Astron. Astrophys. 335, 522-532 (1998)

2. Observations

2.1. Source selection

Our sample was drawn from the analysis of Ophiuchi presented by Comerón et al. 1993 (hereafter CRBR), who used an early version of the method described in Sect. 3 to identify possible brown dwarfs. Refinements of the analysis method and the incorporation of new theoretical isochrones (Burrows et al. 1993; D'Antona & Mazzitelli 1994) essentially confirmed our previous results, which were also independently supported by K-band spectroscopy (Williams et al. 1995). We selected as ISO targets brown dwarf candidates with weak or no infrared excess in the model fit. Fits to the ISOCAM fluxes of objects with strong infrared excess would be dominated by the emission of a circumstellar disk or envelope, and a comparison to theoretical isochrones (which ultimately would yield the mass of the central object) would therefore be very indirect. An exception was made for source 2320.8-1721, since its 2 µm spectrum supports its substellar nature. The list of our primary ISO targets is given in Table 1. The denomination of sources is the same as in CRBR; namely, the first set of digits correspond to the offset in minutes and seconds of right ascension from 16h, and the second set, to the offset in minutes and seconds of declination from (1950.0 coordinates)

Table 1. Candidate brown dwarfs from JHK photometry

The centers of the ISOCAM frames were chosen to include other nearby objects, in most cases low mass stars. This enabled us to apply our method on a well sampled spectral energy distribution for objects over a fairly large range of masses. The results help us assess the validity of the method for the study of the statistical properties of larger samples of objects, often with only JHK photometry available (Comerón et al. 1996)

2.2. ISOCAM observations

Measurements were made in four ISOCAM bands. SW1 (m, ) overlaps with the groundbased window and provides a direct comparison with the groundbased photometry. LW1 (m, ) and LW4 (m, ) should provide accurate photometry beyond the wavelengths accessible from the ground. LW2 (m, ) was also used despite the reduced accuracy due to its wide bandpass, because of the potentially higher sensitivity. However, since this filter includes several interstellar emission features (Sellgren et al. 1985, Boulanger et al. 1988), the signal to noise in this band was not significantly better than with LW1 or LW4, and we will not include the LW2 results in our discussion.

The ISOCAM observations were obtained between March 1996 and August 1997. The log of observations is given in Table 2. Technical details on the instrumental setup and the exposure times, as well as the data reduction procedures, are given in Appendix A. Aperture photometry was performed on the flux-calibrated images and transformed to magnitudes. Zero-magnitude fluxes, , at the central wavelengths of the ISOCAM filters were derived by linearly interpolating the function , determining and from the zero-magnitude fluxes between neighboring positions tabulated by Engelke 1992. The derived values are close to the exponent corresponding to Rayleigh-Jeans behavior. The flux-to-magnitude transformations are thus:

Table 2. Log of ISOCAM observations

The magnitude measurements of the observed sources, combining ground-based and ISO results, are presented in Table 3. Some of the sources of field Oph2 listed in Table 2 have been excluded: 2318.9-1740, 2321.1-1754, and 2322.6-1802 are too bright and extended, and 2321.1-1715 and 2321.2-1719 cannot be distinguished from 2320.8-1721 at the resolution available. The ISOCAM magnitudes quoted for the latter source may therefore contain some contribution from the two fainter companions although, judging from their fainter magnitudes at bands short of L', we consider it unlikely that this contribution is significant. Magnitude uncertainties for ground-based observations are from CRBR. For ISO observations, the uncertainties were estimated by comparing measurements with different apertures and background reference regions. Lower limits for the ISO magnitudes correspond to over the local average background noise level.

Table 3. Ground-based plus ISOCAM infrared photometry of the observed sources

2.3. New ground-based observations

The approximate area of field Oph2 was imaged in April 1997 in JHK using the IRAC2 infrared camera at the ESO-MPI 2.2 m telescope, as well as in the R and I bands using the ESO New Technology Telescope (NTT). A mosaic showing images of this field in R, I, J, K, LW1, and LW4 has been presented elsewhere (Comerón et al. 1997). The R and I photometry is listed in Table 4.

Table 4. R and I photometry of sources in the field Oph2

© European Southern Observatory (ESO) 1998

Online publication: June 18, 1998
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