Myeong-Gu Park

Preheated Advection Dominated Accretion Flow

The advection-dominated accretion flow (ADAF) has been quite successful in explaining a wide variety of accretion-powered astronomical sources. The physical characteristics of ADAF complement the classical thin disk flow quite nicely, and extensive work has been done on it. However, all high-temperature accretion solutions including ADAF are physically thick, so outgoing radiation interacts with the incoming flow. Thus, ADAF solutions share as much or more resemblance with classical spherical accretion flows as with disk flows. This interaction, which has been neglected by most authors, is primarily through the Compton heating process that will typically limit the steady solutions to L 10-2LEdd, where LEdd is the Eddington luminosity. We examine this interaction for the popular ADAF case, but similar conclusions, we expect, would apply for other high-temperature, geometrically thick disks as well. We study the global thermal nature of the flow, with special consideration given to various cooling and, especially, preheating by Comptonizing hot photons produced at smaller radii. We find that without allowance for Compton preheating a very restricted domain of ADAF solution is permitted and with Compton preheating included a new high-temperature PADAF branch appears in the solution space. In the absence of preheating, high-temperature flows do not exist when the mass accretion rate ≡ c2/LEdd 10-1.5. Below this mass accretion rate, a roughly conical region around the hole cannot sustain high-temperature ions and electrons for all flows having 10-4, which may lead to a funnel possibly filled with a tenuous hot outgoing wind. If the flow starts at large radii with the usual equilibrium temperature ~104 K, the critical mass accretion rate is much lower, ~ 10-3.7, above which level no self-consistent ADAF (without preheating) can exist. However, above this critical mass accretion rate, the flow can be self-consistently maintained at high temperature if Compton preheating is considered. These solutions constitute a new branch of solutions as in spherical accretion flows. High-temperature PADAF flows can exist above the critical mass accretion rate in addition to the usual cold thin disk solutions. We also find solutions where the flow near the equatorial plane accretes normally while the flow near the pole is overheated by Compton preheating, possibly becoming a polar wind, solutions which we designate WADAF.

Setting limits on q0 from gravitational lensing

Gravitational lensing by galaxies in a wide variety of cosmological models is considered. For closed models, the lensing depends on the parameter beta(crit). If beta(crit) is greater than zero, a normal lensing case can be obtained with two bright images separated by an angle twice beta(crit) and a third, arbitrarily dim image between them coincident with the position of the lensing galaxy nucleus. As the QSO approaches the antipodal redshift, which can occur in models with large values of the cosmological constant, the cross sections for lensing blow up. An overfocused case where beta(crit) is less than zero can be obtained for a QSO beyond the antipodal redshift. In this case, when a lensing event occurs, only one arbitrarily dim image coincident with the position of the lensing galaxy nucleus is seen. If galaxy rotation curves are always flat or slowly rising, the overfocused case always produces one image. 33 refs.

The Stream-Stream Collision after the Tidal Disruption of a Star around a Massive Black Hole

A star can be tidally disrupted around a massive black hole. It is known that the debris forms a precessing stream, which may collide with itself. The stream collision is a key process in determining the subsequent evolution of the stellar debris: if the orbital energy is efficiently dissipated, the debris will eventually form a circular disk (or torus). In this paper, we have numerically studied such a stream collision resulting from the encounter between a 106 M☉ black hole and a 1 M☉ normal star with a pericenter radius of 100 R☉. A simple treatment for radiative cooling has been adopted for both optically thick and optically thin regions. We have found that approximately 10% to 15% of the initial kinetic energy of the streams is converted into thermal energy during the collision. The spread in angular momentum of the incoming stream is increased by a factor of 2 to 3, and such an increase, together with the decrease in kinetic energy, significantly helps the circularization process. The initial luminosity burst produced by the collision may reach as high as 1041 ergs s-1 in 104 s, after which the luminosity increases again (but slowly this time) to a steady value of a few 1040 ergs s-1 in a few times 105 s. The radiation from the system is expected to be close to Planckian, with an effective temperature of ~105 K.

