Influence of functional groups on the gas chromatographic retention behaviour of QM-siloxanes

The retention time sequences of QM-siloxane derivatives (derivatives of trimethylsilyl esters of silicic acids) of the formula QmM2m+2-nXn (X = H, Cl, C2H5O; m, n = 1,2,3) and of siloxane derivatives containing M, D and T groups were investigated by capillary gas chromatography. The investigation showed that in compounds of the same structural type the retention times increase in the order of the substituents H < Cl < C2H5O. A linear correlation between retention time and molecular weight was observed only within groups of QM derivatives of the same structural type. The retention sequence rule obtained from QM-siloxanes was modified so that it became valid for the investigated compounds, and so that it should be possible to identify unknown siloxane derivatives by the use of gas chromatography

The distance to the Galactic Centre based on Population-II Cepheids and RR Lyrae stars

Context: The distance to the Galactic Centre (GC) is of importance for the distance scale in the Universe. The value derived by Eisenhauer et al. (2005) of 7.62 +- 0.32 kpc based on the orbit of one star around the central black hole is shorter than most other distance estimates based on a variety of different methods. Aim: To establish an independent distance to the GC with high accuracy. To this end Population-II Cepheids are used that have been discovered in the OGLE-II and III surveys. Method: Thirty-nine Pop-II Cepheids have been monitored on 4 nights spanning 14 days. Light curves have been fitted using the known periods from the OGLE data to determine the mean K-band magnitude. It so happens that 37 RR Lyrae stars are in the field-of-views and mean K-band magnitudes are derived for this sample as well. Results: The period-luminosity relation of Pop-II Cepheids in the K-band is determined, and the derived slope of -2.24 +- 0.14 is consistent with the value derived by Matsunaga et al. (2006). Fixing the slope to their more accurate value results in a zero point, and implies a distance modulus to the GC of 14.51 +- 0.12, with an additional systematic uncertainty of 0.07 mag. Similarly, from the RR Lyrae K-band PL-relation we derive a value of 14.48 +- 0.17 (random) +- 0.07 (syst.). The two independent determinations are averaged to find 14.50 +- 0.10 (random) +- 0.07 (syst.), or 7.94 +- 0.37 +- 0.26 kpc.Comment: A&A accepte

The MACHO Project HST Follow-Up: The Large Magellanic Cloud Microlensing Source Stars

We present Hubble Space Telescope (HST) WFPC2 photometry of 13 microlensed source stars from the 5.7 year Large Magellanic Cloud (LMC) survey conducted by the MACHO Project. The microlensing source stars are identified by deriving accurate centroids in the ground-based MACHO images using difference image analysis (DIA) and then transforming the DIA coordinates to the HST frame. None of these sources is coincident with a background galaxy, which rules out the possibility that the MACHO LMC microlensing sample is contaminated with misidentified supernovae or AGN in galaxies behind the LMC. This supports the conclusion that the MACHO LMC microlensing sample has only a small amount of contamination due to non-microlensing forms of variability. We compare the WFPC2 source star magnitudes with the lensed flux predictions derived from microlensing fits to the light curve data. In most cases the source star brightness is accurately predicted. Finally, we develop a statistic which constrains the location of the Large Magellanic Cloud (LMC) microlensing source stars with respect to the distributions of stars and dust in the LMC and compare this to the predictions of various models of LMC microlensing. This test excludes at > 90% confidence level models where more than 80% of the source stars lie behind the LMC. Exotic models that attempt to explain the excess LMC microlensing optical depth seen by MACHO with a population of background sources are disfavored or excluded by this test. Models in which most of the lenses reside in a halo or spheroid distribution associated with either the Milky Way or the LMC are consistent which these data, but LMC halo or spheroid models are favored by the combined MACHO and EROS microlensing results.Comment: 28 pages with 10 included PDF figures, submitted to Ap

Searching for periodicities in the MACHO light curve of LMC X-2

Using the exceptional long-term monitoring capabilities of the MACHO project, we present here the optical history of LMC X-2 for a continuous 6-yr period. These data were used to investigate the previously claimed periodicities for this source of 8.15 h and 12.54 d : we find upper amplitude limits of 0.10 mag and 0.09 mag, respectively.Comment: 4 pages, 4 figures. Minor changes, including title. MNRAS, in pres

