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Masers and Star Formation
A Review Vincent L. Fish
12 March 2007 IAU Symposium 242: Astrophysical Masers and Their Environments Alice Springs, NT, Australia


Introduction
Review of literature regarding masers and star formation since Mangaratiba meeting Overview of maser species: H2O, CH3OH, OH, SiO, H2CO, others References to other talks Maser overlaps Techniques, results General questions, the future Summary


Key Quotes
Mid-infrared surveys "have confirmed that maser emission in general can trace a variety of phenomena associated with massive stars including shocks, outflows, infall and circumstellar disks. No one maser species is linked exclusively to one particular process or phenomenon." (De Buizer et al. 2005) Water, hydroxyl, and methanol masers "do not seem to be associated with different evolutionary stages of massive stars. Instead it appears that they all trace a variety of stellar phenomena throughout many early stages of massive stellar evolution." (same source) Certain species may preferentially be seen in certain environments at certain evolutionary stages (observational biases?), but the exceptions are especially interesting


Wa t e r
Key line at 22.235 GHz Many other lines in (sub)millimetre: Session 10 Liz Humphreys, Friday 11:00 Todd Hunter, Friday 11:45 Nimesh Patel, Friday 12:10 22 GHz line is often very bright, frequently detected 22 GHz much easier to invert than (sub)mm lines ­ gas does not have to be much hotter than dust (Babkovskaia & Poutanen 2004)
(Wannier et al. 1991)


Water: Outflows Everywhere!
High-mass YSOs, low-mass YSOs Jet ejection and deceleration, inner part of molecular outflows, jet/disk interface Where there's a water maser, there's usually an outflow

(Goddi & Moscadelli 2006)

(Honma et al. 2005)


Water: Microstructure
AU scale jet in IRAS 20050+2720 MMS1 (Furuya et al. 2005) Larger scale expanding ring in W75 N VLA 2 contains AU scale substructure (Torrelles et al. 2003; Uscanga et al. 2005)

(Furuya et al. 2005)

(Torrelles et al. 2003)


Water: Other Contexts
Disks as well as outflows (Seth et al. 2002), with expanding shocks (Gallimore et al. 2003) Infall from hot gas after accretion shock (Menten & van der Tak 2004) Bok globules (GСmez et al. 2006; de Gregorio-Monsalvo et al. 2006) ­ bipolar outflows Bright rimmed clouds (Valdettaro et al. 2005)
(Seth et al. 2002)

(de Gregorio-Monsalvo et al. 2006)

(Gallimore et al. 2003)


Wa t e r
Water masers associated with low-mass protostars preferentially close to ionization front, suggesting triggered star formation truncated after ~105 years (Healy et al. 2004) Zeeman measurements: Wouter Vlemmings, Monday 11:55 Magnetic field directions from linear polarization suggest outflows not source of pinched field in W3 IRS5 (Imai et al. 2003) Variability: Periodic variability in some sources, velocity drift, etc. (Brand et al. 2003, 2005; Lekht et al. 2005, 2006) Jan Brand, Tuesday 16:15 Fractal distribution over several orders of magnitude in spatial scales suggests turbulence: Vladimir Strelnitski, Monday 11:10


Methanol: Overview
Methanol transitions divide into Class I and Class II (Menten 1991) Class I and II transitions are rarely seen together, although may come from different regions in same source Inversion in one Class usually results in enhanced absorption in the other Class Lines from both Classes may be excited simultaneously under certain (rare) conditions 6.7 GHz (II) and 25 GHz (I) masers seen together in OMC-1 (Voronkov et al. 2005) Michele Pestalozzi, Monday 15:05
(Voronkov et al. 2005)


Methanol Class I
"Class I methanol masers are created at the interface between molecular outflows and the parent cloud..." (Ellingsen 2006) Usually trace an earlier stage than Class II masers (Ellingsen 2006) Usually trace distant parts of outflows from YSOs (Sobolev et al. 2005) Collisional excitation Class I masers in NGC 6334I(N) associated with interface between outflows and dense gas (Beuther et al. 2005) Oblique shocks parallel to outflow axis? (Wiesemeyer et al. 2004) Weak 84.5/95.2 GHz masers southwest of W3(OH)? (Sutton et al. 2004)