Around-the-Clock Observations of the Q0957+561A,B Gravitationally Lensed Quasar. II. Results for the Second Observing Season

We report on an observing campaign in 2001 March to monitor the brightness of the later arriving Q0957+561B image in order to compare with the previously published brightness observations of the (first-arriving) A image. The 12 participating observatories provided 3543 image frames, which we have analyzed for brightness fluctuations. From our classical methods for time-delay determination, we find a 417.09 ± 0.07 day time delay, which should be free of effects due to incomplete sampling. During the campaign period, the quasar brightness was relatively constant and only small fluctuations were found; we compare the structure function for the new data with structure function estimates for the 1995-1996 epoch and show that the structure function during our observing interval is unusually depressed. We also examine the data for any evidence of correlated fluctuations at zero lag. We discuss the limits of our ability to measure the cosmological time delay if the quasars emitting surface is time resolved, as seems likely.

Toward the Evidence of the Accretion Disk Emission in the Symbiotic Star RR Telescopii

In this Letter, we argue that in the symbiotic star RR Telescopii, the existence of an accretion disk around the hot companion is strongly implied by the characteristic features exhibited by the Raman-scattered O VI lines around 6830 and 7088 A. High degrees of polarization and the double-peaked profiles in the Raman-scattered lines and the single-peak profiles for other emission lines are interpreted as line-of-sight effects, where the H I scatterers near the giant star see an incident double-peaked profile and an observer with a low inclination sees single-peak profiles. It is predicted that different mass concentrations around the accretion disk that is formed by a dusty wind may lead to the disparate ratios of the blue peak strength to the red counterpart observed in the 6830 and 7088 A features. We discuss the evolutionary links between symbiotic stars and bipolar proto-planetary nebulae and conclude that the Raman-scattering processes may play an important role in the investigation of the physical properties of these objects.

Planetary companions orbiting M giants HD 208527 and HD 220074

Aims. The purpose of the present study is to research the origin of planetary companions by using a precise radial velocity (RV) survey. Methods. The high-resolution spectroscopy of the fiber-fed Bohyunsan Observatory Echelle Spectrograph (BOES) at Bohyunsan Optical Astronomy Observatory (BOAO) is used from September 2008 to June 2012. Results. We report the detection of two exoplanets in orbit around HD 208527 and HD 220074 exhibiting periodic variations in RV of 875.5 +/- 5.8 and 672.1 +/- 3.7 days. The RV variations are not apparently related to the surface inhomogeneities and a Keplerian motion of the planetary companion is the most likely explanation. Assuming possible stellar masses of 1.6 +/- 0.4 and 1.2 +/- 0.3 M_Sun, we obtain the minimum masses for the exoplanets of 9.9 +/- 1.7 and 11.1 +/- 1.8 M_Jup around HD 208527 and HD 220074 with an orbital semi-major axis of 2.1 +/- 0.2 and 1.6 +/- 0.1 AU and an eccentricity of 0.08 and 0.14, respectively. We also find that the previously known spectral classification of HD 208527 and HD 220074 was in error: Our new estimation of stellar parameters suggest that both HD 208527 and HD 220074 are M giants. Therefore, HD 208527 and HD 220074 are so far the first candidate M giants to harbor a planetary companion.

COMPTON-HEATED OUTFLOW FROM CONVECTION-DOMINATED ACCRETION FLOWS

Convection-dominated accretion flows (CDAFs) are expected to have a shallower density profile and a higher radiation efficiency compared to advection-dominated accretion flows (ADAFs). Both solutions have been developed to account for the observed properties of the low-luminosity, high-temperature X-ray sources believed to involve accretion onto massive black holes. Self-similar CDAFs also have steeper poloidal density gradients and temperatures close to the virial temperature. All these characteristics make CDAFs more capable of producing polar outflows driven by Compton heating as compared to other classical accretion disks. We investigate the conditions for producing such outflows in CDAFs and look for the mass accretion rate or, equally, the luminosity of CDAFs for which such outflows will exist. When the electron temperature saturates around 1011 K at the inner region, polar outflows are probable for 8 × 10-7 L/LE 4 × 10-5, where LE is the Eddington luminosity. Outflows are well collimated with small opening angles. The luminosity range for which outflow solutions exist is narrower for lower electron temperature flows and disappears completely for electron temperature 6 × 109 K. When the magnetic field is present, we find that outflows are possible if the magnetic field is less than from 10% to 1% of the equipartition field. We also find that outflows are more likely to be produced when the viscosity parameter α is small. The tendency for jetlike collimated outflows to result from these solutions is presumably astrophysically relevant given the high frequency of jets from active galactic nuclei.Convection-dominated accretion flows (CDAF) are expected to have a shallower density profile and a higher radiation efficiency as compared to advection-dominated accretion flows (ADAF). Both solutions have been developed to account for the observed properties of the low luminosity, high temperature X-ray sources believed to involve accretion onto massive black holes. Self-similar CDAFs also have steeper poloidal density gradients and temperatures close to the virial temperature. All these characteristics make CDAFs more capable of producing polar outflows driven by Compton heating as compared to other classical accretion disks. We investigate the conditions for producing such outflows in CDAFs and look for the mass accretion rate, or, equally, the luminosity of CDAFs for which such outflows will exist. When the electron temperature saturates around 10^11 K at the inner region, polar outflows are probable for 8x10^-7 <~ L/L_E <~ 4x10^-5, where L_E is the Eddington luminosity. Outflows are well collimated with small opening angles. The luminosity range for which outflow solutions exist is narrower for lower electron temperature flows and disappears completely for electron temperature <~ 6x10^9 K. When the magnetic field is present, we find that outflows are possible if the magnetic field is less than from 10% to 1% of the equipartition field. We also find that outflows are more likely to be produced when the viscosity parameter alpha is small. The tendency for jet-like collimated outflows for these solutions is presumably astrophysically relevant given the high frequency of jets from AGNs.

