arXiv subjects

Rob P. Olling

Publications and source records attributed to Rob P. Olling.

13 recordsLinked to original sources

arXiv2009

"An Era of Precision Astrophysics: Connecting Stars, Galaxies and the Universe," an Astro2010 Science White Paper

Abridged: The golden age of astrophysics is upon us with both grand discoveries (extra-solar planets, dark matter, dark energy) and precision cosmology. Fundamental understanding of the working of stars and galaxies is within reach, thanks to newly available precision measurements. We highlight the importance of distances and model independent distances and masses. Distances are fundamental in astrophysics and their knowledge can change our perception of phenomena dramatically: e.g., in antiquity, the Heliocentric model was rejected because the predicted stellar parallaxes were not observed. Distance measurements are directly related to the history & fate of the uni- verse as they provide 2 of 3 methods available to date the universe. The 1st method is based on the ages of stars, which can be ascertained if their lumi- nosities (distances) are accurately known. The 2nd method relies on cosmolo- gical methods. To 1st order, the age of the universe is the inverse H_0. As stressed by the previous decadal report, "the fundamental goal of ... astrophysics is to understand how the universe ... galaxies [and] stars ... formed, how they evolved, and what their destiny will be." These questions can be answered partly by micro-arcsecond astrometry: 1) Galactic archeology: a detailed reconstruction of the formation history of the Milky Way and other Local Group galaxies, 2) the very oldest stars in the Milky Way and the age of the Universe, and 3) H_0 and concordance cosmology. These goals are achievable by combining muas-arcsecond astrometry from the proposed SIM-Lite mission supplemented with ground-based spectroscopy. The results of our proposed project will force the biggest reassessment of stellar astrophysics in 50 years, which will affect most branches of astrophysics.

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arXiv2007

Taking the Measure of the Universe: Precision Astrometry with SIM PlanetQuest

Precision astrometry at microarcsecond accuracy has application to a wide range of astrophysical problems. This paper is a study of the science questions that can be addressed using an instrument that delivers parallaxes at about 4 microarcsec on targets as faint as V = 20, differential accuracy of 0.6 microarcsec on bright targets, and with flexible scheduling. The science topics are drawn primarily from the Team Key Projects, selected in 2000, for the Space Interferometry Mission PlanetQuest (SIM PlanetQuest). We use the capabilities of this mission to illustrate the importance of the next level of astrometric precision in modern astrophysics. SIM PlanetQuest is currently in the detailed design phase, having completed all of the enabling technologies needed for the flight instrument in 2005. It will be the first space-based long baseline Michelson interferometer designed for precision astrometry. SIM will contribute strongly to many astronomical fields including stellar and galactic astrophysics, planetary systems around nearby stars, and the study of quasar and AGN nuclei. SIM will search for planets with masses as small as an Earth orbiting in the `habitable zone' around the nearest stars using differential astrometry, and could discover many dozen if Earth-like planets are common. It will be the most capable instrument for detecting planets around young stars, thereby providing insights into how planetary systems are born and how they evolve with time. SIM will observe significant numbers of very high- and low-mass stars, providing stellar masses to 1%, the accuracy needed to challenge physical models. Using precision proper motion measurements, SIM will probe the galactic mass distribution and the formation and evolution of the Galactic halo. (abridged)

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arXiv2007

Accurate Extra-Galactic Distances and Dark Energy: Anchoring the Distance Scale with Rotational Parallaxes

