Volume 552, April 2013
|Number of page(s)||11|
|Section||Interstellar and circumstellar matter|
|Published online||11 April 2013|
CS Cha was first studied by Gauvin & Strom (1992), who found evidence that it harbors a disk with inner holes. It is known to be a spectroscopic binary system, as confirmed by Guenther et al. (2007) (period ≥7 years, minimum mass of the companion ~0.1 M⊙), although previously Takami et al. (2003) suggested this option based on the large gap found in its disk. This previously unknown feature is probably the reason for the spectral type inconsistency found in the literature (Henize & Mendoza 1973; Appenzeller 1977; Rydgren 1980; Appenzeller et al. 1983; Luhman 2004). In this study we used the K6 spectral type found by Luhman (2007). The binary nature makes the disk around CS Cha into a circumbinary disk. The disk has been modeled intensively, initially excluding the effect of the binary system (Espaillat et al. 2007, 2011) and evidence of an inner hole of ~40 AU was found. Espaillat et al. (2011) also pointed out the need for a different mass distribution in CS Cha compared to that of disks around single stars. A more recent analysis by Nagel et al. (2012) also accounted for the binary effect. To reproduce the variations found at the IR wavelengths, the model includes the emission from the inner disk structure generated by the double system, with a ring and streams of material falling from the ring to the circumstellar disks around the individual stars. Another 2MASS source is found at 5′′ away. It is 6 magnitudes weaker than CS Cha in the 2MASS J band and undetected in the rest of the 2MASS bands. Contamination from this object is therefore very unlikely.
CS Cha is located in front of a bright background. Therefore, the SPIRE fluxes are very likely underestimated because the background emission was probably overestimated during the photometry extraction.
This source was cataloged as a K0 star by Rydgren (1980) and was first identified as a disk with a possible inner gap by Gauvin & Strom (1992). Luhman (2007) reviewed its properties, and it was lately confirmed by Kim et al. (2009) as a transitional disk. It has sometimes been referred to as a pretransitional disk, given the small excess found at 3−10 μm. The first modeling results by Kim et al. (2009) suggested an inner disk radius of ~30 AU. Espaillat et al. (2011) modeled this object in detail, noting flux variations from IRS spectra at different epochs on periods shorter than three years. These variations are attributed to changes in the height of the optically thick disk wall (from 0.006 to 0.08 AU), and they do not modify the 10 μm silicate emission feature. SZ is known to be a wide binary (Vogt et al. 2012). A companion is found at ~5′′(projected distance of 845 AU), which could be causing truncation of the outer disk. The contribution of this source to the total measured fluxes in this study is likely to be negligible, since it is 4 magnitudes weaker than SZ Cha in the 2MASS J band. However, the possibility of an increase in its FIR measurements cannot be excluded.
T25 was identified as an M3 star by Luhman et al. (2008) and was found to be a transitional disk by Kim et al. (2009). The lack of IR excess at wavelengths <8 μm indicates that the inner regions of the disk are well depleted of small dust particles. The modeling by Kim et al. (2009) yields an inner radius of 8 AU for the disk. It is the only detected transitional object, together with T35, lacking the silicates feature at 10 μm, another indication of an efficient depletion of small particles in the inner disk region. T25 has no known stellar companions (Nguyen et al. 2012).
Gauvin & Strom (1992) classified this source as an M0 star, and it was later identified as a possible a pretransitional disk by Kim et al. (2009) because it displays weak excess at short IR wavelengths. In this case, the inner disk radius is located at 15 AU (Espaillat et al. 2011). As in T25, there is no sign of silicate emission. The excess at 70 μm is lower than in other cases, but does not resemble the typical Class II SED. It has no confirmed known stellar or substellar companions (Nguyen et al. 2012). However, recent sparse aperture masking observations of this source by Cieza et al. (2013) showed and asymmetry in its K-band flux. On the basis of modeling, these authors found the inner disk radius to be ~8.3 AU. They were also unable to distinguish between the close-companion scenario and the asymmetry being produced by the starlight scattered off the disk itself.
This source was found to be an M0.5 start in Gauvin & Strom (1992). Kim et al. (2009) identified it as a transitional disk with a inner disk radius of 18 AU. As in the other transitional disks in this study, its excess is higher than the expected Class II flux at the PACS bands. It has no known bound companions (Nguyen et al. 2012).
ISO-ChaI 52 is an M4 star (Luhman 2004). Espaillat et al. (2011) proposed it as a transitional disk, finding the source to be an extreme case among their sample: based on variations of its Spitzer spectrum, models require the inner wall height to increase by about 400% (from 0.0006 to 0.003 AU). We also found it to be an outlier in the sense that it has the flattest SED between 12 and 70 μm. No bound companions are known for ISO-ChaI 52 (Nguyen et al. 2012).
CR Cha is an M0.5 star (Gauvin & Strom 1992). Furlan et al. (2009) found it to be an outlier in their sample when comparing the equivalent width of the silicates emission and the spectral slope between 13 and 31 μm: it was beyond the parameter space considered in their study. The explanation given in Furlan et al. (2009) is that this source could be a pretransitional disk. For this reason, Espaillat et al. (2011) included it in their sample of transitional disks. In this study, we found this object to be compatible with a Class II object. It is also located among other Class II objects using the proposed classification method (Fig.2). Therefore, although we cannot completely rule out the possibility that this object is in a pretransitional disk phase given its strong silicates emission, it would be in a very early stage of the transitional phase.
This source was first classified as a K5 object by Gauvin & Strom (1992). It was included in the analysis of Espaillat et al. (2011) for the same reason as CR Cha, and modeled as a pretransitional disk. Comparison with the median SED of the Class II sources shows that WW Cha is well above the median. The SED of WW Cha resembles a typical Class II object, probably still embedded in the core, as suggested by its high extinction (Av ~ 4.8 mag) and its position in the Herschel maps. The dusty environment in which it is located could significantly pollute the photometry and, hence, our conclusions about this object.
T54 is known to be a misclassified transitional disk (Matrà et al. 2012), and therefore we excluded it from our analysis. The Herschel images show contamination from close-by extended emission, which affected our photometry and, hence, our conclusions. The non-transitional nature of this object is also supported by the fact that it would be the only transitional disk in our sample with no excess emission at 70 μm with respect to the median SED Class II disks.
© ESO, 2013
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