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Statistical ages of neutron stars

Ramandeep Gill and Jeremy Heyl further develop the technique introduced by Goldsbury, Heyl and collaborators to measure the ages of white dwarfs in clusters to estimate the ages of neutron stars in the Galaxy. The cooling theory of neutron stars is corroborated by its comparison with observations of thermally emitting isolated neutron stars. An important ingredient for such an analysis is the age of the object, which, typically, is obtained from the spin-down history. This age is highly uncertain if the object's magnetic field varies appreciably over time. Other age estimators, such as supernova remnant ages and kinematic ages, only apply to few handful of neutron stars. We conduct a population synthesis study of the nearby isolated thermal emitters and obtain their ages statistically from the observed luminosity function of these objects. We argue that a more sensitive blind scan of the galactic disk with the upcoming space telescopes can help to constrain the ages to higher accuracy.

Check out the paper at 1305.0930.

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Stellar Dynamics of NGC 6397

Jeremy Heyl, Harvey Richer, Jay Anderson and collaborators use multi-epoch observations with ACS on HST to probe of the stellar dynamics within NGC 6397. We are able to confront analytic models of the globular cluster with the observed stellar proper motions. The measured proper motions probe well along the main sequence from 0.8 to below <m>0.1 \mathrm{M}_\odot</m> as well as white dwarfs younger than one gigayear. The observed field lies just beyond the half-light radius where standard models of globular cluster dynamics (e.g. based on a lowered Maxwellian phase-space distribution) make very robust predictions for the stellar proper motions as a function of mass. The observed proper motions show no evidence for anisotropy in the velocity distribution; furthermore, the observations agree in detail with a straightforward model of the stellar distribution function. We do not find any evidence that the young white dwarfs have received a natal kick in contradiction with earlier results. Using the observed proper motions of the main-sequence stars, we obtain a kinematic estimate of the distance to NGC 6397 of <m>2.2^{+0.5}_{-0.7} \mathrm{kpc}</m> and a mass of the cluster of <m>1.1 \pm 0.1 \times 10^5 \mathrm{M}_\odot</m> at the photometric distance of 2.53 kpc. One of the main-sequence stars appears to travel on a trajectory that will escape the cluster, yielding an estimate of the evaporation timescale, over which the number of stars in the cluster decreases by a factor of e, of about 3 Gyr. The proper motions of the youngest white dwarfs appear to resemble those of the most massive main-sequence stars, providing the first direct constraint on the relaxation time of the stars in a globular cluster of greater than or about 0.7 Gyr.

Check out the paper at 1210.0826.

A Fast Matching Algorithm for Sheared Stellar Samples

Jeremy Heyl has developed new and efficient algorithms for matching stellar catalogues where the transformation between the coordinate systems of the two catalagoues is unknown and may include shearing. Finding a given object whether a star or asterism from the first catalogue in the second is logarithmic in time rather than polynomial, yielding a dramatic speed up relative to a naive implementation. Both acceleration of the matching algorithm and the ability to solve for arbitrary affine transformations not only will allow the registration of stellar catalogues and images that are now impossible to use but also will find applications in machine vision and other imaging applications.

Check out the article at 1304.0838.

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Measuring White-Dwarf Cooling in 47 Tuc

Ryan Goldsbury, Jeremy Heyl, Harvey Richer and collaborators have developed a new technique to measure the white-dwarf cooling curve empirically. Using spectral models, we determine temperatures for 887 objects from Wide Field Camera-3 data, as well as 292 objects from data taken with the Advanced Camera for Surveys. We make the assumption that the rate of white dwarf formation in the cluster is constant. Stellar evolution models are then used to determine the rate at which objects are leaving the main sequence, which must be the same as the rate at which objects are arriving on the white dwarf sequence in our field. The result is an empirically derived relation between temperature (<m>T_{eff}</m>) and time (<m>t</m>) on the white dwarf cooling sequence. Comparing this result to theoretical cooling models, we find general agreement with the expected slopes between 20,000K and 30,000K and between 6,000K and 20,000K, but the transition to the Mestel cooling rate of <m>T_{eff} \propto t^{-0.4}</m> is found to occur at hotter temperatures, and more abruptly than is predicted by any of these models.

Check out the article at 1209.4901.

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No Intermediate-Mass Black Hole in M71

Raminder Samra, Harvey Richer, Jeremy Heyl and collaborators have used Gemini North together with the NIRI-ALTAIR adaptive optics imager in the H and K bands to explore the core of the Galactic globular cluster M71 (NGC 6838). We obtained proper motions for 217 stars and have resolved its internal proper motion dispersion. Using a 3.8 year baseline, the proper motion dispersion in the core is found to be 179 ± 17 µarcsec/yr. We find no evidence of anisotropy in the motions and no radial variation in the proper motions with respect to distance from the cluster center. We also set an upper limit on any central black hole to be 150 solar masses at 90% confidence level.

