The microlensing rate and distribution of free-floating planets towards the Galactic bulge

Ground-based optical microlensing surveys have provided tantalising, if inconclusive, evidence for a significant population of free-floating planets (FFPs). Both ground and space-based facilities are being used and developed which will be able to probe the distrubution of FFPs with much better sensitivity. It is vital also to develop a high-precision microlensing simulation framework to evaluate the completeness of such surveys. We present the first signal-to-noise limited calculations of the FFP microlensing rate using the Besancon Galactic model. The microlensing distribution towards the Galactic centre is simulated for wide-area ground-based optical surveys such as OGLE or MOA, a wide-area ground-based near-IR survey, and a targeted space-based near-IR survey which could be undertaken with Euclid or WFIRST. We present a calculation framework for the computation of the optical and near-infrared microlensing rate and optical depth for simulated stellar catalogues which are signal-to-noise limited, and take account of extinction, unresolved stellar background light and finite source size effects, which can be significant for FFPs. We find that the global ground-based I-band yield over a central 200 deg^2 region covering the Galactic centre ranges from 20 Earth-mass FFPs year^-1 up to 3,500 year^-1 for Jupiter FFPs in the limit of 100% detection efficiency, and almost an order of magnitude larger for a K-band survey. For ground-based surveys we find that the inclusion of finite source and the unresolved background reveals a mass-dependent variation in the spatial distribution of FFPs. For a space-based H-band covering 2 deg^2, the yield depends on the target field but maximizes close to the Galactic centre with around 76 Earth through to 1,700 Jupiter FFPs year^-1. For near-IR space-based surveys the spatial distribution of FFPs is found to be largely insensitive to the FFP mass scale.Comment: 14 pages, submitted to A&A and accepte

Section 510 (b) of the Bankruptcy Code is to be Interpreted Broadly

(Excerpt) One of the cornerstone principles of Chapter 11 under title 11 of the United States Code (“the Bankruptcy Code”) is section 510 (b). Under section 510 (b), a fraud claim by a purchaser of stock in a corporation that subsequently files a petition for relief under the Bankruptcy Code must be subordinated to general unsecured creditors. Although the application of section 510 (b) may seem straightforward, courts have struggled interpreting the ambiguous language of section 510 (b). Prior to Congress enacting section 510 (b), there was significant confusion throughout the bankruptcy community regarding what claims must be subordinated. Many courts struggled in determining whether such claims should be treated pari passu with claims of general unsecured creditors or whether such claims should be subordinated. Congress sought to clarify these issues when it enacted section 510 (b). When creating section 510 (b), Congress relied heavily on the law review article written by two prestigious law school professors, John J. Slain (“Slain”) and Homer Kripke (“Kripke”). According to Slain and Kripke, section 510 (b) was created to reflect “the different degree to which each party assumes a risk of enterprise insolvency.” Factoring in the general creditor’s need for protection, Congress’ rationale for section 510 (b) was that general creditors rely on a cushion of securities when they decide to extend credit. Furthermore, since creditors rely heavily on the protections from mandatory subordination in the event of bankruptcy, whereas security holders freely bargained for an equity interest rather than a debt interest when they decided to invest in the corporation\u27s stock, Congress sought to fairly balance these risks. Because security holders voluntarily forgo the risk of insolvency with the hopes of making a profit, Congress designed section 510 (b) with the intent to provide creditors with more protection

Difference image photometry with bright variable backgrounds

Over the last two decades the Andromeda Galaxy (M31) has been something of a test-bed for methods aimed at obtaining accurate time-domain relative photometry within highly crowded fields. Difference imaging methods, originally pioneered towards M31, have evolved into sophisticated methods, such as the Optimal Image Subtraction (OIS) method of Alard & Lupton (1998), that today are most widely used to survey variable stars, transients and microlensing events in our own Galaxy. We show that modern difference image (DIA) algorithms such as OIS, whilst spectacularly successful towards the Milky Way bulge, may perform badly towards high surface brightness targets such as the M31 bulge. Poor results can occur in the presence of common systematics which add spurious flux contributions to images, such as internal reflections, scattered light or fringing. Using data from the Angstrom Project microlensing survey of the M31 bulge, we show that very good results are usually obtainable by first performing careful photometric alignment prior to using OIS to perform point-spread function (PSF) matching. This separation of background matching and PSF matching, a common feature of earlier M31 photometry techniques, allows us to take full advantage of the powerful PSF matching flexibility offered by OIS towards high surface brightness targets. We find that difference images produced this way have noise distributions close to Gaussian, showing significant improvement upon results achieved using OIS alone. We show that with this correction light-curves of variable stars and transients can be recovered to within ~10 arcseconds of the M31 nucleus. Our method is simple to implement and is quick enough to be incorporated within real-time DIA pipelines. (Abridged)Comment: 12 pages. Accepted for publication in MNRAS. Includes an expanded discussion of DIA testing and results, including additional lightcurve example

