259 research outputs found
Transiting the Sun: The impact of stellar activity on X-ray and ultraviolet transits
Transits of hot Jupiters in X-rays and the ultraviolet have been shown to be
both deeper and more variable than the corresponding optical transits. This
variability has been attributed to hot Jupiters having extended atmospheres at
these wavelengths. Using resolved images of the Sun from NASA's Solar Dynamics
Observatory spanning 3.5 years of Solar Cycle 24 we simulate transit light
curves of a hot Jupiter to investigate the impact of Solar like activity on our
ability to reliably recover properties of the planet's atmosphere in soft
X-rays (94 {\AA}), the UV (131-1700 {\AA}), and the optical (4500 {\AA}). We
find that for stars with similar activity levels to the Sun, the impact of
stellar activity results in the derived radius of the planet in soft X-ray/EUV
to be underestimated by up-to 25% or overestimated by up-to 50% depending on
whether the planet occults active regions. We also find that in up-to 70% of
the X-ray light curves the planet transits over bright star spots. In the far
ultraviolet (1600 & 1700 {\AA}), we find the mean recovered value of the
planet-to-star radius ratio to be over-estimated by up-to 20%. For optical
transits we are able to consistently recover the correct planetary radius. We
also address the implications of our results for transits of WASP-12b and HD
189733b at short wavelengths.Comment: Accepted for publication in Ap
Transiting the Sun. II. The impact of stellar activity on Lyα transits
This work is supported by NASA Origins of the Solar System grant No. NNX13AH79G.High-energy observations of the Sun provide an opportunity to test the limits of our ability to accurately measure the properties of transiting exoplanets in the presence of stellar activity. Here we insert the transit of a hot Jupiter into continuous disk integrated data of the Sunin Lyα from NASA’s Solar Dynamics Observatory/EVE instrument to assess the impact of stellar activity on the measured planet-to-starradius ratio (Rp/R⋆). In 75% of our simulated light curves, we measure the correct radius ratio; however, incorrect values can be measured if there is significant short-term variability in the light curve. The maximum measured value of Rp/R⋆ is 50% larger than the input value,which is much smaller than the large Lyα transit depths that have been reported in the literature, suggesting that for stars with activity levels comparable to the Sun, stellar activity alone cannot account for these deep transits. We ran simulations without a transit and found that stellar activity cannot mimic the Lyα transit of 55 Cancari b, strengthening the conclusion that this planet has a partially transiting exopshere. We were able to compare our simulations to more active stars by artificially increasing the variability in the Solar Lyα lightcurve. In the higher variability data, the largest value of Rp/R⋆ we measured is <3× the input value, which again is not large enough to reproduce the Lyα transit depth reported for the more active stars HD 189733 and GJ 436, supporting the interpretation that these planets have extended atmospheres and possible cometary tails.Publisher PDFPeer reviewe
Exoplanet Transit Variability: Bow Shocks and Winds Around HD 189733b
By analogy with the solar system, it is believed that stellar winds will form
bow shocks around exoplanets. For hot Jupiters the bow shock will not form
directly between the planet and the star, causing an asymmetric distribution of
mass around the exoplanet and hence an asymmetric transit. As the planet orbits
thorough varying wind conditions, the strength and geometry of its bow shock
will change, thus producing transits of varying shape. We model this process
using magnetic maps of HD 189733 taken one year apart, coupled with a 3D
stellar wind model, to determine the local stellar wind conditions throughout
the orbital path of the planet. We predict the time-varying geometry and
density of the bow shock that forms around the magnetosphere of the planet and
simulate transit light curves. Depending on the nature of the stellar magnetic
field, and hence its wind, we find that both the transit duration and ingress
time can vary when compared to optical light curves. We conclude that
consecutive near-UV transit light curves may vary significantly and can
therefore provide an insight into the structure and evolution of the stellar
wind.Comment: 9 Pages, 7 figures. Accepted for publication in Monthly Notices of
The Royal Astronomical Societ
Using Kepler transit observations to measure stellar spot belt migration rates
Planetary transits provide a unique opportunity to investigate the surface
distributions of star spots. Our aim is to determine if, with continuous
observation (such as the data that will be provided by the Kepler mission), we
can in addition measure the rate of drift of the spot belts. We begin by
simulating magnetic cycles suitable for the Sun and more active stars,
incorporating both flux emergence and surface transport. This provides the
radial magnetic field distribution on the stellar surface as a function of
time. We then model the transit of a planet whose orbital axis is misaligned
with the stellar rotation axis. Such a planet could occult spots at a range of
latitudes. This allows us to complete the forward modelling of the shape of the
transit lightcurve. We then attempt the inverse problem of recovering spot
locations from the transit alone. From this we determine if transit lightcurves
