We present chromospheric activity index S HK measurements for 119 995 F, G and K stars with high signal-to-noise ratio (S/N > 80) spectra, extracted from LAMOST DR1 in SDSS g band. The index δS for each of these stars is calculated by the difference between its S HK value and the baseline determined from very inactive stars. The effect of metallicity on measurement of δS varies with stellar T eff . No evident Vaughan-Preston gap appears in our sample. The relation between δS and vertical distance from the Galactic plane is determined for stars with T eff < 5500 K. Stars with higher δS tend to be closer to the Galactic plane. Two open clusters in the DR1 sample, M45 and M67, exhibit the expected general trend that δS decays with age. For stars with T eff > 5500 K, similar δS levels appear in both young and old cluster stars, which supports Pace's suggestion that caution should be exercised when deriving the age of a single star by using its chromospheric activity. Finally, we investigate the relation between δS and the kinematics of our sample.

c 0000 RAS 2 L. Mancini et al. ABSTRACT We present new ground-based, multi-colour, broad-band photometric measurements of the physical parameters, transmission and emission spectra of the transiting extrasolar planet WASP-19 b. The measurements are based on observations of eight transits and four occultations through a Gunn i filter using the 1.54 m Danish Telescope, 14 transits through an R c filter at the PEST observatory and one transit observed simultaneously through four optical (Sloan g ′ , r ′ , i ′ , z ′ ) and three near-infrared (J, H, K) filters, using the GROND instrument on the MPG/ESO 2.2m telescope. The GROND optical light curves have a point-to-point scatter around the best-fitting model between 0.52 and 0.65 mmag rms. We use these new data to measure refined physical parameters for the system. We find the planet to be more bloated (R b = 1.410 ± 0.017 R Jup ; M b = 1.139 ± 0.030 M Jup ) and the system to be twice as old as initially thought. We also used published and archived data sets to study the transit timings, which do not depart from a linear ephemeris. We detected an anomaly in the GROND transit light curve which is compatible with a spot on the photosphere of the parent star. The starspot position, size, spot contrast and temperature were established. Using our new and published measurements, we assembled the planet's transmission spectrum over the 370-2350 nm wavelength range and its emission spectrum over the 750-8000 nm range. By comparing these data to theoretical models we investigated the theoretically predicted variation of the apparent radius of WASP-19 b as a function of wavelength and studied the composition and thermal structure of its atmosphere. We conclude that: (i) there is no evidence for strong optical absorbers at low pressure, supporting the common idea that the planet's atmosphere lacks a dayside inversion; (ii) the temperature of the planet is not homogenized, because the high warming of its dayside causes the planet to be more efficient in re-radiating than redistributing energy to the night side; (iii) the planet seems to be outside of any current classification scheme.

We use the LAMOST spectra of member stars in Pleiades, M34, Praesepe, and Hyades to study how chromospheric activity vary as a function of mass and rotation at different age. We measured excess equivalent widths of Hα, Hβ , and Ca ii K based on estimated chromospheric contributions from old and inactive field dwarfs, and excess luminosities are obtained by normalizing bolometric luminosity, for more than 700 late-type stars in these open clusters. Results indicate two activity sequences in cool spot coverage and Hα excess emission among GK dwarfs in Pleiades and M dwarfs in Praesepe and Hyades, paralleling with well known rotation sequences. A weak dependence of chromospheric emission on rotation exists among ultra fast rotators in saturated regime with Rossby number Ro 0.1. In the unsaturated regime, chromospheric and coronal emission show similar dependence on Ro, but with a shift toward larger Ro, indicating chromospheric emission gets easily saturated than coronal emission, and/or convective turnover time-scales based on X-ray data do not work well with chromospheric emission. More interestingly, our analysis show fully convective slow rotators obey the rotation-chromospheric activity relation similar to hotter stars, confirming the previous finding. We found correlations among Hα, Hβ , and Ca ii K emissions, in which Hα losses are more important than Ca ii K for cooler and more active stars. In addition, a weak correlation is seen between chromospheric emission and photospheric activity that shows dependency on stellar spectral type and activity level, which provides some clues on how spot configuration vary as a function of mass and activity level.

