Time-averaged Mass Accretion Rate Research Articles

Large Fluctuations within 1 au in Protoplanetary Disks

Protoplanetary disks are often assumed to change slowly and smoothly during planet formation. Here, we investigate the time evolution of isolated disks subject to viscosity and a disk wind. The viscosity is assumed to increase rapidly at around 900 K due to thermal ionization of alkali metals, or thermionic and ion emission from dust, and the onset of magnetorotational instability (MRI). The disks generally undergo large, rapid fluctuations for a wide range of time-averaged mass accretion rates. Fluctuations involve coupled waves in temperature and surface density that move radially in either direction through the inner 1.5 au of the disk. Two types of waves are seen with radial speeds of roughly 50 and 1000 cm s−1, respectively. The pattern of waves repeats with a period of roughly 10,000 yr that depends weakly on the average mass accretion rate. Viscous transport due to MRI is confined to the inner disk. This region is resupplied by mass flux from the outer disk driven by the disk wind. Interior to 1 au, the temperature and surface density can vary by a factor of 2–10 on timescales of years to kiloyears. The stellar mass accretion rate varies by 3 orders of magnitude on a similar timescale. This behavior lasts for at least 1 Myr for initial disks comparable to the minimum-mass solar nebula.

No evidence for a strong decrease of planetesimal accretion in old white dwarfs

ABSTRACT A large fraction of white dwarfs are accreting or have recently accreted rocky material from their planetary systems, thereby ‘polluting’ their atmospheres with elements heavier than helium. In recent years, the quest for mechanisms that can deliver planetesimals to the immediate vicinity of their central white dwarfs has stimulated a flurry of modelling efforts. The observed time evolution of the accretion rates of white dwarfs through their multi-Gyr lifetime is a crucial test for dynamical models of evolved planetary systems. Recent studies of cool white dwarf samples have identified a significant decrease of the mass accretion rates of cool, old white dwarfs over Gyr time-scales. Here, we revisit those results using updated white dwarf models and larger samples of old polluted H- and He-atmosphere white dwarfs. We find no compelling evidence for a strong decrease of their time-averaged mass accretion rates for cooling times between 1 and 8 Gyr. Over this period, the mass accretion rates decrease by no more than a factor of the order of 10, which is one order of magnitude smaller than the decay rate found in recent works. Our results require mechanisms that can efficiently and consistently deliver planetesimals inside the Roche radius of white dwarfs over at least 8 Gyr.

Thermal state of transiently accreting neutron stars with additional heating beyond deep crustal heating

Abstract As some neutron star transients require an additional unknown heat source (referred to as “shallow heating”) to explain their high temperatures at the beginning of quiescence, we investigate the effect of shallow heating as well as compressional heating on the thermal state of transiently accreting neutron stars with the use of evolutionary calculations in the present work. Through comparing our theoretical predictions of the equilibrium redshifted luminosities $(L_{\gamma}^{\infty})$ produced by both deep crustal heating and shallow heating/compressional heating for different time-averaged mass-accretion rates $\langle\dot{M}\rangle$ with 35 updated observations of soft X-ray transients, the results show that both shallow heating and compressional heating make significant contributions to the equilibrium redshifted luminosity. The hotter sources (XTE J1701, MAXI J0556, EXO 0748, Aql X-1 etc.) with higher accretion rates are more likely to be explained with the effect of shallow heating or compressional heating. In addition, for a proper shallow heat $q_\mathrm{sh}$ and mass-accretion rate $\dot{M}$, the effect of shallow heating could be simulated by compressional heating.

Quiescent luminosities of accreting neutron stars-possibility of neutrino losses due to strong pion condensations

We construct the quiescent neutron star models in the evolutionary calculations. The X-ray luminosities have been derived in terms of the time-averaged mass accretion rate for various neutron star masses and surface compositions. We compare the quiescent luminosities observed from X-ray transients in low mass X-ray binaries, where the stellar evolutionary calculations of accreting neutron stars include neutrino cooling due to strong pion condensations. Our results based on the evolutionary calculations suggest that stronger cooling process would be necessary to be consistent with observations.

