Suzan Edwards

Circumstellar material associated with solar-type pre-main-sequence stars - A possible constraint on the timescale for planet building

The observed frequency distribution of near-IR excesses for a sample of 83 stars in the Taurus-Auriga star-forming complex is used to provide an estimate of the number of solar-type premain-sequence (PMS) stars surrounded by circumstellar disks. The results suggest that if all such PMS stars are initially surrounded by disks, then disk lifetimes must range from much less than 3 x 10 to the 6th yr to about 10 to the 7th yr, providing a constraint on the time available for planet building. PMS stars with small near-IR excesses, but significant mid-IR and far-IR excesses, provide some evidence for changes in disk structure with time. 55 refs.

A sensitive 10-micron search for emission arising from circumstellar dust associated with solar-type pre-main-sequence stars

The presence and evolutionary timescales of circumstellar disks surrounding solar-type premain-sequence stars are studied using excess IR radiation above photospheric levels. The analysis is based on optical photometry, published near-IR fluxes, IRAS fluxes, and 10-micron flux measurements of 20 stars in Taurus-Auriga obtained with the NASA IR telescope facility. About half of the stars with ages less than 3 Myr show excess 2.2 and 10 micron emission, consistent with emission from optically thick disks extending inward to the stellar surface. At ages of about 10 Myr, less than 10 percent of the sample stars show evidence of dust emission from optically thick disks. It is concluded that the timescale over which disks survive as IR-luminous, optically thick structures is less than 10 Myr. Also, evidence for inner holes in premain-sequence stars surrounded by optically thick disks is discussed. 18 refs.

Forbidden-line emission and infrared excesses in T Tauri stars - Evidence for accretion-driven mass loss?

It is suggested that disk accretion provides the requisite external source to power the winds of T Tauri stars. As a test of this hypothesis, a potential accretion diagnostic for a representative sample of stars is compared to diagnostics of the wind strength. The luminosity of H-alpha, formed in the inner wind, and the luminosity of the forbidden emission lines, formed in the outer wind, are used as wind diagnostics, while the excess infrared luminosity is used as a potential accretion diagnostic. It is found that forbidden-line O I 6300 A and H-alpha line luminosities are correlated with each other over two orders of magnitude; this is interpreted as the indication of a wide range of mass-loss rates among the given sample of T Tauri stars. The luminosity of each of these lines is also found to be correlated with excess infrared luminosity. However, neither the forbidden nor the H-alpha luminosity is well correlated with the photospheric luminosity, leading to a conclusion that it is the disk, not the star, which primarily determines the strength of wind indicators in T Tauri stars. 67 refs.

Probing T Tauri Accretion and Outflow with 1 Micron Spectroscopy

In a high-dispersion 1 μm survey of 39 classical T Tauri stars (CTTSs) veiling is detected in 80% of the stars, and He I λ10830 and Pγ line emission in 97%. On average, the 1 μm veiling exceeds the level expected from previously identified sources of excess emission, suggesting the presence of an additional contributor to accretion luminosity in the star-disk interface region. Strengths of both lines correlate with veiling, and at Pγ there is a systematic progression in profile morphology with veiling. He I λ10830 has an unprecedented sensitivity to inner winds, showing blueshifted absorption below the continuum in 71% of the CTTSs, compared to 0% at Pγ. This line is also sensitive to magnetospheric accretion flows, with redshifted absorption below the continuum found in 47% of the CTTSs, compared to 24% at Pγ. The blueshifted absorption at He I λ10830 shows considerable diversity in its breadth and penetration depth into the continuum, indicating that a range of inner wind conditions exist in accreting stars. We interpret the broadest and deepest blue absorptions as formed from scattering of the 1 μm continuum by outflowing gas whose full acceleration region envelopes the star, suggesting radial outflow from the star. In contrast, narrow blue absorption with a range of radial velocities more likely arises via scattering of the 1 μm continuum by a wind emerging from the inner disk. Both stellar and disk winds are accretion powered, since neither is seen in nonaccreting WTTSs and among the CTTSs helium strength correlates with veiling.

Near-Infrared Classification Spectroscopy: H-Band Spectra of Fundamental MK Standards

We present a catalog of H-band spectra for 85 stars of approximately solar abundance observed at a resolving power of 3000 with the KPNO Mayall 4 m Fourier Transform Spectrometer. The atlas covers spectral types O7-M5 and luminosity classes I-V as defined in the MK system. We identify both atomic and molecular indices and line ratios that are temperature and luminosity sensitive, allowing spectral classification to be carried out in the H-band. The line ratios permit spectral classification in the presence of continuum excess emission, which is commonly found in pre-main-sequence or evolved stars. We demonstrate that with spectra of R = 1000 obtained at signal-to-noise ratio >50, it is possible to derive spectral types within ±2 subclasses for late-type stars. These data are available electronically through the Astronomical Data Center in addition to being served on the World Wide Web.

