3 Results

3.1 Previous surveys

Previous attempts made to search for excited OH from circumstellar envelopes gave only negative or controversial results with the exception of one object. As far as we are aware only a few searches for J= 1/2 and 5/2 OH emission at 4.7 and 6 GHz from stars have been undertaken (see Thacker et al. 1970; Zuckerman et al. 1972; Baudry 1974; Claussen & Fix 1981; Jewell et al. 1985). The latter work was the most sensitive search for excited OH from stars yet performed. Zuckerman et al. (1972) reported weak 6035 MHz ( $^{ 2}{ \Pi }_{3/2} , J=5/2$) emission from NML Cyg and Claussen & Fix (1981) reported weak 4751 MHz ( $^{ 2}{ \Pi }_{ 1/2}{ }, J=1/2$) emission from AU Gem. However, both detections were not confirmed by Jewell et al. (1985). On the other hand, Jewell et al., reported weak 6035 MHz maser emission from the planetary nebula Vy 2-2 appearing at the same velocity, -62 km s^-1, as the peak 1612 MHz maser emission detected by Davis et al. (1979).

3.2 New Effelsberg survey

In Table 1, we list the 65 late type stars observed by us. For all sources, the velocity range of search for emission is given (with the systemic velocity in parenthesis), together with the sensitivity limit achieved in our new survey at 3$\sigma$. The average noise level reached in our survey is (at 3$\sigma$ with a channel width of 0.29 km s^-1) around 80 mJy; in comparison, Jewell et al. (1985) have reached about 230 mJy (with a channel width of 0.06 km s^-1).

Figure 3: The 6 GHz spectra obtained in Dec 1999 for NML Cyg, the rms at 1$\sigma$ is $\sim$6mJy (LCP spectrum). No detection reported.

Figure 4: 6035 MHz OH spectrum obtained in Dec. 1999 from Vy 2-2. The line intensity is in Jy for single polarization, the rms at 1$\sigma$ is $\sim$6mJy.

Of the 65 sources observed, no one exhibits a clear emission or absorption signal. There are however two sources with tentative detections, NML Cyg (see Fig. 3) and Vy 2-2 (Fig. 4). For NML Cyg, we reached the sensitivity of 20 mJy (at 3$\sigma$ level) over the observing LSR velocity range. The 0.8 K (2.2Jy) signal reported by Zuckerman et al. (1972) and lying close to +5 km s^-1, would have been easily detected by us. However, we can not exclude that the emission varies with time. The tentative feature at about -17 km s^-1 (Fig. 4) is only detected at a $\sim$3$\sigma$ level and is therefore not convincing, but we note that 1612 MHz line emission at -18 km s^-1 has been reported previously (see e.g. Engels 1979).

The case of Vy 2-2 is different. With an integrated intensity of $\sim$48 mJy km s^-1, we have obtained a 6$\sigma$ detection. Only the F= 3-3 maser line transition lying at 6035 MHz was detected. No absorption or emission can be observed for the other transitions. Figure 4 shows the observed 6035 MHz spectrum. The parameters and uncertainties (1$\sigma$) of Gaussian fits to the detected features are displayed in Table 2. The derived apparent luminosity is 1.1 Jy km s^-1kpc²(assuming a distance of 3.8 kpc, Bensby & Lundström 2001). The lack of F= 2-2 emission and the narrow F= 3-3 linewidth suggest that the observed F= 3-3 line results from a maser process. However, only interferometric observation could give a definitive proof of it.

Figure 5: IRAS 60 $\mu$m versus the OH 6035 MHz 3$\sigma$ flux density limit (channel 0.25 km s-1). The filled triangle corresponds to the only detection in our sample, Vy 2-2, the detected flux is given.

   

Table 2: Gaussian line parameters of the 6035 MHz OH emission line of Vy 2-2.

