YSO Bulletin
- July 2021 -

- Fruitful Results on V1117 Her -

It is quite possible that by the time you read this, or shortly afterwards, that V1117 Herculis will have entered a second, deeper, fade. This is my prediction, based on previous behaviour of the object. In a paper on this and a similar star RZ Piscium I described them as two UXORs. Since then it appears that RZ Psc is not in fact a classical UXOR but rather a slightly more evolved star with a debris disc; that is a disc which is already well into the planet-forming process, where the dips in light are indeed caused by circumstellar material but which in this case results from violent interactions between the already-forming planets-to-be themselves, rather than simple clumping in the primordial disc as in UXORs proper.
I think it quite likely also that V1117 Her is a star of the same type, or possibly a member of a recently-investigated group of stars, typical of which is J1407, a young star surrounded by forming planets which are in turn surrounded by circumplanetary discs from which satellites are forming. The whole talk can be found on the AAVSO YouTube channel and I cannot recommend it highly enough! Looking at the lightcurve, the magnitude range is almost the same as that of V1117 her - from magnitude 12 to 15. However, the lightcurve of J1407 is much more complicated than that of 'our' star, with a succession of minor dips of various gradients superimposed on the main fade.
The recent fade of V1117 Her (above) shows little difference between the B, V and I values at most stages, and there appears to be some small evidence of the post-minimal blueing (caused by scattering from the disc) seen in typical UXORs. The star does, however, show IR excess which demonstrates, in this context, the presence of warm dust in the system; in fact it was the IR parameter that suggested on discovery that this was a Mira star - clearly an impossibility due to its A-type spectrum. UXORs are also associated with A-type stars - but the problem remains: what is an apparently young star doing at V1117 Her's position, and more crucially considering its proper motion which shows that it is moving towards the galactic plane over time - from even farther outside it!
So... my thanks to all the AAVSO observers who contribute to the picture of this enigmatic object! Given its relatively simpler variations I don't think we have a J1407-type object in degree, but possibly in kind, but with a simpler disc structure in place.
Plesae continue to observe V1117 Her from this moment on - if only to prove my prediction incorrect!

Dippers again!

Dippers are typically low-mass, PMS stars that display dips in their light curves. These dips have been attributed to dusty warps that form in the inner part of the disk. The goal of a study by Roggero et al was to derive the properties of dipper stars in Taurus to assess the physical mechanisms that induce dipper light curves. They used light curves from the K2 mission (the sucessor mission of Kepler) to select a dipper sample among 179 members and possible members of the Taurus star-forming region based on the light-curve morphology. They studied the periodicities by combining periodograms with wavelet analysis and derived the stellar parameters from the photometry, also studying the morphology of the photometric dips. They found a dipper occurrence of 30% in disk-bearing stars observed with K2 that were identified visually by the team.
This represents a lower limit to their true occurrence. About half of the dippers are aperiodic, and most of these are dominated by another type of variability. The chosen sample is of late spectral type (K/M), low mass and moderate mass accretion rates and covers periods of a few days. They observed a transient dipper over a few rotation cycles and a dipper with a changing period. The structure of the dips can be complex and varies strongly over timescales of down to one stellar rotation.
The corotation radii are located at a few stellar radii, and the temperatures at corotation allow dust survival. Many of the systems are seen at moderate to high inclination. They further found that the angular extension of the dusty structure producing the dips is correlated with the stellar period. Magnetospheric accretion, which causes an accretion column and its base to occult the star, can explain most of the observed light curves. Although compatible with the model, many of the stellar inclination angles are moderate and do not exclude mechanisms other than the occultation by an inner disk warp to account for dipper light curves.

Of FUORs and Flares

Among the low-mass pre-main sequence stars, a small group called FU Orionis-type objects (FUors) are notable for undergoing powerful accretion outbursts. V1057 Cyg, a classical example of a FUor, went into outburst around 1969-1970, after which it faded rapidly, making it the fastest fading FUor known. Around 1995, a more rapid increase in fading occurred. Since that time, strong photometric modulations have been present. A largely Hungarian study presents nearly 10 years of source monitoring at Piszkéstető Observatory, complemented with optical/NIR photometry and spectroscopy from the Nordic Optical Telescope, Bohyunsan Optical Astronomy Observatory, TESS, and SOFIA. The light curves show continuation of significant quasi-periodic variability in brightness over the past decade, while spectroscopic observations show strong wind features, shell features, and forbidden emission lines. All of these spectral lines vary with time. They also report the first detection of [S II], [N II], and [O III] lines in the star.

Staying in Hungary (a great centre of YSO studies!) there is a new census of candidate PMS stars in the Cepheus Flare star-forming region, based on Gaia EDR3 parallaxes, proper motions, and colour−magnitude diagrams. The study identified new candidate members of the previously known young stellar groups associated with NGC 7023, L1177, L1217/L1219, L1228, L1235, and L1251, and looked at the 3D structure of the star-forming complex and the distribution of tangential velocities of the young stars. The young stellar groups are located between 330 and 368 pc from the Sun, divide into three kinematic subgroups, and have ages between 1−5 million years. The results confirm the scenario of propagating star formation, suggested by previous studies. In addition to the bulk PMS star population between 330 and 370 pc, there is a scattered and more evolved similar population at around 150−180 pc. They found new candidate members of the nearby Cepheus Association, and identified a new moving group of 46, 15−20 million years old PMS stars located at a distance of 178 pc, around the A0-type star HD 190833. A few pre-main-sequence stars are located at 800−900 pc, indicative of star-forming regions associated with the Galactic local arm above the Galactic latitude of +10°.