Stars Of Late Spectral Type Research Articles

Radio Evidence for Nonthermal Particle Acceleration in Normal Stars

The Very Large Array has been used to detect nonthermal radio emission from nearby stars of late spectral type F, G, K, and M, and has provided unique high-resolution investigations of the Sun's radio emission.

First Byurakan Spectral Sky Survey. Stars of late spectral types. −11° ≤ δ ≤ −7° belt

A 10th list of late-type M and C stars found on plates of the First Byurakan Spectral Sky Survey in the − 11° ≤ δ ≤ − 7° belt with an area of about 1070 deg2 is given. The list contains data on 169 red stars, 117 of which were found for the first time: 8 are new C stars, 3 are Cstar candidates, 104 are M stars, 1 is either an M or an S star, and 1 object on the survey plate cannot be classified. Of the 117 objects, 47 are unidentified IRAS sources. A statistical analysis of the objects that are and are not identified with IRAS sources shows that the identified stars are, with a high probability, brighter and have relatively more massive envelopes. Two stars were found to have fairly large brightness variability (with an amplitude of at least 6m.O). Gasdust shells are assumed to exist around nine of the IRAS sources. The equatorial coordinates, spectral types, and stellar magnitudes, determined on Palomar E maps, are given for the selected objects.

Spectral classification of stars in the second Byurakan Sky Survey. II

Results are given on a spectral classification of 316 stars and objects having a continuous spectrum, selected in the course of the Second Byurakan Spectral Sky Survey. Slit spectra are used, obtained from 1978 to 1994, predominantly on the 6-m telescope of the Special Astrophysical Observatory, Russian Academy of Sciences, with a resolution of 5–15 A. The sample contains objects in the range of stellar magnitudes 10.12 ≤ m(pg) ≤ 19.5. We found 114 DA, 13 DB, 4 DC, and 1 DQ white dwarfs, 57 sdB and 13 sdO subdwarfs, 12 NHB, 10 cataclysmic variables, 76 stars of late spectral types, 3 spectroscopic binary systems, and 13 objects with a continuous spectrum. Several recordings are given for each of these spectral types.

FROM GALACTIC TO EXTRAGALACTIC STRUCTURES: GALAXIES IN THE 'ZONE OF AVOIDANCE' BETWEEN 90 AND 110 DEGREES

ABSTRACT The longitude range 90 degrees ≤ l ≤ 110 degrees near the galactic plane is characterized by two interesting features. Firstly, the northern galactic warp shows its north-most bending. Secondly, the Perseus arm and a spiral arm beyond it (first detected in HI) is located in that region. So the starting point of the thesis has been a galactic one: While searching for optical spiral tracers beyond the Perseus arm by concentrating on high transparency regions I noticed an unexpectedly high number of galaxies at low latitudes. On POSS-E-prints I systematically searched for galaxies in the region 90 degrees ≤ l ≤ 110 degrees, -10 degrees ≤ b ≤ +10 degrees and found more than 1300 mostly uncatalogued galaxies. The survey was done using a binocular microscope at 16-fold magnification. This method allows to include also elliptical galaxies. I have discussed pros and cons of other search strategies like 21cm blind searches or IRAS-flux criteria methods. The smallest diameters of the galaxies are 0.1' For diameters larger than 0.4' the catalogue is complete. 5.5% of the galaxies proved to be IRAS-point sources. The significant north-south asymmetry in the number density has been shown to originate in the galactic extinction distribution. A new cluster of galaxies at a velocity of about 5200 km/s at (l,b) = (106 degrees, -6 degrees) has been detected. The investigated part of the sky marks the northwestern end of the Pisces-Perseus-Supercluster near the galactic plane. A review on the various connections between Pisces-Perseus to other clusters is given. In a joint program with W.K. Huchtmeier from the MPIfR at Bonn, radial velocities, line widths and HI-masses of 57 galaxies have been measured at the Effelsberg 100m radio telescope. With the 1.93m telescope at the OHP I got 33 optical spectra and determined radial velocities. I could trace the PPScl. to at least b = -5 degrees. From literature data, a long chain of galaxies covering all the first galactic quadrant is visible. It runs almost parallel to the galactic equator and at moderate latitudes. The new radial velocities indicate that this chain extends to l = 107 degrees, b = -5 degrees. We classified CGCG 493-2 as a new Seyfert 1.5 galaxy. The 'compact galaxy' UGC 11823 is in fact a star of late spectral type. As a byproduct a number of new galactic objects (like planetary nebulae, bipolar nebulae, HII-regions, SNR's) have been found.

