Early-type Supergiants Research Articles

Some Manifestations of Nonstationarity in the Spectra of Early-Type Supergiants

This is a theoretical study of changes in several characteristics of the spectra of early-type supergiants owing to scattering of continuum radiation. Thomson scattering on free electrons is treated as the scattering mechanism, although the approach used here is quite general. The reason for changes in the spectral class of a star with constant bolometric luminosity and surface temperature is elucidated and the necessary conditions for their realization are introduced. The effect of scattering in the continuum on the magnitude of the intensity jumps in the hydrogen spectrum depending on the degree of ionization is studied. The conditions under which the Schuster mechanism for formation of emission lines sets in are clarified both for an isothermal atmosphere and for an atmosphere with a temperature gradient.

Absolute Magnitudes of Early-type Supergiants from uvbyβ Photometry and Parallax Data

We combine Strömgren–Crawford uvbyβ photometry with Gaia-DR2 parallaxes for a sample of 880 supergiants of O-B9 spectral types to explore the potential of these data in providing physical parameters of such stars. We obtained reddening corrections and photometric absolute magnitudes via the existing uvbyβ calibrations. We calculated Gaia-based absolute magnitudes applying individual photometric reddening corrections for each star. The uvbyβ photometry is particularly useful in this case, since it provides color excess independent of spectral classification, thus overcoming possible MK classification inconsistencies. After evaluating and correcting the effect of luminosity class mis-classifications on the photometric absolute magnitudes, we found a reasonable agreement between the photometric and Gaia-based absolute magnitudes for this sample. At this point, the color–magnitude MV versus (b − y)0 diagrams based on photometry and Gaia show slight discrepancies, which have to be addressed in the light of the next Gaia data releases. The derived precise reddening corrections should further help to refine the physical parameters of these stars and address their connection to OB-associations and regions of star formation.

Effect of Continuum Scattering on Early-type Supergiants Spectra

The effect of radiation scattering in continuum on the frequency distribution of the observed flux in atmospheres of supergiants of the late B and A spectral classes is treated. Thomson scattering on free electrons, which is important for hydrodynamic balance and wind dynamics in extended atmospheres of these stars, is considered as a specific scattering mechanism in the continuum. It is shown how stars of the same bolometric luminosity and with equal radii can belong to different spectral classes. The dependence of the continuum shortwave drift on the density of rarefied plasma has been established. The role of Thomson scattering in different domains of the hydrogen spectrum is revealed.

Fast and slow winds from supergiants and luminous blue variables

We predict quantitative mass-loss rates and terminal wind velocities for early-type supergiants and luminous blue variables (LBVs) using a dynamical version of the Monte Carlo radiative transfer method. First, the observed drop in terminal wind velocity around spectral type B1 is confirmed by the Monte Carlo method at the correct effective temperature of about 21 000 K. This drop in wind velocity is much steeper than would be expected from the drop in escape speed for cooler stars. The results may be particularly relevant for slow winds inferred for some high-mass X-ray binaries. Second, the strength of the mass-loss bi-stability jump is found to be significantly greater than previously assumed. This could this make bi-stability braking more efficient in massive star evolution; in addition, a rotationally induced version of the bi-stability mechanism may now be capable of producing the correct density of outflowing disks around B[e] supergiants, although multi-dimensional modelling including the disk velocity structure is still needed. For LBVs we find that the bi-stability jump becomes larger at higher metallicities, but perhaps surprisingly also larger at lower Eddington parameters. This may have consequences for the role of LBVs in the evolution of massive stars at different metallicities and cosmic epochs. Finally, our predicted low wind velocities may be important for explaining the slow outflow speeds of supernova type IIb/IIn progenitors, for which the direct LBV-SN link was first introduced.

New fast X-ray transient IGR J18462-0223 discovered by the INTEGRAL observatory

Details of the discovery of a new X-ray source, IGR J18462-0223, on October 12, 2007, during a short (several hours), intense (~ 35 mCrab at the peak) outburst of hard radiation by the IBIS/ISGRI gamma-ray telescope onboard the INTEGRAL observatory are given. The detection of another earlier outburst from this source occurred on April 28, 2006, in the archival data of the telescope is reported. We present the results of the source's localization and our spectral/timing analysis of the observational data. The source may turn out to be yet another representative of the continuously growing population of fast X-ray transients, which are the focus of attention because of the identification of their optical counterparts with early-type supergiants.