We investigate how the uncertainty on the Hubble constant (H_0) affects the uncertainty in the Equation of State (EOS) of Dark Energy and the total density of the Universe (Omega_tot). We use the approximate relations between the cosmological parameters [Spergel etal (2007)] and use error-propagation to estimate the effects of improving the CMB parameters and H_0 on the EOS of Dark Energy (DE). First we assume that the additional data does not improve significantly, but decrease the error on H_0 by a factor <~10. Second, we allow improved additional data but current H_0 errors (i.e., the DE Task Force case). In the 1st scenario, improvements of the CMB parameters hardly change the accuracy of the EOS and Omega_tot, unless H_0 can be measured with an accuracy of a few %. We find that a combination of moderate improvements for both H_0 and other data significantly constrains the evolution of dark energy, but at a reduced cost. We review several methods (and their strengths and weaknesses) that might yield extra-galactic distances with errors of about 1%. We review: the Velocity Field method, two Maser methods, two Light Echo techniques, the Binary Star method, and the Rotational Parallax (RP) technique. Because these methods substantially rely on geometry rather than astrophysics or cosmology, their results are quite robust. We focus on the advantages of the RP technique which can provide single-step, bias-free distances to nearby spirals. These distances can be used to improve the zero-point for other methods which in turn allow for a much improved H_0 errors. Achieving an accuracy of ~2% in the distances to M31, M33 and the LMC by the RP method requires proper motions from future astrometric missions (SIM, GAIA and OBSS, or the SKA).

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arXiv2007

Finding Solar System Analogs With SIM and HIPPARCOS: A White Paper for the ExoPlanet Task Force

The astrometric signature imposed by a planet on its primary increases substantially towards longer periods (proportinal to P^2/3), so that long-period planets can be more easily detected, in principle. For example, a one Solar-mass (M_Sun) star would be pulled by roughly 1 mas by a one Jupiter-mass (M_J) planet with a period of one-hundred years at a distance of 20 pc. Such position accuracies can now be obtained with both ground-based and space-based telescopes. The difficulty was that it often takes many decades before a detectable position shift will occur. However, by the time the next generation of astrometric missions such as SIM will be taking data, several decades will have past since the first astrometric mission, HIPPARCOS. Here we propose to use a new astrometric method that employs a future, highly accurate SIM Quick-Look survey and HIPPARCOS data taken twenty years prior. Using position errors for SIM of 4 muas, this method enables the detection and characterization of Solar-system analogs (SOSAs) with periods up to 240 (500) years for 1 (10) M_J companions. Because many tens of thousands nearby stars can be surveyed this way for a modest expenditure of SIM time and SOSAs may be quite abundant, we expect to find many hundreds of extra-solar planets with long-period orbits. Such a data set would nicely complement the short-period systems found by the radial-velocity method. Brown dwarfs and low-mass stellar companions can be found and characterized if their periods are shorter than about 500 years. This data set will provide invaluable constraints on models of planet formation, as well as a database for systems where the location of the giant planets allow for the formation of low-mass planets in the habitable zone. [Abridged]

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arXiv2007

LEAVITT: A MIDEX-class Mission for Finding & Characterizing 10,000 Transiting Planets in the Solar Neighborhood

We propose a MIDEX-class space mission with the goal to find and characterize roughly 10,000 transiting planets. When transits occur, a much more detailed characterization of the planet is possible and so a large data base of transiting planets will provide planets with a large range in periods and radii for follow-up studies. Our survey will be all-sky and focused on stars brighter than V=14.8. Down to V=12, LEAVITT will be able to detect Neptune-sized objects. Because of it's high cadence, LEAVITT is about 100 times more sensitive at detecting transits than GAIA, while it will find more than 20 times as many transits as KEPLER. LEAVITT has multi-band photometric capability implemented via a low-resolution dispersive element which can obtain 0.2% (2 mmag) photometry down to V=14.8. LEAVITT's high multi-band photometric accuracy reduces the number of false-positives significantly.

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arXiv2005

Astrometric Binaries in the Age of the Next Generation of Large (Space) Telescopes

I analyze several catalogs of known visual and spectroscopic binaries and conclude that a large number of binaries is missing in current catalogs. Samples of the best studied (nearby and bright) stars indicate that the true binary fraction may be as high as 95%. A preliminary analysis indicates that these binaries can a ect the astrometry signicantly.

