arXiv subjects

Carlton Pryor

Publications and source records attributed to Carlton Pryor.

18 recordsLinked to original sources

arXiv2016

Proper Motion of the Leo II Dwarf Galaxy Based On Hubble Space Telescope Imaging

This article reports a measurement of the proper motion of Leo II, a dwarf galaxy that is a likely satellite of the Milky Way, based on imaging with the Hubble Space Telescope and Wide Field Camera 3. The measurement uses compact background galaxies as standards of rest in both channels of the camera for two distinct pointings of the telescope and a QSO in one channel for each pointing, resulting in the weighted average of six measurements. The measured proper motion in the the equatorial coordinate system is (mu_alpha, mu_delta) = (-6.9 +- 3.7, -8.7 +- 3.9) mas/century and in the Galactic coordinate system is (mu_l, mu_b) = (6.2 +- 3.9, -9.2 +- 3.7) mas/century. The implied space velocity with respect to the Galactic center is (Pi, Theta, Z) = (-37 +- 38, 117 +- 43, 40 +- 16) km/s or, expressed in Galactocentric radial and tangential components, (V_r, V_tan) = (21.9 +- 1.5, 127 +- 42) km/s. The space velocity implies that the instantaneous orbital inclination is 68 degrees, with a 95% confidence interval of (66 deg, 80 deg). The measured motion supports the hypothesis that Leo II, Leo IV, Leo V, Crater 2, and the globular cluster Crater fell into the Milky Way as a group.

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arXiv2014

Proper Motion of the Draco Dwarf Galaxy Based On Hubble Space Telescope Imaging

We have measured the proper motion of the Draco dwarf galaxy using images at two epochs with a time baseline of about two years taken with the Hubble Space Telescope and the Advanced Camera for Surveys. Wide Field Channel 1 and 2 provide two adjacent fields, each containing a known QSO. The zero point for the proper motion is determined using both background galaxies and the QSOs and the two methods produce consistent measurements within each field. Averaging the results from the two fields gives a proper motion in the equatorial coordinate system of $(μ_α,μ_δ) = (17.7\pm 6.3, -22.1 \pm 6.3)$ mas century$^{-1}$ and in the Galactic coordinate system of $(μ_{\ell},μ_{b}) = (-23.1\pm 6.3, -16.3 \pm 6.3)$ mas century$^{-1}$. Removing the contributions of the motion of the Sun and of the LSR to the measured proper motion yields a Galactic rest-frame proper motion of $(μ_α^{Grf},μ_δ^{Grf}) = (51.4\pm 6.3, -18.7 \pm 6.3)$ mas century$^{-1}$ and $(μ_{\ell}^{Grf},μ_{b}^{Grf}) = (-21.8\pm 6.3, -50.1 \pm 6.3)$ mas century$^{-1}$. The implied space velocity with respect to the Galactic center is $(Π, Θ, Z) = (27\pm 14, 89\pm 25, -212\pm 20)$ km s$^{-1}$. This velocity implies that the orbital inclination is 70 degrees, with a 95% confidence interval of (59 deg, 80 deg), and that the plane of the orbit is consistent with that of the vast polar structure (VPOS) of Galactic satellite galaxies.

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arXiv2012

A Test for Radial Mixing Using Local Star Samples

We use samples of local main-sequence stars to show that the radial gradient of [Fe/H] in the thin disk of the Milky Way decreases with mean effective stellar temperature. Many of these stars are visiting the solar neighborhood from the inner and outer Galaxy. We use the angular momentum of each star about the Galactic center to determine the guiding center radius and to eliminate the effects of epicyclic motion, which would otherwise blur the estimated gradients. We interpret the effective temperature as a proxy for mean age, and conclude that the decreasing gradient is consistent with the predictions of radial mixing due to transient spiral patterns. We find some evidence that the trend of decreasing gradient with increasing mean age breaks to a constant gradient for samples of stars whose main-sequence life-times exceed the likely age of the thin disk.

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arXiv2011

Velocity-space substructure from nearby RAVE and SDSS stars

We extract a sample of disc stars within 200 pc of the Sun from the RAVE and SDSS surveys. Distances are estimated photometrically and proper motions are from ground-based data. We show that the velocity-space substructure first revealed in the Geneva-Copenhagen sample is also present in this completely independent sample. We also evaluate action-angle variables for these stars and show that the Hyades stream stars in these data are again characteristic of having been scattered at a Lindblad resonance. Unfortunately, analysis of such local samples can determine neither whether it is an inner or an outer Lindblad resonance, nor the multiplicity of the pattern.

