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The eclipses of UX Ori type stars by large-scale disc humps are studied in detail. The influence of the hump extension on the eclipse depth and parameters of the linear polarization is modelled and compared with a compact cloud eclipse model. Eclipses were investigated both for the flared disc and for the disc with a puffing-up in the dust sublimation zone. It is shown that the large-scale hump eclipse may be significantly deeper than a compact cloud eclipse and show a greater linear polarization degree compared to it. It is also demonstrated that the hump extension together with the disc puffing-up can strongly affect the degree of polarization and colour index of the star during the eclipse. The position angle of the linear polarization may also change markedly during and after an eclipse by a large scale hump for the model with a puffed-up inner rim. Also, in this model, the maximum degree of the linear polarization can be achieved not at the brightness minimum, but closer to the end of the eclipse.
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The LMC star, SSTISAGE1C J050756.44-703453.9, was first noticed during a survey of EROS-2 lightcurves for stars with large irregular brightness variations typical of the R Coronae Borealis (RCB) class. However, the visible spectrum showing emission lines including the Balmer and Paschen series as well as many Fe II lines is emphatically not that of an RCB star. This star has all of the characteristics of a typical UX Ori star. It has a spectral type of approximately A2 and has excited an H II region in its vicinity. However, if it is an LMC member, then it is very luminous for a Herbig Ae/Be star. It shows irregular drops in brightness of up to 2 mag, and displays the reddening and blueing typical of this class of stars. Its spectrum, showing a combination of emission and absorption lines, is typical of a UX Ori star that is in a decline caused by obscuration from the circumstellar dust. SSTISAGE1C J050756.44-703453.9 has a strong IR excess and significant emission is present out to 500 micron. Monte Carlo radiative transfer modeling of the SED requires that SSTISAGE1C J050756.44-703453.9 has both a dusty disk as well as a large extended diffuse envelope to fit both the mid- and far-IR dust emission. This star is a new member of the UX Ori subclass of the Herbig Ae/Be stars and only the second such star to be discovered in the LMC.
We report the results of a long campaign of time-series photometry on the nova-like variable UX Ursae Majoris during 2015. It spanned 150 nights, with ~1800 hours of coverage on 121 separate nights. The star was in its normal `high state near magnitude V=13, with slow waves in the light curve and eclipses every 4.72 hours. Remarkably, the star also showed a nearly sinusoidal signal with a full amplitude of 0.44 mag and a period of 3.680 +/- 0.007 d. We interpret this as the signature of a retrograde precession (wobble) of the accretion disc. The same period is manifest as a +/-33 s wobble in the timings of mid-eclipse, indicating that the discs centre of light moves with this period. The star also showed strong `negative superhumps at frequencies w_orb+N and 2w_orb+N, where w_orb and N are respectively the orbital and precession frequencies. It is possible that these powerful signals have been present, unsuspected, throughout the more than 60 years of previous photometric studies.
The UX Ori type variables (named after the prototype of their class) are intermediate-mass pre-main sequence objects. One of the most likely causes of their variability is the obscuration of the central star by orbiting dust clouds. We investigate the structure of the circumstellar environment of the UX~Ori star V1026 Sco (HD 142666) and test whether the disk inclination is large enough to explain the UX Ori variability. We observed the object in the low-resolution mode of the near-infrared interferometric VLTI/AMBER instrument and derived H- and K-band visibilities and closure phases. We modeled our AMBER observations, published Keck Interferometer observations, archival MIDI/VLTI visibilities, and the spectral energy distribution using geometric and temperature-gradient models. Employing a geometric inclined-ring disk model, we find a ring radius of 0.15 +- 0.06 AU in the H band and 0.18 +- 0.06 AU in the K band. The best-fit temperature-gradient model consists of a star and two concentric, ring-shaped disks. The inner disk has a temperature of 1257^{+133}_{-53} K at the inner rim and extends from 0.19 +- 0.01 AU to 0.23 +- 0.02 AU. The outer disk begins at 1.35^{+0.19}_{-0.20} AU and has an inner temperature of 334^{+35}_{-17} K. The derived inclination of 48.6^{+2.9}_{-3.6}deg approximately agrees with the inclination derived with the geometric model (49 +- 5deg in the K band and 50 +- 11deg in the H band). The position angle of the fitted geometric and temperature-gradient models are 163 +- 9deg (K band; 179 +- 17deg in the H band) and 169.3^{+4.2}_{-6.7}deg, respectively. The narrow width of the inner ring-shaped model disk and the disk gap might be an indication for a puffed-up inner rim shadowing outer parts of the disk. The intermediate inclination of ~50deg is consistent with models of UX Ori objects where dust clouds in the inclined disk obscure the central star.
The primary star in the young stellar object (YSO) binary CO Ori displays UX Ori-type variability: irregular, high amplitude optical and near-infrared photometric fluctuations where flux minima coincide with polarization maxima. This is attributed to changes in local opacity. In CO Ori A, these variations exhibit a 12.4 yr cycle. Here, we investigate the physical origin of the fluctuating opacity and its periodicity using interferometric observations of CO Ori obtained using VLTI/GRAVITY. Continuum K-band circum-primary and circum-secondary emission are marginally spatially resolved for the first time while Br$gamma$ emission is detected in the spectrum of the secondary. We estimate a spectral type range for CO Ori B of K2-K5 assuming visual extinction, $A_{rm{V}}=2$ and a distance of 430 pc. From geometric modelling of the continuum visibilities, the circum-primary emission is consistent with a central point source plus a Gaussian component with a full-width-half-maximum of 2.31$pm$0.04 milliarcseconds (mas), inclined at 30.2$pm$2.2$^{circ}$ and with a major axis position angle of 40$pm$6$^{circ}$. This inclination is lower than that reported for the discs of other UX Ori-type stars, providing a first indication that the UX Ori phenomena may arise through fluctuations in circumstellar material exterior to a disc, e.g. in a dusty outflow. An additional wide, symmetric Gaussian component is required to fit the visibilities of CO Ori B, signifying a contribution from scattered light. Finally, closure phases of CO Ori A were used to investigate whether the 12.4 yr periodicity is associated with an undetected third component, as has been previously suggested. We rule out any additional companions contributing more than 3.6% to the K-band flux within ~7.3-20 mas of CO Ori A.
We report the discovery of mid-infrared excess emission in the young object RZ Psc. The excess constitutes ~8% of its Lbol, and is well fit by a single 500K black-body implying a dust free region within 0.7AU for optically thick dust. The object displays dust obscuration events (UXOR behaviour) with a time-scale that suggests dusty material on orbits of 0.5AU. We also report a 12.4 year cyclical photometric variability which can be interpreted as due to perturbations in the dust distribution. The system is characterized by a high inclination, marginal extinction (during bright photometric states), a single temperature for the warm dust, and an age estimate which puts the star beyond the formation stage. We propose that the dust occultation events present a dynamical view of an active asteroid belt whose collisional products sporadically obscure the central star.
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