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At least 757 records · Page 42
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NASA NTRS2000

Dispersal of Disk Around Young Stars

Young stars produce sufficient ultraviolet photon luminosity and mechanical luminosity in their winds to significantly affect the structure and evolution of the accretion disks surrounding them. The Lyman continuum photons create a nearly static, ionized, isothermal 10(exp 4) K atmosphere forms above the neutral disk at small distances from the star. Further out, they create a photoevaporative flow which relatively rapidly destroys the disk. The resulting slow (10-50 km per second) ionized outflow, which persists for greater than or equal to 10(exp 5) years for disk masses Md approx. 0.3M*, may explain the observational characteristics of many ultracompact HII regions. We compare model results to the observed radio free-free Spectra and luminosities of ultracompact HII regions and to the interesting source MWC349, which is observed to produce hydrogen masers. We also apply the results to the early solar nebula to explain the the dispersal of the solar nebula and the differences in hydrogen content in the giant planets. Finally, we model the small bright objects ("proplyds") observed in the Orion Nebula as disks around young, low mass stars which are externally illuminated by the UV photons from the nearby massive star.

DeVincenzi, Donald L.↗
NASA NTRS1988

Accretion-driven nonthermal phenomena

Arguments are presented which suggest that accretion disks around black holes and neutron stars may self-generate ordered magnetic fields, from currents associated with orbital motion of the disk material. This leads one to expect electromagnetic extraction of the rotational energy of the disk, and it is proposed that such a magnetic field is the ultimate source of various nonthermal phenomena in accreting ultracompact objects.

Walker, Mark↗
NASA NTRS1990

Radiation shear-stress heating in accretion flows

The effects of radiation shear stresses acting in accretion disks and boundary layers are considered. It is demonstrated that these stresses are dynamically unimportant for nonrelativistic flows. For accretion onto a black hole or nonmagnetic neutron star, where the system is relativistic, radiation shear stresses can contribute substantially to the total luminosity of the system. The spectral and temporal signatures of this form of heating are discussed, and those features which are unique to radiative dissipation are emphasized. The results are in broad agreement with the spectra of the nonmagnetic X-ray binary systems, including Cyg X-1.

Walker, Mark A.↗
NASA NTRS1994

The hard X-ray emission spectra from accretion columns in intermediate polars

We consider the hard (greater than 2 keV) X-ray emission from accretion columns in an intermediate polar system, GK Per, using a simple settling solution. The rate of photon emission per logarithmic energy interval can be fitted with a power law, E(exp -gamma), with gamma approximately 2.0, in agreement with observations. This index is only weakly dependent on the mass accretion rate, dot-M, for dot-M in the range of a few times 10(exp 16-18) g/s. The peak energy of the photon spectra (after photoelectric absorption) is expected to be E(sub p) approximately (5 keV) gamma(exp -1/3) (N(sub H)/10(exp 23)/sq cm)(exp 1/3) where N(sub H) is the hydrogen column density along the line of sight. The observed spectra of GK Per and possibly of V1223 Sgr suggest N(sub H) approximately 10(exp 23)/sq cm. This large N(sub H) may be due to partially ionized preshock column material. Alternatively, we also consider absorption by the cool outer parts of an accretion disk. In this case the photoelectric absorption depth in the disk is a sensitive function of inclination. For GK Per the required inclination is approximately 83 deg. For mass accretion rates larger than a critical rate of approximately 10(exp 18) g/s, X-ray emission from the column accretion is significantly affected by radiation drag. Although the mass accretion rate increases dramatically during outbursts, the observed hard (greater than 2 keV) X-ray luminosity will not rise proportionately. The slope and peak energy of the outburst spectra are only weakly affected. We conclude that the observed X-ray spectra can be explained by this simple analytic solution and that the production of hard X-rays from the accretion shock at the magnetic poles in the intermediate polars is in general agreement with the observations. However, since the X-ray emission and absorption depend on the mass accretion rate in a complicated manner, observed hard X-ray luminosities (greater than 2 keV) are not a good indicator of the mass accretion rate in the X-ray emitting regions.

Yi, Insu↗
NASA NTRS2012

The 0.5 micrometer-2.2 micrometer Scattered Light Spectrum of the Disk Around TW Hya

We present a 0.5-2.2micron scattered light spectrum of the circumstellar disk around TW Hya from a combination of spatially resolved HST STIS spectroscopy and NICMOS coronagraphic images of the disk. \Ve investigate the morphology at the disk at distances> 40 AU over this wide range of wavelengths. We measure the surface brightness, azimuthal symmetry, and spectral character of the disk as a function of radius. We find that the scattering efficiency of the dust is largely neutral to blue over the observed wavelengths. We find a good fit to the data over a wide range of distances from the star if we use a model disk with a partial gap of 30% depth at 80 AU and with steep disk truncation exterior to 100 AU. If the gap is caused by a planetary companion in the process of accreting disk gas, it must be less than 20 Solar mass.

