The Formation Rate of Blue Stragglers in 47 Tucanae

We investigate the effects of changes in the blue straggler formation rate in globular clusters on the blue straggler distribution in the color-magnitude diagram. We find that the blue straggler distribution is highly sensitive to the past formation rate. Comparing our models to new UBV observations of a region close to the core of 47 Tucanae suggests that this cluster may have stopped forming blue straggler formation several Gyr ago. This cessation of formation can be associated with an epoch of primordial binary burning which has been invoked in other clusters to infer the imminence of core collapse.Comment: 17 pages, 9 figures, submitted to the Astrophysical Journa

Hubble Space Telescope times-series photometry of the planetary transit of HD189733: no moon, no rings, starspots

We monitored three transits of the giant gas planet around the nearby K dwarf HD 189733 with the ACS camera on the Hubble Space Telescope. The resulting very-high accuracy lightcurve (signal-to-noise ratio near 15000 on individual measurements, 35000 on 10-minute averages) allows a direct geometric measurement of the orbital inclination, radius ratio and scale of the system: i = 85.68 +- 0.04, Rpl/R*=0.1572 +- 0.0004, a/R*=8.92 +- 0.09. We derive improved values for the stellar and planetary radius, R*=0.755+- 0.011 Rsol, Rpl=1.154 +- 0.017 RJ, and the transit ephemerides, Ttr=2453931.12048 +- 0.00002 + n 2.218581 +- 0.000002$. The HST data also reveal clear evidence of the planet occulting spots on the surface of the star. At least one large spot complex (>80000 km) is required to explain the observed flux residuals and their colour evolution. This feature is compatible in amplitude and phase with the variability observed simultaneously from the ground. No evidence for satellites or rings around HD 189733b is seen in the HST lightcurve. This allows us to exlude with a high probability the presence of Earth-sized moons and Saturn-type debris rings around this planet. The timing of the three transits sampled is stable to the level of a few seconds, excluding a massive second planet in outer 2:1 resonance.Comment: revised version. Significant updates and new figures; to appear in Astronomy and Astrophysic

Detection of atmospheric haze on an extrasolar planet: The 0.55 - 1.05 micron transmission spectrum of HD189733b with the Hubble Space Telescope

The nearby transiting planet HD 189733b was observed during three transits with the ACS camera of the Hubble Space Telescope in spectroscopic mode. The resulting time series of 675 spectra covers the 550-1050 nm range, with a resolution element of ~8 nm, at extremely high accuracy (signal-to-noise ratio up to 10,000 in 50 nm intervals in each individual spectrum). Using these data, we disentangle the effects of limb darkening, measurement systematics, and spots on the surface of the host star, to calculate the wavelength dependence of the effective transit radius to an accuracy of ~50 km. This constitutes the ``transmission spectrum'' of the planetary atmosphere. It indicates at each wavelength at what height the planetary atmosphere becomes opaque to the grazing stellar light during the transit. In this wavelength range, strong features due to sodium, potassium and water are predicted by atmosphere models for a planet like HD 189733b, but they can be hidden by broad absorption from clouds or hazes higher up in the atmosphere. We observed an almost featureless transmission spectrum between 550 and 1050 nm, with no indication of the expected sodium or potassium atomic absorption features. Comparison of our results with the transit radius observed in the near and mid-infrared (2-8 microns), and the slope of the spectrum, suggest the presence of a haze of sub-micron particles in the upper atmosphere of the planet.Comment: 11 pages, MNRAS, accepted, minor correction

Detection of atmospheric haze on an extrasolar planet: the 0.55-1.05 μm transmission spectrum of HD 189733b with the Hubble Space Telescope

