D. L. Welch

Leibniz Institute for Astrophysics Potsdam, Potsdam, Brandenburg, Germany

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Publications (125)424.58 Total impact

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    ABSTRACT: We have identified 5 new R Coronae Borealis (RCB) stars in the Galactic bulge, using the MACHO photometry database, raising the total number of known Galactic RCB stars to 40. We have obtained spectra to confirm the identifications. The fact that four out of the 5 newly identified RCB stars are ``cool'' rather than ``warm'' suggests the preponderance of warm RCB stars among the existing sample may be a selection bias. Using the volume of the surveyed field, we estimate the total number of Galactic RCB stars to be about 700. This project was supported by the NSF/REU grant AS T-0097694 and the Nantucket Maria Mitchell Association.
    12/2004;
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    ABSTRACT: We present a catalog of 450 high signal-to-noise microlensing events observed by the MACHO collaboration between 1993 and 1999. The events are distributed throughout our fields and, as expected, they show clear concentration toward the Galactic center. No optical depth is given for this sample since no blending efficiency calculation has been performed, and we find evidence for substantial blending. In a companion paper we give optical depths for the sub-sample of events on clump giant source stars, where blending is not a significant effect. Several events with sources that may belong to the Sagittarius dwarf galaxy are identified. For these events even relatively low dispersion spectra could suffice to classify these events as either consistent with Sagittarius membership or as non-Sagittarius sources. Several unusual events, such as microlensing of periodic variable source stars, binary lens events, and an event showing extended source effects are identified. We also identify a number of contaminating background events as cataclysmic variable stars. Comment: 34 pages, 9 figures + 3 example lightcurves, all 564 lightcurves will be available at http://wwwmacho.mcmaster.ca, submitted to ApJ, see companion paper by Popowski et al
    The Astrophysical Journal 10/2004; 631(2):906. · 6.73 Impact Factor
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    ABSTRACT: We present the first results from our next-generation microlensing survey, the SuperMACHO project. We are using the CTIO 4m Blanco telescope and the MOSAIC imager to carry out a search for microlensing toward the Large Magellanic Cloud (LMC). We plan to ascertain the nature of the population responsible for the excess microlensing rate seen by the MACHO project. Our observing strategy is optimized to measure the differential microlensing rate across the face of the LMC. We find this derivative to be relatively insensitive to the details of the LMC's internal structure but a strong discriminant between Galactic halo and LMC self lensing. In December 2003 we completed our third year of survey operations. 2003 also marked the first year of real-time microlensing alerts and photometric and spectroscopic followup. We have extracted several dozen microlensing candidates, and we present some preliminary light curves and related information. Similar to the MACHO project, we find SNe behind the LMC to be a significant contaminant - this background has not been completely removed from our current single-color candidate sample. Our follow-up strategy is optimized to discriminate between SNe and true microlensing. Comment: To appear in Proceedings of IAU Symposium 225: Impact of Gravitational Lensing on Cosmology, 6 pages
    Proceedings of the International Astronomical Union 09/2004;
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    D. Lepischak, D. L. Welch
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    ABSTRACT: Eclipsing binary systems potentially allow the direct and precise determination of the important properties of their component stars. An eclipsing binary containing a Cepheid variable which is also a double-lined spectroscopic binary would allow, for the first time, the direct measurement of the absolute luminosity and mass of the Cepheid. The MACHO Project LMC database contains five systems whose light curves show variations due to both eclipses and pulsation but only one has been clearly identified as an intermediate-mass, Population I object. This object, MACHO 81.8997.87 (=OGLELMC_SC16 119952) is a 2.035-d overtone Cepheid in an 800.4-d binary system with an M-type companion. Here we present the results of the analysis of the light curve of this system, the implications for its evolutionary history and discuss the prospects for future observations.
    04/2004; 310:372.
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    ABSTRACT: We present a detailed study of the viewing angles and geometry of the inner LMC (P 4) based on a sample of more than 2000 MACHO Cepheids with complete {VR} KC light curves and single-phase Two Micron All Sky Survey (2MASS) JHK s observations. The sample is considerably larger than any previously studied subset of LMC Cepheids and has an improved areal coverage. Single-epoch random-phase 2MASS photometry is cor-rected using MACHO V light curves to derive mean JHK s magnitudes. We analyze the resulting period-lumi-nosity relations in VRJHK s to recover statistical reddening and distance to each individual Cepheid, with respect to the mean distance modulus and reddening of the LMC. By fitting a plane solution to the derived individual distance moduli, the values of LMC viewing angles are obtained: position angle ¼ 151 :0 AE 2 :4, inclination i ¼ 30 :7 AE 1 :1. In the so-called ring analysis, we find a strong dependence of the derived viewing angles on the adopted center of the LMC, which we interpret as being due to deviations from planar geometry. Analysis of residuals from the plane fit indicates the presence of a symmetric warp in the LMC disk and the bar elevated above the disk plane. Nonplanar geometry of the inner LMC explains a broad range for values of i and in the literature and suggests caution when deriving viewing angles from inner LMC data.
