Velocity-resolved Images Research Articles

The extremely sharp transition between molecular and ionized gas in the Horsehead nebula

Massive stars can determine the evolution of molecular clouds by eroding and photo-evaporating their surfaces with strong ultraviolet (UV) radiation fields. Moreover, UV radiation is relevant in setting the thermal gas pressure in star-forming clouds, whose influence can extend across various spatial scales, from the rims of molecular clouds to entire star-forming galaxies. Probing the fundamental structure of nearby molecular clouds is therefore crucial to understand how massive stars shape their surrounding medium and how fast molecular clouds are destroyed, specifically at their UV-illuminated edges, where models predict an intermediate zone of neutral atomic gas between the molecular cloud and the surrounding ionized gas whose size is directly related to the exposed physical conditions. We present the highest angular resolution (~0.″5, corresponding to 207 au) and velocity-resolved images of the molecular gas emission in the Horsehead nebula, using CO J = 3–2 and HCO+ J = 4−3 observations with the Atacama Large Millimeter/submillimeter Array (ALMA). We find that CO and HCO+ are present at the edge of the cloud, very close to the ionization (H+/H) and dissociation fronts (H/H2), suggesting a very thin layer of neutral atomic gas (<650 au) and a small amount of CO-dark gas (AV = 0.006–0.26 mag) for stellar UV illumination conditions typical of molecular clouds in the Milky Way. The new ALMA observations reveal a web of molecular gas filaments with an estimated thermal gas pressure of Pth = (2.3 – 4.0) × 106 K cm−3, and the presence of a steep density gradient at the cloud edge that can be well explained by stationary isobaric photo-dissociation region (PDR) models with pressures consistent with our estimations. However, in the H II region and PDR interface, we find Pth,PDR > Pth,H II suggesting the gas is slightly compressed. Therefore, dynamical effects cannot be completely ruled out and even higher angular observations will be needed to unveil their role.

Gas dynamics in the inner few AU around the Herbig B[e] star MWC297

We present near-infrared AMBER (R = 12, 000) and CRIRES (R = 100, 000) observations of the Herbig B[e] star MWC297 in the hydrogen Br-gamma-line. Using the VLTI unit telescopes, we obtained a uv-coverage suitable for aperture synthesis imaging. We interpret our velocity-resolved images as well as the derived two-dimensional photocenter displacement vectors, and fit kinematic models to our visibility and phase data in order to constrain the gas velocity field on sub-AU scales. The measured continuum visibilities constrain the orientation of the near-infrared-emitting dust disk, where we determine that the disk major axis is oriented along a position angle of 99.6 +/- 4.8 degrees. The near-infrared continuum emission is 3.6 times more compact than the expected dust-sublimation radius, possibly indicating the presence of highly refractory dust grains or optically thick gas emission in the inner disk. Our velocity-resolved channel maps and moment maps reveal the motion of the Br-gamma-emitting gas in six velocity channels, marking the first time that kinematic effects in the sub-AU inner regions of a protoplanetary disk could be directly imaged. We find a rotation-dominated velocity field, where the blue- and red-shifted emissions are displaced along a position angle of 24 +/- 3 degrees and the approaching part of the disk is offset west of the star. The visibility drop in the line as well as the strong non-zero phase signals can be modeled reasonably well assuming a Keplerian velocity field, although this model is not able to explain the 3 sigma difference that we measure between the position angle of the line photocenters and the position angle of the dust disk. We find that the fit can be improved by adding an outflowing component to the velocity field, as inspired by a magneto-centrifugal disk-wind scenario.

Imaging the spinning gas and dust in the disc around the supergiant A[e] star HD 62623

Context. To progress in the understanding of evolution of massive stars one needs to constrain the mass-loss and determine the phenomenon responsible for the ejection of matter an its reorganization in the circumstellar environment Aims. In order to test various mass-ejection processes, we probed the geometry and kinematics of the dust and gas surrounding the A[e] supergiant HD 62623. Methods. We used the combined high spectral and spatial resolution covered by the VLTI/AMBER instrument. Thanks to a new multiwavelength optical/IR interferometry imaging technique, we reconstructed the first velocity-resolved images with a milliarcsecond resolution in the infrared domain. Results. We managed to disentangle the dust and gas emission in the HD 62623 circumstellar disc.We measured the dusty disc inner inner rim, i.e. 6 mas, constrained the inclination angle and the position angle of the major-axis of the disc.We also measured the inner gaseous disc extension (2 mas) and probed its velocity field thanks to AMBER high spectral resolution. We find that the expansion velocity is negligible, and that Keplerian rotation is a favoured velocity field. Such a velocity field is unexpected if fast rotation of the central star alone is the main mechanism of matter ejection. Conclusions. As the star itself seems to rotate below its breakup-up velocity, rotation cannot explain the formation of the dense equatorial disc. Moreover, as the expansion velocity is negligible, radiatively driven wind is also not a suitable explanation to explain the disc formation. Consequently, the most probable hypothesis is that the accumulation of matter in the equatorial plane is due to the presence of the spectroscopic low mass companion.

