Scientific Organizing Committee: Cornelius Dullemond (MPIA Heidelberg), Carlos Eiroa (Madrid University), Goesta Gahm (Stockholm University), Konstantin Grankin (CrAO), V. Grinin (co-chair) (Pulkovo Observatory), Antonella Natta (co-chair) (Arcetri Astrophysical Observatory), Rene Oudmajer (The University of Leeds), Timo Prusti (EAS), Alla Rostopchina (CrAO), Mario van den Ancker (ESO).
Owing to the specific orientation of circumstellar (CS) disks (almost edge-on) the UX Ori type stars (UXORs) are the excellent laboratory for the studies of CS disks, their structure and dynamical state. The goal of this workshop is to summarize the observational properties of these stars and discuss current models suggested for their explanation. The main topics are: optical photometry, polarimetry and spectroscopy of UXORs; infrared activity of UXORs; spectral energy distribution of UXORs and related objects; the circumstellar disks and the nearest environment of young stars; hydrodynamic processes in CS disks of single and binary stars and the origin of cyclic variability of UXORs; related phenomena in evolved stars: R Crb stars etc.; exotic eclipsing systems: KH 15D, H 187; connections and interrelations between TTS and UXORs studies.
Summary
After a short welcome by the director of the Crimean Astrophysical observatory (CrAO) Alla Rostopchina, Peter Petrov told about the foundation of the Simeiz Observatory 100 years ago. During about half a century it was the Crimean branch of the Pulkovo Observatory and then transformed to the Crimean Astrophysical Observatory. Under the leadership of academician A. Shain the Simeiz Observatory had played a very important role in the development of astrophysics in Russia and in USSR .
The first session was devoted to the optical observations of UXors. In his review “UX Ori stars as a tool for diagnostics of the proto-planetary disks” Vladimir Grinin summarized the main observational properties of these stars and ideas suggested for their explanation. An accent has been made on the analysis of the photopolarimetric activity of UXors. The cause of this activity is a variable circumstellar obscuration of stars by optically thick fragments of their CS disks. Therefore, the level of the UXors activity depends strongly on the orientation of CS disks relatively to an observer. In many cases the large-scale variability of these stars consists of two components: (i) the irregular and (ii) the periodic (or cyclic) variability. Cycles last from a few to ten and more years. They testify strong perturbations in the CS disks produced by the low-mass companions or proto-planets.
This topic was continued in the contributed talk “Search for Keplerian periods in light-curves of UXors and T Tauri stars” by Svetlana Artemenko and co-authors. They analyzed the long-term (10-20 yr) photometric series of 49 T Tauri stars and 13 Herbig Ae stars with the aim to find the most stable periods caused by orbital motion of dust clouds within circumstellar disks. As a result, periods in the range of 25 to 250 days were detected in 12 T Tauri stars. Some of the Herbig Ae stars show cyclic variability with characteristic times within the same range. This indicates existence of stable structures (dust clouds or density waves), probably associated with protoplanets, rotating at Keplerian orbits with semi-major axes within 0.2 to 1.1 AU.
Alla Rostopchina and co-authors presented the catalog of the photopolarimetric activity of UX Ori stars. It is based on the results of the multi-year synchronous UBVRI photometric and polarimetric observations of the large sample of the UXORs. These observations started in CrAO in 1986 and continue up to now.
Dmitry Shakhovskoy presented a talk on a dichroism of aligned non-spherical grains as a possible mechanism of some polarimetric effects observed in UXors.
Konstantin Grankin reviewed the long term photometric variability of 49 CTTs and 13 UXORs type stars over up to 23 years and characterized it from a set of statistical parameters. He identified some various photometric subgroups within this sample, corresponding to specific sources of variability (cold spots, hot spots, circumstellar extinction). It has been shown that most of UXors appear as an homogeneous photometric group characterized by a high level of variability as compared to CTTs. This confirmed that the photometric variability of UXors is dominated by a single process, namely the circumstellar extinction. He has shown that the UXors are divided by two subgroups on the one of statistical parameters, which characterizes the variability in an average light level. The first subgroup of UXors includes six stars with the significant variability in an average light level, the second one includes seven UXors with the moderate variability. All of six stars from the first subgroup demonstrae the ‘blueing effect'. On the contrary, this effect is not present for six stars from the second subgroup.
