Masers and star formation
Astrophysical masers and their environments
Proceedings IAU Symposium No. 242, 2007
J. Chapman & W. Baan, eds.
c© 2007 International Astronomical Union
DOI: 00.0000/X000000000000000X
Masers and star formation
Vincent L. Fish
Jansky Fellow, National Radio Astronomy Observatory, 1003 Lopezville Rd., Socorro, NM
87801, USA
email: vfish@nrao.edu
Abstract. Recent observational and theoretical advances concerning astronomical masers in
star forming regions are reviewed. Major masing species are considered individually and in
combination. Key results are summarized with emphasis on present science and future prospects.
Keywords. masers — stars: formation — radio lines: ISM — ISM: molecules — ISM: jets and
outflows — ISM: kinematics and dynamics
1. Introduction
This review summarizes maser results pertinent to star formation appearing in the
literature since the last maser meeting (IAU Symposium 206). References are drawn
from recent literature when possible.
2. Masing species
2.1. Water (H2O)
The 22.235 GHz water line is the predominant water maser line. Masers in this transition
are very bright, easily observable, and inverted under a wide range of conditions (e.g.,
Babkovskaia & Poutanen 2004). Several millimeter and submillimeter transitions of wa-
ter are also seen as masers. Discussion of these transitions can be found in the section of
these proceedings devoted to millimeter and submillimeter masers.
Water masers are frequently seen in outflows from both high-mass and low-mass YSOs
(Honma et al. 2005; Moscadelli, Cesaroni, & Rioja 2005; Goddi & Moscadelli 2006; Moscadelli et al. 2006).
These jets are seen in deceleration (Imai et al. 2002) and often have substructure on AU
scales (Torrelles et al. 2003; Furuya et al. 2005; Uscanga et al. 2005).
Water masers are sometimes believed to trace disks as well as outflows (Seth, Greenhill, & Holder 2002;
Gallimore et al. 2003), possibly excited by an expanding shock wave. Indeed, shocks
likely are responsible for arc-like maser distributions (Honma et al. 2004) and may ex-
cite masers in accreting material as well (Menten & van der Tak 2004). Water masers
appear in Bok globules (Gómez et al. 2006), likely tracing bipolar molecular outflows
(de Gregorio-Monsalvo et al. 2006), as well as bright rimmed clouds (Valdettaro et al. 2005),
again likely associated with outflows (Urquhart et al. 2006). The location of water masers
near the ionization front of large H ii regions provides evidence in support of triggered
star formation (Healy, Hester, & Claussen 2004). The common thread of all these en-
vironments is the existence of energetic shocks, which fits with conventional wisdom
regarding water maser pumping.
Despite the small Zeeman splitting coefficient of water, line-of-sight magnetic fields
of tens to hundreds of milligauss have been measured in star forming regions via water
maser Zeeman splitting (Sarma et al. 2002; Vlemmings et al. 2006), although direct in-
terpretation of the Stokes V profile as a magnetic field strength may be in error by up to a
http://arxiv.org/abs/0704.0242v1
2 Fish
factor of two depending on local velocity and magnetic field gradients (Vlemmings 2006).
Linear polarization observations of water masers can provide information on the orien-
tation of the magnetic field in the plane of the sky. The hourglass morphology of the
magnetic field in W3 IRS 5 appears to be due to the processes of collapse rather than a
result of the outflow traced by water masers (Imai et al. 2003). Further interpretation of
water maser polarization can be found in the review by Wouter Vlemmings.
Water masers have a fractal distribution over 4 orders of magnitude in spatial scale,
possibly indicating that they appear at the turbulent dissipation scale (Strelnitski et al. 2002;
Ripman & Strelnitski 2006; see also Vladimir Strelnitski’s contribution to these pro-
ceedings). Turbulence may also be responsible for variability, including changes in the
line-of-sight velocity of individual components, of the water masers in some sources
(Lekht et al. 2006a). Larger-scale variations may contribute as well, such as changes in
outflow parameters or cyclic variability of the central star (Pashchenko & Lekht 2005;
Lekht et al. 2006b). An ordered structure is inferred in W31(2) from successive flaring
of features at different velocities (Lekhn, Munitsyn, & Tolmachev 2005).
