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Triggered star formation, HII regions and Spitzer bubbles Mark - - PowerPoint PPT Presentation
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
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Bubbles, bubbles everywhere
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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
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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
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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
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Early discoveries by IRAS & MSX
IRAS (Van Buren & McCray 1988)
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)
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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 µm Two main imaging cameras, IRAC & MIPS GLIMPSE surveyed inner Galaxy with IRAC at 3.6, 4.5, 5.8 and 8.0 µm (~1” resolution) MIPSGAL surveyed same area with MIPS at 24 and 70 µm (at 6” and 18”)
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Spitzer bubbles
Rich discovery of bubbles in GLIMPSE images Almost 600 bubbles identified in Churchwell et al (2006, 2007) Red: 8.0 µm Green: 5.8 µm Blue: 4.5 µm
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Spitzer bubbles
Secondary bubbles Star forming regions UC HII & MYSO identified in the RMS survey (Urquhart et al 2008)
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Spitzer bubbles
Bipolar bubbles, reminiscent of the classic bipolar HII region S106
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Bubble morphology at 8 µm: PAHs
Why are the bubble edges bright at 8 µm?
Tielens 2008
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Bubble morphology at 8 µm: PAHs
Polycyclic Aromatic Hydrocarbons (PAH) Large hydrocarbon molecules (e.g. coronene, C24H12) Abundance of ~10-7 relative to H2 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
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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%
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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 µm emission in bubble centres, 8 µm 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?
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The nature of Spitzer bubbles
Bubble N49 (Watson et al 2008)
24 µm (red), 8 µm (green), 4.5 µm (blue) 20 cm (contours) Consistent with HII region around central O5 star Wind-blown cavity in centre Dynamical age of bubble ~ 105 years YSOs identified at bubble rim - triggered? Similar results for another sample of 5 bubbles (Watson et al 2009)
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Molecular studies with JCMT/HARP
Beaumont & Williams (2010) Archival 20 cm MAGPIS plus new observations of CO/HCO+ with HARP
- n JCMT
Sample of 43 bubbles in total
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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
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Are bipolar bubbles edge on rings?
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Archival studies with ATLASGAL
Most comprehensive study to date is Deharveng et al (2010) ATLASGAL 870 µm continuum to trace cold dust (molecular gas) MAGPIS/VGPS VLA 20 cm continuum to trace ionised gas (free-free) MIPSGAL 24 µm to trace hot dust
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A gallery of bubbles
102 bubbles studied in total 10<l<48 & |b|< 0.8 Hot dust present in many bubbles Small bubbles
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A gallery of bubbles
102 bubbles studied in total 10<l<48 & |b|< 0.8 Hot dust present in many bubbles Large bubbles
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The molecular environment
870 µm concentrated towards outer shell Often clumpy structure Column densities N(H2) ~ 1023 cm-2 Many star formation tracers observed at bubble edges (Lecture 3) 65 bubbles associated with 870 µm continuum 40% associated with a partial shell 28% show morphology consistent with interaction (radiative driven implosion?)
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The molecular environment
6.7 GHz methanol maser shown by cross - identifies massive SF
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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 µm 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
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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
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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?
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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
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Results from The Milky Way Project
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Results from The Milky Way Project
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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)
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