Radio evidence for nonthermal particle acceleration on stars of late spectral type

Radio evidence for nonthermal particle acceleration on stars of late spectral type (G, K, and M) is reviewed, including the pre-main-sequence T Tauri stars, the dwarf M flare stars, and the RS CVn binary stars. Both the long-lasting radio emission and the transient radio flares can be attributed to nonthermal electrons accelerated in magnetically confined coronae, radiating by either incoherent gyrosynchrotron radiation or by coherent processes. Related observations of the Sun provide the framework for interpreting the radio emission of late-type stars. We review recent Very Long Base Interferometry (VLBI) observations of single and binary stars, observations of rapid acceleration and dynamic spectra of dwarf M flare stars, and models for long-lasting radio emission from dwarf M flare stars and RS CVn stars. The VLBI data provide evidence for nonthermal particle acceleration in large-scale magnetospheres several times larger than the stellar radius for T Tauri and RS CVn stars. In contrast, rapid radio flares on dwarf M stars require sources much smaller than the visible star in size, and this is consistent with the VLBI data. Dynamic spectra suggest particle acceleration by relativistic electron beams on dwarf M stars. The long-lived radio radiation of dwarf M stars and RS CVn stars requires more-or-less continuous particle acceleration over periods of hours, perhaps by persistent low-amplitude flaring.

Radio Evidence for Nonthermal Particle Acceleration on Stars of Late Spectral Type

AbstractRadio evidence for nonthermal particle acceleration on stars of late spectral type (G, K, and M) is reviewed, including the pre-main-sequence T Tauri stars, the dwarf M flare stars, and the RS CVn binary stars. Both the long-lasting radio emission and the transient radio flares can be attributed to nonthermal electrons accelerated in magnetically confined coronae, radiating by either incoherent gyrosynchrotron radiation or by coherent processes. Related observations of the Sun provide the framework for interpreting the radio emission of late-type stars. We review recent VLBI observations of single and binary stars, observations of rapid acceleration and dynamic spectra of dwarf M flare stars, and models for long-lasting radio emission from dwarf M flare stars and RS CVn stars. The VLBI data provide evidence for nonthermal particle acceleration in large-scale magneto-spheres several times larger than the stellar radius for T Tauri and RS CVn stars. In contrast, rapid radio flares on dwarf M stars require sources much smaller than the visible star in size, and this is consistent with the VLBI data.Dynamic spectra suggest particle acceleration by relativistic electron beams on dwarf M stars. The long-lived radio radiation of dwarf M stars and RS CVn stars requires more-or-less continuous particle acceleration over periods of hours, perhaps by persistent low-amplitude flaring.Subject headings: acceleration of particles — binaries: close — radio continuum: stars — stars: flare — stars: late-type — stars: pre-main-sequence

EMPIRICAL CONTINUUM MODELING FOR IUE LOW-DISPERSION SPECTRA

ABSTRACT The authors describe a numerical procedure for empirically determining a "continuum" level in low-dispersion spectra from the International Ultraviolet Explorer. They developed the technique for the automated fitting of SWP-LO (1150 Å - 2000 Å) spectra of stars of late spectral type, but it is more widely applicable.

Redshifts of high-temperature emission lines in the far-ultraviolet spectra of late-type stars. II - New, precise measurements of dwarfs and giants

Results are presented from an IUE SWP camera investigation of the occurrence of gasdynamic flows, analogous to the downdrafts of 10 to the 5th K material observed over magnetic active regions of the sun, among stars of late spectral type. The SWP calibration spectra study conducted documents the existence of local, small, persistent distortions of the echelle wavelength scales that are of unknown origin. Attention is given to the enormous widths of the stellar high-excitation emission lines in both the dwarfs and the giants, with respect to the comparatively small, subsonic Doppler shifts; the widths are typically an order of magnitude greater than the redshifts.

A method for the analysis of long series of visual observations of red, small amplitude variable stars

This paper proposes a method that corrects the spurious effects induced by systematic differences in the estimates made by a great number of visual observers. Their own systematic error is calculated individually for each observer and the duly corrected estimates then analyzed using weighted means. The method has been applied to numerous series of visual estimates obtained by GEOS observers on variable stars of late spectral type and a comparison with photoelectric measures shows its efficiency. This method could be especially useful in the analysis of visual series secured on long-period or irregular variables.