2.2. Methanol (CH3OH)
Methanol masers divide into two categories, known as class I and class II, based on
their propensity for certain transitions to produce masers while others are seen in ab-
sorption. Traditionally, class I and class II masers do not mix (e.g., Ellingsen 2005);
however, fine-tuned conditions may rarely excite lines from both classes simultaneously,
as appears to be the case in OMC-1 (Voronkov et al. 2005). For purposes of this review,
class I and class II methanol masers will be treated separately. Improved laboratory data
for rest frequencies have been obtained for many methanol transitions in both classes
(Müller, Menten, & Mäder 2004).
2.2.1. Class I
Class I masers are primarily collisionally pumped. They are typically found in younger
sources that are Class II masers and may trace distant parts of outflows interacting with
dense molecular gas (Beuther et al. 2005; Ellingsen 2006). Comparison of interferometric
maps of the 9.9 and 104.3 GHz transitions with H2 data confirms the outflow association
in IRAS 16547−4247 (Voronkov et al. 2006). Linear polarization suggests that Class I
masers may appear in oblique shocks parallel to the outflow axis and perpendicular to
the magnetic field in OMC-2 (Wiesemeyer, Thum, & Walmsley 2004), although interpre-
tation of methanol polarization may be complicated (e.g., Elitzur 2002).
Many different Class I transitions have been observed. Weak masers at 84.5 and
95.2 GHz to the southwest of W3(OH) provide strong evidence in support of colli-
sional pumping and allow for physical conditions to be inferred (Sutton et al. 2004).
The latter transition is commonly seen as a maser in both Class I and Class II sources
(Minier & Booth 2002). Strong 36 GHz maser emission is believed to be an indicator of an
early evolutionary stage, as may line ratios of other transitions (Gillis, Pratap, & Strelnitski 2005;
Hoffmann, Pratap, & Strelnitski 2006). The intensity ratio of highly-excited 146.6 and
156.8 GHz methanol masers may be a sensitive probe of density and temperature in mas-
ing regions (Lemonias, Strelnitski, & Pratap 2006). Short-timescale variability is seen in
the 44 and 146.6 GHz transitions (Pratap, Hoffmann, & Strelnitski 2006).
2.2.2. Class II
The key Class II maser transitions are at 6668 and 12178 MHz. Class II masers are
often found in an earlier evolutionary stage than ultracompact (UC) H ii regions (e.g.,
Minier et al. 2005), but observations of methanol masers cospatial with both millimeter
Masers and star formation 3
and centimeter continuum emission indicate that Class II masers appear over a wide
range of evolutionary stages (Pestalozzi et al. 2006). While the lower stellar mass limit
for methanol masers is still a subject of research, 6.7 GHz masers do not appear below
approximately 3 solar masses (Minier et al. 2003).
Linear structures of masers with organized velocity structures have led some to con-
clude that methanol masers often trace edge-on disks (Norris et al. 1993, 1998). Obser-
vations of maser proper motions (Minier et al. 2000), shocked H2 (De Buizer 2003), and
SiO (De Buizer et al. 2006) indicate that, in the majority of cases at least, methanol
masers are aligned with an outflow, not a disk. These structures may be explained by
propagation of a shock front into a region with large-scale velocity structure, such as rota-
tion (Dodson, Ojha, & Ellingsen 2004). Disk candidate sources remain (Slysh et al. 2002b;
Pestalozzi et al. 2004; Pillai et al. 2006), although these, too, may turn out to be associ-
ated with outflows when studied at high resolution in the mid infrared (e.g., De Buizer & Minier 2005).
The conclusion to be drawn is that a linear distribution of masers with a velocity gra-
dient does not by itself present convincing evidence that the masers trace an edge-on
disk. An intriguing variant is the possibility of methanol masers tracing a face-on disk in
G23.657−0.127 (Bartkiewicz, Szymczak, & van Langevelde 2005).
