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1.
We present a systematic search for parallax microlensing events among a total of 512 microlensing candidates in the OGLE II data base for the  1997–1999  seasons. We fit each microlensing candidate with both the standard microlensing model and a parallax model that accounts for the Earth's motion around the Sun. We then search for the parallax signature by comparing the χ 2 of the standard and parallax models. For the events which show a significant improvement, we further use the 'duration' of the event and the signal-to-noise ratio as criteria to separate true parallax events from other noisy microlensing events. We have discovered one convincing new candidate, sc33_4505, and seven other marginal cases. The convincing candidate (sc33_4505) is caused by a slow-moving, and likely low-mass, object, similar to other known parallax events. We found that irregular sampling and gaps between observing seasons hamper the recovery of parallax events. We have also searched for long-duration events that do not show parallax signatures. The lack of parallax effects in a microlensing event puts a lower limit on the Einstein radius projected on to the observer plane, which in turn imposes a lower limit on the lens mass divided by the relative lens–source parallax. Most of the constraints are however quite weak.  相似文献   

2.
Extended source size effects have been detected in photometric monitoring of gravitational microlensing events. We study similar effects in the centroid motion of an extended source lensed by a point mass. We show that the centroid motion of a source with uniform surface brightness can be obtained analytically. For a source with a circularly symmetric limb-darkening profile, the centroid motion can be expressed as a one-dimensional integral, which can be evaluated numerically. We find that when the impact parameter is comparable to the source radius, the centroid motion is significantly modified by the finite source size. In particular, when the impact parameter is smaller than the source radius, the trajectories become clover-leaf like. Such astrometric motions can be detected using space interferometers such as the Space Interferometry Mission . Such measurements offer exciting possibilities for determining stellar parameters, such as stellar radius, to excellent accuracy.  相似文献   

3.
Microlensing events are usually selected among single-peaked non-repeating light curves in order to avoid confusion with variable stars. However, a microlensing event may exhibit a second microlensing brightening episode when the source or/and the lens is a binary system. A careful analysis of these repeating events provides an independent way to study the statistics of wide binary stars and to detect extrasolar planets. Previous theoretical studies predicted that 0.5–2 per cent of events should repeat due to wide binary lenses. We present a systematic search for such events in about 4000 light curves of microlensing candidates detected by the Optical Gravitational Lensing Experiment (OGLE) towards the Galactic bulge from 1992 to 2007. The search reveals a total of 19 repeating candidates, with six clearly due to a wide binary lens. As a by-product, we find that 64 events (∼2 per cent of the total OGLE-III sample) have been misclassified as microlensing; these misclassified events are mostly nova or other types of eruptive stars. The number and importance of repeating events will increase considerably when the next-generation wide-field microlensing experiments become fully operational in the future.  相似文献   

4.
Interferometers from the ground and space will be able to resolve the two images in a microlensing event. This will at least partially lift the inherent degeneracy between physical parameters in microlensing events. To increase the signal-to-noise ratio, intrinsically bright events with large magnifications will be preferentially selected as targets. These events may be influenced by finite source size effects both photometrically and astrometrically. Using observed finite source size events as examples, we show that the fringe visibility can be affected by ∼5–10 per cent, and the closure phase by a few degrees – readily detectable by ground and space interferometers. Such detections will offer unique information about the lens–source trajectory relative to the baseline of the interferometers. Combined with photometric finite source size effects, interferometry offers a way to measure the angular sizes of the source and the Einstein radius accurately. Limb-darkening changes the visibility by a small amount compared with a source with uniform surface brightness, marginally detectable with ground-based instruments. We discuss the implications of our results for the plans to make interferometric observations of future microlensing events.  相似文献   

