arXiv subjects

Terrence M. Girard

Publications and source records attributed to Terrence M. Girard.

At least 19 recordsLinked to original sources

arXiv2025

HST Proper Motions of Andromeda V and VI

We measure the absolute proper motions of Andromeda V (And V) and Andromeda VI/Pegasus (And VI) dwarf galaxies, satellites of M31 located near its galactic plane. And VI is located the farthest from M31 among the six satellites with currently measured proper motions. A combination of ACS/WFC and WFPC2 exposures are utilized, spanning a 20-year time baseline. The WFPC2 exposures are processed using a recently developed deep-learning centering procedure as well as the most up-to-date astrometric calibration of the camera. We use on the order of 100 background galaxies per satellite to determine the correction to absolute proper motion. For And V we obtain an absolute proper motion of $(μ_α , μ_δ)_{And\,V} = (26.1\pm21.5, -74.2\pm19.1)~μ$as yr$^{-1}$. For And VI we obtain an absolute proper motion of $(μ_α , μ_δ)_{And\,VI} = (-1.6\pm12.3, -52.6\pm11.2)~μ$as yr$^{-1}$. Orbit integrations and analyses are made for these two Andromeda satellites using two estimates of both the mass and proper motion of M31. It is found that And V has an orbit consistent within errors with alignment with M31's disk and counter orbiting it, although this alignment is not well constrained. And VI's orbit is better determined and is very much consistent with co-orbiting with M31's disk. While currently at a distance of ~280 kpc from M31, And VI will remain beyond a distance of ~ 90 kpc from M31, thus experiencing low tidal influence compared to the other M31 satellites with known orbits. Both satellites are determined to be well-bound to M31.

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arXiv2024

HST Proper Motion of Andromeda III: Another Satellite Co-orbiting The M31 Satellite Plane

We measure the absolute proper motion of Andromeda III using ACS/WFC and WFPC2 exposures spanning an unprecedented 22-year time baseline. The WFPC2 exposures have been processed using a deep-learning centering procedure recently developed as well as an improved astrometric calibration of the camera. The absolute proper motion zero point is given by 98 galaxies and 16 Gaia EDR3 stars. The resulting proper motion is $(μ_α , μ_δ) = (-10.5\pm12.5, 47.5\pm12.5)~μ$as yr$^{-1}$. We perform an orbit analysis of And III using two estimates of M31's mass and proper motion. We find that And III's orbit is consistent with dynamical membership to the Great Plane of Andromeda system of satellites although with some looser alignment compared to the previous two satellites NGC 147 and NGC 185. And III is bound to M31 if M31's mass is $M_{\mathrm{vir}}\geq 1.5\times10^{12}\,M_{\odot}$.

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arXiv2024

Star-Image Centering with Deep Learning II: HST/WFPC2 Full Field of View

We present an expanded and improved deep-learning (DL) methodology for determining centers of star images on HST/WFPC2 exposures. Previously, we demonstrated that our DL model can eliminate the pixel-phase bias otherwise present in these undersampled images; however that analysis was limited to the central portion of each detector. In the current work we introduce the inclusion of global positions to account for the PSF variation across the entire chip and instrumental magnitudes to account for nonlinear effects such as charge transfer efficiency. The DL model is trained using a unique series of WFPC2 observations of globular cluster 47 Tuc, data sets comprising over 600 dithered exposures taken in each of two filters, F555W and F814W. It is found that the PSF variations across each chip correspond to corrections of the order of 100 mpix, while magnitude effects are at a level of about 10 mpix. Importantly, pixel-phase bias is eliminated with the DL model; whereas, with a classic centering algorithm, the amplitude of this bias can be up to 40 mpix. Our improved DL model yields star-image centers with uncertainties of 8-10 mpix across the full field of view of WFPC2.

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arXiv2023

Star-Image Centering with Deep Learning: HST/WFPC2 Images

A Deep Learning (DL) algorithm is built and tested for its ability to determine centers of star images on HST/WFPC2 exposures, in filters F555W and F814W. These archival observations hold great potential for proper-motion studies, but the undersampling in the camera's detectors presents challenges for conventional centering algorithms. Two exquisite data sets of over 600 exposures of the cluster NGC 104 in these filters are used as a testbed for training and evaluation of the DL code. Results indicate a single-measurement standard error of from 8.5 to 11 mpix, depending on detector and filter.This compares favorably to the $\sim20$ mpix achieved with the customary ``effective PSF'' centering procedure for WFPC2 images. Importantly, pixel-phase error is largely eliminated when using the DL method. The current tests are limited to the central portion of each detector; in future studies the DL code will be modified to allow for the known variation of the PSF across the detectors.

