YSO Bulletin
- November 2021 -

- V1117 Herculis again! -

Fade caught just in time

Our enigmatic friend V1117 Herculis has just entered a deep fade, though by the time you read this it is back to maximum again. I quickly messaged AAVSO to issue an alert notice and this has enabled us to confirm the post-minimum behaviour. The fade was observed by our doughty fademeister (that's what I'm calling him anyway!) John Pickett who observes with a DSLR in the TG band (a three-colour value intended for imaging). He recorded V1117 at magnitude 15.028 on October 19th. Bringing up the recent lightcurve in B and V - because a characteristic of UXORs is increasing blueness post-minimum as a result of scattered light from the dusty disc - shows a small but definite blueing at these times, though I think it in bad taste for V1117 to undergo a fade just as the object gets closer to the sun!

Oct 15 (1) B-V = 0.712
Oct 19 (2) B-V = 0.688 (John's obs is the green circle)
Oct 23 (3) B-V = 0.543
Oct 25 (4) B-V = 0.597

A new use for 3-D Printing

I actually heard about this via a link on my smartphone from SciTechDaily, but it is highly relevant to our interests!
Speaking personally as someone with an interest in both science and art, this article also reveals how the two fields may be married to produce not only things of beauty but also an accompanying scientific utility. Using known data about starforming clouds the researchers were able to produce actual tangible models (of course derived from the other sort of model; the mathematical sort) of the areas surrounding forming stars. You can read the entire article here.
The new issue of the Star Formation Newsletter (our big brother) even features the models on its front cover!

Dusty Discs again

Protoplanetary disks in T Tauri stars play a central role in star and planet formation. A study using the GRAVITY survey (Perrault et al) spatially resolved at sub-AU scales the innermost regions of a sample of T Tauri discs to better understand their morphology and composition, and the survey was extended to 27 Herbig [i.e., higher-mass] stars and collected near-IR K-band observations of 17 T Tauri stars, spanning effective temperatures and luminosities in the ranges of 4000-6000 K and 0.4-10 L☉. Best-fit models of the discs' inner rim correspond to wide rings. Extending the Radius-luminosity relation toward the smallest luminosities (0.4-10 L☉) they found the R-L trend was no longer valid, since the K-band sizes measured with GRAVITY were larger than the predicted sizes from sublimation radius computation.

MYSOs

A paper by Frost et al looks at how the rarity and deeply embedded nature of stars with masses larger than 8 solar masses has limited our understanding of their formation. Previous work has shown that complementing spectral energy distributions with interferometric and imaging data can probe the circumstellar environments of massive young stellar objects (MYSOs) well. However, complex studies of single objects often use different approaches in their analysis, therefore the results of these studies cannot be directly compared. The work of this study aimed to obtain the physical characteristics of a sample of MYSOs at 0.01" scales, at 0.1" scales, and as a whole, which enabled a comparison of the characteristics of the sources. They applied the same multi-scale method and analysis to a sample of MYSOs. High-resolution interferometric data, near-diffraction-limited imaging data, and a multi-wavelength spectral energy distribution are combined. By fitting simulated observables derived from 2.5D radiative transfer models of disk-outflow-envelope systems to their observations, the properties of the MYSOs are constrained. It was found that the observables of all the MYSOs can be reproduced by models with disk-outflow-envelope geometries, analogous to the geometry associated with low-mass protostars.
The characteristics of the envelopes and the cavities within them are very similar across the sample. On the other hand, the disks seem to differ between the objects, in particular with regards to what they interpreted as evidence of complex structures and inner holes. This is comparable to the morphologies observed for low-mass YSOs. A strong correlation was found between the luminosity of the central MYSO and the size of the transition disk-like inner hole for the MYSOs, implying that photoevaporation or the presence of binary companions may be the cause.

Why do M dwarfs have more transiting planets?

Mulders et al in their paper propose a planet-formation scenario to explain the elevated occurrence rates of transiting planets around M dwarfs compared to sun-like stars discovered by Kepler. They used a pebble drift and accretion model to simulate the growth of planet cores inside and outside of the snow line. A smaller pebble size interior to the snow line delays the growth of super-Earths, allowing giant planet cores in the outer disk to form first. When those giant planets reach pebble isolation mass they cut off the flow of pebbles to the inner disk and prevent the formation of close-in super-Earths. They applied this model to stars with masses between 0.1 and 2 solar mass, and for a range of initial disk masses and found that the masses of hot super-Earths and of cold giant planets are anti-correlated. The fraction of the simulations that form hot super-Earths is higher around lower-mass stars and matches the exoplanet occurrence rates from Kepler. The fraction of simulations forming cold giant planets is consistent with the stellar mass dependence from radial velocity surveys. A key testable prediction of the pebble accretion hypothesis is that the occurrence rates of super-Earths should decrease again for M dwarfs near the sub-stellar boundary like Trappist-1.