Velocity	Peak flux density	Linewidth
(km s-1)	(mJy)	(km s-1)
-63.0 $\pm$ 0.14	38 $\pm$ 8	1.14 $\pm$ 0.17
-61.6 $\pm$ 0.14	39 $\pm$ 8	0.84 $\pm$ 0.17

This detection is consistent with the results of Jewell et al. (1985) who observed maser emission at nearly the same velocity ($\sim$-62.3 km s^-1) and with about the same line width ($\sim$1.5 km s^-1) but with a peak flux intensity four times stronger (0.15 Jy). The presence of the two features (Fig. 4) is likely real. After splitting our data in two equal parts, the same two components appear. In another data reduction test, we have degraded our spectral resolution. This yields one single feature with a line width of $\sim$2.5 km s^-1, i.e. twice the line width observed by Jewell et al., centered around -62.3 km s^-1. Our observations and data reduction confirm long term OH emission from Vy 2-2.

3.3 Vy 2-2

As is the case for other Galactic planetary nebulae, the distance to Vy 2-2 (G045.4-02.7) is poorly known. Previous attempts to determine the distance have resulted in a wide range of estimates. Those estimates put this object from 1.9 kpc (see Acker 1978) based on an optical calibration to a kinematic distance of 20 kpc (Davis et al. 1979). The most recent estimate, based on a compilation of previous measurements (see Bensby & Lundström 2001) gives a distance of 3.8 kpc. Vy 2-2 is a source of free-free radio continuum radiation and dust-type infrared emission. VLA maps show a slightly elongated continuum source (Seaquist & Davis 1983). The continuum emission originates from a compact (diameter $\sim$ 0.5'') and narrow (thickness $\sim <$ 0 $.\!\!^{\prime\prime}$12) shell of ionized gas. This ionized region is surrounded by an extended halo of over 25'' in radius, detected through its H$\alpha$ line emission (see Miranda & Solf 1991). From the visibility analysis, Christianto & Seaquist (1998) estimate an angular expansion of 1.13 $\pm$ 0.12 mas/yr^-1. This would give for a distance of 3.8 kpc an expansion velocity of about 20 km s^-1, in contradiction with the expansion velocity of 6 km s^-1 measured by Miranda & Solf (1991) in the equatorial plane and qualified to be slow. Taking a systemic velocity for the source of -44.3 $\pm$ 1.0 km s^-1(tentative detection of Knapp & Morris 1985) this would give a blue-shifted velocity for the OH maser of about 20 km s^-1. The inferred expansion velocity is then in good agreement with the value derived by Christianto & Seaquist (1998) for a distance of 3.8 kpc. The kinematic age of the nebula they derived is 213 years and supports the conclusion that this object is a very young planetary nebula. The temperature of the central star is estimated to be greater than 35 000 K (see Zijlstra et al. 1989; Clegg & Walsh 1989). The dust color temperature was estimated by Cohen & Barlow (1974) to be less than 190K.

Jewell et al. (1985) and Cohen et al. (1991) searched without success for $^{ 2}{ \Pi }_{ 1/2}{ }, J=1/2$ maser emission (down to a $3\sigma$ limit of $\sim$0.25 Jy). The 1612 MHz maser emission, the only ground-state maser transition observed, was first detected by Davis et al. (1979). Seaquist & Davis (1983) located the maser at the front edge of the ionized shell, coincident with a shock front and an ionization front, placing the OH maser on the near side of the expanding shell and thus providing an explanation for the blue-shifted maser feature. This is consistent with the fact that OH molecules are effectively produced in the outer parts of circumstellar envelopes due to photoionization of H₂O by interstellar UV photons. The typical abundance for OH molecules relative to H₂ is about 10^-5 and HST observations (see Sahai & Trauger 1998) show a compact bright bipolar source expanding along an axis roughly orthogonal to the bipolar axis. Despite the fact that almost all planetary nebulae appear optically thin at 5 GHz, Vy 2-2 is optically thick (see Purton et al. 1982).