Stars Of Luminosity Classes Research Articles

Photometric Strömgren-Hβ Quantification for O and B Stars of Luminosity Class V

In any photometric system, the relationship between the spectral type and the intrinsic colors is fundamental for the photometric stellar classification and for a reliable derivation of stellar parameters. Such relationships are being constantly verified and improved based on larger and better samples. The capability of the Strömgren-Hβ photometry to provide reddening-free quantities allows the classification of reddened stars over the entire spectral sequence. However, a detailed consistent tabulation of the uvbyβ quantities for the O-B9 part of the main sequence is currently warranted. We combine spectral types and photometry from the literature to provide a new empirical tabulation of the intrinsic color b − y, color differences c1, m1, [c1] and [m1] and the Hβ index for main-sequence (luminosity class V) stars of spectral types O4.5 through B9.5 in terms of spectral sub-types. Our survey is based on more than 1400 stars. We use the established c1 versus b − y intrinsic line and the accepted reddening slopes to obtain dereddened quantities and then calculate their median values as a function of the spectral sub-type. The classification ability of the system in this spectral range is reliable and the photometric quantities are not affected by the stellar rotation.

Absolute magnitudes of OB and Be stars based on Hipparcos parallaxes - III

The absolute visual magnitudes, M v , of A-M stars are based on calculated Hipparcos trigonometric parallaxes. The sample used consists of 30986 unreddened and reddened A-M stars in luminosity classes Ia, Iab, Ib, II, III, IV and V. The colour excesses of the reddened stars were calculated using the mean colour indices, according to the SIMBAD data base and the intrinsic B - V values calibrated for the given spectral types and luminosity classes by Schmidt-Kaler. The values of the total-to-selective extinction, R v = A v /E(B-V), for all the reddened stars were calculated from previously published near-infrared photometric measurements. The calculated visual magnitudes, M v , of A-M stars compare with the earlier determinations of Schmidt-Kaler. The mean absolute magnitudes published by Schmidt-Kaler are generally brighter (except for the stars in luminosity classes V and IV) than those determined in this paper.

The Araucaria Project: VLT Spectra of Blue Supergiants in WLM— Classification and First Abundances

As part of the Araucaria Project, we present the first spectral catalog of supergiant stars in the Local Group dwarf irregular galaxy WLM. In assigning a spectral classification to these stars we accounted for the low metal content of WLM relative to the galactic standards used in the MK process, by using classification criteria developed for B and A supergiants contained in the Small Magellanic Cloud. Our spectral catalog shows that our higher S/N spectroscopic sample of 19 objects contains at least 6 early-B (B0-B5) supergiants and 6 late-B and early-A (B8-A2) stars of luminosity class between Ia and II, as well as an O7~V star and an O9.7 Ia star. The spectra of several of these stars is of sufficiently high quality for a determination of the stellar parameters and abundances. We have acquired also a second set of lower S/N spectra for mostly BA stars, however their quality does not allow a further analysis. We have carried out a quantitative analysis for three early-B supergiants. The mean oxygen abundance we derive is 12+log(O/H)=7.83 +/- 0.12. This value agrees very well with the measurement that is obtained from HII regions. We therefore find no additional evidence for the discrepancy between stellar and nebular oxygen abundances measured for a single A-type supergiant by Venn et al. The analysis of B- and A-type supergiants yields compatible results for nitrogen, silicon and magnesium. We show that the photometric variability of the blue supergiants included in our spectroscopic sample is negligible for the use of these stars as distance indicators.

Projected rotation velocities of several algol-like irregular variable stars with Hα-emission (alivars)

We present the results ofthe determination of projected rotationvelocities of several Algol-like irregularvariable stars with Hα-emission(ALIVARS). The mean rotation velocity ofthe ALIVARS AAis less than that of the starsof luminosity classes V. This fact can beconsidered as an evidence of a ratheradvanced evolution status of the ALIVARS. The ALIVARS do not show inherent featuresof young objects while they demonstrate theproperties of moderately evolved stars.

A Search for Highly Reddened Early‐Type Stars: Optical Photometry and Spectroscopy of Stars in the Stephenson Objective Prism Survey

The Stephenson objective prism survey contains many stars of apparently early spectral type whose colors imply interstellar extinction values that lie in the key range (8 < AV < 30) between those of nearby reddened stars and the Galactic center infrared sources. As part of a wider program to determine the properties of objects in this survey, we present new optical spectroscopy for 45% of the catalog of ~440 stars, placing constraints on their spectral types and luminosity classes. Contamination by intrinsically red stars, suggested by previous infrared observations of much smaller subsamples, is shown to be heavy: around 90% of the survey appears to consist of late-type, mostly K and M, stars of luminosity class I, II, or III. We also identify six S stars, one of which has high photospheric lithium abundance. Fifteen highly reddened early-type stars are identified: these have visual extinctions in the range 6 < AV < 16 mag, confirming their potential for studies of the interstellar medium.

