Sergei Nayakshin, Sergey Sazonov, Rashid Sunyaev
The last decade has seen a dramatic confirmation that an in situ star formation is possible inside the inner parsec of the Milky Way. Here we suggest that giant planets, solid terrestrial-like planets, comets and asteroids may also form in these environments, and that this may have observational implications for Active Galactic Nuclei (AGN). Like in debris discs around main sequence stars, collisions of large solid objects should initiate strong fragmentation cascades. The smallest particles in such a cascade - the microscopic dust - may provide a significant opacity. We put a number of observational and physical constraints on AGN obscuring torii resulting from such fragmentation cascades. We find that torii fed by fragmenting asteroids disappear at both low and high AGN luminosities. At high luminosities, $L \sim L_{\rm Edd}$, where $L_{\rm Edd}$ is the Eddington limit, the AGN radiation pressure blows out the microscopic dust too rapidly. At low luminosities, on the other hand, the AGN discs may avoid gravitational fragmentation into stars and solids. We also note that these fragmentation cascades may be responsible for astrophysically "large" dust particles of approximately micrometer sizes that were postulated by some authors to explain unusual absorption properties of the AGN torii.
Sergei Nayakshin
We point out that protoplanets created in the framework of the Tidal Downsizing (TD) theory for planet formation play a very important role for the evolution of accretion discs hosting them. Since all TD protoplanets are initially as massive as $\sim 10$ Jupiter masses, they are able to open very deep gaps in their discs, and even completely isolate the inner disc flows from the outer ones. Furthermore, in contrast to other planet formation theories, TD protoplanets are mass donors for their protostars. One potentially observable signature of planets being devoured by their protostars are FU Ori like outbursts, and episodic protostar accretion more generally, as discussed by a number of authors recently. Here we explore another observational implication of TD hypothesis: dust poor inner accretion flows, which we believe may be relevant to some of the observed mm-bright transitional discs around protostars. In our model, a massive protoplanet interrupts the flow of the outer dust-rich disc on its protostar, and at the same time looses a part of its dust-poor envelope into the inner disc. This then powers the observed gas-but-no-dust accretion onto the star. Upon a more detailed investigation, we find that this scenario is quite natural for young massive discs but is less so for older discs, e.g., those whose self-gravitating phase has terminated a fraction of a Million year or more ago. This stems from the fact that TD protoplanets of such an age should have contracted significantly, and so are unlikely to loose much mass. Therefore, we conclude that either (i) the population of "transition discs" with large holes and dust-poor accretion is much younger than generally believed; or (ii) there is a poorly understood stage for late removal of dust-poor envelopes from TD planets; (iii) another explanation for the observations is correct.
Sergei Nayakshin, Demosthenes Kazanas, Timothy R. Kallman
Sep 21, 1999·astro-ph·PDF We study the X-ray illumination of an accretion disk. We relax the simplifying assumption of constant gas density used in most previous studies; instead we determine the density from hydrostatic balance. It is found that the thermal ionization instability prevents the illuminated gas from attaining temperatures at which the gas is unstable. In particular, the uppermost layers of the X-ray illuminated gas are found to be almost completely ionized and at the local Compton temperature ($\sim 10^7 - 10^8$ K); at larger depths, the gas temperature drops abruptly to form a thin layer with $T\sim 10^6$ K, while at yet larger depths it decreases sharply to the disk effective temperature. We find that most of the Fe K$α$ line emission and absorption edge are produced in the coolest, deepest layers, while the Fe atoms in the hottest, uppermost layers are generally almost fully ionized, hence making a negligible contribution to reprocessing features in $\sim 6.4-10$ keV energy range. We provide a summary of how X-ray reprocessing features depend on parameters of the problem. The results of our self-consistent calculations are both quantitatively and qualitatively different from those obtained using the constant density assumption. Therefore, we conclude that X-ray reflection calculations should always utilize hydrostatic balance in order to provide a reliable theoretical interpretation of observed X-ray spectra of AGN and GBHCs.