The Sun and Nearby Stars: Microwave Observations at High Resolution

High-resolution microwave observations are providing new insights into the nature of active regions and eruptions on the sun and nearby stars. The strength, evolution, and structure of magnetic fields in coronal loops can be determined by multiple-wavelength observations with the Very Large Array. Flare models can be tested with Very Large Array snapshot maps, which have angular resolutions of better than 1 second of arc in time periods as short as 10 seconds. Magnetic changes that precede solar eruptions on time scales of tens of minutes involve primarily emerging coronal loops and the interactions of two or more loops. Magnetic reconnection at the interface of two closed loops may accelerate electrons and trigger the release of microwave energy in the coronal parts of the magnetic loops. Nearby main-sequence stars of late spectral type emit slowly varying microwave radiation and stellar microwave bursts that show striking similarities to those of the sun.

SPECTROSCOPIC OBSERVATIONS OF PRE-MAIN-SEQUENCE STARS IN THE PLEIADES.

ABSTRACT Spectroscopic observations of three of the relatively old (220 million years) pre-main-sequence stars of the Pleiades cluster are presented. The observations were carried out at a dispersion of 30 A/mm in the blue and 60 A/mm in the red spectral ranges and resolutions of 1.7 and 3.5 A, respectively, with the image dissector scanner at the ESO 3.6-m telescope. All three stars are observed to show H-alpha, H-gamma and Ca II H and K emission, with H-delta emission also observed in H-134 and H-1321 and H-epsilon possibly present in H-1173. Broad emission lines are noted for H-134, with widths (FWHM approximately 110 km/sec) considerably greater than those of older field dMe stars (FWHM approximately 20 km/sec). The emission-line broadening, which may be a relic of the T Tauri phase, is explained by direct production in the line-emitting region. A search for the Li 6707 resonance line reveals an upper limit of 0.6 for the log of Li abundance on the observed stars, consistent with extrapolation results of Zappala (1972) on stars of late spectral type.

The photodetachment cross section of the negative hydrogen ion

The photoionization cross section of the hydrogen negative ion is computed using a discrete-basis-set expansion to represent both the ground and continuum states of the system. This discrete-basis-set representation is used to construct an approximation to the dynamic polarizability, which in turn is used in an analytical continuation procedure for complex values of the frequency. This discretization of the continuum makes the method potentially applicable to larger systems, and a study of the basis-set dependence is made. The final results are in good agreement with more elaborate calculations and with experimental results. The dynamic polarizability in the normal dispersion region is also computed. I. INTRODUCTION The photoionization cross section of the hydrogen negative ion has been the subject of several calculations since Wildt' first suggested that it could be responsible for the visible and infrared opacity in the atmosphere of cooler late spectral type stars. Ghandrasekhar' and his co-workers pioneered these studies in a series of papers investigating the dependence of the cross section on the accuracy of the bound-state wave function. They also investigated the effect of the central field approximation for the free state. ' The importance of,taking into account exchange effects in the free wave function was first investigated by John' using the Hart-Herzberg 20-parameter wave function for the ground state and the 1sexchange approximation continuum function. A number of subsequent calculations have appeared. Geltman' employed the 70-parameter bound-state wave function of Schwartz and variationally determined symmetrized continuum functions, containing terms corresponding to excited states of the hydrogen atom. This allowed him to treat both correlation and distortion of the residual hydrogen atom. Doughty et al. ' employed a close-coupling approach for using a Hartree-Pock eigenfunction expansion for the continuum states which included up to 3d atomic orbitals of the hydrogen atom. Bell and Kingston' used the method of polarized orbitals to obtain a representation of the continuum. More recently Ajmera and Ghung' used the variational method of Kohn-Feshbach to obtain free-state wave functions. A recent review of the photodetachment of H was given by Risley. ' While many of the above calculations have produced good results for H, they are generally not easily extended to large systems, first because such highly accurate wave functions are not available for systems with more electrons, and secondly because the generation of continuum functions by the methods used above becomes very cumbersome '0 Our interest is in developing a method w'hich could be equally applied to larger systems. Thus we use highly accurate wave functions and methods practicable for polyatomic molecules, and w'e avoid

Uranus occults SAO158687

ON March 10, 1977, Uranus will occult SAO 158687, a star of late spectral type with a visual magnitude of ≈ +9.5 (refs 1, 2). The occultation will be visible from land areas surrounding the Indian Ocean1,2. From high-quality light curves of this event one can obtain temperature, pressure and number density profiles of the uranian atmosphere; information about its composition; and by combining occultation timings from several stations the precise diameter and oblateness of the planet can be found. For the atmospheres of Jupiter3–5 and Mars6–9 these quantities have been obtained from recent stellar occultations by these planets. This experience can be used to predict what can be learned from the forthcoming occultation by Uranus.

