High-resolution Follow-up Observations Research Articles

A detailed chemical study of the extreme velocity stars in the galaxy

ABSTRACT Two decades on, the study of hypervelocity stars is still in its infancy. These stars can provide novel constraints on the total mass of the Galaxy and its dark matter distribution. However how these stars are accelerated to such high velocities is unclear. Various proposed production mechanisms for these stars can be distinguished using chemo-dynamic tagging. The advent of Gaia and other large surveys have provided hundreds of candidate hyper velocity objects to target for ground-based high-resolution follow-up observations. We conduct high-resolution spectroscopic follow-up observations of 16 candidate late-type hyper velocity stars using the Apache Point Observatory and the McDonald Observatory. We derive atmospheric parameters and chemical abundances for these stars. We measure up to 22 elements, including the following nucleosynthetic families: $\alpha$ (Mg, Si, Ca, and Ti), light/odd-Z (Na, Al, V, Cu, and Sc), Fe-peak (Fe, Cr, Mn, Co, Ni, and Zn), and neutron capture (Sr, Y, Zr, Ba, La, Nd, and Eu). Our kinematic analysis shows one candidate is unbound, two are marginally bound, and the remainder are bound to the Galaxy. Finally, for the three unbound or marginally bound stars, we perform orbit integration to locate possible globular cluster or dwarf galaxy progenitors. We do not find any likely candidate systems for these stars and conclude that the unbound stars are likely from the the stellar halo, in agreement with the chemical results. The remaining bound stars are all chemically consistent with the stellar halo as well.

A tentative detection of He I in the atmosphere of GJ 1214 b

The He Iλ10833 Å triplet is a powerful tool for characterising the upper atmosphere of exoplanets and tracing possible mass loss. Here, we analysed one transit of GJ 1214 b observed with the CARMENES high-resolution spectrograph to study its atmosphere via transmission spectroscopy around the He Itriplet. Although previous studies using lower resolution instruments have reported non-detections of He Iin the atmosphere of GJ 1214 b, we report here the first potential detection. We reconcile the conflicting results arguing that previous transit observations did not present good opportunities for the detection of He I, due to telluric H2O absorption and OH emission contamination. We simulated those earlier observations, and show evidence that the planetary signal was contaminated. From our single non-telluric-contaminated transit, we determined an excess absorption of 2.10−0.50+0.45% (4.6σ) with a full width at half maximum (FWHM) of 1.30−0.25+0.30Å. The detection of He Iis statistically significant at the 4.6σlevel, but repeatability of the detection could not be confirmed due to the availability of only one transit. By applying a hydrodynamical model and assuming an H/He composition of 98/2, we found that GJ 1214 b would undergo hydrodynamic escape in the photon-limited regime, losing its primary atmosphere with a mass-loss rate of (1.5–18) × 1010g s−1and an outflow temperature in the range of 2900–4400 K. Further high-resolution follow-up observations of GJ 1214 b are needed to confirm and fully characterise the detection of an extended atmosphere surrounding GJ 1214 b. If confirmed, this would be strong evidence that this planet has a primordial atmosphere accreted from the original planetary nebula. Despite previous intensive observations from space- and ground-based observatories, our He Iexcess absorption is the first tentative detection of a chemical species in the atmosphere of this benchmark sub-Neptune planet.

ALMA’s view of the M-dwarf GSC 07396-00759’s edge-on debris disc: AU Mic’s coeval twin

ABSTRACT We present new ALMA Band 7 observations of the edge-on debris disc around the M1V star GSC 07396-00759. At ∼20 Myr old and in the β Pictoris Moving Group along with AU Mic, GSC 07396-00759 joins it in the handful of low-mass M-dwarf discs to be resolved in the sub-mm. With previous VLT/SPHERE scattered light observations, we present a multiwavelength view of the dust distribution within the system under the effects of stellar wind forces. We find the mm dust grains to be well described by a Gaussian torus at 70 au with a full width at half-maximum of 48 au and we do not detect the presence of CO in the system. Our ALMA model radius is significantly smaller than the radius derived from polarimetric scattered light observations, implying complex behaviour in the scattering phase function. The brightness asymmetry in the disc observed in scattered light is not recovered in the ALMA observations, implying that the physical mechanism only affects smaller grain sizes. High-resolution follow-up observations of the system would allow investigation into its unique dust features as well as provide a true coeval comparison for its smaller sibling AU Mic, singularly well-observed amongst M-dwarfs systems.

