Volume 559, November 2013
|Number of page(s)||22|
|Section||Interstellar and circumstellar matter|
|Published online||20 November 2013|
Figure A.1 shows the PACS spectra of the T Tauri star AS 205 and of the Herbig Ae star HD 97048 between 62–190 μm. The main molecular and atomic transitions detected in the whole sample are shown. Figs. A.2 and A.3 show a portion of the PACS spectra of selected sources.
PACS spectrum of the T Tauri star AS 205 (top) and of the Herbig Ae star HD 97048 (bottom). The marks indicate the positions of [O i] (light blue), [C ii] (orange), CO (green), OH (red), H2O (blue) and CH+ (purple) lines.
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PACS spectra of a sub-sample of the program stars between 62−73 μm. Marks and colors as in Fig. A.1. The 69 μm forsterite feature is present in the spectrum of HD 100546.
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Table B.1 reports the line fluxes of the far-IR OH transitions. The line flux uncertainties correspond to the 1σ error. For non-detection the 3σ upper limit is reported. Table B.2 reports the atomic and molecular data of the transitions detected in this paper. Molecular data are taken from the LAMDA database (Schöier et al. 2005).
OH line fluxes.
Atomic and molecular data of the far-IR detected transitions.
Results of the non-LTE simulations with RADEX. (Top) OH rotational diagram of HD 163296 and AS 205 and non-LTE model predictions: three different models are shown for different values of the gas density and a temperature of 400 K and 200 K for HD 163296 and AS 205, respectively. Only models with n ≥ 1010 cm-3 can reproduce the observations. (Middle) OH rotational diagram of HD 100546 and non-LTE model predictions (N = 2 × 1014 cm-2, T = 200 K) in the case in which the infrared radiation field is included in the RADEX simulation to test the effect of infrared pumping: in both cases (with and without radiation field) high gas densities are needed to reproduce the observed rotational diagram. (Bottom) Ratio of Tk to Tex for 2 OH transitions as a function of NOH in two low gas density cases. Even in the low gas density cases, the OH rotational levels are in LTE (Tk = Tex) for large values of NOH when the lines are optically thick.
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OH lines studied here have large critical densities (ncrit ~ 109 − 1010 cm-3) and non-LTE excitation may be important if the gas density is not high enough to thermalize the OH molecules.
To test the assumption of LTE we fit the observed rotational diagram using the non-LTE code RADEX (van der Tak et al. 2007). We used the same fitting procedure as for the LTE case and we repeated the analysis for different values of the gas (H2, collision partner) density (n = 106,108,1010 cm-3). Figure C.1 shows the OH rotational diagram for two test cases: the Herbig Ae HD 163296 and the T Tauri AS 205 disks. We reproduced the slab model using the best-fit parameters found in the LTE case with NOH = 1015 cm-2 and TK = 400 K for HD 163296 and NOH = 8 × 1015 cm-2 and TK = 200 K for AS 205 (Table 6). The non-LTE model predictions are plotted in Fig. C.1 with different colors for the three values of nH2. For low nH2 values (≤108 cm-3) the model fails to reproduce the observed rotational diagram. The gas density must be nH2 ≥ 1010 cm-3 in order to fit the observations. Thus, the OH rotational lines emerge from an high density region where the OH molecules are thermalized and the rotational levels are in LTE.
Infrared pumping can be relevant for the excitation of OH molecules. To test the effects of line pumping we run a grid of RADEX models for HD 100546 providing also the infrared radiation field (between 20 μm–3 mm) in the input parameters.
The radiation field is taken from the full disk thermo-chemical model of Bruderer et al. (2012) who computed the radiation field at each position of the disk for different wavelengths. The radiation field is stronger in the inner region of the disk (r < 20 AU). As input to RADEX we considered the value of the infrared radiation field at a distance of r = 20 AU and height above the midplane z = 4 AU (z/r = 0.2). At larger radii and height (where the far-IR OH lines originate) the radiation field is always fainter. Figure C.1 shows the OH rotational diagram of HD 100546 (middle row) and the RADEX predictions without (left) and with (right) infrared radiation field. The line flux ratios vary in the presence of infrared pumping, but even in this case high gas density (≥1010 cm-3) is needed to reproduce the observed rotational diagram.
