arXiv subjects

B. Scott Gaudi

Publications and source records attributed to B. Scott Gaudi.

238 records · Page 14Linked to original sources

arXiv1996

Planet Parameters in Microlensing Events

A planetary microlensing event occurs when a planet perturbs one of the two images created in a point-mass microlensing event, causing a deviation from the standard Paczyński curve. Determination of the two physical parameters that can be extracted from a planetary microlensing event, the planet/star mass ratio q, and the planet/star separation in units of the stellar Einstein ring, $y_p$, is hampered by several types of degeneracies. There are two distinct and qualitatively different classes of planetary events: major and minor image perturbations. For major image perturbations, there is a potentially crippling continuous degeneracy in q which is of order $δ_d^{-1}$, where $δ_d$ is the maximum fractional deviation of the planetary perturbation. Since the threshold of detection is expected to be $δ_d \sim 5%$, this degeneracy in q can be a factor of $\sim 20$. For minor image perturbations, the continuous degeneracy in q is considerably less severe, and is typically less than a factor 4. We show that these degeneracies can be resolved by observations from dedicated telescopes on several continents together with optical/infrared photometry from one of these sites. There also exists a class of discrete degeneracies. These are typically easy to resolve given good temporal coverage of the planetary event. Unambiguous interpretation of planetary microlensing events requires the resolution of both types of degeneracy. We describe the degeneracies in detail and specify the situations in which they are problematic. We also describe how individual planet masses and physical projected separations can be measured.

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arXiv1996

Satellite Parallaxes of Lensing Events Towards the Galactic Bulge

In order to understand the nature of the lenses that generate microlensing events, one would like to measure their mass, distance, and velocity. Unfortunately, current microlensing experiments measure only one parameter of the events, the characteristic timescale, which is a combination of the underlying physical parameters. Other methods are required to extract additional information. Parallax measurements using a satellite in an Earth-like orbit yield the projected velocity of the lens: ${\bf{\tilde v}} ={\bf{v}}/(1-z)$, where ${\bf{v}}$ is the transverse velocity (speed and direction) of the lens relative to the Earth-source line of sight, and $z$ is the ratio of the distances to the lens and the source. A measurement of ${\bf{\tilde v}}$ could distinguish between lenses belonging to the bulge and disk populations. We show that for photometric precisions of 1% to 2%, it is possible to measure the projected speed, $\tilde v$, to an accuracy of $\leq 10%$ for over 70% of disk lenses and over 60% of bulge lenses. For measuring the projected velocity ${\bf{\tilde v}}$, the percentages are 40% and {30%}, respectively. We find lines of sight $> 2^{\circ}$ away from the ecliptic are preferable, and an Earth-satellite separation in the range $0.7 {\rm{AU}} - 1.9{\rm{AU}}$ is optimal. The requirements of the satellite for measuring the projected velocities of events towards the bulge are similar to those for measurements toward the LMC.

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arXiv1996

Detection Rates for Close Binaries Via Microlensing

Microlensing is one of the most promising methods of reconstructing the stellar mass function down to masses even below the hydrogen-burning limit. The fundamental limit to this technique is the presence of unresolved binaries, which can in principle significantly alter the inferred mass function. Here we quantify the fraction of binaries that can be detected using microlensing, considering specifically the mass ratio and separation of the binary. We find that almost all binary systems with separations greater than $b \sim 0.4$ of their combined Einstein ring radius are detectable assuming a detection threshold of $3\%$. For two M dwarfs, this corresponds to a limiting separation of $\gsim 1 \au$. Since very few observed M dwarfs have companions at separations $\lsim 1 \au$, we conclude that close binaries will probably not corrupt the measurements of the mass function. We find that the detectability depends only weakly on the mass ratio. For those events for which individual masses can be determined, we find that binaries can be detected down to $b \sim 0.2$.

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arXiv1996

Femtolens Imaging of a Quasar Central Engine Using a Dwarf Star Telescope

We show that it is possible to image the structure of a distant quasar on scales of $\sim 1\,$AU by constructing a telescope which uses a nearby dwarf star as its ``primary lens'' together with a satellite-borne ``secondary''. The image produced by the primary is magnified by $\sim 10^5$ in one direction but is contracted by 0.5 in the other, and therefore contains highly degenerate one-dimensional information about the two-dimensional source. We discuss various methods for extracting information about the second dimension including ``femtolens interferometry'' where one measures the interference between different parts of the one-dimensional image with each other. Assuming that the satellite could be dispatched to a position along a star-quasar line of sight at a distance $r$ from the Sun, the nearest available dwarf-star primary is likely to be at $\sim 15\,\pc\,(r/40\,\rm AU)^{-2}$. The secondary should consist of a one-dimensional array of mirrors extending $\sim 700\,$m to achieve 1 AU resolution, or $\sim 100\,$m to achieve 4 AU resolution.

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