3. High-resolution spectra: the radial velocity
Our high-resolution spectroscopic observations of P1724 were obtained with the CfA echelle spectrographs, on the 1.5m Wyeth reflector (Oak Ridge Observatory, Massachusetts), the 1.5m Tillinghast reflector, and the 4.5m Multiple Mirror Telescope (both on Mt. Hopkins, Arizona). We collected a total of 40 spectra over a period of about one year and a half. For each exposure we used Reticon photon-counters to record a single echelle order (45Å) centered at 5187Å , with S/N ratios per resolution element () ranging from 8 to 25.
Radial velocities were obtained by cross-correlation using the IRAF task xcsao (Kurtz et al. 1992), with a template selected from an extensive grid of synthetic spectra based on model atmospheres by Kurucz (1992a,b), calculated for us by J. Morse. These calculated spectra are available for a range of effective temperatures, projected rotational velocities, surface gravities and metal abundances (cf., Nordström et al. 1994, Latham et al. 1996). For our template parameters we adopted the values , , and , which maximize the correlation, and we assumed solar metallicity. Small run-to-run velocity corrections were obtained from multiple exposures of the twilight sky, and applied systematically to correct for instrumental shifts on all telescopes used (cf., Latham 1992). This effectively forces the same velocity zero-point on the three systems. Table 1 lists our RV measurements.
Table 1. CfA radial velocity measurements for P1724.
Although the resolution in , and of our grid of synthetic spectra is designed for the purpose of velocity determinations, and small changes in the template parameters have little effect on the velocities, it is possible to invert the process to obtain estimates of the physical parameters of the star by interpolation, seeking to maximize the correlation averaged over all exposures. We have done this for P1724, and determined the following values for the effective temperature, surface gravity, and projected rotational velocity: , , and .
A power spectrum analysis of our radial velocities clearly indicates the presence of a variation with a main period close to 5.7 days (Fig. 2). In the absence of concomitant photometric variations, one could argue that these velocity changes might be induced by a low-mass companion orbiting P1724, with a minimum mass of (assuming that the primary has a mass of , see Sect. 5). However, brightness fluctuations with precisely the same period strongly suggest that rotational modulation by surface inhomogeneities (spots) is the likely cause (see also Sect. 6).
A sine curve fit through our radial velocity data gives a semi-amplitude of about ( peak-to-peak), quite a significant effect (by virtue of the rapid rotation), implying also a fairly large spot coverage and/or large temperature differences with the surrounding photosphere. The addition of higher harmonics improves the fit only slightly, and the rms scatter remains at about . This is considerably larger than expected from similar material for a single star with similar exposure levels ( ; Nordström et al. 1994), even with a rotational broadening as large as that of P1724. It is likely that part of this excess scatter has to do with changes in the size, shape, temperature, location, or number of the surface features over the span of our observations, which would destroy the strict phase coherence otherwise expected from rotational modulation. Such changes are not at all unexpected in spotted stars.
Given the nature of the velocity variations and the scatter of the observations, our best estimate of the period from a simple sine curve fit to the spectroscopic data is days, nearly identical to the estimate in the previous section. The mean radial velocity from the same fit is , although this is not necessarily the same as the center-of-mass velocity of the star (see next section).
Residuals from the fit show no evidence for further periodicities, from which we conclude that there are no spectroscopic companions to P1724 with orbital periods up to the duration of these observations, i.e. up to days, corresponding to a semi-major axis of , or arc sec at .
© European Southern Observatory (ESO) 1998
Online publication: June 2, 1998