Search

Brown dwarfs are important objects because they may provide a missing link between stars and planets, two populations that have dramatically different formation history. In this paper, we present the candidate binaries with brown dwarf companions that are found by analyzing binary microlensing events discovered during 2004 - 2011 observation seasons. Based on the low mass ratio criterion of q < 0.2, we found 7 candidate events, including OGLE-2004-BLG-035, OGLE-2004-BLG-039, OGLE-2007-BLG-006, OGLE-2007-BLG-399/MOA-2007-BLG-334, MOA-2011-BLG-104/OGLE-2011-BLG-0172, MOA-2011-BLG-149, and MOA-201-BLG-278/OGLE-2011-BLG-012N. Among them, we are able to confirm that the companions of the lenses of MOA-2011-BLG-104/OGLE-2011-BLG-0172 and MOA-2011-BLG-149 are brown dwarfs by determining the mass of the lens based on the simultaneous measurement of the Einstein radius and the lens parallax. The measured mass of the brown dwarf companions are (0.02 +/- 0.01) M_Sun and (0.019 +/- 0.002) M_Sun for MOA-2011-BLG-104/OGLE-2011-BLG-0172 and MOA-2011-BLG-149, respectively, and both companions are orbiting low mass M dwarf host stars. More microlensing brown dwarfs are expected to be detected as the number of lensing events with well covered light curves increases with new generation searches., 10 pages, 9 figures, 4 tables

The microlensing event OGLE-2008-BLG-510 is characterised by an evident asymmetric shape of the peak, promptly detected by the ARTEMiS system in real time. The skewness of the light curve appears to be compatible both with binary-lens and binary-source models, including the possibility that the lens system consists of an M dwarf orbited by a brown dwarf. The detection of this microlensing anomaly and our analysis demonstrates that: 1) automated real-time detection of weak microlensing anomalies with immediate feedback is feasible, efficient, and sensitive, 2) rather common weak features intrinsically come with ambiguities that are not easily resolved from photometric light curves, 3) a modelling approach that finds all features of parameter space rather than just the `favourite model' is required, and 4) the data quality is most crucial, where systematics can be confused with real features, in particular small higher-order effects such as orbital motion signatures. It moreover becomes apparent that events with weak signatures are a silver mine for statistical studies, although not easy to exploit. Clues about the apparent paucity of both brown-dwarf companions and binary-source microlensing events might hide here., 17 pages with 8 figures, MNRAS submitted

Microlensing can provide a useful tool to probe binary distributions down to low-mass limits of binary companions. In this paper, we analyze the light curves of 8 binary lensing events detected through the channel of high-magnification events during the seasons from 2007 to 2010. The perturbations, which are confined near the peak of the light curves, can be easily distinguished from the central perturbations caused by planets. However, the degeneracy between close and wide binary solutions cannot be resolved with a $3\sigma$ confidence level for 3 events, implying that the degeneracy would be an important obstacle in studying binary distributions. The dependence of the degeneracy on the lensing parameters is consistent with a theoretic prediction that the degeneracy becomes severe as the binary separation and the mass ratio deviate from the values of resonant caustics. The measured mass ratio of the event OGLE-2008-BLG-510/MOA-2008-BLG-369 is $q\sim 0.1$, making the companion of the lens a strong brown-dwarf candidate., Comment: 10 figures, 6 tables, 26 pages

We present 2323 High-Amplitude ��-Scuti (HADS) candidates discovered in the Large Magellanic Cloud (LMC) by the SuperMACHO survey (Rest et al. 2005). Frequency analyses of these candidates reveal that several are multimode pulsators, including 119 whose largest amplitude of pulsation is in the fundamental (F) mode and 19 whose largest amplitude of pulsation is in the first overtone (FO) mode. Using Fourier decomposition of the HADS light curves, we find that the period-luminosity (PL) relation defined by the FO pulsators does not show a clear separation from the PL-relation defined by the F pulsators. This differs from other instability strip pulsators such as type c RR Lyrae. We also present evidence for a larger amplitude, subluminous population of HADS similar to that observed in Fornax (Poretti et al. 2008)., 26 pages, 15 figures, AJ accepted

We have analyzed the first 3.75 years of data from TAOS, the Taiwanese American Occultation Survey. TAOS monitors bright stars to search for occultations by Kuiper Belt Objects (KBOs). This dataset comprises 5e5 star-hours of multi-telescope photometric data taken at 4 or 5 Hz. No events consistent with KBO occultations were found in this dataset. We compute the number of events expected for the Kuiper Belt formation and evolution models of Pan & Sari (2005), Kenyon & Bromley (2004), Benavidez & Campo Bagatin (2009), and Fraser (2009). A comparison with the upper limits we derive from our data constrains the parameter space of these models. This is the first detailed comparison of models of the KBO size distribution with data from an occultation survey. Our results suggest that the KBO population is comprised of objects with low internal strength and that planetary migration played a role in the shaping of the size distribution., 18 pages, 16 figures, Aj submitted

The Taiwanese-American Occultation Survey (TAOS) project has collected more than a billion photometric measurements since 2005 January. These sky survey data-covering timescales from a fraction of a second to a few hundred days-are a useful source to study stellar variability. A total of 167 star fields, mostly along the ecliptic plane, have been selected for photometric monitoring with the TAOS telescopes. This paper presents our initial analysis of a search for periodic variable stars from the time-series TAOS data on one particular TAOS field, No. 151 (RA = 17$^{\rm h}30^{\rm m}6\fs$67, Dec = 27\degr17\arcmin 30\arcsec, J2000), which had been observed over 47 epochs in 2005. A total of 81 candidate variables are identified in the 3 square degree field, with magnitudes in the range 8 < R < 16. On the basis of the periodicity and shape of the lightcurves, 29 variables, 15 of which were previously unknown, are classified as RR Lyrae, Cepheid, delta Scuti, SX Phonencis, semi-regular and eclipsing binaries., 20 pages, 6 figures, accepted in The Astronomical Journal

