NESS objectives

Our aims fall into two broad categories: 1) to study the rate and properties of the enriched matter returned to the ISM by evolved stars; and (2) to explore the physics of dust-laden stellar winds, particularly their onset and time evolution. They can be broken down into several more specific objectives.

Determining the present-day mass-return rate to the Galactic ISM

From CO and sub-mm continuum observations, will obtain gas mass-loss rates (MLRs) and dust production rates (DPRs) for all stars in our volume-limited sample. The gas MLRs will independently measure the material return from AGB stars, while the continuum data will trace the cold dust content of these sources, which is not well constrained from the mid-IR data currently used to calculate DPRs.

We will model the CO and 13CO line profiles simultaneously with a non-LTE line radiative transfer (RT) code (Kemper et al. 2003, A&A 407., 609), using modern statistical methods to robustly estimate outflow parameters (MLR, outflow velocity, etc). In addition, we will develop new dust RT models, building on our experience with GRAMS, to fit the optical–sub-mm SEDs and constrain the DPR including any cold component. We will use these data to produce the most precise estimate of the total (gas+dust) evolved-star mass return to the local ISM.

Gas-to-dust ratios and dust properties

The gas-to-dust ratio carries vital information about the dust-condensation efficiency, and is useful when direct measurements of the gas mass are unavailable. A canonical value of 100 or 200 is often used for Galactic stars (e.g., Groenewegen, 2006, A&A 448, 181), based loosely on metallicity. Knapp (1985, ApJ 293, 273) made the only existing measurement of gas-to-dust ratio in evolved stars, finding ratios of 160 for O-rich stars, and 400 for C-rich stars. This proposal will greatly improve on this determination of the gas-to-dust ratio, by systematically measuring both dust and gas MLRs for ∼400 stars. The resulting gas-to-dust ratios are also useful in studying the relationship of mass loss with stellar parameters.

Little is known about sub-mm dust properties in AGB stars. A large, volume-limited sub-mm study will: (i) reveal cold dust not detectable in the mid-IR, improving DPRs; (ii) provide the best constraints on the long-wavelength emissivity of dust formed by evolved stars, which correlates with the dust composition and grain size; and (iii) shed light on the role of amorphous carbon and metallic dust, which have significantly different sub-mm spectral properties from graphite and silicates (e.g. Papoular & Papoular, MNRAS 443, 2974). Existing models, including GRAMS, poorly predict this region of the spectrum; through the advances above, our dataset will resolve the cause of this difference.

Studying the physics of mass loss

To apply our determined mass-return rate to stars outside of the Solar Neighbourhood, we must also understand the stellar mass-loss mechanisms and how they evolve with changing stellar parameters. Canonical theory dictates that magneto-acoustic processes (Dupree et al. 1984, ApJ, 281, L37) give way to a wind driven by radiation pressure on dust grains, enhanced by stellar pulsations (Wood 1979, ApJ, 227, 220; Bowen 1988, ApJ, 329, 299). In environments of differing ages and metallicities, different pulsation strengths and gas-to-dust ratios will mean the efficiency of these processes changes (e.g. McDonald et al. 2011, ApJ, 730, 71). Recent work has identified this transition region as close to the tip of the red giant branch (RGB), where existing observations suggest that a declining wind velocity transitions to a dust-producing, pulsation-driven wind, before the dust-driven, pulsation-enhanced wind takes over (Groenewegen 2014, A&A, 561, L11; McDonald et al. 2016, MNRAS, 456, 4542 & in prep.). In the Milky Way, the transition between these regimes may occur around pulsation periods of 60 and 300 days (McDonald & Zijlstra 2016, ApJ, 823, L38). Our unbiased survey of nearby AGB stars will allow us to quantitatively determine the efficiency of the wind-driving mechanism, which can be applied to other environments where CO-line observations cannot be made. We will also explore changes in dust mineralogy by using VLT/VISIR 10-micron spectra (PI: Zijlstra).

Spatially resolved wind properties and mass-loss histories of AGB stars

Many traditional studies have derived a single time-averaged MLR for individual evolved stars, based on a small number of CO rotational transitions observed at the position of peak emission (e.g. Loup et al. 1993, A&AS 99, 291). However, variable mass loss is needed to explain observed CO line strengths, even at the central position (Kemper et al. 2003; Decin et al. 2007). Meanwhile, infrared observations (Marengo et al. 2001, MNRAS 324, 1117) and deep scattered-light imaging (e.g. Mauron & Huggins 2000, A&A 359, 707) have revealed complex structure in the dust shells, consistent with mass-loss modulations on time scales of centuries, as predicted by Simis et al. (2001 A&A 371, 205).

Ladjal (2010) spatially resolved the circumstellar envelopes of nine evolved stars in continuum emission at 870 μm, using APEX/LABoCa, demonstrating the power of the sub-mm for observing historic mass loss, and recent work based on SCUBA-2 data (Dharmawardena et al. 2018, MNRAS 479, 536) finds extended emission throughout a sample of 15 sources at both 450 and 850 microns. Preliminary work with HARP jiggle maps demonstrates that spatially resolving the line component is also feasible with the JCMT.

