Astronomy and Astrophysics Supplement Series, 1997
the optical. In this case, the detection of the near infrared counterpart is the only way in whic... more the optical. In this case, the detection of the near infrared counterpart is the only way in which we can extend the study of these sources to other spectral ranges and may be crucial to understand the short-lived phase which precedes the formation of a new planetary nebula.
This memo reviews some of the trades regarding a grating and a set of filters for the AMEX missio... more This memo reviews some of the trades regarding a grating and a set of filters for the AMEX mission. 1.
To be published in the Aug. 1996 issue of the Astronomical Journal In a previous paper (Olling 19... more To be published in the Aug. 1996 issue of the Astronomical Journal In a previous paper (Olling 1995, AJ, 110, 591) 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 H I. 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). The observed flaring of the H I beyond the optical disk puts significant constraints on the shape of the dark matter halo, which are almost independent of the stellar mass-tolight ratio. NGC 4244’s dark matter halo is found to be highly flattened with a shortestto-longest axis ratio of 0.2 +0.3 −0.1. 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 H I. A...
The astrometric signature imposed by a planet on its primary increases substantially towards long... more The astrometric signature imposed by a planet on its primary increases substantially towards longer periods ( ∝ P 2/3), so that long-period planets can be more easily detected, in principle. For example, a one Solar-mass (M⊙) star would be pulled by roughly 1 milliarcsec (mas) by a one Jupiter-mass (MJ) planet with a period of one-hundred years at a distance of 20 pc [cf. eqn. (3) below]. 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 1 [e.g., (Edberg et al. 2005)] will be taking data, several decades will have past since the first astrometric mission, HIPPARCOS (ESA 1997). Here we propose to use a new astrometric method that employs a future, highly accurate SIM Quick-Look (SQL) survey and HIPPARCOS data taken twenty years prior. Using a conservative position e...
Axisymmetric models of the Milky Way exhibit strong interrelations between the Galactic constants... more Axisymmetric models of the Milky Way exhibit strong interrelations between the Galactic constants [the Sun’s distance to the Galactic centre (R0), and the local rotation speed (Θ0)], the local stellar columndensity (Σ∗(R0)) and the shortest-to-longest axis ratio of the dark matter halo (q). In this paper we present simple analytical approximations that allow for an efficient search through the vastness of parameter space, and apply this approximation to investigate the consequences of the uncertain gaseous velocity dispersion (σg) on the constraints imposed by the thickness of the Milky Way’s gas layer. The extra degree of freedom does not significantly alter the conclusions drawn in a previous paper on the shape of the Milky Way’s dark matter halo. A significant contribution to the total gas pressure by cosmic rays and magnetic fields beyond the optical disk is thus ruled out. We find that the Milky Way’s dark halo is close to spherical if R0 ∼> 7.1 kpc, while a significantly fl...
We develop Bayesian statistical methods for discovering and assigning probabilities to non-random... more We develop Bayesian statistical methods for discovering and assigning probabilities to non-random (e.g., physical) stellar companions. These companions are either presently bound or were previously bound. The probabilities depend on similarities in corrected proper motion parallel and perpendicular to the brighter component's motion, parallax, and the local phase-space density of field stars. Control experiments are conducted to understand the behavior of false positives. The technique is applied to the Hipparcos Catalogue within 100 pc. This is the first all-sky survey to locate escaped companions still drifting along with each other. In the < 100 pc distance range, ∼220 high probability companions with separations between 0.01-1 pc are found. The first evidence for a population (∼300) of companions separated by 1-8 pc is found. We find these previously unnoticed naked-eye companions (both
The Space Interferometry Mission (SIM PlanetQuest) can provide astrometric data of such high qual... more The Space Interferometry Mission (SIM PlanetQuest) can provide astrometric data of such high quality that it will be possible to determine geometric (luminosity-independent) dis-tances to the nearest spiral galaxies, even though these systems are too far away for direct trigonometric parallax determination. Instead, the method to use is that of the \Rotational
Abstract. I analyze several catalogs of known visual and spectroscopic binaries and conclude that... more Abstract. 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 affect the astrometry significantly. 1. Binarity: Past Present & Future Duquennoy & Mayor (1991, hereafter referred to as DM1991) established the multiplicity of G-type stars within about 22 pc. Their observational dataset consists of thirteen years of radial velocity monitoring, in combination with astrometric, visual and eclipsing binaries, for 164 G-type stars. However, the pre-Hipparcos lists of wide binaries were rather incomplete. Currently, the number of known binaries is about three times larger. In fact, an analysis of the currently available binarity and multiplicity data of nearby Hipparcos-selected G-type stars indicates that as many as 95 % of pr...