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arXiv2022

Milky Way thin and thick disk kinematics with GAIA EDR3 and RAVE DR5

We present a detailed analysis of kinematics of the Milky Way disk in solar neighborhood using GAIA DR3 catalog. To determine the local kinematics of the stellar disks of the Milky Way galaxy we use a complete sample of 278,228 red giant branch (RGB) stars distributed in a cylinder, centered at the Sun with a 1 kpc radius and half-height of 0.5 kpc. We determine separately the kinematical properties of RGB stars for each Galactic hemisphere in search for possible asymmetries. The kinematical properties of the RGB stars reveal the existence of two kinematically distinct components: the thin disk with mean velocities ${V_R}, {V_ϕ}, {V_Z}$ of about -1, -239, 0 km s$^{-1}$ correspondingly and velocity dispersions $σ_R, σ_ϕ, σ_Z$ of 31, 20 and 11 km s$^{-1}$, and the Thick disk with mean velocities components of about +1, -225, 0 km s$^{-1}$, and velocity dispersions of 49, 35, and 22 km s$^{-1}$. We find that up to 500 pc height above/below the galactic plane, Thick disk stars comprise about half the stars of the disk. Such high amount of RGB stars with Thick disk kinematics points at the secular evolution scenario origin for the Thick disk of the Milky Way galaxy.

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arXiv2021

A New Proper Motion Determination of Leo I

We measure the absolute proper motion of Leo I using a WFPC2/HST data set that spans up to 10 years, to date the longest time baseline utilized for this satellite. The measurement relies on ~ 2300 Leo I stars located near the center of light of the galaxy; the correction to absolute proper motion is based on 174 Gaia EDR3 stars and 10 galaxies. Having generated highly-precise, relative proper motions for all Gaia EDR3 stars in our WFPC2 field of study, our correction to the absolute EDR3 system does not rely on these Gaia stars being Leo I members. This new determination also benefits from a recently improved astrometric calibration of WFPC2. The resulting proper-motion value, (mu_alpha, mu_delta) = (-0.007 +- 0.035, -0.119 +-0.026) mas/yr is in agreement with recent, large-area, Gaia EDR3-based determinations. We discuss all the recent measurements of Leo I's proper motion and adopt a combined, multi-study average of (mu_alpha_3meas, mu_delta_3meas) = (-0.036 +- 0.016, -0.130 +- 0.010) mas/yr. This value of absolute proper motion for Leo I indicates its orbital pole is well aligned with that of the Vast Polar Structure, defined by the majority of the brightest dwarf-spheroidal satellites of the Milky Way.

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arXiv2021

A Comprehensive Astrometric Calibration of HST's WFPC2. I. Distortion Mapping

Wide field planetary camera 2 (WFPC2) exposures are already some 20 years older than Gaia epoch observations, or future JWST observations. As such, they offer an unprecedented time baseline for high-precision proper-motion studies, provided the full astrometric potential of these exposures is reached. We have started such a project with the work presented here being its first step. We explore geometric distortions beyond the well-known ones published in the early 2000s. This task is accomplished by using the entire database of WFPC2 exposures in filters F555W, F606W and F814W and three standard astrometric catalogs: Gaia EDR3, 47 Tuc and $ω$Cen. The latter two were constructed using HST observations made with cameras other than WFPC2. We explore a suite of centering algorithms, and various distortion maps in order to understand and quantify their performance. We find no high-frequency systematics beyond the 34th-row correction, down to a resolution of 10 pixels. Low-frequency systematics starting at a resolution of 50-pixels are present at a level of 30-50 millipix (1.4-2.3 mas) for the PC and 20-30 millipix (2-3 mas) for the WF chips. We characterize these low-frequency systematics by providing correction maps and updated cubic-distortion coefficients for each filter.

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arXiv2019

A Kinematically Cold Structure of Candidate Young OB Stars Toward The Anticenter

We combine GALEX and Gaia DR2 catalogs to track star formation in the outskirts of our Galaxy. Using photometry, proper motions and parallaxes we identify a structure of ~ 300 OB-type candidates located between 12 and 15 kpc from the Galactic center that are kinematically cold. The structure is located between l = 120 and 200 degrees, above the plane up to ~700 pc and below the plane to ~ 1 kpc. The bulk motion is disk-like; however we measure a mean upward vertical motion of 5.7 +-0.4 km/s, and a mean outward radial motion of between 8 and 16 km/s. The velocity dispersion along the least dispersed of its proper-motion axes (perpendicular to the Galactic disk) is 6.0 +-0.3 km/s confirming the young age of this structure. While spatially encompassing the outer spiral arm of the Galaxy, this structure is not a spiral arm. Its explanation as the Milky-Way warp is equally unsatisfactory. The structure's vertical extent, mean kinematics and asymmetry with respect to the plane indicate that its origin is more akin to a wobble generated by a massive satellite perturbing the Galaxy's disk. The mean stellar ages in this outer structure indicate the event took place some 200 Myr ago.

