Predicting convective blueshift and radial-velocity dispersion due to granulation for FGK stars

To detect Earth-mass planets using the Doppler method, a major obstacle is to differentiate the planetary signal from intrinsic stellar variability (e.g., pulsations, granulation, spots and plages). Convective blueshift, which results from small-scale convection at the surface of Sun-like stars, is relevant for Earth-twin detections as it exhibits Doppler noise on the order of 1 m/s. Here, we present a simple model for convective blueshift based on fundamental equations of stellar structure. Our model successfully matches observations of convective blueshift for FGK stars. Based on our model, we also compute the intrinsic noise floor for stellar granulation in the radial velocity observations. We find that for a given mass range, stars with higher metallicities display lower radial-velocity dispersion due to granulation, in agreement with MHD simulations. We also provide a set of formulae to predict the amplitude of radial-velocity dispersion due to granulation as a function of stellar parameters. Our work is vital in identifying the most amenable stellar targets for EPRV surveys and radial velocity follow-up programmes for TESS, CHEOPS, and the upcoming PLATO mission.Comment: 11 pages, 5 figures, 3 tables. Submitted, under revie

Constraining planet structure from stellar chemistry: the cases of CoRoT-7, Kepler-10, and Kepler-93

We explore the possibility that the stellar relative abundances of different species can be used to constrain the bulk abundances of known transiting rocky planets. We use high resolution spectra to derive stellar parameters and chemical abundances for Fe, Si, Mg, O, and C in three stars hosting low mass, rocky planets: CoRoT-7, Kepler-10, and Kepler-93. These planets follow the same line along the mass-radius diagram, pointing toward a similar composition. The derived abundance ratios are compared with the solar values. With a simple stoichiometric model, we estimate the iron mass fraction in each planet, assuming stellar composition. We show that in all cases, the iron mass fraction inferred from the mass-radius relationship seems to be in good agreement with the iron abundance derived from the host star's photospheric composition. The results suggest that stellar abundances can be used to add constraints on the composition of orbiting rocky planets.Comment: A&A Letters, in pres

Survey of factors affecting satisfaction and success of residents enrolled in European College of Veterinary Diagnostic Imaging (ECVDI) residency programs

Several published studies have described the success and satisfaction of residents enrolled in human radiology training programs, however comparable studies for veterinary radiology residents are currently lacking. The purpose of this descriptive, survey study was to evaluate resident perceptions of the European College of Veterinary Diagnostic Imaging (ECVDI) residency training programs. A web-based survey was sent to all listed 114 ECVDI residents and received a 57.0% response (65/114). Of these, 88% of respondents enjoyed their training and 73.5% of finishing respondents agreed they felt ready to work as a veterinary radiologist. First-time pass rates for the theoretical and practical components of the board certifying examinations were >80%. Regarding research and exam preparation, 41.5% and 49.2% of respondents, respectively, did not feel they received appropriate guidance. Eleven percent of respondents felt a lack of mentoring and 7.7% reported  not receiving annual appraisals. Findings from this study can be used as background for future studies on perceptions of ECVDI residents and ECVDI residency program directors and for future policy decisions regarding ECVDI residency training programs

Local order around rare earth ions during the devitrification of oxyfluoride glasses.

International audienceErbium L(3)-edge extended x-ray absorption fine structure (EXAFS) measurements were performed on rare earth doped fluorosilicate and fluoroborate glasses and glass ceramics. The well known nucleating effects of erbium ions for the crystallization of cubic lead fluoride (based on x-ray diffraction measurements) and the fact that the rare earth ions are present in the crystalline phase (as indicated by Er(3+) emission spectra) seem in contradiction with the present EXAFS analysis, which indicates a lack of medium range structural ordering around the Er(3+) ions and suggests that the lead fluoride crystallization does not occur in the nearest neighbor distance of the rare earth ion. Molecular dynamics simulations of the devitrification process of a lead fluoride glass doped with Er(3+) ions were performed, and results indicate that Er(3+) ions lower the devitrification temperature of PbF(2), in good agreement with the experimental results. The genuine role of Er(3+) ions in the devitrification process of PbF(2) has been investigated. Although Er(3+) ions could indeed act as seeds for crystallization, as experiments suggest, molecular dynamics simulation results corroborate the experimental EXAFS observation that the devitrification does not occur at its nearest neighbor distance

Melting of Partially Fluorinated Graphene: From Detachment of Fluorine Atoms to Large Defects and Random Coils

The melting of fluorographene is very unusual and depends strongly on the degree of fluorination. For temperatures below 1000 K, fully fluorinated graphene (FFG) is thermo-mechanically more stable than graphene but at T$_m\approx$2800 K FFG transits to random coils which is almost twice lower than the melting temperature of graphene, i.e. 5300 K. For fluorinated graphene (PFG) up to 30 % ripples causes detachment of individual F-atoms around 2000 K while for 40-60 % fluorination, large defects are formed beyond 1500 K and beyond 60% of fluorination F-atoms remain bonded to graphene until melting. The results agree with recent experiments on the dependence of the reversibility of the fluorination process on the percentage of fluorination.Comment: 16 pages, 6 figure