23,914 research outputs found
The nature of solar brightness variations
The solar brightness varies on timescales from minutes to decades.
Determining the sources of such variations, often referred to as solar noise,
is of importance for multiple reasons: a) it is the background that limits the
detection of solar oscillations, b) variability in solar brightness is one of
the drivers of the Earth's climate system, c) it is a prototype of stellar
variability which is an important limiting factor for the detection of
extra-solar planets. Here we show that recent progress in simulations and
observations of the Sun makes it finally possible to pinpoint the source of the
solar noise. We utilise high-cadence observations from the Solar Dynamic
Observatory and the SATIRE model to calculate the magnetically-driven
variations of solar brightness. The brightness variations caused by the
constantly evolving cellular granulation pattern on the solar surface are
computed with the MURAM code. We find that surface magnetic field and
granulation can together precisely explain solar noise on timescales from
minutes to decades, i.e. ranging over more than six orders of magnitude in the
period. This accounts for all timescales that have so far been resolved or
covered by irradiance measurements. We demonstrate that no other sources of
variability are required to explain the data. Recent measurements of Sun-like
stars by CoRoT and Kepler uncovered brightness variations similar to that of
the Sun but with much wider variety of patterns. Our finding that solar
brightness variations can be replicated in detail with just two well-known
sources will greatly simplify future modelling of existing CoRoT and Kepler as
well as anticipated TESS and PLATO data.Comment: This is the submitted version of the paper published in Nature
Astronom
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Migration of Ca II H bright points in the internetwork
The migration of magnetic bright point-like features (MBP) in the lower solar
atmosphere reflects the dispersal of magnetic flux as well as the horizontal
flows of the atmospheric layer they are embedded in. We analyse trajectories of
the proper motion of intrinsically magnetic, isolated internetwork Ca II H MBPs
(mean lifetime 461 +- 9 s) to obtain their diffusivity behaviour. We use
seeing-free high spatial and temporal resolution image sequences of quiet-Sun,
disc-centre observations obtained in the Ca II H 3968 {\AA} passband of the
Sunrise Filter Imager (SuFI) onboard the Sunrise balloon-borne solar
observatory. Small MBPs in the internetwork are automatically tracked. The
trajectory of each MBP is then calculated and described by a diffusion index
({\gamma}) and a diffusion coefficient (D). We further explore the distribution
of the diffusion indices with the help of a Monte Carlo simulation. We find
{\gamma} = 1.69 +- 0.08 and D = 257 +- 32 km^2/s averaged over all MBPs.
Trajectories of most MBPs are classified as super-diffusive, i.e., {\gamma} >
1, with the determined {\gamma} being to our knowledge the largest obtained so
far. A direct correlation between D and time-scale ({\tau}) determined from
trajectories of all MBPs is also obtained. We discuss a simple scenario to
explain the diffusivity of the observed, relatively short-lived MBPs while they
migrate within a small area in a supergranule (i.e., an internetwork area). We
show that the scatter in the {\gamma} values obtained for individual MBPs is
due to their limited lifetimes. The super-diffusive MBPs can be well-described
as random walkers (due to granular evolution and intergranular turbu- lence)
superposed on a large systematic (background) velocity, caused by granular,
mesogranular and supergranular flows.Comment: 10 pages, 7 figures, 3 table
Emissions Trading with Profit-Neutral Permit Allocations
This paper examines the operation of an emissions trading scheme (ETS) in a Cournot oligopoly. We study the impact of the ETS on industry output, price, costs, emissions, and profits. In particular, we develop formulae for the number of emissions permits that have to be freely allocated to firms in order to neutralize any adverse impact the ETS may have on profits. These formulae tell us that the profit impact of the ETS is usually limited. Indeed, under quite general conditions, industry profits are preserved so long as firms are freely allocated a fraction of their total demand for permits, with this fraction being lower than the industry's Herfindahl index.Emissions trading, permit allocation, profit-neutrality, cost pass-through, abatement, grandfathering
Rotation in the Orion Nebula Cluster
