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NASA NTRS1986

Jets, mass outflows and disks associated with young stellar objects

Over the past 3 years, a series was obtained of the CCD images of regions surrounding young stellar objects (YSO's) located in a variety of environments with the goal of providing a census of the properties of mass outflows associated these objects. A catalog describing the optical morphology and kinematics of 38 mass outflows associated with young stellar objects (YSOs) was completed. The direct detection was reported of disks surrounding HL Tau and LI551/IRS 5 -- both sources of optical jets and large-scale mass outflows. In each case, the disk dimensions are on the order of several hundred astronomical units and the disk dust masses approximately 10 to the -7th power solar masses. A program aimed at indirect detection of circumstellar disks associated with YSOs was started. The technique involves comparison of an observed bolometric luminosity (L(bol)) with an independent measure of the true luminosity (L(spec)) of a YSO which derives from the observed strengths of surface-gravity-sensitive spectral features.

Strom, S. E.↗

NASA NTRS1979

Einstein observations of the X-ray structure of Centaurus A - Evidence for the radio-lobe energy source

The X-ray source at the center of the radio galaxy Centaurus A has been resolved into the following components with the imaging detectors on board the Einstein X-ray Observatory: (1) a point source coincident with the infrared nucleus; (2) diffuse X-ray emission coinciding with the inner radio lobes; (3) a 4-arcmin extended region of emission about the nucleus; and (4) an X-ray jet between the nucleus and the NE inner radio lobe. The 2 x 10 to the 39th ergs/s detected from the radio lobes probably arises from inverse Compton scattering of the microwave background. The average magnetic field in the SW lobe is determined to be not less than 4 microgauss. The extended region may be due to emission by a cloud of hot gas, cosmic-ray scattering, or stellar sources. The jet provides strong evidence for the continuous resupply of energy to the lobes from the nucleus.

Schreier, E. J.↗

NASA NTRS1992

Compact jets and the AGN paradigm

The nature of compact, nonthermal jets in active galactic nuclei is reviewed from both observational and theoretical standpoints. Despite rapid variations in brightness, the smallest emission regions in a jet may be relatively far removed from the central engine. Certain models for the formation of a relativistic particle beam that evolves downstream into the observed radio-infrared jet do, however, predict substantial high energy radiation from regions in the beam close to the accretion disk. Observations of such emission, and the general requirement that the jets be well collimated and accelerated to relativistic bulk velocities by the point where they are observed in the radio, can provide indirect constraints on the massive black hole accretion paradigm. Observations of well collimated, semi-relativistic jets in the accreting stellar systems SS 433 and Cyg X-3, as well as slower jets emanating from star forming regions, encourage one to associate jet formation with accretion of gas onto a compact object.

Marscher, Alan P.↗

NASA NTRS2013

A Universal Scaling for the Energetics of Relativistic Jets From Black Hole Systems

Black holes generate collimated, relativistic jets which have been observed in gamma-ray bursts (GRBs), microquasars, and at the center of some galaxies (active galactic nuclei; AGN). How jet physics scales from stellar black holes in GRBs to the supermassive ones in AGNs is still unknown. Here we show that jets produced by AGNs and GRBs exhibit the same correlation between the kinetic power carried by accelerated particles and the gamma-ray luminosity, with AGNs and GRBs lying at the low and high-luminosity ends, respectively, of the correlation. This result implies that the efficiency of energy dissipation in jets produced in black hole systems is similar over 10 orders of magnitude in jet power, establishing a physical analogy between AGN and GRBs.

Nemmen, R. S.↗

NASA NTRS1984

Optically thick ablation fronts

The physical characteristics of optically thick ablation fronts such as interstellar clouds are analyzed. Attention is given to cold clumps in both planar and spherical geometries and modifications caused by accelerations in a gravitational field or by evaporation of the clumps when encountered hot gas. The effects of ablation on the appearance of the Rayleigh-Taylor instability are examined in both linear and nonlinear regimes. The results of the calculations are applied to the astrophysical phenomena of cold clumps immersed in a supersonic flow, optically thick jets, and ablation in stellar envelopes. Evaporation in an optically thick front is projected to be orders of magnitude larger than evaporation in electron-conduction fronts in optically thin conditions. The optically thick processes could then be useful for modeling flows from, e.g., newly formed stars and active galactic nuclei.

