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

1. Introduction

On the basis of its properties at optical wavelengths, V Hya is usually considered to be a normal N type carbon star on the asymptotic giant branch (AGB). Its effective temperature is 2650 K, its spectral type is C6-7,5e and it is a semi-regular variable (SRa) with a period of 529 days (Kholopov et al. 1985; Lambert et al. 1986). The star has a large infrared excess and strong millimeter wavelength molecular line emission, showing that it is losing mass at a fairly high rate, several (Zuckerman et al. 1977; Knapp & Morris 1985; Zuckerman & Dyck 1986; Tsuji et al. 1988; Kahane et al. 1988; Bujarrabal et al. 1994b,c; Stanek et al. 1995). The IRAS colors are also normal for carbon stars with high mass loss rates (cf. van der Veen & Habing 1988).

It has long been known from infrared and molecular line observations that the circumstellar envelope of V Hya is peculiar. The starlight is polarized at near infrared wavelengths, and both the degree of polarization and the position angle change with time, sometimes dramatically (Johnson & Jones 1991; Johnson 1993; Trammell et al. 1994). These observations show that the circumstellar envelope is not spherically symmetric, and that its density structure changes on timescales of months to years. The CO rotational line profiles differ from those seen in most carbon star envelopes; they have a double-horned structure and strong wings, and are considerably broader than lines from other molecular species in the envelope, such as CS and HCN (Bujarrabal et al. 1994b,c; Groenewegen et al. 1996; Knapp et al. in preparation.). Further, the emission at positive velocities with respect to the center of the line appears to arise from gas which is spatially separated from that emitting at negative velocities, suggesting that the outflow is bipolar (Tsuji et al. 1988; Kahane et al. 1988; Kahane et al. 1993, 1996, hereafter KABM). Recent high resolution observations of the stellar spectrum at optical and near infrared wavelengths show unusually broad absorption lines (Barnbaum et al. 1995). Absorption in the co v = 0-1 rotation-vibration line at 4.6 extends to -130 with respect to the stellar velocity (Sahai & Wannier 1988; Sugerman et al. 1997), while the optical spectrum shows forbidden-line emission reminiscent of that seen in Herbig-Haro objects, at a velocity blueshifted by with respect to the photospheric velocity (Lloyd Evans 1991). These data strongly suggest that V Hya is at an unusual evolutionary stage.

The observations described in the present paper were made to measure the CO millimeter-wavelength emission from V Hya with greater velocity resolution, sensitivity and velocity coverage than has been used for previous observations, to examine the emission line shapes and velocity widths, and to investigate the structure of the envelope. These observations discovered that the star has a molecular outflow with a speed . This paper describes this observation and its implications for AGB evolution. The next section discusses the molecular line observations and results and a kinematic model for the circumstellar envelope. Sect. 3 describes the broad-band spectrum of the star and models the infrared continuum emission and the molecular line emission to derive the mass loss rate and dust content of the envelope. Sect. 4 discusses the properties and evolutionary status of the star, and the conclusions are given in Sect. 5.

The basic data for V Hya from the literature are given in Table 1. The effective temperature, spectral type and abundances are from Lambert et al. (1986). The pulsation periods and variable type are from Mayall (1965) and Kholopov et al. (1985). V Hya is an N-type carbon star, and these objects in the Magellanic Clouds have an absolute K magnitude of (Cohen et al. 1981). The apparent K magnitude of (Bergeat et al. 1976) gives a distance of 380 pc. This distance estimate contains no correction for interstellar or circumstellar extinction. The resulting stellar luminosity and stellar radius are as derived by Luttermoser & Brown (1992).

Table 1. Basic data for V Hya

© European Southern Observatory (ESO) 1997

Online publication: April 20, 1998
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