Rene Oudmaijer, in his work on the intermediate mass pre-main sequence Herbig Ae/Be stars, investigates whether they have small scale disks reaching onto the star. Showing this would go a long way into demonstrating that massive stars form via some sort of disk accretion. To this end he used spectropolarimetry, which exploits the fact that stellar continuum photons are scattered by free electrons in a circumstellar disk, while the emission line photons are not. This results in an observable difference between continuum and line polarization, a ``line-effect''. The method is very sensitive to small scales, typically of order few stellar radii. He showed that most Herbig Ae/Be stars show a line effect and thus that the majority of young intermediate mass stars may form through disks. A further result he discussed was that the Herbig Ae stars had a different polarization signature over the emission line than the Herbig Be stars. Whereas the Herbig Be stars showed simple depolarizations, the Herbig Ae stars displayed much more complex profiles, and even instances of enhanced polarization were observed. In fact, the Herbig Ae stars show almost the same polarization behaviour as the T Tauri stars. For T Tauri stars it can shown that the enhanced polarization is due emission from a compact, anisotropic, source on the stellar surface that is scattered off a rotating circumstellar disk. The compact sources are identified with the accretion hot spots and the similarity between the Herbig Ae stars and the T Tauri stars strongly suggest that both types of object form in the same manner. The prediction that the more massive A-type objects may form due to magnetically controlled accretion too has since been supported by the measurements of magnetic fields towards some Herbig Ae stars. The main conclusion is that the current boundary between the different modes of formation between low and high mass stars is at much higher masses than previously thought. In his second talk he presented spectro-astrometry of a large sample of Herbig Ae/Be stars. The technique is extremely sensitive to detecting binary systems, even in conditions when the system is otherwise unresolved.
Mikhail Pogodin and co-authors presented results of a high-resolution spectroscopy of a southern Herbig Ae star HD163296 carried out during 3 nights in May 2002. An analysis of the data revealed manifestation of a layered spatial structure of the wind zone containing layers of preferable generation of local inhomogeneities in the outflowing gas. It has been found that a variability observed in a number of atmospheric lines is likely to be stimulated by accretion onto the star. A spectroscopic investigation of another southern Herbig Ae star HD144432 carried out at the same season with the same instrument, showed a complex picture of variability observed in circumstellar lines in the spectrum of the object (Beskrovnaya et al.). It evidences in favour of a strongly inhomogeneous structure of the circumstellar gas around this star.
Ignacio Mendigutía presented a spectroscopic catalogue of Pre-Main Sequence (PMS) stars. This catalogue is based on 370 spectra of 43 PMS Hα emitters, mainly Herbig Ae/Be, taken during four different observing campaigns, when simultaneous optical and near infrared photopolarimetry were also carried out. The spectroscopic measurements for each observation consist of two parts. First, a complete characterization of the Hα line, by means of the Equivalent Width (EW), the line width and the profile shape classification; second, the EW measurements of the He I 5876 Å, Na D1D2 and [O I] 6300 Å lines, allowing the study of different disk regions and physical processes. A total of 13 stars of different spectral types displayed the UXor phenomena, both at high and low brightness states, and are included in the sample. The cross correlation between the spectroscopic measurements and the simultaneous V-magnitudes reveals, among others, that the EW-V correlation in Hα (and sometimes in [O I] 6300 Å) is a main feature of the UX-Ori type stars, and it can be explained according to the coronagraphic effect. Both the He I 5876 Å and Na D1D2 can display large variations uncorrelated with the V behaviour, possibly indicating that accretion processes and gas blobs transits can occur, letting the stellar brightness measured almost unaffected. The spectroscopic study of PMS stars and all related data will be submitted to be published along 2008.
The results of the long-term spectroscopic monitoring of three Ae Herbig stars in the regions of Hα emission line, the Na I D resonance lines and the OI 7774 triplet lines were presented by Olesya Kozlova. She showed the presence of the long-term spectral variability on the time scale about several years for all stars investigated. This variability is caused by the changing in the structure of the inner regions of their CS gas envelopes. According to the results of modeling of the CS disc structure (for example, Artymowicz and Lubov 1996) the observed variability may be connected with the binarity of the star. So, the long-term spectral variability may be a common feature of the young stars and play an important role in the observed characteristics of their CS envelopes.
