304 research outputs found
Diffraction-limited near-IR imaging at Keck reveals asymmetric, time-variable nebula around carbon star CIT 6
We present multi-epoch, diffraction-limited images of the nebula around the
carbon star CIT 6 at 2.2 microns and 3.1 microns from aperture masking on the
Keck-I telescope. The near-IR nebula is resolved into two main components, an
elongated, bright feature showing time-variable asymmetry and a fainter
component about 60 milliarcseconds away with a cooler color temperature. These
images were precisely registered (~35 milliarcseconds) with respect to recent
visible images from the Hubble Space Telescope (Trammell et al. 2000), which
showed a bipolar structure in scattered light. The dominant near-IR feature is
associated with the northern lobe of this scattering nebula, and the
multi-wavelength dataset can be understood in terms of a bipolar dust shell
around CIT 6. Variability of the near-IR morphology is qualitatively consistent
with previously observed changes in red polarization, caused by varying
illumination geometry due to non-uniform dust production. The blue emission
morphology and polarization properties can not be explained by the above model
alone, but require the presence of a wide binary companion in the vicinity of
the southern polar lobe. The physical mechanisms responsible for the breaking
of spherical symmetry around extreme carbon stars, such as CIT 6 and IRC+10216,
remain uncertain.Comment: 18 pages, 5 figures (one in color), to appear in the Astrophysical
Journa
Interacting Binaries with Eccentric Orbits. Secular Orbital Evolution Due To Conservative Mass Transfer
We investigate the secular evolution of the orbital semi-major axis and
eccentricity due to mass transfer in eccentric binaries, assuming conservation
of total system mass and orbital angular momentum. Assuming a delta function
mass transfer rate centered at periastron, we find rates of secular change of
the orbital semi-major axis and eccentricity which are linearly proportional to
the magnitude of the mass transfer rate at periastron. The rates can be
positive as well as negative, so that the semi-major axis and eccentricity can
increase as well as decrease in time. Adopting a delta-function mass-transfer
rate of 10^{-9} M_\sun {\rm yr}^{-1} at periastron yields orbital evolution
timescales ranging from a few Myr to a Hubble time or more, depending on the
binary mass ratio and orbital eccentricity. Comparison with orbital evolution
timescales due to dissipative tides furthermore shows that tides cannot, in all
cases, circularize the orbit rapidly enough to justify the often adopted
assumption of instantaneous circularization at the onset of mass transfer. The
formalism presented can be incorporated in binary evolution and population
synthesis codes to create a self-consistent treatment of mass transfer in
eccentric binaries.Comment: 16 pages, 8 figures, Accepted by The Astrophysical Journa
Equipotential Surfaces and Lagrangian points in Non-synchronous, Eccentric Binary and Planetary Systems
We investigate the existence and properties of equipotential surfaces and
Lagrangian points in non-synchronous, eccentric binary star and planetary
systems under the assumption of quasi-static equilibrium. We adopt a binary
potential that accounts for non-synchronous rotation and eccentric orbits, and
calculate the positions of the Lagrangian points as functions of the mass
ratio, the degree of asynchronism, the orbital eccentricity, and the position
of the stars or planets in their relative orbit. We find that the geometry of
the equipotential surfaces may facilitate non-conservative mass transfer in
non-synchronous, eccentric binary star and planetary systems, especially if the
component stars or planets are rotating super-synchronously at the periastron
of their relative orbit. We also calculate the volume-equivalent radius of the
Roche lobe as a function of the four parameters mentioned above. Contrary to
common practice, we find that replacing the radius of a circular orbit in the
fitting formula of Eggleton (1983) with the instantaneous distance between the
components of eccentric binary or planetary systems does not always lead to a
good approximation to the volume-equivalent radius of the Roche-lobe. We
therefore provide generalized analytic fitting formulae for the
volume-equivalent Roche lobe radius appropriate for non-synchronous, eccentric
binary star and planetary systems. These formulae are accurate to better than
1% throughout the relevant 2-dimensional parameter space that covers a dynamic
range of 16 and 6 orders of magnitude in the two dimensions.Comment: 12 pages, 10 figures, 2 Tables, Accepted by the Astrophysical Journa
New Data and Improved Parameters for the Transiting Planet OGLE-TR-56b
We report new spectroscopic observations of the recently discovered
transiting planet OGLE-TR-56b with the Keck/HIRES instrument. Our radial
velocity measurements with errors of ~100 m/s show clear variations that are in
excellent agreement with the phasing (period and epoch) derived from the OGLE
transit photometry, confirming the planetary nature of the companion. The new
data combined with measurements from the previous season allow an improved
determination of the mass of the planet, M_p = 1.45 +/- 0.23 M_Jup. All
available OGLE photometry, including new measurements made this season, have
also been analyzed to derive an improved value for the planetary radius of R_p
= 1.23 +/- 0.16 R_Jup. We discuss the implications of these results for the
theory of extrasolar planets.Comment: submitted to ApJ
The `666' collaboration on OGLE transits: I. Accurate radius of the planets OGLE-TR-10b and OGLE-TR-56b with VLT deconvolution photometry
Transiting planets are essential to study the structure and evolution of
extra-solar planets. For that purpose, it is important to measure precisely the
radius of these planets. Here we report new high-accuracy photometry of the
transits of OGLE-TR-10 and OGLE-TR-56 with VLT/FORS1. One transit of each
object was covered in Bessel V and R filters, and treated with the
deconvolution-based photometry algorithm DECPHOT, to ensure accurate
millimagnitude light curves. Together with earlier spectroscopic measurements,
the data imply a radius of 1.22 +0.12-0.07 R_J for OGLE-TR-10b and 1.30 +- 0.05
R_J for OGLE-TR-56b. A re-analysis of the original OGLE photometry resolves an
earlier discrepancy about the radius of OGLE-TR-10. The transit of OGLE-TR-56
is almost grazing, so that small systematics in the photometry can cause large
changes in the derived radius. Our study confirms both planets as inflated hot
Jupiters, with large radii comparable to that of HD 209458 and at least two
other recently discovered transiting gas giants.Comment: Fundamental updates compared to previous version; accepted for
publication in Astronomy & Astrophysic
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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