Effect of Binary Source Companions on the Microlensing Optical Depth Determination toward the Galactic Bulge Field

Currently, gravitational microlensing survey experiments toward the Galactic bulge field utilize two different methods of minimizing blending effect for the accurate determination of the optical depth \tau. One is measuring \tau based on clump giant (CG) source stars and the other is using `Difference Image Analysis (DIA)' photometry to measure the unblended source flux variation. Despite the expectation that the two estimates should be the same assuming that blending is properly considered, the estimates based on CG stars systematically fall below the DIA results based on all events with source stars down to the detection limit. Prompted by the gap, we investigate the previously unconsidered effect of companion-associated events on $\tau$ determination. Although the image of a companion is blended with that of its primary star and thus not resolved, the event associated with the companion can be detected if the companion flux is highly magnified. Therefore, companions work effectively as source stars to microlensing and thus neglect of them in the source star count could result in wrong \tau estimation. By carrying out simulations based on the assumption that companions follow the same luminosity function of primary stars, we estimate that the contribution of the companion-associated events to the total event rate is ~5f_{bi}% for current surveys and can reach up to ~6f_{bi}% for future surveys monitoring fainter stars, where f_{bi} is the binary frequency. Therefore, we conclude that the companion-associated events comprise a non-negligible fraction of all events. However, their contribution to the optical depth is not large enough to explain the systematic difference between the optical depth estimates based on the two different methods.Comment: 4 pages, 1 figure, 1 table, ApJ, submitte

Pollen Rupture and Its Impact on Precipitation in Clean Continental Conditions

Pollen grains emitted from vegetation can rupture, releasing subpollen particles (SPPs) as fine atmospheric particulates. Previous laboratory research demonstrates potential for SPPs as efficient cloud condensation nuclei (CCN). We develop the first model of atmospheric pollen grain rupture and implement the mechanism in regional climate model simulations over spring pollen season in the United States with a CCN‐dependent moisture scheme. The source of SPPs (surface or in‐atmosphere) depends on region and sometimes season, due to the distribution of relative humidity and rain. Simulated concentrations of SPPs are approximately 1–10 or 1–1,000 cm−3, depending on the number of SPPs produced per pollen grain (nspg). Lower nspg (103) produces a negligible effect on precipitation, but high nspg (106) in clean continental CCN background concentrations (100 CCN per cubic centimeter) shows that SPPs suppress average seasonal precipitation by 32% and shift rates from heavy to light while increasing dry days. This effect is smaller (2% reduction) for polluted air.Plain Language SummaryPollen grains emitted by wind from a variety of plants can swell from exposure to high levels of humidity, creating internal pressure that may cause the grains to rupture. Particles that are 10 to a thousand times smaller than pollen grains are released in the process. These subpollen particles (SPPs) have been found in laboratory studies to efficiently collect water on their surfaces, making them potential cloud condensation nuclei (i.e., particles that may grow into cloud droplets). We have developed a numerical model of pollen rupture that interfaces with an atmosphere model to determine (1) how many SPPs are produced during the pollen season from two different sources: rupture of pollen at the surface and rupture of airborne pollen grains; (2) the geographic and vertical distribution of SPPs seasonally; and (3) the impact of SPPs on regional precipitation. We find that the strength of either source in any region or phase of season depends on rain and relative humidity. We also find that SPPs have the potential to suppress seasonal precipitation in clean conditions when anthropogenic pollution is not present depending on how many are released for each pollen grain that ruptures. The magnitude of suppression regionally is dependent on source magnitude of SPPs, as well as the availability of water vapor.Key PointsThe first model of moisture‐induced pollen rupture and release of subpollen particles (SPPs) is coupled to a regional climate modelDuring peak pollen season in the United States, simulated SPPs range from 1 to 1,000 cm−3, depending on the number produced per pollen grain rupturedSPP may have the ability to suppress precipitation regionally in clean continental CCN conditions and induce a negative feedback to SPP productionPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/145502/1/grl57690_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/145502/2/grl57690.pd

The Cuspy LINER Nucleus of the S0/a Galaxy NGC 2681

The nucleus of the bulge-dominated, multiply-barred S0/a galaxy NGC 2681 is studied in detail, using high resolution Hubble Space Telescope FOC and NICMOS imaging and FOS spectroscopy. The ionised gas central velocity dispersion is found to increase by a factor ~2 when narrowing the aperture from R~1.5" (ground) to R~0.1" (FOS). Dynamical modeling of these velocity dispersions suggests that NGC 2681 does host a supermassive black hole (BH) for which one can estimate a firm mass upper limit M_BH < 6*10^7 Solar Masses. This upper limit is consistent with the relation between the central BH mass and velocity dispersion M_BH - sigma known for other galaxies. The emission line ratios place the nucleus of NGC 2681 among LINERs. It is likely that the emission line region comes from a rather mild, but steady, feeding of gas to the central BH in this galaxy. The inner stellar population lacks any measurable color gradient (to a radius of 0.6 kpc) from the infrared to the ultraviolet, consistently with FOC, FOS and IUE data, all indicating that this system underwent a starburst ~1 Gyr ago that encompassed its whole interior, down to its very center. The most likely source of such a widely-distributed starburst is the dumping of tidally-extruded gas from a galaxy neighbor. If so, then NGC 2681 can be considered as the older brother of M82, seen face-on as opposed to the edge-on view we have for M82.Comment: 25 pages, LaTeX, with 10 PostScript figures, to appear in The Astrophysical Journa