We analyzed data accumulated during 2005 and 2006 by the Taiwan-American Occultation Survey (TAOS) in order to detect short-period variable stars (periods of, Comment: Accepted for publication in AJ

We combine all available information to constrain the nature of OGLE-2005-BLG-071Lb, the second planet discovered by microlensing and the first in a high-magnification event. These include photometric and astrometric measurements from Hubble Space Telescope, as well as constraints from higher order effects extracted from the ground-based light curve, such as microlens parallax, planetary orbital motion and finite-source effects. Our primary analysis leads to the conclusion that the host of Jovian planet OGLE-2005-BLG-071Lb is an M dwarf in the foreground disk with mass M= 0.46 +/- 0.04 Msun, distance D_l = 3.3 +/- 0.4 kpc, and thick-disk kinematics v_LSR ~ 103 km/s. From the best-fit model, the planet has mass M_p = 3.8 +/- 0.4 M_Jup, lies at a projected separation r_perp = 3.6 +/- 0.2 AU from its host and so has an equilibrium temperature of T ~ 55 K, i.e., similar to Neptune. A degenerate model less favored by \Delta\chi^2 = 2.1 (or 2.2, depending on the sign of the impact parameter) gives similar planetary mass M_p = 3.4 +/- 0.4 M_Jup with a smaller projected separation, r_\perp = 2.1 +/- 0.1 AU, and higher equilibrium temperature T ~ 71 K. These results from the primary analysis suggest that OGLE-2005-BLG-071Lb is likely to be the most massive planet yet discovered that is hosted by an M dwarf. However, the formation of such high-mass planetary companions in the outer regions of M-dwarf planetary systems is predicted to be unlikely within the core-accretion scenario. There are a number of caveats to this primary analysis, which assumes (based on real but limited evidence) that the unlensed light coincident with the source is actually due to the lens, that is, the planetary host. However, these caveats could mostly be resolved by a single astrometric measurement a few years after the event., Comment: 51 pages, 12 figures, 3 tables, Published in ApJ

We investigate constraints on additional planets orbiting the distant M-dwarf star OGLE-2005-BLG-390L, around which photometric microlensing data has revealed the existence of the sub-Neptune-mass planet OGLE-2005-BLG-390Lb. We specifically aim to study potential Jovian companions and compare our findings with predictions from core-accretion and disc-instability models of planet formation. We also obtain an estimate of the detection probability for sub-Neptune mass planets similar to OGLE-2005-BLG-390Lb using a simplified simulation of a microlensing experiment. We compute the efficiency of our photometric data for detecting additional planets around OGLE-2005-BLG-390L, as a function of the microlensing model parameters and convert it into a function of the orbital axis and planet mass by means of an adopted model of the Milky Way. We find that more than 50 % of potential planets with a mass in excess of 1 M_J between 1.1 and 2.3 AU around OGLE-2005-BLG-390L would have revealed their existence, whereas for gas giants above 3 M_J in orbits between 1.5 and 2.2 AU, the detection efficiency reaches 70 %; however, no such companion was observed. Our photometric microlensing data therefore do not contradict the existence of gas giant planets at any separation orbiting OGLE-2005-BLG-390L. Furthermore we find a detection probability for an OGLE-2005-BLG-390Lb-like planet of around 2-5 %. In agreement with current planet formation theories, this quantitatively supports the prediction that sub-Neptune mass planets are common around low-mass stars., 10 pages, 4 figures, accepted by A&A

We report the successful identification of the type of the supernova responsible for the supernova remnant SNR 0509-675 in the Large Magellanic Cloud (LMC) using Gemini spectra of surrounding light echoes. The ability to classify outbursts associated with centuries-old remnants provides a new window into several aspects of supernova research and is likely to be successful in providing new constraints on additional LMC supernovae as well as their historical counterparts in the Milky Way Galaxy (MWG). The combined spectrum of echo light from SNR 0509-675 shows broad emission and absorption lines consistent with a supernova (SN) spectrum. We create a spectral library consisting of 26 SNe Ia and 6 SN Ib/c that are time-integrated, dust-scattered by LMC dust, and reddened by the LMC and MWG. We fit these SN templates to the observed light echo spectrum using $��^2$ minimization as well as correlation techniques, and we find that overluminous 91T-like SNe Ia with $\dm15, 12 pages, 18 Figures, to be published in ApJ

We present the results from an analysis of Hubble Space Telescope High Resolution Camera data for the Large Magellanic Cloud microlensing event MACHO-LMC-5. By determining the parallax and proper motion of this object we find that the lens is an M dwarf star at a distance of 578(+65/-53)pc with a proper motion of 21.39 +/- 0.04 mas/yr. Based on the kinematics and location of this star it more likely to be part of the Galactic thick disk than thin disk population. We confirm that the microlensing event LMC-5 is a jerk-parallax event., 4 pages, 1 figure, Accepted ApJL, minor changes

We describe a few recent microlensing results from the MACHO Collaboration. The aim of the MACHO Project was the identification and quantitative description of dark and luminous matter in the Milky Way using microlensing toward the Magellanic Clouds and Galactic bulge. We start with a discussion of the HST follow-up observations of the microlensing events toward the LMC detected in the first 5 years of the experiment. Using color-magnitude diagrams we attempt to distinguish between two possible locations of the microlensing sources: 1) in the LMC or 2) behind the LMC. We conclude that unless the extinction is extremely patchy, it is very unlikely that most of the LMC events have sources behind the LMC. During an examination of the HST images of the 13 LMC events we found a very red object next to the source star of event LMC-5. Astrometry, microlensing parallax fit, and a spectrum suggest that in this case we directly image the lens - a low-mass disk star. Then we focus on the majority of events observed by the MACHO Project, which are detected toward the Galactic bulge. We argue that the microlensing optical depth toward the bulge is best measured using events that have clump giant sources, which are almost unaffected by blending. From this sample we derive a low optical depth toward the Galactic bulge of (1.4 +/- 0.3) x 10^{-6}, in good agreement with other observational constraints and with theoretical models. The presence of many long-duration events among the bulge candidates allows us to investigate the microlensing parallax effect. Events with the strongest parallax signal are probably due to massive remnants. Events MACHO-96-BLG-5 and MACHO-98-BLG-6 might have been caused by the 6-solar-mass black holes., Comment: 25 pages, Invited Review, to appear in "Gravitational Lensing: A Unique Tool For Cosmology", Aussois 2003, eds. D. Valls-Gabaud & J.-P. Kneib

