113 research outputs found
Habitable Zones in the Universe
Habitability varies dramatically with location and time in the universe. This
was recognized centuries ago, but it was only in the last few decades that
astronomers began to systematize the study of habitability. The introduction of
the concept of the habitable zone was key to progress in this area. The
habitable zone concept was first applied to the space around a star, now called
the Circumstellar Habitable Zone. Recently, other, vastly broader, habitable
zones have been proposed. We review the historical development of the concept
of habitable zones and the present state of the research. We also suggest ways
to make progress on each of the habitable zones and to unify them into a single
concept encompassing the entire universe.Comment: 71 pages, 3 figures, 1 table; to be published in Origins of Life and
Evolution of Biospheres; table slightly revise
Superoutbursts, superhumps and the tidal-thermal instability model
We include the tidal instability due to the 3:1 resonance in the disc
instability model developed by Hameury et al. (1998) and modified by
Buat-Menard et al. (2001). We confirm earlier results by Osaki (1989) that the
tidal instability can account for the SU UMa light curves. We show that in
ultra-low mass ratio systems such as ER UMa stars and WZ Sge stars, the
superoutburst ends while the disc is still eccentric, as proposed by Hellier
(2001). However, since the disc shrinks rapidly once a cooling wave has
started, the eccentricity should stop shortly after the end of a superoutburst.
This result disagrees with the suggestion by Hellier that decoupling the
thermal and tidal instability in the TTI model can account for late superhumps
and echo outbursts in ultra-low mass ratio systems. We propose instead that ER
UMa short supercycles can be explained either by the alternation of narrow and
wide outbursts similar to those occurring in SS Cyg, or by the effects of
irradiation (Hameury et al. 2000). In both cases, we predict that superhumps
should be permanent, which is suggested by observations (Gao et al. 1999). We
can also reproduce light curves similar to those of EG Cnc, varying the mass
transfer rate in a TTI model including both irradiation and the presence of an
inner hole in the disc.Comment: Astronomy and Astrophysics, submitted. 8 pages, 5 figure
The HARPS search for southern extra-solar planets XI. Super-Earths (5 & 8 M_Earth) in a 3-planet system
This Letter reports on the detection of two super-Earth planets in the Gl581
system, already known to harbour a hot Neptune. One of the planets has a mass
of 5 M_Earth and resides at the ``warm'' edge of the habitable zone of the
star. It is thus the known exoplanet which most resembles our own Earth. The
other planet has a 7.7 M_Earth mass and orbits at 0.25 AU from the star, close
to the ``cold'' edge of the habitable zone. These two new light planets around
an M3 dwarf further confirm the formerly tentative statistical trend for i)
many more very low-mass planets being found around M dwarfs than around
solar-type stars and ii) low-mass planets outnumbering Jovian planets around M
dwarfs.Comment: Revised version resubmitted to A&A Letters, 5 pages, 4 figure
Tidal torques, disc radius variations, and instabilities in dwarf novae and soft X-ray transients
The study of outer disc radius variations in close binary systems is
important for understanding the structure and evolution of accretion discs.
These variations are predicted by models of both quasi steady and
time-dependent discs, and these predictions can be confronted with
observations. We consider theoretical and observational consequences of such
variations in cataclysmic variables and low mass X-ray binaries. We find that
the action of tidal torques, that determine the outer radius at which the disc
is truncated, must be important also well inside the tidal radius. We conclude
that it is doubtful that the tidal-thermal instability is responsible for the
superoutburst/superhump phenomena in dwarf novae, and confirm that it cannot be
the reason for the outbursts of soft X-ray transients. It is likely that tidal
torques play a role during superoutbursts of very-low mass-ratio systems but
they cannot be the main and only cause of superhumps.Comment: 8 pages; accepted for publication in Astronomy and Astrophysic
The International Deep Planet Survey I. The frequency of wide-orbit massive planets around A-stars
Breakthrough direct detections of planetary companions orbiting A-type stars
confirm the existence of massive planets at relatively large separations, but
dedicated surveys are required to estimate the frequency of similar planetary
systems. To measure the first estimation of the giant exoplanetary systems
frequency at large orbital separation around A-stars, we have conducted a
deep-imaging survey of young (8-400 Myr), nearby (19-84 pc) A- and F-stars to
search for substellar companions in the 10-300 AU range. The sample of 42 stars
combines all A-stars observed in previous AO planet search surveys reported in
the literature with new AO observations from VLT/NaCo and Gemini/NIRI. It
represents an initial subset of the International Deep Planet Survey (IDPS)
sample of stars covering M- to B-stars. The data were obtained with
diffraction-limited observations in H- and Ks-band combined with angular
differential imaging to suppress the speckle noise of the central stars,
resulting in typical 5-sigma detection limits in magnitude difference of 12 mag
at 1", 14 mag at 2" and 16 mag at 5" which is sufficient to detect massive
planets. A detailed statistical analysis of the survey results is performed
using Monte Carlo simulations. Considering the planet detections, we estimate
the fraction of A-stars having at least one massive planet (3-14 MJup) in the
range 5-320 AU to be inside 5.9-18.8% at 68% confidence, assuming a flat
distribution for the mass of the planets. By comparison, the brown dwarf (15-75
MJup) frequency for the sample is 2.0-8.9% at 68% confidence in the range 5-320
AU. Assuming power law distributions for the mass and semimajor axis of the
planet population, the AO data are consistent with a declining number of
massive planets with increasing orbital radius which is distinct from the
rising slope inferred from radial velocity (RV) surveys around evolved A-stars.Comment: 20 pages, 10 figures, 7 tables. Accepted for publication in A&
Planetary population synthesis
In stellar astrophysics, the technique of population synthesis has been
successfully used for several decades. For planets, it is in contrast still a
young method which only became important in recent years because of the rapid
