Browsing by Author "Minniti, D"

Now showing 1 - 20 of 26
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    A dwarf irregular galaxy at the edge of the local group: Stellar populations and distance of IC5152
    (1999) Zijlstra, AA; Minniti, D
    We have obtained V- and I-band photometry for about 700 stars in the field of the dwarf irregular galaxy IC 5152, previously considered a possible member of the Local Group. Deep VI color-magnitude diagrams are presented of the main body of this galaxy and of a nearby field. We infer a distance by comparison with the VI color-magnitude diagrams of similar galaxies and with theoretical isochrones. The distance modulus to IC 5152 is found to be m - M-o = 26.15 +/- 0.2, adopting E(V - I) = 0. This distance of D = 1.7 Mpc implies that IC 5152 is not a certain member of the Local Group. We also construct a deep optical luminosity function. By comparison with theoretical isochrones, we find a metallicity Z approximate to 0.002, which is lower than previous estimates. Using this metallicity and distance, IC 5152 now fits the metallicity-luminosity relation for dwarf galaxies. The youngest stars in the held studied have an age of similar to 10(7) Gyr, and there is a substantial population of stars with log t > 7.8-8 yr. The central region of IC 5152 is an active site of star formation, as found by comparing our optical photometry with Hubble Space Telescope ultraviolet images. Several candidate globular clusters are found, as well as a candidate for a potential nucleus.
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    Absolute parameters for eight eclipsing binaries in the Large Magellanic Cloud
    (2005) González, JF; Ostrov, P; Morrell, N; Minniti, D
    A simultaneous light and radial velocity analysis of eight Large Magellanic Cloud (LMC) eclipsing binaries is presented. Combining spectroscopic observations obtained with UVES at the ESO Very Large Telescope and light curves available from the MACHO and OGLE databases, accurate masses and radii for the binary components, along with their spectral types and luminosities, are derived. These determinations allow us to construct the first mass-luminosity relation for late O and early B type stars in the LMC. This mass-luminosity relation looks very similar to that of the Milky Way, in spite of the different metallicities. The good agreement achieved in the comparison with recent theoretical isochrones is encouraging regarding the reliability of star models up to 20 M-..
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    Binary microlensing events from the MACHO Project
    (2000) Alcock, C; Allsman, RA; Alves, D; Axelrod, TS; Baines, D; Becker, AC; Bennett, DP; Bourke, A; Brakel, A; Cook, KH; Crook, B; Crouch, A; Dan, J; Drake, AJ; Fragile, PC; Freeman, KC; Gal-Yam, A; Geha, M; Gray, J; Griest, K; Gurtierrez, A; Heller, A; Howard, J; Johnson, BR; Kaspi, S; Keane, M; Kovo, O; Leach, C; Leach, T; Leibowitz, EM; Lehner, MJ; Lipkin, Y; Maoz, D; Marshall, L; McDowell, D; McKeown, S; Mendelson, H; Messenger, B; Minniti, D; Nelson, C; Peterson, BA; Popowski, P; Pozza, E; Purcell, P; Pratt, MR; Quinn, J; Quinn, PJ; Rhie, SH; Rodgers, AW; Salmon, A; Shemmer, O; Stetson, P; Stubbs, CW; Sutherland, W; Thomson, S; Tomaney, A; Vandehei, T; Walker, A; Ward, K; Wyper, G
    We present the light curves of 21 gravitational microlensing events from the first six years of the MACHO Project gravitational microlensing survey that are likely examples of lensing by binary systems. These events were manually selected from a total sample of similar to 350 candidate microlensing events that were either detected by the MACHO Alert System or discovered through retrospective analyses of the MACHO database. At least 14 of these 21 events exhibit strong (caustic) features, and four of the events are well fit with lensing by large mass ratio (brown dwarf or planetary) systems, although these fits are not necessarily unique. The total binary event rate is roughly consistent with predictions based upon our knowledge of the properties of binary stars, but a precise comparison cannot be made without a determination of our binary lens event detection efficiency. Toward the Galactic bulge, we find a ratio of caustic crossing to noncaustic crossing binary lensing events of 12:4, excluding one event for which we present two fits. This suggests significant incompleteness in our ability to detect and characterize noncaustic crossing binary lensing. The distribution of mass ratios, N(q), for these binary lenses appears relatively flat. We are also able to reliably measure source-face crossing times in four of the bulge caustic crossing events, and recover from them a distribution of lens proper motions, masses, and distances consistent with a population of Galactic bulge lenses at a distance of 7 +/- 1 kpc. This analysis yields two systems with companions of similar to 0.05 M..
