Daniel Proga's Homepage


Classes:
Spring 2014 AST103

AST 103 Fall 2007, Spring 2008 , Spring 2009 , Spring 2011 , Spring 2013, Fall 2013
AST 104 Fall 2005, Spring 2006, Fall 2006, Spring 2007, Spring 2010
AST 714 Spring 2007
AST 723 Fall 2009, Fall 2012
AST 731 Fall 2008, Fall 2010, Spring 2013


UNLV Logo

Contact Information
Department of Physics & Astronomy
University of Nevada, Las Vegas
4505 South Maryland Parkway
Box 454002
Las Vegas, NV 89154-4002
USA

E-mail: dproga@physics.unlv.edu

Telephone: (702) 895 3507
FAX: (702) 895 0804

Education and Degrees
Ph.D., Nicolaus Copernicus Astronomical Center (CAMK), Warsaw, Poland
M.S., Nicolaus Copernicus University , Torun, Poland  

Publications
ADS database
LANL astro-ph

Daniel Proga

I am a Professor in the Department of Physics and Astronomy at University of Nevada, Las Vegas (UNLV).  

My main interests are mass accretion processes onto compact objects and related mass outflows. I use primarily numerical methods for astrophysical fluid dynamics to study effects of radiation and magnetic fields on gas under the influence of gravity. I also work on photoionization and radiative transfer processes.

CURRENT PROJECTS

BLACK HOLE ACCRETION AND RELATED OUTFLOWS:

1) cases with simplified microphysics (applicable to low luminosity AGN and the galactic center, for instance):

Some of the most dramatic phenomena of astrophysics, such as quasars and powerful radio galaxies, are most likely powered by accretion onto supermassive black holes (SMBHs). Nevertheless, SMBHs appear to spend most of their time in a remarkably quiescent state. SMBHs are embedded in the relatively dense environments of galactic nuclei and it is natural to suppose that the gravity due to an SMBH will draw in matter at high rates, leading to a high system luminosity. However, this simple prediction often fails as many systems are much dimmer than one would expect.

One of the key effects that could reduce the mass accretion is gas rotation. Even a relatively slow rotation can result in the centrifugal force strong enough to affect the gas dynamics. In Proga & Begelman ( 2003a ), we considered inviscid 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 studied accretion flows by means of numerical 2D, axisymmetric, hydrodynamical simulations. Later, in the follow-up papers we studied the effects of the adiabatic index, temperature, and 3D, in the hydrodynamical and magnetohydrodynamical limits:
-- HD in 2D - Moscibrodzka & Proga (2008);
-- HD in 3D - Janiuk, Proga, & Kurosaw (2008), and Janiuk, et al. (2009);
-- MHD in 2D - Proga & Begelman (2003b), Proga (2005), and Moscibrodzka & Proga (2009).
We also computed synthetic broad-band spectra based on Proga & Begelman's 2-D MHD simulations for direct comparison of the observations of Sgr A* (Moscibrodzka et al. 2007).

Our main result from the 2-D inviscid case, is that the properties of the accretion flow do not depend as much on the outer boundary conditions (i.e., the amount as well as distribution of the angular momentum) as on the geometry of the non-accreting matter. The material that has too much angular momentum to be accreted forms a thick torus near the equator (see movie 1 and its zoom-in version movie 2). Consequently, the geometry of the polar region, where material is accreted (the funnel), and the mass accretion rate through it are constrained by the size and shape of the torus. Our results show one way in which the mass accretion rate of slightly rotating gas can be significantly reduced compared to the accretion of non-rotating gas (i.e., the Bondi rate), and set the stage for calculations that will take into account the transport of angular momentum and energy.

Figure above shows some results from simulations of a magnetized, slowly rotating flow presented in Proga & Begelman (2003b). Specifically, it shows the density maps overplotted with the direction of the velocity field for four generic stages of the inner accertion flow close to the black hole: (i) the initial stage when both the equatorial torus and the polar funnel accrete (the top left panel; note a torus corona between the torus and the funnel), (ii) the stage when the accretion occurs only through the torus (the top righ panel), (iii) the stage when there is practically no accretion because the torus is pushed away by a very strong poloidal magnetic field forming a cylinder with the black holes inside it (the bottom left panel; this stage is recurrent yet very short-lived), and (iv) the accretion occurs through the torus and through one of the polar region where the low angular momentum material manages to get into the inner flow (the bottom right panel). The last stage is similar to the first stage but there are the following differences: during the fourth stage the polar funnel accretion is only on one side of the equator whereas during the first stage it in on both sides; the fourth stage lasts relatively long at the beginning of the simulations and even repeats whereas the fourth stage is recurrent and last a shorter period of time (much longer than the third stage though). This movie shows how the inner flow changes between the second, third, and fourth stages.
Broad-band spectra predicted by these simulations are presented in Moscibrodzka et al. ( 2007).

