10,430 research outputs found
The Self-Regulated Winds of Long Period Variable Stars
Numerical models of the extended atmospheres of long period variable or Mira
stars have shown that their winds have a very simple, power law structure when
averaged over the pulsation cycle. This structure is stable and robust despite
the pulsational wave disturbances, and appears to be strongly self-regulated.
Observational studies support these conclusions. The models also show that
dust-free winds are nearly adiabatic, with little heating or cooling. The
classical, steady, adiabatic wind solution to the hydrodynamic equations fails
to account for an extensive region of nearly constant outflow velocity. We
investigate analytic solutions which include the effects of wave pressure,
heating, and the resulting entropy changes. Wave pressure is represented by a
term like that in the Reynolds turbulence equation for the mean velocity.
Although the pressure from individual waves is modest, the waves are likely the
primary agent of self-regulation of dust-free winds. In models of dusty winds,
the gas variables also adopt a power law dependence on radius. Heating is
required at all radii to maintain this flow, and grain heating and heat
transfer to the gas are significant. Both hydrodynamic and gas/grain thermal
feedbacks transform the flow towards self-regulated forms. (Abridged)Comment: 14 pgs., 3 figures, accepted for MNRAS, 200
Stochastic Wobble of Accretion Discs and Jets from Turbulent Rocket Torques
Models of accretion discs and their associated outflows often incorporate
assumptions of axisymmetry and symmetry across the disc plane. However, for
turbulent discs these symmetries only apply to averaged quantities and do not
apply locally. The local asymmetries can induce local imbalances in outflow
power across the disc mid-plane, which can in turn induce local tilting
torques. Here we calculate the effect of the resulting stochastic torques on
disc annuli that are a consequence of standard mean field accretion disc
models. The torques induce a random walk of the vector perpendicular to the
plane of each averaged annulus. This random walk is characterized by a radially
dependent diffusion coefficient which we calculate for small angle tilt. We use
the coefficient to calculate a radially dependent time scale for annular tilt
and associated jet wobble. The wobble time depends on the square of the wander
angle so the age of a given system determines the maximum wobble angle. We
apply this to examples of blazars, young stellar objects and binary engines of
pre-planetary nebulae and microquasars. It is noteworthy that for an averaging
time days, we estimate a wobble angle for jets in SS433 of
degrees, not inconsistent with observational data. In general
the non-periodic nature of the stochastic wobble could distinguish it from
faster periodic jet precession.Comment: 12 pages, accepted by MNRA
A model-based approach to recovering the structure of a plant from images
We present a method for recovering the structure of a plant directly from a
small set of widely-spaced images. Structure recovery is more complex than
shape estimation, but the resulting structure estimate is more closely related
to phenotype than is a 3D geometric model. The method we propose is applicable
to a wide variety of plants, but is demonstrated on wheat. Wheat is made up of
thin elements with few identifiable features, making it difficult to analyse
using standard feature matching techniques. Our method instead analyses the
structure of plants using only their silhouettes. We employ a generate-and-test
method, using a database of manually modelled leaves and a model for their
composition to synthesise plausible plant structures which are evaluated
against the images. The method is capable of efficiently recovering accurate
estimates of plant structure in a wide variety of imaging scenarios, with no
manual intervention
Modeling the Extragalactic Background Light and the Cosmic Star Formation History
We present an updated model for the extragalactic background light (EBL) from
stars and dust, over wavelengths approximately 0.1 to 1000 m. This model
uses accurate theoretical stellar spectra, and tracks the evolution of star
formation, stellar mass density, metallicity, and interstellar dust extinction
and emission in the universe with redshift. Dust emission components are
treated self-consistently, with stellar light absorbed by dust reradiated in
the infrared as three blackbody components. We fit our model, with free
parameters associated with star formation rate and dust extinction and
emission, to a wide variety of data: luminosity density, stellar mass density,
and dust extinction data from galaxy surveys; and -ray absorption
optical depth data from -ray telescopes. Our results strongly
constraint the star formation rate density and dust photon escape fraction of
the universe out to redshift , about 90% of the history of the universe.
We find our model result is, in some cases, below lower limits on the EBL
intensity, and below some low- -ray absorption measurements.Comment: 23 pages, 12 figures, 3 tables. Accepted for publication in AAS
journal
POISSON project - II - A multi-wavelength spectroscopic and photometric survey of young protostars in L 1641
Characterising stellar and circumstellar properties of embedded young stellar
objects (YSOs) is mandatory for understanding the early stages of the stellar
evolution. This task requires the combination of both spectroscopy and
photometry, covering the widest possible wavelength range, to disentangle the
various protostellar components and activities. As part of the POISSON project,
we present a multi-wavelength spectroscopic and photometric investigation of
embedded YSOs in L1641, aimed to derive the stellar parameters and evolutionary
stages and to infer their accretion properties. Our database includes
low-resolution optical-IR spectra from the NTT and Spitzer (0.6-40 um) and
photometric data covering a spectral range from 0.4 to 1100 um, which allow us
to construct the YSOs spectral energy distributions (SEDs) and to infer the
main stellar parameters. The SED analysis allows us to group our 27 YSOs into
nine Class I, eleven Flat, and seven Class II objects. However, on the basis of
the derived stellar properties, only six Class I YSOs have an age of ~10^5 yr,
while the others are older 5x10^5-10^6 yr), and, among the Flat sources, three
out of eleven are more evolved objects (5x10^6-10^7 yr), indicating that
geometrical effects can significantly modify the SED shapes. Inferred mass
accretion rates (Macc) show a wide range of values (3.6x10^-9 to 1.2x10^-5
M_sun yr^-1), which reflects the age spread observed in our sample. Average
values of mass accretion rates, extinction, and spectral indices decrease with
the YSO class. The youngest YSOs have the highest Macc, whereas the oldest YSOs
do not show any detectable jet activity in either images and spectra. We also
observe a clear correlation among the YSO Macc, M*, and age, consistent with
mass accretion evolution in viscous disc models.Comment: 61 pages, 16 figures; A&A in pres
Mindfulness interventions reduce blood pressure in patients with non-communicable diseases: A systematic review and meta-analysis
Purpose
Mindfulness based interventions (MBIs) are an emerging area of empirical study, not only in positive psychology, but also in clinical health care. This research aims to synthesize the evidence about whether MBIs reduce blood pressure (BP) in patients with non-communicable diseases (NCDs).