The non-LTE simulations also show that for large values of the column density (NOH ≥ 1018 cm-2) the OH rotational lines are in LTE even at gas densities ≤108 cm-3 (Fig. C.1, bottom). This is due to line opacity which traps the photons and helps to thermalize the gas. However, we can exclude this scenario for most of the sources based on the non-detection of the intra-ladder transitions at 79 μm. These transitions are indeed very sensitive to line opacity and the lines are easily detected for NOH ≳ 1016 cm-2, as in the case of DG Tau and AS 205. This is shown in Fig. C.2 where the ratio of the OH 79 μm to the OH 65 μm lines is shown in the LTE case for different temperatures. In order for the intra-ladder lines to be detected high column density is needed.
Ratio of the OH 79 μm to the OH 65 μm lines from the LTE calculation at 3 different temperatures. The ratio increases rapidly with column density. The dashed lines indicate the observed ratio for AS 205 and HD 163296 (1σ upper limit).
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This section describes an analysis of the atomic lines aiming at addressing the spatial extent of the line. The Herschel/PACS PSF varies substantially from 50 μm to 200 μm. As a consequence the amount of flux in the central spaxel varies from ~70% at 60 μm to 55% at 160 μm. For this reason, line emission can be detected outside the central spaxel (especially in the red part >100 μm). To check whether a line is spatially extended we compute the equivalent width (W) and integrated continuum (Fc) next to the line and check the relative spatial distribution. If the line emission is co-spatial to the continuum emission, then the spatial distribution of the equivalent width will be equal to that of the integrated continuum (Fc) (same PSF). In particular, the distribution of Fc corresponds to the PSF at the given wavelength (assuming that the continuum emission is not spatially resolved).
For the [O i] 63 μm line, FC is measured integrating the spectrum between 64.0−64.5 μm and the equivalent width is measured integrating the spectrum between 63.08−63.30 μm. The only source where off-source oxygen excess emission is DG Tau. Figure D.1 shows the [O i] 63 μm spectral map of DG Tau: the (blue) dashed contours show the distribution of the spectral continuum and the sub-panels shows the [O i] 63 μm spectrum in each spaxel. While the continuum is compact and centered on the central spaxel, the line emission shows an excess emission outside the central spaxel. The maximum excess is measured southward of the central source in agreement with the outflow position.
[O i] 63 μm and [C ii] spectral map in DG Tau showing the spatially extended line emission. The contours represent the spectral continuum measured in the vicinity of the line, the last contour level corresponds to 10% of the continuum peak. The sub-panels show the line spectrum measured in each spaxel.
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For the [C ii] line the line is integrated between 157.530 and 157.970 μm and the continuum flux between 158.5 and 162 μm. Figures D.1 and D.2 show the line spectral map for different sources. The spectral map shows the spectrum (continuum subtracted) in each spaxel. All the sources where [C ii] emission is detected show excess line emission outside the central spaxel. The most clear cases are HD 38120, IRS 48 and DG Tau. This pattern is the result of extended line emission. In the case of AB Aur and HD 97048 the object is mis-pointed and the spatial distribution of the continuum emission deviates from the PSF. Nevertheless, also in these two cases the line emission is not co-spatial with the continuum emission and proves a spatially extended line emission. The case of HD 50138 is less clear.
To estimate the maximum [C ii] emission associated with the protoplanetary disk the extended emission needs to be subtracted. To do this the [C ii] line flux (integrated between 157.60 and 157.98 μm) in each of the 9 central spaxels is calculated. Then the extended emission is determined as the average of the line flux measured in the 8 neighboring spaxels (around the central one) and subtracted from the value measured in the central spaxel. The result is reported in Table 3. In this way, the large scale (>94) [C ii] emission is approximately removed. The value of the [C ii] flux derived by this method must be considered an upper limit to the [C ii] emission arising from the disk as extended emission from a compact remnant envelope may still be present in the central 94 × 94 area of the sky.
© ESO, 2013
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