Top PDF The Dynamics of White Dwarfs, Black Holes and Stellar Cusps

The Dynamics of White Dwarfs, Black Holes and Stellar Cusps

The Dynamics of White Dwarfs, Black Holes and Stellar Cusps

In the light of this work the upcoming Large Synoptic Survey Telescope (LSST) survey is especially promising for discovering ultracool white dwarfs. LSST is expected to have single-visit 5-σ depths in the u, g, r, i, z, and y-bands of 23.9, 25.0, 24.7, 24.0, 23.3, and 22.1, respectively (Ivezic et al., 2008). Each area of the sky is visited times 70, 100, 230, 230, 200, and 200, respectively, giving co-added depths of approximately 26.3, 27.5, 27.7, 27.0, 26.2, and 24.9. Assuming that all ultracool white dwarfs are similar to LHS 3250 and conservatively that they can be detected the single visit 5-σ depth then they would be detectable to ∼ 210 pc, the limiting band being the u-band. This is a factor of 2.4 greater in distance than even the SDSS 2-σ limit of 22.0, and we therefore expect at least a factor of 10 increase in number. Less conservatively the single visit depth in the g and r-bands are ∼ 650 pc and so, these images can be used to calculate the astrometric solution (i.e., position, proper motion, and parallax) of each object. The color-color cuts involving the u, i, and z-bands could be applied to the co-added image increasing the maximum detectable distance to ∼ 650 pc. This would result in a 400-fold increase in number of ultracool white dwarf detections over SDSS, or of order 4000 ultracool white dwarfs.
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HiPERCAM: A high-speed quintuple-beam CCD camera for the study of rapid variability in the universe

HiPERCAM: A high-speed quintuple-beam CCD camera for the study of rapid variability in the universe

One of the best ways of studying compact stellar remnants is through their multi-colour photometric variabil- ity. The dynamical timescales of black holes, neutron stars and white dwarfs range from milliseconds to seconds. This means that the rotation and pulsation of these objects, and the motion of any material in close proximity to them (e.g. in an accretion disc), tends to occur on such short timescales. Hence, only by observing at high speeds can the variability of compact objects be resolved, thereby revealing a wealth of information, such as their structure, radii, masses and emission mechanisms. 3
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Fake plunges are very eccentric real EMRIs in disguise - …they dominate the rates and are blissfully ignorant of angular momentum barriers

Fake plunges are very eccentric real EMRIs in disguise - …they dominate the rates and are blissfully ignorant of angular momentum barriers

This is what makes an experiment such as eLISA [31, 32] so appealing from the viewpoint of an astrophysicist, since it can be envisaged as a magnifying glass that will look much deeper and with more detail in the areas of interest. However, this is only true for compact stars. Whilst main-sequence stars are tidally disrupted when approaching the central MBH, compact objects (stellar black holes, neutron stars, and white dwarfs) slowly spiral into the MBH and are swallowed after some ∼105 orbits in the eLISA band. At the closest approach to the MBH, the system emits a burst of GWs which contains information about the spacetime geometry, the masses of the system, and the spins of the MBH. We can regard each such burst as a snapshot of the system. This is what makes EMRIs so appealing: a set of ∼105 bursts of GWs radiated by one system will tell us with the utmost accuracy about the system itself, it will test general relativity [33], it will tell us about the distribution of dark objects in galactic nuclei and globular clusters and, thus, we will have a new understanding of the physics of the process. Besides, new phenomena, unknown and unanticipated, are likely to be discovered.
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Prompt emission from tidal disruptions of white dwarfs by intermediate mass black holes

Prompt emission from tidal disruptions of white dwarfs by intermediate mass black holes

could live in the centers of galaxies that fail to feed their BHs [1] or in globular clusters [2–6]. IMBH can form as a result of a collapse of a massive star [7–9] or a massive cloud [10] or grow from a stellar mass BH. While the presence of stellar mass BHs and SMBHs is established, only tentative candidates of IMBHs exist [1,11,12] and the debates of the nature of the candidates are ongoing (e.g. [13]). More IMBH candidates with qualitatively di ff erent observational signatures could help identify those objects.