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    Deep Hubble Space Telescope STIS color-magnitude diagrams of the dwarf irregular galaxy WLM
    (2000) Rejkuba, M; Minniti, D; Gregg, MD; Zijlstra, AA; Alonso, MV; Goudfrooij, P
    We have obtained V- and I-band photometry for 1886 stars down to I = 27 and y = 28 in the held of the dwarf irregular galaxy WLM, using deep Space Telescope Imaging Spectrograph (STIS) CL- and LP-band images taken in the parallel mode with the Hubble Space Telescope. The photometry is used to study the horizontal branch identified in WLM. The horizontal branch, extending blueward from the red giant clump, is an unambiguous signature of an old population. We demonstrate that it is possible to reach the horizontal branch of an old population at a distance of similar or equal to 1 Mpc using STIS, with relatively short exposure times. From the VI color-magnitude diagram, we obtain an accurate distance modulus (m - M)(0) = 24.95 +/- 0.13 for WLM by using the Y magnitude of the horizontal branch, and by adopting E(V-I)= 0.03. The implications are that (1) WLM formed stars at high redshift and (2) the old population of WLM can be representative of a protogalactic fragment, related to those accreted to form the Milky Way halo.
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    Difference image analysis of galactic microlensing. II. Microlensing events
    (1999) Alcock, C; Allsman, RA; Alves, D; Axelrod, TS; Becker, AC; Bennett, DP; Cook, KH; Drake, AJ; Freeman, KC; Griest, K; Lehner, MJ; Marshall, SL; Minniti, D; Peterson, BA; Pratt, MR; Quinn, PJ; Stubbs, CW; Sutherland, W; Tomaney, A; Vandehei, T; Welch, DL
    The MACHO collaboration has been carrying out difference image analysis (DIA) since 1996 with the aim of increasing the sensitivity to the detection of gravitational microlensing. This is a preliminary report on the application of DIA to galactic bulge images in one field. We show how the DIA technique significantly increases the number of detected lensing events, by removing the positional dependence of traditional photometry schemes and lowering the microlensing event detection threshold. This technique, unlike PSF photometry, gives the unblended colors and positions of the microlensing source stars. We present a set of criteria for selecting microlensing events from objects discovered with this technique. The 16 pixel and classical microlensing events discovered with the DIA technique are presented.
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    Extragalactic globular clusters in the near infrared - I. A comparison between M87 and NGC 4478
    (2002) Kissler-Patig, M; Brodie, JP; Minniti, D
    We compare optical and near infrared colours of globular clusters in M87, the central giant elliptical in Virgo, and NGC 4478, an intermediate luminosity galaxy in Virgo, close in projection to M87. Combining V and I photometry obtained with the WFPC2 on HST and K-s photometry obtained with the NIRC on Keck 1, we find the broad range in colour and previously detected bi-modality in M87. We confirm that NGC 4478 only hosts a blue sub-population of globular clusters and now show that these clusters' V-I and V-K colours are very similar to those of the halo globular clusters in Milky Way and M31. Most likely, a metal-rich sub-population never formed around this galaxy (rather than having formed and been destroyed later), probably because its metal-rich gas was stripped during its passage through the centre of the Virgo cluster.
