1,721,094 research outputs found
Atmospheric Habitable Zones in Y Dwarf Atmospheres
Full text linkWe use a simple organism lifecycle model to explore the viability of an atmospheric habitable zone (AHZ), with temperatures that could support Earth-centric life, which sits above an environment that does not support life. We illustrate this idea using the object WISE J085510.83{0714442.5, which is a cool, free-foating brown dwarf. We allow organisms to adapt to their atmospheric environment (described by temperature, convection, and gravity) by adopting dierent growth strategies that maximize their chance of survival and proliferation. We assume a constant upward vertical velocity through the AHZ. We found that the organism growth strategy is most sensitive to the magnitude of the atmospheric convection. Stronger convection supports the evolution of more massive organisms. For a purely radiative environment we nd that evolved organisms have a mass that is an order of magnitude smaller than terrestrial microbes, thereby dening a dynamical constraint on the dimensions of life that an AHZ can support. Based on a previously dened statistical approach we infer thatthere are of order 109 Y brown dwarfs in the Milky Way, and likely a few tens of these objects are within ten parsecs from Earth. Our work also has implications for exploring life in the atmospheres of temperate gas giants. Consideration of the habitable volumes in planetary atmospheres signicantly increases the volume of habitable space in the galaxy
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Planet formation and the early evolution of self-gravitating protoplanetary discs
Full text linkWhen a Giant Molecular Cloud (GMC) collapses to form a stellar core,
conservation of angular momentum will lead to the formation of a protoplanetary
disc, with an initial mass potentially of the order of its stellar host. If a massive
disc forms, then the disc’s self-gravity will play a crucial role in the earliest
stages of its evolution; driving its viscous evolution, and potentially leading
to the formation of wide orbit, giant planets and brown dwarfs through disc
fragmentation.
I begin this thesis by placing improved constraints on the conditions required
for disc fragmentation, specifically focusing on how the disc’s environment may
influence its evolution and eventual fate.
Recent results from direct imaging surveys suggest that wide orbit giant planets
and brown dwarfs are found more frequently around higher mass stars. I use
Smoothed Particle Hydrodynamics (SPH) simulations to show that a disc’s
susceptibility to fragmentation is dependent on the mass of its host star. I
demonstrate that discs around higher mass stars may fragment for lower disc-to-star mass ratios, making them favourable sites for the formation of wide orbit,
massive objects, such as those found in direct imaging surveys. Low mass stars
may support high mass discs, in principle providing large reservoirs of material
for core accretion planet formation.
Results from direct imaging surveys also find that stars hosting close in giant
planets or brown dwarfs display an excess of outer binary companions, with
indications that some of these objects may have formed through the gravitational
instability (GI). I use SPH to simulate a suite of self-gravitating discs with a
binary companion, and show that there is a narrow region of parameter space
where intermediate separation companions may trigger fragmentation. Short
separation encounters are destructive, whilst wide orbit companions have little effect. The range of binary separations found to favour the formation of short
period, giant planets is consistent with results from direct imaging surveys.
Although numerical models suggest that GI may dominate a disc’s early
evolution, it is still unclear from observations whether massive, self-gravitating
discs exist in nature. Recent high-resolution infrared imaging of protoplanetary
discs have given rise to unparalleled observations of their substructure, including
rings, gaps and spirals, providing us with crucial insights to the earliest stages of
planet formation.
Observations of the protoplanetary disc surrounding AB Aurigae have revealed
the possible presence of two massive planets in the process of forming. The
young measured age for the system places strict time constraints on the planet’s
formation histories. I use analytic core accretion models to show that their
expected core accretion formation timescales are longer than the system’s current
age. Using SPH and viscous evolution models of self-gravitating discs, I show that
a proto-AB Aurigae disc could have been massive enough to fragment in the past,
with typical fragment masses consistent with the masses of the protoplanets which
have been observed in the disc.
