Real-time difference imaging analysis of moa galactic bulge observations during 2000
01 Nov 2001-Monthly Notices of the Royal Astronomical Society (Oxford University Press)-Vol. 327, Iss: 3, pp 868-880
TL;DR: In this paper, the authors describe observations carried out by the MOA group of the Galactic bulge during 2000 that were designed to detect efficiently gravitational microlensing of faint stars in which the magnification is high and/or of short duration.
Abstract: We describe observations carried out by the MOA group of the Galactic bulge during 2000 that were designed to detect efficiently gravitational microlensing of faint stars in which the magnification is high and/or of short duration. These events are particularly useful for studies of extrasolar planets and faint stars. Approximately 17 deg2 were monitored at a sampling rate of up to six times per night. The images were analysed in real time using a difference imaging technique. 20 microlensing candidates were detected, of which eight were alerted to the microlensing community whilst in progress. Approximately half of the candidates had high magnifications (≳10), at least one had very high magnification (≳50), and one exhibited a clear parallax effect. The details of these events are reported here, together with details of the on-line difference imaging technique. Some nova-like events were also observed and these are described, together with one asteroid.
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Institut d'Astrophysique de Paris1, Heidelberg University2, European Southern Observatory3, University College London4, University of St Andrews5, University of Tasmania6, Niels Bohr Institute7, University of Warsaw8, University of Notre Dame9, University of Canterbury10, Space Telescope Science Institute11, Lawrence Livermore National Laboratory12, University of Rijeka13, University of Vienna14, University of Toulouse15, NASA Exoplanet Science Institute16, Osaka University17, University of Concepción18, University of Cambridge19
TL;DR: It is concluded that stars are orbited by planets as a rule, rather than the exception, and that of stars host Jupiter-mass planets 0.5–10 au (Sun–Earth distance) from their stars.
Abstract: Most known extrasolar planets (exoplanets) have been discovered using the radial velocity or transit methods. Both are biased towards planets that are relatively close to their parent stars, and studies find that around 17–30% of solar-like stars host a planet. Gravitational microlensing on the other hand, probes planets that are further away from their stars. Recently, a population of planets that are unbound or very far from their stars was discovered by microlensing. These planets are at least as numerous as the stars in the Milky Way. Here we report a statistical analysis of microlensing data (gathered in 2002–07) that reveals the fraction of bound planets 0.5–10 au (Sun–Earth distance) from their stars. We find that 17^(+16)_(-9)% of stars host Jupiter-mass planets (0.3–10 M_J, where M_J = 318 M_⊕ plus and M_⊕ plus is Earth’s mass). Cool Neptunes (10–30 M_⊕ plus) and super-Earths (5–10 M_⊕ plus) are even more common: their respective abundances per star are 52^(+22)_(-29)% and 62^(+35)_(-73)% . We conclude that stars are orbited by planets as a rule, rather than the exception.
623 citations
TL;DR: The discovery of a population of unbound or distant Jupiter-mass objects is reported, which are almost twice as common as main-sequence stars, based on two years of gravitational microlensing survey observations towards the Galactic Bulge.
Abstract: Gravitational microlensing observations in the direction of the Galactic Bulge have come up with a surprising result: the discovery of ten previously unknown extrasolar planets that are not bound to host stars. These seemingly free-ranging Jupiter-mass objects could be in very distant orbits around host stars, but no hosts could be detected within a distance of 10 astronomical units from the free-floating planets. It seems possible, therefore, that planet scattering is a routine part of the planet formation process.
560 citations
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01 May 2011TL;DR: In this paper, the authors present an overview of the solar system and its evolution, including the formation and evolution of stars, asteroids, and free-floating planets, as well as their internal and external structures.
Abstract: 1. Introduction 2. Radial velocities 3. Astrometry 4. Timing 5. Microlensing 6. Transits 7. Imaging 8. Host stars 9. Brown dwarfs and free-floating planets 10. Formation and evolution 11. Interiors and atmospheres 12. The Solar System Appendixes References Index.
