4 Discussion and conclusion

4.1 Nature of the candidates

   

Table 1: Photometric data of the selected candidates ($I-J \ge 2$).

IDa	$\alpha$(J2000)	$\delta$(J2000)	Ib	Jb	$K_{\rm s}^b$	AV	Ref.c
C1$\;\:$	12 43 53.1	-77 29 44.6	$16.97\pm0.12$	$14.67\pm0.12$	$13.41\pm0.19$	0.7
C2$\;\:$	12 46 08.7	-77 01 02.7	$17.77\pm0.44$	$14.75\pm0.13$	$13.43\pm0.17$	1.1
C3$\;\:$	12 47 11.7	-76 39 29.8	$16.49\pm0.16$	$14.34\pm0.11$	$13.31\pm0.17$	0.7
C4$\;\:$	12 48 28.5	-76 47 11.6	$17.31\pm0.20$	$14.53\pm0.11$	$12.74\pm0.12$	1.5	1
C5$\;\:$	12 50 22.2	-76 56 37.9	$16.51\pm0.08$	$11.49\pm0.06$	$8.51\pm0.06$	6.0
C6$\;\:$	12 50 39.7	-76 10 39.1	$17.32\pm0.12$	$14.70\pm0.12$	$12.98\pm0.14$	1.0	1
C7$\;\:$	12 51 10.5	-77 30 54.0	$17.26\pm0.03$	$15.14\pm0.04$	$13.42\pm0.08$	0.2
C8$\;\:$	12 51 28.3	-76 54 00.3	$17.67\pm0.15$	$14.56\pm0.12$	$12.44\pm0.12$	5.6	5
C9$\;\:$	12 51 29.0	-76 54 55.5	$14.86\pm0.04$	$11.29\pm0.06$	$9.01\pm0.06$	5.8
C10$\;\:$	12 51 30.5	-76 54 37.7	$14.42\pm0.04$	$10.54\pm0.05$	$8.13\pm0.06$	5.5
C11$\;\:$	12 52 11.0	-76 52 53.3	$15.73\pm0.06$	$10.95\pm0.05$	$8.04\pm0.07$	3.9
C12$\;\:$	12 52 15.5	-76 52 25.6	$17.51\pm0.14$	$14.64\pm0.12$	$13.05\pm0.16$	3.5
C13$\;\:$	12 52 30.6	-77 15 13.1	$12.21\pm0.02$	$9.10\pm0.05$	$7.25\pm0.08$	4.1	1,2
C14$\;\:$	12 53 23.4	-76 59 11.3	$17.81\pm0.17$	$14.90\pm0.14$	$13.19\pm0.18$	2.1
C15$\;\:$	12 53 29.7	-76 58 34.9	$16.88\pm0.10$	$14.41\pm0.12$	$12.96\pm0.16$	1.9
C16$\;\:$	12 53 29.8	-77 10 56.8	$16.74\pm0.10$	$13.43\pm0.09$	$11.41\pm0.10$	6.6	5
C17$\;\:$	12 53 38.9	-77 15 53.2	$15.25\pm0.05$	$11.63\pm0.07$	$9.41\pm0.08$	5.1	1
C18$\;\:$	12 54 00.2	-76 24 25.1	$11.52\pm0.02$	$8.12\pm0.05$	$6.55\pm0.09$	1.0
C19$\;\:$	12 54 05.2	-76 52 51.3	$16.13\pm0.07$	$13.50\pm0.09$	$11.88\pm0.11$	2.6
C20$\;\:$	12 54 27.1	-76 53 14.3	$16.86\pm0.11$	$14.26\pm0.11$	$12.65\pm0.14$	2.7
C21$\;\:$	12 54 36.2	-76 44 45.2	$17.99\pm0.18$	$14.90\pm0.14$	$13.28\pm0.19$	2.8
C22$\;\:$	12 54 37.3	-76 47 50.6	$16.37\pm0.09$	$13.87\pm0.10$	$12.40\pm0.11$	2.5	1
C23$\;\:$	12 54 42.6	-76 41 25.8	$17.06\pm0.12$	$13.73\pm0.09$	$11.79\pm0.10$	4.4
C24$\;\:$	12 54 45.2	-76 42 19.5	$14.11\pm0.04$	$10.99\pm0.05$	$8.93\pm0.07$	4.2
C25$\;\:$	12 54 47.4	-76 52 32.5	$16.89\pm0.11$	$14.22\pm0.11$	$12.54\pm0.12$	3.0
C26$\;\:$	12 54 50.8	-76 46 32.4	$16.80\pm0.10$	$14.23\pm0.11$	$12.74\pm0.14$	2.6	5
C27$\;\:$	12 55 03.3	-76 43 07.8	$15.21\pm0.05$	$12.26\pm0.07$	$10.33\pm0.07$	3.8
C28$\;\:$	12 55 08.7	-76 45 10.4	$16.04\pm0.07$	$13.44\pm0.08$	$11.74\pm0.09$	2.6
