New faint optical spectrophotometric standards: hot white dwarfs from the Sloan Digital Sky Survey

The spectral energy distributions for pure-hydrogen (DA) hot white dwarfs can be accurately predicted by model atmospheres. This makes it possible to define spectrophotometric calibrators by scaling the theoretical spectral shapes with broad-band photometric observations – a strategy successfully exploited for the spectrographs onboard the Hubble Space Telescope ( HST ) using three primary DA standards. Absolute fluxes for non-DA secondary standards, introduced to increase the density of calibrators in the sky, need to be referred to the primary standards, but a far better solution would be to employ a network of DA stars scattered throughout the sky. We search for blue objects in the sixth data release of the Sloan Digital Sky Survey (SDSS) and fit DA model fluxes to identify suitable candidates. Reddening needs to be considered in the analysis of many of these stars. We propose a list of nine pure-hydrogen white dwarfs with absolute fluxes with estimated uncertainties below 3 per cent, including four objects with estimated errors <2 per cent, as candidates for spectrophotometric standards in the range  14 < g < 18  , and provide model-based fluxes scaled to match the SDSS broad-band fluxes for each. We apply the same method to the three HST DA standards, linking the zero point of their absolute fluxes to ugr magnitudes transformed from photometry obtained with the US Naval Observatory 1-m telescope. For these stars, we estimate uncertainties of <1 per cent in the optical, finding good consistency with the fluxes adopted for HST calibration.     

Molecules in white dwarfs

Molecular dissociation equilibrium calculations were done for the model atmospheres of DA and non-DA white dwarfs. Our calculations show that He ₂ ⁺ and HeH⁺ appear as most abundant molecules in the atmospheres of non-DA white dwarfs while H₂ and H ₂ ⁺ are most abundant molecules in DA white dwarfs. It is suggested that these molecules should be searched for in the atmospheres of white dwarfs.     

白矮星研究的某些进展

综合叙述了白矮星诞生率研究的近况，详细介绍了ＤＡ，非ＤＡ型白矮星质量，质量分布以及确定质量的方法，对白矮星光度函数作了较为全面的回顾，指出了一些目前白矮星研究工作中仍存在的问题。     

O VI in the local interstellar medium

We report preliminary results of a search for O VI absorption in the spectra of ～100 hot DA white dwarfs observed by the FUSE satellite. We have carried out a detailed analysis of the radial velocities of interstellar and (where present) stellar absorption lines for the entire sample of stars. In many cases, the velocity differences between the interstellar and photospheric components are below the resolution of the FUSE spectrographs. However, in a significant number of cases the interstellar and photospheric contributions can be separated. In the majority of stars where we find O VI absorption lines, the material is clearly associated with the stellar photosphere and not the interstellar medium. There are a small number of lines-of-sight where the gas is interstellar in nature but the stars are located beyond the boundaries of the local cavity.     

Relation of Velocity Distribution and Mass for DA White Dwarfstwo

White dwarfs are the evolutionary endpoint of the low-and-medium mass stars. In the studies of white dwarfs, the mass of white dwarf is an important physical parameter. In this paper, we give an analysis about the velocity distribution of DA white dwarfs in the Sloan Digital Sky Survey (SDSS), and hope to find the relation between mass and velocity distribution of white dwarfs. We get the radial velocity and tangential velocity of every DA white dwarf according to their proper motion and spectral shift. Through analyzing the velocity distribution of DA white dwarfs, we find that the small-mass white dwarfs, which are produced from the single-star evolution channel, have a relatively large velocity dispersion.     