We present a method for solving the lightcurve of an eclipsing binary system which 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 ID's 6.6454.5, 78.6338.24 and 81.8997.87. A best-fit value is obtained for each system's 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 Vir 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., Comment: 35 pages, 14 tables, 6 figures, web address for photometry included, minor changes to abstract and author list, comments and references added to sections 3 and 5, accepted for publication in ApJ, direct scientific correspondence to D. Lepischak and D.L. Welch

We present the photometric properties of 1279 blue variable stars within the LMC. Photometry is derived from the MACHO database. The lightcurves of the sample exhibit a variety of quasi-periodic and aperiodic outburst behavior. A characteristic feature of the photometric variation is that the objects are reddest when at maximum outburst. A subset of 102 objects were examined spectroscopically. Within this subset, 91% exhibited Balmer emission in at least one epoch, in some cases with spectacular spectral variability. The variability observed in the sample is consistent with the establishment and maintenance of the Be phenomenon., Comment: 19 pages, AJ accepted

We present preliminary results of the analysis of 5 years of MACHO data on the Galactic bulge microlensing events with clump giants as sources. This class of events allows one to obtain robust conclusions because relatively bright clump stars are not strongly affected by blending. We discuss: 1) the selection of `giant' events, 2) the distribution of event durations, 3) the anomalous character of event durations and optical depth in the MACHO field 104 centered on (l,b) = (3.1, -3.0). We report the preliminary average optical depth of (2.0 +/- 0.4) x 10^{-6} (internal) at (l,b) = (3.9, -3.8), and present a map of the spatial distribution of the optical depth. When field 104 is removed from the sample, the optical depth drops to (1.4 +/- 0.3) x 10^{-6}, which is in excellent agreement with infrared-based models of the central Galactic region., Comment: 3 pages, 1 color figure, style file AATS.sty included; in ASP Conference Series, Vol. 245, Astrophysical Ages and Times Scales, eds. T. von Hippel, C. Simpson, and N. Manset (San Francisco: Astronomical Society of the Pacific)

We present photometry and analysis of the microlensing alert MACHO 96-LMC-2. The ~3% photometry provided by the Global Microlensing Alert Network follow--up effort reveals a periodic modulation in the lightcurve. We attribute this to binarity of the lensed source. Microlensing fits to a rotating binary source magnified by a single lens converge on two minima, separated by delta chi^2 ~ 1. The most significant fit X1 predicts a primary which contributes ~100% of the light, a dark secondary, and an orbital period (T) of 9.2 days. The second fit X2 yields a binary source with two stars of roughly equal mass and luminosity, and T = 21.2 days. The lensed object appears to lie on the upper LMC main sequence. We estimate the mass of the primary component of the binary system, M ~2 M_sun. For the preferred model X1, we explore the range of dark companions by assuming 0.1 M_sun and 1.4 M_sun objects in models X1a and X1b, respectively. We find lens velocities projected to the LMC in these models of v^hat_X1a = 18.3 +/- 3.1 km/s and v^hat_X1b = 188 +/- 32 k/ms. In both these cases, a likelihood analysis suggests an LMC lens is preferred over a Galactic halo lens, although only marginally so in model X1b. We also find v^hat_X2 = 39.6 +/- 6.1 k/ms, where the likelihood for the lens location is strongly dominated by the LMC disk. In all cases, the lens mass is consistent with that of an M-dwarf. The LMC self-lensing rate contributed by 96-LMC-2 is consistent with model self-lensing rates. (Abridged), 23 pages, including 3 tables and 6 figures; Accepted for publication in The Astrophysical Journal

More 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 9 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 & first overtone) variables this study increased the number of the known double-mode stars in the LMC to 181. During the total 6.5yr time span of the data, 10% of the stars show strong period changes. 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, in addition, they are symmetric in their frequency spacing relative to the central component. This latter type of variables is associated with their relatives among the fundamental pulsators, known as Blazhko variables. Their high frequency (~20%) among the fundamental mode variables versus the low occurrence rate of their first overtone counterparts makes it more difficult to explain Blazhko phenomenon by any theory depending mainly on the role of aspect angle or magnetic field. Current theoretical models invoke nonradial pulsation components in these stars., 20 pages, 21 figures (bitmapped), 7 tables, to appear in Ap.J

The MACHO project is a search for dark matter in the form of massive compact halo objects (MACHOs). The project has photometrically monitored tens of millions of stars in the Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and Galactic bulge in search of rare gravitational microlensing events caused by these otherwise invisible objects. In 5.7 years of observations toward the LMC some 13-17 microlensing events have been observed by the MACHO survey, allowing powerful statements to be made about the nature of the dark population in the halo of our Galaxy. A critical component of these statements is an accurate determination of the survey's detection efficiency. The detection efficiency is a complicated function of temporal sampling, stellar crowding (the luminosity function), image quality, photometry, time-series analysis, and criteria used to select the microlensing candidates. Such a complex interdependence is most naturally solved using a Monte Carlo approach. Here we describe the details of the Monte Carlo used to calculate the efficiency presented in the MACHO 5.7-year LMC results. Here we correct several shortcomings of past determinations, including (1) adding fainter source stars (2.5 magnitudes below our faintest detected "stars"), (2) an up-to-date luminosity function for the LMC, (3) better sampling of real images in both stellar density and observing conditions, (4) an improved scheme for adding artificial microlensing onto a random sample of real lightcurves, and many other improvements. [Abridged], 32 pages, Latex with 16 postscript figures, submitted to ApJS

