Orionis Stars Research Articles

Flares on opposite sides of the star: Observational aspects

The problem of the flare taking place on opposite sides of a star is considered. Such a screened flare, diffused through the star's atmosphere (chromosphere), may also be registered. The theoretical light curve for diffused flare event is derived, which differs strongly from the usual flare light curves. The light curve of diffused flare is characterized first of all by its very slow rise of brightness. This result opens quite a new direction to understand the nature of the so-called ‘slow’ flares, observed often among the UV Cet-type stars as well as flare stars in aggregates. All slow flares can be interpreted as quite ordinary flares of quite ordinary flare stars — taking place, however, on the opposite sides of the star. The results of interpretation of some slow flare events of YY Gem and three flare stars in Orion are presented. An attempt is made for the determination of actual amplitudes of screened flares taking place on the opposite sides of a star.

Analysis of the light curves of the Fuor V 1057 Cyg

Observations made by the author and taken from the literature have been used to construct composite standardized light curves of the FU Orion star (fuor) V 1057 Cyg in the UBVR bands up to 1982. The circumstellar reddening and the radius and temperature of the star before and after the outburst have been determined. During the outburst, the star increased its radius from 3.5 to 17 solar radii and its temperature from 4600 K to 9000 K and ejected a dust shell which subsequently remained unchanged. Since the outburst, the star has cooled without changing its radius, and the various signs of peculiarity have gradually weakened. The observed track of the fuor in the color-magnitude diagram shows a tendency for the star to be displaced to the region of yellow giants.

Decaying stellar magnetic fields, magnetic braking - Evidence from magnetic observations in Orion OB1

view Abstract Citations (80) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Decaying stellar magnetic fields, magnetic braking : evidence from magnetic observations in Orion OB1. Borra, E. F. Abstract The magnetic fields of 13 Ap and helium-weak stars in the Orion OB 1 association are observed and compared to those of older magnetic stars. It is found that the magnetic stars in Orion have magnetic fields stronger than those of the older stars by a factor of three, in agreement with a fossil origin of the fields. The short e-folding time of decay, which is estimated to be of the order of 100 million years, is seen as evidence for the view that internal mass motions are present in upper-main-sequence stars, with potentially important effects on their evolution. The periods of variation, although uncertain, suggest that young magnetic stars rotate faster than their older counterparts. This difference implies magnetic braking on the main sequence. Publication: The Astrophysical Journal Pub Date: October 1981 DOI: 10.1086/183654 Bibcode: 1981ApJ...249L..39B Keywords: A Stars; Magnetic Flux; Magnetic Stars; Orion Constellation; Stellar Evolution; Stellar Magnetic Fields; B Stars; Decay Rates; Kolmogoroff-Smirnoff Test; Main Sequence Stars; Stellar Rotation; Stellar Structure; Astrophysics full text sources ADS | data products SIMBAD (17)

Far-ultraviolet spectra and flux distributions of some Orion stars

Far-ultraviolet (950-1800 A) spectra with about 2 A resolution were obtained of a number of stars in Orion during a sounding-rocket flight 1975 December 6. These spectra have been reduced to absolute flux distributions with the aid of preflight calibrations. The derived fluxes are in good agreement with model-atmosphere predictions and previous observations down to about 1200 A. In the 1200-1080 A range, the present results are in good agreement with model predictions but fall above the rocket measurements of Brune, Mount and Feldman. Below 1080 A, our measurements fall below the model predictions, reaching a deviation of a factor of 2 near 1010 A and a factor of 4 near 950 A. The present results are compared with those of Brune et al. via Copernicus U2 observations in this spectral range, and possible sources of discrepancies between the various observations and model-atmosphere predictions are discussed. Other aspects of the spectra, particularly with regard to spectral classification, are briefly discussed.

The theory of mass loss from hot stars

Although some evidence had been available earlier, it was the work of Morton (1967) that proved that the OB Supergiants are losing mass. He obtained spectra with a rocket borne spectrograph of the three stars in Orion's belt, δ, ε and ζ Orionis. These stars are all supergiants of spectral type 09.5 or BO. The spectra showed resonance lines of ions such as C III, C IV, Si IV, N V with P-Cygni type profiles. The deepest part of the absorption profile corresponded to a velocity away from the star of 1400 km s−1, and the absorption profile extended out to wavelengths corresponding to velocities of 2000 km s−1 away from the star. Since these velocities are much greater than the escape velocities from the surface of these stars, it is clear that matter is escaping from them. With a number of simplifying assumptions Morton deduced that the rate of mass loss is about 10−6 M⊙ yr−1. This order of magnitude has remained unchanged by the many high quality observations made since that time.