Singly and doubly deuterated formaldehyde in massive star-forming regions

Context. Deuterated molecules are good tracers of the evolutionary stage of star-forming cores. During the star formation process, deuterated molecules are expected to be enhanced in cold, dense pre-stellar cores and to deplete after protostellar birth. Aims. In this paper, we study the deuteration fraction of formaldehyde in high-mass star-forming cores at different evolutionary stages to investigate whether the deuteration fraction of formaldehyde can be used as an evolutionary tracer. Methods. Using the APEX SEPIA Band 5 receiver, we extended our pilot study of the J = 3 →2 rotational lines of HDCO and D2CO to eleven high-mass star-forming regions that host objects at different evolutionary stages. High-resolution follow-up observations of eight objects in ALMA Band 6 were performed to reveal the size of the H2CO emission and to give an estimate of the deuteration fractions HDCO/H2CO and D2CO/HDCO at scales of ~6″ (0.04–0.15 pc at the distance of our targets). Results. Our observations show that singly and doubly deuterated H2CO are detected towards high-mass protostellar objects (HMPOs) and ultracompact H II regions (UC H II regions), and the deuteration fraction of H2CO is also found to decrease by an order of magnitude from the earlier HMPO phases to the latest evolutionary stage (UC H II), from ~0.13 to ~0.01. We have not detected HDCO and D2CO emission from the youngest sources (i.e. high-mass starless cores or HMSCs). Conclusions. Our extended study supports the results of the previous pilot study: the deuteration fraction of formaldehyde decreases with the evolutionary stage, but higher sensitivity observations are needed to provide more stringent constraints on the D/H ratio during the HMSC phase. The calculated upper limits for the HMSC sources are high, so the trend between HMSC and HMPO phases cannot be constrained.

Three microlensing planets with no caustic-crossing features

Aims. We search for microlensing planets with signals exhibiting no caustic-crossing features, considering the possibility that such signals may be missed due to their weak and featureless nature. Methods. For this purpose, we reexamine the lensing events found by the KMTNet survey before the 2019 season. From this investigation, we find two new planetary lensing events, KMT-2018-BLG-1976 and KMT-2018-BLG-1996. We also present the analysis of the planetary event OGLE-2019-BLG-0954, for which the planetary signal was known but no detailed analysis had previously been presented. We identify the genuineness of the planetary signals by checking various interpretations that can generate short-term anomalies in lensing light curves. Results. From Bayesian analyses conducted with the constraint from available observables, we find that the host and planet masses are (M1, M2) ~ (0.65 M⊙, 2 MJ) for KMT-2018-BLG-1976L, ~(0.69 M⊙, 1 MJ) for KMT-2018-BLG-1996L, and ~(0.80 M⊙, 14 MJ) for OGLE-2019-BLG-0954L. The estimated distance to OGLE-2019-BLG-0954L, 3.63−1.64+1.22 kpc, indicates that it is located in the disk, and the brightness expected from the mass and distance matches the brightness of the blend well, indicating that the lens accounts for most of the blended flux. The lens of OGLE-2019-BLG-0954 may be resolved from the source by conducting high-resolution follow-up observations in and after 2024.

Searching for r-process-enhanced stars in the LAMOST survey I: the method

The abundance patterns of r-process-enhanced stars contain key information required to constrain the astrophysical site(s) of r-process nucleosynthesis, and to deepen our understanding of the chemical evolution of our Galaxy. To expand the sample of known r-process-enhanced stars, we have developed a method to search for candidates in the LAMOST medium-resolution (R ∼ 7500) spectroscopic survey by matching the observed spectra to synthetic templates around the Eu ii line at 6645.1 Å. We obtain a sample of 13 metal-poor (−2.35 < [Fe/H] < −0.91) candidates from 12 209 unique stars with 32 774 mediumresolution spectra. These candidates will be further studied by high-resolution follow-up observations in the near future. We describe some extensions of this effort to include larger samples of stars, in particular at lower metallicity, using the strength of the Ba ii line at 6496.9 Å.