C29$\;\:$	12 55 14.4	-76 22 00.5	$12.41\pm0.03$	$10.10\pm0.05$	$8.64\pm0.06$	0.9
C30$\;\:$	12 55 15.7	-76 56 33.1	$15.45\pm0.06$	$12.27\pm0.07$	$10.14\pm0.07$	2.7	1
C31$\;\:$	12 55 20.7	-77 00 35.9	$15.18\pm0.05$	$12.11\pm0.07$	$10.13\pm0.07$	2.7
C32$\;\:$	12 55 24.0	-76 38 17.6	$17.44\pm0.15$	$13.88\pm0.10$	$11.63\pm0.09$	4.2
C33$\;\:$	12 55 25.7	-77 00 46.5	$15.31\pm0.05$	$12.58\pm0.07$	$10.84\pm0.08$	2.4
C34$\;\:$	12 55 25.7	-76 38 10.9	$17.47\pm0.14$	$14.24\pm0.11$	$12.23\pm0.11$	4.2
C35$\;\:$	12 57 25.9	-77 23 35.2	$17.04\pm0.13$	$14.18\pm0.10$	$12.33\pm0.11$	3.8
C36$\;\:$	12 57 29.3	-76 12 06.5	$15.00\pm0.04$	$12.49\pm0.07$	$10.92\pm0.08$	2.1
C37$\;\:$	12 57 31.5	-76 43 04.4	$15.58\pm0.06$	$12.55\pm0.07$	$10.47\pm0.07$	2.9	1
C38$\;\:$	12 57 41.6	-76 12 51.2	$15.96\pm0.07$	$13.07\pm0.08$	$11.13\pm0.08$	2.3
C39$\;\:$	12 57 54.5	-77 30 43.5	$15.75\pm0.06$	$12.73\pm0.07$	$10.73\pm0.07$	4.6
C40$\;\:$	12 58 31.4	-77 13 16.4	$17.73\pm0.17$	$14.00\pm0.10$	$11.49\pm0.08$	8.8
C41$\;\:$	12 59 09.8	-76 51 03.5	$16.82\pm0.10$	$14.04\pm0.10$	$11.19\pm0.09$	0.1	1
C42$\;\:$	12 59 10.7	-77 39 19.1	$12.91\pm0.02$	$9.86\pm0.05$	$8.08\pm0.06$	2.0
C43$\;\:$	12 59 30.3	-77 21 04.6	$17.27\pm0.12$	$13.97\pm0.09$	$11.66\pm0.09$	5.2
C44*	12 59 42.8	-77 12 12.8	$15.68\pm0.06$	$11.27\pm0.05$	$8.29\pm0.05$	11.7
C45$\;\:$	12 59 51.8	-77 27 18.9	$16.20\pm0.08$	$12.86\pm0.07$	$10.56\pm0.07$	4.5
C46*	12 59 52.1	-77 20 15.0	$14.30\pm0.03$	$10.97\pm0.06$	$8.59\pm0.06$	6.2
C47*	13 00 24.1	-77 10 22.4	$17.18\pm0.12$	$12.71\pm0.07$	$9.51\pm0.06$	9.7
C48*	13 00 59.3	-77 14 02.7	$16.40\pm0.08$	$11.77\pm0.09$	$8.05\pm0.08$	7.9
C49*	13 01 21.4	-77 13 25.2	$17.62\pm0.15$	$13.52\pm0.11$	$10.90\pm0.10$	5.6
C50$\;\:$	13 02 22.9	-77 34 49.5	$15.42\pm0.05$	$12.77\pm0.10$	$11.26\pm0.11$	2.0
C51$\;\:$	13 03 09.1	-77 55 59.5	$14.46\pm0.04$	$11.83\pm0.07$	$10.38\pm0.08$	2.3
C52$\;\:$	13 05 05.1	-77 40 31.3	$17.34\pm0.14$	$13.98\pm0.10$	$11.71\pm0.10$	2.8
C53$\;\:$	13 05 08.6	-77 33 42.5	$14.68\pm0.04$	$12.02\pm0.06$	$9.87\pm0.07$	2.0	3
C54$\;\:$	13 05 30.9	-77 35 11.9	$17.10\pm0.11$	$14.75\pm0.12$	$13.03\pm0.17$	1.8
C55$\;\:$	13 05 32.7	-77 35 26.2	$14.60\pm0.04$	$11.89\pm0.06$	$10.03\pm0.06$	1.8
C56$\;\:$	13 05 57.1	-77 43 00.5	$13.74\pm0.03$	$10.05\pm0.04$	$7.61\pm0.05$	6.0
C57$\;\:$	13 06 04.8	-77 46 28.3	$16.75\pm0.09$	$13.54\pm0.08$	$11.38\pm0.08$	6.0
C58$\;\:$	13 06 11.1	-77 56 26.4	$17.64\pm0.15$	$14.92\pm0.13$	$13.04\pm0.17$	3.5
C59$\;\:$	13 06 30.8	-77 00 24.0	$15.96\pm0.07$	$12.59\pm0.07$	$10.52\pm0.07$	3.5