There is evidence to support the hypothesis that most methanol masers are tracing
shocked regions, often in the presence of outflows. Methanol masers appear preferentially
near radio sources with a spectral index indicative of an outflow (Zapata et al. 2006).
Mid-infrared images of some sources indicate that masers are found along the shocked
material on the surface of an outflow cavity (De Buizer 2006, 2007). In some sources,
methanol masers appear near but offset from UCH ii regions, suggesting that they appear
in the shocked molecular gas outside the ionization front, similar to hydroxyl masers
(Phillips & van Langevelde 2005).
The 6.7 GHz transition is a popular line for Galactic maser surveys. Several surveys
were reported on during IAU Symposium 206. Details of the Arecibo and Parkes multi-
beam 6.7 GHz surveys can be found in the section of these proceedings devoted to Galac-
tic maser surveys. Unsurprisingly, their distribution correlates well with Galactic struc-
ture (Pestalozzi, Minier, & Booth 2005; Pestalozzi et al. 2007). The 12.2 GHz line, when
it occurs, is almost always weaker than 6.7 GHz emission (B laszkiewicz & Kus 2004).
Both lines have also been the subject of monitoring studies (e.g., Goedhart, Gaylard, & van der Walt 2005),
which find variability in a large fraction of sources including periodic variability and a
time delay between features possibly due to light travel time (Goedhart, Gaylard, & van der Walt 2005;
Goedhart et al. 2005). Based on comparison of spectra over a period of a decade, the life-
time of an individual 6.7 GHz maser feature is about 150 years (Ellingsen 2007), while the
lifetime of the 6.7 GHz maser phase in a source is a few× 104 years (Codella et al. 2004;
van der Walt 2005), similar to the lifetime of the OH maser phase (e.g., Fish & Reid 2006).
Numerous other Class II transitions have been observed. New maser sources have
been found in rare transitions at 85.5, 86.6, and 107.0 GHz (Minier & Booth 2002;
Ellingsen et al. 2003) and a torsionally-excited line at 44.9 GHz (Voronkov, Austin, & Sobolev 2002).
Several weak maser lines near 165 GHz have also been detected (Salii & Sobolev 2006).
Emission in the 19.9 GHz transition is usually weak and correlates well with 6035 MHz
OH masers (Ellingsen et al. 2004). A search for 23.1 GHz emission resulted in no new
detections beyond the previously known maser in NGC 6334F (Cragg et al. 2004). Ob-
servations of these less common methanol maser transitions can help constrain physical
parameters in maser models. Improved collisional rate data has also allowed refinement
of methanol models, which slightly affects predicted excitation conditions and bright-
ness temperatures but not which transitions are expected to produce detectable Class II
masers (Cragg, Sobolev, & Godfrey 2005).
4 Fish
2.3. Hydroxyl (OH)
Hydroxyl masers are usually studied in sources with associated UCH ii regions (e.g.,
Fish et al. 2005) but are also found toward less evolved massive protostellar objects
(Edris, Fuller, & Cohen 2007). A different class of OH masers is seen at the ends of
the jet in the W3 TW object (Argon, Reid, & Menten 2003).
Hydroxyl masers around more evolved sources are usually seen in expansion (some-
times very rapid; see Stark et al. 2007) ahead of the ionization front of a UCH ii region
(Fish & Reid 2006). Sometimes the masers appear to trace a molecular disk or torus
(Slysh et al. 2002a; Hutawarakorn & Cohen 2005; Edris et al. 2005; Nammahachak et al. 2006).
Masers are often seen along arcs or filaments (Cohen et al. 2006), with extended filamen-
tary emission especially common at 4.7 GHz (Palmer, Goss, & Devine 2003).