The local stellar kinematics of the Milky Way, parameterized by the Oort constants A and B, depen... more 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 (Θ0), and the distance to the Galactic center (R0). 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 ∼ Θ0/R dependence, in the Solar neighborhood and other locations in the Galaxy. These models may explain the ∼40 % difference between the values for 2AR0 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=H87) and distant Cepheids (d = 0.5−6 kpc; Feast & Whitelock 1997=FW97). However, a consiste...
I present a new method of deriving the shape of the dark matter (DM) halos of spiral galaxies. Th... more 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. So far, determinations of the flaring of the gas layer (i.e. the increase of the thickness with galactocentric radius) have been used to determine the mass-to-light ratio, M/L, of the stellar disk of several edge-on galaxies. In this paper I describe a method which can be used to determine the shape of DM-halos. This technique will be applied in a forthcoming paper. I show that the model predictions of the gas layer width are best calculated using a global approach, in which the potential arising from the 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 calcula...
To be published in the Aug. 1996 issue of the Astronomical Journal I present sensitive high resol... more To be published in the Aug. 1996 issue of the Astronomical Journal I present sensitive high resolution VLA B, C, and D array observations of the almost edge-on Scd galaxy NGC 4244 in the 21-cm spectral line of neutral atomic hydrogen. The gas layer of NGC 4244 is rather symmetric in all respects, i.e. the surface density distribution, flaring and warping. This symmetry allows for a reliable determination of the rotation curve, despite the fact that the galaxy is close to edge-on. The rotation curve rises slowly in the inner 6 kpc, is roughly constant at 100 km s −1 out to 10 kpc, and decreases in Keplerian fashion by 15 % at the last measured point at 14 kpc. The rotation curve 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 high resolution H I observations. This procedure uses the apparent widths at many azimuths (many...
In a previous paper 26 a method was developed to determine the shapes of dark matter halos of spi... more In a previous paper 26 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 27,28 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 H I. 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 R0 = 7.1 kpc, Θ0 = 180 km s −1, while the stellar disk is ∼ 80 % of maximal. 1
We investigate how the current and future uncertainty on the Hubble constant affects the uncertai... more We investigate how the current and future uncertainty on the Hubble constant affects the uncertainty in the Equation of State of Dark Energy (w) and the total density of the Universe (Ωtot). We start with the approximate linear relations between the cosmological parameters as presented by (Spergel et al. 2006), and use the standard error-propagation relations to estimate the effects of improving the Cosmic Microwave Background (CMB) parameters as well as the Hubble constant (H0) on our knowledge of the Equation of State (EOS) of Dark Energy. Because we do not assume a flat universe, we also estimate the attainable accuracy on Ωtot and the spatial curvature of the Universe. In one limiting case, we assume that the constraints provided by additional data (galaxy clustering, weak lensing and so forth) do not improve significantly, while the error on the Hubble constant is decreased by a factor up to ten. The other limiting case of significantly improved additional data with current H0 ...
The local stellar kinematics of the Milky Way offer a useful tool for studying the rotation curve... more The local stellar kinematics of the Milky Way offer a useful tool for studying the rotation curve of the Galaxy. These kinematics – usually parameterized by the Oort constants A and B – depend on the local gradient of the rotation curve as well as its absolute value (Θ0), and the Sun’s distance to the Galactic center (R0). The density of interstellar gas in the Milky Way is shown to vary non-monotonically with radius, and so contributes significantly to the local gradient of the rotation curve. We have therefore calculated mass models for the Milky Way that include this component, and have derived the corresponding radial variation in the Oort constants. Between 0.9R0 and 1.2R0 the Oort functions A(R) and B(R) differ significantly from the general ∼ Θ0/R dependence. Various previously-inexplicable observations are shown to be consistent with these new predictions. For example, these models may explain the ∼40 % difference between the values for 2AR0 derived from radial velocity data...
