Ch. Ginski

Transit timing variation in exoplanet WASP-3b★

Photometric follow-ups of transiting exoplanets may lead to discoveries of additional, less massive bodies in extrasolar systems. This is possible by detecting and then analysing variations in transit timing of transiting exoplanets. We present photometric observations gathered in 2009 and 2010 for exoplanet WASP-3b during the dedicated transit-timing-variation campaign. The observed transit timing cannot be explained by a constant period but by a periodic variation in the observations minus calculations diagram. Simplified models assuming the existence of a perturbing planet in the system and reproducing the observed variations of timing residuals were identified by three-body simulations. We found that the configuration with the hypothetical second planet of the mass of ∼15 M⊕, located close to the outer 2:1 mean motion resonance is the most likely scenario reproducing observed transit timing. We emphasize, however, that more observations are required to constrain better the parameters of the hypothetical second planet in WASP-3 system. For final interpretation not only transit timing but also photometric observations of the transit of the predicted second planet and the high precision radial-velocity data are needed.

The Young Exoplanet Transit Initiative (YETI)

We present the Young Exoplanet Transit Initiative (YETI), in which we use several 0.2 to 2.6-m telescopes around the world to monitor continuously young (≤100 Myr), nearby (≤1 kpc) stellar clusters mainly to detect young transiting planets (and to study other variability phenomena on time-scales from minutes to years). The telescope network enables us to observe the targets continuously for several days in order not to miss any transit. The runs are typically one to two weeks long, about three runs per year per cluster in two or three subsequent years for about ten clusters. There are thousands of stars detectable in each field with several hundred known cluster members, e.g. in the first cluster observed, Tr-37, a typical cluster for the YETI survey, there are at least 469 known young stars detected in YETI data down to R = 16.5 mag with sufficient precision of 50 millimag rms (5 mmag rms down to R = 14.5 mag) to detect transits, so that we can expect at least about one young transiting object in this cluster. If we observe ∼10 similar clusters, we can expect to detect ∼10 young transiting planets with radius determinations. The precision given above is for a typical telescope of the YETI network, namely the 60/90-cm Jena telescope (similar brightness limit, namely within ±1 mag, for the others) so that planetary transits can be detected. For targets with a periodic transit-like light curve, we obtain spectroscopy to ensure that the star is young and that the transiting object can be sub-stellar; then, we obtain Adaptive Optics infrared images and spectra, to exclude other bright eclipsing stars in the (larger) optical PSF; we carry out other observations as needed to rule out other false positive scenarios; finally, we also perform spectroscopy to determine the mass of the transiting companion. For planets with mass and radius determinations, we can calculate the mean density and probe the internal structure. We aim to constrain planet formation models and their time-scales by discovering planets younger than ∼100 Myr and determining not only their orbital parameters, but also measuring their true masses and radii, which is possible so far only by the transit method. Here, we present an overview and first results (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Multi-site campaign for transit timing variations of WASP-12 b: possible detection of a long-period signal of planetary origin

Aims. The transiting planet WASP-12 b was identified as a potential target for transit-timing studies because a departure from a linear ephemeris has been reported in the literature. Such deviations could be caused by an additional planet in the system. We attempt to confirm the claimed variations in transit timing and interpret their origin. Methods. We organised a multi-site campaign to observe transits by WASP-12 b in three observing seasons, using 0.5–2.6-metre telescopes. Results. We obtained 61 transit light curves, many of them with sub-millimagnitude precision. The simultaneous analysis of the best-quality datasets allowed us to obtain refined system parameters, which agree with values reported in previous studies. The residuals versus a linear ephemeris reveal a possible periodic signal that may be approximated by a sinusoid with an amplitude of 0.00068 ± 0.00013 d and period of 500 ± 20 orbital periods of WASP-12 b. The joint analysis of timing data and published radial velocity measurements results in a two-planet model that explains observations better than do single-planet scenarios. We hypothesise that WASP-12 b might not be the only planet in the system, and there might be the additional 0.1 MJup body on a 3.6-d eccentric orbit. A dynamical analysis indicates that the proposed two-planet system is stable on long timescales.

Planetary transit observations at the University Observatory Jena: TrES-2

We report on observations of several transit events of the transiting planet TrES-2 obtained with the Cassegrain-Teleskop-Kamera at the University Observatory Jena. Between March 2007 and November 2008 ten different transits and almost a complete orbital period were observed. Overall, in 40 nights of observation 4291 exposures (in total 71.52 h of observation) of the TrES-2 parent star were taken. With the transit timings for TrES-2 from the 34 events published by the TrES-network, the Transit Light Curve project and the Exoplanet Transit Database plus our own ten transits, we find that the orbital period is P = (2.470614 ± 0.000001) d, a slight change by ∼0.6 s compared to the previously published period. We present new ephemeris for this transiting planet. Furthermore, we found a second dip after the transit which could either be due to a blended variable star or occultation of a second star or even an additional object in the system. Our observations will be useful for future investigations of timing variations caused by additional perturbing planets and/or stellar spots and/or moons (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Transit timing of TrES-2: a combined analysis of ground- and space-based photometry

