C. C. Batalha

Variability of Southern T Tauri Stars. II. The Spectral Variability of the Classical T Tauri Star TW Hydrae

We present the analysis of 42 spectra of the Classical T Tauri star TW Hya observed with the FEROS echelle spectrograph over 2 yr. We determined the rotational and radial velocities of TW Hya, obtaining v sin i = 5 ± 2 km s-1 and vrad = 12.5 ± 0.5 km s-1. The star exhibits strong emission lines that show substantial variety and variability in their profile shapes. Emission lines such as Hα, Hβ, and He I show both outflow and infall signatures, which change on different timescales. The system displays periodic variations in line and veiling intensities, but the stellar rotation period remains uncertain. We see evidence of a variation in the mass accretion rate over a 1 yr period from the Na D line profiles that are well fitted by magnetospheric accretion models with moderate mass accretion rates (10-9 up to 10-8 M☉ yr-1). The lower values inferred from the models are close to the average mass accretion rate obtained from the veiling estimates (~2 × 10-9 M☉ yr-1), but the veiling results are consistent with a constant mass accretion rate within the errors of the calculations. The Hα, He I, Na D, and Hβ emission-line equivalent widths corrected from veiling correlate well with each other and are correlated with the veiling, indicating the same mechanism should be powering them and suggesting an origin related to the accretion process. The wings of the main emission lines are generally correlated, except when the Balmer lines exhibit properties suggesting a strong contribution from a wind. The blueward absorption components of the Balmer lines, most likely from a wind, are not correlated with veiling. The spectroscopic analysis allows us to infer the inclination of the stellar rotation axis (i = 18° ± 10°) that matches the current estimations of the disk orientation (0° < i < 15°). A magnetospheric dipole axis that is misaligned with the stellar/disk rotation axis could produce the observed photometric variability, and we tend to favor a low inclination but not a totally face-on geometry for the system. TW Hya exhibits typical spectral characteristics of many classical T Tauri stars in Taurus despite its older age, indicating that active accretion disks can readily survive up to 10 Myr.

Hamilton echelle spectra of young stars. I. Optical veiling

An extensive set of veiling measurements of T Tauri stars covering almost the entire optical spectrum is presented. These are based on Hamilton echelle spectra obtained during 1987-1989. A full range of T Tauri stars, from weak to continuum stars is studied; in some cases, several spectra of a given target are seen in which the veiling has changed, occasionally dramatically. Veilings are determined using two new methods: one which concentrates on the residual intensities of selected spectral lines, and the other which compares the autocorrelation functions for a T Tauri star and a veiled standard. The general shape of the veiling is often as predicted by accretion disk models: almost flat in the red and increasing in the blue. Cooler stars have generally higher veiling, implying that the veiling power is relatively independent of stellar mass. The relation between veiling and the narrow and broad emission lines supports a nonstellar origin for the veiling and broad line components, and a stellar origin for the narrow lines. 42 refs.

The Narrow Emission Lines of T Tauri Stars

We present the first comprehensive study of the narrow emission lines of T Tauri stars (TTS). These narrow lines have been reported in the literature as originating in the stellar atmosphere and having Gaussian-type profiles centered at the stellar rest velocity, with a base width not larger than 50 km s. Here, we concentrate on the Ca n lines XX8498, 8542, and 8662 and the helium line X5876. After applying veiling corrections, the average narrow component line emission is found to be larger than that found in active main-sequence stars: up to several times larger for classical T Tauri stars with strong rates of disk accretion. More striking is the finding that the resulting line emission strengths of these lines correlate with veiling. The correlation is confirmed on individual stars for which observations at several epochs exist and for which veiling varies widely on relatively short timescales. We also find a correlation between the narrow emission fluxes and the near-infrared excesses for stars with low levels of veiling, which includes the few weak-lined TTS of the sample. We discuss possible formation sites for the narrow emission lines in the classical TTS, and we present simple models to explain the observations. In these models, the excess line emission found for the stars with higher accretion rates is assumed to originate in localized regions near the magnetic footpoints of the accretion column. We refer to these hypothetical regions in the atmosphere collectively as the “hot chromosphere” since we assume they are additionally heated by the reprocessed energy of the colliding gas in the accretion process. Computing two chromospheric models, one representing the typical weak TTS chromosphere and the other representing the best guess at the “hot chromosphere,” we find the following. The “hot chromosphere” is characterized by a steep temperature gradient beginning at low continuum optical depths in order to give simultaneously the large observed central flux and the relatively narrow baselines ( 50-60 km s ~ ). The chromosphere temperature rise is not similar to the earlier deep chromosphere models in which a sudden chromospheric temperature rise is appended to the photosphere at relatively large mass column. For the most extreme cases (i.e., largest line fluxes), 20%, at most, of the star’s surface must be covered by “hot chromospheric” regions. Subject headings: line: profiles — stars: chromospheres — stars: emission-line, Be — stars: pre-main-sequence

The atmospheres of T Tauri stars. II: Chromospheric line fluxes and veiling

A set of high-resolution calibrated spectra of southern T Tauri Stars (TTS), comprising objects with Hα equivalent widths ranging from 3 to 40 A are used to derive one-component plane-parallel chromospheric models. Our motivation is to discover how much emission activity can truly be ascribed to stellar activity, using real calibrated stellar data. We find that simple models can reproduce the Ca II line at 8542 A for most of the stars. The chromospheric enhancements are comparable to the ones found among active main-sequence stars of similar spectral type. These models cannot account for the veiling observed in TTS. Likewise, realistic TTS chromospheres are unable to create the observed Balmer jumps

The pre-main sequence spectroscopic binary AK SCO

We present an analysis of 32 high-resolution echelle spectra of the premain sequence spectroscopic binary AK Sco obtained during 1998 and 2000, as well as a total of 71 photoelectric radial-velocity observations from the period 1986–1994. These data allow considerable improvement of the period and other orbital parameters of AK Sco. Our analysis also includes eight series of photometric observations in the uvby and Geneva seven-color systems. No eclipses or other periodic variations are seen in the photometry, but the well-determined HIPPARCOS parallax allows us to constrain the orbital inclination of the system and obtain physical parameters for the two stars.