arXiv subjects

J. Anderson

Publications and source records attributed to J. Anderson.

At least 577 records · Page 32Linked to original sources

arXiv2007

The ACS Survey of Galactic Globular Clusters. II. Stellar Evolution Tracks, Isochrones, Luminosity Functions, and Synthetic Horizontal-Branch Models

The ACS Survey of Galactic Globular Clusters, an HST Treasury Project, will deliver high quality, homogeneous photometry of 65 globular clusters. This paper introduces a new collection of stellar evolution tracks and isochrones suitable for analyzing the ACS Survey data. Stellar evolution models were computed at [Fe/H]= -2.5, -2.0, -1.5, -1.0, -0.5, and 0; [alpha/Fe]= -0.2, 0, 0.2, 0.4, 0.6, and 0.8; and three initial He abundances for masses from 0.1 to 1.8 Msun and ages from 2 to 15 Gyr. Each isochrone spans a wide range in luminosity from Mv~14 up to the tip of the red giant branch. These are complemented by a set of He-burning tracks that extend from the zero age horizontal branch to the onset of thermal pulsations on the asymptotic giant branch. In addition, a set of computer programs are provided that make it possible to interpolate the isochrones in [Fe/H], generate luminosity functions from the isochrones, and create synthetic horizontal branch models. The tracks and isochrones have been converted to the observational plane with two different color-Teff transformations, one synthetic and one semi-empirical, in ground-based B, V, and I, and F606W and F814W for both ACS-WFC and WFPC2 systems. All models and programs presented in this paper are available from http://stellar.dartmouth.edu/~models/

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arXiv2007

An Extrasolar Planet Census with a Space-based Microlensing Survey

A space-based gravitational microlensing exoplanet survey will provide a statistical census of exoplanets with masses down to 0.1 Earth-masses and orbital separations ranging from 0.5AU to infinity. This includes analogs to all the Solar System's planets except for Mercury, as well as most types of planets predicted by planet formation theories. Such a survey will provide results on the frequency of planets around all types of stars except those with short lifetimes. Close-in planets with separations < 0.5 AU are invisible to a space-based microlensing survey, but these can be found by Kepler. Other methods, including ground-based microlensing, cannot approach the comprehensive statistics on the mass and semi-major axis distribution of extrasolar planets that a space-based microlensing survey will provide. The terrestrial planet sensitivity of a ground-based microlensing survey is limited to the vicinity of the Einstein radius at 2-3 AU, and space-based imaging is needed to identify and determine the mass of the planetary host stars for the vast majority of planets discovered by microlensing. Thus, a space-based microlensing survey is likely to be the only way to gain a comprehensive understanding of the nature of planetary systems, which is needed to understand planet formation and habitability. The proposed Microlensing Planet Finder (MPF) mission is an example of a space-based microlensing survey that can accomplish these objectives with proven technology and a cost that fits comfortably under the NASA Discovery Program cost cap.

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arXiv2007

A Triple Main Sequence in the Globular Cluster NGC 2808

Accurate photometry with HST/ACS shows that the main sequence of the globular cluster NGC 2808 splits into three separate branches. The three MS branches may be associated with complexities of the cluster's horizontal branch and of its abundance distribution. We attribute the MS branches to successive rounds of star formation, with different helium abundances; we discuss possible sources of helium enrichment. Some other massive globulars also appear to have complex populations; we compare them with NGC 2808.

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arXiv2007

The multiplicity of the subgiant branch of Omega Centauri: Evidence for prolonged star formation