5.
We determine the most likely values of the free parameters of an N -body model for the Galaxy developed by Fux via a discrete–discrete comparison with the positions on the sky and line-of-sight velocities of an unbiased, homogeneous sample of OH/IR stars. Via Monte Carlo simulation, we find the plausibility of the best-fitting models, as well as the errors on the determined values. The parameters that are constrained best by these projected data are the total mass of the model and the viewing angle of the central bar, although the distribution of the latter has multiple maxima. The other two free parameters, the size of the bar and the (azimuthal) velocity of the Sun, are less well-constrained. The best model has a viewing angle of ∼ 44°, a semimajor axis of 2.5 kpc (corotation radius 4.5 kpc, pattern speed 46 km s−1 kpc−1), a bar mass of 1.7×1010 M⊙ and a tangential velocity of the local standard of rest of 171 km s−1. We argue that the lower values that are commonly found from stellar data for the viewing angle (∼25°) arise when too few coordinates are available, when the longitude range is too narrow or when low latitudes are excluded from the fit. The new constraints on the viewing angle of the Galactic bar from stellar line-of-sight velocities decrease further the ability of the distribution of the bar to account for the observed microlensing optical depth toward Baade's window: our model reproduces only half the observed value. The signal of triaxiality diminishes quickly with increasing latitude, fading within approximately 1 scaleheight (≲3°). This suggests that Baade's window is not a very appropriate region in which to sample bar properties.  相似文献   

6.
Recently, Heyrovský & Sasselov investigated the sensitivity of single-lens gravitational microlensing event light curves to spots and found that, during source transit, spots can cause deviations in amplification larger than 2 per cent, and thus be detectable. In this paper, we explore the feasibility of spot detection from the observations of binary-lens microlensing events instead of single-lens events. For this we investigate the sensitivity of binary-lens event light curves to spots and compare it with that of single-lens events. From this investigation, we find that during caustic crossings the fractional amplification deviations of light curves from those of spotless source events are equivalent to those of single-lens events, implying that spots can also be detected with a similar photometric precision to that required for spot detection by observing single-lens events. We discuss the relative advantages of observing binary-lens events over the observations of single-lens events in detecting stellar spots.  相似文献   

7.
In recent years, it has been shown that microlensing is a powerful tool for examining the atmospheres of stars in the Galactic bulge and Magellanic Clouds. The high gradient of magnification across the source during both small impact parameter events and caustic crossings offers a unique opportunity for determining the surface brightness profile of the source. Furthermore, models indicate that these events can also provide an appreciable polarization signal: arising from differential magnification across the otherwise symmetric source. Earlier work has addressed the signal from a scattering photosphere for both point mass lenses and caustic crossings. In a previous paper, polarimetric variations from point lensing of a circumstellar envelope were considered, as would be suitable for an extended envelope around a red giant. In this work, we examine the polarization in the context of caustic crossing events, the scenario that represents the most easily accessible situation for actually observing a polarization signal in Galactic microlensing. Furthermore, we present an analysis of the effectiveness of using the polarimetric data to determine the envelope properties, illustrating the potential of employing polarimetry in addition to photometry and spectroscopy with microlensing follow-up campaigns.  相似文献   

8.
The measurement of isotopic ratios provides a privileged insight both into nucleosynthesis and into the mechanisms operating in stellar envelopes, such as gravitational settling. In this article, we give a few examples of how isotopic ratios can be determined from high‐resolution, high‐quality stellar spectra. We consider examples of the lightest elements, H and He, for which the isotopic shifts are very large and easily measurable, and examples of heavier elements for which the determination of isotopic ratios is more difficult. The presence of 6Li in the stellar atmospheres causes a subtle extra depression in the red wing of the 7Li 670.7 nm doublet which can only be detected in spectra of the highest quality. But even with the best spectra, the derived 6Li abundance can only be as good as the synthetic spectra used for their interpretation. It is now known that 3D non‐LTE modelling of the lithium spectral line profiles is necessary to account properly for the intrinsic line asymmetry, which is produced by convective flows in the atmospheres of cool stars, and can mimic the presence of 6Li. We also discuss briefly the case of the carbon isotopic ratio in metal‐poor stars, and provide a new determination of the nickel isotopic ratios in the solar atmosphere. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

