Silicon Monoxide: Detection of Maser Emission from the Second Vibrationally Excited State

Abstract

The J = 1-0 rotational transition of the second vibrational state (v = 2) of SiO has been detected in late M-type variable stars and in the center of the Orion Nebula. This transition of SiO requires 3520° K of excitation for pumping the observed maser line. The ground {v = 0) and third {v = 3) vibrational states were not detected. Subject headings: masers-molecules-Orion Nebula-variable stars In 1973 December we detected maser emission from the center of the Orion Nebula which we suggested might be a rotational transition (a = 1, / = 2-1) in the first vibrationally excited state of silicon monoxide (Snyder and Buhl 1974a). With the subsequent detection of two more rotational transitions of SiO (a = 1, J = 3-2; and a = 1, / = 1-0) this identification was confirmed (Davis et al. 1974; Thaddeus et al. 1974). During 1974 January and March we made a survey of sources of maser emission from SiO (v-1, / = 2-1) at 3.5 mm wavelength and found that the only sources of this line outside of Orion were a number of late M-type variable stars, many of which have OH and H 2 0 maser emission (Kaifu, Buhl, and Snyder 1974). In this Letter we report on the detection of the second vibrationally excited state of SiO (v = 2, J = 1-0) as well as the first vibrationally excited state (a = 1, / = 1-0) in a number of sources. The observations were made from 1974 June 1 to June 4 with the 36-foot (11m) telescope of the National Radio Astronomy Observatory at Kitt Peak. A 7-mm-wavelength mixer receiver was used which has a single-sideband system temperature of 1500° K. The spectra were taken with a 256-channel filter bank which has a resolution of 100 kHz corresponding to a velocity resolution of 0.7 km s-1. Temperature calibration was done with a chopping wheel in front of the feed. This method of calibration corrects for telescope blockage, ohmic loss, and feed spillover, yielding an antenna temperature Ta (Ulich 1974). Observations of Orion were done with the dome blocking the telescope to prevent solar heating of the surface. Hence, the temperature scales displayed for Orion have been increased * The NRAO is operated by Associated Universities, Inc., under contract with the National Science Foundation. t Visiting Fellow on leave from the by 1.67 to correct for dome loss. Other sources were observed through the open slit of the dome. Velocities in this paper are given with respect to the local standard of rest. The aperture efficiency of the telescope at this wavelength is 50 percent, and the half-power beamwidth is 160". We made observations on the first three vibrationally excited states of 28 SiO (v = 1, 2, and 3, / = 1-0) as well as the ground state (a = 0, / = 1-0). The frequencies for these transitions are 43.122 GHz (?; = 1), 42.82051 GHz (v = 2) (Lovas 1974), 42.51933 GHz (v = 3), and 43.42379 GHz (v = 0) (Lovas and Krupenie 1974). All of the frequencies have been measured in the laboratory. The first vibrationally excited state is 1231 cm-1 above the ground state, the second is 2449 cm" 1 above the ground state, and the third is 3655 cm" 1 above the ground state. These correspond to thermal excitation temperatures of 1770°, 3520°, and 5260° K, respectively. In figure 1 we show the spectra obtained for the a = 0, 1, 2, and 3,J= 1-0 transitions of SiO in the star W Hya, an M8e variable, which also exhibits OH and H 2 0 maser emission (Wilson 1973; Schwartz and Barrett 1970). In this star the v = 1 and 2 lines of SiO are present in about equal intensity while the v = 0 and 3 lines are at least a factor of 20 weaker. The temperature scales for the a = 0 and 3 lines are expanded by a factor of ^10 to show the noise level. The negative results for these lines represent an integration time of 60 and 20 min, respectively. This star had the largest peak intensity of any star surveyed at 3.5 mm (Kaifu et al. 1974); however, we did not look at o Cet (Mira). The 3.5-mm spectrum for W Hya (v = 1, J = 2-1) shows a main peak (Ta = 10° K) at +39 km s" 1 (velocity width = 5 km s" 1) with a smaller blended peak at +44 km s" 1 (Ta = 2° K). In figure 1 the a = 1, J = 1-0 line shows mainly the +39 km s" 1 feature with a slightly asymmetric profile indicating the presence of L97