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NAOS-CONICA Performance in a Crowded Field – the Arches Cluster A

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NAOS-CONICA Performance in a Crowded Field – the Arches Cluster A NAOS-CONICA Performance in a Crowded Field – the Arches Cluster A. STOLTE1, W. BRANDNER1, E.K. GREBEL1, DON F. FIGER2, F. EISENHAUER3, R. LENZEN1, Y. HARAYAMA3 1Max-Planck-Institut für Astronomie, Heidelberg, Germany 2Space Telescope Science Institute, Baltimore, USA 3Max-Planck-Institut für Extraterrestrische Physik, Garching, Germany 1. NAOS-CONICA – Near-Infrared Way. With an age of only ∼ 2–3 Myr it field (Schödel et al. 2002 and Ott et al. Camera with Adaptive Optics contains a rich massive stellar popula- in this issue), NIR wavefront sensing tion with about 150 O stars located in with NIR bright reference targets (K ∼10 NAOS-CONICA is a near-infrared the cluster centre (Figer et al. 1999), mag) is capable of improving the per- imaging camera and spectrograph and several more found in the immedi- formance in a highly extincted region. (CONICA) attached to an adaptive optics ate vicinity. The core density has been All images were taken in auto-jitter 5 –3 (AO) system (NAOS) for wavefront cor- estimated to be 3 ⋅ 10 Mᓃ pc , and the mode. The auto-jitter template allows 4 rections (Lenzen et al. 1998, Rousset et total stellar mass exceeds 10 Mᓃ automatic, random offsets between in- al. 2000). The AO system is designed to (Figer et al. 1999). The massive cluster dividual exposures to facilitate back- deliver diffraction-limited observations at centre contains ∼ 12 Wolf-Rayet stars ground subtraction and avoid after- an 8-metre-class telescope. NAOS is (Cotera et al. 1996, see also Blum et al. glows at the position of bright sources. equipped with two Shack-Hartmann 2001), intertwined with an intermediate- A set of additional exposures with short wavefront sensors operating at visual mass main-sequence population. At the detector integration times were taken to and NIR wavelengths, respectively. faint end, where pre-main-sequence obtain photometry of the bright cluster Most existing AO systems feed near-in- stars would be expected, the stellar stars (8 × 1 s DIT in H, 8 × 0.5 s DIT in frared (NIR) cameras, but operate at vi- population is severely contaminated by Ks). Seeing conditions varied during the sual wavelengths. In addition to the fact bulge stars, as Arches is located in the Arches observations. In H-band, the that the spectral energy distribution of central region of the Milky Way at a pro- seeing ranged from 0.45 to 0.85, while bright stars peaks at optical wave- jected distance of only 25 pc (roughly during K-band observations the seeing lengths, visual detectors offer higher 10 arcminutes) from the Galactic degraded from 0.8 to 1.3. On April 4, sensitivity and lower read-out noise Centre (Fig. 1). the moon was only 6° from our target, than NIR detectors, thus allowing one With a distance of about 8 kpc and a and increased background fluctuations to use fainter targets for wavefront foreground extinction in the visual of 24 decreased the detectability of faint sensing. The restriction to visual refer- to 34 mag, Arches is a very challenging sources in the cluster vicinity. ence sources, however, excludes object. The high foreground extinction The data were reduced within IRAF, deeply embedded objects located in re- obscures the faint cluster population at using twilight flat fields taken during gions with active star formation or high visual wavelengths, so that NIR obser- Comm 3 and combined with the nacop foreground extinction such as the vations, where extinction is less severe, twflat algorithm in eclipse (Devillard Galactic Centre. With wavefront sen- are necessary to detect the cluster pop- 1997). Eclipse is the ESO pipeline re- sors operating both at visual and NIR ulation. Given these extreme proper- duction package, which allows special wavelengths, NAOS combines the ad- ties, and the existence of data sets ob- treatment of particular instrument char- vantages of visual, faint reference tar- tained with HST/NICMOS and the acteristics adapted to the available gets with the option to penetrate highly Gemini/ Hokupa’a AO system allowing ESO instrumentation. Sky frames were obscured regions containing bright in- a detailed comparison, the Arches clus- created from the cluster observations frared sources. NAOS-CONICA was ter was chosen as NAOS-CONICA themselves, which is not ideal, but was commissioned on Yepun (UT4) in the commissioning target to test the NACO sufficient for our technical study. Star- course of Period 69. For more detailed performance on a dense, crowding-lim- light was excluded from these skies by information on NACO performance and ited stellar field. rejecting the bulk of the bright pixels. As first light, see Brandner et al. (2002) in the CONICA detector has a significant The Messenger 107. 