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Stellar, Circumstellar and Interstellar Physics

Team Lead: Janet Drew

 

Advancing our knowledge and understanding of the stellar and diffuse interstellar content of the Milky Way continues to occupy a central position in astronomy: advances in characterising stellar and interstellar processes working now and locally support progress in understanding the appearance, content, and evolutionary histories of galaxies in the wider, younger Universe. Uniquely, it is in the Milky Way that we easily access individual examples of the least well-described, short-lived phases in stellar evolution, down to sub-solar-mass objects  – and study at maximum angular resolution the relationship between stars and the interstellar medium. Topics of current significance include environmental effects in the formation of stars, extinction and ISM mapping, the lifecycle of high-mass stars, mass-loss phenomena across the HRD, the consequences of binary interaction, the approach to end states of cooling white dwarfs, X-ray emitting binaries and gravitational wave sources.

This programme is about Galactic Plane (GP) science, favouring the higher-mass and young/old extremes of stellar evolution, rather than the more frequent older stars that are the natural focus of Galactic archaeology. It is also about the interstellar medium on the large scale – both the ionised interstellar medium (ISM), made visible via Ha and other optical emission lines, and also the ISM seen in absorption against background starlight. The R=5000 mode of WEAVE will provide unprecedented access to the physical and chemical properties of the gaseous and dusty ISM, along with sufficient kinematic resolution to discern the contributions from major cloud components lying along every sightline sampled.  For target selection, SCIP will draw on Gaia data releases, and also the European Galactic Plane Surveys (EGAPS) – comprising the IPHAS and UVEX surveys executed on the INT, and VPHAS+ at the VST – mapping the full Plane within |b|<5o (Drew et al 2005, Groot et al 2009, Drew et al 2014).  The latter u/g/r/i/Ha photometric surveys are on course to capturing more than half a billion point sources in the Milky Way’s disc – its main mass component – from which well-tuned, maximal samples of the target object types can be extracted down to at least V~19.  A defining feature of the SCIP survey, supported by the EGAPS surveys, is that the samples studied will correct previous bias favouring clustered environments – for the first time high-precision target selection across the wide field is possible and is being undertaken.

Spectroscopy at R~5000, spanning almost the entire optical range, provides many types of information that cannot be derived from photometry alone: these include systemic/orbital radial velocities, sound stellar parameters and metallicity, first evidence of chemical peculiarity, interstellar absorption features, a broad suite of constraints on circumstellar/nebular matter, mass transfer/loss signatures, and signs of magnetic activity.  At the higher resolution available, R = 20000, observations in the Cygnus region and Galactic Anticentre can take this further through the measurement of individual element abundances, the determination of better constraints on binarity, and access to enhanced precision of radial velocity measurement that can expose more subtle localised velocity dispersion.

The current SCIP footprint, in Galactic coordinates, is shown below:

Figure 1: The SCIP footprint broken down into the constituent tiles envisaged for the R=5000 survey, plotted against Galactic coordinates. The colouring indicates the expected target numbers per tile.  A second visit will be sought to those tiles edged in lime green in order to double the exposure on fainter O and early B targets.  The sub-regions outlined in blue represent the target regions of the R=20000 Cygnus and HR Anticentre surveys. (Final footprint currently under discussion.)

 

The main target groups for SCIP are, in roughly descending order of expected target densities: OBA stars, including emission line stars and RSGs; ionised nebulae and the diffuse ISM; young stars across the wide field; WDs and compact binaries; Cepheids.

 

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