105 research outputs found
Asteroid Belts in Debris Disk Twins: VEGA and FOMALHAUT
Vega and Fomalhaut, are similar in terms of mass, ages, and global debris
disk properties; therefore, they are often referred as "debris disk twins". We
present Spitzer 10-35 um spectroscopic data centered at both stars, and
identify warm, unresolved excess emission in the close vicinity of Vega for the
first time. The properties of the warm excess in Vega are further characterized
with ancillary photometry in the mid infrared and resolved images in the
far-infrared and submillimeter wavelengths. The Vega warm excess shares many
similar properties with the one found around Fomalhaut. The emission shortward
of ~30 um from both warm components is well described as a blackbody emission
of ~170 K. Interestingly, two other systems, eps Eri and HR 8799, also show
such an unresolved warm dust using the same approach. These warm components may
be analogous to the solar system's zodiacal dust cloud, but of far greater. The
dust temperature and tentative detections in the submillimeter suggest the warm
excess arises from dust associated with a planetesimal ring located near the
water-frost line and presumably created by processes occurring at similar
locations in other debris systems as well. We also review the properties of the
2 um hot excess around Vega and Fomalhaut, showing that the dust responsible
for the hot excess is not spatially associated with the dust we detected in the
warm belt. We suggest it may arise from hot nano grains trapped in the magnetic
field of the star. Finally, the separation between the warm and cold belt is
rather large with an orbital ratio >~10 in all four systems. In light of the
current upper limits on the masses of planetary objects and the large gap, we
discuss the possible implications for their underlying planetary architecture,
and suggest that multiple, low-mass planets likely reside between the two belts
in Vega and Fomalhaut.Comment: 14 pages, accepted for publication in Ap
The ASK Network: developing a virtuous cycle of subsurface data and knowledge exchange
Knowledge of the subsurface is essential in delivering successful construction and regeneration projects. Inadequate understanding of subsurface ground conditions can constrain effective development of urban areas and is a key factor in project delay and overspending. Improving this situation demands much better use, and re-use, of subsurface data and knowledge. The establishment of ASK subsurface data and knowledge exchange network has led to substantial improvements in how urban subsurface data is reported and exchanged between the public and private sectors. Implementation of the GSPEC standardised digital data reporting format has improved the integrity and accessibility of data. ASK and GSPEC are enabling the expansion and exchange of high quality systematic subsurface datasets, improving development of robust 3D ground models which can be used to promote more cost effective and better informed ground engineering investigations, and monitoring and regulation of resources in the urban environment. The work underway in Glasgow is acting as a standard for change, both within the UK and Europe
Unraveling the Mystery of Exozodiacal Dust
Exozodiacal dust clouds are thought to be the extrasolar analogs of the Solar System's zodiacal dust. Studying these systems provides insights in the architecture of the innermost regions of planetary systems, including the Habitable Zone. Furthermore, the mere presence of the dust may result in major obstacles for direct imaging of earth-like planets. Our EXOZODI project aims to detect and study exozodiacal dust and to explain its origin. We are carrying out the first large, near-infrared interferometric survey in the northern (CHARA/FLUOR) and southern (VLTI/PIONIER) hemispheres. Preliminary results suggest a detection rate of up to 30% around A to K type stars and interesting trends with spectral type and age. We focus here on presenting the observational work carried out by our tea
PHL 5038AB: Is the brown dwarf causing pollution of its white dwarf host star?
We present new results on PHL 5038AB, a widely separated binary system composed of a white dwarf and a brown dwarf, refining the white and brown dwarf parameters and determining the binary separation to be AU. New spectra of the white dwarf show calcium absorption lines suggesting the hydrogen-rich atmosphere is weakly polluted, inferring the presence of planetesimals in the system, which we determine are in an S-type orbit around the white dwarf in orbits closer than 17-32 AU. We do not detect any infrared excess that would indicate the presence of a disc, suggesting all dust present has either been totally accreted or is optically thin. In this system, we suggest the metal pollution in the white dwarf atmosphere can be directly attributed to the presence of the brown dwarf companion disrupting the orbits of planetesimals within the system
Debris Disks: Probing Planet Formation
Debris disks are the dust disks found around ~20% of nearby main sequence
stars in far-IR surveys. They can be considered as descendants of
protoplanetary disks or components of planetary systems, providing valuable
information on circumstellar disk evolution and the outcome of planet
formation. The debris disk population can be explained by the steady
collisional erosion of planetesimal belts; population models constrain where
(10-100au) and in what quantity (>1Mearth) planetesimals (>10km in size)
typically form in protoplanetary disks. Gas is now seen long into the debris
disk phase. Some of this is secondary implying planetesimals have a Solar
System comet-like composition, but some systems may retain primordial gas.
Ongoing planet formation processes are invoked for some debris disks, such as
the continued growth of dwarf planets in an unstirred disk, or the growth of
terrestrial planets through giant impacts. Planets imprint structure on debris
disks in many ways; images of gaps, clumps, warps, eccentricities and other
disk asymmetries, are readily explained by planets at >>5au. Hot dust in the
region planets are commonly found (<5au) is seen for a growing number of stars.
This dust usually originates in an outer belt (e.g., from exocomets), although
an asteroid belt or recent collision is sometimes inferred.Comment: Invited review, accepted for publication in the 'Handbook of
Exoplanets', eds. H.J. Deeg and J.A. Belmonte, Springer (2018
Circumstellar discs: What will be next?