We report on a search for long duration microlensing events towards the Large Magellanic Cloud. We find none, and therefore put limits on the contribution of high mass objects to the Galactic dark matter. At 95% confidence level we exclude objects in the mass range 0.3 solar masses to 30.0 solar masses from contributing more than 4 times 10^11 solar masses to the Galactic halo. Combined with earlier results, this means that objects with masses under 30 solar masses 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 solar masses contribute less than 40% of the dark matter., Comment: 6 pages, latex with 3 postscript figures, submitted to ApJ Letters

We present UBV photometry of the eclipsing binary Harvard Variable 2274 in the Large Magellanic Cloud (LMC). The stellar parameters of this binary system were calculated by Guinan et al. (1998a) who gave both a reddening towards HV 2274 of E(B-V) = 0.083 +/- 0.006 and a distance modulus to the LMC, mu_{LMC} = 18.42 +/- 0.007. The reddening of this system was also determined by Udalski et al. (1998) who found E(B-V) = 0.149 +\- 0.015. With Udalski et al. (1998) B and V photometry Guinan et al.(1998b) obtained E(B-V) = 0.120 +\- 0.009 and mu_{LMC} = 18.30 +\- 0.07. Using our UBV photometry, we derive a reddening of E(B-V)= 0.088 +\- 0.025, consistent with the original value of Guinan et al. (1998a) and supporting a longer distance modulus to the LMC of about mu_{LMC} = 18.40 +\- 0.07. We stress the uncertainties inherent in ground-based UBV photometry and the concomitant uncertainties in determining distances based upon such photometry., submitted to AJ, revised version, results unchanged, omission in acknowledgements corrected

In this paper, we will review major new results regarding classical Cepheids, in the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC). Specifically, we discuss recent work regarding multimode Cepheids and describe new observations of a W Vir star (HV 5756) and a Cepheid which are each in eclipsing binary systems. An additional interesting pulsating supergiant in an eclipsing system is also identified. Ephemerides for eclipses for the three systems are provided., Comment: 4 pages, 1 Encapsulated PostScript figure. To appear in the ASP Conference Series, Proceedings of IAU Symposium 190, "New Views of the Magellanic Clouds"

This paper has been withdrawn, Comment: This paper has been withdrawn by the author

We present photometric observations and analysis of the second microlensing event detected towards the Small Magellanic Cloud (SMC), MACHO Alert 98-SMC-1. This event was detected early enough to allow intensive observation of the lightcurve. These observations revealed 98-SMC-1 to be the first caustic crossing, binary microlensing event towards the Magellanic Clouds to be discovered in progress. Frequent coverage of the evolving lightcurve allowed an accurate prediction for the date of the source crossing out of the lens caustic structure. The caustic crossing temporal width, along with the angular size of the source star, measures the proper motion of the lens with respect to the source, and thus allows an estimate of the location of the lens. Lenses located in the Galactic halo would have a velocity projected to the SMC of v^hat ~1500 km/s, while an SMC lens would typically have v^hat ~60 km/s. We have performed a joint fit to the MACHO/GMAN data presented here, including recent EROS data of this event. These joint data are sufficient to constrain the time for the lens to move an angle equal to the source angular radius; 0.116 +/- 0.010 days. We estimate a radius for the lensed source of 1.4 +/- 0.1 R_sun. This yields a projected velocity of v^hat = 84 +/- 9 km/s. Only 0.15% of halo lenses would be expected to have a v^hat value at least as small as this, while 31% of SMC lenses would be expected to have v^hat as large as this. This implies that the lensing system is more likely to reside in the SMC than in the Galactic halo., 16 pages, including 3 tables and 3 figures; submitted to The Astrophysical Journal

Using the novel data set of the MACHO project, we show that three ``Blazhko Effect'' RR Lyrae stars show nearly-pure amplitude modulation of a single pulsation mode. This is strong observational evidence that the ``Oblique Pulsator Model'' is the correct solution to this 90-year-old problem., Comment: 31 pages, 9 Encapsulated PostScript figures. Submitted to the Astrophysical Journal

The MACHO Project monitors millions of stars in the Large Magellanic Cloud, the Small Magellanic Cloud and the bulge of the Milky Way searching for the gravitational microlensing signature of baryonic dark matter. This Project has yielded surprising results. An analysis of two years of data monitoring the Large Magellanic Cloud points to {$\sim 50%$} of the mass of the Milky Way's halo in compact objects of {$\sim 0.5 M_{\odot}$}. An analysis of one year of monitoring the bulge has yielded more microlensing than predicted without the invocation of a massive bar or significant disk dark matter. The huge database of light curves created by this search is yielding information on extremely rare types of astrophysical variability as well as providing temporal detail for the study of well known variable astrophysical phenomena. The variable star catalog created from this database is previewed and example light curves are presented., 10 pages, 18 PostScript figures, uses conf_iap.sty. To appear in the Proceedings of the 12th IAP Colloquium "Variable Stars and the Astrophysical Returns of Microlensing Surveys"

We describe the search for delta Scuti stars in the MACHO database of bulge fields. Concentrating on a sample of high amplitude delta Scutis, we examine the light curves and pulsation modes. We also discuss their spatial distribution and evolutionary status using mean colors and absolute magnitudes., 6 pages, PostScript file. To appear in IAU Symp. 189 on "Fundamental Stellar Properties: Confrontation between Theory and Observations", eds. T. Bedding et al. (Kluwer: Dordrecht), 1997