The POINT-AGAPE Survey: Comparing Automated Searches of Microlensing Events toward M31

Searching for microlensing in M31 using automated superpixel surveys raises a number of difficulties which are not present in more conventional techniques. Here we focus on the problem that the list of microlensing candidates is sensitive to the selection criteria or "cuts" imposed and some subjectivity is involved in this. Weakening the cuts will generate a longer list of microlensing candidates but with a greater fraction of spurious ones; strengthening the cuts will produce a shorter list but may exclude some genuine events. We illustrate this by comparing three analyses of the same data-set obtained from a 3-year observing run on the INT in La Palma. The results of two of these analyses have been already reported: Belokurov et al. (2005) obtained between 3 and 22 candidates, depending on the strength of their cuts, while Calchi Novati et al. (2005) obtained 6 candidates. The third analysis is presented here for the first time and reports 10 microlensing candidates, 7 of which are new. Only two of the candidates are common to all three analyses. In order to understand why these analyses produce different candidate lists, a comparison is made of the cuts used by the three groups...Comment: 28 pages, 24 figures, 9 table

The Origin of Primordial Dwarf Stars and Baryonic Dark Matter

I present a scenario for the production of low mass, degenerate dwarfs of mass $>0.1 M_{\odot}$ via the mechanism of Lenzuni, Chernoff & Salpeter (1992). Such objects meet the mass limit requirements for halo dark matter from microlensing surveys while circumventing the chemical evolution constraints on normal white dwarf stars. I describe methods to observationally constrain this scenario and suggest that such objects may originate in small clusters formed from the thermal instability of shocked, heated gas in dark matter haloes, such as suggested by Fall & Rees (1985) for globular clusters.Comment: TeX, 4 pages plus 2 postscript figures. To appear in Astrophysical Journal Letter

Is the Large Magellanic Cloud a Large Microlensing Cloud?

An expression is provided for the self-lensing optical depth of the thin LMC disk surrounded by a shroud of stars at larger scale heights. The formula is written in terms of the vertical velocity dispersion of the thin disk population. If tidal forcing causes 1-5 % of the disk mass to have a height larger than 6 kpc and 10-15 % to have a height above 3 kpc, then the self-lensing optical depth of the LMC is $0.7 - 1.9 \times 10^{-7}$, which is within the observational uncertainties. The shroud may be composed of bright stars provided they are not in stellar hydrodynamical equilibrium. Alternatively, the shroud may be built from low mass stars or compact objects, though then the self-lensing optical depths are overestimates of the true optical depth by a factor of roughly 3. The distributions of timescales of the events and their spatial variation across the face of the LMC disk offer possibilities of identifying the dominant lens population. In propitious circumstances, an experiment lifetime of less than 5 years is sufficient to decide between the competing claims of Milky Way halos and LMC lenses. However, LMC disks can sometimes mimic the microlensing properties of Galactic halos for many years and then decades of survey work are needed. In this case observations of parallax or binary caustic events offer the best hope for current experiments to deduce the lens population. The difficult models to distinguish are Milky Way halos in which the lens fraction is low (< 10 %) and fattened LMC disks composed of lenses with a typical mass of low luminosity stars or greater. A next-generation wide-area microlensing survey, such as the proposed ``SuperMACHO'' experiment, will be able to distinguish even these difficult models with just a year or two of data.Comment: 25 pages, 4 figures, The Astrophysical Journal (in press

Novae In External Galaxies From The POINT-AGAPE Survey And The Liverpool Telescope

We have recently begun a search for Classical Novae in M31 using three years of multicolour data taken by the POINT-AGAPE microlensing collaboration with the 2.5m Isaac Newton Telescope (INT) on La Palma. This is a pilot program leading to the use of the Liverpool Telescope (LT) to systematically search for and follow novae of all speed classes in external galaxies to distances up to around 5Mpc.Comment: 5 pages, 4 figures, To appear in the Proceedings of the Conference ``Classical Nova Explosions'', M. Hernanz and J. Jose eds., American Inst. of Physics, 200

Gamma ray astronomy and baryonic dark matter

Recently, Dixon et al. have re-analyzed the EGRET data, finding a statistically significant diffuse $\gamma$-ray emission from the galactic halo. We show that this emission can naturally be explained within a previously-proposed model for baryonic dark matter, in which $\gamma$-rays are produced through the interaction of high-energy cosmic-ray protons with cold $H_2$ clouds clumped into dark clusters - these dark clusters supposedly populate the outer galactic halo and can show up in microlensing observations. Our estimate for the halo $\gamma$-ray flux turns out to be in remarkably good agreement with the discovery by Dixon et al. We also address future prospects to test our predictions.Comment: 9 pages, 1 figure included, to appear in ApJ 510, L103 (1999