1. SMA Observations of 321 GHz water maser emission in Cepheus A
Nimesh A. Patel
Harvard-Smithsonian Center for Astrophysics
Submillimeter Array
Collaborators:
Salvador Curiel (UNAM)
Qizhou Zhang (CfA)
T. K. Sridharan (CfA)
Paul T. P. Ho (CfA, ASIAA)
Jose M. Torrelles (CSIC-IEEC)
I will talk about a specific application of the way we can use submillimeter masers as high-spatial resolution probes of physical conditions as just described by Liz Humphreys. My collaborators are listed here... These results are reported in our paper which will come out next week in ApJ Letters.
Patel, Curiel, Zhang et al. (2007), ApJ 658,L55

2. Neufeld and Melnick (1991) ApJ 368,215
This is the energy level diagram of ortho and para water showing the masing transitions, from Neufeld and Melnick’s paper on calculations of level populations for submillimeter masers.
Neufeld and Melnick (1991) ApJ 368,215

3. A zoom of this plot showing the transitions relevant to us right now...

4. ...shows that the 321 GHz maser transition is about 1200 K above the well studied 22 GHz transition (which, to remind you, is itself more than 600 K above the ground level). Thus, observing the 321 GHz maser allows us to trace highly excited gas.

5. Discovery of submillimeter H2O masers:
321 GHz: K. Menten, G. Melnick & G. Phillips, 1990, ApJ, 350, L41
The 321 GHz maser emission was first observed by Karl Menten, Gary Melnick and Tom Phillips in They observed 5 sources, all of them massive star-forming regions, except one- VY CMa. Here is a figure from their paper showing the 321 GHz lines along with 22 GHz lines. Note the weak and narrow maser lines and also lack of similarity between 22 GHz and 321 GHz line profiles. The 22 GHz data were obtained about one month after the submillimeter data.

6. Motivation: To map the 321 GHz water maser emission with the SMA
To use the 22/321 GHz line ratio to probe the physical conditions in regions of massive star-formation with high spatial resolution.
To understand the physics of the excitation of submillimeter water maser transitions.

7. Cepheus A massive star-forming region
contours: VLA 1.3cm radio continuum, + 22 GHz 0.1”resolution
Torrelles, Gomez, Garay et al. 1998, ApJ 509,262
725 AU
Now I will give some background on the source we observed- Cepheus A is a well known high-mass star-forming region. Here is a plot from Torrelles et al., showing the 1.3cm continuum emission as contours, observed with the VLA, and 22 GHz masers shown as plus symbols. About 10 radio continuum sources are known, first observed by Hughes and Wouterlout, hence called HW sources. HW2 is the most dominant source in the region.

8. VLA radio continuum 3.6 cm 1.3 cm SMA (ext. array) 0.8”
Disk and jet in Cepheus A HW2
SMA (ext. array)
0.8”
330 GHz continuum
CH3CN
VLA
radio continuum
3.6 cm
1.3 cm
We observed this source with the SMA and here is the main figure from our Nature paper of This is 330 GHz continuum emission mapped with the SMA at 0.8” resolution. The red and blue contours show VLA observations of the thermal jet, which was subsequently found to have very large proper motions of about 500 km/s as shown by Curiel et al. last year. The green contours show methyl cyanide emission. We have interpreted this elongated structure seen to be coincident with the thermal jet, to be representing a circumstellar disk. However, I would like to point out other references with alternative interpretation: Brogan et al. PPV paper presents SMA compact configuration data showing extended emission that we have missed in our observations. Comito et al. presented observations from PdBI interpreted as multiple sources along the elongated structure around the jet. Martin-Pintado et al. found a new source about 0.5” east of the HW2 source, within the disk-structure we show here. Hence, this region appears to be quite complex and we can ask how observations of masers help diagnose the structure and kinematics in this region.
Brogan et al. (2005 PPV)
Martin-Pintado et al. (2005, ApJ, 628,L61)
Comito et al. (2005 IAU#231)
Patel, Curiel, Sridharan et al., 2005, Nature, 427,109

9. 200 AU See also: Posters on methanol masers VLBI observations by
K. Torstensson &
H. J. van Langevelde
(JIVE)
Sugiyama et al.
(JVN)
Gallimore et al.
(2003) ApJ 586, 306
MERLIN observations
This figure is from Wouter Vlemmings et al.’s recent paper and it summarizes most of the previous 22 GHz water maser observations. One may add to this plot MERLIN observations by Gallimore et al. The ellipse represents the position and orientation of the HW2 thermal jet. I also note that in this meeting, we have two poster papers presenting results of methanol maser observations in this region, again showing alternative interpretations regarding the disk associated with HW2.
200 AU
Vlemmings, Diamond, van Langevelde & Torrelles, 2006, A&A, 448,597

