arXiv subjects

Penny D. Sackett

Publications and source records attributed to Penny D. Sackett.

31 records · Page 2Linked to original sources

arXiv1999

Planetary Microlensing: Present Status and Long-term Goals

Massive gravitational microlensing programs were begun about a decade ago as a means to search for compact baryonic dark matter in the Galaxy, but before the first events were detected the technique was also proposed as a means of detecting extra-solar planets in our Galaxy. Current microlensing planet searches, which have been underway for four years, are sensitive to jovian-mass planets orbiting a few to several AU from their parent Galactic stars. Within two years, sufficient data should be in hand to characterize or meaningfully constrain the frequency of massive planets in this range of parameter space, nicely complementing information about planets at smaller orbital radii now being provided by radial velocity searches. In principle, the technique could be pushed to smaller planetary masses, but only if a larger number of faint microlensed sources can be monitored with higher precision and temporal sampling. The VST on Paranal, with spectroscopic follow-up with the VLT, may be the ideal instrument for such an ambitious program.

astro-ph↗

arXiv1999

Detection Efficiencies of Microlensing Datasets to Stellar and Planetary Companions

Microlensing light curves are now being monitored with the precision required to detect small perturbations due to planetary companions of the primary lens. Microlensing is complementary to other planetary search techniques in its potential for measuring the statistical frequency of planets beyond the solar neighborhood. We present an algorithm to analyze the efficiency with which the presence of binaries can be detected in real microlensing datasets. Such an analysis is required in order to draw statistical inferences about lensing companions. We apply the method to artificial (but realistic) data to explore the dependence of detection efficiencies on observational parameters, the impact parameter of the event, the finite size of the background source, the amount of unlensed (blended) light, and the criterion used to define a detection. We find that: (1) the integrated efficiency depends strongly on the impact parameter distribution of the monitored events, (2) calculated detection efficiencies are robust to changes in detection criterion for strict criteria and large mass ratios, (3) finite sources can dramatically alter detection efficiencies to companions with small mass ratios, and (4) accurate determination of the blended light fraction is crucial for the accurate determination of the detection efficiency of individual events. Suggestions are given for addressing complications associated with computing accurate detection efficiencies of real datasets.

astro-ph↗

arXiv1999

The Shape of Dark Matter Halos

Techniques for inferring the radial and geometric form of dark matter halos and the results they have produced to date are reviewed. Dark halos appear to extend to at least ~50 kpc with total enclosed masses that rise linearly with radius R. Whether this behavior can be extrapolated to distances as large as 200 kpc and beyond is controversial; results at this radius are model-dependent. Observationally, the geometrical form of the dark halo can be characterized by the equatorial axis ratio b/a (ovalness) and vertical-to-equatorial axis ratio c/a (flattening) of the total density. Different techniques consistently yield b/a > 0.7 (and thus b/a > 0.9 for the potential) at R~20 kpc, with more axisymmetric values, b/a >~ 0.8, being more likely. Results are less consistent for the vertical flattening, perhaps due to the difference in the spatial regions probed by different techniques or inappropriate assumptions. Techniques that probe furthest from the stellar plane z~15 kpc consistently implicate substantially flattened c/a = 0.5 +/- 0.2 dark halos. These axis ratios are in acceptable agreement with expectations from N-body simulations of cold dark matter mixed with ~10% dissipational gas.

astro-ph↗

arXiv1998

Searching for Unseen Planets via Occultation and Microlensing

The fields of occultation and microlensing are linked historically. Early this century, occultation of the Sun by the Moon allowed the apparent positions of background stars projected near the limb of the Sun to be measured and compared with their positions six months later when the Sun no longer influenced the light path to Earth. The measured shift in the stellar positions was consistent with lensing by the gravitational field of the Sun during the occultation, as predicted by the theory of general relativity. This series of lectures explores the principles, possibilities and challenges associated with using occultation and microlensing to discover and characterize unseen planets orbiting distant stars. The two techniques are complementary in terms of the information that they provide about planetary systems and the range of system parameters to which they are most sensitive. Although the challenges are large, both microlensing and occultation may provide avenues for the discovery of extra-solar planets as small as Earth.

