arXiv subjects

Simon Hodgkin

Publications and source records attributed to Simon Hodgkin.

39 records · Page 3Linked to original sources

arXiv2006

The Monitor project: Searching for occultations in young open clusters

The Monitor project is a photometric monitoring survey of nine young (1-200Myr) clusters in the solar neighbourhood to search for eclipses by very low mass stars and brown dwarfs and for planetary transits in the light curves of cluster members. It began in the autumn of 2004 and uses several 2 to 4m telescopes worldwide. We aim to calibrate the relation between age, mass, radius and where possible luminosity, from the K-dwarf to the planet regime, in an age range where constraints on evolutionary models are currently very scarce. Any detection of an exoplanet in one of our youngest targets (<=10Myr) would also provide important constraints on planet formation and migration timescales and their relation to proto-planetary disc lifetimes. Finally, we will use the light curves of cluster members to study rotation and flaring in low-mass pre-main sequence stars. The present paper details the motivation, science goals and observing strategy of the survey. We present a method to estimate the sensitivity and number of detections expected in each cluster, using a simple semi-analytic approach which takes into account the characteristics of the cluster and photometric observations, using (tunable) best-guess assumptions for the incidence and parameter distribution of putative companions, and we incorporate the limits imposed by radial velocity follow-up from medium and large telescopes. We use these calculations to show that the survey as a whole can be expected to detect over 100 young low and very low mass eclipsing binaries, and approx. 3 transiting planets with radial velocity signatures detectable with currently available facilities.

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arXiv2006

The Monitor project: Rotation of low-mass stars in the open cluster M34

We report on the results of a V and i-band time-series photometric survey of M34 (NGC 1039) using the Wide Field Camera (WFC) on the Isaac Newton Telescope (INT), achieving better than 1% precision per data point for 13 <~ i <~ 17. Candidate cluster members were selected from a V vs V-I colour-magnitude diagram over 14 < V < 24 (0.12 <~ M/Msun <~ 1.0), finding 714 candidates, of which we expect ~ 400 to be real cluster members (taking into account contamination from the field). The mass function was computed, and found to be consistent with a log-normal distribution in dN/dlogM. Searching for periodic variable objects in the candidate members gave 105 detections over the mass range 0.25 < M/Msun < 1.0. The distribution of rotation periods for 0.4 < M/Msun < 1.0 was found to peak at ~ 7 days, with a tail of fast rotators down to periods of ~ 0.8 days. For 0.25 < M/Msun < 0.4 we found a peak at short periods, with a lack of slow rotators (eg. P >~ 5 days, consistent with the work of other authors (eg. Scholz & Eisloffel 2004) at very low masses. Our results are interpreted in the context of previous work, finding that we reproduce the same general features in the rotational period distributions. A number of rapid rotators were found with velocities ~ a factor of two lower than in the Pleiades, consistent with models of angular momentum evolution assuming solid body rotation without needing to invoke core-envelope decoupling.

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arXiv2002

The luminosity function of the Virgo Cluster from M_B=-22 to M_B=-11

We measure the galaxy luminosity function (LF) for the Virgo Cluster between blue magnitudes M_B = -22 and M_B = -11 from wide-field CCD imaging data. The LF is only gradually rising for -22 < M_B < -16. Between M_B = -16 and M_B = -14 it rises steeply, with a logarithmic slope alpha ~ -1.6. Fainter than M_B = -14, the LF flattens again. This LF is shallower (although turning up at brighter absolute magnitudes) than the R-band LF recently measured by Phillipps et al. (1998), who found alpha ~ -2.2 fainter than M_R = -13. It is similar, however, to the LF determined from the Virgo Cluster Catalog by Sandage et al. (1985). A few faint galaxies are found which Sandage, et al. missed because their surface-brightness threshold for detection was too high, but these do not dominate the luminosity function at any magnitude. Most of the faint galaxies we find are dwarf elliptical, alternatively called dwarf spheroidal, galaxies. The most important potential source of systematic error is that we may have rejected some high surface-brightness galaxies from the cluster sample because we think that they are background galaxies. This is quite different from what has conventionally been regarded as the most serious source of systematic error in this kind of study: that we are missing many LOW surface-brightness galaxies because they are never visible above the sky.

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