3 and 12-mm Compact Array observations of masers in star-forming regions and exoplanetary systems
Astrophysical masers have been widely studied in recent years and have been shown to be invaluable probes of the details of the environment in which they are found. Masers arise from various astronomical regions such as star-forming regions, circumstellar envelopes of AGB stars, comets, proto-planetary nebulae, starburst galaxies and AGNs. In this newsletter, we present very recent Australia Telescope Compact Array (ATCA) observations of 3-mm methanol masers in G345.01+1.79, an exceptional massive star-forming region, as well as the attempt to detect water-vapour masers in exoplanetary systems with the fully upgraded array at 12 mm.
6.7 and 12.2-GHz methanol masers are now recognized as excellent tracers of the physical conditions in massive star-forming regions, at scales from 1 to 1000 AU (1 milliarcsecond to 1 arcsecond at 1 kpc). VLBI observations have shown, for instance, that 6.7 and 12.2-GHz methanol masers arise within 3000 AU around the massive stellar object (Minier et al., 2001) and that the masing regions have linear dimension from 1 to 100 AU (Minier et al., 2002). As masers require very specific density and temperature to switch on, their observation at high resolution would reveal the nature of the physical conditions in the inner part of the protostellar envelope.
Methanol masers also exist at higher frequencies in the 3-mm range. They were predicted by Sobolev et al., (1997) and were observed at 85.5, 86.6, 86.9, 107.0 and 108.8 GHz (e.g. Cragg et al., 2001; Minier and Booth, 2002). In the southern hemisphere G345.01+1.79, a massive star-forming region, exhibits strong methanol emission at 6.7, 12.2, 85.5, 86.6, 86.9, 107.0, 108.8 and 156.6 GHz. The millimetre emission lines have been interpreted as maser features based on their narrow line-widths, their velocity coincidence with the 6.7 and 12.2-GHz maser features and their LSR velocities distinct from that of the quasi-thermal emission seen at 108.8 GHz (Cragg et al., 2001). Ellingsen et al. (2003) have recently combined the multi-frequency maser observations with the Sobolev maser-model and proposed to constrain density, dust and gas temperatures of the masing medium.
We observed 85.5 and 86.9-GHz methanol masers in G345.01+1.79 with the ATCA in October 2002. Three antennas were used in the 750A configuration. 85.5 and 86.9-GHz masers were easily detected at the same velocity as the 6.7 and 12.2-GHz masers (Figure 1a).
Figure 1a: Methanol masers in G345.01+1.79. (a) Positions of the 6.7 and 12.2-GHz masers (white circle) and of the 85.5 and 86.9-GHz methanol masers (yellow square) overlaid on the radio continuum emission (colour scale and grey contours). The ATCA 3-mm maser spectra are shown at the bottom of the figure.
Surprisingly, the 3-mm masers coincide in velocity and position with the cm masers, and hence coincide in space in G345.01+1.79. This is only possible for a narrow range of physical conditions according to the maser model (Figure 1b). Figure 1b suggests that the environment of the masing medium in G345.01+1.79 contains warm dust (250 K), cool gas (40 K) and a large methanol column-density.
Figure 1b: Modelling of the maser suitable conditions (Ellingsen et al., 2003). The contour diagrams show where methanol masers become active as function of density and temperature. Contours are labelled with the maser frequency in GHz and represent a brightness temperature threshold of 106 K. For instance, all masers are quenched for hydrogen densities greater than 108 cm-3. G345.01+1.79 corresponds to point F in the diagram for which all transitions could be detected.
Figure 2: Examples of water-vapour maser spectra in G345.01+1.79, a massive star-forming region and in NGC 2024-FIR5, a low-mass class 0 object in the Orion B region. The spectra were obtained with the fully upgraded ATCA at 22 GHz after averaging all baseline data.
Water masers are also found in star-forming regions (Figure 2), but in this newsletter we present a search for water-vapour masers in a new class of astronomical objects, the extra-solar systems.