9.
We present very low-mass stellar models as computed using non-grey model atmospheres for selected assumptions about the stellar metallicities. The role of atmospheres is discussed and the models are compared with models based on the Eddington approximation, and with similar models that have appeared in the recent literature. Theoretical predictions concerning both the HR diagram location and the mass–luminosity relation are presented and discussed in terms of expectations in selected photometric bands. Comparison with available observational data concerning both galactic globular clusters and dwarfs in the solar neighbourhood reveals a satisfactory agreement together with the existence of some residual mismatches.  相似文献   

10.
Cold, dense clouds of gas have been proposed to explain the dark matter in Galactic haloes, and have also been invoked in the Galactic disc as an explanation for the excess faint submillimetre sources detected by SCUBA. Even if their dust-to-gas ratio is only a small percentage of that in conventional gas clouds, these dense systems would be opaque to visible radiation. We examine the possibility that the data sets of microlensing experiments searching for massive compact halo objects can also be used to search for occultation signatures by such clouds. We compute the rate and time-scale distribution of stellar transits by clouds in the Galactic disc and halo. We find that, for cloud parameters typically advocated by theoretical models, thousands of transit events should already exist within microlensing survey data sets. We examine the seasonal modulation in the rate caused by the Earth's orbital motion and find it provides an excellent probe of whether detected clouds are of disc or halo origin.  相似文献   

11.
1 INTRODUCTIONObservations of the most famous starburst galaxy M82 with the High-Resolution Cameraon board the Chandra X-Ray Observatory, showed that there are nine sources in the celltral11 x 11 region, but no son-roe was detected at the galactic center (Matsumoto et al. 2001).Comparing the observations on 1999 October 28 and those on 2000 January 20, the authorsfound an extremely large time variability of the source CXO M82 J095550.2 694047, which islocated gll away from the galact…  相似文献   

12.
The far-UV spectrum of the T Tauri stars (TTSs) provides important clues about the structure of the stellar atmospheres, winds and accretion shocks. The IUE ( International Ultraviolet Explorer ) Final Archive contains the most complete data base for such studies. A new extraction system, the IUE Newly Extracted Spectra ( ines ), has been developed to overcome the disadvantages of the extraction system used in the IUE Final Archive, the Signal Weighted Extraction Technique ( swet ). We have compared the ines spectra of the whole sample of TTSs in the far-UV range (1200–2000 Å) with the swet low-resolution spectra available in the IUE Final Archive. Although in most of the cases there is a good agreement between both samples, an important enhancement of the ines line fluxes with respect to the swet line fluxes is reported for particular spectra. The line fluxes are enhanced by as much as a factor of ∼2.5 in some objects, which is significant for variability studies of TTSs because the variations of the UV lines are typically of this order. The emission-measure distributions built to study the atmospheres of these stars are based on the UV emission line fluxes, so the new system is susceptible to introduce changes in these models. Moreover, the non-linear enhancement of the ines line fluxes produces variations in diagnostic line ratios usually taken as temperature and density tracers in late-type stars. These line ratios can vary by as much as a factor of 3 when the ines data are compared with the swet , with the subsequent variation of the physical parameters derived from them.  相似文献   

13.
We outline a method for fitting binary-lens caustic-crossing microlensing events based on the alternative model parametrization proposed and detailed by Cassan. As an illustration of our methodology, we present an analysis of OGLE-2007-BLG-472, a double-peaked Galactic microlensing event with a source crossing the whole caustic structure in less than three days. In order to identify all possible models we conduct an extensive search of the parameter space, followed by a refinement of the parameters with a Markov Chain Monte Carlo algorithm. We find a number of low-  χ2  regions in the parameter space, which lead to several distinct competitive best models. We examine the parameters for each of them, and estimate their physical properties. We find that our fitting strategy locates several minima that are difficult to find with other modelling strategies and is therefore a more appropriate method to fit this type of event.  相似文献   