OH masers and magnetic fields near the cometary H ii region G34.3+0.2

The mechanism responsible for exciting high-order acoustic oscillations in rapidly oscillating Ap stars is still an open issue. Recently, Balmforth et al. (hereafter BCDGV) proposed a model according to which high-frequency oscillations may be excited in roAp stars if the intensities of the magnetic fields present in these stars are sufficiently large to suppress convection at least in some region of their envelopes. Using models similar to those proposed by BCDGV, we predict the theoretical edges of the instability strip appropriate to roAp stars and compare them with the observations. Moreover, we discuss our results in the light of some of the systematic differences found between roAp stars, noAp stars and Ap stars in general. We suggest that a combination of intrinsic differences between these types of stars and a bias related to the frequencies of the unstable oscillations might hold the explanation to some of the differences observed.     

The effect of photospheric heavy elements on the hot DA white dwarf temperature scale

Using the latest non-local thermodynamic equilibrium (non-LTE) synthetic spectra and stellar model calculations, we have evaluated the potential effect of the presence of heavy elements in the photospheres of hot H-rich DA white dwarfs. In particular, we have examined their influence on the effective temperature and surface gravity perceived from analysis of the Balmer line profiles. It is apparent that both the inclusion of non-LTE effects in the models and significant quantities of heavy elements act independently to lower the value of T _eff determined from a particular spectrum. Hence, the true effective temperatures of the heavy element-rich DA white dwarfs, currently estimated to be above 55 000 K, are apparently lower than previously reported from pure-H LTE analyses, by some 4000–7000 K. We do not see any similar influence on measurements of log g . This work concentrates on a group of relatively bright well-studied objects, for which heavy element abundances are known. As a consequence of this, establishment of correct temperatures for all other hot white dwarfs will require a programme of far-UV spectroscopy in order to obtain the essential compositional information. Since only stars with effective temperatures lying notionally in the range from ≈ 55 000 to 70 000 K (52 000–62 000 K when the non-LTE effects and heavy elements are taken into account) have been considered here, important questions remain regarding the magnitude of any similar effects in even hotter white dwarfs and pre-white dwarfs. The resulting implications for the plausibility of the evolutionary link between the main hot DA population and their proposed precursors, the H-rich central stars of planetary nebulae, need to be investigated.     

Investigation of the motions of fast stars based on observations with the Pulkovo normal astrograph

Astrometric CCD observations of 1123 stars with large proper motions (μ > 300 mas yr⁻¹) from the LSPM (I/298) catalog in the declination zone +30°–+70° have been carried out with the Pulkovo normal astrograph since 2006. The observational program includes mostly stars that previously have not entered into high-accuracy projects to determine the proper motions. Our studies are aimed at determining new proper motions of fast stars in the HCRF/UCAC3 system and searching for stars with invisible companions in the immediate Galactic neighborhoods of the Sun. Having analyzed about 10 000 CCD frames, we have obtained the equatorial coordinates of 414 program stars in the HCRF/UCAC3 system at an accuracy level of 10–50 mas and determined their new proper motions. To derive the proper motions, we have used the data from several star catalogs and surveys (M2000, CMC14, 2MASS, SDSS) as early epochs. The epoch differences range from 5 to 13 years (on average, about 10 years); the mean accuracy of the derived proper motions is 4–5 mas yr⁻¹. For 70 stars, we have revealed significant differences between the derived proper motions and those from the LSPM and I/306A catalogs (these proper motions characterize the mean motion of the photocenter in 50 years or more). Apart from systematic errors, these differences can result from the existence of invisible components of the program stars.     