Reddening and polarization of young stars - Intracluster or circumstellar

view Abstract Citations (9) References (9) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Reddening and polarization of young stars: intracluster or circumstellar? Breger, M. Abstract Intracluster dust and circumstellar shells have both been used in the recent literature to explain most of the observed polarization of the highly reddened early-type stars in Orion. Because of the apparent contradiction, the physical arguments used are examined. We find that the main arguments against the intracluster hypothesis (and in favor of the circumstellar hypothesis) are afflicted by numerical error, are inappropriate, or are inconclusive. While both effects cause polarization and reddening of the early-type stars, intracluster dust effects are considerably more important. For the late-type stars in Orion the relative importance of these two mechanisms still needs to be determined. Publication: The Astrophysical Journal Pub Date: July 1978 DOI: 10.1086/156248 Bibcode: 1978ApJ...223..180B Keywords: Early Stars; Interstellar Matter; Linear Polarization; Polarized Light; Star Clusters; Stellar Envelopes; Astronomical Photometry; Cosmic Dust; Ubv Spectra; Astrophysics; Interstellar Reddening:Stellar Spectra; Polarization:Stellar Spectra full text sources ADS |

Ultraviolet spectrophotometry from Gemini 11 of stars in Orion

view Abstract Citations (5) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Ultraviolet spectrophotometry from Gemini 11 of stars in Orion. Morgan, T. H. ; Spear, G. G. ; Kondo, Y. ; Henize, K. G. Abstract Ultraviolet spectrophotometry in the wavelength region 2600-3600 A is reported for the bright early-type stars beta, eta, gamma, delta, iota, epsilon, sigma, zeta, and kappa Ori. The results are in good agreement with other observations, and, with the possible exception of the supergiants, are in good agreement with recent line-blanketed model atmospheres. There is evidence that the supergiants possess a small ultraviolet deficiency shortward of 3000 A relative to main-sequence stars of similar spectral type. The most extreme example of this phenomenon is the star kappa Ori. Publication: The Astrophysical Journal Pub Date: April 1975 DOI: 10.1086/153523 Bibcode: 1975ApJ...197..371M Keywords: Early Stars; Main Sequence Stars; Spaceborne Astronomy; Stellar Spectrophotometry; Supergiant Stars; Ultraviolet Spectra; Astronomical Models; Gemini 11 Flight; Stellar Atmospheres; Stellar Spectra; Ubv Spectra; Astronomy full text sources ADS | data products SIMBAD (11)

Slow flare in Pleiades

In the course of observations of Pleiades on the 40'' Schmidt telescope of Byurakan Observatory, one slow and two rapid flares were detected in star No. 103. The rise time to the maximum of the flare was ~40 min and the total duration was about 5 h. The brightness magnitudes and photographs of the flare are shown. A statistical analysis of a number of pre-maximum images of flare stars in Pleiades and Orion, taken with different exposures, was carried out. The slow flare stars in Orion and Pleiades are compared. (auth)

ON Circumstellar Gas Emission among Pre-Main Stars in IC Orionis and NGC 2264

view Abstract Citations (3) References (7) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS ON Circumstellar Gas Emission among Pre-Main Stars in IC Orionis and NGC 2264 Smith, Myron A. Abstract Photometric and spectrophotometric observations of pre-main-sequence A-ty'pe stars in the Ic Orion is association are reported, confirming by analogy the suggestion of Strom et a!. of Balmer emission in such stars. Actually, the small scatter in the (Ac1, AM,) diagram for these stars would suggest somewhat smaller amounts of visual extinction by circumstellar dust than inferred (and observed) in the case of NGC 2264. Based on a reexamination of gas and dust emission data for NGC 2264, it is concluded that no general correlation between circumstellar gas and dust can be demonstrated. It is argued that a spread in age rather than such a correspondence is most important in explaining the observed range of AC1 in both stellar groups. Publication: The Astrophysical Journal Pub Date: September 1972 DOI: 10.1086/151663 Bibcode: 1972ApJ...176..617S full text sources ADS |

Energy Distributions and Spectra of Orion B Stars

New MK spectral types and energy distributions are presented for B stars in Orion for which far ultraviolet flux excesses have recently been discovered. Significant differences between HD spectral energy distributions show the Orion late B stars to have smaller Balmer discontinuities than do field stars of the same spectral types. For the late B stars, these effects cause the 1500 A fluxes to be under-estimated by approximately 0.5 mag. No comparable systematic effects were found for the early B stars.

