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Context. The instability strip (IS) of classical Cepheids has been extensively studied theoretically. Comparing the theoretical IS edges with those obtained empirically, using the most recent Cepheids catalogs available, can provide us with insights into the physical processes that determine the position of the IS boundaries. Aims. We aim to investigate the empirical positions of the IS of the classical Cepheids in the Large Magellanic Cloud (LMC), considering any effect that increases its width, to obtain intrinsic edges that can be compared with theoretical models. Methods. We used data of classical fundamental-mode (F) and first-overtone (1O) LMC Cepheids from the OGLE-IV variable star catalog, together with a recent high-resolution reddening map from the literature. Our final sample includes 2058 F and 1387 1O Cepheids. We studied their position on the Hertzsprung-Russell diagram and determined the IS borders by tracing the edges of the color distribution along the strip. Results. We obtained the blue and red edges of the IS in V- and I-photometric bands, in addition to log Teff and log L. The results obtained show a break located at the Cepheids' period of about three days, which was not reported before. We compared our empirical borders with theoretical ones published in the literature, obtaining a good agreement for specific parameter sets. Conclusions. The break in the IS borders is most likely explained by the depopulation of second- and third-crossing classical Cepheids in the faint part of the IS, since blue loops of evolutionary tracks in this mass range do not extend blueward enough to cross the IS at the LMC metallicity. Results from the comparison of our empirical borders with theoretical ones prove that our empirical IS is a useful tool for constraining theoretical models. [ABSTRACT FROM AUTHOR]

In the era of precision cosmology, it is essential to determine the Hubble constant to an accuracy of three per cent or better. At present, its uncertainty is dominated by the uncertainty in the distance to the Large Magellanic Cloud (LMC), which, being our second-closest galaxy, serves as the best anchor point for the cosmic distance scale. Observations of eclipsing binaries offer a unique opportunity to measure stellar parameters and distances precisely and accurately. The eclipsing-binary method was previously applied to the LMC, but the accuracy of the distance results was lessened by the need to model the bright, early-type systems used in those studies. Here we report determinations of the distances to eight long-period, late-type eclipsing systems in the LMC, composed of cool, giant stars. For these systems, we can accurately measure both the linear and the angular sizes of their components and avoid the most important problems related to the hot, early-type systems. The LMC distance that we derive from these systems (49.97 ± 0.19 (statistical) ± 1.11 (systematic) kiloparsecs) is accurate to 2.2 per cent and provides a firm base for a 3-per-cent determination of the Hubble constant, with prospects for improvement to 2 per cent in the future. [ABSTRACT FROM AUTHOR]

ABSTRACT High-precision () new JHK observations of 226 of the brightest and nearest red clump stars in the solar neighbourhood are used to determine distance moduli for the Large Magellanic Cloud (LMC). The resulting K- and H-band values of 18.47 ± 0.02 and 18.49 ± 0.06 imply that any correction to the K-band Cepheid PL relation due to metallicity differences between Cepheids in the LMC and the solar neighbourhood must be quite small. [ABSTRACT FROM AUTHOR]

We present a catalogue with coordinates and photometric data of 2446 Be star candidates in the Large Magellanic Cloud (LMC), based on a search of the OGLE II data base. The I-band light curves of these stars show outbursts in 24 per cent of the sample (Type-1 stars), high and low states in 10 per cent, periodic variations in 6 per cent (Type-3 stars), and stochastic variations in 60 per cent of the cases. We report on the result of the statistical study of light curves of Type-1 and Type-3 stars in the LMC, and the comparison with the previously reported results of the Small Magellanic Cloud (SMC) sample. We find a statistically significant difference between amplitude, duration and asymmetry distributions of outbursts in both galaxies. Outbursts of SMC Type-1 stars are usually brighter, longer and with a slower decline. We find a bimodal distribution of periods of Type-3 stars in both galaxies, probably related to the recently discovered double periodic blue variables. We find also period and amplitude distributions of Type-3 LMC stars statistically different from those of the SMC stars. Our findings above suggest that the mechanisms causing the observed photometric variability of Type-1 and Type-3 stars could depend on metallicity. Moreover, they suggest that the outbursts are not primarily caused by stellar winds. [ABSTRACT FROM AUTHOR]

