264 research outputs found
On the Origin of the Eccentricities of Extrasolar Planets
We develop a phenomenological theory that aims to account for the origin of
the large eccentricities of extrasolar planets and that of the small
eccentricities in the solar system, the preference for apsidal alignment in
non-resonant multiplanet systems, and the origin of the similarities in the
eccentricity distribution of extra-solar planets and that of spectroscopic
binary stars. We show that if a physical process is weakly dependent on the
local dynamics of the companion and imparts a small relative acceleration to
the star-companion system, the eccentricity of the companion's orbit is excited
to large values depending on the direction and duration of acceleration. A
natural candidate for such processes are asymmetric stellar jets and star-disk
winds. When the acceleration originates from a precessing jet, large
eccentricities can be excited by the resonance of the jet's precession
frequency with the induced acceleration's excitation frequency even for nearly
perpendicular jets. Precession also reduces the eccentricity amplitude far
inside the resonance radius. The acceleration's strength is best constrained in
multiplanet systems because of the companions' mutual gravitational
perturbations, while the acceleration's duration is bounded by the condition
that the residual velocity imparted to the star is smaller than the stellar
velocity dispersion in the Galaxy. In the outer parts of the star-companion
system where the acceleration excitation time is comparable to or smaller than
the orbital period, significant radial migration takes place which may have
important consequences for the dynamics of the minor body populations in the
solar system. The theory is illustrated with the Andromedae binary
system.Comment: 41 pages, 14 figures, accepted for publication in the Astronomical
Journa
HeI 1.083 micron emission and absorption in DG Tau: line excitation in jet, hot wind, and accretion flow
We present long-slit spectroscopy and spectro-astrometry of HeI 1.083 micron
emission in the T Tauri star, DG Tau. We identify three components in the HeI
feature: (1) a blueshifted emission component atv -200 km s^-1, (2) a bright
emission component at zero-velocity with a FWZI of ~500 km s^-1, and (3) a
blueshifted absorption feature at velocities between -250 and -500 km s^-1. The
position and velocity of the blueshifted HeI emission coincide with a
high-velocity component (HVC) of the [FeII] 1.257 micron emission, which arises
from a jet within an arcsecond of the star. The presence of such a high
excitation line (excitation energy ~ 20 eV) within the jet supports the
scenario of shock heating. The bright HeI component does not show any spatial
extension, and it is likely to arise from magnetospheric accretion columns.
The blueshifted absorption shows greater velocities than that in H-alpha,
suggesting that these absorption features arise from the accelerating wind
close to the star.Comment: 10 pages including 2 figures, accepted for publication in the
Astrophysical Journal Letter
A Survey for HO Megamasers in Active Galactic Nuclei -- II. A Comparison of Detected and Undetected Galaxies
A survey for H2O megamaser emission from 354 active galaxies has resulted in
the detection of 10 new sources, making 16 known altogether. The galaxies
surveyed include a distance-limited sample (covering Seyferts and LINERs with
recession velocities < 7000 km s^{-1}) and a magnitude-limited sample (covering
Seyferts and LINERs with m(B) <= 14.5). In order to determine whether the
H2O-detected galaxies are "typical" AGN or have special properties which
facilitate the production of powerful masers, we have accumulated a database of
physical, morphological, and spectroscopic properties of the observed galaxies.
The most significant finding is that H2O megamasers are detected only in
Seyfert 2 and LINER galaxies, not Seyfert 1s. Implications for this finding are
discussed. LINERs are detected at a similar rate to Seyfert 2s, constituting a
strong argument that at least some nuclear LINERs are AGN rather than
starbursts, since starbursts have not been detected as H2O megamasers. We
preferentially detect H2O emission from the nearer galaxies and from those
which are apparently brighter at mid- and far-infrared and centimeter radio
wavelengths. There is also a possible trend for the H2O-detected galaxies to be
more intrinsically luminous in nuclear 6 cm radio emission than the undetected
ones, though these data are incomplete. We find evidence that Seyfert 2s with
very high (N(H) > 10^{24} cm^{-2}) X-ray absorbing columns of gas are more
often detected as H2O maser emitters than Seyfert 2s with lower columns. It may
be that the probability of detecting H2O maser emission in Seyfert galaxies
increases with increasing column of cool gas to the nucleus, from Seyfert 1s
through NLXGs to Seyfert 2s.Comment: 19 pages, 35 figures, 3 of the tables are contained in separate LaTeX
files: Table 1-a, 1-b, and 3. Also, figure captions are contained in a
separate file which must be compiled with plain TeX, not LaTe
Data model issues in the Cherenkov Telescope Array project
The planned Cherenkov Telescope Array (CTA), a future ground-based
Very-High-Energy (VHE) gamma-ray observatory, will be the largest project of
its kind. It aims to provide an order of magnitude increase in sensitivity
compared to currently operating VHE experiments and open access to guest
observers. These features, together with the thirty years lifetime planned for
the installation, impose severe constraints on the data model currently being
developed for the project.