Fundamental parameters of B supergiants from the BCD system

Effective temperatures of early-type supergiants are important to test stellar atmosphere- and internal structure-models of massive and intermediate mass objects at different evolutionary phases. However, these Teff values are more or less discrepant depending on the method used to determine them. We aim to obtain a new calibration of the Teff parameter for early-type supergiants as a function of observational quantities that are highly sensitive to the ionization balance in the photosphere and its gas pressure, independent of the interstellar extinction, and as much as possible model-independent. The observational quantities that best address our aims are the (lambda1, D) parameters of the BCD spectrophotometric system. They describe the energy distribution around the Balmer discontinuity, which is highly sensitive to Teff and logg. We perform a calibration of the (lambda1, D) parameters into Teff using effective temperatures derived with the bolometric-flux method for 217 program stars, whose individual absolute relative uncertainties are on average 0.05. The final comparison of calculated with obtained Teff values in the (lambda1,D) calibration further show that the latter have total relative uncertainties, which on average are of the order of 0.05 for all spectral types and luminosity classes. The effective temperatures of OB supergiants derived in this work agree on average within some 2000 K with other determinations found in the literature, except those issued from wind-free non-LTE plane-parallel models of stellar atmospheres, which produce effective temperatures that can be overestimated by up to more than 5000 K near Teff = 25000 K.

The serendipitous discovery of a stellar ring at mid-infrared wavelengths

We present images and photometry of a stellar ring-like structure G052.4-00.0 discovered during visual inspection of the Spitzer GLIMPSE Atlas. Although G052.4-00.0 may correspond to one of a large variety of sources, it is argued that it is most likely to be associated with an early-type supergiant at a distance of ∼ 8 kpc from the Sun. Where this is the case, then the radius of the ring would be of the order of ≈ 0.3 pc, comparable to those of similar structures about other supergiant stars, and the extinction A v would be of the order of ∼21 mag.

The Missing Luminous Blue Variables and the Bistability Jump

We discuss an interesting feature of the distribution of luminous blue variables (LBVs) on the H-R diagram, and we propose a connection with the bistability jump seen in the winds of early-type supergiants. There appears to be a deficiency of quiescent LBVs on the S Doradus instability strip at luminosities between log(L/Lsolar)~=5.6 and 5.8. The upper boundary, interestingly, is also where the temperature-dependent S Doradus instability strip intersects the bistability jump at about Teff~=21,000 K. Because of increased opacity, winds of early-type supergiants are slower and denser on the cool side of the bistability jump, and we postulate that this may trigger optically thick winds that inhibit quiescent LBVs from residing there. We conduct numerical simulations of radiation-driven winds for a range of temperatures, masses, and velocity laws at log(L/Lsolar)=5.7 to see what effect the bistability jump should have. We find that for relatively low stellar masses, the order-of-magnitude increase in the wind density at the bistability jump leads to the formation of a modest to strong pseudophotosphere that might alter a star's apparent position on the H-R diagram. The effect is strongest for LBVs approaching 10 Msolar, where the pseudophotospheres are sufficiently extended to make an early B-type star appear as a yellow hypergiant. Thus, the proposed mechanism will be most relevant for LBVs that are post-red supergiants [curiously, the upper boundary at log(L/Lsolar)~=5.8 coincides with the upper luminosity limit for red supergiants]. Further work is obviously needed, especially with regard to a possible evolutionary connection between the ``missing'' LBVs and the most luminous red supergiants and yellow hypergiants. Specifically, yellow hypergiants such as IRC +10420 and rho Cas occupy the same luminosity range as the missing LBVs and show apparent temperature variations at constant luminosity. If these yellow hypergiants do eventually become Wolf-Rayet stars, we speculate that they may skip the normal LBV phase, at least as far as their apparent positions on the H-R diagram are concerned.

