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J. Anderson

Publications and source records attributed to J. Anderson.

At least 127 records · Page 7Linked to original sources
arXiv2018

Hubble Space Telescope Proper motion (HSTPROMO) Catalogs of Galactic Globular Clusters. VI. Improved data reduction and internal-kinematic analysis of NGC 362

We present an improved data-reduction technique to obtain high-precision proper motions (PMs) of globular clusters using Hubble Space Telescope data. The new reduction is superior to the one presented in the first paper of this series for the faintest sources in very crowded fields. We choose the globular cluster NGC 362 as a benchmark to test our new procedures. We measure PMs of 117 450 sources in the field, showing that we are able to obtain a PM precision better than 10 $μ$as yr$^{-1}$ for bright stars. We make use of this new PM catalog of NGC 362 to study the cluster's internal kinematics. We investigate the velocity-dispersion profiles of the multiple stellar populations hosted by NGC 362 and find new pieces of information on the kinematics of first- and second-generation stars. We analyze the level of energy equipartition of the cluster and find direct evidence for its post-core-collapsed state from kinematic arguments alone. We refine the dynamical mass of the blue stragglers and study possible kinematic differences between blue stragglers formed by collisions and mass transfer. We also measure no significant cluster rotation in the plane of the sky. Finally, we measure the absolute PM of NGC 362 and of the background stars belonging to the Small Magellanic Cloud, finding a good agreement with previous estimates in the literature. We make the PM catalog publicly available.

astro-ph.SR↗
arXiv2018

New variable stars in the Galactic Bulge: I. The bright regime

We report the detection of 1143 variable stars towards the Galactic bulge, including 320 previously uncatalogued variables, using time-series photometry extracted from data obtained with the VIMOS imager at the Very Large Telescope. Observations of the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS) field in the Galactic Bulge were taken over 2 years between March and October at a cadence of $\sim$ 4 days, enabling the detection of variables with periods up to $\sim$100 days. Many of these were already known, but we detected a significant number of new variables, including 26 Cepheids, a further 18 Cepheid candidates, and many contact binaries. Here we publish the catalog of the new variables, containing coordinates, mean magnitudes as well as periods and classification; full light curves for these variables are also made available electronically.

astro-ph.SR↗
arXiv2018

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters - XV. The dynamical clock: reading cluster dynamical evolution from the segregation level of blue straggler stars

The parameter A+, defined as the area enclosed between the cumulative radial distribution of blue straggler stars (BSSs) and that of a reference population, is a powerful indicator of the level of BSS central segregation. As part of the Hubble Space Telescope UV Legacy Survey of Galactic globular clusters (GCs), here we present the BSS population and the determination of A+ in 27 GCs observed out to about one half-mass radius. In combination with 21 additional clusters discussed in a previous paper this provides us with a global sample of 48 systems (corresponding to \sim 32\% of the Milky Way GC population), for which we find a strong correlation between A+ and the ratio of cluster age to the current central relaxation time. Tight relations have been found also with the core radius and the central luminosity density, which are expected to change with the long-term cluster dynamical evolution. An interesting relation is emerging between A+ and the ratio of the BSS velocity dispersion relative to that of main sequence turn-off stars, which measures the degree of energy equipartition experienced by BSSs in the cluster. These results provide further confirmation that BSSs are invaluable probes of GC internal dynamics and A+ is a powerful dynamical clock.

astro-ph.GA↗
arXiv2018

Shock location and CME 3D reconstruction of a solar type II radio burst with LOFAR

Type II radio bursts are evidence of shocks in the solar atmosphere and inner heliosphere that emit radio waves ranging from sub-meter to kilometer lengths. These shocks may be associated with CMEs and reach speeds higher than the local magnetosonic speed. Radio imaging of decameter wavelengths (20-90 MHz) is now possible with LOFAR, opening a new radio window in which to study coronal shocks that leave the inner solar corona and enter the interplanetary medium and to understand their association with CMEs. To this end, we study a coronal shock associated with a CME and type II radio burst to determine the locations at which the radio emission is generated, and we investigate the origin of the band-splitting phenomenon.

