A new method of correcting radial velocity time series for inhomogeneous convection

Magnetic activity strongly impacts stellar RVs and the search for small planets. We showed previously that in the solar case it induces RV variations with an amplitude over the cycle on the order of 8 m/s, with signals on short and long timescales. The major component is the inhibition of the convective blueshift due to plages. We explore a new approach to correct for this major component of stellar radial velocities in the case of solar-type stars. The convective blueshift depends on line depths; we use this property to develop a method that will characterize the amplitude of this effect and to correct for this RV component. We build realistic RV time series corresponding to RVs computed using different sets of lines, including lines in different depth ranges. We characterize the performance of the method used to reconstruct the signal without the convective component and the detection limits derived from the residuals. We identified a set of lines which, combined with a global set of lines, allows us to reconstruct the convective component with a good precision and to correct for it. For the full temporal sampling, the power in the range 100-500~d significantly decreased, by a factor of 100 for a RV noise below 30 cm/s. We also studied the impact of noise contributions other than the photon noise, which lead to uncertainties on the RV computation, as well as the impact of the temporal sampling. We found that these other sources of noise do not greatly alter the quality of the correction, although they need a better noise level to reach a similar performance level. A very good correction of the convective component can be achieved providing very good RV noise levels combined with a very good instrumental stability and realistic granulation noise. Under the conditions considered in this paper, detection limits at 480~d lower than 1 MEarth could be achieved for RV noise below 15 cm/s.Comment: Accepted in A&A 18 July 201

Using the Sun to estimate Earth-like planets detection capabilities.I. Impact of cold spots

Stellar spots may in some cases produce radial velocity (RV) signatures similar to those of exoplanets. To further investigate the impact of spots, we aim at studying the detectability of Earth mass planets in the habitable zone (HZ) of solar type stars, if covered by spots similar to the sunspots. We have used the Sunspots properties recorded over one solar cycle between 1993 and 2003 to build the RV curve that a solar type star seen edge-on would show, if covered by such spots with Tsun -Tspot = 550K. We also simulate the RV of such a spotted star surrounded by an Earth mass planet located in the HZ. Under present assumptions, the detection of a 1 M Earth planet located between 0.8 and 1.2 AU requires an intensive monitoring (weekly or better), during several years of low activity phasis. The temporal sampling is more crucial than the precision of the data (assuming precisions in the range [1-10] cm/s). Cooler spots may become a problem for such detections. Also, we anticipate that plages, not considered in this paper, could further complicate or even compromise the detections

Evolution of the rates of mass wasting and fluvial sediment transfer from the epicentral area of the 1999, Mw 7.6 earthquake

The 1999 Chichi earthquake (Mw=7.6) triggered more than 20,000 landslides in the epicentral area in central west Taiwan, and subsequent typhoons have caused an even larger number of slope failures. As a result, the suspended sediment load of the epi- central Choshui River has increased dramatically. Measurements of suspended sedi- ment at a downstream gauging station indicate that the unit sediment concentration increased about six times due to the earthquake, and decreased exponentially due to flushing by subsequent typhoons. The e-folding time scale of the seismic perturbation of sediment transfer in the Choshui River is 3-5 years. Based on this estimate of the de- cay of the erosional response to the earthquake, a mass balance can be calculated for the earthquake, including co-seismic uplift and subsidence, post-seismic relaxation, and erosion. This mass balance shows that the Chi-Chi earthquake has acted to build ridge topography in the hanging wall of the fault, but in the far field, some destruc- tion of topography has occurred. However, our estimate of seismically-driven erosion may be incomplete. A detailed analysis of landsliding in the Chenyoulan tributary of the Choshui River indicates that most co-and post seismic landslide debris remains on hillslopes within the catchment. Recent typhoons have continued to cause high rates of landsliding high in the landscape, but rates of mass wasting near the stream net- work have decreased. The full geomorphic response to the Chi-Chi earthquake may be much larger, and more protracted than indicated by river gauging data