The Optical Gravitational Lensing Experiment. Catalog of stellar proper motions in the OGLE-II Galactic bulge fields

We present a proper motion (\mu) catalogue of 5,080,236 stars in 49 Optical Gravitational Lensing Experiment II (OGLE-II) Galactic bulge (GB) fields, covering a range of -11 deg. <l< 11 deg. and -6 deg. <b<3 deg., the total area close to 11 square degrees. The proper motion measurements are based on 138 - 555 I-band images taken during four observing seasons: 1997-2000. The catalogue stars are in the magnitude range 11 < I < 18 mag. In particular, the catalogue includes Red Clump Giants (RCGs) and Red Giants in the GB, and main sequence stars in the Galactic disc. The proper motions up to \mu = 500 mas/yr were measured with the mean accuracy of 0.8-3.5 mas/yr, depending on the brightness of a star. This catalogue may be useful for studying the kinematic of stars in the GB and the Galactic disk.Comment: 13 pages, 16 figures, MNRAS in pres

The MACHO Project Hubble Space Telescope Follow-Up: Preliminary Results on the Location of the Large Magellanic Cloud Microlensing Source Stars

We attempt to determine whether the MACHO microlensing source stars are drawn from the average population of the LMC or from a population behind the LMC by examining the HST color-magnitude diagram (CMD) of microlensing source stars. We present WFPC2 HST photometry of eight MACHO microlensing source stars and the surrounding fields in the LMC. The microlensing source stars are identified by deriving accurate centroids in the ground-based MACHO images using difference image analysis (DIA) and then transforming the DIA coordinates to the HST frame. We consider in detail a model for the background population of source stars based on that presented by Zhao, Graff & Guhathakurta. In this model, the source stars have an additional reddening = 0.13 mag and a slightly larger distance modulus ~ 0.3 mag than the average LMC population. We also investigate a series of source star models, varying the relative fraction of source stars drawn from the average and background populations and the displacement of the background population from the LMC. Due to the small number of analyzed events the distribution of probabilities of different models is rather flat. A shallow maximum occurs at a fraction s_LMC ~ 0.8 of the source stars in the LMC. This is consistent with the interpretation that a significant fraction of observed microlensing events are due to lenses in the Milky Way halo, but does not definitively exclude other models.Comment: revised version, results slightly changed, accepted by Ap

MACHO 96-LMC-2: Lensing of a Binary Source in the LMC and Constraints on the Lensing Object

We present photometry and analysis of the microlensing alert MACHO 96-LMC-2. The ~3% photometry provided by the Global Microlensing Alert Network follow--up effort reveals a periodic modulation in the lightcurve. We attribute this to binarity of the lensed source. Microlensing fits to a rotating binary source magnified by a single lens converge on two minima, separated by delta chi^2 ~ 1. The most significant fit X1 predicts a primary which contributes ~100% of the light, a dark secondary, and an orbital period (T) of 9.2 days. The second fit X2 yields a binary source with two stars of roughly equal mass and luminosity, and T = 21.2 days. The lensed object appears to lie on the upper LMC main sequence. We estimate the mass of the primary component of the binary system, M ~2 M_sun. For the preferred model X1, we explore the range of dark companions by assuming 0.1 M_sun and 1.4 M_sun objects in models X1a and X1b, respectively. We find lens velocities projected to the LMC in these models of v^hat_X1a = 18.3 +/- 3.1 km/s and v^hat_X1b = 188 +/- 32 k/ms. In both these cases, a likelihood analysis suggests an LMC lens is preferred over a Galactic halo lens, although only marginally so in model X1b. We also find v^hat_X2 = 39.6 +/- 6.1 k/ms, where the likelihood for the lens location is strongly dominated by the LMC disk. In all cases, the lens mass is consistent with that of an M-dwarf. The LMC self-lensing rate contributed by 96-LMC-2 is consistent with model self-lensing rates. (Abridged)Comment: 23 pages, including 3 tables and 6 figures; Accepted for publication in The Astrophysical Journa