2 Observations and data reduction

 

 

Table 1: Observations of G11.11-0.40

Date	$\lambda$	Tel./Instr.	FOV$^{\rm a}$	PSF	Ref. Star$^{\rm b}$	Limiting
				FWHM		Mag.$^{\rm c}$
1999 Sep.	J/H/K'	CA3.5(ALFA/OMEGA)	40 $^{\prime \prime }$	0 $.\!\!^{\prime\prime}$5/0 $.\!\!^{\prime\prime}$5/0 $.\!\!^{\prime\prime}$4$^{\rm d}$	FS28 (10.744/10.644/10.602)	19.5/19.8/19.1
1998 Jul.	Br$\gamma$/Cont/H2(1-0)S1	ESO2.2m/IRAC2b	130 $^{\prime \prime }$	1 $.\!\!^{\prime\prime}$5	HD 210427 (8.852)	$\sim$17.5
1998 Mar.	1.3mm	ESOSEST(Bolo)	4$^\prime$	23 $^{\prime \prime }$	Uranus	50mJy/beam

$^{\rm a}$ Field of view for single frames.
$^{\rm b}$ Brightness given in magnitudes for the observed wavelength.
$^{\rm c}$ Derived from background noise for point sources with given PSF (1$\sigma$ detection).
$^{\rm d}$ Measured on target, the resolution on the WFS source is 0 $.\!\!^{\prime\prime}$19 (Strehl ratio 17%) in K'. The seeing was 0 $.\!\!^{\prime\prime}$8 in K'.

 

 

Table 2: Observations of G341.21-0.21

Date	$\lambda$	Tel./Instr.	FOV$^{\rm a}$	PSF	Ref. Star$^{\rm b}$	Limiting
				FWHM		Mag.$^{\rm c}$
1999 Mar.	K' Pol	ESONTT / SOFI	4.9$^\prime$	0 $.\!\!^{\prime\prime}$5	-	-
1998 Jul.	Br$\gamma$/Cont/H2(1-0)S1	ESO2.2m/IRAC2b	130 $^{\prime \prime }$	1 $.\!\!^{\prime\prime}$5	HD 210427 (8.852)	$\sim$17.5
1996 Oct.	N	ESO2.2(MANIAC)	43 $.\!\!^{\prime\prime}$5	1 $.\!\!^{\prime\prime}$1	$\alpha$Sco (-4.54)	6.3
1998 Mar.	1.3mm	ESOSEST(Bolo)	4$^\prime$	23 $^{\prime \prime }$	Uranus	50mJy/beam
1995 Aug.	H/K'	ESO3.6(ADONIS/SHARP2)	13 $^{\prime \prime }$	0 $.\!\!^{\prime\prime}$39/0 $.\!\!^{\prime\prime}$25$^{\rm d}$	HD 97048/HD 2811 (6.19/7.067)

$^{\rm a}$ Field of view for single frames.
$^{\rm b}$ Brightness given in magnitudes for the observed wavelength.
$^{\rm c}$ Derived from background noise for point sources with given PSF (1$\sigma$ detection).
$^{\rm d}$ The seeing was 0 $.\!\!^{\prime\prime}$7 in K'.

2.1 Adaptive optics NIR imaging

2.1.1 G11.11-0.40

G11.11 was observed as part of the ALFA science demonstration run in September 1999. This programme, designed to show the science capabilities of the new AO system ALFA (Hippler et al. 1998) on the 3.5m telescope on Calar Alto (Spain), allowed flexible scheduling of the observations to ensure optimal meteorological conditions. J, H, and K' images were taken in seeing conditions around 0 $.\!\!^{\prime\prime}$8 (K'). ALFA was locked on the m_V=10.8mag star GSC 0627201030. 5$\times$5 subapertures on the wavefront sensor were used, allowing the correction of 18 modes at a speed of 100Hz. In spite of an airmass of 2.4 during the observations, a maximum Strehl ratio of 17% was reached in K', yielding a FWHM of 0 $.\!\!^{\prime\prime}$19 on the reference star. However, due to the target's distance of $\sim$20 $^{\prime \prime }$ from that star, this resolution degrades to 0 $.\!\!^{\prime\prime}$44 at the location of G11. In H and J these values are 0 $.\!\!^{\prime\prime}$52 and 0 $.\!\!^{\prime\prime}$55. In each band, a mosaic of five frames taken at slightly different positions were obtained. The total integration time amounted to one minute per band at each position. To achieve a photometric calibration and access the seeing value, open loop images of the UKIRT standard star FS28 were taken.

2.1.2 G341.21-0.21

For this object, near-infrared H and K' imaging was performed in August 1995 using ESO's AO system ADONIS (Beuzit et al. 1994) on the 3.6m telescope at La Silla/Chile. In both bands, data were taken at three positions to derive the sky background directly from the images. The total integration time was 6 min per band. The seeing during the observations was around 0 $.\!\!^{\prime\prime}$7 in K', the airmass was 1.06. The m_V=12.7 mag star GSC0788001158, located at a distance of 12 $^{\prime \prime }$, served as the wavefront sensor star. The higher order AO corrections achieved a resolution of 0 $.\!\!^{\prime\prime}$3 in K' and 0 $.\!\!^{\prime\prime}$28 in H, respectively. In both bands, the Strehl number is approximately 6%. For photometric calibration, images of the standard star HD 2811 were taken.