Konigl, A.↗

NASA NTRS1986

Enhancement of features in galaxy images

Several image-enhancement techniques useful for morphological analysis of galactic or cometary images are described and compared. Such techniques can be used to search for, and investigate the properties of dust lanes, stellar disks or rings, jets, shells, tidal distortions, etc. Applications of the techniques are illustrated on CCD images of the peculiar galaxy Arp 230; this object has a rich morphology, indicative of a merger of two disk galaxies.

Djorgovski, S.↗

NASA NTRS1988

Mass loss from rapidly rotating magnetic protostars

It is proposed that bipolar outflows from young stellar objects originate from a protostar rotating at breakup at its equator because it is being spun up by an adjoining accretion disk. Mass outflow at an appreciable fraction of the infall rate from a surrounding molecular cloud core onto the star and disk can be driven centrifugally if the protostar has a sufficiently strong magnetic field. The expansion of the flow toward the rotational poles may provide a collimation mechanism for focusing an ordinary stellar wind into optical jets.

Shu, Frank H.↗

NASA NTRS1988

The formation of interstellar jets by the convergence of supersonic conical flows

The interaction of a stellar wind with a nonuniform environment leads, under the assumption of steady state, to the formation of an ovoidal cavity with acute ending tips. The stellar wind recollected by the walls of the cavity ends up being deposited at the tips. Here, it is shown that this focusing effect leads to the formation of a narrow cylindrical stream or 'jet' of reshocked stellar wind matter moving directly away from the star with a large velocity. A 'typical' T Tauri star may produce jets with densities of 1000-10,000/cu cm, velocities about 100 km/s, widths about 0.00001-0.001 pc over a length of 0.001-0.01 pc. The opening angles are 7 deg or less.

Canto, J.↗

NASA NTRS1994

Magnetocentrifugally driven flows from young stars and disks. 1: A generalized model

We propose a generalized model for stellar spin-down, disk accretion, and truncation, and the origin of winds, jets, and bipolar outflows from young stellar objects. We consider the steady state dynamics of accretion of matter from a viscous and imperfectly conducting disk onto a young star with a strong magnetic field. For an aligned stellar magnetosphere, shielding currents in the surface layers of the disk prevent stellar field lines from penetrating the disk everywhere except for a range of radii about pi = R(sub x), where the Keplerian angular speed of rotation Omega(sub x) equals the angular speed of the star Omega(sub *). For the low disk accretion rates and high magnetic fields associated with typical T Tauri stars, R(sub x) exceeds the radius of the star R(sub *) by a factor of a few, and the inner disk is effectively truncated at a radius R(sub t) somewhat smaller than R(sub x). Where the closed field lines between R(sub t) and R(sub x) bow sufficiently inward, the accreting gas attaches itself to the field and is funneled dynamically down the effective potential (gravitational plus centrifugal) onto the star. Contrary to common belief, the accompanying magnetic torques associated with this accreting gas may transfer angular momentum mostly to the disk rather than to the star. Thus, the star can spin slowly as long as R(sub x) remains significantly greater than R(sub *). Exterior to R(sub x) field lines threading the disk bow outward, which makes the gas off the mid-plane rotate at super-Keplerian velocities. This combination drives a magnetocentrifugal wind with a mass-loss rate M(sub w) equal to a definite fraction f of the disk accretion rate M(sub D). For high disk accretion rates, R(sub x) is forced down to the stellar surface, the star is spun to breakup, and the wind is generated in a manner identical to that proposed by Shu, Lizano, Ruden, & Najita in a previous communication to this journal. In two companion papers (II and III), we develop a detailed but idealized theory of the magnetocentrifugal acceleration process.