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arXiv2003

The Oort Constants Measured from Proper Motions

The Oort constants describe the local variations of the stellar streaming field. Classically, they are determined from stellar proper motions. We discuss problems arising in this procedure. A large, hitherto overlooked, source of systematic error arises from longitudinal variations of the mean stellar parallax. Together with the solar reflex motion, these variations create contributions to the longitudinal proper motions mu_l*(l) that are indistinguishable from the Oort Constants at the 20% level. Fortunately, we can correct for this "mode mixing" using the latitudinal proper motions mu_b(l). We use ~10^6 stars from the ACT/Tycho-2 catalogs with proper motion errors of ~ 3 mas/yr. We find significant deviations from expectations based on a smooth axisymmetric equilibrium disk, in particular non-zero C for old red giant stars. We also find variations of the Oort constants with the asymmetric drift of the sub-sample. These correlations are different in nature than those expected for an axisymmetric Galaxy. The most reliable tracers for the ``true'' Oort constants are red giants, which are old enough to be in equilibrium and distant enough to be unaffected by possible local anomalies. For these stars we find, A ~ 16, B ~- 17, A - B ~ 33, and C ~- 10 km/s/kpc with internal errors of about 1-2 and external error of perhaps the same order. These values are consistent with our knowledge of the Milky Way (flat rotation curve and Omega=A - B ~ 28 +/- 2). Based on observations made with the ESA Hipparcos astrometry satellite. (Abridged)

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arXiv1997

Refining the Oort Constants: the case for a smaller Milky Way

The local stellar kinematics of the Milky Way, parameterized by the Oort constants A and B, depend on the local gradient of the rotation curve, its absolute value (Theta_0), and the distance to the Galactic center (R_0). The surface density of interstellar gas in the Milky Way varies non-monotonically with radius, and so contributes significantly to the local gradient of the rotation curve, and the Oort constants. Because of this, the Oort functions A(R) and B(R) differ significantly from the dominant ~Theta_0/R dependence, in the Solar neighborhood and other locations in the Galaxy. These models may explain the ~40% difference between the values for 2 A R_0 derived from radial velocity data originating in the inner and outer Galaxy (Merrifield 1992). Incorporating these local non-linearities explains the significant differences between the Oort constants derived from nearby stars (d <= 1 kpc; Hanson 1987) and distant Cepheids (d=0.5-6 kpc; Feast & Whitelock 1997). However, a consistent picture only emerges if one adopts small values for the Galactic constants: R_0 = 7.1 +/- 0.4 kpc, and Theta_0 = 184 +/- 8 km/s. These values are consistent with most kinematical methods of determining R_0, including the proper motion of Sgr A* (Backer 1996), the direct determination of R_0 using water masers (7.2 +/- 0.7 kpc; Reid 1993), and constraints set by the shape of the Milky Way's dark halo (Olling & Merrifield 1997b).

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arXiv1996

The Flattened Dark Matter Halos of NGC 4244 and the Milky Way

In a previous paper (Paper I) a method was developed to determine the shapes of dark matter halos of spiral galaxies from the flaring and velocity dispersion of the gas layer. Here I present the results for the almost edge-on Scd galaxy NGC 4244 (Paper II Paper III) and preliminary results for the Milky Way. NGC 4244's dark matter halo is found to be highly flattened with a shortest-to-longest axis ratio of 0.2 +0.3-0.1. If the dark matter is disk-like, the vertical velocity dispersion of the dark matter must be ~20% larger than the measured tangential dispersion in the HI. The flaring of the Milky Way's gas layer, the local column of identified stars and the total column within 1.1 kpc from the plane are consistent with a moderately flattened dark halo (E7-E0) and galactic constants of \RSUN = 7.1 kpc, \VSUN = 180 \kms, while the stellar disk is ~80% of maximal.

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arXiv1996

The Flattened Dark Matter Halo of NGC 4244

In a previous paper (Olling 1995, astro-ph/9505002) a method was developed to determine the shapes of dark matter halos of spiral galaxies from an accurate determination of the rotation curve, the flaring of the gas layer and the velocity dispersion in the HI. Here I report the results for the almost edge-on Scd galaxy NGC 4244 (Olling 1996a,astro-ph/9605110; 1996b, astro-ph/9605111). The observed flaring of the HI beyond the optical disk puts significant constraints on the shape of the dark matter halo, which are almost independent of the stellar mass-to-light ratio. NGC 4244's dark matter halo is found to be highly flattened with a shortest-to-longest axis ratio of 0.2 (-0.1,+0.3). If the dark matter is disk-like, the data presented in this paper imply that the vertical velocity dispersion of the dark matter must be 10% - 30% larger than the measured tangential dispersion in the HI.