In this contribution we analyze the challenges faced by the CTA data model
development and present the requirements imposed to face them. While the full
data model is still not completed we show the organization of the work, status
of the design, and an overview of the prototyping efforts carried out so far.
We also show examples of specific aspects of the data model currently under
development.Comment: In Proceedings of the 34th International Cosmic Ray Conference
(ICRC2015), The Hague, The Netherlands. All CTA contributions at
arXiv:1508.0589
Further Indications of Jet Rotation in New Ultraviolet and Optical HST/STIS Spectra
We present survey results which suggest rotation signatures at the base of
T-Tauri jets. Observations were conducted with the Hubble Space Telescope
Imaging Spectrograph at optical and near ultraviolet wavelengths (NUV). Results
are presented for the approaching jet from DG Tau, CW Tau, HH 30 and the
bipolar jet from TH 28. Systematic asymmetries in Doppler shift were detected
across the jet, within 100 AU from the star. At optical wavelengths, radial
velocity differences were typically 10 to 25 (+/-5) km/s, while differences in
the NUV range were consistently lower at typically 10 (+/-5) km/s. Results are
interpreted as possible rotation signatures. Importantly, there is agreement
between the optical and NUV results for DG Tau. Under the assumption of steady
magnetocentrifugal acceleration, the survey results lead to estimates for the
distance of the jet footpoint from the star, and give values consistent with
earlier studies. In the case of DG Tau, for example, we see that the higher
velocity component appears to be launched from a distance of 0.2 to 0.5 AU from
the star along the disk plane, while the lower velocity component appears to
trace a wider part of the jet launched from as far as 1.9 AU. The results for
the other targets are similar. Therefore, if indeed the detected Doppler
gradients trace rotation within the jet then, under the assumption of steady
MHD ejection, the derived footpoint radii support the existence of magnetized
disk winds. However, since we do not resolved the innermost layers of the flow,
we cannot exclude the possibility that there also exists an X-wind or stellar
wind component.Comment: 22 pages, 21 figures, accepted by The Astrophysical Journa
On the Flaring of Jet-sustaining Accretion Disks
Jet systems with two unequal components interact with their parent accretion
disks through the asymmetric removal of linear momentum from the star-disk
system. We show that as a result of this interaction, the disk's state of least
energy is not made up of orbits that lie in a plane containing the star's
equator as in a disk without a jet. The disk's profile has the shape of a
sombrero curved in the direction of acceleration. For this novel state of
minimum energy, we derive the temperature profile of thin disks. The flaring
geometry caused by the sombrero profile increases the disk temperature
especially in its outer regions. The jet-induced acceleration disturbs the
vertical equilibrium of the disk leading to mass loss in the form of a
secondary wind emanating from the upper face of the disk. Jet time variability
causes the disk to radially expand or contract depending on whether the induced
acceleration increases or decreases. Jet time variability also excites vertical
motion and eccentric distortions in the disk and affects the sombrero profile's
curvature. These perturbations lead to the heating of the disk through its
viscous stresses as it tries to settle into the varying state of minimum
energy. The jet-disk interaction studied here will help estimate the duration
of the jet episode in star-disk systems and may explain the origin of the
recently observed one-sided molecular outflow of the HH 30 disk-jet system.Comment: 18 pages, 4 figures, accepted for publication in the Astrophysical
Journa
Hubble Space Telescope Observations of Element Abundances in Low-redshift Damped Lyman-alpha Galaxies and Implications for the Global Metallicity-Redshift Relation
Most models of cosmic chemical evolution predict that the mass-weighted mean
interstellar metallicity of galaxies should rise with time from a low value
solar at to a nearly solar value at . In the
absence of any selection effects, the damped Lyman-alpha absorbers (DLAs) in
quasar spectra are expected to show such a rise in global metallicity. However,
it has been difficult to determine whether or not DLAs show this effect,
primarily because of the very small number of DLA metallicity measurements at
low redshifts. In an attempt to put tighter constraints on the low-redshift end
of the DLA metallicity-redshift relation, we have observed Zn II and Cr II
lines in four DLAs at , using the Space Telescope Imaging
Spectrograph (STIS) onboard the Hubble Space Telescope (HST). These
observations have provided the first constraints on Zn abundances in DLAs with
. In all the three DLAs for which our observations offer meaningful
constraints on the metallicity, the data suggest that the metallicities are
much lower than the solar value. These results are consistent with recent
imaging studies indicating that these DLAs may be associated with dwarf or low
surface brightness galaxies. We combine our results with higher redshift data
from the literature to estimate the global mean metallicity-redshift relation
for DLAs. We find that the global mean metallicity shows at most a slow
increase with decreasing redshift. ...(Please see the paper for the complete
abstract).Comment: 56 pages, including 13 figures. Accepted for publication in the
Astrophysical Journa
Three-Dimensional Simulations of Jets from Keplerian Disks: Self--Regulatory Stability
We present the extension of previous two-dimensional simulations of the
time-dependent evolution of non-relativistic outflows from the surface of
Keplerian accretion disks, to three dimensions. The accretion disk itself is
taken to provide a set of fixed boundary conditions for the problem. The 3-D
results are consistent with the theory of steady, axisymmetric, centrifugally
driven disk winds up to the Alfv\'en surface of the outflow. Beyond the
Alfv\'en surface however, the jet in 3-D becomes unstable to non-axisymmetric,
Kelvin-Helmholtz instabilities. We show that jets maintain their long-term
stability through a self-limiting process wherein the average Alfv\'enic Mach
number within the jet is maintained to order unity. This is accomplished in at
least two ways. First, poloidal magnetic field is concentrated along the
central axis of the jet forming a ``backbone'' in which the Alfv\'en speed is
sufficiently high to reduce the average jet Alfv\'enic Mach number to unity.