Benjamín Montesinos presented the results of a study devoted to determine the absolute parameters for a sample of 30 Herbig Ae/Be and Vega-type stars. The accurate knowledge of the stellar parameters is needed as a previous step to model the surrounding disks, because the input energy from the star will determine the radiation received by the disk and hence its energy balance, geometry and other processes. The effective temperature, Teff, is computed by fitting the photospheric part of the spectral energy distribution (SED) with Kurucz models. The stellar gravity, log g, is determined from mid-resolution observations of the Balmer lines Hβ, Hγ and Hδ, comparing the observed profiles with synthetic spectra. For this kind of stars the width of the Balmer lines is very sensitive to the gravity, therefore an accurate determination of this parameter is feasible. Once Teff and log g are estimated, the metallicity [M/H] can be computed by comparing high resolution spectra with a grid of synthetic spectra. One has to be extremely careful at this step given the large variability of some spectral lines. Given the high value of v sin i for many stars, only an average of [M/H] can be estimated. With these three parameters in hand, one can place the stars in a HR diagram gravity-effective temperature and superimpose the appropriate set of PMS evolutionary tracks and isochrones for that particular metallicity. This allows us to extract the mass, age and luminosity. An important feature of the whole process is that it is {distance-independent} in the sense that the distance to the object, which is in most of the occasions unknown, is not used anywhere. The complete work has been recently submitted to Astronomy & Astrophysics.
Rene Oudmaijer reported that the Herbig Ae/Be stars he observed had a binary fraction of 70%, the largest such fraction ever measured for Herbig Ae/Be stars with a single method. Clearly, any theory of the formation of massive stars needs to be able to produce binary objects. A further constraint that he derived from this work was that the binaries appeared aligned with the disks around the primary objects, such as those found in spectropolarimetry. This lends support to fragmentation as the main mode for the formation of binaries with a massive primary. He presented his efforts to split the spectra of
these binaries which have separations less than the seeing.
Jerome Bouvier reviewed the process of magnetospheric accretion in classical T Tauri stars and addressed the relationship between this process and the UX Ori phenomenon. Based mostly on the prototypical case AA Tau, he showed how the various timescales of variability and the various diagnostics (spectroscopic, photometric, and polarimetric) could be understood within a single framework, namely: magnetically-controled accretion of material from the inner disk edge onto the star through a large-scale, inclined stellar magnetic field. Indeed, AA Tau shows many of the signatures usually assigned to UXOrs : increased polarization when faint, the so-called ``blueing'' effect, and large amplitude photometric variability. All these properties can be ascribed to the inner edge of the dusty disk being warped by the inclined stellar magnetosphere, thus occulting the stellar photosphere as it rotates across the line of sight, and this results in eclipses being observed every week or so. While these effects are best seen in highly-inclined systems such as AA Tau (i~75 deg.), Bouvier showed that other well-known T Tauri stars, such as SU Aur, DK Tau, and GW Ori, for instance, also appear to exhibit, at least at times, UXor-type variability, often superimposed onto the usual CTTS variability. This suggests that the connection between CTTS and UXor variability may be stronger than previously thought and certainly deserve further investigation.
Bill Herbst reviewed the properties of the unusual eclipsing system KH 15D. He presented photometry through the 2007/8 season and compared with the model predictions from Winn et al. (2006). The star is not fading out quite as rapidly as expected so a small adjustment to those model parameters will be required. It is likely that star A (the only currently visible star) will have disappeared entirely by early next season and the system will continue to be seen only by reflected light.
In a second talk, Herbst discussed the nature of the reflected light. He showed that, as in the case of UXors, the star gets bluer when fainter, but unlike UXors it never shows any reddening whatsoever. In the case of KH 15D there are several possible causes for the bluing: 1) Star A itself has a bluer and redder hemisphere caused by its spots and the redder hemisphere is eclipsed first, 2) star B is bluer than star A so the reflected light from it is also bluer, 3) there may be some scattering from small grains in a halo around the system that contribute to the blueness, and 4) the sand-sized grains responsible for much of the scattering may have a composition that imparts a blue spectral character to the reflected light. Herbst also showed that the reflected light was red-shifted by the exact amount expected if it were coming from the far side of the same circumbinary disk that caused the occultation. From the spectra it is furthermore possible to divide the light into reflected and transmitted components and thereby deduce the opacity of the disk. This argument suggests a typical grain size of about 1 mm , consistent with the lack of observed reddening. Evidently, KH 15D is a somewhat evolved disk in a transition state between a Classical T Tauri star disk and a debris disk. The grains have grown substantially from their interstellar medium size (microns or less) but have not yet produced objects large enough to have debris-producing impacts.
