YSO Bulletin
- February 2022 -

- Biggest protostar in action -

...Forever Blowing Bubbles

NGC7538 lies on the Cep-Cas border near the 'bubble nebula' in the high-mass starforming area Cas OB2, but the object is blowing its own bubble as well (non-UK readers note that "I'm forever blowing bubbles" was a popular song from 1918 which has become the anthem of West Ham United FC!) and a recent paper has sought to answer the question How do expanding HII regions interact with their environmental cloud?
Using SOFIA it was found that there are multiple expanding bubbles which require powerful stellar winds from the central high-mass O type stars to instigate the activity. NGC 7538 S, the star driving this activity, is actually the lagest known protostar with a luminosity of about 15,000 solar, surrounded by a large, nearly edge-on disc with a radius of 15,000 AU and a disk mass of about 100 M☉

Close Encounters!

Close encounters between young stellar objects in star forming clusters are expected to dramatically perturb circumstellar discs. Such events are witnessed in numerical simulations of star formation, but few direct observations of ongoing encounters have been made. A study in Nature Astronomy reports ALMA and JVLA observations towards the sub-million year old binary protostar Z CMa in dust continuum and molecular line emission. A point source 4700 AU from the binary has been discovered at both millimeter and centimetre wavelengths. It is located along the extension of a 2000 AU streamer structure previously found in scattered light imaging, whose counterpart in dust and gas emission is also newly identified. Comparison with simulations shows signposts of a rare flyby event in action. Z CMa is a "double burster", as both binary components undergo accretion outbursts, which may be facilitated by perturbations to the host disc by flybys.
Attentive readers will not need to be told about a similar flyby on RW Aur a few years back which caused such interest in both professional and amateur communities. Please observe this object which is perfectly well-placed at the moment. Good-quality imaging may also be useful for all you 'pretty picture' enthusiasts out there! Z CMa is an extrememly interesting system; a binary with a high-mass HAeBe primary with a lower-mass companion that shows FUOR episodes. The Wikipedia article is a good outline.

More on FUORs

Discs around young stellar objects (YSOs) consist of material that thermally emits the energy provided by a combination of passive heating from the central star, and active, viscous heating due to mass accretion. FU Ori stars are YSOs with substantially enhanced accretion rates in their inner disc regions. As a disc transitions from standard low-state, to FU Ori-like high-state accretion, the outburst manifests through photometric brightening over a broad range of wavelengths. A recent study presents results for the expected amplitudes of the brightening between ~4000 Å and 8 μm – the wavelength range where FU Ori type outburst events are most commonly detected. Their model consists of an optically thick passive+active steady-state accretion disc with low and high accretion states.

Odysseus returns!

(One for all you classicists)... The Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES) Director’s Discretionary Program of low-mass pre-main-sequence stars, coupled with forthcoming data from ALMA and JWST, will provide the foundation to revolutionize our understanding of the relationship between young stars and their protoplanetary discs, and this is of course that AAVSO observers are heavily involved with. A comprehensive evaluation of the physics of disk evolution and planet formation requires understanding the intricate relationships between mass accretion, mass outflow, and disc structure. Here they describe the ODYSSEUS Survey and present initial results of the classical T Tauri Star CVSO 109 in Orion OB1b as a demonstration of the science that will result from the survey. ODYSSEUS will analyze the ULLYSES spectral database, ensuring a uniform and systematic approach in order to (1) measure how the accretion flow depends on the accretion rate and magnetic structures, (2) determine where winds and jets are launched and how mass-loss rates compare with accretion, and (3) establish the influence of FUV radiation on the chemistry of the warm inner regions of planet-forming disks. ODYSSEUS will also acquire and provide contemporaneous observations at X-ray, optical, NIR, and millimeter wavelengths to enhance the impact of the ULLYSES data. Our goal is to provide a consistent framework to accurately measure the level and evolution of mass accretion in protoplanetary disks, the properties and magnitudes of inner-disk mass loss, and the influence of UV radiation fields that determine ionization levels and drive disk chemistry.

V1318 Cyg in Outburst

Magakian et al. (2019) called attention to the current bright state of LkHa 225 South (V1318 Cyg), which over the past two decades has changed from > 20m to < 13m. A paper by Hillenbrand et al presents recent optical photometric monitoring that shows colourless, non-sinusoidal, periodic brightness variations. The oscillations occur every 43 days, and have an amplitude of around 0.7m with some variation among cycles. We also present new flux-calibrated optical and near-infrared spectroscopy, which we model in terms of a keplerian disk. Additional high dispersion spectra demonstrate the similarity of the absorption line pattern to some categories of "mixed temperature" accretion outburst objects. At blue wavelengths, the object has a pure absorption spectrum and is a good spectral match to the FU Ori stars V1515 Cyg and V1057 Cyg. At red optical and infrared wavelengths, however, the spectrum is more similar to Gaia 19ajj, showing emission in TiO, CO, and metals. Sr II lines indicate a low surface gravity atmosphere. There are also signatures of a strong wind/outflow. V1318 was moderately bright in the early 1950’s as well as in the late 1980’s, with evidence for deep fades during intervening epochs. The body of evidence suggests that this is another case of a source with episodically enhanced accretion that causes brightening by orders of magnitude, and development of a hot absorption spectrum and warm wind. It is similar to Gaia 19ajj, but also reminiscent in its long brightening time and brightness oscillation near peak, to the embedded sources L1634 IRS7 and ESO Ha 99.