Astron. Astrophys. 329, L53-L56 (1998)

2. Observations

We obtained data with the ISOPHOT instrument (Lemke et al., 1996) on board of the ISO satellite (Kessler et al. 1996) and used AOT (Astronomical Observation Template) PHT03 at 25 µm and PHT22 at 60, 90, 135, and 180 µm.  Cnc is 4.7 arcmin away from the MIII star HD 75716 which is a relatively bright IRAS source (IRAS 08494+2826) with fluxes of 13.4 and 3.5 Jy at 12 and 25 µm respectively. Calibration observations performed at a later stage on brighter targets have shown that straylight from HD 75716 as an infrared source is negligible. However, detection of this star by ISOPHOT would cause a significant detector responsivity drift which could hinder accurate measurement of the weaker fluxes of  Cnc. We therefore designed a 3 points raster scan observation with scan direction approximately perpendicular to the vector defined by HD 75716 and  Cnc (scan position angle is 135 degrees). The second raster pointing is centered on  Cnc, the first and third pointings measure the background level. For each filter measurement, two identical calibration measurements of a grey body internal to ISOPHOT (the fine calibration source, FCS) were collected at the beginning and end of each raster scan. A summary of the observations is presented in Table 1.

Table 1. Summary of observations

Per raster step an exposure time of 128 sec was applied. For the C_60 and C_90 measurements we used the 3x3 C100 detector array. The target was therefore measured by the center pixel of the array during the integration on the second raster position. For the C_135 and C_180 measurements the 2x2 C200 detector array was used. Data reduction was performed with the ISOPHOT interactive analysis software package (PIA). The short measurements (32 sec) of the FCS were corrected for detector responsivity transients. For the longer integrations on the raster points (128 sec) this was not necessary. The target flux in each pixel was obtained by subtracting the average flux of the two background positions from the flux at the target position. The absolute calibration of the data utilized the pairs of FCS measurements related to each filter. At the time of writing this paper the accuracy in the absolute calibration of the FCS is better than 30% for P_25, C_60 and C_90. At the longer wavelengths the uncertainty is higher.

The background subtracted flux per pixel for the 60 and 90 µm measurements is presented in Fig. 1. For both wavelength bands all fluxes are larger than zero suggesting that the flux in the second pointing is systematically higher than the two background pointings. Since we expect the flux from the target predominantly in pixel 5 this observation indicates either that the background at the target position is higher or the presence of a non-linear long-term signal drift during the raster measurement. To correct for this we determined the mean flux among all pixels except pixel 5 and used this value as the new zero background level. Pixel 5 was excluded because it could contain emission from the star.

Fig. 1. The background subtracted flux per pixel. Upper panel: 60 µm, lower panel: 90 µm. The matrix in the upper panel depicts the relative pixel positions on the sky. The target was centered on pixel 5. The horizontal dotted lines are the zero levels obtained by taking the average of all pixels except for pixel 5.

At 60 µm we find a significantly higher flux for pixel 5 and 6. At 90 µm the flux in these pixels is also higher, but is not significant. In both cases there are reasons to assume that the flux in pixel 6 is not reliable. It has been known for some time that (i) pixel 6 has a dark signal which is 4 to 6 times higher than the other pixels - a 10% variation in the dark signal would cause a flux variation of 70 mJy - and (ii) pixel 6 can show a signal drift which behaves differently compared to the other pixels. Since the background subtracted fluxes are very small compared to the total signal (5% for pixel 5), these anomalies in pixel 6 can show up in the difference signal. We therefore omit pixel 6 in further analysis.

Both the 135 and 180 µm observations exhibit background levels that are higher than the brightness level of the source position. We conclude that the star was not detected at these wavelengths. The uncertainty is dominated by the cirrus confusion noise.

The derived flux densities are given in Table 2, together with measurements and limits from IRAS. The uncertainties are the statistical errors obtained from the error propagation in the signals including FCS signals. The upper limit at 90 µm is 5 times the statistical error. The upper limits in the C_135 and C_180 bands are 5 times the estimated cirrus confusion noise on the target position obtained from the cartesian sum of the background subtracted flux of each pixel. Absolute calibration uncertainties are not included. The IRAS limits at 60 and 100 µm are taken from the IRAS Point Source Catalogue (IPSC). The IRAS Faint Source Catalogue (IFSC) gives an upper limit of 105 mJy at 60 µm, in conflict with our own measurement of 170mJy. However, we believe the IFSC limit is not accurate in this case, probably corrupted by the influence of the nearby IR source IRAS 08494+2826. The mJy limit from the IPSC is consistent with our measurement.

Table 2. Measured flux densities for  Cnc

The ISO measurements are plotted in Fig. 2 as squares. Also shown are the measurements from IRAS, plotted as triangles.

Fig. 2. The spectral energy distribution of  Cnc. The dashed line is a Kurucz atmosphere fitted to the UBVJHKL and IRAS 12 µm fluxes. The solid line indicates the flux of the disk model with icy grains described in the text. The dotted line shows a different model fit with ice-free dust grains. Triangular points are colour corrected measurements from the IRAS faint source catalogue. The limit shown at 60 and 100 µm are taken from the IRAS point source catalogue. Squares are the new ISO measurements (also colour corrected). Open symbols with arrows indicate upper limits.

© European Southern Observatory (ESO) 1998

Online publication: December 16, 1997
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