O and Of Stars
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My intention here is to discuss the ‘high temperature’ portion of this symposium and call attention to those stars that are called Of. There are some similarities in spectral appearance to WR stars, e.g. emission lines. I should first like to define what I think are the essential differences among four groups of hot stars; O stars : Stars that have only absorption lines in the visible spectrum. Type O is distinguished from type B by the presence of He ii 4541 at MK dispersion. It may be that some (supergiants) O stars will have emission lines in the rocket UV region but this description will be primarily concerned with ground based observations. Of stars : These are O type stars that also have λλ 4634,40 N iii in emission above the continuum. In addition to normal O star absorption lines and N iii emission, they may also have other lines in emission. I will discuss this further below. Oe stars : These are O type stars that have emission in the hydrogen lines (or at least at H α ), but with no emission in N iii or in other lines. I personally think that this small class of objects is related to the Be stars in their evolutionary status and in their emission mechanism. WR stars : These stars are primarily characterized by emission lines. The only absorption lines seen are violet shifted (P Cyg type). Although in some cases emission lines appear which are similar to those found in some Of stars, the latter types always have some unshifted absorption lines present. Several Of stars have P Cyg profiles in some lines.Keywords:
O-type star
We present low-resolution optical spectra for 29 X-ray sources identified as either massive star candidates or low-mass pre-main-sequence (PMS) star candidates in the clusters Trumpler 16 and Trumpler 14 of the Carina Nebula. Spectra of two more objects (one with an X-ray counterpart, and one with no X-ray counterpart), not originally our targets, but found close (∼3'') to two of our targets, are presented as well. Twenty early-type stars, including an O8 star, seven B1–B2 stars, two B3 stars, a B5 star, and nine emission-line stars, are identified. Eleven T Tauri stars, including eight classical T Tauri stars (CTTSs) and three weak-lined T Tauri stars, are identified. The early-type stars in our sample are more reddened compared to the previously known OB stars of the region. The Chandra hardness ratios of our T Tauri stars are found to be consistent with the Chandra hardness ratios of T Tauri stars of the Orion Nebula Cluster. Most early-type stars are found to be nonvariable in X-ray emission, except the B2 star J104518.81–594217.9, the B3 star J104507.84–594134.0, and the Ae star J104424.76–594555.0, which are possible X-ray variables. J104452.20–594155.1, a CTTS, is among the brightest and the hardest X-ray sources in our sample, appears to be a variable, and shows a strong X-ray flare. The mean optical and near-infrared photometric variability in the V and Ks bands, of all sources, is found to be ∼0.04 and 0.05 mag, respectively. The T Tauri stars show significantly larger mean variation, ∼0.1 mag, in the Ks band. The addition of one O star and seven B1–B2 stars reported here contributes to an 11% increase of the known OB population in the observed field. The 11 T Tauri stars are the first ever confirmed low-mass PMS stars in the Carina Nebula region.
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Blue supergiant
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view Abstract Citations (2) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS HD 51956: A Supergiant in an Interacting Binary Parsons, Sidney B. ; Ake, Thomas B. Abstract IUE ultraviolet spectral observations reveal that the F supergiant HD 51956 contains a hot companion star partially obscured by warm plasma. Variable absorption features are analogous to those in the strongly interacting system HD 207739. The F type component is apparently transferring matter to a subluminous B type star which originally was the primary component. Publication: The Astrophysical Journal Pub Date: November 1990 DOI: 10.1086/185863 Bibcode: 1990ApJ...364L..13P Keywords: Binary Stars; Companion Stars; Hot Stars; Supergiant Stars; Ultraviolet Spectra; Variable Stars; B Stars; High Temperature Plasmas; Iue; Astrophysics; STARS: BINARIES; STARS: INDIVIDUAL HENRY DRAPER NUMBER: HD 51956; STARS: SUPERGIANTS; STARS: VARIABLES; ULTRAVIOLET: SPECTRA full text sources ADS | data products SIMBAD (3) MAST (1) INES (1)
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The author discusses the search for low-mass pre-main-sequence (PMS) stars associated with X-ray sources in regions of star formation. The survey to date has revealed at least 30 low-mass PMS stars in the Taurus-Auriga region, and a comparable number in Ophiuchus. These stars are the naked T Tauri stars, unveiled versions of the well-known classical T Tauri stars. The author discusses the properties of these newly discovered PMS stars and their relation to the classical T Tauri stars. He concludes that the naked T Tauri stars are the true low-mass PMS stars, and that the observable characteristics defining the classical T Tauri stars are due to the interaction of an underlying, fairly normal star with a dominant circumstellar environment. He discusses the impact the naked T Tauri stars are likely to have on our understanding of the PMS evolution of low-mass stars.
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We survey fluorescent H_2 emission in HST STIS spectra of the classical T Tauri stars (CTTSs) TW Hya, DF Tau, RU Lupi, T Tau, and DG Tau, and the weak-lined T Tauri star (WTTS) V836 Tau. From each of those sources we detect between 41 and 209 narrow H_2 emission lines, most of which are pumped by strong Lyα emission. H_2 emission is not detected from the WTTS V410 Tau. The fluorescent H_2 emission appears to be common to circumstellar environments around all CTTSs, but high spectral and spatial resolution STIS observations reveal diverse phenomenon. Blueshifted H_2 emission detected from RU Lupi, T Tau, and DG Tau is consistent with an origin in an outflow. The H_2 emission from TW Hya, DF Tau, and V836 Tau is centered at the radial velocity of the star and is consistent with an origin in a warm disk surface. The H_2 lines from RU Lupi, DF Tau, and T Tau also have excess blueshifted H_2 emission that extends to as much as -100 km s^(-1). The strength of this blueshifted component from DF Tau and T Tau depends on the upper level of the transition. In all cases, the small aperture and attenuation of H_2 emission by stellar winds restricts the H_2 emission to be formed close to the star. In the observation of RU Lupi, the Lyα emission and the H_2 emission that is blueshifted by 15 km s^(-1) are extended to the SW by ~0.07, although the faster H_2 gas that extends to ~100 km s^(-1) is not spatially extended. We also find a small reservoir of H_2 emission from TW Hya and DF Tau consistent with an excitation temperature of ~2.5 × 10^4 K.
