117 research outputs found
Accretion Disks around Young Stars: Lifetimes, Disk Locking and Variability
We report the findings of a comprehensive study of disk accretion and related
phenomena in four of the nearest young stellar associations spanning 6-30
million years in age, an epoch that may coincide with the late stages of planet
formation. We have obtained ~650 multi-epoch high-resolution optical spectra of
100 low-mass stars that are likely members of the eta Chamaeleontis (~6 Myr),
TW Hydrae (~8 Myr), beta Pictoris (~12 Myr) and Tucanae-Horologium (~30 Myr)
groups. Our data were collected over 12 nights between 2004 December - 2005
July on the Magellan Clay 6.5m telescope. Based on H line profiles,
along with a variety of other emission lines, we find clear evidence of
on-going accretion in three out of 11 eta Cha stars and two out of 32 TW Hydrae
members. None of the 57 beta Pic or Tuc-Hor members shows measurable signs of
accretion. Together, these results imply significant evolution of the disk
accretion process within the first several Myr of a low-mass star's life. While
a few disks can continue to accrete for up to ~10 Myr, our findings suggest
that disks accreting for beyond that timescale are rather rare. This result
provides an indirect constraint on the timescale for gas dissipation in inner
disks and, in turn, on gas giant planet formation. All accretors in our sample
are slow rotators, whereas non-accretors cover a large range in rotational
velocities. This may hint at rotational braking by disks at ages up to ~8 Myr.
Our multi-epoch spectra confirm that emission-line variability is common even
in somewhat older T Tauri stars, among which accretors tend to show
particularly strong variations. Thus, our results indicate that accretion and
wind activity undergo significant and sustained variations throughout the
lifetime of accretion disks.Comment: to appear in The Astrophysical Journa
Atomic alignment and Diagnostics of Magnetic Fields in Diffuse Media
We continue our studies of atomic alignment in diffuse media, in
particularly, in interstellar and circumstellar media, with the goal of
developing new diagnostics of magnetic fields in these environments. We
understand atomic alignment as alignment of atoms or ions in their ground
state. Such atoms are sensitive to weak magnetic fields. In particular, we
provide predictions of the polarization that arises from astrophysically
important aligned atoms (ions) with fine structure of the ground level, namely,
OI and SII and Ti II. Unlike our earlier papers which dealt with weak fields
only, a substantial part of our current paper is devoted to the studies of
atomic alignment when magnetic fields get strong enough to affect the emission
from the excited level, i.e. with the regime when the magnetic splitting is
comparable to the line-width. This is a regime of Hanle effect modified by the
atomic alignment. Using an example of emission and absorption lines of SII ion
we demonstrate how polarimetric studies can probe magnetic fields in
circumstellar regions and accretion disks. In addition, we show that atomic
alignment induced by anisotropic radiation can induce substantial variations of
magnetic dipole transitions within the ground state, thus affecting abundance
studies based on this emission. Moreover, the radio emission is polarized,
provides a new way to study magnetic fields, e.g. at the epoch of Universe
reionization.Comment: Minor changes, accepted to Ap
The Magnetic Fields of Classical T Tauri Stars
We report new magnetic field measurements for 14 classical T Tauri stars
(CTTSs). We combine these data with one previous field determination in order
to compare our observed field strengths with the field strengths predicted by
magnetospheric accretion models. We use literature data on the stellar mass,
radius, rotation period, and disk accretion rate to predict the field strength
that should be present on each of our stars according to these magnetospheric
accretion models. We show that our measured field values do not correlate with
the field strengths predicted by simple magnetospheric accretion theory. We
also use our field strength measurements and literature X-ray luminosity data
to test a recent relationship expressing X-ray luminosity as a function of
surface magnetic flux derived from various solar feature and main sequence star
measurements. We find that the T Tauri stars we have observed have weaker than
expected X-ray emission by over an order of magnitude on average using this
relationship. We suggest the cause for this is actually a result of the very
strong fields on these stars which decreases the efficiency with which gas
motions in the photosphere can tangle magnetic flux tubes in the corona.Comment: 25 pages, 5 figure
Molecular jets driven by high-mass protostars: a detailed study of the IRAS 20126+4104 jet
We present here an extensive analysis of the protostellar jet driven by IRAS
20126+4104, deriving the kinematical, dynamical, and physical conditions of the
H2 gas along the flow. The jet has been investigated by means of near-IR H2 and
[FeII] narrow-band imaging, high resolution spectroscopy of the 1-0S(1) line
(2.12 um), NIR (0.9-2.5 um) low resolution spectroscopy, along with ISO-SWS and
LWS spectra (from 2.4 to 200 um). The flow shows a complex morphology. In
addition to the large-scale jet precession presented in previous studies, we
detect a small-scale wiggling close to the source, that may indicate the
presence of a multiple system. The peak radial velocities of the H2 knots range
from -42 to -14 km s^-1 in the blue lobe, and from -8 to 47 km s^-1 in the red
lobe. The low resolution spectra are rich in H_2 emission, and relatively faint
[FeII] (NIR), [OI] and [CII] (FIR) emission is observed in the region close to
the source. A warm H2 gas component has an average excitation temperature that
ranges between 2000 K and 2500 K. Additionally, the ISO-SWS spectrum reveals
the presence of a cold component (520 K), that strongly contributes to the
radiative cooling of the flow and plays a major role in the dynamics of the
flow. The estimated L(H2) of the jet is 8.2+/-0.7 L_sun, suggesting that
IRAS20126+4104 has an accretion rate significantly increased compared to
low-mass YSOs. This is also supported by the derived mass flux rate from the H2
lines (Mflux(H2)~7.5x10^-4 M_sun yr^-1). The comparison between the H2 and the
outflow parameters strongly indicates that the jet is driving, at least