This prospective chapter gives our view on the evolution of the study of
circumstellar discs within the next 20 years from both observational and
theoretical sides. We first present the expected improvements in our knowledge
of protoplanetary discs as for their masses, sizes, chemistry, the presence of
planets as well as the evolutionary processes shaping these discs. We then
explore the older debris disc stage and explain what will be learnt concerning
their birth, the intrinsic links between these discs and planets, the hot dust
and the gas detected around main sequence stars as well as discs around white
dwarfs.Comment: invited review; comments welcome (32 pages
Groundwater resources in the Indo-Gangetic Basin : resilience to climate change and abstraction
Groundwater within the Indo‐Gangetic Basin (IGB) alluvial aquifer system forms one of the
world’s most important and heavily exploited reservoirs of freshwater. In this study we
have examined the groundwater system through the lens of its resilience to change – both
from the impact of climate change and increases in abstraction. This has led to the
development of a series of new maps for the IGB aquifer, building on existing datasets held
in Pakistan, India, Nepal and Bangladesh, a review of approximately 500 reports and papers,
and three targeted field studies on under‐researched topics within the region. The major
findings of the study are described below.
The IGB groundwater system
1. The IGB alluvial aquifer system comprises a large volume of heterogeneous
unconsolidated sediment in a complex environmental setting. Annual rainfall varies
from 2000mm in the Bengal basin, and the
system is dissected by the major river systems of the Indus, Ganges and Brahmaputra.
The groundwater system has been modified by the introduction of large scale canal
irrigation schemes using water from the Indus and Ganges since the 19th and early 20th
centuries.
2. High yielding tubewells can be sustained in most parts of the alluvial aquifer system;
permeability is often in the range of 10 – 60 m/d and specific yield (the drainable
porosity) varies from 5 – 20%, making it highly productive.
3. High salinity and elevated arsenic concentrations exist in parts of the basin limiting the
usefulness of the groundwater resource. Saline water predominates in the Lower Indus,
and near to the coast in the Bengal Delta, and is also a major concern in the Middle
Ganges and Upper Ganges (covering much of the Punjab Region in Pakistan, southern
Punjab, Haryana and parts of Uttar Pradesh in India). Arsenic severely impacts the
development of shallow groundwater in the fluvial influenced deltaic area of the Bengal
Basin.
4. Recharge to the IGB aquifer system is substantial and dynamic, controlled by monsoonal
rainfall, leakage from canals, river infiltration and irrigation returns. Recharge from
rainfall can occur even with low annual rainfall (350 mm) and appears to dominate
where rainfall is higher (> 750 mm). Canal leakage is also highly significant and
constitutes the largest proportion of groundwater recharge in the drier parts of the
aquifer, partially mitigating the effects of abstraction on groundwater storage.
5. Deep groundwater (>150 m) in the Bengal basin has strategic value for water supply,
health and economic development. Excessive abstraction poses a greater threat to the
quality of this deep groundwater than climate change. Heavy pumping may induce the
downward migration of arsenic in parts of Bangladesh, and of saline water in coastal
regions, but field evidence and modelling both suggest that deep groundwater
abstraction for public water supply in southern Bangladesh is in general secure against
widespread ingress of arsenic and saline water for at least 100 years
Characteristics of high-intensity groundwater abstractions from weathered crystalline bedrock aquifers in East Africa
Weathered crystalline bedrock aquifers sustain water supplies across the tropics, including East Africa. Although well yields are commonly <1 L s−1, more intensive abstraction occurs and provides vital urban and agricultural water supplies. The hydrogeological conditions that sustain such high abstraction from crystalline bedrock aquifers remain, however, poorly characterised. Five sites of intensive groundwater abstraction (multiple boreholes yielding several L s−1 or more) were investigated in Uganda and Tanzania. Analysis of aquifer properties data indicates that the sites have transmissivities of 10–1,000 m2 day−1, which is higher than generally observed in deeply weathered crystalline bedrock aquifers. At four of the five sites, weathered bedrock (saprolite) is overlain by younger superficial sediments, which provide additional storage and raise the water table within the underlying aquifer. Residence-time indicators suggest that: (1) abstracted water derives, in part, from modern recharge (within the last 10–60 years); and (2) intensive abstraction is sustained by recharge occurring over several decades. This range of encountered residence times indicates a degree of resilience to contemporary climate variability (e.g. short-term droughts), although the long-term sustainability of intensive abstractions remains uncertain. Evidence from one site in Tanzania (Makutapora) highlights the value of multi-decadal groundwater-level records in establishing the long-term viability of intensive groundwater abstraction, and demonstrates the influence of intra-decadal climate variability in determining the magnitude and frequency of recharge
Accretion of Planetary Material onto Host Stars
Accretion of planetary material onto host stars may occur throughout a star's
life. Especially prone to accretion, extrasolar planets in short-period orbits,
while relatively rare, constitute a significant fraction of the known
population, and these planets are subject to dynamical and atmospheric
influences that can drive significant mass loss. Theoretical models frame
expectations regarding the rates and extent of this planetary accretion. For
instance, tidal interactions between planets and stars may drive complete
orbital decay during the main sequence. Many planets that survive their stars'
main sequence lifetime will still be engulfed when the host stars become red
giant stars. There is some observational evidence supporting these predictions,
such as a dearth of close-in planets around fast stellar rotators, which is
consistent with tidal spin-up and planet accretion. There remains no clear
chemical evidence for pollution of the atmospheres of main sequence or red
giant stars by planetary materials, but a wealth of evidence points to active
accretion by white dwarfs. In this article, we review the current understanding
of accretion of planetary material, from the pre- to the post-main sequence and
beyond. The review begins with the astrophysical framework for that process and
then considers accretion during various phases of a host star's life, during
which the details of accretion vary, and the observational evidence for
accretion during these phases.Comment: 18 pages, 5 figures (with some redacted), invited revie
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