Finding the most variable stars in the Orion Belt with the All Sky Automated Survey

We look for high‐amplitude variable young stars in the open clusters and associations of the Orion Belt. We use public data from the ASAS‐3 Photometric V ‐band Catalogue of the All Sky Automated Survey, infrared photometry from the 2MASS and IRAS catalogues, proper motions, and the Aladin sky atlas to obtain a list of the most variable stars in a survey area of side 5° centred on the bright star Alnilam (ε Ori) in the centre of the Orion Belt. We identify 32 highly variable stars, of which 16 had not been reported to vary before. They are mostly variable young stars and candidates (16) and background giants (8), but there are also field cataclysmic variables, contact binaries, and eclipsing binary candidates. Of the young stars, which typically are active Herbig Ae/Be and T Tauri stars with Hα emission and infrared flux excess, we discover four new variables and confirm the variability status of another two. Some of them belong to the well known σ Orionis cluster. Besides, six of the eight giants are new variables, and three are new periodic variables (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Gamma-ray bursts: optical afterglows in the deep Newtonian phase

Gamma-ray burst remnants become trans-relativistic typically in days to tens of days, and they enter the deep Newtonian phase in tens of days to months, during which the majority of shock-accelerated electrons will no longer be highly relativistic. However, a small portion of electrons are still accelerated to ultra-relativistic speeds and are capable of emitting synchrotron radiation. The distribution function for electrons is re-derived here so that synchrotron emission from these relativistic electrons can be calculated. Based on the revised model, optical afterglows from both isotropic fireballs and highly collimated jets are studied numerically, and compared to analytical results. In the beamed cases, it is found that, in addition to the steepening due to the edge effect and the lateral expansion effect, the light curves are universally characterized by a flattening during the deep Newtonian phase.     

Distribution of early-type stars and dusty matter in the direction of the stellar cluster NGC 1893

The distribution of 255 O-B9-A2, K-G stars and interstellar dust in the direction of the stellar cluster NGC 1893 is studied using V, B-V, and U-B photometric data. Sixteen groups of stars (associations) are discovered at various distances. The first group includes 9 stars of different spectral classes of later types in the sun’s neighborhood lying at a distance of 110 pc. The next 3 groups, at distances of 420, 890, and 14300 pc, are type B associations and the remaining twelve groups are OB associations. They are designated as Aur 0.11, Aur B 0.43, Aur B 0.89, Aur OB 1.4, Aur OB 2.6, AurOB 3.8, Aur OB 4.6, Aur OB 5.4, Aur OB 6.1, Aur OB 7.4, Aur OB 9.3, Aur OB11.6, Aur OB14.3, Aur OB 17.9, Aur OB 25.9, and Aur OB 31.3. For most of these stars the absorption lies within the range from 0m.45 to 5m.41. Such high absorption may be caused by circumstellar absorption as well as by the diffuse nebula IC 410. The dusty matter is distributed nonuniformly in the Aur 0.11, Aur B 0.43, and Aur B 0.89 associations. There is no dust in the space between the associations. There is essentially no dust within the groups (associations) at distances greater than 0.9 kpc. (See Table 2.) __________ Translated from Astrofizika, Vol. 50, No. 2, pp. 243–251 (May 2007).     

Signature of differential rotation in solar disk‐integrated chromospheric line emission

UARS SOLSTICE data have been subjected to Fourier and wavelet analyses in order to search for the signature of the solar rotation law in the disk‐integrated irradiance of UV lines. Lyman‐α, Mg II, and Ca II data show a different behaviour. In the SOLSTICE data there are significant temporal variations of the rotation rate of the UV tracers over 5—6 years. Often several distinct rotation periods appear almost simultaneously. Beside the basic period around 27 days there are signals at 32—35 days corresponding to the rotation rate at very high latitudes. For more than 5 years during another period of the solar cycle the rotational behaviour is quite different; there is an indication of differential rotation of active regions in these Ca II ground‐based data. The data contain a wealth of information about the solar differential rotation, but it proves difficult to disentangle the effects of the different emitting sources.     

Evolution of the symbiotic star AS 338 after its strong outburst in 1983

The photometric UBV observations of AS 338 that we began after its outburst in 1983 are presented. They were accompanied by yearly spectroscopic observations and by occasional estimations of the star’s infrared JHKL magnitudes. In June 1993, the star’s optical spectrum was extended to the ultraviolet via IUE observations of AS 338. Collectively, the above observations make it possible to trace the evolution of stellar activity over a period of 15 years in various spectral ranges. In particular, a short-time return of the hot component of AS 338 to the state when He II lines reappeared in the star’s spectrum was noted in 1993. At this time, a blend of the C IV λλ5802 and 5812 lines, which is typical of Wolf-Rayet spectra, was detected in it. In June 1993, the temperature of the hot component was T _h ≈ 8.8 × 10⁴ K, and the ratio of its bolometric flux to that of the red giant was F _{h, bol}/F _{g, bol} ≈ 1.0. In August, its temperature increased to ～1.0×10⁵ K, while the bolometric flux dropped by a factor of ～1.5(F _{h, bol}/F _{g, bol} ≈ 0.7). In the B-V, U diagram, the points referring to this so-called quiescent state form a separate group shifted in B-V from all the remaining ones located in a horizontal strip with $\Delta U \approx 3\mathop .\limits^m 5$ and $\Delta (B - V) \approx 0\mathop .\limits^m 4$ . This allows us to diagnose the state of the hot component without spectroscopic observations of the star. In October 1993, the hot component flared up again. The main brightness rise took no more than 19 days. The outburst occurred shortly before eclipse egress of the hot component, whose duration was ～0.01P _orb. In December 1993, F _{h, bol}/F _{g, bol}≤1.5 at maximum light. During the recurrent, even stronger outburst in April 1995, F _{h, bol}/F _{g, bol}≤3.4. The Hαline during outbursts has a P Cyg profile and broad wings stretching to velocities of ±1500 km s^?1. The color temperature of the active hot component at short optical wavelengths and in the ultraviolet lies in the range of effective temperatures for hot supergiants. Nevertheless, it always produces an H II region in the circumstellar envelope that is larger in size than this binary system.     

Numerical simulations of protostellar encounters — II. Coplanar disc–disc encounters

It is expected that an average protostar will undergo at least one impulsive interaction with a neighbouring protostar whilst a large fraction of its mass is still in a massive, extended disc. Such interactions must have a significant impact upon the evolution of the protostars and their discs.   We have carried out a series of simulations of coplanar encounters between two stars, each possessing a massive circumstellar disc, using an SPH code that models gravitational, hydrodynamic and viscous forces. We find that during a coplanar encounter, disc material is swept up into a shock layer between the two interacting stars, and the layer then fragments to produce new protostellar condensations. The truncated remains of the discs may subsequently fragment; and the outer regions of the discs may be thrown off to form circumbinary disc-like structures around the stars. Thus coplanar disc–disc encounters lead efficiently to the formation of multiple star systems and small- N clusters, including substellar objects.