Late-type Dwarf Stars Research Articles

Microwave emission from the coronae of late-type dwarf stars

VLA microwave observations of 14 late-type dwarf and subgiant stars and binary systems are examined. In this extensive set of observations, four sources at 6 cm (Chi-1 Ori, UV Cet, YY Gem, and Wolf 630AB) were detected and low upper limits for the remaining stars were found. The microwave luminosities of the nondetected F-K dwarfs are as small as 0.01 those of the dMe stars. The detected emission is slowly variable in all cases and is consistent with gyroresonant emission from thermal electrons spiraling in magnetic fields of about 300 gauss if the source sizes are as large as R/R(asterisk) = 3-4. This would correspond to magnetic fields that are probably in the range 0.001-0.0001 gauss at the photospheric level. An alternative mechanism is gyrosynchrotron emission from a relatively small number of electrons with effective temperature.

Outer atmospheres of cool stars. XV - High-dispersion ultraviolet studies of active chromosphere G-K dwarfs

IUE ultraviolet echelle spectra of three late-type active chromosphere dwarf stars, Chi-1 Orionis, Xi Bootis, and Epsilon Eridani, are compared with previously published observations of the quiet chromosphere dwarfs Alpha Centauri A and Alpha Centauri B. The spectra indicate a gross strengthening of chromospheric and transition-region line fluxes from quiet to active dwarf stars without a fundamental change in their line shapes. Qualitatively the same spectroscopic behavior is seen in detail on the solar surface between regions of strong and weak magnetic fields. It is argued that the correspondence between the stellar and solar behavior is not accidental, but instead affirms that enhanced chromospheric and coronal emission on late-type main sequence stars is a result of increased surface coverage of magnetic active regions.

On the Stellar Rotation-Activity Connection

I discuss the connection between the rotation rate of late-type dwarf stars and their level of surface “activity” (as deduced from visible, UV, and X-ray observations) from both theoretical and observational perspectives.

First detection of nonflare microwave emissions from the coronae of single late-type dwarf stars

Results are presented of a search for nonflare microwave radiation from the coronae of nearby late-type dwarf stars comparable to the sun: single stars without evidence for either a large wind or circumstellar envelope. The observing program consisted of flux measurements of six stars over a 24-h period with the VLA in the C configuration at a wavelength of 6 cm with 50 MHz bandwidth. Positive detections at 6 cm were made for Chi 1 Ori (0.6 mJy) and the flare star UV Cet (1.55 mJy), and upper limits were obtained for the stars Pi 1 UMa, Xi Boo A, 70 Oph A and Epsilon Eri. It is suggested that Chi 1 Ori, and possibly UV Cet, represent the first detected members of a new class of radio sources which are driven by gyroresonance emission, i.e. cyclotron emission from nonrelativistic Maxwellian electrons.

The nature of the Giclas +4 stars

view Abstract Citations (10) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The nature of the Giclas + 4 stars. Weistrop, D. Abstract Broadband BVRI photometry has been obtained for 130 stars in the Lowell Observatory proper motion survey with color class +4. The sample is complete for the 75 +4 class stars between RA = 2h31m and RA = 12h20m in the Northern Hemisphere. The absolute magnitudes and distances of the stars are estimated using the R magnitude-(R-I) relation. Most of the stars are normal late-type dwarf stars. The B-V color distribution for the +4 class stars is similar to that for +3 class stars, suggesting there is no significant difference between the two color classes. Several individual stars which may be subluminous or otherwise of interest have been identified, as have several stars within 25 pc which are not included in published catalogs of nearby stars. Publication: The Astronomical Journal Pub Date: August 1981 DOI: 10.1086/113001 Bibcode: 1981AJ.....86.1220W Keywords: Astronomical Catalogs; Dwarf Stars; Late Stars; Stellar Spectrophotometry; Broadband; Colorimetry; Stellar Motions; Ubv Spectra; Astronomy full text sources ADS | data products SIMBAD (147) GCPD (1)

Lithium in flare stars

We determine the lithium abundance by anon-LTE method (KE) of a group of late-type dwarf stars. The majority of these stars have a Li abundance less than or equal to the solar value. Only one exception (Gliese 182) has a high Li abundance with a value near to that of the interstellar Li. If these results are typical abundances of this type of stars, we can eliminate them as sources of interstellar Li enrichment. We do not find a general relation between the presence of Li and chromospheric activity. Gliese 182 is a Li-rich star with high flare incidence activity and may be a member of a very young kinematical group formed by other non-Li stars that show similar behaviour.

