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Register a new userDeciphering the 3-D Orion Nebula-II: A low-ionization region of multiple velocity components southwest of Theta1OriC confounds interpretation of low velocity resolution studies of temperature, density, and abundance
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We establish that there are two velocity systems along lines-of-sight that contribute to the emission-line spectrum of the the brightest parts of the Orion Nebula. These overlie the Orion-S embedded molecular cloud southwest of the dominant ionizing star (Theta1OriC). Examination of 10x10 samples of high spectral resolution emission-line spectra of this region reveals it to be of low ionization, with velocities and ionization different from the central part of the Nebula. These properties jeopardize earlier determinations of abundance and physical conditions since they indicate that this region is much more complex than has been assumed in analyzing earlier spectroscopic studies and argue for use of very high spectral resolution or known simple regions in future studies.
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Based on imaging and spectroscopic data, we develop a 3-D model for the Huygens Region of the Orion Nebula. Theta1OriC , the hottest star in the Trapezium, is surrounded by a wind-blown Central Bubble that opens SW into the Extended Orion Nebula. Outside of this feature lies a layer of ionized gas at about 0.4 pc from Theta1OriC. Both of these features are moving rapidly away from Theta1OriC with an expansion age for the Central Bubble of only 15,000 yrs.
In order to determine if the material ablated from high-velocity clouds (HVCs) is a significant source of low-velocity high ions (C IV, N V, and O VI) such as those found in the Galactic halo, we simulate the hydrodynamics of the gas and the time-dependent ionization evolution of its carbon, nitrogen, and oxygen ions. Our suite of simulations examines the ablation of warm material from clouds of various sizes, densities, and velocities as they pass through the hot Galactic halo. The ablated material mixes with the environmental gas, producing an intermediate-temperature mixture that is rich in high ions and that slows to the speed of the surrounding gas. We find that the slow mixed material is a significant source of the low-velocity O VI that is observed in the halo, as it can account for at least ~1/3 of the observed O VI column density. Hence, any complete model of the high ions in the halo should include the contribution to the O VI from ablated HVC material. However, such material is unlikely to be a major source of the observed C IV, presumably because the observed C IV is affected by photoionization, which our models do not include. We discuss a composite model that includes contributions from HVCs, supernova remnants, a cooling Galactic fountain, and photoionization by an external radiation field. By design, this model matches the observed O VI column density. This model can also account for most or all of the observed C IV, but only half of the observed N V.
We present results from integral field optical spectroscopy with the Potsdam Multi-Aperture Spectrograph of the Herbig-Haro (HH) object HH 204, with a spatial sampling of 1 x 1 arcsec^2. We have obtained maps of different emission lines, physical conditions and ionic abundances from collisionally excited lines. The ionization structure of the object indicates that the head of the bow shock is optically thick and has developed a trapped ionization front. The density at the head is at least five times larger than in the background ionized gas. We discover a narrow arc of high T_e([N II]) values delineating the southeast edge of the head. The temperature in this zone is about 1,000 K higher than in the rest of the field and should correspond to a shock-heated zone at the leading working surface of the gas flow. This is the first time this kind of feature is observed in a photoionized HH object. We find that the O^+ and O abundance maps show anomalous values at separate areas of the bow shock probably due to: a) overestimation of the collisional de-excitation effects of the [O II] lines in the compressed gas at the head of the bow shock, and b) the use of a too high T_e([N II]) at the area of the leading working surface of the flow.
We have extended the membership and determined the 3-D structure of the large (0.19 pc) HH~269 sequence of shocks in the Orion Nebula. All of the components lie along a track that is highly tilted to the plane-of-the-sky and emerge from within the Orion-S embedded molecular cloud. Their source is probably either the highly obscured mm 9 source associated with a high N2H+ density core (more likely) or the more distant star COUP 632 (less likely). The former must be located in the Photon Dominated Region (PDR) underlying the ionized surface of the Orion South Cloud, while the latter would be embedded within the cloud. The flows seem to be episodic, with intervals of 1900 to 2600 years or 700 to 2600 years if COUP 632 is the source.
We have used widely spaced in time Hubble Space Telescope images to determine tangential velocities of features associated with outflows from young stars. These observations were supplemented by groundbased telescope spectroscopy and from the resultant radial velocities, space velocities were determined for many outflows. Numerous new moving features were found and grouped into known and newly assigned Herbig Haro objects. It was found that stellar outflow is highly discontinuous, as frequently is the case, with long-term gaps of a few hundred years and that these outflow periods are marked by staccato bursts over periods of about ten years. Although this has been observed in other regions, the Orion Nebula Cluster presents the richest display of this property. Most of the large scale Herbig Haro objects in the brightest part of the Orion Nebula appear to originate from a small region northeast of the strong Orion-S radio and infrared sources. With the possible exception of HH 203, we are not able to identify specific stellar sources, but do identify candidate sources for several other bright Herbig Haro objects. We find that there are optical features in the BN-KL region that can be related to the known large scale outflow that originates there. We find additional evidence for this outflow originating 500 to 1000 years ago.
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