YSO Bulletin
- March 2021 -

- TW Hya kicks off new Pro-Am campaign -

It starts here!

The promised observational campaign on Southern YSOs begins with a session on the important T Tau object TW Hydrae. This is an interesting system from several viewpoints, since it is the closest YSO to us as well as being part of a starforming area known as the TW Hydrae association. TW Hya is a typical T Tauri object, being comparable to the sun in size (it is slightly larger) but not so much in both mass and luminosity, since it is fainter than the sun on both these counts. It has a protoplanetary disc which we observe face on - that is, we are looking 'down' on TW Hya's pole. Its variations are therefore less than some other YSOs and the amplitude is not unlike that of T Tauri itself, normally just over half a magnitude in a fairly leisurely fashion.
However, observers should be aware of other 'gentle' variables like SU Aur which can always surprise us with unpredictable behaviour!

More YSO news

HD 38206 is a 5.7m A0V star in Lepus, in the Columba association, hosting a debris disc first discovered by IRAS. Further observations by Spitzer and Herschel showed that the disc has two components, likely analogous to the asteroid and Kuiper belts of the Solar System. The young age of this star makes it a prime target for direct imaging planet searches. Possible planets in the system can be constrained using the debris disc. A recent MNRAS paper by Booth et al presented the first ALMA observations of the system's Kuiper belt and the team detected an extended disc of dust, peaking at around 180 au with a width of 140 au. The disc is close to edge on and shows tentative signs of an asymmetry, best fit by an eccentricity of ~0.25. They used the fitted parameters to determine limits on the masses of planets interior to the cold belt, and determined that a minimum of four planets are required, each with a minimum mass of 0.64 MJ, in order to clear the gap between the asteroid and Kuiper belts of the system. If we make the assumption that the outermost planet is responsible for the stirring of the disc, the location of its inner edge and the eccentricity of the disc, then we can more tightly predict its eccentricity, mass and semimajor axis to be e_p=0.34^+0.20−0.13, m_p=0.7^+0.5−0.3MJ and a_p=76⁺¹²⁻¹³.

A paper in A&A (with the SFN's own João Alves as a coauthor) outlined the first study of the 3D dynamics of the gas in the entire southern Orion cloud complex. They used the parallaxes and proper motions of YSOs from Gaia DR2 as a proxy for gas distance and proper motion, and the gas radial velocities from archival CO data in order to compute the space motions of the different star-forming clouds in the complex, including subregions in Orion A, Orion B, and two outlying cometary clouds. From the analysis of the clouds' orbits in space and time, they found that they were closest about 6 Myr ago and are moving radially away from roughly the same region in space. This coherent 100-pc scale radial motion supports a scenario where the entire complex is reacting to a major feedback event, which they named the Orion-BB (big blast) event. This event, which they tentatively associate with the recently discovered Orion X stellar population, shaped the distribution and kinematics of the gas observed today, although it is unlikely to have been the sole major feedback event in the region.
They argue that the dynamics of most of the YSOs carry the memory of the feedback-driven star formation history in Orion and that the majority of the young stars in this complex are a product of large-scale triggering, which can raise the star formation rate by at least an order of magnitude, as for the head of Orion A (the Integral Shape Filament). Their results imply that a feedback, compression, and triggering process lies at the genesis of the Orion Nebula Cluster and NGC2023/2024 in Orion B, thus confirming broadly the classical feedback-driven scenario proposed in Elmegreen & Lada (1977). The space motions of the well-known young compact clusters, σ Orionis and NGC 1977, are consistent with this scenario. A momentum estimate suggests that the energy of a few to several supernovae is needed to power the coherent 3D gas motion measured in the paper.

More details

Dr. Fred Walter of Stonybrook University writes: "Part II of the ODYSSEUS pre-main sequence star program starts next month [i.e., March]. ODYSSEUS is an international collaborative campaign to understand the processes involved in the formation of low mass stars, their circumstellar disks, and their planets, via the time-variability of these systems", and it is in this regard that he is requesting AAVSO observations. He continues: "The HST will monitor the 10 million year old K7 pre-main sequence star TW Hya intensively, obtaining 12 sets of ultra-violet spectra spread over about 13 days, 7-19 March 2021, simultaneously with TESS observations. TESS will observe this part of the sky nearly continuously for 27 days, from 7 March through 2 April, with a 10 minute cadence. TESS data, both in terms of length of coverage and data quality, cannot be matched from the ground, but TESS is a single channel photometer operating in the red. We need ground-based optical photometry to place the TESS magnitudes in context.

• Are stellar variations due to enhanced accretion events or long-lived flares, which case the star to become bluer
• are they due to variable circumstellar extinction, in which case the star will become redder
• are they due to obscuration of the photosphere by optically-thick irregularities in the circumstellar disk, in which case the color should not change?

"Changes in extinction in particular are important for interpreting the UV spectra, which are very sensitive to the absorption. TW Hya is fairly bright, with a mean V ~ 11, and a historical range from 10.6 to 11.3 (from VSX). The typical day-to-day range of variability is ±0.1 mag. I seek to extract colors accurate to better than 2% (0.02 mag), which requires CCD photometry accurate to better than 0.01 mag. Observations are requested from 1 March through 5 April. Dr. Walter continues:
"Highest priority will be during the HST observations, but those exact times are not known yet". They will be announced in the relevant YSO forum thread.
Next Highest priority are nightly observations 6 through 19 March inclusive, spanning the time of the HST observations. No more than one observation is needed per night.
Lower priority would be from 20 March through 2 April, when only TESS is on the target.
The observations before the HST window and after the TESS window are to provide a baseline for each observer's data. Please observe in 4 bands (B,V and either Cousins RI (preferred) or Sloan ri). When I learn exactly when the HST observations will be scheduled, I will provide that information. But contemporaneous photometry more than suffices for our purposes."

Another look at FU Orionis

FU Orionis is the archetypal FUor star, a subclass of young stellar object that undergo rapid brightening events, often gaining 4-6 magnitudes on timescales of days. This brightening is often associated with a massive increase in accretion; one of the most ubiquitous processes in astrophysics from planets and stars to super-massive black holes. Multi-band interferometric observations were made of the FU Ori circumstellar environment by Labdon et al, including the first J-band interferometric observations of a YSO. The aim was to investigate the morphology and temperature gradient of the innermost regions of the accretion disk around FU Orionis and to characterise the heating mechanisms of the disk and comment on potential outburst triggering processes.
Recent upgrades to the MIRC-X instrument at the CHARA array allowed the first dual-band J and H observations of YSOs. Using baselines up to 331 m, they presented high angular resolution data of a YSO covering the near-infrared bands J, H, and K. The unprecedented spectral range of the data allowed them to apply temperature gradient models to the innermost regions of FU Ori.
They spatially resolved the innermost astronomical unit of the disk and determined the exponent of the temperature gradient of the inner disk to T = r−0.74±0.02. This agrees with theoretical work that predicts T=r−0.75 for actively accreting, steady state disks, a value only obtainable through viscous heating within the disk. They found a disk which extends down to the stellar surface at 0.015±0.007 au where the temperature is found to be 5800±700 K indicating boundary layer accretion, and which is inclined at 32±4° with a minor-axis position angle of 34±11°