[en] The transition from early Of stars to WN-type objects is poorly understood. O-type supergiants with emission lines (OIf⁺) are considered to be intermediate between these two classes. The scope of this paper is to investigate the spectral variability of three Of⁺ supergiants. We constituted spectral time series of unprecedented quality for our targets (∼200 spectra in total), essentially in the blue domain, covering timescales from a few hours up to a few years. Temporal Variance Spectrum and Fourier analyses were performed in order to characterize their spectral variability. We report on a correlated significant line profile variability in the prominent He II λ4686 and Hβ lines most likely related to the strong stellar winds. The variability pattern is similar for the three stars investigated (HD 14947, HD 15570, and HD 16691), and the main differences are more quantitative than qualitative. However, the reported timescales are somewhat different, and the most striking variability pattern is reported for HD 16691. We did not find any clear evidence for binarity, and we focus mainly on an interpretation based on a single-star scenario. We show that the behavior of the three stars investigated in this study present strong similarities, pointing to a putative common scenario, even though a few differences should be noted. Our preferred interpretation scheme is that of Large-Scale Corotating Structures modulating the profile of the lines that are produced in the strong stellar wind.

[en] On UT 2009 January 16, we observed a white light megaflare on the dM4.5e star YZ CMi as part of a long-term spectroscopic flare-monitoring campaign to constrain the spectral shape of optical flare continuum emission. Simultaneous U-band photometric and 3350-9260 A spectroscopic observations were obtained during 1.3 hr of the flare decay. The event persisted for more than 7 hr and at flare peak, the U-band flux was almost 6 mag brighter than in the quiescent state. The properties of this flare mark it as one of the most energetic and longest-lasting white light flares ever to be observed on an isolated low-mass star. We present the U-band flare energetics and a flare continuum analysis. For the first time, we show convincingly with spectra that the shape of the blue continuum from 3350 A to 4800 A can be represented as a sum of two components: a Balmer continuum as predicted by the Allred et al. radiative hydrodynamic flare models and a T∼ 10,000 K blackbody emission component as suggested by many previous studies of the broadband colors and spectral distributions of flares. The areal coverage of the Balmer continuum and blackbody emission regions vary during the flare decay, with the Balmer continuum emitting region always being significantly (∼3-16 times) larger. These data will provide critical constraints for understanding the physics underlying the mysterious blue continuum radiation in stellar flares.

[en] Complete text of publication follows. Descriptive study of the variations in the geomagnetic indices and the correlation with solar activities in history has been studied. Analytical studies of the effects of Geomagnetic storms on the earth's atmosphere. Historical studies of the last fifteen solar cycles and its effects on Earth's climate. The Earth's global temperature variation during the last two thousand years has been studied too.

[en] Michigan InfraRed Combiner-eXeter (MIRC-X) is a new highly sensitive six-telescope interferometric imager installed at the CHARA Array that provides an angular resolution equivalent of up to a 330 m diameter baseline telescope in J- and H-band wavelengths ($\frac{\mathrm{\lambda <!--\; ??\; -->}}{2B}\sim <!--\; ???\; -->0.6$ mas). We upgraded the original Michigan InfraRed Combiner (MIRC) instrument to improve sensitivity and wavelength coverage in two phases. First, a revolutionary sub-electron noise and fast-frame-rate C-RED ONE camera based on an SAPHIRA detector was installed. Second, a new-generation beam combiner was designed and commissioned to (i) maximize sensitivity, (ii) extend the wavelength coverage to J band, and (iii) enable polarization observations. A low-latency and fast-frame-rate control software enables high-efficiency observations and fringe tracking for the forthcoming instruments of the CHARA Array. Since mid-2017, MIRC-X has been offered to the community and has demonstrated best-case H-band sensitivity down to 8.2 correlated magnitude. MIRC-X uses single-mode fibers to coherently combine the light from six telescopes simultaneously with an image-plane combination scheme and delivers a visibility precision better than 1%, and closure phase precision better than 1°. MIRC-X aims at (i) imaging protoplanetary disks, (ii) detecting exoplanets with precise astrometry, and (iii) imaging stellar surfaces and starspots at an unprecedented angular resolution in the near-infrared. In this paper, we present the instrument design, installation, operation, and on-sky results, and demonstrate the imaging capability of MIRC-X on the binary system ι Peg. The purpose of this paper is to provide a solid reference for studies based on MIRC-X data and to inspire future instruments in optical interferometry.