Water is the most common triatomic molecule in the universe and the basis of life on Earth. Doppler radial-velocity surveys have detected about 100 planets orbiting nearby solar-type stars including 12 planetary systems. The proximity and availability of water on or near these exoplanets is an important piece of our emerging picture of how our Solar System compares to these newly detected planetary systems, and more speculatively what the prospects for water-based life are.
22-GHz water-maser emission has been reported coming from Jupiter induced by the Shoemaker-Levy comet collision (Cosmovici et al., 1996) and more recently from several exoplanet host stars (Cosmovici et al., 2002). However, null results have also been reported for five exoplanet host stars (Greenhill, IAU circular 7985), of which Upsilon Andromedae and Epsilon Eridani were thought to emit water-maser emission.
The mechanisms that might generate 22-GHz water-maser emission include (but are not limited to) cometary impacts in atmospheres of giant planets, particularly in younger stellar systems in which much more massive and frequent impacts are expected. Whether the required conditions (column density, temperature...) are present to produce water masing is unknown or unlikely, but seven years ago, the presence of hot Jupiter-like planets around nearby stars also seemed unlikely.
Observations of water masers from exoplanetary systems would give us a new detailed window through which to explore them. Details extractable from maser detection include answers to the following questions: what is the velocity of the masing source (e.g. planets or comets)? Which part of the planetary system is compatible with the column densities of water, a pumping mechanism and a lack of collisional thermalization that would otherwise quench the maser? By combining observations and models, the physical conditions of the exoplanetary systems may be probed.
The possible detections of water masers by Cosmovici et al. in Upsilon Andromedae and Epsilon Eridani were made with a single-dish telescope (Medicina-32m) whose primary beam was equal to 100 arcsecond at 22 GHz. This is considerably larger than the diameter of planet orbits (typically a few AU) for planetary systems at 10 to 100 parsec.
In April 2003, we used the newly upgraded ATCA system at 12 mm in the EW352-baseline configuration and with antenna 6. Twenty exoplanetary systems were searched for water masers down to sensitivity of 20 30 mJy/beam (rms). An angular resolution of about 10 arcseconds was obtained, allowing us to probe the inner part of the planetary systems. Such a small beam would clearly establish the connection between water maser and exoplanets in case of detection. In addition, the high sensitivity of ATCA at 12 mm allows us to reach rms of 20 mJy/beam in 30 minutes. This is important since masers from a planetary atmosphere would be highly variable as the planet rotates (only 10-hour sidereal period for Jovian planets).
A 3-s detection was made toward HD47536, a K1III giant star located at 123 pc, which is probably orbited by a giant planet (Setiawan et al., 2003). The detected line is narrow and peaks at a radial velocity of 37 km/s, which would correspond to nearly the maximum Keplerian velocity of the candidate planet around HD47356 (Figure 3). Interestingly, the line varies in velocity and intensity during the 30-minute observing scan. The line was not re-detected with Tidbinbilla in May 2003. Given the potential variability of such a line, it is not surprising. No obvious detection was made toward the other exoplanets although a few marginal detections would need further analysis.
Figure 3: Possible detection of a 22.2-GHz water-vapour maser in HD47536, a K giant star with a 5-9 MJ companion. The spectrum has been generated by averaging the data taken on all the baselines over 18 minutes.
Our observations have confirmed that water masers in planetary systems are rare phenomena and would require constant monitoring to be able to model their origin.
With ATCA at 3 and 12 mm, we are now able to probe the conditions in various astronomical objects in the southern sky with the help of maser phenomena. Future research in relation with H2O, CH3OH masers, but also with 24-GHz NH3 masers and 86-GHz SiO masers will make full use of the newly upgraded ATCA.
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Vincent Minier in collaboration with Simon Ellingsen, Cormac Purcell, Michael Burton, Tony Wong, Dinah Cragg, Andrej Sobolev (methanol masers) and Charley Lineweaver and Ray Norris (water masers).