Planetary Microlensing at High Magnification

Simulations of planetary microlensing at high magnification that were carried out on a cluster computer are presented. It was found that the perturbations due to two-thirds of all planets occur in the time interval [-0.5t_FWHM, 0.5t_ FWHM] with respect to the peak of the microlensing light curve, where t_FWHM is typically about 14 hours. This implies that only this restricted portion of the light curve need be intensively monitored for planets, a very significant practical advantage. Nearly all planetary detections in high magnification events will not involve caustic crossings. We discuss the issues involved in determining the planetary parameters in high magnification microlensing events. Earth mass planets may be detected with 1-m class telescopes if their projected orbital radii lie within about 1.5 - 2.5 AU. Giant planets are detectable over a much larger region. For multi-planet systems the perturbations due to individual planets can be separated under certain conditions. The size of the source star needs to be determined independently, but the presence of spots on the source star is likely to be negligible, as is the effect of planetary motion during an event.Comment: 12 pages, 13 embedded figures, accepted for publication by MNRA

The interaction of planetary nebulae and their AGB progenitors with the interstellar medium

Interaction with the Interstellar Medium (ISM) cannot be ignored in understanding planetary nebula (PN) evolution and shaping. In an effort to understand the range of shapes observed in the outer envelopes of PNe, we have run a comprehensive set of three-dimensional hydrodynamic simulations, from the beginning of the asymptotic giant branch (AGB) superwind phase until the end of the post--AGB/PN phase. A 'triple-wind' model is used, including a slow AGB wind, fast post--AGB wind and third wind reflecting the linear movement through the ISM. A wide range of stellar velocities, mass-loss rates and ISM densities have been considered. We find ISM interaction strongly affects outer PN structures, with the dominant shaping occuring during the AGB phase. The simulations predict four stages of PN--ISM interaction whereby the PN is initially unaffected (1), then limb-brightened in the direction of motion (2), then distorted with the star moving away from the geometric centre (3) and finally so distorted that the object is no longer recognisable as a PN and may not be classed as such (4). Parsec-size shells around PN are predicted to be common. The structure and brightness of ancient PNe is largely determined by the ISM interaction, caused by rebrightening during the second stage; this effect may address the current discrepancies in Galactic PN abundance. The majority of PNe will have tail structures. Evidence for strong interaction is found for all known planetary nebulae in globular clusters.Comment: 22 pages, 16 figures, accepted by MNRAS (consists of 14 page journal paper and 8 page online-only appendix). Email C Wareing for high quality PDF versio

Improving the Prospects for Detecting Extrasolar Planets in Gravitational Microlensing in 2002

Gravitational microlensing events of high magnification have been shown to be promising targets for detecting extrasolar planets. However, only a few events of high magnification have been found using conventional survey techniques. Here we demonstrate that high magnification events can be readily found in microlensing surveys using a strategy that combines high frequency sampling of target fields with online difference imaging analysis. We present 10 microlensing events with peak magnifications greater than 40 that were detected in real-time towards the Galactic Bulge during 2001 by MOA. We show that Earth mass planets can be detected in future events such as these through intensive follow-up observations around the event peaks. We report this result with urgency as a similar number of such events are expected in 2002.Comment: 11 pages, 3 embedded ps figures including 2 colour, revised version accepted by MNRA

Quantifying the contamination by old main-sequence stars in young moving groups: the case of the Local Association

The associations and moving groups of young stars are excellent laboratories for investigating stellar formation in the solar neighborhood. Previous results have confirmed that a non-negligible fraction of old main-sequence stars is present in the lists of possible members of young stellar kinematic groups. A detailed study of the properties of these samples is needed to separate the young stars from old main-sequence stars with similar space motion, and identify the origin of these structures. We used stars possible members of the young (~ 10 - 650 Myr) moving groups from the literature. To determine the age of the stars, we used several suitable age indicators for young main sequence stars, i.e., X-ray fluxes and other photometric data. We also used spectroscopic data, in particular the equivalent width of the lithium line Li I and Halpha, to constrain the range of ages of the stars. By combining photometric and spectroscopic data, we were able to separate the young stars (10 - 650 Myr) from the old (> 1 Gyr) field ones. We found, in particular, that the Local Association is contaminated by old field stars at the level of ~30%. This value must be considered as the contamination for our particular sample, and not of the entire Local Association. For other young moving groups, it is more difficult to estimate the fraction of old stars among possible members. However, the level of X-ray emission can, at least, help to separate two age populations: stars with <200 Myr and stars older than this. Our results are consistent with a scenario in which the moving groups contain both groups of young stars formed in a recent star-formation episode and old field stars with similar space motion. Only by combining X-ray and optical spectroscopic data is it possible to distinguish between these two age populations.Comment: 7 pages, 7 figures. Accepted for publication in A&

A refurbished convergent point method for finding moving groups in the Hipparcos Catalogue

