183 research outputs found
The Origin of Warped, Precessing Disks in X-Ray Binaries
The radiation-driven warping instability discovered by Pringle holds
considerable promise as the mechanism responsible for producing warped,
precessing accretion disks in X-ray binaries. This instability is an inherently
global mode of the disk, thereby avoiding the difficulties with earlier models
for the precession. Here we follow up earlier work to study the linear behavior
of the instability in the specific context of a binary system. We treat the
influence of the companion as an orbit-averaged, quadrupole torque on the disk.
The presence of this external torque allows the existence of solutions in which
the direction of precession of the warp is retrograde with respect to disk
rotation, in addition to the prograde solutions which exist in the absence of
external torques.Comment: 12 pages, 3 figure
Strong Evidence for a Buried AGN in UGC 5101: Implications for LINER-Type Ultra-Luminous Infrared Galaxies
We report on the results of 3--4 m spectroscopy of the ultra-luminous
infrared galaxy (ULIRG) UGC 5101. It has a cool far-infrared color and a
LINER-type optical spectrum, and so, based on a view gaining some currency,
would be regarded as dominated by star formation. However, we find that it has
strong 3.4 m carbonaceous dust absorption, low-equivalent-width 3.3 m
polycyclic aromatic hydrocarbon (PAH) emission, and a small 3.3 m PAH to
far-infrared luminosity ratio. This favors an alternative scenario, in which an
energetically dominant AGN is present behind obscuring dust. The AGN is
plausibly obscured along all lines of sight (a `buried AGN'), rather than
merely obscured along our particular line of sight. Such buried AGNs have
previously been found in thermal infrared studies of the ULIRGs IRAS 08572+3915
and IRAS F001837111, both classified optically as LINERs. We argue that
buried AGNs can produce LINER-type optical spectra, and that at least some
fraction of LINER-type ULIRGs are predominantly powered by buried AGNs.Comment: 11 pages, 1 figure, accepted by ApJ Lette
Has Blending Compromised Cepheid-Based Determinations of the Extragalactic Distance Scale?
We examine the suggestion that half of the HST Key Project- and
Sandage/Saha-observed galaxies have had their distances systematically
underestimated, by 0.1-0.3 mag in the distance modulus, due to the
underappreciated influence of stellar profile blending on the WFC chips. The
signature of such an effect would be a systematic trend in (i) the Type Ia
supernovae corrected peak luminosity and (ii) the Tully-Fisher residuals, with
increasing calibrator distance, and (iii) a differential offset between PC and
WFC distance moduli, within the same galaxy. The absence of a trend would be
expected if blending were negligible (as has been inherently assumed in the
analyses of the aforementioned teams). We adopt a functional form for the
predicted influence of blending that is consistent with the models of Mochejska
et al. and Stanek & Udalski, and demonstrate that the expected correlation with
distance predicted by these studies is not supported by the data. We conclude
that the Cepheid-based extragalactic distance scale has not been severely
compromised by the neglect of blending.Comment: 14 pages, 2 figures, 1 table, LaTeX, accepted for publication in
Astrophysical Journal Letters, also available at
http://casa.colorado.edu/~bgibson/publications.htm
The Escape of Ionizing Photons from the Galaxy
The Magellanic Stream and several high velocity clouds have now been detected
in optical line emission. The observed emission measures and kinematics are
most plausibly explained by photoionization due to hot, young stars in the
Galactic disk. The highly favorable orientation of the Stream allows an
unambiguous determination of the fraction of ionizing photons, F_esc, which
escape the disk. We have modelled the production and transport of ionizing
photons through an opaque interstellar medium. Normalization to the Stream
detections requires F_esc = 6%, in reasonable agreement with the flux required
to ionize the Reynolds layer. Neither shock heating nor emission within a hot
Galactic corona can be important in producing the observed H-alpha emission. If
such a large escape fraction is typical of L_* galaxies, star-forming systems
dominate the extragalactic ionizing background. Within the context of this
model, both the three-dimensional orientation of the Stream and the distances
to high-velocity clouds can be determined by sensitive H-alpha observations.Comment: 4 pages; LaTeX2e, emulateapj.sty, apjfonts.sty; 4 encapsulated PS
figures. For correct labels, may need to print Fig. 3 separately due to psfig
limitation. Astrophysical Journal (Letters), accepte
Radiation-Driven Warping: The Origin of Warps and Precession in Accretion Disks
A geometrically thin, optically thick, warped accretion disk with a central
source of luminosity is subject to non-axisymmetric forces due to radiation
pressure; the resulting torque acts to modify the warp. In a recent paper,
\cite{pri96} used a local analysis to show that initially planar accretion
disks are unstable to warping driven by radiation torque. Here we extend this
work with a global analysis of the stable and unstable modes. We confirm
Pringle's conclusion that thin centrally-illuminated accretion disks are
generically unstable to warping via this mechanism; we discuss the
time-evolution and likely steady-state of such systems and show specifically
that this mechanism can explain the warping of the disk of water masers in NGC
4258 and the 164-day precession period of the accretion disk in SS 433.
Radiation-driven warping and precession provides a robust mechanism for
producing warped, precessing accretion disks in active galactic nuclei and
X-ray binary systems.Comment: 16 pages, latex, 3 figure
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