Monitoring SiO Masers in TX Cam
• The Star
TX Cam is a pulsating star, a cool Mira Variable of spectral type M8-M10, located at the constellation of Camelopardalis. It's period is 557.4 days and its distance is 390pc. Like all AGB stars, its relatively small temperature allows the formation of molecules in its circumstellar envelope. From all the molecules found in its atmosphere, only silicon monoxide (SiO) exhibits maser emission, firsty observed in 1977. Since then, maser emission from other transitions and isotopes has been also observed. The importance of circumstellar SiO masers is their location. They are located in a narrow area between the surface of the star and the dust condensation region thus, their study can reveal the properties and dynamics of this area.
Click on the image to enlarge: The constelation of Camelopardalis (the Giraffe) in the night sky.
• The Data
We used the Very Long Baseline Array (VLBA) to observe the v=1, J=1→0 SiO maser at 43 GHz. The source was observed in biweekly intervals between the 24th May 1997 to the 9th September 1999 and then in monthly intervals until the 21st May 2000. Given the period of TX Cam, this corresponds to almost two complete stellar cycles from phases 0.68 to 2.64. Data were recorded in dual polarisation over 128 channels with 4 MHz bandwidth each at a rest frequency of 3.122027 GHz. Each scan was 6-8 hours long, 2.5 hours devoted to TX Cam and the rest to the calibrators 3C 454.3, 0359+509 and 0609-157. The data consistency demanded a uniform reduction for all the data sets so, an automated procedure was developed within AIPS. The final images were 1028x1028 pixels, with pixel spacing of 0.1 mas and restoring beam of 540x420 μas and converted in an animated movie. For missing or bad data the frames were linearly interpolated.
Click on image to enlarge: The 10 VLBA radio-telescopes.
• Results
The main properties of the v=1, J=1→0 are summerised below:
•The emission is confined in a ring-like structure, at a distance of 2-4 R* from its surface.
•The amplification is tangential since there is no emission inside the ring.
•The ring is well defined in its inner part with an uncertain boundary since the emission is changing from well defined maser spots to defuse emission with increasing radius.
•The flow is generally ordered but there are features that deviate from this behaviour.
•There is a lag of ~10% between the radio and optical. The variability follows that of the light curve.
•The ring undergoes expansion during the first cycle but in the second cycle, both expansion and contraction are observed.
•The gravitational acceleration for the first and second cycle were found to be 1.51x10-7 km/sec2 and 1.53x10-7 km/sec2 respectively. The initial velocities at the inner part of the ring were estimated for the first cycle as ~6.3 km/sec and for the second as ~7.3 km/sec given a distance of 390 pc for TX Cam.
•The width of the ring follows the pulsation mode and varies between 3.1 and 4.7AU.
•The maser shell is not rotating
•There is a velocity gradient along the filamentary emission.
•Most of the individual components also follow the behaviour of the ring. Some are kinematically complex, demonstrating three-dimentional kinematic behaviour.
•The lifetime of components follow a Gaussian distribution with a maximum of 200 days.
•There are several features that can be attributed to the existence of shock waves and their interaction with the circumstellar material. A graphical calculation gave us an estimate of the shock wave velocity of 12.5 km/sec.
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• The Movie - Latest Version (122 Frames)
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• The Movie - Older Version (73 Frames)
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• Papers Published so Far
• Observations of a Ring Structure in SiO Maser Emission from Late-Type Stars
Diamond P.J., Kemball A.J., Junor W., Zensus A., Benson J and Dhawan, ApJ, 1994, 430, L61
• Imaging the Magnetic Field in the Atmosphere of TX Camelopardalis
Kemball A.J., Diamond P.J., ApJ, 1997, 481, L111
• A Movie of a Star: Multiepoch Very Long Baseline Array Imaging of the SiO Masers Toward the Mira Variable TX Cam
Diamond P.J., Kemball A.J., ApJ, 2003, 599, 1372
• Multi-Epoch Imaging Polarimetry of the SiO Masers in the Extanded Atmosphere of the Mira TX Cam
Kembal A.J., Diamond P.J., Gonidakis I., Mitra M., Yim K., Pan K-C., Chiang H-F, ApJ, 2009, 698, 1721
• Kinematics of the v=1, J=1→0 Masers at 43 GHz towards TX Cam - A New 73-Frame Movie
Gonidakis I., Piamond P.J., Kembal A.J., MNRAS, 2010, 406, 395
• Electric Vector Rotation of π/2 in Polarised Circumstellar SiO Maser Emission
Kembal A.J., Diamond P.J., Pichter L., Gonidakis I., Xue R., ApJ, Accepted for publication, arXiv:1110.5094
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