Triggered star formation, HII regions and Spitzer bubbles Mark - PowerPoint PPT Presentation

Triggered star formation, HII regions and Spitzer bubbles Mark Thompson

Bubbles, bubbles everywhere

Bubbles, bubbles everywhere

Why are bubbles important? Bubbles trace the location of energy input to the ISM • Supernovae • Stellar winds • HII regions Understand the physics of feedback in the ISM • Ionisation/heating • Momentum transfer/shocks The role of feedback in triggering new generations of stars

Outline of the lectures 1.Observational surveys for infrared bubbles • Spitzer bubbles & the Milky Way Project 2.Theory of bubble formation & triggered star formation • HII regions & wind-blown bubbles • Collect & Collapse and Radiative-Driven Implosion 3.The star-forming environment of bubbles • Sequential star formation • Statistical studies

Outline of the lectures 1.Observational surveys for infrared bubbles • Spitzer bubbles & the Milky Way Project 2.Theory of bubble formation & triggered star formation • HII regions & wind-blown bubbles • Collect & Collapse and Radiative-Driven Implosion 3.The star-forming environment of bubbles • Sequential star formation • Statistical studies

Early discoveries by IRAS & MSX In late 70s/early 80s bubbles identified in narrow-band images and HI Infrared discoveries from late 80s with IRAS, ISO & MSX Warm dust and/or PAH emission from swept up shells MSX (Green & Cohen 2001) IRAS (Van Buren & McCray 1988)

Spitzer GLIMPSE In 2005 Spitzer carried out the GLIMPSE survey of the inner Milky Way Ten times better angular resolution than MSX & much greater sensitivity 0.85m IR telescope operating from 3 to 180 µ m Two main imaging cameras, IRAC & MIPS GLIMPSE surveyed inner Galaxy with IRAC at 3.6, 4.5, 5.8 and 8.0 µ m (~1” resolution) MIPSGAL surveyed same area with MIPS at 24 and 70 µ m (at 6” and 18”)

Spitzer bubbles Rich discovery of bubbles in GLIMPSE images Almost 600 bubbles identified in Churchwell et al (2006, 2007) Red: 8.0 µ m Green: 5.8 µ m Blue: 4.5 µ m

Spitzer bubbles Secondary bubbles Star forming regions UC HII & MYSO identified in the RMS survey (Urquhart et al 2008)

Spitzer bubbles Bipolar bubbles, reminiscent of the classic bipolar HII region S106

Bubble morphology at 8 µ m: PAHs Tielens 2008 Why are the bubble edges bright at 8 µ m?

Bubble morphology at 8 µ m: PAHs Polycyclic Aromatic Hydrocarbons (PAH) Large hydrocarbon molecules (e.g. coronene, C 24 H 12 ) Abundance of ~10 -7 relative to H 2 PAH features dominate mid-IR spectra of almost all objects with associated dust/gas and illuminated by UV photons PAHs excited by UV illumination but destroyed by FUV Hence, good tracers of molecular cloud exterior

Bubble identification is mostly by eye All 591 bubbles in Churchwell et al (2006, 2007) are identified by eye Resulting catalogue is biased & incomplete, possibly by as much as 50%

Galactic distribution & properties Churchwell et al. found: 1. No dependence of bubble properties on Galactic longitude or latitude 2. Scale height of ~0.6 degrees - similar to HII regions 3. 24 µ m emission in bubble centres, 8 µ m towards edges 4. Correlations with HII regions from Paladini et al (2003) catalogue 5. No correlation with PNe catalogues or W-R stars 6. Only 3 SNRs positionally associated with bubbles Does this mean the bubbles are likely to be HII regions?

The nature of Spitzer bubbles 24 µ m (red), 8 µ m (green), 4.5 µ m (blue) 20 cm (contours) Consistent with HII region around central O5 star Wind-blown cavity in centre Dynamical age of bubble ~ 10 5 years YSOs identified at bubble rim - triggered? Similar results for another sample of 5 bubbles (Watson et al 2009) Bubble N49 (Watson et al 2008)

Molecular studies with JCMT/HARP Beaumont & Williams (2010) Archival 20 cm MAGPIS plus new observations of CO/HCO+ with HARP on JCMT Sample of 43 bubbles in total

Bubbles or “rings”? CO gas does not seem not consistent with bubble morphology “Blow-out” from front and back surfaces? But, if so then should expect to see more bipolar bubbles, which we don’t

Are bipolar bubbles edge on rings?

Archival studies with ATLASGAL Most comprehensive study to date is Deharveng et al (2010) ATLASGAL 870 µ m continuum to trace cold dust (molecular gas) MAGPIS/VGPS VLA 20 cm continuum to trace ionised gas (free-free) MIPSGAL 24 µ m to trace hot dust

A gallery of bubbles 102 bubbles studied in total 10< l <48 & | b |< 0.8 Hot dust present in many bubbles Small bubbles

A gallery of bubbles 102 bubbles studied in total 10< l <48 & | b |< 0.8 Hot dust present in many bubbles Large bubbles

The molecular environment 870 µ m concentrated towards outer shell Often clumpy structure Column densities N(H 2 ) ~ 10 23 cm -2 Many star formation tracers observed at bubble edges (Lecture 3) 65 bubbles associated with 870 µ m continuum 40% associated with a partial shell 28% show morphology consistent with interaction (radiative driven implosion?)

The molecular environment 6.7 GHz methanol maser shown by cross - identifies massive SF

Spitzer bubbles are mostly HII regions 86% of the bubbles enclose ionised gas & many bubbles have Radio Recombination Line detections (Anderson et al 2010) Hence they are mostly HII regions Hot dust common within the bubbles - emission mechanism not clear More than half the 24 µ m emission can come from within the bubble Dense molecular condensations seen around over 50% of bubbles Active signs of star formation towards ~ 1/4 of bubbles

Getting bubblier: The Milky Way Project Spitzer bubbles identified by eye from GLIMPSE using ~10 “observers” Issues of incompleteness - many bubbles likely to have been missed How to address completeness? Simple: turn it into a game! The Zooniverse: www.zooniverse.org Online Science for the Public Simpson et al 2012, Kendrew et al 2012

Getting bubblier: The Milky Way Project Each user presented with random GLIMPSE image & asked to draw a bubble Bubble parameters go into a database >35,000 volunteers >12,000 classified images But how many bubbles?

Combining individual bubble candidates Each candidate used to draw a “heat map” Each bubble must be ID’d by > 5 users Weighted averages used to measure size, ellipticity, thickness Hit Ratio for each bubble

Results from The Milky Way Project

Results from The Milky Way Project

Results from The Milky Way Project Total of 5106 bubbles identified in Milky Way Project (Simpson et al 2012) 3744 large bubbles with elliptical fits 1362 small bubbles with only circular fits Estimate >94% completeness rate from discovery rate Scale height similar to Churchwell bubbles ⇒ HII regions? But beware “observer bias” 35,000 observers = 35,000 biases?? Bright neighbour effect Observer fatigue Must use the “hit ratio” to select well categorised bubbles (see Lecture 3)

Summary Most Spitzer bubbles are HII regions Outer rims traced by 8 µ m PAH emission Centre of HII regions often associated with hot dust (UV absorption...) Morphology of molecular surroundings suggests rings not bubbles Active star formation towards a number of bubbles Citizen Science can lead to a mostly complete catalogue (but beware caveats) Coming up: theory of bubbles & triggered star formation...