Context.Type Ia supernovae (Ia-SNe) are thought to arise from the thermonuclear explosions of white dwarfs (WDs). The progenitors of such explosions are still highly debated; in particular the ...conditions leading to detonations in WDs are not well understood in most of the suggested progenitor models. Nevertheless, direct head-on collisions of two WDs were shown to give rise to detonations and produce Ia-SNe – like explosions, and were suggested as possible progenitors. Aims.The rates of such collisions in dense globular clusters are far below the observed rates of type Ia SNe, but it was suggested that quasi-secular evolution of hierarchical triples could produce a high rate of such collisions. With regular secular evolution, the expected Ia-SNe rate from isolated triples is orders of magnitude below the observed rate. Here we aim to test if the rate of WD collisions in triples can be significantly enhanced if quasi-secular evolution is taken into account. Methods.We used detailed triple stellar evolution populations synthesis models coupled with dynamical secular evolution to calculate the rates of WD-WD collisions in triples and their properties. We explored a range of models with different realistic initial conditions and derived the expected SNe total mass, mass-ratio and delay time distributions for each of the models. Results.We find that the SNe rate from WD-WD collisions is of the order of 0.1% of the observed Ia-SNe rate across all our models, and the delay-time distribution is almost uniform in time, and is inconsistent with observations. Conclusions.We conclude that SNe from WD-WD collisions in isolated triples can at most provide for a small fraction of Ia-SNe, and can not serve as the main progenitors of such explosions.
ABSTRACT
Normal type Ia supernovae (SNe) are thought to arise from the thermonuclear explosion of massive (>0.8 M⊙) carbon–oxygen white dwarfs (WDs), although the exact mechanism is debated. In some ...models, helium accretion on to a carbon–oxygen (CO) WD from a companion was suggested to dynamically trigger a detonation of the accreted helium shell. The helium detonation then produces a shock that after converging on itself close to the core of the CO WD, triggers a secondary carbon detonation, and gives rise to an energetic explosion. However, most studies of such scenarios have been done in one or two dimensions, and/or did not consider self-consistent models for the accretion and the He donor. Here, we make use of detailed 3D simulation to study the interaction of a He-rich hybrid $0.69\, \mathrm{M_\odot }$ HeCO WD with a more massive $0.8\, \mathrm{M_\odot }$ CO WD. We find that accretion from the hybrid WD on to the CO WD gives rise to a helium detonation. However, the helium detonation does not trigger a carbon detonation in the CO WD. Instead, the helium detonation burns through the accretion stream to also burn the helium shell of the donor hybrid HeCO WD. The detonation of its massive helium shell then compresses its CO core, and triggers its detonation and full destruction. The explosion gives rise to a faint, likely highly reddened transient, potentially observable by the Vera Rubin survey, and the high-velocity ($\sim \! 1000\, \mathrm{km s^{-1}}$) ejection of the heated surviving CO WD companion. Pending on uncertainties in stellar evolution, we estimate the rate of such transient to be up to $\sim \! 10{{\ \rm per\ cent}}$ of the rate of type Ia SNe.
Abstract Recently, several noninteracting black hole–stellar binaries have been identified in Gaia data—for example, Gaia BH1, where a Sun-like star is in a moderately eccentric ( e = 0.44) 185 days ...orbit around a black hole. This orbit is difficult to explain through binary evolution. The present-day separation suggests the progenitor system would have undergone an episode of common-envelope evolution, but a common envelope should shrink the period below the observed one. Since the majority of massive stars form in higher-multiplicity systems, a triple evolution scenario is more likely for the progenitors of BH binaries. Here we show that such systems can indeed be more easily explained via evolution in hierarchical triple systems. von Zeipel–Lidov–Kozai oscillations or instabilities can delay the onset of the common-envelope phase in the inner binary of the triple, so that the black hole progenitor and low-mass star are more widely separated when it begins, leading to the formation of wider binaries. There are also systems with similar periods but larger eccentricities, where the BH progenitor is a merger product of the inner binary in the triple. Such mergers lead to a more top-heavy black hole mass function.
