1. Max Voronkov Software Scientist – ASKAP 14th December 2010
Image credit: ALMA
Masering ALMA
Max Voronkov
Software Scientist – ASKAP
14th December 2010

2. What is a maser?
A spectral line formed under special conditions (population inversion)
Narrow lines and high brightness temperature (for strong masers, i.e. <-1)
Possible in a limited number of transitions
Sensitive to physical conditions
It is harder to create high-frequency maser
Bright masers are often used as tools:
to locate targets, to measure parallax, etc
Pumping process involves a delicate balance between radiative and collisional transitions. It is not understood well for some masers

3. Where do we find them? Star-forming regions in our Galaxy
High-mass: OH, H2O, CH3OH (both classes), a few SiO, NH3 and formaldehyde
Low- and intermediate-mass: OH, H2O, CH3OH (class I)
Supernova remnants: OH
Late-type stars and circumstellar environment
OH, H2O, SiO, SiS, possibly HCN and HC3N
Extragalactic masers (also known as kilomasers, megamasers, etc)
Star-forming regions in LMC and nearby galaxies (OH, H2O, class II CH3OH)
Late-type stars in LMC (SiO, OH)
Galactic nuclei (H2O)

4. Two classes of methanol masers
Class I methanol (CH3OH) masers
Collisional excitation (e.g. by shocks)
Regions of star formation (low-mass ones as well)
Usually offset from YSOs (up to a parsec)
Many maser spots scattered over tens of arcsec
Widespread masers: 36, 44, 84, 95 GHz
Rare/weak: 9.9, 23.4, series at 25, GHz
Class II methanol (CH3OH) masers
Radiational excitation (by infrared from YSO)
Regions of high mass star formation only
Located at the nearest vicinity of YSOs
Usually just one maser spot at the arcsec scale
Widespread masers: 6.7, 12 GHz
Rare/weak: 19.9, 23, 85/86, 37/38, 107, 108 GHz

5. Masers as evolutionary clocks
Image credit: Cormac Purcell
Image credit: Simon Ellingsen
Ellingsen (2006): class I masers tend to be deeply embedded younger.
However recent data show that a significant number of class I masers trace relatively evolved stage of high-mass star-formation
Whether class I masers can precede class II masers is unclear

6. I  methanol
From now on, I will concentrate on (Galactic) methanol masers
For a good review of submillimetre masers on other molecules see
E.M.L. Humphreys, 2007, IAUS 242, 471
I will discuss separately class I and class II methanol masers
what we already know from low frequency observations
which transitions can be observed with ALMA

7. My understanding of early ALMA
Masers have narrow lines. For methanol we aim at 0.1 km/s resolution
ALMA band
Frequency
Required resolution
Correlator mode 3
GHz
40 kHz
62.5 MHz/2048 channels (mode 18) 6
GHz
90 kHz 7
GHz
125 kHz
As above, or 500 MHz/4096 channels (mode 9) 9
GHz
240 kHz
500 MHz/4096 channels (mode 9)
The full 2 GHz bandwidth mode is not very useful for masers

8. G9.62+0.20E: strong class II maser
Fine structure at VLBI resolutions
(12 GHz maser)
6.7 GHz and 12 GHz masers are the strongest class II methanol masers
107 and 108 GHz transitions are popular weaker masers
Image: Goedhart et al. (2005)

9. ATCA and CABB at 3mm band
Frequency below
105 GHz!

10. Methanol maser series Red is class I Green is class II
Interestingly, all but one class II maser series go downwards and eventually terminate at the lowest possible level for that particular series
Class I masers are more important for ALMA

11. Class II methanol maser series J1-(J+1)o A+
Most widespread class II methanol maser series
Expect a single cluster of spots at arcsec scale (class II maser)
Starts with strongest and most common maser at 6.7 GHz (5000 Jy)
But not very impressive for early ALMA except for 107 GHz transition
Similar situation is for the second strongest 12.2 GHz transition. Only 108 GHz seems to be useful for early ALMA
Transition
Approximate frequency
51-60 A+
6.7 GHz
31-40 A+
107 GHz
21-30 A+
157 GHz
11-20 A+
206 GHz
Known masers

12. Class II methanol maser series J-2-(J+1)-1 E
Weak class II methanol maser series (bright in the isotropic case, i.e. no beaming and same optical depth in all directions)
Expect a single cluster of spots at arcsec scale (class II maser)
Several sources known at 37 GHz (strongest ~ 300 Jy), to be followed up with ATCA in March (PI: S.Ellingsen)
Transition
Approximate frequency E
37 GHz E
85 GHz E
230 GHz E
279 GHz
Known masers

13. Class II methanol maser series J1-(J+1)2 A-
Rare class II methanol maser series (narrow range of density and methanol abundance, quite high densities of cm-3)
Expect a single cluster of spots at arcsec scale (class II maser)
Only two reliable detections: W33(OH) (~10 Jy at 23.1 GHz) and NGC6334-I (~35 Jy at 23.1 GHz) + possible maser in NGC 7538 (0.5 Jy)
Transition
Approximate frequency A+
23.1 GHz
72-81 A+
111 GHz
42-51 A+
247 GHz
32-41 A+
294 GHz
22-31 A+
340 GHz
Known maser