Sergei Nayakshin, Roland Svensson
Jul 12, 2000·astro-ph·PDF It is currently believed that the ``standard'' accretion disk theory under-predicts the observed X-ray luminosity from Soft X-ray Transients (SXT) in quiescence by as much as 4 to 6 orders of magnitude. This failure of the standard model is considered to be an important argument for the existence of the alternative mode of accretion -- Advection Dominated Accretion Flows (ADAF) in astrophysics, since these flows allow a much higher level of X-ray emission in quiescence, in agreement with the observations. Here we point out that, in stark contrast to steady-state standard disks, such disks in quiescence (being non-steady) produce most of the X-ray emission very far from the last stable orbit. Taking this into account, these disks can accommodate the observed X-ray luminosities of SXTs rather naturally. Our theory predicts that Fe K-alpha lines from standard accretion disks in quiescence should be narrow even though the cold disk goes all the way down to the last stable orbit.
Sergei Nayakshin
Oct 19, 2004·astro-ph·PDF The extremely hot and tenuous accretion flow in the immediate vicinity of Sgr A* is believed to be invisible (too dim) in the X-ray band, except for short X-ray flares. Here we point out that during pericenter passages, close brightest stars irradiate the inner region of the accretion flow, providing a plenty of optical/UV photons. These seed photons are Compton up-scattered by the hot electrons of the accretion flow to higher frequencies, some into the X-ray band, potentially making the innermost accretion flow much brighter in X-rays than usual. We propose to use coordinated near infra-red and X-ray observations of close star passages to put constraints onto Sgr A* accretion theories. The absence of a noticeable change in the steady emission of Sgr A* as observed by Chandra in the year 2002, when the star named S2 passed through a pericenter of its orbit, already rules out the hotter of the ``standard'' Advection-Dominated Accretion Flows. The less dense accretion flows, in particular the model of Yuan et al. (2003), passes the test and is constrained to accretion rates no larger than $\sim 10^{-7}$ Solar masses per year.
Sergei Nayakshin, Jorge Cuadra, Volker Springel
We numerically model fragmentation of a gravitationally unstable gaseous disc under conditions that may be appropriate for the formation of the young massive stars observed in the central parsec of our Galaxy. In this study, we adopt a simple prescription with a locally constant cooling time. We find that, for cooling times just short enough to induce disc fragmentation, stars form with a top-heavy Initial Mass Function (IMF), as observed in the Galactic Centre (GC). For shorter cooling times, the disc fragments much more vigorously, leading to lower average stellar masses. Thermal feedback associated with gas accretion onto protostars slows down disc fragmentation, as predicted by some analytical models. We also simulate the fragmentation of a gas stream on an eccentric orbit in a combined Sgr A* plus stellar cusp gravitational potential. The stream precesses, self-collides and forms stars with a top-heavy IMF. None of our models produces large enough co-moving groups of stars that could account for the observed ``mini star cluster'' IRS13E in the GC. In all of the gravitationally unstable disc models that we explored, star formation takes place too fast to allow any gas accretion onto the central super-massive black hole. While this can help to explain the quiescence of `failed AGN' such as Sgr A*, it poses a challenge for understanding the high gas accretion rates infered for many quasars.
Sergei Nayakshin
Nov 19, 2014·astro-ph.EP·PDF Tidal Downsizing (TD) is a recently developed planet formation theory that supplements the classical Gravitational disc Instability (GI) model with planet migration inward and tidal disruptions of GI fragments in the inner regions of the disc. Numerical methods for a detailed population synthesis of TD planets are presented here. As an example application, the conditions under which GI fragments collapse faster than they migrate into the inner $a\sim$ few AU disc are considered. It is found that most gas fragments are tidally or thermally disrupted unless (a) their opacity is $\sim 3$ orders of magnitude less than the interstellar dust opacity at metallicities typical of the observed giant planets, or (b) the opacity is high but the fragments accrete large dust grains (pebbles) from the disc. Case (a) models produce very low mass solid cores ($M_{\rm core} < 0.1$ Earth masses) and follow a negative correlation of giant planet frequency with host star metallicity. In contrast, case (b) models produce massive solid cores, correlate positively with host metallicity and explain naturally while giant gas planets are over-abundant in metals.
