The Cygnus-X region contains a massive star formation complex with the richest known concentration of massive protostars and the largest OB associations in the nearest 2 kpc. This unbiased survey of 24 sq degrees in Cygnus-X with the IRAC and MIPS instruments will have the sensitivity to detect young stars to a limit of 0.5 Msolar. With this survey we will:
- analyze the evolution of high mass protostars with a large and statistically robust sample at a single distance
- study the role of clustering in high mass star formation,
- study low mass star formation in a massive molecular cloud complex dominated by the energetics of ~100 O-stars,
- assess what fraction of all young low mass stars in the nearest 2 kpc are forming in this one massive complex, and
- provide an unbiased survey of the region and produce a legacy data set which can be used in conjunction with future studies of this region (e.g., with Herschel and JWST).
The Cygnus-X survey will be an important step in constructing one of Spitzer's greatest legacies: surveying with high sensitivity and spatial resolution a representative sample of Galactic star forming regions, from Bok globules to complexes containing millions of solar masses of gas and hundreds of O-stars. The data will be made available to the astronomical community in the form of images, source catalogs, and 3-70 μm photometry.
High Mass Star Formation
All potential sites of star formation in Cygnus-X the densest clumps within molecular clouds are detected in a complete, unbiased survey (~12 deg2) using the CS 2-1 and N2H+ 1-0
(Schneider et al. 2007). Most of the clumps show substructure, revealed by wide-field 1.2 mm
continuum imaging (3 deg2) with the MAMBO-2 camera (11'' resolution). In total, 129 protostellar dense cores (diameter ~0.1 pc; mass 4-900 Msolar; Motte et al. 2005, 2007) were detected from which 40 are likely to be the precursors of massive OB stars. Half of these are high-luminosity IR sources (like DR21, W75N, AFGL2591, and S106IR), but the other half are either weak or even not detected by MSX at 8 μm. These IR-quiet objects have very powerful outflows traced by intense SiO emission and therefore represent a newly recognized evolutionary stage of high-mass star formation.
With this Spitzer survey, we will address the following questions:
- What is the nature of the IR-quiet protostars? The IR-quiet sources are not detected by MSX, but Spitzer is ~600x more sensitive at 8 μm. Two of the 17 IR-quiet cores in Cygnus-X are detected in existing MIPS data at a low level flux at 24 μm, but the vast majority are not covered in previous observations. The deep Spitzer observations are needed to detect these objects between 3 and 70 μm and study their SEDs.
- How do the SEDs of high-mass protostars evolve with time? We will use Spitzer photometry from 3 to 70 μm together with our 350 μm (SHARC2) and 1.2 mm (MAMBO-2) data, and ultimately, with Herschel fluxes from 75 to 520 μm, UKIDSS J, H, K photometry, and SCUBA2 fluxes at 850 μm.
This will produce SEDs spanning 1 to 1200 μm and provide well-constrained luminosities of high-mass protostars. These SEDs will be invaluable for guiding the development of models of high mass protostellar evolution.
- What is the duration of the observed evolutionary stages? The unbiased approach will provide unique results on the statistical properties of high-mass stars from their earliest phases of formation (IR-quiet protostars) to the main sequence (OB stars). With the hypothesis that star formation is a continuous process in Cygnus-X, the duration of the different stages (IR-quiet and IR-bright protostars, HII regions) relative to OB star lifetimes will be derived.
- What is the incidence of clusters around high-mass protostars? Spitzer has the sensitivity and resolution to detect clusters of low mass stars around the high mass protostars. We will determine the number and density of low mass stars surrounding the massive stars as a function of their evolutionary stage. With this we can assess whether clusters are necessary for the onset of massive star formation or if low and high mass stars form coevally in clusters, thereby testing theories that require clusters of low mass stars to form massive stars (e.g., Bonnell et al. 2004).
Low Mass Star Formation
Although high mass stars may dominate the energetics of the Cygnus-X complex, the vast majority of stars produced in all star forming regions are low mass stars; these stars contain most of the stellar mass. The huge concentration of molecular mass and high mass star formation in the Cygnus-X region suggest that it is the largest nursery of low mass stars in the nearest 2 kpc, potentially producing 105 stars more than all the molecular clouds within 500 pc of the Sun. Using methods developed by our team members to identify and classify young stellar objects (Allen et al. 2007), we will produce a census of the low mass protostars and pre-main sequence stars in the complex. These observations will be sensitive to 1 Myr pre-main sequence stars with masses of 0.2 Msolar.
With this census, we will measure the number and spatial distribution of protostars and pre-main sequence stars with disks. In contrast to massive stars, low mass stars form in a range of environments. Spitzer observations show that even for molecular cloud complexes within OB associations, as many as 40-60% of the low-mass stars are forming away from dense clusters (Allen et al. 2007; Megeath et al. 2007, Gutermuth et al. 2007). We will study the demographics of where low mass stars form in Cygnus-X (isolation, groups, or clusters); this will inform studies of more distant OB associations in the Galaxy and others.
The Role of Feedback in a Massive OB star/Molecular Cloud Complex
The evolution of massive star forming complexes such as Cygnus-X is driven by the energetics of the massive stars. Spitzer observations of bright-rimmed clouds in smaller OB associations such as W5 (Koenig et al. 2007) show clusters of young stars and protostars concentrated near the cloud surfaces. This geometry suggests a wave of induced star formation. We will search for similar structures, and assess the fraction of stars being formed at the cloud surfaces by triggering. These data will provide unique insights into processes inside starburst galaxies, where swarms of super star clusters may be compressing the associated ISM (Keto et al. 2005).
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Region surveyed with MIPS (outlined in red), overlayed on Av image calculated using 2MASS