132 research outputs found
Indirect Detection of Forming Protoplanets via Chemical Asymmetries in Disks
We examine changes in the molecular abundances resulting from increased
heating due to a self-luminous planetary companion embedded within a narrow
circumstellar disk gap. Using 3D models that include stellar and planetary
irradiation, we find that luminous young planets locally heat up the parent
circumstellar disk by many tens of Kelvin, resulting in efficient thermal
desorption of molecular species that are otherwise locally frozen out.
Furthermore, the heating is deposited over large regions of the disk, AU
radially and spanning azimuthally. From the 3D chemical
models, we compute rotational line emission models and full ALMA simulations,
and find that the chemical signatures of the young planet are detectable as
chemical asymmetries in observations. HCN and its isotopologues are
particularly clear tracers of planetary heating for the models considered here,
and emission from multiple transitions of the same species is detectable, which
encodes temperature information in addition to possible velocity information
from the spectra itself. We find submillimeter molecular emission will be a
useful tool to study gas giant planet formation in situ, especially beyond
AU.Comment: 14 pages, 14 figures, accepted for publication in Ap
Exploring the Origins of Carbon in Terrestrial Worlds
Given the central role of carbon in the chemistry of life, it is a
fundamental question as to how carbon is supplied to the Earth, in what form
and when. We provide an accounting of carbon found in solar system bodies, in
particular a comparison between the organic content of meteorites and that in
identified organics in the dense interstellar medium (ISM). Based on this
accounting identified organics created by the chemistry of star formation could
contain at most ~15% of the organic carbon content in primitive meteorites and
significantly less for cometary organics, which represent the putative
contributors to starting materials for the Earth. In the ISM ~30% of the
elemental carbon is found in CO, either in the gas or ices, with a typical
abundance of ~10^-4 (relative to H2). Recent observations of the TW Hya disk
find that the gas phase abundance of CO is reduced by an order of magnitude
compared to this value. We explore a solution where the volatile CO is
destroyed via a gas phase processes, providing an additional source of carbon
for organic material to be incorporated into planetesimals and cometesimals.
This chemical processing mechanism requires warm grains (> 20 K), partially
ionized gas, and sufficiently small <10 micron grains, i.e. a larger total
grain surface area, such that freeze-out is efficient. Under these conditions
static (non-turbulent) chemical models predict that a large fraction of the
carbon nominally sequestered in CO can be the source of carbon for a wide
variety of organics that are present as ice coatings on the surfaces of warm
pre-planetesimal dust grains.Comment: 19 pages, 7 figures, to appear in Faraday Disc., vol 168, 2014, DOI:
10.1039/C4FD00003
Radionuclide Ionization in Protoplanetary Disks: Calculations of Decay Product Radiative Transfer
We present simple analytic solutions for the ionization rate
arising from the decay of short-lived radionuclides (SLRs)
within protoplanetary disks. We solve the radiative transfer problem for the
decay products within the disk, and thereby allow for the loss of radiation at
low disk surface densities; energy loss becomes important outside
for typical disk masses M. Previous studies of
chemistry/physics in these disks have neglected the impact of ionization by
SLRs, and often consider only cosmic rays (CRs), because of the high CR-rate
present in the ISM. However, recent work suggests that the flux of CRs present
in the circumstellar environment could be substantially reduced by relatively
modest stellar winds, resulting in severely modulated CR ionization rates,
, equal to or substantially below that of SLRs
( s). We compute the net ionizing
particle fluxes and corresponding ionization rates as a function of position
within the disk for a variety of disk models. The resulting expressions are
especially simple for the case of vertically gaussian disks (frequently assumed
in the literature). Finally, we provide a power-law fit to the ionization rate
in the midplane as a function of gas disk surface density and time. Depending
on location in the disk, the ionization rates by SLRs are typically in the
range s.Comment: 7 pages, 4 figures, accepted to Ap
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