    The Astrophysical Journal 03/2004; 601(1):260. · 6.73 Impact Factor
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    ABSTRACT: The lightcurve of the Large Magellanic Cloud (LMC) variable star MACHO 81.8997.87 shows evidence for photometric variations due to both stellar pulsation, with a 2.035 day period, and eclipsing behavior, with an 800.4 day period. The primary star of the system has been identified as a first-overtone Cepheid but the nature of the secondary star has not been determined. Here we present multicolor BVI photometry of a primary eclipse of the system and fit a model to the complete lightcurve to produce an updated set of elements. These results are combined with 2MASS JHK photometry to give further insight into the identity of the companion star. We find that the companion is most consistent with a late-K or an early-M giant but also that there are a number of problems with this interpretation. The prospects for future observations of this system are also discussed. Comment: 7 pages, 5 figures, 6 tables, submitted to ApJ
    The Astrophysical Journal 02/2004; · 6.73 Impact Factor
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    ABSTRACT: We present time-series VI photometry of the metal-rich globular cluster NGC 6496. Our color-magnitude diagrams show a red-clump horizontal branch morphology consistent with typical metal-rich globular clusters like 47 Tuc. We detected at least six variable stars in the viscinity of NGC 6496. All represent new discoveries, since the cluster had not been properly searched before for stellar variability. We present light curves and cluster membership analysis for these stars. This work was funded by a grant from the NSF.
    12/2003;
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    ABSTRACT: We present time-series VI photometry of the metal-rich globular cluster NGC 6637. Our color-magnitude diagrams show a predominantly red-clump horizontal branch morphology with hints of a blue horizontal branch extension as seen in NGC 6388 and NGC 6441. We discovered at least four new long-period variable stars in addition to recovering the nine variable stars already discovered. We discuss the cluster membership probabilities of the variables, and present their light curves. This work was funded by a grant from the NSF.
    12/2003;
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    ABSTRACT: We present time-series VI photometry of the metal-rich globular cluster NGC 6316. Our color-magnitude diagrams show a predominantly red-clump horizontal branch morphology with no evidence of the blue extensions seen in NGC 6388 and NGC 6441. Our data are in good agreement with published estimates of the reddening and metallicity provided the cluster has an absolute distance modulus of 15.50 mag. We have discovered more than one dozen long-period variable stars in our field of view, and argue that at least seven of them are members of NGC 6316. We have also discovered four RR Lyrae variables, although only one of them probably belongs to the cluster. The specific frequency of RR Lyrae stars in NGC 6316 is found to be less than 0.4, making it clear that this cluster is a normal, metal-rich globular and is not an analog of NGC 6388 and NGC 6441. We recommend long-term photometric monitoring of NGC 6316 to clarify the nature of the RR Lyrae member candidate as well as the pulsation classes of the long-period variables. Comment: 33 pages, 5 figures, 6 tables. Accepted for publication in the Astronomical Journal
    The Astronomical Journal 04/2003; · 4.97 Impact Factor
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    ABSTRACT: We present a method for solving the light curve of an eclipsing binary system that contains a Cepheid variable as one of its components as well as the solutions for three eclipsing Cepheids in the Large Magellanic Cloud (LMC). A geometric model is constructed in which the component stars are assumed to be spherical and on circular orbits. The emergent system flux is computed as a function of time, with the intrinsic variations in temperature and radius of the Cepheid treated self- consistently. Fitting the adopted model to photometric observations, incorporating data from multiple bandpasses, yields a single parameter set best describing the system. This method is applied to three eclipsing Cepheid systems from the MACHO project LMC database: MACHO 6.6454.5, 78.6338.24, and 81.8997.87. A best-fit value is obtained for each systems orbital period and inclination and for the relative radius, color, and limb-darkening coefficients of each star. Pulsation periods and parameterizations of the intrinsic color variations of the Cepheids are also obtained, and the amplitude of the radial pulsation of each Cepheid is measured directly. The system 6.6454.5 is found to contain a 4.97 day Cepheid, which cannot be definitely classified as type I or type II, with an unexpectedly brighter companion. The system 78.6338.24 consists of a 17.7 day, W Virginis class type II Cepheid with a smaller, dimmer companion. The system 81.8997.87 contains an intermediate-mass, 2.03 day overtone Cepheid with a dimmer, red giant secondary.
