The LONG-LIVED DISKS IN THE Η CHAMAELEONTIS CLUSTER

Abstract

We present IRS spectra and revised MIPS photometry for the 18 members of the η Chamaeleontis cluster. Aged 8 Myr, the η Cha cluster is one of the few nearby regions within the 5-10 Myr age range, during which the disk fraction decreases dramatically and giant planet formation must come to an end. For the 15 low-mass members, we measure a disk fraction ∼50%, high for their 8 Myr age, and 4 of the 8 disks lack near-IR excesses, consistent with the empirical definition of "transition" disks. Most of the disks are comparable to geometrically flat disks. The comparison with regions of different ages suggests that at least some of the "transition" disks may represent the normal type of disk around low-mass stars. Therefore, their flattened structure and inner holes may be related to other factors (initial masses of the disk and the star, environment, binarity), rather than to pure time evolution. We analyze the silicate dust in the disk atmosphere, finding moderate crystalline fractions (∼10-30%) and typical grain sizes ∼1-3 µm, without any characteristic trend in the composition. These results are common to other regions of different ages, suggesting that the initial grain processing occurs very early in the disk lifetime (<1 Myr). Large grain sizes in the disk atmosphere cannot be used as a proxy for age, but are likely related to higher disk turbulence. The dust mineralogy varies between the 8-12 µm and the 20-30 µm features, suggesting high temperature dust processing and little radial mixing. Finally, the analysis of IR and optical data on the B9 star η Cha reveals that it is probably surrounded by a young debris disk with a large inner hole, instead of being a classical Be star.

Young stars inϵChamaleontis and their disks: disk evolution in sparse associations

“…Spitzer observations of clusters with ages of several Myr, such as IC 348, NGC 2362, η Cha, and the Coronet cluster, suggest that, qualitatively, two evolutionary paths exist for going from a primordial to a debris disk configuration (Lada et al 2006;Sicilia-Aguilar et al 2008;Currie & Kenyon 2009;Sicilia-Aguilar et al 2009;Currie & Sicilia-Aguilar 2011). Each path shows a characteristic behavior of the SED:…”

“…In the η Cha association, 18 members have been observed with Spitzer. The disk frequency is estimated to be ∼44 +12 −11 % (8/18) (Megeath et al 2005;Sicilia-Aguilar et al 2009). The sources RECX-3 and RECX-4 only show excess emission at wavelengths longer than 10 μm.…”

“…Nearby pre-main sequence (PMS) associations at ∼50-300 pc are well suited for detailed investigations of young stars and their disks, since their members can be observed with high signal-to-noise ratio (S/N) throughout the electromagnetic spectrum. Several PMS associations have been studied with the Spitzer Space Telescope, e.g., MBM 12, Cha, and η Cha (Meeus et al 2009;Megeath et al 2005;Bouwman et al 2006;Sicilia-Aguilar et al 2009) using the infrared array camera (IRAC, Fazio et al 2004), the multiband imaging photometer for Spitzer (MIPS, Rieke et al 2004), and the infrared spectrograph (IRS, Houck et al 2004). The Spitzer data allow characterization of the disks around low-mass Based on observations performed at ESO's La Silla-Paranal observatory under programme 076.C-0470.…”

“…Analysis of the spectral features in the Spitzer/IRS spectra provides constraints on the chemical composition and grain sizes (see, e.g., Henning 2010, for a recent review). The dust properties derived from Spitzer/IRS spectra of the members of the MBM 12 and η Cha associations suggest that dust processing in the disks occurs very early and that radial mixing is not efficient (Meeus et al 2009;Sicilia-Aguilar et al 2009). Because the η Cha association is much older than MBM 12, it is highly desirable to study an association at an intermediate age in order to draw a more complete picture of disk evolution in sparse stellar associations.…”

Circumstellar disks and planets

“…Some disks display inner depleted regions (the so-called transitional disks, see e.g. Brown et al, 2009;Sicilia-Aguilar et al, 2009;Muzerolle et al, 2010). Some others have or are believed to feature intermediate large walls (as opposed to the inner rims at the dust sublimation radius, typically a fraction of an AU) at the limit between an inner and an outer massive, optically thick disk (Bouwman et al, 2003;Thalmann et al, 2010;Verhoeff et al, 2011).…”