388 research outputs found
Astrobiology: An Astronomer's Perspective
In this review we explore aspects of the field of astrobiology from an
astronomical viewpoint. We therefore focus on the origin of life in the context
of planetary formation, with additional emphasis on tracing the most abundant
volatile elements, C, H, O, and N that are used by life on Earth. We first
explore the history of life on our planet and outline the current state of our
knowledge regarding the delivery of the C, H, O, N elements to the Earth. We
then discuss how astronomers track the gaseous and solid molecular carriers of
these volatiles throughout the process of star and planet formation. It is now
clear that the early stages of star formation fosters the creation of water and
simple organic molecules with enrichments of heavy isotopes. These molecules
are found as ice coatings on the solid materials that represent microscopic
beginnings of terrestrial worlds. Based on the meteoritic and cometary record,
the process of planet formation, and the local environment, lead to additional
increases in organic complexity. The astronomical connections towards this
stage are only now being directly made. Although the exact details are
uncertain, it is likely that the birth process of star and planets likely leads
to terrestrial worlds being born with abundant water and organics on the
surface.Comment: 40 pages, 11 figures to be published in: XVII Special Courses at the
National Observatory of Rio de Janeiro. AIP Conference Proceedings, Volume
TB
Evidence of fast pebble growth near condensation fronts in the HL Tau protoplanetary disk
Water and simple organic molecular ices dominate the mass of solid materials
available for planetesimal and planet formation beyond the water snow line.
Here we analyze ALMA long baseline 2.9, 1.3 and 0.87 mm continuum images of the
young star HL Tau, and suggest that the emission dips observed are due to rapid
pebble growth around the condensation fronts of abundant volatile species.
Specifically, we show that the prominent innermost dip at 13 AU is spatially
resolved in the 0.87 mm image, and its center radius is coincident with the
expected mid-plane condensation front of water ice. In addition, two other
prominent dips, at distances of 32 and 63 AU, cover the mid-plane condensation
fronts of pure ammonia or ammonia hydrates and clathrate hydrates (especially
with CO and N) formed from amorphous water ice. The spectral index map of
HL Tau between 1.3 and 0.87 mm shows that the flux ratios inside the dips are
statistically larger than those of nearby regions in the disk. This variation
can be explained by a model with two dust populations, where most of solid mass
resides in a component that has grown into decimeter size scales inside the
dips. Such growth is in accord with recent numerical simulations of volatile
condensation, dust coagulation and settling.Comment: 6 pages, 3 figures, Accepted for publication in the Astrophysical
Journal Letter
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