A discussion on infared astronomy - Near infrared photometry of late-type stars

A multi-band photoelectric photometer for observations in the ultraviolet, blue and visible and the infrared bands W (1.06μm), X (1.13 μm, Y (1.63 μm) and Z (2.21 μm) has been constructed and applied to both stellar and planetary observations. The results of photometry obtained for 61 stars are presented. The observations at λ = 1.63 μm are shown to exhibit an excess flux due to a minimum in the H opacity in accordance with the predictions of model atmosphere studies. Bolometric corrections are derived for stars of late spectral type from integration of the observed absolute spectral irradiance curve. A simple photometric method for the measurement of stellar diameters is proposed based upon the absolute irradiance observed at 2.21 μm and the 2.21 μm flux at the surface of the star calculated from model atmospheres. Angular diameters derived by this technique are consistent with interferometric results; and, when combined with the bolometric corrections, effective temperatures are found.

Binary Frequency as a Probable Index of Secular Changes in the Main Sequence.

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Binary Frequency as a Probable Index of Secular Changes in the Main Sequence. Hynek, J. Allen Abstract As Aitken pointed out long ago, stars of late spectral type have a lower proportion of visual companions than do stars of early spectral type. This appears to apply to spectroscopic binaries as well. According to present theories of stellar evolution, this implies a secular change in the frequency of binaries along the main sequence. The reality of the binary frequency imbalance between giant and main sequence stars has been re-examined with emphasis on possible selection factors and on its relation to space motions of the stars concerned. The discussion is limited to the Yale Bright Stars, for which stars there exists maximum homogeneity of observations of multiplicity, radial velocity, and spectra. It is concluded that observational selection, difference in Population, and space motions are not sufficient to explain the deviation from a "steady-state" main sequence implied by the binary-poor nature of the present giant population. The more recently formed stars appear to have a marked increase in the number of binary stars. Publication: The Astronomical Journal Pub Date: 1965 DOI: 10.1086/109640 Bibcode: 1965AJ.....70S.140H full text sources ADS |

Luminosity classification of stars of late spectral type from spectra of moderate dispersion

Extensive classification has been undertaken, on the MK system, of spectrograms of G and K type taken at the David Dunlap Observatory for radial velocity determinations. An oscilloscopic microphotometer which was developed at this observatory has provided a rapid means of effecting classification to a decimal subdivision of the MK luminosity classes. Tests indicate that the probable error of a luminosity obtained by this method is about one-tenth of an MK interva.

Bright-Line Stars among the Taurus Dark Clouds.

On QbjeCtiVe-priSm photographs of three areas of the Taurus dark clouds, 40 faint stars with Ha emission were discovered. Slit spectrograms of 35 of these stars were examined for the purpose of finding spectral ,peculiarities which might be attributed to interaction between the stars and the surrounding clouds. Except for two stars of type Be which are probably not involved in the clouds and one peculiar A4e star, the stars are main-sequence stars of late spectral type and low luminosity. The stars are con- centrated in small regions in or near areas of great obscuration. If these stars are of normal luminosity and are at the distance of the clouds, their apparent magnitudes are generally too faint as a result of space absorption. The spectra show many of the characteristics previously found in the T Tauri variables. The hydrogen and Ca ii (H and K) bright lines are extremely strong in spectral types dGe-dM2.5e. Emission lines of He I, Fe I, Fe II, and [S ii] decrease in intensity in later spectral types in the two Taurus areas but are practic3lly absent in stars of the area in Orion. Related stars-A few stars with similar spectra, located in other regions of the sky are discussed for comparison with the Taurus stars. Radi~al velocities.-The approximate radial velocities from emission and absorption lines were meas- ured. The differences, emission minus absorption, seem to be greatest for the earlier types, which also show the strongest T Tauri characteristics

On the motions, parallaxes and luminosities of the long-period variables and other stars of late spectral types

On the motions, parallaxes and luminosities of the long-period variables and other stars of late spectral types

A Determination of the Solar Motion and the Stream-Motion from Radial Velocities and Absolute Magnitudes of Stars of Late Spectral Types

A Determination of the Solar Motion and the Stream-Motion from Radial Velocities and Absolute Magnitudes of Stars of Late Spectral Types