    The Astrophysical Journal 07/2002; 573:338. · 6.73 Impact Factor
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    ABSTRACT: 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 Msolar from contributing more than 4×1011 Msolar to the Galactic halo. Combined with earlier results, this means that objects with masses under 30 Msolar 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 Msolar contribute less than 40% of the dark matter.
    The Astrophysical Journal 03/2001; 550:L169-L172. · 6.73 Impact Factor
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    ABSTRACT: We present a ∼5-yr optical light curve of the recurrent Be/X-ray transient A0538–66 obtained as a by-product of the MACHO Project. These data reveal both a long-term modulation at P=420.8±0.8 d and a short-term modulation at 16.6510±0.0022 d which, within errors, confirms the previously found orbital period. Furthermore, the orbital activity is only seen at certain phases of the 421-d cycle, suggesting that the long-term modulation is related to variations in the Be star envelope.
    Monthly Notices of the Royal Astronomical Society 02/2001; 321(4):678 - 684. · 5.52 Impact Factor
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    ABSTRACT: We report the discovery of eight new R Coronae Borealis (RCB) stars in the Large Magellanic Cloud (LMC) using the MACHO project photometry database. The discovery of these new stars increases the number of known RCB stars in the LMC to thirteen. We have also discovered four stars similar to the Galactic variable DY Per. These stars decline much more slowly and are cooler than the RCB stars. The absolute luminosities of the Galactic RCB stars are unknown since there is no direct measurement of the distance to any Galactic RCB star. Hence, the importance of the LMC RCB stars. We find a much larger range of absolute magnitudes (M(V) = -2.5 to -5 mag) than inferred from the small pre-MACHO sample of LMC RCB stars. It is likely that there is a temperature - M(V)relationship with the cooler stars being intrinsically fainter. Cool (~5000 K) RCB stars are much more common than previously thought based on the Galactic RCB star sample. Using the fairly complete sample of RCB stars discovered in the MACHO fields, we have estimated the likely number of RCB stars in the Galaxy to be ~3,200. The SMC MACHO fields were also searched for RCB stars but none were found. Comment: 36 pages, Latex plus 16 additional tables. ApJ, in press
    The Astrophysical Journal 02/2001; 554(1):298. · 6.73 Impact Factor
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    ABSTRACT: We present the preliminary results of our astrometric study of stellar motions along the lines of sight of the Magellanic Clouds and the Galactic bulge. We find that we are able to select stars with proper motions as small as 0.03 inch/yr from five years of PSF photometry due to the characteristic nature of the shapes the light curves of HFM stars. This shape arises from the proper motion of the object relative to the initial fixed centroid location where all photometry of the object is performed. By selecting such light curves and performing astrometry on candidate HPM stars we have discovered 154 new high proper motion (HPM) stars in 50{sup {open_square}}{sup o} from amongst the {approx} 55 million of stars observed by the MACHO project in these fields. These objects have proper motions as high as 0.5 inch/yr, luminosities ranging from V {approx} 13 to V {approx} 19, and V-R colours between 0.3 and 1.45.
    10/2000
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    ABSTRACT: We present the microlensing optical depth toward the Galactic bulge based on the detection of 99 events found in our Difference Image Analysis (DIA) survey. This analysis encompasses 3 yr of data, covering ~17 million stars in ~4 deg 2 , to a source-star baseline magnitude limit of V = 23. The DIA technique improves the quality of photometry in crowded fields, and allows us to detect more microlensing events with faint source stars. We find that this method increases the number of detection events by 85% compared with the standard analysis technique. DIA light curves of the events are presented, and the microlensing fit parameters are given. The total microlensing optical depth is estimated to be τ total = 2.43 ##IMG## [http://ej.iop.org/images/0004-637X/541/2/734/img1.gif] img1.gif × 10 -6 , averaged over eight fields centered at l = 2 ##IMG## [http://ej.iop.org/icons/Entities/fdg.gif] fdg 68 and b = -3 ##IMG## [http://ej.iop.org/icons/Entities/fdg.gif] fdg 35. For the bulge component, we find τ bulge = 3.23 ##IMG## [http://ej.iop.org/images/0004-637X/541/2/734/img2.gif] img2.gif × 10 -6 , assuming a 25% stellar contribution from disk sources. These optical depths are in good agreement with the past determinations of the MACHO and OGLE groups, and are higher than predicted by contemporary Galactic models. We show that our observed event timescale distribution is consistent with the distribution expected from normal mass stars, if we adopt the Scalo stellar mass function as our lens mass function. However, we note that since there is still disagreement about the exact form of the stellar mass function, there is uncertainty in this conclusion. Based on our event timescale distribution, we find no evidence for the existence of a large population of brown dwarfs in the direction of the Galactic bulge.
    The Astrophysical Journal 07/2000; 541(2):734. · 6.73 Impact Factor