3. NAOS-CONICA fraction of time-varying hot pixels, a We have used the CONICA JHK im- Commissioning Data cosmic ray rejection routine was used aging mode with the S27 camera in to obtain a bad pixel mask. This mask combination with the NAOS visual Arches H and Ks images were ob- was applied during the drizzling wavefront sensor for performance tests tained during Commissioning 3 of process, in which images are shifted in the crowded stellar field of the dense, NAOS-CONICA in March and April and combined. The 7 exposures with massive star cluster Arches. The S27 2002. 20 exposures with integration the highest resolution were combined camera with a pixel scale of 27 milliarc- times of 1 min (2 × 30 s detector inte- in Ks to one 7 min integrated frame, and second (mas) and corresponding field gration time (DIT)) each were obtained the 14 best exposures in H, where see- of view (FOV) of 27″ × 27″ is optimized in H, and 15 × 1 min exposures in Ks (4 ing conditions were much more stable, for diffraction-limited observations in × 15 s DIT). As the NIR wavefront sen- could be used to obtain a 14 min H- the J to K band wavelength range. At sor was not available during Comm 3, a band image. During the K-band obser- 2.2 µm, the diffraction limit of an 8-m blue foreground giant with V = 16 mag vations, the AO performance changed telescope is 57 mas, such that the S27 (V–R ∼1 mag) served as the reference with time as the seeing worsened. As camera allows Nyquist sampling. source for the visual wavefront sensor. crowding is the limiting factor on the Although the brightest star in the Arches field, the gain in resolution by 2. Why Arches? Arches field, this star is already close to rejecting the large fraction of frames the limiting magnitude (∼17 mag) of the with lower quality supersedes the loss The Arches cluster is one of the few visual wavefront sensor. As has been in photometric depth due to the shorter starburst clusters found in the Milky demonstrated on the Galactic Centre resulting total integration time on the fi- 9 Figure 1: A JHK composite 2MASS image of the Galactic Plane, including the Arches Cluster and the Galactic Centre, is shown in the left panel. The Arches cluster is the little blob of stars pointed out in the North of the image. The CONICA field of view (FOV) is indicated. The NAOS-CONICA H and Ks two-colour composite taken during Commissioning 3 is shown in the right panel. The FOV of this mosaic is 25″ × 27″, approximately the FOV of the CONICA S27 camera. The dense popu- lation in the Arches cluster centre has been resolved with NACO for the first time. nal image (see also Tessier et al. 1994). density gradient over small spatial in a very dense stellar field, background The best K-band photometry was ob- scales. fluctuations and saturation peaks fre- tained when combining only the frames quently cause false detections. where the best AO performance was 4. Luminosity Functions As sources in the Arches cluster are achieved. The resulting images have hidden behind ∼ 30 mag of foreground Strehl ratios of 14% in H and 20 % in K, In Figure 2, luminosity functions extinction, reddening influences the de- yielding a spatial resolution of ∼ 85 mas (LFs) of the NACO commissioning data tection of stars more severely at short- in both filters. are compared to HST/NICMOS LFs of er wavelengths. Consequently, the H- Standard DAOPHOT PSF fitting pho- the same field. Arches was observed band LF shown in Fig. 2 represents the tometry was used to obtain the final with NICMOS2 for 256 s integration photometric performance more realisti- magnitudes. A constant PSF function time in the broadband filters F110W, cally, as the K-band LF is artificially has been used, as isolated PSF stars F160W and F205W corresponding to J, truncated by the requirement that are rare in the dense Arches field. A H, and K. The NICMOS detection limits sources have to be detected in H as Penny function, which has a Gaussian were approximately 21 mag in H well. core with elongated Lorentzian wings, (F160W), and 20 mag in K (F205W). While the field star LF is comparable yielded the best fit to the data. Within a factor of 2 longer integration for NACO and NICMOS down to the The NACO data were calibrated time in K and a factor of 3.3 in H, NACO NICMOS detection limit of H ∼ 21 mag, using existing HST/NICMOS photom- reaches detection limits of 22 mag in H NACO clearly gains a large number of etry of the same field (Figer et al. 1999). and 21 mag in K, one magnitude deep- sources in the cluster centre. Here, the All calibrated data shown below are er than the NICMOS observations. significantly better resolution of 85 mas in the NICMOS photometric system.
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