2 Observations and data processing

2.1 Presentation of the sample

For a full description and discussion of the sample we refer to Paper I; here we briefly describe the properties of the 25 $\mu$m sample. The stars were selected on the following selection criteria: (1) all stars are within 25 pc of the Sun; (2) a spectral type later than B9 but earlier than M, and only dwarfs of type IV-V or V; (3) a predicted photospheric flux at 60 $\mu$m of at least 30 mJy; (4) no (optical) binaries within the aperture; (5) no known variables.

The third selection criterion implies that the minimum photospheric flux at 25 $\mu$m is of the order of 90 mJy. During the mission, some stars were added but these have not biased the statistics (cf. Paper I, Sect. 5.4).

In Table 1 we have listed the properties of the stars of the sample observed at 25 $\mu$m. Columns 1 to 3 describe the catalogue names of the stars, including the ISO observation identifier. Columns 4 to 6 list the optical stellar parameters from the Hipparcos Catalogue (Perryman et al. 1997), the spectral types in Col. 6 come from the machine-readable version of the Hipparcos Catalogue. Column 7 gives the age of the star from Lachaume et al. (1999). Columns 8 and 9 give the predicted flux density from Eq. (1) (below) and IRAS flux density, respectively. Columns 10 and 11 list flux density plus uncertainty obtained from the ISO data. Columns 12 and 13 give the adopted flux density plus uncertainty as described in Sect. 3.1. Finally, Col. 14 lists possible flags - "E'' indicating a significant excess, and "N'' indicating that the star was not included in the 60 $\mu$m list of Paper I due to instrumental reasons and visibility constraints during the ISO mission.

   

Table 1: The stars of the sample at 25 $\mu$m.