We present metallicity measurements based on GIRAFFE@VLT spectra of 80 subgiant-branch stars of the Galactic globular cluster Omega Centauri. The VLT spectroscopic data are complemented by ACS/HST and WFI@ESO2.2m high-accuracy color-magnitude diagrams. We have obtained the [Fe/H] abundance for each of the 80 target stars, and the abundances of C, N, Ca, Ti, and Ba for a subset of them, by comparison with synthetic spectra. We show that stars with [Fe/H]<-1.25 have a large magnitude spread on the flat part of the SGB. We interpret this as empirical evidence for an age spread. We have identified four distinct stellar groups within the SGB region: (i) an old, metal-poor group ([Fe/H]~-1.7); (ii) an old, metal-rich group ([Fe/H]~-1.1); (iii) a young (up to 4--5 Gyr younger than the old component) metal-poor group ([Fe/H]~-1.7); (iv) a young, intermediate-metallicity ([Fe/H]~-1.4) group, on average 1--2 Gyr younger than the old metal-poor population, and with an age spread that we cannot properly quantify with the present sample. In addition, a group of SGB stars are spread between the intermediate-metallicity and metal-rich branches of the SGB. The spread in age within each population establishes that the progenitor of Omega Cen system must have had a composite nature.

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arXiv2007

The Distance to the Isolated Neutron Star RX J0720.4-3125

We have used a set of dedicated astrometric data from the Hubble Space Telescope to measure the parallax and proper motion of the nearby neutron star RX J0720.4-3125. At each of eight epochs over two years, we used the High Resolution Camera of the Advanced Camera for Surveys to measure the position of the B=26.6 target to a precision of ~2 mas (~0.07 pix) relative to 22 other stars. From these data we measure a parallax of pi=2.8+/-0.9 mas (for a distance of 360+170-90 pc) and a proper motion of mu=107.8+/-1.2 mas/yr. Exhaustive testing of every stage of our analysis suggests that it is robust, with a maximum systematic uncertainty on the parallax of 0.4 mas. The distance is compatible with earlier estimates made from scaling the optical emission of RX J0720.4-3125 relative to the even closer neutron star RX J1856.5-3754. The distance and proper motion imply a transverse velocity of 180+90-40 km/s, comparable to velocities observed for radio pulsars. The speed and direction suggest an origin for RX J0720.4-3125 in the Trumpler 10 OB association ~0.7 Myr ago, with a possible range of 0.5-1.0 Myr given by the uncertainty in the distance.

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arXiv2006

The ACS Survey of Galactic Globular Clusters. I. Overview and Clusters Without Previous HST Photometry

We present the first results of a large ACS Survey of Galactic globular clusters. This Hubble Space Telescope (HST) Treasury project is designed to obtain photometry with S/N > ~10 for main sequence stars with masses > ~0.2Msun in a sample of globulars using the Advanced Camera for Surveys (ACS) Wide Field Channel. Here we focus on clusters without previous HST imaging data. These include NGC 5466, 6779, 5053, 6144, Palomar 2, E 3, Lynga 7, Palomar 1, and NGC 6366. Our CMDs extend reliably from the horizontal branch to as much as seven magnitudes fainter than the main sequence turnoff and represent the deepest CMDs published to-date for these clusters. Using fiducial sequences for three standard clusters (M92, NGC 6752, and 47 Tuc) with well-known metallicities and distances, we perform main sequence fitting on the target clusters in order to obtain estimates of their distances and reddenings. These comparisons along with fitting the cluster main sequences to theoretical isochrones yield ages for the target clusters. We find that the majority of the clusters have ages that are consistent with the standard clusters at their metallicities. The exceptions are E 3 which appears ~2 Gyr younger than 47 Tuc, and Pal 1, which could be as much as 8 Gyr younger than 47 Tuc.

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arXiv2006

The absolute motion of the peculiar cluster NGC6791

We present improved values of the three components of the absolute space velocity of the open cluster NGC6791. One HST ACS/WFC field with two-epoch observations provides astrometric measurements of objects in a field containing the cluster center. Identification of 60 background galaxies with sharp nuclei allows us to determine an absolute reference point, and measure the absolute proper motion of the cluster. We find (mu_alpha cos(delta), mu_delta)_J2000.0 = (-0.57+/-0.13, -2.45+/-0.12)mas/yr, and adopt V_rad = -47.1+/-0.7km/s from the average of the published values. Assuming a Galactic potential, we calculate the Galactic orbit of the cluster for various assumed distances, and briefly discuss the implications on the nature and the origin of this peculiar cluster.