Context. The Baade-Wesselink (BW) method, which combines linear and angular diameter variations, is the most common method to determine the distances to pulsating stars. However, the projection factor, p-factor, used to convert radial velocities into pulsation velocities, is still poorly calibrated. This parameter is critical on the use of this technique, and often leads to 5-10% uncertainties on the derived distances. Aims. We focus on empirically measuring the p-factor of a homogeneous sample of 29 LMC and 10 SMC Cepheids for which an accurate average distances were estimated from eclipsing binary systems. Methods. We used the SPIPS algorithm, which is an implementation of the BW technique. Unlike other conventional methods, SPIPS combines all observables, i.e. radial velocities, multi-band photometry and interferometry into a consistent physical modelling to estimate the parameters of the stars. The large number and their redundancy insure its robustness and improves the statistical precision. Results. We successfully estimated the p-factor of several Magellanic Cloud Cepheids. Combined with our previous Galactic results, we find the following P-p relation: -0:08±0:04(log P - 1:18) + 1:24±0:02. We find no evidence of a metallicity dependent p-factor. We also derive a new calibration of the period-radius relation, log R = 0:684±0:007(log P-0:517)+1:489±0:002, with an intrinsic dispersion of 0.020. We detect an infrared excess for all stars at 3:6 μm and 4:5 μm, which might be the signature of circumstellar dust. We measure a mean offset of Δm3.6 = 0:057 ± 0:006 mag and Δm4.5 = 0:065 ± 0:008 mag. Conclusions. We provide a new P-p relation based on a multi-wavelength fit that can be used for the distance scale calibration from the BW method. The dispersion is due to the LMC and SMC width we took into account because individual Cepheids distances are unknown. The new P-R relation has a small intrinsic dispersion: 4.5% in radius. This precision will allow us to accurately apply the BW method to nearby galaxies. Finally, the infrared excesses we detect again raise the issue of using mid-IR wavelengths to derive period-luminosity relation and to calibrate the Hubble constant. These IR excesses might be the signature of circumstellar dust, and are never taken into account when applying the BW method at those wavelengths. Our measured offsets may give an average bias of ~2:8% on the distances derived through mid-IR P-L relations. [ABSTRACT FROM AUTHOR]

We analyse long-term optical photometry for two Galactic recurrent novae (V745 Sco and V3890 Sgr) and one Large Magellanic Cloud object (Nova LMC 1990b) observed over several years by the Optical Gravitational Lensing Experiment (OGLE) sky survey. We do not find variability with the previously claimed orbital period of V745 Sco. This voids previous findings based on this value (e.g. the distance determination). The quiescence variability of this object is dominated by semiregular pulsations of the red giant secondary (with periods of 136.5 and 77.4 d). The photometry of Nova LMC 1990b reveals an unnoticed eruption in 2010 and eclipse-like variability in quiescence with a period of 1.26432(8) d. The photometric properties make this object very similar to U Sco. Finally, we describe the eruptions of two likely symbiotic novae, V5590 Sgr and OGLE-2011-BLG-1444. The secondary of V5590 Sgr is a Mira star with pulsation period of 236 d. [ABSTRACT FROM PUBLISHER]

We present a variability study of 4646 massive stars in the Small Magellanic Cloud (SMC) with known spectral types from the catalog of Bonanos et al. (2010) using the light curves from the OGLE-III database. The goal is to exploit the time domain information available through OGLE-III to gain insight into the processes that govern the evolution of massive stars. This variability survey of massive stars with known spectral types is larger than any previous survey by a factor of 7. We find that 60% of our sample (2766 stars) show no significant variability and 40% (1880 stars) exhibit variability distributed as follows: 807 stars display low-amplitude stochastic variability with fluctuations in the I-band of up to 0.05 mag, 443 stars present irregular variability of higher amplitude (76% of these are reported as variables for the first time), 205 are eclipsing binaries (including 101 newly discovered systems), 50 are candidate rotating variables, 126 are classical Cepheids, 188 stars exhibit short-term sinusoidal periodicity (P < 3 days) making them candidate "slowly pulsating B stars" and non-radial Be pulsators, and 61 periodic stars exhibit longer periods. We demonstrate the wealth of information provided in the time domain, by doubling the number of known massive eclipsing binary systems and identifying 189 new candidate early-type Be and 20 Oe stars in the SMC. In addition, we find that ~80% of Be stars are photometrically variable in the OGLE-III time domain and provide evidence that short-term pulsating stars with additional photometric variability are rotating close to their break-up velocity. [ABSTRACT FROM AUTHOR]

