Browsing by Author "Welch, DL"
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- ItemDifference 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, DLThe 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.
- ItemIndependent distance determinations to Milky Way cepheids in open clusters and associations .3. CV Mon in Anon vandenBergh(1996) Gieren, WP; Mermilliod, JC; Matthews, JM; Welch, DLAs part of a program aimed at comparing the galactic Cepheid distance scales from cluster ZAMS fitting and from independent Baade-Wesselink techniques, we have used new, high-quality light and radial velocity curves of the Cepheid CV Mon supposed to be a member of the sparse cluster Anon van den Bergh to obtain its distance and mean radius from the visual surface brightness method, We find the distance to CV Mon to be 2160+/-200 pc which is similar to 20% larger than the ZAMS-fitting distance to the cluster determined by Turner [JRASC, 72, 248 (1978)] but cluster membership of CV Mon is still possible given the uncertainties of both determinations. Our surface brightness analysis yields a mean radius of 53.5+/-3.9 R. for CV Mon. The Cepheid's absolute visual magnitude is -3.67+/-0.36 mag, whose large uncertainty is dominated by the relatively uncertain absorption correction. Use of near infrared photometry would reduce the uncertainty in the absolute magnitude. We find rather clear evidence that CV Mon is pulsating in the fundamental radial mode. There is no evidence, neither from the existing photometry nor from radial velocity observations obtained over eleven years, that there is a binary companion to the Cepheid. (C) 1996 American Astronomical Society.
- ItemMacho 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, DLWe 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.
- ItemThe 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, DLWe 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.
- ItemThe MACHO project Large Magellanic Cloud variable-star inventory.: IX.: Frequency analysis of the first-overtone RR Lyrae stars and the indication for nonradial pulsations(2000) Alcock, C; Allsman, R; Alves, DR; Axelrod, T; Becker, A; Bennett, D; Clement, C; Cook, KH; Drake, A; Freeman, K; Geha, M; Griest, K; Kovács, G; Kurtz, DW; Lehner, M; Marshall, S; Minniti, D; Nelson, C; Peterson, B; Popowski, P; Pratt, M; Quinn, P; Rodgers, A; Rowe, J; Stubbs, C; Sutherland, W; Tomaney, A; Vandehei, T; Welch, DLMore than 1300 variables classified provisionally as first-overtone RR Lyrae pulsators in the MACHO variable-star database of the Large Magellanic Cloud (LMC) have been subjected to standard frequency analysis. Based on the remnant power in the prewhitened spectra, we found 70% of the total population to be monoperiodic. The remaining 30% (411 stars) are classified as one of nine types according to their frequency spectra. Several types of RR Lyrae pulsational behavior are clearly identified here for the first time. Together with the earlier discovered double-mode (fundamental and first-overtone) variables, this study increased the number of known double-mode stars in the LMC to 181. During the total 6.5 yr time span of the data, 10% of the stars showed strong period changes. The size, and in general also the patterns of the period changes, exclude a simple evolutionary explanation. We also discovered two additional types of multifrequency pulsators with low occurrence rates of 2% for each. In the first type, there remains one closely spaced component after prewhitening by the main pulsation frequency. In the second type, the number of remnant components is two; they are also closely spaced, and are symmetric in their frequency spacing relative to the central component. This latter type of variables are associated with their relatives among the fundamental pulsators, known as Blazhko variables. Their high frequency (approximate to 20%) among the fundamental-mode variables versus the low occurrence rate of their first-overtone counterparts makes it more difficult to explain the Blazhko phenomenon by any theory depending mainly on the role of aspect angle or magnetic field. None of the current theoretical models are able to explain the observed close frequency components without invoking nonradial pulsation components in these stars.
- ItemThe MACHO project sample of galactic bulge high-amplitude δ Scuti stars(2000) Alcock, C; Allsman, RA; Alves, DR; Axelrod, TS; Becker, AC; Bennett, DP; Cook, KH; Freeman, KC; Geha, M; Griest, R; Lehner, MJ; Marshall, SL; McNamara, BJ; Minniti, D; Nelson, C; Peterson, BA; Popowski, P; Pratt, MR; Quinn, PJ; Rodgers, AW; Sutherland, W; Templeton, MR; Vandehei, T; Welch, DLWe have detected 90 objects with periods and light-curve structures similar to those of held delta Scuti stars using the Massive Compact Halo Object (MACHO) Project database of Galactic bulge photometry. If we assume similar extinction values for all candidates and absolute magnitudes similar to those of other field high-amplitude delta Scuti stars (HADS), the majority of these objects lie in or near the Galactic bulge. At least two of these objects are likely foreground delta Scuti stars, one of which may be an evolved nonradial pulsator, similar to other evolved, disk-population delta Scuti stars. We have analyzed the light curves of these objects and find that they are similar to the light curves of held delta Scuti stars and the delta Scuti stars found by the Optical Gravitational Lens Experiment (OGLE). However, the amplitude distribution of these sources lies between those of low- and high-amplitude delta Scuti stars, which suggests that they may be an intermediate population. We have found nine double-mode HADS with frequency ratios ranging from 0.75 to 0.79, four probable double- and multiple-mode objects, and another four Objects with marginal detections of secondary modes. The low frequencies (5-14 cycles day(-1)) and the observed period ratios of similar to 0.77 suggest that the majority of these objects are evolved stars pulsating in fundamental or first overtone radial modes.