The SPITZER GLIMPSE (Galactic Legacy Infrared Mid-Plane Survey Extrordinare) programme is the first of a number of sensitive infrared surveys covering the inner Galactic plane at high resolution and in an unbiased manner (Churchwell et al 2001). The northern half of GLIMPSE covers the region spanning l=10o - 65o, |b|< 1o at wavelengths of 3.6 - 8.0 microns, which preferentially selects warm and dusty embedded sources. The companion SPITZER MIPSGAL survey (Carey et al 2005) has imaged the same region at 25 and 70 microns, where the bulk of the energy from massive young stellar objects is emitted, and is hence sensitive to cooler and more deeply embedded young stellar objects. Completing the infrared picture of Galactic star formation is the UKIDSS project (UK IR Deep Sky Survey, Lawrence et al 2007). UKIDSS is a near-infrared survey operating in the J, H and K bands and is sensitive down to the 18th magnitude. The combined data from these surveys are driving the detailed characterisation of the Galactic population via their infrared colours (e.g., Longmore et al 2007). A complementary picture of the molecular and atomic interstellar medium (ISM) is being provided by the BU-FCRAO Galactic Ring Survey for CO (Jackson et al 2006) and the VGPS HI survey (Stil et al 2006). Similarly, the ongoing IPHAS H-alpha survey (Drew et al 2005) probes H-alpha in emission towards nebulae and in absorption towards stars.
Conspicuous by its absence here is a comparable radio-continuum survey for compact ionised gas in the ISM. From a star formation perspective, the presence or absence of free-free emission is vital to distinguish evolved ultra-compact HII (UCHII) regions from their younger counterparts with similar thermal SEDs. No previous radio survey of the Galactic plane is comparable with the resolution or coverage of the SPITZER GLIMPSE data. The CORNISH (Co-Ordinated Radio `N' Infrared Survey for High-mass star formation) project aims to deliver a uniform, sensitive and high-resolution radio survey to address key questions in high-mass star formation, as well as many other areas of astrophysics.
The following are the key science goals of CORNISH.
Ultra-Compact HII Regions
The proposed survey will deliver a complete and unbiased sample of UCHIIs in this region of the plane. Such a sample enables us to tackle the questions of:
- The structure of our galaxy as traced by massive star forming regions.
- The frequency of potentially triggered or sequential massive star formation compared to spontaneous isolated gravitational collapse via combined CORNISH/GLIMPSE datasets.
- The evolution of HII regions and their role in cloud dispersal and determining the star formation efficiency.
- The upper IMF across a range of galactic environments.
UCHIIs are the best tracers of massive star formation in a galaxy and always provide one of the clearest signatures of spiral structure. A clear elucidation of the spiral structure can be used to test the currently adopted 4-arm pattern for our galaxy (Taylor and Cordes 1993).
Analysis of the distribution of projected separations of the UCHIIs from the centres of their associated molecular clouds can be used to assess whether massive stars form predominately on the edges of molecular clouds or in the centre.
Our large, uniform area survey will also provide the final word on the distribution of morphologies and the lifetimes of the UCHII phase. The `lifetime debate' has raged ever since Wood & Churchwell (1989) concluded from their pointed survey that the large numbers of UCHIIs meant that their expansion must be constrained by some mechanism to prolong their lifetime. With the new sensitive high resolution IR surveys we will be able to directly detect all the OB stars across the galaxy. Comparison of the relative numbers of the UCHIIs found with the number of field OB stars straightforwardly delivers the lifetime.
Massive Young Stellar Objects
It is well known that the IR colours of UCHIIs and MYSOs are very similar. Hence, radio observations are crucial in distinguishing the usually bright, extended UCHIIs from the weak, unresolved or undetected MYSOs. Again the number density of MYSOs as a function of luminosity will provide definitive answers on the lifetimes of this important phase.
The location of the MYSOs relative to previous episodes of OB star formation will enable us to address directly the question of sequential star formation. Nearby UCHIIs and older OB star clusters will be visible in the radio and IR survey data whilst larger, more evolved H II regions are known from single-dish radio surveys. The identification of a large, well-selected sample of MYSOs is important so that systematic studies of their properties can be made free from selection biases.
Planetary and Proto-planetary Nebulae
Zijlstra and Pottasch (1991) estimated that there should be about 23,000 planetary nebula (PN) in the galaxy. Of the more compact ones that we are sensitive too in these co-ordinated radio and IR surveys we can expect about a thousand in the GLIMPSE region. A complete sample over a large region of the plane will constrain the density of PN and hence their formation rate in the Galaxy. This has implications for models of post-AGB evolution and mass-loss which are not well constrained (i.e how many post-AGB stars actually become central stars of PN?). A complete sample is important for determining the fraction of O- and C-rich PN, the properties of the progenitor population, its evolution and its Galactic distribution (e.g. Casassus & Roche 2001).
Young compact PN that are heavily reddened by line-of-sight extinction in the galactic plane will have IR colours very similar to compact HII regions (Lumsden et al. 2002). The radio survey will detect many new PN that have been hidden from view in previous optical searches. The complementary surveys will be able to distinguish the PN from UCHIIs since they should not be embedded in molecular clouds or be associated with young clusters.