astro-ph.SR↗
arXiv2018

Multifrequency behaviour of the anomalous events of PSR J0922+0638

PSR J0922+0638 (B0919+06) shows unexplained anomalous variations in the on-pulse phase, where the pulse appears to episodically move to an earlier longitude for a few tens of rotations before reverting to the usual phase for approximately several hundred to more than a thousand rotations. These events, where the pulse moves in phase by up to 5$^{\circ}$, have been previously detected in observations from $\sim$300 to 2000 MHz. We present simultaneous observations from the Effelsberg 100-m radio telescope at 1350 MHz and the Bornim (Potsdam) station of the LOw Frequency ARray at 150 MHz. Our observations present the first evidence for an absence of the anomalous phase-shifting behaviour at 150 MHz. Instead, the observed intensity at the usual pulse-phase typically decreases, often showing a pseudo-nulling feature corresponding to the times when phase shifts are observed at 1350 MHz. The presence of weak emission at the usual pulse-phase supports the theory that these shifts may result from processes similar to the 'profile-absorption' expected to operate for PSR J0814+7429 (B0809+74). A possible mechanism for this could be intrinsic variations of the emission within the pulsar's beam combined with absorption by expanding shells of electrons in the line of sight.

astro-ph.HE↗
arXiv2018

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters - XIV. Multiple stellar populations within M 15 and their radial distribution

In the context of the Hubble Space Telescope UV Survey of Galactic Globular Clusters (GCs), we derived high-precision, multi-band photometry to investigate the multiple stellar populations in the massive and metal-poor GC M 15. By creating for red-giant branch (RGB) stars of the cluster a 'chromosome map', which is a pseudo two-colour diagram made with appropriate combination of F275W, F336W, F438W, and F814W magnitudes, we revealed colour spreads around two of the three already known stellar populations. These spreads cannot be produced by photometric errors alone and could hide the existence of (two) additional populations. This discovery increases the complexity of the multiple-population phenomenon in M 15. Our analysis shows that M 15 exhibits a faint sub-giant branch (SGB), which is also detected in colour-magnitude diagrams (CMDs) made with optical magnitudes only. This poorly-populated SGB includes about 5% of the total number of SGB stars and evolves into a red RGB in the m_F336W vs. m_F336W-m_F814W CMD, suggesting that M 15 belongs to the class of Type II GCs. We measured the relative number of stars in each population at various radial distances from the cluster centre, showing that all of these populations share the same radial distribution within statistic uncertainties. These new findings are discussed in the context of the formation and evolution scenarios of the multiple populations.

astro-ph.SR↗
arXiv2018

Multiple stellar populations in Magellanic Cloud clusters. VI. A survey of multiple sequences and Be stars in young clusters

The split main sequences (MSs) and extended MS turnoffs (eMSTOs) detected in a few young clusters have demonstrated that these stellar systems host multiple populations differing in a number of properties such as rotation and, possibly, age.We analyze Hubble Space Telescope photometry for thirteen clusters with ages between ~40 and ~1000 Myrs and of different masses. Our goal is to investigate for the first time the occurrence of multiple populations in a large sample of young clusters. We find that all the clusters exhibit the eMSTO phenomenon and that MS stars more massive than ~1.6 solar masses define a blue and red MS, with the latter hosting the majority of MS stars. The comparison between the observations and isochrones suggests that the blue MSs are made of slow-rotating stars, while the red MSs host stars with rotational velocities close to the breakup value. About half of the bright MS stars in the youngest clusters are H-alpha emitters. These Be stars populate the red MS and the reddest part of the eMSTO thus supporting the idea that the red MS is made of fast rotators. We conclude that the split MS and the eMSTO are a common feature of young clusters in both Magellanic Clouds. The phenomena of a split MS and an eMSTO occur for stars that are more massive than a specific threshold which is independent of the host-cluster mass. As a by-product, we report the serendipitous discovery of a young SMC cluster, GALFOR1.