Mean colors and magnitudes of RR Lyrae stars in 24 fields towards the Galactic bulge from the MACHO database are presented. Accurate mean reddenings are computed for these fields on the basis of the mean colors. The distribution along the line of sight of the RR Lyrae population is examined on the basis of the mean magnitudes, and it is shown that the bulk of the RR Lyrae population is not barred. Only the RR Lyrae in the inner fields closer to the Galactic center (l-4) show evidence for a bar. The red giant clump stars in the MACHO fields, however, clearly show a barred distribution, confirming the results of previous studies. Given the different spatial distribution, the RR Lyrae and the clump giants trace two different populations. The RR Lyrae would represent the inner extension of the Galactic halo in these fields., 29 pages including 11 PostScript figures, uses aaspp4.sty. To appear in The Astrophysical Journal

We report the first discovery of a gravitational microlensing candidate towards a new population of source stars, the Small Magellanic Cloud (SMC). The candidate event's light curve shows no variation for 3 years before an upward excursion lasting $ \sim 217$ days that peaks around January 11, 1997 at a magnification of $ \sim 2.1$. Microlensing events towards the Large Magellanic Cloud and the Galactic bulge have allowed important conclusions to be reached on the stellar and dark matter content of the Milky Way. The SMC gives a new line-of-sight through the Milky Way, and is expected to prove useful in determining the flattening of the Galactic halo., Comment: 5 pages, Latex, 2 postscript figures, Published in: ApJ Lett. 491, L11, 1997

The recent suggestion that the microlensing events observed towards the Large Magellanic Cloud are due to an intervening Sgr-like dwarf galaxy is examined. A search for foreground RR Lyrae in the MACHO photometry database yields 20 stars whose distance distribution follow the expected halo density profile. Cepheid and red giant branch clump stars in the MACHO database are consistent with membership in the LMC. There is also no evidence in the literature for a distinct kinematic population, for intervening gas, or for the turn-off of such a hypothetical galaxy. We conclude that if the lenses are in a foreground galaxy, it must be a particularly dark galaxy., Comment: 11 pages, 4 PostScript figures, uses aaspp4.sty. Submitted to The Astrophysical Journal Letters

We review the pulsating stars contained in the top 24 fields of the MACHO bulge database, with special emphasis on the red semiregular stars. Based on period, amplitude and color cuts, we have selected a sample of 2000 semiregular variables with $15, Comment: 12 pages, 8 PostScript figures, uses jd24.sty. Invited review for IAU-JD24, to appear in: Pulsating Stars - Recent Developments in Theory and Observation, 1998, eds. M. Takeuti, D. Sasselov, (Universal Academy Press: Tokyo)

We report the discovery of 20 1H/2H and 7 F/1H beat Cepheids in the SMC by the MACHO Project. We utilize the 20 1H/2H stars to determine lightcurve shape for the SMC second-overtone (2H) mode of Cepheid pulsation. We predict, similar to the findings of Alcock et al. (1997, ApJ, submitted), that 2H Cepheids will have nearly or purely sinusoidal light variations; that the P-L relation for 2H Cepheids will not be distinguishable from the P-L relation for 1H Cepheids within photometric accuracy; and that 2H stars may be discernable from F and 1H stars using the amplitude-period diagram and Fourier parameter progressions for periods P < 0.7 days, our current sample 2H period limit., 15 pages, 4 Encapsulated PostScript figures. Submitted to the Astronomical Journal

We report the discovery of RV Tauri stars in the Large Magellanic Cloud. In light and colour curve behaviour, the RV Tauri stars appear to be a direct extension of the type II Cepheids to longer periods. A single period- luminosity-colour relationship is seen to describe both the type II Cepheids and the RV Tauri stars in the LMC. We derive the relation: $V_{\circ} = 17.89 (\pm 0.20) - 2.95 (\pm 0.12) \log_{10}P + 5.49 (\pm 0.35) \bar{(V-R)_{\circ}}$, valid for type II Cepheids and RV Tauri stars in the period range $0.9 < \log_{10}P < 1.75$. Assuming a distance modulus to the Large Magellanic Cloud of 18.5, the relation in terms of the absolute luminosities becomes: $M_{V} = -0.61 (\pm 0.20) - 2.95 (\pm 0.12) \log_{10}P + 5.49 (\pm 0.35) \bar{(V-R)_{\circ}}$., 35 pages, 20 PostScript figures. Submitted to the Astronomical Journal

We present the lightcurves of two microlensing events from the MACHO Project data that are likely to be due to lenses with masses similar to Jupiter's mass. Although the MACHO Project survey data are not sufficient to definitively establish the identification of planetary mass lenses in these cases, observations by microlensing follow-up networks such as GMAN and PLANET should be able to definitively determine the planetary nature of similar events which may occur in the near future., 5 pages, Latex, 5 postscript figures included, To appear in the Proceedings of "Planets Beyond the Solar System and the Next Generation of Space Missions", held at STScI, October 16-18, 1996

The MACHO project is searching for dark matter in the form of massive compact halo objects (Machos), by monitoring the brightness of millions of stars in the Magellanic Clouds to search for gravitational microlensing events. Analysis of our first 2.3 years of data for 8.5 million stars in the LMC yields 8 candidate microlensing events, well in excess of the $\approx 1$ event expected from lensing by known low-mass stars. The event timescales range from 34 to 145 days, and the estimated optical depth is $\sim$ 2 x 10^{-7}, about half of that expected from a `standard' halo. Likelihood analysis indicates the typical lens mass is $0.5^{+0.3}_{-0.2} \Msun$, suggesting they may be old white dwarfs., Latex, 12 pages, uses sprocl.sty (included), with 5 Postscript figures. To appear in proceedings of workshop on ``Identification of Dark Matter'', Sheffield, Sep. 1996

