Volume 537, January 2012
|Number of page(s)||14|
|Section||Stellar structure and evolution|
|Published online||19 December 2011|
The VISIR N8.6 μm catalogue of the 47 Tuc central region.
In general, all objects reported in Table 3 were visible by eye in the VISIR images, and had an optical HST counterpart (with mF606W ≤ 13.0). One exception is target ID#98781, which showed an irregular shape in the VISIR N8.6 μm images and had an HST counterpart (within a 1-pixel matching radius) that is fainter than mF606W ~ 13.0. This target fell in field#3 at (RA, Dec)J2000 = (6.0323672, − 72.0677501) and had mF606W, mF814W, N8.6 μm, J-, and K-magnitudes of 16.100, 15.423, 8.022, 14.809 and 14.225, respectively. The irregular shape of the target hinted at a background galaxy, and this was confirmed after building the SED of that source (see Fig. A.1). The SED fitting was performed with the Multi-wavelength Analysis of Galaxy Physical Properties (MAGPHYS code) presented in Cunha et al. (2008). This indicates a z ~ 0.3 redshift galaxy with a total mass of ~2 × 1011 M⊙ and a quite significant star-formation rate of ~150 M⊙/yr.
Red symbols show the SED of the peculiar VISIR target (ID#98781) with faint HST counterpart. The black line reports the best fit obtained with the MAGPHYS code, corresponding to a z ~ 0.3 redshift galaxy.
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Another peculiar red target (ID#89862) is that seen in Fig. 9. Located at (RA, Dec)J2000 = (6.06149167, − 72.07903611), it has mF606W, mF814W, N8.6 μm, J-, and K-magnitudes of 11.120, 9.765, 7.052, 10.953 and 9.680, respectively. Indeed, it is V19 as reported in Table 3. In the optical mF814W, (mF606W − mF814W) colour − magnitude diagram, this star shows up as a bright AGB candidate, located to the left side of the RGB mean loci. On the other hand, and in the N8.6 μm vs. (mF606W − N8.6 μm) diagram (see Fig. 10), it is located very close to RGB tip. Basically, the near-infrared J- and K-magnitudes (as reported in the Salaris et al. catalogue) show a significant colour excess. We repeated the VISIR/HST/SOFI coordinate matching and visually inspected the location of this star in the images. This test confirmed our initial identification, and showed no particular indication of possible blending or mismatch caused by a faint unresolved companion. Moreover, the analysis on the optical V mean magnitude of this variable did not indicate significant variations.
In conclusion, the Salaris et al. (2007) photometry of V19 shows a rather fainter than expected J- and K-magnitudes. Indeed, the 2MASS J- and K-photometry of V19 (8.695 and 7.567, respectively) is ~2 mag brighter than that reported in Salaris et al. (2007). After excluding a possible coordinate mismatch, we caution that the Salaris et al. (2007) catalogue (see Sect. 2.4) suffered photometric saturation around K ≃ 8.0. This throws some doubt on the Salaris et al. (2007) photometry of V19 and, consequently, on its position in the Fig. 9. Lastly, this variable will be subject of a future investigation.
© ESO, 2012
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