In order to test the reliability of determinations of the shapes of galaxies ’ dark matter halos,... more In order to test the reliability of determinations of the shapes of galaxies ’ dark matter halos, we have made such measurements for the Milky Way by two independent methods. First, we have combined the measurements of the over-all mass distribution of the Milky Way derived from its rotation curve and the measurements of the amount of dark matter in the solar neighborhood obtained from stellar kinematics to determine the flattening of the dark halo. Second, we have used the established technique based on the variation in thickness of the Milky Way’s Hi layer with radius: by assuming that the Hi gas is in hydrostatic equilibrium in the gravitational potential of a galaxy, one can use the observe flaring of the gas layer to determine the shape of the dark halo. These techniques are found to produce a consistent estimate for the flattening of the dark matter halo, with a shortest-to-longest axis ratio of q ∼ 0.8, but only if one adopts somewhat non-standard values for the distance to t...
currently studying a mission called SIM Lite, as a more cost-effective way to do the precision as... more currently studying a mission called SIM Lite, as a more cost-effective way to do the precision astrometry recommended by the both of the last two Astronomy and Astrophysics Decadal Surveys (Bahcall 1991; McKee & Taylor 2001). SIM Lite is a variant on the design of SIM PlanetQuest (Nemati & Shao 2008) and inherits the same level of technical maturity. The project has carried the design and development of SIM PlanetQuest nearly through NASA Phase B. SIM Lite has the same functionality as SIM PlanetQuest, which allows it to tackle the breadth of science areas described in the PASP paper. Because of the success of the technology and instrument testbed programs, SIM PlanetQuest greatly exceeds the performance goals set for the mission; so even though a shorter baseline (6m vs. 9m) reduces the astrometric accuracy, SIM Lite still meets the original performance goals.
We explore several exciting projects that could be achieved by Full-Sky Astrometric Mapping Explo... more We explore several exciting projects that could be achieved by Full-Sky Astrometric Mapping Explorer (FAME), or similar astrometric satellites, by including a limited number of faint objects (15 < R � 18) in its observing list, that is, by going beyond the current R < 15 limit. Observing some 50,000 quasars will improve the definition of the reference frame in each direction from 17 µasyr −1 to 8 µasyr −1, allowing the proper motion of the Galactic center (the reflex motion of the Sun) to be determined with an accuracy of 0.1%. It will also allow precision tests of non-axisymmetry of the Galaxy and very accurate proper motions of the Magellanic Clouds. Quasar observations also offer a powerful check on any unmodeled parallax systematics. Observing proper motions of 30,000 faint field blue horizontal branch (BHB) stars will allow stellar halo rotation to be mapped to beyond 35 kpc with errors of a few km s −1. Halo substructures producing clumps in the velocity space will be de...
Astronomy and Astrophysics Supplement Series, 1997
the optical. In this case, the detection of the near infrared counterpart is the only way in whic... more the optical. In this case, the detection of the near infrared counterpart is the only way in which we can extend the study of these sources to other spectral ranges and may be crucial to understand the short-lived phase which precedes the formation of a new planetary nebula.
This memo reviews some of the trades regarding a grating and a set of filters for the AMEX missio... more This memo reviews some of the trades regarding a grating and a set of filters for the AMEX mission. 1.
To be published in the Aug. 1996 issue of the Astronomical Journal In a previous paper (Olling 19... more To be published in the Aug. 1996 issue of the Astronomical Journal In a previous paper (Olling 1995, AJ, 110, 591) 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 H I. 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). The observed flaring of the H I beyond the optical disk puts significant constraints on the shape of the dark matter halo, which are almost independent of the stellar mass-tolight ratio. NGC 4244’s dark matter halo is found to be highly flattened with a shortestto-longest axis ratio of 0.2 +0.3 −0.1. 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 H I. A...
The astrometric signature imposed by a planet on its primary increases substantially towards long... more The astrometric signature imposed by a planet on its primary increases substantially towards longer periods ( ∝ P 2/3), so that long-period planets can be more easily detected, in principle. For example, a one Solar-mass (M⊙) star would be pulled by roughly 1 milliarcsec (mas) by a one Jupiter-mass (MJ) planet with a period of one-hundred years at a distance of 20 pc [cf. eqn. (3) below]. 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 1 [e.g., (Edberg et al. 2005)] will be taking data, several decades will have past since the first astrometric mission, HIPPARCOS (ESA 1997). Here we propose to use a new astrometric method that employs a future, highly accurate SIM Quick-Look (SQL) survey and HIPPARCOS data taken twenty years prior. Using a conservative position e...