can be used to measure spot belt locations as a function of time. We find that
for low-activity stars such as the Sun, the 3.5 year Kepler window is
insufficient to determine this drift rate. For more active stars, it may be
difficult to distinguish subtle differences in the nature of flux emergence,
such as the degree of overlap of the "butterfly wings". The rate and direction
of drift of the spot belts can however be determined for these stars. This
would provide a critical test of dynamo theory.Comment: 5 pages. Accepted for publication in Monthly Notices of the Royal
Astronomical Society Letter
Effective detergent/ chlorophyll ratio and detergent concentration in the aqueous phase during solubilization of Phormidium laminosum membranes
AbstractExperiments of turbidity decrease induced by detergents were systematically performed to characterize the solubilization of Phormidium laminosum membrane fragments. SDS, Triton X-100 and a mixture of octyl glucoside/decyl maltoside/lithium dodecyl sulfate (OG/DM/LiDS, in a molar ratio of 4.19:2.54:1) were used. The detergent concentration in the aqueous phase (Dw) and the effective detergent/chlorophyll ratio in mixed aggregates (Re) were determined. Both parameters increased during the solubilization and in an exponential way in the range from 10 to 90% solubilization. At detergent concentrations which caused the complete solubilization, DW values were close to the described critical micellar concentrations (cmc), but solubilization started at concentrations well below the cmc. At the onset of solubilization five molecules of SDS, one of Triton X-100 and three of the mixture OG/DM/LiDS, per chlorophyll molecule, saturated the membrane fragments. The increase of DW and Re values was characterized by two constants. This permits the design of a model to predict the detergent concentration which produces a desired solubilization of thylakoid membrane fragments for a given chlorophyll concentration
Searching for spectroscopic binaries within transition disk objects
S.A. Kohn acknowledges the support of NSF REU grant AST-1004107 through Northern Arizona University and Lowell Observatory. J. Llama acknowledges support from NASA Origins of the Solar System grant No. NNX13AH79G and from STFC grant ST/M001296/1. This research made use of the SIMBAD database, operated at CDS, Strasbourg, France.Transition disks (TDs) are intermediate stage circumstellar disks characterized by an inner gap within the disk structure. To test whether these gaps may have been formed by closely orbiting, previously undetected stellar companions, we collected high-resolution optical spectra of 31 TD objects to search for spectroscopic binaries (SBs). Twenty-four of these objects are in Ophiuchus and seven are within the Coronet, Corona Australis, and Chameleon I star-forming regions. We measured radial velocities for multiple epochs, obtaining a median precision of 400 ms−1. We identified double-lined SB SSTc2d J163154.7–250324 in Ophiuchus, which we determined to be composed of a K7(±0.5) and a K9(±0.5) star, with orbital limits of a < 0.6 au and P < 150 days. This results in an SB fraction of in Ophiuchus, which is consistent with other spectroscopic surveys of non-TD objects in the region. This similarity suggests that TDs are not preferentially sculpted by the presence of close binaries and that planet formation around close binaries may take place over similar timescales to that around single stars.Publisher PDFPeer reviewe
Testing the recovery of stellar rotation signals from Kepler light curves using a blind hare-and-hounds exercise
We present the results of a blind exercise to test the recoverability of
stellar rotation and differential rotation in Kepler light curves. The
simulated light curves lasted 1000 days and included activity cycles, Sun-like
butterfly patterns, differential rotation and spot evolution. The range of
rotation periods, activity levels and spot lifetime were chosen to be
representative of the Kepler data of solar like stars. Of the 1000 simulated
light curves, 770 were injected into actual quiescent Kepler light curves to
simulate Kepler noise. The test also included five 1000-day segments of the
Sun's total irradiance variations at different points in the Sun's activity
cycle.
Five teams took part in the blind exercise, plus two teams who participated
after the content of the light curves had been released. The methods used
included Lomb-Scargle periodograms and variants thereof, auto-correlation
function, and wavelet-based analyses, plus spot modelling to search for
differential rotation. The results show that the `overall' period is well
recovered for stars exhibiting low and moderate activity levels. Most teams
reported values within 10% of the true value in 70% of the cases. There was,
however, little correlation between the reported and simulated values of the
differential rotation shear, suggesting that differential rotation studies
based on full-disk light curves alone need to be treated with caution, at least
for solar-type stars.
The simulated light curves and associated parameters are available online for
the community to test their own methods.Comment: Accepted for publication in MNRAS. Accepted, 13 April 2015. Received,
26 March 2015; in original form, 9 November 201
Immobilization of recombinant human endostatin to different supports: potential applications in biomedicine
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