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    Extragalactic globular clusters in the near-infrared - II. The globular clusters systems of NGC 3115 and NGC 4365
    (2002) Puzia, TH; Zepf, SE; Kissler-Patig, M; Hilker, M; Minniti, D; Goudfrooij, P
    We combine near-infrared photometry obtained with the VLT/ISAAC instrument and archival HST/WFPC2 optical images to determine VIK magnitudes and colours of globular clusters in two early-type galaxies, NGC 3115 and NGC 4365. The combination of near-IR and optical photometry provides a way to lift the age-metallicity degeneracy. For NGC 3115, the globular cluster colours reveal two major sub-populations, consistent with previous studies. By comparing the V-I, V-K colours of the NGC 3115 globular clusters with stellar populations models, we find that the colour difference between the two greater than or similar to 10 Gyr old major sub-populations is primarily due to a difference in metallicity. We find Delta[Fe/H] = 1.0 +/- 0.3 dex and the blue (metal-poor) and red (metal-rich) globular cluster sub-populations being coeval within 3 Gyr. In contrast to the NGC 3115 globular clusters, the globular cluster system in NGC 4365 exhibits a more complex age and metallicity structure. We find a significant population of intermediate-age very metal-rich globular clusters along with an old population of both metal-rich and metal-poor clusters. Specifically, we observe a large population of globular clusters with red V K colours but intermediate V I colours, for which all current stellar population models give ages and metallicities in the range similar to2-8 Gyr and similar to0.5 Z(.) -3 Z(.), respectively. After 10 Gyr of passive evolution, the intermediate-age globular clusters in NGC 4365 will have colours which are consistent with the very metal-rich population of globular clusters in giant elliptical galaxies, such as M 87. Our results for both globular cluster systems are consistent with previous age and metallicity studies of the diffuse galactic light. In addition to the major globular cluster populations in NGC 3115 and NGC 4365 we report on the detection of objects with extremely red colours (V-K greater than or similar to3.8 mag), whose nature could not ultimately be revealed with the present data.
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    Hubble Space Telescope observations of globular cluster systems along the Hubble sequence of spiral galaxies
    (2003) Goudfrooij, P; Strader, J; Brenneman, L; Kissler-Patig, M; Minniti, D; Huizinga, JE
    We have studied the globular cluster systems of seven giant, edge-on spiral galaxies using Hubble Space Telescope imaging in V and I . The galaxy sample covers the Hubble types Sa to Sc, allowing us to study the variation of the properties of globular cluster systems along the Hubble sequence. The photometry reaches similar to1.5 mag beyond the turn-over magnitude of the globular cluster luminosity function for each galaxy. Specific frequencies of globular clusters (S-N values) were evaluated by comparing the numbers of globular clusters found in our WFPC2 pointings with those in our Milky Way that would be detected in the same spatial region if placed at the distance of the target galaxies. Results from this method were found to be consistent with the more commonly used method of constructing radial distribution functions of globular clusters. The S-N values of spirals with B /T less than or similar to 0.3 (i.e. spirals with a Hubble type later than about Sb) are consistent with a value of S-N = 0.55 +/- 0.25. We suggest that this population of globular clusters represents a 'universal', old halo population that is present around each galaxy. Most galaxies in our sample have S-N values that are consistent with a scenario in which globular cluster systems are made up of (i) the aforementioned halo population plus (ii) a population that is associated with bulges, which grows approximately linearly with the mass of the bulge. Such scenarios include the 'merger scenario' for the formation of elliptical galaxies as well as the 'multi-phase collapse' scenario, but it seems inconsistent with the 'secular evolution' scenario of Pfenniger & Norman, in which bulges are formed from disc stars by means of the redistribution of angular momentum through bar instabilities and/or minor perturbations. However, there is one bulge-dominated spiral galaxy in our sample (NGC 7814) with a low S-N value that is consistent with those of the latest-type spirals. This means that the 'secular evolution' scenario can still be viable for some bulge-dominated spirals. Thus, our results suggest that the formation histories of galaxy bulges of early-type spirals can be significantly different from one galaxy to another.