Observations of magnetic fields on two late-type dwarf stars

view Abstract Citations (145) References (9) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Observations of magnetic fields on two late-type dwarf stars. Robinson, R. D. ; Worden, S. P. ; Harvey, J. W. Abstract The detection of magnetic fields on the G8 V star Xi Boo A and on the K0 B star 70 Oph A is reported. A method has been developed and applied which provides estimates of the total field strength and fraction of the visible stellar surface covered by fields. The essence of this method is to perform the Fourier deconvolution of a nonmagnetically sensitive line profile from a magnetically sensitive one. This method is insensitive to magnetic field geometry and does not depend on polarization measurements. Results indicate fields of 2550 + or 390 gauss covering 20-45% Xi Boo A, and that fields of 1880 + or - 350 gauss may cover 10% of 70 Oph A. Comparison observations of solar active regions show fields of 1800 + or - 550 gauss covering 10% of the solar active regions, in line with previous work. Tests of solar quiet regions and sunspots also support the validity of the stellar measurements. Publication: The Astrophysical Journal Pub Date: March 1980 DOI: 10.1086/183217 Bibcode: 1980ApJ...236L.155R Keywords: Dwarf Stars; Late Stars; Magnetic Stars; Stellar Magnetic Fields; Field Strength; Line Spectra; Solar Activity; Zeeman Effect; Astrophysics full text sources ADS | data products SIMBAD (6)

OBSERVATIONS OF SEVERAL BY DRACONIS VARIABLES.

ABSTRACT Eight late-type dwarf stars were surveyed with the KPNO no. 4 0.4-m telescope during the period 1978 May 10 to 17 for photometric variability of the BY Draconis type. Five of the stars were not variable at the 0.02-magnitude level. The appearance of variations of a very substantial amplitude in BD + 36 deg 2322 is new. The object has developed at one point in a full range of at least 0.1 mag. The data are inadequate to establish a period, but a simple sine-wave of period 3.2 days seems to phase the limited observations well. This appearance of BY Dra-like variations in a previously thought nonvariable object suggests that most if not all of the dMe stars are subject to the syndrome. The primary purpose of the observations was to establish rotational phases, but this attempt was frustrated by bad weather, although a few records were obtained which yield a negative result.

STELLAR SURFACE PHENOMENA: HALPHA VARIABILITY IN DME STARS.

ABSTRACT Image-tube spectrograms (resolution 1 A) of the Ha emission line in 15 late-type dwarf stars are examined for evidence of equivalent-width variability on a time scale of days, weeks, and years. Scattered data show Ha variations in three known BY Draconis variables: YY Gem, Gl 182, and BD +48" 1958A. A much more extensive series of spectrograms of BY I)ra itself obtained during 1972 and 1976 shows only a marginal correlation between Ha EW and rotational phase. Criteria for detection of such variability are noted. Key words: dMe stars-BY Draconis variables-Ha emission-starspots