Multitransition overlaps are of special interest because of their ability to constrain
physical conditions in models. The 4765 MHz line is observed to be the strongest line of
the 6 cm triplet but is usually only weakly inverted and often spatially extended (e.g.,
Palmer, Goss, & Whiteoak 2004; Harvey-Smith & Cohen 2005). A histogram of 18 cm
emission resembles 4.7 GHz lineshapes in W49A, suggesting that the high-gain 18 cm
emission and low-gain 6 cm emission have similar velocity distributions, even if the
4660 MHz emission is spatially separate (Palmer & Goss 2005). Much is made of over-
laps between 4765 and 1720 MHz masers (Palmer, Goss, & Devine 2003; Niezurawska
et al. 2004, 2005). It should be noted that 6035 MHz maser emission correlates more
strongly with 4765 MHz than does 1720 MHz, even if the velocities do not always agree
(Dodson & Ellingsen 2002; Smits 2003). Masers in the 1720 MHz transition also appear
to correlate with 1665 and 6035 MHz OH masers and 6.7 GHz methanol masers, at least
to arcsecond accuracy (Caswell 2004a). Masers in the 6030 MHz transition are almost
always accompanied by stronger emission at 6035 MHz (Caswell 2003), with excellent
spatial coincidence and agreement of magnetic field strengths with each other and with
1665 MHz masers (Etoka, Cohen, & Gray 2005). Masers in the highly-excited 13441 MHz
transition are rare but are always accompanied by 6035 MHz masers at the same velocity,
although the intensities in the two transitions do not show a high degree of correlation
(Baudry & Desmurs 2002; Caswell 2004c). The ground-state, satellite line transitions at
1612 and 1720 MHz are usually conjugate with respect to absorption and maser emis-
sion (Szymczak & Gérard 2004). Sources do exist in which both transitions are inverted,
though not in direct spatial overlap (e.g., Wright, Gray, & Diamond 2004).
Magnetic fields as strong as 40 mG are seen in OH masers (Slysh & Migenes 2006;
Fish & Reid 2007). Magnetic fields are highly ordered in star forming regions (Fish & Reid 2006)
and support pictures in which the processes of star formation do not tangle field lines
significantly. While magnetic field strengths are usually stable from epoch to epoch,
monotonic decay of the field in a Zeeman group in Cep A continues to be observed
(Bartkiewicz et al. 2005). High spectral resolution observations support the conventional
assumption that the Zeeman splitting coefficient appropriate for σ±1 components should
be assumed when measuring magnetic fields at 1612 and 1720 MHz (Fish, Brisken, & Sjouwerman 2006).
Linear polarization is of limited usefulness in determining the full, three-dimensional
orientation of the magnetic field, likely due to a combination of Faraday rotation and
anisotropic magnetohydronamic turbulence (Watson et al. 2004; Fish & Reid 2006).
Extreme variability is occasionally seen in OH. The 1665 MHz maser in W75 N briefly
flared to nearly 1 kJy to become the brightest OH maser in the sky (Alakoz et al. 2005).
The 4765 MHz transition is highly time-variable (Palmer, Goss, & Whiteoak 2004): the
maser in Mon R2 flared to nearly 80 Jy before disappearing (Smits 2003) and reap-
Masers and star formation 5
pearing (Fish et al. 2006b). Short-timescale variability is seen in the ground-state lines
(Ramachandran, Deshpande, & Goss 2006; also Miller Goss in these proceedings).
2.4. Formaldehyde (H2CO)
Formaldehyde masers are seen near a handful of several massive YSOs, with several new
detections in recent years (Araya et al. 2005, 2006). They have both a compact and an ex-
tended component with velocity gradients (Hoffman et al. 2003; Hoffman, Goss, & Palmer 2007).
A short-duration flare has been detected toward one source (Araya et al. 2007). Further
details can be found in the review by Esteban Araya.