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arXiv2018

The Origins of Young Stars in the Direction of the Leading Arm of the Magellanic Stream: Abundances, Kinematics, and Orbits

We explore the origins of the young B-type stars found by Casetti-Dinescu et al.(2014) at the outskirts of the Milky-Way disk in the sky region of Leading Arm of the Magellanic Stream. High-resolution spectroscopic observations made with the MIKE instrument on the Magellan Clay 6.5m telescope for nine stars are added to the previous sample analyzed by Zhang et al. (2017). We compile a sample of fifteen young stars with well-determined stellar types, ages, abundances and kinematics. With proper motions from Gaia DR2 we also derive orbits in a realistic Milky-Way potential. We find that our previous radial-velocity selected LA candidates have substantial orbital angular momentum. The substantial amount of rotational component for these stars is in contrast with the near-polar Magellanic orbit, thus rendering these stars unlikely members of the LA. There are four large orbital-energy stars in our sample. The highest orbital-energy one has an age shorter than the time to disk crossing, with a birthplace $z=2.5$~kpc and $R_{\rm GC}\sim 28$~kpc. Therefore, the origin of this star is uncertain. The remaining three stars have disk runaway origin with birthplaces between 12 and 25 kpc from the Galactic center. Also, the most energetic stars are more metal poor ([Mg/H] =$-0.50\pm0.07$) and with larger He scatter ($σ_{\rm [He/H]} = 0.72$) than the inner disk ones ([Mg/H] $=0.12\pm0.36$, $σ_{\rm [He/H]} = 0.15$). While the former group's abundance is compatible with that of the Large Magellanic Cloud, it could also reflect the metallicity gradient of the MW disk and their runaway status via different runaway mechanisms.

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arXiv2018

In-Situ Star Formation in the Outskirts of the Large Magellanic Cloud: Gaia DR2 Confirmation

We explore the Gaia DR2 proper motions of six young, main-sequence stars, members of the Large Magellanic Cloud (LMC) reported by Moni Bidin et al. (2017). These stars are located in the outskirts of the disk, between 7 and 13 degrees from the LMC's center where there is very low H I content. Gaia DR2 proper motions confirm that four stars formed locally, in situ, while two are consistent with being expelled via dynamical interactions from inner, more gas-rich regions of the LMC. This finding establishes that recent star formation occurred in the periphery of the LMC, where thus far only old populations are known.

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arXiv2017

Proper Motion of the Sextans Dwarf Galaxy from Subaru Suprime-Cam Data

We have measured the absolute proper motion of the Sextans dwarf spheroidal galaxy using Subaru Suprime-Cam images taken at three epochs, with a time baseline of ~ 10 years. We astrometrically calibrate each epoch by constructing distortion-correction maps from the best available Subaru Suprime-Cam dithered data sets and from Gaia DR1 positions. The magnitude limit of the proper-motion study is V ~ 24. The area covered is 26.7 x 23.3 arcmin, which is still within the core radius of Sextans. The derived proper motion is (mu_a, mu_d) = (-0.409 +/- 0.050, -0.047 +/- 0.058) mas/yr. The direction of motion is perpendicular to the major axis of the galaxy. Our measurement, combined with radial velocity and distance from the literature, implies a low eccentricity orbit, with a moderate inclination to the Galactic plane, and a period of 3 Gyr. Sextans is now some 0.4 Gyr away from its pericenter ( r_peri ~ 75 kpc), moving toward its apocenter (r_apo ~ 132 kpc). Its orbit is inconsistent with membership to the vast polar structure of Galactic satellites.