HD	HIP	ISO_id	V	B-V	Spect.	age	$F_{\nu}^{\rm pred}$	$F_{\nu}^{\rm IRAS}$	$F_{\nu}^{\rm ISO}$	$\Delta F_{\nu}^{\rm ISO}$	$F_{\nu}^{\rm ad}$	$\Delta F_{\nu}^{\rm ad}$	Excess
			mag	mag		Gyr	Jy	Jy	Jy	Jy	Jy	Jy	Flag
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)	(11)	(12)	(13)	(14)
693	910	37500901	4.89	0.487	F5V	5.13	0.231	0.174	0.254	0.013	0.214	0.040
1581	1599	85900104	4.23	0.576	F9V	6.46	0.511	0.496	0.491	0.011	0.494	0.008
4628	3765	39502507	5.74	0.890	K2V	7.94	0.241		0.209	0.016	0.209	0.016
4813	3909	38701510	5.17	0.514	F7IV-V	1.38	0.189	0.221	0.220	0.030	0.220	0.021
7570	5862	38603613	4.97	0.571	F8V	3.16	0.256	0.246	0.232	0.029	0.239	0.020
9826	7513	42301519	4.10	0.536	F8V	2.88	0.530	0.519	0.532	0.021	0.526	0.015
10700	8102	39301216	3.49	0.727	G8V	7.24	1.373	1.542	1.096	0.023	1.319	0.223
10780	8362	45701319	5.63	0.804	K0V	2.82	0.223	0.149	0.176	0.003	0.163	0.013
12311	9236	17902322	2.86	0.290	F0V	0.81	0.972	0.999	1.108	0.048	1.053	0.055
13445	10138	81301125	6.12	0.812	K0V	5.37	0.145	0.131	0.161	0.032	0.146	0.022
13709	10320	40101128	5.27	-0.01	A0V	0.34	0.049	0.112	0.076	0.026	0.094	0.018	N
14412	10798	40101731	6.33	0.724	G8V	7.24	0.100	0.107	0.151	0.016	0.129	0.022
14802	11072	40301534	5.19	0.608	G2V	5.37	0.225	0.244	0.266	0.039	0.255	0.028
15008	11001	15700537	4.08	0.034	A3V	0.45	0.168	0.174	0.138	0.001	0.156	0.018
17051	12653	41102840	5.40	0.561	G3IV	3.09	0.169	0.163	0.223	0.048	0.193	0.034
17925	13402	28302143	6.05	0.862	K1V	0.08	0.171	0.189	0.202	0.023	0.196	0.016
19373	14632	81001846	4.05	0.595	G0V	3.39	0.627	0.600	0.674	0.018	0.637	0.037
20630	15457	79201552	4.84	0.681	G5Vvar	0.30	0.361	0.334	0.433	0.031	0.383	0.050
20766	15330	27506149	5.53	0.641	G2V	4.79	0.176	0.202	0.077	0.018	0.139	0.063
20807	15371	57801755	5.24	0.600	G1V	7.24	0.212	0.209	0.242	0.017	0.225	0.016
22001	16245	69100658	4.71	0.410	F5IV-V	2.04	0.231	0.242	0.251	0.012	0.247	0.008
22484	16852	79501561	4.29	0.575	F9V	5.25	0.482	0.486	0.536	0.036	0.511	0.025
26965	19849	84801864	4.43	0.820	K1V	7.24	0.697	0.816	0.711	0.022	0.763	0.053
30495	22263	83901667	5.49	0.632	G3V	0.21	0.179	0.160	0.161	0.010	0.160	0.007
33262	23693	58900870	4.71	0.526	F7V	2.95	0.296	0.314	0.236	0.020	0.275	0.039
34411	24813	83801473	4.69	0.630	G0V	6.76	0.373	0.343	0.269	0.016	0.306	0.037
37394	26779	83801976	6.21	0.840	K1V	0.34	0.141	0.146	0.156	0.006	0.151	0.005
38392		70201401	6.15	0.940	K2V	0.87	0.183	0.153	0.207	0.027	0.180	0.027
38393	27072	70201304	3.59	0.481	F7V	1.66	0.755	0.701	0.849	0.031	0.775	0.074
38678	27288	69202307	3.55	0.104	A2Vann	0.37	0.328	0.826	0.748	0.040	0.787	0.039	E
39060	27321	70201079	3.85	0.171	A3V	0.28	0.294	6.294	5.810	0.205	6.052	0.242	E
43834	29271	19000682	5.08	0.714	G5V	7.24	0.309	0.286	0.334	0.039	0.310	0.027
48915	32349	72301710	-1.4	0.009	A0m...		25.28	24.264	20.222	0.625	22.243	2.021
50281	32984	71802113	6.58	1.071	K3V	2.63	0.162		0.185	0.029	0.185	0.029
74576	42808	15600188	6.58	0.917	K2V	0.81	0.117	0.284	0.140	0.018	0.212	0.072	N
84737	48113	14300497	5.08	0.619	G2V	5.37	0.255	0.234	0.261	0.004	0.247	0.014	N
88230	49908	14500801	6.60	1.326	K8V	7.59	0.285	0.418	0.407	0.007	0.412	0.005	E, N
90839	51459	13100204	4.82	0.541	F8V	1.18	0.276	0.279	0.392	0.094	0.335	0.067	N
95128	53721	18000207	5.03	0.624	G0V	6.31	0.270	0.222	0.236	0.023	0.229	0.016	N
97603	54872	16900610	2.56	0.128	A4V	0.68	0.868		0.894	0.021	0.894	0.021	N

   

Table 1: continued.

