Herschel/HIFI observations of molecular emission in protoplanetary nebulae and young planetary nebulae⋆,⋆⋆
1 Observatorio Astronómico Nacional (IGN), Ap 112, 28803 Alcalá de Henares, Spain
2 Observatorio Astronómico Nacional (IGN), Alfonso XII N ◦ 3, 28014 Madrid, Spain
3 European Space Astronomy Centre, ESA, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
4 CAB, INTA-CSIC, Ctra de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
5 Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
6 Sterrenkundig Instituut Anton Pannekoek, University of Amsterdam, Science Park 904, 1098 Amsterdam, The Netherlands
7 Onsala Space Observatory, Dept. of Radio and Space Science, Chalmers University of Technology, 43992 Onsala, Sweden
8 Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
9 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
10 The Johns Hopkins University, 3400 North Charles St, Baltimore, MD 21218, USA
11 N. Copernicus Astronomical Center, Rabiańska 8, 87-100 Toruń, Poland
12 Joint ALMA Observatory, El Golf 40, Las Condes, Santiago, Chile
13 Department of Astrophysics/IMAPP, Radboud University Nijmegen, Nijmegen, The Netherlands
14 Astronomical Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands
15 Department of Astronomy, AlbaNova University Center, Stockholm University, 10691 Stockholm, Sweden
Received: 7 July 2011
Accepted: 27 September 2011
Aims. We aim to study the physical conditions, particularly the excitation state, of the intermediate-temperature gas in protoplanetary nebulae and young planetary nebulae (PPNe, PNe). The information that the observations of the different components deliver is of particular importance for understanding the evolution of these objects.
Methods. We performed Herschel/HIFI observations of intermediate-excitation molecular lines in the far-infrared/submillimeter range in a sample of ten nebulae. The high spectral resolution provided by HIFI allows the accurate measurement of the line profiles. The dynamics and evolution of these nebulae are known to result from the presence of several gas components, notably fast bipolar outflows and slow shells (that often are the fossil AGB shells), and the interaction between them. Because of the diverse kinematic properties of the different components, their emissions can be identified in the line profiles. The observation of these high-energy transitions allows an accurate study of the excitation conditions, particularly in the warm gas, which cannot be properly studied from the low-energy lines.
Results. We have detected FIR/sub-mm lines of several molecules, in particular of 12CO, 13CO, and H2O. Emission from other species, like NH3, OH, H218O, HCN, SiO, etc., has been also detected. Wide profiles showing sometimes spectacular line wings have been found. We have mainly studied the excitation properties of the high-velocity emission, which is known to come from fast bipolar outflows. From comparison with general theoretical predictions, we find that CRL 618 shows a particularly warm fast wind, with characteristic kinetic temperature Tk ≳ 200 K. In contrast, the fast winds in OH 231.8+4.2 and NGC 6302 are cold, Tk ~ 30 K. Other nebulae, like CRL 2688, show intermediate temperatures, with characteristic values around 100 K. We also discuss how the complex structure of the nebulae can affect our estimates, considering two-component models. We argue that the differences in temperature in the different nebulae can be caused by cooling after the gas acceleration (that is probably caused by shocks); for instance, CRL 618 is a case of very recent acceleration, less than ~100 yr ago, while the fast gas in OH 231.8+4.2 was accelerated ~1000 yr ago. We also find indications that the densest gas tends to be cooler, which may be explained by the expected increase of the radiative cooling efficiency with the density.
Key words: stars: AGB and post-AGB / circumstellar matter / stars: mass-loss / planetary nebulae: general
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
Appendix A is available in electronic form at http://www.aanda.org
© ESO, 2012