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arXiv2017

Chemical Composition of Young Stars in the Leading Arm of the Magellanic System

Chemical abundances of eight O- and B-type stars are determined from high-resolution spectra obtained with the MIKE instrument on the Magellan 6.5m Clay telescope. The sample is selected from 42 candidates of membership in the Leading Arm of the Magellanic System. Stellar parameters are measured by two independent grids of model atmospheres and analysis procedures, confirming the consistency of the stellar parameter results. Abundances of seven elements (He, C, N, O, Mg, Si, and S) are determined for the stars, as are their radial velocities and estimates of distances and ages. Among the seven B-type stars analyzed, the five that have radial velocities compatible with membership to the LA have an average [Mg/H] of $-0.42\pm0.16$, significantly lower than the average of the remaining two [Mg/H] = $-0.07\pm0.06$ that are kinematical members of the Galactic disk. Among the five LA members, four have individual [Mg/H] abundance compatible with that in the LMC. Within errors, we can not exclude the possibility that one of these stars has a [Mg/H] consistent with the more metal-poor, SMC-like material. The remaining fifth star has a [Mg/H] close to MW values. Distances to the LA members indicate that they are at the edge of the Galactic disk, while ages are of the order of $\sim 50-70$ Myr, lower than the dynamical age of the LA, suggesting a single star-forming episode in the LA. V$_{\rm LSR}$ the LA members decreases with decreasing Magellanic longitude, confirming the results of previous LA gas studies.

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arXiv2016

Proper Motion of the Draco Dwarf Galaxy from Subaru Suprime-Cam Data

We have measured the absolute proper motion of the Draco dwarf spheroidal galaxy using Subaru Suprime-Cam images taken at three epochs, with time baselines of 4.4 and 7 years. The magnitude limit of the proper-motion study is i=25, thus allowing for thousands of background galaxies and Draco stars to be used to perform extensive astrometric tests and to derive the correction to an inertial reference frame. The derived proper motion is (mu_alpha, mu_delta) =(-0.284 +- 0.047, -0.289 +-0.041) mas/yr. This motion implies an orbit that takes Draco to a pericenter of $\sim 20$ kpc; a somewhat disruptive orbit suggesting that tides might account for the rising velocity-dispersion profile of Draco seen in line-of-sight velocity studies. The orbit is only marginally consistent with Draco's membership to the vast polar structure of Galactic satellites, in contrast to a recent HST proper-motion measurement that finds alignment very likely. Our study is a test case to demonstrate that deep imaging with mosaic cameras of appropriate resolution can be used for high-accuracy, ground-based proper-motion measurement. As a useful by-product of the study, we also identify two faint brown-dwarf candidates in the foreground field.

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arXiv2015

Hubble Space Telescope Astrometry of the Procyon System

The nearby star Procyon is a visual binary containing the F5 IV-V subgiant Procyon A, orbited in a 40.84 yr period by the faint DQZ white dwarf Procyon B. Using images obtained over two decades with the Hubble Space Telescope, and historical measurements back to the 19th century, we have determined precise orbital elements. Combined with measurements of the parallax and the motion of the A component, these elements yield dynamical masses of 1.478 +/- 0.012 Msun and 0.592 +/- 0.006 Msun for A and B, respectively. The mass of Procyon A agrees well with theoretical predictions based on asteroseismology and its temperature and luminosity. Use of a standard core-overshoot model agrees best for a surprisingly high amount of core overshoot. Under these modeling assumptions, Procyon A's age is ~2.7 Gyr. Procyon B's location in the H-R diagram is in excellent agreement with theoretical cooling tracks for white dwarfs of its dynamical mass. Its position in the mass-radius plane is also consistent with theory, assuming a carbon-oxygen core and a helium-dominated atmosphere. Its progenitor's mass was 1.9-2.2 Msun, depending on its amount of core overshoot. Several astrophysical puzzles remain. In the progenitor system, the stars at periastron were separated by only ~5 AU, which might have led to tidal interactions and even mass transfer; yet there is no direct evidence that these have occurred. Moreover the orbital eccentricity has remained high (~0.40). The mass of Procyon B is somewhat lower than anticipated from the initial-to-final-mass relation seen in open clusters. The presence of heavy elements in its atmosphere requires ongoing accretion, but the place of origin is uncertain.

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arXiv2014

Recent Star Formation in the Leading Arm of the Magellanic Stream

Strongly interacting galaxies undergo a short-lived but dramatic phase of evolution characterized by enhanced star formation, tidal tails, bridges and other morphological peculiarities. The nearest example of a pair of interacting galaxies is the Magellanic Clouds, whose dynamical interaction produced the gaseous features known as the Magellanic Stream trailing the pair's orbit about the Galaxy, the Bridge between the Clouds, and the Leading Arm, a wide and irregular feature leading the orbit. Young, newly formed stars in the Bridge are known to exist, giving witness to the recent interaction between the Clouds. However, the interaction of the Clouds with the Milky Way is less well understood. In particular, the Leading Arm must have a tidal origin, however no purely gravitational model is able to reproduce its morphology and kinematics. A hydrodynamical interaction with the gaseous hot halo and disk of the Galaxy is plausible as suggested by some models and supporting neutral hydrogen observations. Here we show for the first time that young, recently formed stars exist in the Leading Arm, indicating that the interaction between the Clouds and our Galaxy is strong enough to trigger star formation in certain regions of the Leading Arm --- regions in the outskirts of the Milky Way disk (R ~ 18 kpc), far away from the Clouds and the Bridge.

