Dusty wind of W Hya. Multi-wavelength modelling of the present-day and recent mass-loss

Low- and intermediate-mass stars go through a period of intense mass-loss at the end of their lives in a phase known as the asymptotic giant branch (AGB). During the AGB a significant fraction of their initial mass is expelled in a stellar wind. This process controls the final stages of their evolution and contributes to the chemical evolution of galaxies. However, the wind-driving mechanism of AGB stars is not yet well understood, especially so for oxygen-rich sources. Characterizing both the present-day mass-loss and wind structure and the evolution of the mass-loss rate of such stars is paramount to advancing our understanding of this processes. We modelled the dust envelope of W Hya using an advanced radiative transfer code. The dust model was analysed in the light of a previously calculated gas-phase wind model and compared to measurements available in the literature, such as infrared spectra, infrared images, and optical scattered light fractions. We find that the dust spectrum of W Hya can partly be explained by a gravitationally bound dust shell that probably is responsible for most of the amorphous Al$_2$O$_3$ emission. The composition of the large ($sim$,0.3,$mu$m) grains needed to explain the scattered light cannot be constrained, but probably is dominated by silicates. Silicate emission in the thermal infrared was found to originate from beyond 40 AU from the star and we find that they need to have substantial near-infrared opacities to be visible at such large distances. The increase in near-infrared opacity of the dust at these distances roughly coincides with a sudden increase in expansion velocity as deduced from the gas-phase CO lines. Finally, the recent mass loss of W Hya is confirmed to be highly variable and we identify a strong peak in the mass-loss rate that occurred about 3500 years ago and lasted for a few hundred years.

The evolution of rotating very massive stars with LMC composition

We present a dense model grid with tailored input chemical composition appropriate for the Large Magellanic Cloud. We use a one-dimensional hydrodynamic stellar evolution code, which accounts for rotation, transport of angular momentum by magnetic fields, and stellar wind mass loss to compute our detailed models. We calculate stellar evolution models with initial masses of 70-500 Msun and with initial surface rotational velocities of 0-550 km/s, covering the core-hydrogen burning phase of evolution. We find our rapid rotators to be strongly influenced by rotationally induced mixing of helium, with quasi-chemically homogeneous evolution occurring for the fastest rotating models. Above 160 Msun, homogeneous evolution is also established through mass loss, producing pure helium stars at core hydrogen exhaustion independent of the initial rotation rate. Surface nitrogen enrichment is also found for slower rotators, even for stars that lose only a small fraction of their initial mass. For models above 150 MZAMS, and for models in the whole considered mass range later on, we find a considerable envelope inflation due to the proximity of these models to their Eddington limit. This leads to a maximum zero-age main sequence surface temperature of 56000 K, at 180 Msun, and to an evolution of stars in the mass range 50-100 Msun to the regime of luminous blue variables in the HR diagram with high internal Eddington factors. Inflation also leads to decreasing surface temperatures during the chemically homogeneous evolution of stars above 180 Msun. The cool surface temperatures due to the envelope inflation in our models lead to an enhanced mass loss, which prevents stars at LMC metallicity from evolving into pair-instability supernovae. The corresponding spin-down will also prevent very massive LMC stars to produce long-duration gamma-ray bursts, which might, however, originate from lower masses.

Rotational velocities of single and binary O-type stars in the Tarantula Nebula

Rotation is a key parameter in the evolution of massive stars, affecting their evolution, chemical yields, ionizing photon budget, and final fate. We determined the projected rotational velocity, $v_esin i$, of $sim$330 O-type objects, i.e. $sim$210 spectroscopic single stars and $sim$110 primaries in binary systems, in the Tarantula nebula or 30 Doradus (30,Dor) region. The observations were taken using VLT/FLAMES and constitute the largest homogeneous dataset of multi-epoch spectroscopy of O-type stars currently available. The most distinctive feature of the $v_esin i$ distributions of the presumed-single stars and primaries in 30 Dor is a low-velocity peak at around 100,$rm{km s^{-1}}$. Stellar winds are not expected to have spun-down the bulk of the stars significantly since their arrival on the main sequence and therefore the peak in the single star sample is likely to represent the outcome of the formation process. Whereas the spin distribution of presumed-single stars shows a well developed tail of stars rotating more rapidly than 300,$rm{km s^{-1}}$, the sample of primaries does not feature such a high-velocity tail. The tail of the presumed-single star distribution is attributed for the most part -- and could potentially be completely due -- to spun-up binary products that appear as single stars or that have merged. This would be consistent with the lack of such post-interaction products in the binary sample, that is expected to be dominated by pre-interaction systems. The peak in this distribution is broader and is shifted toward somewhat higher spin rates compared to the distribution of presumed-single stars. Systems displaying large radial velocity variations, typical for short period systems, appear mostly responsible for these differences.

