Period, epoch and prediction errors of ephemeris from continuous sets of timing measurements

Space missions such as Kepler and CoRoT have led to large numbers of eclipse or transit measurements in nearly continuous time series. This paper shows how to obtain the period error in such measurements from a basic linear least-squares fit, and how to correctly derive the timing error in the prediction of future transit or eclipse events. Assuming strict periodicity, a formula for the period error of such time series is derived: sigma_P = sigma_T (12/( N^3-N))^0.5, where sigma_P is the period error; sigma_T the timing error of a single measurement and N the number of measurements. Relative to the iterative method for period error estimation by Mighell & Plavchan (2013), this much simpler formula leads to smaller period errors, whose correctness has been verified through simulations. For the prediction of times of future periodic events, the usual linear ephemeris where epoch errors are quoted for the first time measurement, are prone to overestimation of the error of that prediction. This may be avoided by a correction for the duration of the time series. An alternative is the derivation of ephemerides whose reference epoch and epoch error are given for the centre of the time series. For long continuous or near-continuous time series whose acquisition is completed, such central epochs should be the preferred way for the quotation of linear ephemerides. While this work was motivated from the analysis of eclipse timing measures in space-based light curves, it should be applicable to any other problem with an uninterrupted sequence of discrete timings for which the determination of a zero point, of a constant period and of the associated errors is needed.Comment: Astronomy and Astrophysics, accepte

Starburst ages in HII galaxies

The age of the starbursts of a few H2 galaxies is derived from optical photometry and compared with previous results from radio continuum spectra. H2 galaxies are gas rich and characterized by the dominance of giant H2 regions, blue stellar colors, high surface brightness, and narrow emission lines. The presently observed high abundances of bright young stars must be the result of a recent -or even ongoing- starburst. The question, whether the observed starburst is the first one or if it is a recurrent phenomenon, is still open. If the starburst is the first one, H2 galaxies can be interpreted as being among the most unevolved galaxies known - their star formation characteristics possibly paralleling that of normal galaxies early in their evolution. A sample of five H2 galaxies was observed with B, R, and I broadband filters. They have been observed previously at several radio continuum frequencies. The photometric data are given. The results of the photometry and a comparison with radio continuum spectra are given

TEE, a simple estimator for the precision of eclipse and transit minimum times

Context: Transit or eclipse timing variations have proven to be a valuable tool in exoplanet research. However, no simple way to estimate the potential precision of such timing measures has been presented yet, nor are guidelines available regarding the relation between timing errors and sampling rate. Aims: A `timing error estimator' (TEE) equation is presented that requires only basic transit parameters as input. With the TEE, it is straightforward to estimate timing precisions both for actual data as well as for future instruments, such as the TESS and PLATO space missions. Methods: A derivation of the timing error based on a trapezoidal transit shape is given. We also verify the TEE on realistically modeled transits using Monte Carlo simulations and determine its validity range, exploring in particular the interplay between ingress/egress times and sampling rates. Results: The simulations show that the TEE gives timing errors very close to the correct value, as long as the temporal sampling is faster than transit ingress/egress durations and transits with very low S/N are avoided. Conclusions: The TEE is a useful tool to estimate eclipse or transit timing errors in actual and future data-sets. In combination with an equation to estimate period errors (Deeg 2015), predictions for the ephemeris precision of long-coverage observations are possible as well. The tests for the TEE's validity-range led also to implications for instrumental design: Temporal sampling has to be faster than transit in- or egress durations, or a loss in timing-precision will occur. An application to the TESS mission shows that transits close to its detection limit will have timing uncertainties that exceed 1 hour within a few months after their acquisition. Prompt follow-up observations will be needed to avoid a `loosing' of their ephemeris.Comment: Accepted by A&A. Version 2 with updated timing uncertainties of TESS mission due to correction of a figure in Sullivan et al. 201

