Protostellar holes: Spitzer Space Telescope observations of the protostellar binary IRAS16293-2422

Mid-infrared (23-35 micron) emission from the deeply embedded "Class 0" protostar IRAS16293-2422 is detected with the Spitzer Space Telescope infrared spectrograph. A detailed radiative transfer model reproducing the full spectral energy distribution (SED) from 23 micron to 1.3 mm requires a large inner cavity of radius 600 AU in the envelope to avoid quenching the emission from the central sources. This is consistent with a previous suggestion based on high angular resolution millimeter interferometric data. An alternative interpretation using a 2D model of the envelope with an outflow cavity can reproduce the SED but not the interferometer visibilities. The cavity size is comparable to the centrifugal radius of the envelope and therefore appears to be a natural consequence of the rotation of the protostellar core, which has also caused the fragmentation leading to the central protostellar binary. With a large cavity such as required by the data, the average temperature at a given radius does not increase above 60-80 K and although hot spots with higher temperatures may be present close to each protostar, these constitute a small fraction of the material in the inner envelope. The proposed cavity will also have consequences for the interpretation of molecular line data, especially of complex species probing high temperatures in the inner regions of the envelope.Comment: Accepted for publication in ApJ Letter

Herschel/HIFI observations of molecular emission in protoplanetary nebulae and young planetary nebulae

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 (CO)-C-12, (CO)-C-13, and H2O. Emission from other species, like NH3, OH, (H2O)-O-18, 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 T-k greater than or similar to 200 K. In contrast, the fast winds in OH 231.8+4.2 and NGC 6302 are cold, T-k similar to 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 similar to 100 yr ago, while the fast gas in OH 231.8+4.2 was accelerated similar to 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

MERLIN observations of OH/IR stars

We present MERLIN OH 1612 MHz observations towards 3 OH/IR stars, OH15.7+0.8, OH38.3+1.9 and IRAS 19319+2214. The main objective of the project is to investigate if their complicated OH 1612 MHz spectral properties can be related to the phenomenon of highly time-variable mass loss found towards a handful of carbon stars, where detached shells with an overall spherical symmetry are present. Despite extensive searches, no oxygen-rich star with such a shell has been found in any molecular emission. Based on the MERLIN data however, we find that the circumstellar structure of the three OH/IR stars appears to be more complicated than that of detached shells. Signs of bipolarity in the outflows of OH38.3+1.9 and IRAS 19319+2214 could indicate that these are early post-AGB objects

Circumstellar water vapour in M-type AGB stars: radiative transfer models, abundances, and predictions for HIFI

Context.Surprisingly high amounts of H2O have recently been reported in the circumstellar envelope around the M-type asymptotic giant branch star W Hya. This has lead to the speculation that evaporation of icy cometary or planetary bodies might be an effective ongoing mechanism in such systems. However, substantial uncertainties remain, as the required radiative transfer modelling is difficult due to high optical depths, sub-thermal excitation and the sensitivity to the combined radiation field from the central star and dust grains. Aims.By performing a detailed radiative transfer analysis, we determine fractional abundances of circumstellar H2O in the envelopes around six M-type asymptotic giant branch stars. The models are also used to predict H2O spectral line emission for the upcoming Herschel/HIFI mission. Methods.We use Infrared Space Observatory Long Wavelength Spectrometer spectra to constrain the circumstellar fractional abundance distribution of ortho-H2O, using a non-local thermal equilibrium, and non-local, radiative transfer code based on the accelerated lambda iteration formalism. The mass-loss rates and kinetic temperature structures for the sample stars are determined through radiative transfer modelling of CO line emission based on the Monte-Carlo method. The density and temperature profiles of the circumstellar dust grains are determined through spectral energy distribution modelling using the publicly available code Dusty. Results.The determined ortho-H2O abundances lie between 2$\times$10-4 and 1.5$\times$10-3 relative to H2, with the exception of WX Psc, which has a much lower estimated ortho-H2O abundance of only 2$\times$10-6, possibly indicating H2O adsorption onto dust grains or recent mass-loss-rate modulations. The estimated abundances are uncertain by, at best, a factor of a few. Conclusions.The high water abundance found for the majority of the sources suggests that either the “normal” chemical processes are very effective in producing H2O, or else non-local thermal equilibrium atmospheric chemistry, grain surface reactions, or a release of H2O (e.g. from icy bodies like Kuiper belt objects) play a role. However, more detailed information on the physical structure and the velocity field of the region where the water vapour lines are formed is required to improve abundance estimates. We provide predictions for ortho-H2O lines in the spectral window of Herschel/HIFI. These spectrally resolved lines cover a wide range of excitation conditions and will provide valuable additional information on the physical and chemical properties of the inner stellar wind where H2O is abundant