Showing papers by "Lee Hartmann published in 1984"

Rotation, convection, and magnetic activity in lower main-sequence stars

Abstract: Rotation periods are reported for 14 main-sequence stars, bringing the total number of such stars with well-determined rotation periods to 41. It is found that the mean level of their Ca n H and K emission (averaged over 15 years) is correlated with rotation period, as expected. However, there is a further dependence of the emission on spectral type. When expressed as the ratio of chromospheric flux to total bolometric flux, the emission is well correlated with the parameter Pohs/Tc, where Pohs is the observed rotation period and tc(B—V) is a theoretically-derived convective overturn time, calculated assuming a mixing length to scale height ratio a ~ 2. This finding is consonant with general predictions of dynamo theory, if the relation between chromospheric emission and dynamo-generated magnetic fields is essentially independent of rotation rate and spectral type for the stars considered. The dependence of mean chromospheric emission on rotation and spectral type is essentially the same for stars above and below the Vaughan-Preston “gap,” thus casting doubt on explanations of the gap in terms of a discontinuity in dynamo characteristics. Subject headings: Ca n emission — convection — stai

Cool stars, stellar systems and the Sun

Abstract: These conference proceedings focus on cool stars, stellar systems and the Sun. Individual papers deal with star clusters, star evolution, and star models. Particular attention is paid to binary stars.

Rotational velocities of low-mass stars in the Hyades

Abstract: High-resolution echelle spectra have been used to estimate rotational velocities for K and M dwarfs in the Hyades. All of the K dwarfs have rotational velocities less than the instrumental limit of 10 km/s. Most of the M dwarfs with (R - I)K larger than 1.0 have detectable rotational velocities, with v sin i between 10 and 20 km/s. Combining these data with results from the younger Pleiades and Alpha Persei clusters, it is found that G dwarfs spin down from about 100 km/s to about 10 km/s in less than 40 million yr, whereas M dwarfs take an order of magnitude longer to spin down.

On the extended chromosphere of Alpha Orionis

Abstract: A detailed examination of the extended chromosphere of Alpha Orionis was performed in an attempt to characterize the matter-emitting processes of red giants and supergiants. The problem was approached using an Alfven wave driven stellar wind model to compute the density, temperature, and velocity structure of the wind. It is shown that the mechanical energy fluxes necessary to maintain the observed chromospheric temperatures are commensurate with the momentum deposition necessary to extend the chromosphere. The chromospheric model constructed is, however, inconsistent with the kinematics of the observed symmetric profiles, which could correspond to a fountain-type model that described infalling, rather than escaping, ejected material such as He.

An analysis of the Vaughan-Preston survey of chromospheric emission

Abstract: Etude des proprietes statistiques d'un releve de Voughan-Preston de l'emission chromospherique d'etoiles proches, a l'aide de modeles du declin de l'activite chromospherique avec un âge croissant

Chromospheres and mass loss in metal-deficient giant stars

Abstract: Semiempirical atmospheric models indicate that the characteristic emission in the wings of the H-alpha line observed in Population II giant stars can arise naturally within static chromospheres. Radial expansion gives an asymmetric, blueshifted H-alpha core accompanied by greater emission in the red line wing than in the blue wing. Wind models with extended atmospheres suggest mass loss rates much smaller than 2 x 10 to the -9th solar mass per yr. Thus H-alpha provides no evidence that steady mass loss can significantly affect the evolution of stars on the red giant branch of globular clusters.

The photometric variability of solar-type stars. iv. detection of rotational modulation among hyades stars.

Abstract: High-precision differential photometric observations of 32 Hyades stars, spectral type F4 V to K1 V, were obtained at Lowell Observatory over a five-month season during late 1982 and early 1983. Contemporaneous Ca II H+K emission flux measurements for 30 of these stars were made at Mount Wilson Observatory. Fifteen (possibly 18) of these stars, all later than type F7 V, were found to be photometrically variable. Highly significant correlations between continuum and Ca II emission variations were found in several cases. The photometric variations for nine of the stars were periodic. The authors have interpreted this as rotational modulation, since the measured periods agreed well with rotation periods predicted from measurements of the mean chromospheric emission fluxes for these stars.

