The dance of dust: Investigating young stellar object dipper variability

TL;DR

This study presents the first multi-epoch spectroscopic analysis of 16 irregular YSO dippers in Upper Scorpius using VLT/X-Shooter, aiming to connect dip events to inner-disc dust dynamics. By deriving intrinsic photospheres through Class III templates anchored by Gaia multi-epoch photometry, the authors quantify dip properties across 400–900 nm, finding that many dips are produced by dust with grain sizes well above ISM values and, in several cases, substantial scattering. The results reveal strong variability in dust properties from dip to dip and source to source, implying rapid inner-disc dust growth and transport processes—likely driven by disc winds, unstable accretion columns, or disc warps. Near-infrared behavior shows limited excess in most epochs, with occasional intrinsic K-band variability and scenarios where the same dust obscures both the photosphere and hot inner-disc emission. Overall, the work highlights the extreme dynamism of the inner disc in irregular dippers and proposes observational avenues to further constrain dip-driving mechanisms and dust substructure dynamics.

Abstract

The dipper subclass of YSOs are characterised by frequent dips in their light curves. Irregular dippers do not show periodic signatures and have dips accounting for significant proportions of their photospheric flux. Given the short timescales on which these dips occur, their driving mechanisms are linked to the inner circumstellar disc dynamics. We present the first multi-epoch analysis of 16 irregular dippers observed with X-Shooter. Investigating the properties of their dips, and in particular the analysis of the dust characteristics, we aim to understand the root of their variability, and get a glimpse of the inner disc behaviour. We employed a novel approach to measure the properties of the dips, by combining class III templates with Gaia photometry to construct the intrinsic photospheres. We measured several dip properties including the depth of the dips, near-infrared (NIR) excesses, and their optical depths as a function of wavelength. We record 20 significant dips that range in their dip properties and show no relation to one another. In almost all cases, the low optical depths and small NIR excesses are observed. Comparison of their optical depths with grain opacity models show that the dips can be explained by the presence of dust substructures containing processed grains obscuring their photospheres and/or their discs. These grain distributions can have maximum sizes as large as 20$μm$ and in many cases have almost grey-like extinction, while some require a strong scattering component. The findings highlight the extent of the irregularity of dippers, but also link it to the dust dynamics in the inner regions of circumstellar discs. The dust substructures causing the variability require processed dust grains to be lifted above the disc into the line of sight. Possible lifting mechanisms including disc winds, unstable accretion columns, and disc warps are discussed.

Figures (12)

• Figure 1: Ratios of the observed to photospheric flux at 550 nm ($R_{550}$) for each star (see colour legend) are shown as a function of the spectral type. When not visible, the errors are smaller than the size of the dots. Dotted and dashed grey lines represent R$_{550}$ thresholds of 1.0, 0.85 respectively. No relation between spectral type and R$_{550}$ is observed.
• Figure 2: $\beta$ (eq. \ref{['eq: beta']}) values as a function of $\tau_{550}$ for objects with R$_{550}$<0.85. Multiple dips of the same object are colour coded according their shortened 2MASS ID (see \ref{['tab: master_table']}). Dashed blue line represents the value expected for ISM reddening under typical conditions, assuming the Cardelli extinction law Cardelli89 with $R_V$=3.1.
• Figure 3: $\beta$ colour maps from dust opacity models. Each figure represents a grid of dust size distributions varying minimum and maximum grain sizes ($n(a)\propto a^{-q}$ with q=3.0). Coloured by the values of $\beta$ derived from the opacities for a) pure absorption ($\beta_{abs}$), b)scattering ($\beta_{sca}$, c) extinction ($\beta_{ext}$), and d) effective extinction ($\beta_{eff}$; see text). Contours are marked by black lines at $\beta$ = 0.0 and $\beta_{ISM} =1.38$). Pattern effects are artefacts of the interpolation used.
• Figure 4: Optical depth as function of wavelength for three representative dips. The observed values are shown by grey dots with error bars. The red dashed line shows the extinction opacity (absorption + scattering) for the grain size distribution that best fit the data in the wavelength interval 400-900 nm (limits shown by the vertical grey dotted lines), normalised to the observed value at 700 nm. The values of $a_{min}, a_{max}, q$ are given on top of each panel; the corresponding grain size distributions ($n(a)$) are displayed in the inset plots.
• Figure 5: Excess emission at 2.1 $\mu m$, $\epsilon_K$ shown as a function of R$_{550}$. The top panel plots the ratio between the observed flux and the intrinsic photospheric one ($\epsilon_K=\frac{F_{obs,K}}{F_{ph,K}}-1$). The bottom panel shows the ratio between the observed flux corrected for the extinction derived at optical wavelengths and the photospheric flux ($\epsilon_K=\frac{F_{obs,K}e^{\tau}}{F_{ph,K}}-1$). The grey area covers a $\pm$20% region around $\epsilon_K$ = 0. Vertical error bars on $\epsilon_K$ are smaller than the size of the points.