Wildlife-livestock interactions and risk areas for cross-species spread of bovine tuberculosis

The transmission of diseases between livestock and wildlife can be a hindrance to effective disease control. Maintenance hosts and contact rates should be explored to further understand the transmission dynamics at the wildlife-livestock interface. Bovine tuberculosis (BTB) has been shown to have wildlife maintenance hosts and has been confirmed as present in the African buffalo (Syncerus caffer) in the Queen Elizabeth National Park (QENP) in Uganda since the 1960s. The first aim of this study was to explore the spatio-temporal spread of cattle illegally grazing within the QENP recorded by the Uganda Wildlife Authority (UWA) rangers in a wildlife crime database. Secondly, we aimed to quantify wildlife-livestock interactions and cattle movements, on the border of QENP, using a longitudinal questionnaire completed by 30 livestock owners. From this database, 426 cattle sightings were recorded within QENP in 8 years. Thirteen (3.1%) of these came within a 300 m–4 week space-time window of a buffalo herd, using the recorded GPS data. Livestock owners reported an average of 1.04 (95% CI 0.97–1.11) sightings of Uganda kob, waterbuck, buffalo or warthog per day over a 3-month period, with a rate of 0.22 (95% CI 0.20–0.25) sightings of buffalo per farmer per day. Reports placed 85.3% of the ungulate sightings and 88.0% of the buffalo sightings as further than 50 m away. Ungulate sightings were more likely to be closer to cattle at the homestead (OR 2.0, 95% CI 1.1–3.6) compared with the grazing area. Each cattle herd mixed with an average of five other cattle herds at both the communal grazing and watering points on a daily basis. Although wildlife and cattle regularly shared grazing and watering areas, they seldom came into contact close enough for aerosol transmission. Between species infection transmission is therefore likely to be by indirect or non-respiratory routes, which is suspected to be an infrequent mechanism of transmission of BTB. Occasional cross-species spillover of infection is possible, and the interaction of multiple wildlife species needs further investigation. Controlling the interface between wildlife and cattle in a situation where eradication is not being considered may have little impact on BTB disease control in cattle

Reconstructing the solar integrated radial velocity using MDI/SOHO

Searches for exoplanets with radial velocity techniques are increasingly sensitive to stellar activity. It is therefore crucial to characterize how this activity influences radial velocity measurements in their study of the detectability of planets in these conditions. In a previous work we simulated the impact of spots and plages on the radial velocity of the Sun. Our objective is to compare this simulation with the observed radial velocity of the Sun for the same period. We use Dopplergrams and magnetograms obtained by MDI/SOHO over one solar cycle to reconstruct the solar integrated radial velocity in the Ni line 6768 \AA. We also characterize the relation between the velocity and the local magnetic field to interpret our results. We obtain a stronger redshift in places where the local magnetic field is larger (and as a consequence for larger magnetic structures): hence we find a higher attenuation of the convective blueshift in plages than in the network. Our results are compatible with an attenuation of this blueshift by about 50% when averaged over plages and network. We obtain an integrated radial velocity with an amplitude over the solar cycle of about 8 m/s, with small-scale variations similar to the results of the simulation, once they are scaled to the Ni line. The observed solar integrated radial velocity agrees with the result of the simulation made in our previous work within 30%, which validates this simulation. The observed amplitude confirms that the impact of the convective blueshift attenuation in magnetic regions will be critical to detect Earth-mass planets in the habitable zone around solar-like stars.Comment: 17 pages, 11 figures, accepted in Astronomy and Astrophysic

Variability of stellar granulation and convective blueshift with spectral type and magnetic activity. I. K and G main sequence stars