Zeeman Measures of Sharp-Lined Early B Stars

view Abstract Citations (25) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Zeeman Measures of Sharp-Lined Early B Stars Conti, Peter S. Abstract Ieasures of Zeeman-analyzed spectrograms of early B stars in Orion are presented. There is no evidence that any of the stars studied have detectable fields. These results do not confirm the presence of magnetic fields in these stars reported by Sargent, Sargent, and Strittmatter. Publication: The Astrophysical Journal Pub Date: February 1970 DOI: 10.1086/150346 Bibcode: 1970ApJ...159..723C full text sources ADS | data products SIMBAD (8)

Far-ultraviolet photometry of orion stars

Far-ultraviolet photometric data for early type stars in Orion, in the 1050–1180 and 1230–1350 A wavelength ranges, were obtained in an Aerobee rocket flight on 30 January 1969. The results corrected for interstellar extinction, appear in good agreement with model atmospheres in the case of main-sequence stars. Bright giant and supergiant stars, however, appear to be up to one magnitude fainter than main-sequence stars of similar spectral class in the 1050–1180 A range.

Far-Ultraviolet Spectrophotometry of Bright Stars in Orion

view Abstract Citations (6) References (12) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Far-Ultraviolet Spectrophotometry of Bright Stars in Orion Stuart, Frank E. Abstract A 12-inch telescope and low-resolution grating spectrophotometer were flown on an Aerobee rocket to obtain absolute vacuum-ultraviolet spectra of bright stars in Orion Nineteen stars, including the Trape- zium, were observed in the spectral range XX1050-2180. A 55 A region around Ly-a was purposely oc- culted to reduce the background due to geocoronal Ly-a light and to test for diffuse sources. The data shortward of Ly-a are not absolute, but are useful for relative comparison between different stars Absolute intensities were generally somewhat higher than those reported by others, although differ- ences were within the error limits of the calibration Agreement of observed spectra with a B 1.5 V model was generally good. A rapid divergence for wavelengths below 1800 A between observed data for an A3 star (i3 En) and a model was noted. Variation of the general slope of the spectra of the earlier-type stars with luminosity class can be observed. A large ultraviolet excess was found for 42 On and probably for BS 1923 Evidence is presented to indicate that in 42 On the excess is probably due to a diffuse glow around the star Publication: The Astrophysical Journal Pub Date: September 1969 DOI: 10.1086/150152 Bibcode: 1969ApJ...157.1255S full text sources ADS | data products SIMBAD (20)

Rocket Observations of Orion Stars with an All-Reflective Ultraviolet Spectrograph

Far UV spectra of O and B stars in Orion photographed from Aerobee rocket, describing wavelength measurements and line identifications

Is Interstellar Hydrogen capable of Maser Action at 21 Centimetres?

THREE sets of observations suggest that the density of interstellar neutral hydrogen may be less than the value usually inferred from 21 cm emission line measurements. This possible discrepancy is found in observations of (a) the incoming high velocity neutral hydrogen clouds which, according to Oort1, deposit material on to the galactic disk at an alarmingly large rate; (b) the ultraviolet spectra of stars in Orion which show an unexpectedly weak interstellar Lyman-α absorption line2,3; and (c) the X-ray background at ∼ 50 A which suggests that the opacity of the Galaxy is less than would be estimated from the observed column density of neutral hydrogen4.

The Far-Ultraviolet Spectra of Six Stars in Orion

Aerobee rocket sounding of far UV spectra of six stars in Orion, extrapolating mass ejection from resonance absorption lines

Helium in the Galactic Disk B Stars

Greenstein and Munch1, and Sargent and Searle2, have recently shown that the atmospheric helium abundance in the halo and globular cluster B stars is low compared with that for normal B stars in the Galaxy. Greenstein, Truran and Cameron3 have suggested that the low abundance may be the result of gravitational diffusion of helium from the atmosphere to a lower layer within the star. They consider that normally mixing currents set up by stellar rotation would counteract such diffusion. In contrast to galactic disk B stars, most of the halo stars appear to be slow rotators. This result is apparently confirmed by Sargent and Strittmatter4, who have discussed the weak helium line stars in Orion and conclude that they are intrinsically slow rotators, and that the anomaly is an atmospheric one.

Far-Ultraviolet Spectra of Six Stars in Orion.

