D. L. Welch

McMaster University, Hamilton, Ontario, Canada

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Publications (141)424.58 Total impact

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    ABSTRACT: We have begun a 180 orbit project using the Advanced Camera for Surveys to search for and phase Cepheid variables in two spiral galaxies in the core of the Coma cluster. A direct application of this canonical primary distance indicator at Coma's recession velocity of 7000 km/s will measure the far-field Hubble constant free of many of the systematic uncertainties which beset current determinations relying on secondary indicators. Establishing the far-field H0 with Cepheids will provide one of the strongest links in the extragalactic distance scale. Reducing the uncertainty in the far-field H0 will help constrain the equation of state of the dark energy. We will directly calibrate the fiducial fundamental plane of elliptical galaxies in Coma, as well as the Thomson optical depth of the cluster for comparison with the 30 to 300 Ghz Sunyaev-Zeldovich spectrum.A S/N=5-10 or better can be reached for Cepheids with periods of 40d to 70d at mean light in 5 orbits with the F606W filter if H0 is approximately 70 km/s/Mpc. Each of the two target galaxies will be observed 12 times, for a duration of 5 orbits, optimally spaced for periods of 40-70d, providing robust detection and phasing information. An additional 6 observational visits using the F814W filter are planned. We will present a progress report of the first few epochs of data which will be in hand by the time of the January 2007 AAS meeting.
    01/2006;
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    ABSTRACT: The SuperMACHO collaboration has discovered light echoes from four ancient supernovae (SNe) in the Large Magellanic Cloud using difference image analysis (Rest et. al, 2005). These light echoes offer one of the most effective means to probe the 3-D structure of the dust in the interstellar medium in the LMC, as they illuminate sheets and bands of the dust at different distances from the SN. As the light echos "move", they continuously map out different structures, providing the opportunity to map out large structures over time. We find that the light echoes cross-correlate very well with dust structures detected in the infrared in images from the Spitzer space telescope. We also cross-correlated these with structures found in the ATCA HI velocity maps.
    01/2006;
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    ABSTRACT: Microlensing event MACHO 97-SMC-1 is one of the rare microlensing events for which the source is a variable star, simply because most variable stars are systematically eliminated from microlensing studies. Using observational data for this event, we show that the intrinsic variability of a microlensed star is a powerful tool for constraining the nature of the lens by breaking the degeneracy between the microlens parallax and the blended light. We also present a statistical test for discriminating the location of the lens based on the chi2 contours of the vector Lambda, the inverse of the projected velocity. We find that while SMC self-lensing is somewhat favored over halo lensing, neither location can be ruled out with good confidence.
    01/2006;
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    ABSTRACT: We have carried out a new search for variable stars in the metal-rich bulge globular cluster NGC 6304 ([Fe/H] = -0.59) using CCD observations obtained at CTIO. We used two data sets: one was taken on the 0.9m in May and June of 1996, and the second was taken on the 1m Yalo telescope in February and March of 2002. We have identified and obtained BVI light curves for 11 RR Lyrae stars, including 6 RRab and 5 RRc stars within the tidal radius of the cluster, and partial light curves for several long-period variables. Most of the RR Lyrae stars had been previously discovered, although not always recognized to be RR Lyrae type variables. We are able to exclude several RR Lyrae stars as probable field stars. In light of the large number of long-period RRab stars recently discovered within the metal-rich globular clusters NGC 6388 and NGC 6441, it is noteworthy that two of the possible RRab have periods greater than 0.8 days. The nature of these long-period RR Lyrae and the question of their membership will be discussed.
    Memorie della Societa Astronomica Italiana. 01/2006;
  • G. C. Clayton, D. L. Welch
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    ABSTRACT: We present new spectroscopic and photometric observations of the interesting Type II-P supernova, SN 2002hh, in NGC 6946. Gemini/GMOS-N has been used to acquire visible spectra and also g'r'i' photometry covering 5 epochs between August 2004 and October 2005, following the evolution of the supernova from 650 to 1050 d since its initial explosion. Supernova spectra obtained 3 years after outburst are rare. In addition, data have been obtained at several epochs in the JHK bands using the Steward 90" with the 256x256 imager and with Gemini/NIRI. Dust emission from SN 2002hh has been detected at mid-infrared wavelengths by SST/IRAC and confirmed by higher angular resolution Gemini/Michelle observations (Barlow et al. 2005, ApJ, 627, L113). There is a pre-existing optically thick dust shell having a mass of ˜0.1 Msun, suggesting a massive M supergiant or luminous blue variable precursor. However, the formation of new dust in the ejecta of SN 2002hh has not been ruled out. The IR emission from any such new dust would be swamped by the emission from the existing circumstellar dust. The new data, presented here, are being used to investigate the late-time evolution of SN 2002hh and whether new dust has been formed in its ejecta. In particular, we are looking for changes in the H-alpha emission line profiles and for variations in brightness due to changes in the extinction and emission due to dust. This study is partially supported by the Natural Sciences and Engineering Research Council of Canada (NSERC).
    11/2005; 37:1433.
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    D. Lepischak, D. L. Welch