Vela OB1: Probable New Members and Hertzsprung-Russell Diagram

Several dozen stars are identified as probable previously unrecognized members of the association Vel OB1, including the Vela X-1 binary pulsar system. Based on a variable extinction analysis of 70 stars, the distance and ratio of total to selective absorption of the association are determined to be 1750 pc and 3.70, respectively. Vel OB1 may be significantly larger than previously appreciated, perhaps up to 600 by 300 pc in the sense of longitude by latitude. This physical extent, and the fact that the H-R diagram includes both low-mass evolved yellow supergiants and massive, early-type supergiants, indicates that Vel OB1 is probably not coeval.

A search for OH maser emission from post-asymptotic giant branch stars

We present a survey for OH maser emission at 18 cm in the direction of 196 IRAS point sources. The primary aim of the survey was to detect OH maser emission from post-asymptotic giant branch (post-AGB) stars. OH emission and/or absorption was detected in the direction of 77 sources of which 57 OH detections were previously unknown, giving an overall detection rate of 39 per cent. Fourteen maser sources associated with evolved stars or planetary nebulae were found, corresponding to 7 per cent of the total observed. These included three planetary nebulae (two new OH detections), nine post-AGB stars (three new OH detections) and two cold OH/IR stars (both previously known). Two sources were of uncertain identification (both new OH detections). The remainder of the OH sources were identified with young stellar objects and with Hii regions. The survey included the detection of OH maser emission from HD 101584, an early-type supergiant at high Galactic latitude. For several of the post-AGB stars, changes have occurred in the OH spectral profiles over a period of several years. This is interpreted as evidence for rapid changes in the stellar winds, due either to interactions with stellar companions, novae-like events or rapid stellar evolution.

Stellar winds in A-type supergiants

A-type supergiants are just at the boundary between the early type supergiants, with strong stellar winds driven by radiation pressure and the cool supergiants in which the origin of mass loss is the dissipation of mechanical energy. The mechanisms involved in the mass loss processes in A supergiants as a whole, are still uncertain.

The Puppis region and the last crusade for faint OB stars

A search for faint OB stars over an area of 43 sq. deg in the Puppis region resulted in the discovery of 397 OB stars and 5 early type supergiants; the blue magnitude range of the survey is about 12.0 < B < 15.0 mag. About 73% of the stars found had not been previously recognized as OB stars; about 12% of them show hydrogen line emission. UBV photoelectric and photographic measures of OB stars from this list and from the Luminous Stars Survey are presented. The galactic distribution of the OB stars in the region shows concentrations around the open clusters Ruprecht 44 and Ruprecht 55, and the presence of an important grouping of young stars located far below the plane

H-alpha observations of early-type stars

Results are presented of a first session of a program run at the Catania Astrophysical Observatory to study and monitor the H-alpha emission from early-type stars, and the properties of their mass loss are determined. Twelve early-type supergiants in the Cygnus region were observed repeatedly in 1988 July. From these observations, evidence is found for short-term variability of the mass-loss rate. Also, a comparison with literature data reveals strong long-term variations for some of the stars. In addition, simplified models were developed to describe the H-alpha profile resulting from the combination of photospheric absorption with emission and scattering in the wind. By fitting the observed profiles with the models, the mass-loss rates and the velocity fields are determined for all stars with accuracies generally better than 25 percent.

The signature of a black hole transit

A black hole transiting a companion star in a binary system will produce a time-varying intensity profile as observed at the Earth because of the Einstein photometric effect (gravitational lens phenomenon). If the transited star is an early-type supergiant with electron scattering as its dominant atmospheric opacity source, then variable linear polarization will also result from the destruction of the circular symmetry of the observed stellar disk. The simultaneous variation of the three Stokes parametersI, Q, andU may be thought of as the signature of a black hole transit. Monte-Carlo calculations show that the effect has the properties expected from qualitative considerations. The amplitude of the photometric and polarimetric light curves in a typical X-ray binary is too small to be observed with present instrumentation. A black hole transit might be detectable in a binary having a large separation of the components. The signature is also masked in close binaries by the much larger variability caused by the changing aspect of the tidally distorted OB star. The polarization induced by tidal distortion always produces a derived inclination of 90° when the standard method of analyzing the data is used. This effect may contribute to the unrealistically large values of inclination derived from polarimetric observations for the Cyg XR-1/HDE226868 system.

