Skip to main page content
Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2017 Jan 25;4(1):160652.
doi: 10.1098/rsos.160652. eCollection 2017 Jan.

The Transiting Dust Clumps in the Evolved Disc of the Sun-like UXor RZ Psc

Affiliations
Free PMC article

The Transiting Dust Clumps in the Evolved Disc of the Sun-like UXor RZ Psc

Grant M Kennedy et al. R Soc Open Sci. .
Free PMC article

Abstract

RZ Psc is a young Sun-like star, long associated with the UXor class of variable stars, which is partially or wholly dimmed by dust clumps several times each year. The system has a bright and variable infrared excess, which has been interpreted as evidence that the dimming events are the passage of asteroidal fragments in front of the host star. Here, we present a decade of optical photometry of RZ Psc and take a critical look at the asteroid belt interpretation. We show that the distribution of light curve gradients is non-uniform for deep events, which we interpret as possible evidence for an asteroidal fragment-like clump structure. However, the clumps are very likely seen above a high optical depth midplane, so the disc's bulk clumpiness is not revealed. While circumstantial evidence suggests an asteroid belt is more plausible than a gas-rich transition disc, the evolutionary status remains uncertain. We suggest that the rarity of Sun-like stars showing disc-related variability may arise because (i) any accretion streams are transparent and/or (ii) turbulence above the inner rim is normally shadowed by a flared outer disc.

Keywords: circumstellar matter; debris discs; protoplanetary discs; variable stars.

Figures

Figure 1.
Figure 1.
WASP and KELT-North data. Photometry is shown in dimensionless form, relative to a quiescent level of 1, and was converted from observed magnitudes as described in the text.
Figure 2.
Figure 2.
WASP and KELT-North data, focusing on dimming events. The vertical and horizontal scales in each subpanel are the same.
Figure 3.
Figure 3.
Flux density distribution of RZ Psc, including 2MASS, WISE, AKARI, VISIR and IRAS data and their (approximate) year of observation. The dark blue line shows a stellar photosphere model at the approximate stellar temperature of 5350 K, and the light blue line a 500 K blackbody. The latter is not a fit, but an approximate continuum level that illustrates that the WISE 3.4 and 22 μm photometry cannot both be accounted for with a single blackbody, if the silicate feature seen with VISIR was present in 2010.
Figure 4.
Figure 4.
WISE epoch photometry at 3.4, 4.6, 12 and 22 μm (W1, W2, W3 and W4, in magnitudes). Panel (a) shows the time variation in W1 over 5.5 years. Panels (bd) show how W1 correlates with W2, W3 and W4, which do not have observations at all W1 epochs. The dashed lines show the slope expected for constant disc flux variation with wavelength (the variation is smaller in W1/2, because the total flux is not dominated by the disc).
Figure 5.
Figure 5.
Discrete autocorrelation function for yearly WASP and KELT-North data, computed for lags between 10 and 155 days. The second and third lines from the top show all data, and 2006-excluded data. The topmost line shows the number of years that show a peak more than 3σ above the clipped DACF mean within each 5 day bin. The peak at 70 days is 7 years, the dashed line is zero and the y-axis scale is shown to the right.
Figure 6.
Figure 6.
Power at a given period from the iterative event finding. The solid line shows the power from the data, and is the same in each panel. The dashed lines show the mean and ±1σ power from simulated dimming events, which from top to bottom are: random, periodic between 64 and 75 days with three repeats, and periodic at 64 days with 10 repeats.
Figure 7.
Figure 7.
Gradient and minimum flux measured from individual nights’ observations. Open circles have gradients significantly different from zero, and a symbol size proportional to the inverse of the gradient uncertainty. Dots are consistent with zero slope. The lines show the gradients implied by the velocities for circular orbits at 0.3, 1, 10 and 100 AU.
Figure 8.
Figure 8.
Semi-major axes estimated from the light curve gradients in figure 7. Open circles have gradients significantly different from zero, and a symbol size proportional to the inverse of the gradient uncertainty. Dots are consistent with zero slope. The stellar radius (grey dashed line) has been estimated as solar. Because gradients may have lower minimum fluxes, they can move down along lines parallel to the solid lines. Points near or above the blue line are consistent with zero light curve gradient.
Figure 9.
Figure 9.
Clump properties assuming circular orbits for a range of dimming event durations (as labelled). The brown region marks the approximate location of the RZ Psc asteroid belt (or the inner edge of a more extended disc). The blue-shaded region shows the range of Herbig Ae inner disc edge radii. The dashed line shows where a clump has an azimuthal extent similar to the scale height of a typical gas-rich disc.
Figure 10.
Figure 10.
The spectral energy distribution of RZ Psc in comparison with other young disc-hosting stars. All SEDs are normalized to a common flux density at H band. The triangles are AKARI and IRAS upper limits for RZ Psc. Measurements were made at different times, so the apparent discrepancy between the WISE detection at 22 μm and the AKARI upper limit at 18μm is an indicator of IR variability (see §3.2).
Figure 11.
Figure 11.
Cartoon shows possible origins of dippers and UXors, and why Sun-like stars may only rarely show analogous behaviour. In each case the star, magnetic dipole and rotation axis are shown at the left (the stellar magnetic field is not necessarily always tilted with respect to the disc). Possible disc structures viewed edge-on to the right. (a) Low-mass stars (dippers) are occulted by co-rotating material that is accreting onto the star, and the dust sublimation radius is interior to co-rotation [34]. (b) Sun-like stars are rarely seen as dippers or UXors because (i) dust sublimates outside co-rotation (represented by the grey accretion column in the upper half [34]) or (ii) material lifted by turbulence is shadowed by the outer disc (spiral in the lower half). (c) Herbig Ae/Be stars (UXors) are occulted by turbulence that appears above self-shadowed discs [33].

Similar articles

See all similar articles

References

    1. Haisch KE Jr, Lada EA, Lada CJ. 2001. Disk frequencies and lifetimes in young clusters. Astrophys. J. Lett. 553, L153–L156. (doi:10.1086/320685) - DOI
    1. Backman DE, Paresce F. 1993. Main-sequence stars with circumstellar solid material: the VEGA phenomenon. In Protostars and planets III (eds EH Levy, JI Lunine), pp. 1253–1304. Arizona University Press. Available at http://adsabs.harvard.edu/abs/1993prpl.conf.1253B.
    1. Wyatt MC, Dent WRF. 2002. Collisional processes in extrasolar planetesimal discs: dust clumps in Fomalhaut’s debris disc. Mon. Not. R. Astron. Soc 334, 589–607. (doi:10.1046/j.1365-8711.2002.05533.x) - DOI
    1. Krivov AV. 2010. Debris disks: seeing dust, thinking of planetesimals and planets. Res. Astron. Astrophys. 10, 383–414. (doi:10.1088/1674-4527/10/5/001) - DOI
    1. Fabrycky D, Tremaine S. 2007. Shrinking binary and planetary orbits by kozai cycles with tidal friction. Astrophys. J. 669, 1298–1315. (doi:10.1086/521702) - DOI

LinkOut - more resources

Feedback