Viscous hydrodynamics simulations of circumbinary accretion discs: variability, quasi-steady state and angular momentum transfer

We carry out numerical simulations of circumbinary discs, solving the viscous hydrodynamics equations on a polar grid covering an extended disc outside the binary co-orbital region. We use carefully controlled outer boundary conditions and long-term integrations to ensure that the disc reaches a quasi-steady state, in which the time-averaged mass accretion rate on to the binary, |$\langle \dot{M}\rangle$|⁠, matches the mass supply rate at the outer disc. We focus on binaries with comparable masses and a wide range of eccentricities (eB). For eB ≲ 0.05, the mass accretion rate of the binary is modulated at about five times the binary period; otherwise, it is modulated at the binary period. The inner part of the circumbinary disc (r ≲ 6aB) generally becomes coherently eccentric. For low and high eB, the disc line of apsides precesses around the binary, but for intermediate eB (0.2–0.4), it instead becomes locked with that of the binary. By considering the balance of angular momentum transport through the disc by advection, viscous stress and gravitational torque, we determine the time-averaged net angular momentum transfer rate to the binary, |$\langle \dot{J}\rangle$|⁠. The specific angular momentum, |$l_0 = \langle \dot{J}\rangle /\langle \dot{M}\rangle$|⁠, depends non-monotonically on eB. Contrary to previous claims, we find that l0 is positive for most eB, implying that the binary receives net angular momentum, which may cause its separation to grow with time. The minimum l0 occurs at intermediate eB (0.2–0.4), corresponding to the regime where the inner eccentric disc is apsidally aligned with the binary.

DISCOVERY OF THE THIRD TRANSIENT X-RAY BINARY IN THE GALACTIC GLOBULAR CLUSTER TERZAN 5

We report and study the outburst of a new transient X-ray binary (XRB) in Terzan 5, the third detected in this globular cluster, Swift J174805.3-244637 or Terzan 5 X-3. We find clear spectral hardening in Swift/XRT data during the outburst rise to the hard state, thanks to our early coverage (starting at L_X ~ 4x10^{34} ergs/s) of the outburst. This hardening appears to be due to the decline in relative strength of a soft thermal component from the surface of the neutron star (NS) during the rise. We identify a {Type I X-ray burst} in Swift/XRT data with a long (16 s) decay time, indicative of {hydrogen burning on the surface of the} NS. We use Swift/BAT, Maxi/GSC, Chandra/ACIS, and Swift/XRT data to study the spectral changes during the outburst, identifying a clear hard-to-soft state transition. We use a Chandra/ACIS observation during outburst to identify the transient's position. Seven archival Chandra/ACIS observations show evidence for variations in Terzan 5 X-3's non-thermal component, but not the thermal component, during quiescence. The inferred long-term time-averaged mass accretion rate, from the quiescent thermal luminosity, suggests that if this outburst is typical and only slow cooling processes are active in the neutron star core, such outbursts should recur every ~10 years.

X-ray softening in the new X-ray transient XTE J1719−291 during its 2008 outburst decay

The X-ray transient XTE J1719−291 was discovered with Rossi X-ray Timing Explorer/Proportional Counter Array (RXTE/PCA) during its outburst in 2008 March, which lasted at least 46 d. Its 2–10 keV peak luminosity is 7 × 1035 erg s−1 assuming a distance of 8 kpc, which classifies the system as a very faint X-ray transient. The outburst was monitored with Swift, RXTE, Chandra and XMM–Newton. We analysed the X-ray spectral evolution during the outburst. We fitted the overall data with a simple power-law model corrected for absorption and found that the spectrum softened with decreasing luminosity. However, the XMM–Newton spectrum cannot be fitted with a simple one-component model, but it can be fitted with a thermal component (blackbody or disc blackbody) plus a power-law model affected by absorption. Therefore, the softening of the X-ray spectrum with decreasing X-ray luminosity might be due to a change in photon index or alternatively it might be due to a change in the properties of the soft component. Assuming that the system is an X-ray binary, we estimated a long-term time-averaged mass accretion rate of ∼ 7.7 × 10−13 M⊙ yr−1 for a neutron star as a compact object and ∼ 3.7 × 10−13 M⊙ yr−1 in the case of a black hole. Although no conclusive evidence is available about the nature of the accretor, based on the X-ray/optical luminosity ratio we tentatively suggest that a neutron star is present in this system.