Sergei Nayakshin
Feb 26, 2015·astro-ph.EP·PDF Core Accretion (CA), the de-facto accepted theory of planet formation, requires formation of massive solid cores as a prerequisite for assembly of gas giant planets. The observed metallicity correlations of exoplanets are puzzling in the context of CA. While gas giant planets are found preferentially around metal-rich host stars, planets smaller than Neptune orbit hosts with a wide range of metallicities. We propose an alternative interpretation of these observations in the framework of a recently developed planet formation hypothesis called Tidal Downsizing (TD). We perform population synthesis calculations based on TD, and find that the connection between the populations of the gas giant and the smaller solid-core dominated planets is non linear and not even monotonic. While gas giant planets formed in the simulations in the inner few AU region follow a strong positive correlation with the host star metallicity, the smaller planets do not. The simulated population of these smaller planets shows a shallow peak in their formation efficiency at around the Solar metallicity. This result is driven by the fact that at low metallicities the solid core's growth is damped by the scarcity of metals, whereas at high metallicities the fragments within which the cores grow contract too quickly, cutting the core's growth time window short. Finally, simulated giant gas planets do not show a strong host star metallicity preference at large separations, which may explain why one of the best known directly imaged gas giant planet systems, HR 8799, is metal poor.
Sergei Nayakshin, Seung-Hoon Cha
Jun 20, 2013·astro-ph.EP·PDF It is well known that massive protoplanetary disc are gravitationally unstable beyond tens of AU from their parent star. The eventual fate of the self-gravitating gas clumps born in the disc is currently not understood, although the range of uncertainty is well known. If clumps migrate inward rapidly, they are tidally disrupted, which may leave behind giant or terrestrial like planets. On the other hand, if clumps migrate less rapidly, they tend to accrete gas, becoming proto brown dwarfs or low mass companions to the parent star. Here we argue that radiative feedback of contracting clumps (protoplanets) on their discs is an important effect that has been overlooked in previous calculations. We show analytically that temperature in clump's vicinity may be high enough to support a quasi-static atmosphere if the clump mass is below a critical value, $M_{\rm cr} \sim 6$ Jupiter masses ($M_J$). This may arrest further gas accretion onto the clump and thus promote formation of planets rather than low mass companions. We use numerical simulations to evaluate these analytical conclusions by studying migration and accretion of gas clumps as a function of their initial mass, $M_i$. Simulations neglecting the radiative preheating effect show that gas clumps with mass less than $\sim 2 M_J$ migrate inward rapidly; more massive clumps result in low mass companions. In contrast, simulations that include radiative preheating from the clump show that clumps as massive as $8 M_J$ migrate inward rapidly and end up tidally disrupted in the inner disc. We conclude that, with other parameters being equal, previous simulations neglecting radiative feedback from self-gravitating clumps over-estimated the population of brown dwarfs and low mass stellar companions and under-estimated the population of planets.
Sergei Nayakshin, Fernando Cruz Sáenz de Miera, Ágnes Kóspál
May 16, 2024·astro-ph.EP·PDF Recent imaging observations with ALMA and other telescopes found widespread signatures of planet presence in protoplanetary discs at tens of au separations from their host stars. Here we point out that the presence of very massive planets at 0.1 au sized orbits can be deduced for protostars accreting gas at very high rates, when their discs display powerful Thermal Instability bursts. Earlier work showed that a massive planet modifies the nature of this instability, with outbursts triggered at the outer edge of the deep gap opened by the planet. We present simulations of this effect, finding two types of TI outbursts: downstream and upstream of the planet, which may or may not be causally connected. We apply our model to the outburst in Gaia20eae. We find that the agreement between the data and our disc thermal instability model is improved if there is a planet of 6 Jupiter masses orbiting the star at 0.062 au separation. Gaia20eae thus becomes the second episodically erupting star, after FU Ori, where the presence of a massive planet is strongly suspected. Future observations of similar systems will constrain the mode and the frequency of planet formation in such an early epoch.