The wind of W Hya as seen by Herschel. II. The molecular envelope of W Hya

The evolution of low- and intermediate-mass stars on the asymptotic giant branch (AGB) is mainly controlled by the rate at which these stars lose mass in a stellar wind. Understanding the driving mechanism and strength of the stellar winds of AGB stars and the processes enriching their surfaces with products of nucleosynthesis are paramount to constraining AGB evolution and predicting the chemical evolution of galaxies. In a previous paper we have constrained the structure of the outflowing envelope of W Hya using spectral lines of the $^{12}$CO molecule. Here we broaden this study by modelling an extensive set of H$_{2}$O and $^{28}$SiO lines observed by the three instruments on board Herschel using a state-of-the-art molecular excitation and radiative transfer code. The oxygen isotopic ratios and the $^{28}$SiO abundance profile can be connected to the initial stellar mass and to crucial aspects of dust formation at the base of the stellar wind, respectively. The modelling of H$_{2}$O and $^{28}$SiO confirms the properties of the envelope model of W Hya derived from $^{12}$CO lines. We find an H$_2$O ortho-to-para ratio of 2.5,$^{+2.5}_{-1.0}$, consistent with what is expected for an AGB wind. The O$^{16}$/O$^{17}$ ratio indicates that W Hya has an initial mass of about 1.5 M$_odot$. Although the ortho- and para-H$_{2}$O lines observed by HIFI appear to trace gas of slightly different physical properties, a turbulence velocity of $0.7pm0.1$ km s$^{-1}$ fits the HIFI lines of both spin isomers and those of $^{28}$SiO well. The ortho- and para-H$_2^{16}$O and $^{28}$SiO abundances relative to H$_{2}$ are $(6^{+3}_{-2}) times 10^{-4}$, $(3^{+2}_{-1}) times 10^{-4}$, and $(3.3pm 0.8)times 10^{-5}$, respectively. Assuming a solar silicon-to-carbon ratio, the $^{28}$SiO line emission model is consistent with about one-third of the silicon atoms being locked up in dust particles.

The wind of W Hya as seen by Herschel - I. The CO envelope

Asymptotic giant branch (AGB) stars lose their envelopes by means of a stellar wind whose driving mechanism is not understood well. Characterizing the composition and thermal and dynamical structure of the outflow provides constraints that are essential for understanding AGB evolution, including the rate of mass loss and isotopic ratios. We modeled the CO emission from the wind of the low mass-loss rate oxygen-rich AGB star W Hya using data obtained by the HIFI, PACS, and SPIRE instruments onboard the Herschel Space Observatory and ground-based telescopes. $^{12}$CO and $^{13}$CO lines are used to constrain the intrinsic $^{12}$C/$^{13}$C ratio from resolved HIFI lines. The acceleration of the outflow up to about 5.5 km/s is quite slow and can be represented by a beta-type velocity law with index 5. Beyond this point, acceleration up the terminal velocity of 7 km/s is faster. Using the J=10-9, 9-8, and 6-5 transitions, we find an intrinsic $^{12}$C/$^{13}$C ratio of $18pm10$ for W Hya, where the error bar is mostly due to uncertainties in the $^{12}$CO abundance and the stellar flux around 4.6 $mu$m. To match the low-excitation CO lines, these molecules need to be photo-dissociated at about 500 stellar radii. The radial dust emission intensity profile measured by PACS images at 70 $mu$m shows substantially stronger emission than our model predicts beyond 20 arcsec. The initial slow acceleration of the wind implies inefficient wind driving in the lower part of the envelope. The final injection of momentum in the wind might be the result of an increase in the opacity thanks to the late condensation of dust species. The derived intrinsic isotopologue ratio for W Hya is consistent with values set by the first dredge-up and suggestive of an initial mass of 2 M$_odot$ or more. However, the uncertainty in the main-sequence mass derived based on this isotopologic ratio is large.