Debes, John H.↗
NASA NTRS1990

Nearly simultaneous optical, ultraviolet, and x ray observations of three PG quasars

Nearly simultaneous optical, ultraviolet, and x ray observations of three low redshift quasars are presented. The EXOSAT x ray spectra span the range of observed spectral indices for quasars from the canonical 0.7 energy index typical of Seyfert galaxies for PG0923+129 (Mrk 705) to the steep spectral indices frequently seen in higher luminosity quasars with an index of 1.58 for PG0844+349 (Ton 951). None of the quasars exhibits any evidence for a soft x ray excess. This is consistent with accretion disk spectra fit to the IR through UV continua of the quasars -- the best fitting disk spectra peak at approximately 6 eV with black hole masses in the range 5 x 10(exp 7) to 1 x 10(exp 9) solar mass and mass accretion rates of approximately 0.1 times the Eddington-limited rate. These rather soft disk spectra are also compatible with the observed optical and ultraviolet line ratios.

Kriss, Gerard A.↗
NASA NTRS2000

Disk Evaporation in Star Forming Regions

Young stars produce sufficient ultraviolet photon luminosity and mechanical luminosity in their winds to significantly affect the structure and evolution of the accretion disks surrounding them. The Lyman continuum photons create a nearly static, ionized, isothermal 10(exp 4) K atmosphere forms above the neutral disk at small distances from the star. Further out, they create a photoevaporative flow which relatively rapidly destroys the disk. The resulting slow (10-50 km/s) ionized outflow, which persists for approx. greater than 10(exp 5) years for disk masses M(sub d) approx. 0.3M(sub *), may explain the observational characteristics of many ultracompact HII regions. We compare model results to the observed radio free-free spectra and luminosities of ultracompact HII regions and to the interesting source MWC349, which is observed to produce hydrogen masers. We apply the results to Ae and Be stars in order to determine the lifetimes of disks around such stars. We also apply the results to the early solar nebula to explain the the dispersal of the solar nebula and the differences in hydrogen content in the giant planets. Finally, we model the small bright objects ("proplyds") observed in the Orion Nebula as disks around young, low mass stars which are externally illuminated by the UV photons from the nearby massive star Theta(sup 1) C.

Hollenbach, David↗
NASA NTRS1998

Disk Destruction by Winds and Radiation

Young stars produce sufficient ultraviolet photon luminosity and mechanical luminosity in their winds to significantly affect the structure and evolution of the accretion disks surrounding them. The Lyman continuum photons create a nearly static, ionized, isothermal 10(exp 4) K atmosphere forms above the neutral disk at small distances from the star. Further out, they create a photoevaporative flow which relatively rapidly destroys the disk. The resulting slow (10-50 km/s) ionized outflow, which persists for approx. greater than 10(exp 5) years for disk masses M(sub d) approx. 0.3M(sub *), may explain the observational characteristics of many ultracompact HII regions. We compare model results to the observed radio free-free spectra and luminosities of ultracompact HII regions and to the interesting source MWC349, which is observed to produce hydrogen masers. We also apply the results to the early solar nebula to explain the dispersal of the solar nebula and the differences in hydrogen content in the giant planets. Finally, we model the small bright objects ("proplyds") observed in the Orion Nebula as disks around young, low mass stars which are externally illuminated by the UV photons from the nearby massive star Theta(sup 1)C.

Hollenbach, David↗
NASA NTRS2018

Magnetized Disk Winds in NGC 3783

We analyze a 900 kilosecond stacked Chandra/HETG (High-Energy Transmission Grating) spectrum of NGC 3783 in the context of magnetically driven accretion-disk wind models in an effort to provide tight constraints on the global conditions of the underlying absorbers. Motivated by the earlier measurements of its absorption measure distribution (AMD) indicating X-ray-absorbing ionic columns that decrease slowly with decreasing ionization parameter, we employ 2-dimension (2-D) magnetohydrodynamic (MHD) disk wind models to describe the global outflow. We compute its photoionization structure along with the wind kinematic properties, allowing us to further calculate in a self-consistent fashion the shapes of the major X-ray absorption lines. With the wind radial density profile determined by the AMD, the profiles of the ensemble of the observed absorption features are determined by the two global parameters of the MHD wind; i.e., disk inclination theta (sub obs) and wind density normalization n (sub o). Considering the most significant absorption features in the approximately 1.8-20 angstrom range, we show that the MHD wind is best described by n(r) approximately equal to 6.9 times 10 (sup 11) (r/r (sub o)) (sup - 1.15) cubic centimeters and theta (sub obs). We argue that winds launched by X-ray heating or radiation pressure, or even MHD winds but with steeper radial density profiles, are strongly disfavored by data. Considering the properties of Fe K-band absorption features (i.e., Fe XXV and Fe XXVI), while typically prominent in the active galactic nucleus X-ray spectra, they appear to be weak in NGC 3783. For the specific parameters of our model obtained by fitting the AMD and the rest of the absorption features, these features are found to be weak, in agreement with observations.

