Volume 565, May 2014
|Number of page(s)||6|
|Published online||30 April 2014|
Characterizing the inner circumstellar (CS) disk(s) is more challenging, because of the limited resolution of the interferometer. Not all system characteristics can be directly fitted. Using our spectral-type estimates for Aa and Ab1+2 and the resolved photometric values from the literature for Aa and Ab, we derived the relative contributions of the photospheres and of the CS material (IR excess), and use these inputs to model the VLTI data.
We first assumed that all the emission in the I band is purely photospheric in origin and we adopted an optical extinction ratio RV = 5 (extinction laws from Mathis 1990), and similar extinction values AV = 0.3 for Ab1 and 0.45 for Ab2 from Hartigan & Kenyon (2003). In the H band, we derive a fractional excess emission Fd/Ftot = 0.32 ± 0.16 for Aa, and Fd/Ftot = 0.61 ± 0.12 for Ab1+Ab2 (insensitive to the spectral type adopted for Ab2, see Table 2).
We then fit the PIONIER data with analytical models that consist of one (or two) star(s) plus a geometrically thin, circular ring to mimic the bright inner rim of the CS disk(s). For GG Tau Aa, a fully resolved component may contribute up to 7% of the emission (V2< 1 at short baselines, see Fig. 2). Such extended components (or “halos”) are common around young stars, and can reach ~20% of the total emission (e.g., Akeson et al. 2005). It has been proposed that scattered light (at the disk surface or by a residual envelope) might explain this visibility drop (e.g., Monnier et al. 2006; Pinte et al. 2008). Fixing the total excess flux ratio to 32% (model-2 in Table 3), we infer a ring radius in the range 0.05–0.1 au, a value consistent with the expected grain sublimation distance for a disk around a 0.38 L⊙ star (Pinte et al. 2008). Our observation of Aa is thus consistent with an unresolved photosphere accounting for 68% of the H-band emission, surrounded by a canonical circumprimary disk whose inner rim bright edge remains marginally resolved by the VLTI. This disk is also known to be large and massive enough to produce detectable mm emission (Piétu et al. 2011).
In the 32 mas binary Ab, the presence of at least one CS disk is indirectly attested by the detection of the 10 μm silicate feature (Skemer et al. 2011) and classical accretion tracers
(White et al. 1999). PIONIER visibilities do not show a clear drop at long baselines (Fig. 2), although the data are more noisy than for the brighter Aa. This indicates that any disk-like emission remains mostly unresolved. A fit of PIONIER data (CP + V2) with two point sources and a halo-like component (see Table 3, model-4) yields Fhalo ~ 20%, FAb1 ~ 70% (star + unresolved dust emission), FAb2 ~ 10%, and FAb1/FAb2 = 4.3 ± 2.0. The discrepancy with the observed NaCo contrast (1.6 ± 0.4) is hard to explain. We cannot exclude that it has an instrumental origin: the modest Strehl ratio ~ 30% of the VLTI adaptive optics and tip-tilt correction residuals make the FOV correction delicate. However, if this discrepancy is real, it might be linked to the spatial extent and location of the halo emission in Ab. We propose that the halo might be located around Ab2, with a spatial extent in the range 10–30 mas radius (i.e., 1.5–4.5 au). It would thus be fully resolved on VLTI baselines, but would remain unresolved for NaCo, and would thus only contribute to the NaCo-SAM closure phase. This halo emission could partly originate from the complex geometry of streamers in the gravitationally unstable zone around the close binary. This is suggested by the detection of an extended warm H2 emission around Ab, which most likely traces accretion shocks of inflowing material towards the CS disk(s) (Beck et al. 2012). If the halo and Ab2 emissions were co-located, the VLTI contrast (after correction for FOV-attenuation) would thus become FAb1/ (FAb2 + Fhalo) = 1.4 ± 0.6 (model-4), in better agreement with the NaCo-SAM value. Finally, from the photometry constraints (literature data, see Table 2), we estimate that the H-band dust excess amounts to 60 ± 10% of the total emission in Ab. Because Ab2 (star+disk) can only account for ~ 10% of the total flux, most of the remaining 40% excess flux should arise from the Ab1 CS environment (disk?). Theoretical simulations of binary system formation (e.g., Bate & Bonnell 1997) suggest that the primary star usually accretes more material than the secondary, and in some cases the circumsecondary disk may not even be present. Although the limited spatial information and data quality do not allow us to fully constrain the CS environments in the Ab close binary, the current data set seems consistent with this scenario.
© ESO, 2014
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