Galactic OB Stars Research Articles

What are the mass-loss rates of O stars?

Empirical mass-loss rates of 28 luminous galactic OB stars are derived from thermal radio emission and from Hα recombination radiation. The velocity fields of all stars had previously been analyzed in the IUE ultraviolet spectral region. The results of these ultraviolet line-profile analyses (shape of the velocity law and terminal velocity) are combined with the radio and Hα data for a precise and consistent determination of the stellar mass-loss rates. It is found that radio and Hα rates agree within the observational errors. This suggests that significant clumping in the wind is unlikely due to the very different radius of origin of radio and Hα radiation within the wind

A survey of radio emission from Galactic OB stars

Radio continuum observations of 88 O-type and early B-type stars are examined. Results are presented for a subset of these stars which forms a distance-limited sample of all OB stars within 2.5 kpc of the sun. Mass loss rates are derived for HD 15570, HD 166734, HD 151804, HD 152408, Alpha Cam, HD 169454, and Zeta Sco. All of these very luminous OB stars are found to be losing mass at a rate on the order of 10 to the -5 solar masses/yr. Multifrequency observations confirm the free-free interpretation for Zeta Pup and provide evidence for variability in the free-free sources P Cygni and Cyg OB2 No. 12.

H-alpha as a tracer of mass loss from OB stars

view Abstract Citations (167) References (35) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS H alpha as a Tracer of Mass Loss from OB Stars Leitherer, Claus Abstract This paper investigates the use of the H-alpha emission from stellar winds of OB stars to determine the stellar mass-loss rate. The power in H-alpha emitted by the wind can be parameterized in terms of the temperature and the density field of the wind. A simple expression is derived which relates the observed H-alpha luminosity to the stellar mass-loss rate, the stellar radius, the velocity law, and the stellar effective temperature. This expression is calibrated for the influence of the velocity law using a sample of Galactic OB stars with UV mass-loss rates. Consequently, the results depend on the validity of the UV rates. The derived velocity law for O stars turns out to be in agreement with the radiation-pressure-driven wind theory. There is evidence for a dependence of the velocity-law gradient on spectral type. The results for B stars, however, are more uncertain due to the dependence on the adopted mass accretion rate/L relation. Application of the calibrated H-alpha luminosity/mass-loss rate relation to a sample of 149 galactic OB stars shows that mass accretion rate can be reliably determined from H-alpha. Due to the moderate amount of observing time required to derive mass accretion rate from H-alpha, this method may be applied successfully to investigate mass-loss effects in extra-Galactic stars. Publication: The Astrophysical Journal Pub Date: March 1988 DOI: 10.1086/166097 Bibcode: 1988ApJ...326..356L Keywords: B Stars; Early Stars; H Alpha Line; O Stars; Stellar Mass Ejection; Stellar Winds; Ultraviolet Radiation; Astrophysics; LINE PROFILES; STARS: EARLY-TYPE; STARS: MASS LOSS full text sources ADS | data products SIMBAD (149)

The Effect of Mass-Loss On the Evolution of HII Regions in the L.M.C.

Empirical calibrations based on prior studies of Galactic OB Stars are used to determine the integrated stellar wind mechanical luminosity (Lw) and the integrated Lyman continuum photon luminosity (S*) for 10 OB clusters in the L.M.C. These values of Lw and S*, together with narrow band H-α surface photometry of an overlapping sample of 15 L.M.C. HII regions, are used to show that the large shell-like HII regions, in the L.M.C. are stellar wind bubbles 3 to 5 million years old. In order to reproduce the general properties of these HII shells, a thick cushion of shocked stellar wind gas must have been present on the inside of the shell for most of the life time of the nebula, and the shell itself must be ram pressure confined by the HI/molecular cloud out of which it formed.

Copernicus observations of the N V resonance doublet in 53 early-type stars

view Abstract Citations (35) References (36) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Copernicus observations of the N V resonance doublet in 53 early-type stars. Abbott, D. C. ; Bohlin, R. C. ; Savage, B. D. Abstract UV spectra in the wavelength interval 1170-1270 A are presented for 53 early-type stars ranging in spectral type from O6.5 V to B2.5 IV. The sample includes four Wolf-Rayet stars, seven known Oe-Be stars, and six galactic halo OB stars. A qualitative analysis of the stellar N v doublet reveals that: (1) N v is present in all stars hotter and more luminous than type B0 for the main sequence, B1 for giants, and B2 for supergiants; (2) shell components of N v and an unidentified absorption feature at 1230 A are present in about half of the stars; (3) the column density of N v is well correlated with bolometric luminosity over the spectral range O6 to B2; and (4) the ratio of emission to absorption equivalent width is a factor of 2 smaller in the main sequence stars than in supergiants, which suggests that the wind structure changes as a star evolves. For several stars, this ratio is too small to be explained by traditional wind models. Publication: The Astrophysical Journal Supplement Series Pub Date: March 1982 DOI: 10.1086/190781 Bibcode: 1982ApJS...48..369A Keywords: Early Stars; Nitrogen Ions; Resonance Lines; Stellar Evolution; Stellar Spectra; Stellar Winds; Ultraviolet Spectra; B Stars; Hertzsprung-Russell Diagram; Main Sequence Stars; O Stars; Stellar Envelopes; Stellar Luminosity; Stellar Temperature; Wolf-Rayet Stars; Astrophysics full text sources ADS | data products SIMBAD (83) MAST (2)