The Luminosity of the Central Parsec of the Galaxy

The luminosity of the central 5 pc of the Galaxy -encompassing the inner regions of the rotating ring of dust and gas which surrounds the galactic center - emerges primarily at infrared wavelengths in the form of thermal emission from heated dust. The nature and location of the sources which heat the dust can be inferred from the spatial and temperature distribution of the thermal infrared emission (λ&gt;20um), from studies of the ionized gas in this region, and from direct imaging in the near infrared. These observations show that the principal heating sources within this 5-pc region are concentrated within the central parsec of the Galaxy and indicate that the luminosity of these sources is within a factor of two of 107 LO. The near-infrared observations of the compact sources at the galactic center do not reveal a single dominant source but suggest instead that the several components of the IRS-16 complex, taken together, may contribute the bulk of the luminosity; however, the data also permit a single object to dominate the energetics of this region. We draw attention to the striking morphological similarities between the galactic center and the innermost regions of the 30 Doradus nebula in the Large Magellanic Cloud and speculate that the luminosity sources in the galactic center may resemble the early-type supergiants in 30 Doradus.

MISCELLANEOUS SPECTROSCOPIC NOTES

ABSTRACT Results of slit-spectrograph observations are reported for approximately 260 stars. The data presented range from recognition of many new Ap, Am, and other unusual stars to H-alpha observations of early-type supergiants and Be stars. The material discussed was obtained over the past 40 years at a number of U.S. observatories and at the Dominion Astrophysical Observatory in Victoria, B.C.

Spectroscopy of unusual emission-line stars

view Abstract Citations (5) References (14) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Spectroscopy of Unusual Emission-Line Stars Bopp, Bernard W. Abstract New spectroscopic observations are reported for ten stars that have been identified in the literature as having Hα emission with suspected F, G, or K spectral types. Three of the stars are shown to be Be stars, two are confirmed as early-type supergiants, three show composite (F or K+B) spectra, one is a "post-T Tauri" star, and one is an ordinary F star without emission. Publication: The Astronomical Journal Pub Date: May 1988 DOI: 10.1086/114750 Bibcode: 1988AJ.....95.1543B Keywords: H Alpha Line; Stellar Spectra; Stellar Spectrophotometry; Supergiant Stars; T Tauri Stars; Absorption Spectra; Charge Coupled Devices; Dwarf Stars; Astrophysics; STARS: ATMOSPHERES; STARS: ABUNDANCES; STARS: SPECTRAL CLASSIFICATION full text sources ADS | data products SIMBAD (12)

The visually brightest early-type supergiants in the spiral galaxies NGC 2403, M81, and M101

The visually brightest early-type supergiants in the spiral galaxies NGC 2403, M81, and M101

On the puzzling line spectrum of the B[e]-supergiant R 4 of the SMC

The early-type supergiants of the Magellanic Clouds (MC's) include a class of objects, the B[e] -supergiants, which are described in detail in Zickgraf et al. (1985, 1986) who suggested a two-component wind model consisting of a cool, dense and slowly expanding disk-like equatorial wind component and a hot line-driven fast polar wind. This model could explain many properties of most of the B[e] -supergiants as e. g. the occurrence of “shell” absorption lines. R 4 of the SMC, however, although well fitting to the general scheme of B[e) -supergiants exhibits several pecularities of its line spectrum, which make this particular star an exceptional case among the B[e] -supergiants of the MC's.

Gas in and toward the Magellanic Clouds

Attention is given to the Ca II K absorption spectra of 48 early-type supergiants in the SMC and LMC, with a view to the Clouds' possible fragmentary nature and the distance of the gas responsible for the absorption lines at 60 and 120 km/sec. The comparison of the spectra obtained with H I measurements toward the Clouds indicates that in both Clouds, the velocity components in the neutral hydrogen distribution lie at different distances along the line of sight. Good agreement is found between the radial velocity shown in absorbing material and the radial velocity spread predicted by Murai and Fujimoto's (1980) dynamical model of the formation of the Magellanic Stream by tidal stripping. It is speculated that both the SMC and LMC are extended and fragmented along the line of sight.