A bimodal correlation between host star chromospheric emission and the surface gravity of hot Jupiters

The chromospheric activity index logR'HK of stars hosting transiting hot Jupiters appears to be correlated with the planets' surface gravity. One of the possible explanations is based on the presence of condensations of planetary evaporated material located in a circumstellar cloud that absorbs the CaII H&K and MgII h&k resonance line emission flux, used to measure chromospheric activity. A larger column density in the condensations, or equivalently a stronger absorption in the chromospheric lines, is obtained when the evaporation rate of the planet is larger, which occurs for a lower gravity of the planet. We analyze here a sample of stars hosting transiting hot Jupiters tuned in order to minimize systematic effects (e.g., interstellar medium absorption). Using a mixture model, we find that the data are best fit by a two-linear-regression model. We interpret this result in terms of the Vaughan-Preston gap. We use a Monte Carlo approach to best take into account the uncertainties, finding that the two intercepts fit the observed peaks of the distribution of logR'HK for main-sequence solar-like stars. We also find that the intercepts are correlated with the slopes, as predicted by the model based on the condensations of planetary evaporated material. Our findings bring further support to this model, although we cannot firmly exclude different explanations. A precise determination of the slopes of the two linear components would allow one to estimate the average effective stellar flux powering planetary evaporation, which can then be used for theoretical population and evolution studies of close-in planets.Comment: 23 pages, 4 figures, 1 table, accepted for publication in ApJ

On the Excitation of Double Giant Resonances in Heavy Ion Reactions

The interplay of nuclear and Coulomb processes in the inelastic excitation of single- and double-phonon giant resonances in heavy ion collisions is studied within a simple reaction model. Predominance of the Coulomb excitation mechanism on the population of the single-phonon and, on the contrary, predominance of the nuclear excitation for the double-phonon is evidenced. The effect of the spreading of the strength distribution of the giant resonances on the excitation process is analyzed, showing sizeable modifications in the case of Coulomb dominated processes.Comment: Accepten in Nuclear Physics A. 10 eps figures and source file in an uncompressed tar packag

Long-term radial-velocity variations of the Sun as a star: The HARPS view

Stellar radial velocities play a fundamental role in the discovery of extrasolar planets and the measurement of their physical parameters as well as in the study of stellar physical properties. We investigate the impact of the solar activity on the radial velocity of the Sun using the HARPS spectrograph to obtain measurements that can be directly compared with those acquired in the extrasolar planet search programs. We use the Moon, the Galilean satellites, and several asteroids as reflectors to measure the radial velocity of the Sun as a star and correlate it with disc-integrated chromospheric and magnetic indexes of solar activity that are similar to stellar activity indexes. We discuss in detail the systematic effects that affect our measurements and the methods to account for them. We find that the radial velocity of the Sun as a star is positively correlated with the level of its chromospheric activity at about 95 percent significance level. The amplitude of the long-term variation measured in the 2006-2014 period is 4.98 \pm 1.44 m/s, in good agreement with model predictions. The standard deviation of the residuals obtained by subtracting a linear best fit is 2.82 m/s and is due to the rotation of the reflecting bodies and the intrinsic variability of the Sun on timescales shorter than the activity cycle. A correlation with a lower significance is detected between the radial velocity and the mean absolute value of the line-of-sight photospheric magnetic field flux density. Our results confirm similar correlations found in other late-type main-sequence stars and provide support to the predictions of radial velocity variations induced by stellar activity based on current models.Comment: 11 pages, 7 figures, 2 tables, 1 Appendix; accepted by Astronomy and Astrophysic

Planet-Induced Emission Enhancements in HD 179949: Results from McDonald Observations

We monitored the Ca II H and K lines of HD 179949, a notable star in the southern hemisphere, to observe and confirm previously identified planet induced emission (PIE) as an effect of star-planet interaction. We obtained high resolution spectra (R ~ 53,000) with a signal-to-noise ratio S/N >~ 50 in the Ca II H and K cores during 10 nights of observation at the McDonald Observatory. Wide band echelle spectra were taken using the 2.7 m telescope. Detailed statistical analysis of Ca II K revealed fluctuations in the Ca II K core attributable to planet induced chromospheric emission. This result is consistent with previous studies by Shkolnik et al. (2003). Additionally, we were able to confirm the reality and temporal evolution of the phase shift of the maximum of star-planet interaction previously found. However, no identifiable fluctuations were detected in the Ca II H core. The Al I lambda 3944 A line was also monitored to gauge if the expected activity enhancements are confined to the chromospheric layer. Our observations revealed some variability, which is apparently unassociated with planet induced activity.Comment: 11 pages, 11 figures, 5 tables; Publications of the Astronomical Society of Australia (in press

Measuring stellar differential rotation with high-precision space-borne photometry