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    Macho project limits on black hole dark matter in the 1-30 M⊙ range
    (2001) Alcock, C; Allsman, RA; Alves, DR; Axelrod, TS; Becker, AC; Bennett, DP; Cook, KH; Dalal, N; Drake, AJ; Freeman, KC; Geha, M; Griest, K; Lehner, MJ; Marshall, SL; Minniti, D; Nelson, CA; Peterson, BA; Popowski, P; Pratt, MR; Quinn, PJ; Stubbs, CW; Sutherland, W; Tomaney, AB; Vandehei, T; Welch, DL
    We report on a search for long-duration microlensing events toward the Large Magellanic Cloud. We find none and therefore put limits on the contribution of high-mass objects to the Galactic dark matter. At a 95% confidence level, we exclude objects in the mass range of 0.3-30.0 M-circle dot from contributing more than 4 x 10(11) M-circle dot to the Galactic halo. Combined with earlier results, this means that objects with masses under 30 M-circle dot cannot make up the entire dark matter halo if the halo is of typical size. For a typical dark halo, objects with masses under 10 M-circle dot contribute less than 40% of the dark matter.
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    New metallicities of RR Lyrae stars in ω Centauri
    (2006) Sollima, A; Borissova, J; Catelan, M; Smith, HA; Minniti, D; Cacciari, C; Ferraro, FR
    We present new spectroscopic metal abundances for 74 RR Lyrae (RRL) stars in omega Cen obtained with FLAMES. The well-known metallicity spread is visible among the RRL variables. The metal-intermediate (MI) RRL stars ([Fe/H] similar to -1.7) are fainter than the bulk of the dominant metal-poor population ([Fe/H] similar to -1.7), in good agreement with the corresponding zero-age horizontal-branch models with cosmological helium abundance. This result Y = 0.246 conflicts with the hypothesis that the progenitors of the MI RRL stars correspond to the anomalous blue main-sequence stars, which share a similar metallicity but whose properties are currently explained by assuming for them a large helium enhancement. Therefore, in this scenario, the coexistence within the cluster of two different populations with similar metallicities ([Fe/H] similar to -1.7) and different helium abundances has to be considered.
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    On-axis spatially resolved spectroscopy of low redshift quasar host galaxies
    (2002) Courbin, F; Letawe, G; Magain, P; Wisotzki, L; Jablonka, P; Jahnke, DK; Kuhlbrodt, B; Alloin, D; Meylan, G; Minniti, D; Burud, I
    We present the first result of a comprehensive spectroscopic study of quasar host galaxies. On-axis, spatially resolved spectra of low redshift quasars have been obtained with FORS1, mounted on the 8.2 m ESO Very Large Telescope, Antu. The spectra are spatially deconvolved using a spectroscopic version of the "MCS deconvolution algorithm". The algorithm decomposes two dimensional spectra into the individual spectra of the central point-like nucleus and of its host galaxy. Applied to HE 1503+0228 at z = 0.135 (M-B = -23.0), it provides us with the spectrum of the host galaxy between 3600 Angstrom and 8500 Angstrom (rest-frame), at a mean resolving power of 700. The data allow us to measure several of the important Lick indices. The stellar populations and gas ionization state of the host galaxy of HE 1503+0228 are very similar to the ones measured for normal non-AGN galaxies. Dynamical information is also available for the gas and stellar components of the galaxy. Using deconvolution and a deprojection algorithm, velocity curves are derived for emission lines, from the center up to 400 away from the nucleus of the galaxy. Fitting a simple three-components mass model (point mass, spherical halo of dark matter, disk) to the position-velocity diagram, we infer a mass of M(r < 1 kpc) = (2.0 +/- 0.3) x 10(10) M-. within the central kiloparsec of the galaxy, and a mass integrated over 10 kpc of M(r < 10 kpc) = (1.9 +/- 0.3) x 10(11) M-., with an additional 10% error due to the uncertainty on the inclination of the galaxy. This, in combination with the analysis of the stellar populations indicates that the host galaxy of HE 1503+0228 is a normal spiral galaxy.