Check out the article at 1203.5336.

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Betelgeuse

Anand Thirumalai and Jeremy Heyl present calculations for a magnetised hybrid wind model for Betelegeuse (α Orionis). The model is a direct application of our previously derived theory, combining a canonical Weber-Davis (WD) stellar wind with dust grains in the envelope of an AGB star. The resulting hybrid picture provides a mechanism for solving the problem of lifting stellar material up from the photosphere and into the circumstellar envelope. It also predicts wind velocities in agreement with current estimates. Our approach reveals that magnetic fields in supergiant stars like Betelgeuse may play a vital role in determining the nature of the stellar outflow and consequently, opens a new avenue of investigation in the field of hybrid stellar winds.

Check out the paper at 1109.5148.

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Axions and White Dwarfs

Axions are hypothetical pseudo-scalar particles that were borne out of the Peccei-Quinn solution to the strong CP problem. From astrophysical considerations it was realized that if these particles were to exist in our Universe, then their mass must lie in the range 10^-6 to 10^-2 eV. Several laboratory experiments have been conducted to find the axion based on its fortuitous coupling to electromagnetic radiation, but to no avail as the photon-axion coupling strength is very weak. Thus, only upper limits on how strongly it couples to radiation could be obtained.

In the recent article, Ramandeep Gill and Jeremy Heyl use a novel method to improve upon current constraints on the photon-axion coupling strength using magnetic white dwarfs. The figure shows constraints on axion mass from Cosmology and SN 1987A measurements, on photon-axion coupling strength from horizontal branch stars, magnetic WDs, and other lab experiments. The KSVZ and DFSZ lines are derived from two theoretical models that give the scaling of coupling strength with axion mass.

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Bernstein modes

In a recent paper (http://arxiv.org/abs/0906.4811) Ramandeep Gill and Jeremy Heyl present the dispersion relation for relativistic Bernstein modes. These electrostatic waves may play an important role in pulsar magnetospheres so understanding their behaviour in the strong relativistic plasma surrounding a neutron star is crucial.

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The modes are restricted to smaller wavenumbers and lower frequencies at the plasma becomes hotter. Furthermore, there is little remnant of the cyclotron resonances in the strongly relativistic plasma.

Cosmic rays from magnetars

Jeremy Heyl, Ramandeep Gill and Lars Hernquist have found that magnetars and pulsars can account for the excess abundance of cosmic-ray electrons and positrons at high energies over those expected from other astrophysical sources such as supernova remnants. They argue in 1005.1003 that pair-winds produced either by magnetic-field decay (in the case of magnetars) or spin down (in the case of pulsars) explains the excess high-energy leptons and increases positron fraction above 10 GeV.

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Magnetized dusty wind

Anand Thirumalai has calculated for the first time the properties of winds with both dust and magnetized appropriate for asymptotic-giant-branch stars in 1006.2181. He has applied this model to the AGB star Mira (1206.0044) as well as the supergiant Betelgeuse (1109.5148). These stars giant stars consume hydrogen and helium in two separate shells in their interiors while the carbon ash accumulates in their centres to ultimate form a white dwarf. Meanwhile their atmospheres fly off in a powerful, dusty, magnetized wind.

The graph below show several plausible hybrid winds. The blue curve denotes the run of temperature with radius. The green curves trace several dust-free winds from the surface and the red curve is the critical dusty wind. At the dust formation radius the solution jumps from a green curve to the red curve. The location of the dust formation determines which dust-free solution the flow follows.

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Diffractive microlensing

Microlensing and occultation are generally studied in the geometric optics limit. However, diffraction may be important when recently discovered Kuiper-Belt objects (KBOs) or interstellar planets occult distant stars. In particular the effects of diffraction become more important as the wavelength of the observation and the distance to the object increase.

The effects of gravitational lensing on a piece of graph paper. The Einstein radius is about three units in the most lensed frame. The final frame shows the effects of diffraction where the wavelength of radiation is about equal to the Schwarzschild radius of the lens. <youtube>FE1Lgdqm7Qo</youtube>


Magnetar waves

Dan Mazur has discovered a class of waves that can travel through the strongly magnetized plasmas surrounding magnetars without changing their shapes. Quantum-field theory predicts that waves without plasma will steepen to form shocks. The dispersive properties of the plasma can stabilize these non-linear waves. Instead of a single electric component and a perpendicular magnetic component, these waves consist of a electric and three magnetic components all pointing in the same direction, and similarly another single magnetic and three electric components pointing in another direction orthogonal to the first quartet. Check out the paper at 1002.2915. Here is what the Fourier transform of the wave looks like (primary component in red and secondary in blue).