Homogeneous observations and careful analysis of transit light curves can lead to the identification of transit timing variations (TTVs). TrES-2 is one of few exoplanets, which offer the matchless possibility to combine long-term ground-based observations with continuous satellite data. Our research aimed at the search for TTVs that would be indicative of perturbations from additional bodies in the system. We also wanted to refine the system parameters and the orbital elements. We obtained 44 ground-based light curves of 31 individual transit events of TrES-2. Eight 0.2 - 2.2-m telescopes located at six observatories in Germany, Poland and Spain were used. In addition, we analysed 18 quarters (Q0-Q17) of observational data from NASAs space telescope Kepler including 435 individual transit events and 11 publicly available ground-based light curves. Assuming different limb darkening (LD) laws we performed an analysis for all light curves and redetermined the parameters of the system. We also carried out a joint analysis of the ground- and space-based data. The long observation period of seven years (2007-2013) allowed a very precise redetermination of the transit ephemeris. For a total of 490 transit light curves of TrES-2, the time of transit mid-point was determined. The transit times support neither variations on long time-scale nor on short time-scales. The nearly continuous observations of Kepler show no statistically significant increase or decrease in the orbital inclination i and the transit duration D. Only the transit depth shows a slight increase which could be an indication of an increasing stellar activity. In general, system parameters obtained by us were found to be in agreement with previous studies but are the most precise values to date.

A Possible Detection of Occultation by a Proto-Planetary Clump in GM Cephei

GM Cephei (GM Cep), in the young ({approx}4 Myr) open cluster Trumpler 37, has been known to be an abrupt variable and to have a circumstellar disk with a very active accretion. Our monitoring observations in 2009-2011 revealed that the star showed sporadic flare events, each with a brightening of {approx}< 0.5 mag lasting for days. These brightening events, associated with a color change toward blue, should originate from increased accretion activity. Moreover, the star also underwent a brightness drop of {approx}1 mag lasting for about a month, during which time the star became bluer when fainter. Such brightness drops seem to have a recurrence timescale of a year, as evidenced in our data and the photometric behavior of GM Cep over a century. Between consecutive drops, the star brightened gradually by about 1 mag and became blue at peak luminosity. We propose that the drop is caused by the obscuration of the central star by an orbiting dust concentration. The UX Orionis type of activity in GM Cep therefore exemplifies the disk inhomogeneity process in transition between the grain coagulation and the planetesimal formation in a young circumstellar disk.

Investigation of a transiting planet candidate in Trumpler 37: An astro-physical false positive eclipsing spectroscopic binary star *

We report our investigation of the first transiting planet candidate from the YETI project in the young (∼4 Myr old) open cluster Trumpler 37. The transit-like signal detected in the lightcurve of F8V star 2M21385603+5711345 repeats every 1.364894 + 0.000015 days, and has a depth of 54.5 + 0.8 mmag in R. Membership in the cluster is supported by its mean radial velocity and location in the color-magnitude diagram, while the Li diagnostic and proper motion are inconclusive in this regard. Follow-up photometric monitoring and adaptive optics imaging allow us to rule out many possible blend scenarios, but our radial-velocity measurements show it to be an eclipsing single-lined spectroscopic binary with a late-type (mid-M) stellar companion, rather than one of planetary nature. The estimated mass of the companion is 0.15–0.44 M⊙. The search for planets around very young stars such as those targeted by the YETI survey remains of critical importance to understand the early stages of planet formation and evolution.

Search for transiting exoplanets and variable stars in the open cluster NGC 7243

We report results of the first five observing campaigns for the open stellar cluster NGC 7243 in the frame of project Young Exoplanet Transit Initiative (YETI). The project focuses on the monitoring of young and nearby stellar clusters, with the aim to detect young transiting exoplanets and to study other variability phenomena on time-scales from minutes to years. After five observing campaigns and additional observations during 2013 and 2014, a clear and repeating transit-like signal was detected in the light curve of J221550.6+495611. Furthermore, we detected and analysed 37 new eclipsing binary stars in the studied region. The best fit parameters and light curves of all systems are given. Finally, we detected and analysed 26 new, presumably pulsating variable stars in the studied region. The follow-up investigation of these objects, including spectroscopic measurements of the exoplanet candidate, is currently planned. (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A new flare star member candidate in the Pleiades cluster

We present a new flare star, which was discovered during our survey on a selected field at the edge of the Pleiades cluster. The field was observed in the period 2007–2010 with three different CCD-cameras at the University Observatory Jena with telescopes from 25 to 90 cm. The flare duration is almost one hour with an amplitude in the R-band of about 1.08 mag. The location of the flare star in a color-magnitude diagram is consistent with age and distance of the Pleiades. In the optical PSF of the flare star there are two 2MASS objects (unresolved in most images in the optical Jena PSF), so it is not yet known which one of them is responsible for this flare. The BVRIJHK colors yield spectral types of M1 and M2 with extinction being Av = 0.231 ± 0.024 mag and Av = 0.266 ± 0.020 for those two stars, consistent with the Pleiades cluster (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Variability of young stars: determination of rotational periods of weak-line T Tauri stars in the Cepheus-Cassiopeia star-forming region

We report on observation and determination of rotational periods of ten weak-line T Tauri stars in the Cepheus-Cassiopeia star-forming region. Observations were carried out with the Cassegrain-Teleskop-Kamera (CTK) at University Observatory Jena between 2007 June and 2008 May. The periods obtained range between 0.49 d and 5.7 d, typical for weak-line and post T Tauri stars (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)