Using Stellar Densities to Evaluate Transiting Exoplanetary Candidates

One of the persistent complications in searches for transiting exoplanets is the low percentage of the detected candidates that ultimately prove to be planets, which significantly increases the load on the telescopes used for the follow-up observations to confirm or reject candidates. Several attempts have been made at creating techniques that can pare down candidate lists without the need of additional observations. Some of these techniques involve a detailed analysis of light curve characteristics; others estimate the stellar density or some proxy thereof. In this paper, we extend upon this second approach, exploring the use of independently-calculated stellar densities to identify the most promising transiting exoplanet candidates. We use a set of CoRoT candidates and the set of known transiting exoplanets to examine the potential of this approach. In particular, we note the possibilities inherent in the high-precision photometry from space missions, which can detect stellar asteroseismic pulsations from which accurate stellar densities can be extracted without additional observations.Comment: 33 pages, 9 figure

UTM, a universal simulator for lightcurves of transiting systems

The Universal Transit Modeller (UTM) is a light-curve simulator for all kinds of transiting or eclipsing configurations between arbitrary numbers of several types of objects, which may be stars, planets, planetary moons, and planetary rings. Applications of UTM to date have been mainly in the generation of light-curves for the testing of detection algorithms. For the preparation of such test for the Corot Mission, a special version has been used to generate multicolour light-curves in Corot's passbands. A separate fitting program, UFIT (Universal Fitter) is part of the UTM distribution and may be used to derive best fits to light-curves for any set of continuously variable parameters. UTM/UFIT is written in IDL code and its source is released in the public domain under the GNU General Public License.Comment: 4 pages, 2 figures, to appear in Proc. of'Transiting Planets', IAU Symposium 25

Deep CCD Photometry and the Initial Mass Function of the Core of the OB Cluster Berkeley 86

Based on photometry of deep CCD frames of the central region of the OB cluster Berkeley 86, we derive the cluster mass function. The absence of current star formation, and the cluster\u27s young age of about 6Myrs, leads to the conclusion that the initial mass function (IMF) and the current mass function are identical for stars with m\u3c 10msun_. In the range of 1.2-20msun_, an IMF with a slope of {GAMMA}=-1.3+/-0.3 is found. This value agrees well with other recent determinations of young clusters IMFs which are close to the classical Salpeter IMF with {GAMMA}=-1.35. Sections of the IMF of Berkeley 86 that are significantly steeper, or flatter, are most likely the result of a dip in the star\u27s mass distribution in the range of 3.5-10msun_. Similar dips may have led to steep IMFs over narrow mass ranges, as reported in the literature for some other clusters. No sign for a low mass turn-over in the IMF of Berkeley 86 is found for masses extending down to 0.85 msun _ (Refer to PDF file for exact formulas)

The orbital phases and secondary transit of Kepler-10b - A physical interpretation based on the Lava-ocean planet model -

The Kepler mission has made an important observation, the first detection of photons from a terrestrial planet by observing its phase curve (Kepler-10b). This opens a new field in exoplanet science: the possibility to get information about the atmosphere and surface of rocky planets, objects of prime interest. In this letter, we apply the Lava-ocean model to interpret the observed phase curve. The model, a planet with no atmosphere and a surface partially made of molten rocks, has been proposed for planets of the class of CoRoT-7b, i.e. rocky planets very close to their star (at few stellar radii). Kepler-10b is a typical member of this family. It predicts that the light from the planet has an important emission component in addition to the reflected one, even in the Kepler spectral band. Assuming an isotropical reflection of light by the planetary surface (Lambertian-like approximation), we find that a Bond albedo of \sim50% can account for the observed amplitude of the phase curve, as opposed to a first attempt where an unusually high value was found. We propose a physical process to explain this still large value of the albedo. The overall interpretation can be tested in the future with instruments as JWST or EChO. Our model predicts a spectral dependence that is clearly distinguishable from that of purely reflected light, and from that of a planet at a uniform temperature.Comment: Accepted in ApJ Letters, 17 pages, 3 figure