A four-year baselineSwiftstudy of enigmatic X-ray transients located near the Galactic center

We report on continued monitoring observations of the Galactic center carried out by the X-ray telescope aboard the Swift satellite in 2008 and 2009. This campaign revealed activity of the five known X-ray transients AX J1745.6-2901, CXOGC J174535.5-290124, GRS 1741-2853, XMM J174457-2850.3 and CXOGC J174538.0-290022. All these sources are known to undergo very faint X-ray outbursts with 2-10 keV peak luminosities of Lx,peak~1E34-1E36 erg/s, although the two confirmed neutron star low-mass X-ray binaries AX J1745.6-2901 and GRS 1741-2853 can also become brighter (Lx,peak~1E36-1E37 erg/s). We discuss the observed long-term lightcurves and X-ray spectra of these five enigmatic transients. In 2008, AX J1745.6-2901 returned to quiescence following an unusually long accretion outburst of more than 1.5 years. GRS 1741-2853 was active in 2009 and displayed the brightest outburst ever recorded for this source, reaching up to a 2-10 keV luminosity of Lx~1E37 (D/7.2 kpc)^2 erg/s. This system appears to undergo recurrent accretion outbursts approximately every 2 years. Furthermore, we find that the unclassified transient XMM J174457-2850.3 becomes bright only during short episodes (days) and is often found active in between quiescence (Lx~1E32 erg/s) and its maximum outburst luminosity of Lx~1E36 erg/s. CXOGC J174535.5-290124 and CXOGC J174538.0-290022, as well as three other very-faint X-ray transients that were detected by Swift monitoring observations in 2006, have very low time-averaged mass-accretion rates of ~< 2E-12 Msun/yr. Despite having obtained two years of new data in 2008 and 2009, no new X-ray transients were detected.

Unveiling the hard X-ray spectrum from the ‘burst-only’ source SAX J1753.5-2349 in outburst

Abstract Discovered in 1996 by BeppoSAX during a single type-I burst event, SAX J1753.5−2349 was classified as ‘burst-only’ source. Its persistent emission, either in outburst or in quiescence, had never been observed before 2008 October, when SAX J1753.5−2349 was observed for the first time in outburst. Based on INTEGRAL observations, we present here the first high-energy emission study (above 10 keV) of a so-called ‘burst-only’. During the outburst, the SAX J1753.5−2349 flux decreased from 10 to 4 mCrab in 18–40 keV, while it was found being in a constant low/hard spectral state. The broad-band (0.3–100 keV) averaged spectrum obtained by combining INTEGRAL/IBIS and Swift/XRT data has been fitted with a thermal Comptonization model and an electron temperature ≳24 keV inferred. However, the observed high column density does not allow the detection of the emission from the neutron star surface. Based on the whole set of observations of SAX J1753.5−2349, we are able to provide a rough estimate of the duty cycle of the system and the time-averaged mass-accretion rate. We conclude that the low to very low luminosity of SAX J1753.5−2349 during outburst may make it a good candidate to harbour a very compact binary system.

The applicability of the viscous α-parameterization of gravitational instability in circumstellar disks

We study numerically the applicability of the effective-viscosity approach for simulating the effect of gravitational instability (GI) in disks of young stellar objects with different disk-to-star mass ratios ξ. We adopt two α-parameterizations for the effective viscosity based on Lin and Pringle [Lin, D.N.C., Pringle, J.E., 1990. ApJ 358, 515] and Kratter et al. [Kratter, K.M., Matzner, Ch.D., Krumholz, M.R., 2008. ApJ 681, 375] and compare the resultant disk structure, disk and stellar masses, and mass accretion rates with those obtained directly from numerical simulations of self-gravitating disks around low-mass (M∗∼1.0M⊙) protostars. We find that the effective viscosity can, in principle, simulate the effect of GI in stellar systems with ξ≲0.2–0.3, thus corroborating a similar conclusion by Lodato and Rice [Lodato, G., Rice, W.K.M., 2004. MNRAS 351, 630] that was based on a different α-parameterization. In particular, the Kratter et al.’s α-parameterization has proven superior to that of Lin and Pringle’s, because the success of the latter depends crucially on the proper choice of the α-parameter. However, the α-parameterization generally fails in stellar systems with ξ≳0.3, particularly in the Classes 0 and I phases of stellar evolution, yielding too small stellar masses and too large disk-to-star mass ratios. In addition, the time-averaged mass accretion rates onto the star are underestimated in the early disk evolution and greatly overestimated in the late evolution. The failure of the α-parameterization in the case of large ξ is caused by a growing strength of low-order spiral modes in massive disks. Only in the late Class II phase, when the magnitude of spiral modes diminishes and the mode-to-mode interaction ensues, may the effective viscosity be used to simulate the effect of GI in stellar systems with ξ≳0.3. A simple modification of the effective viscosity that takes into account disk fragmentation can somewhat improve the performance of α-models in the case of large ξ and even approximately reproduce the mass accretion burst phenomenon, the latter being a signature of the early gravitationally unstable stage of stellar evolution [Vorobyov, E.I., Basu, S., 2006. ApJ 650, 956]. However, further numerical experiments are needed to explore this issue.

