139 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
On biases in the predictions of stellar population synthesis models
Sampling fluctuations in stellar populations give rise to dispersions in
observables when a small number of sources contribute effectively to the
observables. This is the case for a variety of linear functions of the spectral
energy distribution (SED) in small stellar systems, such as galactic and
extragalactic HII regions, dwarf galaxies or stellar clusters. In this paper we
show that sampling fluctuations also introduce systematic biases and
multi-modality in non-linear functions of the SED, such as luminosity ratios,
magnitudes and colours. In some cases, the distribution functions of rational
and logarithmic quantities are bimodal, hence complicating considerably the
interpretation of these quantities in terms of age or evolutionary stages.
These biases can be only assessed by Monte Carlo simulations. We find that
biases are usually negligible when the effective number of stars, Neff, which
contribute to a given observable is larger than 10. Bimodal distributions may
appear when Neff is between 10 and 0.1. Predictions from any model of stellar
population synthesis become extremely unreliable for small Neff values,
providing an operational limit to the applicability of such models for the
interpretation of integrated properties of stellar systems. In terms of stellar
masses, assuming a Salpeter Initial Mass Function in the range 0.08 -- 120 Mo,
Neff=10 corresponds to about 10**5 Mo (although the exact value depends on the
age and the observable). This bias may account, at least in part, for claimed
variations in the properties of the stellar initial mass function in small
systems, and arises from the discrete nature of small stellar populations.Comment: 16 pages, 14 figures. Accpeted for publication in MNRA
Microwave ISM Emission Observed by WMAP
We investigate the nature of the diffuse Galactic emission in the Wilkinson
Microwave Anisotropy Probe (WMAP) temperature anisotropy data. Substantial
dust-correlated emission is observed at all WMAP frequencies, far exceeding the
expected thermal dust emission in the lowest frequency channels (23, 33, 41
GHz). The WMAP team (Bennett et al.) interpret this emission as dust-correlated
synchrotron radiation, attributing the correlation to the natural association
of relativistic electrons produced by SNae with massive star formation in dusty
clouds, and deriving an upper limit of 5% on the contribution of Draine &
Lazarian spinning dust at K-band (23 GHz). We pursue an alternative
interpretation that much, perhaps most, of the dust-correlated emission at
these frequencies is indeed spinning dust, and explore the spectral dependence
on environment by considering a few specific objects as well as the full sky
average. Models similar to Draine & Lazarian spinning dust provide a good fit
to the full-sky data. The full-sky fit also requires a significant component
with free-free spectrum uncorrelated with \Halpha, possibly hot (~million K)
gas within 30 degrees of the Galactic center.Comment: ApJ in press (accepted 5 Dec 2003), version 2: corrected typos and
added references. 23 pages, 5 figures, 2 tables. Free-free haze map is
available at http://skymaps.inf
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