View Online | Back Issues

YSO Bulletin
- October 2021 -

- Mixed Bag Month -

Observers Corner

I plan to feature an individual YSO in this occasional series, and will kick off with a rather strange object that could do with more observations. It is a particularly noteworthy star as it is in the same field as a more famous object. Our star is BM And, located near Z And, prototype of its class. BM And is classed as an UXOR/ROT. The 'rot' part indicates that some of the variations are due to regions on the star's surface being brighter (or of course darker) than the rest, thus giving rise to small, regular light changes. The rotation period in this case is about 18½ days but one characteristic of UXORs is that they show larger light variations than can be accounted for by mere rotational effects. The quoted range is 11.63 - 14.02 visual; a much more UXOR-like amplitude!

BM And is in a rather attractive cluster of stars, some of which are nebulous, as can be seen from the picture. However, while most UXORs disovered so far belong to early spectral types, that of BM And is type K, which we associate more with lower-mass objects. It is thought the mass of BM And is only slightly greater than that of the Sun. Also, the type tends to be associated with large amounts of circumstellar material than that found around BM And in a 1997 study by Natta et al. Since the star is not excessively faint it will make a good target for modestly-sized scopes, while of course CCD users can follow the smaller variations effectively, since other processes in the circumstellar environment may have other effects.

Hot spots on GM Aur

Magnetospheric accretion models predict that matter from protoplanetary disks accretes onto the star via funnel flows which follow the stellar field lines and shock on the stellar surface leaving a hot spot with a density gradient. Previous work has inferred different densities in the hot spot, but has not been sensitive to the radial density distribution. Attempts have been made to measure this with X-ray observations, but X-ray emission only traces a fraction of the hot spot and also coronal emission. A paper by Espaillat et al reports periodic ultraviolet and optical light curves of the accreting star GM Aur that display a time lag of about 1 day between their peaks.
The periodicity arises as the source of the ultraviolet and optical emission moves into and out of view as it rotates along with the star. The time-lag indicates a difference in the spatial distribution of ultraviolet and optical brightness over the stellar surface. Within the framework of the magnetospheric accretion model, this indicates a radial density gradient in a hot spot on the stellar surface, since different density parts of the hot spot are expected to emit radiation at different wavelengths. These results are the first observational confirmation of the magnetospheric accretion model’s prediction of a density gradient in the hot spot and demonstrate the insights gained from focusing on the wavelengths where the bulk of the accretion energy can be observed.

J1407 again!

In 2007, the young star 1SWASP J140747.93-394542.6 (V1400 Cen) underwent a complex series of deep eclipses over 56 days. This was attributed to the transit of a ring system filling a large fraction of the Hill sphere of an unseen substellar companion. Subsequent photometric monitoring has not found any other deep transits from this candidate ring system, but if there are more substellar companions and they are coplanar with the potential ring system, there is a chance that they will transit the star as well. This young star is active and the light curves show a 5% modulation in amplitude with a dominant rotation period of 3.2 days due to star spots rotating in and out of view.
A recent study led by Matthew Kenworthy (who recently gave an AAVSO webinar on these stars) modelled and removed the rotational modulation of the J1407 light curve and searched for additional transit signatures of substellar companions orbiting around J1407. They combined the photometry of J1407 from several observatories, spanning a 19 year baseline, removed the rotational modulation by modeling the variability as a periodic signal, whose periodicity changes slowly with time over several years due to the activity cycle of the star. A Transit Least Squares (TLS) analysis searched for any periodic transiting signals within the cleaned light curve. An activity cycle of J1407 with a period of 5.4 years was found, but a TLS search did not find any plausible periodic eclipses in the light curve, from 1.2% amplitude at 5 days up to 1.9% at 20 days. This sensitivity is confirmed by injecting artificial transits into the light curve and determining the recovery fraction as a function of transit depth and orbital period.
J1407 is confirmed as a young active star with an activity cycle consistent with a rapidly rotating solar mass star. With the rotational modulation removed, the TLS analysis ruled out transiting companions with radii larger than about 1 Jupiter.

Water, water (not) everywhere!

Millimeter continuum imaging of protoplanetary disks reveals the distribution of solid particles and the presence of substructures (gaps and rings) beyond 5-10 au, while infrared (IR) spectra provides access to abundances of gaseous species at smaller disk radii. Building on recent observational findings of an anti-correlation between the inner disk water luminosity and outer dust disk radius, a recent study investigated the dynamics of icy solids that drift from the outer disk and sublimate their ice inside the snow line, enriching the water vapor that is observed in the IR. Their modelling explored a range of conditions (gap location, particle size, disk mass, and alpha-viscosity) under which gaps in the outer disk efficiently block the inward drift of icy solids. It found that inner-disk vapor enrichment is highly sensitive to the location of a disk gap, yielding for each particle size a radial "sweet spot" that reduces the inner-disk vapor enrichment to a minimum.
For pebbles of 1-10 mm in size, which carry the most mass, this sweet spot is at 7-15 AU, suggesting that inner gaps may have a key role in reducing ice delivery to the inner disk, and may not allow the formation of Earths and super-Earths. This highlights the importance of observationally determining the presence and properties of inner gaps in disks.