 

Table 1: continued.

IDa	$\alpha$(J2000)	$\delta$(J2000)	Ib	Jb	$K_{\rm s}^b$	AV	Ref.c
C60$\;\:$	13 06 56.5	-77 23 09.5	$13.59\pm0.03$	$10.89\pm0.05$	$9.78\pm0.06$	2.4	4
C61$\;\:$	13 06 57.4	-77 23 41.5	$13.56\pm0.03$	$11.01\pm0.05$	$8.90\pm0.06$	2.6	1,4
C62$\;\:$	13 07 18.1	-77 40 53.0	$16.07\pm0.07$	$13.17\pm0.07$	$11.52\pm0.08$	4.1
C63$\;\:$	13 07 21.4	-77 42 55.6	$17.81\pm0.20$	$14.69\pm0.12$	$12.64\pm0.12$	5.6	5
C64$\;\:$	13 07 41.2	-78 05 43.3	$13.17\pm0.03$	$10.38\pm0.05$	$8.63\pm0.06$	2.0
C65$\;\:$	13 08 22.8	-78 07 04.6	$14.95\pm0.04$	$12.21\pm0.06$	$10.66\pm0.07$	1.7
C66$\;\:$	13 08 27.2	-77 43 23.3	$16.53\pm0.09$	$13.57\pm0.08$	$12.26\pm0.10$	3.7
C67$\;\:$	13 08 46.8	-78 06 44.5	$16.50\pm0.09$	$13.84\pm0.09$	$12.40\pm0.12$	1.0	1
C68$\;\:$	13 09 02.4	-78 08 38.6	$15.95\pm0.07$	$13.23\pm0.07$	$11.64\pm0.08$	0.9
C69$\;\:$	13 09 14.6	-78 08 06.3	$17.62\pm0.18$	$14.75\pm0.12$	$12.81\pm0.15$	0.8
C70$\;\:$	13 09 21.6	-78 07 47.8	$17.81\pm0.20$	$15.04\pm0.14$	$13.29\pm0.19$	0.7
X1$\;\:$	12 59 10.1	-77 12 13.7	$14.29\pm0.03$	$11.23\pm0.06$	$9.19\pm0.06$	11.5
X2$\;\:$	13 02 42.3	-77 11 11.1	$15.68\pm0.03$	$14.75\pm0.09$	$13.50\pm0.13$	1.4
X3$\;\:$	13 02 49.2	-77 05 44.6	$15.31\pm0.05$	$14.28\pm0.13$	$13.33\pm0.19$	0.2
X4$\;\:$	13 03 09.0	-77 00 32.8	$13.18\pm0.03$	$12.53\pm0.08$	$11.96\pm0.11$	0.7
X5$\;\:$	13 03 23.9	-77 16 37.1	$15.45\pm0.05$	$14.64\pm0.12$	$13.46\pm0.22$	0.9
X6$\;\:$	13 03 30.5	-76 44 51.4	$15.57\pm0.05$	$15.00\pm0.14$	$13.50\pm0.22$	0.0

^a C = Colour selected candidate; X = X-ray selected candidate; * = object detected by ISO (P. Persi, private communication).
^b Observed magnitudes. The intrinsic photometric uncertainty is given for each star.
^c REFERENCES: (1): IRAS; (2): Larson et al. (1998); (3): Hartigan (1993); (4): Schwartz (1977); (5): Vilas-Boas et al. (1994).

