3,735 research outputs found
Episodic accretion: the interplay of infall and disc instabilities
Using zoom-simulations carried out with the adaptive mesh-refinement code
RAMSES with a dynamic range of up to we
investigate the accretion profiles around six stars embedded in different
environments inside a (40 pc) giant molecular cloud, the role of mass
infall and disc instabilities on the accretion profile, and thus on the
luminosity of the forming protostar. Our results show that the environment in
which the protostar is embedded determines the overall accretion profile of the
protostar. Infall on to the circumstellar disc may trigger gravitational disc
instabilities in the disc at distances of around ~10 to ~50 au leading to rapid
transport of angular momentum and strong accretion bursts. These bursts
typically last for about ~10 to a ~100 yr, consistent with typical orbital
times at the location of the instability, and enhance the luminosity of the
protostar. Calculations with the stellar evolution code mesa show that the
accretion bursts induce significant changes in the protostellar proper- ties,
such as the stellar temperature and radius. We apply the obtained protostellar
properties to produce synthetic observables with RADMC3D and predict that
accretion bursts lead to ob- servable enhancements around 20 to 200 m in
the spectral energy distribution of Class 0 type young stellar objects.Comment: 17 pages, 14 figures, accepted by MNRA
Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments with Dynamics 1.0: A General Circulation Model for Simulating the Climates of Rocky Planets
Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments
with Dynamics (ROCKE-3D) is a 3-Dimensional General Circulation Model (GCM)
developed at the NASA Goddard Institute for Space Studies for the modeling of
atmospheres of Solar System and exoplanetary terrestrial planets. Its parent
model, known as ModelE2 (Schmidt et al. 2014), is used to simulate modern and
21st Century Earth and near-term paleo-Earth climates. ROCKE-3D is an ongoing
effort to expand the capabilities of ModelE2 to handle a broader range of
atmospheric conditions including higher and lower atmospheric pressures, more
diverse chemistries and compositions, larger and smaller planet radii and
gravity, different rotation rates (slowly rotating to more rapidly rotating
than modern Earth, including synchronous rotation), diverse ocean and land
distributions and topographies, and potential basic biosphere functions. The
first aim of ROCKE-3D is to model planetary atmospheres on terrestrial worlds
within the Solar System such as paleo-Earth, modern and paleo-Mars,
paleo-Venus, and Saturn's moon Titan. By validating the model for a broad range
of temperatures, pressures, and atmospheric constituents we can then expand its
capabilities further to those exoplanetary rocky worlds that have been
discovered in the past and those to be discovered in the future. We discuss the
current and near-future capabilities of ROCKE-3D as a community model for
studying planetary and exoplanetary atmospheres.Comment: Revisions since previous draft. Now submitted to Astrophysical
Journal Supplement Serie
Simulating the Cascading Effects of an Extreme Agricultural Production Shock: Global Implications of a Contemporary US Dust Bowl Event
Higher temperatures expected by midcentury increase the risk of shocks to crop production, while the interconnected nature of the current global food system functions to spread the impact of localized production shocks throughout the world. In this study, we analyze the global potential impact of a present-day event of equivalent magnitude to the US Dust Bowl, modeling the ways in which a sudden decline in US wheat production could cascade through the global network of agricultural trade. We use observations of country-level production, reserves, and trade data in a Food Shock Cascade model to explore trade adjustments and country-level inventory changes in response to a major, multiyear production decline. We find that a 4-year decline in wheat production of the same proportional magnitude as occurred during the Dust Bowl greatly reduces both wheat supply and reserves in the United States and propagates through the global trade network. By year 4 of the event, US wheat exports fall from 90.5 trillion kcal before the drought to 48 trillion to 52 trillion kcal, and the United States exhausts 94% of its reserves. As a result of reduced US exports, other countries meet their needs by leveraging their own reserves, leading to a 31% decline in wheat reserves globally. These findings demonstrate that an extreme production decline would lead to substantial supply shortfalls in both the United States and in other countries, where impacts outside the United States strongly depend on a country's reserves and on its relative position in the global trade network
Pinwheels in the sky, with dust: 3D modeling of the Wolf-Rayet 98a environment
The Wolf-Rayet 98a (WR 98a) system is a prime target for interferometric
surveys, since its identification as a "rotating pinwheel nebulae", where
infrared images display a spiral dust lane revolving with a 1.4 year
periodicity. WR 98a hosts a WC9+OB star, and the presence of dust is puzzling
given the extreme luminosities of Wolf-Rayet stars. We present 3D hydrodynamic
models for WR 98a, where dust creation and redistribution are self-consistently
incorporated. Our grid-adaptive simulations resolve details in the wind
collision region at scales below one percent of the orbital separation (~4 AU),
while simulating up to 1300 AU. We cover several orbital periods under
conditions where the gas component alone behaves adiabatic, or is subject to
effective radiative cooling. In the adiabatic case, mixing between stellar
winds is effective in a well-defined spiral pattern, where optimal conditions
for dust creation are met. When radiative cooling is incorporated, the
interaction gets dominated by thermal instabilities along the wind collision
region, and dust concentrates in clumps and filaments in a volume-filling
fashion, so WR 98a must obey close to adiabatic evolutions to demonstrate the
rotating pinwheel structure. We mimic Keck, ALMA or future E-ELT observations
and confront photometric long-term monitoring. We predict an asymmetry in the
dust distribution between leading and trailing edge of the spiral, show that
ALMA and E-ELT would be able to detect fine-structure in the spiral indicative
of Kelvin-Helmholtz development, and confirm the variation in photometry due to
the orientation. Historic Keck images are reproduced, but their resolution is
insufficient to detect the details we predict.Comment: Accepted for publication in mnra
Report from the Tri-Agency Cosmological Simulation Task Force
The Tri-Agency Cosmological Simulations (TACS) Task Force was formed when
Program Managers from the Department of Energy (DOE), the National Aeronautics
and Space Administration (NASA), and the National Science Foundation (NSF)
expressed an interest in receiving input into the cosmological simulations
landscape related to the upcoming DOE/NSF Vera Rubin Observatory (Rubin),
NASA/ESA's Euclid, and NASA's Wide Field Infrared Survey Telescope (WFIRST).
The Co-Chairs of TACS, Katrin Heitmann and Alina Kiessling, invited community
scientists from the USA and Europe who are each subject matter experts and are
also members of one or more of the surveys to contribute. The following report
represents the input from TACS that was delivered to the Agencies in December
2018.Comment: 36 pages, 3 figures. Delivered to NASA, NSF, and DOE in Dec 201
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