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Jarrod Hurley

Publications and source records attributed to Jarrod Hurley.

29 records · Page 2Linked to original sources
arXiv2009

Populating the Galaxy with pulsars -- II. Galactic dynamics

We produce synthetic populations of pulsars within our Galaxy and calculate the resulting scale heights as well as the radial and space velocity distributions of the pulsars. Results are presented for isolated pulsars, binary pulsars and millisecond pulsars. We also test the robustness of the outcomes to variations in the assumed form of the Galactic potential, the birth distribution of binary positions, and the strength of the velocity kick given to neutron stars at birth. We find that isolated pulsars have a greater scale height than binary pulsars. This is also true when restricted to millisecond pulsars unless we allow for low-mass stars to be ablated by radiation from their pulsar companion in which case the isolated and binary scale heights are comparable. Double neutron stars are found to have a large variety of space velocities, in particular, some systems have speeds similar to the Sun. We look in detail at the predicted Galactic population of millisecond pulsars with black hole companions, including their formation pathways, and show where the short-period systems reside in the Galaxy. Some of our population predictions are compared in a limited way to observations but the full potential of this aspect will be realised in the near future when we complete our population synthesis code with the selection effects component.

astro-ph.GA↗
arXiv2008

Populating the Galaxy with pulsars I: stellar & binary evolution

The computation of theoretical pulsar populations has been a major component of pulsar studies since the 1970s. However, the majority of pulsar population synthesis has only regarded isolated pulsar evolution. Those that have examined pulsar evolution within binary systems tend to either treat binary evolution poorly or evolve the pulsar population in an ad-hoc manner. Thus no complete and direct comparison with observations of the pulsar population within the Galactic disk has been possible to date. Described here is the first component of what will be a complete synthetic pulsar population survey code. This component is used to evolve both isolated and binary pulsars. Synthetic observational surveys can then be performed on this population for a variety of radio telescopes. The final tool used for completing this work will be a code comprised of three components: stellar/binary evolution, Galactic kinematics and survey selection effects. Results provided here support the need for further (apparent) pulsar magnetic field decay during accretion, while they conversely suggest the need for a re-evaluation of the assumed \textit{typical} MSP formation process. Results also focus on reproducing the observed $P\dot{P}$ diagram for Galactic pulsars and how this precludes short timescales for standard pulsar exponential magnetic field decay. Finally, comparisons of bulk pulsar population characteristics are made to observations displaying the predictive power of this code, while we also show that under standard binary evolutionary assumption binary pulsars may accrete much mass.

astro-ph↗
arXiv2008

A Deep HST Study of the Globular Cluster NGC 6397: Reduction Methods

We describe here the reduction methods that we developed to study the faintest red dwarfs and white dwarfs in an outer field of NGC6397, which was observed by \hst for 126 orbits in 2005. The particular challenge of this data set is that the faintest stars are not readily visible in individual exposures, so special care must be taken to combine the information in all the exposures in order to identify and measure them. Unfortunately, it is hard to find the faintest stars without also finding a large number of faint galaxies, so we developed specialized tools to distinguish between the point-like stars and the barely resolved galaxies. We found that artificial-star tests, while obviously necessary for completeness determination, can also play an important role in helping us optimize our finding and measuring algorithms. Although this paper focuses on this data set specifically, many of the techniques are new and might find application in other work, particularly when a large number of images is available for a single field.

astro-ph↗
arXiv2007

Furnishing the Galaxy with Pulsars

The majority of pulsar population synthesis studies performed to date have focused on isolated pulsar evolution. Those that have incorporated pulsar evolution within binary systems have tended to either treat binary evolution poorly of evolve the pulsar population in an ad-hoc manner. Here we present the first model of the Galactic field pulsar population that includes a comprehensive treatment of both binary and pulsar evolution. Synthetic observational surveys mimicking a variety of radio telescopes are then performed on this population. As such, a complete and direct comparison of model data with observations of the pulsar population within the Galactic disk is now possible. The tool used for completing this work is a code comprised of three components: stellar/binary evolution, Galactic kinematics and survey selection effects. Here we give a brief overview of the method and assumptions involved with each component. Some preliminary results are also presented as well as plans for future applications of the code.

astro-ph↗
arXiv2007

Deep ACS Imaging in the Globular Cluster NGC 6397: The Cluster Color Magnitude Diagram and Luminosity Function

We present the CMD from deep HST imaging in the globular cluster NGC 6397. The ACS was used for 126 orbits to image a single field in two colors (F814W, F606W) 5 arcmin SE of the cluster center. The field observed overlaps that of archival WFPC2 data from 1994 and 1997 which were used to proper motion (PM) clean the data. Applying the PM corrections produces a remarkably clean CMD which reveals a number of features never seen before in a globular cluster CMD. In our field, the main sequence stars appeared to terminate close to the location in the CMD of the hydrogen-burning limit predicted by two independent sets of stellar evolution models. The faintest observed main sequence stars are about a magnitude fainter than the least luminous metal-poor field halo stars known, suggesting that the lowest luminosity halo stars still await discovery. At the bright end the data extend beyond the main sequence turnoff to well up the giant branch. A populous white dwarf cooling sequence is also seen in the cluster CMD. The most dramatic features of the cooling sequence are its turn to the blue at faint magnitudes as well as an apparent truncation near F814W = 28. The cluster luminosity and mass functions were derived, stretching from the turn off down to the hydrogen-burning limit. It was well modeled with either a very flat power-law or a lognormal function. In order to interpret these fits more fully we compared them with similar functions in the cluster core and with a full N-body model of NGC 6397 finding satisfactory agreement between the model predictions and the data. This exercise demonstrates the important role and the effect that dynamics has played in altering the cluster IMF.