Methanol Class I
Enhanced 36 GHz maser emission indicates younger evolutionary stage? (Hoffmann et al. 2006) 146.6/156.8 GHz intensity ratio may be a diagnostic of density and temperature (Lemonias et al. 2006) Short-term variability at 44 GHz (Pratap et al. 2006) Interferometric observations of 9.9 and 104 GHz masers, association with 2.12 m H2 indicates outflow (Voronkov et al. 2006) Some survey work at Haystack
(Voronkov et al. 2006)


Methanol Class II
Key transitions: 6668, 12178 MHz Many trace infrared dark clouds and hot molecular cores Often found in earlier evolutionary stage than ultracompact HII regions (Minier et al. 2005; Ellingsen 2006): Andrej Sobolev, Tuesday 11:00 Overlap with both UCHII regions and mm peaks ­ wide range of evolutionary stages in massive star formation (Pestalozzi et al. 2006) No masers toward sites of low-mass star formation (Minier et al. 2003) Trace Galactic structure well (Pestalozzi et al. 2005)


Methanol Class II: Disks?
Most linear structures with velocity gradients are not disks based on H2 location (De Buizer 2003) and maser motions (Minier et al. 2000) Candidate sources remain (Voronkov et al. 2003; Pestalozzi et al. 2004; Pillai et al. 2006) Shock fronts propagating into material with ordered velocity? (Dodson et al. 2004)

(Dodson et al. 2004)

(Norris et al. 1998)


Methanol Class II: Outflows and Shocks
Methanol masers associated with radio spectral indices typical of outflows (Zapata et al. 2006) Mid infrared suggests that masers are tracing walls of outflow cavities (de Buizer 2006): James De Buizer, Monday 17:00 Shocked neutral gas around UCHII regions, similar to OH (Phillips & van Langevelde 2005) Ring-shaped morphology in G23.657-0.127: radial shock? face-on disk? (Bartkiewicz et al. 2005)
(Bartkiewicz et al. 2005) (Phillips & van Langevelde 2005) (De Buizer 2006)


Methanol Class II: Monitoring/Surveys
Many maser sources are variable (Goedhart 2004) Periodic flares in G9.62+0.20 (Goedhart et al. 2003, 2005): Sharmila Goedhart, Monday 16:45 Surveys galore! (6.7 GHz) Toru, Onsala surveys Arecibo: Jagadheep Pandian, Tuesday 14:50 Parkes multibeam: James Green, Tuesday 16:00 12.2 GHz survey toward 6.7 GHz maser sources (Blaszkiewicz & Kus 2004): 12.2 GHz line usually weaker and rarer Other survey talks: James Caswell, Tuesday 14:00 Simon Ellingsen, Tuesday 15:15

(Goedhart et al. 2005)


Methanol Class II: Other Transitions
New detection of torsionally-excited 44.9 GHz maser (Voronkov et al. 2002) New detections of masers at 85.5 and 86.6 GHz (Ellingsen et al. 2003), 165 GHz (Salii & Sobolev 2006) New detections of 19.9 GHz masers: close association with cm emission and OH 6030/6035 MHz lines (Ellingsen et al. 2004) No new detections in rare 23.1 GHz maser line (Cragg et al. 2004) Observations of these and other transitions can help constrain physical parameters in maser models Better molecular data have not changed which lines are predicted to have masers, but do (slightly) change predicted brightness temperatures and excitation conditions (Cragg et al. 2005)


Hydroxyl
OH usually studied in sources with associated UCHII regions OH common also toward high-mass protostellar objects, often in association with Class II methanol, water masers (Edris et al. 2007) OH masers seen at ends of jet in W3 TW object (Argon et al. 2003) Nearby sources can host hundreds of detectable masers (Wright et al. 2004; Cohen et al. 2006; Fish et al. 2006)

(Edris et al. 2007)

(Argon et al. 2003)