A new photometric study of T Tauri stars in the infrared

We have compiled infrared photometric data from the literature of practically all T Tauri stars found up to date including 444 classical T Tauri stars (CTTSs), 1698 weak-line T Tauri stars (WTTSs) and 1258 not classified T Tauri stars (3400 in total) in addition to 196 post-T Tauri stars (PTTSs). From this data bank we extract the infrared characteristics of the different groups and discuss different origins of the infrared radiation. The observational data are taken from the AKARI, IRAS, WISE and 2MASS missions. We show that in the wavelength range 1–140 μm, all T Tauri stars have infrared excesses. CTTSs have more infrared excess than WTTSs, while PTTSs have little or no infrared excess. We found that in the 1–3 μm wavelength range the infrared emission of T Tauri stars is mainly due to thermal radiation from the photosphere and hot dust grains from circumstellar envelopes. In the 3–140 μm wavelength range the infrared emission of T Tauri stars is mainly due to radiation from dusty/gaseous disks surrounding the stars. In addition, we also make a comparison between T Tauri stars and Herbig AeBe stars (HAeBe). There are some differences between these two kinds of objects in that for HAeBe stars the infrared radiation as a rule originates in dusty/gaseous disks in the 1–3 μm wavelength range, while in the range 3–12 μm it is possibly due to PAH emission for about half of HAeBe stars. In other wavelength ranges both kinds of stars have similar infrared characteristics indicating emission from dusty/gaseous disks.     

Chemically peculiar stars in the orion OB1 association. I. occurrence frequency, spatial distribution, and kinematics

A total of 85 CP stars of various types are identified among 814 members of the OriOB1 association. The fraction of CP stars decreases with age for different cluster subgroups: from 15.1% in the youngest subgroup (b) to 7.7% in the oldest one (a). Individual comments are provided for each of the 85 stars, where we analyze the physical parameters and distance of the objects. All the 23 Am stars identified as a result of this study are found to have heliocentric distances between 100 and 300 pc and appear not to be members of the OrionOB1 association, but foreground objects. We identified 59 Bp stars, which account for 13.4% of the total number of B-type stars in the association. The fraction of peculiar B-type stars in the OriOB1 association is found to be twice higher than that of peculiar A-type stars. The same is true for field stars. The association contains 22 magnetic stars, out of which 21 are Bp stars and only one is an Ap star. Seventeen of these stars are objects with anomalous helium lines. Magnetic stars show a well-defined tendency to concentrate in the central region of the association (in Orion’s Belt), which contains most of these objects. No significant differences are found between the field strengths in the B-type stars of the association and Bp-type field stars, although there is a noticeable trend for He-rich stars to have stronger fields compared to He-weak stars. We identified 17 binaries, which make up 20% of the total number of peculiar stars studied, which is the standard ratio for CP stars. Except for one HgMn star (HD35548), the radial velocities and proper motions of our identified objects are consistent with the corresponding parameters of normal B-type stars.     

The Coma Morphology Due to an Extended Active Region and the Implications for the Spin State of Comet Hale-Bopp

We have compared the kinematics and metallicity of the main-sequence binary and single uvby F stars from the Hipparcos catalog to see if the populations of these stars originate from the same statistical ensemble. The velocity dispersions of the known unresolved binary F stars have been found to be dramatically smaller than those of the single F stars. This suggests that the population of these binaries is, in fact, younger than that of the single stars, which is further supported by the difference in metal abundance: the binaries turn out to be, on average, more metal rich than the single stars. So, we conclude that the population of these binaries is indeed younger than that of the single F stars. Comparison of the single F stars with the C binaries (binary candidates identified in Suchkov & McMaster) has shown, on the other hand, that the latter stars are, on average, older than the single F stars. We suggest that the age difference between the single F stars, known unresolved binaries, and C binaries is associated with the fact that stellar evolution in a binary systems depends on the binary components' mass ratio and separation, with these parameters being statistically very different for the known binaries and C binaries (e.g., mostly substellar secondaries in C binaries vs. stellar secondaries in known binaries). In general we conclude that the populations of known binaries, C binaries, and single F stars do not belong to the same statistical ensemble. The implications of the discovered age difference between these populations along with the corresponding differences in kinematics and metallicity should be important not only for understanding the evolution of stars but also for the history of star formation and the evolution of the local Galactic disk.     

Stark Broadening in Compact Stars: Xe VI Lines

We will consider Stark broadening of non hydrogenic spectral lines in the impact approximation in compact stars: pre-white dwarf and white dwarf atmospheres. In order to show an example, Stark broadening parameters have been calculated, using the impact semiclassical perturbation approach for four Xe VI spectral lines. Obtained results have been used to demonstrate the influence of Stark broadening in DA and DB white dwarf atmospheres.     