A high-resolution study of Herbig-Haro objects 1 and 2

Abstract: Observations have been conducted of Herbig-Haro objects 1 and 2 with high spectral and spatial resolution in order to test shock models for the line emission. A detailed bow shock model has been constructed which yields line widths and optical and UV spectra in reasonable agreement with observation for maximum shock velocities approximately 200 km/s. Such velocities are comparable to the transverse velocities of the knots in HH 1 and 2 inferred from proper motion studies. The observations suggest that HH objects are rapidly moving, dense clouds shocking with less dense interstellar medium. The large energy requirements of these objects, combined with their small apparent size as seen from the exciting star, seem to require unreasonably large stellar mass loss rates. However, it is possible that the HH objects subtended a much larger solid angle at the star when originally accelerated.

A study of the dependence of Mg II emission on the rotational periods of main-sequence stars

Abstract: International Ultraviolet Explorer satellite measurements of the Mg II chromospheric emission from a sample of late-type dwarfs with known rotational periods are presented. The ratio of chromospheric to photospheric luminosity for the late-G and K type stars in this sample can be fitted within the probable errors by a single function of rotational period. The chromospheric emission of early-G and F dwarfs appears to depend upon rotation in a qualitatively different way, which suggests that the rapid variation of convective zone properties in this spectral range affects the dissipation of energy in the chromosphere. Mg II h and k emission correlates fairly well with Ca II emission. However, the data sample is not large enough at present to show conclusively that Mg II emission exhibits precisely the same dependence on rotation and spectral type as the Ca II H and K line fluxes.

How to find galaxies at high redshift.

Abstract: The UV and optical spectra presented for the blue star-forming galaxies Mrk 357, Mrk 496, and Mrk 26 have sufficiently large redshifts to make Lyman-alpha observations possible. Weak Lyman-alpha emission is observed in Mrk 357 and Mrk 469, with a flux value comparable to that of H-beta, and no detectable Lyman-alpha emission is detected in the case of Mrk 26. All three objects exhibit strong UV continuum emission, which is characteristic of hot stars, and although little evidence of dust is found, the observational constraints on extinction and neutral hydrogen column densities allow substantial reductions of the Lyman-alpha flux by dust absorption in conjunction with large line optical depths. These results imply that primeval galaxy detection calls for attention to be given to redshifted continuum emission from massive galaxies.

Cool stars, stellar systems, and the sun : proceedings of the Third Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun, held in Cambridge, Massachusetts, October 5-7, 1983

