Issue
A&A
Volume 518, July-August 2010
Herschel: the first science highlights
Article Number L114
Number of page(s) 5
Section Letters
DOI https://doi.org/10.1051/0004-6361/201014622
Published online 16 July 2010
A&A 518, L114 (2010)

Herschel: the first science highlights

LETTER TO THE EDITOR

Herschel-SPIRE spectroscopy of the DR21 molecular cloud core[*]

G. J. White1,2, A. Abergel4, L. Spencer3, N. Schneider5, D. A. Naylor3, L.D. Anderson8, C. Joblin17,18, P. Ade7, P. André 5, H. Arab4, J.-P. Baluteau8, J.-P. Bernard17,18, K. Blagrave9, S. Bontemps5,6, F. Boulanger4, M. Cohen10, M. Compiegne9, P. Cox11, E. Dartois4, G. Davis12, R. Emery2, T. Fulton4, B. Gom4, M. Griffin7, C. Gry8, E. Habart4, M. Huang14, S. Jones3, J. M. Kirk7, G. Lagache4, S. Leeks2, T. Lim2, S. Madden5, G. Makiwa3, P. Martin9, M.-A. Miville-Deschênes4, S. Molinari14, H. Moseley15, F. Motte5, K. Okumura5, D. Pinheiro Gonçalves9, E. Polehampton2,4, T. Rodet16, J. A. Rodón8, D. Russeil8, P. Saraceno14, S. Sidher2, B. M. Swinyard2, D. Ward-Thompson7, A. Zavagno8

1 - Department of Physics & Astronomy, The Open University, UK
2 - Space Science Department, Rutherford Appleton Laboratory, Chilton, UK 
3 - Institute for Space Imaging Science, University of Lethbridge, Lethbridge, Alberta, Canada 
4 - Institut d'Astrophysique Spatiale, CNRS/Université Paris-Sud 11, 91405 Orsay, France - Laboratoire AIM, CEA/IRFU 
5 - CNRS/INSU - Université Paris Diderot, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France 
6 - CEA, Laboratoire AIM, Irfu/SAp, Orme des Merisiers, 91191 Gif-sur-Yvette, France 
7 - Department of Physics and Astronomy, Cardiff University, Cardiff, UK 
8 - Laboratoire d'Astrophysique de Marseille, UMR6110 CNRS, 38 rue F. Joliot-Curie, 13388 Marseille, France 
9 - Canadian Institute for Theoretical Astrophysics, Toronto, Ontario, M5S 3H8, Canada 
10 - University of California, Radio Astronomy Laboratory, Berkeley, 601 Campbell Hall, US Berkeley CA 94720-3411, USA 
11 - Institut de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, 38406 Saint-Martin-d'Hères, France 
12 - Joint Astronomy Centre, University Park, Hilo, USA 
13 - National Astronomical Observatories (China) 
14 - Istituto di Fisica dello Spazio Interplanetario, INAF, Via del Fosso del Cavaliere 100, 00133 Roma, Italy 
15 - NASA-Goddard SFC, USA 
16 - Laboratoire des Signaux et Systèmes (CNRS Supélec Université Paris-Sud 11), Plateau de Moulon, 91192 Gif-sur-Yvette, France 
17 - Université de Toulouse; UPS; CESR; 9 avenue du colonel Roche, 31028 Toulouse Cedex 4, France 
18 - CNRS; UMR5187; 31028 Toulouse, France

Received 31 March 2010 / Accepted 11 May 2010

Abstract
We present far-infrared spectra and maps of the DR21 molecular cloud core between 196 and 671 $\mu $m, using the ${\it Herschel}$-SPIRE spectrometer. Nineteen molecular lines originating from CO, 13CO, HCO+ and H2O, plus lines of [N  II] and [CI] were recorded, including several transitions not previously detected. The CO lines are excited in warm gas with $T_{\rm kin} \sim 125$ K and $n_{\rm {H_2}}$ $\sim7 \times 10^4$ cm-3, CO column density $N{\rm (CO)} \sim 3.5 \times 10^{18}$ cm-2 and a filling factor of $\sim $12$\%$, and appear to trace gas associated with an outflow. The rotational temperature analysis incorporating observations from ground-based telescopes reveals an additional lower excitation CO compoment which has a temperature $\sim $78 K and $N{\rm (CO)} \sim 4.5\times10^{21}$ cm-2.