The nearby transiting planet HD 189733b was observed during three transits with the Advanced Camera for Surveys of the Hubble Space Telescope in spectroscopic mode. The resulting time-series of 675 spectra covers the 550-1050 nm range, with a resolution element of ∼8 nm, at extremely high accuracy (signal-to-noise ratio up to 10 000 in 50-nm intervals in each individual spectrum). Using these data, we disentangle the effects of limb darkening, measurement systematics and spots on the surface of the host star, to calculate the wavelength dependence of the effective transit radius to an accuracy of ∼50 km. This constitutes the ‘transmission spectrum' of the planetary atmosphere. It indicates at each wavelength at what height the planetary atmosphere becomes opaque to the grazing stellar light during the transit. In this wavelength range, strong features due to sodium, potassium and water are predicted by atmosphere models for a planet like HD 189733b, but they can be hidden by broad absorption from clouds or hazes higher up in the atmosphere. We observed an almost featureless transmission spectrum between 550 and 1050 nm, with no indication of the expected sodium or potassium atomic absorption features. Comparison of our results with the transit radius observed in the near and mid-infrared (2-8 μm), and the slope of the spectrum, suggest the presence of a haze of submicrometre particles in the upper atmosphere of the plane

Hubble Space Telescope Transmission Spectroscopy of the Exoplanet HD 189733b: High-altitude atmospheric haze in the optical and near-UV with STIS

We present Hubble Space Telescope optical and near-ultraviolet transmission spectra of the transiting hot-Jupiter HD189733b, taken with the repaired Space Telescope Imaging Spectrograph (STIS) instrument. The resulting spectra cover the range 2900-5700 Ang and reach per-exposure signal-to-noise levels greater than 11,000 within a 500 Ang bandwidth. We used time series spectra obtained during two transit events to determine the wavelength dependance of the planetary radius and measure the exoplanet's atmospheric transmission spectrum for the first time over this wavelength range. Our measurements, in conjunction with existing HST spectra, now provide a broadband transmission spectrum covering the full optical regime. The STIS data also shows unambiguous evidence of a large occulted stellar spot during one of our transit events, which we use to place constraints on the characteristics of the K dwarf's stellar spots, estimating spot temperatures around Teff~4250 K. With contemporaneous ground-based photometric monitoring of the stellar variability, we also measure the correlation between the stellar activity level and transit-measured planet-to-star radius contrast, which is in good agreement with predictions. We find a planetary transmission spectrum in good agreement with that of Rayleigh scattering from a high-altitude atmospheric haze as previously found from HST ACS camera. The high-altitude haze is now found to cover the entire optical regime and is well characterised by Rayleigh scattering. These findings suggest that haze may be a globally dominant atmospheric feature of the planet which would result in a high optical albedo at shorter optical wavelengths.Comment: 14 pages, 14 figures, 4 tables, accepted to MNRAS, revised version has minor change

Photometric variability in the old open cluster M 67. II. General Survey

We use differential CCD photometry to search for variability in BVI among 990 stars projected in and around the old open cluster M 67. In a previous paper we reported results for 22 cluster members that are optical counterparts to X-ray sources; this study focuses on the other stars in our observations. A variety of sampling rates were employed, allowing variability on time scales ranging from \sim 0.3 hours to \sim 20 days to be studied. Among the brightest sources studied, detection of variability as small as sigma approx 10 mmag is achieved (with > 3 sigma confidence); for the typical star observed, sensitivity to variability at levels sigma approx 20 mmag is achieved. The study is unbiased for stars with 12.5 < B < 18.5, 12.5 < V < 18.5, and 12 < I < 18 within a radius of about 10 arcmin from the cluster centre. In addition, stars with 10 < BVI < 12.5 were monitored in a few small regions in the cluster. We present photometry for all 990 sources studied, and report the variability characteristics of those stars found to be variable at a statistically significant level. Among the variables, we highlight several sources that merit future study, including stars located on the cluster binary sequence, stars on the giant branch, blue stragglers, and a newly discovered W UMa system.Comment: 12 pages, including 6 figures and 5 tables. Tables 1 and 3 only available in electronic version of paper. Accepted by A&