The Cover

Larson, John A.

doi:10.1001/jama.1986.03380180014001

Cited by 2 publications

(3 citation statements)

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“…Both components are populated by stars following a logarithmic MF in the range 0.1M ⊙ ≤ M l ≤ 1.2M ⊙ . These distributions produce an event duration distribution which is consistent with observed ML timescales (see BKR, Figure 3), though other evidence points towards a disk MF which exhibits some flattening below 1 M ⊙ (Scalo 1986;Larson 1986;Gould, Bahcall, & Flynn 1995).…”

Section: Expected Fractions Of Parallax-shifted Eventssupporting

confidence: 76%

“…In the above calculations we assumed a logarithmic MF down to 0.1 M ⊙ for disk lenses, which is consistent with current ML observations. However, other evidence points to a disk MF which turns over at masses near 0.4 M ⊙ (Scalo 1986;Larson 1986;Gould, Bahcall, & Flynn 1995). Thus, given the sensitivity of F P SE to disk lenses, we also consider the HST MF, log φ = 1.35 − 1.34 log(M/M ⊙ ) − 1.85 [log(M/M ⊙ )] 2 in the range from 0.1 M ⊙ ≤ M l ≤ 5.0 M ⊙ , as well as a composite power-law MF with dN/dM = 0 for 0.1 (Mihalas & Binney 1982).…”

Section: Expected Fractions Of Parallax-shifted Eventsmentioning

confidence: 99%

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Rates for Parallax‐shifted Microlensing Events from Ground‐based Observations of the Galactic Bulge

Buchalter

Kamionkowski

1997

ApJ

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The parallax e †ect in ground-based microlensing (ML) observations consists of a distortion to the standard ML light curve arising from the EarthÏs orbital motion. This can be used to partially remove the degeneracy among the system parameters in the event timescale,In most cases, the resolution in t 0 . current ML surveys is not accurate enough to observe this e †ect, but parallax could conceivably be detected with frequent follow-up observations of ML events in progress, providing the photometric errors are small enough. We calculate the expected fraction of ML events where the shape distortions will be observable by such follow-up observations, adopting Galactic models for the lens and source distributions that are consistent with observed microlensing timescale distributions. We study the dependence of the rates for parallax-shifted events on the frequency of follow-up observations and on the precision of the photometry. For example, we Ðnd that for hourly observations with typical photometric errors of 0.01 mag, 6% of events where the lens is in the bulge, and 31% of events where the lens is in the disk (or B10% of events overall), will give rise to a measurable parallax shift at the 95% conÐdence level. These fractions may be increased by improved photometric accuracy and increased sampling frequency. While long-duration events are favored, the surveys would be e †ective in picking out such distortions in events with timescales as low as days. We study the dependence of these fractions on t 0 B 20 the assumed disk mass function and Ðnd that a higher parallax incidence is favored by mass functions with higher mean masses. Parallax measurements yield the reduced transverse speed, which gives both v8 , the relative transverse speed and lens mass as a function of distance. We give examples of the accuracies with which may be measured in typical parallax events. Fitting ML light curves, which may be shapev8 distorted (e.g., by parallax, blending, etc.), with only the three standard ML parameters can result in inferred values for these quantities that are signiÐcantly in error. Using our model, we study the e †ects of such systematic errors and Ðnd that, due primarily to blending, the inferred timescales from such Ðts, for events with disk lenses, tend to shift the event duration distribution by B10% toward shorter Events t 0 . where the lens resides in the bulge are essentially una †ected. In both cases, the impact parameter distribution is depressed slightly at both the low and high ends.

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Section: Expected Fractions Of Parallax-shifted Eventssupporting

confidence: 76%

Section: Expected Fractions Of Parallax-shifted Eventsmentioning

confidence: 99%

Rates for Parallax‐shifted Microlensing Events from Ground‐based Observations of the Galactic Bulge

Buchalter

Kamionkowski

1997

ApJ

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“…If the assumed bulge MF is also adopted for the disk, the resulting duration distribution is consistent with MACHO and OGLE observations in the sense of fitting the main peak (see ZRS Figure 2). However, the present-day MF of the disk is believed to follow a power-law behavior which flattens out around sub-solar masses (Scalo 1986, Larson 1986). Scalo (1986) finds a flattening in the MF below 1.0M ⊙ with a maximum near 0.3M ⊙ , while HST observations by Gould, Bahcall, & Flynn (1995) find a maximum near 0.5M ⊙ .…”

Section: Disk Modelmentioning

confidence: 99%

Rates for Color-shifted Microlensing Events

Buchalter¹,

Kamionkowski²,

Rich³

1996

ApJ

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If the objects responsible for gravitational microlensing of Galactic-bulge stars are faint dwarfs, then blended light from the lens will distort the shape of the microlensing light curve and shift the color of the observed star during the microlensing event. In most cases, the resolution in current microlensing surveys is not accurate enough to observe this color-shift effect. However, such signatures could conceivably be detected with frequent followup observations of microlensing events in progress, providing the photometric errors are small enough. We calculate the expected rates for microlensing events where the shape distortions will be observable by such followup observations, assuming that the lenses are ordinary low-mass main-sequence stars in a rapidly rotating bar and in the disk. We adopt Galactic models consistent with observed microlensing timescale distributions, and consider separately the cases of self-lensing of the bulge, lensing of the bulge by the disk, and self-lensing of the disk, further differentiating between events where the source is a giant or a main-sequence star. We study the dependence of the rates for color-shifted microlensing events on the frequency of followup observations and on the precision of the photometry for a variety of waveband pairings. We find that for hourly observations in B and K with typical photometric errors of 0.01 mag, 28% of the events where a main-sequence bulge star is lensed, and 7% of the events where the source is a bulge giant, will give rise to a measurable color shift at the 95% confidence level. For observations in V and I, the fractions become 18% and 5%, respectively,

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Cited by 2 publications

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Rates for Parallax‐shifted Microlensing Events from Ground‐based Observations of the Galactic Bulge

Rates for Parallax‐shifted Microlensing Events from Ground‐based Observations of the Galactic Bulge

Rates for Color-shifted Microlensing Events

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