3 System properties

3.1 Classification, distance, interstellar lines and reddening

The 3300-9100 Å spectra of StH$\alpha$ 190 and MKK standards in MZ01 allows us to classify the cool giant as G7 III. Comparing MHZ photometry with the intrinsic colours from Fitzgerald (1970) and absolute magnitudes from Schmidt-Kaler (1982), the reddening turns out to be E_B-V=0.10 and the distance d=575 pc. SN93 estimated an identical E_B-V=0.10 from the 2175 Å interstellar hump in the IUE spectra of StH$\alpha$ 190.

Multiple interstellar components are superposed on the rotationally broadened stellar NaI doublet (cf. Fig. 1). Their $RV_\odot$ are $-18.1\ (\pm 0.5)$, $-10.0\ (\pm 0.5)$ and $-0.9\ (\pm 0.4)$ km s^-1, with 0.159, 0.051 and 0.112 ($\pm$0.003) Å as equivalent widths for the 5889 component, respectively. They are unresolved on the Asiago spectra and the blend has $RV_\odot = -10.8\ (\pm 0.5)$ km s^-1. StH$\alpha$ 190 is at $b=-36^\circ$ so our line-of-sight exits the galactic dust layer (assumed to reach $\bigtriangleup z \sim 100$ pc over the galactic plane) at the projected distance of $\sim$170 pc, where the effect of the galactic rotation on the radial velocity does not exceed 2 km s^-1. None of the three interstellar lines shares the velocity of the StH$\alpha$ 190 circumstellar material (see bottom line in Table 1), so they have to originate in distinct clouds with RV dispersion similar to that of extreme Pop I objects (12.5 km s^-1, Binney & Merrifield 1998). If we use the NaI vs. E_B-V calibration of Munari & Zwitter (1997), the equivalent widths of the three interstellar NaI components correspond to E_B-V=0.049, 0.016 and 0.035, respectively, giving a total E_B-V=0.10, a value identical to what above derived by independent methods.

Figure 2: Evolution of the HeI, H$\alpha$ and [OIII] emission line profiles over a 4 month period (see dates on the right and Table 1); S = SARG, A = Asiago spectra. The higher resolution of SARG spectra is the reason for the apparent variability of the [OIII] line, which is actually pretty constant. The vertical scale is constant for each line (expansion factors 1:7.2:7.2), the continuum is normalised and the line profiles are truncated

3.2 Radial velocities and orbital motion

Heliocentric velocities of the main emission lines and the stellar NaI absorption which traces the G7 III star are listed in Table 1. The fairly constant $RV_\odot$ of emission lines indicates that their formation regions do not follow the orbital motion and are circumstellar in origin.

The stellar NaI doublet clearly shows orbital motion velocity shifts, even if the observations did not cover a full orbital cycle. Lower limit to the velocity amplitude ( $\bigtriangleup RV_\odot \sim30$ km s^-1) is remarkable for a symbiotic star and it is the largest recorded so far (see Table 4 of Belczynski et al. 2000). This suggests a large orbital inclination, an unusually massive hot companion and a short orbital period.

3.3 Photometric variability

StH$\alpha$ 190 has been detected by Tycho at the limit of its sensitivity range during 78 passages distributed over 17 dates (from Dec. 27, 1989 to Dec. 14, 1992). Automatic analysis of Tycho data summarized in the Hipparcos Catalogue did not detected variability of StH$\alpha$ 190 over the large noise in the $B_{\rm T}$ and $V_{\rm T}$ data. Our recent IR photometry reported in Sect. 2 is in excellent agreement with the older W95 data.

However, if a detailed search for variability and periodicities is performed on Tycho and W95 data as well as on the radial velocities of Table 1, an average periodicity of $171\pm 5$ days (and its $115\pm 7$ yearly alias) is found. If this corresponds to the orbital period it would be the shortest known among all symbiotic stars, followed by those of two other yellow symbiotics TX CVn (199 days) and BD-21.3873 (282 days) and the recurrent symbiotic nova T CrB (228 days; cf. Belczynski et al. 2000 and references therein).

3.4 Rotational velocity

The width of the stellar NaI absorption lines in Fig. 1 corresponds to $V_{\rm rot}$ sin $\,i \sim 105$ km s^-1, which translates into a 5 days rotation period for the G7 III star, much less than the possible 171 day orbital period. Such a rotational velocity is very high: from the catalogue of rotational velocities of Bernacca & Perinotto (1973) the mean value for the 288 giants between G2 K2 is $V_{\rm rot}=9.7$ km s^-1, with 92% of them having $V_{\rm rot}\leq 10$ km s^-1. The high $V_{\rm rot}$sin$\,i$ further strengthens the idea of a high orbital inclination for StH$\alpha$ 190.