The Magellanic Quasars Survey (MQS) has now increased the number of quasars known behind the Magellanic Clouds by almost an order of magnitude. All survey fields in the Large Magellanic Cloud (LMC) and 70% of those in the Small Magellanic Cloud (SMC) have been observed. The targets were selected from the third phase of the Optical Gravitational Lensing Experiment (OGLE-III) based on their optical variability, mid-IR, and/or X-ray properties. We spectroscopically confirmed 758 quasars (565 in the LMC and 193 in the SMC) behind the clouds, of which 94% (527 in the LMC and 186 in the SMC) are newly identified. The MQS quasars have long-term (12 yr and growing for OGLE), high-cadence light curves, enabling unprecedented variability studies of quasars. The MQS quasars also provide a dense reference grid for measuring both the internal and bulk proper motions of the clouds, and 50 quasars are bright enough (I ≲ 18 mag) for absorption studies of the interstellar/intergalactic medium of the clouds. [ABSTRACT FROM AUTHOR]

The early B supergiant Large Magellanic Cloud (LMC) star BI 108 is photometrically variable with a unique light curve; two strong periods are present in an almost precise 3:2 resonance. We collected spectroscopic data at the Very Large Telescope (VLT) with the Ultraviolet and Visual Echelle Spectrograph (UVES), sampling the supercycle of 10.733 d in 10 epochs. We find spectral signatures for an SB2 system consisting of two massive B1 supergiants orbiting at the orbital period of 5.366 d. The shorter periodicity resembles the light curve of an eclipsing binary with periodicity 3.578 d that is not detected in the data. We discuss possible causes for the short periodicity and conclude that the quadruple system is the more plausible hypothesis. [ABSTRACT FROM AUTHOR]

We present a new Bayesian approach to constrain the intrinsic parameters (stellar mass and age) of the eclipsing binary system--CEP0227--in the Large Magellanic Cloud (LMC). We computed several sets of evolutionary models covering a broad range in chemical compositions and in stellar mass. Independent sets of models were also constructed either by neglecting or by including a moderate convective core overshooting (βov = 0.2) during central hydrogen-burning phases. Sets of models were also constructed either by neglecting or by assuming a canonical (η = 0.4,0.8) or an enhanced (η = 4) mass-loss rate. The most probable solutions were computed in three different planes: luminosity-temperature, mass-radius, and gravity-temperature. By using the Bayes factor, we found that the most probable solutions were obtained in the gravity-temperature plane with a Gaussian mass prior distribution. The evolutionary models constructed by assuming a moderate convective core overshooting (βov = 0.2) and a canonical mass-loss rate (η = 0.4) give stellar masses for the primary (Cepheid)--M = 4.14-0.05+0.04 Mʘ--and for the secondary--M = 4.15-0.05+0.04 Mʘ--that agree at the 1% level with dynamical measurements. Moreover, we found ages for the two components and for the combined system--t = 151-3+4 Myr--that agree at the 5% level. The solutions based on evolutionary models that neglect the mass loss attain similar parameters, while those ones based on models that either account for an enhanced mass loss or neglect convective core overshooting have lower Bayes factors and larger confidence intervals. The dependence on the mass-loss rate might be the consequence of the crude approximation we use to mimic this phenomenon. By using the isochrone of the most probable solution and a Gaussian prior on the LMC distance, we found a true distance modulus--18.53-0.02+0.02 mag--and a reddening value--E(B - V) = 0.142-0.010+0.005 mag--that agree quite well with similar estimates in the literature. [ABSTRACT FROM AUTHOR]

