Astron. Astrophys. 326, 318-328 (1997)

4. Discussion

The molecular line observations of V Hya discussed in the previous section show that it is a member of an unusual group of evolved stars, those with very fast molecular winds. To date, eight evolved stars with molecular outflows at speeds exceeding 50 are known. These are listed in Table 5, which gives the star name, the outflow speed(s) of the molecular wind(s), the spectral type of the central star, and references to molecular line data.

Table 5. Evolved stars with molecular wind velocities

These stars have several other common properties. First, they are almost all evolved well past the AGB. CRL 618, NGC 3132, , M1-92 and are planetary nebulae with hot nuclei, while HD 101584 and CRL 2688 have central stars much hotter than does the typical AGB star. Only OH 231.8+4.2, like V Hya, has a cool central star, of type M6-M9. A second common property among the stars in Table 5 is that almost all of the circumstellar envelopes have well-developed bipolar structure, and when sufficient resolution is used to make the molecular line observations, the winds themselves are also found to be bipolar (e.g. Bujarrabal et al. 1992, 1994a and references in Table 5).

The molecular line profiles for most of these stars show outflows at more than one velocity, as is the case for V Hya. Typically, the line profiles have a central component of roughly parabolic shape and width about 10 - 30 and, centered at the same velocity, weaker, much broader line wings. Two of the stars in Table 5, HD 101584 and , show only the fast molecular wind. The mechanical luminosity (and momentum flux) of the fast wind is generally much larger than that of the slower wind, and can approach a significant fraction of the stellar luminosity (Table 3). It seems reasonable to identify the parabolic component with the wind shed while the star was on the AGB and to associate the fast wind with evolution beyond the AGB.

Fig. 10 shows the IRAS color-color diagram for V Hya and the stars in Table 5. The colors for CRL 2688, which is not in the IRAS Point Source Catalogue, were approximately found from photometry in the compilation by Gezari et al. (1993). V Hya is the only member of this group of objects with IRAS colors corresponding to those of AGB stars; the other stars all have very cold IRAS colors. Because the IRAS colors are those of very dense, hollow circumstellar shells, cold IRAS sources are considered to be highly evolved and indeed to be protoplanetary nebulae (not all cold IRAS sources have fast molecular winds, however; e.g. Likkel et al. 1987; Loup et al. 1990).

Fig. 10. IRAS color-color diagram for evolved stars with molecular wind speeds in excess of 50 . The point labeled " " corresponds to the colors of a zero magnitude infinite temperature black body. The dashed lines outline the region of the color-color diagram containing normal mass-losing carbon-rich and oxygen-rich AGB stars (van der Veen and Habing's 1988). Region I: photospheres, no circumstellar emission. Regions II and IIIa: oxygen rich envelopes. Region VII: carbon stars. Regions VIa and VIb: cool stars with detached shells.

However, while V Hya's envelope appears to have IRAS colors like those of AGB stars, the lines of molecules like HCN, CS and , while present, are much weaker relative to the CO lines than in most carbon stars (Bujarrabal et al. 1994b,c). This chemistry resembles that of the CRL 618 envelope (Gammie et al. 1989). Since the emission from these species arises preferentially from the dense inner circumstellar envelope, the relative weakness of these molecular lines suggests that the inner region of V Hya's envelope is hollow due to the cessation of mass loss or the ionization/dissociation of the inner envelope.

As noted above, only V Hya and OH231.8+4.2 among the objects in Table 5 are cool stars. Like V Hya, OH231.8+4.2 has a Herbig-Haro like optical spectrum (Reipurth 1987), indicative of shock-induced emission, and this is probably due to the fast molecular wind colliding with the surrounding, more slowly moving circumstellar material. These comparisons, and Fig. 10, suggest that V Hya is at the very beginning phases of evolution away from the AGB.

V Hya is a member of a small class of doubly-periodic semi-regular variable stars (Lloyd Evans 1985, 1995), with variations of amplitude and 529 day period superimposed on variations with a much larger amplitude () and longer-period (17 - 18 years, Mayall 1965). The deep minima are much weaker at longer wavelengths. Moreover, Johnson (1993) reports a strong increase in the V-band polarization from 0.7% to 10% at the time of the optical fading. Lloyd Evans (1993, 1995) suggests that the deep minima are associated with episodes of dust formation, such as those seen in Type b RV Tau variables. These doubly-periodic stars are believed to have started to cross the HR diagram, and their relationship with post-AGB stars has been discussed by Gingold (1986), Bujarrabal et al. (1990), and Lloyd-Evans (1995).

V Hya has the broadest photospheric absorption lines in a large sample of carbon stars measured by Barnbaum et al. (1995). These authors suggest that the line is rotationally broadened, due to envelope spin-up during a short-lived common envelope binary phase. However, such broad lines are a distinctive property of post-AGB stars (e.g. Bakker et al. 1996a, b), with macroturbulence considered to be the main broadening mechanism (Gray 1992).

Thus, the photometric, polarization, spectroscopic and circumstellar envelope features of V Hya are not unusual when compared with those of post-AGB stars and protoplanetary nebulae. However, V Hya is unusual in this group of stars in having a low effective temperature and IRAS colors like those of AGB stars. It may therefore represent the earliest phase of evolution beyond the AGB yet found.

If this is so, the presence of a very fast molecular wind in V Hya's envelope shows that this phenomenon takes place immediately upon entering the post-AGB phase of evolution and may, to all intents and purposes, initiate it. The low mass and short dynamical time (1000 years) of the circumstellar envelope suggest that evolution away from the AGB is also accompanied by a short period of copious mass loss. The mechanism causing fast bipolar molecular winds in single stars is not obvious, though it may imply the dumping of large amounts of energy into the stellar envelope as the star moves away from the AGB.

In conclusion, the body of observational data for V Hya indicates that it is at an unusual and short-lived evolutionary phase. If it is just beginning its evolution away from the AGB, as the observations suggest, the molecular line observations show that the fast, powerful molecular winds which are present in several post-AGB stars are initiated at the very beginning of post-AGB evolution.

© European Southern Observatory (ESO) 1997

Online publication: April 20, 1998
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