Helium Emission from Classical T Tauri Stars: Dual Origin in Magnetospheric Infall and Hot Wind

High-resolution emission-line profiles of He I and He II in 31 classical T Tauri stars are analyzed with the aim of probing the environs of the star-disk interface in accreting low-mass young stars. The diagnostic power of the helium lines lies in their high-excitation potentials, which restrict their formation to a region either of high temperature or close proximity to a source of ionizing radiation. The He I profiles are decomposed into kinematic components that support the paradigm of magnetically controlled accretion from the disk onto the stellar surface but also require a significant contribution from a hot wind. A narrow component, seen in 28/31 stars, is characterized by relatively uniform line widths and centroid velocities among all the helium lines. Our analysis supports previous conclusions that this feature is consistent with formation in the decelerating postshock gas at the magnetosphere footpoint. A broad component, seen in 22/31 stars, displays a diversity of kinematic properties. Our analysis suggests that in many stars the He I broad component is itself composite. At one extreme are stars where the broad component is redshifted in excess of 8 km s-1, as would occur if helium emission arises primarily from polar angles less than 547 in the funnel flow. At the other extreme are stars where the broad component is blueshifted in excess of -30 km s-1, requiring an origin in outflowing gas. The additional occurrence of maximum blue wing velocities exceeding -200 km s-1 in 14 stars leads us to argue that hot winds are present in about half of our sample. The relation between the narrow component and the optical veiling differs between the stars with or without a hot helium wind, suggesting that when the hot wind is present the luminosity and temperature of the accretion shock are reduced. A comparison of broad component helium emission with standard outflow indicators leads us to suggest that there are two sources of inner wind in T Tauri accretion disk systems: one a hot polar/coronal wind that prevails in stars with high veiling, and the other a more widespread cool disk wind that is likely launched at the magnetosphere/disk boundary.

Forbidden line and H-alpha profiles in T Tauri star spectra - A probe of anisotropic mass outflows and circumstellar disks

The results of a high-resolution spectroscopic study of 10 T Tauri stars (TTS) and two Herbig emission stars are presented based on red echelle spectra including the lines of forbidden O I 6300 A, forbidden N II 6584 A, forbidden S II 6716, 6731 A, as well as H-alpha. The forbidden lines display a continuous progression of profile types. The velocity structure in the forbidden lines is critically examined and compared to computed line profiles for a number of different wind models. Constant velocity spherical or conical winds fail to reproduce the observed line profiles, which are better explained by a wind with a latitude-dependent velocity field. A prediction of the wind model correlating the velocity of the reversal at H-alpha and the forbidden line velocity structure as a function of the view angle to the star is explored. Estimates of the average densities and sizes for the TTS forbidden emission regions are presented, and mass-loss rates are computed. IRAS far-IR fluxes are used to estimate the disk sizes.

Optical excess emission in T Tauri stars

We present a set of simultaneous high-resolution spectroscopic and spectrophotometric observations of 22 K7-M1 T Tauri stars in the Taurus-Auriga dark cloud. Analysis of the high-resolution data makes it possible to separate the optical excess (veiling) emission from the photospheric fluxes in these stars. The amount of optical excess emission at 5500 A ranges from undetectable (≤10% of the photosphere) to as much as 10 times the photospheric flux in the most extreme objects

Accretion Rates for T Tauri Stars Using Nearly Simultaneous Ultraviolet and Optical Spectra

We analyze the accretion properties of 21 low-mass T Tauri stars using a data set of contemporaneous near-UV (NUV) through optical observations obtained with the Hubble Space Telescope Imaging Spectrograph and the ground-based Small and Medium Aperture Research Telescope System, a unique data set because of the nearly simultaneous broad wavelength coverage. Our data set includes accreting T Tauri stars in Taurus, Chamaeleon I, η Chamaeleon, and the TW Hydra Association. For each source we calculate the accretion rate (Ṁ) by fitting the NUV and optical excesses above the photosphere, produced in the accretion shock, introducing multiple accretion components characterized by a range in energy flux (or density) for the first time. This treatment is motivated by models of the magnetospheric geometry and accretion footprints, which predict that high-density, low filling factor accretion spots coexist with low-density, high filling factor spots. By fitting the UV and optical spectra with multiple accretion components, we can explain excesses which have been observed in the near-IR. Comparing our estimates of Ṁ to previous estimates, we find some discrepancies; however, they may be accounted for when considering assumptions for the amount of extinction and variability in optical spectra. Therefore, we confirm many previous estimates of the accretion rate. Finally, we measure emission line luminosities from the same spectra used for the Ṁ estimates, to produce correlations between accretion indicators (Hβ, Ca II K, C II], and Mg II) and accretion properties obtained simultaneously.

Modeling T tauri winds from He I λ10830 profiles

The high opacity of He I λ10830 makes it an exceptionally sensitive probe of the inner wind geometry of accreting T Tauri stars. In this line, blueshifted absorption below the continuum results from simple scattering of stellar photons, a situation that is readily modeled without definite knowledge of the physical conditions and recourse to multilevel radiative transfer. We present theoretical line profiles for scattering in two possible wind geometries, a disk wind and a wind emerging radially from the star, and compare them to observed He I λ10830 profiles from a survey of classical T Tauri stars. The comparison indicates that subcontinuum blueshifted absorption is characteristic of disk winds in ~30% of the stars and of stellar winds in ~40%. We further conclude that for many stars the emission profile of helium likely arises in stellar winds, increasing the fraction of accreting stars inferred to have accretion-powered stellar winds to ~60%. Stars with the highest disk accretion rates are more likely to have stellar wind than disk wind signatures and less likely to have redshifted absorption from magnetospheric funnel flows. This suggests the possibility that when accretion rates are high, disks can extend closer to the star, magnetospheric accretion zones can be reduced in size, and conditions can arise that favor radially outflowing stellar winds.