Finally, I use Monte Carlo radiative transfer models to generate observational
predictions of self-gravitating discs using ALMA. I develop an existing 3D semi-analytic model to include a prescription for dust trapping in the disc’s spirals. I
make predictions about the disc properties which may drive spirals that could be
visible to ALMA, in particular focusing on the impact of dust trapping. I also
use these models to analyse 3 discs from the DSHARP survey, and discuss the
plausibility of their observed spirals being the result of GI
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Mineral false-positives in the search for exoplanet reflectance biosignatures: in the context of the ever-advancing field of astrobiology
No full textFor the foreseeable future, the great distances that separate us from even the most nearby
star systems indicate that all measurements of exoplanets must be made through remote
sensing techniques. Current technology, like the James Webb Space Telescope (JWST), can
obtain transmission spectra of exoplanet atmospheres that help astrobiologists and
astronomers determine these planets’ atmospheric conditions. Future missions in the coming
decades, like LUVOIR and HabEx, will make it possible to resolve disk-integrated
reflectance spectra from exoplanet surfaces in addition to atmospheric features where we
could potentially identify habitable and life-bearing worlds. Among surface features, the
“vegetation red edge” (VRE), a sharp step-like feature in the reflectance spectrum
characteristic of light-harvesting organisms, is the paradigm of a biosignature that might be
detectable on exoplanets. However, similar spectral features in biomass have been found
from abiotic sources.
In astrobiology, microbes and minerals are closely linked – minerals record environmental
conditions, supply the surfaces needed to support and preserve organic material, and act as
templates for biogeochemical reactions that define life. Current research has limitations due
to the lack of comprehensive experimentation in capturing reflectance spectra of minerals
and microbes, accounting for exoplanet atmospheres, and simulating realistic observations
with telescopic noise sources. As astrobiologists interested in the distribution of life in the
universe, we also wish to learn from exoplanet spectra how planetary habitability changes
through time. Mars (for example) once had liquid water on its surface, and because there
might be exoplanets that once had water or even life, observing hydrated minerals may tell
us that a planet might have been once habitable, even if it is now desiccated.
This project prepares for future missions in astrobiology by helping to make the crucial
distinction between biology and abiotic processes that mimic life, while examining potential
evidence for past planetary habitability. First, by examining NASA’s ECOSTRESS
(Ecosystem Spaceborne Thermal Radiometer Experiment on Space Station) spectral
database and extracting all available mineral data. Then, taking the microbial spectral data
from Hegde et al. (2015) to plot both groups onto a principal component analysis (PCA).
The PCA allowed me to study overlaps between spectral features and where several minerals
were identified with edge-like features between 500 and 700 nm. After choosing a selection
of minerals to examine, I acquired hemispherical reflectance measurements using ASD
FieldSpec 4 spectroradiometers from the University of Edinburgh and Cornell University
for more reliable comparisons. Finally, I integrated astronomy with mineral spectra by
applying the effects of telescope instrument sensitivity, resolution, and noise using NASA’s
Planetary Spectrum Generator (PSG) to produce simulated spectroscopic fingerprints of
minerals analogous to those we might find on exoplanets. Finally, the use of EXO-Prime2
provided by collaborators at Cornell University allowed me to compare our mineral
measurements with a modern Earth atmosphere. The results highlight several challenges to
the future detection of reflectance biosignatures on exoplanets
Characterising weather and rotation on substellar worlds
Full text linkPhotometric variability monitoring is sensitive to atmospheric inhomogeneities
as they rotate in and out of view, and is a key probe of atmospheric features in
brown dwarfs and giant exoplanets. The population of young, low-gravity brown
dwarfs are an excellent test bed for characterising the gravity dependence of brown
dwarf atmospheres while also providing a critical analogue to the population of
directly-imaged planets. This thesis aims to expand our current understanding of
the variability of brown dwarfs with a focus on the role of gravity on variability
properties. In Chapter 2 I present the detection of photometric variability in the
L7 planetary-mass object PSO 318.5-22, as well as subsequent multi-wavelength
ground-based and space-based monitoring. I use the followup observations to
measure the rotational period of PSO 318.5-22 and to investigate the horizontal
and vertical structure of its atmosphere. In Chapter 3 I supplement the existing
brown dwarf variability data in the literature with new rotational velocities to
reveal relations between inclination angle, variability amplitude and colour. These
new relations probe the latitudinal cloud structure of brown dwarfs for the first
time and will inform future searches for variability on both free-floating exoplanets
and directly-imaged companions. In Chapter 4 I present Spitzer monitoring of
the three lowest-mass members of the AB Doradus moving group. I report mid-IR
variability in two late-L exoplanet analogues, W0047 and 2M2244 and place upper
limits on the variability of the T5.5 planetary-mass object SDSS 1110. I measure
the rotational periods and inclination angles of W0047 and 2M2244 and find that
they are consistent with the relations reported in Chapter 3 for the field dwarf
sample. Finally, in Chapter 5 I present the first large survey for photometric
variability in young low-gravity brown dwarfs. Theory and observations have
shown that gravity plays an important role in the atmospheric properties of L
and T types objects. Surface gravity significantly affects the height at which
condensate clouds form in the atmosphere (Marley et al., 2012) and thus we can
expect that gravity will have an effect on variability. I detect variability in 6
low-gravity brown dwarfs, 4 of which are reported in this thesis for the first time.