Methods
Relevant studies were identified via PubMed, the Cochrane Library, Embase and the CINAHL database between 2009 and 2019. The papers selected focused on mindfulness and the effect of these on the BP of patients with NCDs. The change in SBP and DBP were meta-analyzed, stratified by type of intervention (Breathing awareness meditation (BAM), Mindfulness Meditation (MM), and Mindfulness-based Stress Reduction (MBSR).
Results
Fourteen articles met eligibility criteria and were included in the final review. Among the studies using the type and duration of intervention, systolic BP was reduced after the mindfulness-based stress reduction for 8 weeks (-6.90 mmHg [95% CI: -10.82, -2.97], p < .050), followed by the breathing awareness meditation for 12 weeks (-4.10 mmHg [95% CI: -7.54, -0.66], p < .050) and the mindfulness-based intervention for 8 weeks (-2.69 mmHg [95% CI: -3.90, -1.49], p < .050) whereas diastolic BP was reduced after the mindfulness-based stress reduction for 8 weeks (-2.45 mmHg [95% CI: -3.74, -1.17], p < .050) and the mindfulness-based intervention for 8 weeks (-2.24 mmHg [95% CI: -3.22, -1.26], p < .050).
Conclusion
MBIs can provide effective alternative therapies to assist in blood pressure reduction for patients with NCDs
Star and protoplanetary disk properties in Orion's suburbs
(Note: this is a shortened version of the original "structured" A&A format
abstract.) We performed a large optical spectroscopic and photometric survey of
the Lynds~1630N and 1641 clouds. We provide a catalog of 132 confirmed young
stars in L1630N and 267 such objects in L1641. We identify 28 transition disk
systems, 20 of which were previously unknown, as well as 42 new transition disk
candidates for which we have broad-band photometry but no optical spectroscopy.
We estimate mass accretion rates M_acc from the equivalent widths of the
H_alpha, H_beta, and HeI 5876\AA emission lines, and find a dependence on
stellar mass of M_acc propto Mstar^alpha, with alpha~3.1 in the subsolar mass
range that we probe. An investigation of a large literature sample of mass
accretion rate estimates yields a similar slope of alpha~2.8 in the subsolar
regime, but a shallower slope of alpha~2.0 if the whole mass range of 0.04
M_sun-5 Msun is included. Among the transition disk objects, the fraction of
stars that show significant accretion activity is relatively low compared to
stars with still optically thick disks (26\pm11% vs. 57\pm6%, respectively).
However, those transition disks that do show significant accretion have the
same median accretion rate as normal optically thick disks of 3-4*10^{-9}
M_sun/yr. We find that the ages of the transition disks and the WTTSs without
disks are statistically indistinguishable, and both groups are significantly
older than the CTTSs. These results argue against disk-binary interaction or
gravitational instability as mechanisms causing a transition disk appearance.
Our observations indicate that disk lifetimes in the clustered population are
shorter than in the distributed population. We propose refined Halpha
equivalent width criteria to distinguish WTTSs from CTTSs.Comment: 52 pages, 16 tables, 29 figures. Accepted by A&A. Table numbering
error correcte
The outburst of an embedded low-mass YSO in L1641
Strong outbursts in very young and embedded protostars are rare and not yet
fully understood. They are believed to originate from an increase of the mass
accretion rate onto the source. We report the discovery of a strong outburst in
a low-mass embedded young stellar object (YSO), namely 2MASS-J05424848-0816347
or [CTF93]216-2, as well as its photometric and spectroscopic follow-up. Using
near- to mid-IR photometry and NIR low-resolution spectroscopy, we monitor the
outburst, deriving its magnitude, duration, as well as the enhanced accretion
luminosity and mass accretion rate. [CTF93]216-2 increased in brightness by
~4.6, 4.0, 3.8, and 1.9 mag in the J, H, Ks bands and at 24 um, respectively,
corresponding to an L_bol increase of ~20 L_sun. Its early spectrum, probably
taken soon after the outburst, displays a steep almost featureless continuum,
with strong CO band heads and H_2O broad-band absorption features, and Br gamma
line in emission. A later spectrum reveals more absorption features, allowing
us to estimate T_eff~3200 K, M~0.25 M_sun, and mass accretion rate~1.2x10^{-6}
M_sun yr^{-1}. This makes it one of the lowest mass YSOs with a strong outburst
so far discovered.Comment: To be published in A&A letter; 5 pages, 4 figure
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