3 Results

  
3.1 Extraction of 25 $\mu$m excess stars

The predicted, IRAS, and recalibrated ISO fluxes are listed in Table 1. The IRAS and ISO fluxes have been colour corrected assuming a stellar photosphere, the colour correction factors are 1.40 and 1.28 for IRAS and ISO, respectively. The table includes the adopted flux $F_{\nu}^{\rm ad}$ at 25 $\mu$m, determined in the following way:

 

$$F_{\nu}^{\rm ad} = (F_{\nu}^{\rm ISO}+F_{\nu}^{\rm IRAS})/2,$$ (2)

$${\Delta}F_{\nu}^{\rm ad} = {\rm max}({\Delta}F_{\nu}^{\rm ISO}/\sqrt{2},~ (F_{\nu}^{\rm ISO}-F_{\nu}^{\rm IRAS})/2).$$

We make the conservative assumption that for most of the individual measurements the uncertainty in the IRAS flux is as large as that of ISO. We checked the validity of this assumption by analysing the distribution of residual fluxes $F_{\nu}-F_{\nu}^{\rm pred}$ for the IRAS and ISO measurements separately (cf. Fig. 3). We found that both distributions have similar FWHM, of about 80 mJy, which suggests similar uncertainties for the IRAS and ISO data. In case no IRAS flux is available the ISO flux and uncertainty were adopted.

Figure 2: V-[25] versus B-V colour for all stars in the sample. The solid line is the photospheric emission as predicted by Eq. (1). The dashed line is the relationship for K and M dwarfs with B-V > 0.8 derived by Mathioudakis & Doyle (1993).

The visual-infrared colour-colour diagram for the stars in the sample is presented in Fig. 2. The predicted photospheric flux follows closely the distribution of points in the sample indicating that Eq. (1) is applicable. The good match also indicates that most of the 25 $\mu$m flux densities are predominantly photospheric. The sample contains 5 stars with B-V >1.0 (Table 1), these stars are all K dwarfs and have V-[25] above the prediction. On the other hand, the relation derived by Mathioudakis & Doyle (1993) for K and M dwarfs predicts values of V-[25] which are too high for the three stars with highest V-[25].

To assess the photometric quality of the sample we present a histogram of the difference $F_{\nu}^{\rm ad}-F_{\nu}^{\rm pred}$ in Fig. 3. The distribution is strongly peaked and suggests a normal distribution close to zero for the majority of stars in the sample.

Figure 3: Distribution of deviations from the predicted fluxes. The dashed bins give the number of stars below and above the given flux limits. The solid line is a normal distribution based on $\sigma~=$ 35 mJy, mean = 8 mJy.

The parameters of the normal distribution have been derived as follows. Initially, an intermediate mean and standard deviation was derived of the stars in the interval $\vert F_{\nu}^{\rm ad}-F_{\nu}^{\rm pred}\vert <~100$ mJy. Judging from the histogram we decided that the stars falling outside this interval must be outliers. Subsequently, all stars were rejected which are more than 2.6 standard deviations away from the mean (i.e. $\leq$1% probability of occurrence). This yields an interval of $-81~{\rm mJy}< F_{\nu}^{\rm ad}-F_{\nu}^{\rm pred} <~97$ mJy. The mean of the remaining stars is 8 mJy with a dispersion of 35 mJy. The normal distribution has been included in Fig. 3. Application of a Kolmogorov-Smirnov test showed that the distribution in the given interval is normal with a significance level of 5%.

This analysis indicates that for the majority of the stars in the sample, the 25 $\mu$m fluxes are consistent with the expected photospheric fluxes. The overall scatter between the observations and expectations is 35 mJy. Judging from the individual uncertainties of the 25 $\mu$m fluxes, we conclude that most of the scatter must come from the infrared measurements and that the predicted fluxes which are only based on optical data are very accurate, within 8 mJy for the sample average. In addition, there is no indication that the distribution is non-normal, suggesting that there is no statistical evidence for a surplus of positive excesses in the distribution for $F_{\nu}^{\rm ad}-F_{\nu}^{\rm pred} <~97$ mJy.

Figure 4: The properties of the stars more than 3 $\sigma$ away from the peak of the distribution presented in Fig. 3. The error bars present ${\pm }3{\times }{\Delta }F_{\nu }^{\rm ad}$. The star HD 39060 ($\beta$ Pic) is not included due to its off-scale positive excess of a lot.

A total of 11 (=14%) targets fall outside the ${\pm}3~\sigma$ interval, and 2 out of these 11 targets are below the expected flux. For these outlying stars we have plotted in Fig. 4 the ratio $F_{\nu}^{\rm ad}/F_{\nu}^{\rm pred}$. In order to indicate the significance of the deviation we have included ${\pm}3~{\Delta}F_{\nu}^{\rm ad}$ error bars, highlighting the uncertainties in the individual measurements.

Figure 4 indicates that 7 stars (6 stars plus $\beta$ Pic) with $F_{\nu}^{\rm ad}/F_{\nu}^{\rm pred}~>~0$ have fluxes which are more than $3{\Delta}F_{\nu}^{\rm ad}$ above the predicted values. These stars have been flagged in Table 1.

3.2 Infrared excess from K dwarfs

Two of the 7 excess stars are classified as K dwarfs (Table 1). It is likely that Eq. (1) does not apply for these type of stars but rather the relation derived by Mathioudakis & Doyle (1993), see also Fig. 2. Indeed, for HD 88230 Mathioudakis & Doyle (1993) predict a photospheric flux of 513 mJy which is above $F_{\nu}^{\rm ad}=412$ mJy observed by us. For HD 191408 the value for B-V is low, in the regime where the difference between Eq. (1) and the relation by Mathioudakis & Doyle is small. The measured 25 $\mu$m flux of 462 mJy for HD 191408 is still more than $3{\Delta}F_{\nu}^{\rm ad}$ above the flux of 383 mJy expected for K dwarfs. In conclusion, we reject the detection of an excess in HD 88230.

3.3 List of excess stars in the sample

We have listed resulting the excess stars in Table 2. The excess emission at 25 $\mu$m depends on the shape of the spectral energy distribution and the response of the filterband. We only determined the in-band excess emission for the ISO observations.

 

 

Table 2: Stars showing significant deviations from the predicted photospheric flux at 25 $\mu$m. The dust temperatures $T_{\rm d}$ are derived from the 25/60 flux ratio (Sect. 4.1).

HD	name	${F_{\nu}^{\rm star}}$	Excess	$T_{\rm d}$
		Jy	Jy	K
38678	$\zeta$ Lep	0.33	$0.54\pm0.05$	$122\pm6$
39060	$\beta$ Pic	0.29	$7.06\pm0.26$	$82\pm1$
102647	$\beta$ Leo	1.16	$0.39\pm0.07$	$83\pm5$
172167	$\alpha$ Lyr	6.35	$2.41\pm0.39$	$81\pm4$
191408*		0.34	$0.17\pm0.03$	(-)
216956	$\alpha$ PsA	3.26	$0.20\pm0.07$	$49\pm1$

^* Not a Vega-like excess star, see Sect. 4.1

If the excess emission is not point-like but comes from a region which is a significant fraction of the beam profile, then the averaging of the two flux measurements is not valid. In all positive excess cases except for HD 191408, the IRAS excess at 25 $\mu$m is larger than the ISO excess. This might indicate that the excess emission is extended and has partly been resolved by ISO (cf. Fig. 5).