Abstract: Recent observations of T Tauri stars.- Results from ultraviolet observations of T Tauri stars.- X-ray emission from pre-main sequence stars.- Infrared and molecular line observations of young stars and associated dense gas.- Photometric monitoring of T Tauri stars, RS CVn stars and related objects.- Angular momentum loss during pre-main sequence contraction.- Atmospheres and winds of T Tauri stars.- Post-T Taubi stars in T associations.- The formation of cool stars from cloud cores.- A dynamical origin for the stellar birthline.- Rotational velocities of low mass stars in intermediate age open clusters.- Measurements of rotational velocities of F and G stars.- High-resolution observations of cool stars with the ESO Coude' Echelle Spectrometer.- The rapidly-rotating field M-dwarf Gliese 890.- Evolution of rotation in main-sequence stars.- On the origins of chromospheric activity in late-type stars.- Time series measurements of chromospheric emission and possible evidence for differential rotation.- Deterministic time series modeling of Ca II H & K stellar index data.- The evolution of coronal activity in main sequence cool stars.- Coronal structure and rotation in late-type stars.- A search for secular luminosity variations in the Pleiades cluster.- Solar flux variations and concepts for a stellar photometric satellite.- Solar-cycle temperature variations in sunspots.- HRTS ultraviolet images of the solar chromosphere and transition zone.- The NSO initiative to pursue the solar-stellar connection on the McMath Solar Telescope.- Laboratory identification of the 3P1,2 - 5S 2 O intersystem lines of S III.- Transition region - coronal models: A comparison with XUV observations.- X-ray transients and flares from active stellar systems.- Long-term behavior of stellar flare activity in AD Leo.- Radio observations of active stars: Direct evidence for polarity reversals?.- Capella HL.- Lithium depletion in solar-type stars.- Nitrogen abundances in disk and halo dwarfs.- Four W ursae majoris contact binaries in the old galactic cluster NGC 188.- Does the physical state of a mantle indicate the stage of evolution of a star?.- A review of the first observations of stars using the Infrared Astronomical Satellite (IRAS).- RS CVn binary systems.- Starspots, differential rotation and a possible six-year spot cycle on Lambda Andromedae.- Visual and infrared photometry of the long-period RS CVn system HR 7275.- H? observations of the RS CVn star, RT LAC: Evidence for gas streaming: Preliminary results.- Near infrared photometry of some RS CVn systems.- The puzzle of the UV continua of the Hyades giants.- Multicolor photometry of the active G giant FK Com.- Rotational modulation and high speed streams in FK Comae Berenices: Evidence for a massive, highly-evolved binary system.- Outer atmospheres of giant and supergiant stars.- Envelopes of red giant stars.- Radiative transfer in an expanding spherical medium.- Asymptotic giant branch stars as producers of carbon and of neutron-rich isotopes.- Chromospheres in metal deficient field stars.- Interpretation of the observed MG II emission from N-type carbon stars.- Introductory remarks to the session on alpha orionis and related topics.- Multiband photoelectric photometry of Betelgeuse.- Lower atmospheric changes in Betelgeuse deduced from optical continua polarization measurements.- Photoionization of the circumstellar shell of Betelgeuse by chromospheric radiation.- Better determinations of mass loss rates for red giants and supergiants.- Sulphur I emission lines in the EUV spectrum of ? GRU (M3 II).- OH maser emission as a probe of circumstellar envelope structure.- Preliminary results from studies of high resolution line spectra of late-type giant and supergiant stars.- Discovery of a hot, interactive companion to the S star HD 35155.

Chromospheres in metal deficient field stars

Abstract: The chromospheric structure of red giant stars with metal deficient atmospheres presents a fundamental observational and theoretical problem for atmospheric theory. Moreover, the occurrence and rate of mass loss from these stars have consequences for the theory of stellar evolution of low mass stars in globular clusters. This paper reports some results from a systematic program to assess the presence and character of chromospheric line profiles and their variations, and to obtain quantitative measurements of radiative losses from chromospheres. Such data are needed to constrain semi-empirical models of these atmospheres. Three metal deficient field stars have been observed both in the ultraviolet and optical spectral regions: HD 165195, HD 110281, and HD 232078. High dispersion spectra near the Ha transition were obtained with the echelle spectrograph and reticon detector at the F. L. Whipple Observatory of SAO. These stars were observed twice in 1983, separated by a two month interval. IUE spectra were also obtained using the long wavelength cameras in low dispersion mode. Observations of the three stars are discussed below. HD 165195 The Ha cores show strong asymmetry indicating outward motion both in the May 1983 and July 1983 spectra. The velocity of the core center is blue-shifted by -3.4 and -1.8 km s -I, but these -i values differ from zero just at the 3 km s uncertainty of the velocity measurement. No emission is present in the May observations whereas emission on the red wing of the line (modified by telluric absorption) is present in July. This contrasts with Mallia and Pagel's (1978) discovery of a blue emission wing in August 1977. The IUE long wavelength, low resolution spectra: LWR 11703 (6 Oct. 1981); LWP 1909 (27 June 1983); and LWR 16543 (7 Aug. 1983) show broad absorption in the Mg II region (A2800), no sign of an emission core (although at low resolution, this would be hard to detect in such a hot star) and no indication of changes in the line profile beyond the noise in the spectrum. An upper limit to the Mg II flux is given in Table i.