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arXiv2006

Absolute motions of globular clusters. II. [HST astrometry and VLT radial velocities in NGC6397]

In this paper we present a new, accurate determination of the three components of the absolute space velocity of the Galactic globular cluster NGC6397 (l 338d, b -12d). We used three HST/WFPC2 fields with multi-epoch observations to obtain astrometric measurements of objects in three different fields in this cluster. The identification of 33 background galaxies with sharp nuclei allowed us to determine an absolute reference point and measure the absolute proper motion of the cluster. The third component has been obtained from radial velocities measured on spectra from the multi-fiber spectrograph FLAMES at UT2-VLT. We find [mu_alpha cos(delta), mu_delta](J2000.0) = [+3.39 +/- 0.15, -17.55 +/- 0.15] mas/yr, and V_rad = +18.36 +/- 0.09 (+/-0.10) km/s. Assuming a Galactic potential, we calculate the cluster orbit for various assumed distances, and briefly discuss the implications.

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arXiv2006

Microlens OGLE-2005-BLG-169 Implies Cool Neptune-Like Planets are Common

We detect a Neptune mass-ratio (q~8e-5) planetary companion to the lens star in the extremely high-magnification (A~800) microlensing event OGLE-2005-BLG-169. If the parent is a main-sequence star, it has mass M~0.5 M_sun implying a planet mass of ~13 M_earth and projected separation of ~2.7 AU. When intensely monitored over their peak, high-magnification events similar to OGLE-2005-BLG-169 have nearly complete sensitivity to Neptune mass-ratio planets with projected separations of 0.6 to 1.6 Einstein radii, corresponding to 1.6--4.3 AU in the present case. Only two other such events were monitored well enough to detect Neptunes, and so this detection by itself suggests that Neptune mass-ratio planets are common. Moreover, another Neptune was recently discovered at a similar distance from its parent star in a low-magnification event, which are more common but are individually much less sensitive to planets. Combining the two detections yields 90% upper and lower frequency limits f=0.37^{+0.30}_{-0.21} over just 0.4 decades of planet-star separation. In particular, f>16% at 90% confidence. The parent star hosts no Jupiter-mass companions with projected separations within a factor 5 of that of the detected planet. The lens-source relative proper motion is μ~7--10 mas/yr, implying that if the lens is sufficiently bright, I<23.8, it will be detectable by HST by 3 years after peak. This would permit a more precise estimate of the lens mass and distance, and so the mass and projected separation of the planet. Analogs of OGLE-2005-BLG-169Lb orbiting nearby stars would be difficult to detect by other methods of planet detection, including radial velocities, transits, or astrometry.

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arXiv2005

Color--Magnitude Diagrams and Luminosity Functions down to the Hydrogen Burning Limit. III. A Preliminary HST Study of NGC6791

Using HST ACS/WFC images, we derive the color--magnitude diagram of the old, metal-rich open cluster NGC 6791 to nearly 29th magnitude in V, which is the neighborhood of the hydrogen-burning limit.Comparison with isochrones leads to a discussion of the distance modulus, the reddening, and the age of the cluster. By making a statistical correction for field stars we derive a preliminary luminosity function, and a very tentative mass function. The white dwarf sequence is clearly shown, and has been discussed in a separate paper.

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arXiv2005

Star-Cluster Astrometry with Ground-Based Wide Field Imagers

We show the astrometric potential of the Wide Field Imager at the focus of the MPI-ESO 2.2m Telescope. Currently, we are able to measure the position of a well-exposed star with a precision of $\sim$4 mas/frame in each coordinate (under 0.8 arcsec seeing conditions). We present some preliminary results here.

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arXiv2005

The white dwarf cooling sequence in NGC 6791

In the old, populous, and metal-rich open cluster NGC 6791 we have used deep HST/ACS images to track the white dwarf cooling sequence down to m_F606W~28.5. The white dwarf luminosity function shows a well defined peak at m_F606W~27.4, and a bending to the blue in the color--magnitude diagram. If this peak corresponds to the end of the white dwarf cooling sequence the comparison with theoretical isochrones provides a cluster age estimate of ~2.4 Gyr, in sharp contrast with the age of 8--9 Gyr inferred from the main-sequence turn-off. If the end is at fainter magnitudes, the peak at m_F606W~27.4 is even more enigmatic. We discuss possible causes, none of them very convincing.