Sergei Nayakshin
Jul 23, 2010·astro-ph.EP·PDF We hypothesise that planets are made by tidal downsizing of migrating giant planet embryos. The proposed scheme for planet formation consists of these steps: (i) a massive young protoplanetary disc fragments at R ~ several tens to hundreds of AU on gaseous clumps with masses of a few Jupiter masses; (ii) the clumps cool and contract, and simultaneously migrate closer in to the parent star; (iii) as earlier suggested by Boss (1998), dust sediments inside the gas clumps to form terrestrial mass solid cores; (iv) if the solid core becomes more massive than ~ 10 Earth masses, a massive gas atmosphere collapses onto the solid core; (v) when the gas clumps reach the inner few AU from the star, tidal shear and evaporation due to stellar irradiation peel off the outer metal-poor envelope of the clump. If tidal disruption occurs quickly, while the system is still in stage (iii), a terrestrial planet core is left. If it happens later, in stage (iv), a metal rich gas giant planet with a solid core emerges from the envelope.
Sergei Nayakshin
Feb 16, 2010·astro-ph.CO·PDF The observed super-massive black hole (SMBH) mass -- galaxy velocity dispersion ($M_{\rm cmo} - σ$) correlation, and the similar correlation for nuclear star clusters, may be established when winds/outflows from the CMO ("central massive object") drive gas out of the potential wells of classical bulges. Timescales of growth for these objects may explain why smaller bulges appear to host preferentially NCs while larger ones contain SMBHs only. Despite much recent progress, feedback processes in bulge/galaxy formation are far from being understood. Our numerical simulations show that that understanding how the CMO feeds is as important a piece of the puzzle as understanding how its feedback affects its host galaxy.
Sergei Nayakshin, Chris Power
Nov 12, 2009·astro-ph.CO·PDF The observed super-massive black hole (SMBH) mass -- galaxy velocity dispersion ($M_{\rm bh} - σ$) correlation may be established when winds/outflows from the SMBH drive gas out of the potential wells of classical bulges. Here we present numerical simulations of this process in a static isothermal potential. Simple spherically symmetric models of SMBH feedback at the Eddington luminosity can successfully explain the $M_{\rm bh} - σ$ and nuclear cluster mass $M_{\rm NC}-σ$ correlations, as well as why larger bulges host SMBHs while smaller ones host nuclear star clusters. However these models do not specify how SMBHs feed on infalling gas whilst simultaneously producing feedback that drives gas out of the galaxy. More complex models with rotation and/or anisotropic feedback allow SMBHs to feed via a disc or regions not exposed to SMBH winds, but in these more realistic cases it is not clear why a robust $M_{\rm bh} - σ$ relation should be established. In fact, some of the model predictions contradict observations. For example, an isotropic SMBH wind impacting on a disc (rather than a shell) of aspect ratio $H/R \ll 1$ requires the SMBH mass to be larger by a factor $\sim R/H$, which is opposite to what is observed. We conclude that understanding how a SMBH feeds is as important a piece of the puzzle as understanding how its feedback affects its host galaxy. Finally, we note that in aspherical cases the SMBH outflows induce differential motions in the bulge. This may pump turbulence that is known to hinder star formation in star forming regions. SMBH feedback thus may not only drive gas out of the bulge but also reduce the fraction of gas turned into stars.
Sergei Nayakshin, James E. Owen, Vardan Elbakyan
Disc accretion rate onto low mass protostar FU Ori suddenly increased hundreds of times 85 years ago and remains elevated to this day. We show that the sum of historic and recent observations challenges existing FU Ori models. We build a theory of a new process, Extreme Evaporation (EE) of young gas giant planets in discs with midplane temperatures exceeding 30, 000 K. Such temperatures are reached in the inner 0.1 AU during thermal instability bursts. In our 1D time-dependent code the disc and an embedded planet interact through gravity, heat, and mass exchange. We use disc viscosity constrained by simulations and observations of dwarf novae instabilities, and we constrain planet properties with a stellar evolution code. We show that dusty gas giants born in the outer self-gravitating disc reach the innermost disc in a $\sim$ 10,000 years with radius of $\sim 10 R_J$. We show that their EE rates are $\sim 10^{-5}$ Msun/yr; if this exceeds the background disc accretion activity then the system enters a planet-sourced mode. Like a stellar secondary in mass-transferring binaries, the planet becomes the dominant source of matter for the star, albeit for $\sim$ O(100) years. We find that a $\sim$ 6 Jupiter mass planet evaporating in a disc fed at a time-averaged rate of $\sim 10^{-6}$ Msun/yr appears to explain all that we currently know about FU Ori accretion outburst. More massive planets and/or planets in older less massive discs do not experience EE process. Future FUOR modelling may constrain planet internal structure and evolution of the earliest discs.
