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

4. Results

A summary of the best fitting parameters for each source is given in Table 5. The quality of the fit was estimated from the average of the quadratic sum of the residuals, most of them weighted with the inverse of the estimated error in magnitude. We gave only half of this weight to the residuals in R and I, because the combination of the cool temperatures of our objects and the strong extinction cause large flux gradients in this region, and the effective extinction over the filter width thus becomes a complicated function of the spectral energy distribution of the source and the transmission curve of the filter. A smaller weight was also given to the measurements longwards from SW1, due to the fact that our modeling of the intrinsic infrared excess probably gives only a rough approximation to the true spectral energy distribution in the disk emission-dominated region

Table 5. Best fitting parameters to the available photometry

In Table 5, we give for each object the visual extinction (assuming ; Rieke & Lebofsky 1985), spectral index n, temperature T, and mass M. Also indicated is whether an "old" or a "young" isochrone provide the best fit, where the separation between these cases is set at years. This age is about half of the estimated age of the Ophiuchi complex given by Wilking et al. 1989, which is roughly consistent with that estimated by Comerón et al. 1996 in their reanalysis of the CRBR data. Although the isochrones considered were spaced by intervals of years, the quality of the fits was insensitive to changes between consecutive isochrones; therefore, we feel that it is not meaningful to give more precise values of the best fitting age. The mass given by the fit, on the contrary, does have some sensitivity to the chosen isochrone, especially at the earliest evolutionary stages, and choice of one or another particular isochrone within the same age group may occasionally move a star from the brown dwarf to the stellar domain or vice versa. The masses listed in Table 5 correspond to ages of years for the young group, and years in the old group. In some cases for which the quality of the fit is similar for ages in the "young" or the "old" range, it has been possible to make a choice based on spectra obtained by Williams et al. 1995 and, more recently, by Meyer et al. 1998. Such cases are noted in the discussion of individual objects below.

The best fitting spectral energy distributions given by the models for the objects listed in Table 5 are illustrated in Fig. 1. Also plotted are the contributions of the stellar photosphere without circumstellar excess in the cases when such excess is required to obtain a good fit.

Fig. 1. Best fits to the available photometry. The solid line is the fitted spectral energy distribution of a star surrounded by circumstellar material, obscured by foreground extinction. The dotted line is the intrinsic spectral energy distribution of the star without the circumstellar excess, obscured by the same amount of foreground extinction. The frequency is given in Hz, and in erg s-1 cm-2

The required circumstellar excess is fairly insensitive to the chosen isochrone over the whole range of ages considered here. However, even moderate deviations (by a few tenths) in the slope of the excess, n, from the values quoted in Table 5 produce clear departures from the observed fluxes at the longest wavelengths. These deviations cannot be removed by varying the level of foreground extinction or the temperature of the central object. The constraints on n are particularly strong for the objects detected at LW1 or LW4, which stresses the importance of the good spectral coverage provided by ISO in improving the object/excess/extinction decomposition. To illustrate this point, Fig. 2 shows the best fits to 2320.8-1721 at wavelengths longer than 1.25 µm obtained by imposing three different values of the spectral index: (the best fitting one; solid line), (dotted line), and (dashed line). In each case the extinction and photospheric temperature have been optimized. It is clear that only produces an acceptable fit to the shortest and the longest wavelengths simultaneously, even if the fluxes longward from LW1 suggest that more excess may be required to fit this region. However, a better model fit at long wavelengths would produce discrepances exceeding a factor of 2 in the photosphere-dominated near infrared, where the JHK fluxes are generally measured with a greater accuracy and the spectral energy distribution modeling is much more reliable. Our fits give more weight to the short wavelength measurements, as explained above; departures at the longer wavelengths are most probably due to our simplified way of modeling the excess of circumstellar origin, coupled with the smaller photometric accuracy.

Fig. 2. Comparison between fits to the observed spectral energy distribution of source 2320.8-1721. The solid line corresponds to the best fitting value , the dashed line to , and the dotted line to . To expand the vertical scale so that the differences can be clearly seen, the two data points in R and I have been left out of the plot.