We introduce a method of measuring a lower limit to the amplitude of surface differential rotation from high-precision, evenly sampled photometric time series. It is applied to main-sequence late-type stars whose optical flux modulation is dominated by starspots. An autocorrelation of the time series was used to select stars that allow an accurate determination of starspot rotation periods. A simple two-spot model was applied together with a Bayesian information criterion to preliminarily select intervals of the time series showing evidence of differential rotation with starspots of almost constant area. Finally, the significance of the differential rotation detection and a measurement of its amplitude and uncertainty were obtained by an a posteriori Bayesian analysis based on a Monte Carlo Markov Chain approach. We applied our method to the Sun and eight other stars for which previous spot modelling had been performed to compare our results with previous ones. We find that autocorrelation is a simple method for selecting stars with a coherent rotational signal that is a prerequisite for successfully measuring differential rotation through spot modelling. For a proper Monte Carlo Markov Chain analysis, it is necessary to take the strong correlations among different parameters that exist in spot modelling into account. For the planet-hosting star Kepler-30, we derive a lower limit to the relative amplitude of the differential rotation of \Delta P / P = 0.0523 \pm 0.0016. We confirm that the Sun as a star in the optical passband is not suitable for measuring differential rotation owing to the rapid evolution of its photospheric active regions. In general, our method performs well in comparison to more sophisticated and time-consuming approaches.Comment: Accepted to Astronomy and Astrophysics, 15 pages, 13 figures, 4 tables and an Appendi

Certified accuracy of rainfall data as a standard requirement in scientific investigations

International audienceThis paper elaborates on the rationale behind the proposed standard limits for the accuracy of rainfall intensity measurements obtained from tipping-bucket and other types of rain gauges. Indeed, based on experimental results obtained in the course of international instrument Intercomparison initiatives and specific laboratory tests, it is shown here that the accuracy of operational rain gauges can be reduced to the limits of ±1% after proper calibration and correction. This figure is proposed as a standard accuracy requirement for the use of rain data in scientific investigations. This limit is also proposed as the reference accuracy for operational rain gauge networks in order to comply with quality assurance systems in meteorological observations

Daily variability of Ceres' Albedo detected by means of radial velocities changes of the reflected sunlight

Bright features have been recently discovered by Dawn on Ceres, which extend previous photometric and Space Telescope observations. These features should produce distortions of the line profiles of the reflected solar spectrum and therefore an apparent radial velocity variation modulated by the rotation of the dwarf planet. Here we report on two sequences of observations of Ceres performed in the nights of 31 July, 26-27 August 2015 by means of the high-precision HARPS spectrograph at the 3.6-m La Silla ESO telescope. The observations revealed a quite complex behaviour which likely combines a radial velocity modulation due to the rotation with an amplitude of approx +/- 6 m/s and an unexpected diurnal effect. The latter changes imply changes in the albedo of Occator's bright features due to the blaze produced by the exposure to solar radiation. The short-term variability of Ceres' albedo is on timescales ranging from hours to months and can both be confirmed and followed by means of dedicated radial velocity observations.Comment: 5 pag, 1fig, two tables, MNRAS Letters 201

Sectoral r modes and periodic RV variations of Sun-like stars

Radial velocity (RV) measurements are used to search for planets orbiting late-type main-sequence stars and confirm the transiting planets. The most advanced spectrometers are approaching a precision of $\sim 10$ cm/s that implies the need to identify and correct for all possible sources of RV oscillations intrinsic to the star down to this level and possibly beyond. The recent discovery of global-scale equatorial Rossby waves in the Sun, also called r modes, prompted us to investigate their possible signature in stellar RV measurements. R modes are toroidal modes of oscillation whose restoring force is the Coriolis force and propagate in the retrograde direction in a frame that corotates with the star. The solar r modes with azimuthal orders $3 \leq m \lesssim 15$ were identified unambiguously because of their dispersion relation and their long e-folding lifetimes of hundreds of days. Here we simulate the RV oscillations produced by sectoral r modes with $2 \leq m \leq 5$ assuming a stellar rotation period of 25.54 days and a maximum amplitude of the surface velocity of each mode of 2 m/s. This amplitude is representative of the solar measurements, except for the $m=2$ mode which has not yet been observed. Sectoral r modes with azimuthal orders $m=2$ and $3$ would produce RV oscillations with amplitudes of 76.4 and 19.6 cm/s and periods of 19.16 and 10.22 days, respectively, for a star with an inclination of the rotation axis $i=60^{\circ}$. Therefore, they may produce rather sharp peaks in the Fourier spectrum of the radial velocity time series that could lead to spurious planetary detections. Sectoral r~modes may represent a source of confusion in the case of slowly rotating inactive stars that are preferential targets for RV planet search. The main limitation of the present investigation is the lack of observational constraint on the amplitude of the $m=2$ mode on the Sun.Comment: 7 pages; 4 figures; 1 table; accepted to Astronomy & Astrophysic