preliminary maps of the emission, estimate the flux and size of the maser spots, and discuss their detectability on baselines from VSOP-2 to ground telescopes.
Approaching Micro-Arcsecond Resolution with VSOP-2: Astrophysics and Technology c 2009 ASP Conference Series, Vol. 402, Y. Hagiwara, E. Fomalont, M. Tsuboi, and Y. Murata, eds.
Detectability of Circumstellar SiO Maser Emission on VSOP-2 Baselines F. Colomer, V. Bujarrabal, R. Soria Ruiz, R. Dodson, J. Alcolea, J.-F. Desmurs Observatorio Astron´ omico Nacional, Apartado 112, E-28803 Alcal´ a de Henares, Spain Abstract. We have studied compact circumstellar SiO maser emission at 86 GHz with the Global Millimeter VLBI Array (GMVA), which provides the same spatial resolution as the VSOP-2 to ground baselines at 43 GHz. We present preliminary maps of the emission, estimate the flux and size of the maser spots, and discuss their detectability on baselines from VSOP-2 to ground telescopes.
1.
Introduction
VLBI observations of SiO masers are providing extremely valuable information on the inner circumstellar shells around AGB stars, as well as on the pumping mechanisms responsible for this emission often observed in AGB envelopes. The J=1–0 maser lines (in the v=1 and v=2 vibrationally excited states), at 7 mm wavelength, systematically yield ring-like flux distributions, at about 1014 cm from the star (equivalent to a few stellar radii). The structure and dynamics of these inner shells has been studied with a resolution that is equivalent to about 2 1012 cm, at the typical distance to these objects. (See e.g. Diamond and Kemball 2003, Desmurs et al. 2000.) The comparison of the brightness distribution for different lines is particularly fruitful. The v=1 and v=2 J=1–0 line distribution is composed of a number of spots that are often shifted by a few mas, and spatial coincidence between both lines is rare (Desmurs et al. 2000, Soria-Ruiz et al. 2004), a result that remains to be theoretically explained. For both lines, the clumps are distributed in ring-like structures, but the v=2 often occupies regions slightly closer to the star. These authors concluded that the relative spatial distribution of v=1,2 spots is an argument in favor of radiative excitation models. SiO v ≥ 1 J=2–1 lines at 3mm wavelength are also strong masers. It is interesting to compare the relative positioning of J=1–0 and J=2–1 lines with theoretical predictions. Both collisional and radiative models clearly predict ‘maser chains’ across the (excited) vibrational ladders, in such a way that the inversion of the different-J transitions in the same v state is mutually reinforced (e.g. Lockett and Elitzur 1992, Bujarrabal 1994). Although the maser phenomenon itself tends in general to amplify differences of any kind, it is difficult to avoid the conclusion that theory does predict that always the v=1 J=1–0 and 2–1 lines must come from the same clumps. However, observations of stars like IRC+10011, R Leo, TX Cam and the S-type star χ Cyg, have shown that the v=1 J=1–0 and 2–1 lines are not coincident at all, in fact they occupy quite different regions in the circumstellar shell (Phillips et al. 2003, Soria-Ruiz et al. 2004, Soria-Ruiz et al. 2006, Soria-Ruiz et al. 2007). It is practically impossible 404
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to reconcile this result with the general features of theoretical models of SiO masers, at least under the standard scenarios. The case of the v=2 J=2–1 SiO line is peculiar. Surprisingly this line is very weak, much weaker for instance than the v=1 J=2–1 one (see e.g. Olofsson et al. 1985, Bujarrabal et al. 1996). The v=2 J=2–1 line has been detected only in a few AGB stars, particularly including S-type stars. It has been argued that the reason for this behavior is a frequency overlap between two H2 O and SiO infrared lines (the H2 O transition ν=0 127,5 – ν2 =1 116,6 and the SiO transition v=1 J=0 – v=2 J=1), which modifies the available photon field at this frequency and leads to an overpopulation of the v=2 J=1 level. In S-type stars, the water vapor abundance would be relatively low and this phenomenon would less efficiently affect the SiO maser emission. Soria-Ruiz et al. (2004) have proposed that this overlap effect may be also the explanation of the surprising relative spatial distributions of the v=1 J=1–0 and 2–1 lines. Numerical simulations show that the overlap effect is so strong that tends to couple the excitation of the v=1 J=1–0 and v=2 J=1–0 masers, breaking the expected v=1 maser chain and the strong relation between the v=1 J=1–0 and J=2–1 maser pumping. Therefore the study of several SiO maser lines, in particular the v=1,2 J=1–0 and 2–1 transitions, may provide the information to study in general the pumping of SiO masers and, in particular, to confirm whether the line overlap plays a role in the excitation of these masers. The launch of VSOP2 is a unique opportunity to produce ultimate resolution maps of the 43 GHz sources, in comparison with those obtained with the GMVA at 86 GHz with the same spatial resolution.