Key words: ISM: lines and bands - infrared: ISM - ISM: molecules

1 Introduction

We report observations of the far-IR spectrum of the DR21 molecular cloud core obtained with the Herschel satellite between 196 and 671 $\mu $m . The DR21 HII-region/molecular cloud is part of the Cygnus X complex of molecular clouds located at a distance of 1.7 kpc (Schneider et al. 2006). This region has been subject to numerous studies at different wavelengths (Richardson et al. 1988; Wilson & Mauersberger 1990; Liechti & Walmsley 1997; Schneider et al. 2006, 2010; Jakob et al. 2007). The main DR21 cloud core has a mass of $\sim $20 000 $M_{\odot}$ (Richardson et al. 1989), and contains one of the most energetic star formation outflows detected, with an outflow mass of $\sim $3000 $M_{\odot}$ (Garden et al. 1991; Cruz-Gonz ${\acute{\rm a}}$lez et al. 2010).

2 SPIRE observations

2.1 Spectra

We present science demonstration phase (SDP) observations obtained with ESA's Herschel Space Observatory (Pilbratt et al. 2010), using the Spectral and Photometric Imaging REceiver (SPIRE - Griffin et al. 2010). The calibration and characteristics of SPIRE have been described by Swinyard et al. (2010). SPIRE was operated as an imaging Fourier-transform spectrometer (FTS) in the high resolution mode ( ${\lambda}/{\Delta\lambda} = 1000$ (=300 km s-1at 250 $\mu $m ) sampling across an approximately circular field of view with an unvignetted diameter of 2.6$^\prime $. This means that the line profiles are unresolved.

The sky footprint is formed by two detector arrays: the 19 pixel SLW array (671-303 $\mu $m ) and the 37 pixel SSW array (313-194 $\mu $m ), with beam widths varying from 17 $^{\prime \prime }$ at 194 $\mu $m to 42 $^{\prime \prime }$ at 671 $\mu $m, with uncertainties of $\pm$7-10% (Griffin et al. 2010). The integration time was 1065 s, summed from two seperate observations. The current best estimates of the absolute uncertainties for the FTS detectors are 10-20% for the SSW detectors, and $\sim $30% for the SLW detectors (Swinyard et al. 2010).

\begin{figure}
\par\includegraphics[width=9cm,clip]{14622fg1.eps}\\
\includegraphics[width=9cm,clip]{14622fg2.eps} %
\end{figure} Figure 1:

(Upper picture) Spectrum of DR21 with the SLW detector array, with beam sizes varying from 42.0 $^{\prime \prime }$ at 671.1 $\mu $m to 37.3 $^{\prime \prime }$ at 316.5 $\mu $m, and (lower picture) with the SSW detector array, with beam sizes varying from 18.7 $^{\prime \prime }$ at 281.7 $\mu $m to 16.8 $^{\prime \prime }$ at 194.2 $\mu $m.

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The unapodised FTS spectra provide the highest spectral resolution, with a classical instrumental sinc function line shape. A spectral line fitting routine was developed for extracting line parameters (Jones et al. 2009). This fits a continuum (either a low order polynomial or a blackbody variant) using the Levenberg-Marquardt least squares method. The fitting procedure weights the spectral intensity at a given frequency of an averaged spectrum by the statistical uncertainty at that frequency, returning line centers, intensities, line widths and their associated fit errors.

2.2 Maps

The SPIRE observations sparsely sample the field of view, although there are calibration uncertainties for the outer ring of detectors at the edges of both arrays that are not yet fully characterised. To provide a first look at the relative distributions in the various species, we have interpolated the fluxes of individual lines, although this spatial information is not used in subsequent line modeling.