[en] In order to observe flare stars of nearby open clusters and star associations, astronomers of many countries employ the same method. This method of photographic photometry by wide-field telescopes is only partly standardized. Differences in the various steps of the method, in the fields of view of the telescopes and in sky brightness of the observatories participating in the common work can make the statistical evaluation of the results more difficult and can lead to delusive conclusions. A critical evaluation of the problematic points of the observations and reports is given. Statistical results of flare star observations in the Pleiades field made during the past two decades are summarized. It is shown that the estimated number of flare stars in the region observed as published by scientists of the Byurakan Astrophysical Observatory is too low. A model of the flare star system of the field is presented. It is composed of seven differently populated subsystems with various mean flare frequencies. The strength of this model is shown on the basis of the photographic observations of the last decade. The total number of flare stars in the Pleiades field is estimated to be higher than 2500 and the expected number of flare-up observations in order to discover all of them is given (nearly 23000). (author)

[en] Optical and infrared emission lines from H II regions are an important diagnostic used to study galaxies, but interpretation of these lines requires significant modeling of both the internal structure and dynamical evolution of the emitting regions. Most of the models in common use today assume that H II region dynamics are dominated by the expansion of stellar wind bubbles, and have neglected the contribution of radiation pressure to the dynamics, and in some cases also to the internal structure. However, recent observations of nearby galaxies suggest that neither assumption is justified, motivating us to revisit the question of how H II region line emission depends on the physics of winds and radiation pressure. In a companion paper we construct models of single H II regions including and excluding radiation pressure and winds, and in this paper we describe a population synthesis code that uses these models to simulate galactic collections of H II regions with varying physical parameters. We show that the choice of physical parameters has significant effects on galactic emission line ratios, and that in some cases the line ratios can exceed previously claimed theoretical limits. Our results suggest that the recently reported offset in line ratio values between high-redshift star-forming galaxies and those in the local universe may be partially explained by the presence of large numbers of radiation-pressure-dominated H II regions within them.

[en] With the development of global navigation satellite system (GNSS) precise point positioning (PPP) technology, higher positioning accuracy is required in some applications. An important error source in PPP is the residual higher-order (i.e. second- and third-order) ionospheric error after the first-order ionospheric error has been removed by dual-frequency observation combinations. Generally, higher-order ionospheric errors are negligible; but at high ionospheric activities, higher-order ionospheric errors can reach a few centimeters, which must be considered in high-precision positioning. In this study, a quad-constellation PPP approach with higher-order ionospheric corrections is proposed. The temporal variations of the higher-order ionospheric errors for GPS, GLONASS, BDS and Galileo are analyzed and their effects on quad-constellation PPP are evaluated. The GNSS observations collected at different solar activities are used to analyze the effect of the higher-order ionospheric errors and the results indicate that their magnitudes can reach almost 2 cm for their first frequency signals at high solar activity. The occurrence of geomagnetic storms further increases the higher-order ionospheric errors by a few millimeters. Multi-GNSS datasets collected at low, middle and high latitude stations at different ionospheric activities are processed and the results indicate that the higher-order ionospheric delay can affect the 3D position solutions of the quad-constellation PPP at a maximum of 6 mm. (paper)

[en] An analytical model is developed for the formation of comets from grain collisions in cold, dense shells of material swept by a protostellar wind. The wind moves into the circumstellar envelope and generates a two-shock flow pattern, forming a rapidly cooling thin, dense outwardly moving shell which within 1000 yr attains a density of 10-100 trillion/cu m in motions sufficiently turbulent as to produce many collisions. The growth rate of the subsequently formed grains is estimated. It is shown that a velocity dispersion under 0.01 m/sec for large grains would lead to the formation of kilometer sized bodies, i.e., prospective comet nuclei. The process would be ubiquitous around young stars. 54 references

[en] Observations of bipolar outflows, including jets often with clumpy concentrations of matter, have been made for a wide variety of astronomical systems. In most but not all of the systems, an accretion disk is present. It is proposed that the general process responsible for bipolar ejection involves the conversion of rotational energy into magnetic energy, usually in the form of a polar magnetic torus, deep in the interiors of the systems involved. If the buoyancy of the torus results in draining the field lines of most of the matter which they thread, then the acceleration of the remaining matter in the toroidal bubble may produce velocities in excess of the escape velocity from the surface of the system. It is contemplated that this process will be repeated many times in most systems. A discussion is given of the application of these ideas to protostars, to stars evolved beyond the main sequence, to neutron stars, and to black holes on both stellar and galactic scales

[en] Synchronous three-telescope UBV photographic colorimetry of Pleiades and Orion stellar flares obtained at Biurakan Astrophysical Observatory and Abastumani Astrophysical Observatory during 86 observing hours in 1980 and 1981 is presented. The data are compiled in tables and discussed in terms of color differences appearing at different stages of a flare. A total of 32 flares are observed (25 in the Pleiades and 7 in Orion), and four new flare stars are identified in each region. 12 references