* Astronomy

Members Login
Post Info TOPIC: NGC7027


L

Posts: 131433
Date:
NGC 7027
Permalink  
 


Title: Modelling the physical and excitation conditions of the molecular envelope of NGC 7027
Authors: M. Santander-García, V. Bujarrabal, J. Alcolea

The link between the shaping of bipolar planetary nebulae and the mass ejection activity of their central stars is still poorly understood. Appropriately characterising the evolution of the shells ejected during the late stages of evolution is vital to gain insight into the mechanism of nebular shaping.
Herschel/HIFI provides an invaluable tool by opening a new window from which to probe warm molecular gas (~50-1000 K). We present a radiative-transfer, spatio-kinematic modelling of the molecular envelope of the young planetary nebula NGC 7027 in high- and low-J 12CO and 13CO transitions observed by Herschel/HIFI and IRAM 30-m, and discuss the structure and dynamics of the molecular envelope. We have developed a code which, used along with the existing SHAPE software, implements spatio-kinematic modelling with accurate non-LTE calculations of line excitation and radiative transfer in molecular species. We have used this code to build a relatively simple "Russian doll" model of the molecular envelope of NGC 7027.
The model nebula consists of four nested, mildly bipolar shells plus a pair of high-velocity blobs. The innermost shell is the thinnest and shows a significant jump in physical conditions (temperature, density, abundance and velocity) with respect to the adjacent shell. This is a clear indication of a shock front in the system. Each of the high-velocity blobs is divided into two sections with different physical conditions. The presence of H2O in NGC 7027, a C-rich nebula, is likely due to photo-induced chemistry from the hot central star. The computed molecular mass of the nebula is 1.3 solar masses, compatible with estimates from previous works.

Read more (230kb, PDF)



__________________


L

Posts: 131433
Date:
RE: NGC7027
Permalink  
 


Title: A spectral line survey of NGC 7027 at millimeter wavelengths
Authors: Yong Zhang, Sun Kwok, Dinh-V-Trung

We report on a recent spectral line survey of the planetary nebula (PN) NGC 7027 using the Arizona Radio Observatory (ARO) 12m telescope and the Heinrich Hertz Submillimeter Telescope (SMT) at millimetre wavelengths. The spectra covering the frequency ranges 71--111GHz, 157--161GHz, and 218--267GHz were obtained with a typical sensitivity of rms<8mK. A total of 67 spectral lines are detected, 21 of which are identified with 8 molecular species, 32 with recombination lines from hydrogen and helium, and 14 remains unidentified. As the widths of emission lines from CO, other neutral molecules, molecular ions, as well as recombination of H+ and He+ are found to be different with each other, the line strengths and profiles are used to investigate the physical conditions and chemical processes of the neutral envelope of NGC 7027. The column densities and fractional abundances relative to H2 of the observed molecular species are calculated and compared with predictions from chemical models. We found evidence for overabundance of N2H+ and underabundance of CS and HNC in NGC 7027, suggesting that X-ray emission and shock wave may play an important role in the chemistry of the hot molecular envelope of the young PN.

Read more (677kb, PDF)

__________________


L

Posts: 131433
Date:
Permalink  
 

Title: The Evolution of NGC 7027 at Radio Frequencies: A New Determination of the Distance and Core Mass
Authors: Albert Zijlstra, P.A.M. van Hoof, R.A. Perley

We present the results of a 25-year program to monitor the radio flux evolution of the planetary nebula NGC7027. We find significant evolution of the spectral flux densities. The flux density at 1465 MHz, where the nebula is optically thick, is increasing at a rate of 0.251±0.015 % per year, caused by the expansion of the ionised nebula. At frequencies where the emission is optically thin, the spectral flux density is changing at a rate of -0.145±0.005 % per year, caused by a decrease in the number of ionising photons coming from the central star. A distance of 980±100 pc is derived. By fitting interpolated models of post-AGB evolution to the observed changes, we find that over the 25-yr monitoring period, the stellar temperature has increased by 3900±900 K and the stellar bolometric luminosity has decreased by 1.75±0.38 %. We derive a distance-independent stellar mass of 0.655+-0.01 solar masses adopting the Bloecker stellar evolution models, or about 0.04 solar masses higher when using models of Vassiliadis & Wood which may provide a better fit. A Cloudy photoionisation model is used to fit all epochs at all frequencies simultaneously. The differences between the radio flux density predictions and the observed values show some time-independent residuals of typically 1 %. A possible explanation is inaccuracies in the radio flux scale of Baars et al. We propose an adjustment to the flux density scale of the primary radio flux calibrator 3C286, based on the Cloudy model of NGC7027. We also calculate precise flux densities for NGC7027 for all standard continuum bands used at the VLA, as well as for some new 30GHz experiments.

Read more (179kb, PDF)

__________________
Page 1 of 1  sorted by
Quick Reply

Please log in to post quick replies.



Create your own FREE Forum
Report Abuse
Powered by ActiveBoard