Feast and Famine: regulation of black hole growth in low-redshift galaxies

We analyze the observed distribution of Eddington ratios as a function of supermassive black hole mass for a large sample of nearby galaxies drawn from the Sloan Digital Sky Survey. We demonstrate that there are two distinct regimes of black hole growth in nearby galaxies. The first is associated with galaxies with significant star formation in their central kiloparsec regions, and is characterized by a broad log-normal distribution of accretion rates peaked at about one percent of the Eddington limit. In this regime, the Eddington ratio distribution is independent of the mass of the black hole and shows no further dependence on the central stellar population of the galaxy. The second regime is associated with galaxies with old central stellar populations, and is characterized by a power-law distribution function of Eddington ratios. In this regime, the time-averaged mass accretion rate onto black holes is proportional to the mass of stars in the galaxy bulge, with a constant of proportionality that depends on the mean stellar age of the stars. This result is once again independent of black hole mass. We show that both the slope of the power-law and the decrease in the accretion rate onto black holes in old galaxies are consistent with population synthesis model predictions of the decline in stellar mass loss rates as a function of mean stellar age. Our results lead to a very simple picture of black hole growth in the local Universe. If the supply of cold gas in a galaxy bulge is plentiful, the black hole regulates its own growth at a rate that does not further depend on the properties of the interstellar medium. Once the gas runs out, black hole growth is regulated by the rate at which evolved stars lose their mass.

Time Variability of Accretion Flows: Effects of the Adiabatic Index and Gas Temperature

We report on the current phase of our study of rotating accretion flows onto black holes. We consider hydrodynamical (HD) accretion flows with a spherically symmetric density distribution at the outer boundary but with spherical symmetry broken by the introduction of a small, latitude-dependent angular momentum. We study accretion flows by means of numerical two-dimensional, axisymmetric, HD simulations for a variety of values for the adiabatic index γ and the gas temperature at infinity c∞. Our work is an extension of work done by Proga and Begelman who consider models for only γ = 5/3. Our main result is that the flow properties such as the topology of the sonic surface and time behavior strongly depend on γ but little on c∞. In particular, for 1 < γ < 5/3, the mass accretion rate shows large-amplitude, slow time variability which is a result of mixing between slow and fast rotating gas. This temporal behavior differs significantly from that in models with γ ≲ 5/3 where the accretion rate is relatively constant and from that in models with γ ≳ 1 where the accretion exhibits small-amplitude quasi-periodic oscillations. The key parameter responsible for the differences is the sound speed of the accretion flow which in turn determines whether the flow is dominated by gas pressure, radiation pressure, or rotation. Despite these differences, the time-averaged mass accretion rate in units of the corresponding Bondi rate is a weak function of γ and c∞.

