244 research outputs found
Magnetically induced termination of giant planet formation
Here a physical model for terminating giant planet formation is outlined and
compared to other methods of late-stage giant planet formation. As has been
pointed out before, gas accreting into a gap and onto the planet will encounter
the planetary dynamo-generated magnetic field. The planetary magnetic field
produces an effective cross section through which gas is accreted. Gas outside
this cross section is recycled into the protoplanetary disk, hence only a
fraction of mass that is accreted into the gap remains bound to the planet.
This cross section inversely scales with the planetary mass, which naturally
leads to stalled planetary growth late in the formation process. We show that
this method naturally leads to Jupiter-mass planets and does not invoke any
artificial truncation of gas accretion, as has been done in some previous
population synthesis models. The mass accretion rate depends on the radius of
the growing planet after the gap has opened, and we show that so-called
hot-start planets tend to become more massive than cold-start planets. When
this result is combined with population synthesis models, it might show
observable signatures of cold-start versus hot-start planets in the exoplanet
population.Comment: 8 pages, 3 figures, accepted for publication in Astronomy &
Astrophysic
Composition of Early Planetary Atmospheres II: Coupled Dust and Chemical Evolution in Protoplanetary Disks
We present the next step in a series of papers devoted to connecting the
composition of the atmospheres of forming planets with the chemistry of their
natal evolving protoplanetary disks. The model presented here computes the
coupled chemical and dust evolution of the disk and the formation of three
planets per disk model. Our three canonical planet traps produce a Jupiter near
1 AU, a Hot Jupiter and a Super-Earth. We study the dependency of the final
orbital radius, mass, and atmospheric chemistry of planets forming in disk
models with initial disk masses that vary by 0.02 above and below our
fiducial model (). We compute C/O and C/N for the
atmospheres formed in our 3 models and find that C/O
C/O, which does not vary strongly between different planets formed
in our model. The nitrogen content of atmospheres can vary in planets that grow
in different disk models. These differences are related to the formation
history of the planet, the time and location that the planet accretes its
atmosphere, and are encoded in the bulk abundance of NH. These results
suggest that future observations of atmospheric NH and an estimation of the
planetary C/O and C/N can inform the formation history of particular planetary
systems.Comment: Accepted for publication in MNRA
Preservice Social Studies Teachers’ Conceptions of and Experiences with Discussion as a Pedagogical Approach: A Case Study
An extensive body of empirical data emphasizes the numerous benefits of incorporating discussion into the social studies classroom. Therefore, it is necessary to better understand how educators view discussion and what experiences they have with in throughout their college courses. The authors conducted a single-case study at a large southeastern university that sought to explore how 12 preservice social studies teachers experience discussion in their college-level coursework. The study also sought to discover the extent to which the participants valued discussion within their coursework and whether they considered discussion as a practical approach for their own classroom. Findings suggest that the participants mostly experience lecture in their lower-level core curriculum courses as well as their teacher preparation coursework. Additionally, the study demonstrates that the participants valued discussion as a pedagogical approach, but they viewed it as a less practical strategy than more traditional forms of pedagogy such as lecturing
Preservice Social Studies Teachers’ Conceptions of and Experiences with Discussion as a Pedagogical Approach: A Case Study
An extensive body of empirical data emphasizes the numerous benefits of incorporating discussion into the social studies classroom. Therefore, it is necessary to better understand how educators view discussion and what experiences they have with in throughout their college courses. The authors conducted a single-case study at a large southeastern university that sought to explore how 12 preservice social studies teachers experience discussion in their college-level coursework. The study also sought to discover the extent to which the participants valued discussion within their coursework and whether they considered discussion as a practical approach for their own classroom. Findings suggest that the participants mostly experience lecture in their lower-level core curriculum courses as well as their teacher preparation coursework. Additionally, the study demonstrates that the participants valued discussion as a pedagogical approach, but they viewed it as a less practical strategy than more traditional forms of pedagogy such as lecturing
Planet formation in the PDS 70 system: Constraining the atmospheric chemistry of PDS 70b and c
Understanding the chemical link between protoplanetary disks and planetary
atmospheres is complicated by the fact that the popular targets in the study of
disks and planets are widely separated both in space and time. The 5 Myr PDS 70
systems offers a unique opportunity to directly compare the chemistry of a
giant planet's atmosphere to the chemistry of its natal disk. To that end, we
derive our current best physical and chemical model for the PDS 70 disk through
forward modelling of the CO, CO, and CH emission radial
profiles with the thermochemical code DALI and find a volatile C/O ratio above
unity in the outer disk. Using what we know of the PDS 70 disk today, we
analytically estimate the properties of the disk as it was 4 Myr in the past
when we assume that the giant planets started their formation, and compute a
chemical model of the disk at that time. We compute the formation of PDS 70b
and PDS 70c using the standard core accretion paradigm and account for the
accretion of volatile and refractory sources of carbon and oxygen to estimate
the resulting atmospheric carbon-to-oxygen number ratio (C/O) for these
planets. Our inferred C/O ratio of the gas in the PDS 70 disk indicates that it
is marginally carbon rich relative to the stellar C/O = 0.44 which we derive
from an empirical relation between stellar metallicity and C/O. Under the
assumption that the disk has been carbon rich for most of its lifetime, we find
that the planets acquire a super-stellar C/O in their atmospheres. If the
carbon-rich disk is a relatively recent phenomenon (i.e. developed after the
formation of the planets at Myr) then the planets should have close to
the stellar C/O in their atmospheres. This work lays the groundwork to better
understand the disk in the PDS 70 system as well as the planet formation
scenario that produce its planets.Comment: 18 pages, 14 figures, 5 tables, accepted for publication in A&
Diverse outcomes of planet formation and composition around low-mass stars and brown dwarfs
The detection of Earth-size exoplanets around low-mass stars -- in stars such
as Proxima Centauri and TRAPPIST-1 -- provide an exceptional chance to improve
our understanding of the formation of planets around M stars and brown dwarfs.