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arXiv2014

High proper motion X-ray binaries from the Yale Southern Proper Motion Survey

We discuss the results of cross-correlating catalogs of bright X-ray binaries with the Yale Southern Proper Motion catalog (version 4.0). Several objects already known to have large proper motions from Hipparcos are recovered. Two additional objects are found which show substantial proper motions, both of which are unusual in their X-ray properties. One is IGR J17544-2619, one of the supergiant fast X-ray transients. Assuming the quoted distances in the literature for this source of about 3 kpc are correct, this system has a peculiar velocity of about 275 km/sec -- greater than the velocity of a Keplerian orbit at its location of the Galaxy, and in line with the expectations formed from suggestions that the supergiant fast X-ray transients should be highly eccentric. We discuss the possibility that these objects may help explain the existence of short gamma-ray bursts outside the central regions of galaxies. The other is the source 2A~1822-371, which is a member of the small class of objects which are low mass X-ray binaries and long (i.e. $>$ 100 millisecond) X-ray pulsars. This system also shows both an anomalously high X-ray luminosity and a large orbital period derivative for a system with its orbital period, and some possible indications of an eccentric orbit. A coherent picture can be developed by adding in the proper motion information in which this system formed in the Perseus spiral arm of the Galaxy about 3 Myr ago, and retains a slightly eccentric orbit which leads to enhanced mass transfer.

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arXiv2013

Space Velocities of Southern Globular Clusters VII. NGC 6397, NGC 6626 (M 28) and NGC 6656 (M 22)

We have measured the absolute proper motions of globular clusters NGC 6397, NGC 6626 (M 22) and NGC 6656 (M 28) as part of our ongoing Southern Proper-Motion Program. The reference system is the ICRS via Hipparcos stars for these three low galactic latitude clusters. Formal errors range between ~0.3 and 0.7 mas/yr. Notable is the result for NGC 6397 which differs by 2.5 mas/yr from two HST determinations, while agreeing with previous ground-based ones. We determine orbits for all three clusters in an axisymmetric and barred model of the Galaxy and discuss these in the context of globular-cluster formation. M 22 is a well-known cluster with an iron abundance spread; such clusters are now believed to have formed in massive parent systems that can retain ejecta of core-collapsed SNe. We find that the five currently-accepted globular clusters with iron/calcium abundance spread show orbits unrelated to each other, thus suggesting at least five independent, massive progenitors that have contributed to the build-up of the Milky-Way halo.

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arXiv2012

The Origin of the Virgo Stellar Substructure

We present three-dimensional space velocities of stars selected to be consistent with membership in the Virgo stellar substructure. Candidates were selected from SA 103, a single 40x40 arcmin field from our proper motion (PM) survey in Kapteyn's Selected Areas (SAs), based on the PMs, SDSS photometry, and follow-up spectroscopy of 215 stars. The signature of the Virgo substructure is clear in the SDSS color-magnitude diagram (CMD) centered on SA 103, and 16 stars are identified that have high Galactocentric-frame radial velocities (V_GSR > 50 km/s) and lie near the CMD locus of Virgo. The implied distance to the Virgo substructure from the candidates is 14+/-3 kpc. We derive mean kinematics from these 16 stars, finding a radial velocity V_GSR = 153+/-22 km/s and proper motions (mu_alpha*cos(delta), mu_delta) = (-5.24, -0.91)+/-(0.43, 0.46) mas/yr. From the mean kinematics of these members, we determine that the Virgo progenitor was on an eccentric (e ~ 0.8) orbit that recently passed near the Galactic center (pericentric distance R_p ~ 6 kpc). This destructive orbit is consistent with the idea that the substructure(s) in Virgo originated in the tidal disruption of a Milky Way satellite. N-body simulations suggest that the entire cloud-like Virgo substructure (encompassing the "Virgo Overdensity" and the "Virgo Stellar Stream") is likely the tidal debris remnant from a recently-disrupted massive (~10^9 M_sun) dwarf galaxy. The model also suggests that some other known stellar overdensities in the Milky Way halo (e.g., the Pisces Overdensity and debris near NGC 2419 and SEGUE 1) are explained by the disruption of the Virgo progenitor.

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