The first direct detection of a gravitational micro-lens toward the Galactic bulge

We present a direct detection of the gravitational lens that caused the microlensing event MACHO-95-BLG-37. This is the first fully resolved microlensing system involving a source in the Galactic bulge, and the second such system in general. The lens and source are clearly resolved in images taken with the High Resolution Channel of the Advanced Camera for Surveys on board the Hubble Space Telescope (HST) ~9 years after the microlensing event. The presently available data are not sufficient for the final, unambiguous identification of the gravitational lens and the microlensed source. While the light curve models combined with the high resolution photometry for individual objects indicate that the source is red and the lens is blue, the color-magnitude diagram for the line of sight and the observed proper motions strongly support the opposite case. The first scenario points to a metal-poor lens with mass M = ~0.6 M_Sun at the distance D_l = ~4 kpc. In the second scenario the lens could be a main-sequence star with M = 0.8 - 0.9 M_Sun about half-way to the Galactic bulge or in the foreground disk, depending on the extinction.Comment: Accepted for publication in Ap

Influence of Vertical Heterogeneities in the Canopy Microenvironment on Interannual Variability of Carbon Uptake in Temperate Deciduous Forests

Vegetation structure and function are key design choices in terrestrial models that affect the relationship between carbon uptake and environmental drivers. Here, we investigate how representing canopy vertical structure in a terrestrial biosphere model- that is, micrometeorological, leaf area, and leaf water profiles- influences carbon uptake at five U.S. temperate deciduous forest sites in July. Specifically, we test whether the interannual variability (IAV) of gross primary productivity (GPP) responds differently to four abiotic environmental drivers- air temperature, relative humidity, incoming shortwave radiation, and soil moisture- using either a Community Land Model multilayer canopy model (CLM- ml) or a big- leaf model (CLM4.5/CLM5). We conclude that vertical leaf area and microclimatic profiles (temperature, humidity, and wind) do not impact GPP IAV compared to a single- layer model when plant hydraulics is excluded. However, with a mechanistic representation of plant hydraulics there is vertically varying water stress in CLM- ml, and the sensitivity of carbon uptake to particular climate variables changes with height, resulting in dampened canopy- scale GPP IAV relative to CLM4.5. Dampening is due to both a reduced dependence on soil moisture and opposing climatic forcing on different leaf layers. Such dampening is not evident in the single- layer representation of plant hydraulic water stress implemented in the recently released CLM5. Overall, both model representations of the canopy fail to accurately simulate observed GPP IAV and this may be related by their inability to capture the upper range of observed hourly GPP and diffuse light- GPP relationships that cannot be resolved by canopy structure alone.Key PointsExplicitly simulated leaf area and microclimatic profiles do not affect gross primary productivity (GPP) interannual variability compared to a - big- leaf- simplificationMultilayer plant hydraulics lead to vertically varying water stress, altering leaf- layer responses to interannual climate variationsAll model simulations underestimate hourly GPP compared to FLUXNET estimates, adversely impacting simulated GPP interannual variabilityPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/156484/2/jgrg21710_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/156484/1/jgrg21710.pd

A SAURON study of stars and gas in Sa bulges

We present results from our ongoing effort to understand the morphological and kinematical properties of early-type galaxies using the integral-field spectrograph SAURON. We discuss the relation between the stellar and gas morphology and kinematics in our sub-sample of 24 representative Sa spiral bulges. We focus on the frequency of kinematically decoupled components and on the presence of star formation in circumnuclear rings.Comment: 6 pages, 3 figures; To appear in the proceedings of the "Island Universes: Structure and Evolution of Disk Galaxies" conference held in Terschelling, Netherlands, July 2005, ed. R. de Jong. A high resolution version is available at http://www.strw.leidenuniv.nl/~jfalcon/JFB_terschelling.pdf.g

Molecular Gas in Candidate Double-Barred Galaxies II. Cooler, Less Dense Gas Associated with Stronger Central Concentrations

We have performed a multi-transition CO study of the centers of seven double-barred galaxies that exhibit a variety of molecular gas morphologies to determine if the molecular gas properties are correlated with the nuclear morphology and star forming activity. Near infrared galaxy surveys have revealed the existence of nuclear stellar bars in a large number of barred or lenticular galaxies. High resolution CO maps of these galaxies exhibit a wide range of morphologies. Recent simulations of double-barred galaxies suggest that variations in the gas properties may allow it to respond differently to similar gravitational potentials. We find that the 12CO J=3-2/J=2-1 line ratio is lower in galaxies with centrally concentrated gas distributions and higher in galaxies with CO emission dispersed around the galactic center in rings and peaks. The 13CO/12CO J=2-1 line ratios are similar for all galaxies, which indicates that the J=3-2/J=2-1 line ratio is tracing variations in gas temperature and density, rather than variations in optical depth. There is evidence that the galaxies which contain more centralized CO distributions are comprised of molecular gas that is cooler and less dense. Observations suggest that the star formation rates are higher in the galaxies containing the warmer, denser, less centrally concentrated gas. It is possible that either the bar dynamics are responsible for the variety of gas distributions and densities (and hence the star formation rates) or that the star formation alone is responsible for modifying the gas properties.Comment: 27 pages + 6 figures; to appear in the April 20, 2003 issue of Ap