Binary interaction dominates the evolution of massive stars

The presence of a nearby companion alters the evolution of massive stars in binary systems, leading to phenomena such as stellar mergers, X-ray binaries and gamma-ray bursts. Unambiguous constraints on the fraction of massive stars affected by binary interaction were lacking. We simultaneously measured all relevant binary characteristics in a sample of Galactic massive O stars and quantified the frequency and nature of binary interactions. Over seventy per cent of all massive stars will exchange mass with a companion, leading to a binary merger in one third of the cases. These numbers greatly exceed previous estimates and imply that binary interaction dominates the evolution of massive stars, with implications for populations of massive stars and their supernovae.

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.

Rotational Mixing in Magellanic Clouds B Stars - Theory versus Observation

We have used VLT FLAMES data to constrain the uncertain physics of rotational mixing in stellar evolution models. We have simulated a population of single stars and find two groups of observed stars that cannot be explained: (1) a group of fast rotating stars which do not show evidence for rotational mixing and (2) a group of slow rotators with strong N enrichment. Binary effects and fossil magnetic fields may be considered to explain those two groups. We suggest that the element boron could be used to distinguish between rotational mixing and the binary scenario. Our single star population simulations quantify the expected amount of boron in fast and slow rotators and allow a comparison with measured nitrogen and boron abundances in B-stars.

A parameter study of self-consistent disk models around Herbig AeBe stars

We present a parameter study of self-consistent models of protoplanetary disks around Herbig AeBe stars. We use the code developed by Dullemond and Dominik, which solves the 2D radiative transfer problem including an iteration for the vertical hydrostatic structure of the disk. This grid of models will be used for several studies on disk emission and mineralogy in followup papers. In this paper we take a first look on the new models, compare them with previous modeling attempts and focus on the effects of various parameters on the overall structure of the SED that leads to the classification of Herbig AeBe stars into two groups, with a flaring (group I) or self-shadowed (group II) SED. We find that the parameter of overriding importance to the SED is the total mass in grains smaller than 25um, confirming the earlier results by Dullemond and Dominik. All other parameters studied have only minor influences, and will alter the SED type only in borderline cases. We find that there is no natural dichotomy between group I and II. From a modeling point of view, the transition is a continuous function of the small dust mass. We also show that moderate grain growth produces spectra with weak or no 1um feature, both for flaring (Group I) and non-flaring (Group II) sources. The fact that sources with weak features have been found mostly in Group I sources is therefore surprising and must be due to observational biases or evolutionary effects.

The variable mass loss of the AGB star WX Psc as traced by the CO J=1-0 through 7-6 lines and the dust emission

Low and intermediate mass stars lose a significant fraction of their mass through a dust-driven wind during the Asymptotic Giant Branch (AGB) phase. Recent studies show that winds from late-type stars are far from being smooth. Mass-loss variations occur on different time scales, from years to tens of thousands of years. The variations appear to be particularly prominent towards the end of the AGB evolution. The occurrence, amplitude and time scale of these variations are still not well understood. The goal of our study is to gain insight into the structure of the circumstellar envelope (CSE) of WX Psc and map the possible variability of the late-AGB mass-loss phenomenon. We have performed an in-depth analysis of the extreme infrared AGB star WX Psc by modeling (1) the CO J=1-0 through 7-6 rotational line profiles and the full spectral energy distribution (SED) ranging from 0.7 to 1300 micron. We hence are able to trace a geometrically extended region of the CSE. Both mass-loss diagnostics bear evidence of the occurrence of mass-loss modulations during the last ~2000 yr. In particular, WX Psc went through a high mass-loss phase (Mdot~5e-5 Msun/yr) some 800 yr ago. This phase lasted about 600 yr and was followed by a long period of low mass loss (Mdot~5e-8 Msun/yr). The present day mass-loss rate is estimated to be ~6e-6 Msun/yr. The AGB star WX Psc has undergone strong mass-loss rate variability on a time scale of several hundred years during the last few thousand years. These variations are traced in the strength and profile of the CO rotational lines and in the SED. We have consistently simulated the behaviour of both tracers using radiative transfer codes that allow for non-constant mass-loss rates.