Abstract
Here we propose a mechanism for efficiently growing intermediate mass black holes (IMBH) in discs around supermassive black holes. Stellar mass objects can efficiently agglomerate when ...facilitated by the gas disc. Stars, compact objects and binaries can migrate, accrete and merge within discs around supermassive black holes. While dynamical heating by cusp stars excites the velocity dispersion of nuclear cluster objects (NCOs) in the disc, gas in the disc damps NCO orbits. If gas damping dominates, NCOs remain in the disc with circularized orbits and large collision cross-sections. IMBH seeds can grow extremely rapidly by collisions with disc NCOs at low relative velocities, allowing for super-Eddington growth rates. Once an IMBH seed has cleared out its feeding zone of disc NCOs, growth of IMBH seeds can become dominated by gas accretion from the active galactic nucleus (AGN) disc. However, the IMBH can migrate in the disc and expand its feeding zone, permitting a super-Eddington accretion rate to continue. Growth of IMBH seeds via NCO collisions is enhanced by a pile-up of migrators.
We highlight the remarkable parallel between the growth of IMBH in AGN discs with models of giant planet growth in protoplanetary discs. If an IMBH becomes massive enough it can open a gap in the AGN disc. IMBH migration in AGN discs may stall, allowing them to survive the end of the AGN phase and remain in galactic nuclei. Our proposed mechanisms should be more efficient at growing IMBH in AGN discs than the standard model of IMBH growth in stellar clusters. Dynamical heating of disc NCOs by cusp stars is transferred to the gas in an AGN disc helping to maintain the outer disc against gravitational instability. Model predictions, observational constraints and implications are discussed in a companion paper (Paper II).
ABSTRACT
The majority of massive black holes (MBHs) likely hosted gas discs during their lifetimes. These could either be long-lived active galactic nuclei (AGN) discs, or shorter-lived discs formed ...following singular gas infall events, as was likely the case in our own Galactic Centre. Stars and compact objects in such environments are therefore expected to interact with the gaseous disc as they go through it, and potentially become aligned and fully embedded within it. The interactions of embedded stars with the gas could give rise to a plethora of physical processes affecting the stars, including growth through accretion of gas, migration in the disc, stellar captures, and mergers with other stars. The impact of such processes strongly depends on the population of stars that eventually align with the disc and become embedded in it. Here we make use of analytic tools to analyze the alignment process, accounting for both geometric drag and gas dynamical friction. We find that up to $\sim \! 50~{{\ \rm per\ cent}}$ of main sequence stars and stellar mass black holes in the central 0.1 pc can align with AGN disc in the Galactic Centre and similar galactic nuclei. The orbits of aligned stars are typically circularized and are prograde with respect to the AGN disc. Furthermore, alignment and accretion are intimately linked, and the capture of stars by an AGN disc can potentially explain the origin of the young stellar disc in the Galactic Centre with a top-heavy mass function, even without the need for a star-formation event.
Rates of Stellar Tidal Disruption Stone, N. C.; Vasiliev, E.; Kesden, M. ...
Space science reviews,
04/2020, Letnik:
216, Številka:
3
Journal Article
Recenzirano
Odprti dostop
Tidal disruption events occur rarely in any individual galaxy. Over the last decade, however, time-domain surveys have begun to accumulate statistical samples of these flares. What dynamical ...processes are responsible for feeding stars to supermassive black holes? At what rate are stars tidally disrupted in realistic galactic nuclei? What may we learn about supermassive black holes and broader astrophysical questions by estimating tidal disruption event rates from observational samples of flares? These are the questions we aim to address in this Chapter, which summarizes current theoretical knowledge about rates of stellar tidal disruption, and compares theoretical predictions to the current state of observations.