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arXiv2006

Proper Motions of Dwarf Spheroidal Galaxies from Hubble Space Telescope Imaging. V: Final Measurement for Fornax

The measured proper motion of Fornax, expressed in the equatorial coordinate system, is $(μ_α,μ_δ)=(47.6\pm 4.6,-36.0\pm 4.1)$ mas century$^{-1}$. This proper motion is a weighted mean of four independent measurements for three distinct fields. Each measurement uses a quasi-stellar object as a reference point. Removing the contribution of the motion of the Sun and of the Local Standard of Rest to the measured proper motion produces a Galactic rest-frame proper motion of $(μ_α^{\mbox{\tiny{Grf}}}, μ_δ^{\mbox{\tiny{Grf}}}) = (24.4\pm 4.6,-14.3\pm 4.1)$ mas century$^{-1}$. The implied space velocity with respect to the Galactic center has a radial component of $V_{r}=-31.8 \pm 1.7$ km s$^{-1}$ and a tangential component of $V_{t}=196 \pm 29$ km s$^{-1}$. Integrating the motion of Fornax in a realistic potential for the Milky Way produces orbital elements. The perigalacticon and apogalacticon are 118 (66, 137) kpc and 152 (144, 242) kpc, respectively, where the values in the parentheses represent the 95% confidence intervals derived from Monte Carlo experiments. The eccentricity of the orbit is 0.13 (0.11, 0.38), and the orbital period is 3.2 (2.5, 4.6) Gyr. The orbit is retrograde and inclined by $101^{\circ}$ ($94^{\circ}$, $107^{\circ}$) to the Galactic plane. Fornax could be a member of a proposed ``stream'' of galaxies and globular clusters, however the membership of another proposed galaxy in the stream, Sculptor, has been previously ruled out. Fornax is in the Kroupa-Theis-Boily plane that contains eleven of the Galactic satellite galaxies, but its orbit will take it out of that plane.

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arXiv2006

HST Proper Motions and Stellar Dynamics in the Core of the Globular Cluster 47 Tucanae

We have used HST imaging of the central regions (R<100 arcsec, about 5 core radii) of the globular cluster 47 Tucanae to derive proper motions and U- and V-band magnitudes for 14,366 cluster members. We also present a catalogue of astrometry and F475W photometry for nearly 130,000 stars in a rather larger central area. These data are made available in their entirety, in the form of downloadable electronic tables. We use them first to obtain a new estimate for the position of the cluster center and to define the stellar density profile into essentially zero radius. We then search in particular for any very fast-moving stars, such as might be expected to result from very close stellar encounters. Likely fewer than 0.1% (and no more than about 0.3%) of stars have total speeds above the nominal central escape velocity in 47 Tuc, and at lower speeds the velocity distribution is described very well by a regular King model. Considerations of only the proper-motion velocity dispersion then lead to a number of results: (1) Blue stragglers in the core of 47 Tuc have a velocity dispersion lower than that of the cluster giants by a factor of sqrt{2}. (2) The velocity distribution in the cluster center is essentially isotropic, as expected. (3) Using a sample of radial velocities for stars in the core, we estimate the distance to 47 Tuc: D = 4.0 +/- 0.35 kpc. And (4) we infer a 1-sigma upper limit of M<1000-1500 solar masses for any central, intermediate-mass black hole. We can neither confirm nor refute the hypothesis that 47 Tuc might lie on an extension of the M-sigma relation observed for supermassive black holes in galaxy bulges. [Abridged]

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arXiv2002

Hubble Space Telescope Evidence for an Intermediate-Mass Black Hole in the Globular Cluster M15: II. Kinematical Analysis and Dynamical Modeling