KMT-2018-BLG-0748Lb: sub-Saturn microlensing planet orbiting an ultracool host

Aims. We announce the discovery of a microlensing planetary system, in which a sub-Saturn planet is orbiting an ultracool dwarf host. Methods. We detected the planetary system by analyzing the short-timescale (tE ~ 4.4 days) lensing event KMT-2018-BLG-0748. The central part of the light curve exhibits asymmetry due to negative deviations in the rising side and positive deviations in the falling side. Results. We find that the deviations are explained by a binary-lens model with a mass ratio between the lens components of q ~ 2 × 10−3. The short event timescale, together with the small angular Einstein radius, θE ~ 0.11 mas, indicate that the mass of the planet host is very small. The Bayesian analysis conducted under the assumption that the planet frequency is independent of the host mass indicates that the mass of the planet is Mp = 0.18−0.10+0.29 MJ, and the mass of the host, Mh = 0.087−0.047+0.138 M⊙, is near the star–brown dwarf boundary, but the estimated host mass is sensitive to this assumption about the planet hosting probability. High-resolution follow-up observations would lead to revealing the nature of the planet host.

Revisiting MOA 2013-BLG-220L: A Solar-type Star with a Cold Super-Jupiter Companion

Abstract We present the analysis of high-resolution images of MOA-2013-BLG-220, taken with the Keck adaptive optics system six years after the initial observation, identifying the lens as a solar-type star hosting a super-Jupiter-mass planet. The masses of planets and host stars discovered by microlensing are often not determined from light-curve data, while the star–planet mass ratio and projected separation in units of Einstein ring radius are well measured. High-resolution follow-up observations after the lensing event is complete can resolve the source and lens. This allows direct measurements of flux, and the amplitude and direction of proper motion, giving strong constraints on the system parameters. Due to the high relative proper motion, mas yr−1, the source and lens were resolved in 2019, with a separation of 77.1 ± 0.5 mas. Thus, we constrain the lens flux to . By combining constraints from the model and Keck flux, we find the lens mass to be at . With a mass ratio of the planet’s mass is determined to be at a separation of . The lens mass is much higher than the prediction made by Bayesian analysis that assumes all stars have an equal probability to host a planet of the measured mass ratio, and suggests that planets with mass ratios of a few times 10−3 are more common orbiting massive stars. This demonstrates the importance of high-resolution follow-up observations for testing theories like these.

OGLE-2015-BLG-1649Lb: A Gas Giant Planet around a Low-mass Dwarf

Abstract We report the discovery of an exoplanet from the analysis of the gravitational microlensing event OGLE-2015-BLG-1649 that challenges the core accretion model of planet formation and appears to support the disk instability model. The planet/host-star mass ratio is q = 7.2 × 10−3 and the projected separation normalized to the angular Einstein radius is s = 0.9. We conducted high-resolution follow-up observations using the Infrared Camera and Spectrograph (IRCS) camera on the Subaru telescope and are able to place an upper limit on the lens flux. From these measurements we are able to exclude all host stars greater than or equal in mass to a G-type dwarf. We conducted a Bayesian analysis with these new flux constraints included as priors resulting in estimates of the masses of the host star and planet. These are M L = 0.34 ± 0.19 M ⊙ and , respectively. The distance to the system is . The projected star–planet separation is . The estimated relative lens-source proper motion, ∼7.1 mas yr−1, is fairly high and thus the lens can be better constrained if additional follow-up observations are conducted several years after the event.

OGLE-2016-BLG-0156: Microlensing Event with Pronounced Microlens-parallax Effects Yielding a Precise Lens Mass Measurement

Abstract We analyze the gravitational binary-lensing event OGLE-2016-BLG-0156, for which the lensing light curve displays pronounced deviations induced by microlens-parallax effects. The light curve exhibits three distinctive widely separated peaks and we find that the multiple-peak feature provides a very tight constraint on the microlens-parallax effect, enabling us to precisely measure the microlens parallax . All the peaks are densely and continuously covered from high-cadence survey observations using globally located telescopes and the analysis of the peaks leads to the precise measurement of the angular Einstein radius . From the combination of the measured and , we determine the physical parameters of the lens. It is found that the lens is a binary composed of two M dwarfs with masses M 1 = 0.18 ± 0.01 M ⊙ and M 2 = 0.16 ± 0.01 M ⊙ located at a distance . According to the estimated lens mass and distance, the flux from the lens comprises an important fraction, ∼25%, of the blended flux. The bright nature of the lens combined with the high relative lens-source motion, μ = 6.94 ± 0.50 mas yr−1, suggests that the lens can be directly observed from future high-resolution follow-up observations.