14.
Using the stellar photometry catalogue based on the latest data release (DR4) of the Sloan Digital Sky Survey (SDSS), a study of the Galactic structure using star counts is carried out for selected areas of the sky. The sample areas are selected along a circle at a Galactic latitude of +60°, and 10 strips of high Galactic latitude along different longitudes. Direct statistics of the data show that the surface densities of ℓ from 180° to 360° are systematically higher than those of ℓ from 0° to 180°, defining a region of overdensity (in the direction of Virgo) and another one of underdensity (in the direction of Ursa Major) with respect to an axisymmetric model. It is shown by comparing the results from star counts in the ( g − r ) colour that the density deviations are due to an asymmetry of the stellar density in the halo. Theoretical models for the surface density profile are built and star counts are performed using a triaxial halo of which the parameters are constrained by observational data. Two possible reasons for the asymmetric structure are discussed.  相似文献   

15.
We present high-cadence, high-precision multiband photometry of the young, M1Ve, debris disc star, AU Microscopii. The data were obtained in three continuum filters spanning a wavelength range from 4500 to 6600 Å, plus Hα, over 28 nights in 2005. The light curves show intrinsic stellar variability due to star-spots with an amplitude in the blue band of 0.051 mag and a period of 4.847 d. In addition, three large flares were detected in the data which all occur near the minimum brightness of the star. We remove the intrinsic stellar variability and combine the light curves of all the filters in order to search for transits by possible planetary companions orbiting in the plane of the nearly edge-on debris disc. The combined final light curve has a sampling of 0.35 min and a standard deviation of 6.8 mmag. We performed Monte Carlo simulations by adding fake transits to the observed light curve and find with 95 per cent significance that there are no Jupiter mass planets orbiting in the plane of the debris disc on circular orbits with periods,   P ≤ 5  d. In addition, there are no young Neptune like planets (with radii 2.5 times smaller than the young Jupiter) on circular orbits with periods,   P ≤ 3  d.  相似文献   

16.
Sulphur is a volatile α ‐element which is not locked into dust grains in the interstellar medium (ISM). Hence, its abundance does not need to be corrected for dust depletion when comparing the ISM to the stellar atmospheres. The abundance of sulphur in the photosphere of metal‐poor stars is a matter of debate: according to some authors, [S/Fe] versus [Fe/H] forms a plateau at low metallicity, while, according to other studies, there is a large scatter or perhaps a bimodal distribution. In metal‐poor stars sulphur is detectable by its lines of multiplet 1 at 920 nm, but this range is heavily contaminated by telluric absorptions, and one line of the multiplet is blended by the hydrogen Paschen ζ line. We study the possibility of using multiplet 3 (at 1045 nm) for deriving the sulphur abundance because this range, now observable at the VLT with the infra‐red spectrograph CRIRES, is little contaminated by telluric absorption and not affected by blends at least in metal‐poor stars. We compare the abundances derived from multiplets 1 and 3, taking into account NLTE corrections and 3D effects. Here we present the results for a sample of four stars, although the scatter is less pronounced than in previous analysis, we cannot find a plateau in [S/Fe], and confirm the scatter of the sulphur abundance at low metallicity (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

17.
We study the proposed use of parallax microlensing in the direction of the Large Magellanic Cloud (LMC) to separate the effects of the mass function of dark massive halo objects (MHOs or 'machos') on the one hand, and their spatial distribution and kinematics on the other. This disentanglement is supposed to allow a much better determination of the two than could be achieved entirely on the basis of the durations of events. We restrict our treatment to the same class of power-law spherical models for the halo of MHOs studied in a previous paper by Marković 38 Sommer-Larsen, and assume that one can eliminate microlensing events caused by massive objects outside the halo (e.g., the LMC halo). Whereas the duration-based error in the average MHO mass, μ¯ ≡  M ¯/M, exceeds (at N  = 100 events) μ¯ by a factor of 2 or more, parallax microlensing remarkably brings it down to 15–20 per cent of μ¯, regardless of the shape of the mass function. In addition, the slope α of the mass function, d n /dμ ∝ μα, can be inferred relatively accurately (σα < 0.4) for a broader range, −3 < α < 0. The improvement in the inference of the halo structure is also significant: the index γ of the density profile ( ρ ∼  R −γ) can be obtained with the error σγ < 0.4. While in a typical situation the errors for the parameters specifying the velocity dispersion profile are of about the same magnitude as the parameters themselves, virtually all the uncertainty is 'concentrated' in linear combinations of the parameters that may have little influence on the profile, thus allowing its reasonably accurate inference.  相似文献   