astro-ph↗
arXiv2007

NBODY meets stellar population - The HYDE-PARC Project

N-body simulations give us a rough idea of how the shape of a simulated object appears in three-dimensional space. From an observational point of view this may give us a misleading picture. The faint stars may be the most common stars in the system but the morphological information obtained by observations of an object may be dominated by the color properties of the bright stars. Due to dynamical effects, such as energy equipartition, different masses of stars may populate different regions in the object. Since stars are evolving in mass the stellar evolution may also influence the dynamics of the system. Hence, if one is interested in simulating what the morphology will look like through a telescope, one needs to simulate in addition evolving stars and weight them by their luminosity. Therefore we need to combine simulations of the dynamical evolution and a stellar population synthesis at the same time. For the dynamical evolution part we use a parallel version of a direct N-body code, NBODY6++. This code also includes stellar evolution. We link the parameters from this stellar evolution routine to the BaSeL 2.0 stellar library. For isolated clusters we found results assimilable to standard stellar population synthesis codes such as the PEGASE code. For clusters in a tidal field we found that the integrated colors become relatively bluer due to energy equipartition effects. In the time shortly before dissolution of the cluster the stellar M/L ratio becomes lower compared to isolated clusters. We compared the results of our simulations to integrated spectra of galactic globular clusters. For the cluster NGC 1851 we found good agreement between simulation and observation. For extragalactic clusters in M81 and M31 we looked at medium band spectral energy distributions and found for some clusters also a good agreement.

astro-ph↗
arXiv2007

The White Dwarf Cooling Sequence of NGC6397

We present the results of a deep Hubble Space Telescope (HST) exposure of the nearby globular cluster NGC6397, focussing attention on the cluster's white dwarf cooling sequence. This sequence is shown to extend over 5 magnitudes in depth, with an apparent cutoff at magnitude F814W=27.6. We demonstrate, using both artificial star tests and the detectability of background galaxies at fainter magnitudes, that the cutoff is real and represents the truncation of the white dwarf luminosity function in this cluster. We perform a detailed comparison between cooling models and the observed distribution of white dwarfs in colour and magnitude, taking into account uncertainties in distance, extinction, white dwarf mass, progenitor lifetimes, binarity and cooling model uncertainties. After marginalising over these variables, we obtain values for the cluster distance modulus and age of μ_0 = 12.02 \pm 0.06 and T_c = 11.47 \pm 0.47Gyr (95% confidence limits). Our inferred distance and white dwarf initial-final mass relations are in good agreement with other independent determinations, and the cluster age is consistent with, but more precise than, prior determinations made using the main sequence turnoff method. In particular, within the context of the currently accepted ΛCDM cosmological model, this age places the formation of NGC6397 at a redshift z=3, at a time when the cosmological star formation rate was approaching its peak.

astro-ph↗
arXiv2007

Probing the Faintest Stars in a Globular Star Cluster

NGC 6397 is the second closest globular star cluster to the Sun. Using 5 days of time on the Hubble Space Telescope, we have constructed the deepest ever color-magnitude diagram for this cluster. We see a clear truncation in each of its two major stellar sequences. Faint red main sequence stars run out well above our observational limit and near to the theoretical prediction for the lowest mass stars capable of stable hydrogen-burning in their cores. We also see a truncation in the number counts of faint blue stars, namely white dwarfs. This reflects the limit to which the bulk of the white dwarfs can cool over the lifetime of the cluster. There is also a turn towards bluer colors in the least luminous of these objects. This was predicted for the very coolest white dwarfs with hydrogen-rich atmospheres as the formation of H2 causes their atmospheres to become largely opaque to infrared radiation due to collision-induced absorption.

astro-ph↗
arXiv2006

McScatter: a Simple Three-Body Scattering Package with Stellar Evolution

We describe a simple computer package which illustrates a method of combining stellar dynamics with stellar evolution. Though the method is intended for elaborate applications (especially the dynamical evolution of rich star clusters) it is illustrated here in the context of three-body scattering, i.e. interactions between a binary star and a field of single stars. We describe the interface between the dynamics and the two independent packages which describe the internal evolution of single stars and binaries. We also give an example application, and introduce a stand alone utility for the visual presentation of simulation results.

astro-ph↗
arXiv1998

The Binary Second Sequence in Cluster Colour--Magnitude Diagrams

We show how the second sequence seen lying above the main sequence in cluster colour magnitude diagrams results from binaries with a large range of mass ratios and not just from those with equal masses. We conclude that the presence of a densely populated second sequence, with only sparse filling in between it and the single star main sequence, does not necessarily imply that binary mass ratios are close to unity.

astro-ph↗