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Pulsations in white dwarfs: Selected topics

Pulsations in white dwarfs: Selected topics

right hand side of equation (17) always positive; in other word the convective flux perturbation drives pulsations in the white dwarf as long as the pulsation period is much longer than the convection turnover time. This is the convection mechanism for g-mode pulsations in DA variables, which was found by Brickhill [2] and further discussed by Goldreich & Wu [12]. The cause of the convective driving can be considered as the effect of partial ionization of hydrogen which absorbs energy in the compressed phase and release it in the expanded phase.

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Supermassive Black Holes

Supermassive Black Holes

It is evident from Table 1 and Table 2 that the largest and smallest black holes are similar, in that their density is very low. Moreover, the force of gravity at the surface is weak. It is roughly equal to the force here on Earth. Leptons near the surface would be vulnerable to ambient radiation, and it is an open question whether such black holes could achieve equilibrium in the first place. Such con- siderations provide loose upper and lower limits for the black hole mass. To date, the largest and smallest observed masses are 2 10 × 10 M

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The Genesis of Magnetic Fields in White Dwarfs

The Genesis of Magnetic Fields in White Dwarfs

While the full suite of mass-loss rates described by Hurley, Pols & Tout (2000) was used it was found that, in order to generate su ffi cient low-mass WDs, it was necessary to take η = 1.0 for Reimers’ mass-loss parameter so that value was used throughout the work. Alternatively, su ffi cient low-mass WDs were found to be formed with smaller η if the Galactic disc were somewhat older. Meng et al. (2008) produce them with η = 0.25 in populations of 12 Gyr in age. However, the recent work of Kilic et al. (2017) has convincingly shown that the age of the Galactic disc cannot be greater than 10 Gyr. The metallicity was taken to be solar (Z = 0.02) in all the calculations. From all evolved systems those that could generate single HFMWDs were selected. To this end all pairs of WDs that merge outside any CE and leave a single WD remnant were extracted. These are the double degenerate (DD) WD WD mergers. Added to these are WD remnants of systems that underwent at least one CE phase and merged dur- ing the last CE phase and satisfy two further criteria. Firstly, either one or both of the stars must have a degenerate core before merging and secondly, there must be no further core burning before the remnant WD is exposed. It was assumed that such a core burning would be convective and destroy any frozen-in high magnetic field. The e ff ective number of actual binary systems was calculated by assuming that the primary stars are distributed according to Salpeter’s IMF (Initial Mass Function, Salpeter, 1955) N(M) dM ∝ M −2.35 dM, where N(M) dM is the number of stars with masses between M and M + dM, and that the secondary stars follow a flat mass ratio distribution for q ≤ 1, e.g. Ferrario (2012). Each binary system was then evolved with BSE from ZAMS to an age of 9.5 GYr (assumed age of the Galactic disk, e.g. Oswalt et al. (1996); Liu & Chaboyer (2000); Kilic et al. (2017). All binary systems in both populations, i.e. those becoming HFMWDs and those that do not, were given a weighting according to the Salpeter IMF. It was then possible to calculate the incidence of HFMWDs in the total WD population once the populations had been time integrated through to the Galactic disk age. The output from BSE for each binary system consisted of a time table of evolution through various stellar types (See table 2.1: “Stellar types distin- guished within the BSE algorithms”). By interrogating the output timetable for each system which became a HFMWD it was possible to extract the percentages of stellar companion types immediately preceding the last CE event in which the stellar merger giving rise to the high magnetic field occurred or alternatively which systems gave a DD merger post-CE. Similarly, it was possible to distinguish the WD types emerging from the CE.
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The Macroeconomic Black Holes

The Macroeconomic Black Holes

The idea to write this paper is to observe how the black markets can generate a negative impact on the final GDP in any country. In our case, we represent the black markets by the macroeconomic black holes. It is to observe how the black markets can generate considerable outflow from the initial GDP. In the process to analyze and visualize the impact of the macroeconomic black holes (See Figure 1 and 2) on the outflow of economic growth, we propose a new indicator that counteract on the performance of the GDP. The basic premise in the construction of macroeconomic black holes depend on the “the black markets outflow circumference (BMO-Circumference) ” . To build the BMO-Circumference, we suggest first to find the diameter “☼ Y i ” (See Expression 1). It is equal to the total sum of the drugs smuggling growth rate under the application of multi-dimensional partial differentiation in real time ( ☼ X 1 ),
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The properties of cool DZ white dwarfs

