Astron. Astrophys. 361, 167-174 (2000)

2. Observations

2.1. Velocity variations

Most of the velocity data of SRVs that will be discussed have been published elsewhere (Hinkle et al. 1997, Lebzelter 1999). We selected stars that have a reasonable number of velocity and light curve measurements. For comparison we have also included the short period mira RT Cyg. This star has a period similar to the SRVs but with a significantly larger amplitude in V. The velocity amplitude is also similar to that of other miras (Lebzelter et al. 1999). The final sample covers a large range in period and amplitude. Both types, SRa and SRb, are included. Table 1 gives an overview on the origin of the data and the instruments used to obtain them as well as fundamental properties of the stars.

Table 1. Properties of stars selected for the present investigation. Columns 3, 4, 5 and 6 give general properties of the objects from the GCVS4. Columns 7 and 8 list the source of the velocity data, where CF stands for the Coudé Feed telescope on Kitt Peak. References are encoded as follows: HLS97 = Hinkle et al. 1997; LHH99 = Lebzelter et al. 1999; LZ99 = Lebzelter 1999.

FTS spectra were obtained over a broad wavelength interval always covering the entire 1.6 µm H window and frequently the entire 1.6-2.5 µm H and K region. We present a single velocity from these measurements. The Coudé Feed spectrograph with the NICMASS detector covers a very small spectral region, about 2 percent of the H band window (Lebzelter 1999). The Coudé Feed observations were always obtained in two wavelength regions, both around 1.6 µm, that were analyzed separately. In the figures of this paper these two sets of NICMASS observations will be marked by two different symbols.

Previously unpublished velocity data for the semiregular variable V450 Aql are also presented. Observations were obtained with the FTS at the 4 m Mayall telescope on Kitt Peak. The data are listed in Table 2. Details on the data reduction techniques can be found in Hinkle et al. (1982). The excitation temperature and column density were derived from the CO lines for two observations as described in Hinkle et al. (1982). As has been found for other similar SRVs, the uncertainty of these numbers (=3300200 and log(NL[CO])=22.80.1) is too large to allow derivation of stellar variations in low amplitude variables.

Table 2. FTS measurements for V450 Aql. Spectra have been apodized by Norton & Beer (1976) function I2. Resolution (theor. FWHM) of the spectra is 0.07 cm^-1.

Some confusion is found in the literature concerning the spectral type of V450 Aql. Most authors agree that the spectral type is M5 to M5.5III (e.g. Keenan & McNeil 1989), while the Hipparcos Input Catalogue lists M8V, apparently based on a measurement by McCuskey (1949). From its IRAS colours V450 Aql clearly belongs to the blue SRVs (Kerschbaum & Hron 1992).

The literature values for the radial velocity of V450 Aql scatter between -52 km s^-1 (Feast, Woolley & Yilmaz 1972) and -50.6 km s^-1 (Jones & Fisher 1984). We will use an average value of -51.3 km s^-1. All velocity measurements we obtained for this object (Table 2) are slightly below this literature value. V450 Aql therefore exhibits the same behavior found already for a number of SRVs by Lebzelter (1999), namely a systematic shift between the literature values and the measured CO velocities.

2.2. Light curve data

The measurement and long-term monitoring of long-period variable magnitudes is one field in astronomy where the contribution of amateur astronomers is crucial. The investigation presented in this paper was made possible by the rich data base of individual magnitude estimates provided by associations of amateur astronomers. Three sources of visual data were utilized, namely those of the Association Francaise des Observateurs d'Etoiles Variables (AFOEV ¹), the Variable Star Observers' League in Japan (VSOLJ ²) and the American Association of Variable Star Observers (AAVSO International Database which includes part of AFOEV and VSOLJ observations). The possible effects of overlapping data series (i.e. the common estimates can be included twice or thrice) were avoided, because where the (most complete) AAVSO data were available we did not mix them with the other ones. On the other hand, there are practically no common observations in the AFOEV and VSOLJ data base.

In order to decrease the effects of the observational scatter, we applied similar data handling as used in Kiss et al. (1999). That means data averaging in 5- or 10-day bins depending on the actual period value. Further noise-filtering was done by applying a Gaussian smoothing with a FWHM of 4 and 8 days (80 percent of the bin). This was necessary in the very low-amplitude regime, where the usual accuracy of individual estimates (about 0.3 mag) would obscure the light variations. The use of this smoothing was illustrated in a comparison with simultaneous photoelectric measurements (see Fig. 2 in Kiss et al. 1999) and turned out to be very effective in the case of light curves with numerous and densely distributed data. The estimated precision (in the visual system) is somewhat better than 0.1 mag. However, since there are no independent and simultaneous well-calibrated photometric data, the accuracy can only be estimated.

© European Southern Observatory (ESO) 2000

Online publication: September 5, 2000
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