Hydroxyl: Morphology & Distribution
Shocked neutral gas ahead of expanding UCHII region (Fish & Reid 2006), sometimes in very rapid expansion (Stark et al. 2007) Molecular disk or torus (Edris et al. 2005; Nammahachak et al. 2006) Masers in lines and arcs (Cohen et al. 2006) Filaments, especially 6 cm (Palmer et al. 2003)
(4765: Palmer et al. 2003) (13441: Baudry & Diamond 1998)

(Cohen et al. 2006)


Hydroxyl: Transitions & Overlaps
4.7 GHz lines usually weakly inverted, 4765 MHz usually strongest of triplet (Palmer et al. 2004); spectrum of 4765 and histogram of ground-state lines are similar (Palmer & Goss 2005) 4765/1720 overlaps (Palmer et al. 2003; Niezurawska et al. 2005); 4765 correlates better with 6035, though often at different velocity (Dodson & Ellingsen 2002; Smits 2003) 6030 almost always weaker than 6035 at same velocity (Caswell 2003), very close spatial association (Etoka et al. 2005; Fish & Sjouwerman, in preparation), often found with 1665 1612/1720 exhibit conjugate absorption and emission (Szymczak & GИrard 2004), although spatially-separated masers in both transitions in some sources (e.g., W3(OH): Wright et al. 2004) Multi-transition overlaps can constrain physical conditions in models


Hydroxyl: Magnetic Fields
Magnetic fields up to 40 mG (Slysh & Migenes 2006; Fish & Reid 2007) Magnetic field decay in Cep A continues (Bartkiewicz et al. 2005) Anisotropic MHD turbulence and Faraday rotation may suppress components and circularize components; full three-dimensional magnetic field reconstruction impossible (Watson et al. 2004; Fish & Reid 2006) Detailed study of 1612 and 1720 MHz transitions demonstrates that only ±1 components seen, important for Zeeman splitting coefficient (Fish et al. 2006)
(Fish et al. 2006) (Bartkiewicz et al. 2005)


Hydroxyl: Variability & Pumping
Short-term variability: Miller Goss, Tuesday 10:15 Huge 1665 MHz flare in W75 N up to nearly 1 kJy (Alakoz et al. 2005) 4765 MHz time variable (Palmer et al. 2004), extreme flaring rare but does occur (Mon R2: Smits 2003) FIR pumping line observations (He & Chen 2004, He 2005), although detectability limited by instrument spectral resolution Detailed pump modelling by Gray (2007): Malcolm Gray, Monday 9:30
(Gray 2007)

(Smits 2003)


Silicon Monoxide
SiO masers are common in evolved stars, rare in star forming regions Found in bipolar outflows, closer to exciting source than H2O masers In Orion, J=21 masers located farther from protostar than J=10 (Doeleman et al. 2004)

(W51 IRS2: Eisner et al. 2002)

(Orion Source I: Greenhill et al. 2004)


Silicon Monoxide
Five VLBA epochs of SiO v = 1 and 2 masers in Orion Source I Top arms redshifted, bottom arms blueshifted ­ implies rotation Masers trace edge of confining funnel at disk/wind interface? wind from highly inclined, flared disk? Disk-mediated accretion model for high-mass star formation
(movie courtesy Lynn Matthews)


Formaldehyde
H2CO masers appear in vicinity of massive YSOs Partially resolved at VLBI scales (Hoffman et al. 2003, 2007) Associations with other species: H2O, CH3OH, NH3 (Hoffman et al. 2003, 2007; Araya et al. 2005) New detections (Araya et al. 2005, 2006) Flare event (Araya et al. 2007) New insights into excitation mechanism Esteban Araya, Tuesday 9:00
(Araya et al. 2007)


Other Species
Ammonia (NH3): New (6,6) maser in NGC 6334 I (Beuther et al. 2007) Acetaldehyde (CH3CHO): 1065 MHz toward Sgr B2 (Chengalur & Kanekar 2003) Methylidyne (CH): No new observations, but Herschel and SOFIA will allow observations of 560 m line

