems have been successfully used for reconstructing the paleoclimate of last interglacial continental Europe. However, all previously investigated speleothems are restricted to southern Europe or the Alps, leaving large parts of northwestern Europe undocumented. To better understand regional climate changes over the past, a larger spatial coverage of European last interglacial continental records is essential, and speleothems, because of their ability to obtain excellent chronologies, can provide a major contribution. Here, we present new, high-resolution data from a stalagmite (Han-9) obtained from the Han-sur-Lesse Cave in Belgium. Han-9 formed between 125.3 and  ∼  97 ka, with interruptions of growth occurring at 117.3–112.9 and 106.6–103.6 ka. The speleothem was investigated for its growth, morphology and stable isotope (δ13C and δ18O) composition. The speleothem started growing relatively late within the last interglacial, at 125.3 ka, as other European continental archives suggest that Eemian optimum conditions were already present during that time. It appears that the initiation of Han-9 growth is caused by an increase in moisture availability, linked to wetter conditions around 125.3 ka. The δ13C and δ18O proxies indicate a period of relatively stable conditions after 125.3 ka; however, at 120 ka the speleothem δ18O registered the first signs of regionally changing climate conditions, being a modification of ocean source δ18O linked to an increase in ice volume towards the Marine Isotope Stage (MIS) 5e–5d transition. At 117.5 ka, drastic vegetation changes are recorded by Han-9 δ13C immediately followed by a cessation of speleothem growth at 117.3 ka, suggesting a transition to significantly dryer conditions. The Han-9 record covering the early Weichselian displays larger amplitudes in both isotope proxies and changes in stalagmite morphology, evidencing increased variability compared to the Eemian. Stadials that appear to be analogous to those in Greenland are recognized in Han-9, and the chronology is consistent with other European (speleothem) records. Greenland Stadial 25 is reflected as a cold/dry period within Han-9 stable isotope proxies, and the second interruption in speleothem growth occurs simultaneously with Greenland Stadial 24
Magneto-rotational overstability in accretion disks
We present analytical and numerical studies of magnetorotational
instabilities occuring in magnetized accretion disks. In these studies we make
use of the linearised compressible MHD equations. These calculations are
performed for general radially stratified disks in the cylindrical limit. In
particular, we investigate the influence of nonvanishing toroidal magnetic
field component on the growth rate and oscillation frequency of
magnetorotational instabilities in Keplerian disks. We find the persistence of
these instabilities in accretion disks close to equipartition. Our calculations
show that these eigenmodes become overstable (complex eigenvalue), due to the
presence of a toroidal magnetic field component, while their growth rate
reduces slightly. Furthermore, we demonstrate the presence of
magneto-rotational overstabilities in weakly magnetized sub-Keplerian rotating
disks. We show that the growth rate scales with the rotation frequency of the
disk. These eigenmodes also have a nonzero oscillation frequency, due to the
presence of the dominant toroidal magnetic field component. The overstable
character of the MRI increases as the rotation frequency of the disk decreases.Comment: 11 pager, 18 Postscript figures, accepted for publication in
Astronomy & Astrophysic
First-generation college students’ motives to start university education:An investment in self- development, one’s economic prospects or to become a role model?
In this article, we engage with the emerging literature that studies the increased enrolment of first-generation college students (FGCS), that is, students from households where neither parent has obtained a bachelor’s /master’s degree. Our article answers two research questions. First, data from 2,338 first-year students are used to investigate the extent to which FGCS differ from continuing-generation college students (CGCS) concerning the reason why one enrols in university education. Second, to what degree do these motives explain differences in study choice? Our results show that FCGS, compared to CGCS, more strongly endorsed the economic investment motive and what we call the social investment motive, that is, the motivation to become a role model for one’s community. In addition, our findings reveal that the choice for more economically rewarding fields of study is related to these motives to start a university education. In the conclusion, we discuss the implications of our findings
A Search for Photometric Rotation Periods in Low-Mass Stars and Brown Dwarfs in the Pleiades
We have photometrically monitored (Cousins Ic) eight low mass stars and brown
dwarfs which are probable members of the Pleiades. We derived rotation periods
for two of the stars - HHJ409 and CFHT-PL8 - to be 0.258 d and 0.401 d,
respectively. The masses of these stars are near 0.4 and 0.08 Msun,
respectively; the latter is the second such object near the hydrogen-burning
boundary for which a rotation period has been measured. We also observed HHJ409
in V; the relative amplitude in the two bands shows that the spots in that star
are about 200 K cooler than the stellar effective temperature of 3560 K and
have a filling factor on the order of 13%. With one possible exception, the
remaining stars in the sample do not show photometric variations larger than
the mean error of measurement. We also examined the M9.5V disk star 2MASSJ0149,
which had previously exhibited a strong flare event, but did not detect any
photometric variation.Comment: 13 pages, four figures. Accepted for publication in A
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