The approximately 1500 Cepheid variables in 22 LMC fields of the MACHO Project survey have been calibrated and analysed. In this paper, we report improved period ratios for a total of 73 beat Cepheids and provide a first look at the Fourier decomposition parameters for both singly- and doubly-periodic Cepheids. We also note an unusual amplitude-changing pulsator., 8 pages, 6 Encapsulated PostScript figures, uses conf_iap.sty. To appear in the Proceeding of the 12th IAP Colloquium "Variable Stars and the Astrophysical Returns of Microlensing Surveys". Author list corrected

The MACHO Project is a search for dark matter in the form of massive compact halo objects (Machos). Photometric monitoring of millions of stars in the Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and Galactic bulge is used to search for gravitational microlensing events caused by these otherwise invisible objects. Analysis of the first 2.1 years of photometry of 8.5 million stars in the LMC reveals 8 candidate microlensing events. This is substantially more than the number expected ($\sim 1.1$) from lensing by known stellar populations. The timescales ($\that$) of the events range from 34 to 145 days. We estimate the total microlensing optical depth towards the LMC from events with $2 < \that < 200$ days to be $��_2^{200} = 2.9 ^{+1.4}_{-0.9} \ten{-7}$ based upon our 8 event sample. This exceeds the optical depth, $��_{\rm backgnd} = 0.5 \ten{-7}$, expected from known stars, and the difference is to be compared with the optical depth predicted for a ``standard" halo composed entirely of Machos: $��_{halo} = 4.7\ten{-7}$. Likelihood analysis gives a fairly model independent estimate of the halo mass in Machos within 50 kpc of $2.0^{+1.2}_{-0.7} \ten{11} \msun$, about half of the ``standard halo" value. We also find a most probable Macho mass of $0.5^{+0.3}_{-0.2}\msun$, although this value is strongly model dependent. Additionally, the absence of short duration events places stringent upper limits on the contribution of low-mass Machos: objects from $10^{-4} \msun$ to $0.03 \msun$ contribute $\simlt 20\%$ of the ``standard" dark halo., Latex, 54 pages, uses aas2pp4.sty and astrobib.sty, with 24 out of 26 Postscript figures in gzipped tar file. 2 extra greyscale figures and/or full paper available from ftp://igpp.llnl.gov/pub/macho/LMC2/ Submitted to ApJ, June 1996

We report the discovery of 73 double-mode RR Lyrae (RRd) stars in fields near the bar of the LMC. The stars are detected among the MACHO database of short-period variables that currently contains about 7900 RR Lyrae stars. Fundamental periods (P_0) for these stars are found in the range 0.46-0.55 days and first overtone-to-fundamental period ratios are found to be in the range 0.742 < P_1/P_0 < 0.748. A significant fraction of our current sample have period ratios smaller than any previously discovered RRd variables. We present mean magnitudes, colors, and lightcurve properties for all LMC RRd stars detected to date. The range in period ratios is unexpectedly large. We present a determination of absolute magnitudes for these stars based primarily on pulsation theory and the assumption that all observed stars are at the fundamental blue edge (FBE) of the instability strip. Comparison of the calibrated MACHO V and R_KC photometry with these derived absolute magnitudes yields an absorption-corrected distance modulus to the LMC of 18.57 +/- 0.19 mag which is in good agreement with that found (18.5) through comparison of galactic and LMC Cepheids. Adopting this luminosity calibration, we derive an increase in the distance modulus, and thus a reduction in the age found via isochrone fitting for M15 of about 33% and discuss the implications for cosmology., 22 pages (text and figures), 4 embedded Postscript figures, uses aasms4.sty and apjpt4.sty, one three-page landscape TeX table. Submitted for publication in the ApJ. Updated abstract to correct error. Added landscape PostScript version of Table 1

We report the discovery of 30 type a,b RR Lyrae (RRab) which are likely members of the Sagittarius (Sgr) dwarf galaxy. Accurate positions, periods, amplitudes and magnitudes are presented. Their distances are determined with respect to RRab in the Galactic bulge found also in the MACHO 1993 data. For R$_{\odot} = 8$ kpc, the mean distance to these stars is $D = 22 \pm 1$ kpc, smaller than previous determinations for this galaxy. This indicates that Sgr has an elongated main body extending for more than 10 kpc, which is inclined along the line of sight, with its northern part (in Galactic coordinates) closer to us. The size and shape of Sgr give clues about the past history of this galaxy. If the shape of Sgr follows the direction of its orbit, the observed spatial orientation suggests that Sgr is moving away from the Galactic plane. Also, Sgr stars may be the sources of some of the microlensing events seen towards the bulge., Comment: 15 pages including 4 PostScript figures, uses aaspp4.sty. Also available at http://wwwmacho.anu.edu.au/ or http://wwwmacho.mcmaster.ca/

We report the discovery of two new R Coronae Borealis (RCB) stars in the Large Magellanic Cloud (LMC) using the MACHO project photometry database. The identification of both stars has been confirmed spectroscopically. One is a cool RCB star (T_eff about 5000 K) characterized by very strong Swan bands of C_2 and violet bands of CN, and weak or absent Balmer lines, G-band and 12C-13C bands. The second star is an example of a hot RCB star of which only 3 were previously known to exist in the Galaxy and none in the LMC. Its spectrum is characterized by several C II lines in emission. Both stars have shown deep declines of Delta V > 4 mag in brightness. The new stars are significantly fainter at maximum light than the three previously known LMC RCB stars. The amount of reddening toward these stars is somewhat uncertain but both seem to have absolute magnitudes, M_V, about half a magnitude fainter than the other three stars. Estimates of M_Bol find that the hot RCB star lies in the range of the other three stars while the cool RCB star is fainter. The two cool LMC RCB stars are the faintest at M_Bol. The discovery of these two new stars brings to five the number of known RCB stars in the LMC and demonstrates the utility of the MACHO photometric database for the discovery of new RCB stars., 29 pages, 8 embedded Postscript figures, uses aasms4.sty. Accepted for publication in the Oct 10 ApJ. Also available at http://wwwmacho.anu.edu.au/ or http://wwwmacho.mcmaster.ca/ Title and authorlist corrected

