ALMA observations of TiO$_2$ around VY Canis Majoris

Titanium dioxide, TiO$_2$, is a refractory species that could play a crucial role in the dust-condensation sequence around oxygen-rich evolved stars. To date, gas phase TiO$_2$ has been detected only in the complex environment of the red supergiant VY CMa. We aim to constrain the distribution and excitation of TiO$_2$ around VY CMa in order to clarify its role in dust formation. We analyse spectra and channel maps for TiO$_2$ extracted from ALMA science verification data. We detect 15 transitions of TiO$_2$, and spatially resolve the emission for the first time. The maps demonstrate a highly clumpy, anisotropic outflow in which the TiO$_2$ emission likely traces gas exposed to the stellar radiation field. A roughly east-west oriented, accelerating bipolar-like structure is found, of which the blue component runs into and breaks up around a solid continuum component. A distinct tail to the south-west is seen for some transitions, consistent with features seen in the optical and near-infrared. We find that a significant fraction of TiO$_2$ remains in the gas phase outside the dust-formation zone and suggest that this species might play only a minor role in the dust-condensation process around extreme oxygen-rich evolved stars like VY CMa.

PO and PN in the wind of the oxygen-rich AGB star IK Tau

Phosphorus-bearing compounds have only been studied in the circumstellar environments (CSEs) of the asymptotic giant branch (AGB) star IRC +10216 and the protoplanetary nebula CRL 2688, both C-rich objects, and the O-rich red supergiant VY CMa. The current chemical models cannot reproduce the high abundances of PO and PN derived from observations of VY CMa. No observations have been reported of phosphorus in the CSEs of O-rich AGB stars. We aim to set observational constraints on the phosphorous chemistry in the CSEs of O-rich AGB stars, by focussing on the Mira-type variable star IK Tau. Using the IRAM 30m telescope and the Submillimeter Array (SMA), we observed four rotational transitions of PN (J=2-1,3-2,6-5,7-6) and four of PO (J=5/2-3/2,7/2-5/2,13/2-11/2,15/2-13/2). The IRAM 30m observations were dedicated line observations, while the SMA data come from an unbiased spectral survey in the frequency range 279-355 GHz. We present the first detections of PN and PO in an O-rich AGB star and estimate abundances X(PN/H2) of about 3x10^-7 and X(PO/H2) in the range 0.5-6.0x10^-7. This is several orders of magnitude higher than what is found for the C-rich AGB star IRC +10216. The diameter (<=0.7) of the PN and PO emission distributions measured in the interferometric data corresponds to a maximum radial extent of about 40 stellar radii. The abundances and the spatial occurrence of the molecules are in very good agreement with the results reported for VY CMa. We did not detect PS or PH3 in the survey. We suggest that PN and PO are the main carriers of phosphorus in the gas phase, with abundances possibly up to several 10^-7. The current chemical models cannot account for this, underlining the strong need for updated chemical models that include phosphorous compounds.

On the physical structure of IRC+10216. Ground-based and Herschel observations of CO and CCH

The C-rich AGB star IRC+10216 undergoes strong mass loss, and quasi-periodic density enhancements in the circumstellar matter have been reported. CO is ubiquitous in the CSE, while CCH emission comes from a spatially confined shell. With the IRAM 30m telescope and Herschel/HIFI, we recently detected unexpectedly strong emission from the CCH N=4-3, 6-5, 7-6, 8-7, and 9-8 transitions, challenging the available chemical and physical models. We aim to constrain the physical properties of IRC+10216s CSE, including the effect of episodic mass loss on the observed emission. In particular, we aim to determine the excitation region and conditions of CCH and to reconcile these with interferometric maps of the N=1-0 transition. Via radiative-transfer modelling, we provide a physical description of the CSE, constrained by the SED and a sample of 20 high-resolution and 29 low-resolution CO lines. We further present detailed radiative-transfer analysis of CCH. Assuming a distance of 150pc, the SED is modelled with a stellar luminosity of 11300Lsun and a dust-mass-loss rate of 4.0times10^{-8}Msun/yr. Based on the analysis of 20 high resolution CO observations, an average gas-mass-loss rate for the last 1000yrs of 1.5times10^{-5}Msun/yr is derived. This gives a gas-to-dust-mass ratio of 375, typical for an AGB star. The gas kinetic temperature throughout the CSE is described by 3 powerlaws: it goes as r^{-0.58} for r<9R*, as r^{-0.40} for 9<=r<=65R*, and as r^{-1.20} for r>65R*. This model successfully describes all 49 CO lines. We show the effect of wind-density enhancements on the CCH-abundance profile, and the good agreement of the model with the CCH N=1-0 transition and with the lines observed with the 30m telescope and HIFI. We report on the importance of radiative pumping to the vibrationally excited levels of CCH and the significant effect this has on the excitation of all levels of the CCH-molecule.

Snooping around the big dog: VY CMa as seen with Herschel/HIFI

In the framework of the HIFISTARS guaranteed time key programme, we measured more than 70 molecular emission lines with high signal-to-noise ratio towards VY CMa using the high-resolution HIFI spectrometer on board the Herschel satellite. The kinematic information obtained from the measured water lines supports the hypothesis of multiple outflow components. The observed high-intensity maser lines give no indication for strong polarisation.

Probing the mass-loss history of AGB and red supergiant stars from CO rotational line profiles - II. CO line survey of evolved stars: derivation of mass-loss rate formulae

We aim to (1) set up simple and general analytical expressions to estimate mass-loss rates of evolved stars, and (2) from those calculate estimates for the mass-loss rates of asymptotic giant branch (AGB), red supergiant (RSG), and yellow hypergiant stars in our galactic sample. Rotationally excited lines of CO are a very robust diagnostic in the study of circumstellar envelopes (CSEs). When sampling different layers of the CSE, observations of these molecular lines lead to detailed profiles of kinetic temperature, expansion velocity, and density. A state-of-the-art, nonlocal thermal equilibrium, and co-moving frame radiative transfer code that predicts CO line intensities in the CSEs of late-type stars is used in deriving relations between stellar and molecular-line parameters, on the one hand, and mass-loss rate, on the other. We present analytical expressions for estimating the mass-loss rates of evolved stellar objects for 8 rotational transitions of the CO molecule, apply them to our extensive CO data set covering 47 stars, and compare our results to those of previous studies. Our expressions account for line saturation and resolving of the envelope, thereby allowing accurate determination of very high mass-loss rates. We argue that, for estimates based on a single rotational line, the CO(2-1) transition provides the most reliable mass-loss rate. The mass-loss rates calculated for the AGB stars range from 4x10^-8 Msun/yr up to 8x10^-5 Msun/yr. For RSGs they reach values between 2x10^-7 Msun/yr and 3x10^-4 Msun/yr. The estimates for the set of CO transitions allow time variability to be identified in the mass-loss rate. Possible mass-loss-rate variability is traced for 7 of the sample stars. We find a clear relation between the pulsation periods of the AGB stars and their derived mass-loss rates, with a levelling off at approx. 3x10^-5 Msun/yr for periods exceeding 850 days.