174 research outputs found
Calibrating the Cepheid Period-Luminosity relation with the VLTI
The VLTI is the ideal instrument for measuring the distances of nearby
Cepheids with the Baade-Wesselink method, allowing an accurate recalibration of
the Cepheid Period-Luminosity relation. The high accuracy required by such
measurement, however, can only be reached taking into account the effects of
limb darkening, and its dependence on the Cepheid pulsations. We present here
our new method to compute phase- and wavelength-dependent limb darkening
profiles, based on hydrodynamic simulation of Classical Cepheid atmospheres.Comment: 3 pages, 2 postscript figures, uses eas.cls LaTeX class file, to
appear in the proc. Eurowinter School "Observing with the VLTI", Feb 3-8
2002, Les Houches (France
An X-Ray Jet from a White Dwarf - Detection of the Collimated Outflow from CH Cygni with Chandra
Most symbiotic stars consist of a white dwarf accreting material from the
wind of a red giant. An increasing number of these objects have been found to
produce jets. Analysis of archival Chandra data of the symbiotic system CH
Cygni reveals faint extended emission to the south, aligned with the optical
and radio jets seen in earlier HST and VLA observations. CH Cygni thus contains
only the second known white dwarf with an X-ray jet, after R Aquarii. The
X-rays from symbiotic-star jets appear to be produced when jet material is
shock-heated following collision with surrounding gas, as with the outflows
from some protostellar objects and bipolar planetary nebulae.Comment: 4 & a bit pages, 4 figures, accepted by ApJL; uses emulateapj.cls and
revtex4. Minor changes following referees report, & shortened to meet page
limi
The Highest Resolution Chandra View of Photoionization and Jet-Cloud Interaction in the Nuclear Region of NGC 4151
We report high resolution imaging of the nucleus of the Seyfert 1 galaxy NGC
4151 obtained with a 50 ks Chandra HRC observation. The HRC image resolves the
emission on spatial scales of 0.5", ~30 pc, showing an extended X-ray
morphology overall consistent with the narrow line region (NLR) seen in optical
line emission. Removal of the bright point-like nuclear source and image
deconvolution techniques both reveal X-ray enhancements that closely match the
substructures seen in the Hubble Space Telescope [OIII] image and prominent
knots in the radio jet. We find that most of the NLR clouds in NGC 4151 have
[OIII] to soft X-ray ratio ~10, despite the distance of the clouds from the
nucleus. This ratio is consistent with the values observed in NLRs of some
Seyfert 2 galaxies, which indicates a uniform ionization parameter even at
large radii and a density decreasing as as expected for a nuclear wind
scenario. The [OIII]/X-ray ratios at the location of radio knots show an excess
of X-ray emission, suggesting shock heating in addition to photoionization. We
examine various mechanisms for the X-ray emission and find that, in contrast to
jet-related X-ray emission in more powerful AGN, the observed jet parameters in
NGC 4151 are inconsistent with synchrotron emission, synchrotron self-Compton,
inverse Compton of CMB photons or galaxy optical light. Instead, our results
favor thermal emission from the interaction between radio outflow and NLR gas
clouds as the origin for the X-ray emission associated with the jet. This
supports previous claims that frequent jet-ISM interaction may explain why jets
in Seyfert galaxies appear small, slow, and thermally dominated, distinct from
those kpc scale jets in the radio galaxies.Comment: Accepted for publication in ApJ. 28 pages, 9 figures, 3 table
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