2) cases with sophisticated microphysics of very high density and temperature fluids (applicable to gamma-ray bursts):

As a illustration of a universal nature of accretion onto a black hole, we can consider gamma-ray bursts (GRBs). In Proga, MacFadyen , Armitage , & Begelman ( 2003) we present results from axisymmetric, time-dependent magnetohydrodynamic (MHD) simulations of the collapsar model for gamma-ray bursts. We begin the simulations after the 1.7 solar mass iron core of a 25 solar mass presupernova star has collapsed and study the ensuing accretion of the 7 solar mass helium envelope onto the central black hole formed by the collapsed iron core. We consider a spherically symmetric progenitor model, but with spherical symmetry broken by the introduction of a small, latitude-dependent angular momentum and a weak radial magnetic field. Our MHD simulations include a realistic equation of state, neutrino cooling, photodisintegration of helium, and resistive heating. Our main conclusion is that, within the collapsar model, MHD effects alone are able to launch, accelerate and sustain a strong polar outflow. We also find that the outflow is Poynting flux-dominated, and note that this provides favorable initial conditions for the subsequent production of a baryon-poor fireball.
These simulations are relevant to the early as well as late evolution of GRBs (e.g., Proga & Zhang 2006 , Janiuk & Proga 2008 , and Janiuk, Moderski, & Proga 2008 ).

Maps of logarithmic density and toroidal magnetic field overplotted with the direction of the poloidal velocity at t=0.2735 s. The length scale is in units of the BH radius (i.e., r'=r/R_S and z'=z/R_S)

As above, but a zoom-in version.

Movie1 , movie2 , and movie3 show the time evolution (of the density) in our entire computational domain (out to 1000 black hole radii), in the inner part of the domain (out to 100 black hole radii), and the innermost part of the domain (out to 40 black hole radii), respectively.

MASS OUTFLOWS FROM ACCRETION DISKS:

Luminous accretion disks are believed to exist in many astrophysical environments: for example, in active galactic nuclei (AGN); in cataclysmic variables (CVs); and in young stellar objects (YSOs). Invariably, such disks are associated with mass outflows and winds. An obvious, and conceptually simple, mechanism for powering disk winds is radiation pressure on spectral lines: several time-independent solutions for the steady-state structure of radiation-driven disk winds have been proposed.

Using numerical techniques (the hydrodynamical code ZEUS code by J.M. Stone and M.L. Norman), we constructed a set of time-dependent two-dimensional radiation-driven disk wind models to identify a parameter domain where mass loss is significant. Our numerical approach is motivated by a desire to account for the multi-dimensional character of the disk wind problem from first principles .

We find that there exists a significant parameter domain in which time-variable behavior is inevitable. Specifically, whenever the disk's luminosity dominates the total luminosity of the system, the outflow is subject to large-amplitude velocity and density fluctuations, although on timescales of order a few flow times, the average properties of the wind are constant. To obtain a steady outflow, it is necessary to add into the total radiation field, a significant radial component, such as that from a bright central star. Our solutions agree qualitatively with the kinematics of outflows in CVs inferred from spectroscopic observations (Proga, Stone & Drew 1998 ; 1999 ).

The time-dependent evolution of the wind is best illustrated in this movie of the density for the model in the top left panel (the case where the disk contribution dominates the total luminosity of the system). See also this movie of the density for the model in the bottom left panel (the case where the central object and disk luminosities are the same).

My other work on mass outflows involves also testing my theoretical models against observations. One way of doing this is to compute synthetic spectra based on multidimensional dynamical models and compared with observations.

Line profiles for our steady state disk wind (the bottom left panel in the figure above) as a function of inclination angle, i. The figure compares the line profiles for the model dashed lines, with the line profiles for the same model but with the slow wind material between the disk equator and the 'fast stream' assumed to be optically thin in the line, solid lines. See Proga et al. ( 2002) for details.

We applied our disk wind models in the most detail first to CVs, as these systems are the best place to test the physics of such models. It is very encouraging that recent HST observations of BZ Cam indicate variable small-scale structuring present in this system (Prinja et al. 2000a , b).

We also applied a similar approach to model dynamics of massive YSOs and B[e] stars. The main modification in these applications was that we allowed an outflow also from a central star. Our models offer great promise in explaining the extreme mass loss signatures of massive YSOs (Drew, Proga, & Stone 1998) and B[e] stars (Oudmaijer, Proga, Drew & de Winter 1998). For example, in Oudmaijer et al., we showed that our model is consistent with the original two-wind concept for B[e] stars suggested by Zickgraf et al. (1985), and exhibits kinematic properties that may well explain the observed spectral features in those stars.

