Towards a Sharper Picture of R136 with SPHERE Extreme Adaptive Optics
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Astronomical Science DOI: 10.18727/0722-6691/5023 Towards a Sharper Picture of R136 with SPHERE Extreme Adaptive Optics Zeinab Khorrami1, 2 Local Group. In the heart of 30 Doradus, its better-than-nominal performance that Farrokh Vakili1 the most massive and compact star clus- surpasses previous NAOS-CONICA Thierry Lanz1 ter R136 is located. This cluster hosts the (NACO) and Multi-conjugate Adaptive Maud Langlois 3, 4 most massive stars known in the Local optics Demonstrator (MAD) observations. Eric Lagadec1 Universe (Crowther et al., 2016; Crowther The air mass during these observations Michael R. Meyer 5, 6 et al., 2010). R136 provides a unique ranged from 1.54 to 1.67. The average Raffaele Gratton7 opportunity to study the formation of Strehl ratio (SR) in the J- and Ks-bands Jean-Luc Beuzit8 massive stars and their feedback on clus- was determined to be 0.40 ± 0.05 and David Mouillet 8 ter formation and evolution. 0.75 ± 0.03, respectively. We used the second-generation Very In order to qualify our data, we compared 1 School of Physics and Astronomy, Large Telescope (VLT) SpectroPolari- the reduced Ks-band images with pub- Cardiff University, United Kingdom metric High-contrast Exoplanet REsearch lished images of R136 from HST (with the 2 Université Côte d’Azur, OCA, CNRS, (SPHERE) instrument to observe the Wide Field Planetary Camera 3, WFC3) Lagrange, France central core of R136 in the near-infrared. in the V-band and with the MAD imaging 3 Université de Lyon, Université Lyon 1, Using the small field of view (FoV) of the (Campbell et al., 2010) in the Ks-band CNRS, CRAL UMR5574, Saint-Genis SPHERE InfraRed Dual-band Imager and (Figure 2). This comparison confirms that Laval, France Spectrograph (IRDIS), more than one our data present better spatial resolution 4 Université Aix Marseille, CNRS, thousand sources have been detected in and point-spread function (PSF) sampling Laboratoire d’Astrophysique de Ks- and J-band data in the core of R136 which are more suitable for applying Marseille, UMR 7326, France (10.9 × 12.3 arcseconds) covering almost deconvolution techniques. For the photo- 5 Institute for Astronomy, ETH Zurich, 2.7 x 3.1 pc at the distance of R136 metry, we used the Starfinder package Switzerland (50 kpc). (Diolaiti et al., 2000), implemented in the 6 Department of Astronomy, University of Interactive Data Language (IDL). Star- Michigan, Ann Arbor, U.S.A. SPHERE’s extreme adaptive optics (AO) finder is designed for the analysis of AO 7 INAF — Astronomical Observatory of system results in the same resolution with images of crowded fields such as the Padua, Italy the VLT Melipal telescope in the Ks-band Galactic Centre (Pugliese et al., 2002). It 8 Université Grenoble Alpes, CNRS, as the Hubble Space Telescope (HST) enables determination of the empirical Institut de Planétologie et yields in the visible (0.055 arcseconds). local PSF from several isolated sources in d’Astrophysique de Grenoble, France In the J-band the resolution of 0.035 arc- the image and uses this PSF to extract seconds exceeds that of the HST, and other stellar sources across the FoV. does so with a better pixel sampling of The SPHERE extreme adaptive optics 12.25 milli-arcseconds per pixel. Three well-isolated stars (no neighbours instrument was used to observe the cen- within 0.47 arcseconds) were used to tral core of the Large Magellanic Cloud, extract the PSF in the J- and Ks-band R136, in the near-infrared. This chal- Observations data separately. The extracted PSFs from lenging observation demonstrated the the J- and Ks-band data were used as capabilities of SPHERE for imaging dis- We collected data in Guaranteed Time input for the stellar source detection by tant clusters. More than one thousand Observation (GTO) runs to image R136 Starfinder. The full width half maximum sources have been detected in Ks- and using the classical imaging mode of IRDIS (FWHM) of the extracted PSFs are 54.71 J-band images in the small field of view (Langlois et al., 2014). Observations were and 65.16 milli-arcseconds in the J- and of IRDIS covering almost 2.7 × 3.1 pc of performed in September 2015, with high Ks-band data, respectively. As a conse- the core of R136. Based on isochrone dynamic and high