Fukumura, Keigo↗
NASA NTRS1990

Nonlinear evolution of protostellar disks and light modulations in young stellar objects

An evolutionary model of dynamical processes in protostellar disks is described and illustrated with graphs of typical results. The effective transport mechanisms are discussed, including thermal convection, nonaxisymmetric gravitational instabilities in the outer regions of disks, and wave propagation. Consideration is then given to the stages of dynamical evolution, FU Ori outburst phenomena, unsteady accretion-disk flows, and nonlinear feedback as a mechanism to modulate mass transfer. The simulations show that mass redistribution is determined by angular-momentum transfer, which in turn is regulated by the effective viscosity generated by convectively driven turbulence. Significant mass transfer occurs as a result of mixing of infalling material with disk gas and is affected by the tidal torque associated with the growth of nonaxisymmetric disturbances in the outer disk. The time scale for disk evolution is found to be about 1 Myr.

Lin, D. N. C.↗
NASA NTRS1994

X-ray emission from 4U 2129+47 (= V1727 Cygni) in quiescence

Observations with the ROSAT HRI allow detection of weak X-ray flux from the low-mass X-ray binary (LMXB) 4U 2129+47 during its current quiescent state. The quiescent luminosity is similar to that seen in several other quiescent LMXBs containing neutron stars. The quiescent X-ray light curve may not show the eclipse seen when the source was in its high state, which would indicate that the enhanced vertical structure present in the disk during the high state has collapsed. This in turn provides support for the idea that the vertical structure in LMXB accretion disks is a consequence of high X-ray luminosity. A comparison of the absorption of low-energy X-rays due to the interstellar medium (determined from Einstein IPC observations) and the optical extinction does not rule out the triple system hypothesis.

Garcia, Michael R.↗
NASA NTRS1994

Star-disk collisions and the origin of the broad lines in quasars

Quasars and other active galactic nuclei (AGNs) are most likely powered by a massive black hole surrounded by an accretion disk and inhabiting the core of a dense stellar cluster. Star orbits intersect the disk. Hence, star-disk collisions are a daily (if not more frequent) occurrence. We consider the consequences of such collisions and show that the 'star tails' of the disk material, accelerated by the encounter with the passing star, can travel well above the disk and, therefore, are a natural candidate for the observed broad emission-line regions.

Zurek, W. H.↗
NASA NTRS2012

Evidence for Ultra-Fast Outflows in Radio-Quiet AGNs: III - Location and Energetics

Using the results of a previous X-ray photo-ionization modelling of blue-shifted Fe K absorption lines on a sample of 42 local radio-quiet AGNs observed with XMM-Newton, in this letter we estimate the location and energetics of the associated ultrafast outflows (UFOs). Due to significant uncertainties, we are essentially able to place only lower/upper limits. On average, their location is in the interval approx.0.0003-0.03pc (approx.10(exp 2)-10(exp 4)tau(sub s) from the central black hole, consistent with what is expected for accretion disk winds/outflows. The mass outflow rates are constrained between approx.0.01- 1 Stellar Mass/y, corresponding to approx. or >5-10% of the accretion rates. The average lower-upper limits on the mechanical power are logE(sub K) approx. or = 42.6-44.6 erg/s. However, the minimum possible value of the ratio between the mechanical power and bolometric luminosity is constrained to be comparable or higher than the minimum required by simulations of feedback induced by winds/outflows. Therefore, this work demonstrates that UFOs are indeed capable to provide a significant contribution to the AGN r.osmological feedback, in agreement with theoretical expectations and the recent observation of interactions between AGN outflows and the interstellar medium in several Seyferts galaxies .