The Hipparcos data allow a major step forward in the research of `moving groups' in the Solar neighbourhood, as the common motion of group members causes converging proper motions. Previous knowledge on these coherent structures in velocity space has always been limited by the availability, reliability, and accuracy of ground-based proper motion measurements. A refurbishment of Jones' convergent point method is presented which takes full advantage of the quality of the Hipparcos data. The original implementation of this method determines the maximum likelihood convergent point on a grid on the sky and simultaneously selects group members from a given set of stars with positions and proper motions. The refurbished procedure takes into account the full covariance matrix of the Hipparcos measurements instead of standard errors only, allows for internal motions of the stars, and replaces the grid-based approach by a direct minimization. The method is tested on Monte Carlo simulations of moving groups, and applied to the Hyades. Despite the limited amount of data used by the convergent point method, the results for stars in and around the cluster- centre region agree very well with those of the recent comprehensive study by Perryman et al. (1998).Comment: 14 pages, 7 Postscript figures, LaTeX using mn.sty and psfig.sty; accepted for publication in MNRA

Microlensing optical depth towards the Galactic bulge from MOA observations during 2000 with Difference Image Analysis

We analyze the data of the gravitational microlensing survey carried out by by the MOA group during 2000 towards the Galactic Bulge (GB). Our observations are designed to detect efficiently high magnification events with faint source stars and short timescale events, by increasing the the sampling rate up to 6 times per night and using Difference Image Analysis (DIA). We detect 28 microlensing candidates in 12 GB fields corresponding to 16 deg^2. We use Monte Carlo simulations to estimate our microlensing event detection efficiency, where we construct the I-band extinction map of our GB fields in order to find dereddened magnitudes. We find a systematic bias and large uncertainty in the measured value of the timescale $t_{\rm Eout}$ in our simulations. They are associated with blending and unresolved sources, and are allowed for in our measurements. We compute an optical depth tau = 2.59_{-0.64}^{+0.84} \times 10^{-6} towards the GB for events with timescales 0.3<t_E<200 days. We consider disk-disk lensing, and obtain an optical depth tau_{bulge} = 3.36_{-0.81}^{+1.11} \times 10^{-6}[0.77/(1-f_{disk})] for the bulge component assuming a 23% stellar contribution from disk stars. These observed optical depths are consistent with previous measurements by the MACHO and OGLE groups, and still higher than those predicted by existing Galactic models. We present the timescale distribution of the observed events, and find there are no significant short events of a few days, in spite of our high detection efficiency for short timescale events down to t_E = 0.3 days. We find that half of all our detected events have high magnification (>10). These events are useful for studies of extra-solar planets.Comment: 65 pages and 30 figures, accepted for publication in ApJ. A systematic bias and uncertainty in the optical depth measurement has been quantified by simulation

Kinematics of Stellar Populations with RAVE Data

We study the kinematics of the Galactic thin and thick disk populations using stars from the RAVE survey's second data release together with distance estimates from Breddels et al. (2009). The velocity distribution exhibits the expected moving groups present in the solar neighborhood. We separate thick and thin disk stars by applying the X (stellar-population) criterion of Schuster et al. (1993), which takes into account both kinematic and metallicity information. For 1906 thin disk and 110 thick disk stars classified in this way, we find a vertical velocity dispersion, mean rotational velocity and mean orbital eccentricity of (sigma_W, Vphi, e)_thin = (18\pm0.3 km/s, 223\pm0.4 km/s, 0.07\pm0.07) and (sigma_W, Vphi, e)_thick = (35\pm2 km/s, 163\pm2 km/s, 0.31\pm0.16), respectively. From the radial Jeans equation, we derive a thick disk scale length in the range 1.5-2.2 kpc, whose greatest uncertainty lies in the adopted form of the underlying potential. The shape of the orbital eccentricity distribution indicates that the thick disk stars in our sample most likely formed in situ with minor gas-rich mergers and/or radial migration being the most likely cause for their orbits. We further obtain mean metal abundances of _thin = +0.03 \pm 0.17, and _thick = -0.51\pm0.23, in good agreement with previous estimates. We estimate a radial metallicity gradient in the thin disk of -0.07 dex/kpc, which is larger than predicted by chemical evolution models where the disk grows insideout from infalling gas. It is, however, consistent with models where significant migration of stars shapes the chemical signature of the disk, implying that radial migration might play at least part of a role in the thick disk's formation.Comment: 27 pages, 7 figures, accepted for publication in New Astronom

The OSACA Database and a Kinematic Analysis of Stars in the Solar Neighborhood

We transformed radial velocities compiled from more than 1400 published sources, including the Geneva--Copenhagen survey of the solar neighborhood (CORAVEL-CfA), into a uniform system based on the radial velocities of 854 standard stars in our list. This enabled us to calculate the average weighted radial velocities for more than 25~000 HIPPARCOS stars located in the local Galactic spiral arm (Orion arm) with a median error of +-1 km/s. We use these radial velocities together with the stars' coordinates, parallaxes, and proper motions to determine their Galactic coordinates and space velocities. These quantities, along with other parameters of the stars, are available from the continuously updated Orion Spiral Arm CAtalogue (OSACA) and the associated database. We perform a kinematic analysis of the stars by applying an Ogorodnikov-Milne model to the OSACA data. The kinematics of the nearest single and multiple main-sequence stars differ substantially. We used distant (r\approx 0.2 kpc) stars of mixed spectral composition to estimate the angular velocity of the Galactic rotation -25.7+-1.2 km/s/kpc, and the vertex deviation,l=13+-2 degrees, and detect a negative K effect. This negative K effect is most conspicuous in the motion of A0-A5 giants, and is equal to K=-13.1+-2.0 km/s/kpc.Comment: 16 pages, 8 figure