astro-ph.SR↗
arXiv2018

The Optical/Near-Infrared Extinction Law In Highly Reddened Regions

A precise extinction law is a critical input when interpreting observations of highly reddened sources such as young star clusters and the Galactic Center (GC). We use Hubble Space Telescope observations of a region of moderate extinction and a region of high extinction to measure the optical and near-infrared extinction law (0.8 $μ$m -- 2.2 $μ$m). The moderate extinction region is the young massive cluster Westerlund 1 (Wd1; A$_{Ks} \sim$ 0.6 mag), where 453 proper motion-selected main-sequence stars are used to measure the shape of the extinction law. To quantify the shape we define the parameter $\mathcal{S}_{1/λ}$, which behaves similarly to a color excess ratio but is continuous as a function of wavelength. The high extinction region is the GC (A$_{Ks} \sim$ 2.5 mag), where 819 red clump stars are used to determine the normalization of the law. The best-fit extinction law is able to reproduce the Wd1 main sequence colors, which previous laws misestimate by 10%-30%. The law is inconsistent with a single power law, even when only the near-infrared filters are considered, and has A$_{F125W}$/A$_{Ks}$ and A$_{F814W}$/A$_{Ks}$ values that are 18% and 24% larger than the commonly used \citet{Nishiyama:2009fc} law, respectively. Using the law we recalculate the Wd1 distance to be 3896 $\pm$ 328 pc from published observations of eclipsing binary W13. This new extinction law should be used for highly reddened populations in the Milky Way, such as the Quintuplet cluster and Young Nuclear Cluster. A python code is provided to generate the law for future use.

astro-ph.GA↗
arXiv2018

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. XIII. ACS/WFC Parallel-Field Catalogues

As part of the Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters, 110 parallel fields were observed with the Wide Field Channel of the Advanced Camera for Surveys, in the outskirts of 48 globular clusters, plus the open cluster NGC 6791. Totalling about $0.3$ square degrees of observed sky, this is the largest homogeneous Hubble Space Telescope photometric survey of Galalctic globular clusters outskirts to date. In particular, two distinct pointings have been obtained for each target on average, all centred at about $6.5$ arcmin from the cluster centre, thus covering a mean area of about $23\,{\rm arcmin^{2}}$ for each globular cluster. For each field, at least one exposure in both F475W and F814W filters was collected. In this work, we publicly release the astrometric and photometric catalogues and the astrometrised atlases for each of these fields.

astro-ph.GA↗
arXiv2018

New Cataclysmic Variables and other Exotic Binaries in the Globular Cluster 47 Tucanae

We present 22 new (+3 confirmed) cataclysmic variables (CVs) in the non core-collapsed globular cluster 47 Tucanae (47 Tuc). The total number of CVs in the cluster is now 43, the largest sample in any globular cluster so far. For the identifications we used near-ultraviolet (NUV) and optical images from the Hubble Space Telescope, in combination with X-ray results from the Chandra X-ray Observatory. This allowed us to build the deepest NUV CV luminosity function of the cluster to date. We found that the CVs in 47 Tuc are more concentrated towards the cluster center than the main sequence turnoff stars. We compared our results to the CV populations of the core-collapsed globular clusters NGC 6397 and NGC 6752. We found that 47 Tuc has fewer bright CVs per unit mass than those two other clusters. That suggests that dynamical interactions in core-collapsed clusters play a major role creating new CVs. In 47 Tuc, the CV population is probably dominated by primordial and old dynamically formed systems. We estimated that the CVs in 47 Tuc have total masses of approx. 1.4 M_sun. We also found that the X-ray luminosity function of the CVs in the three clusters is bimodal. Additionally, we discuss a possible double degenerate system and an intriguing/unclassified object. Finally, we present four systems that could be millisecond pulsar companions given their X-ray and NUV/optical colors. For one of them we present very strong evidence for being an ablated companion. The other three could be CO- or He-WDs.

astro-ph.SR↗
arXiv2018

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters - XII. The RGB Bumps of multiple stellar populations

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters is providing a major breakthrough in our knowledge of Globular Clusters (GCs) and their stellar populations. Among the main results, we discovered that all the studied GCs host two main discrete groups consisting of first generation (1G) and second generation (2G) stars. We exploit the multiwavelength photometry from this project to investigate, for the first time, the Red Giant Branch Bump (RGBB) of the two generations in a large sample of GCs. We identified, with high statistical significance, the RGBB of 1G and 2G stars in 26 GCs and found that their magnitude separation as a function of the filter wavelength follows comparable trends. The comparison of observations to synthetic spectra reveals that the RGBB luminosity depends on the stellar chemical composition and that the 2G RGBB is consistent with stars enhanced in He and N and depleted in C and O with respect to 1G stars. For metal-poor GCs the 1G and 2G RGBB relative luminosity in optical bands mostly depends on helium content, Y. We used the RGBB observations in F606W and F814W bands to infer the relative helium abundance of 1G and 2G stars in 18 GCs, finding an average helium enhancement $Δ$Y=0.011$\pm$0.002 of 2G stars with respect to 1G stars. This is the first determination of the average difference in helium abundance of multiple populations in a large number of clusters and provides a lower limit to the maximum internal variation of helium in GCs.