The maps of selected species are shown in Fig. 2. The CO lines in both detector arrays show a prominent central peak, with extensions to the east and west along the well known outflow. This has been assumed to be associated with outflowing gas with ( $T_{\rm ex}\sim 2000$ K and $H({\rm H}_2) \sim 1\times 10^{19}$ cm-2 from Garden et al. 1991). However, as will be seen in the high resolution JCMT observations (Fig. 3), the emission traced in the SPIRE maps is also clearly visible in the relatively low excitation CO ${\it J} = 3{-}2$ data, suggesting that there may be a mixture of low and high excitation gas present. This is confirmed in Fig. 3, where similar extensions of the ambient gas are present in the JCMT CO j32 map, and that of Schneider et al. (2010). This is not unexpected, as this outflow appears to have a very large mass of several thousand $M_{\odot}$, and presumably the high velocity gas phase overlaps (or may co-exist with) ambient material. The SPIRE maps also show that the 3P1-3P0  atomic carbon line has a similar spatial distribution to that of CO. By contrast, the H2O and [N  II] lines appear to be more compact and centred close to the DR21 cloud core, although the [N  II] distribution is elongated to the east - observations with higher signal to noise and better sampling are needed for more detailed comparison.

\begin{figure}
\par\includegraphics[angle=90,width=9cm,clip]{14622fg3.eps} %
\end{figure} Figure 2:

The SPIRE maps of selected species toward DR21. Since the data points do not fully sample the mapped area, this map was constructed by interpolating between the data points. The (0,0) position is located at RA (2000) = 20$^{\rm h}$39$^{\rm m}$01 $.\!\!^{\rm s}$18, Dec (2000) = +42$^{\circ }$19$^\prime $43 $.\!\!^{\prime\prime}$66.

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Table 1:   Fluxes measured in the central pixel.

2.3 JCMT CO J = 3-2 observations

CO j32 JCMT archival data (programme M07AU01) with a 15 $^{\prime \prime }$ beam and spectral of 0.05 km s-1 are shown in Fig. 3, from a 4.5 h integration using the HARP array receiver. The area covered by the SPIRE footprint (Fig. 2) is shown as a white square. The JCMT observations clearly trace the outflow which runs from the NE-SW from DR21 from the centre of the white box. The JCMT map also reveals a prominent north-south ridge that includes CO peaks associated with the well-studied sources DR21(OH) and DR21-FIR1. Around the DR21 core, a bipolar structure close to the systemic velocity is coincident with distribution of high velocity gas and shocked H2 (Garden et al. 1991).

\begin{figure}
\par\includegraphics[width=8cm,clip]{14622fg4.eps} %
\end{figure} Figure 3:

CO ${\it J}$ = 3-2 integrated emission map of DR21 between velocities of - $40{-}\pm 40$ km s-1. The lowest contour is drawn at 1 K km s-1 (5$\sigma $ as observed in a 61 kHz ($\sim $0.05 km s-1) spectral channel), and the contour intervals are incremented in steps of 50 K km s-1. The peak antenna temperature and velocity integrated temperatures at the DR21 peak are 39.2 K and 726 K km s-1 respectively. The temperature scale is calibrated in antenna temperature $T_{\rm a}$$^{\rm *}$ units, which is related to the main beam brightness temperature $T_{\rm mb}~$by $T_{\rm mb}~$= $T_{\rm a}$$^{\rm *}$/ $\eta _{\rm mb}$, where the main beam efficiency, $\eta _{\rm mb}$ = 0.63. Further details of HARP calibration are given in Buckle et al. (2010).

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3 Modeling the CO lines

The most extensive modeling of the CO toward DR21 is by Richardson et al. (1986, 1988), Wilson et al. (1990), Schneider et al. (2006) and Jakob et al. (2007). Richardson et al. (1986, 1988) presented a multiphase model with gas densities spanning the range 103-106 cm-3, and gas temperatures in the low temperature component $\ga$30 K. Jakob et al. (2007) confirmed this using KOSMA and ISO observations, finding and additional warm phase component with $T_{\rm kin} \sim80{-}150$ K and clump density $n_{\rm {H_2}}$ $\sim $ 104-106 cm-3.