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SGR flares

Several SGR hyperflares have exhibit a much smaller precursor either seconds or minutes before the energy burst. Ramandeep Gill has examined several possible trigger mechanisms for SGR hyperflares that causally connect the hyperflare to the preceding trigger. Check out the article at 1002.3662.

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Magnetized-atom breakthrough

Anand Thirumalai and Jeremy Heyl present the state-of-the-art calculations of atoms in strong magnetic fields in a series of papers on the arXiv: arXiv:0806.3113 and arXiv:0903.0020. Strong magnetic fields distort the structure of atoms.  To the left is depicted the ground state and first excited state of hydrogen in no magnetic field and in a field for a typical neutron star.   The magnetic field points up and down. Notice how much the electron is squished along and across the vertical direction. Anand Thirumalai and Jeremy Heyl have developed a software suite to calculate accurately the structure of magnetized atoms in arbitrarily strong magnetic fields  even in intermediate regimes where neither nucleus and magnetic field dominates.   So far they have built neutral hydrogen, helium and lithium.  Not only do the results agree with the zero field results but are more accurate than previous work even for strong fields and up to 10,000 times faster.  Below is the wavefunction for the less bound electron of helium in magnetic fields ranging from 6 × 107 G to 6 × 1010 G.  The Earth's magnetic field is about 0.1 G.

We have now produce a new version to extend these results to even stronger fields. Check it out at 1203.3437.

The left panels give the ground and excited state of hydrogen in no magnetic field. The right panels give the same states in a neutron-star field of 1012 G.

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Crustal (paradigm) shift

Kelsey Hoffman has discovered that the atmospheres of unsullied neutron stars probably do not consist of iron. Rather their composition depends on the cooling history of the star and can range from silicon to chromium. Check out the article at 0901.4174. The following graph shows the evolution of composition with temperature for a star with the thin crust (e.g. a quark star).

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White dwarf kicks

In two recent papers (arXiv:0803.2704 and arXiv:0901.1872) Jeremy Heyl and Matthew Penrice argue that if white dwarfs get a velocity boost of a few kilometers per second at their births, the energy in these kicks would dominate the energy budget of young globular clusters and delay the onset of core collapse, making globular clusters today appear puffier.

Learn more about white-dwarf kicks from the complete series of papers (so far):

  • Constraining white-dwarf kicks in globular clusters: arXiv:0706.0900
  • Orbital evolution with white-dwarf kicks: arXiv:0706.0912
  • Constraining white-dwarf kicks in globular clusters : II. Observational Significance: arXiv:0709.3118
  • Constraining white-dwarf kicks in globular clusters : III. Cluster Heating: arXiv:0803.2704
  • Constraining white-dwarf kicks in globular clusters : IV. Retarding Core Collapse: arXiv:0901.1872

Magnetar birthrate

Ramandeep Gill has calculated the birthrate of magnetars by simulating the entire ROSAT All-Sky Survey. About one in ten supernovae result in a magnetar, or a birthrate of 0.22 per century. Check out our paper for more details.

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To obtain this estimate they simulated the entire ROSAT All-Sky Survey (RASS) for each known magnetar to determine the portion of the Galaxy over which the magnetar could have been detected in the RASS. The image depicts the results for AXP 4U 0142+61.

SGR emission

Jeremy Heyl and Lars Hernquist have found that waves travelling through the plasma surrounding a magnetar may form shocks that can naturally produce the observed hard x-ray emission from these objects. To learn more please read our recent papers.

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Double pendulum fractal

A double pendulum consists of two pendulums with the rod on the lower pendulum hanging from the bob of the other. It is a classic example of a chaotic system in which what the system does depends very sensitively on the initial conditions. You can learn more about the physics on the double pendulum on our wiki or see the resulting fractal in greater detail with our fractal explorer.

We can ask a simple question: if we start with the two pendulums making a particular angle with the vertical, how long will it be before either pendulum flips through the vertical? In the picture, green is a short time, followed by red, purple, blue and white.

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Stringy black holes

Jeremy Heyl has found that if a string is supported along a cycloid, transverse oscillations of the string cannot propagate through the bottom of the cycloid, forming a horizon there (gr-qc/0602065).

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