News from the 30m Telescope

Travel to the 30-m telescope

If on your visit to our observatory you have lost or forgotten your toothbrush, shampoo or other hygiene articles, do not hesitate to ask our kitchen staff who have a small supply to help out in such kind of emergencies.

Starting February 5, 2005 Granada airport will have its first international flight connection: Ryan Air (www.ryanair.com) will offer a daily low price direct connection between Granada and London Stansted.

Rainer Mauersberger

Errors in the frequency calculations for the wideband single pixel receivers at Pico Veleta

The A230 and B230 receivers (in use since 1998 at Pico Veleta Observatory) and the C150, D150, C270 and D270 receivers (installed in 1999) and also HERA can be used for instantaneous bandwidths of up to 1 GHz. Since the use of the maximum instantaneous bandwidth of 1 GHz is incompatible with certain backend combinations, there is also a so called "narrowband mode" with 500 MHz instantaneous bandwidths for any of the above mentioned receivers. In the OBS control software, this narrow band mode is in fact the default.

Both modes require different settings of the IFs, and therefore different methods to calculate the sky frequency. It turned out that there has been an error in the calculation of the sky frequency of the above mentioned receivers when used in narrowband mode. This error was present from the installation of these receivers until recently, when the problem was detected and fixed (October 21th, 2004).

The calculational error concerns the Doppler correction. The error is thus small when the Earth moves perpendicular to the viewing direction and largest (i.e. 25 kHz for a source close to the ecliptic) 3 months later. The Doppler correction due to the rotation of the Earth is two order of magnitude smaller, and can therefore be neglected in almost any conceivable case. In the worst case, a CS(3-2) line could appear to move by 0.1 km/s between two observations performed 6 months apart. We are aware that this error is not negligible for some kinds of projects and apologize for the inconvenience this may cause. We are willing to offer our help to recover the correct frequencies and ask anybody whose data is affected to send me (mauers@iram.es) the corresponding project number, dates of observations and range of scan numbers and receivers used.

Rainer Mauersberger

The confusion limit toward some astronomical molecular line sources

Table 1: The confusion limit reached by the IRAM 30-m telescope toward some molecular line sources

Source	$\lambda$	weakest	$t_{\rm int}^{a)}$	Ref.$^b$
$\Delta v$		detect. Lines
	mm	mK	h
IRC+10216	3	1.5	28	1)
28kms$^{-1}$	2	3.5	10	1, 2)
	1.3	$\la 2$	80	2)
Orion-KL	3	20	0.8	3)
5kms$^{-1}$	2	50	0.3	4)
	1.3	70	0.4	3)
SgrB2(N)	3	$\sim 20$	0.4	3)
$\sim$ 17kms$^{-1}$	2	$\sim 30$	0.4	3)
	1.3	$\sim 40$	0.5	3)

a) integration time (on+off) with present receivers at the IRAM 30-m telescope under normal winter conditions (good summer conditions), one polarization to obtain an rms of 1/3 $T_{\rm mb}$ with a velocity resolution of 1/5 the line width (one polarization receiver only); b)References: 1) Mauersberger, 2004, A&A 426, 219 2) Ziurys et al. 2002, ApJ 564, L45; 3) unpublished data; 4) Mauersberger et al. 1988, A&A 205, 235; c) weaker lines can be identified if one makes use of the unique line shapes of the spectra of this source (Cernicharo et al. 2004, ApJ 615, L145).

The following discussion is based on Mauersberger et al. (2004, A&A 426, 219). From their Section 5.1 it is evident that in the 2 and 3mm spectra of IRC+10216 presented in the article they are close to the confusion limit, where an increase of integration time does not yield much further information. It is interesting to investigate where line confusion begins to play a role for other favorite objects of molecule hunters such as the Orion Hot Core, SgrB2. We limit our discussion to the IRAM-30m telescope. For an extrapolation of our results to other telescopes with a higher or lower resolution it is necessary to take into account the detailed source structure and chemical and physical conditions within the regions observed (Comito & Schilke 2002 A&A 395, 357).

The definition of the useful observing time or rms to be reached is by no means unique and depends on whether one is interested in a mere detection of a line or whether one also wants to obtain some detailed information on the line shape. Here we try to be pragmatic: we have investigated spectra of several molecular line sources made with the IRAM 30-m telescope at 3mm, 2mm and 1.3mm wavelength made with such a long integration that $\ga 50\%$ of the spectral range observed is covered with features. We have determined the antenna temperature of the weakest unequivocal line features (knowing the typical line width and shape in the sources studied, but without sophisticated excitation analyses). The results are given in Table 1.

We also give an indication of the necessary rms for a 3$\sigma$ detection with a velocity resolution pf 1/5 of the full line widths typical for these sources, and the corresponding necessary integration time with the 30-m telescope and its present receivers (including all observing overheads, using one polarization only). In Orion, part of the confusion arises because of the large line widths observed in the outflow source. This can e.g. be prevented by observing at a carefully selected position offset from the outflow source (Combes et al. 1996, A&A 308, 618).

It will in certain cases be possible to identify lines that are weaker than the limits in Tab. 1

• if one carefully selects regions of the spectra which are relatively free of blending lines,
• if one can estimate the strengths of known blenders by an excitation analysis,
• if one makes use of information on the line shape and
• if one can confirm any such detections by observing several lines of a certain molecule.

Any observing proposal where lines close to the detection limit are requested should address these difficulties.

Rainer Mauersberger