2. Observations and data reduction
The single dMe stars YZ CMi and AD Leo were observed on 1997 April 18/19 and 1996 December 12, respectively, with the VLBA and the phased-up VLA as a joint VLBI system yielding an angular resolution of better than one milliarcsecond (mas).
The VLA was also available in its normal interferometer mode and was used for total flux measurements at very small baseline length which allowed us to monitor the changes in total flux density and polarisation of both stars through the observations. 3C286 was used for the flux calibration of both stars.
The VLBI observations used a bandwidth of 8 MHz in both left and right circular polarization at 8.41 GHz (3.6 cm) and two bit sampling, giving a data rate of 128 Mbit/s. In order to reliably image such weak sources ( mJy) the phase referencing technique was used (Beasley & Conway 1995) in which we switched between the target and a bright calibrator (quasar) in cycles of two to three minutes. For our targets YZ CMi and AD Leo these calibrators were 0736+017 and 1022+194 respectively (see Table 2), at 2.1 and 1.4 degrees separation from the target.
Table 2. Adopted positions for the correlation of the primary and secondary calibrators of each target star. The primary calibrators were used as reference sources in the phase referencing, the secondary calibrators as check sources (see Sect. 2). All positions come from Johnston et al.(1995).
The amplitude calibration was performed with AIPS, followed by editing in DIFMAP (in order to flag more precisely single, bad visibilities of the VLBA data) and by a continued analysis in AIPS (including all the mapping). We first made hybrid maps of our calibrators using closure phase methods in order to determine their structure. We then determined the atmospheric contributions to the phase of the calibrator data. With these solutions it was possible to phase-correct the stellar data. We then made preliminary wide maps in order to find the stars and phase-rotated the data in order to bring the target stars to the image phase center. Finally we deconvolved the stellar images using CLEAN.
As a check of the reliability of the phase calibration, our observations also included a second bright calibrator for each star (0743-006 for YZ CMi, and 1013+208 for AD Leo, see Table 2). The maps of these secondary calibrators, made using the phase solutions found towards the primary calibrators showed a dynamic range of about 20:1. Since the separation between the two calibrators is approximately the same as that between the star and each calibrator, we can conclude that our stellar images have also a 20:1 dynamic range and hence are in practice noise limited rather than dynamic range limited. We also produced selfcalibration maps and datasets of the secondary calibrators. The plots of the fringe amplitude against u,v -distance show the same fall off than the ones produced from the phase referenced datasets. This confirms the value of the dynamic range of the images of the targets.
The source centroid positions of the phase referenced maps of the secondary calibrators were within 0.5 and 0.2 mas of the correlated positions (a-priori positions) for 0743-006 and 1013+208 respectively (see Table 2). This is consistent with the claimed accuracy of those correlated positions (0.5 mas, Johnston et al.1995). This test gives us confidence in the astrometric accuracy of our stellar positions (see Sect. 3.4).
© European Southern Observatory (ESO) 2000
Online publication: December 17, 1999