We will perform detailed mapping of a sample of 46 bright, nearby AGB stars with a wide range of DPRs in continuum and CO lines. This study will provide the first census of extended sub-mm emission from local AGB stars. By interpreting these data with existing RT tools, we will robustly constrain the dust and gas mass-loss histories of the sample. This will reveal the prevalence of mass-loss variability, and variations in the gas-to-dust ratio as a function of time. Furthermore, this will be sensitive to large-scale deviations from circular symmetry and detached shells. By setting a limit to the outer boundary of the dust shell, the sub-mm continuum data provides an upper limit on the duration of the dust-producing phase.

Our proposed mapping observations will also place constraints on CO photodissociation by the interstellar UV radiation field (Mamon et al. 1988, ApJ 328, 797; McDonald et al. 2015, MNRAS 453, 4324) by spatially resolving the extent of CO emission. The exact location of the dissociation radius depends on the efficacy of CO shielding (self-shielding, and shielding by H, H2 and dust; Li et al. 2014, 2016).

Galactic AGB stars as a population

Our 852-star sample, covering many orders of magnitude in DPR, will enable the most comprehensive exploration and analysis of the relationships between stellar and outflow properties on a population-wide scale. This will yield new constraints for AGB-star models from the onset of mass loss to the superwind phase.

Large-scale determinations of wind expansion velocities are usually restricted to bright stars in the Galactic Bulge (e.g., Blommaert et al. 1999 IAUS 191, 511) and the Magellanic Clouds (e.g., Marshall et al. 2004 MNRAS 355, 1348), due to the known distance to these populations. Studies on smaller samples including nearby stars have also been performed – e.g, De Beck et al. (2010, A&A 523, 18) determined the expansion speeds and MLRs for ∼50 sources by combining JCMT observations with Herschel data. These studies find that the outflow speed spans a large range (2—40 km/s), and that it varies with stellar parameters such as the period, luminosity, circumstellar chemistry, and gas-to-dust ratio (e.g., Wood et al. 1992 ApJ 397, 552; Marshall et al. 2004, MNRAS 355, 1348; de Beck et al. 2010). The gas-to-dust ratio, in turn, depends on metallicity (e.g., van Loon 2000, A&A 354, 125). The CO line data obtained by our program will, for the first time, provide expansion speeds and gas-to-dust ratios for a large number of nearby AGB stars, which will lend itself to robust inference of population-wide relationships such as the dependence of these quantities on the circumstellar chemistry, period, and metallicity.

We will also be able to probe the variation of the gas-to-dust ratio with gas MLR, which will provide key insights into the efficiency of the dust-condensation process. Further comparison with outflow velocities will elucidate its role in wind driving for different classes of stars. By combining these data with detailed analysis of mid-infrared spectra (e.g. Jones et al. 2017, MNRAS 470, 3250) or ALMA observations (e.g. De Beck et al. 2015, A&A 580, 36), we will constrain the dust condensation sequence, providing a new window on nucleation theory and the wind-driving problem in O-rich AGB stars. Similar analyses may help to elucidate the origin of the 30 micron feature by exploring whether these sources have systematically different sub-mm properties (mass loss rate, gas-to-dust ratio, etc.) to normal AGB stars.

The census of MLRs from this project and known stellar parameters will be compared with stellar evolution and population synthesis models to calibrate MLR formulae (e.g., Vassiliadis & Wood 1993, ApJ 413, 641) used in models. Improved distance estimates from future Gaia releases will also allow us to compare the luminosity function of nearby AGB stars with the results of stellar population synthesis models (e.g., Bruzual et al. 2013, IAUS 295, 282).

Measuring 13C/12C

The isotopic ratios of C are a key tracer of the evolutionary state of AGB stars and the nucleosynthetic processes they undergo (Karakas & Lattanzio, 2014, PASA 31, 30). The most abundant isotope, 12C is produced in the triple-alpha process in He core-burning on the horizontal branch and shell-burning on the AGB, and is brought to the surface by third dredge-up, but may be converted to 14N by hot bottom burning (Wannier 1980, ARA&A 18, 399). Intermediate-mass stars convert 12C into 13C via the CNO cycle during the main sequence and during interpulse phases on the AGB (e.g., de Beck et al. 2010). Consequently, 13C/12C is expected to decrease with time for more massive AGB stars, however a large range of 13C/12C is observed across the range of AGB stars (de Beck et al., 2010; Ohnaka & Tsuji, 1999, A&A 345, 233). In particular, 13C is enhanced compared to models for low-mass AGB stars, suggesting extra, as yet unknown mixing processes (Abia & Isern, 1997, MNRAS 289L, 11).

We will derive 13C/12C for the largest sample to date; nearly a factor of 10 larger than that of de Beck et al. (2010). As well as providing robust constraints to stellar evolution models, we will explore the dependence of 13C/12C on other stellar and outflow properties across the full population of local AGB stars.