The properties of cool DZ white dwarfs

The extreme pressures and temperatures expected in dense white dwarf interiors indicated they should be composed of a fully ionised plasma (Saha, 1920; Edding- ton, 1926). With the simultaneous development of atomic theory and quantum mechanics, it became clear that the white dwarf interiors, unlike ‘normal’ stars, could not be modelled as classical ideal-gases (Eddington, 1926). Fowler (1926) ap- plied the newly developed Fermi-Dirac statistics (Fermi, 1926; Dirac, 1926) to the electrons in white dwarf interiors, resolving how material could exist in such a dense state. At these densities, the average separation between electrons is shorter than their thermal de-Broglie wavelength, and so the electron gas becomes degenerate. The electrons are then forced to occupy the lowest available energy states in both physical- and momentum-space. From this Fowler (1926) explained that the appar- ent force required to oppose gravitational collapse arose from statistical means. By reducing the available volume, and hence the available states in physical-space, elec- trons would be forced into higher momentum-states. The high-momentum of these spacially confined electrons thus manifests itself as a pressure, balancing further gravitational collapse. While white dwarf interiors are generally considered to be “hot”, the average thermal energy per electron is much lower than the Fermi-energy, and thus the degenerate electron gas can be modeled as being at zero temperature. Further development along these lines explained that as more mass is added to a white dwarf a greater deal of pressure is required to oppose gravitational col- lapse. To provide the increased degeneracy pressure the star therefore decreases in radius, forcing the electrons into the necessary higher momentum states.
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A new magnetic white dwarf: PG 2329+267

A new magnetic white dwarf: PG 2329+267

We have discovered that the white dwarf PG 2329+267 is magnetic, and, assuming a centred dipole structure, has a dipole magnetic field strength of approximately 2.3 MG. This makes it one of only approximately 4 per cent of isolated white dwarfs with a detectable magnetic field. Linear Zeeman splitting, as well as quadratic Zeeman shifts, is evident in the hydrogen Balmer sequence and circular spectropolarimetry reveals , 10 per cent circular polarization in the two displaced j components of Ha. We suggest from comparison with spectra of white dwarfs of known mass that PG 2329+267 is more massive than typical isolated white dwarfs, in agreement with the hypothesis that magnetic white dwarfs evolve from magnetic chemically peculiar Ap and Bp type main-sequence stars.
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Building Positive Future Orientations: The Role of Natural Mentors in the Lives of African American Girls

Building Positive Future Orientations: The Role of Natural Mentors in the Lives of African American Girls

dynamic interactions. In a follow-up effort, Mason et al. (2009a) confirmed these findings, once again concluding that at least 75% of the O-type stars in clusters and associations are part of binary or multiple systems. Kobulnicky & Fryer (2007) compared the observed radial velocity of early-type stars in the Cygnus OB2 association with expectations from Monte-Carlo models to conclude that binary fractions are likely greater than 80% and that the companion mass distribution follows the Initial Mass Function (IMF) for only 60% of the companions, with the remainder having q ∼ 1. At the other end of the main sequence, Fischer & Marcy (1992) find a smaller binary fraction of 42% ± 9% among M-dwarfs, con- sistent with the earlier efforts of Henry & McCarthy (1990), and smaller than the DM91 results for solar type stars. Thus, multiplicity frequency appears to drop with the mass of the primary, perhaps due to the shrinking mass available for companions or because lower- mass stars tend to be older and hence have a greater opportunity to lose companions from dynamical interactions.
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Micro black holes in the laboratory