Stellar model chromospheres. VIII - 70 Ophiuchi A /K0 V/ and Epsilon Eridani /K2 V/

view Abstract Citations (57) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Stellar model chromospheres. VIII. 70 Ophiuchi A (K0 V) and epsilon Eridani (K2 V). Kelch, W. L. Abstract Model atmospheres for the late-type active-chromosphere dwarf stars 70 Oph A and Epsilon Eri are computed from high-resolution Ca II K line profiles as well as Mg II h and k line fluxes. A method is used which determines a plane-parallel homogeneous hydrostatic-equilibrium model of the upper photosphere and chromosphere which differs from theoretical models by lacking the constraint of radiative equilibrium (RE). The determinations of surface gravities, metallicities, and effective temperatures are discussed, and the computational methods, model atoms, atomic data, and observations are described. Temperature distributions for the two stars are plotted and compared with RE models for the adopted effective temperatures and gravities. The previously investigated T min/T eff vs. T eff relation is extended to Epsilon Eri and 70 Oph A, observed and computed Ca II K and Mg II h and k integrated emission fluxes are compared, and full tabulations are given for the proposed models. It is suggested that if less than half the observed Mg II flux for the two stars is lost in noise, the difference between an active-chromosphere star and a quiet-chromosphere star lies in the lower-chromospheric temperature gradient. Publication: The Astrophysical Journal Pub Date: June 1978 DOI: 10.1086/156212 Bibcode: 1978ApJ...222..931K Keywords: Chromosphere; Dwarf Stars; Late Stars; Stellar Atmospheres; Stellar Models; Stellar Spectra; Binary Stars; H Lines; K Lines; Line Spectra; Metallic Stars; Photoionization; Photosphere; Spectrum Analysis; Stellar Mass; Stellar Temperature; Temperature Distribution; Astrophysics; Line Profiles:Stellar Chromospheres; Stellar Chromospheres: Models full text sources ADS | data products SIMBAD (2) Related Materials (12) Part 1: 1973ApJ...180..473L Part 2: 1974ApJ...192...93A Part 3: 1975ApJ...200..660A Part 4: 1975ApJ...201..212A Part 5: 1976ApJ...210..199A Part 6: 1978ApJ...220..619L Part 7: 1978ApJ...220..962K Part 9: 1979ApJ...229..700K Part 10: 1979ApJS...41...47L Part 11: 1979ApJS...41..481L Part 12: 1981ApJ...246..502G Part 13: 1982ApJ...258..740G

Broad-band infrared colors and CO and H2O absorption indices for late-type dwarf stars

Broad-band infrared colors and CO and H2O absorption indices for late-type dwarf stars

Studies of late-type dwarfs. II - A photometric metal abundance index and application to galactic evolution

view Abstract Citations (27) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Studies of late-type dwarfs. II. A photometric metal abundance index and application to galactic evolution. Hartwick, F. D. A. Abstract Photometric and radial-velocity data are presented for 37 southern late-type dwarf stars. With data from the literature for a larger sample of stars, it is shown that late-type subdwarfs have deficiencies of light in the blue region of the spectrum when compared with normal stars of the same (R - I) color. A photometric index that is a measure of the blue deficiency is defined and shown to correlate with the ultraviolet excess. The photometric index is also shown to correlate with the dynamical quantities of the star's orbit in the Galaxy. Publication: The Astrophysical Journal Pub Date: June 1977 DOI: 10.1086/155307 Bibcode: 1977ApJ...214..778H Keywords: Abundance; Dwarf Stars; Galactic Evolution; Late Stars; Radial Velocity; Stellar Spectrophotometry; Chemical Evolution; Correlation; Stellar Motions; Astrophysics full text sources ADS | data products SIMBAD (39) GCPD (1) Related Materials (5) Part 1: 1976ApJ...208..776H Part 3: 1982ApJ...253..237C Part 4: 1981AJ.....86.1200D Part 5: 1983ApJ...266..747V Part 6: 1984ApJ...286..269H