We explore the formation of such planets with a population synthesis code based
on a planetesimal-driven model previously used to study the formation of the
Jovian satellites. Because the discs have low mass and the stars are cool, the
formation is an inefficient process that happens at short periods, generating
compact planetary systems. Planets can be trapped in resonances and we follow
the evolution of the planets after the gas has dissipated and they undergo
orbit crossings and possible mergers. We find that formation of planets above
Mars mass and in the planetesimal accretion scenario, is only possible around
stars with masses and discs of . We find that planets above Earth-mass form around stars with
masses larger than while planets larger than do
not form in our model, even not under the most optimal conditions (massive
disc), showing that planets such as GJ 3512b form with another, more efficient
mechanism. Our results show that the majority of planets form with a
significant water fraction; that most of our synthetic planetary systems have
1, 2, or 3 planets, but those with 4, 5, 6, and 7 planets are also common,
confirming that compact planetary systems with many planets should be a
relatively common outcome of planet formation around small stars.Comment: published in MNRA
An improved synthesis of (2E,4Z)-6-(benzyloxy)-4-bromohexa-2,4-dien-1-ol
An improved synthesis of (2E,4Z)-6-(benzyloxy)-4-bromohexa-2,4-dien-1-ol has been devised. This new route increases the throughput and yield of the diene product by circumventing a low yielding preparation of boronic acid intermediate as well as removing the need to use multi-gram quantities of highly toxic thallium salts. In the process of developing this new route, a higher yielding preparation of ( E)-3-hydroxyprop-1-enylboronic acid was also achieved. (c) 2007 Elsevier Ltd. All rights reserved
Genome of Drosophila suzukii, the spotted wing drosophila.
Drosophila suzukii Matsumura (spotted wing drosophila) has recently become a serious pest of a wide variety of fruit crops in the United States as well as in Europe, leading to substantial yearly crop losses. To enable basic and applied research of this important pest, we sequenced the D. suzukii genome to obtain a high-quality reference sequence. Here, we discuss the basic properties of the genome and transcriptome and describe patterns of genome evolution in D. suzukii and its close relatives. Our analyses and genome annotations are presented in a web portal, SpottedWingFlyBase, to facilitate public access
Cold Dust but Warm Gas in the Unusual Elliptical Galaxy NGC 4125
Data from the Herschel Space Observatory have revealed an unusual elliptical galaxy, NGC 4125, which has strong and extended submillimeter emission from cold dust but only very strict upper limits to its CO and Hi emission. Depending on the dust emissivity, the total dust mass is 2-5 x 10(6) M-circle dot. While the neutral gas-to-dust mass ratio is extremely low (= 10(4) K faster than the dust is evaporated. If galaxies like NGC 4125, where the far-infrared emission does not trace neutral gas in the usual manner, are common at higher redshift, this could have significant implications for our understanding of high redshift galaxies and galaxy evolution.Canadian Space AgencyNatural Sciences and Engineering Research Council of CanadaAgenzia Spaziale Italiana (ASI) I/005/11/0BMVIT (Austria)ESA-PRODEX (Belgium)CEA/CNES (France)DLR (Germany)ASI/INAF (Italy)CICYT/MCYT (Spain)CSA (Canada)NAOC (China)CEA, (France)CNES (France)CNRS (France)ASI (Italy)MCINN (Spain)SNSB (Sweden)STFC (UK)NASA (USA)National Aeronautics and Space AdministrationAstronom
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