2.5. Silicon monoxide (SiO)
While SiO masers are commonly seen in evolved stars, they are rare in star form-
ing regions. They are seen in bipolar outflows in W51 IRS 2 and Orion KL source I
and appear much closer to the central source than do water masers (Eisner et al. 2002;
Greenhill et al. 2004). As is the case in evolved stars, the maser species SiO, water, and
OH occur at progressively larger distances from source I (Cohen et al. 2006). While OH
masers are ubiquitous throughout Orion, there is a “zone of avoidance” associated with
source I in which they do not appear but SiO and water masers do. Interestingly, the
v = 1, J = 2 → 1 masers are found closer to the protostar in source I than are the
J = 1 → 0 masers, a finding that is difficult to understand in the context of SiO maser
pumping models (Doeleman et al. 2004).
2.6. Other species
Few other new maser species or transitions have been reported in the literature since the
last meeting. The first (J,K) = (6, 6) ammonia (NH3) maser has been detected centered
on a millimeter peak in NGC 6334 I (Beuther et al. 2007). Weakly inverted acetaldehyde
(CH3CHO) has been detected in the 111 → 110 transition at 1065.075 MHz toward Sgr
B2 (Chengalur & Kanekar 2003).
3. Multi-species associations
Methanol, OH, and water masers are frequently in the same source, although water and
methanol masers usually originate in different regions (Beuther et al. 2002; Caswell 2004b;
Edris et al. 2005; Szymczak, Pillai, & Menten 2005). Most 6.7 GHz methanol maser sources
have associated OH masers, almost always at 1665 MHz and frequently at 1667 MHz
as well (Szymczak & Gérard 2004), while the correlation between 6.7 GHz methanol
masers and 22 GHz water masers is less strong (e.g., Breen et al. 2007). The distribu-
tions of 6.7 GHz methanol masers and 6.0 GHz OH masers in W3(OH) are very similar,
although direct overlap of the two species is rare (Etoka, Cohen, & Gray 2005). Simi-
lar phenomena are also seen between 6.7 GHz methanol and 1.6/4.7 GHz OH masers
(Harvey-Smith & Cohen 2006).
All three species correlate more strongly with mid infrared emission than centimeter or
near infrared emission, and all are frequently found in linear groupings (De Buizer et al. 2005).
However, the luminosity of water masers correlates less strongly with far infrared lumi-
nosity than is the case for methanol and OH masers, likely because water masers are
not predominantly pumped by infrared photons (Szymczak, Pillai, & Menten 2005), al-
though they may not be pumped entirely by collisions either (Liu, Forster, & Sun 2005;
Liu et al. 2007). In any case, the existence of either masers (water and methanol) or out-
flows towards a UCH ii region is an excellent predictor of the other (Codella et al. 2004),
indicating that both masers and outflows are usually detectable in the UCH ii phase.
6 Fish
4. Observational advances
4.1. Proper motions and geometric distances
Evidence in support of the kinematic interpretation of maser motions continues to pile
up, with reports of the persistence of spot shapes in methanol (Moscadelli et al. 2002)
and water masers (Goddi et al. 2006) and the inferred average overdensity of masers as
compared to non-masing material (Fish, Reid, & Menten 2005; Fish et al. 2006a). In ad-
dition to tracing internal source motions, maser proper motions can be used to obtain
geometric parallax distances and measurements of Galactic rotation. Recent years have
seen this technique used for both methanol and water masers in W3(OH) to obtain dis-
tances accurate to a few percent (Xu et al. 2006; Hachisuka et al. 2006). Further details
can be found in proceedings in the Galactic structure session.
4.2. Spectral resolution
Very high spectral line observations at VLBI (very long baseline interferometry) spa-
tial resolution have been obtained toward masers in several species in star forming re-
gions. Detailed line profile analyses of water masers conclude that the near-Gaussian
lineshapes indicate that they occur in hot (∼ 1200 K) gas with small beaming angles
(Watson, Sarma, & Singleton 2002). Similar observations of 12.2 GHz methanol masers
(Moscadelli et al. 2003) and the ground-state quartet of OH in W3(OH) (Fish, Brisken, & Sjouwerman 2006)
find similarly Gaussian spectral profiles as well as maser spot positional gradients as a
function of velocity (equivalently, velocity gradients). The positional gradients show no
clear large-scale spatial organization but have similar magnitudes in methanol and three
of the four OH transitions. It is possible that these positional gradients represent turbu-
lent motions on very small spatial scales. If so, it is important to understand the char-
acteristics of the turbulence, since observed maser properties, including variability, can
be highly sensitive to turbulence in the masing region (Böger, Kegel, & Hegmann 2003;
Sobolev, Watson, & Okorokov 2003; Silant’ev et al. 2006). Similar polarization charac-
teristics in Class II methanol features at different velocities may indicate that velocity gra-
dients induce velocity redistribution (Wiesemeyer, Thum, & Walmsley 2004), which may
play a critical role in preventing saturated rebroadening (e.g., Nedoluha & Watson 1988).
