Entry 17

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Authors

  • Gijs van Malsen

This figure represents a sample of Young Stellar Objects (YSO) in the Chameleon I star forming region. The sample is adapted from Szücs et al. (2010). The idea of that paper and the current work is to investigate the vertical structure of protoplanetary circumstellar disks around low mass YSOs. These are stars with spectral type K and later and brown dwarfs, which generally start at spectral type M6. The sample is divided into two groups: One containing low mass stars (from spectral type K4 to M4.624) and one containing Very Low Mass Objects (VLMOs, from M4.625 to M9), including brown dwarfs.The hypothesis is that these disks should be more vertically extended around low mass objects, due to first theoretical principles (i.e. gravitational interaction with the star). However, the finding from observations is that these disks are in fact emph{flatter} around low mass YSOs. This is clear from the figure when one compares the colored versus the grey areas in the upper part of the figure. It is suspected that this difference in scale height is due to dust settling. This means that the dust in the disk is decoupled from the gas and settles to a smaller vertical scale height. Consequently, what we measure in infrared photometric observations is not the full (gas) disk, but the dust component. This figure aims to exemplify this phenomenon.

The lower part shows the median values for the flaring index \(\beta\) and reference scale height \(H_0\). These are related to the height of the disk as a function of the disk radius \(H(r)\) as:

\[H(r) = H_0 \frac{r^{\beta - 1}}{r_{0}^{\beta}}\]

Here \(r_0\) is the reference radius where \(H_0\) is defined at 100 au. The values from these plots are used to visualize the upper part as well as take a closer look at the distribution of values in the sample in order to evaluate how well the median values represent the entire sample. The results are acquired by performing radiative transfer simulations with the Monte Carlo 3D-radiative transfer code MCFOST (Pinte et al. (2006)).

All values shown in this plot are still tentative as the research is ongoing. The research is part of a project aimed at attaining a Master’s degree in astrophysics from the Radboud University Nijmegen, the Netherlands and is being conducted in collaboration with researchers from the University of Amsterdam and the Universidad de Chile.

Figure Caption

Top: A cross-section of the “median disks” as constructed with the results of MCFOST radiative transfer modeling (Pinte et al. (2006)). The left side of the “disk” represents the low mass star sample and the right side is the VLMO sample. The grey underlay is four times the pressure scale height based on the fully flared model in Szücs et al. (2010). This height was chosen as a proxy for the height at which the disk would theoretically intercept the starlight. The analytic perscription provided in Dullemond & Dominik (2004) (see their equations (6) and (20)) was used to calculate this height. The x-axis ranges from \(r_{in}\) to 300 au. \(r_{in}\) is the median inner radius of the disk in au for the two groups, respectively. The y-axis is represented in \(H/r\) and is a measure of the amount of flaring at a certain radius.

Bottom: Scatter plots of the best-fit scale height \(H_0\) (in au) and flaring index \(\beta\) (dimensionless) values for the individual objects. The x axis shows the spectral types of the objects, ranging from K4 to M9. The dashed lines in both figures indicate the median value for the low mass (left, red) group and the VLMO group (right, blue). The transparent bars are the uncertainties given as 1.4826 \(\cdot\) MAD, where MAD is the median absolute deviation. This gives an indication of the typical deviation from the median. The vertical dashed line indicates the seperation of the two samples at spectral type M4.6.25. The color gradient indicates the distribution of the \(\chi_{r}^{2}\) values of the fits. The objects which have stars as markers are the objects for which \(\chi_{r}^{2}\) are smaller than 10.0.

References

Dullemond, C. P., & Dominik, C. 2004, A&A, 421, 1075

Pinte, C., Ménard, F., Duchêne, G., & Bastien, P. 2006, A&A, 459, 797

Szücs, L., Apai, D., Pascucci, I., & Dullemond, C. P. 2010, ApJ, 720, 1668

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