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We present the results of the UBVI C variability survey in the young open cluster NGC 6611 based on observations obtained during 34 nights spanning one year. In total, we found 95 variable stars. Most of these stars are classified as periodic and irregular pre-main sequence (PMS) stars. The analysis of the JHK S 2MASS photometry and four-colour IRAC photometry revealed 165 Class II young stellar sources, 20 of which are irregular variables and one is an eclipsing binary. These classifications, complemented by JHK UKIDSS photometry and riH α VPHAS photometry, were used to identify 24 candidates for classical T Tauri stars and 30 weak-lined T Tauri stars. In addition to the PMS variables, we discovered eight δ Scuti candidates. None of these were previously known. Furthermore, we detected 17 eclipsing binaries where two were previously known. Based on the proper motions provided by the Gaia EDR3 catalogue, we calculated the cluster membership probabilities for 91 variable stars. For 61 variables, a probability higher than 80 per cent was determined, which makes them cluster members. Only 25 variables with a probability less than 20 per cent were regarded to be non-members. [ABSTRACT FROM AUTHOR]

ABSTRACT In this fourth part of the series presenting the Optical Gravitational Lensing Experiment (OGLE) microlensing studies of the dark matter halo compact objects (MACHOs), we describe results of the OGLE-III monitoring of the Small Magellanic Cloud (SMC). Three sound candidates for microlensing events were found and yielded the optical depth τSMC-OIII= 1.30 ± 1.01 × 10−7, consistent with the expected contribution from Galactic disc and SMC self-lensing. We report that event OGLE-SMC-03 is most likely a thick-disc lens candidate, the first of such type found towards the SMC. In this paper we also combined all OGLE Large Magellanic Cloud and SMC microlensing results in order to refine the conclusions on MACHOs. All but one of the OGLE events are most likely caused by the lensing by known populations of stars; therefore, we concluded that there is no need for introducing any special dark matter compact objects in order to explain the observed event rates. Potential black hole event indicates that similar lenses can contribute only about 2 per cent to the total mass of the halo, which is still in agreement with the expected number of such objects. [ABSTRACT FROM AUTHOR]

We report the result of the analysis of the light curve of a caustic-crossing binary-lens microlensing event OGLE-2009-BLG-023/MOA-2009-BLG-028. Even though the event was observed solely by survey experiments, we could uniquely determine the mass of the lens and distance to it by simultaneously measuring the Einstein radius and lens parallax. From this, we find that the lens system is composed of M-type dwarfs with masses and located in the Galactic disc with a distance of toward the Galactic bulge direction. The event demonstrates that physical lens parameters of binary-lens events can be routinely determined from future high-cadence lensing surveys and thus microlensing can provide a new way to study Galactic binaries. [ABSTRACT FROM AUTHOR]

ABSTRACT In the third part of the series presenting the Optical Gravitational Lensing Experiment (OGLE) microlensing studies of the dark matter halo compact objects (MACHOs), we describe results of the OGLE-III monitoring of the Large Magellanic Cloud (LMC). This unprecedented data set contains almost continuous photometric coverage over 8 years of about 35 million objects spread over 40 deg2. We report a detection of two candidate microlensing events found with the automated pipeline and an additional two, less probable, candidate events found manually. The optical depth derived for the two main candidates was calculated following a detailed blending examination and detection efficiency determination and was found to be τ= (0.16 ± 0.12) × 10−7. If the microlensing signal we observe originates from MACHOs, then it means their masses are around 0.2 M⊙ and they comprise only f= 3 ± 2 per cent of the mass of the Galactic halo. However, the more likely explanation of our detections does not involve dark matter compact objects at all and relies on the natural effect of self-lensing of LMC stars by LMC lenses. In such a scenario, we can almost completely rule out MACHOs in the subsolar mass range with an upper limit at f < 7 per cent reaching its minimum of f < 4 per cent at M= 0.1 M⊙. For masses around M= 10 M⊙, the constraints on the MACHOs are more lenient with f∼ 20 per cent. Owing to limitations of the survey, there is no reasonable limit found for heavier masses, leaving only a tiny window of mass spectrum still available for dark matter compact objects. [ABSTRACT FROM AUTHOR]