astro-ph↗

arXiv1998

Microlensing by Multiple Planets in High Magnification Events

Microlensing is increasingly gaining recognition as a powerful method for the detection and characterization of extra-solar planetary systems. Naively, one might expect that the probability of detecting the influence of more than one planet on any single microlensing light curve would be small. Recently, however, Griest & Safizadeh (1998) have shown that, for a subset of events, those with minimum impact parameter $u_{min} \lsim 0.1$ (high magnification events), the detection probability is nearly 100% for Jovian mass planets with projected separations in the range 0.6--1.6 of the primary Einstein ring radius $R_E$, and remains substantial outside this zone. In this Letter, we point out that this result implies that, regardless of orientation, all Jovian mass planets with separations near 0.6--1.6$R_E$ dramatically affect the central region of the magnification pattern, and thus have a significant probability of being detected (or ruled out) in high magnification events. The probability, averaged over all orbital phases and inclination angles, of two planets having projected separations within 0.6--$1.6R_E$ is substantial: 1-15% for two planets with the intrinsic orbital separations of Jupiter and Saturn orbiting around 0.3--1.0$M_\odot$ parent stars. We illustrate by example the complicated magnification patterns and light curves that can result when two planets are present, and discuss possible implications of our result on detection efficiencies and the ability to discriminate between multiple and single planets in high magnification events.

astro-ph↗

arXiv1998

Red Clump Morphology as Evidence Against a New Intervening Stellar Population as the Primary Source of Microlensing Toward the LMC

We examine the morphology of the color-magnitude diagram (CMD) for core helium-burning (red clump) stars to test the recent suggestion by Zaritsky & Lin (1997) that an extension of the red clump in the Large Magellanic Cloud (LMC) toward brighter magnitudes is due to an intervening population of stars that is responsible for a significant fraction of the observed microlensing toward the LMC. Using our own CCD photometry of several fields across the LMC, we confirm the presence of this additional red clump feature, but conclude that it is caused by stellar evolution rather than a foreground population. We do this by demonstrating that the feature (1) is present in all our LMC fields, (2) is in precise agreement with the location of the blue loops in the isochrones of intermediate age red clump stars with the metallicity and age of the LMC, (3) has a relative density consistent with stellar evolution and LMC star formation history, and (4) is present in the Hipparcos CMD for the solar neighborhood where an intervening population cannot be invoked. Assuming there is no systematic shift in the model isochrones, which fit the Hipparcos data in detail, a distance modulus of $μ_{LMC} = 18.3$ provides the best fit to our dereddened CMD.

astro-ph↗

arXiv1997

Planet Detection via Microlensing

Microlensing is the most promising method to study the statistical frequency of extra-solar planets orbiting typical (random) stars in the Milky Way, even those several kiloparsecs from Earth. The lensing zone corresponds to orbital separations of a few times the Earth-Sun distance (AU) -- a good match to many planets in our own Solar System -- and the probability of detection is a rather weak function of planetary mass. Microlensing is thus a perfect complement to radial velocity and astrometric techniques that allow the detailed study of nearby planets with larger masses and smaller orbital separations. This report forms Appendix C of the Final Report of the European Southern Observatory (ESO) Working Group on the Detection of Extrasolar Planets, which contains recommendations to ESO for designing a competitive strategy in the field of extrasolar planets. The full report is available from ESO as document SPG-VLTI-97/002.

astro-ph↗

arXiv1997

Does the Milky Way have a Maximal Disk?

The Milky Way is often considered to be the best example of a spiral for which the dark matter not only dominates the outer kinematics, but also plays a major dynamical role in the inner galaxy: the Galactic disk is therefore said to be ``sub-maximal.'' This conclusion is important to the understanding of the evolution of galaxies and the viability of particular dark matter models. The Galactic evidence rests on a number of structural and kinematic measurements, many of which have recently been revised. The new constraints indicate not only that the Galaxy is a more typical member of its class (Sb-Sc spirals) than previously thought, but also require a re-examination of the question of whether or not the Milky Way disk is maximal. By applying to the Milky Way the same definition of ``maximal disk'' that is applied to external galaxies, it is shown that the new observational constraints are consistent with a Galactic maximal disk of reasonable $M/L$. In particular, the local disk column can be substantially less than the oft-quoted required $Σ_{\odot} \approx 100 \msolar pc^{-2}$ - as low as $40 \msolar pc^{-2}$ in the extreme case - and still be maximal, in the sense that the dark halo provides negligible rotation support in the inner Galaxy. This result has possible implications for any conclusion that rests on assumptions about the potentials of the Galactic disk or dark halo, and in particular for the interpretation of microlensing results along both LMC and bulge lines of sight.

astro-ph↗

arXiv1995

The Distribution of Dark Mass in Galaxies: Techniques, Puzzles, and Implications for Lensing