We initially constructed a rotational temperature diagram for the SPIRE CO and 13CO lines. These were augmented with the JCMT CO line from Sect. 2.3, plus IRAM CO j21 observations (Schneider et al. 2010), with suitable beam size corrections. The rotational temperature diagram is shown in Fig. 4.

\begin{figure}
\par\includegraphics[width=9cm,clip]{14622fg5.eps} %
\end{figure} Figure 4:

CO and 13CO line rotational temperature diagram.

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\begin{figure}
\par\includegraphics[width=9cm,clip]{old/14622fg6.ps} \vspace*{-2mm}
\end{figure} Figure 5:

LVG analysis of the CO and 13CO lines toward DR21. The SPIRE data points, (J = 4-3 to 13-12) are shown as filled circles, and the ISO data points (Jakob et al. 2007) as squares, along with 30% absolute error bars. The CO J = 3-2 line was obtained from the JCMT data, convolved to a resolution of 17 $^{\prime \prime }$ to match the beamsize of the nearby SPIRE lines. The filled circles are the SPIRE data, open sqares are ISO data from Jakob et al. (2007), open triangle is determined from the JCMT data in this paper, and the crosses are the SPIRE 13CO data. The top solid curve shows the fit to the predicted SPIRE CO line intensities, and the lower dashed one is for 13CO with an assumed CO/13CO abundance of 67, an estimated filling factor of 12%. As might be expected, the fit for a single tempeature lies intermediate between the two $T_{\rm rot}~$values inferred from Fig. 4.

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Both species show evidence for two gas components, a lower temperature phase with a rotational temperature $T_{\rm rot}~$ = 78 K and total CO column density $N{\rm (CO)} \sim 4.5\times10^{21}$ cm-2, in addition to a higher temperature component with $T_{\rm rot}~$= 185 K and $N{\rm (CO)} \sim 9.7 \times 10^{17}$ cm-2. The 13CO  lines are more limited and noisy, with the SPIRE lines indicating an intermediate temperature phase having $T_{\rm rot}~$= 109 K and $N(^{13}{\rm CO})\sim 8.8 \times 10^{16}$ cm-2. The data for 13CO also show evidence for a low temperature component, although this relies on comparison with low frequency ground based data (JCMT, IRAM) obtained with different beam sizes. Such a result is expected, since the observations probe deeper into the PDR of each clump in 13CO than in CO.

There are several problems with the rotational temperature approach, including wavelength dependent beam size corrections, opacity and calibration errors. These uncertainties can however be mitigated by i) taking ratios of the various CO line intensities on a single detector and using these to constrain the excitation conditions though our LVG modeling, and ii) using observations from the central pixel where the SSW and SLW beams are coincident and the calibration is well determined. This approach particularly mitigates against the beam size and calibration errors, since only flux ratios are being used to estimate the excitation conditions.

The model fit was made to the CO and 13CO lines using the off-line version of the RADEX LVG code (Van der Tak et al. 2007). The line ratios observed on the same detectors (hence the beam sizes are similar) were used to restrict the likely excitation conditions. It proved difficult to find an unique single temperature model that simultaneously predicted the relative intensities of both isotopologues. However, the SPIRE data can be approximately reproduced by a single phase moderate temperature gas with $T_{\rm kin}$ $\sim 125$ K, volume density $\sim $ $7 \times 10^4$ cm-3, with $N{\rm (CO)} \sim 3.5 \times 10^{18}$ cm-2, filling factor $\sim $12%, and a [C]/[13C] ratio of 65. This model does however slightly overpredict the low-J 13CO 4-3 - 6-5 line intensities, compared to the J = 7-6 line. Changing the temperature and density from these conditions considerably worsened the high-J CO line fits, although a more complex multiphase model, with appropriate (and uncertain) beam size corrections would improve the fit of the low J-lines. We have not attempted to fit to a PDR-model, as the data and calibration quality need to be improved if tests between models are to be made, and that this is beyond this first look paper.