Shu, Frank↗

NASA NTRS2020

Status of NASA’s stellar astrometry testbeds for exoplanet detection: Science and technology overview

Accurate measurement of exoplanetary masses is a critical step in addressing key aspects of NASA's science vision. Measuring masses of Earth-analogs around FGK stars out to 10 pc requires sub-microarcsecond astrometric accuracy, which is not within the capabilities of current instrumentation. Thus, new technology will be required to build an astrometric instrument capable of achieving such performance. This will immediately empower the possibility for dedicated astrometric missions, and perhaps most enticing, it will enable astrometric observing modes to be added (with relatively low cost and impact) to any mission boasting a sufficiently stable direct imaging platform. In this paper, we provide an overview of the scientifi?c goals and technology utilized on two of NASA's astrometry testbeds dedicated to advancing stellar astrometry for exoplanet detection. The ?first one, located at the Jet Propulsion Laboratory (JPL), is dedicated to imaging stellar astrometry on sparse ?fields. The goal of this testbed is to mature the Diffractive Pupil (DP) technology to TRL-5, demonstrating high-?fidelity performance in a relevant environment. This testbed operates in a vacuum tank at the High Contrast Imaging Testbed (HCIT) at JPL. The second testbed, located at NASA Ames Research Center, is dedicated to advancing narrow angle relative astrometry to detect exoplanets around nearby binary stars. The key technology in this testbed is a DP specially designed to measure the angle between two sources on the sky. This testbed operates in air and aims to bring this technology to TRL-4.

Guyon, Olivier↗

NASA NTRS1987

Observations of jets from low-luminosity stars. II - SVS 12, HH 30, near HL Tauri, and HH 34

Spatially resolved spectral scans were obtained for four Herbig-Haro jets emanating from low-luminosity premain-sequence stars. There appears to be a general tendency for the excitation and electron density to diminish along these jets. For three jets, the electron density dependence is close to 1/r. HH 30's scattered stellar continuum showed much weaker Fe II lines in 1985 than in 1979, indicative of variable stellar activity. The most distant knot in HH 34's jet has the lowest excitation of any known HH object, with S II-line/H-alpha = 12.

Cohen, Martin↗

NASA NTRS1984

A wind-type model for the generation of astrophysical jets

Wind-type solutions for the generation of astrophysical jets from active galactic nuclei and stellar sources, such as those associated with SS 433 and protostellar objects, are discussed. Acceleration, collimation, and morphology are consistently interpreted in terms of a flow starting from the galactic or stellar core inside the 'throat' of a thick accretion disk.

Ferrari, A.↗

NASA NTRS1998

New Mysteries at Chiron

Considered as a comet, Chiron is unusual in two respects: (1) it exhibits outbursts at great distances from the sun (nearly up to its aphelion distance of 18.9 AU), and (2) its nucleus is much larger than any other known comet. It is similar in size, however, to the recently discovered Kuiper belt objects, leading to the conjecture that Chiron is closely related to these objects, but its chaotic orbit has brought it much closer to the sun. Our work with the Kuiper Airborne Observatory resulted in the first stellar occultation by Chiron observed simultaneously at visible and infrared wavelengths. We detected four features in the coma, with different degrees of certainty. Our conclusions about Chiron from this work and a previous stellar occultation are: (1) the jet-like features observed provide evidence that the coma material originates from just a few, small active areas, rather than uniform sublimation; (2) a bound coma has possibly been detected; (3) the particle radii in at least one of the jet-like features are larger than 0.25 gm; (4) material in Chiron's orbit plane has likely been detected; and (5) the radius of Chiron's nucleus lies between 83 and 156 km.

Elliot, James L.↗

NASA NTRS2000

Interferometry for X-Ray Astronomy

With direct imaging, the nature of distant astronomical objects and the physical mechanisms that control them can be constrained and understood. From Galileo's observations of the solar system, to Hubble Space Telescope's imaging of distant galaxies, improved astronomical imaging has always brought scientific understanding. The x-ray band of the spectrum, where exotic objects can have extremely high surface brightness, is ideally suited for high resolution imaging, but has lacked ultra-high quality telescopes. We report a practical x-ray interferometer that features high efficiency, affordable mirrors, adjustable baseline, and can be scaled to a full size observatory. Our prototype system, with just under one millimeter of baseline, created fringes at 1.25 keV with angular resolution of 100 milli-arcseconds. With a larger version of this interferometer in orbit it will be possible to resolve stars, black holes and other compact constituents of the universe. We can study the environments of pulsars, image and then model relativistic blast waves, study the space-time metric near the surface of a black hole, watch the physical formation of astrophysical jets, and study the dynamos of stellar coronae.