partially, the outflow. As already found for low-mass protostellar jets, the
measured H2 outflow luminosity is tightly related to the source bolometric
luminosity.Comment: 16 pages, 12 figures. A&A accepte
Tropical Andean Forests Are Highly Susceptible to Nutrient Inputs - Rapid Effects of Experimental N and P Addition to an Ecuadorian Montane Forest
Tropical regions are facing increasing atmospheric inputs of nutrients, which will have unknown consequences for the structure and functioning of these systems. Here, we show that Neotropical montane rainforests respond rapidly to moderate additions of N (50 kg ha−1 yr−1) and P (10 kg ha−1 yr−1). Monitoring of nutrient fluxes demonstrated that the majority of added nutrients remained in the system, in either soil or vegetation. N and P additions led to not only an increase in foliar N and P concentrations, but also altered soil microbial biomass, standing fine root biomass, stem growth, and litterfall. The different effects suggest that trees are primarily limited by P, whereas some processes—notably aboveground productivity—are limited by both N and P. Highly variable and partly contrasting responses of different tree species suggest marked changes in species composition and diversity of these forests by nutrient inputs in the long term. The unexpectedly fast response of the ecosystem to moderate nutrient additions suggests high vulnerability of tropical montane forests to the expected increase in nutrient inputs
Semi-Analytic Stellar Structure in Scalar-Tensor Gravity
Precision tests of gravity can be used to constrain the properties of
hypothetical very light scalar fields, but these tests depend crucially on how
macroscopic astrophysical objects couple to the new scalar field. We develop
quasi-analytic methods for solving the equations of stellar structure using
scalar-tensor gravity, with the goal of seeing how stellar properties depend on
assumptions made about the scalar coupling at a microscopic level. We
illustrate these methods by applying them to Brans-Dicke scalars, and their
generalization in which the scalar-matter coupling is a weak function of the
scalar field. The four observable parameters that characterize the fields
external to a spherically symmetric star (the stellar radius, R, mass, M,
scalar `charge', Q, and the scalar's asymptotic value, phi_infty) are subject
to two relations because of the matching to the interior solution, generalizing
the usual mass-radius, M(R), relation of General Relativity. We identify how
these relations depend on the microscopic scalar couplings, agreeing with
earlier workers when comparisons are possible. Explicit analytical solutions
are obtained for the instructive toy model of constant-density stars, whose
properties we compare to more realistic equations of state for neutron star
models.Comment: 39 pages, 9 figure
Multiwavelength Intraday Variability of the BL Lac S5 0716+714
We report results from a 1 week multi-wavelength campaign to monitor the BL
Lac object S5 0716+714 (on December 9-16, 2009). In the radio bands the source
shows rapid (~ (0.5-1.5) day) intra-day variability with peak amplitudes of up
to ~ 10 %. The variability at 2.8 cm leads by about 1 day the variability at 6
cm and 11 cm. This time lag and more rapid variations suggests an intrinsic
contribution to the source's intraday variability at 2.8 cm, while at 6 cm and
11 cm interstellar scintillation (ISS) seems to predominate. Large and
quasi-sinusoidal variations of ~ 0.8 mag were detected in the V, R and I-bands.
The X-ray data (0.2-10 keV) do not reveal significant variability on a 4 day
time scale, favoring reprocessed inverse-Compton over synchrotron radiation in
this band. The characteristic variability time scales in radio and optical
bands are similar. A quasi-periodic variation (QPO) of 0.9 - 1.1 days in the
optical data may be present, but if so it is marginal and limited to 2.2
cycles. Cross-correlations between radio and optical are discussed. The lack of
a strong radio-optical correlation indicates different physical causes of
variability (ISS at long radio wavelengths, source intrinsic origin in the
optical), and is consistent with a high jet opacity and a compact synchrotron
component peaking at ~= 100 GHz in an ongoing very prominent flux density
outburst. For the campaign period, we construct a quasi-simultaneous spectral
energy distribution (SED), including gamma-ray data from the FERMI satellite.
We obtain lower limits for the relativistic Doppler-boosting of delta >= 12-26,
which for a BL\,Lac type object, is remarkably high.Comment: 16 pages, 15 figures, table 2; Accepted for Publication in MNRA
Region of magnetic dominance near a rotating black hole
This is a brief contribution in which a simplified criterion of the relevance
of the test-particle approximation describing motion of material near a
magnetized black hole is discussed. Application to processes of the dissipative
collimation of astronomical jets (as proposed by de Felice and Curir, 1992) is
mentioned.Comment: 11 pages, to appear in General Relativity and Gravitation, also
available (with additional illustrations) at
http://otokar.troja.mff.cuni.cz/user/karas/au_www/karas/papers.ht
Multiwavelength periodicity study of Markarian 501
Context: Active Galactic Nuclei are highly variable emitters of
electromagnetic waves from the radio to the gamma-ray regime. This variability
may be periodic, which in turn could be the signature of a binary black hole.
Systems of black holes are strong emitters of gravitational waves whose
amplitude depends on the binary orbital parameters as the component mass, the
orbital semi-major-axis and eccentricity.
Aims: It is our aim to prove the existence of periodicity of the AGN
Markarian 501 from several observations in different wavelengths. A
simultaneous periodicity in different wavelengths provides evidence for bound
binary black holes in the core of AGN.
Methods: Existing data sets from observations by Whipple, SWIFT, RXTE and
MAGIC have been analysed with the Lomb-Scargle method, the epoch folding
technique and the SigSpec software.
Results: Our analysis shows a 72-day period, which could not be seen in
previous works due to the limited length of observations. This does not
contradict a 23-day period which can be derived as a higher harmonic from the
72-day period.Comment: 9 pages, 15 figures, accepted at A&
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