Axisymmetric models of the Milky Way exhibit strong interrelations between the Galactic constants... more Axisymmetric models of the Milky Way exhibit strong interrelations between the Galactic constants [the Sun’s distance to the Galactic centre (R0), and the local rotation speed (Θ0)], the local stellar columndensity (Σ∗(R0)) and the shortest-to-longest axis ratio of the dark matter halo (q). In this paper we present simple analytical approximations that allow for an efficient search through the vastness of parameter space, and apply this approximation to investigate the consequences of the uncertain gaseous velocity dispersion (σg) on the constraints imposed by the thickness of the Milky Way’s gas layer. The extra degree of freedom does not significantly alter the conclusions drawn in a previous paper on the shape of the Milky Way’s dark matter halo. A significant contribution to the total gas pressure by cosmic rays and magnetic fields beyond the optical disk is thus ruled out. We find that the Milky Way’s dark halo is close to spherical if R0 ∼> 7.1 kpc, while a significantly fl...
We develop Bayesian statistical methods for discovering and assigning probabilities to non-random... more We develop Bayesian statistical methods for discovering and assigning probabilities to non-random (e.g., physical) stellar companions. These companions are either presently bound or were previously bound. The probabilities depend on similarities in corrected proper motion parallel and perpendicular to the brighter component's motion, parallax, and the local phase-space density of field stars. Control experiments are conducted to understand the behavior of false positives. The technique is applied to the Hipparcos Catalogue within 100 pc. This is the first all-sky survey to locate escaped companions still drifting along with each other. In the < 100 pc distance range, ∼220 high probability companions with separations between 0.01-1 pc are found. The first evidence for a population (∼300) of companions separated by 1-8 pc is found. We find these previously unnoticed naked-eye companions (both
The Space Interferometry Mission (SIM PlanetQuest) can provide astrometric data of such high qual... more The Space Interferometry Mission (SIM PlanetQuest) can provide astrometric data of such high quality that it will be possible to determine geometric (luminosity-independent) dis-tances to the nearest spiral galaxies, even though these systems are too far away for direct trigonometric parallax determination. Instead, the method to use is that of the \Rotational
Abstract. I analyze several catalogs of known visual and spectroscopic binaries and conclude that... more Abstract. 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 affect the astrometry significantly. 1. Binarity: Past Present & Future Duquennoy & Mayor (1991, hereafter referred to as DM1991) established the multiplicity of G-type stars within about 22 pc. Their observational dataset consists of thirteen years of radial velocity monitoring, in combination with astrometric, visual and eclipsing binaries, for 164 G-type stars. However, the pre-Hipparcos lists of wide binaries were rather incomplete. Currently, the number of known binaries is about three times larger. In fact, an analysis of the currently available binarity and multiplicity data of nearby Hipparcos-selected G-type stars indicates that as many as 95 % of pr...
The local stellar kinematics of the Milky Way, parameterized by the Oort constants A and B, depen... more 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 (Θ0), and the distance to the Galactic center (R0). 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 ∼ Θ0/R dependence, in the Solar neighborhood and other locations in the Galaxy. These models may explain the ∼40 % difference between the values for 2AR0 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=H87) and distant Cepheids (d = 0.5−6 kpc; Feast & Whitelock 1997=FW97). However, a consiste...
I present a new method of deriving the shape of the dark matter (DM) halos of spiral galaxies. Th... more 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. So far, determinations of the flaring of the gas layer (i.e. the increase of the thickness with galactocentric radius) have been used to determine the mass-to-light ratio, M/L, of the stellar disk of several edge-on galaxies. In this paper I describe a method which can be used to determine the shape of DM-halos. This technique will be applied in a forthcoming paper. I show that the model predictions of the gas layer width are best calculated using a global approach, in which the potential arising from the 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 calcula...
To be published in the Aug. 1996 issue of the Astronomical Journal I present sensitive high resol... more To be published in the Aug. 1996 issue of the Astronomical Journal I present sensitive high resolution VLA B, C, and D array observations of the almost edge-on Scd galaxy NGC 4244 in the 21-cm spectral line of neutral atomic hydrogen. The gas layer of NGC 4244 is rather symmetric in all respects, i.e. the surface density distribution, flaring and warping. This symmetry allows for a reliable determination of the rotation curve, despite the fact that the galaxy is close to edge-on. The rotation curve rises slowly in the inner 6 kpc, is roughly constant at 100 km s −1 out to 10 kpc, and decreases in Keplerian fashion by 15 % at the last measured point at 14 kpc. The rotation curve 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 high resolution H I observations. This procedure uses the apparent widths at many azimuths (many...