The neutron star soft X-ray transient 1H 1905+000 in quiescence

In this paper, we report on our analysis of a ∼25 ks. Chandra X-ray observation of the neutron star soft X-ray transient (SXT) 1H 1905+000 in quiescence. Furthermore, we discuss our findings of the analysis of optical photometric observations which we obtained using the Magellan telescope and photometric and spectroscopic observations which we obtained using the Very Large Telescope (VLT) at Paranal. The X-ray counterpart of 1H 1905+000 was not detected in our Chandra data, with a 95 per cent confidence limit to the source count rate of 1.2 × 10−4 counts s−1. For different spectral models this yields an upper limit on the luminosity of 1.8 × 1031 erg s−1 (for an upper limit on the distance of 10 kpc). This luminosity limit makes 1H 1905+000 the faintest neutron star SXT in quiescence observed to date. The neutron star luminosity is so low that it is similar to the lowest luminosities derived for black hole (BH) SXTs in quiescence. This low luminosity for a neutron star SXT challenges the hypothesis presented in the literature that BH SXTs in quiescence have lower luminosities than neutron star SXTs as a result of the presence of a BH event horizon. Furthermore, the limit on the neutron star luminosity obtained less than 20 yr after the outburst has ceased, constrains the thermal conductivity of the neutron star crust. Finally, the neutron star core must be so cold that unless the time-averaged mass accretion rate is lower than 2 × 10−12 M⊙ yr−1, core cooling has to proceed via enhanced neutrino emission processes. The time-averaged mass accretion rate can be derived from binary evolution models if the orbital period of the system is known. Our optical observations show that the optical counterpart discovered when the source was in outburst has faded. Near the outburst optical position we find two stars with a separation of 0.7 arcsec and I= 19.3 ± 0.1 and 21.3 ± 0.1. VLT optical spectroscopy revealed that the spectrum of the brighter of the two sources is a G5–7V star. However, the outburst astrometric position of the optical counterpart does not coincide with the position of the G5–7V star nor with that of the fainter star. We derive a limit on the absolute I-band magnitude of the quiescent counterpart of MI > 7.8 assuming the source is at 10 kpc. This is in line with 1H 1905+000 being an ultracompact X-ray binary, as has been proposed based on the low-outburst V-band absolute magnitude.

The Far‐Ultraviolet Spectrum of Z Camelopardalis in Quiescence and Standstill

We have obtained FUSE (905-1187 A) spectra of the nonmagnetic cataclysmic variable Z Cam during a period of quiescence. The spectrum resembles that of a hot (57,000 K) metal-enriched white dwarf. A high effective temperature is consistent with the expectation that dwarf nova systems that show standstills, as Z Cam does, have higher than normal time-averaged mass accretion rates. It is also consistent with current estimates for the mass and distance to Z Cam. A white dwarf model in which 29% of the surface has a temperature of 72,000 K and the rest of the surface is at 26,000 K also reproduces the spectral shape and the continuum flux level at the nominal distance and white dwarf mass, and is a somewhat better statistical fit to the data. A second component could be due to a rotating accretion belt, a remnant of an outburst that ended 11 days prior to the FUSE observation, or ongoing accretion. We favor the uniform-temperature model for Z Cam, largely because the data do not require anything more complicated. We have compared the quiescent spectrum with an archival spectrum of Z Cam in standstill, obtained with ORFEUS. The standstill spectrum is described well by a disk accreting at disk = 7 × 1016 g s-1, where the errors depend on the assumed inclination. The quiescent spectra cover a full orbital period and are time-resolved to ~500 s. No variability was observed in the continuum during the observation, but the depth of many of the absorption lines increased between orbital phases 0.65 and 0.81. We attribute this effect to absorption by material associated with the accretion stream, which is easier to understand if the inclination is near the maximum allowed value of 68°.

The faint neutron star soft X-ray transient SAX J1810.8–2609 in quiescence

We present the analysis of a 35-ksec-long Chandra observation of the neutron star soft X-ray transient (SXT) SAX J1810.8-2609. We detect three sources in the field of view. The position of one of them is consistent with the location of the ROSAT error circle of SAX J1810.8-2609. The accurate Chandra position of that source coincides with the position of the proposed optical counterpart, strengthening the identification as the counterpart. We detected the neutron star SXT system in quiescence at an unabsorbed luminosity of similar to1 x 10(32) erg s(-1) (assuming a distance of 4.9 kpc). This luminosity is at the low end of quiescent luminosities found in other neutron star SXTs. This renders support to the existence of a group of faint soft X-ray transients of which the accreting millisecond X-ray pulsar SAX J1808.4-3658 is the most prominent member. The quiescent spectrum of SAX J1810.8-2609 is well-fitted with an absorbed power law with a photon index of 3.3 +/- 0.5. With a value of 3.3 x 10 21 cm 2, the galactic absorption is consistent with the value derived in outburst. Because the spectra of quiescent neutron star SXTs are often fitted with an absorbed blackbody or neutron star atmosphere plus power-law model, we also fitted the spectrum using those fitting functions. Both models provide a good fit to the data. If cooling of the neutron star core and/or crust is responsible for the soft part of the spectrum, the time-averaged mass accretion rate must have been very low (similar to5.7 x 10(13) M-circle dot yr(-1); assuming standard core cooling only) or the neutron star must be massive. We also discuss the possibility that the thermal spectral component in neutron stars in quiescence is produced by residual accretion.

