Karl-Heinz Böhm

TIME-DEPENDENT ACCRETION BY MAGNETIC YOUNG STELLAR OBJECTS AS A LAUNCHING MECHANISM FOR STELLAR JETS

A time-dependent jet launching and collimating mechanism is presented. Initial results of numerical simulations of the interaction between an aligned dipole rotator and a conducting circumstellar accretion disk that is initially threaded by the dipole field show that differential rotation between the disk and the star leads to the rapid expansion of the magnetic loops that connect the star to the disk. The expansion of these magnetic loops above and below the disk produces a two-component outflow. A hot, well-collimated outflow is generated by the convergent flow of attached plasma toward the rotation axis, while a cool, slower outflow is produced on the disk side of the expanding loop. The expanding loop, which later forms a plasmoid, defines the boundary between the jetlike flow and the disk wind. Episodic magnetic reconnection above and below the disk releases the jet plasma from the system and allows the process to repeat, reinforcing the hot, well-collimated outflow.

Jets from Accreting Magnetic Young Stellar Objects. I. Comparison of Observations and High-Resolution Simulation Results

High-resolution numerical magnetohydrodynamic (MHD) simulations of a new model for the formation of jets from magnetic accreting young stellar objects (YSOs) are presented and compared with observations. The simulation results corroborate a previously laid out conceptual mechanism for forming jets, wherein the interaction of the stellar magnetosphere with a surrounding accretion disk leads to an outflow. The high resolution of the numerical simulation allows optical condensations, which form in the region close to the star to be seen. The optical condensations and the episodic behavior of the jet are effects that are inherent to the jet-launching mechanism itself. A disk wind arises as well. The simulated outflow is compared with observations, and it is shown that simulated images in the forbidden lines [S II] λλ6716+6731 have morphology consistent with recent observations of the jet source HH 30. Furthermore, velocity spectra of the simulated outflow in [S II] λλ6716+6731 and mass weighted by n clearly show a two-component outflow, in agreement with observed outflows from T Tauri stars such as DG Tauri. The mechanism produces a highly collimated fast jet and a slower disk wind. While the match between existing observations and the simulated system are not perfect (the time- and size scales of the jet differ from those in HH 30 by an order of magnitude), the morphology associated with both imagery and velocity spectra of the jet are matched well. A companion paper lays out the physics that control the timescale for knot production and defines the controlling parameters of the jet-launching mechanism in general.

Simulation‐based Investigation of a Model for the Interaction between Stellar Magnetospheres and Circumstellar Accretion Disks

We examine, parametrically, the interaction between the magnetosphere of a rotating young stellar object and a circumstellar accretion disk using 2.5-dimensional (cylindrically symmetric) numerical magnetohydrodynamic simulations. The interaction drives a collimated outflow, and we find that the jet formation mechanism is robust. For variations in initial disk density of a factor of 16, variations of stellar dipole strength of a factor of 4, and various initial conditions with respect to the disk truncation radius and the existence of a disk field, outflows with similar morphologies were consistently produced. Second, the system is self-regulating, where the outflow properties depend relatively weakly on the parameters above. The large-scale magnetic field structure rapidly evolves to a configuration that removes angular momentum from the disk at a rate that depends most strongly on the field and weakly on the rotation rate of the footpoints of the field in the disk and the mass outflow rate. Third, the simulated jets are episodic, with the timescale of jet outbursts identical to the timescale of magnetically induced oscillations of the inner edge of the disk. To better understand the physics controlling these disk oscillations, we present a semianalytical model and confirm that the oscillation period is set by the spin-down rate of the disk inner edge. Finally, our simulations offer strong evidence that it is indeed the interaction of the stellar magnetosphere with the disk, rather than some primordial field in the disk itself, that is responsible for the formation of jets from these systems.

Collimation of a central wind by a disc-associated magnetic field

We present the results of time-dependent, numerical magnetohydrodynamic simulations of a realistic young stellar object outflow model with the addition of a disk-associated magnetic field. The outflow produced by the magnetic star-disk interaction consists of an episodic jet plus a wide-angle wind with an outflow speed comparable to that of the jet (100–200 km s-1). An initially vertical field of ≪ 0.1 Gauss, embedded in the disk, has little effect on the wind launching mechanism, but we show that it collimates the entire flow (jet + wide wind) at large (several AU) distances. The collimation does not depend on the polarity of the vertical field. We also discuss the possible origin of the disk-associated field.

An interpretation of observations of HH 1 in terms of a time-dependent bow-shock model

The Herbig-Haro (H-H) object HH 1 shows a structure of flow-contrast condensations embedded in a more diffuse background. In a previous paper, it was suggested that it might be possible to explain this structure in terms of a time-dependent, radiating bow-shock model. The paper presents a more complete comparison between the predictions from these models and: (1) the observed proper motions (Herbig and Jones 1981), (2) monochromatic H-alpha and forbidden O III 5007 images, and (3) H-alpha position-velocity diagrams obtained for HH 1. This comparison shows that many of the observational characteristics of HH 1 can indeed be explained with models of a single, time-dependent, radiating bow shock. It appears that it would be more difficult to explain some of these observational results with models in which each condensation is assumed to represent a separate bow shock. 31 references.

