16,455 research outputs found
Habitable Zone Lifetime of Exoplanets around Main Sequence Stars
Funding: Dean's Scholarship at the University of East Anglia.The potential habitability of newly discovered exoplanets is initially assessed by determining whether their orbits fall within the circumstellar habitable zone of their star. However, the habitable zone (HZ) is not static in time or space, and its boundaries migrate outward at a rate proportional to the increase in luminosity of a star undergoing stellar evolution, possibly including or excluding planets over the course of the star’s main sequence lifetime. We describe the time that a planet spends within the HZ as its ‘‘habitable zone lifetime.’’ The HZ lifetime of a planet has strong astrobiological implications and is especially important when considering the evolution of complex life, which is likely to require a longer residence time within the HZ. Here, we present results from a simple model built to investigate the evolution of the ‘‘classic’’ HZ over time, while also providing estimates for the evolution of stellar luminosity over time in order to develop a ‘‘hybrid’’ HZ model. These models return estimates for the HZ lifetimes of Earth and 7 confirmed HZ exoplanets and 27 unconfirmed Kepler candidates. The HZ lifetime for Earth ranges between 6.29 and 7.79 · 109 years (Gyr). The 7 exoplanets fall in a range between ∼1 and 54.72 Gyr, while the 27 Kepler candidate planets’ HZ lifetimes range between 0.43 and 18.8 Gyr. Our results show that exoplanet HD 85512b is no longer within the HZ, assuming it has an Earth analog atmosphere. The HZ lifetime should be considered in future models of planetary habitability as setting an upper limit on the lifetime of any potential exoplanetary biosphere, and also for identifying planets of high astrobiological potential for continued observational or modeling campaigns.Publisher PDFPeer reviewe
Protostellar half-life: new methodology and estimates
(Abridged) Protostellar systems evolve from prestellar cores, through the
deeply embedded stage and then disk-dominated stage, before they end up on the
main sequence. Knowing how much time a system spends in each stage is crucial
for understanding how stars and associated planetary systems form, because a
key constraint is the time available to form such systems. Equally important is
understanding what the spread in these time scales is. The most commonly used
method for inferring protostellar ages is to assume the lifetime of one
evolutionary stage, and then scale this to the relative number of protostars in
the other stages, i.e., assuming steady state. This method does not account for
the underlying age distribution and apparent stochasticity of star formation,
nor that relative populations are not in steady state. To overcome this, we
propose a new scheme where the lifetime of each protostellar stage follows a
distribution based on the formalism of sequential nuclear decay. The main
assumptions are: Class 0 sources follow a straight path to Class III sources,
the age distribution follows a binomial distribution, and the star-formation
rate is constant. The results are that the half-life of Class 0, Class I, and
Flat sources are (2.4+/-0.2)%, (4.4+/-0.3)%, and (4.3+/-0.4)% of the Class II
half-life, respectively, which translates to 47+/-4, 88+/-7, and 87+/-8 kyr,
respectively, for a Class II half-life of 2 Myr for protostars in the Gould
Belt clouds with more than 100 protostars. The mean age of these clouds is
1.2+/-0.1 Myr, and the star formation rate is (8.3+/-0.5)x10^-4 Msun/yr. The
critical parameters in arriving at these numbers are the assumed half-life of
the Class II stage, and the assumption that the star-formation rate and
half-lives are constant. This method presents a first step in moving from
steady-state to non-steady-state solutions of protostellar populations.Comment: Accepted for publication in A&
Initial Conditions of Planet Formation: Lifetimes of Primordial Disks
The statistical properties of circumstellar disks around young stars are
important for constraining theoretical models for the formation and early
evolution of planetary systems. In this brief review, I survey the literature
related to ground-based and Spitzer-based infrared (IR) studies of young
stellar clusters, with particular emphasis on tracing the evolution of
primordial (``protoplanetary'') disks through spectroscopic and photometric
diagnostics. The available data demonstrate that the fraction of young stars
with optically thick primordial disks and/or those which show spectroscopic
evidence for accretion appears to approximately follow an exponential decay
with characteristic time ~2.5 Myr (half-life = 1.7 Myr). Large IR surveys of
~2-5 Myr-old stellar samples show that there is real cluster-by-cluster scatter
in the observed disk fractions as a function of age. Recent Spitzer surveys
have found convincing evidence that disk evolution varies by stellar mass and
environment (binarity, proximity to massive stars, and cluster density).