FUSE observations of PG1342+444: new insights into the nature of the hottest DA white dwarfs

We present Far Ultraviolet Spectroscopic Explorer ( FUSE ) observations of the very hot  (T_eff≈60 000 K)  DA white dwarf PG1342+444, describing our data reduction and analysis techniques. The spectrum reveals a number of photospheric absorption lines from high ionization species along with numerous interstellar features. The photospheric detections include the 1031.9- and 1037.0-Å O  vi lines which are seen for the first time in a hot DA atmosphere and are usually associated with the much hotter PG1159 stars and so-called O  vi  central stars of planetary nebulae. Estimates of the stellar effective temperature made independently using both the Balmer and Lyman series lines are in disagreement  (T_eff≈67 000  and ≈54 000 K respectively), when taking into account just the statistical uncertainties in the analyses. However, the presence of weak absorption from the C  iii multiplet near 1176 Å, which is predicted to be much stronger if the star were as cool as the Lyman measurement suggests, leads us to favour the higher temperature. PG1342+444 appears to have enhanced C, Fe and Ni abundances in its atmosphere compared with all the other G191−B2B-like DA white dwarfs, which might affect the temperature structure of the atmosphere if not homogeneously distributed, as assumed in this study.     

Chromospheric changes in K stars with activity

We study the differences in chromospheric structure induced in K stars by stellar activity, to expand our previous work for G stars, including the Sun as a star. We selected six stars of spectral type K with  0.82 < B − V < 0.90  , including the widely studied Epsilon Eridani and a variety of magnetic activity levels. We computed chromospheric models for the stars in the sample, in most cases in two different moments of activity. The models were constructed to obtain the best possible match with the Ca  ii K and the Hβ observed profiles. We also computed in detail the net radiative losses for each model to constrain the heating mechanism that can maintain the structure in the atmosphere. We find a strong correlation between these losses and   S _Ca II  , the index generally used as a proxy for activity, as we found for G stars.     

Magnetic B-type stars of the main sequence. I. Problem formulation and selection of objects for observations

This paper reviews the current state of the problem of magnetism in massive Main Sequence stars. Chemically peculiar Bp stars with enhanced silicon lines and anomalous helium lines in their spectra are shown to be the most promising targets for the observational verification of various mechanisms of the formation and subsequent evolution of magnetic fields in CP stars. A catalog of magnetic Bp stars, containing 125 objects is prepared. Applying different criteria, we compiled a variety of magnetic star samples, which were then used to analyze magnetic fields in objects of different ages. The results of this analysis show that massive stars generally have stronger fields in all the samples studied, and thus confirm earlier results based on smaller star samples. No tight relation is observed and the parameters of individual objects show a very large scatter about the mean relation. The strongest and most complex fields are found in the youngest Bp stars with ages below 30 Myr. Magnetic Bp stars generally rotate slower than normal B-type stars, except for the hottest objects with enhanced helium lines, which have normal rotation velocities. No systematic differences are found between the angular rotation velocities of Bp stars with anomalous helium and silicon lines. We discuss various criteria, which can be used to observationally test the alternative mechanisms of formation and evolution of magnetic fields in CP stars and, in particular, to quantitatively compare not only the magnitudes, but also the topology of fields in objects of different ages.     

Spectral observations of blue stellar objects from FBS in the zone δ = +43°

Results of spectral observations of blue stellar objects from the FBS are presented for the purpose of classifying them, discovering new interesting objects, and studying the FBS sample as a whole. 99 FBS objects in the band with central declination δ = +43° were observed on the 2.6-m telescope at the Byurakan Observatory in 1987–1991 and have been digitized using a professional scanner and processed by MIDAS in a way similar to CCD spectra. 12 objects were also observed with the BAO-2.6 and OHP-1.93 telescopes using modern techniques during 1997–2000; some were repeated observations for confirming or correcting the classifications. A new planetary nebula, 7 white dwarfs, 78 hot subdwarfs, 9 HBB stars, and 6 stars in classes F-G were discovered. Proper motions in the range 57–84 mas/year were obtained for three stars (DA, DAB, and sdB). Three stars (a white dwarf and two subdwarfs) are identified with x-ray sources. The spectra of the 30 most interesting objects are given. __________ Translated from Astrofizika, Vol. 49, No. 3, pp. 391–405 (August 2006).     

