D. L. Welch

McMaster University, Hamilton, Ontario, Canada

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Publications (244)758.74 Total impact

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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.
    Full-text · Article · Oct 2006 · The Astrophysical Journal
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    ABSTRACT: We present late-time optical and mid-infrared observations of the Type II supernova 2003gd in the galaxy NGC 628. Mid-infrared excesses consistent with cooling dust in the ejecta are observed 499 to 678 days after outburst and are accompanied by increasing optical extinction and growing asymmetries in the emission-line profiles. Radiative-transfer models show that up to 0.02 solar masses of dust has formed within the ejecta, beginning as early as 250 days after outburst. These observations show that dust formation in supernova ejecta can be efficient and that massive-star supernovae could have been major dust producers throughout the history of the universe.
    Full-text · Article · Aug 2006 · Science
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    ABSTRACT: The MACHO Project sought evidence for Massive Compact Halo Objects (MACHO) by surveying for the gravitational effort of the objects on light, microlensing.
    No preview · Conference Paper · Jul 2006
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    ABSTRACT: In recent years, light echoes have been discovered around some nearby extragalactic supernovae (SNe) well after the explosion. However, to date no light echoes of historical SNe of Galactic or extragalactic origin have been discovered. The SuperMACHO project has discovered echoes from three ancient SNe in the Large Magellaninc Cloud. Spectra of the light echoes taken with GMOS on Gemini South indicate that one of them is an overluminous SN Ia.
    Full-text · Article · Jan 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.
    Preview · Article · Jan 2006
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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.
    No preview · Article · Jan 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.
    No preview · Article · Jan 2006
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    ABSTRACT: The light from historical supernovae could in principle still be visible as scattered-light echoes centuries after the explosion. The detection of light echoes could allow us to pinpoint the supernova event both in position and age and, most importantly, permit the acquisition of spectra to determine the 'type' of the supernova centuries after the direct light from the explosion first reached Earth. Although echoes have been discovered around some nearby extragalactic supernovae, targeted searches have not found any echoes in the regions of historical Galactic supernovae. Here we report three faint variable-surface-brightness complexes with high apparent proper motions pointing back to three of the six smallest (and probably youngest) previously catalogued supernova remnants in the Large Magellanic Cloud, which are believed to have been thermonuclear (type Ia) supernovae. Using the distance and apparent proper motions of these echo arcs, we estimate ages of 610 and 410 years for two of them.
    Full-text · Article · Jan 2006 · Nature
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    ABSTRACT: Several long period eclipsing binary systems with Cepheid components have been discovered in the Magellanic Clouds from the OGLE and MACHO microlensing photometry programs (Udalski et al. 1999; Alcock et al. 2002). The astrophysical and cosmological importance of finding a Cepheid as a member of an eclipsing binary is considerable. If an eclipsing binary is a double-line system, the mass, radius, and luminosity can be directly determined from the analysis of the light and radial velocity curves. From spectrophotometry, the stellar temperatures and chemical abundances, as well as ISM reddenings and precise distances, can also be secured. Moreover, the study of Cepheids in eclipsing binaries offers an important opportunity to investigate the structure and evolution of Cepheids as well as tests of pulsational theories. These systems provide opportunity to minimize the dependence of the Cosmic Distance Scale and Hubble's constant (Ho) on uncertainties in assumed "zero-points". In this poster we discuss the progress being made in our ongoing study of two important LMC eclipsing binaries containing Cepheid components. These stars are: the 14.5 mag, 397d LMC EB (SC21 40876 = MACHO 6.6454.5), which contains a classical Cepheid with a pulsation period of 4.97d and the 17th mag, 801d LMC EB (SC16 119952) which contains a Cepheid with a 2.03d period. We discuss the initial results of the analysis of HST-STIS FUV to near-IR spectrophotometry as well as the analysis of their light curves and (partial) radial velocity observations. The study of these systems represents an unprecedented opportunity to probe the distance to the LMC, to "self-calibrate" the P-L and P-L-C relations, to test the Baade-Wesselink pulsational parallax method, and to directly determine fundamental physical properties and ages of the Cepheids themselves. This research is supported by NASA HST Grant GO-09176 and NSF/RUI Grant AST05-07542 which we very gratefully acknowledge.
    No preview · Article · Nov 2005
  • 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).
    No preview · Article · Nov 2005
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    ABSTRACT: The SuperMACHO project, an NOAO survey, is in its 5th year of searching for microlensing toward the Large Magellanic Cloud (LMC) using the CTIO 4m+MOSAIC (Rest et al. 2005, AJ, in press). The project samples 68 fields covering 24 square degrees over the LMC in a broad VR filter and reaches VR ˜23 with an uncertainty of 0.1 mag. Each field was imaged 20-30 times with a typical separation of 2 days over 3 months for the past 4 years. Image subtraction analysis is used to identify microlensing candidates and other variable sources in these crowded fields where more than 16 million difference image detections are found in the 2 TB of image data. With the difference image detections, we have built a prototype database in MySQL and created objects using simple clustering methods. A current total of 84,836 variable objects with light curves characterized by various statistics (e.g., period estimates by SuperSmoother, Fourier coefficients) are being investigated. In cross-matching to MACHO, 21,425 sources are re-confirmed as known variables, with the remaining being new and/or fainter detections. A large sample of Delta Scuti variables has been identified, extending beyond the MACHO set, providing a new catalog of good signal-to-noise light curves for analysis. Similarly, MACHO and SuperMACHO light curves for RR Lyrae have been compared to explore period and phase evolution behavior. The eclipsing binary catalog from MACHO is extended nearly 3 mag deeper. We will present a summary of these and other variable stars found in more detail.