Evgeni Semkov presented data from multicolor photometry and from archival photographic observations of V 1184 Tau. The recent photometric data suggest that the period of strong light variations that started in 2003 continues up to the present, at least 5 years. The data from the archival photographic plates demonstrate that an unknown minimum of brightness exists in the approximate period 1980-1985. V 1184 Tau is a G type low-mass star whose spectrum is similar to WTT stars, but its photometric behavior is typical of the UX Orionis variable stars. Assuming the obscuration from orbiting dust clouds as a reason for a deep minimum, the approximate period of obscurations and the interval between two deep minima was estimated. The calculations give a 25-28 year period between the two minima and approximately 8-10 year duration of the minima.
Anne Dutrey presented the review “Observations of disks with mm/submm arrays”. Since the early 90's, millimeter arrays have allowed radioastronomers to image proto-planetary disks encountered around low-mass (T Tauri) and intermediate-mass (Herbig Ae) young stars. Observations of the thermal emission of the dust and of the CO rotation lines provide important constraints on the disk geometry and physics. Several results were presented in order to illustrate which kind of information can be deduced from the interferometric mm/submm data. In particular, the CO properties of the disk surrounding CQ Tau, an UX Ori-like star mapped with the IRAM array were performed.
In the next talk “Circumstellar Disks and Planet Formation: an ALMA perspective” Anne Dutrey told about the current state of the Atacama Large Millimeter Array (ALMA). This project will enter in operation in the Atacama desert, in Chile, in 2010. The full main array consisting of 50 antennas of 12 m in diameter will be complete in 2012. With a unique sensitivity and angular resolution in the mm and submm domain, ALMA will become a powerful instrument to unveil the physics and chemistry of inner circumstellar disks surrounding young stars.
Timo Prusti reviewed the studies of mid- and far-infrared variability of UXors. He pointed out the difficulties to work in the near-infrared where stellar and disk emission combined with extinction due to absorption and scattering complicates the interpretation. The infrared variability can confirm the basic result in the optical wavelengths: the UXor phenomenon is associated with physical changes in the circumstellar environment. The fundamental strength of infrared studies is the relaxation of the need to have the event occurring at the line of sight to the star. Despite the scarcity of observational infrared variability data it is possible to conclude that the UXor phenomenon is rather caused by few events at a given time, whatever they are, than an ensemble of events. He concluded the review by urging for better observational data to describe the infrared variability first better than has been done till now.
Bernadette Rodgers presented results of a large Near-infrared Adaptive Optics survey for multiplicity in Herbig Ae/Be stars, including many UXOR stars. We have imaged 125 stellar systems so far, with an observed binary frequency of at least 65%. The survey is sensitive to companions between 0.1 and ~10 arcseconds and to a delta-K of 9 magnitudes beyond 1 arcsecond. Based on a statistical analysis used to assess the probability that detected stars are physical companions, over 200 probable companions have been identified. The companions vary widely in distance from the parent star and in delta-K magnitude. Follow-up spectroscopy of approximately 15% of the stars, finds that early B stars are most likely to have companions with similar spectral type, while A stars tend to have lower mass companions. The sample includes at least 21 known UXors, which show a detected binary frequency >80% (17/21). In binary properties, the UXor systems seem to be well-mixed with the rest of the sample and do not stand out as a distinct sub-group.
Hydrodynamic processes in young binaries can be a source of cyclic variability of circumstellar extinction. The results of the SPH modeling of such processes were presented by Natalia Sotnikova and co-authors. The calculations show that periodic variations in column density in projection onto the primary component take place at low inclinations of the binary plane to the line of sight and can result in periodic extinction variations. Three periodic components can exist in general case. The first component has a period equal to the orbital one and is attributable to the streams of matter penetrating into the inner regions of the binary. The second component has a period that is a factor of 5-8 longer than the orbital one and is related to the density waves generated in a circumbinary (CB) disk. The third, longest period is attributable to the precession of the inner asymmetric region of CB disk. The relationship between the amplitudes of these cycles depends on the model parameters as well as on the inclination and orientation of the binary in space. At a dust-to-gas ratio of 1 : 100 and a mass extinction coefficient of 250 cm2/g, the amplitude of the brightness variations of the primary component in the V-band can reach 1 m at a mass accretion rate onto the binary components of 10-8 MSun yr-1 and a 10o inclination of the binary plane to the line of sight. These results can be applied to the interpretation of the cyclic variability of UX Ori stars.
