The death of stars, both at high and low mass, remains one of the least understood phases of the stellar life cycle. The complex interplay of different physical mechanisms such as mass loss, pulsation and nuclear burning, along with highly nonlinear processes such as convection and dust formation, make this one of the most intriguing yet challenging phases of stellar evolution to model. The material returned from evolved stars drives the chemical enrichment of the interstellar medium (ISM) of galaxies. Stellar evolution models suggest that low- to intermediate mass (1-8 M☉) post-main sequence stars, e.g. asymptotic giant branch (AGB) stars, dominate chemical enrichment today (Karakas, 2016, MemSAI 87, 229). Their importance, along with the ease of observing the highly luminous stars in this phase, continues to drive great progress in our understanding of the final stages of stellar evolution.
Nevertheless, many key questions remain open. These include the total mass returned to the ISM by evolved stars, the physics that drives the onset of mass loss, the fraction of the ejected mass that condenses into dust, and the role of variations in the rate of mass loss over time. Thanks to recent developments in astrometry from Spitzer and Gaia, and the capabilities of telescopes like the JCMT, APEX, SMA and ALMA, we are now perfectly positioned to tackle these problems.
Mid-infrared observations reveal the dust-forming regions around AGB stars, but such studies of the Galactic evolved-star population are hampered by extinction, complicating the determination of the distance to sources and their dust content. However, the AGB population in the Magellanic Clouds is well-studied, and the distributions of their luminosities and dust-production rates (DPRs) are well known. Spitzer observations have revealed the global AGB-star DPR in these galaxies (e.g. Riebel et al. 2012, ApJ 753, 71; Srinivasan et al. 2016, MNRAS 457, 2814). The most evolved, heavily dust-enshrouded AGB stars (the so-called ‘extreme’ AGB stars; e.g., Boyer et al. 2012, ApJ 748, 40) are found to be the main dust producers.
The mass loss from AGB stars can also be measured from CO observations (e.g. Kemper et al. 2003, A&A 407, 609; Decin et al. 2007, A&A 475, 233), as models show that its abundance relative to the main constituent of the stellar wind, H2, is stable across a range of C/O ratios (Cherchneff, 2006, A&A 456, 1001). Excepting the brightest objects in the Magellanic Clouds (Matsuura et al. 2016, MNRAS 462, 2995; Groenewegen et al. 2016, A&A 596, A50), such measurements can only been made for Galactic sources, and the fainter, low-J rotational transitions of CO can only be systematically determined for Solar Neighbourhood objects. However, such studies have either consisted of individual objects or observationally biased samples (e.g., de Beck et al. 2010 A&A 523, 18).
The most recent systematic study of the Galactic dust budget is dated (Gehrz, 1989, IAU Symp. 135, 445), and the only determination of gas-to-dust ratios known to us was made over 30 years ago (Knapp 1985, ApJ 293, 273) by separately determining gas and dust mass-loss rates (MLRs). Studies of the dependence of the mass loss process on stellar parameters (e.g., period, luminosity, chemistry) in nearby stars also suffer from small numbers, and/or are biased towards objects with high MLRs. As we cannot observe all Galactic AGB stars, a sample delineated only by geometry is the best way to circumvent potential biases and accurately characterize the population. The major limitation in designing such a study has been the uncertain distances to Galactic evolved stars.
The ongoing Gaia mission is remedying this situation. The Tycho–Gaia Astrometric Solution (TGAS), part of the first Gaia Data Release (Gaia DR1), provides reliable parallaxes for AGB stars out to ∼1 kpc, and improvements in future data releases will expand this parallax set further. This advance allows, for the first time, for the construction of reliable volume-limited samples of Galactic AGB stars with enough sources to facilitate robust statistical inference over a large parameter space. The advanced observational facilities of the JCMT provide unparalleled sensitivity to the gas and dust in the envelopes of evolved stars, facilitating observations of large, statistically significant samples in a reasonable amount of time. This presents a unprecedented opportunity to a examine the processes driving mass-loss from evolved stars and their role in Galactic evolution.
NESS targets a volume-limited sample of mass-losing AGB stars within 3 kpc of the Sun to derive the dust and gas return rates in the Solar Neighborhood, and constrain the physics underlying these processes. With this ambitious program we aim to fill that gap in our knowledge, exploiting newly available Gaia data, taking a multi-pronged approach.
With the JCMT in the North and APEX in the South, we will observe our sample in 850 micron continuum emission, thus extending the infrared spectral energy distribution to the submm, allowing us to detect the coldest dust tracing historical mass loss. Second, we will target our sample in the CO(3-2) and CO(2-1) transitions to determine the gas mass loss rate. A subsample of nearby targets will be mapped in both continuum and CO lines, making it possible to resolve variations in the dust mass-loss history and gas-to-dust ratio. We will also observe selected bright targets in the 13CO (2-1) and (3-2) lines to measure the 13C/12C ratio. It remains very challenging to systematically measure CO gas mass-loss rates outside the Milky Way, and NESS therefore represents our best chance to derive properties like the gas-to-dust ratio in stellar outflows, as well as characterize the physical conditions in the outflowing dusty gas, in the near future.
The sample is divided into five mass-loss rate bins over 6 orders of magnitude, with similar numbers of objects per bin. High mass-loss objects are rarer but more luminous, and the sample will therefore be complete out to 3 kpc in that bin. In the lowest mass-loss rate bin we can only observe objects in the direct vicinity of the Sun, out to 250 pc. Measuring the 13C/12C ratio will provide constraints on existing nucleosynthesis models. Further science goals include the detection of non-sphericity of the circumstellar environment; the determination of the outer radius of the CO emission, which is set by the CO dissociation due to the interstellar radiation field; and correlation of the dust mineralogy as detected in the mid-infrared with the mass-loss properties.
Coordinators: Peter Scicluna (TW), Iain Mcdonald (UK), Jinhua He (CN), Jan Cami (CA), Hiroko Shinnaga (JP), Hyosun Kim (KR)