Supernovae are thought to arise from two different physical processes. The cores of massive, short-lived stars undergo gravitational core collapse and typically eject a few solar masses during their ...explosion. These are thought to appear as type Ib/c and type II supernovae, and are associated with young stellar populations. In contrast, the thermonuclear detonation of a carbon-oxygen white dwarf, whose mass approaches the Chandrasekhar limit, is thought to produce type Ia supernovae. Such supernovae are observed in both young and old stellar environments. Here we report a faint type Ib supernova, SN 2005E, in the halo of the nearby isolated galaxy, NGC 1032. The ‘old’ environment near the supernova location, and the very low derived ejected mass (∼0.3 solar masses), argue strongly against a core-collapse origin. Spectroscopic observations and analysis reveal high ejecta velocities, dominated by helium-burning products, probably excluding this as a subluminous or a regular type Ia supernova. We conclude that it arises from a low-mass, old progenitor, likely to have been a helium-accreting white dwarf in a binary. The ejecta contain more calcium than observed in other types of supernovae and probably large amounts of radioactive 44Ti.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We present a study of the properties of the host galaxies of unusual transient objects of two types, both being subluminous compared with the major classes of supernovae. Those of one type exhibit ...unusually strong calcium features, and have been termed 'Ca-rich'. Those of the second type, with SN2002cx as the prototype and SN2008ha as the most extreme example to date, have some properties in common with the first, but show typically lower ejecta velocities and different early spectra. We confirm important differences in the environments of the two types, with the Ca-rich transients preferentially occurring in galaxies dominated by old stellar populations. Quantitatively, the association of the Ca-rich transients with regions of ongoing star formation is well matched to that of Type Ia supernovae. The SN2002cx-like transients are very different, with none of the present sample occurring in an early-type host, and a statistical association with star-formation regions similar to that of Type II-P supernovae, and therefore a delay time of 30-50 Myr.
Pebble accretion is a promising process for decreasing growth timescales of planetary cores, allowing gas giants to form at wide orbital separations. However, nebular turbulence can reduce the ...efficiency of this gas-assisted growth. We present an order-of-magnitude model of pebble accretion that calculates the impact of turbulence on the average velocity of small bodies, the radius for binary capture, and the sizes of the small bodies that can be accreted. We also include the effect of turbulence on the particle scale height, which has been studied in previous works. We find that turbulence does not prevent rapid growth in the high-mass regime: the last doubling time to the critical mass to trigger runaway gas accretion (M ∼ 10 M⊕) is well within the disk lifetime, even for strong ( 10−2) turbulence. We find that, while the growth timescale is quite sensitive to the local properties of the protoplanetary disk, there are large regimes of parameter space over which large cores grow in less than the disk lifetime, if appropriately sized small bodies are present. Instead, the effects of turbulence are most pronounced for low planetary masses. For strong turbulence, the growth timescale is longer than the gas disk lifetime until the core reaches masses . A "flow isolation mass," at which binary capture ceases, emerges naturally from our model framework. We comment that the dependence of this mass on orbital separation is similar to the semimajor axis distribution of solar system cores.
We present new observations of the nuclear star cluster in the central parsec of the Galaxy with the adaptive optics assisted, integral field spectrograph SINFONI on the ESO/VLT. Our work allows the ...spectroscopic detection of early- and late-type stars to mK >= 16, more than 2 mag deeper than our previous data sets. Our observations result in a total sample of 177 bona fide early-type stars. We find that most of these Wolf Rayet (WR), O-, and B-stars reside in two strongly warped disks between 08 and 12'' from Sgr A*, as well as a central compact concentration (the S-star cluster) centered on Sgr A*. The later type B-stars (mK >15) in the radial interval between 08 and 12''seem to be in a more isotropic distribution outside the disks. The observed dearth of late-type stars in the central few arcseconds is puzzling, even when allowing for stellar collisions. The stellar mass function of the disk stars is extremely top heavy with a best-fit power law of dN/dm m -0.45+/- 0.3. WR/O-stars were formed in situ in a single star formation event ~6 Myr ago, this mass function probably reflects the initial mass function (IMF). The mass functions of the S-stars inside 08 and of the early-type stars at distances beyond 12'' are compatible with a standard Salpeter/Kroupa IMF (best-fit power law of dN/dm m -2.15+/- 0.3).
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