Micro black holes in the laboratory

Quantum gravity is an ongoing attempt to reconcile quantum mechanics with general relativity, aiming to unify into a single consistent model all known ob- servable interactions in the universe, at both subatomic and astronomical scales. Despite decades of efforts and the formulation of many candidate theories, our un- derstanding of quantum gravity is far from being complete. On the experimental side, we do not yet have observations to confirm or reject theoretical formulations, due to the extreme energy required to observe quantum gravity effects. A possi- ble way out of this puzzling situation is given by a prototype of quantum gravity, namely a model theory that can be used to test the concepts and processes we are following in the path to quantum gravity. This is quantum field theory (QFT) in curved space-time, the simplest theoretical arena for studying particle physics in the presence of gravitational effects. QFT in curved space, which is often regarded as the semi-classical limit of quantum gravity, has a robust theoretical prediction: a black hole emits quantum thermal radiation like a black body at a temperature proportional to the inverse of its mass, T ∝ 1/M . This result, known as black hole evaporation since Hawking’s early work6, tells us that astrophysical black holes have negligible thermal properties, while only smaller size black holes could have temperatures relevant for experiments on Earth. One can show that the black hole evaporation time would exceed the age of the universe for black holes with sizes bigger than 10 −16 m. Thus microscopic black holes provide the best avenue for the
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A menagerie of hairy black holes

A menagerie of hairy black holes

which have no counterpart in asymptotically flat space. These nodeless solutions are of particular interest because it can be proven that at least some of them are lin- early stable under spherically symmetric perturbations [7]. When the gauge group is su (N), the gauge field is described by N − 1 independent functions ω j (r), corre- sponding to N − 1 matter degrees of freedom. Since there are stable solutions for any N, there is therefore no limit on the amount of stable gauge field “hair” with which a black hole in adS can be dressed.

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Infinitely coloured black holes

Infinitely coloured black holes

Motivated by these considerations, we would like in this note to take a different view- point, and consider black holes in standard Einstein four-dimensional space-times, which however capture some of the essential features of the above-mentioned two-dimensional stringy black holes, namely the infinite-dimensional character of the gauge hair, as well as its area-preserving nature, which was argued to be responsible for two-dimensional quantum coherence [4]. At this point we should stress that the W -hair of ref. [4] was quantum hair [6], arguably measured by Aharonov-Bohm type experiments, and there- fore pertaining to phases in the respective matter wave functions, whereas in this paper we are concerned with purely classical hair. Nevertheless, some of the features we find, specifically the holographic properties, may be related to the issue of quantum coherence in the sense of ref. [4].
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On the linear stability of solitons and hairy black holes with a negative cosmological constant: the even-parity sector

On the linear stability of solitons and hairy black holes with a negative cosmological constant: the even-parity sector

Unless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version - refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher’s website.
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Fundamental parameter accuracy of DA and DB white dwarfs in Gaia Data Release 2

Fundamental parameter accuracy of DA and DB white dwarfs in Gaia Data Release 2

Gaia photometric parameters allow the derivation of stel- lar masses by adopting the white dwarf mass-radius rela- tions described in Section 2. Despite their incompleteness, our adopted samples are expected to present a reasonable picture of the field white dwarf mass distribution for a mag- nitude limited survey, similarly to what has been done in the pre-Gaia era (see, e.g., Tremblay et al. 2016). Gaia selected samples will be more appropriate in the future to study the astrophysical implications of the white dwarf mass distri- bution, but such samples currently have very poor spectro- scopic completeness beyond 20 pc, leading to potentially er- roneous interpretations given the strong colour differences between cool DA and DC white dwarfs (El-Badry et al. 2018; Gentile Fusillo et al. 2018).
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A search for pulsations in helium white dwarfs

A search for pulsations in helium white dwarfs

We employed a variable-aperture algorithm to optimize the S/N of our photometry in order to deal with the varying PSFs. For each observation, a single representative frame was selected. For every star in the image we varied the size of the photometric aperture to find the size that optimized the S/N of the measure- ment. The optimum apertures were then scaled by the average FWHM of 2D Gaussians fitted to each star. For all other images the average FWHM of the stellar PSFs was measured and then scaled using the appropriate optimum scaling. Therefore, as the seeing fluctuated during an observation, the apertures were ad- justed to be larger or smaller (a technique similar to that used in Deeg & Doyle 2001).
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General Relativistic Gas Dynamics in the Central Cavity of Binary Black Holes

