Clumpy dust rings around non-accreting young stars

Abstract

We investigate four young, but non-accreting, very low mass stars in Orion, which show irregular eclipses by circumstellar dust. The eclipses are not recurring periodically, are variable in depth, lack a flat bottom, and their duration is comparable to the typical timescale between eclipses. The dimming is associated with reddening consistent with dust extinction. Taken together this implies the presence of rings around these four stars, with radii ranging from 0.01 to 40 AU, comprised of optically thin dust clouds. The stars also show IR excess indicating the presence of evolved circumstellar disks, with orders of magnitude more material than needed for the eclipses. However, the rings need to cover an opening angle of about 20 degrees to explain how common these variable stars are in the coeval population in the same region, which is more extended than a typical disk. Thus, we propose that the rings may not be part of the disks, but instead separate structures with larger scale heights. To be sustained over years, the rings need to be replenished by dust from the disk or gravitationally bound to an object (e.g., planets or planetesimals). These four stars belong to a growing and diverse class of post-T Tauri stars with dips or eclipses in their lightcurves. Dusty rings with scale heights exceeding those of disks may be a common phenomenon at stellar ages between 5 and 10 Myr, in the transition from accretion disks to debris disks. These structures could be caused by migrating planets and may be signposts for the presence of young planetary systems.

Time-resolved photometry of the young dipper RX J1604.3-2130A: Unveiling the structure and mass transport through the innermost disk

“…Figure 9 shows the color variation as a function of the magnitude variation for several combinations of bands. Considering the standard interstellar extinction laws (R V =3.1-5.5, see for instance Schlegel et al 1998;Stoughton et al 2002;Cardelli et al 1989), the slopes of the color variations up to ∆g'∼1 mag are quite consistent with the standard extinction vectors. There are some places where the color variation becomes suddenly smaller, for instance around ∆g'∼1 mag and (especially in the g' vs r'−z' diagram), around ∆g'∼0.2 mag.…”

“…The typical depth of the extinction event in g' is 1.2 mag. Assuming standard interstellar extinction (Schlegel et al 1998), this is equivalent to A V =1 mag, or N H =1.8e21 cm −2 (Predehl & Schmitt 1995). For an average particle weight of 1.36 m H (for solar metallicity neutral gas, e.g.…”

Time-resolved photometry of the young dipper RX J1604.3-2130A

“…The observed small changes and displacements of the shadows are consistent with a "wavy" or warped disk (Pinilla et al 2018), which is also consistent with other observations (Grinin et al 2008;McGinnis et al 2015), models of self-shadowed UXor disks (Dullemond et al 2003), and the non-axisymmetric or clumpy structures observed in gas and dust in the inner disks of some young stars (e.g., Sicilia-Aguilar et al 2012;Siwak et al 2014;Scholz et al 2019). A warp at the point where the disk and the star are connected at the basis of the accretion column (see, e.g., the models of Alencar et al 2012Alencar et al , 2018McGinnis et al 2015;Bodman et al 2017) could also explain the observations, as shown in the cartoon in Fig.…”

The one that got away: a unique eclipse in the young brown dwarf Roque 12

“…There is a growing class of evolved stars showing eclipses or 'dips' in the lightcurve due to circumstellar material, although there is no IR excess indicating the presence of a debris or accretion disk. The most prominent one might be Boyajian's Star found in the Kepler data set (Boyajian et al 2016), but there are other examples at younger ages (Stauffer et al 2017), see also Scholz et al (2019) for a discussion. A brown dwarf in σ Orionis might also belong into that category (Elliott et al 2017).…”

“…The uniqueness of the eclipse could be explained by material spiralling or falling into the central object. As shown in Scholz et al (2019), only very little dust (<< M Earth ) is needed to cause substantial absorption along the line of sight. That means, this explanation does not necessarily conflict with the lack of infrared excess.…”