1,320 research outputs found
HAZMAT VI: The Evolution of Extreme Ultraviolet Radiation Emitted from Early M Star
Quantifying the evolution of stellar extreme ultraviolet (EUV, 100 -- 1000
) emission is critical for assessing the evolution of
planetary atmospheres and the habitability of M dwarf systems. Previous studies
from the HAbitable Zones and M dwarf Activity across Time (HAZMAT) program
showed the far- and near-UV (FUV, NUV) emission from M stars at various stages
of a stellar lifetime through photometric measurements from the Galaxy
Evolution Explorer (GALEX). The results revealed increased levels of
short-wavelength emission that remain elevated for hundreds of millions of
years. The trend for EUV flux as a function of age could not be determined
empirically because absorption by the interstellar medium prevents access to
the EUV wavelengths for the vast majority of stars. In this paper, we model the
evolution of EUV flux from early M stars to address this observational gap. We
present synthetic spectra spanning EUV to infrared wavelengths of 0.4
0.05 M stars at five distinct ages between 10 and 5000 Myr, computed
with the PHOENIX atmosphere code and guided by the GALEX photometry. We model a
range of EUV fluxes spanning two orders of magnitude, consistent with the
observed spread in X-ray, FUV, and NUV flux at each epoch. Our results show
that the stellar EUV emission from young M stars is 100 times stronger than
field age M stars, and decreases as t after remaining constant for a few
hundred million years. This decline stems from changes in the chromospheric
temperature structure, which steadily shifts outward with time. Our models
reconstruct the full spectrally and temporally resolved history of an M star's
UV radiation, including the unobservable EUV radiation, which drives planetary
atmospheric escape, directly impacting a planet's potential for habitability.Comment: 23 pages, 15 figures, accepted to Ap
HAZMAT. V. The Ultraviolet and X-ray Evolution of K Stars
Knowing the high-energy radiation environment of a star over a planet's
formation and evolutionary period is critical in determining if that planet is
potentially habitable and if any biosignatures could be detected, as UV
radiation can severely change or destroy a planet's atmosphere. Current efforts
for finding a potentially habitable planet are focused on M stars, yet K stars
may offer more habitable conditions due to decreased stellar activity and more
distant and wider habitable zones (HZ). While M star activity evolution has
been observed photometrically and spectroscopically, there has been no
dedicated investigation of K-star UV evolution. We present the first
comprehensive study of the near-UV, far-UV, and X-ray evolution of K stars. We
used members of young moving groups and clusters ranging in age from 10 - 625
Myr combined with field stars and their archived GALEX UV and ROSAT X-ray data
to determine how the UV and X-ray radiation evolve. We find that the UV and
X-ray flux incident on a HZ planet is 5 - 50 times lower than that of HZ
planets around early-M stars and 50 - 1000 times lower than those around late-M
stars, due to both an intrinsic decrease in K dwarf stellar activity occurring
earlier than for M dwarfs and the more distant location of the K dwarf HZ.Comment: 27 pages, 19 figure
HAZMAT. VIII. A Spectroscopic Analysis of the Ultraviolet Evolution of K Stars: Additional Evidence for K Dwarf Rotational Stalling in the First Gigayear
Efforts to discover and characterize habitable zone planets have primarily
focused on Sun-like stars and M dwarfs. K stars, however, provide an appealing
compromise between these two alternatives that has been relatively unexplored.
Understanding the ultraviolet (UV) environment around such stars is critical to
our understanding of their planets, as the UV can drastically alter the
photochemistry of a planet's atmosphere. Here we present near-UV and far-UV
\textit{Hubble Space Telescope}'s Cosmic Origins Spectrograph observations of
39 K stars at three distinct ages: 40 Myr, 650 Myr, and 5 Gyr. We find
that the K star (0.6 -- 0.8 M) UV flux remains constant beyond 650
Myr before falling off by an order of magnitude by field age. This is distinct
from early M stars (0.3 -- 0.6 M), which begin to decline after only
a few hundred Myr. However, the rotation-UV activity relation for K stars is
nearly identical to that of early M stars. These results may be a consequence
of the spin-down stalling effect recently reported for K dwarfs, in which the
spin-down of K stars halts for over a Gyr when their rotation periods reach
10 d, rather than the continuous spin down that G stars experience.