Focusing on the L0-L8/5 objects in the survey, I find a variability occurrence
rate of 30+16/-8 %. I reanalyse the results of Radigan (2014) and find a variability
occurrence rate of 11+13/-4 % for the field dwarf sample. This is the first quantitative
indication that the young objects are more likely to be variable than their higher
mass counterparts. The work presented in this thesis has provided crucial insight
into the role of gravity on variability properties, and acts as a key pathfinder for
future studies of variability on directly-imaged planets
Geochemistry of Enceladus and implications for life detection
Full text linkEnceladus, a moon of Saturn, is one of the most promising candidates for
the search for life beyond Earth. The Cassini-Huygens mission revealed that
Enceladus has a thick crust composed of water ice. Beneath this crust there
is a subsurface liquid water ocean that erupts into space through jets near
the south pole, forming a plume of ice and gas. It is suggested that this
ocean may be habitable and future missions to Enceladus will likely involve
life detection experiments on ejected plume material or of the surface around
the plume source.
A limitation to habitability on Enceladus is the freezing point of water; however, the presence of dissolved salts extends this freezing point to
lower temperatures. On Earth, frozen environments such as sea-ice, snow
and glacial surfaces, and subglacial lakes contain microbial ecosystems with
complex dynamics. The presence of ice does not mean water is unavailable
and liquid brine networks can extend throughout the ice, providing an extensive micro-environment for microbial life to inhabit. As a result, it is
suggested that the icy crust of Enceladus, especially around the warmer,
thinner southern pole, may contain accessible habitats close to the surface.
Furthermore, the surface is likely connected to the ocean across short to
geological timescales and relict habitable regions may be detectable on the
surface.
Many questions still remain about the phase behaviour of Enceladus type brines at low temperatures and the evolution of physiochemical param eters as these solutions freeze. This thesis explores the cryogeochemistry of
Enceladus-type Na-Cl-CO3 solutions and how microscale freezing dynamics
can reveal information about planetary scale processes, and ultimately, the
habitability of Enceladus. We present, for the first time, results that significantly improve our understanding of Enceladus’s geochemistry and that will
inform future life detection based missions.
We first explore the cryomineralogy of Na-Cl-CO3 solutions using powder
x-ray diffraction and show that a mixture of hydrohalite and hydrated sodium
carbonate minerals form. Several minor phases exist that we are unable to
identify but that will be important to investigate further for future missions.
Additionally, we look at the microscale freezing dynamics of these solutions
using cryomicroscopy. Based on our results, we suggest that behaviour of
carbonate minerals will have important implications for ocean CO2 dynamics
which impacts our understanding of the predicted pH of the ocean.
Next, we explored how this system behaves in three-dimensions using x-ray computed microtomography. We show that the relative precipitation of
these salt phases and the ice will affect where they are found on Enceladus,
and ultimately, their presence or absence can be used as an indicator of
thermal history analogous to igneous and metamorphic petrology. Areas
with precipitated salts on the surface may contain vital information about
Enceladus’s interior processes and they may be the best place to find evidence
of life.