In a previous paper 26 a method was developed to determine the shapes of dark matter halos of spi... more In a previous paper 26 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 27,28 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 H I. 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 R0 = 7.1 kpc, Θ0 = 180 km s −1, while the stellar disk is ∼ 80 % of maximal. 1
We investigate how the current and future uncertainty on the Hubble constant affects the uncertai... more We investigate how the current and future uncertainty on the Hubble constant affects the uncertainty in the Equation of State of Dark Energy (w) and the total density of the Universe (Ωtot). We start with the approximate linear relations between the cosmological parameters as presented by (Spergel et al. 2006), and use the standard error-propagation relations to estimate the effects of improving the Cosmic Microwave Background (CMB) parameters as well as the Hubble constant (H0) on our knowledge of the Equation of State (EOS) of Dark Energy. Because we do not assume a flat universe, we also estimate the attainable accuracy on Ωtot and the spatial curvature of the Universe. In one limiting case, we assume that the constraints provided by additional data (galaxy clustering, weak lensing and so forth) do not improve significantly, while the error on the Hubble constant is decreased by a factor up to ten. The other limiting case of significantly improved additional data with current H0 ...
The local stellar kinematics of the Milky Way offer a useful tool for studying the rotation curve... more The local stellar kinematics of the Milky Way offer a useful tool for studying the rotation curve of the Galaxy. These kinematics – usually parameterized by the Oort constants A and B – depend on the local gradient of the rotation curve as well as its absolute value (Θ0), and the Sun’s distance to the Galactic center (R0). The density of interstellar gas in the Milky Way is shown to vary non-monotonically with radius, and so contributes significantly to the local gradient of the rotation curve. We have therefore calculated mass models for the Milky Way that include this component, and have derived the corresponding radial variation in the Oort constants. Between 0.9R0 and 1.2R0 the Oort functions A(R) and B(R) differ significantly from the general ∼ Θ0/R dependence. Various previously-inexplicable observations are shown to be consistent with these new predictions. For example, these models may explain the ∼40 % difference between the values for 2AR0 derived from radial velocity data...
In order to test the reliability of determinations of the shapes of galaxies ’ dark matter halos,... more In order to test the reliability of determinations of the shapes of galaxies ’ dark matter halos, we have made such measurements for the Milky Way by two independent methods. First, we have combined the measurements of the over-all mass distribution of the Milky Way derived from its rotation curve and the measurements of the amount of dark matter in the solar neighborhood obtained from stellar kinematics to determine the flattening of the dark halo. Second, we have used the established technique based on the variation in thickness of the Milky Way’s Hi layer with radius: by assuming that the Hi gas is in hydrostatic equilibrium in the gravitational potential of a galaxy, one can use the observe flaring of the gas layer to determine the shape of the dark halo. These techniques are found to produce a consistent estimate for the flattening of the dark matter halo, with a shortest-to-longest axis ratio of q ∼ 0.8, but only if one adopts somewhat non-standard values for the distance to t...
currently studying a mission called SIM Lite, as a more cost-effective way to do the precision as... more currently studying a mission called SIM Lite, as a more cost-effective way to do the precision astrometry recommended by the both of the last two Astronomy and Astrophysics Decadal Surveys (Bahcall 1991; McKee & Taylor 2001). SIM Lite is a variant on the design of SIM PlanetQuest (Nemati & Shao 2008) and inherits the same level of technical maturity. The project has carried the design and development of SIM PlanetQuest nearly through NASA Phase B. SIM Lite has the same functionality as SIM PlanetQuest, which allows it to tackle the breadth of science areas described in the PASP paper. Because of the success of the technology and instrument testbed programs, SIM PlanetQuest greatly exceeds the performance goals set for the mission; so even though a shorter baseline (6m vs. 9m) reduces the astrometric accuracy, SIM Lite still meets the original performance goals.
We explore several exciting projects that could be achieved by Full-Sky Astrometric Mapping Explo... more We explore several exciting projects that could be achieved by Full-Sky Astrometric Mapping Explorer (FAME), or similar astrometric satellites, by including a limited number of faint objects (15 < R � 18) in its observing list, that is, by going beyond the current R < 15 limit. Observing some 50,000 quasars will improve the definition of the reference frame in each direction from 17 µasyr −1 to 8 µasyr −1, allowing the proper motion of the Galactic center (the reflex motion of the Sun) to be determined with an accuracy of 0.1%. It will also allow precision tests of non-axisymmetry of the Galaxy and very accurate proper motions of the Magellanic Clouds. Quasar observations also offer a powerful check on any unmodeled parallax systematics. Observing proper motions of 30,000 faint field blue horizontal branch (BHB) stars will allow stellar halo rotation to be mapped to beyond 35 kpc with errors of a few km s −1. Halo substructures producing clumps in the velocity space will be de...
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Papers by Robert Olling