Eighteen fields in the Orion Nebula Cluster (ONC) have been monitored for one
or more observing seasons from 1990-99 with a 0.6-m telescope at Wesleyan
University. Photometric data were obtained in Cousins I on 25-40 nights per
season. Results from the first 3 years of monitoring were analyzed by Choi &
Herbst (1996; CH). Here we provide an update based on 6 more years of
observation and the extensive optical and IR study of the ONC by Hillenbrand
(1997) and Hillenbrand et al. (1998). Rotation periods are now available for
134 ONC members. Of these, 67 were detected at multiple epochs with identical
periods by us and 15 more were confirmed by Stassun et al. (1999) in their
study of Ori OBIc/d. The bimodal period distribution for the ONC is confirmed,
but we also find a clear dependence of rotation period on mass. This can be
understood as an effect of deuterium burning, which temporarily slows the
contraction and thus spin-up of stars with M <0.25 solar masses and ages of ~1
My. Stars with M <0.25 solar masses have not had time to bridge the gap in the
period distribution at ~4 days. Excess H-K and I-K emission, as well as CaII
infrared triplet equivalent widths (Hillenbrand et al. 1998), show weak but
significant correlations with rotation period among stars with M >0.25 solar
masses. Our results provide new observational support for the importance of
disks in the early rotational evolution of low mass stars. [abridged]Comment: 18 pages of text, 17 figures, and 4 tables; accepted for publication
in The Astronomical Journa
Estimating the masses of extra-solar planets
All extra-solar planet masses that have been derived spectroscopically are
lower limits since the inclination of the orbit to our line-of-sight is unknown
except for transiting systems. It is, however, possible to determine the
inclination angle, i, between the rotation axis of a star and an observer's
line-of-sight from measurements of the projected equatorial velocity (v sin i),
the stellar rotation period (P_rot) and the stellar radius (R_star). This
allows the removal of the sin i dependency of spectroscopically derived
extra-solar planet masses under the assumption that the planetary orbits lie
perpendicular to the stellar rotation axis. We have carried out an extensive
literature search and present a catalogue of v sin i, P_rot, and R_star
estimates for exoplanet host stars. In addition, we have used Hipparcos
parallaxes and the Barnes-Evans relationship to further supplement the R_star
estimates obtained from the literature. Using this catalogue, we have obtained
sin i estimates using a Markov-chain Monte Carlo analysis. This allows proper
1-sigma two-tailed confidence limits to be placed on the derived sin i's along
with the transit probability for each planet to be determined. While a small
proportion of systems yield sin i's significantly greater than 1, most likely
due to poor P_rot estimations, the large majority are acceptable. We are
further encouraged by the cases where we have data on transiting systems, as
the technique indicates inclinations of ~90 degrees and high transit
probabilities. In total, we estimate the true masses of 133 extra-solar
planets. Of these, only 6 have revised masses that place them above the 13
Jupiter mass deuterium burning limit. Our work reveals a population of
high-mass planets with low eccentricities and we speculate that these may
represent the signature of different planetary formation mechanisms at work.Comment: 40 pages, 6 tables, 2 figures. Accepted for publication in the
Monthly Notices of the Royal Astronomical Society after editing of Tables 1 &
6 for electronic publication. Html abstract shortened for astro-ph submissio
RR Lyrae Stars In The GCVS Observed By The Qatar Exoplanet Survey
We used the light curve archive of the Qatar Exoplanet Survey (QES) to
investigate the RR Lyrae variable stars listed in the General Catalogue of
Variable Stars (GCVS). Of 588 variables studied, we reclassify 14 as eclipsing
binaries, one as an RS Canum Venaticorum-type variable, one as an irregular
variable, four as classical Cepheids, and one as a type II Cepheid, while also
improving their periods. We also report new RR Lyrae sub-type classifications
for 65 variables and improve on the GCVS period estimates for 135 RR Lyrae
variables. There are seven double-mode RR Lyrae stars in the sample for which
we measured their fundamental and first overtone periods. Finally, we detect
the Blazhko effect in 38 of the RR Lyrae stars for the first time and we
successfully measured the Blazhko period for 26 of them.Comment: Accepted IBV
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