4.1. Discussion of individual objects

2317.3-1925
The fit implies a mass for an age of years and close to the stellar/substellar borderline for any age within the range expected for Ophiuchus sources. The inferred temperature, around 2850 K, varies little with the chosen isochrone. The spectrum of this object presented by Williams et al. 1995 led them to place the temperature slightly below 3000K, in agreement with the uncertainties of our fit. More recent K-band spectra lead Meyer et al. 1998 to assign a M7.5 spectral type to this object. We can consider therefore that all these results are in good agreement.

The ISOCAM measurements are poorly fitted ¹, but the detection at LW1 and LW4 suggest a cold infrared excess. In fact, the large LW4 flux suggests a spectral index shallower (i.e., with a smaller absolute value) than -2.2 which, if extrapolated to shorter wavelengths, would imply a smaller photospheric contribution at K and an accordingly lower mass and luminosity.

The JHK photometry of CRBR and that obtained by Strom et al. 1995 agree to better than 0.05 mag in each filter, indicating the source is not strongly variable.

2317.5-1729
The fit is nearly insensitive to the age over the considered range, and indicates a mass in the interval 0.20 - 0.24 , with almost no circumstellar excess. This mass is in agreement with the rather early spectral type M5 found by Meyer et al.1998. The new infrared observations from La Silla in April 1997 suggest that this object may be now somewhat bluer than at the time of the observations reported by CRBR: the K magnitude is approximately the same, but the source is brighter by about 0.4 and 0.3 mag in J and H, respectively. Other objects in the same frame (2320.8-1708 and 2320.8-1721) do not show such a difference. This source was also detected at I with the NTT. The I magnitude is in good agreement with the extrapolation of the CRBR + ISO fluxes from longer wavelengths, but falls somewhat low if the most recent JHK values are used. A trend for I to fall below the best fitting spectral energy distribution is also observed in the other two sources observed in this band.

2320.0-1915 = GY5
This object appears to be a low mass star or massive brown dwarf with a moderate excess and light extinction. Fits to the photometry do not allow a clear choice between a young or an old age. A temperature of K provides a good fit if the object has an age of years. The 2 µm spectrum indicates a temperature slightly above 3000K (Williams et al. 1995; Meyer et al. 1998 classify it as M7), still consistent with our fit but perhaps indicating that a somewhat older isochrone could be more appropriate; in that case, the mass would be around 0.10 . An alternative explanation may be the veiling of spectral features by circumstellar emission, as suggested by the infrared excess derived from ISOCAM observations. The photometry is in agreement with that of Strom et al. 1995, but our J is brighter than that of Greene & Young 1992 by 0.48 magnitudes, out of the quoted uncertainties. However, our K magnitude agrees with that of Greene & Young.

2320.8-1708 = GY10
Best fits are obtained with masses between 0.1 and 0.2 , depending on the age; a wide range of ages provides similarly acceptable fits. The residuals are slightly better assuming an old age, but the spectral features (Williams et al. 1995) tend to favour a temperature, and hence an age and mass, on the lower side. This is confirmed by the very late spectral type, M8.5, estimated by Meyer et al. 1998. Agreement between these works and our fits is found if the age is around years, as in that case our best fit is obtained for a temperature of 2900 K. Overall, the photometry produces a good fit with nearly no circumstellar excess. This object was detected at 1.3 mm by André & Montmerle 1994, with a flux of 75 mJy which may imply the existence of a reservoir of cold dust in its vicinity. Possible variability is suggested by the differences of 0.2 magnitudes in all bands between our JHK photometry and that obtained by Strom et al. 1995. The values listed in Table 3 for this object are in better agreement with those of Greene & Young 1992, but the photometric accuracy of that work is lower. Our new observations carried out in April 1997 yield magnitudes equal, within errors, to those of CRBR.