527 citations
Ohio State University1, California Institute of Technology2, Mount Stromlo Observatory3, University of São Paulo4, INAF5, Chungbuk National University6, Carnegie Learning7, Weizmann Institute of Science8, University of Warsaw9, University of Notre Dame10, Massey University11, Osaka University12, Nagoya University13, Kyoto Sangyo University14
TL;DR: In this paper, a detailed elemental abundance analysis of 32 dwarfs and subgiant stars in the Galactic bulge is presented, based on high-resolution spectra obtained during gravitational microlensing events.
Abstract: Based on high-resolution spectra obtained during gravitational microlensing events we present a detailed elemental abundance analysis of 32 dwarf and subgiant stars in the Galactic bulge. Combined with the sample of 26 stars from the previous papers in this series, we now have 58 microlensed bulge dwarfs and subgiants that have been homogeneously analysed. The main characteristics of the sample and the findings that can be drawn are: (i) the metallicity distribution (MDF) is wide and spans all metallicities between [Fe/H] = −1.9 to +0.6; (ii) the dip in the MDF around solar metallicity that was apparent in our previous analysis of a smaller sample (26 microlensed stars) is no longer evident; instead it has a complex structure and indications of multiple components are starting to emerge. A tentative interpretation is that there could be different stellar populations at interplay, each with a different scale height: the thin disk, the thick disk, and a bar population; (iii) the stars with [Fe/H] ≲ −0.1 are old with ages between 10 and 12 Gyr; (iv) the metal-rich stars with [Fe/H] ≳ −0.1 show a wide variety of ages, ranging from 2 to 12 Gyr with a distribution that has a dominant peak around 4−5 Gyr and a tail towards higher ages; (v) there are indications in the [α/Fe]−[Fe/H] abundance trends that the “knee” occurs around [Fe/H] = −0.3 to −0.2, which is a slightly higher metallicity as compared to the “knee” for the local thick disk. This suggests that the chemical enrichment of the metal-poor bulge has been somewhat faster than what is observed for the local thick disk. The results from the microlensed bulge dwarf stars in combination with other findings in the literature, in particular the evidence that the bulge has cylindrical rotation, indicate that the Milky Way could be an almost pure disk galaxy. The bulge would then just be a conglomerate of the other Galactic stellar populations (thin disk, thick disk, halo, and ...?), residing together in the central parts of the Galaxy, influenced by the Galactic bar.
470 citations
TL;DR: The OGLE-2003-BLG-262 microlensing event as mentioned in this paper, a relatively short (tE ¼ 12:5 � 0:1 day) microlens event generated by a point-mass lens transiting the face of a K giant source in the Galactic bulge, is the only published event to date in which the lens transits the source.
Abstract: We analyze OGLE-2003-BLG-262, a relatively short (tE ¼ 12:5 � 0:1 day) microlensing event generated by a point-mass lens transiting the face of a K giant source in the Galactic bulge. We use the resulting finite-source effects to measure the angular Einstein radius, � E ¼ 195 � 17 � as, and so constrain the lens mass to the FWHM interval 0:08 < M =M� < 0:54. The lens-source relative proper motion isrel ¼ 27 � 2k m s � 1 kpc � 1 .B oth values are typical of what is expected for lenses detected toward the bulge. Despite the short duration of the event, we detect marginal evidence for a ''parallax asymmetry'' but argue that this is more likely to be induced by acceleration of the source, a binary lens, or possibly by statistical fluctuations. Although OGLE-2003-BLG-262 is only the second published event to date in which the lens transits the source, such events will become more common with the new OGLE-III survey in place. We therefore give a detailed account of the analysis of this event to facilitate the study of future events of this type.
396 citations
Cites methods from "Real-time difference imaging analys..."
...…Search (MPS, Rhie et al. 1999), and the Microlensing Follow-Up Network (µFUN, http://www.astronomy.ohio-state.edu/∼microfun/) intensively monitor alerts from EWS and also from the Microlensing Observations in Astrophysics collaboration (MOA, Bond et al. 2001), primarily to search for planets....