18.
The presence of heavy elements in the atmospheres of the hottest H-rich DA white dwarfs has been the subject of considerable interest. While theoretical calculations can demonstrate that radiative forces, counteracting the effects of gravitational settling, can explain the detections of individual species, the predicted abundances do not accord well with observation. However, accurate abundance measurements can only be based on a thorough understanding of the physical structure of the white dwarf photospheres, which has proved elusive. Recently, the availability of new non-local thermodynamic equilibrium model atmospheres with improved atomic data has allowed self-consistent analysis of the extreme ultraviolet (EUV), far UV and optical spectra of the prototypical object G191-B2B. Even so, the predicted and observed stellar fluxes remain in serious disagreement at the shortest wavelengths (below ≈190 Å), while the inferred abundances remain largely unaltered. We show here that the complete spectrum of G191-B2B can be explained by a model atmosphere where Fe is stratified, with increasing abundance at greater depth. This abundance profile may explain the difficulties in matching observed photospheric abundances, usually obtained by analyses using homogeneous model atmospheres, to the detailed radiative levitation predictions, particularly as the latter are only strictly valid for regions deeper than where the EUV/far UV lines and continua are formed. Furthermore, the relative depletion of Fe in the outer layers of the atmosphere may be evidence for radiatively driven mass-loss in G191-B2B.  相似文献   

19.
We describe the discovery of the longest microlensing event ever observed, OGLE-1999-BUL-32, also independently identified by the MACHO collaboration as MACHO-99-BLG-22. This unique event has an Einstein radius crossing time of 640 d. The high-quality data obtained with difference image analysis shows a small but significant parallax signature. This parallax effect allows one to determine the Einstein radius projected on to the observer plane as     . The transverse velocity projected on to the observer plane is about 79 km s−1. We argue that the lens is likely to have a mass of at least a few solar masses, i.e. it could be a stellar black hole. The black hole hypothesis can be tested using the astrometric microlensing signature with the soon-to-be installed Advanced Camera for Surveys on board the Hubble Space Telescope . Deep X-ray and radio images may also be useful for revealing the nature of the object.  相似文献   

20.
The disciplines of asteroseismology and extrasolar planet science overlap methodically in the branch of high‐precision photometric time series observations. Light curves are, amongst others, useful to measure intrinsic stellar variability due to oscillations, as well as to discover and characterize those extrasolar planets that transit in front of their host stars, periodically causing shallow dips in the observed brightness. Both fields ultimately derive fundamental parameters of stellar and planetary objects, allowing to study for example the physics of various classes of pulsating stars, or the variety of planetary systems, in the overall context of stellar and planetary system formation and evolution. Both methods typically also require extensive spectroscopic follow‐up to fully explore the dynamic characteristics of the processes under investigation. In particularly interesting cases, a combination of observed pulsations and signatures of a planet allows to characterize a system's components to a very high degree of completeness by combining complementary information. The planning of the relevant space missions has consequently converged with respect to science cases, where at the outset there was primarily a coincidence in instrumentation and techniques. Whether space‐ or ground‐based, a specific type of stellar pulsations can themselves be used in an innovative way to search for extrasolar planets. Results from this additional method at the interface of stellar pulsation studies and exoplanet hunts in a beyond‐mainstream area are presented (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

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