Effects of a stochastic initial mass function on the upper main sequence band

The theoretical (Mb versus Log Te) HR diagram for the brightest galactic OB stars shows an upper boundary for the luminosity, which is characterized by a decreasing luminosity with decreasing effective temperature (Humphreys and Davidson, 1979). The existence of this limit was interpreted by Chiosi et al. (1978) as due to the effect of mass loss by stellar wind on the evolution of most massive stars in core H-burning phase. In fact, evolutionary models calculated at constant mass cover a wider and wider range in effective temperature as the initial mass increases during the main sequence phase. On the contrary, sufficiently high mass-loss rates make the evolutionary sequences of most massive stars (M 60⩾Mʘ) shrink toward the zero age main sequence whenever, due to mass loss, CNO processed material is brought to the surface (Chiosi et al., 1978; de Loore et al., 1978; Maeder, 1980).

Stellar winds from hot supergiants

Spectrographic data of high dispersion are described from 65 very luminous galactic OB stars. Their absolute magnitudes and masses are estimated. Almost all show evidence for mass outflow, and the flow rate is empirically calibrated from mass-loss models for a few of them. Approximate contours of mass-loss on the H-R diagram are suggested which support the zero point of the individual rates. These are as high as 10/sup -5/ M/sub sun/ yr/sup -1/ from a 60 M/sub sun/ supergiant. Anomalously high mass-loss rates found in a few stars may be connected with duplicity of the objects. (AIP)

A RECALIBRATION OF THE ABSOLUTE MAGNITUDES OF EARLY TYPE STARS CLASSIFIED ON THE MK SYSTEM

Galactic clusters and OB stars frequently provide the data for estimates of the numerical values of galactic parameters. Conclusions drawn from such values are numerically correct only to the extent that the distance scale of the specific type of objects used for the discussion is correct. In particular, conclusions based on agreement or disagreement of galactic parameter values derived from clusters and field OB stars are significant only to the extent that the distance scales of the clusters and the field stars are consistent. The distance scale of the galactic clusters is now generally based upon the unevolved main sequence, the zero point of which has been established from accurate kinematic parallax determinations of members of the Hyades cluster. Johnson has discussed practical details of fitting observed cluster main sequences to the standard zero-age main sequence.1 In the procedure one assumption is implicit: variations in chemical composition or related effects are not sufficiently great to produce any significant displacement between the unevolved main sequence of the observed cluster and the standard zero-age main sequence. In what follows we shall take it as axiomatic that this assumption is fulfilled. Further, we accept the standard zeroage main sequence as being well established in the Mv, B - V plane, and assert that it provides a means of determining accurate cluster distances. We ask : Is the distance scale of the OB stars, derived from the currently used absolute magnitudes of the MK spectral types and luminosity classes, consistent with that of the galactic clusters, based upon the standard zero-age main sequence?

Observations of early-type stars in extragalactic nebulae.

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Observations of early-type stars in extragalactic nebulae. Code, A. D. ; Houck, T. E. Abstract The application of a particular class of celestial objects as a distance indicator in extragalactic systems tacitly assumes that this class is essentially similar from one system to another. A program has been undertaken to investigate the similarity of early-type stars of high luminosity in nearby galaxies since they are sufficiently luminous so that one may obtain both spectrographic and photometric observations. They offer in addition the possibility of investigating the interstellar reddening within these systems. Three-color observations have been obtained of a number of blue stars in an outer spiral arm 75' from the center of M31, and spectral types in addition to colors have been determined for a number of OB stars in the Large and Small Magellanic Clouds. On the basis of a distance modulus of 23.8 mag. the luminosities deduced for these blue stars in M31 are somewhat lower than the presently accepted luminosities of early-type galactic super- giants. Several of the objects are bluer than galactic OB stars and suggest that the galactic absorption derived from the secant law is an overestimate. The general lack of dust in this arm, inferred from the extragalactic nebulae visible through the arm, provides a reasonable explanation for the absence of very luminous supergiants. Spectra of the early-type stars in the Large and Small Magellanic Clouds are similar to those of normal high luminosity galactic OB stars. The photometric data for the Clouds indicate that there is no significant interstellar absorption within our own galaxy in that direction; however, absorptions in excess of I mag. occur within the Large Cloud. This absorption apparently follows the same law of reddening as in our own galaxy. The stars in the Small Cloud fall close to the intrinsic line for galactic supergiants. The spectral types inferred from the photometric data, however, do not agree well with those obtained directly. This discrepancy could be explained by assuming a law of interstellar reddening nearly parallel with the intrinsic line for galactic super- giants. Finally, on the basis of a distance modulus of 18.3 mag. for the clouds, the mean absolute visual magnitudes of the stars observed in the Large and Small Cloud, respectively, are - 8.o and -7.3 mag. Washburn Observatory, University of Wisconsin, Madison, Wis. Publication: The Astronomical Journal Pub Date: 1956 DOI: 10.1086/107386 Bibcode: 1956AJ.....61Q.173C full text sources ADS |