Bipolar Molecular Outflows from High‐Mass Protostars

We report observations of the bipolar molecular outflows associated with the luminous (~2 × 104 L☉) far-IR sources IRAS 21519+5613 and IRAS 22506+5944, as well the dust and molecular gas condensations on which these outflows appear to be centered. The observations were made in 12CO, 13CO, C18O, and continuum at 3 mm with the BIMA array and in 12CO and 13CO with the NRAO 12 m telescope to recover extended emission filtered out by the interferometric array. We find that the outflow associated with each IRAS source shows a clear bipolar morphology in 12CO, with properties (i.e., total mass of order 10-100 M☉, mass-outflow rate ≳10-3 M☉, dynamical timescale 104-105 yr, and energetics) comparable with those of other massive outflows associated with luminous young stellar objects. Each outflow appears to be centered on a dust and gas condensation with a mass of 200-300 M☉, likely marking the location of the driving source. The outflow lobes of both sources are fully resolved along their major but not minor axes, and they have collimation factors that may be comparable with young low-mass stars. The mass-velocity diagrams of both outflows change in slope at a velocity of ~10 km s-1, suggesting that the high-velocity component (HVC) may drive the low-velocity component (LVC). Although the HVC of IRAS 21519+5613 shows evidence for deceleration, no such signature is seen in the HVC of IRAS 22506+5944. Neither HVC has a momentum supply rate sufficient to drive their corresponding LVCs, although it is possible that the HVC is more highly excited and hence its thrust underestimated. Like for other molecular outflows the primary driving agent cannot be ionized gas, leaving atomic gas as the other remaining candidate. Neither IRAS 21519+5613 nor IRAS 22506+5944 exhibits detectable free-free emission, which together with the observed properties of their molecular outflows and surrounding condensations make them credible candidates for high-mass protostars. The mass-accretion rate required to produce their observed IRAS luminosity is ≳10-4 M☉ yr-1, which is more than sufficient to quench the development of an UC-H II region. On the other hand, the individual IRAS sources may be associated with a group of stars whose dominant member is a main-sequence star that is responsible for the observed outflow. Such a star would be required to have a spectral type of ~B2 (luminosity of ~3000 L☉) or later to not excite a detectable UC-H II region; the time-averaged mass-accretion rate needed to produce this star is then 10-3 to 10-4 M☉ yr-1. Thus, regardless of the evolutionary stage of the outflow driving source, the inferred mass-accretion rate is much higher than that allowed by simple inside-out collapse models but can be accommodated by recently proposed variants.

Cataclysmic Variables: An Empirical Angular Momentum Loss Prescription from Open Cluster Data

We apply the angular momentum loss rates inferred from open cluster stars to the evolution of cataclysmic variables (CVs). We show that the angular momentum prescriptions used in earlier CV studies are inconsistent with the measured rotation data in open clusters. The timescale for angular momentum loss () above the fully convective boundary is ~2 orders of magnitude longer than inferred from the older model, and the observed angular momentum loss properties show no evidence for a change in behavior at the fully convective boundary. This provides evidence against the hypothesis that the period gap is caused by an abrupt change in the angular momentum loss law when the secondary becomes fully convective. Furthermore, the empirical angular momentum loss law implies a timescale for CV evolution that is comparable to a Hubble time; for the same reason, it will be more difficult to produce CVs from the products of common envelope evolution, and it implies a lower space density of CVs. The predicted loss rate for short-period CVs is consistent with the observed period minimum (1.3 hr). We infer the time-averaged mass accretion rate and derive the mass-period relation for different evolutionary states of the secondary. The steady-state accretion rates are significantly lower than the claimed observational rates; we discuss some possible explanations. The mass-period relationship is more consistent with evolved secondaries than with unevolved secondaries above the period gap. Implications for the CV period gap are discussed, including the possibility that two populations of secondaries could produce the gap.