Yuri Efimov presented the review of the photometric, polarimetric and speckle-interferometric study of the R CrB phenomenon observed in several types of variable stars. The most data are mainly based on his observations of R CrB itself during its 9 deep minima events from 1972 till 2004. It was shown that there is the simple exponential relation connecting the brightness of the star and its degree of polarization. This relation allows us to estimate an expected degree of polarization during the very beginning of the event. A similarity and difference in the R CrB and the UX Ori phenomena (variations in the brightness and polarization observed) are shown. An analysis of available data leads to the base model of the asymmetric patchy dust envelope with rotating dust clouds replenished with the matter ejected from the star.
The influence of the dusty disk winds on the circumstellar extinction in single and binary young stars was considered in the talk by Larisa Tambovtseva. She argued that the dusty disk wind from T Tauri stars (TTSs) could be opaque for the ultraviolet and optical radiation of the star and capable to absorb its noticeable fraction. Calculations show that at the accretion rates from 10-8 to 10-6 MSun per year this fraction for TTSs may range from 20% to 40% of a total luminosity of the star correspondingly. This means that the disk wind in TTSs can play the same role as the puffed inner rim considered in the modern models of accretion disks. In Herbig Ae stars inner regions of the disk winds (r < 0.5 AU) are free of dust since there dust grains sublimate under the effect of the radiation of the star. Therefore, in this case a fraction of the absorbed radiation by the disk wind is significantly less, and may be compared with the effect of the puffed-up inner rim only at accretion rates greater than 10-6 MSun yr-1. Due to a structural inhomogeneity of the disk wind, its optical depth towards an observer may be variable resulting in the photometric activity of the young stars. For the same reason, one can observe moving shadows from the gas and dust streams on the highly resolved circumstellar disk images. She presented two models of the low velocity disc winds with the aim of investigating an origin of a low velocity outflow in the 12 CO J=1-2 molecule line (Pety et al. 2006) revealed in HH 30. Following Andlada et al. (2006) she treated HH 30 as a binary system. Two cases have been considered: i) the orbital period P = 53 yrs and ii) P < 1 yr. Calculations showed that in the first case the outflow cone had a spiral-like structure due to summing the velocities of the orbital motion and the disc wind. Such a structure contradicts the observations. In the second case, the outflow cone demonstrates symmetry relatively to the system axis and agrees well with the observations.
Mario van den Ancker summarized in the review talk “UX Orionis Stars: An Infrared Perspective” our current knowledge about the mineralogy of disks around UX Orionis stars (based on ISO and more recent Spitzer results). He presented also some recent results on molecular emission from UX Orionis stars based on ground-based data. In his second talk he gives a comparison of the distribution of gas and dust in protoplanetary disks.
Arkadi Arkharov and co-authors presented the results of the near infrared (JHK) photometry of the unusual UXor star V 1184 Tau. According to observations by Semkov (2004) the brightness of this star in the visual wavelengths decreased strongly in 2003, and the star did not return to the normal state up to now. The NIR photometry has shown that the J and H fluxes decreased strongly also in 2003. At the same time the K flux did not practically change. Such a behavior of the infrared fluxes cannot be explained by extinction event in a single star but can be accountable if V 1184 Tau is a binary system with a variable optical component and a stable infrared component.
Victor Shenavrin and his co-authors presented the results of the multi-year near infrared (JHKL) and optical observations of the UX Ori type stars. Several photometrically low-active Herbig Ae stars (AB Aur, MWC 480 and some others) have been observed for comparison. It is shown that in several UXors (CO Ori, CQ Tau etc.) the optical variability correlates well with the NIR variability up to the L band. In some cases (SV Cep and RR Tau) there were observed the photometrical events in which the optical variability anti-correlated with the NIR variability. The largest amplitudes of the NIR variability have been observed in the photometrically low-active HAE stars AB Aur and MWC 480. Thus, the activity of UXors at the NIR wavelengths demonstrates a large diversity in properties and this circumstance needs the theoretical analysis.