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Figure 1. Telescopes participating in the Global Millimeter VLBI Array (GMVA) at 86 GHz, and uv coverage obtained in R Cas.
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GMVA Observations and Results
We have studied the v=1 J=2–1 SiO maser emission at 86 GHz in the circumstellar envelope of the AGB star R Cas. The observations were performed in 2004 and 2005 with the Global Millimeter VLBI Array (GMVA), comprising a total of 13 radio telescopes in Europe and USA (see Fig. 1). The goal of the observations was to produce a map of the emission with the highest spatial resolution ever achieved, thanks to the participation of very sensitive telescopes in Europe (Effelsberg, and the IRAM telescopes at Pico Veleta and Plateau de Bure) that would ensure detection of emission in the transcontinental baselines. The results shown in Fig. 2 and Fig. 3 are compatible with a 0.1 mas maser spot, maximum brightness temperature TB = 4.7 · 1010 K, and maximum opacity attained by the SiO maser of τ = −20. RCAS GC024 Freq = 86.2300 GHz, Bw = 16.000 MHz Phas deg
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Figure 2. Total (left) and cross-correlated (right) flux of the v=1 J=2–1 SiO emission towards R Cas as detected by the GMVA (only the transcontinental baselines VLBA-Effelsberg are displayed).
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Figure 3. Map of the v = 1 J=2–1 SiO emission towards R Cas. Left: with the whole GMVA; Right: only transcontinental VLBA-Effelsberg baselines.
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In parallel, because this very high spatial resolution is comparable to that expected in VSOP2-to-ground baselines at 43 GHz, we were expecting to test the detectability of any compact SiO emission and the feasibility of SiO maser studies with VSOP-2. Even if these are still preliminary, it is demonstrated that there exist ultra-compact SiO maser spots that are detectable, even with the very high spatial resolution of the GMVA and VSOP-2 instruments.
Figure 4. Detectability of a 300 Jy SiO (Gaussian & logarithmic) maser spot of 0.1 mas size on VSOP2-VLBA (dark) and VSOP2-phased VLA (blue) baselines.
References Bujarrabal V., 1994, A&A 285, 953 Bujarrabal V., Alcolea J., S´ anchez Contreras C., Colomer F., 1996, A&A 314, 883 Desmurs J.-F., Bujarrabal V., Colomer F., Alcolea J., 2000, A&A 360, 189 Diamond P.J., Kemball A.J., 2003, ApJ 599, 1372 Lockett P., Elitzur M., 1992, ApJ 399, 704 ˚, 1985, A&A 150, 169 Olofsson H., Rydbeck O.E.H., Nyman L.-A Phillips R.B., Straughn A.H., Doeleman S.S., Lonsdale C.J., 2003, ApJ, 588, L108 Soria-Ruiz R., Alcolea J., Colomer F., Bujarrabal V., Desmurs J-F., Marvel K.M., Diamond P.J., 2004, A& A 426, 131 Soria-Ruiz R., Colomer F., Alcolea J., Bujarrabal V., Desmurs J-F., Marvel K.B., 2006, Proceedings of the 8th European VLBI Network Symposium held in Torun (Poland) on September 26-29, 2006. Proceedings of Science, p.48 Soria-Ruiz R., Alcolea J., Colomer F., Bujarrabal V., Desmurs J-F., 2007, A& A 468, L1
