YSO Bulletin
- October 2023 -

- Three Amigos! -

Activity Alert

Some of our old friends have shown remarkable activity recently. First up is V1117 Her, which our fade expert John Pickett caught plummeting to 15.5 in the second week of the month. After a very short time there it recovered almost as quickly and is now (26th Oct) back at maximum. This event happened just before the star became too low in the sky for many observers (me included) and we look forward to its return next year!
V730 Cep continues to enthral in the midst of its record-breaking faint spell. Because the star is a recent discovery (2000) we don't know whether this fading has occurred before or not - but no 'modern' observer has ever seen this behaviour. Historical photographs may not be a possibility because, as we know, V730 lies very close to an 8th-mag star whose light will probably have overwhelmed that of V730. Important Note: I have just learnt that the variable has a 16.3 companion 1 arcsecond away, and as V730 is currently fluctuating around that magnitude some observers may have (a) estimated V730 and its companion as the combined light of the two or (b) made an uncertain observation due to the appearance of the possibly-resolved pair. But whatever the result it cannot be doubted that we are now in a new phase of activity in this fascinating object, which is apparently going through an AA Tauri phase where the protracted faint state is caused, in AA Tauri's case, by a warped circumstellar disc.
Still in Cepheus, BG Cep has been showing a lot of variation recently, again mainly from the results of John (Tri-colour green band, so not too dissimilar to visual) and the AAVSO lightcurve shows a rapid fade to a minimum of 14.0. Both the decline and recovery are punctuated by equally-rapid but smaller variations. Definitely one to follow!

HM Lup

HM Lup is a young M-type star that accretes material from a circumstellar disk through a magnetosphere. A recent study (using AAVSO observations via the ULLYSES programme) looked at the inner disk structure of HM Lup to characterize its variability. They used spectroscopic data from HST and several other sources, together with photometric data from TESS and AAVSO. The 2021 TESS light curve shows variability typical for young stellar objects of the "accretion burster" type. The spectra cover the temporal evolution of the main burst in the 2021 TESS light curve.
They compared the strength and morphology of emission lines from different species and ionization stages, determined the mass accretion rate from selected emission lines and from the UV continuum excess emission at different epochs, and also examined its relation to the photometric light curves. The emission lines in the optical spectrum of HM Lup delineate a temperature stratification along the accretion flow.
The shape and periodicity of the 2019 and 2021 TESS light curves, when qualitatively compared to predictions from magnetohydrodynamic models, suggest that HM Lup was in a regime of unstable ordered accretion during the 2021 TESS observation due to an increase in the accretion rate. Although HM Lup is not an extreme accretor, it showed enhanced emission in the metallic species during this high accretion state that is produced by a density enhancement in the outer part of the accretion flow.

CPV's

Complex periodic variables (CPVs) are stars that exhibit highly structured and periodic optical light curves. Previous studies have indicated that these stars are typically disk-free pre-main-sequence M dwarfs with rotation periods ranging from 0.2 to 2 days. To advance our understanding of these enigmatic objects, Bouma et al conducted a blind search using TESS 2-minute data of selected 65,760 K and M dwarfs within 150 pc, finding 50 high-quality CPVs, and subsequently determined that most are members of stellar associations. Among the new discoveries are the brightest, closest (around 20 pc!), and oldest (about 200 Myr) CPVs known. One exceptional object, LP 12-502, exhibited up to eight flux dips per cycle. Some of these dips coexisted with slightly different periods, and the shortest-duration dips precisely matched the expected timescale for transiting small bodies at the corotation radius. Broadly, our search confirms that CPVs are mostly young (<150 Myr) and low-mass (<0.4 M☉). The flux dips characteristic of the class have lifetimes of around 100 cycles, although stellar flares seem to induce sudden dip collapse once every few months. The most plausible explanation for these phenomena remains corotating concentrations of gas or dust. The gas or dust is probably entrained by the star’s magnetic field, and the sharp features could result from a multipolar field topology, a hypothesis supported by correspondences between the light curves of CPVs and of rapidly rotating B stars known to have multipolar magnetic fields.

Edge-on Discs

Scattered light imaging of protoplanetary disks provides key insights on the geometry and dust properties in the disk surface. A study by Duchêne et al includes JWST long-wavelength images of a 1000 au-radius edge-on protoplanetary disk surrounding an 0.4 M☉ young star in Taurus, 2MASS J04202144+2813491 (Tau 042021). These observations represent the longest wavelengths at which a protoplanetary disk is spatially resolved in scattered light. Combining these observations with HST optical images and ALMA continuum and CO mapping they find that the changes in the scattered light disk morphology are remarkably small across a factor of 30 in wavelength, indicating that dust in the disk surface layers is characterized by an almost gray opacity law. Using radiative transfer models, they conclude that grains up to >10 μm in size are fully coupled to the gas in this system, whereas those >100 μm are strongly settled towards the midplane.
Further analyses of these observations, and similar ones of other edge-on disks, will provide strong empirical constraints on disk dynamics and evolution and grain growth models. In addition, the 7.7 and 12. μm JWST images reveal an X-shaped feature located above the warm molecular layer traced by CO line emission. The highest elevations at which this feature is detectable roughly match the maximal extent of the disk in visible wavelength scattered light as well as of an unusual kinematic signature in CO. They propose that these phenomena could be related to a disk wind entraining small dust grains.