Vladimir Grinin and co-authors presented the results of the photometric and spectroscopic observations of the remarkable WTTS V718 Per (H187). This star is periodically undergone to long-lasting eclipses caused by variable amounts of the circumstellar dust in the line-of-sight to the star. The photometric observations at the visual wavelengths range (VRI) were made in 2004-2007. They cover fully the extended eclipse of this star and show that it was almost identical to the previous one observed by Cohen et al. (2003). The eclipse duration (3.5 yrs) and the time interval between them (4.7 yrs) show that the size of the extended dust (or gas and dust) cloud which periodically screens the star is comparable with the size of its orbit. The near-infrared (JHK) observations show that the amplitude of the eclipse decreases with increasing wavelength and the extinction law is near to the "standard" extinction law in the interstellar medium. The best fit of the observed SED to the SED of the standard star of the same spectral type gives the extinction AV = 4.7m. The comparison of the SED with the IR fluxes at 3.6, 5.8 and 8.0 μm from the Spitzer data base shows that V718 Per has a weak excess at λ ≥ 5 μm. The combination of the weak IR excess and the large AV suggests that the star is apparently surrounded by a low-mass circumstellar disk. The disk is observed almost edge-on and has a gap in the inner part. The analysis of two high resolution (R = 45000) spectra of H 187 obtained with the Keck telescope in 2005 and 2006 with help of G. Herbig did not reveal any spectroscopic signatures of the secondary companion. We could also not find any changes in the radial velocity of the star on the time interval between two successive observations (about one quarter of the period) exceeding 80 m sec-1. The latter means that the mass of the unseen companion, which is responsible for the orbital motion of the extended dust cloud, cannot be more then 6 MJup. Therefore, it is likely that V718 Per is surrounded by remnants of the protoplanetary disk and has a planetary system which was formed very recently.
Natalia Katysheva and co-authors told about colorimetric properties of UXors and cataclysmic variable stars (CVS). The comparative analysis has shown that the color-magnitude diagrams in these two groups of the objects are very different. This means that in the case of UXors the contribution of the hot gas emission (the main source of CVS variability) is negligible.
Nina Beskrovnaya and co-authors presented the poster dedicated to a variability in the Herbig A7e star HD144432 based on the new high-resolution echelle data obtained in May, 8-10 2002 at the ESO (1.52m telescope + FEROS). The night-to-night and hour-to-hour variability of the emission Balmer lines, DNaI doublet and CaII (K+H) doublet in the spectrum of the object is similar to that observed in typical Herbig Ae stars with signs of stellar wind. The P Cyg-profiles of the Hα line transformed from type II to type III during 3 nights. Short-term variations of the Hα , Hβ and CaII lines in the blue part of the residual spectra in the form of “standing waves” were observed each night. The DNaI profiles demonstrate a multi-component structure including the emission component and a number of local absorptions originating in the remote wind. The HeI 5876 line was very variable, changing its shape from the two-peaked profile to the single emission. At the end of the second observing night this line completely disappeared, but it was seen again during the next night.
Nataly Efimova and co-authors presented the results of the near-IR (JHK) observations of the two UXor stars VX Cas and V517 Cyg. The objects have shown the unusual photometrical behavior: 1) During the first two seasons the brightening of the VX Cas was accompanied with the reddening in J-H and H-K colors. The following seasons show the opposite effect: the IR colors become bluer with the J-magnitude increasing. 2) In the case of V517 Cyg the J and H magnitudes correlate well while the J and K magnitude anti-correlate. The behavior of color tracks (J-H,J) and (H-K,J) is strongly differ from the standard extinction law. The poly-semantic behavior of VX Cas does not keep within the most generally accepted self-shadow model. But for V517 Cyg this model is applicable if one takes into account that the main contribution in the J and H fluxes give the radiation of the star itself.
Alexander Rosenbush presented results of the observations of the 4th Great visual light minimum of R Coronae Borealis. This minimum is a sequence of separate minima each of which is a consequence of a formation of a spherical dust shell (Rosenbush, ASP Conf. Ser. 391, 39, 2008). The new dust shell is formed in each following 40-50-days due to pulsation of the star. Light curve of such minimum can be described by a simple analytical formula.
In conclusion Vladimir Grinin noted that the workshop participants have considered and discussed practically all important topics on the UXor's physics and related phenomena. He stressed that the dense in time monitoring of the UXors with the help of small robotic telescopes can give the unique information about the fine structure of the innermost parts of their CS discs.