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    Optical counterparts of X-ray point sources observed by Chandra in NGC 5128
    (2004) Minniti, D; Rejkuba, M; Funes, JG; Akiyama, S
    Very Large Telescope images in BVI are used to identify the optical counterparts to bright Chandra X-ray points sources discovered by Kraft and coworkers. Of a total of 111 X-ray point sources with L-X > 2 x 10(36) ergs s(-1) present in a 56 arcmin(2) field centered on this galaxy, 58 have optical counterparts. On the basis of the sizes, optical magnitudes, and colors, 20 new globular cluster counterparts of X-ray sources are identified, and three are identified on the basis of their sizes. This brings the total number of globular cluster X-ray sources in this galaxy to 33 and establishes that 30% of the X-ray point sources in NGC 5128 are associated with globular clusters. These X-ray globular clusters occupy the brightest end of the globular cluster luminosity function, indicating that bright low-mass X-ray binaries are preferentially found in massive clusters. Most of the globular clusters with X-ray sources have red colors, with 1.0 < V - I < 1.5, indicating that low-mass X-ray binaries are preferentially formed in metal-rich clusters. The NGC 5128 X-ray globular cluster sources are brighter compared with the Milky Way sources: there are 24 globular clusters with X-ray sources of L-X > 10(37) ergs s(-1). There is, however, no globular cluster X-ray source in NGC 5128 as bright as expected for an accreting black hole. In addition, 31 optical counterparts of X-ray point sources that are not associated with globular clusters are identified. Finally, 53 X-ray point sources (48% of the population) do not have any optical counterparts down to the faintest magnitude limits (B = 25).
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    Proper motions of dwarf spheroidal galaxies from Hubble Space Telescope imaging.: I.: Method and a preliminary measurement for Fornax
    (2002) Piatek, S; Pryor, C; Olszewski, EW; Harris, HC; Mateo, M; Minniti, D; Monet, DG; Morrison, H; Tinney, CG
    This article presents and discusses a method for measuring the proper motions of the Galactic dwarf spheroidal galaxies using images taken with the Hubble Space Telescope. The method involves fitting an effective point-spread function to the image of a star or quasi-stellar object to determine its centroid with an accuracy of about 0.005 pixels (0.25 mas)-an accuracy sufficient to measure the proper motion of a dwarf spheroidal galaxy, using images separated by just a few years. The data consist of images, dithered to reduce the effects of undersampling, taken at multiple epochs with the Space Telescope Imaging Spectrograph or the Wide Field Planetary Camera. The science fields are in the directions of the Carina, Fornax, Sculptor, and Ursa Minor dwarf spheroidal galaxies, and each has at least one quasi-stellar object whose identity has been established by other studies. The rate of change with time of the centroids of the stars of the dwarf spheroidal with respect to the centroid of the quasi-stellar object is the proper motion. Four independent preliminary measurements of the proper motion of Fornax for three fields agree within their uncertainties. The weighted average of these measurements is mu(alpha) = 49 +/- 13 mas century(-1) and mu(delta) = -59 +/- 13 mas century(-1). The Galactocentric velocity derived from the proper motion implies that Fornax is near perigalacticon, may not be bound to the Milky Way, and is not a member of any of the proposed streams of galaxies and globular clusters in the Galactic halo. If Fornax is bound, the Milky Way must have a mass of at least (1.6 +/- 0.8) x 10(12) M.