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view Abstract Citations (146) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Periodic light variations in four pre-main-sequence K stars. Rydgren, A. E. ; Vrba, F. J. Abstract Four pre-main-sequence K stars with weak line emission (HD 283447, V410 Tau, and two of the X-ray stars studied by Feigelson and Kriss, 1981) have been monitored with UBVRI photometry during seven consecutive nights in 1981 October. All four stars show quasi-sinusoidal light variations, apparently due to the presence of large starspots. The light variations have amplitudes of from 0.06 to 0.23 mag in yellow light and periods ranging from 1.9 to 4.1 days. Several significant UV flares were also observed. The observations indicate that these young stars possess magnetic surface activity similar to that seen in the RS CVn and BY Dra stars. The observed stars fall within rather than below the T Tauri band in the H-R diagram and have deduced rotational velocities comparable to those of typical T Tauri stars. Relatively rapid rotation in a late-type pre-main-sequence star is apparently not a sufficient condition for producing the strong line and continuum emission which characterizes true T Tauri stars. Publication: The Astrophysical Journal Pub Date: April 1983 DOI: 10.1086/160858 Bibcode: 1983ApJ...267..191R Keywords: K Stars; Late Stars; Main Sequence Stars; Stellar Rotation; Stellar Spectrophotometry; Variable Stars; Angular Velocity; Emission Spectra; Light Curve; Line Spectra; Starspots; T Tauri Stars; Ubv Spectra; X Ray Sources; Astrophysics full text sources ADS | data products SIMBAD (12)
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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.
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We have completed a high-resolution optical study of 14 stars classified as Vega-like, having an IR excess attributable to dust emission. Surface lithium abundances were measured for the four G- and K-type stars of the sample, to test the suggestion that these Vega-like stars are intermediate in evolutionary state between pre-main-sequence objects and established main-sequence stars. Abundances ranged from a very high value in the G5e star HD 143006, implying a very low age of only 1 Myr, to below the limit of measurement for the K2V star HD 23362, which we conclude to be already well established on the main sequence. The emission-line characteristics of all the stars in our sample were studied to compare with those seen in the classical pre-main-sequence Herbig Ae/Be (HAeBe) stars and T Tauri stars. Activity levels ranged from stars showing little or no activity, such as HD 23362, to those exhibiting extensive activity, such as the A9/F0Ve star HD 144432, which showed distinctive P Cyg profiles in its spectrum, and HD 143006, which is young enough to be included in the T Tauri class of stars. The A2/3e star HD 35187 shows evidence of variability in its Hα and He i λ5876 lines, with four other A-type stars in our sample also showing evidence of He i λ5876 activity in the form of emission or absorption. We interpret the excess absorption and/or emission in the λ5876 line as providing direct evidence for ongoing accretion activity on these systems. We find that the emission characteristics of the Hα, Nα i D, He i and Ca ii K lines are not significantly different from those of HAeBe stars and T Tauri stars. Some of our sample have also been previously classified as pre-main-sequence or candidate pre-main-sequence stars, which would seem to suggest that there is no distinct boundary between Vega-like stars and the HAeBe and T Tauri stars. The surface gravities of the A- and F-type stars in our sampleimply that they have already reached the main sequence, consistent with the short time-scales to reach the main sequence predicted for stars of their mass and the fact that they are not located close to star formation regions. On the other hand, the ages derived for the three emission-line G- and K-type stars in our sample imply that they have probably not yet reached the main sequence. It is likely that these emission-line Vega-like stars represent the intermediate stage between classical pre-main-sequence stars with 'dusty' IR excesses and stars that are well established on the main sequence. We also studied our sample for evidence of optical circumstellar gas absorption features. Of the 14 stars, seven show evidence for narrow absorption lines in their spectra. Most of these appear to be of interstellar origin. One of these stars, HD 144432, has a narrow absorption component in the absorption trough of its Na i D P Cyg profile, at a heliocentric velocity of −91 km s−1, which excludes an interstellar origin and therefore more likely originates in its circumstellar environment. We also detect narrow absorption lines in the spectrum of HD 158643 (51 Oph) arising from excited-state Fe ii lines, which can only come from the circumstellar environment of the star.
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Some observations of the emission features at H α and H β in the spectra of the Be stars η Tauri, β Canis Minoris, the peculiar Be star K Draconis, and the shell stars ζ Tauri and 48 Librae are presented. It is shown that the widths of the emission lines of the Be stars are consistent with the hypothesis, postulated some years ago by Struve, that the emission lines of Be stars originate in a rotating equatorial bulge surrounding the rapidly rotating star. However, this hypothesis is not adequate to explain all the emission features of the spectra of shell stars where H α is shown to have very extensive wings. It is proposed that the excessive broadening of the emission at H α in shell spectra comes from material in tenuous streamers at comparatively large distances from the rapidly rotating star. These streamers are thought to be moving at great speeds in random directions and are only visible in H α radiation. The equatorial bulge and inner shell of the shell star are thought to give rise to the ordinary shell spectrum of absorption and emission lines.
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