In solar-type stars, the attenuation of convective blueshift by stellar magnetic activity dominates the RV variations over the low amplitude signal induced by low mass planets. Models of stars that differ from the Sun will require a good knowledge of the attenuation of the convective blueshift to estimate its impact on the variations. It is therefore crucial to precisely determine not only the amplitude of the convective blueshift for different types of stars, but also the dependence of this convective blueshift on magnetic activity, as these are key factors in our model producing the RV. We studied a sample of main sequence stars with spectral types from G0 to K2 and focused on their temporally averaged properties: the activity level and a criterion allowing to characterise the amplitude of the convective blueshift. We find the differential velocity shifts of spectral lines due to convection to depend on the spectral type, the wavelength (this dependence is correlated with the Teff and activity level), and on the activity level. This allows us to quantify the dependence of granulation properties on magnetic activity for stars other than the Sun. The attenuation factor of the convective blueshift appears to be constant over the considered range of spectral types. We derive a convective blueshift which decreases towards lower temperatures, with a trend in close agreement with models for Teff lower than 5800 K, but with a significantly larger global amplitude. We finally compare the observed RV variation amplitudes with those that could be derived from our convective blueshift using a simple law and find a general agreement on the amplitude. Our results are consistent with previous results and provide, for the first time, an estimation of the convective blueshift as a function of Teff, magnetic activity, and wavelength, over a large sample of G and K main sequence stars

Mechanistic studies on DNA damage by minor groove binding copper–phenanthroline conjugates

Copper–phenanthroline complexes oxidatively damage and cleave nucleic acids. Copper bis-phenanthroline and copper complexes of mono- and bis-phenanthroline conjugates are used as research tools for studying nucleic acid structure and binding interactions. The mechanism of DNA oxidation and cleavage by these complexes was examined using two copper–phenanthroline conjugates of the sequence-specific binding molecule, distamycin. The complexes contained either one or two phenanthroline units that were bonded to the DNA-binding domain through a linker via the 3-position of the copper ligand. A duplex containing independently generated 2-deoxyribonolactone facilitated kinetic analysis of DNA cleavage. Oxidation rate constants were highly dependent upon the ligand environment but rate constants describing elimination of the alkali-labile 2-deoxyribonolactone intermediate were not. Rate constants describing DNA cleavage induced by each molecule were 11–54 times larger than the respective oxidation rate constants. The experiments indicate that DNA cleavage resulting from β-elimination of 2-deoxyribonolactone by copper–phenanthroline complexes is a general mechanism utilized by this family of molecules. In addition, the experiments confirm that DNA damage mediated by mono- and bis-phenanthroline copper complexes proceeds through distinct species, albeit with similar outcomes

The status of intranetwork magnetic field observations with THEMIS

Weak magnetic fields inside network cells are an important component of the solar magnetic field and they have been observed by a number a groups in the past few years, which yielded important clues to their behavior. However it is important to confirm these different results and to state with greater precision the properties of intranetwork magnetic features (vector magnetic field, lifetime...). In this paper we describe some of the multi-line spectro-polarimetric observations made with the THEMIS telescope in June 2000 in order to study the intranetwork magnetic fields. We highlight the data processing and some limitations we met when performing this study

Comparison of different exoplanet mass detection limit methods using a sample of main-sequence intermediate-type stars

The radial velocity (RV) technique is a powerful tool for detecting extrasolar planets and deriving mass detection limits that are useful for constraining planet pulsations and formation models. Detection limit methods must take into account the temporal distribution of power of various origins in the stellar signal. These methods must also be able to be applied to large samples of stellar RV time series We describe new methods for providing detection limits. We compute the detection limits for a sample of ten main sequence stars, which are of G-F-A type, in general active, and/or with detected planets, and various properties. We use them to compare the performances of these methods with those of two other methods used in the litterature. We obtained detection limits in the 2-1000 day period range for ten stars. Two of the proposed methods, based on the correlation between periodograms and the power in the periodogram of the RV time series in specific period ranges, are robust and represent a significant improvement compared to a method based on the root mean square of the RV signal. We conclude that two of the new methods (correlation-based method and local power analysis, i.e. LPA, method) provide robust detection limits, which are better than those provided by methods that do not take into account the temporal sampling.Comment: 18 pages, 15 figures Accepted in Astronomy & Astrophysic