OGLE-2017-BLG-0039: Microlensing Event with Light from a Lens Identified from Mass Measurement

Abstract We present an analysis of the caustic-crossing binary microlensing event OGLE-2017-BLG-0039. Thanks to the very long duration of the event, with a time scale t E ∼ 130 days, the microlens parallax is measured precisely despite its low value of . Analysis of the well-resolved caustic crossings during the source star’s entrance and exit of the caustic yields an angular Einstein radius of mas. The measured and indicate that the lens is a binary composed of two stars with masses and ∼0.15 M ⊙, and is located at a distance of ∼6 kpc. From the color and brightness of the lens estimated from its determined mass and distance, it is expected that ∼2/3 of the I-band blended flux comes from the lens. Therefore, the event is a rare case of a bright lens event for which high-resolution follow-up observations can confirm its nature.

Measuring the Recoverability of Close Binaries in Gaia DR2 with the Robo-AO Kepler Survey

Abstract We use the Robo-AO survey of Kepler planetary candidate host stars, the largest adaptive optics survey yet performed, to measure the recovery rate of close stellar binaries in Gaia DR2. We find that Gaia recovers binaries down to 1″ at magnitude contrasts as large as six; closer systems are not resolved, regardless of secondary brightness. Gaia DR2 binary detection does not have a strong dependence on the orientation of the stellar pairs. We find 177 nearby stars to Kepler planetary candidate host stars in Gaia DR2 that were not detected in the Robo-AO survey, almost all of which are faint (G &gt; 20); the remainder were largely targets observed by Robo-AO in poor conditions. If the primary star is the host, the impact on the radii estimates of planet candidates in these systems is likely minimal; many of these faint stars, however, could be faint eclipsing binaries that are the source of a false positive planetary transit signal. With Robo-AO and Gaia combined, we find that 18.7 ± 0.7% of Kepler planet candidate hosts have nearby stars within 4″. We also find 36 nearby stars in Gaia DR2 around 35 planetary candidate host stars detected with K2. The nearby star fraction rate for K2 planetary candidates is significantly lower than that for the primary Kepler mission. The binary recovery rate of Gaia will improve initial radius estimates of future Transiting Exoplanet Survey Satellite planet candidates significantly; however, ground-based high-resolution follow-up observations are still needed for precise characterization and confirmation. The sensitivity of Gaia to closely separated binaries is expected to improve in later data releases.

OGLE-2017-BLG-0537: A Microlensing Event with a Resolvable Lens in ≲5 years from High-resolution Follow-up Observations

Abstract We present an analysis of the binary-lens microlensing event OGLE-2017-BLG-0537. The light curve of the event exhibits two strong caustic-crossing spikes among which the second caustic crossing was resolved by high-cadence surveys. It is found that the lens components with a mass ratio ∼0.5 are separated in projection by , where θ E is the angular Einstein radius. Analysis of the caustic-crossing part yields mas and a lens-source relative proper motion of μ = 12.4 ± 1.1 mas yr−1. The measured μ is the third highest value among the events with measured proper motions and is ∼3 times higher than the value of typical Galactic bulge events, making the event a strong candidate for follow-up observations to directly image the lens by separating it from the source. From the angular Einstein radius combined with the microlens parallax, it is estimated that the lens is composed of two main-sequence stars with masses M 1 ∼ 0.4 M ⊙ and M 2 ∼ 0.2 M ⊙ located at a distance of D L ∼ 1.2 kpc. However, the physical lens parameters are not very secure due to the weak microlens-parallax signal, thus we cross-check the parameters by conducting a Bayesian analysis based on the measured Einstein radius and event timescale, combined with the blending constraint. From this, we find that the physical parameters estimated from the Bayesian analysis are consistent with those based on the measured microlens parallax. Resolving the lens from the source can be done in about 5 years from high-resolution follow-up observations and this will provide a rare opportunity to test and refine the microlensing model.

