Volume 567, July 2014
|Number of page(s)||7|
|Section||Interstellar and circumstellar matter|
|Published online||07 July 2014|
Summary of HIFI observations conducted in 2011 and 2012.
Summary of Effelsberg observations conducted in 2011 and 2012.
Map of 621 GHz emission corresponding to dataset 2-H in Table A.1. The offsets from (RA[J2000], Dec[J2000]) are given in the upper left of each panel in seconds of arc. The horizontal and vertical axes bordering the entire set of 15 panels correspond roughly to these respective offsets for the positions at which the spectra shown in the individual panels were observed. The fine vertical scales on the individual panels, run from [− 0.2to2.5] degrees Kelvin; the width of the individual panels cover a Vlsr range of [− 60to80] km s-1 roughly centred on the vertical line marking Vlsr = 12 km s-1, the velocity of the narrow 621 GHz maser feature.
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The source for the variability of the broad pedestal component in the 621 GHz spectral line requires explanation. More precisely, we need to determine whether some time-varying physical mechanism and/or pointing errors are responsible for the change that occurs in the pedestal between 2011 to 2012.
As the first and last set of observations involved a small map, we examined two possibilities. First, whether the increase in emission from the broad component is common to all areas of the map, and second, if not, whether there is a noticeable offset in the spatial positioning. Figures B.1 and B.2 illustrate the horizontally polarised versions of the two small maps taken during the first and last epochs of our observations. In Table A.1, this corresponds to observation numbers 2-H and 6b-H, respectively. From 2011 and 2012, the intensity of the maser line itself seems to systematically decrease between the maps of Figs. B.1 and B.2. At the same time, in many panels toward the map centre, the aforementioned broad component appears to get stronger. This trend differs at points farther from the centre, where there is more of a decrease, especially in the line wings of the bottom left panels. This latter point seems to indicate systematic changes in the source. However, one possible explanation for the increase in the broad component may be due to the proximity of the Orion “hot core” (HC) whose molecular line emissions were studied extensively by Beuther et al. (2005). The HC is located at (RA[J2000] , Dec[J2000] ) only about 6 arcsec removed from Orion KL. It is displaced from our pointing direction by only 20″ in Right Ascension and 30″ in Declination, and thus lies at an offset of only 36 s of arc from our prime pointing direction. This small offset, barely exceeding
the FWHM of our beam at 621 GHz, implies that even a relatively small error in pointing could effect a large apparent variability, given that the HC would lie on the flank of our beam profile.
In Sect. 3.2, we stated that the adjustment in the absolute pointing error (APE) improved the pointing accuracy of HIFI but retained some of its uncertainty. This is especially true of the mapped observations, which have additional pointing uncertainties due to relative offsetting and jitter, that may total for OTF maps. Both mapping and pointed modes can be further misdirected from their intended pointing during telescope switching from OFF to ON source. Indeed the APE was verified during Herschel photometric operation and was never proven to exactly match while in spectroscopic observations. In general, HIFI has shown itself able to resolve source structure to better than 1″ at its high-frequency end. Nevertheless pointing errors of order 3″ have occasionally been observed on Herschel.
As we are combining data across several observation days, it is easy to conceive of pointing errors of this order. Together with the large source gradient shown in Figs. B.1 and B.2 and the presence of the HC in the vicinity of our source, the effect of pointing errors must be considered a likely source of variability in the broad spectral component. Quantitatively, if one is to look more closely at, for example, Fig. B.2, there is a drastic, ~1.5 K increase in the broad component as one moves from the centre position to that directly beneath. Following the discussion from above, a reasonable pointing error of 3″, or 23% of the map step size, would thus correspond to an increase of ~0.35 K in the broad pedestal. An offset of this magnitude from the centre position would therefore result in the amplitude of the broad component rising to ~0.85 K, or ~1.7 times the intensity.
© ESO, 2014
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