DENIS Survey of AGB and Tip-RGB Stars in the LMC Bar West and Optical Center Fields
We have observed with the DENIS instrument two regions of 22’ × 23’, located in the Bar of the LMC, and overlapping the Bar West and Optical Center fields studied by Blanco et al. (1980, BMB) in a prism-objective survey. Observations were performed simultaneously in the I, J, and K S bands. There were 376 AGB stars identified in previous studies in these 2 fields. We typically find back 97% of them in the DENIS data. We show that the BMB’s M and C stars follow two different sequences in the (I–J,J–K S ) colour-colour diagram. For M stars, the [I–J] colour is an indicator of the M subtype, increasing from 0.9 to 1.8 as one goes from MO to M7 spectral types, while the [J–K S ] colour remains almost constant (from 1.0 to 1.35). Conversely, C stars are located on a “reddening” branch, with [J–K S ] ranging from 1.0 to about 2.2. M and C stars discovered by BMB have no or faint mass-loss. The (I–J,J–K S ) diagram allows to define a simple criterion to select stars of spectral types M and C. With this criterion, we find a total of 1177 new AGB and tip-RGB stars candidates, and have calculated the bolometric luminosities of all of them as well as previously known stars, assuming a distance modulus of 18.55. 90% of the new candidates are blue sources, not searched for in a complete way by BMB. Their luminosity distribution shows a very clear peak at M bol ≃ -3.5, that we interpret as the tip of the Red Giant Branch. A second group is formed by stars for which the BMB survey should have been complete (i.e. relatively red C stars and M stars with spectral type later than M5.5). We find, however, 23% and 38% more such stars than they do in the Bar West and Optical Center, respectively.
Finally, we also discover in this study 42 red sources, with 2.2 < [J-K S ] < 4.2. These stars experience mass-loss at significant rates. Note that there might be redder obscured AGB stars in the fields, but that the DENIS limiting magnitudes do not allow to detect sources with [J-K S ] larger than about 4. Only 4 of them have an IRAS counterpart. It is interesting to note that we discover 42 mass-lossing AGB stars in 0.14 square degrees, while only about 50 have been discovered in the whole LMC using the IRAS data. The luminosity distribution of these red sources is very similar to the one of the BMB’s C stars, ranging from M bol ≃ -3.5 to -6.
KeywordsSpectral Type Magellanic Cloud Distance Modulus Large Magellanic Cloud IRAS Source
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- Fouqué P. et al., 1997, in preparationGoogle Scholar