Radio Galaxy Zoo: A Search for Hybrid Morphology Radio Galaxies

Abstract Hybrid morphology radio sources (HyMoRS) are a rare type of radio galaxy that display different Fanaroff–Riley classes on opposite sides of their nuclei. To enhance the statistical analysis of HyMoRS, we embarked on a large-scale search of these sources within the international citizen science project, Radio Galaxy Zoo (RGZ). Here, we present 25 new candidate hybrid morphology radio galaxies. Our selected candidates are moderate power radio galaxies ( W Hz−1 sr−1) at redshifts . Hosts of nine candidates have spectroscopic observations, of which six are classified as quasars, one as high- and two as low-excitation galaxies. Two candidate HyMoRS are giant ( Mpc) radio galaxies, one resides at the center of a galaxy cluster, and one is hosted by a rare green bean galaxy. Although the origin of the hybrid morphology radio galaxies is still unclear, this type of radio source starts depicting itself as a rather diverse class. We discuss hybrid radio morphology formation in terms of the radio source environment (nurture) and intrinsically occurring phenomena (nature; activity cessation and amplification), showing that these peculiar radio galaxies can be formed by both mechanisms. While high angular resolution follow-up observations are still necessary to confirm our candidates, we demonstrate the efficacy of the RGZ in the pre-selection of these sources from all-sky radio surveys, and report the reliability of citizen scientists in identifying and classifying complex radio sources.

MOA-2016-BLG-227Lb: A Massive Planet Characterized by Combining Light-curve Analysis and Keck AO Imaging

Abstract We report the discovery of a microlensing planet—MOA-2016-BLG-227Lb—with a large planet/host mass ratio of q ≃ 9 × 10−3. This event was located near the K2 Campaign 9 field that was observed by a large number of telescopes. As a result, the event was in the microlensing survey area of a number of these telescopes, and this enabled good coverage of the planetary light-curve signal. High angular resolution adaptive optics images from the Keck telescope reveal excess flux at the position of the source above the flux of the source star, as indicated by the light-curve model. This excess flux could be due to the lens star, but it could also be due to a companion to the source or lens star, or even an unrelated star. We consider all these possibilities in a Bayesian analysis in the context of a standard Galactic model. Our analysis indicates that it is unlikely that a large fraction of the excess flux comes from the lens, unless solar-type stars are much more likely to host planets of this mass ratio than lower mass stars. We recommend that a method similar to the one developed in this paper be used for other events with high angular resolution follow-up observations when the follow-up observations are insufficient to measure the lens–source relative proper motion.

Spectroscopic signatures of extratidal stars around the globular clusters NGC 6656 (M 22), NGC 3201, and NGC 1851 from RAVE

Stellar population studies of globular clusters have suggested that the brightest clusters in the Galaxy might actually be the remnant nuclei of dwarf spheroidal galaxies. If the present Galactic globular clusters formed within larger stellar systems, they are likely surrounded by extra-tidal halos and/or tails made up of stars that were tidally stripped from their parent systems. The stellar surroundings around globular clusters are therefore one of the best places to look for the remnants of an ancient dwarf galaxy. Here an attempt is made to search for tidal debris around the supernovae enriched globular clusters M22 and NGC 1851 as well as the kinematically unique cluster NGC 3201. The stellar parameters from the Radial Velocity Experiment (RAVE) are used to identify stars with RAVE metallicities, radial velocities and elemental-abundances consistent with the abundance patterns and properties of the stars in M22, NGC 1851 and NGC 3201. The discovery of RAVE stars that may be associated with M22 and NGC 1851 are reported, some of which are at projected distances of ~10 degrees away from the core of these clusters. Numerous RAVE stars associated with NGC 3201 suggest that either the tidal radius of this cluster is underestimated, or that there are some unbound stars extending a few arc minutes from the edge of the cluster's radius. No further extra-tidal stars associated with NGC 3201 could be identified. The bright magnitudes of the RAVE stars make them easy targets for high resolution follow-up observations, allowing an eventual further chemical tagging to solidify (or exclude) stars outside the tidal radius of the cluster as tidal debris. In both our radial velocity histograms of the regions surrounding NGC 1851 and NGC 3201, a peak of stars at 230 km/s is seen, consistent with extended tidal debris from omega Centauri.

THE BEST AND BRIGHTEST METAL-POOR STARS

The chemical abundances of large samples of extremely metal-poor (EMP) stars can be used to investigate metal-free stellar populations, supernovae, and nucleosynthesis as well as the formation and galactic chemical evolution of the Milky Way and its progenitor halos. However, current progress on the study of EMP stars is being limited by their faint apparent magnitudes. The acquisition of high signal-to-noise spectra for faint EMP stars requires a major telescope time commitment, making the construction of large samples of EMP star abundances prohibitively expensive. We have developed a new, efficient selection that uses only public, all-sky APASS optical, 2MASS near-infrared, and WISE mid-infrared photometry to identify bright metal-poor star candidates through their lack of molecular absorption near 4.6 microns. We have used our selection to identify 11,916 metal-poor star candidates with V < 14, increasing the number of publicly-available candidates by more than a factor of five in this magnitude range. Their bright apparent magnitudes have greatly eased high-resolution follow-up observations that have identified seven previously unknown stars with [Fe/H] <~ -3.0. Our follow-up campaign has revealed that 3.8^{+1.3}_{-1.1}% of our candidates have [Fe/H] <~ -3.0 and 32.5^{+3.0}_{-2.9}% have -3.0 <~ [Fe/H] <~ -2.0. The bulge is the most likely location of any existing Galactic Population III stars, and an infrared-only variant of our selection is well suited to the identification of metal-poor stars in the bulge. Indeed, two of our confirmed metal-poor stars with [Fe/H] <~ -2.7 are within about 2 kpc of the Galactic Center. They are among the most metal-poor stars known in the bulge.