We analyze HST/STIS spectra (see Paper I) of the central region of the dense globular cluster M15. We infer the velocities of 64 individual stars, two-thirds of which have their velocity measured for the first time. This triples the number of stars with measured velocities in the central 1 arcsec of M15 and doubles the number in the central 2 arcsec. Combined with existing ground-based data we obtain the radial profiles of the projected kinematical quantities. The RMS velocity sigma_RMS rises to 14 km/s in the central few arcsec, somewhat higher than the values of 10-12 km/s inferred previously from ground-based data. To interpret the results we construct dynamical models based on the Jeans equation, which imply that M15 must have a central concentration of non-luminous material. If this is due to a single black hole, then its mass is M_BH = (3.9 +/- 2.2) x 10^3 solar masses. This is consistent with the relation between M_BH and sigma_RMS that has been established for galaxies. Also, the existence of intermediate-mass black holes in globular clusters is consistent with several scenarios for globular cluster evolution proposed in the literature. Therefore, these results may have important implications for our understanding of the evolution of globular clusters, the growth of black holes, the connection between globular cluster and galaxy formation, and the nature of the recently discovered `ultra-luminous' X-ray sources in nearby galaxies. Instead of a single black hole, M15 could have a central concentration of dark remnants (e.g., neutron stars) due to mass segregation. However, the best-fitting Fokker-Planck models that have previously been constructed for M15 do not predict a central mass concentration that is sufficient to explain the observed kinematics.[ABRIDGED]

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arXiv2002

Addendum: "Hubble Space Telescope Evidence for an Intermediate-Mass Black Hole in the Globular Cluster M15: II. Kinematical Analysis and Dynamical Modeling"

It has been reported that there is an error in the figure in Dull et al. (1997, D97) that shows the radial M/L profile in Fokker-Planck models of M15. We discuss how this modifies the interpretation of our kinematical data. These imply the existence of a dark and compact mass component near the center of M15, either a single black hole (BH) or a collection of dark remnants that have sunk to the cluster center due to mass segregation. We previously showed that the latter interpretation is in conflict with the D97 M/L profile, which supported the BH interpretation. We repeat our analysis here with the corrected D97 profile. Models without a BH are now found to be statistically acceptable (within 1-sigma), although inclusion of a BH still provides a marginally better fit. It does not necessarily follow that dark remnants are now the preferred interpretation of the data. The D97 models, as well as N-body models by Baumgardt et al. (2002), assume that all neutron stars are retained during cluster evolution. This conflicts with predictions of the neutron-star retention rate (typically below 10 per cent) based on pulsar kick velocities. The presence of a single BH therefore continues to be a viable interpretation of the data. The best fit BH mass with the corrected D97 M/L profile is M_BH = 1.7^{+2.7}_{-1.7} x 10^3 solar masses, and with a constant M/L it is M_BH = 3.2^{+2.2}_{-2.2} x 10^3 solar masses. A model that includes both neutron star escape and mass segregation would probably yield a value between these numbers. This agrees with the correlation between velocity dispersion and BH mass inferred for galaxies. However, with the presently available models and data it is neither uniquely implied nor ruled out that M15 has an intermediate-mass BH.

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arXiv2002

Structure of the Draco Dwarf Spheroidal Galaxy

This article studies the structure of the Draco dwarf spheroidal galaxy with an emphasis on the question of whether the spatial distribution of its stars has been affected by the tidal interaction with the Milky Way, using R- and V-band CCD photometry for eleven fields. The article reports coordinates for the center, a position angle of the major axis, and the ellipticity. It also reports the results of searches for asymmetries in the structure of Draco. These results, and searches for a ``break'' in the radial profile and for the presence of principal sequences of Draco in a color-magnitude diagram for regions more than 50 arcmin from the center, yield no evidence that tidal forces from the Milky Way have affected the structure of Draco.

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arXiv2000

Stars of the Draco Dwarf Spheroidal Galaxy Beyond its Measured Tidal Boundary

We report R- and V-band photometry derived from CCD imaging for objects in nine fields in and around the Draco dwarf spheroidal galaxy. The most distant fields are about 1.3 degrees from the center. We use these data to search for Draco stars outside of its measured tidal boundary. The search involves three methods: 1) Plotting color-magnitude diagrams for individual fields, for sections of fields, and for combined fields and sections. A color-magnitude diagram can reveal a population of Draco stars by the presence of the expected principal sequences. 2) Measuring field-to-field fluctuations and 3) measuring intra-field fluctuations in the surface density of objects located near the Draco principal sequences in the color-magnitude diagram. We find evidence for the presence of Draco stars immediately beyond the measured tidal boundary of Draco and place an upper limit on the number of such stars in more distant fields that lie close to the extension of its major axis. The best evidence is the presence of the Draco principal sequences in the color-magnitude diagram for some combined fields and sections of fields. The measurements of the field-to-field fluctuations in the stellar surface density confirm this result.