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arXiv1996

NGC 4244: A Low Mass Galaxy with a Falling Rotation Curve and a Flaring Gas Layer

I present sensitive high resolution HI VLA B+C+D array observations of the edge-on Scd galaxy NGC 4244. The gas layer of NGC 4244 is rather symmetric in all respects, i.e. the surface density distribution, flaring and warping, so that the rotation curve (RC) is determined accurately, despite the fact that i~90^o. The RC rises slowly in the inner 6 kpc, is ~ constant at 100 km/s out to 10kpc, and decreases in Keplerian fashion by 15% at the last measured point at 14 kpc. The RC constrains the stellar mass-to-light ratio to lie between 50 and 100% of the ``maximum-disk'' value. A new technique is presented to determine simultaneously the inclination and the thickness of the gas layer from HI observations which uses the apparent widths at many channels and works for i >= 60^o. The inclination of the un- warped disk is about 84.5 degrees: beyond D25/2 the inclination changes gra- dually till 82.5^o, while at large radii the disk seems to warp back to the plane defined by the inner disk. The gaseous velocity dispersion is roughly constant within the optical disk (8.5+/-1 km/s) and increases slightly beyond. The thickness of the gas layer increases gradually from 0.4 to 1.5 kpc. In an accompanying paper (AJ, Aug. 1996; astro-ph/9605111). I use the measurements presented in this paper to infer that the dark matter halo of NGC 4244 is highly flattened.

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arXiv1996

The Highly Flattened Dark Matter Halo of NGC 4244

In a previous paper (Olling 1995, \aj, 110, 591; astro-ph/9505002) a method was developed to determine the shapes of dark matter halos of spiral galaxies from an accurate determination of the rotation curve, the flaring of the gas layer and the velocity dispersion in the HI. Here this method is applied to the almost edge-on Scd galaxy NGC 4244 for which the necessary parameters are determined in the accompanying paper (AJ, Aug. 1996; astro-ph/9605110). The observed flaring of the HI beyond the optical disk puts significant constraints on the shape of the dark matter halo, which are almost independent of the stellar mass-to-light ratio. NGC 4244's dark matter halo is found to be highly flattened with a shortest-to-longest axis ratio of 0.2 (-0.1)(+0.3). If the dark matter is disk-like, the data presented in this paper imply that the vertical velocity dispersion of the dark matter must be 10% - 30% larger than the measured tangential dispersion in the HI. Alternatively, the measured flaring curve is consistent with a round halo if the gaseous velocity dispersion ellipsoid is anisotropic. In that case the vertical dispersion of the gas is 50 - 70% of the measured tangential velocity dispersion.

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arXiv1995

On the usage of Flaring Gas Layers to determine the Shape of Dark Matter Halos

I present a new method of deriving the shape of the dark matter (DM) halos of spiral galaxies. The method relies on the comparison of model predictions with high spectral and spatial resolution HI observations of the gas layer. The potential arising from the {\em total} mass distribution of the galaxy is used in the calculation of the vertical distribution of the gas. I developed a new algorithm to calculate the force field of an arbitrary, azimuthally symmetric, density distribution. This algorithm is used to calculate the forces due to the radially truncated stellar disk as well as of the flaring gas layer. I use a simple two-parameter family of disk-halo models which have essentially the same observed equatorial rotation curve but different vertical forces. This mass model is composed of a stellar disk with constant M/L, and a DM-halo with a given axial ratio. I approximate the radial force due to the gaseous disk, and iteratively determine the vertical force due to the global distribution of the gas. The thickness of the gaseous disk is sensitive to both the flattening of the DM-halo and the self-gravity of the gas, but not to the particular choice of disk-halo decomposition. I show that the determination of the thickness of the gas layer is not restricted to edge-on galaxies, but can be measured for moderately inclined systems as well.

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