10. Bipolar molecular outflow observed in HCO+
blueshifted +
redshifted
The outflow associated with HW2 was first observed in CO by Rodriguez, Ho and Moran in 1980, and is one of the most powerful outflow sources known. Here I am showing this bipolar outflow as traced by HCO+ emission, from Gomez et al. Note that the blue shifted lobe is towards the northeast.
0.07 pc +
HW2
Gomez, Sargent, Torrelles et al. 1999, ApJ, 514, 287
CO outflow: Rodriguez, Ho & Moran, 1980, ApJ, 240, L149

11. extended configuration, 0.8’’x0.7’’
Observations:
SMA
30 August 2004
extended configuration, 0.8’’x0.7’’
rms noise ~350 mJy/beam
This is a brief summary of our observations. The VLA observations followed the SMA observations after 43 days.
VLA
12 October 2004
A-array 0.09’’x0.07’’
rms noise ~3 mJy/beam

12. Cepheus-A 321 GHz masers in HW2 and HW3c
-19
-21
-11
-3.5 to +1
km/s
Here is the main result showing the 321 GHz masers mapped with the SMA, shown with large symbols with error bars showing the formal uncertainty in positions. The 22 GHz masers observed with the VLA are shown as crosses. Only two of the HW radio continuum sources, HW2 and HW3c, show the 321 GHz maser emission. The numbers are line of sight velocity , indicating that the 321 GHz masers are not only along the jet, but also have similar kinematic signature, consistent with the large scale bipolar outflow shown earlier. Note that the 321 GHz masers are not well coinciding with 22 GHz masers in general; only 3 out of 9 masers coincide in position as well as velocity.

13. This are the spectra of 321 and 22 GHz masers
This are the spectra of 321 and 22 GHz masers. Note that in HW2, 321 GHz masers seem to prefer higher velocities w.r.t. the systemic velocity shown here by dashed line.

14. Neufeld & Melnick 1990
How can we use the observed line ratio of 22 to 321 GHz masers in regions where the masers coincide, to infer physical conditions. This is a plot of temperature vs xi, where xi was defined by Elitzur, Hollenbach and McKee in their 1989 paper. It is basically water abundance times the square of gas density divided by the velocity gradient. For our purpose we can assume this represents the square of the density. We see from this plot that for maser emission comparable in intensity in both the transitions, we need very high temperatures, exceeding 2000 K. The 321 GHz masers are extinguished on the right of the right-most contour shown here.

15. ALI Modelling population inversion of submm. water maser transitions
Yates, Field & Gray (1997)
MNRAS, 285, 303
ALI
no LVG approximation
22 GHz line is inverted
over a broad range of
parameters. K
n=10^8 - 10^9 cm^-3
This is a similar calculation by Yates et al. (the only other theoretical model published for 321 GHz masers), which has a broader range of parameters but comes up with essentially similar results. Note that the 22 GHz masers cover a broader range of parameters, hence they should always be ON even in the hotter regions where 321 GHz masers show up. But the case where we only see 321 GHz masers is not predicted or allowed by either of these models.

16. time-variability of 22 GHz masers
Over time scales of months:
highly luminous sources less variable
brighter (>10 Jy) masers less variable
proper motion studies with VLBA
time-variability of 22 GHz masers
Cepheus A maser monitoring:
One obvious problem in comparision between 321 GHz and 22 GHz masers is due to possible time-variations in the maser emissions. However, for the 22 GHz masers, it appears that highly luminous sources are less variables. Also, brighter masers close to the systemic velocity of the source are less likely to be varying much in intensity. Finally, we note that over time scales of 1-2 months, we have carried out VLBA multi-epoch studies to measure proper motions in this source and we have been able to derive proper motions for a majority of the observed maser spots, which would be difficult if the emission was varying significantly with time. I am grateful to Andrej Sobolev for this reference to the paper by Lekht et al. which reports time monitoring of 22 GHz masers of this specific source, supporting the above statements.
Lekht et al. 1983, Sov. Astron. 26, 170

17. Cepheus-A 321 GHz masers Conclusions
321 GHz masers detected in HW2 and HW3c in the Cepheus-A high- mass star-forming region
3 out of 9 submillimeter masers are associated with 22 GHz masers
submillimeter masers trace the outflow jet, presumably arising in hot gas (1000~2000 K)
Multi-transition observations of H2O masers can be used to constrain theoretical models of excitation
Submillimeter masers, along with cm masers can be used to probe physical conditions such as temperature in star-forming regions
I am summarizing the conclusions here. (Read out each one, if time permits).

18. Future work
Other high-mass star-forming regions: W75N, W3(OH), NGC6334
Masers in 400 GHz band
Simultaneous observations with VLA
Anuj Sarma and I have submitted an SMA proposal to observe W3(OH) and W3IRS5.

19. I am grateful to Jim Moran, Gary Melnick, David Neufeld, Liz Humphreys and the SMA staff. Thank you.
The SMA project is a collaboration between SAO and ASIAA, Taiwan.
We thank the SMA staff for their help with observations.
We thank Jim Moran, Gary Melnick, David Neufeld and Liz Humphreys for helpful discussions.