Sergei Nayakshin
Jan 17, 2020·astro-ph.EP·PDF Recent ALMA observations indicate that the majority of bright protoplanetary discs show signatures of young moderately massive planets. I show that this result is paradoxical. The planets should evolve away from their observed states by radial migration and gas accretion in about 1\% of the system age. These systems should then hatch tens of giant planets in their lifetime, and there should exist a very large population of bright planet-less discs; none of this is observationally supported. An alternative scenario, in which the population of bright ALMA discs is dominated by secondary discs recently rejuvenated by deposition of new gas, is proposed. The data are well explained if the gaseous mass of the discs is comparable to a Jovian planet mass, and they last a small fraction of a Million years. Self-disruptions of dusty gas giant protoplanets, previously predicted in the context of the Tidal Downsizing theory of planet formation, provide a suitable mechanism for such injections of new fuel, and yield disc and planet properties commensurate with ALMA observations. If this scenario is correct, then the secondary discs have gas-to-dust ratios considerably smaller than 100, and long look ALMA and NIR/optical observations of dimmer targets should uncover dusty, not yet disrupted, gas clumps with sizes of order an AU. Alternatively, secondary discs could originate from late external deposition of gas into the system, in which case we expect widespread signatures of warped outer discs that have not yet come into alignment with the planets.
Sergei Nayakshin
Nov 15, 2013·astro-ph.GA·PDF We study effects of AGN feedback outflows on multi-phase inter stellar medium (ISM) of the host galaxy. We argue that SMBH growth is dominated by accretion of dense cold clumps and filaments. AGN feedback outflows overtake the cold medium, compress it, and trigger a powerful starburst -- a positive AGN feedback. This predicts a statistical correlation between AGN luminosity and star formation rate at high luminosities. Most of the outflow's kinetic energy escapes from the bulge via low density voids. The cold phase is pushed outward only by the ram pressure (momentum) of the outflow. The combination of the negative and positive forms of AGN feedback leads to an $M-σ$ relation similar to the result of King (2003). Due to porosity of cold ISM in the bulge, SMBH influence on the low density medium of the host galaxy is significant even for SMBH well below the $M-σ$ mass. The role of SMBH feedback in our model evolves in space and time with the ISM structure. In the early gas rich phase, SMBH accelerates star formation in the bulge. During later gas poor (red-and-dead) phases, SMBH feedback is mostly negative everywhere due to scarcity of the cold ISM.
Sergei Nayakshin, Walter Dehnen, Jorge Cuadra, Reinhard Genzel
Nov 30, 2005·astro-ph·PDF It is believed that young massive stars orbiting Sgr A* in two stellar discs on scales of 0.1-0.2 parsecs were formed either farther out in the Galaxy and then quickly migrated inward, or in situ in a massive self-gravitating disc. Comparing N-body evolution of stellar orbits with observational constraints, we set upper limits on the masses of the two stellar systems. These masses turn out to be few times lower than the expected total stellar mass estimated from the observed young high-mass stellar population and the standard galactic IMF. If these stars were formed in situ, in a massive self-gravitating disc, our results suggest that the formation of low-mass stars was suppressed by a factor of at least a few, requiring a top-heavy initial mass function (IMF) for stars formed near sgr A*.