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arXiv2004

HST/ACS Imaging of Omega Centauri: Optical Counterpart for the Quiescent Low-Mass X-Ray Binary

We report the discovery of an optical counterpart to a quiescent neutron star in the globular cluster Omega Centauri (NGC 5139). The star was found as part of our wide-field imaging study of Omega Cen using the Advanced Camera for Surveys (ACS) on Hubble Space Telescope. Its magnitude and color (R_625 = 25.2, B_435 - R_625 = 1.5) place it more than 1.5 magnitudes to the blue side of the main sequence. Through an H-alpha filter it is ~ 1.3 magnitudes brighter than cluster stars of comparable M_625 magnitude. The blue color and H-alpha excess suggest the presence of an accretion disk, implying that the neutron star is accreting from a binary companion and is thus a quiescent low-mass X-ray binary. If the companion is a main-sequence star, then the faint absolute magnitude (M_625 ~ 11.6) constrains it to be of very low mass (M <~ 0.14 Msolar). The faintness of the disk (M_435 ~ 13) suggests a very low rate of accretion onto the neutron star. We also detect 13 probable white dwarfs and three possible BY Draconis stars in the 20" x 20" region analyzed here, suggesting that a large number of white dwarfs and active binaries will be observable in the full ACS study.

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arXiv2004

The double main sequence of Omega Centauri

Recent, high precision photometry of Omega Centauri, the biggest Galactic globular cluster, has been obtained with Hubble Space Telescope. The color magnitude diagram reveals an unexpected bifurcation of colors in the main sequence (MS). The newly found double MS, the multiple turnoffs and subgiant branches, and other sequences discovered in the past along the red giant branch of this cluster add up to a fascinating but frustrating puzzle. Among the possible explanations for the blue main sequence an anomalous overabundance of helium is suggested. The hypothesis will be tested with a set of FLAMES@VLT data we have recently obtained (ESO DDT program), and with forthcoming ACS@HST images.

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arXiv2004

Omega Centauri: The Population Puzzle Goes Deeper

We present HST observations that show a bifurcation of colors in the middle main sequence of the globular cluster Omega Centauri. We see this in three different fields, observed with different cameras and filters. We also present high precision photometry of a central ACS field which shows a number of main-sequence turnoffs and subgiant branches. The double main sequence, the multiple turnoffs and subgiant branches, and other population sequences discovered in the past along the red giant branch of this cluster add up to a fascinating but frustrating puzzle. We suggest various explanations, none of them very conclusive.

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arXiv2003

HST Astrometry of M4 and the Galactic Constant V_0/R_0

From multi-epoch WFPC2/HST observations we present astrometric measurements of stars in the Galactic globular cluster M4 (NGC 6121) and in the foreground/background. The presence of an extragalactic point source allows us to determine the absolute proper motion of the cluster, and, through use of the field stars in this region only 18 degree from the Galactic center, to measure the difference between the Oort constants, A-B. We find: (mu_alpha cos dec, mu_dec)_J2000 = (-13.21 +/- 0.35, -19.28 +/- 0.35) mas/yr, and A-B = V_0/R_0 = 27.6 +/- 1.7 km / s / kpc.

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arXiv2003

Accurate Internal Proper Motions of Globular Clusters

We have undertaken a long term program to measure high precision proper motions of nearby Galactic globular cluster (GC) stars using multi-epoch observations with the WFPC2 and the ACS cameras on-board the Hubble Space Telescope. The proper motions are used to study the internal cluster kinematics, and to obtain accurate cluster distances. In this paper, we also show how the proper motions of the field stars projected in the direction of the studied clusters can be used to set constraints on the Galaxy kinematics.

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arXiv2003

HST astrometry: the Galactic constant Theta_0/R_0

From multi-epoch WFPC2/HST observations we present astrometric measurements of the absolute motion of the bulge stars. The presence of an extragalactic point-source candidate allows us to measure the difference between the Oort constants, A-B = Theta_0/R_0. We find: Theta_0/R_0 = 27.4 +/- 1.8 km/s/kpc.

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