Equations of general relativistic radiation hydrodynamics from a tensor formalism

Radiation interacts with matter via exchange of energy and momentum. When matter is moving with a relativistic velocity or when the background space‐time is strongly curved, rigorous relativistic treatment of hydrodynamics and radiative transfer is required. Here, we derive fully general relativistic radiation hydrodynamic equations from a covariant tensor formalism. The equations can be applied to any three-dimensional problems and are rather straightforward to understand compared to the comoving frame-based equations. The current approach is applicable to any space‐time or coordinates, but in this work we specifically choose the Schwarzschild space‐time to show explicitly how the hydrodynamic and the radiation moment equations are derived. Some important aspects of relativistic radiation hydrodynamics and the difficulty with the radiation moment formalism are discussed as well.

Analysis of Historical Meteor and Meteor shower Records: Korea, China and Japan

We have compiled and analyzed historical Korean meteor and meteor shower records in three Korean official history books, Samguksagi which covers the three Kingdoms period (57 B.C.–A.D. 935), Goryeosa of Goryeo dynasty (A.D. 918–1392), and Joseonwangjosillok of Joseon dynasty (A.D. 1392–1910). We have found 3861 meteor and 31 meteor shower records. We have confirmed the peaks of Perseids and an excess due to the mixture of Orionids, north-Taurids, or Leonids through the Monte Carlo test. The peaks persist from the period of Goryeo dynasty to that of Joseon dynasty, for almost one thousand years. Korean records show a decrease of Perseids activity and an increase of Orionids/north-Taurids/Leonids activity. We have also analyzed seasonal variation of sporadic meteors from Korean records. We confirm the seasonal variation of sporadic meteors from the records of Joseon dynasty with the maximum number of events being roughly 1.7 times the minimum. The Korean records are compared with Chinese and Japanese records for the same periods. Major features in Chinese meteor shower records are quite consistent with those of Korean records, particularly for the last millennium. Japanese records also show Perseids feature and Orionids/north-Taurids/Leonids feature, although they are less prominent compared to those of Korean or Chinese records.

Thermal Properties of Two-dimensional Advection-dominated Accretion Flow

We study the thermal structure of the widely adopted two-dimensional advection-dominated accretion flow (ADAF) of Narayan & Yi. The critical radius for a given mass accretion rate, outside of which the optically thin hot solutions do not exist in the equatorial plane, agrees with those of one-dimensional studies. However, we find that, even within the critical radius, there always exists a conical region of the flow, around the pole, which cannot maintain the assumed high electron temperature, regardless of the mass accretion rate, in the absence of radiative heating. This could lead to a torus-like advection inflow shape since, in general, the ions too will cool down. We also find that Compton preheating is generally important and, if the radiative efficiency, defined as the luminosity output divided by the mass accretion rate times the velocity of light squared, is above ~4 × 10-3, the polar region of the flow is preheated above the virial temperature by Compton heating, which may result in time-dependent behavior or outflow while accretion continues in the equatorial plane. Thus, under most relevant circumstances, ADAF solutions may be expected to be accompanied by polar outflow winds. While preheating instabilities exist in ADAF, as for spherical flows, the former are to some extent protected by their characteristically higher densities and higher cooling rates, which reduce their susceptibility to Compton driven overheating.