(Beuther et al. 2007)

(Turner 1988)


Multi-Species Associations
Methanol, OH, and water masers frequently found in same source, although water often spatially separate (Caswell 2004; Edris et al. 2005, 2007; Szymczak et al. 2005) 55% of sources with 6668 MHz methanol masers have OH masers, almost always in the 1665 MHz transition and often at 1667 MHz as well (Szymczak & GИrard 2004) 6668 MHz methanol & 6035 MHz OH masers: similar distribution in W3(OH), but direct overlap rare (Etoka et al. 2005)

(Szymczak et al. 2005)

(Etoka et al. 2005)


Multi-Species Associations: Context
Water, OH, and methanol masers correlate more strongly with MIR emission than with cm or NIR emission (De Buizer et al. 2005) Water maser luminosity less correlated with FIR luminosity than are methanol and OH maser luminosity (Szymczak et al. 2005), likely because methanol and OH need FIR photons for pumping Existence of one of either masers (water and 6.7 GHz methanol) or outflows is an excellent predictor of the other (Codella et al. 2004)

(De Buizer et al. 2005)


Distances via Parallax
Kinematic interpretation of maser motions correct: Persistent spot shapes (Moscadelli et al. 2002; Goddi et al. 2006) Higher magnetic fields suggest higher densities than non-masing material (Fish et al. 2005, 2006) Precision astrometry requires close calibrators; known calibrators often not close enough (Xu et al. 2006b) Parallax distances to W3(OH) Water masers: 2.04±0.07 kpc (Hachisuka et al. 2006) Methanol masers: 1.95±0.04 kpc (Xu et al. 2006a) Probing Galactic structure with maser parallaxes and motions: Mark Reid, Thursday 9:00 Mareki Honma, Thursday 10:10


Spectral Resolution
Very high spectral resolution VLBI samples lineshapes and velocity gradients Implications for turbulence, beaming and saturation, physical conditions, velocity redistribution, maser modelling
(Watson et al. 2002)

(Moscadelli et al. 2003) (Fish et al. 2006)


General Questions
What is "a" maser? Complex geometry, filamentary structure, connection between low- and high-gain material, etc. Turbulence ­ How does it affect observed maser features? How can we use maser observations to measure and constrain its parameters? Polarization effects, especially of non-Zeeman origin ­ Amplification of polarized continuum? Curved magnetic fields? AlfvИn waves? Faraday depolarization? Hyperfine degeneracy? (Elitzur 2002) Moshe Elitzur, Monday 09:30 Wouter Vlemmings, Monday, 11:55 What does maser variability tell us about maser-scale and source-scale phenomena?


Future Prospects
Galactic structure: Mark Reid, Thursday 09:00 Galactic/ISM magnetic fields: JinLin Han, Monday 14:40 Richard Crutcher, Monday 14:00 Connections between disk/jet region and larger-scale outflows Higher-resolution contextual information from IR/submm IR/submm pumping lines New methanol transitions and targeted surveys of Class I masers Blind surveys of 1665/1667 MHz OH masers Many more... Future instruments: Session 11 ­ Karl Menten, Friday 14:00 ALMA: Alwyn Wootten, Friday 15:10 VSOP-2: Yasuhiro Murata, Friday 16:00 RadioAstron SKA: Anne Green, Friday 14:45


Summary
Masers are found in all sorts of environments: outflows, shocks, infall, disks The conditions that allow one species to produce a detectable maser frequently produce detectable masers in other species as well ­ multi-transition and multi-species associations and overlaps are an important line of research Some masers fit into an evolutionary sequence, others do not ­ exceptional cases may be exceptionally interesting There has been an impressive array of scientific results and insights since Mangaratiba; the future is especially promising thanks to new instruments (at radio/submillimeter and other wavelengths)


Closing Thoughts
Ellingsen (2004): masers are "the Bart Simpson of star formation research" (under-achievers due to complexity of massive star forming regions, lack of complementary high-resolution data) Common misconception, at least outside the maser astronomy community Impressive range of results, not just astronomy but astrophysics

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