Spectral line formation in a mesoturbulent atmosphere

The formation of spectral lines in a turbulent atmosphere with a spatially correlated velocity field is examined. A new approach for solving this problem is proposed which is not based on the Fokker-Planck formalism. The invariant imbedding method makes it possible to reduce the problem of finding the mean radiant intensity observed in a line to solving a system of differential equations. This possibility is based on determining the mean intensity of the radiation emerging from the medium for a fixed value of the turbulent velocity at its boundary. A separate integral equation is derived for this quantity. The dependence of the line profile, integrated intensity, and width on the mean correlation length and the average value of the hydrodynamic velocity is studied. It is shown that the transition from a microturbulent regime to a macroturbulent regime occurs within a comparatively narrow range of variation in the correlation length. The proposed method yields a solution to the problem for a family of inhomogeneous atmospheres with different optical thicknesses, which makes it easy to determine the radiation field inside the turbulent medium. This approach can be generalized in various ways, in particular, it can be applied without significant changes to the case where the correlation length depends on position within the atmosphere. __________ Translated from Astrofizika, Vol. 50, No. 2, pp. 219–232 (May 2007).     

The orbit of the double-line spectroscopic binary system HR 7112

We present velocity observations, obtained with the E. W. Fick Observatory 0.6 m telescope, of the 7th magnitudeK1 star HR 7112. Our observations show that HR 7112 is a double-line spectroscopic binary system composed of almost equal mass stars with a period of 215.6 days. The orbit has an eccentricitye = 0.142 and the stars orbit each other with a mean semi-major axis distance of approximately 0.4sin(i) AU. The masses of the stars are consistent with the classification as giant stars. Tidal effects are known to circularise the orbits of giant stars and this process is apparently underway in this system.     

Long-term flux variations in Cen X-3: clues from flux-dependent orbital modulation and pulsed fraction

We have investigated the long-term flux variation in Cen X-3 using orbital modulation and pulsed fraction in different flux states using observations made with the All-Sky Monitor and the Proportional Counter Array on board the Rossi X-ray Timing Explorer . In the high state, the eclipse ingress and egress are found to be sharp whereas in the intermediate state the transitions are more gradual. In the low state, instead of eclipse ingress and egress, the light curve shows a smooth flux variation with orbital phase. The orbital modulation of the X-ray light curve in the low state shows that the X-ray emission observed in this state is from an extended object. The flux-dependent orbital modulations indicate that the different flux states of Cen X-3 are primarily due to varying degree of obscuration. Measurement of the pulsed fraction in different flux states is consistent with the X-ray emission of Cen X-3 having one highly varying component with a constant pulsed fraction and an unpulsed component and in the low state, the unpulsed component becomes dominant. The observed X-ray emission in the low state is likely to be due to scattering of X-rays from the stellar wind of the companion star. Though we cannot ascertain the origin and nature of the obscuring material that causes the aperiodic long-term flux variation, we point out that a precessing accretion disc driven by radiative forces is a distinct possibility.     

Radial velocity measurements from LAMOST medium-resolution spectroscopic observations: a pointing towards the Kepler field

Radial velocity is one of the key measurements in understanding the fundamental properties of stars, stellar clusters and the Galaxy. A plate of stars in the Kepler field was observed in May of 2018 with the medium-resolution spectrographs of LAMOST, aiming to test the performance of this new system which is the upgraded equipment of LAMOST after the first five-year regular survey. We present our analysis on the radial velocity measurements(RVs) derived from these data. The results show that slight and significant systematic errors exist among the RVs obtained from the spectra collected by different spectrographs and exposures, respectively. After correcting the systematic errors with different techniques, the precision of RVs reaches ～1.3,～1.0,～0.5 and ～0.3 km s^-1 at S/Nr = 10, 20, 50 and 100, respectively. Comparing with the RVs of standard stars from the APOGEE survey, our RVs are calibrated with a zero-point shift of～7 km s^-1. The results indicate that the LAMOST medium-resolution spectroscopic system may provide RVs with a reasonable accuracy and precision for the selected targets.     

Radial velocities of population II binary stars. I.

Radial velocities for 114 Hipparcos stars, mostly high transverse velocity binaries without previous radial velocity measurements, are published. Measurements are made with the CORAVEL‐type radial velocity spectrometer. The accuracy of measurements is better than 1 km/s. Two stars, BD +30° 2129A and HD 117466AB are found to be radial velocity variables, and three more stars, HD 119515A, HD 131597AB, and HD 153344, are possible radial velocity variables.     