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arXiv1999

CFHT Adaptive Optics Observations of the Central Kinematics in M15

We have used an Imaging Fabry-Perot Spectrophotometer with the Adaptive Optics Bonnette on the Canada-France-Hawaii Telescope to measure stellar radial velocities in the globular cluster M15. An average seeing of 0.15" full-width at half maximum, with the best-seeing image having 0.09", allowed us to measure accurately the velocities for five stars within 1" of the center of M15. Our estimate of the second moment of the velocity distribution inside a radius of 2" is 11.5 km/s, the same value we find out to a radius of about 6". However, the projected net rotation does increase dramatically at small radii, as our previous observations led us to suspect. The rotation amplitude inside a radius of 3.4" is v = 10.4 +- 2.7 km/s and the dispersion after removing the rotation is sigma = 10.3 +- 1.4 km/s, so v/sigma = 1 in this region. In addition, the position angle (PA) of the projected rotation axis differs by 100 degrees from that of the net cluster rotation at larger radii. Current theoretical models do not predict either this large an increase in the rotation amplitude or such a change in the PA. However, a central mass concentration, such as a black hole, could possibly sustain such a configuration. The rotation increase is consistent with the existence of a central dark mass concentration equal to 2500 M_solar.

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arXiv1998

HST Photometry of the Globular Cluster M4

This paper presents a detailed description of the acquisition and processing of a large body of imaging data for three fields in the globular cluster M4 taken with the Wide Field and Planetary Camera 2 aboard the Hubble Space Telescope. Analysis with the ALLFRAME package yielded the deepest photometry yet obtained for this cluster. The resulting data-set for 4708 stars (positions and calibrated photometry in V, I, and, in two fields, U) spanning approximately six cluster core radii is available on the AAS CD-ROM (or email a request to RAI). The scientific analysis is deferred to three companion papers, which investigate the significant white dwarf population discovered and the main sequence population.

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arXiv1997

Mass Segregation in Young LMC Clusters I. NGC 2157

We have carried out WFPC2 V- and I-band imaging of the young LMC cluster NGC 2157. Construction of a color-magnitude diagram and isochrone fitting yields an age of tau = 100 000 000 yrs, a reddening E(B-V) = 0.1 and a distance modulus of 18.4 mag. Our data covers the mass range 0.75 < m < 5.1 solar masses. We find that the cluster mass function changes significantly from the inner regions to the outer regions, becoming steeper (larger number of low mass stars relative to high mass stars) at larger radii. The age of NGC 2157 is comparable to its two-body relaxation timescale only in the cluster core. The observed steepening of the mass function at larger radii is therefore most likely an initial condition of the cluster stars. Such initial conditions are predicted in models of cluster star formation in which dissipative processes act more strongly upon more massive stars.

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arXiv1997

White Dwarfs in Globular Clusters: HST Observations of M4

Using WFPC2 on the Hubble Space Telescope, we have isolated a sample of 258 white dwarfs (WDs) in the Galactic globular cluster M4. Fields at three radial distances from the cluster center were observed and sizeable WD populations were found in all three. The location of these WDs in the color-magnitude diagram, their mean mass of 0.51($ \pm 0.03$)M$_{\odot}$, and their luminosity function confirm basic tenets of stellar evolution theory and support the results from current WD cooling theory. The WDs are used to extend the cluster main-sequence mass function upward to stars that have already completed their nuclear evolution. The WD/red dwarf binary frequency in M4 is investigated and found to be at most a few percent of all the main-sequence stars. The most ancient WDs found are about 9 Gyr old, a level which is set solely by the photometric limits of our data. Even though this is less than the age of M4, we discuss how these cooling WDs can eventually be used to check the turnoff ages of globular clusters and hence constrain the age of the Universe.

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arXiv1996

Fabry-Perot Observations of Globular Clusters III: M15

We have used an Imaging Fabry-Perot Spectrophotometer with the Sub-arcsecond Imaging Spectrograph on the CFHT to measure velocities for 1534 stars in the globular cluster M15. Combined with previous velocity samples, the total number of stars with measured velocities in M15 is 1597. The velocity dispersion profile for M15 remains flat at a value of 11 km/s from a radius of 0.4' into our innermost reliable point at 0.02' (0.06 pc). This profile and the previously-published surface brightness profile can be equally well represented either by a stellar population whose M/L varies with radius from 1.7 in solar units at large radii to 3 in the central region, or by a population with a constant M/L of 1.7 and a central black hole of 1000 M_sol. A non-parametric mass model that assumes no black hole, no rotation, and isotropy constrains the mass density of M15 to better than 30% at a radius of 0.07 parsecs. Using the assumption of local thermodynamic equilibrium, we estimate the present-day mass function and infer a significant number of 0.6-0.7 M_sol objects in the central few parsecs, 85% of which may be in the form of stellar remnants. Not only do we detect rotation; we find that the position angle of the projected rotation axis in the central 10" is 100 degrees different from that of the whole sample. We also detect an increase in the amplitude of the rotation at small radii. Although this increase needs to be confirmed with better-seeing data, it may be the result of a central mass concentration.