astro-ph.SR↗
arXiv2018

The HST large programme on $ω$ Centauri - II. internal kinematics

In this second installment of the series, we look at the internal kinematics of the multiple stellar populations of the globular cluster $ω$ Centauri in one of the parallel Hubble Space Telescope (HST) fields, located at about 3.5 half-light radii from the center of the cluster. Thanks to the over 15-year-long baseline and the exquisite astrometric precision of the HST cameras, well-measured stars in our proper-motion catalog have errors as low as $\sim 10\ μ$as yr$^{-1}$, and the catalog itself extends to near the hydrogen-burning limit of the cluster. We show that second-generation (2G) stars are significantly more radially anisotropic than first-generation (1G) stars. The latter are instead consistent with an isotropic velocity distribution. In addition, 1G have excess systemic rotation in the plane of the sky with respect to 2G stars. We show that the six populations below the main-sequence (MS) knee identified in our first paper are associated to the five main population groups recently isolated on the upper MS in the core of cluster. Furthermore, we find both 1G and 2G stars in the field to be far from being in energy equipartition, with $η_{\rm 1G}=-0.007\pm0.026$ for the former, and $η_{\rm 2G}=0.074\pm0.029$ for the latter, where $η$ is defined so that the velocity dispersion $σ_μ$ scales with stellar mass as $σ_μ\propto m^{-η}$. The kinematical differences reported here can help constrain the formation mechanisms for the multiple stellar populations in $ω$ Centauri and other globular clusters. We make our astro-photometric catalog publicly available.

astro-ph.GA↗
arXiv2018

The HST large programme on $ω$ Centauri -- III. Absolute proper motion

In this paper we report a new estimate of the absolute proper motion (PM) of the globular cluster NGC 5139 ($ω$ Cen) as part of the HST large program GO-14118+14662. We analyzed a field 17 arcmin South-West of the center of $ω$ Cen and computed PMs with an epoch span of $\sim$15.1 years. We employed 45 background galaxies to link our relative PMs to an absolute reference-frame system. The absolute PM of the cluster in our field is: $(μ_α\cosδ, μ_δ) = (-3.341 \pm 0.028 , -6.557 \pm 0.043)$ mas yr$^{-1}$. Upon correction for the effects of viewing perspective and the known cluster rotation, this implies that for the cluster center of mass $(μ_α\cosδ, μ_δ) = (-3.238 \pm 0.028, -6.716 \pm 0.043)$ mas yr$^{-1}$. This measurement is direct and independent, has the highest random and systematic accuracy to date, and will provide an external verification for the upcoming Gaia Data Release 2. It also differs from most reported PMs for $ω$ Cen in the literature by more than 5$σ$, but consistency checks compared to other recent catalogs yield excellent agreement. We computed the corresponding Galactocentric velocity, calculated the implied orbit of $ω$ Cen in two different Galactic potentials, and compared these orbits to the orbits implied by one of the PM measurements available in the literature. We find a larger (by about 500 pc) perigalactic distance for $ω$ Cen with our new PM measurement, suggesting a larger survival expectancy for the cluster in the Galaxy.

