Constraining the Milky Way potential with Double White Dwarfs

The 21st European Workshop on White Dwarfs was held in Austin, TX from July 23rd to 27th of 2018The upcoming LISA mission is the only experiment that will allow us to study the Milky Way’s structure using gravitational wave signals from Galactic double white dwarfs (DWDs). The total number of expected detections exceeds 105. Furthermore, up to a hundred DWDs can be simultaneously detected in both gravitational and optical radiation (e.g. with Gaia and LSST as eclipsing), making DWDs ideal sources for performing a multi-messenger tomography of the Galaxy. We show that LISA will detect DWDs everywhere, mapping also the opposite side of the Galaxy. This complete coverage will : (1) provide precise and unbiased constraints on the scale radii of the Milky Way’s bulge and disc, and (2) allow us to compute the rotation curve and derive competitive estimates for the bulge and disc masses, when combining gravitational wave and optical observations.Astronom

A lower limit on the halo mass to form supermassive black holes

Article / Letter to editorSterrewach

Discovering intermediate massive black holes through tidally disrupted stars

Stars are spheres of gas held together by self-gravity. When flying by a black hole, however, the star self-binding force can be overwhelmed by the black hole tides and the star can be torn apart. This is a physically rich and fascinating event which will be described by first. introducing the concept. of black hole from a mathematical point of view. We will then dive into the physics of the tidal disruption and proceed describing the accompanying electromagnetic flare and gravitational wave burst in the frequency range of the Laser Interferometer Space Antenna. This empowers such events to discover the elusive black holes with mass intermediate between the solar and the million/billion solar masses

Magnetic field evolution in tidal disruption events

When a star gets tidally disrupted by a supermassive black hole, its magnetic field is expected to pervade its debris. In this paper, we study this process via smoothed particle magnetohydrodynamical simulations of the disruption and early debris evolution including the stellar magnetic field. As the gas stretches into a stream, we show that the magnetic field evolution is strongly dependent on its orientation with respect to the stretching direction. In particular, an alignment of the field lines with the direction of stretching induces an increase of the magnetic energy. For disruptions happening well within the tidal radius, the star compression causes the magnetic field strength to sharply increase by an order of magnitude at the time of pericentre passage. If the disruption is partial, we find evidence for a dynamo process occurring inside the surviving core due to the formation of vortices. This causes an amplification of the magnetic field strength by a factor of \u2dc10. However, this value represents a lower limit since it increases with numerical resolution. For an initial field strength of 1 G, the magnetic field never becomes dynamically important. Instead, the disruption of a star with a strong 1 MG magnetic field produces a debris stream within which magnetic pressure becomes similar to gas pressure a few tens of hours after disruption. If the remnant of one or multiple partial disruptions is eventually fully disrupted, its magnetic field could be large enough to magnetically power the relativistic jet detected from Swift J1644+57. Magnetized streams could also be significantly thickened by magnetic pressure when it overcomes the confining effect of self-gravity

Joint constraints on the Galactic dark matter halo and Galactic Centre from hypervelocity stars

Article / Letter to editorSterrewach

Synergies between Gaia and LISA missions for Galactic multi-messenger studies

Ultra-short period Galactic binaries are unique multi-messenger tracers of the Milky Way. They can be detected in large numbers through electromagnetic radiation by Gaia and through gravitational waves by the upcoming LISA mission. First, we revise the current census of known multi-messenger Galactic binaries by computing their GW signals using updated distances from the Gaia Data Release 2. Our work confirms thirteen guaranteed multi-messenger sources: nine AM CVns, three detached double white dwarfs (DWD) and one hot subdwarf. Next, we forecast the detection prospects for DWDs with both Gaia and LISA using a binary population synthesis technique. We predict respectively hundreds and tens of thousands detections by Gaia and LISA, with an overlap of several tens.  We show that synergies between Gaia and LISA observations of DWDs allow the study of the Milky Way baryonic structure. The success of this synergy is due to LISA's ability to localise binaries through virtually the whole Galactic plane, thus mapping its shape. While observations of LISA's electromagnetic counterparts observed by Gaia yield the information on their motion; tracing the underlying total enclosed mass. We envisage that multi-messenger observations will ensure the best science return of the LISA mission for Galactic studies.Instrumentatio

Bright vigorous winds as signposts of supermassive black hole birth

High Energy Astrophysic

Light or heavy supermassive black hole seeds: the role of internal rotation in the fate of supermassive stars

Supermassive black holes are a key ingredient of galaxy evolution. However, their origin is still highly debated. In one of the leading formation scenarios, a black hole of ˜100 M⊙ results from the collapse of the inner core of a supermassive star (≳104-5 M⊙), created by the rapid accumulation (≳0.1 M⊙ yr-1) of pristine gas at the centre of newly formed galaxies at z ˜ 15. The subsequent evolution is still speculative: the remaining gas in the supermassive star can either directly plunge into the nascent black hole or part of it can form a central accretion disc, whose luminosity sustains a surrounding, massive, and nearly hydrostatic envelope (a system called a `quasi-star'). To address this point, we consider the effect of rotation on a quasi-star, as angular momentum is inevitably transported towards the galactic nucleus by the accumulating gas. Using a model for the internal redistribution of angular momentum that qualitatively matches results from simulations of rotating convective stellar envelopes, we show that quasi-stars with an envelope mass greater than a few 105 M_{⊙} × (black hole mass/100 M_{⊙})^{0.82} have highly sub-Keplerian gas motion in their core, preventing gas circularization outside the black hole's horizon. Less massive quasi-stars could form but last for only ≲104 yr before the accretion luminosity unbinds the envelope, suppressing the black hole growth. We speculate that this might eventually lead to a dual black hole seed population: (I) massive (>104 M⊙) seeds formed in the most massive (>108 M⊙) and rare haloes; (II) lighter (˜102 M⊙) seeds to be found in less massive and therefore more common haloes.Interstellar matter and star formationHigh Energy Astrophysic

Enthalpy as internal energy in plug flow reactor models: A long-lasting assumption defeated and its effects on models predictions in dynamic regime

In this paper, a general dynamic model of a pseudo-homogeneous catalytic plug flow reactor (PFR) is developed, which does not apply the traditional assumption of negligible difference between enthalpy and internal energy inside its energy balance. Such a model is then compared to a second dynamic PFR model, whose energy conservation equation identifies internal energy with enthalpy. The aim is that of quantitatively investigating the real suitability of the identification of these two thermodynamic quantities (internal energy and enthalpy) in PFR modeling problems. The Claus process is selected as a meaningful case study for the aforementioned purposes

A lower limit on the halo mass to form supermassive black holes

High Energy AstrophysicsGalaxie