Since their central star has not yet reached 30,000 K, Proto-PN (PPN) may be distinguished from PN as they are radio quiet. PPN hold the key to understanding the envelope ejection process and testing the interacting winds models for the shaping of PNe (e.g., Mellema & Frank 1995). Our survey will deliver a large and well-selected sample of PPN and young PN that can be used to investigate this transition in detail as a function of stellar mass. If we take the birth rate of PN to be about 1 per year and the PPN lifetime of 1500 years then we expect about 250 PPN in our survey region.
Evolved OB Stars
Mass-loss from the later evolutionary stages of massive stars (eg. OB supergiants, Wolf-Rayet stars, and Luminous Blue Variables) are of key importance in the evolution of our own and other galaxies. Thermal radio emission is the preferred method for determining mass-loss rates for the evolved massive stars since it arises at large distances from the underlying stars where the outflow has attained a constant velocity.
Since they still inhabit the densest parts of the plane, there are likely to be many evolved OB stars that are hidden from view by high line of sight extinction. Recent near-IR surveys of the Galactic Centre region have revealed some of the most massive stellar clusters yet discovered - the Quintuplet and Galactic Centre clusters (Glass et al. 1990; Nagata et al. 1990). Other areas of the plane are completely extinguished at near- and even mid-IR wavelengths. Our radio survey will reveal the most massive OB stars hidden from previous searches.
Many of these massive stars have non-thermal components in addition to the thermal emission from their stellar wind (c.f. Beiging et al. 1989) and hence will be detected out to greater distances. The source of the non-thermal emission is a unknown. This survey will provide a larger sample of massive stars exhibiting non-thermal emission (distinguished from purely thermal ones via follow-up multi-wavelength observations) with which to address the question of the origin of the non-thermal emission in OB-type stars.
Sensitive radio observations have revealed the ubiquity of high energy processes across the radio H-R diagram (see the comprehensive review by Guedel 2002). As well as dynamo effects in convective envelopes, high-resolution radio observations have revealed large-scale magnetospheric structures in various stellar types, including both low- and intermediate-mass pre-main sequence stars. Radio observations reveal dozens of such objects in Orion, and Chandra now finds over 1000 active stars in Orion (Garmire et al. 2000). With 5 GHz flux densities reaching several 10's mJy, these sources will be detected to several kpc. They will be detected in the co-ordinated IR surveys, but do not distinguish themselves as active in their IR properties at all. Follow-up high resolution observations with MERLIN and VLBI (and e-MERLIN in the future) will be able to make more detailed investigations of the structure, polarisation and variability of these objects.
Mass transfer in close binaries often leads to strong activity in radio and other wave-bands. RS CVn stars, cataclysmic variables and others are likely to be detected in the survey. However, the most important class of active binary are the so-called micro-quasars where the accreting object is a black-hole or neutron star (Mirabel & Rodriguez 1999; Fender 2002). These produce relativistic jets analogous to AGN where ejections of plasma can be followed in real time. Ejections of discrete knots of emission at 0.98c have been seen in GRS1915+105 (Fender et al. 1999) associated with the onset of flaring in soft X-rays. These objects are perhaps the best laboratories for studying the jet formation process, since the time-scales are short and it is possible to associate the ejection of features in the jets with changes in the X-ray behaviour.
It has been recently established that these jet sources come in two types. The first is the self-absorbed outflows in `hard' X-ray states (Fender 2001). Examples of such systems are the famous jet source SS 433, and the classical black hole candidate Cygnus X-1, which has also been shown recently to produce a jet (Stirling et al. 2001). The other type exhibit optically thin, transient emission during outbursts (Fender & Kuulkers 2001). There may be a short-lived evolutionary regime in which the jet, and its consequent impact upon the nascent star cluster, is the only clear diagnostic of such an evolutionary phase. Our survey has the resolution to resolve such jet structures. At high accretion rates this survey may expect to detect such sources throughout the disc of our galaxy. Candidates for these objects will stand out in combined radio-IR colour plots. These non-thermal type sources are more easily identified at lower frequencies. The 20 cm VLA surveys by Becker et al. (1990), Helfand et al (1992) and Giveon et al. (2005), which have field centre detection thresholds of about 5-10 mJy will aid identification of some steep spectrum (&alpha < 1.0) sources, but others will require further 20 cm observations to identify them.
There will be a significant number of background extragalactic sources detected during the survey. Using equation A2 in Anglada et al. (1998) at our completeness level we would expect to detect about 2,500 extragalactic sources in the 110 square degree area. As well as penetrating the deepest part of the zone of avoidance these sources will be useful in other respects. The most compact ones, which our high resolution survey will reveal, are potential phase calibrators for use in future interferometric studies. There is currently a lack of good phase calibrators in the plane and this has a serious effect on high frequency interferometry where some sources cannot be observed at all. The fact that our survey is also at 5 GHz rather than 1.4 GHz means that we will preferentially pick up extragalactic sources with rather flat spectra, which again will be more useful as potential mm interferometric phase calibrators.
These Galactic plane surveys can also deliver a set of extragalactic background sources that can act as pencil beam probes of molecular clouds in follow-up absorption line studies (Greaves & Nyman 1996).