Volume 558, October 2013
|Number of page(s)||10|
|Section||Interstellar and circumstellar matter|
|Published online||15 October 2013|
An example of the transfer functions at a particular wavelength is presented in Fig. A.1. It can be clearly seen that in two cases (2012/04/06 and 2013/03/04), the visibilities of HD 85567 are the same or similar to the calibrators, indicating a compact source. In the other two cases (2012/05/06 and 2012/12/29), the visibilities of HD 85567 are noticeably lower than the calibrators, indicating a resolved source.
Examples of the transfer functions and raw science data at a wavelength of λ = 2.2 μm. In the visibility plots, open symbols mark calibrator observations and filled symbols mark the observations of HD 85567. In the closure phase plots, the observations of HD 85567 are marked by star symbols and calibrator data by filled circles. A frame selection of 10 percent was used, but the behaviour is independent of the frame selection used.
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The distribution of the fringe S/Ns of the science and calibrator observations are presented in Fig. B.1. The two cases where HD 85567 appears resolved (2012-05-06 and 2012-12-29) are considered first.
It can be seen that in these cases, the S/N distributions of the calibrators extend to significantly higher values than the distribution of the HD 85567 observations. This is particularly apparent in the case of the observations conducted on 2012-05-06. On this occasion, the fringe S/N distribution of HD 85567 on the UT2-UT3 baseline peaks at ~2 while the HD 84177 distribution has a skewed distribution peaking at ~18. This disparity could be due to the fact that although the K-band magnitudes of the target and calibrators are similar, HD 85567 is 1 magnitude dimmer than the calibrators in the H-band. The fringe tracking is performed in the H-band. Therefore, the performance of the fringe tracking was likely higher when observing the calibrators than when observing the target. Furthermore, the observations of the target were associated with worse seeing than the calibrator observations. This could also reduce the fringe tracking performance. Consequently, the high S/N of the calibrator observations is likely due the fringe tracking performing best when observing the calibrators. This hypothesis is substantiated by the FINITO data recorded by the RMNREC software. In the two cases where the target visibilities appear significantly lower than those of the calibrators, the rms of the FINITO phases associated with the target is up to 40 percent larger than that associated with calibrator observations.
Fringe S/N distributions for each observing date and each baseline. The filled black histogram is the fringe S/N distribution of the observations of HD 85567. The distributions filled with vertical blue and horizontal red lines mark the fringe S/Ns associated with the calibrators HD 85313 and HD 84177 respectively.
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The case of the two dates when the target appears close to unresolved (2012-04-06 and 2013-03-04) are now discussed.
In the case of the observations conducted on 2012-04-06, it can be seen that the target and calibrator fringe S/N distributions are almost identical. This suggests that the calibration is accurate. However, as can be seen, the typical S/Ns are relatively low; all the distributions peak below 5. Therefore, these observations are relatively noisy. It can be seen that the observations on 2013-03-04 are of higher quality with typical S/Ns of approximately 10. In this case, the target and calibrator distributions are not identical but there is considerable overlap, which, in conjunction with the high S/N, suggests that calibration in this case should be reliable. The superior S/N of the HD 85567 observations conducted on 2013-03-04 is attributed to the AMBER intervention of January 2013 which improved the sensitivity of AMBER, particularly in the H-band where fringe tracking is conducted.
The disparity between the fringe S/N distributions of the calibrator and science observations of 2012-05-06 and 2012-12-29 could result in an inaccurate calibration. This could occur in the following manner. The lower fringe S/N of the target observations could result in the target fringes exhibiting a lower fringe contrast than the calibrators, thus making the target appearing more resolved than the calibrators, even if this is not the case. Indeed, as noted above, the raw visibilities of HD 85567 obtained on these two dates are lower than those of the calibrators. We surmise that the difference between the target and calibrator observations on these dates is a bias caused by the fringe tracking performance degrading when observing the target.
We conclude that only the observations when the fringe S/N distributions of the target and calibrator observations are similar can be calibrated. In principle, the observations of 2012-04-06 and 2013-03-04 offer reliable calibration. However, since the fringe S/Ns of the observations conducted 2012-04-06 are relatively low, this paper focuses exclusively on the observations obtained on the date 2013-03-04.
© ESO, 2013
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