2320.8-1721 = GY11
The likely substellar nature of this object, already pointed out by Rieke & Rieke 1990, is supported by the new ISOCAM observations. A detailed discussion and spectroscopic observations of 2320.8-1721 in the 2 µm band can be found in Williams et al. 1995. Our fitted temperature is in good agreement with theirs. This object has been also observed by Meyer et al. 1998, who classify it as M6.5. Although Williams et al. 1995 suggested possible variability, based on the differences in equivalent width of the overlying extended H₂ emission, the JHK photometry of CRBR agrees with that of Greene & Young 1992, Strom et al. 1995, and with the new photometry obtained by us in April 1997.

Our best fit is obtained with a moderate age ( years) and a spectral index , yielding . The slow early evolution of objects with such a mass makes this fit nearly mass-independent within the range of ages considered here. A fit implying a mass greater than 0.08 would be possible only by assuming an age greater than years, which seems to be ruled out by the estimated age of the Ophiuchi complex (de Geus 1992). For this source, such an age is particularly unlikely because of the well-determined infrared excess (see Fig. 1), which would be expected to be uncommon in objects older than years. Moreover, a fit with an age in excess of years would require a temperature above 3000 K, in disagreement with the feature strengths in the 2 µm spectrum.

Despite its youth and the evidence for circumstellar matter around it, the possibility of further accretion raising the mass of this object much above its present value seems very unlikely in view of its non-detection at 1.3 mm by André & Montmerle 1994.

2321.6-1918
The photometry of this object listed in Table 3 can be fitted fairly well by a massive brown dwarf of 0.07 with a moderate circumstellar excess. Slightly better fits are obtained for an old age, but the improvement in the quality of the fits is only marginal. The best fitting mass remains substellar for ages below years, although due to its proximity to the stellar/substellar boundary the brown dwarf character remains doubtful.

An intriguing aspect of this object is the large discrepancy in the measured values of H between CRBR and Strom et al. 1995, amounting to 1.15 magnitudes; however, the difference in K is only 0.27 magnitudes, in agreement within the uncertainties. Follow-up observations should be carried out to confirm such apparent extreme variations in color.

2322.6-1233
The non-detections of this object at SW1, LW4, and, especially, at LW1 rule out a significant circumstellar excess. The mild excess adopted here, , maximizes the contribution to the near infrared fluxes by the central source, therefore giving an upper limit to the mass of 0.05 - 0.06 for ages between and years. The object is thus another possible brown dwarf.

2331.1-1952 = GY64
The ground-based and ISOCAM photometry for this object is very similar to that of 2320.8-1721, with 2331.1-1952 being only a little brighter at short wavelengths (except perhaps for J, where the CRBR measurement is rather uncertain) and fainter at longer wavelengths. For this reason, a similar fit with a somewhat decreased circumstellar excess produces good results for 2331.1-1952 too, implying that it is also a good brown dwarf candidate. The best fitting mass, 0.05 , goes over the substellar limit only if the age is of order years or more.

This object is promising for further spectroscopic followup. It is only moderately obscured, like 2320.8-1721, making it a promising target for follow-up spectroscopic observations in the visible. At longer wavelengths, on the other hand, the small intrinsic infrared excess should decrease the importance of veiling which otherwise complicates the interpretation of spectra in the 2 µm region (Luhman & Rieke 1998). Its spectrum in this region has been recently obtained by Meyer et al. 1998, and the spectral type M8.5 derived by these authors strongly support the brown dwarf character.

2349.8-2601 = GY141
Rieke & Rieke 1990 and CRBR classified this object as a likely foreground M dwarf, based on its blue colors. Its detection at 4.5 µm by ISOCAM is therefore surprising. However, Luhman, Liebert, & Rieke (1997) have found it to have strong H emission and photospheric absorptions that include both giant and dwarf characteristics, indicative of a lower surface gravity than that of an evolved foreground dwarf. The object therefore appears to be a member of the Ophiuchi star forming cluster, but one which has escaped in our direction from the molecular cloud.

A considerable infrared excess, , is required to produce an overall fit to the available photometry, although with rather large residuals over the JHK bands. The non-detection at LW4 is still consistent with such an excess. The temperature derived from the fit, 2450K, compares well with that measured spectroscopically by Luhman et al. (1997), 2700 150K. The fitted mass is for an age below years, again in reasonably good agreement with the estimate of 0.045 0.015 derived by Luhman et al. (1997) by placing the object on the HR diagram and using the Burrows tracks as we have.