astro-ph.GA↗
arXiv2017

Deep HST Imaging in 47 Tucanae: A Global Dynamical Model

Multi-epoch observations with ACS and WFC3 on HST provide a unique and comprehensive probe of stellar dynamics within 47 Tucanae. We confront analytic models of the globular cluster with the observed stellar proper motions that probe along the main sequence from just above 0.8 to 0.1M$_\odot$ as well as white dwarfs younger than one gigayear. One field lies just beyond the half-light radius where dynamical models (\eg lowered Maxwellian distributions) make robust predictions for the stellar proper motions. The observed proper motions in this outer field show evidence for anisotropy in the velocity distribution as well as skewness; the latter is evidence of rotation. The measured velocity dispersions and surface brightness distributions agree in detail with a rotating, anisotropic model of the stellar distribution function with mild dependence of the proper-motion dispersion on mass. However, the best fitting models under-predict the rotation and skewness of the stellar velocities. In the second field, centered on the core of the cluster, the mass segregation in proper motion is much stronger. Nevertheless the model developed in the outer field can be extended inward by taking this mass segregation into account in a heuristic fashion. The proper motions of the main-sequence stars yield a mass estimate of the cluster of $1.31 \pm 0.02 \times 10^6 \mathrm{M}_\odot$ at a distance of 4.7 kpc. By comparing the proper motions of a sample of giant and sub-giant stars with the observed radial velocities we estimate the distance to the cluster kinematically to be $4.29 \pm 0.47$ kpc.

astro-ph.GA↗
arXiv2017

The Association of a J-burst with a Solar Jet

Context. The Sun is an active star that produces large-scale energetic events such as solar flares and coronal mass ejections and numerous smaller-scale events such as solar jets. These events are often associated with accelerated particles that can cause emission at radio wavelengths. The reconfiguration of the solar magnetic field in the corona is believed to be the cause of the majority of solar energetic events and accelerated particles. Aims. Here, we investigate a bright J-burst that was associated with a solar jet and the possible emission mechanism causing these two phenomena. Methods. We used data from the Solar Dynamics Observatory (SDO) to observe a solar jet, and radio data from the Low Frequency Array (LOFAR) and the Nançay Radioheliograph (NRH) to observe a J-burst over a broad frequency range (33-173 MHz) on 9 July 2013 at ~11:06 UT. Results. The J-burst showed fundamental and harmonic components and it was associated with a solar jet observed at extreme ultraviolet wavelengths with SDO. The solar jet occurred at a time and location coincident with the radio burst, in the northern hemisphere, and not inside a group of complex active regions in the southern hemisphere. The jet occurred in the negative polarity region of an area of bipolar plage. Newly emerged positive flux in this region appeared to be the trigger of the jet. Conclusions. Magnetic reconnection between the overlying coronal field lines and the newly emerged positive field lines is most likely the cause of the solar jet. Radio imaging provides a clear association between the jet and the J-burst which shows the path of the accelerated electrons.

astro-ph.SR↗
arXiv2017

The Single-Phase ProtoDUNE Technical Design Report

ProtoDUNE-SP is the single-phase DUNE Far Detector prototype that is under construction and will be operated at the CERN Neutrino Platform (NP) starting in 2018. ProtoDUNE-SP, a crucial part of the DUNE effort towards the construction of the first DUNE 10-kt fiducial mass far detector module (17 kt total LAr mass), is a significant experiment in its own right. With a total liquid argon (LAr) mass of 0.77 kt, it represents the largest monolithic single-phase LArTPC detector to be built to date. It's technical design is given in this report.

physics.ins-det↗
arXiv2017

Hubble Space Telescope Proper Motion (HSTPROMO) Catalogs of Galactic Globular Clusters. V. The rapid rotation of 47 Tuc traced and modeled in three dimensions

High-precision proper motions of the globular cluster 47 Tuc have allowed us to measure for the first time the cluster rotation in the plane of the sky and the velocity anisotropy profile from the cluster core out to about 13'. These profiles are coupled with prior measurements along the line of sight and the surface-brightness profile, and fit all together with self-consistent models specifically constructed to describe quasi-relaxed stellar systems with realistic differential rotation, axisymmetry and pressure anisotropy. The best-fit model provides an inclination angle i between the rotation axis and the line-of-sight direction of 30 deg, and is able to simultaneously reproduce the full three-dimensional kinematics and structure of the cluster, while preserving a good agreement with the projected morphology. Literature models based solely on line-of-sight measurements imply a significantly different inclination angle (i=45 deg), demonstrating that proper motions play a key role in constraining the intrinsic structure of 47 Tuc. Our best-fit global dynamical model implies an internal rotation higher than previous studies have shown, and suggests a peak of the intrinsic V/sigma ratio of ~0.9 at around two half-light radii, with a non-monotonic intrinsic ellipticity profile reaching values up to 0.45. Our study unveils a new degree of dynamical complexity in 47 Tuc, which may be leveraged to provide new insights into the formation and evolution of globular clusters.

astro-ph.GA↗