4 Modeling the H2O and [N II] lines

An objective of this study was to detect the [N  II] 205 $\mu $m line, and to compare it with the [C  II]157 line which has $n_{\rm crit} = 46$ cm-3, $T_{\rm e} = 8000$ K. This has a nearly identical critical density for excitation in ionised regions. Their line ratio is directly related to the N+/C+ abundance ratio, and this ratio traces the fraction of the observed [C  II] emission that arises from ionized regions (Oberst et al. 2006). Taking the SPIRE upper limit of $7.5 \times 10^{-8}$ W m-2 sr-1 with Jakob et al. (2007), the ratio of the 122/205 $\mu $m lines is $\ge$1.9, which is only adequate to constrain the ionised gas density to be $\ge$$\sim $30 cm-3. The [C  II]/[N  II]205 ratio using the Jakob et al. (2007) tabulation is $\ge$5.6. Given current uncertainties and lack of an [N  II]122 flux, it is necessary to await improved data. We note that the [N  II] extension to the east (see Fig. 2) coincides with a hole in the excited H2 emission image (Cruz-Gonz ${\acute{\rm a}}$lez et al. 2010), which may indicate there is a cavity of ionised gas. However clarification will require future observations with better sampling.

In Fig. 6 we show a section of the spectrum with the 398.6 $\mu $m para-H2O line, and the HCO+ J=6-5, 7-6 and 8-7 lines. Putting the SPIRE sensitivity into perspective, Jakob et al. (2007) report that the integrated CI 3P1-3P0  intensity measured from the KOSMA telescope with an 80 $^{\prime \prime }$ beam is 46.6 K km s-1, and main beam brightness temperature $\sim $25 K. By comparison the same line observed with SPIRE has a peak S/N ratio of $\ga$14 as seen in a single SPIRE channel. We also used RADEX to compute an LVG solution for the 211-202 para-H2O line at 398.5 $\mu $m. Assuming similar excitation to that from the CO solution, for an abundance X[H2O] = $\sim $$\times$ 10-8 and line width of 40 km s-1 (Hjalmarson et al. 2003), we predict that the SPIRE flux should be 2.4 $\times$ 10-8 W m-2 sr-1, which agrees with the measured value of 2.33 $\pm$ 0.3 $\times$ 10-8 W m-2 sr-1.

\begin{figure}
\par\includegraphics[width=9cm,clip]{14622fg7.eps}\vspace*{-2mm}
%
\end{figure} Figure 6:

Continuum subtracted spectrum showing the 398.6 $\mu $m para-H2O, and HCO+ j76 and j87 lines. Some instrumental features remain - notably a broad bump close to the CH+ line at 347.9 $\mu $m. Although it is possible to flatten this spectrum further, we chose only to fit a third order polynomial to the entire 671-319 $\mu $m range of the SLW detector.

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5 Conclusions

We have presented the SPIRE spectrum of a star-forming molecular core, DR21, showing for the first time the complete CO and 13CO band head from J = 4-3 to 13-12, along with their maps at far-infrared wavelengths. A rotational temperature analysis shows two gas phases with $T_{\rm rot}~$ $\sim80$ K and CO column density $\sim $ $4.5\times 10^{18}$ cm-2, and $T_{\rm rot}~$= 185 K and $N{\rm (CO)}\sim10^{18}$ cm-2respectively, although the 13CO $T_{\rm rot}~$is intermediate between these two. Simple LVG modeling shows the presence of warm (125 K) and dense ( $n_{\rm {H_2}}$$\sim $$\times$ 104 cm-3) gas, which is traced by in the SW extension in the CO and CI maps. The observed flux from the 398.6 $\mu $m H2O line is consistent with these values.

Acknowledgements

SPIRE has been developed by a consortium of institutes led by Cardiff Univ. (UK) and including Univ. Lethbridge (Canada); NAOC (China); CEA, LAM (France); IFSI, Univ. Padua (Italy); IAC (Spain); Stockholm Observatory (Sweden); Imperial College London, RAL, UCL-MSSL, UKATC, Univ. Sussex (UK); Caltech, JPL, NHSC, Univ. Colorado (USA). This has been supported by national funding agencies: CSA (Canada); NAOC (China); CEA, CNES, CNRS (France); ASI (Italy); MCINN (Spain); Stockholm Observatory (Sweden); STFC (UK); and NASA (USA).

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Footnotes

... core[*]
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

All Tables

Table 1:   Fluxes measured in the central pixel.