2351.8-2553 = GY146
No good quality fits to the observed fluxes of this object are possible using tracks for low mass stars. The very red is suggestive of a background source or an intermediate-mass star located at the opposite edge of the cloud. CRBR tentatively classified it as a possible Class I source, but weak fluxes at the ISOCAM wavelengths argue against that interpretation.

2404.5-2152 = GY202
A fit can be obtained with any isochrone in the age range of Ophiuchi objects and with a moderate excess. However, the non-detection in SW1 (consistent with the large uncertainty in the L' measurement) is rather puzzling, and might be indicative of strong water absorption. Otherwise, the characteristics of this source are similar to those of 2331.1-1952. In addition, 2404.5-2152 was observed by Williams et al. 1995, who favoured a temperature slightly below 3000K from the strength of the features identified in the 2 µm spectrum, and by Meyer et al. 1998, who confirmed such a low temperature, classifying its spectrum as M7. Our best fitting temperature of 2750 K is in good agreement with the spectroscopic observations, and discards possible fits with masses above 0.08 , which would require ages above years and temperatures over 3000 K. All this makes 2404.5-2152 an excellent candidate to be a brown dwarf, although it would be a relatively massive one.

2408.6-2229 = GY218
This faint object has not been detected in , SW1, or LW4, suggesting its spectral index is not far from . The fit, to JHK photometry alone, gives a mass of 0.03 , independent of the age over the range from 2.5 to years. Even smaller masses are derived for ages under years, with similar residuals to the fit. The low fluxes and blue color suggest that this object may be the least massive of our sample, with the possible exception of 2349.8-2601.

4.2. Comparison with other work

The well-constrained fits made possible by combining groundbased and ISOCAM photometry make it of interest to compare the temperatures derived by isochrone fitting with spectral determinations. Details are given in the discussion of individual sources. Of seven objects measured with both techniques, extending from 0.2 down to 0.03 , excellent agreement is achieved in all cases. In addition, although all differences are within the expected errors, there is also no discernible trend - that is, the residuals show no bias toward high or low temperatures from the fitting technique compared with spectroscopy. A noteworthy comparison is 2349.8-2601, whose optical spectrum is analyzed in detail by Luhman et al. (1997). Both the temperature and the substellar mass they derive agree with our values from isochrone fitting.

Given the good agreement between spectroscopy and the broadband fits, we can test the quality of the fits based on groundbased infrared photometry alone. Fig. 3 compares the masses derived for the objects studied here with those of CRBR. A good overall agreement exists, despite the more limited wavelength coverage of CRBR and their use of older models of stellar interiors and atmospheres. The only object for which the mass estimate is significantly changed is 2317.5-1729, for which CRBR assumed a strong circumstellar excess unconfirmed by the ISOCAM observations in LW1 and LW4 (see discussion above). The present results thus support the usefulness of fits to ground-based photometry, or even JHK only, to study the mass function of embedded populations. At the same time, confirmation of the substellar nature of objects with masses suspected to be near the brown dwarf limit but with strong excesses appears to require observations over a wide range of wavelengths (or spectroscopy).

Fig. 3. Comparison between the masses listed in Table 5 and those derived for the same objects by Comerón et al. 1993. Units are in solar masses. Two objects have been excluded from this sample: 2349.8-2601, considered as a probable foreground M dwarf by CRBR, and 2351.8-2553, which CRBR considered a possible Class I object and may be a background source.

In the alternate method of Strom et al. (1995), the JHK color-color diagram is used to deredden the sources and estimate their J luminosities, which are finally compared with the theoretical isochrones to obtain masses. The results have been compared with those of CRBR by Williams et al. (1995). The agreement between the two approaches is very good. Thus, the validity of their method is also supported by the ISOCAM observations and analysis.

© European Southern Observatory (ESO) 1998

Online publication: June 18, 1998
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