The Space Density of M Dwarfs - An Observational Program

In spite of extensive proper motion surveys, there remains some uncertainty about the space density and the kinematics of late-type dwarf stars. This uncertainty is primarily due to the unknown kinematic bias introduced by the proper motion discovery technique.A spectrographic search, especially one using an objective prism on a Schmidt telescope, can identify significant numbers of intrinsically (and actually) faint stars, free from any kinematic bias. To find faint and cool stars, one chooses an objective prism with low dispersion and emulsions sensitive to the red and near photographic infrared. At low dispersions, in this wavelength region, there are no reliable luminosity criteria. Thus Sanduleak conducted his survey in the direction of the north galactic pole, with the expectation that very few remote giants would contaminate his results. A catalog of 273 probable dwarf stars of type M3 and later (Sanduleak, N. 1976, A.J., 81, 350) based on his objective prism survey in the direction of the north galactic pole is now available. Thanks to W.J. Luyten (1976, Proper Motion Survey with the Forty-Eight Inch Schmidt Telescope XLVI (Univ. Minnesota, Minneapolis)) proper motions have been measured for all of these stars.

Selective Excitation in the Flare of 1958 August 7

view Abstract Citations (5) References (10) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Selective Excitation in the Flare of 1958 August 7 Cowley, Charles ; Marlborough, J. M. Abstract The Fe i line X4063 is enhanced in a region of the flare of 1958 August 7. The enhancement is inter- preted as a selective-excitation effect similar to that observed in late-type dwarf stars. The appearance of this selective excitation is used to obtain information on the temperature and electron density in the region of the flare where the line arises Publication: The Astrophysical Journal Pub Date: November 1969 DOI: 10.1086/150239 Bibcode: 1969ApJ...158..803C full text sources ADS |

A sensitive photometric luntinositv discriminant for late-type dwarf and giant stars

view Abstract Citations (4) References (7) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A sensitive photometric luntinositv discriminant for late-type dwarf and giant stars Neff, John S. Abstract Filter photometry of late-type giants and dwarfs with intermediate bandpass interference filters located at 0.33, 0.37, 0.47, and 0.55 is presented. The locations of unreddened dwarfs and giants as well as little reddened supergiants on several two-color diagrams are shown. Giants and dwarfs later than CS on the MK system, with similar m.47-m.55 colors, can have ultraviolet colors .33- . that can differ by as much as 0W2 with the giants showing a deficiency relative to the dwarfs. It is suggested that late-type dwaris can be separated from giants by photometric observations of this type. Publication: The Astronomical Journal Pub Date: April 1966 DOI: 10.1086/109905 Bibcode: 1966AJ.....71..202N full text sources ADS | data products SIMBAD (3)

The Possible Connexion Between T Tauri Stars and UV Ceti Stars

As is well known, very rapid and non-periodic changes in brightness have been discovered in several late-type dwarf stars in recent years. In the vicinity of the sun, within a radius not exceeding 10 parsecs, nine or ten such objects have been found and named ‘flare’ stars because of their extraordinarily rapid variations. The prototype of these flare stars is UV Ceti. For the purpose of the present discussion, we shall call these objects ‘classical’ flare stars.

Hα Emission Stars and Peculiar Objects in the Orion Nebula.

view Abstract Citations (126) References (7) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Hα Emission Stars and Peculiar Objects in the Orion Nebula. Haro, Guillermo Abstract Two hundred and fifty-five stars showing emission in Ha have been observed in an area of 3.5 square degrees which contains the brightest part of the Orion Nebula. The distribution and frequency of these emission stars seem to be correlated with the general shape and differing intensity of the bright nebulosities of this region. At the same time the existence of a dense stellar group provisionally called the "Orion Nebula Cluster' `-centered at the Trapezium stars is strikingly revealed. Fourteen peculiar objects, most of them stars imbedded in nebulous material, have been found in the same area. One of these is probably a new faint cometary nebula. The apparent coexistence and concentricity observed between the Nebuia and the densest group of emission stars suggests a tentative hypothesis of the formation of late-type dwarf stars from the Orion nebular material. Publication: The Astrophysical Journal Pub Date: January 1953 DOI: 10.1086/145669 Bibcode: 1953ApJ...117...73H full text sources ADS | data products SIMBAD (270)

The spectra of the late-type dwarf stars

The spectra of the late-type dwarf stars

Special characteristics of a few late-type dwarf stars.

Special characteristics of a few late-type dwarf stars.