All Figures

  \begin{figure}
\par\includegraphics[width=9cm,clip]{14622fg1.eps}\\
\includegraphics[width=9cm,clip]{14622fg2.eps} %
\end{figure} Figure 1:

(Upper picture) Spectrum of DR21 with the SLW detector array, with beam sizes varying from 42.0 $^{\prime \prime }$ at 671.1 $\mu $m to 37.3 $^{\prime \prime }$ at 316.5 $\mu $m, and (lower picture) with the SSW detector array, with beam sizes varying from 18.7 $^{\prime \prime }$ at 281.7 $\mu $m to 16.8 $^{\prime \prime }$ at 194.2 $\mu $m.

Open with DEXTER
In the text

  \begin{figure}
\par\includegraphics[angle=90,width=9cm,clip]{14622fg3.eps} %
\end{figure} Figure 2:

The SPIRE maps of selected species toward DR21. Since the data points do not fully sample the mapped area, this map was constructed by interpolating between the data points. The (0,0) position is located at RA (2000) = 20$^{\rm h}$39$^{\rm m}$01 $.\!\!^{\rm s}$18, Dec (2000) = +42$^{\circ }$19$^\prime $43 $.\!\!^{\prime\prime}$66.

Open with DEXTER
In the text

  \begin{figure}
\par\includegraphics[width=8cm,clip]{14622fg4.eps} %
\end{figure} Figure 3:

CO ${\it J}$ = 3-2 integrated emission map of DR21 between velocities of - $40{-}\pm 40$ km s-1. The lowest contour is drawn at 1 K km s-1 (5$\sigma $ as observed in a 61 kHz ($\sim $0.05 km s-1) spectral channel), and the contour intervals are incremented in steps of 50 K km s-1. The peak antenna temperature and velocity integrated temperatures at the DR21 peak are 39.2 K and 726 K km s-1 respectively. The temperature scale is calibrated in antenna temperature $T_{\rm a}$$^{\rm *}$ units, which is related to the main beam brightness temperature $T_{\rm mb}~$by $T_{\rm mb}~$= $T_{\rm a}$$^{\rm *}$/ $\eta _{\rm mb}$, where the main beam efficiency, $\eta _{\rm mb}$ = 0.63. Further details of HARP calibration are given in Buckle et al. (2010).

Open with DEXTER
In the text

  \begin{figure}
\par\includegraphics[width=9cm,clip]{14622fg5.eps} %
\end{figure} Figure 4:

CO and 13CO line rotational temperature diagram.

Open with DEXTER
In the text

  \begin{figure}
\par\includegraphics[width=9cm,clip]{old/14622fg6.ps} \vspace*{-2mm}
\end{figure} Figure 5:

LVG analysis of the CO and 13CO lines toward DR21. The SPIRE data points, (J = 4-3 to 13-12) are shown as filled circles, and the ISO data points (Jakob et al. 2007) as squares, along with 30% absolute error bars. The CO J = 3-2 line was obtained from the JCMT data, convolved to a resolution of 17 $^{\prime \prime }$ to match the beamsize of the nearby SPIRE lines. The filled circles are the SPIRE data, open sqares are ISO data from Jakob et al. (2007), open triangle is determined from the JCMT data in this paper, and the crosses are the SPIRE 13CO data. The top solid curve shows the fit to the predicted SPIRE CO line intensities, and the lower dashed one is for 13CO with an assumed CO/13CO abundance of 67, an estimated filling factor of 12%. As might be expected, the fit for a single tempeature lies intermediate between the two $T_{\rm rot}~$values inferred from Fig. 4.

Open with DEXTER
In the text

  \begin{figure}
\par\includegraphics[width=9cm,clip]{14622fg7.eps}\vspace*{-2mm}
%
\end{figure} Figure 6:

Continuum subtracted spectrum showing the 398.6 $\mu $m para-H2O, and HCO+ j76 and j87 lines. Some instrumental features remain - notably a broad bump close to the CH+ line at 347.9 $\mu $m. Although it is possible to flatten this spectrum further, we chose only to fit a third order polynomial to the entire 671-319 $\mu $m range of the SLW detector.

Open with DEXTER
In the text


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