General Relativistic Gas Dynamics in the Central Cavity of Binary Black Holes

time-averaged state includes only one such crossing. In practice, streams are continually pulled from the edge of the circumbinary. Therefore, the quasi-modulated stream-disk impacts would have a time period of strengthening associated with the overall size of the lump. However, this broadening would not alter the frequencies discussed. This quasi-periodic modulation of the stream momentum flux could help to explain the asymmetry in the mini-disks. To confirm this, one would need to perform a longer simulation and average over a longer period of time; taking care to include precisely two sets of streams departing from the lump. However, we note that the binary evolves appreciably over as little as a single binary orbital period at the separations considered in this Chapter ( ˙ a/a ∝ a 4 ). Therefore, in terms of single binary separation, the system will be marked by an individual mini-disk being singled out for streams departing from the lump. Further studies will be necessary to explore the inter-play of the time periods of modulating accretion stream flux due to the lump with the orbital and inspiral periods of the binary. More specifically, how these various timescales can alter the mini-disk dynamics.
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On the evolution of magnetic white dwarfs

On the evolution of magnetic white dwarfs

Gaia will provide precise parallaxes for more than 100,000 white dwarfs, including all known magnetic white dwarfs ( Torres et al. 2005; Carrasco et al. 2014 ) , and spectroscopic follow-ups will identify even more magnetic objects. Gaia will establish the fi rst homogeneous mass distribution and cooling sequence of magnetic remnants. Given the ubiquitous presence of magnetic white dwarfs in the high-mass regime, it is critical to understand these objects to recover the Galactic star formation history and initial mass function in the ∼ 3 – 8 M e range ( Tremblay et al. 2014 ) . Magnetic remnants can also be used to constrain stellar evolution at intermediate masses ( Külebi et al. 2013b ) and study possible populations of mergers ( Badenes & Maoz 2012; Wegg & Phinney 2012 ) . It is therefore essential, at this stage, to build precise model atmospheres and evolution sequences for these peculiar degenerate stars. It has been suggested for a long time that convection is completely inhibited in HFMWDs ( Wickramasinghe & Martin 1986; Valyavin et al. 2014 ) , although this has not yet been veri fi ed with realistic simulations. Furthermore, Kepler et al. ( 2013 ) suggest that small undetected magnetic fi elds could impact the mass distribution of cool convective white dwarfs.
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Bogdan, Akos
  

(2010):


	X-ray emission from nearby early-type galaxies and origin of Type Ia Supernovae.


Dissertation, LMU München: Fakultät für Physik

Bogdan, Akos (2010): X-ray emission from nearby early-type galaxies and origin of Type Ia Supernovae. Dissertation, LMU München: Fakultät für Physik

We study the origin of unresolved X-ray emission from the bulge of M31 based on archival Chandra and XMM-Newton observations. We demonstrate that three different components are present: (i) Broad- band emission from a large number of faint sources – mainly accreting white dwarfs and active binaries, associated with the old stellar population, similar to the Galactic Ridge X-ray emission of the Milky Way. The X-ray to K-band luminosity ratios are compatible with those for the Milky Way and for M32, in the 2 − 10 keV band it is (3.6 ± 0.2) · 10 27 erg s − 1 L − ⊙ 1 . (ii) Soft emission from ionized gas with temperature of about ∼ 300 eV and mass of ∼ 2 · 10 6 M ⊙ . The gas distribution is significantly extended along the minor axis of the galaxy suggesting that it may be outflowing in the direction perpendicular to the galactic disk. The mass and energy supply from evolved stars and type Ia supernovae is sufficient to sustain the outflow. We also detect a shadow cast on the gas emission by spiral arms and the 10-kpc star-forming ring, confirming significant extent of the gas in the “vertical” direction. (iii) Hard extended emission from spiral arms, most likely associated with young stellar objects and young stars located in the star-forming regions. The L X /SFR ratio equals ∼ 9 · 10 38 (erg/s)/(M ⊙ /yr) which is about ∼ 1/3 of the HMXBs contribution,
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