A note on some unusual velocity values of the Be star o And

Die Radialgeschwindigkeiten für die Ca II (K)-, Na (D)- und Balmer-Linien wurden für den Be Stern o And bestimmt. Die zeitlichen Variationen der Geschwindigkeiten, besonders die der Ca II (K)-Linie, können durch die Existenz eines Begleiters erklärt werden.     

On variations in the fine-structure constant and stellar pollution of quasar absorption systems

At redshifts   z _abs≲ 2  , quasar absorption-line constraints on space–time variations in the fine-structure constant, α, rely on the comparison of Mg  ii and Fe  ii transition wavelengths. One potentially important uncertainty is the relative abundance of Mg isotopes in the absorbers, which, if different from solar, can cause spurious shifts in the measured wavelengths and, therefore, α. Here we explore chemical evolution models with enhanced populations of intermediate-mass (IM) stars, which, in their asymptotic giant branch phase, are thought to be the dominant factories for heavy Mg isotopes at the low metallicities typical of quasar absorption systems. By design, these models partially explain recent Keck/HIRES evidence for a smaller α in   z _abs < 2  absorption clouds than on Earth. However, such models also overproduce N, violating observed abundance trends in high- z _abs damped Lyman-α (DLA) systems. Our results do not support the recent claim of Ashenfelter et al. that similar models of IM-enhanced initial mass functions (IMFs) may simultaneously explain the HIRES varying-α data and DLA N abundances. We explore the effect of the IM-enhanced model on Si, Al and P abundances, finding it to be much less pronounced than for N. We also show that the ¹³C/¹²C ratio, as measured in absorption systems, could constitute a future diagnostic of non-standard models of the high-redshift IMF.     

Spectral line profile variations in the γ Doradus variable HD 164615: non-radial pulsations versus star-spots

High-resolution spectral data of the Fe  II 5318 Å line in the γ Doradus star HD 164615 are presented. These show systematic changes in the spectral lineshapes with the photometric period of 0.8133 d which are modelled using either non-radial pulsations or cool star-spots. The non-radial modes that can fit the lineshape changes have m degree of 2–4. However, only the m = 2 mode seems to be consistent with the amplitude of the radial velocity variations measured for this star. The star-spot model, although it can qualitatively fit the lineshape changes, is excluded as a possible hypothesis on the basis of (1) poorer fits to the observed spectral line profiles, (2) an inability to account for the large changes in the spectral linewidth as a function of phase, (3) a predicted radial velocity curve that looks qualitatively different from the observed one, and (4) a predicted photometric curve that is a factor of 5 larger than the observed light curve (and with the wrong qualitative shape). Finally, a 'Doppler image' (assuming cool spots) derived from a sequence of synthetic line profiles having non-radial pulsations results in an image that is almost identical to the Doppler image derived for HD 164615. These results provide strong evidence that non-radial pulsations are indeed the explanation for the variability of HD 164615 as well as the other γ Dor variables.     

Search for Variability in the Equivalent Width of the HeI D<Subscript>3</Subscript> Spectrum Line in Several Stars of Later Spectral Type

High resolution spectra with a high signal/noise ratio have been obtained in the region of the HeI D line for 13 dwarfs of spectral types A5 through K0. The variability in the equivalent width of this spectrum line was studied for five of these stars over a period of several hundred days. Significant variability was observed for only one of these stars, θ Cyg. __________ Translated from Astrofizika, Vol. 48, No. 4, pp. 553–564 (November 2005).     

Spectral analysis of water vapour in cool stars

M-star spectra, at wavelengths beyond 1.35 μm, are dominated by water vapour, yet terrestrial water vapour makes it notoriously difficult to obtain accurate measurement from ground-based observations. We have used the short-wavelength spectrometer on the Infrared Space Observatory at four wavelength settings to cover the  2.5–3.0 μm  region for a range of M stars. The observations show a good match with previous ground-based observations and with synthetic spectra based on the Partridge & Schwenke line list, although not with the SCAN line list. We have used a least-squared minimization technique to systematically find best-fitting parameters for the sample of stars. The temperatures that we find indicate a relatively hot temperature scale for M dwarfs. We consider that this could be a consequence of problems with the Partridge & Schwenke line list which leads to synthetic spectra predicting water bands that are too strong for a given temperature. Such problems need to be solved in the next generation of water vapour line lists, which will extend the calculation of water vapour to higher energy levels with the good convergence necessary for reliable modelling of hot water vapour. Then water bands can assume their natural role as the primary tool for the spectroscopic analysis of M stars.     

