Showing papers by "Lee Hartmann published in 1989"
TL;DR: In this paper, the rotational velocities of pre-main-sequence stars in the Taurus-Auriga molecular cloud were measured and the results confirm earlier findings that angular-momentum loss proceeds very efficiently in the earliest stages of star formation.
Abstract: New rotational-velocity measurements for pre-main-sequence stars in the Taurus-Auriga molecular cloud are reported. Rotational velocities or upper limits of 10 km/s are now available for 90 percent of the T Tauri stars with V less than 14.7 in the catalog of Cohen and Kuhi. Measurements of 'continuum emission' stars, thought to be accreting high-angular-momentum material from a circumstellar disk, show that these objects are not especially rapid rotators. The results confirm earlier findings that angular-momentum loss proceeds very efficiently in the earliest stages of star formation, and suggest that stars older than about one million yr contract to the main sequence at nearly constant angular momentum. The slow rotation of T Tauri stars probably requires substantial angular-momentum loss via a magnetically coupled wind.
130 citations
TL;DR: In this article, a method for separating the "veiling" continuum often present in T Tauri stars from the underlying photospheric spectrum is described, by means of goodness-of-fit tests.
Abstract: A method for separating the 'veiling' continuum often present in T Tauri stars from the underlying photospheric spectrum is described. Echelle observations from 5100 to 6800 A of the partially veiled T Tauri star BP Tau were analyzed to determine the shape of the veiling spectrum. The residuals of the fit indicate the deviation of the veiling spectrum from a simple continuum and identify the location and strength of any emission-line components. It is shown, by means of goodness-of-fit tests, that the spectrum of BP Tau can be decomposed into a normal stellar spectrum plus a smooth veiling continuum with only a few emission lines superposed. The continuum dominates the veiling spectrum in this spectral region; the veiling does not arise from numerous deep photospheric absorption lines that are filled in by weak emission.
116 citations
TL;DR: In this paper, it is suggested that the pre-main-sequence object Z CMa is a luminous accretion disk, similar in many respects to the FU Orionis variables.
Abstract: It is suggested that the pre-main-sequence object Z CMa is a luminous accretion disk, similar in many respects to the FU Orionis variables. Z CMa shows the broad, doubled optical absorption lines expected from a rapidly rotating accretion disk. The first overtone CO absorption detected in Z CMa is blue-shifted, suggesting line formation in a disk wind. Accretion at rates about 0.001 solar mass/yr over 100 yr is required to explain the luminosity of Z CMa. The large amount of material accreted (0.1 solar mass/yr) indicates that Z CMa is in a very early stage of stellar evolution, possibly in an initial phase of massive disk accretion.
73 citations
TL;DR: The rotational velocity distribution for low-mass stars in the Alpha Per cluster is compared with recently derived rotational velocities for T Tauri stars, placing strong constraints on the mechanisms for angular momentum loss during pre-main-sequence evolution.
Abstract: About 30 new, low-mass members of the young open cluster Alpha Persei are identified via a proper-motion study and subsequent photometric and spectroscopic observations. Membership in the cluster is confirmed for a number of the fainter proper-motion candidates from Heckman, Dieckvoss, and Kox (1956). Coordinates, finding charts, BVRI photometry, and rotational velocities are provided for most of the stars. At least two of the stars show peculiar H-alpha emission profiles, with weak but very broad emission wings, and relatively narrow absorption reversals. The rotational velocity distribution for low-mass stars in the Alpha Per cluster are compared with recently derived rotational velocity distributions for T Tauri stars, placing strong constraints on the mechanisms for angular momentum loss during pre-main-sequence evolution.
66 citations
TL;DR: In this article, seven or eight new, late-type members of the poor open cluster IC 2391 are identified, and membership is confirmed for two other stars. The new members fall approximately along a 3 x 10 to the 7th yr isochrone, which is the age estimated for the cluster on the basis of it super main-seqence turnoff.