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References
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TL;DR: A simple model of microlensing by massive objects that might be present in the halo of the Galaxy is presented in this article, where it is shown that in any nearby galaxy one star out of a million is strongly microlensed by a "dark" object located in the Galactic halo, if the hale is made up of objects more massive than about 10 to the -8th solar mass.
Abstract: A simple model of microlensing by massive objects that might be present in the halo of the Galaxy is presented. It is shown that in any nearby galaxy one star out of a million is strongly microlensed by a 'dark' object located in the Galactic halo, if the halo is made up of objects more massive than about 10 to the -8th solar mass. Monitoring the brightness of a few million stars in the Magellanic Clouds over a time scale between two hours and two years may lead to a discovery of 'dark halo' objects between 10 to the -6th and 10 to the -2nd solar mass, or it may put strong upper limits on the number of such objects.
1,437 citations
"Real-time difference imaging analys..." refers methods in this paper
...…= Fbase Nref ∑ i A(u(ti)) (9) The amplification, A(u), expressed in terms of the distance, u, of the lens star from the line-of-sight to the source star expressed in units of the Einstein radius is given by (Paczyński 1986) A(u) = u2 + 2 u √ u2 + 4 (10) where u(t) = √ u2min + ( t − tmax tE )2 ....
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TL;DR: In this article, a point-spread function (PSF) fitting photometry program, DOPHOT, is described and the quality of the resulting photometry is assessed via reductions of an 'artificial' globular cluster generated from a list of stars with known magnitudes and colors.
Abstract: The design considerations and operational features of DOPHOT, a point-spread function (PSF) fitting photometry program, are described. Some relevant details of the PSF fitting are discussed. The quality of the photometry returned by DOPHOT is assessed via reductions of an 'artificial' globular cluster generated from a list of stars with known magnitudes and colors. Results from comparative tests between DOPHOT and DAOPHOT using this synthetic cluster and real data are also described.
896 citations
"Real-time difference imaging analys..." refers methods in this paper
...The Dophot procedure was applied to the corresponding MOA reference images and the resulting star positions were cross-referenced with those of stars with UBVI measurements....
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...The photometry measurements were based on profile fitting software such as Dophot (Schechter et al. 1993) which determines fluxes by integrating over the profile of a given star on a given image....
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...The reference flux may be measured by applying a profile fitting program such as Dophot to the reference image....
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TL;DR: In this article, a microlensing experiment was conducted to determine whether the dark matter halo of our Galaxy is made up of massive compact halo objects (MACHOs), known collectively as brown dwarfs or "jupiters" (bodies too small to produce their own energy by fusion), neutron stars, old white dwarfs, or black holes.
Abstract: THERE is now abundant evidence for the presence of large quantities of unseen matter surrounding normal galaxies, including our own1,2. The nature of this ’dark matter‘ is unknown, except that it cannot be made of normal stars, dust or gas, as they would be easily detected. Exotic particles such as axions, massive neutrinos or other weakly interacting massive particles (collectively known as WIMPs) have been proposed3,4, but have yet to be detected. A less exotic alternative is normal matter in the form of bodies with masses ranging from that of a large planet to a few solar masses. Such objects, known collectively as massive compact halo objects5 (MACHOs), might be brown dwarfs or ‘jupiters’ (bodies too small to produce their own energy by fusion), neutron stars, old white dwarfs or black holes. Paczynski6 suggested that MACHOs might act as gravitational microlenses, temporarily amplifying the apparent brightness of background stars in nearby galaxies. We are conducting a microlensing experiment to determine whether the dark matter halo of our Galaxy is made up of MACHOs. Here we report a candidate for such a microlensing event, detected by monitoring the light curves of 1.8 million stars in the Large Magellanic Cloud for one year. The light curve shows no variation for most of the year of data taking, and an upward excursion lasting over 1 month, with a maximum increase of ∼2 mag. The most probable lens mass, inferred from the duration of the candidate lensing event, is ∼0.1 solar mass.
