9,364 research outputs found
Cored Apple Bipolarity : A Global Instability to Convection in Radial Accretion?
We propose that the prevalence of bipolarity in Young Stellar Objects is due
to the fine tuning that is required for spherical accretion of an ambient
medium onto a central node.It is shown that there are two steady modes that are
more likely than radial accretion, each of which is associated with a
hyperbolic central point in the meridional stream lines, and consequently with
either an equatorial inflow and an axial ejection or vice versa. In each case
the stream lines pass through a thick accretion torus, which is better thought
of as a standing pressure wave rather than as a relatively inert Keplerian
structure.We base our arguments on a simple analytic example,which is topologi
cally generic,wherein each bipolarmode is created by the rebound of accreting
matter under the action of the thermal,magnetic,turbulent and centrifugal
pressures created in the flow. In both bipolar modes the presence of non-zero
angular momentum implies axial regions wherein the pressure is first reduced
below the value at infinity and then becomes negative, where the solution fails
because rotating material can not enter this region without suction.The model
thus has empty stems where the activity of the central source must dominate.So
the basic engine of the bipolar flow discussed here is simply the rebound of
freely falling material from a thick pressure disc into an axial low pressure
region.The low mass,high velocity outflow must be produced in this region by an
additional mechanism. This is reminiscent of the cored apple structure observed
recently in the very young bipolar source VLA 1623.Comment: PostScript, 10 page
New constraints on a triaxial model of the Galaxy
We determine the values of parameters of an N-body model for the Galaxy
developed by Fux via comparison with an unbiased, homogeneous sample of OH/IR
stars. Via Monte-Carlo simulation, we find the plausibilities of the
best-fitting models, as well as their errors. The parameters that are
constrained best by these projected data are the total mass of the model and
the viewing angle of the central Bar, although the distribution of the latter
has multiple maxima. The best model has a viewing angle of 44 degrees,
semi-major axis of 2.5 kpc, a bar mass of 1.7E10 solar masses and a tangential
velocity of the local standard of rest of 171 km/s . We argue that the lower
values that are commonly found from stellar data for the viewing angle (around
25 degrees) arise when too few coordinates are available, when the longitude
range is too narrow or when low latitudes are excluded from the fit. The new
constraints on the viewing angle of the galactic Bar from stellar line-of-sight
velocities decrease further the ability of the Bar's distribution to account
for the observed micro-lensing optical depth toward Baade's window : our model
reproduces only half the observed value. The signal of triaxiality diminishes
quickly with increasing latitude, fading within approximately one scaleheight.
This suggests that Baade's window is not a very appropriate region to sample
Bar properties.Comment: 10 pages, 8 figures, TeX, accepted for publication in MNRA
Discovery of a high-redshift Einstein ring
We report the discovery of a partial Einstein ring of radius 1.48arcsec
produced by a massive (and seemingly isolated) elliptical galaxy. The
spectroscopic follow-up at the VLT reveals a 2L* galaxy at z=0.986, which is
lensing a post-starburst galaxy at z=3.773. This unique configuration yields a
very precise measure of the mass of the lens within the Einstein radius,
(8.3e11 +- 0.4)/h70 Msolar. The fundamental plane relation indicates an
evolution rate of d [log (M/L)B] / dz = -0.57+-0.04, similar to other massive
ellipticals at this redshift. The source galaxy shows strong interstellar
absorption lines indicative of large gas-phase metallicities, with fading
stellar populations after a burst. Higher resolution spectra and imaging will
allow the detailed study of an unbiased representative of the galaxy population
when the universe was just 12% of its current age.Comment: 5 pages, 3 figures, accepted in A&A Le
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