HD	HIP	ISO_id	V	B-V	Spect.	age	$F_{\nu}^{\rm pred}$	$F_{\nu}^{\rm IRAS}$	$F_{\nu}^{\rm ISO}$	$\Delta F_{\nu}^{\rm ISO}$	$F_{\nu}^{\rm ad}$	$\Delta F_{\nu}^{\rm ad}$	Excess
			mag	mag		Gyr	Jy	Jy	Jy	Jy	Jy	Jy	Flag
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)	(11)	(12)	(13)	(14)
101501	56997	17200316	5.31	0.723	G8Vvar	1.55	0.255		0.308	0.027	0.308	0.027	N
102365	57443	25400519	4.89	0.664	G3/G5V	7.24	0.333	0.351	0.374	0.016	0.362	0.011	N
102647	57632	18401422	2.14	0.090	A3Vvar	0.24	1.160	1.659	1.467	0.055	1.563	0.096	E
106591	59774	14301528	3.32	0.077	A3Vvar	0.48	0.378	0.386	0.422	0.016	0.404	0.018
110833	62145	60000525	7.01	0.936	K3V	12.60	0.082		0.090	0.022	0.090	0.022
112758	63366	40100128	7.54	0.769	K0V	5.89	0.036		0.075	0.036	0.075	0.036	N
114710	64394	21501031	4.23	0.572	G0V	3.63	0.507	0.461	0.436	0.041	0.449	0.029
114762	64426	39000531	7.30	0.525	F9V	11.22	0.027		0.030	0.021	0.030	0.021	N
115383	64792	24101534	5.19	0.585	G0Vs	3.80	0.215	0.191	0.208	0.021	0.199	0.015	N
117176	65721	39600734	4.97	0.714	G5V	7.59	0.342	0.384	0.315	0.006	0.349	0.034	N
120136	67275	39400137	4.50	0.508	F7V	1.38	0.346	0.346	0.322	0.007	0.334	0.012	N
126660	70497	19501343	4.04	0.497	F7V	2.95	0.516	0.517	0.558	0.039	0.537	0.027
128167	71284	26900346	4.47	0.364	F3Vwvar	1.70	0.261	0.295	0.314	0.043	0.305	0.030
134083	73996	08901049	4.93	0.429	F5V	1.82	0.197	0.176	0.217	0.009	0.197	0.020
139664	76829	09000155	4.64	0.413	F5IV-V	1.12	0.248	0.493	0.253	0.032	0.373	0.120
142373	77760	28100658	4.60	0.563	F9V	8.51	0.354	0.408	0.388	0.035	0.398	0.025
142860	78072	08901261	3.85	0.478	F6V	3.24	0.591	0.630	0.763	0.031	0.697	0.067
149661	81300	80700364	5.77	0.827	K2V	2.09	0.206	0.224	0.173	0.014	0.198	0.026
154088	83541	45801567	6.59	0.814	K1V	7.24	0.094		0.089	0.009	0.089	0.009
156026	84478	09401670	6.33	1.144	K5V	0.63	0.239		0.266	0.017	0.266	0.017
157214	84862	09101273	5.38	0.619	G0V	7.24	0.193	0.196	0.182	0.023	0.189	0.016
157881	85295	09202576	7.54	1.359	K7V	5.25	0.130	0.112	0.185	0.022	0.148	0.036
160691	86796	45800282	5.12	0.694	G5V	6.17	0.286	0.321	0.244	0.036	0.282	0.038
166620	88972	36901485	6.38	0.876	K2V	7.24	0.130	0.133	0.136	0.011	0.135	0.007
172167	91262	08900788	0.03	-0.00	A0Vvar	0.35	6.351	8.079	8.234	0.305	8.156	0.215	E
173667	92043	10600291	4.19	0.483	F6V	2.40	0.436	0.430	0.445	0.011	0.437	0.008
185144	96100	28801094	4.67	0.786	K0V	5.50	0.522	0.553	0.554	0.017	0.553	0.012
185395	96441	54801897	4.49	0.395	F4V	1.29	0.274	0.221	0.236	0.029	0.229	0.021
187642	97649	13001001	0.76	0.221	A7IV-V	1.23	5.723	5.757	5.166	0.179	5.462	0.296
191408	99461	34301404	5.32	0.868	K2V	7.24	0.339	0.450	0.474	0.027	0.462	0.019	E
192310	99825	18300407	5.73	0.878	K3V		0.237	0.289	0.311	0.025	0.300	0.018
197692	102485	13501110	4.13	0.426	F5V	2.00	0.408	0.367	0.387	0.034	0.377	0.024
203280	105199	08900313	2.45	0.257	A7IV-V	0.89	1.314	1.220	1.262	0.029	1.241	0.021
203608	105858	10902816	4.21	0.494	F6V	10.47	0.439	0.459	0.459	0.017	0.459	0.012
207129	107649	13500819	5.57	0.601	G2V	6.03	0.156	0.143	0.269	0.030	0.206	0.063
209100	108870	18300322	4.69	1.056	K5V	1.29	0.894	0.980	1.155	0.048	1.067	0.087
215789	112623	14401525	3.49	0.083	A3V	0.54	0.328	0.354	0.384	0.032	0.369	0.023
216956	113368	35300828	1.17	0.145	A3V	0.22	3.259	3.419	3.414	0.057	3.417	0.041	E
217014	113357	37401640	5.45	0.666	G5V	5.13	0.200	0.175	0.179	0.031	0.177	0.022
219134	114622	09102431	5.57	1.000	K3Vvar		0.353	0.387	0.349	0.028	0.368	0.020
222368	116771	37800834	4.13	0.507	F7V	3.80	0.485	0.526	0.524	0.039	0.525	0.028

  Notes:  HD106591 was observed twice, the ISO flux density is the weighted average; the second ISO_id is 33700128.