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    Proper motions of dwarf spheroidal galaxies from Hubble Space Telescope imaging.: II.: Measurement for Carina
    (2003) Piatek, S; Pryor, C; Olszewski, EW; Harris, HC; Mateo, M; Minniti, D; Tinney, CG
    This article presents and discusses a measurement of the proper motion for the Carina dwarf spheroidal galaxy (dSph) from images in two distinct fields in the direction of Carina taken with the Hubble Space Telescope, at three epochs. Each field contains a confirmed quasi-stellar object that is the reference point for measuring the proper motion of the dSph. The consecutive epochs are 1-2 yr apart. The components of the measured proper motion for Carina, expressed in the equatorial coordinate system, are mu(alpha)=22+/-9 mas century(-1) and mu(delta)=15+/-9 mas century(-1). The quoted proper motion is a weighted mean of two independent measurements and has not been corrected for the motions of the Sun and of the local standard of rest. Given the proper motion and its uncertainty, integrating the family of possible orbits of Carina in a realistic gravitational potential for the Milky Way indicates that Carina is bound gravitationally to the Milky Way and is close to apogalacticon. The best estimate of, and the 95% confidence interval for, the apogalacticon of the orbit is 102 kpc and (102, 113) kpc, for the perigalacticon is 20 kpc and (3.0, 63) kpc, and for the orbital period is 1.4 Gyr and (1.3, 2.0) Gyr. Carina does not seem to be on a polar orbit. The best estimate of the inclination of the orbit with respect to the Galactic plane is 39degrees, but the 95% confidence interval is so wide, (23degrees, 102degrees), that it includes a polar orbit. We are unable to confirm or to rule out the membership of Carina in a "stream'' of galaxies in the Galactic halo because the difference between the observed and predicted directions of the proper motion is 1.6 times the uncertainty of the difference. Carina must contain dark matter to have survived the tidal interaction with the Milky Way until the present. The triggering of star formation by perigalacticon passages and crossings of the Galactic disk do not explain the history of star formation in Carina.
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    Proper motions of dwarf spheroidal galaxies from Hubble Space Telescope imaging.: III.: Measurement for Ursa Mminor
    (2005) Piatek, S; Pryor, C; Bristow, P; Olszewski, EW; Harris, HC; Mateo, M; Minniti, D; Tinney, CG
    This article presents a measurement of the proper motion of the Ursa Minor dwarf spheroidal galaxy determined from images taken with the Hubble Space Telescope in two distinct fields. Each field contains a quasi-stellar object that serves as the "reference point.'' The measured proper motion for Ursa Minor, expressed in the equatorial coordinate system, is (mu(alpha); mu(delta)) = ( - 50 +/- 17; 22 +/- 16) mas century(-1). Removing the contributions of the solar motion and the motion of the local standard of rest yields the proper motion in the Galactic rest frame: (mu(Grf)(alpha), mu(Grf)(delta)) = (- 8 +/- 17, 38 +/- 16) mas century(-1). The implied space velocity with respect to the Galactic center has a radial component of V-r = - 75 +/- 44 km s(-1) and a tangential component of V-t = 144 +/- 50 km s(-1). Integrating the motion of Ursa Minor in a realistic potential for the Milky Way produces orbital elements. The perigalacticon and apogalacticon are 40 ( 10, 76) and 89 ( 78, 160) kpc, respectively, where the values in the parentheses represent the 95% confidence intervals derived from Monte Carlo experiments. The eccentricity of the orbit is 0.39 (0.09, 0.79), and the orbital period is 1.5 (1.1, 2.7) Gyr. The orbit is retrograde and inclined by 124 degrees (94 degrees, 136 degrees) to the Galactic plane. Ursa Minor is not a likely member of a proposed stream of galaxies on similar orbits around the Milky Way, nor is the plane of its orbit coincident with a recently proposed planar alignment of galaxies around the Milky Way. Comparing the orbits of Ursa Minor and Carina shows no reason for the different star formation histories of these two galaxies. Ursa Minor must contain dark matter to have a high probability of having survived disruption by the Galactic tidal force until the present.