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arXiv1996

Spectroscopic Binaries in Globular Clusters. II. A Search for Long-Period Binaries in M22

A catalog of 383 radial velocities for red giants in the globular cluster M22 has been compiled from the literature and from new observations accumulated between 1972 and 1994. This 22-year baseline is the longest available for any sample of globular cluster stars. Using 333 repeat velocities for 109 cluster members, we have carried out a search for spectroscopic binaries with periods in the range 0.2 -- 40 years and with mass ratios between 0.1 and 1.0. Although the velocities for these evolved stars show clear evidence for atmospheric motions, no star is convincingly found to exhibit a velocity variation greater than 7 km/s. By comparing the observed velocity variations to those found in a series of Monte-Carlo simulations, we estimate the cluster binary fraction to be X = 0.01 (circular orbits) and X = 0.03 (thermal orbits). These results are to be compared to the corresponding binary fraction of X = 0.12 for nearby solar-type stars having similar mass ratios and periods. We speculate that both the relative abundances of short- and long-period binaries in globular clusters and the large differences in measured binary fractions for clusters with high binary ionization rates (M22, Omega Cen) compared to those for clusters with low ionization rates (M71, M4, NGC 3201) point to a frequency-period distribution in which soft binaries have been disrupted by stellar encounters. Finally, we note that none of the three CH stars in our survey shows evidence for velocity variations; this is in stark contrast to field CH stars, virtually all of which are binaries. We argue that binaries in M22 which have binding energies similar to field CH stars are unlikely to have been disrupted by stellar encounters and suggest that the cluster CH stars are otherwise normal red giants which lie in the carbon-enriched tail of the cluster metallicity

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arXiv1996

Age Gradient and the Second Parameter Problem in the Galactic Halo

We establish a framework for determing absolute ages of Galactic globular clusters and then use these ages to investigate the age-metallicity and age-Galactocentric distance relations for the 36 clusters with the most reliable age data. The clusters span Galactocentric distances from 4 through 100 kpc and cover a metallicity range from $[Fe/H] = -0.6$ to $-2.3$. Adopting currently plausible choices for the relation between cluster metallicity and horizontal-branch luminosity, and alpha-enhancement ratios, we find that the majority of the globular clusters form an age distribution with a dispersion $σ(t)$ about $10^9$ years, and a total age spread smaller than 4 Gyr. Clusters in the lowest metallicity group ($[Fe/H] < -1.8$) appear to be the same age to well within 1 Gyr at all locations in the Milky Way halo, suggesting that star formation began throughout the halo nearly simultaneously in its earliest stages. We find no statistically significant correlation between mean cluster age and Galactocentric distance (no age gradient) from 4 to 100 kpc. The correlation between cluster age and horizontal-branch type suggests that causes in addition to metallicity and age are required to understand the distribution of stars along the horizontal branches in globular cluster color-magnitude diagrams.

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arXiv1995

Hubble Space Telescope Observations of White Dwarfs in the Globular Cluster M4

With the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope, we have discovered in M4 (NGC 6121, C 1620-264) the first extensive sequence of cooling white dwarfs seen in a globular cluster. Adopting a distance modulus of (m-M)_V = 12.65 and a reddening of E(B-V) = 0.37, we show that the sequence, which extends over 9 < M_U < 13, is comprised of white dwarfs of mass \sim 0.5 M_{\odot}. The total mass loss from the present turnoff to the white dwarf sequence is 0.31 M_{\odot} and the intrinsic dispersion in the mean mass appears to be < 0.05 M_{\odot}. Both the location of the white dwarf cooling sequence in the cluster color-magnitude diagram and the cumulative luminosity function attest to the basic correctness and completeness of the physics in theoretical models for the upper three magnitudes of the observed white dwarf cooling sequence. To test the theory in globular clusters at cooling ages beyond \sim 3 \times 10^8 years will require deeper and more complete data.

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