Abstract: Seven or eight new, late-type members of the poor open cluster IC 2391 are identified, and membership is confirmed for two other stars. The new members fall approximately along a 3 x 10 to the 7th yr isochrone, which is the age estimated for the cluster on the basis of it super main-seqence turnoff. Echelle spectra were obtained for the most probable cluster members. Most show H-alpha in emission and a strong Li 6707 A absorption line, and a few are rapid rotators. The Li abundances for cluster stars cooler than the sun are considerably less than the primordial Li abundance, providing the first direct evidence for substantial premain-sequence Li burning. The rotational velocities show a range from about 15 to 150 km/s, with a distribution of rotational velocities not significantly different from that observed for low-mass stars in the Pleiades.
51 citations
TL;DR: IUE observations in the 1150-3250-A region were obtained of GK Per during the rise, at the maximum, and during the decline of the 2.5-mag optical outburst in 1981.
Abstract: IUE observations in the 1150-3250-A region were obtained of GK Per during the rise, at the maximum, and during the decline of the 2.5-mag optical outburst in 1981. The results support previous predictions for the interaction of an accretion disk with the magnetic field of an accretion star. The luminosity at minimum is found to be 2.3 solar luminosities, with 1/3 of this being attributed to the cool secondary star.
33 citations
TL;DR: In this article, low-resolution, ultraviolet spectra of the pre-main-sequence objects Z Canis Majoris, V1057 Cygni, and FU Orionis are presented.
Abstract: Low-resolution, ultraviolet spectra of the pre-main-sequence objects Z Canis Majoris, V1057 Cygni, and FU Orionis are presented. Ultraviolet absorption features indicate spectral types of A5 I for FU Ori and F5 I for Z CMa. These results are in reasonable agreement with predictions of simple accretion disk models. The lack of significant veiling of ultraviolet absorption features implies that a hot boundary layer at the inner edge of the disk contributes less than 20 percent of the radiation at 2600-3200 A, which is significantly smaller than expected. It is suggested that accretion has significantly modified the structure of the underlying pre-main-sequence stars in these systems (perhaps by expanding the photosphere or by spinning up a radiative photosphere close to breakup velocity, or both), or that the boundary layer region is much more extended above the disk midplane than standard thin disk theories predict.
31 citations
TL;DR: In this paper, it is suggested that the winds of T T Tauri stars are heated at distances of 50-100 AU by oblique shocks with circumstellar disks, producing the observed high-velocity forbidden-line emission.
Abstract: It is suggested that the winds of T Tauri stars are heated at distances of 50-100 AU by oblique shocks with circumstellar disks, producing the observed high-velocity forbidden-line emission. The low shock velocities characteristic of this model provide low-excitation emission and line profiles in reasonable agreement with observations. The blueshifted character of the observed emission is explained by a combination of disk occultation of the receding flow, plus a modest collimation of the observable wind by the disk envelope. Instabilities at the wind-disk interface may be important in producing the observed emission.
28 citations
TL;DR: In this article, high-spectral-resolution 2-micron spectroscopy of two FU Ori variables, V1057 Cyg and Fu Ori, confirms that the ratio of IR-to-optical v sin i in these objects is significantly smaller than unity.
Abstract: High-spectral-resolution 2-micron spectroscopy of two FU Ori variables, V1057 Cyg and FU Ori, confirms that the ratio of IR-to-optical v sin i in these objects is significantly smaller than unity. The observed ratio is slightly larger than predictions of simple accretion-disk models but is consistent with strict Keplerian rotation given the uncertainties in the models. Blueshifted CO absorption is observed in FU Ori and in V1057 Cyg, suggesting mass loss at rates less than about 10 to the -7th solar mass/yr from the outer disk regions. The data suggest mass ejection in FU Ori is dominated by flows from inner disk regions. The mass-loss rate in FU Ori appears to be variable. 25 refs.
16 citations
4 citations