Volume 531, July 2011
|Number of page(s)||17|
|Section||Interstellar and circumstellar matter|
|Published online||20 June 2011|
During the night of March 26 2005, Q-band imaging was performed using the VLT Imager and Spectrometer for the mid-IR (VISIR, Lagage et al. 2004). For photometric calibration and PSF determination, the standard stars HD 50310 and HD 150798 were observed before and after the science observation respectively. Standard chopping and nodding was employed to remove the atmospheric background emission. The imaging was performed with the Q2 filter (λc = 18.72 μm) in the small field mode (pixel field of view = 0.075″). The observing conditions were fair, airmass ~1.4, and the optical seeing ~0.8″. The achieved sensitivity was 80 before and ~100 mJy/10σ1h after the science observation. The full width at half maximum (FWHM) of the point spread function (PSF) was ~0.5″. The data reduction was performed with a dedicated pipeline, which corrects for various instrumental signatures (see Pantin et al. 2008, 2009; Pantin 2010). The reduced image is presented in Fig. A.1
VISIR Q band image at 18.7 μm. North is up and east is left, with a 3 × 3″ field of view. The scale is logarithmic in intensity.
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The VISIR image (Fig. A.1) shows a resolved disk that is almost completely spherical, consistent with previous imaging at this wavelength (Liu et al. 2003). Deviations from spherical symmetry are down to the 10% level, which means that inclination must be small. Due to the relatively large size of the PSF at this wavelengths, it is difficult to get a direct constraint on the inclination out of the image. Comparison with a set of inclined, PSF-convolved model images show that it is consistent with an inclination less than ~40 degrees and a position angle along the south-east to north-west.
Due to the increased sensitivity with respect to previous observations, the image shows resolved emission up to ~1.4″, corresponding to a physical radius of 145 AU at 103 pc. To characterize this emission, we construct a surface brightness profile. Because the disk is not seen face-on, we construct the surface brightness profile from the image by averaging the surface brightness over an elliptical annulus – rather than a circular one. The shape of the annuli correspond to a disk inclination of 42° and a position angle of 145°. The semi-major axis of this ellipse corresponds to the distance from the star at which the radiation is emitted (radius). Annuli are 0.075″ wide, the size of one pixel on the VISIR chip. They are thus much smaller than the width of the VISIR PSF.
Radial emission profiles in the Q band at 18.7 μm, calculated assuming elliptic annuli (see text for details). Displayed are: VISIR image (diamonds) and reference PSF (grey line). The errors on the PSF are displayed as horizontal error bars, those on the noise as vertical ones. Overplotted are PSF-convolved disk models with a varying surface density power law of p = 0.0, 1.0 and 2.0 (dotted, solid and dashed respectively). NOTE: radius refers to the semi-major axis of the ellipse, which reflects the physical radius where the radiation is emitted.
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Comparing the measured radial intensity profiles to that of the VISIR PSF6 (Fig. A.2), we see that the disk is clearly resolved. The profile itself is not easily characterized by simple power-laws as is often the case for scattered light images, due to the different emission mechanism (thermal versus scattered light) as well the relatively large size of the PSF.
Instead, we recognize two components. The profile up to 0.5″ is well described by a Gaussian with a FWHM of 0.73″. After quadratic subtraction of the PSF, we derive a spatial scale of 0.33″ for this central component, corresponding to a physical scale of FWHM = 34 AU at a distance of 103 pc. This number agrees well with the value of 34 ± 2 found by Liu et al. (2003) with direct imaging at the same wavelength. This component therefore originates in a region just behind the disk wall, which has a spatial scale of ~26 AU.
The second component – outside of 0.5″– comes from farther out in the disk, and has not been analysed before at this wavelength. Extended emission is seen up to 1.4″, corresponding to a physical radius of 145 AU at 103 pc. This emission probes the thermal continuum emission from the outer disk surface. The bump around 1.0″ corresponds to the first diffraction ring in the VISIR PSF, and does not represent a real structure in the disk. Because the radial profile traces the disk surface over almost an order of magnitude in radius, we can use it to put limits on the surface density of small grains, which we will do in the next section.
To test if the assumed power law index of p = 1 for the surface density profile matches our VISIR image, we compare model images of varying SDP with the observed image, keeping the dust mass and outer radius fixed, as well as the inner disk surface density. Because the pixel size is much smaller than the VISIR PSF, we convolve it with our model images before constructing the radial intensity profiles, and take into account the errors on the width of PSF (FWHM = 0.509 ± 0.008″) in the fitting procedure.
Because we solve for the hydrostatic disk structure, the shape of the radial profile depends only on the surface density distribution (Fig. A.2). The model is best fit for a surface density power law with index .
© ESO, 2011
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