Recently, I considered a new generation of disk wind models: a hybrid of line-driven and MHD driven wind model. I used ideal MHD to compute numerically the evolution of Keplerian disks, varying the magnetic field strengths and the luminosity of the disk, the central accreting object or both. I find that the magnetic fields very quickly start deviating from purely axial due to the magnetorotational instability. This leads to fast growth of the toroidal magnetic field as field lines wind up due to the disk rotation. As a result the toroidal field dominates over the poloidal field above the disk and the gradient of the former drives a slow and dense disk outflow, which conserves specific angular momentum. Depending on the strength of the magnetic field relative to the system luminosity the disk wind can be radiation- or MHD driven. The pure radiation-driven wind consists of a dense, slow outflow that is bounded on the polar side by a high-velocity stream. The mass-loss rate is mostly due to the fast stream. As the magnetic field strength increases first the slow part of the flow is affected, namely it becomes denser and slightly faster and begins to dominate the mass-loss rate. In very strong magnetic field or pure MHD cases, the wind consists of only a dense, slow outflow without the presence of the distinctive fast stream so typical to pure radiation-driven winds.

An example of a MHD/LD driven disk wind. The model paramters related to line-driving are the same as for the pure LD model presented in the top left panel in the four panel figure above.

I have also incorporated into my model radiation-driven wind model a self-consistent evaluation of the ionization structure of the wind. I have computed using this version of the model the structure of line-driven winds from accretion disks in AGNs.

The framework of our two-dimensional hydrodynamical calculations for a line-driven disk wind. The drawing is not to scale. We assume the disk is flat, Keplerian, geometrically thin, and optically thick. The radiation pressure dominated disk is represented by the blue regions while the gas pressure dominated disk is represented by the red regions. The black hole is represented by the black circle while the X-ray source, is marked by the yellow region. The dashed line perpendicular to the disk is the disk rotational axis. Our computational domain is marked by solid lines. The theta= 90 degrees axis, the dashed line perpendicular to the rotational axis, is located above the disk midplane, the dot-dashed line. The offset of the theta=90 degrees axis from the disk midplane is given by the disk pressure scale height.

A sequence of density maps for the model from our fiducial model after 13.3, 14.6, and 16.47~years (left, middle, and right panel). Note both the time-dependent fine structure near the base of the wind and the time-dependent large scale structure associated with the fast stream at the polar angle about 75 degrees.

Results of this project will provide basic dynamical and kinematical properties of disk winds in systems such as BAL QSOs [see Proga, Stone & Kallman (2000) , and movies of the density: movie 1 : the fiducial model, movie 2 : the zoom-in movie 1, and movie 3 : the overionized "failed" wind]. These results will help to clarify the nature of QSOs in general. We use this kind of simulations to predict synthetic line profiles (Proga & Kalman 2004 ) and synthetic broad-band spectra (Schurch et al. 2009 ; Sim et al. 2010). Therefore I will be able to directly compare our models with observations from HST, CHANDRA, XMM and other facilities. X-ray observations should allow us to better understand the central engine, BAL and BEL regions, and warm absorbers in AGNs (Dorodnitsyn, Kallman & Proga 2008a, b ). The tools developed to model disk winds in AGNs can be used to study other systems which are strong X-ray sources (e.g., X-ray binaries, Proga & Kallman 2002 - movie , Proga 2006 , 2009 , and Luketic et al 2010 ).

PAST PROJECTS

  • SYMBIOTIC STARS:

    Symbiotics are interacting binary stars composed of an evolved red giant and a hot companion star. The hot companion is usually a white dwarf, sometimes a main sequence star, and rarely a neutron star. The hot component generates energy by accreting material lost by the red giant. Starting with the work for my Ph. D. thesis, I have been interested in the physical structure of symbiotic stars (see Proga, Kenyon , Raymond & Mikolajewska 1996 and Proga et at. 1998 ). In particular, I have worked of illumination effects in those stars. As in many other close binary systems, symbiotic stars have light curve that display the ``reflection effect'' in which the hot component star heats up the facing hemisphere of the red giant. The higher effective temperature of the heated hemisphere produces a characteristic sinusoidal light variation. Aside from this simple photospheric display, illumination can have a significant impact on spectroscopic analyses. For example, radiation from a hot secondary can distort absorption line profiles and thus cause errors in effective temperature or gravity estimates and in radial velocity curve used for orbits. In some cases, this extra radiation might cause the heated atmosphere to expand. Extra mass-loss from the extended atmospheres of illuminated red dwarfs is important in low mass X-ray binary systems (LMXB's), CVs, some symbiotic stars, because it can significantly affect the evolution of the binary system. Illumination effects can also be very important in studies of extrasolar giant planets if the radiation of their parent star is intense.

    To tackle the problem of illumination I have constructed a non-LTE photoionization code which handles both low and high ionization state conditions and calculates a spectrum for a wide wavelength range. I included many opacity sources and forbidden line subroutines that are important in a red giant atmosphere. The model assumes radiative, and statistical equilibria for the red giant photosphere or wind and solves the radiative transfer equation with a local escape probability method. I computed non-LTE level populations for a variety of ions and predict the variation of emission line fluxes as functions of the temperature and luminosity of the hot component.

    My models generally match observations of the symbiotic stars EG And and AG Peg. The optically thick cross-section of the red giant wind as viewed from the hot component is a crucial parameter in these models. Winds with cross-sections of 2-3 red giant radii reproduce the observed fluxes. My models favor winds with acceleration regions that either lie far from the red giant photosphere or extend for 2-3 red giant radii.

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