Sergei Nayakshin
Nov 30, 2004·astro-ph·PDF Orientation of parsec-scale accretion disks in AGN is likely to be nearly random for different black hole feeding episodes. Since AGN accretion disks are unstable to self-gravity on parsec scales, star formation in these disks will create young stellar disks, similar to those recently discovered in our Galactic Center. The disks blend into the quasi-spherical star cluster enveloping the AGN on time scales much longer than a likely AGN lifetime. Therefore, the gravitational potential within the radius of the black hole influence is at best axi-symmetric rather than spherically symmetric. Here we show that as a result, a newly formed accretion disk will be warped. For the simplest case of a potential resulting from a thin stellar ring, we calculate the disk precession rates, and the time dependent shape. We find that, for a realistic parameter range, the disk becomes strongly warped in few hundred orbital times. We suggest that this, and possibly other mechanisms of accretion disk warping, have a direct relevance to the problem of AGN obscuration, masing warped accretion disks, narrow Fe K-alpha lines, etc.
Sergei Nayakshin
Feb 19, 2004·astro-ph·PDF We study the accretion flow of a hot gas captured by the black hole gravity in the presence of a thin cold accretion disk. Such geometrical arrangement is expected in Active Galactic Nuclei (AGN) and in galactic X-ray binary systems because both hot and cold gases are present in the black hole vicinity. Previous astrophysical literature concentrated on the evaporation of the cold disk in the classical heat conduction limit. Here we consider the inverse process, i.e. condensation of the hot gas onto the cold disk. We find two distinct condensation regimes. (i) In the classical thermal conduction limit, the radiative cooling in the hot gas itself force condensation above a certain critical accretion rate. Most of the flow energy in this case is re-emitted as X-ray radiation. (ii) Below a certain minimum accretion rate, the hot electrons are collisionless and the classical heat flux description becomes invalid. We use the ``non-local'' heat flux approach borrowed from the terrestrial laser heated plasma experiments. Due to their very large mean free path, the hot particles penetrate deep into the cold disk where the radiative losses are significant enough to enable condensation. In this case the hot flow energy is inconspicuously re-radiated by the transition layer in many UV and especially optical recombination lines (e.g., Ly$α$, $Hα$, H$β$) as well as via the optically thick disk emission. We derive an approximate analytical solution for the dynamics of the hot condensing flow. If the cold disk is inactive, i.e. accumulating mass for a future accretion outburst, then the two-phase flows appear radiatively inefficient. These condensing solutions may be relevant to \sgra, low luminosity AGN, and transient binary accreting systems in quiescence.
Sergei Nayakshin
Quasar accretion disks are believed to form stars by self-gravity. Low Luminosity Active Galactic Nuclei (LLAGN) are much dimmer galactic centers, and are often believed to be quasars that ran out of gaseous fuel. LLAGN accretion disks should thus co-exist with thousands to millions of stars or proto-stars left from the previous stronger accretion activity. In principle, these stars may produce several important effects: (i) contribute to the optical/UV spectra of some LLAGN; (ii) reprocessing of the stellar radiation in the dusty disks could dominate the LLAGN infra-red spectra; (iii) deplete the (accretion) gas disk much faster than it can accrete onto the supper-massive black hole (SMBH); (iv) stars, individually or in groups, may slow down and modulate the accretion flow significantly due to their inertia. In this way they may produce the LLAGN cut-off disks; (v) alternatively, frequent enough stellar collisions and resulting stellar disruptions could keep the inner disk empty. Here we explore these ideas. We find that, despite ``low'' luminosities of LLAGN, unrealistically high stellar densities are required to make a sizable radiative contribution to the (HST) optical/UV spectra of these galactic nuclei. Stellar contribution to the infrared spectrum is more likely. Further, if LLAGN are in a quasi steady-state for as long as 10^7 years or more, too high stellar densities would again be required to significantly affect the dynamics of accretion flow. However, if LLAGN are ``short''-lived phenomena, e.g. t < 10^5 years, quiescent states of quiescence-outburst cycles, then embedded stars may be much more important through the mass effects (iii) -- (v). With observations of LLAGN becoming progressively better, it will be more and more difficult to neglect the presence of close stars in and around nuclear accretion disks.