We report the discovery of excess K-band radiation from a metal-rich DAV white dwarf star, WD1150-153. Our near infrared spectroscopic observations show that the excess radiation cannot be explained by a (sub)stellar companion, and is likely to be caused by a debris disk similar to the other DAZ white dwarfs with circumstellar debris disks. We find that the fraction of DAZ white dwarfs with det...

We describe new, deep MIPS photometry and new high signal-to-noise optical spectroscopy of the 2.5 Myr-old IC 348 Nebula. To probe the properties of the IC 348 disk population, we combine these data with previous optical/infrared photometry and spectroscopy to identify stars with gas accretion, to examine their mid-IR colors, and to model their spectral energy distributions. IC 348 contains man...

We review the present knowledge of disk accretion in young low mass stars, and in particular, the mass accretion rate ˙ M and its evolution with time. The methods used to obtain mass accretion rates from ultraviolet excesses and emission lines are described, and the current best estimates of ˙ M for Classical T Tauri stars and for objects still surrounded by infalling envelopes are given. We ar...

Recent observations show a strong correlation between stellar mass and accretion rate in young stellar and sub-stellar objects, with the scaling Ṁacc ∝ M 2 ∗ holding over more than four orders of magnitude in accretion rate. We explore the consequences of this correlation in the context of disk evolution models. We note that such a correlation is not expected to arise from variations in disk an...

We compute the properties of a geometrically thin, steady accretion disk surrounding a central rotating, magnetized star. The magnetosphere is assumed to entrain the disk over a wide range of radii. The model is simplified in that we adopt two (alternate) ad hoc, but plausible, expressions for the azimuthal component of the magnetic field as a function of radial distance. We find a solution for...

We construct a model for supercritical accretion disks with winds, where the mass-accretion rate highly exceeds the Eddington rate and a significant fraction of the accreting gas is expelled as a superwind, under a self-similar treatment. The mass-accretion rate Ṁ decreases with radius r as Ṁ ∝ r + 1/2, where s (≥ −1/2) is some arbitrary constant. The surface density Σ and the disk central temp...

We analyse the ionization state of the magnetohydrodynamically turbulent protoplanetary disks and propose a new mechanism of sustaining ionization. First, we show that in the quasi-steady state of turbulence driven by magnetorotational instability in a typical protoplanetary disk with dust grains the amount of energy dissipation should be sufficient for providing the ionization energy that is r...

If supermassive black holes (SMBHs) are the energy sources that power quasars and active galactic nuclei, then QSO SDSS 1148+5251, the quasar with the highest redshift (zQSO = 6.43), hosts a supermassive black hole formed within ∼ 0.9 Gyr after the Big Bang. This requirement places constraints on the cosmological formation of SMBHs, believed to grow from smaller initial seeds by a combination o...

The radiative efficiencies of accretion flows become significantly lower than that of standard thin disks, if they are accreting at rates lower than a critical accretion rate ṁcrit. The transition of a standard thin disk to a radiatively inefficient accretion flow (RIAF) is expected to occur, while ṁ ∼ ṁcrit (ṁ = Ṁ/ṀEdd). We simulate the distributions of Eddington ratio λL B λ/LEdd for active g...

The discovery of hundreds of young, bright stars within a parsec from the massive black hole Sgr A at the center of the Galaxy presents a challenge to star formation theories. The requisite Roche densities for gravitational collapse of gas clouds are most naturally achieved in accretion disks. The water maser sources in Keplerian rotation in the nuclei of NGC4258, NGC1068, and the Circinus Gala...