2.1.3 Common reduction scheme

All frames were subject to the standard NIR reduction scheme for sky-subtraction, flat-fielding, bad pixel removal and photometric calibration. Sky frames were constructed by stacking the image frames taken at different mosaic positions directly and adjusting the mean background level through an additive constant. Then, for each pixel, the minimum (G341) or the next higher value (G11) was selected to efficiently remove all sources from the sky frame. Photometric calibration was achieved by a simple procedure of multiplying the frames with the known counts-(per unit time)-to-flux ratio of the standard frame. No corrections for colour terms or atmospheric extinction were introduced. This limits the accuracy of our photometry to about 0.1mag, which is sufficient for our purposes. Astrometric calibration was made by comparison to the corresponding images from the digitized sky survey (DSS). For G11, a set of five stars with their positions known from the DSS was fitted to produce the astrometric reference frame. For G341, 7 stars were fitted in the larger scale narrow-band continuum image (see below). The astrometric frame of that image was then transferred to the high-resolution images via comparison of 4 stars.

2.2 NIR narrow-band observations

Both targets were observed in June 1998 using the IRAC2b NIR-camera at ESO's 2.2m telescope on La Silla/Chile. Four filters were used with central wavelengths 2.105  m (BP4), 2.121  m (BP5, used as H₂(1-0)S1 filter), 2.148  m (BP7), and 2.164  m (BP8, used as Br filter). The effective wavelengths were taken from the IRAC2b manual. The use of lens C resulted in a pixel scale of 0 $.\!\!^{\prime\prime}$507 per pixel. A mosaic of five frames was taken with the object once in the centre of the detector and once in the centre of each quadrant. The total integration time on source added up to 20 min. The same procedure was performed for the standard star HD 210427 with 10 min of total integration time. Sky subtraction, flat-fielding, bad pixel removal and photometric calibration were achieved in the same way as for the broad-band images. The procedure of subtracting the continuum for photometry of the Br$\gamma$ emission is described in Feldt et al. (1999). Astrometric calibration was achieved in the same way as for the broad-band images via a comparison with the DSS.

2.3 Polarimetric observations of G11.11-0.40

Polarimetry of G11 was carried out in March 1999 on ESO's NTT on La Silla/Chile. The images where taken using SOFI'S NB $2.195\,\ensuremath{\mu} {\rm m}$ filter ( $\Delta \lambda = 0.03$ m) and a Wollaston prism as polarizer. Two perpendicular polarization orientations were mapped on the detector simultaneously. Two more orientations were obtained after offsetting the NTT's de-rotator by 45$^{\rm o}$. For each orientation of the de-rotator, 5 frames of 180s integration time with slight offsets in between were taken. The procedure was repeated using the NBBr filter of SOFI. Sky frames were constructed from the offset frames in the usual way. Flatfielding, sky subtraction, and bad-pixel removal followed standard IR techniques. No photometric calibration was done for these data. Polarization vectors were computed from the sum of the continuum and Br images using the light ratios in the four polarization orientations. Instrumental effects were calibrated out by declaring the polarization of 23 field stars to be zero, thereby computing an offset to the resulting Stokes vectors which yield the intrinsic instrumental polarization.

2.4 N-band observations of G341.21-0.21

G341 was subject to observations using the MANIAC (Böker et al. 1997) mid-infrared camera at ESO's 2.2m telescope on La Silla/Chile in March 1998. MANIAC's N-band filter ( $\lambda = 10.5\,\ensuremath{\mu} {\rm m}, \Delta\lambda=5.0\,\ensuremath{\mu} {\rm m}$) was used. The total integration time on source was 640s. This includes the combination of the two chopping beams. For photometric calibration, $\alpha$ Sco was observed as a standard star. As G341 is the only source visible in the N-band frame, the astrometric reference frame had to be established by identifying the maximum of the emission with the maximum of the broad-band K'-emission from the target.

2.5 1.3 mm continuum observations

The continuum radiation of both targets was mapped in March 1998 at the 15m SEST telescope at La Silla/Chile. The detector system was the $^{\rm 3}$He-cooled single-channel bolometer system (Kreysa 1990). The equivalent bandwidth of the bolometer is $\approx$50GHz centred on a frequency of $\nu_0 = 236\,$GHz ( ${\rm\lambda_0 = 1.27~mm}$). The effective beam size at this wavelength is $\theta_{\rm b} = 23$ $^{\prime \prime }$.

The source was mapped four times with the "double beam'' technique described first by Emerson et al. (1979). To generate the dual beams, a focal plane chopper with a chopping frequency of 6 Hz was used. Chopping was done in azimuth. The chopper throw was 67 $^{\prime \prime }$. The map rows were generated by moving the telescope continuously along the direction of the beam separation (i.e. in azimuth) with a scanning velocity of 8 $^{\prime \prime }$/s and an elevation separation between adjacent scans of 8 $^{\prime \prime }$.

Calibration maps of the planet Uranus (adopted brightness temperature 96K; Griffin & Orton 1993) were obtained with the same technique and parameters as used for the two science targets. The atmospheric transmission was measured by sky dips. The telescope pointing was found to be repeatable within $\pm$5 $^{\prime \prime }$.

Data reduction was performed with the SEST standard software and with the software package MOPSI (written by R. Zylka) which use the NOD2 and GAG libraries.