Spatially resolved observations of warm ionized gas and feedback in local ULIRGs★

We present VLT/VIMOS-IFU emission-line spectroscopy of a volume limited sample of 18 southern ULIRGs selected with z<0.09 and dec<10. By covering a wide range of ULIRG types, this dataset provides an important set of templates for comparison with high-redshift galaxies. We employed an automated Gaussian line fitting program to decompose the emission line profiles of Halpha, [NII], [SII], and [OI] into individual components, and chart the Halpha kinematics, and the ionized gas excitations and densities. 11/18 of our galaxies show evidence for outflowing warm ionized gas with speeds between 500 and a few 1000 km/s, with the fastest outflows associated with systems that contain an AGN. Our spatially resolved spectroscopy has allowed us to map the outflows, and in some cases determine for the first time to which nucleus the wind is associated. In three of our targets we find line components with widths >2000 km/s over spatially extended regions in both the recombination and forbidden lines; in two of these three, they are associated with a known Sy2 nucleus. Eight galaxies have clear rotating gaseous disks, and for these we measure rotation velocities, virial masses, and calculate Toomre Q parameters. We find radial gradients in the emission line ratios in a significant number of systems in our study. We attribute these gradients to changes in ionizing radiation field strength, most likely due to an increasing contribution of shocks with radius. We conclude with a detailed discussion of the results for each individual system, with reference to the existing literature. Our observations demonstrate that the complexity of the kinematics and gas properties in ULIRGs can only be disentangled with high sensitivity, spatially resolved IFU observations. Many of our targets are ideal candidates for future high spatial resolution follow-up observations.

Neutral hydrogen sub-structure in the Galactic Halo outside the solar circle

21-cm studies of the Galactic extra-planar gas layer can help us understand the origin of the gaseous Galactic halo and its interaction with the Milky Way disk. The 100-m Effelsberg radio telescope was utilized to detect and analyze a population of Hi halo clouds belonging to this layer, at Galactocentric radii R > 8.5kpc, following Galactic rotation. This sample of clouds has similar properties to the population detected by Lockman with the GBT at R < 8.5kpc. High resolution follow-up observations resolve them into a number of small, dense, cold arc-minute sized Hi cores. Their study provides evidence for a thermal equilibrium with the extended warmer envelopes. Single dish observations with the 300-m Arecibo telescope disclosed a sample of Hi clouds with similar properties. A power spectrum analysis of the Arecibo maps probes the characteristics of turbulence as a dominant physical process in the halo region.

Near‐Field Microlensing from Wide‐Field Surveys

We estimate the rate of near-field microlensing events expected from all-sky surveys and investigate the properties of these events. Under the assumption that all lenses are composed of stars, our estimation of the event rate ranges from \Gamma_{tot}~0.2 yr^{-1}$ for a survey with a magnitude limit of V_{lim}=12 to \Gamma_{tot}~20 yr^{-1} for a survey with V_{lim}=18. We find that the average distances to source stars and lenses vary considerably depending on the magnitude limit, while the dependencies of the average event time scale and lens-source transverse speed are weak and nearly negligible, respectively. We also find that the the average lens-source proper motion of events expected even from a survey with V_{lim}=18 would be >~ 40 mas yr^{-1}, implying that the source and lens of a significant fraction of near-field events could be resolved from high-resolution follow-up observations. From the investigation of the variation of the event characteristics depending on the position of the sky, we find that the average distances to source stars and lenses become shorter, the lens-source transverse speed increases, and the time scale becomes shorter as the the galactic latitude of the field increases. Due to the concentration of events near the galactic plane, we find that >~ 50 % of events would be detected in the field with b <= 20^\circ.