Second, the onset of higher order Kelvin-Helmholtz ``flute'' modes (m \ge 2)
reduce the efficiency with which the jet material is accelerated, and transfer
kinetic energy of the outflow into the stretched, poloidal field lines of the
distorted jet. This too has the effect of increasing the Alfv\'en speed, and
thus reducing the Alfv\'enic Mach number. The jet is able to survive the onset
of the more destructive m=1 mode in this way. Our simulations also show that
jets can acquire corkscrew, or wobbling types of geometries in this relatively
stable end-state, depending on the nature of the perturbations upon them.
Finally, we suggest that jets go into alternating periods of low and high
activity as the disappearance of unstable modes in the sub-Alfv\'enic regime
enables another cycle of acceleration to super-Alfv\'enic speeds.Comment: 57 pages, 22 figures, submitted to Ap
The nature of the soft X-ray source in DG Tau
The classical T Tauri star DG Tau shows all typical signatures of X-ray
activity and, in particular, harbors a resolved X-ray jet. We demonstrate that
its soft and hard X-ray components are separated spatially by approximately 0.2
arcsec by deriving the spatial offset between both components from the event
centroids of the soft and hard photons utilizing the intrinsic
energy-resolution of the Chandra ACIS-S detector. We also demonstrate that this
offset is physical and cannot be attributed to an instrumental origin or to low
counting statistics. Furthermore, the location of the derived soft X-ray
emission peak coincides with emission peaks observed for optical emission
lines, suggesting that both, soft X-rays and optical emission, have the same
physical origin.Comment: 5 pages, 3 figures, accepted for publication as A&A Lette
The radio luminosity, black hole mass and Eddington ratio for quasars from the Sloan Digital Sky Survey
We investigate the \mbh- \sigma_* relation for radio-loud quasars with
redshift in Data Release 3 of the Sloan Digital Sky Survey (SDSS). The
sample consists of 3772 quasars with better model of H and \oiii lines
and available radio luminosity, including 306 radio-loud quasars, 3466
radio-quiet quasars with measured radio luminosity or upper-limit of radio
luminosity (181 radio-quiet quasars with measured radio luminosity). The virial
supermassive black hole mass (\mbh) is calculated from the broad \hb line, the
host stellar velocity dispersion () is traced by the core \oiii
gaseous velocity dispersion, and the radio luminosity and the radio loudness
are derived from the FIRST catalog. Our results are follows: (1) For
radio-quiet quasars, we confirm that there is no obvious deviation from the
\mbh- \sigma_* relation defined in inactive galaxies when \mbh uncertainties
and luminosity bias are concerned. (2) We find that radio-loud quasars deviate
much from the \mbh- \sigma_* relation respect to that for radio-quiet
quasars. This deviation is only partly due to the possible cosmology evolution
of the \mbh- \sigma_* relation and the luminosity bias. (3) The radio
luminosity is proportional to
\mbh^{1.28^{+0.23}_{-0.16}}(\lb/\ledd)^{1.29^{+0.31}_{-0.24}} for radio-quiet
quasars and \mbh^{3.10^{+0.60}_{-0.70}}(\lb/\ledd)^{4.18^{+1.40}_{-1.10}} for
radio-loud quasars. The weaker correlation of the radio luminosity dependence
upon the mass and the Eddington ratio for radio-loud quasars shows that other
physical effects would account for their radio luminosities, such as the black
hole spin.Comment: 15 pages, 8 figures, 2 tables, accepted for publication in ChJA
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