Perhaps most significantly for understanding the planeticity of stars, the disk
fraction decay timescale appears to vary by stellar mass, ranging from ~1 Myr
for >1.3 Msun stars to ~3 Myr for <0.08 Msun brown dwarfs. The exponential
decay function may provide a useful empirical formalism for estimating very
rough ages for YSO populations and for modeling the effects of disk-locking on
the angular momentum of young stars.Comment: 8 pages, 1 figure, invited review, Proceedings of the 2nd Subaru
International Conference "Exoplanets and Disks: Their Formation and
Diversity", Keauhou - Hawaii - USA, 9-12 March 200
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Photochemistry in the arctic free troposphere: NOx budget and the role of odd nitrogen reservoir recycling
On the rotation periods of the components of the triple system TYC9300-0891-1AB/TYC9300-0529-1 in the Octans Association
Stellar rotation depends on different parameters. The range of values of
these parameters causes the dispersion in the rotation period distributions
observed in young stellar clusters/associations. We focus our investigation on
the effects of different circumstellar environments on stellar rotation. More
specifically, we are searching in stellar Associations for visual triple
systems where all stellar parameters are similar, with the only exceptions of
the unknown initial rotation period, and of the circum-stellar environment, in
the sense that one of the two about equal-mass components has a close-by third
'perturber' component. In the present study we analyse the 35-Myr old visual
triple system TYC 9300-0891-1AB + TYC 9300-0529-1 in the young Octans stellar
association consisting of three equal-mass K0V components. We collected from
the literature all information that allowed us to infer that the three
components are actually physically bound forming a triple system and are
members of the Octans Association. We collected broad-band photometric
timeseries in two observation seasons. We discovered that all the components
are variable, magnetically active, and from periodogram analysis we found the
unresolved components TYC 9300-0891-1AB to have a rotation period P = 1.383d
and TYC 9300-0529-1 a rotation period P = 1.634d. TYC 9300-0891-1A, TYC
9300-0891-1B, and TYC 9300-0529-1 have same masses, ages, and initial chemical
compositions. The relatively small 16% rotation period difference measured by
us indicates that all components had similar initial rotation periods and disc
lifetimes, and the separation of 157AU between the component A and the
'perturber' component B (or vice-versa) has been sufficiently large to prevent
any significant perturbation/shortening of the accretion-disc lifetime.Comment: Accepted by New Astronomy 201
Bayesian Ages for Early-Type Stars from Isochrones Including Rotation, and a Possible Old Age for the Hyades
We combine recently computed models of stellar evolution using a new
treatment of rotation with a Bayesian statistical framework to constrain the
ages and other properties of early-type stars. We find good agreement for
early-type stars and clusters with known young ages, including beta Pictoris,
the Pleiades, and the Ursa Majoris Moving Group. However, we derive a
substantially older age for the Hyades open cluster (750+/-100 Myr compared to
625+/-50 Myr). This older age results from both the increase in main-sequence
lifetime with stellar rotation and from the fact that rotating models near the
main-sequence turnoff are more luminous, overlapping with slightly more massive
(and shorter-lived) nonrotating ones. Our method uses a large grid of
nonrotating models to interpolate between a much sparser rotating grid, and
also includes a detailed calculation of synthetic magnitudes as a function of
orientation. We provide a web interface at www.bayesianstellarparameters.info
where the results of our analysis may be downloaded for individual early-type
(B-V<~0.25) Hipparcos stars. The web interface accepts user-supplied parameters
for a Gaussian metallicity prior and returns posterior probability
distributions on mass, age, and orientation.Comment: 11 pages, 6 figures, ApJ accepted. Error fixed: ages -> ~15% younger.
bayesianstellarparameters.info update
Dust Evolution and the Formation of Planetesimals
The solid content of circumstellar disks is inherited from the interstellar
medium: dust particles of at most a micrometer in size. Protoplanetary disks
are the environment where these dust grains need to grow at least 13 orders of
magnitude in size. Our understanding of this growth process is far from
complete, with different physics seemingly posing obstacles to this growth at
various stages. Yet, the ubiquity of planets in our galaxy suggests that planet
formation is a robust mechanism. This chapter focuses on the earliest stages of
planet formation, the growth of small dust grains towards the gravitationally
bound "planetesimals", the building blocks of planets. We will introduce some
of the key physics involved in the growth processes and discuss how they are
expected to shape the global behavior of the solid content of disks. We will
consider possible pathways towards the formation of larger bodies and conclude
by reviewing some of the recent observational advances in the field.Comment: 43 pages, 6 figures. Chapter in International Space Science Institute
(ISSI) Book on "The Disk in Relation to the Formation of Planets and their
Proto-atmospheres", published in Space Science Reviews by Springe
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