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    Proper motions of dwarf spheroidal galaxies from Hubble Space Telescope imaging.: IV.: Measurement for sculptor
    (2006) Piatek, S; Pryor, C; Bristow, P; Olszewski, EW; Harris, HC; Mateo, M; Minniti, D; Tinney, CG
    This article presents a measurement of the proper motion of the Sculptor dwarf spheroidal galaxy determined from images taken with the Hubble Space Telescope using the Space Telescope Imaging Spectrograph in the imaging mode. Each of two distinct fields contains a quasi-stellar object that serves as the "reference point.'' The measured proper motion of Sculptor, expressed in the equatorial coordinate system, is (mu(alpha), mu(delta)) (9 +/- 13; 2 +/- 13) mas century(-1). Removing the contributions from the motion of the Sun and the motion of the local standard of rest produces the proper motion in the Galactic rest frame: (mu(Grf)(alpha), mu(Grf)(delta)) = (-23 +/- 13; 45 +/- 13) mas century(-1). The implied space velocity with respect to the Galactic center has a radial component of V-r = 79 +/- 6 km s(-1) and a tangential component of V-t = 198 +/- 50 km s(-1). Integrating the motion of Sculptor in a realistic potential for the Milky Way produces orbital elements. The perigalacticon and apogalacticon are 68 (31, 83) and 122 (97, 313) kpc, respectively, where the values in the parentheses represent the 95% confidence interval derived from Monte Carlo experiments. The eccentricity of the orbit is 0.29 (0.26, 0.60), and the orbital period is 2.2 (1.5, 4.9) Gyr. Sculptor is on a polar orbit around the Milky Way: the angle of inclination is 86 degrees (83 degrees, 90 degrees).
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    Searching for periodicities in the MACHO light curve of LMC X-2
    (2000) Alcock, C; Allsman, RA; Alves, DR; Axelrod, TS; Becker, AC; Bennett, DP; Charles, PA; Cook, KH; Drake, AJ; Freeman, KC; Geha, M; Griest, K; Groot, P; Lehner, MJ; Marshall, SL; McGowan, KE; Minniti, D; Nelson, CA; Peterson, BA; Popowski, P; Pratt, MR; Quinn, PJ; Sutherland, W; Tomaney, AB; Vandehei, T; van Paradijs, J
    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 limits of 0.10 mag and 0.09 mag, respectively.
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    The Araucaria Project
    (2004) Gieren, W; Pietrzynski, G; Walker, A; Bresolin, F; Minniti, D; Kudritzki, RP; Udalski, A; Soszynski, I; Fouqué, P; Storm, J; Bono, G
    In a previous paper, we reported on the discovery of more than a hundred new Cepheid variables in the Sculptor Group spiral galaxy NGC 300 from wide-field images taken in the B and V photometric bands at ESO/La Silla. In this paper, we present additional VI data, derive improved periods and mean magnitudes for the variables, and construct period-luminosity relations in the V, I, and the reddening-independent V - I Wesenheit bands using 58 Cepheid variables with periods between 11 and 90 days. We obtain tightly defined relations, and by fitting the slopes determined for the LMC Cepheids by the OGLE II Project we obtain reddening-corrected distances to the galaxy in all bands, which show a slight offset to each other in the sense that the Wesenheit relation yields the smallest distance, whereas the I- and V-band distances are larger by 0.094 and 0.155 mag, respectively. We adopt as our best value the distance derived from the reddening-free Wesenheit magnitudes, which is 26.43 +/- 0.04( random) +/- 0.05( systematic) mag. The distance moduli from both the V and I bands agree perfectly with the Wesenheit value if one assumes an additional reddening of E( B - V) = 0.05 mag intrinsic to NGC 300, in addition to the Galactic foreground reddening toward NGC 300 of 0.025 mag. Such a modest intrinsic reddening is supported by recent Hubble Space Telescope (HST) images of NGC 300, which show that this galaxy is relatively dustfree but also reveal that there must be some dust absorption in NGC 300. We argue that our current distance result for NGC 300 is the most accurate that has so far been obtained using Cepheid variables and that it is largely free from systematic effects due to metallicity, blending, and sample selection. It agrees very well with the recent distance determination from the tip of the red giant branch method obtained from HST data by Butler and coworkers, and it is consistent with the Cepheid distance to NGC 300 that was derived by Freedman and coworkers from CCD photometry of a smaller sample of stars.