We use the spectra of Pleiades and field stars from LAMOST DR2 archive to study how spottedness and activity vary as a function of mass at young ages. We obtained standard TiO band strength by measuring TiO bands near 7050Å from LAMOST spectra (R≈1800) for large sample of field GKM dwarfs with solar metallicity. Analysis show that active dwarfs, including late G-and early K-type, have extra TiO absorption compare to inactive counterparts, indicating the presence of cool spots on their surface. Active late K-and M-dwarfs show deeper TiO2 and shallower TiO4 compare to inactive stars at a given TiO5, which could be partly explained through cool spots. We estimated cool spot fractional coverage for 304 Pleiades candidates by modelling their TiO2 (&TiO5) band strength with respect to standard value. Results show that surface of large fraction of K-and M-type members have very large spot coverage (∼ 50%). We analysed a correlation between spot coverage, rotation and the amplitude of light variation, and found spot coverage on slow rotators (R o > 0.1) increases with decreasing Rossby Number R o . Interestingly, we detected a saturation-like feature for spot coverage in fast rotators with a saturation level of 40% − 50%. In addition, spot distribution in hotter fast rotators show more symmetrical compare to slow rotators. More interestingly, we detected large spot coverage in many M type members with no or little light variation. In bigger picture, these results provide important constraints for stellar dynamo on these cool active stars.

We present time-series photometric observations of thirteen transits in the planetary systems WASP-24, WASP-25 and WASP-26. All three systems have orbital obliquity measurements, WASP-24 and WASP-26 have been observed with Spitzer, and WASP-25 was previously comparatively neglected. Our light curves were obtained using the telescope-defocussing method and have scatters of 0.5 to 1.2 mmag relative to their best-fitting geometric models. We used these data to measure the physical properties and orbital ephemerides of the systems to high precision, finding that our improved measurements are in good agreement with previous studies. High-resolution Lucky Imaging observations of all three targets show no evidence for faint stars close enough to contaminate our photometry. We confirm the eclipsing nature of the star closest to WASP-24 and present the detection of a detached eclipsing binary within 4.25 arcmin of WASP-26.

We present photometric observations of four transits in the WASP-17 planetary system, obtained using telescope defocusing techniques and with scatters reaching 0.5 mmag per point. Our revised orbital period is 4.0 ± 0.6 s longer than previous measurements, a difference of 6.6σ , and does not support the published detections of orbital eccentricity in this system. We model the light curves using the JKTEBOP code and calculate the physical properties of the system by recourse to five sets of theoretical stellar model predictions. The resulting planetary radius, R b = 1.932 ± 0.052 ± 0.010 R Jup (statistical and systematic errors, respectively), provides confirmation that WASP-17 b is the largest planet currently known. All 14 planets with radii measured to be greater than 1.6 R Jup are found around comparatively hot (T eff > 5900 K) and massive (M A > 1.15 M ) stars. Chromospheric activity indicators are available for eight of these stars, and all imply a low activity level. The planets have small or zero orbital Based on data collected by MiNDSTEp with the Danish 1.54-m telescope at the ESO La Silla Observatory.

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We report three new barium (Ba) dwarfs lying in Sirius-like systems, which provides direct evidence that Ba dwarfs are companions to white dwarfs (WDs). Atmospheric parameters, stellar masses, and chemical abundances of 25 elements, including light, α, Fe-peak and s-process elements, are derived from high resolution and high S/N spectra. Enhancement of s-process elements with [s/Fe] ratios between 0.4 and 0.6 confirm them as mild barium stars. The estimated metallicities (−0.31, −0.06, 0.13) of BD+68• 1027, RE J0702+129 and BD+80 • 670 (3.59 M ⊙ ), which confirms the predicted range of progenitor AGB masses (1.5 ∼ 4 M ⊙ ) for unseen WDs around Ba dwarfs. Surface abundances of s-process elements in RE J0702+129 and BD+80• 670 are compared with AGB models and they are in close agreement, within predicted accretion efficiencies and pollution factors for Ba stars. These results support that the origin of s-process overabundances in Ba dwarfs is similar to Ba giants via McClure hypothesis in which Ba stars accumulate s-process elements through mass transfer from their host companions during AGB phase.