arXiv subjects

A. Raga

Publications and source records attributed to A. Raga.

22 records · Page 2Linked to original sources

arXiv2002

Unveiling the structure of the planetary nebula M 2-48: Kinematics and physical conditions

The kinematics and physical conditions of the bipolar planetary nebula M 2-48 are analysed from high and low dispersion long-slit spectra. Previous CCD narrow-band optical observations have suggested that this nebula is mainly formed by a pair of symmetric bow-shocks, an off-center semi-circular shell, and an internal bipolar structure. The bipolar outflow has a complex structure, characterised by a series of shocked regions located between the bright core and the polar tips. There is an apparent kinematic discontinuity between the bright bipolar core and the outer regions. The fragmented ring around the bright bipolar region presents a low expansion velocity and could be associated to ejection in the AGB-PN transition phase, although its nature remains unclear. The chemical abundances of the central region are derived, showing that M 2-48 is a Type I planetary nebula (PN).

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arXiv1998

A 3-mode, Variable Velocity Jet Model for HH 34

Variable ejection velocity jet models can qualitatively explain the appearance of successive working surfaces in Herbig-Haro (HH) jets. This paper presents an attempt to explore which features of the HH-34 jet can indeed be reproduced by such a model. From previously published data on this object, we find evidence for the existence of a 3-mode ejection velocity variability, and then explore the implications of such a variability. From simple, analytic considerations it is possible to show that the longer period modes produce a modulation on the shorter period modes, resulting in the formation of ``trains'' of multiple knots. The knots observed close to the source of HH-34 could correspond to such a structure. Finally, a numerical simulation with the ejection velocity variability deduced from the HH-34 data is computed. This numerical simulation shows a quite remarkable resemblance with the observed properties of the HH-34 jet.

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arXiv1997

Highly Collimated Molecular Hydrogen Jets Near IRAS 05487+0255: NIR Imaging and Spectroscopy

We present new narrow-band near-infrared images together with K band spectra of highly collimated bipolar jets close to the IRAS 05487+0255 source. The jets are located at 50" West of the Herbig-Haro 110 outflow. The jets are not visible at optical wavelengths, and therefore, do not fall into the `standard' Herbig-Haro object classification scheme. Nevertheless, they belong to an ever growing group of molecular hydrogen jets associated with YSOs which are optically undetected. The jets are very well collimated, with a length-to-width ratio of 10-20. The spectra of the jet and counter-jet in the K-band show a limited number of molecular hydrogen emission lines which makes it difficult to obtain an accurate excitation temperature. We estimate Tex = 1104+/-67 K and Tex = 920+/- 156 K for the red and blue jet components respectively. The radial velocities of the jet and counter-jet, based on the shift of the (1,0) S(1) 2.121 micron line, are -275+/- 50 km/s and 180+/- 50 km/s respectively, suggesting an angle of 30 to 45 degrees between the jet and the line of sight. The molecular hydrogen emission of the entire jet extends for at least 40" or 0.1 pc at the distance of Orion. If the flow velocity is comparable to that of the radial velocities, then the dynamical age of the system is quite short (about 500 yrs), consistent with a young jet arising from an embedded source. Entrainment in a turbulent mixing layer may explain this morphology and spectral character.

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arXiv1996

An Axisymmetric, Radiative Bow Shock Model with a Realistic Treatment of Ionization and Cooling

We have chosen a reduced set of 18 ionization rate equations (for ions of H, C, N, O, S and Ne), which allow us to obtain a moderately accurate estimate of the non-equilibrium radiative cooling function. We evaluate the accuracy of this approach by comparing our cooling function with previous calculations, computed with a more extended set of ions, for the case of gas that cools from a high temperature at constant density. We also compute steady, plane shock models, which we find to compare well with models calculated with much more detailed microphysics. Using our reduced set of rate equations, we present a simulation of a radiative bow shock formed by a supersonic, plane stream impinging on a rigid sphere. The parameters for the calculation are chosen as to approximately represent typical values found for Herbig-Haro objects, and to give a cooling distance to bow shock radius ratio of 1/10. This simulation is done with an adaptive grid code, which allows a reasonably high resolution (with greater than or equal to 25 points) of the standoff distance between the bow shock and the rigid obstacle. Contrary to the standard expectation, we find that the bow shock standoff distance in the stagnation region is considerably shorter than the cooling distance behind a plane-parallel shock. Also, the centrifugal pressure is found to play an important role in determining the structure of the recombin- ation region. This appears to partially invalidate previous bow shock models based on a ``quasi-1D'' approach, at least for the particular parameters chosen for the present simulation. Finally, we present tabulations of the cooling rates that have been used (for the different species), in order to facilitate the inclusion of this treatment in other gasdynamic codes.

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