increase of the number of known extrasolar planets, and the associated growth
of statistical observational constraints. With planetary population synthesis,
the theory of planet formation and evolution can be put to the test against
these constraints. In this review of planetary population synthesis, we first
briefly list key observational constraints. Then, the work flow in the method
and its two main components are presented, namely global end-to-end models that
predict planetary system properties directly from protoplanetary disk
properties and probability distributions for these initial conditions. An
overview of various population synthesis models in the literature is given. The
sub-models for the physical processes considered in global models are
described: the evolution of the protoplanetary disk, the planets' accretion of
solids and gas, orbital migration, and N-body interactions among concurrently
growing protoplanets. Next, typical population synthesis results are
illustrated in the form of new syntheses obtained with the latest generation of
the Bern model. Planetary formation tracks, the distribution of planets in the
mass-distance and radius-distance plane, the planetary mass function, and the
distributions of planetary radii, semimajor axes, and luminosities are shown,
linked to underlying physical processes, and compared with their observational
counterparts. We finish by highlighting the most important predictions made by
population synthesis models and discuss the lessons learned from these
predictions - both those later observationally confirmed and those rejected.Comment: 47 pages, 12 figures. Invited review accepted for publication in the
'Handbook of Exoplanets', planet formation section, section editor: Ralph
Pudritz, Springer reference works, Juan Antonio Belmonte and Hans Deeg, Ed
Planet Populations as a Function of Stellar Properties
Exoplanets around different types of stars provide a window into the diverse
environments in which planets form. This chapter describes the observed
relations between exoplanet populations and stellar properties and how they
connect to planet formation in protoplanetary disks. Giant planets occur more
frequently around more metal-rich and more massive stars. These findings
support the core accretion theory of planet formation, in which the cores of
giant planets form more rapidly in more metal-rich and more massive
protoplanetary disks. Smaller planets, those with sizes roughly between Earth
and Neptune, exhibit different scaling relations with stellar properties. These
planets are found around stars with a wide range of metallicities and occur
more frequently around lower mass stars. This indicates that planet formation
takes place in a wide range of environments, yet it is not clear why planets
form more efficiently around low mass stars. Going forward, exoplanet surveys
targeting M dwarfs will characterize the exoplanet population around the lowest
mass stars. In combination with ongoing stellar characterization, this will
help us understand the formation of planets in a large range of environments.Comment: Accepted for Publication in the Handbook of Exoplanet
Trigonometric Regressive Spectral Analysis Reliably Maps Dynamic Changes in Baroreflex Sensitivity and Autonomic Tone: The Effect of Gender and Age
BACKGROUND: The assessment of baroreflex sensitivity (BRS) has emerged as prognostic tool in cardiology. Although available computer-assisted methods, measuring spontaneous fluctuations of heart rate and blood pressure in the time and frequency domain are easily applicable, they do not allow for quantification of BRS during cardiovascular adaption processes. This, however, seems an essential criterion for clinical application. We evaluated a novel algorithm based on trigonometric regression regarding its ability to map dynamic changes in BRS and autonomic tone during cardiovascular provocation in relation to gender and age. METHODOLOGY/PRINCIPAL FINDINGS: We continuously recorded systemic arterial pressure, electrocardiogram and respiration in 23 young subjects (25+/-2 years) and 22 middle-aged subjects (56+/-4 years) during cardiovascular autonomic testing (metronomic breathing, Valsalva manoeuvre, head-up tilt). Baroreflex- and spectral analysis was performed using the algorithm of trigonometric regressive spectral analysis. There was an age-related decline in spontaneous BRS and high frequency oscillations of RR intervals. Changes in autonomic tone evoked by cardiovascular provocation were observed as shifts in the ratio of low to high frequency oscillations of RR intervals and blood pressure. Respiration at 0.1 Hz elicited an increase in BRS while head-up tilt and Valsalva manoeuvre resulted in a downregulation of BRS. The extent of autonomic adaption was in general more pronounced in young individuals and declined stronger with age in women than in men. CONCLUSIONS/SIGNIFICANCE: The trigonometric regressive spectral analysis reliably maps age- and gender-related differences in baroreflex- and autonomic function and is able to describe adaption processes of baroreceptor circuit during cardiovascular stimulation. Hence, this novel algorithm may be a useful screening tool to detect abnormalities in cardiovascular adaption processes even when resting values appear to be normal
The autonomic nervous system as a therapeutic target in heart failure: a scientific position statement from the Translational Research Committee of the Heart Failure Association of the European Society of Cardiology
Despite improvements in medical therapy and device-based treatment, heart failure (HF) continues to impose enormous burdens on patients and health care systems worldwide. Alterations in autonomic nervous system (ANS) activity contribute to cardiac disease progression, and the recent development of invasive techniques and electrical stimulation devices has opened new avenues for specific targeting of the sympathetic and parasympathetic branches of the ANS. The Heart Failure Association of the European Society of Cardiology recently organized an expert workshop which brought together clinicians, trialists and basic scientists to discuss the ANS as a therapeutic target in HF. The questions addressed were: (i) What are the abnormalities of ANS in HF patients? (ii) What methods are available to measure autonomic dysfunction? (iii) What therapeutic interventions are available to target the ANS in patients with HF, and what are their specific strengths and weaknesses? (iv) What have we learned from previous ANS trials? (v) How should we proceed in the future
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