Precise determination of circumstellar disk lifetimes: Disk evolution in a single star-forming region

Abstract

Determining how long circumstellar disks last is key to understanding the timescale of planet formation. Typically, this is done by measuring the fraction of young stars with infrared-excess, a sign of circumstellar material, in stellar clusters of different ages. However, comparing data from different star-forming regions at different distances introduces uncertainties and biases because of the different sample completeness and environment. This study addresses these challenges by analyzing 33 clusters, aged 3 to 21 million years (PARSEC isochrones), within the Scorpius-Centaurus OB association, sampling the stellar IMF from the hydrogen burning limit to about 8 M$_\odot$. By using $\mathit{Gaia}$, 2MASS, and WISE data, we identified stars with infrared-excess through color-color diagrams and spectral energy distributions, ensuring a consistent selection of disk-bearing sources. Our results indicate a disk lifetime of $5.8 \pm 0.3$ Myr, about a factor of two longer than most previous estimates, suggesting that planet formation may have more time than previously thought. We also find that an exponential decay model best describes disk dispersal. These findings emphasize the importance of studying disk evolution in a single star-forming region to reduce uncertainties and refine our understanding of planet formation timelines.

Figures (12)

• Figure 1: Plot of galactic longitude versus galactic latitude showing the region of Sco-Cen. The sources are shown based on their selection, blue circles for disk-bearing and orange crosses for disk-less sources.
• Figure 2: Disk fraction versus age. The black data points represent the stellar clusters, on the left for the CCD selection, in the middle for the SED selection and on the right for the Luhman selection. The Luhman selection only includes 25.0 out of 33.0 stellar clusters because the study by Luhman_2022 encompasses a smaller region compared to Ratzenboeck_2023a. Shown in green-blue in the right plot are the missing stellar clusters (from the CCD selection). In the left plot, the red line represents the median of the fiducial+CCD result, which is also shown in the middle and right plot with reduced transparency. The orange dash-dotted line in the middle plot gives the median of the fiducial+SED result. The blue dotted line is the median of the fiducial+Luhman result. The gray area shows the $1$-$\sigma$ confidence interval for each median result. The uncertainties in ages are from Ratzenboeck_2023b and the disk fraction uncertainties are derived from the underlying counting process which can be modeled as a simple Bernoulli trial.
• Figure 3: Violin plot showing the posterior probability density functions (PDFs) of the disk decay time parameter $\tau$. The x-axis denotes the decay time in Myr, while the y-axis labels each model and disk selection method. The number $N$ displayed next to each distribution indicates the number of sources used in the corresponding fit; this number remains constant for the top three rows, where only the model configuration varies. Posterior distributions are grouped and color-coded: red indicates variations in prior assumptions (with the fiducial model highlighted at the top), green represents the supplementary disk selection methods (SED, Luhman), and blue corresponds to subsamples based on stellar multiplicity (single vs. candidate binaries). See Table \ref{['tab:parameter']} for the corresponding numerical results.
• Figure 4: Selection of disk candidates obtained from a combination of four different infrared CCDs. The upper left panel shows the $W123$ CCD selection ($W1 - W2$ versus $W2 - W3$), the upper right panel the $W124$ CCD selection ($W1 - W2$ versus $W2 - W4$), the lower left panel the $HKW2$ CCD selection ($H - K_S$ versus $K_S - W2$), and the lower right the $HKW3$ CCD selection ($H - K_S$ versus $K_S - W3$). In each plot, the blue circles show sources selected as disk-bearing and the orange crosses are disk-less. The borders are the dashed, black lines in each diagram. In each plot, the extinction vector is shown, scaled to a value of $A_{K_S}$ given in the plot above the extinction vector.
• Figure 5: Histogram of the spectral index $\alpha$ for all sources obtained through SED fitting. The orange bars indicate disk-less sources, the blue one disk-bearing. The selection border at -2.0 is shown as black, dashed line.