		      HD110833 is not in the list of Lachaume et al. (1999), the age is estimated by us according to Lachaume et al. (1999)

  

2.2 IRAS data

The IRAS data were obtained from the IRAS faint source catalog (IFSC, Moshir et al. 1989). When no IFSC data are available we have taken data from the IRAS point source catalog.

2.3 Predicted photospheric fluxes

In order to decide whether a star has any excess emission at 25 $\mu$m we have used the relation derived by Plets (1997):

 

V-[25] = -0.03 + 2.99 (B-V) - 1.06  (B-V)² + 0.47  (B-V)³, (1)

which is based on the analysis of photometric data obtained in the optical and with IRAS of a very large sample of stars. The relation is valid for B-V as high as 1.6 mag. However, investigation of the V-[25] versus B-Vdiagrams by Mathioudakis & Doyle (1993) showed that K dwarfs may have higher infrared fluxes than predicted by Eq. (1) in case B-V > 1.2 mag. We will take these findings into consideration when assessing stars with significant infrared excess.

Equation (1) is tied to the IRAS calibration. We adopted a flux density of 6.73 Jy for [25]=0 mag, this value is consistent with the IRAS photometric calibration (IRAS Explanatory Supplement).

For the analysis of likely excess stars we needed to estimate the photospheric flux at 12 $\mu$m. We used the relationship similar to Eq. (1) derived by Waters et al. (1987). The photospheric flux densities in the far-infrared ( $\lambda~\geq~60$ $\mu$m) were estimated by assuming that for a given star the magnitude longward of 60 $\mu$m is identical to the 60 $\mu$m magnitude as given in Paper I.

The values of V and B-V were taken from the Hipparcos Catalogue (Perryman et al. 1997). At the low flux end with $F_{\nu}(25~{\rm {\mu}m})<$ 300 mJy, an uncertainty of m_V= 0.01 corresponds to about 2 mJy and B-V= 0.01 corresponds to about 5 mJy. Consequently, the statistical uncertainty in the "predicted'' flux density according to Eq. (1) is estimated to be of the order of 2-3%.

2.4 ISO data processing

The ISOPHOT data at 25 $\mu$m (Lemke et al. 1996) were collected throughout the ISO mission (Leech & Pollock 2000) with observation template AOT PHT03 in triangular chopped mode (Klaas et al. 1994). The chopper throw was 60'' and the aperture used was 52''. The on-target exposure time was 128 s and an equal amount of time was spent on the two background positions.

The data were processed using ISOPHOT interactive analysis PIA Version 8.1 (Gabriel et al. 1997). All standard signal corrections were applied. A generic chopper pattern of two plateaux of 4 "source plus background'' and 4 "background'' signals were derived. For the signal difference we have taken the average of the last two signals of each plateau. The signals were converted to flux densities under the assumption that the responsivity of the detector has the same value at the beginning of each ISO revolution, and changes with orbital phase due to ionising radiation according to an empirical function tabulated in the "Cal G'' table PPRESP (Laureijs et al. 2001).

To check the ISO results we have correlated the ISO fluxes with the predicted fluxes and found a tight correlation. However, the correlation is not along the line of unit slope but along a power law where the high fluxes ( $F_{\nu}>~1$ Jy) are systematically underestimated and the lower fluxes ( $F_{\nu}<~300$ mJy) are systematically overestimated with respect to the model predictions, see Fig. 1.

Figure 1: Comparison between the photometric data of ISO and IRAS. Left panel: the correlation between ISO and IRAS. For the ISOPHOT data at 25 $\mu$m we assumed a default detector responsivity and standard calibrations. Right panel: the same correlation after correction of the ISO fluxes to obtain an overall match with the predicted fluxes. The solid lines represent the lines of unit slope.

Assuming that the majority of the stars in our sample have no significant excess emission at 25 $\mu$m we recalibrated the ISO fluxes to the predicted fluxes so that the correlation between subsamples of the two data sets scatters around the line of unit slope. The method is illustrated in Fig. 1 where the correlations are given between the ISO and IRAS fluxes before and after the correction. A detailed description of the recalibration is given elsewhere (Laureijs & Jourdain de Muizon 2000). It should be stressed that the recalibration systematically changes the fluxes in the ISO sample as a whole and does not affect the relative scatter amoung the individual observations. Also, the Plets (1997) predictions are based on the IRAS calibration, whereas the ISOPHOT calibration is based on a different photometric system. The recalibration ensures that the fluxes of the ISO observations are consistent with the IRAS calibration.