Tombesi, F.↗
NASA NTRS2014

XMM-Newton and Swift Observations of WZ Sagittae: Spectral and Timing Analysis

WZ Sagittae is the prototype object of a subclass of dwarf novae with rare and long (super)outbursts, in which a white dwarf primary accretes matter from a low mass companion. High-energy observations offer the possibility of a better understanding of the disk-accretion mechanism in WZ Sge-like binaries.

binaries: general aEuro" white dwarfs aEuro" X-ray↗
NASA NTRS2005

X-ray Observations of the Black Hole Transient 4U 1630-47 During 2 Years of X-ray Activity

The black hole candidate (BHC) X-ray transient 4U 1630-47 continuously produced strong X-ray emission for more than 2 years during its 2002-2004 outburst, which is one of the brightest and longest outbursts ever seen from this source. We use more than 300 observations made with the Rossi X-Ray Timing Explorer (RM E) to study the source throughout the outburst, along with hard X-ray images from the International Gamma-Ray Astrophysics Laboratory (INTEGRAL), which are critical for interpreting the RXTE data in this crowded field. The source exhibits extreme behaviors, which can be interpreted as an indication that the system luminosity approaches the Eddington limit. For 15 observations, fitting the spectral continuum with a disk-blackbody plus power-law model results in measured inner disk temperatures between 2.7 and 3.8 key and such temperatures are only rivaled by the brightest BHC systems, such as GRS 1915+105 and XTE 51550-564. If the high temperatures are caused by the dominance of electron scattering opacity in the inner regions of the accretion disk, it is theoretically required that the source luminosity be considerably higher than 20% of the Eddington limit. We detect a variety of high-amplitude variability, including hard 10-100 s flares, which peak at levels as much as 2-3 times higher than nonflare levels. This flaring occurs at the highest disk luminosities in a regime in which the source deviates from the L(sub disk) infinity T(sup 4)(sub in) relationship that is seen at lower luminosities, possibly suggesting that we are seeing transitions between a Shakura & Sunyaev disk and a "slim" disk, which is predicted to occur at very high mass accretion rates. The X-ray properties in 2002-2004 are significantly different from those seen during the 1998 outburst, which is the only outburst with detected radio jet emission. Our results support the "jet line" concept recently advanced by Fender and coworkers. Our study allows for a test of the quantitative McClintock & Remillard spectral state definitions, and we find that these definitions alone do not provide a complete description of the outburst. Finally, for several of the observations, the high-energy emission is dominated by the nearby sources IGR 516320-4751 and IGR 516358-4726, and we provide information on when these sources were bright and on the nature of their energy spectra

Tomsick, John A.↗
NASA NTRS1986

Continuum variability of the optically-active radio-quiet quasar GQ Comae

The use of thermal accretion disk models to explain the continuum shape and variability of GQ Comae is outlined. The mass is estimated at 7 x 10 to the 8th solar masses and its accretion rate at 0.1-0.2 solar masses per year. The effects of assuming different values of the inclination angle of the disk, and of the specific angular momentum of the central black hole, are explored. Additional uncertainties in the emergent fluxes in these models are discussed.

Sitko, Michael L.↗
NASA NTRS1994

The evidence for clumpy accretion in the Herbig Ae star HR 5999

Analysis of IUE high- and low-dispersion spectra of the young Herbig Ae star HR 5999 (HD 144668) covering 1978-1992 has revealed dramatic changes in the Mg II h and k (2795.5, 2802.7 A) emission profiles, changes in the column density and distribution in radial velocity of accreting gas, and flux in the Ly(alpha), O I and C IV emission lines, which are correlated with the UV excess luminosity. We also observe variability in the spectral type inferred from the UV spectral energy distribution, ranging from A5 IV-III in high state to A7 III in the low state. The trend of earlier inferred spectral type with decreasing wavelength and with increasing UV continuum flux has previously been noted as a signature of accretion disks in lower mass pre-main sequence stars (PMS) and in systems undergoing FU Orionis-type outbursts. Our data represent the first detection of similar phenomena in an intermediate mass (M equal to or greater than 2 solar mass) PMS star. Recent IUE spectra show gas accreting toward the star with velocities as high as +300 km/s, much as is seen toward beta Pic, and suggest that we also view this system through the debris disk. The absence of UV lines with the rotational broadening expected given the optical data (A7 IV, upsilon sin i = 180 plus or minus 20 km/s) for this system also suggests that most of the UV light originates in the disk, even in the low continuum state. The dramatic variability in the column density of accreting gas, consistent with clumpy accretion, such as has been observed toward beta Pic, is a hallmark of accretion onto young stars, and is not restricted to the clearing phase, since detectable amounts of accretion are present for stars, and is not restricted to the clearing phase, since detectable amounts of accretion are present for stars with 0.5 less than t(sub age) less than 2.8 Myr. The implications for models of beta Pic and similar systems are briefly discussed.

Perez, M. R.↗