Cash, Webster↗

NASA NTRS2012

Comparison of Ejection Events in the Jet and Accretion Disc Outflows in 3C 111

We present a comparison of the parameters of accretion disc outflows and the jet of the broad-line radio galaxy 3C 111 on sub-pc scales. We make use of published X-ray observations of ultra-fast outflows (UFOs) and new 43 GHz VLBA images to track the jet knots ejection. We find that the superluminal jet coexists with the mildly relativistic outflows on sub-pc scales, possibly indicating a transverse stratification of a global flow. The two are roughly in pressure equilibrium, with the UFOs potentially providing additional support for the initial jet collimation. The UFOs are much more massive than the jet, but their kinetic power is probably about an order of magnitude lower, at least for the observations considered here. However, their momentum flux is equivalent and both of them are powerful enough to exert a concurrent feedback impact on the surrounding environment. A link between these components is naturally predicted in the context of MHD models for jet/outflow formation. However, given the high radiation throughput of AGNs, radiation pressure should also be taken into account. From the comparison with the long-term 2-10 keV RXTE light curve we find that the UFOs are preferentially detected during periods of increasing flux. We also find the possibility to place the UFOs within the known X-ray dips-jet ejection cycles, which has been shown to be a strong proof of the disc-jet connection, in analogue with stellar-mass black holes. However, given the limited number of observations presently available, these relations are only tentative and additional spectral monitoring is needed to test them conclusively.

Tombesi, F.↗

NASA NTRS2017

Significant and Variable Linear Polarization During the Prompt Optical Flash of GRB 160625B

Measurement of polarized light provides a direct probe of magnetic fields in collimated outflows (jets) of relativistic plasma from accreting stellar-mass black holes at cosmological distances. These outflows power brief and intense flashes of prompt gamma-rays known as Gamma Ray Bursts (GRBs), followed by longer-lived afterglow radiation detected across the electromagnetic spectrum. Rapid-response polarimetric observations of newly discovered GRBs have probed the initial afterglow phase. Linear polarization degrees as high as Π∼30% are detected minutes after the end of the prompt GRB emission, consistent with a stable, globally ordered magnetic field permeating the jet at large distances from the central source. In contrast, optical and gamma-ray observations during the prompt phase led to discordant and often controversial results, and no definitive conclusions on the origin of the prompt radiation or the configuration of the magnetic field could be derived. Here we report the detection of linear polarization of a prompt optical flash that accompanied the extremely energetic and long-lived prompt gamma-ray emission from GRB 160625B. Our measurements probe the structure of the magnetic field at an early stage of the GRB jet, closer to the central source, and show that the prompt GRB phase is produced via fast cooling synchrotron radiation in a large-scale magnetic field advected from the central black hole and distorted from dissipation processes within the jet.

High-energy astrophysics↗

NASA NTRS2017

Significant and Variable Linear Polarization During the Prompt Optical Flash of GRB 160625B

High-energy astrophysics↗

NASA NTRS1995

Models for classical gamma-ray bursts

The origin of gamma-ray bursts continues to be a great mystery. Here we review some relevant observations and a number of recent models. While no clear solution exists at the present time, the parameter space for Galactic halo models is becoming very constrained. Cosmological models on the other hand require both enormous total energy and the concentration of that energy into a small mass. This implies compact objects, probably accreting black holes of stellar size. We review some of the physics of such accreting black holes and point out that the jet formed from accretion into a rapidly accreting black hole of stellar mass might precess. This precession, coupled to beaming, could impose additional time structure on the burst and its spectrum. In the event of a 'failed' supernova model, the wind of the Wolf-Rayet star prior to the event could provide the beam dump where the jet generates gamma-rays. Enduring emission that grows harder with time might be expected for several hours as the density in the vicinity of the black hole declines.

Hartmann, D. H.↗