angular resolution quence, 1110 and 1059 sources were fitting of the colour- magnitude diagram, imaging in J- and Ks-bands, over a FoV detected in the J- and Ks-bands, respec- ages of 1 and 1.5 Myr for the inner of 10.9 × 12.3 arcseconds, centred on tively, with a signal-to-noise ratio (S/N) 3-arcsecond core and the outer core of the core of the cluster (Figure 1). The see- better than two. The AO correction R136 fit our data best. The mass func- ing was 0.63 ± 0.1 arcseconds during efficiency degrades as a function of dis- tion slope is –0.96 ± 0.22 over the mass the observations and the night was rated tance from R136a1, which is the refer- range of 3 to 300 M⊙. Using SPHERE as clear. Less than 10 % of the sky (above ence star for the AO loop. At the borders data, we have gone one step further in 30 degree elevation) was covered by of the FoV, the isoplanatic limits are partially resolving the core of R136, but clouds, and the transparency variations approached. Overall, the PSF is not cen- this is certainly not the final step and were lower than 10 %. tro-symmetric at large distances from higher resolution is still required. R136a1. We took these distortions into The SPHERE data consist of 300 frames, account to estimate the local statis tical each of 4.0 s exposure, in the two IRDIS errors, which become significant for 30 Doradus, in the Large Magellanic broad-band Ks and J filters (BB-Ks, BB-J). those sources that are more distant from Cloud (LMC), is one of the most massive The Wolf-Rayet star R136a1 was used to the centre of the image than typically and optically brightest H II regions in the guide the AO loop of SPHERE, confirming 3 arcseconds. 32 The Messenger 168 – June 2017 NASA, Paresce ESA, F. (INAF-IASF, Bologna, Italy), R. O'Connell (University of Virginia,and Charlottesville), the Wide Field Camera 3 Science Oversight Committee Figure 1. (Upper left) Zooming into the core of R136 Figure 2. (Lower) Comparison of images of the R136 the lower row of images are zoomed into the very (275 arcseconds), from a large-field BVR image with core with the highest available angular resolution. core of R136 (FoV of 1.5 × 1.5 arcseconds). From FORS1 to the core of R136 (upper right) from an HST The FoV of all the images in the upper row is the left to right: HST/WFC3 (V-band), MAD (Ks-band), WFC3 JH-band image (59 arcsecond field-of-view), same as the IRDIS data (10.9 × 12.3 arcseconds); and SPHERE/IRDIS (Ks-band). showing the area imaged by SPHERE IRDIS. The Messenger 168 – June 2017 33 Astronomical Science Khorrami Z. et al., Towards a Sharper Picture of R136 with SPHERE Extreme AO Full FoV r < 3ೀ r > 3ೀ Age and extinction 11 7611 11 6 To estimate the stellar ages and the 12 12 12 extinction in the core of R136, we used 6 13 13 5 13 5 the effective temperature (Teff) and lumi- nosity (log L) of 54 stars that were stud- 14 14 14 ied spectroscopically by Crowther et 5 al. (2016). We also chose a grid of isoch- 15 15 4 15 4 rones at different ages (from 0.1 up to 8 Myr) with the LMC metallicity (Z = 0.006) 16 16 16 from the latest sets of PARSEC evolution- 4 ary models (Bressan et al., 2012), a theo- J 17 17 3 17 3 retical library1 that includes the latest set of stellar phases from pre-main sequence 18 3 18 18 to main sequence and covering stellar masses from 0.1 to 350 M⊙. By fitting the 19 19 2 19 2 isochrones to each star, we estimated its 2 age and intrinsic colour with error bars. 20 20 20 +1. 2 An age range of 1.8 – 0.8 Myr is the most 21 21 1 21 1 probable for these stars. The extinction 1 in J- and Ks-bands is 0.45 ± 0.5 and 0.6 Myr 0.6 Myr 0.6 Myr 22 1 Myr 22 1 Myr 22 1 Myr 0.2 ± 0.5 mag, respectively. 1.5 Myr 1.5 Myr 1.5 Myr 2 Myr 2 Myr 2 Myr 23 0 23 0 23 0 Figure 3 (upper) shows the colour-magni- –1 012 –1 012 –1 012 tude diagram (CMD) of detected sources J–Ks J–Ks J–Ks in J- and Ks-band IRDIS data with their error bars. The CMD is plotted for the 1000 whole FoV (818 sources), in the very core 0.6 Myr of the cluster (r < 3 arcseconds), and 1.0 Myr 1.5 Myr outside (r > 3 arcseconds), from left to right. The PARSEC isochrones at four dif- Γ0.6 Myr=–0.88 +/– 0.12 ferent ages (0.6, 1.0, 1.5 and 2.0 Myr) Γ1.0 Myr=–0.90 +/– 0.13 Γ =–0.98 +/– 0.18 are also plotted in this figure using a dis- 100 1.5 Myr tance modulus of 18.49 to R136 and central extinction values in J (0.45 mag) and K (0.2 mag).