A Variable-Velocity, Precessing Jet Model for HH 32

HH 32 has a bright, strongly redshifted lobe with a system of scattered condensations. We propose that these condensations correspond to internal working surfaces in a variable ejection velocity, precessing jet. From a three-dimensional numerical simulation, we obtain predictions of [O II] λ3726+λ3729, [O III] λ5007, [O I] λ6300, Hα, [N II] λ6583, and [S II] λ6716+λ6731 intensity maps, for an orientation angle of ~70° between the outflow axis and the plane of the sky (as appropriate for HH 32). We also obtain predictions of radial velocity channel maps. We then compare these predictions with previously published observations of HH 32 and discuss the strengths and weaknesses of the model.

Kinematics of the HH 111 Jet from the Space Telescope Imaging Spectrograph

We present a long-slit spectrum of the Herbig-Haro object HH 111, obtained with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. This spectrum has a spectral resolution of ≈50 km s-1, so it gives a good picture of the kinematical properties of the observed object at very high angular resolution. We find that some of the knots along the jet are associated with sudden drops in the radial velocity (modulus), confirming that the emission from the knots is formed in shocks. We interpret the observed position-velocity diagrams in terms of a model of a jet from a variable source, and we attempt to carry out a reconstruction of the ejection velocity variability necessary for reproducing the observed kinematical structure.

Optical and Near-Infrared Study of the Cepheus E Outflow, A Very Low-Excitation Object

We present images and spectra of the Cepheus E (Cep E) region at both optical and infrared wavelengths. Only the brightest region of the southern lobe of the Cep E outflow reveals optical emission, suggesting that the extinction close to the outflow source plays an important role in the observed difference between the optical and IR morphologies. Cep E is a unique object since it provides a link between the spectroscopic properties of the optical Herbig-Haro (HH) objects and those of deeply embedded outflows. The observed H2 infrared lines allow us to determine an excitation temperature of ~2300 K, an Ortho-to-Para ratio of ~3, and an H2 (1, 0)/(2, 1) S(1) line ratio of ~9. These results are consistent with the values observed for HH objects with detected NIR emission lines, with shock excitation as the main mechanism for their formation, and also with the values observed for embedded, NIR flows. The optical spectroscopic characteristics of Cep E (HH 377) appear to be similar to the ones of low-excitation HH objects. However, the electron density determined from the [S II] λλ6731/6717 line ratio for this object (ne = 4100 cm-3), and the [O I] λ6300/Hα, [S II] λλ(6717 + 6731)/Hα ratios are higher than the values of all of the previously studied low-excitation HH objects. This result is likely to be the consequence of an anomalously high environmental density in the HH 377 outflow. The ionization fraction obtained for HH 377 is xe ~ 1%. From this result, together with the observed [O I] λ6300/Hα line ratio, we conclude that the observed Hα line emission is collisionally excited. From a comparison with shock models, we also conclude that the extinction toward HH 377 is very low. Comparing the observed Hβ and Hα fluxes of HH 377 with model predictions, we determine a shock speed between 15 and 20 km s-1, although somewhat higher velocities also produce spectra with line ratios that qualitatively agree with the observations of HH 377.

Collimation of a Central Wind by a Disk-Associated Magnetic Field

We present the results of time-dependent, numerical magnetohydrodynamic simulations of a realistic young stellar object outflow model with the addition of a disk-associated magnetic field. The outflow produced by the magnetic star-disk interaction consists of an episodic jet plus a wide-angle wind with an outflow speed comparable to that of the jet (100–200 km s-1). An initially vertical field of ≪ 0.1 Gauss, embedded in the disk, has little effect on the wind launching mechanism, but we show that it collimates the entire flow (jet + wide wind) at large (several AU) distances. The collimation does not depend on the polarity of the vertical field. We also discuss the possible origin of the disk-associated field.

The electron density and temperature distributions predicted by bow shock models of Herbig-Haro objects

The observable spatial electron density and temperature distributions for series of simple bow shock models, which are of special interest in the study of Herbig-Haro (H-H) objects are computed. The spatial electron density and temperature distributions are derived from forbidden line ratios. It should be possible to use these results to recognize whether an observed electron density or temperature distribution can be attributed to a bow shock, as is the case in some Herbig-Haro objects. As an example, the empirical and predicted distributions for H-H 1 are compared. The predicted electron temperature distributions give the correct temperature range and they show very good diagnostic possibilities if the forbidden O III (4959 + 5007)/4363 wavelength ratio is used. 44 refs.