Large-amplitude periodic variations in the angular diameter of R Leonis

We report the first direct detection of long-term periodic diameter variations in a Mira variable. Angular diameter measurements of the 313-d period variable R Leonis at 833 nm and 940 nm obtained between 1996 February and 1997 June using the Cambridge Optical Aperture Synthesis Telescope (COAST) and the William Herschel Telescope (WHT) show a cyclic modulation of the apparent stellar diameter by approximately 35 per cent. The agreement between these new data and archival measurements from 1992 January suggests coherence in the modulation over a 5-yr period. Our data are consistent with recent models which suggest that, in photometric bands with only weak to moderate molecular contamination, periodic variations in stellar diameter of order 50 per cent can be maintained. The measurements indicate that the apparent stellar diameter was largest at visual phase 0.5 and that any phase shifts between the visual light curve and those at 833 and 940 nm were at most 0.05. The large offset (∼ 0.25) between the phase of the observed diameter maximum and that predicted for the photospheric continuum diameter variations suggests that our observations are more sensitive to the changing temperature structure of the outer atmosphere than to the deeper continuum-forming layers.     

Measurements of the Ca II infrared triplet lines of young stellar objects

Equivalent widths and line widths of Ca II infrared triplet emission lines were measured in the high-resolution optical spectra of 39 young stellar objects.We found that the equivalent widths of the emission lines decrease with stellar evolution.It has often been claimed that strong chromospheric activity is generated by a dynamo process caused by fast rotation of the photosphere.However,we found no clear correlation between the strength of the Ca II lines and the stellar rotation velocity.Instead,we found that the objects with high mass accretion rates had stronger Ca II emission lines.This correlation supports the turbulent chromosphere model or the magnetic accretion theory for classical T Tauri stars.We also noticed that the equivalent widths of Ca II lines in transitional disk objects are one-tenth of those in classical T Tauri stars,even if the masses of the circumstellar disks are comparable.     

Cyclic variations in the angular diameter of χ Cygni

We report the direct detection of cyclic diameter variations in the Mira variable χ Cygni. Interferometric observations made between 1997 July and 1999 September, using the Cambridge Optical Aperture Synthesis Telescope (COAST) and the William Herschel Telescope (WHT), indicate periodic changes in the apparent angular diameter at a wavelength of 905 nm, with amplitude 45 per cent of the smallest value. The star appears largest at minimum light. Measurements made at a wavelength of 1.3 μm over the same period suggest much smaller size changes. This behaviour is consistent with a model in which most of the apparent diameter variation at 905 nm is caused by a large increase in the opacity of the outer atmospheric layers (which is mostly owing to titanium oxide) near minimum light, rather than by physical motions of the photosphere. The 1.3-μm waveband is relatively uncontaminated by TiO, and so much smaller size changes would be expected in this band. The latest non-linear pulsational models predict maximum physical size close to maximum light, and increases in opacity near minimum light that are too small to reproduce the diameter variation seen at 905 nm. This suggests either that the phase-dependence of the model pulsation is incorrect, or that the opacities in the models are underestimated. Future interferometric monitoring in uncontaminated near-infrared wavebands should resolve this question.     

Spectral Observations of the Novalike Star TT Ari

Spectral observations of the novalike star TT Ari at the Byurakan Observatory in September 2001 are reported. In most cases these results are in good agreement with earlier observational data. Some interesting differences from the earlier results were obtained, in particular the absence of emission and the presence of very weak absorption in the H line. Another interesting difference that was recorded is the absence of an anti-P Cygni H line profile along with rapid variations in the P Cygni profile during our observations.     

Long-Term Spectral Variability of the Herbig Ae-Star HD 179218

The results of high-resolution long-term spectral monitoring of Herbig Ae star HD 179218 in the region of emission H line and the sodium resonance doublet Na I D are presented. The received data show the existence of short-term variability (about 10 days) and long-term wave-like variability of equivalent width EW, intensity of circumstellar (CS) emission I and parameters of H emission profile. The analysis of these data allows us to suppose the existence of the global variability of parameters of accretion disk.     

Spectropolarimetry of hot,luminous stars

I review polarimetric observations of presumably single, hot, luminous stars. The stellar types discussed are OB stars, B[e] supergiants, Luminous Blue Variables (LBV), Wolf-Rayet (W-R) stars, and type II supernovae (SN). It is shown that variable, intrinsic polarization is a common phenomenon in that part of the Hertzsprung-Russell (HR) diagram which these stars occupy. However, much observational work remains to be done before we can answer the most basic, statistical questions about the polarimetric properties of different groups of hot, luminous stars. Insight into the diagnostic power of polarization observations has been gained, but cannot be exploited without detailed models. Thus, while polarimetric observations do tell us that the mass-loss processes of all types of massive stars are time-dependent and anisotropic, the significance that this might have for the accuracy of their stellar parameters and evolutionary paths remains elusive.