14. Class II methanol maser series J2-(J+1)1 A-
Rare class II methanol maser series (it is not very clear to me why)
Expect a single cluster of spots at arcsec scale (class II maser)
Only one known source - W3(OH)
Lack of extensive searches (so these masers may be more common than we think they are)
Transition
Approximate frequency
82-91 A-
28.9 GHz
72-81 A-
81 GHz
42-51 A-
235 GHz
32-41 A-
285 GHz
22-31 A-
335 GHz
Known maser

15. Class II methanol maser series J2-(J-1)3 A±
Rare class II methanol maser series (seem to require rather low densities about 104 cm-3)
Pair of maser transitions for A+ and A- methanol at close frequencies
Expect a single cluster of spots at arcsec scale (class II maser)
Several sources known at 38/87 GHz
The only class II series which does not terminate
Transition
Approximate frequency
62-53 A±
38 GHz
72-63 A±
87 GHz A±
233 GHz A±
281 GHz A±
330 GHz A±
624 GHz A±
673 GHz
Known masers

16. G357.97-0.16 (good maser target for ALMA)
Red contour shows 12mm continuum (50% of the peak)
Squares are class II methanol masers at 6.7 GHz
Crosses are water masers
Circle shows position of rare class I masers
Background is 8.0µm Spitzer IRAC image
Northern source has an OH maser, the associated H2O maser has a large velocity spread with almost continuous emission across 180 km/s

17. G343.12-0.06 (outflow association)
Some maser spots are associated with an outflow traced by H2 emission
Rare masers are confined to a single spot near the brightest H2 knot

18. G309.38-0.13 (high-velocity feature at 36 GHz)
Background: Spitzer IRAC data
Red: 8.0 µm, green: 4.5 µm, blue: 3.6 µm
Excess of 4.5 µm may be a signature of
Shocks
Red contours: peak of the 36 GHz
emission across the velocity cube
Circles/crosses: maser spots (36/44 GHz)
36 GHz is one of the widespread
Class I maser transitions

19. Association with expanding Hii regions?
Class I masers may be associated with ionisation shocks driven by an expanding HII region into surrounding molecular cloud
This result is currently based on observations of 9.9 GHz masers (need higher temperature and density to form than 36/44 GHz) but should apply to other class I methanol masers as well
Another possible example
(but it has an outflow as well)
Crosses: 9.9 GHz masers
Open boxes: 6.7 GHz maser (Caswell 2010)
Contours: 8.6 GHz continuum
Grayscale: Spitzer 4.5µm
Grayscale: NH3 (Ho et al. 1986; Garay et al. 1998) G
W33-Met (G ) G

20. Class I methanol maser series J0-(J-1)1 A+
The most widespread and strong class I methanol maser series
Strongest known is ~ 500 Jy at 44 GHz
Spots (sub-arcsec size) scattered over a large area of 1 arcmin or more
Low contamination from thermal emission
Transition
Approximate frequency
70-61 A+
44 GHz
80-71 A+
95 GHz A+
251 GHz A+
303 GHz A+
356 GHz A+
623 GHz A+
677 GHz
Known masers

21. Class I methanol maser series J-1-(J-1)0 E
Second strongest class I methanol maser series, widespread
Spots (sub-arcsec size) scattered over a large area of 1 arcmin or more
Lower excitation energy than for J0-(J-1)1 A+, so thermal emission is more common (at least at 36 GHz)
Transition
Approximate frequency E
36 GHz E
84 GHz E
133 GHz E
229 GHz E
279 GHz E
327 GHz E
623 GHz E
673 GHz
Known masers

22. Class I methanol maser series J-1-(J-1)-2 E
Rare class I methanol maser series, traces strongest shocks
Usually just a single maser spot (sub-arcsec size)
Only few sources are known
Strongest maser in G (~70 Jy at 9.9 GHz)
Transition
Approximate frequency E
9.9 GHz E
104.3 GHz E
243 GHz E
287 GHz E
331 GHz E
608 GHz E
642 GHz E
675 GHz E
706 GHz
Known masers

23. Class I methanol maser series J1-(J-1)2 A-
Rare class I methanol maser series, traces strongest shocks
Very new maser series, properties are not well understood yet
Expect a single maser spot (sub-arcsec size)
Only two sources known: G (~20 Jy at 23.4 GHz) and G (~7 Jy at 23.4 GHz)
Transition
Approximate frequency A-
23.4 GHz A-
76.3 GHz A-
237 GHz A-
291 GHz A-
346 GHz A-
625 GHz A-
681 GHz
Known maser

24. What would I do with ALMA?
Look for various high-frequency class I methanol masers
There are several good targets studied in detail with ATCA
Tracing kinematics and morphology of shocks (outflows, expanding HII regions) at high resolution using widespread masers (e.g. 229 GHz)
Start with known widespread maser transition, e.g. 229 GHz
Constraining maser models
Many new transitions are very important, currently number of transitions ~ number of unknowns in the model
What is the highest frequency methanol maser?
Applicable to both class I and class II, but more important for class I
Follow up of the southern class II targets at 107/108 GHz
Accurate positions, high-resolution maps to compare with 6.7-GHz
Easy project with guaranteed success
In addition, masers are good test targets

25. Thank you Australia Telescope National Facility Max Voronkov
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Australia Telescope National Facility
Max Voronkov
Software Scientist (ASKAP)
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Thank you