Finally, using fluorescence and polarised light cryomicroscopy coupled
with cell staining, we show how microbial cells physically behave in these
frozen environments and how they may be trapped within ice or salts and
transported to the surface. We explore how controlled boiling of Enceladus’s
ocean may lead to sputtering and dispersal of microbial material into the
plume and how this may impact plume sampling. Our results suggest that
larger plume particles will be more likely to contain cells, and as most large
particles fall back to the surface, a lander mission would be best suited to
finding life.
We show that there are many answers to be found with lab-based empirical studies of simple cryogenic systems using modern techniques. Improving
our understanding of cryogeochemistry will provide a solid foundation for
future missions to frozen environments beyond Earth and ultimately will
provide context to information gained from the exploration of the surface
and subsurface of icy moons
Detection and characterisation of young planetary-mass objects: novel techniques and optimised survey strategies
Full text linkYoung, low-mass brown dwarfs can be similar in size and composition to young,
giant exoplanets. Many exist without host stars and are uncontaminated by
starlight, making them useful analogues for studying planets in solar systems.
Increasing the population of well-studied brown dwarfs and exoplanets will
improve our understanding of the underlying distribution of planets, and of which
formation scenarios are viable. Young star-forming regions, such as Serpens and
Taurus, are ideal targets when looking for populations of planetary-mass brown
dwarfs, as they are relatively nearby, young and active in star formation.
In this thesis, I present surveys, past and future, of nearby star-forming regions,
conducted in the hope of finding new, very low-mass brown dwarf and planetary-mass members. I also focus on the characterisation of newly-identified individual
objects, and of populations as a whole. I aim to demonstrate how custom-designed narrowband photometric filters can be incredibly effective at selecting
brown dwarf members of young regions for spectroscopic follow-up.
In Chapter 2, I present a survey of the Serpens star-forming region using the novel
W-band technique. I obtain photometry using the Wide-field Infrared Camera
(WIRCAM) on the Canada-France-Hawaii Telescope (CFHT), and the custom-designed W-band filter, which is centred on the 1.45 µm absorption feature
present in brown dwarf atmospheres. I then describe a spectroscopic follow-up campaign, covering J−, H− and K−bands. Finally, I describe a subset
of observations using the Hubble Space Telescope (HST), obtained to identify
possible low-mass companions or binary components. Using this photometric,
spectroscopic, and high-resolution imaging data, I identify five likely-members
of Serpens Core and Serpens South, four of which are consistent with having
spectral types of M5 or later.
In Chapter 3, I describe a future direct imaging survey, optimised to detect
young, giant planets using a custom filter and a target list informed by our
current understanding of the underlying planet distribution. The survey will
use the Near Infrared Camera System (NIX), a high-contrast imager, part of
the Enhanced Resolution Imager and Spectrograph (ERIS) instrument that has
recently been installed at the Very Large Telescope (VLT). I present the ‘spectral
shape’ technique, which uses the custom-designed K−peak filter to efficiently
identify promising targets for follow-up observations. I discuss possible targets
for such a survey, and conclude that a nearby, young star-forming region is an
ideal target to maximise the yield of planet and brown dwarf detections.
Finally, in Chapter 4 I use an additional W-band data set to investigate the the
form of the initial mass function (IMF) in the Taurus star-forming region, and the
question of the possible environmental dependence of the IMF. I combine CFHT
and Gaia photometry to isolate likely Taurus members from field contaminants.
Using the isolated cluster population, I run multiple Monte Carlo Markov Chain
simulations to assess the likely form of the IMF. I use different IMF functional
forms (broken power law and log-normal) and Taurus star-formation histories,
and find evidence for a spread of stellar ages in Taurus from 1–10 Myr. I also
find that both functional forms provide a reasonable fit to the data (with a
slight preference for the broken power law), and that the best-fit IMF parameters
extracted are consistent with literature values for other clusters and the general
Galactic population, supporting the theory of a universal IMF
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