Chromospheric activity properties and search for subdwarfs and extreme subdwarfs based on LAMOST stellar spectral survey

Big data obtained from a stellar spectroscopic survey carried out using the Large Sky Area Multi-object Fiber Spectroscopic Telescope (LAMOST, also known as Guo Shou Jing telescope) provide important information for studying chromospheric activity, variability of chromospheric activity, and chromospheric statistical properties, and for searching subdwarfs and extreme subdwarfs. Using as chromospheric activity indicator the H_α line, we detected 6391 active M stars among the 99741 stars in the M-star catalogue of the LAMOST survey. We also obtained the relationship between the fraction of active stars and the spectral types, which is consistent to previous results. We also studied the effects of activity on broadband photometric colors, and we did not see significant differences between active and inactive M stars. Using as spectroscopic molecular indicators the CaH123 and TiO5 lines, we found 1288 subdwarfs (including 120 active subdwarfs). We also found 15 extreme subdwarf (2 active extreme subdwarf) candidates. Our subdwarf candidates are slightly redder by about 0.05 mag in g-r compared with dwarfs using the g-r and r-i, and g-r and i-z color diagrams. Using our active M-star catalogue, we found that 898 stars exhibited H_α emission in at least two exposures (170 of them in at least three exposures). Among these 170 stars, 163 of them show variability in H_α emission on long timescales (more than 2.5 h). Furthermore, 34 stars show variability over short timescales (less than 2.5 h), and 29 actives show variability over both short and long time scales.     

The fraction of double degenerates among DA white dwarfs

We present the results of a radial velocity survey designed to measure the fraction of double degenerates among DA white dwarfs. The narrow core of the H line was observed twice or more for 46 white dwarfs yielding radial velocities accurate to a few km s¹. This makes our survey the most sensitive to the detection of double degenerates undertaken to date. We found no new double degenerates in our sample, though H emission from distant companions is seen in two systems. Two stars known to be double degenerates prior to our observations are included in the analysis. We find a 95 per cent probability that the fraction of double degenerates among DA white dwarfs lies in the range [0.017, 0.19].     

Spectral classification of stars in the second byurakan sky survey. I

Results are given on a spectral classification of 316 stars and objects having a continuous spectrum, selected in the course of the Second Byurakan Spectral Sky Survey. Slit spectra are used, obtained from 1978 to 1994, predominantly on the 6-m telescope of the Special Astrophysical Observatory, Russian Academy of Sciences, with a resolution of 5–15 Å. The sample contains objects in the range of stellar magnitudes 10.12 ≤ m(pg) ≤ 19.5. We found 114 DA, 13 DB, 4 DC, and 1 DQ white dwarfs, 57 sdB and 13 sdO subdwarfs, 12 NHB, 10 cataclysmic variables, 76 stars of late spectral types, 3 spectroscopic binary systems, and 13 objects with a continuous spectrum. Several recordings are given for each of these spectral types.     

The UV-Excess survey of the northern Galactic plane

The UV-Excess survey of the northern Galactic plane images a  10°× 185°  wide band, centred on the Galactic equator using the 2.5-m Isaac Newton Telescope in four bands (       i  5875) down to  ∼21–22  mag (     in       i  5875). The setup and data reduction procedures are described. Simulations of the colours of main-sequence stars, giant, supergiants, DA and DB white dwarfs and AM Canum Venaticorum stars are made, including the effects of reddening. A first look at the data of the survey (currently 30 per cent complete) is given.