4.3. Infrared pumping lines
Molecular infrared transitions observations, particularly of OH, are essential for measur-
ing maser pump efficiencies and may help place observational constraints on the radiative
pump cycles of some models (e.g., Gray 2007). Archival Infrared Space Observatory data
have been searched for the 34.6 and 53.3 µm pumping lines of OH with limited success,
due to the low spectral resolution of the instruments (He & Chen 2004; He 2005). Her-
schel and SOFIA will have the spectral resolution and frequency coverage required to
observe pumping lines of OH as well as the critical 560 µm line of methylidyne (CH).
5. Further remarks
Two quotes from De Buizer et al. (2005) serve to summarize the common themes of
the observations over the past five years. The first is 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.” Indeed, while certain maser species may preferentially turn up
in a particular context (possibly a result of observational biases), the set of all observa-
tions of any one maser species resist being pigeonholed into a particular phenomenon.
Masers and star formation 7
The second quote is that water, OH, and methanol masers “do not seem to be associated
with different early 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.” As is clear from §3, different maser species are commonly found together,
independent of the evolutionary stage of the source. While proposed sequences in which
certain masers turn on before others may be useful for statistical evaluation of evolu-
tionary phases, important exceptions to such sequences exist. Those oddball masers that
do not seem to fit present standard paradigms should be studied in especial detail, since
we cannot predict beforehand what will be thereby learned about their environment or
about maser processes in general.
It was only a few years ago that Ellingsen (2004) referred to masers as “the Bart Simp-
son of star formation research,” noting that they are “under-achievers” in comparison
with masers in other environments due to the lack of sensitive, high resolution observa-
tions at complementary wavelengths. While this may once have been true, recent maser
observations have made great advancements in probing a wide range of dynamic struc-
tures relevant to star formation. Maser VLBI allows observations of small- and large-scale
morphologies, magnetic fields, and motions on AU scales and is showing great promise
as a tool to trace Galactic structure. Maser models for some species are becoming suf-
ficiently refined to provide good constraints on physical conditions. The community is
beginning to appreciate the role of turbulence and the ability to probe its properties
using maser observations. Synergies with mid infrared instruments have clarified many
of the mysteries of linear structures with velocity gradients. We have entered the era of
greatly improved far infrared instrumentation, and the ALMA (Atacama Large Millime-
ter Array) era, with unprecedented sensitivity and angular resolution at submillimeter
wavelengths, will begin in a few years. Further advancements in radio instrumentation,
including new space VLBI missions and the SKA (Square Kilometre Array), will provide
even greater insights. It is perhaps more correct to state that maser observations are at
the vanguard of star formation research: yesterday’s observations can be explained by
complementary data and theory today, and today’s observations lay the groundwork for
the breakthroughs that will be achieved in the context of tomorrow.
Acknowledgements
The National Radio Astronomy Observatory is a facility of the National Science Foun-
dation operated under cooperative agreement by Associated Universities, Inc.
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	Introduction
	Masing species
	Water (H2O)
	Methanol (CH3OH)
	Class I
	Class II
	Hydroxyl (OH)
	Formaldehyde (H2CO)
	Silicon monoxide (SiO)
	Other species
	Multi-species associations
	Observational advances
	Proper motions and geometric distances
	Spectral resolution
	Infrared pumping lines
	Further remarks