The primary goal of this paper is to provide evidence that can prove true or false the hypothesis that dark matter in the Galactic halo can clump into stellar-mass compact objects. If such objects exist, they would act as lenses to external sources in the Magellanic Clouds, giving rise to an observable effect of microlensing. We present the results of our search for such events, based on data from the second phase of the OGLE survey (1996–2000) towards the Small Magellanic Cloud (SMC). The data set we used comprises 2.1 million monitored sources distributed over an area of 2.4 deg2. We found only one microlensing event candidate, however its poor-quality light curve limited our discussion of the exact distance to the lensing object. Given a single event, taking blending (crowding of stars) into account for the detection-efficiency simulations and deriving the Hubble Space Telescope ( HST)-corrected number of monitored stars, the microlensing optical depth is . This result is consistent with the expected SMC self-lensing signal, with no need to introduce dark matter microlenses. Rejecting the unconvincing event leads to an upper limit on the fraction of dark matter in the form of massive compact halo objects (MACHOs) of per cent for deflector masses around 0.4 and per cent for masses between 0.003 and 0.2 (95 per cent confidence limit). Our result indicates that the Milky Way's dark matter is unlikely to be clumpy and to form compact objects in the subsolar-mass range. [ABSTRACT FROM AUTHOR]

We present the results from the Optical Gravitational Lensing Experiment II (OGLE-II) survey (1996–2000) towards the Large Magellanic Cloud (LMC), which has the aim of detecting the microlensing phenomena caused by dark matter compact objects in the Galactic halo [massive compact halo objects (MACHOs)]. We use high-resolution Hubble Space Telescope images of the OGLE fields and derive the correction for the number of monitored stars in each field. This also yields blending distributions which we use in ‘catalogue-level’ Monte Carlo simulations of the microlensing events in order to calculate the detection efficiency of the events. We detect two candidates for microlensing events in the All Stars Sample, which translates into an optical depth of . If both events were due to MACHO, the fraction of mass of compact dark matter objects in the Galactic halo would be per cent. This optical depth, however, along with the characteristics of the events seems to be consistent with the self-lensing scenario, i.e. self-lensing alone is sufficient to explain the observed microlensing signal. Our results indicate the non-detection of MACHOs lensing towards the LMC with an upper limit on their abundance in the Galactic halo of 19 per cent for and 10 per cent for masses between 0.01 and . [ABSTRACT FROM AUTHOR]

Searches for extrasolar planets have uncovered an astonishing diversity of planetary systems, yet the frequency of solar system analogs remains unknown. The gravitational microlensing planet search method is potentially sensitive to multiple-planet systems containing analogs of all the solar system planets except Mercury. We report the detection of a multiple-planet system with microlensing. We identify two planets with masses of 0.71 and 0.27 times the mass of Jupiter and orbital separations of 2.3 and 4.6 astronomical units orbiting a primary star of mass 0.50 solar mass at a distance of 1.5 kiloparsecs. This system resembles a scaled version of our solar system in that the mass ratio, separation ratio, and equilibrium temperatures of the planets are similar to those of Jupiter and Saturn. These planets could not have been detected with other techniques; their discovery from only six confirmed microlensing planet detections suggests that solar system analogs may be common. [ABSTRACT FROM AUTHOR]

We present 97 QSO candidates in 48 Galactic bulge fields of the Optical Gravitational Lensing Experiment II (OGLE-II) covering∼11 deg2, which are selected via their variability. We extend light curves of variable objects which were detected in a 3-yr baseline in the OGLE-II variable star catalogue to fourth year. We search for objects that are faintand slowly variable over 4 yr in this catalogue by using the variogram/structure function. Finding the QSOs in such stellar-crowded and high extinction fields is challenging, but should be useful for the astrometric reference frame. Spectroscopic follow-up observations are required to confirm these QSO candidates. Follow-up observations are being prepared for four of these fields (BUL_SC1, 2, 32 and 45). Follow-up observations for other fields are strongly encouraged. The complete list and light curves of all 97 candidates are available in electronic format at. [ABSTRACT FROM AUTHOR]

We present a proper-motion (μ) catalogue of 5 080 236 stars in 49 Optical Gravitational Lensing Experiment II (OGLE-II) Galactic bulge (GB) fields, covering a range of and , the total area close to 11 deg2. The proper-motion measurements are based on 138–555 I-band images taken during four observing seasons: 1997–2000. The catalogue stars are in the magnitude range . In particular, the catalogue includes red clump giants and red giants in the GB, and main-sequence stars in the Galactic disc. The proper motions up to were measured with a mean accuracy of 0.8–3.5 mas yr−1, depending on the brightness of a star. This catalogue may be useful for studying the kinematics of stars in the GB and the Galactic disc. [ABSTRACT FROM AUTHOR]