The MACHO collaboration reports on the analysis of our first year LMC data, 9.5 million light curves with an average of 235 observations each. Automated selection procedures give 3 events consistent with microlensing. We evaluate our experimental detection efficiency using a range of Monte- Carlo simulations. Using a `standard' halo density profile we find that a halo comprised entirely of Machos in the mass range 3 \ten{-4} to 0.06 \msun would predict > 15 detected events in this dataset; thus a standard spherical halo cannot be dominated by objects in this mass range. Assuming all three events are microlensing of halo objects and fitting a naive spherical halo model to our data yields a Macho halo fraction f =0.19+0.16-0.10, a total mass in Machos (inside 50 kpc) of 7.6+6-4 \ten{10} \msun, and a microlensing optical depth 8.8+7-5 \ten{-8} (68\% CL). Exploring a wide range of halo models we find that our constraints on the Macho fraction are quite model-dependent, but constraints on the total mass in Machos within 50 kpc are quite secure., 34 pages, uuencoded compressed tarred postscript, 14 figures included; Entire package including large image and important lightcurve figures (not included in above package) available via anonymous ftp at ftp://igpp.llnl.gov/pub/macho/LMC1/LMC1_all.uu, submitted to Astrophys. J

The detection of microlensing has opened the way for the development of new methods in galactic astronomy. This series of papers investigates what microlensing can teach us about the structure and shape of the dark halo. In this paper we present formulas for the microlensing rate, optical depth and event duration distributions for a simple set of axisymmetric disk-halo models. The halos are based on the "power--law models" which have simple velocity distributions. Using these models, we show that there is a large uncertainty in the predicted microlensing rate because of uncertainty in the halo parameters. For example, models which reproduce the measured galactic observables to within their errors still differ in microlensing rate towards the Magellanic Clouds by more than a factor of ten. We find that while the more easily computed optical depth correlates well with microlensing rate, the ratio of optical depth to rate can vary by a factor of two (or greater if the disk is maximal). Comparison of microlensing rates towards the Large and Small Magellanic Clouds (LMC and SMC) and M31 can be used to aid determinations of the halo flattening and rotation curve slope. For example, the ratio of microlensing rates towards the LMC and SMC is $\sim 0.7-0.8$ for E0 halos and $\sim 1.0 - 1.2$ for E7 halos (c.f. Sackett \& Gould 1993). Once the flattening has been established, the ratio of microlensing rates towards M31 and the LMC may help to distinguish between models with rising, flat or falling rotation curves. Comparison of rates along LMC and galactic bulge lines-of-sight gives useful information on the halo core radius, although this may not be so easy to extract in practice. Maximal disk models provide substantially smaller halo optical depths, shorter event durations and even larger model uncertainties., 19 pages, tex, 12 figures(uuencoded + text.ps)

We report the discovery of 45 beat Cepheids in the Large Magellanic Cloud (LMC) using the MACHO Project photometry database. The variables which are pulsating simultaneously in two radial modes are shown to break cleanly into two period-ratio groups, providing the first unambiguous evidence that the second overtone is indeed excited in real Cepheids. Thirty stars are beating in the fundamental and first overtone mode (F/1H, with a period ratio in the neighborhood of 0.72), and fifteen stars are beating in the first and second overtone (1H/2H, with a period ratio near 0.80). The F/1H period ratios are systematically higher than known Galactic beat Cepheids, indicating a metallicity dependence whose sense is in agreement with theory. Beat Cepheids in the LMC are found to select the 1H/2H mode for fundamental periods shorter than 1.25 days. We find the fraction of Cepheids excited in two modes to be about 20\% for stars with fundamental periods shorter than 2.5 days. We fail to confirm any of the proposed beat Cepheid candidates common to our sample from the surveys of Andreasen (1987) and Andreasen \& Petersen (1987). We also present finder charts and find several of the beat Cepheids to be in or near LMC clusters., Plain TeX, 21 page Tex output; title page (1 page), Table 1 (2 pages), Figures (13 pages) available at ftp://igpp.llnl.gov/pub/macho/ OR at http://meteor.anu.edu.au/~pjq/Publications/Cepheids/welch.html

We provide a status report on our search for dark matter in our Galaxy in the form of massive compact halo objects (or Machos), using gravitational microlensing of background stars. This search uses a very large format CCD camera on the dedicated 1.27m telescope at Mt.~Stromlo, Australia, and has been taking data for almost 3 years. At present, we are in the midst of analyzing our second year data for 8 million stars in the Large Magellanic Cloud. We find more microlensing events than expected from known stellar populations suggesting that Machos are indeed present in the Galactic halo, but the observed microlensing rate toward the LMC is too small to allow for a halo dominated by sub-stellar Machos. Our observations of the Galactic bulge have also yielded substantially more microlensing events than anticipated including a number of exotic ``deviant" microlensing events. The implications of these results are discussed., 12 pages, uuencoded gzip compressed postscript (356 kb). Figures included. To appear in the Proceedings of the Astronomical Society of the Pacific Symposium on Clusters, Lensing, and the Future of the Universe

The MACHO experiment is searching for dark matter in the halo of the Galaxy by monitoring more than 20 million stars in the LMC and Galactic bulge for gravitational microlensing events. The hardware consists of a 50 inch telescope, a two-color 32 megapixel ccd camera, and a network of computers. On clear nights the system generates up to 8 GB of raw data and 1 GB of reduced data. The computer system is responsible for all realtime control tasks, for data reduction, and for storing all data associated with each observation in a data base. The subject of this paper is the software system that handles these functions. It is an integrated system controlled by Petri nets that consists of multiple processes communicating via mailboxes and a bulletin board. The system is highly automated, readily extensible, and incorporates flexible error recovery capabilities. It is implemented with C++ in a Unix environment., Comment: 36 pages, 6 Postscript figures, uuencoded compressed tar. Submitted to PASP