view Abstract Citations (20) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Far-Ultraviolet Spectra of Six Stars in Orion. Morton, Donald C. Abstract An Aerobee rocket carried two ultraviolet objective spectrographs to 186 km on the morning of 13 October 1965 and pointed them towards Orionis within 1 ~. The cameras and stabilization system were similar to those used to obtain spectra of stars in Scorpius in June 1965. This time one of the f/2 Schmidt cameras had a lithium fluoride corrector, while the other used calcium fluoride as be- fore. An unexpected shock during re-entry severed the mirror and film cassettes from the calcium fluoride camera so that films from only the lithium fluoride one were recovered. Development of these films showed ultraviolet spectra of the six OB stars in Orion listed below with their wavelength ranges. Spec- Wavelength Star V B-V trum range Velocity 1 Ori2.20 -0.21 09.511 1200 to 1415A -1400 km sec~ Ori 1.70 -0.19 BOla 1220 1595 -1900 ~On 2.05 09.51b 1200 1740 -1900 ~ Ori3.35 -0.19 BO.5V 1220 1400 (weak) Ori 2.77 -0.25 09111 1235 2100 -3000: Ori 2.04 -0.18 BO.51a 1630 2795 An exposure of 180 sec gave the best spectra, wiil~ a resolution of about 3 A, somewhat poorer than the 1 A obtained on the previous flight. Nevertheless, many absorption lines and a few emission lines are definitely visible in the spectra. Zero-order images of other stars on the same film permit an absolute calibration of wavelengths within 1 A. Emission lines of Si IV at 1402.7 A and C IV at 1549.5 A are identified in ~ and E. Strong absorption lines of both components of the Si IV doublet and the unresolved C IV doublet are found in ~, E, and t shifted to shorter wavelengths by several angstroms. The Si IV doublet is also present in ~, shifted in a similar way. The velocities corresponding to these shifts are indicated in the table. The value for is uncertain because of poor lines and a lack of unshifted features for checking the wavelength scale. These displaced absorption features must be formed by gas escaping from the stars at high velocities indicating a significant mass loss from these early type giants and supergiants. Rough limits on the rate of mass loss can be obtained by assuming that 4~e broadening of the lines 15 entirely by either Maxwellian Doppler velocities or radiation damping. For the widths of the lines in ~ Orionis the Doppler model gives ~10-~ M0 yr-1 while damping gives ~10-~ M0 yr-'. Absorption lines about 8 A wide are also visible at 1216 A in ~ and 8, and are attributed to Lyman-~ absorption by interstellar hydrogen atoms. Assuming broadening by radiation damping, the width corresponds to ~lO~~ atoms Cm-- 1n il~e line of sight. Publication: The Astronomical Journal Pub Date: 1966 DOI: 10.1086/110137 Bibcode: 1966AJ.....71S.172M full text sources ADS | data products SIMBAD (6)

On the origin of comets.

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the origin of comets. Robinson, L. V. Abstract The slow disintegration of comets, shown by the loss of tails and the emergence of meteor swarms, apparently justifies a conclusion that they have not always been members of the solar system and that they may have some time been picked up by the sun as aggregates of cosmic debris. Then, the fact that the B stars in Orion are moving almost directly away from the sun affords good evidence that the solar system must have passed through some of the nebulosity which pervades those regions. The mean radial velocity, about 23.2 km/sec, and distance, 560 light-years, of these stars suggest that the sun passed through these realms some 7,500,000 years ago and picked up masses of material since then represented by comets. Wright-Patterson Air Force Base, Dayton, Ohio. Publication: The Astronomical Journal Pub Date: 1953 DOI: 10.1086/106903 Bibcode: 1953AJ.....58Q.226R full text sources ADS |

The Night Sky in September

THE autumnal equinox on September 23 brings equality of day and night the world over; thereafter (until December 22) the nights increase their duration in the northern hemisphere. The moon is new on September 13 and full (the Harvest Moon) on September 28. Jupiter is in conjunction with the moon on September 1 and 28: Saturn on September 3 and 30: Mars on September 23. Mars, Jupiter and Saturn are the triad of evening stars. Mars, post-opposition, decreases from mag. – 1·8 to mag. – 1·1. Jupiter is in opposition on September 27 and reaches its greatest brightness (mag. – 2·5). Close groupings of Jupiter's four inner satellites are seen at Oh. l5m. on September 5 (satellite II in eclipse), 6, 13, 23, and 30. The ring system of Saturn has now reached its most open phase for this year. The variable star, Algol, is well placed for observation all night. The Pleiades rise in the late evenings “like a swarm of fireflies tangled in a silver braid”. The chief interest, however, on clear moonless nights in autumn is the region of the Milky Way. Apart from its remarkable structure— star clouds, dark rifts and “coal sacks”, and gaseous nebulæ, the galactic plane is also the preferential region for classes of stars of peculiar interest, nova; Cepheid variables and O-type stars. In the direction of the rich fields of Sagittarius lies the centre of our stellar system more than 30,000 light years away and obscured from sight by absorbing tracts of cosmic dust and of gas. Tracing a path slightly inclined to the Milky Way is a belt of bright stars that includes the brightest stars in Orion, Taurus, Cassiopeia, Cygnus and Lyra. These stars and others of the brighter B-type and A-type are representatives of arelatively small local cluster that is some 2,000 light years in diameter and contains the solar system. The average star density of the Milky Way fields is about 40,000 stars per square degree; towards the poles of the galaxy the density falls to about 1,200 stars. The 100-inch telescope at the Mount Wilson Observatory has recorded with exposures of 200 minutes on fields at the north galactic polo as many recognizablo nebula; per unit area of the sky as stars ! These remote stellar systems are exemplified by the great Andromeda Nebula visible to the unaided eye as a hazy patch near the star v Andromedæ.