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    ABSTRACT: We present the microlensing optical depth towards the Galactic bulge based on the detection of 99 events found in our Difference Image Analysis (DIA) survey. This analysis encompasses three years of data, covering ~ 17 million stars in ~ 4 deg^2, to a source star baseline magnitude limit of V = 23. The DIA technique improves the quality of photometry in crowded fields, and allows us to detect more microlensing events with faint source stars. We find this method increases the number of detection events by 85% compared with the standard analysis technique. DIA light curves of the events are presented and the microlensing fit parameters are given. The total microlensing optical depth is estimated to be tau_(total)= 2.43^(+0.39/-0.38) x 10^(-6) averaged over 8 fields centered at l=2.68 and b=-3.35. For the bulge component we find tau_(bulge)=3.23^(+0.52/-0.50) x 10^(-6) assuming a 25% stellar contribution from disk sources. These optical depths are in good agreement with the past determinations of the MACHO Alcock et al. (1997) and OGLE Udalski et al. (1994) groups, and are higher than predicted by contemporary Galactic models. We show that our observed event timescale distribution is consistent with the distribution expected from normal mass stars, if we adopt the stellar mass function of Scalo (1986) as our lens mass function. However, we note that as there is still disagreement about the exact form of the stellar mass function, there is uncertainty in this conclusion. Based on our event timescale distribution we find no evidence for the existence of a large population of brown dwarfs in the direction of the Galactic bulge. Comment: Updated references and corrected optical depth values. tau_tot= [2.91(+0.47/-0.45) -> 2.43^(+0.39/-0.38)] x 10^(-6) tau_bul = [3.88(+0.63/-0.60) -> 3.23^(+0.52/-0.50)] x 10^(-6)
    The Astrophysical Journal 02/2000; 541(2):734. · 6.73 Impact Factor
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    ABSTRACT: We present the first massive frequency analysis of the 1200 first overtone RR Lyrae stars in the Large Magellanic Cloud observed in the first 4.3 yr of the MACHO project. Besides the many new double-mode variables, we also discovered stars with closely spaced frequencies. These variables are most probably nonradial pulsators.
    01/2000; 203:313-314.
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    ABSTRACT: We describe a photometric survey for variable stars in twelve metal-rich globular clusters, and provide results for the first two clusters.
    01/2000;
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    ABSTRACT: We present the first results of the analysis of 22 Blazhko stars. We find: 1) Blazhko RRab stars that are nearly pure amplitude modulators; 2) Blazhko RRab stars that have both amplitude and phase modulation; 3) A Blazhko RRab star that has an abrupt period change; 4) Proof of the Blazhko effect in RRc stars. Our data show the character of the amplitude and phase modulations of the light curves over the Blazhko cycles far better than has been previously possible.
    01/2000; 203:291-298.
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    ABSTRACT: To date, the MACHO project has identified approximately 20 microlensing events towards the Large Magellanic Cloud (LMC), each event appearing as a temporary brightening in MACHO photometry of LMC stars. Due to crowding in our ground-based photometry, these ``stars'' are usually not single stars at all, but rather blended objects made up of several fainter stars. By combining the technique of difference imaging with high resolution HST/WFPC2 images we can identify which of the many resolved HST stars constituting one MACHO object was the microlensed star. Difference imaging is used to locate the centroid of the lensed flux in the ground-based MACHO image to an accuracy of around 0.1 to 0.2 arcseconds. We then derive coordinate transformations between the MACHO and HST frames and, in most cases, can unambiguously identify the lensed star. This information allows us to verify ground-based estimates of parameters important to the calculation of the optical depth such as the unlensed, baseline flux, which is critical in determining the Einstein Ring crossing time. It also allows us to verify that lensed stars do not cluster to any particular area of the color-magnitude diagram. Finally, the unblended colors allow us to comment on LMC self-lensing by determining if the lensed stars are more reddened than their neighbours, and therefore more likely to lie on the far side of the LMC disk.
    11/1999; 31:1444.

Publication Stats

1k Citations
424.58 Total Impact Points

Institutions

  • 2014
    • Leibniz Institute for Astrophysics Potsdam
      Potsdam, Brandenburg, Germany
  • 1996–2014
    • McMaster University
      • Department of Physics and Astronomy
      Hamilton, Ontario, Canada
  • 1998–2005
    • University of California, Berkeley
      Berkeley, California, United States
    • Massachusetts Institute of Technology
      Cambridge, Massachusetts, United States
  • 1998–2003
    • University of Michigan
      • Department of Astronomy
      Ann Arbor, Michigan, United States
  • 2001
    • University of Santiago, Chile
      CiudadSantiago, Santiago, Chile
    • University of California, San Diego
      • Department of Physics
      San Diego, CA, United States
  • 1999
    • University of Notre Dame
      • Department of Physics
      Washington, D. C., DC, United States
  • 1997
    • California Institute of Technology
      • Department of Astronomy
      Pasadena, California, United States