We quadruple the number of quasars known behind the Large Magellanic Cloud (LMC) from 56 (42 in the Optical Gravitational Lensing Experiment (OGLE)-III LMC fields) to 200 by spectroscopically confirming 169 (144 new) quasars from a sample of 845 observed candidates in four ~3 deg² Anglo-Australian Telescope/AAOmega fields south of the LMC center. The candidates were selected based on their Spitzer mid-infrared colors. X-ray emission, and/or optical variability properties in the database of the OGLE microlensing survey. The contaminating sources can be divided into 115 young stellar objects (YSOs), 17 planetary nebulae (PNe), 39 Be and 24 blue stars, 68 red stars, and 12 objects classed as either YSO/PN or blue star/YSO. There are also 402 targets with either featureless spectra or too low signal-to-noise ratios for source classification. Our quasar sample is 50% (30%) complete at I = 18.6 mag (19.3 mag). The newly discovered active galactic nuclei (AGNs) provide many additional reference points for proper motion studies of the LMC, and the sample includes 10 bright AGNs (I < 18 mag) that are potentially suitable for absorption line studies. Their primary use, however, is for detailed studies of quasar variability, as they all have long-term, high cadence, continuously growing light curves from the microlensing surveys of the LMC. Completing the existing Magellanic Quasars Survey fields in the LMC and Small Magellanic Cloud should yield a sample of ~700 well-monitored AGNs, and expanding it to the larger regions covered by the OGLE-IV survey should yield a sample of ~3600 AGNs. [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]

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]

We investigate the nature of OGLE 05155332−6925581, one of the brightest members of the enigmatic group of double periodic variables (DPVs) recently found in the Magellanic Clouds. The modelling of archival orbital light curves (LCs), along with the analysis of the radial velocities (RVs) suggest that this object is a semidetached binary with the less massive star transferring matter to the more massive and less evolved star, in an Algol-like configuration. We find evidence for additional orbital variability and Hα emission, likely caused by an accretion disc around the primary star. As in the case of β Lyr the circumprimary disc seems to be more luminous than the primary, but we do not detect orbital period changes. We find that the LC follows a loop in the colour–magnitude diagram during the long cycle; the system is redder when brighter and the rising phase is bluer than during decline. Infrared excess is also present. The source of the long-term periodicity is not eclipsed, indicating its circumbinary origin. Strong asymmetries, discrete absorption components (DACs) and a γ shift are new and essential observational properties in the infrared H i lines. The DACs strength and RV follow a saw-teeth pattern during the orbital cycle. We suggest that the system experiences supercycles of mass outflow feeding a circumbinary disc. Mass exchange and mass loss could produce comparable but opposite effects in the orbital period on a long time-scale, resulting in a quasi-constancy of this parameter. [ABSTRACT FROM AUTHOR]

We report the discovery of a young stellar bridge that forms a continuous connection between the Magellanic Clouds. This finding is based on number density maps for stellar populations found in data gathered by OGLE-IV that fully cover over 270 deg2 of the sky in the Magellanic Bridge area. This is the most extensive optical survey of this region to date. We find that the young population is present mainly in the western half of the MBR, which, together with the newly discovered young population in the eastern Bridge, form a continuous stream of stars connecting both galaxies along δ ∼ –73.5 deg. The young population distribution is clumped, with one of the major densities close to the SMC and the other fairly isolated and located approximately mid-way between the Clouds, which we call the OGLE island. These overdensities are well matched by H I surface density contours, although the newly found young population in the eastern Bridge is offset by ∼2 deg north from the highest H I density contour. We observe a continuity of red clump stars between the Magellanic Clouds which represent an intermediate-age population. Red clump stars are present mainly in the southern and central parts of the Magellanic Bridge, below its gaseous part, and their presence is reflected by a strong deviation from the radial density profiles of the two galaxies. This may indicate either a tidal stream of stars, or that the stellar halos of the two galaxies overlap. On the other hand, we do not observe such an overlap within an intermediate-age population represented by the top of the red giant branch and the asymptotic giant branch stars. We also see only minor mixing of the old populations of the Clouds in the southern part of the Bridge, represented by the lowest part of the red giant branch. [ABSTRACT FROM AUTHOR]