These results imply that exoplanets orbiting K dwarfs may experience a stronger
UV environment than thought, weakening the case for K stars as hosts of
potential "super-habitable" planets.Comment: 18 pages, 7 figure
HAZMAT. IV. Flares and Superflares on Young M Stars in the Far Ultraviolet
M stars are powerful emitters of far-ultraviolet light. Over long timescales, a significant, possibly dominant, fraction of this emission is produced by stellar flares. Characterizing this emission is critical to understanding the atmospheres of the stars producing it and the atmospheric evolution of the orbiting planets subjected to it. Ultraviolet emission is known to be elevated for several hundred million years after M stars form. Whether or not the same is true of ultraviolet flare activity is a key concern for the evolution of exoplanet atmospheres. Hubble Space Telescope (HST) observations by the HAZMAT program (HAbitable Zones and M dwarf Activity across Time) detected 18 flares on young (40 Myr) early M stars in the Tucana-Horologium association over 10 hr of observations, 10 having energy >1030 erg. These imply that flares on young M stars are 100-1000× more energetic than those occurring at the same rate on “inactive,” field age M dwarfs. However, when energies are normalized by quiescent emission, there is no statistical difference between the young and field age samples. The most energetic flare observed, dubbed the “Hazflare,” emitted an energy of 1032.1 erg in the FUV, 30× more energetic than any stellar flare previously observed in the FUV with HST’s COS or STIS spectrographs. It was accompanied by 15,500 ± 400 K blackbody emission bright enough to designate it as a superflare (E > 1033 erg), with an estimated bolometric energy of {10}{33.6-0.2+0.1} erg. This blackbody emitted {18}-1+2% of its flux in the FUV (912-1700 Å), where molecules are generally most sensitive to photolysis. Such hot superflares in young, early M stars could play an important role in the evolution of nascent planetary atmospheres. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555
LSST Science Book, Version 2.0
A survey that can cover the sky in optical bands over wide fields to faint
magnitudes with a fast cadence will enable many of the exciting science
opportunities of the next decade. The Large Synoptic Survey Telescope (LSST)
will have an effective aperture of 6.7 meters and an imaging camera with field
of view of 9.6 deg^2, and will be devoted to a ten-year imaging survey over
20,000 deg^2 south of +15 deg. Each pointing will be imaged 2000 times with
fifteen second exposures in six broad bands from 0.35 to 1.1 microns, to a
total point-source depth of r~27.5. The LSST Science Book describes the basic
parameters of the LSST hardware, software, and observing plans. The book
discusses educational and outreach opportunities, then goes on to describe a
broad range of science that LSST will revolutionize: mapping the inner and
outer Solar System, stellar populations in the Milky Way and nearby galaxies,
the structure of the Milky Way disk and halo and other objects in the Local
Volume, transient and variable objects both at low and high redshift, and the
properties of normal and active galaxies at low and high redshift. It then
turns to far-field cosmological topics, exploring properties of supernovae to
z~1, strong and weak lensing, the large-scale distribution of galaxies and
baryon oscillations, and how these different probes may be combined to
constrain cosmological models and the physics of dark energy.Comment: 596 pages. Also available at full resolution at
http://www.lsst.org/lsst/sciboo
Measurement of the cross-section and charge asymmetry of bosons produced in proton-proton collisions at TeV with the ATLAS detector
This paper presents measurements of the and cross-sections and the associated charge asymmetry as a
function of the absolute pseudorapidity of the decay muon. The data were
collected in proton--proton collisions at a centre-of-mass energy of 8 TeV with
the ATLAS experiment at the LHC and correspond to a total integrated luminosity
of 20.2~\mbox{fb^{-1}}. The precision of the cross-section measurements
varies between 0.8% to 1.5% as a function of the pseudorapidity, excluding the
1.9% uncertainty on the integrated luminosity. The charge asymmetry is measured
with an uncertainty between 0.002 and 0.003. The results are compared with
predictions based on next-to-next-to-leading-order calculations with various
parton distribution functions and have the sensitivity to discriminate between
them.Comment: 38 pages in total, author list starting page 22, 5 figures, 4 tables,
submitted to EPJC. All figures including auxiliary figures are available at
https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/STDM-2017-13
A Study of Time-Dependent CP-Violating Asymmetries and Flavor Oscillations in Neutral B Decays at the Upsilon(4S)
We present a measurement of time-dependent CP-violating asymmetries in
neutral B meson decays collected with the BABAR detector at the PEP-II
asymmetric-energy B Factory at the Stanford Linear Accelerator Center. The data
sample consists of 29.7 recorded at the
resonance and 3.9 off-resonance. One of the neutral B mesons,
which are produced in pairs at the , is fully reconstructed in
the CP decay modes , , , () and , or in flavor-eigenstate
modes involving and (). The flavor of the other neutral B meson is tagged at the time of
its decay, mainly with the charge of identified leptons and kaons. The proper
time elapsed between the decays is determined by measuring the distance between
the decay vertices. A maximum-likelihood fit to this flavor eigenstate sample
finds . The value of the asymmetry amplitude is determined from
a simultaneous maximum-likelihood fit to the time-difference distribution of
the flavor-eigenstate sample and about 642 tagged decays in the
CP-eigenstate modes. We find , demonstrating that CP violation exists in the neutral B meson
system. (abridged)Comment: 58 pages, 35 figures, submitted to Physical Review
Search for chargino-neutralino production with mass splittings near the electroweak scale in three-lepton final states in √s=13 TeV pp collisions with the ATLAS detector
A search for supersymmetry through the pair production of electroweakinos with mass splittings near the electroweak scale and decaying via on-shell W and Z bosons is presented for a three-lepton final state. The analyzed proton-proton collision data taken at a center-of-mass energy of √s=13 TeV were collected between 2015 and 2018 by the ATLAS experiment at the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb−1. A search, emulating the recursive jigsaw reconstruction technique with easily reproducible laboratory-frame variables, is performed. The two excesses observed in the 2015–2016 data recursive jigsaw analysis in the low-mass three-lepton phase space are reproduced. Results with the full data set are in agreement with the Standard Model expectations. They are interpreted to set exclusion limits at the 95% confidence level on simplified models of chargino-neutralino pair production for masses up to 345 GeV
Imagining the future at the global and national scale: a comparative study of British and Dutch press coverage of Rio 1992 and Rio 2012
Climate change and imagined futures are intricately linked, discussed by policymakers and reported in the media. In this article we focus on the construction of future expectation in the press coverage of the 1992 and 2012 United Nations conferences in Rio de Janeiro in British and Dutch national newspapers. We use a novel combination of methods, semantic co-word networks and metaphor analysis to analyse imagined futures. Our findings show that between 1992 and 2012 there was a switch from future-oriented hope to past-oriented disappointment regarding implementing international agreements on climate change policy. While the UK focused on global issues, the Netherlands focused on national (including colonial) and local ones, reflecting different views and expectations about the future of climate change adaptation and mitigation
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