    No preview · Article · Nov 2005
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    ABSTRACT: Recent studies, using OGLE data for LMC Cepheids in the optical, strongly suggest that the period–luminosity (PL) relation for the Large Magellanic Cloud (LMC) Cepheids shows a break or non-linearity at a period of 10 d. In this paper we apply statistical tests, the chi-squared test and the F-test, to the Cepheid data from the MACHO project to test for a non-linearity of the V- and R-band PL relations at 10 d, and extend these tests to the near-infrared (JHK-band) PL relations with 2MASS data. We correct the extinction for these data by applying an extinction map towards the LMC. The statistical test we use, the F-test, is able to take account of small numbers of data points and the nature of that data on either side of the period cut at 10 d. With our data, the results we obtained imply that the VRJH-band PL relations are non-linear around a period of 10 d, while the K-band PL relation is (marginally) consistent with a single-line regression. The choice of a period of 10 d, around which this non-linearity occurs, is consistent with the results obtained when this ‘break’ period is estimated from the data. We show that robust parametric (including least-squares, least absolute deviation, robust regression) and non-parametric regression methods, which restrict the influence of outliers, produce similar results. Long-period Cepheids are supplemented from the literature to increase our sample size. The photometry of these long-period Cepheids is compared with our data and no trend with period is found. Our main results remain unchanged when we supplement our data set with these long-period Cepheids. By examining our data at maximum light, we also suggest arguments as to why errors in reddening are unlikely to be responsible for our results. The non-linearity of the mean V-band PL relation as seen in both of the OGLE and the MACHO data, using different extinction maps, suggests that this non-linearity is real.
    Full-text · Article · Oct 2005 · Monthly Notices of the Royal Astronomical Society
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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.
    Preview · Article · Oct 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.
    Full-text · Article · Aug 2005 · The Astronomical Journal
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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.
    No preview · Article · Dec 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
    Full-text · Article · Oct 2004 · The Astrophysical Journal
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    ABSTRACT: Using 7 years of MACHO survey data, we present a new determination of the optical depth to microlensing towards the Galactic bulge. We select the sample of 62 microlensing events (60 unique) on clump giant sources and perform a detailed efficiency analysis. We use only the clump giant sources because these are bright bulge stars and are not as strongly affected by blending as other events. Using a subsample of 42 clump events concentrated in just 4.5 square degrees, we find tau = 2.17^{+0.47}_{-0.38} x 10^{-6} at (l,b) = (1.50, -2.68), somewhat smaller than found in most previous MACHO studies, but in excellent agreement with recent theoretical predictions. We also present the optical depth in each of the 19 fields in which we detected events, and find limits on optical depth for fields with no events. The errors in optical depth in individual fields are dominated by Poisson noise. We measure optical depth gradients (1.06 +/- 0.71) x 10^{-6} deg^{-1} and (0.29 +/- 0.43) x 10^{-6} deg^{-1} in the galactic latitude b and longitude l directions, respectively. Finally, we discuss the possibility of anomalous duration distribution of events in the field 104 centered on (l,b) = (3.11, -3.01) as well as investigate spatial clustering of events in all fields. Comment: 42 pages, 15 figures (6 of them in color), 12 tables; results unchanged, discussion of blending strengthened including addition of new appendix; to be published in ApJ, Vol. 630, Sept. 10, 2005 issue
    Full-text · Article · Oct 2004 · The Astrophysical Journal
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    ABSTRACT: On September 6th we reported (Barlow et al.; IAUC 8400) the 3.6--8.0-um detection of the Type~II supernova SN 2002hh (located in the face-on spiral galaxy NGC 6946 at a distance of 6~Mpc), in archival Spitzer Space Telescope (SST) SINGS Legacy IRAC images taken on June 10.76 2004, when the 8.0-um flux of the supernova was 20.0 mJy. Since then we have also measured its flux in a SINGS Legacy MIPS 24-um image, taken one month after the IRAC data, which became available this week. SN 2002hh's IRAC+MIPS energy distribution can be fit by a 300K blackbody. Its equivalent blackbody emission radius of 1x10^17 cm is too large for the emitting dust to have been freshly condensed in the supernova (SN) ejecta, so it appears that the dust may have been formed and ejected in an earlier stage of evolution of the massive progenitor star and flash heated by the SN event. Gemini-N Director's Discretionary Time has been awarded to us this week for high angular resolution 3-20-um observations of SN 2002hh, to be be carried out in the next few weeks. We have also submitted a request for HST DD Time, with the aim of obtaining high angular resolution observations of SN 2002hh and its surrounds at optical and near-IR wavelengths, using the ACS and NICMOS (PI: B. Sugerman). Here we request SST Director's Discretionary Time for follow-up IR observations of the likely transient dust emission from this object. We request that IRS spectroscopy, together with IRAC+MIPS photometry, be obtained as early as possible (the supernova could fade to a level several times weaker than on June 10th 2004 and still produce good signal-to-noise ratios throughout most of the IRS spectral domain). As well as allowing the object's overall energy distribution to be constrained, the spectra will enable any dust emission or absorption features to be identified (an Av of 5.0 local to the SN has been deduced), together with any ionic emission lines. Accompanying short-duration IRAC 4-band and MIPS 24-um imaging will provide higher angular resolution than the IRS
    No preview · Article · Sep 2004
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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
    Full-text · Article · Sep 2004 · Proceedings of the International Astronomical Union
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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.
    Preview · Article · Apr 2004