From the comparison of these objects with known TTS and young brown dwarfs (Fig. 3), we expect that most of our candidates are low-mass young stars such as young brown dwarfs and/or low-mass TTS. However, the true nature of the candidates can be unambiguously confirmed only after a positive spectroscopic test such as the detection of the H$\alpha$ emission line and/or the Li I absorption line, signatures of the youth of the stars. We will therefore observe these candidates spectroscopically. H$\alpha$ emission is not unique to young low-mass objects. The detection of Li  I from the photosphere can confirm the youth of the object (Martín et al. 1999). The detection of features such as CaH, TiI, NaI will allow the evaluation of the temperature and surface gravity (Kirkpatrick et al. 1991).

4.2 Evolutionary status

We use the evolutionary tracks for low-mass stars modeled by Baraffe et al. (1998) to estimate the mass and age of our low-mass star candidates. Figure 3 shows the J versus I-J color-magnitude diagram of our 70 candidates selected by $I-J \ge 2$ of which 4 are previously known TTS and six candidates cross-identified with ROSAT X-ray sources. We also plot the previously known TTS of the Cha II cloud identified with DENIS sources and the brown dwarfs of the Cha I detected and spectroscopically confirmed by Comerón et al. (2000). We used our extinction map (Fig. 1) to correct the reddening of all the objects shown in Fig. 3. We include the isochrones at 1 Myr, 10 Myr and 100 Myr from the (Baraffe et al. 1998) model. The selection criterion of sources with $I-J \ge 2$ does not allow us to distinguish low-mass candidates and stars located just behind the cloud. Their location in the color-magnitude diagram shows that they are separated by their luminosity. The location of low-mass candidates (except for the 15 bright candidates) in Fig. 3 suggests that they, indeed, have masses < $0.2~M_{\odot}$, and ages between 1 and 10 Myr. Although we selected these sources only with the $I-J \ge 2$ criterion, they are remarkably close to the evolutionary tracks of the model. This confirms that they probably are young brown dwarfs. Fifteen $I-J \ge 2$ candidates are bright sources ( $K_{\rm s} \sim$ 8). Their infrared color excess $J-K_{\rm s}$ is $\sim$1.5-2 and $I-J \sim 2{-}4$. They do not appear to be field giants (expected to be uniformly distributed), but are located near the cloud cores in the extinction map. The number of giant field stars can be estimated using the so-called Besançon model (Robin & Crézé 1986). For the total Cha II area, the number of expected giant stars is about 400. A consequence of underestimating the extinction near the densest cores would be to select some giant stars near the cores. Only a spectroscopic follow-up can identify the nature of these objects.

4.3 Conclusion

We present the first extinction map of the Cha II cloud using the DENIS I and J band. We use this map to deredden all stars detected in 3 bands $IJK_{\rm s}$, which allows us to select low-mass young stars embedded in the cloud cores.

The location of our dereddened candidates selected with $I-J \ge 2$ in the color-magnitude diagram suggests that 51 of our candidates are probably low-mass TTS and/or brown dwarfs. However, a spectroscopic follow-up is necessary to confirm the true nature of these objects.

This selection is independent of the characteristics of the dark cloud and can thus be applied to all other star formation regions observed by DENIS in the southern hemisphere.

Acknowledgements

We are grateful to Dr. T. Montmerle for his helpful comments and to the anonymous referee for suggestions that improved this paper markedly. We warmly thank the members of the DENIS consortium whose work made these results possible. The DENIS project is partly funded by the European Commission through SCIENCE and Human Capital and Mobility grants. It is also supported in France by INSU, the Education Ministry and CNRS, in Germany by the Land of Baden-Würtenberg, in Spain by DGICYT, in Italy by CNR, in Austria by the Fonds zur Förderung der Wissenschaftlichen Forschung and Bundesministerium für Wissenschaft und Forschung.