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    ABSTRACT: We present the initial results from an in-depth re-examination of the MACHO project Large Magellanic Cloud database to identify and characterize stellar variability near the intersection of the instability strip and the main sequence. This dataset's long time-series and uniform photometry is an unprecedented resource for describing the frequency and regions of incidence of various radial and non-radial modes of excitation. The raw MACHO photometry has been investigated to identify factors responsible for most of the residual photometric variance and increase the sensitivity to small amplitudes. We present details of the search method used, the Delta Scuti variables detected thus far and discuss the implications for the completed survey.
    10/2005;
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    ABSTRACT: We have identified five new R Coronae Borealis (RCB) stars in the Galactic bulge using the MACHO Project photometry database, raising the total number of known Galactic RCB stars to about 40. We have obtained spectra to confirm the identifications. The fact that four out of the five newly identified RCB stars are ``cool'' (T(eff) < 6000 K) rather than ``warm'' (T(eff) > 6000 K) suggests that the preponderance of warm RCB stars among the existing sample is a selection bias. These cool RCB stars are redder and fainter than their warm counterparts and may have been missed in surveys done with blue plates. Based on the number of new RCB stars discovered in the MACHO bulge fields, there may be ~250 RCB stars in the reddened "exclusion" zone toward the bulge.
    The Astronomical Journal 08/2005; · 4.97 Impact Factor
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    ABSTRACT: SN1987A formed spectacular light echoes which have been used to study the dust geometry in the Large Magellanic Cloud (LMC). Because the surface brightness of echoes often does not decrease rapidly with time, we have searched our 4 year SuperMACHO images of the LMC for other echoes of supernovae and other variable objects. Much to our surprise, we found echo arcs whose apparent vector motions clearly point back to three well defined positions in the LMC. Even more surprisingly, these three positions correspond to 3 of the 6 Supernova Remnants (SNRs) that are listed (Hughes et al 1995) both as the youngest SNRs and as possible Type Ia events in the LMC. Gemini spectroscopy obtained via DD time in March 2005 of the brightest echo arc suggests that the type Ia classification is correct. We ask for GMOS-S time to obtain higher quality spectra of all three events to classify the supernova events.
    NOAO Proposal. 08/2005;
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    Doug Welch, Geoff Clayton
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    ABSTRACT: Over 50% of dust production in the Galaxy is from old stellar systems (AGB stars & Type Ia SNe). However, the origin of dust in high-redshift galaxies is expected to be much different. Such galaxies have been observed to contain abundant dust even though they are quite young. After only 1 Gyr, there has not been time for low-mass stars to evolve to the AGB. Using estimates from our Galaxy, the timescale for injection of stardust into the ISM by AGB stars is about 2.5 Gyr. Evidence from pre-solar dust grains implies that Type Ia SNe are also significant sources for interstellar dust locally, but these too are old systems that may not have had time to evolve by the epoch of our observations of high-redshift galaxies. Dust production in high-redshift galaxies therefore is expected to be dominated by Type II SNe. The condensation of dust in SN ejecta is still not well understood. One reason is that detection of newly-formed dust in SNe in nearby galaxies has been extremely rare. In August 2004, we began a program with Gemini/GMOS to study dust formation in Type II SNe. We were able to obtain spectra, as well as VRI photometry, of 14 Type II SNe with ages of 200 to 650 days. Dust formation in the SNe ejecta is indicated by a sudden decrease in continuum brightness in the visible due to increased dust extinction. This is accompanied by the development of asymmetric, blue-shifted emission-line profiles, caused by dust forming in the ejecta, and preferentially extinguishing redshifted emission from material on the far side of the SN. We will present early results from this study as well as the late-time evolution of the spectrum of the Type II supernova 2002hh in NGC 6946 obtained in August, October, and December 2004 with Gemini North and GMOS.
    The Journal of the Royal Astronomical Society of Canada. Royal Astronomical Society of Canada 07/2005; 99:145.
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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: The era of large survey datasets has arrived, and the era of large survey telescope projects is upon us. Many of these new telescope projects will not only produce large datasets, they will produce datasets that require real-time astronomical analysis, including object detection, photometry, and classification. These datasets promise to open new horizons in the exploration of the time domain in astrophysical systems on large scales. But to fulfill this promise, the projects must design and develop data management systems on a much larger scale (many Terabytes per day continuously) than has previously been achieved in astronomy. Working together, NOAO and the University of Washington are developing prototype pipeline systems to explore the issues involved in real-time time-variability analysis. These efforts are not simply theoretical exercises, but rather are driven by NOAO Survey programs which are generating large data flows. Our survey projects provide a science-driven testbed of data management strategies needed for future initiatives such as the Large Synoptic Survey Telescope and other large-scale astronomical data production systems.
    Proc SPIE 12/2002;
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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

Publication Stats

1k Citations
424.58 Total Impact Points

Institutions

  • 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