657 citations
"Real-time difference imaging analys..." refers background in this paper
...The lower right image is the same subtracted image with the positions of saturated stars masked out. approach was adopted by Alcock et al. (1999a) in their reanalysis of a subset of the MACHO data....
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...Recent re-analyses of the MACHO database using DIA have increased the microlensing detection rate and improved the quality of the photometry (Alcock et al. 1999a; 1999b; 2000)....
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...The observational strategy employed by MOA for the Galactic Bulge in 2000 was somewhat different from that of the established survey groups MACHO, OGLE, and EROS....
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...The approach adopted by the MACHO collaboration was to divide their images into small sub-rasters and solve for the convolution kernel separately on each of these sub-rasters assuming no significant variations in the kernel occurred on their length scales (Alcock et al. 1999a)....
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...This is a large fraction of high magnification events when compared with the corresponding fractions from the MACHO and OGLE surveys....
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TL;DR: Alcock et al. as discussed by the authors detected two possible microlensing events in the Large Magellanic Cloud (LMC) by observing the brightness of three million stars for over three years, and they detected three stars for which the brightness increased by two magnitudes over an interval of roughly one month.
Abstract: THE flat rotation curves of spiral galaxies, including our own, indicate that they are surrounded by unseen haloes of ‘dark matter’1,2. In the absence of a massive halo, stars and gas in the outer portions of a galaxy would orbit the centre more slowly, just as the outer planets in the Solar System circle the Sun more slowly than the inner ones. So far, however, there has been no direct observational evidence for the dark matter, or its characteristics. Paczynski3suggested that dark bodies in the halo of our Galaxy can be detected when they act as gravitational ‘microlenses’, amplifying the light from stars in nearby galaxies. The duration of such an event depends on the mass, distance and velocity of the dark object. We have been monitoring the brightness of three million stars in the Large Magellanic Cloud for over three years, and here report the detection of two possible microlensing events. The brightening of the stars was symmetrical in time, achromatic and not repeated during the monitoring period. The timescales of the two events are about thirty days and imply that the masses of the lensing objects lie between a few hundredths and one solar mass. The number of events observed is consistent with the number expected if the halo is dominated by objects with masses in this range. In 1986, Bohdan Paczynski suggested that dark, compact objects in the halo of our Galaxy — the local cousins of similar objects conjectured to contribute dark matter to the haloes of other galaxies — could be detected on Earth, as their gravitation might occasionally amplify the light arriving from more distant stars. In 1993, in surveys of several million stars in the Large Magellanic Cloud, C. Alcock et al. and E. Aubourg et al. observed several candidate events. In total, the two groups detected three stars for which the brightness increased by two magnitudes over an interval of roughly one month.
585 citations
"Real-time difference imaging analys..." refers background in this paper
...The observational strategy employed by MOA for the Galactic Bulge in 2000 was somewhat different from that of the established survey groups MACHO, OGLE, and EROS....
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...More than 500 microlensing events, most of them alerted in real time, have been identified by the microlensing survey groups MACHO (Alcock et al. 1993), OGLE (Udalski et al. 1992), and EROS (Aubourg et al. 1993)....
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TL;DR: In this article, a procedure for estimating the ranges of parameters that describe the spectra of X-rays from clusters of galaxies is presented, which can be applied in more general problems of parameter estimation.
Abstract: A procedure for estimating the ranges of parameters that describe the spectra of X-rays from clusters of galaxies is presented. The applicability of the method is proved by statistical simulations of cluster spectra; such a proof is necessary because of the nonlinearity of the spectral functions. Implications for the spectra of the Perseus, Coma, and Virgo clusters are discussed. The procedure can be applied in more general problems of parameter estimation.
581 citations
"Real-time difference imaging analys..." refers background in this paper
...These formally correspond to confidence limits of 68%, 95%, and 99.73% respectively for the simultaneous estimation of two interesting parameters (Avni 1976)....
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