Gravitational lensing is one of a number of methods used to probe the distribution of dark mass in the Universe. On galactic scales, complementary techniques include the use of stellar kinematics, kinematics and morphology of the neutral gas layer, kinematics of satellites, and morphology and temperature profile of X-ray halos. These methods are compared, with emphasis on their relative strengths and weaknesses in constraining the distribution and extent of dark matter in the Milky Way and other galaxies. It is concluded that (1) the extent of dark halos remains ill-constrained, (2) halos need not be isothermal, and (3) the dark mass is probably quite flattened.

astro-ph↗

arXiv1994

Another Flattened Dark Halo: Plar Ring Galaxy A0136-0801

Knowledge of the shape of dark matter halos is critical to our understanding of galaxy formation, dynamics, and of the nature of dark matter itself. Polar ring galaxies (PRGs) --- early-type galaxies defined by their outer rings of gas, dust and stars on orbits nearly perpendicular to those of the central host --- provide a rare probe of the vertical-to-radial axis ratio $ (q_ρ = c/a)$ of dark halos. We present a Fabry-Perot velocity field for the H$α$ gas in the kinematically-confirmed PRG \gal. By comparing ring orbits evolved in a generalized mass model to the observed ring velocity field and morphology of \gal, we conclude that $q_ρ\sim 0.5$ and rule out a spherical geometry.

astro-ph↗

arXiv1994

A Faint Luminous Halo that May Trace the Dark Matter around Spiral Galaxy NGC~5907

The presence of unseen halos of ``dark matter'' has long been inferred from the high rotation speeds of gas and stars in the outer parts of spiral galaxies$^{1}$. The volume density of this dark matter decreases less quickly from the galactic center than does that of the luminous mass (such as that in stars), meaning that the dark matter dominates the mass far from the center$^{1,2}$. While searching for faint starlight away from the plane of the edge-on disk galaxy \gal$^{3}$, we have found that the galaxy is surrounded by a faint luminous halo. The intensity of light from this halo falls less steeply than any known luminous component of spiral galaxies, but is consistent with the distribution of dark mass inferred from the galaxy's rotation curve.

astro-ph↗

arXiv1994

The Flattened Dark Halo of Polar Ring Galaxy NGC 4650A: A Conspiracy of Shapes?

Kinematics and photometry of the polar ring galaxy NGC 4650A, including new observations of the rotation and velocity dispersion of its central stellar disk, are used to infer the presence of a dark matter halo and to measure its shape. Fits to the observed disk and polar ring rotation curves from detailed mass and photometric modeling rule out a spherical dark halo. The best fit models have halos with isodensity surfaces that are flattened to a shape between E6 and E7 (axis ratios between 0.4 and 0.3); the asymptotic equatorial speeds of these models are in excellent agreement with the I-band Tully-Fisher relation. This degree of dark halo flattening is larger than that expected from N-body collapse simulations of dissipationless dark matter. Since the kinematics and surface brightness profile of the central luminous body indicate that its light has an intrinsic axis ratio $c/a \lsim 0.4$, in NGC 4650A the radial ``conspiracy" between the dark and luminous components that leads to flat rotation curves may extend to the {\it shape} of the mass distribution as well.

astro-ph↗

arXiv1993

MACHOs in a Flattened Halo

If massive compact halo objects (\ms) are detected in ongoing searches, then \tsmctlmc, the ratio of the optical depth toward the Small and Large Magellanic Clouds, will be a robust indicator of the flattening of the Galactic dark matter halo. For a spherical halo, \tsmctlmc\ is about 1.45, independent of details of the shape of the Galactic rotation curve, the assumed mass of the Galactic disk and spheroid, and the truncation distance (if any) of the dark halo. For an E6 halo (axis ratio $c/a = 0.4$), the ratio of optical depths is \tsmctlmc\ $\sim 0.95$, again independent of assumptions about Galactic parameters. This ratio can be measured with a precision as good as $\sim 10\%$ depending on the typical mass of the MACHOs. If the halo is highly flattened (\eg E6) and closely truncated (\eg at twice the solar galactocentric radius), then the optical depth toward the LMC can be reduced by a factor of about two. For these extreme parameters, and the assumption of a heavy Galactic disk and spheroid, the upper limit of the \m\ mass range to which ongoing experiments are sensitive is reduced from ${\cal O}(10^6)\,M_\odot$ to ${\cal O}(10)\,M_\odot$.

astro-ph↗