Publication Stats

5k Citations
758.74 Total Impact Points

Institutions

  • 1990-2015
    • McMaster University
      • Department of Physics and Astronomy
      Hamilton, Ontario, Canada
  • 1996-2005
    • University of California, Berkeley
      • Department of Physics
      Berkeley, California, United States
    • University of Washington Seattle
      • Department of Physics
      Seattle, Washington, United States
  • 2003
    • University of Michigan
      • Department of Astronomy
      Ann Arbor, Michigan, United States
    • Max Planck Institute for Astrophysics
      Arching, Bavaria, Germany
  • 2001
    • University of California, San Diego
      San Diego, California, United States
  • 2000
    • University of California, Santa Cruz
      • Department of Astronomy and Astrophysics
      Santa Cruz, California, United States
  • 1999
    • The University of Sheffield
      • Department of Physics and Astronomy
      Sheffield, England, United Kingdom
    • Louisiana State University
      • Department of Physics & Astronomy
      Baton Rouge, Louisiana, United States
  • 1995-1999
    • Pontifical Catholic University of Chile
      CiudadSantiago, Santiago Metropolitan, Chile
  • 1997
    • California Institute of Technology
      • Department of Astronomy
      Pasadena, California, United States
  • 1993
    • Carnegie Institute
      Pasadena, Texas, United States
  • 1992
    • University of Victoria
      Victoria, British Columbia, Canada
  • 1983-1988
    • University of Toronto
      Toronto, Ontario, Canada
  • 1987
    • Pasadena City College
      Pasadena, Texas, United States