Chromospheric Activity in Dwarf and Evolved Late A‐ and Early F‐Type Stars

Chromospheric activity in late A- and early F-type field stars of luminosity classes III through V has been investigated using the helium D3 absorption feature. This feature shows a detection boundary near b - y = 0.19 (B - V = 0.29). This color index corresponds to a dividing line in activity levels as determined from the C II λ1335 chromospheric emission line. On the red side of this boundary, stars exhibit strong or moderately strong C II emission and strong or moderately strong D3 absorption. However, on the blue side, D3 absorption does not conclusively appear, while several stars show moderately strong C II emission. The data suggest that D3 is sensitive to the boundary at B - V = 0.29, but they also suggest limitations in the use of D3 as an activity indicator in the late A-type stars. To within observational errors, the D3 boundary appears at the same color index for the full range of luminosity classes explored, in contradiction with some acoustic energy calculations. In addition, the strength of D3 absorption shows no significant trend with luminosity class or the Stromgren δc1 index, with a wide range of activity levels at a given luminosity or surface gravity.

Determination of the effective temperatures OF B, A, and F stars of luminosity classes III–V from the slope of the spectrum in the Balmer continuum

A new method is proposed for the determination of effective temperatures, which is based on the linear relationship between the slope of the spectrum in the Balmer continuum and the effective temperatures for B, A, and F stars of luminosity classes III–V. Comparison of the effective temperatures found by this method and those found by other authors shows no systematic differences in the stellar temperatures.

X-ray emission at the low-mass end - Results from an extensive Einstein Observatory survey

We have used available IPC data and a critical compilation of cataloged optical data to measure the 0.16-3.5 keV X-ray emission from 88 K and 169 M stars of luminosity classes IV, V, and VI within 25 pc from the Sun. The IPC detected 54 out of the 88 K stars, 70 out of the 138 M stars with M(v) less than 13.4, and 15 out of the 31 fainter M stars. We have identified a subsample of surveyed stars that is statistically representative of the population of K and M stars in the solar neighborhood. On the basis of this subsample (1) we have shown the occurrence of a drop in the level of X-ray emission for M stars later than approximately M5; (2) we have built unbiased maximum likelihood X-ray luminosity functions for the K, early M, and late M stars; (3) we have confirmed, both for K and M stars, the decrease of X-ray luminosity with increasing stellar age in the range of ages of disk population stars: and (4) we have shown that no obvious correlation is present between X-ray and bolometric luminosities in the entire representative samples of K and M stars, but only within flare stars which also seem to mark a saturation in X-ray luminosity level.

Shell Stars in the Geneva Photometric System

L. Divan (1979) showed that the Balmer jump is greater in a Be-shell star than in a star of same spectral type and we will consider if this property is also present in A and F-shell stars by studying their data in the Geneva photometric system. Two photometric parameters in this system are correlated with the Balmer jump: d=(U-Bl) -1.430 (B1-B2) and Δ=(U-B2) -0.832 (B2-G). In addition to the Balmer jump, Δ is also sensitive to blanketing. Both parameters are reddening free. A study of the Be stars of luminosity class V or IV classified by Slettebak (1982) shows that, in a d vs Δ diagram, they are in the area of main sequence stars or a little above, while the majority of Be-shell stars are above the Be star sequence. Pleione is well known for presenting a Be phase followed by a shell phase.

Rotation, Expansion and Duplicity of Be Stars

In recent years there is a general tendency to divide Be stars into different groups. Bidelman's (1976) division on supergiants, rapidly rotating single stars, interacting binaries, early type nebular variables and quasi (or young) planetary nebulae is often quoted. Another example is the classification by Lesh (1968) or a very recent classification by Jaschek et al (1980), limited a priori to “normal” Be stars i.e. stars of luminosity classes V to III. I do not think such an approach is recommendable. There is still no general agreement as to the nature of the Be phenomenon and all such classifications must be more or less descriptive. The possibility that the hydrogen emission observed in the spectra of apparently very different objects has always the same physical cause, and that the differences between these objects are caused only by continuously varying physical and/or geometrical parameters should seriously be considered, along with all other concepts. At the moment, I find it more promising to study Be stars in context of all early type emission objects, including supergiants, P Cyg stars, Beta Cep objects, WR stars, symbiotic stars and novae. Such a viewpoint is not new – it was expressed already by Struve (1942).

Two Dimensional Spectral Classification of Early Type Stars by Low Dispersion Spectrophotometry

Spectrophotometric data of early type stars (spectral intensity distributions from 3100 to 6000 Å and equivalent widths of Hβ, Hγ and Hδ) have been analysed, looking for the best parameters for a two dimensional spectral classification. It was found that the best correlation with MK classification is given by the equivalent width of Hβ and the Balmer discontinuity.The classification scheme is well defined for stars from B0 to A1 of luminosity classes V to III. No stars of luminosity class II were observed. Luminosity class I is clearly separated from the less luminous stars, but the number of supergiants observed is too small to give a clear separation of spectral types.