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    The macho project Hubble Space Telescope follow-up
    (2001) Alcock, C; Allsman, RA; Alves, DR; Axelrod, TS; Becker, AC; Bennet, DP; Cook, KH; Dalal, N; Drake, AJ; Freeman, KC; Geha, M; Griest, K; Lehner, MJ; Marshall, SL; Minniti, D; Nelson, CA; Peterson, BA; Popowski, P; Pratt, MR; Quinn, PJ; Stubbs, CW; Sutherland, W; Tomaney, AB; Vandehei, T
    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 Hubble Space Telescope (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 of [E(B - V)] = 0.13 mag and a slightly larger distance modulus, [Delta mu] similar to 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. Because of 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) similar to 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.
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    The Macho project Large Magellanic Cloud variable star inventory.: XI.: Frequency analysis of the fundamental-mode RR Lyrae stars
    (2003) Alcock, C; Alves, DR; Becker, A; Bennett, D; Cook, KH; Drake, A; Freeman, K; Geha, M; Griest, K; Kovács, G; Lehner, M; Marshall, S; Minniti, D; Nelson, C; Peterson, B; Popowski, P; Pratt, M; Quinn, P; Rodgers, A; Stubbs, C; Sutherland, W; Vandehei, T; Welch, DL
    We have frequency-analyzed 6391 variables classified earlier as fundamental-mode RR Lyrae (RR0) stars in the MACHO database on the Large Magellanic Cloud (LMC). The overwhelming majority (i.e., 96%) of these variables have been proved to be indeed RR0 stars, whereas the remaining ones have fallen into one of the following categories: single- and double-mode Cepheids, binaries, first-overtone and double-mode RR Lyrae stars, and nonclassified variables. Special attention has been paid to the properties of the amplitude- and phase-modulated RR0 stars ( the Blazhko stars). We found altogether 731 Blazhko variables showing either a doublet or an equidistant triplet pattern at the main pulsation component in their frequency spectra. This sample overwhelmingly exceeds the number of Blazhko stars known in all other systems combined. The incidence rate of the Blazhko variables among the RR0 stars in the LMC is 11.9%, which is 3 times higher than their rate among the first-overtone RR Lyrae stars. No difference is found in the average brightness between the single- mode and Blazhko variables. However, the latter ones show a somewhat lower degree of skewness in their average light curves and a concomitant lower total amplitude in their modulation-free light curves. From the frequency spectra we found that variables with larger modulation amplitudes at the higher frequency side of the main pulsation component are 3 times more common than the ones showing the opposite amplitude pattern. A search for a modulation component with the Blazhko period in the average brightness of the individual variables showed the existence of such a modulation with an overall amplitude of approximate to 0.006 mag. On the other hand, a similar search for quadruple modulation patterns around the main pulsation component has failed to clearly detect such components at the approximate to 0.004 mag level. This means that the amplitudes of the quadruple components ( if they exist) should be, on average, at least 10 times smaller than those of the triplet components. This finding and the existence of Blazhko variables with highly asymmetric modulation amplitudes not only question the validity of the magnetic oblique rotator model but also put stringent constraints on models based on mode-coupling theories.