Velocity Structure in the Orion Nebula. I. Spectral Mapping in Low-Ionization Lines

High-dispersion echelle spectroscopy in optical forbidden lines of O^0, S^+, and S^2+ is used to construct velocity-resolved images and electron density maps of the inner region of the Orion nebula with a resolution of 10 km s-1 x 3" x 2". Among the objects and regions newly discovered in this study are (1) the Diffuse Blue Layer: an extended layer of moderately blue-shifted, low-density, low-ionization emission in the southeast region of the nebula; (2) the Red Bay: a region to the east of the Trapezium where the usual correlation between velocity and ionization potential is very weak, and where the emitting layer is very thick; and (3) HH 873: a new redshifted jet to the southwest of the Trapezium.

Kinematics of Optical Outflows in the Orion Nebula. I. The Giant Outflow HH 400 and the Irradiated Jet HH 502

We present narrowband filter and high-resolution, velocity-resolved Fabry-Perot images of outflows in the southern portion of the Orion Nebula. HH 400 is a giant, low-velocity, redshifted bow shock located about 10' south of the core of the Orion Nebula. Its axis of symmetry passes close to the Trapezium cluster 1.5 pc to the north. The most likely point of origin is a young stellar object in the OMC-1 cloud core. HH 400 may be the remnant of a parsec-scale bipolar outflow powered by one of the young stars forming within this region. The radial velocity of gas in the limb-brightened rim of HH 400 is low, with redshifted speeds ranging from 8 to 20 km s-1 with respect to the emission from the Orion Nebula. The shape of the bow indicates that it lies close to the plane of the sky. For an inclination angle of 30° to the plane of the sky and assuming that the plasma is flowing mostly along the axis of symmetry, the visible gas at the rim of HH 400 has a mean velocity of about 30 km s-1, a mass of about 3 × 10-2 M⊙, and a dynamical age of about 5 × 104 yr, assuming the source lies in the OMC-1 cloud core. The estimated mass flux in the HH 400 bow is about 10-6 M⊙ yr-1. The bent bipolar irradiated jet HH 502 is superposed on the western rim of HH 400. We resolve the spatial and velocity structure of the jet and its multiple bow shocks. The jet consists of a chain of photoionized segments separated by wide gaps; bow shocks lie at the leading edges of these jet segments. The mean radial velocities of the jet segments decrease with increasing distance from the source. The large radial velocity dispersions of the gas at the tips of the HH 502 internal working surfaces and the small spreading angles of the HH 502 jet segments, combined with their low radial velocities, indicate that this flow lies close to the plane of the sky. Assuming that the jet is fully ionized, that it spreads at the Mach angle, and that the internal sound speed in the photoionized gas is about 10 km s-1, the jet must have a space velocity of about 400 km s-1. Finally, we present velocity-resolved images of the bow shocks in HH 540, a flow that may originate from the large protoplanetary disk 181-826. Several additional high-velocity features identified in the Fabry-Perot data trace additional jets and outflows in this portion of the Orion Nebula.

New Observations of LBV Environments

Two observational programs which provide new information about particular LBVs through investigations of their immediate surroundings are described. (1) Digital spectral classification of OB supergiants in compact groups apparently associated with Radcliffe 127 and S Doradus has revealed several interesting objects and indicates which of them are likely to be generically related to the LBVs. (2) Velocity-resolved images of the Eta Carinae shell show qualitatively new features, which will contribute substantially to the interpretation of its complex spatial/kinematical structure.

Velocity-resolved images of molecular hydrogen in the colliding galaxy NGC 6240 - Evidence of a global shock

view Abstract Citations (40) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Velocity-Resolved Images of Molecular Hydrogen in the Colliding Galaxy NGC 6240: Evidence of a Global Shock Herbst, T. M. ; Graham, J. R. ; Beckwith, S. ; Tsutsui, K. ; Soifer, B. T. ; Matthews, K. Abstract Images of the nuclear region of NGC 6240 taken in the 2.122 micron, ν= 1-0 S(1) quadrupole line of molecular hydrogen demonstrate that the line emission is extended and distinct from the two nuclei producing most of the continuum emission. A region ~ 2" southwest of the southern nucleus produces almost 20% of the total line radiation, yet has no detectable 2 micron continuum. The H_2_ lines are asymmetric, with very little emission on the short-wavelength side of the line peaks. These results indicate that a global phenomenon, such as the dissipative collision of two disks of molecular material, is taking place. The presence of shock-excited material at the same velocity throughout the central kpc of the galaxy and the asymmetry in the S(1) line profile support this conclusion. Global shocks may power the far-infrared luminosity of NGC 6240 as well; if so, the strong H_2_ and infrared emission can persist for no more than ~ 10^6^ yr. Publication: The Astronomical Journal Pub Date: June 1990 DOI: 10.1086/115455 Bibcode: 1990AJ.....99.1773H Keywords: Galactic Nuclei; Hydrogen; Interacting Galaxies; Interstellar Gas; Emission Spectra; Far Infrared Radiation; Resonance Fluorescence; Shock Waves; Starburst Galaxies; Astrophysics; GALAXIES: INTERACTIONS; GALAXIES: INDIVIDUAL full text sources ADS | data products SIMBAD (5) NED (1)