We define new families of noncommutative symmetric functions and quasi-symmetric functions depending on two matrices of parameters, and more generally on parameters associated with paths in a binary tree. Appropriate specializations of both matrices then give back the two-vector families of Hivert, Lascoux, and Thibon and the noncommutative Macdonald functions of Bergeron and Zabrocki. Résumé. ...

A new type of object called “Very Low Luminosity Objects (VeLLOs)” has been discovered by the Spitzer Space Telescope. VeLLOs might be substellar objects forming by accretion. However, some VeLLOs are associated with strong outflows, indicating the previous existence of massive accretion. The thermal history, which significantly affects the chemistry, between substellar objects with a continuou...

Relativistic jets are a fundamental aspect of accretion onto black holes on all scales. They can carry away a large fraction of the available accretion power in collimated flows which later energise particles in the ambient medium. The removal of this accretion power and angular momentum must have a dramatic effect on the overall process of accretion. In their most spectacular form they are ass...

Introduction: Early models of accretion of Mars [1] suggested long time scales and led to assumed cold post-accretion initial temperatures [2]. But later accretion models [3] gave much shorter time scales and led to the general acceptance of a hot Mars heated by the kinetic energy of impacting planetesimals. Evidence from isotopic studies also seems to require early melting; in particular, Hf-W...

The theory of coronal evaporation predicts the formation of an inner hole in the cool thin accretion disk for mass accretion rates below a certain value (≈ 1/50 of the Eddington mass accretion rate) and the sudden disappearance of this hole when the mass accretion rate rises above that value. The inner edge of the standard thin disk then suddenly shifts inward vvvvfrom about a few hundred Schwa...

We examine the X-ray radio correlation in Galactic black hole sources. We highlight some of the results which extend the flux-flux relations to sources with very high accretion rates. Some of the recent results indicate that the synchrotron process is unlikely to be the mechanism responsible for the X-ray emission, particularly at high accretion rates. We present a truncated accretion disk scen...

We have analyzed all the observations of BP Tauri taken by the International Ultraviolet Explorer in the low resolution (△λ ∼ 6Å), long wavelength (from λ = 1850Å to λ = 3350Å) range. This dataset contains 61 spectra. We observe variability in the ultraviolet continuum of △mcont. ∼ 1 magnitude and variability in the Mg II line flux of △mMgII ∼ 0.8 magnitudes. Moreover, these spectra do not show...

We consider the effects of eccentricity on the fragmentation of gravitationally unstable accretion disks, using numerical hydrodynamics. We find that eccentricity does not affect the overall stability of the disk against fragmentation, but significantly alters the manner in which such fragments accrete gas. Variable tidal forces around an eccentric orbit slow the accretion process, and suppress...

Accretion rates of order 10−8 M⊙yr are observed in young protostars of approximately a solar mass with evidence of circumstellar disks. The accretion rate is significantly lower for protostars of smaller mass, approximately proportional to the second power of the stellar mass, Ṁaccr ∝ M2. The traditional view is that the observed accretion is the consequence of the angular momentum transport in...