We present Hubble Space Telescope (HST) WFPC2 photometry of 13 microlensed source stars from the 5.7 year Large Magellanic Cloud (LMC) survey conducted by the MACHO Project. 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. None of these sources is coincident with a background galaxy, which rules out the possibility that the MACHO LMC microlensing sample is contaminated with misidentified supernovae or AGN in galaxies behind the LMC. This supports the conclusion that the MACHO LMC microlensing sample has only a small amount of contamination due to non-microlensing forms of variability. We compare the WFPC2 source star magnitudes with the lensed flux predictions derived from microlensing fits to the light curve data. In most cases the source star brightness is accurately predicted. Finally, we develop a statistic which constrains the location of the Large Magellanic Cloud (LMC) microlensing source stars with respect to the distributions of stars and dust in the LMC and compare this to the predictions of various models of LMC microlensing. This test excludes at > 90% confidence level models where more than 80% of the source stars lie behind the LMC. Exotic models that attempt to explain the excess LMC microlensing optical depth seen by MACHO with a population of background sources are disfavored or excluded by this test. Models in which most of the lenses reside in a halo or spheroid distribution associated with either the Milky Way or the LMC are consistent which these data, but LMC halo or spheroid models are favored by the combined MACHO and EROS microlensing results., 28 pages with 10 included PDF figures, submitted to ApJ

We report on our search for microlensing towards the Large Magellanic Cloud (LMC). Analysis of 5.7 years of photometry on 11.9 million stars in the LMC reveals 13 - 17 microlensing events. This is significantly more than the $\sim$ 2 to 4 events expected from lensing by known stellar populations. The timescales ($\that$) of the events range from 34 to 230 days. We estimate the microlensing optical depth towards the LMC from events with $2 < \that < 400$ days to be 1.2 ^{+0.4}_ {-0.3} \ten{-7}$, with an additional 20% to 30% of systematic error. The spatial distribution of events is mildly inconsistent with LMC/LMC disk self-lensing, but is consistent with an extended lens distribution such as a Milky Way or LMC halo. Interpreted in the context of a Galactic dark matter halo, consisting partially of compact objects, a maximum likelihood analysis gives a MACHO halo fraction of 20% for a typical halo model with a 95% confidence interval of 8% to 50%. A 100% MACHO halo is ruled out at the 95% C.L. for all except our most extreme halo model. Interpreted as a Galactic halo population, the most likely MACHO mass is between $ 0.15 \msun$ and $ 0.9 \msun$, depending on the halo model, and the total mass in MACHOs out to 50 kpc is found to be 9+4-3 10^{10} msun, independent of the halo model. These results are marginally consistent with our previous results, but are lower by about a factor of two. Besides a larger data set, this work also includes an improved efficiency determination, improved likelihood analysis, and more thorough testing of systematic errors, especially with respect to the treatment of potential backgrounds to microlensing, such as supernovae in galaxies behind the LMC. [Abridged], 53 pages, Latex with 12 postscript figures, submitted to ApJ

We present preliminary results of the analysis of 5 years of MACHO data on the Galactic bulge microlensing events with clump giants as sources. In particular, we discuss: 1) the selection of `giant' events, 2) distribution of impact parameters, 3) distribution of event durations, 4) the concentration of long duration events in MACHO field 104 centered on (l,b) = (3.1,-3.0). We report the preliminary average optical depth of (2.0 +/- 0.4) x 10^{-6} (internal) at (l,b) = (3.9,-3.8). We discuss future work and prospects for building a coherent model of the Galaxy., 8 pages, 4 figures. To appear in ``Microlensing 2000: A New Era of Microlensing Astrophysics,'' an ASP conference proceedings, eds. J.W. Menzies and P.D. Sackett

Real-time detection of microlensing has moved from proof of concept in 1994 to a steady stream of events this year. Global dissemination of these events by the MACHO and OGLE collaborations has made possible intensive photometric and spectroscopic followup from widely dispersed sites confirming the microlensing hypothesis. Improved photometry and increased temporal resolution from followup observations greatly increases the possibility of detecting deviations from the standard point-source, point-lens, inertial motion microlensing model. These deviations are crucial in understanding individual lensing systems by breaking the degeneracy between lens mass, position and velocity. We report here on GMAN (Global Microlensing Alert Network), the coordinated followup of MACHO alerts., 6 pages, uuencoded compressed postscript, to appear int the Proceedings of the IAU Symposium 173: "Astrophysical Applications of Gravitational Lensing'', Kluwer Academic Publishers, Eds.: C. S. Kochanek and J. N. Hewitt

We present an analysis of the period distribution of $\sim 1800$ Cepheids in the Large Magellanic Cloud, based on data obtained by the MACHO microlensing experiment and on a previous catalogue by Payne-Gaposchkin. Using stellar evolution and pulsation models, we construct theoretical period-frequency distributions that are compared to the observations. These models reveal that a significant burst of star formation has occurred recently in the LMC ($\sim 1.15\times 10^8$ years). We also show that during the last $\sim 10^8$ years, the main center of star formation has been propagating from SE to NW along the bar. We find that the evolutionary masses of Cepheids are still smaller than pulsation masses by $\sim 7$ % and that the red edge of the Cepheid instability strip could be slightly bluer than indicated by theory. There are $\sim 600$ Cepheids with periods below $\sim 2.5$ days cannot be explained by evolution theory. We suggest that they are anomalous Cepheids; a number of these stars are double-mode Cepheids., Comment: 29 pages, 4 figures, uses AAS LATeX 4.0. To appear in Astronomical Journal Feb 1999