Radial velocity measurements

Fig. 1. The color-magnitude diagram of NGC 188 from [2] with the location of the detached eclipsing binary V12 [3] overplotted. From radial-velocity measurements we find (assuming an inclination of 90 degrees since we do not yet have photometry of the eclipses) that the masses of the two components are 1.06 and 1.08 solar masses. We estimate that we will be able to reach a precision of 1% in the mass estimate. We are in the process of acquiring eclipse photometry such that the radii and orbital inclination can be determined. Since both components are very close to the cluster turnoff their masses and radii can be directly used to give a very accurate age estimate for the cluster by comparing to isochrones in the (mass, radius) plane and requiring that they both lie on the same isochrone.

FLAMES/GIRAFFE stars of magnitudes V 16.5 - 18.5 that have photometry consistent with cluster membership and we have confirmed (or not) the membership by means of radial velocity measurements. This has given us a sample of 138 likely cluster members (out of 220), with a maximum of 9 possible spurious objects. 

Spectroscopy The WFI images were used to select samples of stars on the RGB in Scl to take spectra in the Ca II triplet region with VLT/FLAMES. This resulted in radial velocity measurements and metallicity estimates for more than 400 stars in Scl over the fields outlined in Fig. 1, of which 300 have a high membership probability. 

Thus, we have demonstrated, in the form of a diagram, the crosstalk between transverse velocity and a radial velocity measured by red shift. As we shall se in Section XII, the crosstalk between the radial velocity and the apparent transverse velocity is demonstrated by the use of another but similar diagram. By adding those two diagrams together we will finally find a method of evaluating the true velocity of the light source. 

Fig. 6.2 Principle of radial velocity measurement. Note that the spectral lines of the star are blueshifted when it approaches towards the observer and redsUfted when its distance to the observer increase...

Figure 43.25 Resultant shaft velocity vector based on radial vibration measurements...

Before the slit. Motion of the image delivered by the telescope with respect to the slit causes both a loss of throughput and an error in the barycentre of the spectral lines recorded on the detector, unless the object uniformly fills the slit (which implies low throughput). This can cause errors in measurement of radial velocities. For MOS, there is the particular problem of variations in the image scale or rotations of the mask. These can cause errors which depend on position in the field resulting in spurious radial trends in the data. Fibre systems are almost immune to this problem because the fibres scramble posifional information. 

Abstract. We use intermediate resolution (II, 19 300) spectroscopic observations in the spectral region including the Li 6708 A line to study 341 stars in the star forming region (SFR) NGC 6530. Based on the optical color-magnitude diagrams (CMD), they are G, K and early M type pre-main sequence (PMS) cluster candidates. 72% of them are probable cluster members since are X-ray sources detected in a Chandra-ACIS observation ([2]). We use our spectroscopic measurements to confirm cluster membership by means of radial velocities and to investigate the Li abundance of cluster members. 

The DART large programme at ESO made v ei and [Fe/H] measurements from FLAMES spectroscopy of 401 red giant branch (RGB) stars in the Sculptor (Scl) dSph [6]. The relatively high signal/noise, S/N ( 10-20 per pixel) resulted in both accurate metallicities ( 0.1 dex from internal errors) and radial velocities ( 2 km/s). This is the first time that a large sample of accurate velocities and metallicities have been measured in a dwarf galaxy. 

There are indications that the presence of two populations is a common feature of dSph galaxies. Our preliminary analysis of HB stars, Vhei and [Fe/H] measurements in the other galaxies in our sample (Fornax and Sextans dSph Battaglia et al., in prep) also shows very similar characteristics to Scl, especially in the most metal poor component. Pure radial velocity studies [15], [16] have also considered the possibility that kinematically distinct components exist in Ursa Minor, Draco and Sextans dSph galaxies. 

Radial velocities were measured by cross-correlation, using a synthetic spectrum as template. Individual spectra were shifted to rest wavelength and coadded. Effective temperatures were derived from the (V — I)o colours by means of the Alonso calibration [8], We assumed log g = 2.0 for all stars (estimated from isochrones) and with these parameters we fed the spectra to our automatic procedure for the determination of abundances [9], We found that the S/N ratio was too low to be able to determine reliably the microturbulent velocities, the weak Fe I lines could not be measured on many spectra. This resulted in a marked dependence of derived abundances on microturbulent velocities. It is well known that microturbulence is not a truly independent parameter but correlates with surface gravity and, more mildly also with effective temperature. By considering the large sample of stars studied by [10] one can be convinced that for all stars with 1.5 < logg < 3.0 (20 stars) there is no marked dependence from either Tefi or log g, and the mean value of the microturbulent velocity is 1.6 kms 1. For this reason we fixed the microturbulent velocity at 1.6 kms-1. 

Fig. 1 (left panel) shows the radial velocity distribution of selected stars. We estimate a main peak of Vr 220 7 km s 1, in good agreement with previous measurements based on RGB stars [2]. A secondary peak appears at Vr 180 7 km s-1. This dichotomy is shown in the right panel of Fig. 1, where the radial velocity is plotted as s function of the distance from the centre. 

Frequency modulated continuous wave (FMCW) radar is most commonly used to measure range R and range (radial) velocity of a target [42, 43]. The most common structure of a homodyne FMCW radar is presented in Figure 2. 

Even stronger constraints are potentially available from the high-mass X-ray binary Vela X-l. This source contains a 20 M0 star, and radial velocity variations from the star have been measured as well as periodic timing variations from X-ray pulses. The orbital period is 8.96 days and the eccentricity of... 

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