Pre-Main Sequence Stars

The photometric and high-energy signatures of magnetic-field generation have been observed in several T Tau stars. 

White dwarfs are the remnant of the evolution of low and intermediate ( 5 Mq) mass stars, formed by core contraction during the last stages of nuclear burning. Since they reach densities in excess of 10 g cm and typically have radii of order 0.01 R they were expected, by simple theory, to be able to possess very strong surface fields. Any weak field remnant in the core at the time of contraction, should the field be able to survive this stage of evolution, would be amplified by flux-freezing to very large values, of 

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The current status of HF abundances from infrared spectroscopy in samples of red-giants from different Galactic stellar populations are summarized in Figure 1. The abundance results displayed in this figure are from Cunha et al. (2003), plus new results for stars at the lowest metallicities, as well as two Orion pre-main-sequence stars. The run of fluorine with metallicity is now probed between oxygen abundances from roughly 7.7 to 8.7. 

Fig. 1. The Galactic fluorine abundances obtained to date. Three samples are represented the disk of the Milky Way (crosses), including the two young Orion pre-main-sequence stars (open circles), and u> Centauri giants (filled circles).

The uncertainty in the age of pre main sequence stars is therefore of the order of the thermal timescale at the luminosity of D-burning smaller than a few times 105 yr for normal T Tauri, and larger than 106 yr for very low mass stars and brown dwarfs (BD). In fact, comparing observations spanning a wide range of masses we could even constrain the models, for example we can ascertain whether the Stahler et al. (1986) picture of collapse is valid also in the BD regime, or... 

The birth of a protostar and its life as a pre-main-sequence star, its descent to the main sequence and death, starting with a red giant leading to planetary nebula and ending in white and black dwarfs. This sequence varies with mass... 

Tracks followed by 1,3, and 5 M pre-main sequence stars as they evolve toward the main sequence are shown on an H-R diagram. Pre-main sequence stars shine primarily by conversion of gravitational potential energy to heat, although energy released by burning of deuterium and other elements also plays a role. 

Artistic rendering of four observed stages of star formation, (a) Class 0 object a deeply embedded hydrostatic core surrounded by a dense accretion disk. Strong bipolar jets remove angular momentum, (b) Class I object protostar in the later part of the main accretion phase, (c) Class II object or T Tauri star pre-main-sequence star with optically thick protoplanetary disk, (d) Class III object or naked T Tauri star star has an optically thin disk and thus can be directly observed. Some may have planets. 

Class I obj ects also have bipolar outflows, but they are less powerful and less well collimated than those of Class 0 objects. This stage lasts 100 000 to 200 000 years. Class //objects, also known as classical T Tauri stars, are pre-main-sequence stars with optically thick proto-planetary disks. They are no longer embedded in their parent cloud, and they are observed in optical and infrared wavelengths. They still exhibit bipolar outflows and strong stellar winds. This stage lasts from 1-10 million years. Class ///objects are the so-called weak line or naked T-Tauri stars. They have optically thin disks, perhaps debris disks in some cases, and there are no outflows or other evidence of accretion. They are observed in the visible and near infrared and have strong X-ray emission. These stars may have planets around them, although they cannot be observed. 

Class II. Sources with spectral index —1.5 < cyir < 0. These are pre-main-sequence stars with observable accretion discs (classical T Tauri stars). 

FU Orionis star (FU Ori-type star or Fuor) a pre-main-sequence star that recently underwent an extreme brightening at visual wavelengths, typically of five magnitudes or more. They are named after their prototype FU Orionis. These stars rapidly brighten, then remain almost steady or slowly decline by a magnitude or two over a period of decades. It is proposed that the brightening is driven by a runaway disk accretion, a result of an instability. 

For a protostar evolving to the main sequence, theoretical models can be used to predict a few observational quantities such as surface temperature and luminosity. However, the models are not sufficient to predict accurately the ultraviolet flux emitted by a pre-main sequence star. Thus we employ ultraviolet observations of young solar-like stars as the best estimate of the UV flux emitted by the young sun. 

Waelkens et al., 1996) massive pre-main sequence stars surrounded with disks of dust and gas. Herbig Ae/Be stars even show transient gas features in their spectra that have been interpreted as comets falling into the star (Beust et al., 1994). The presence of the olivine feature in comets and circumstellar disk systems and the lack of it in interstellar and molecular clouds, the parental materials for star and planetary formation, is somewhat of a conundrum. A common astronomical interpretation is that interstellar grains are amorphous silicates and when warmed in a circumstellar disk environment, they anneal to produce crystalline materials. The other possibility is that olivine in comets and disks condenses from vapor produced by evaporation of original interstellar materials. 

Feigelson E. D. and Kriss G. A. (1989) Soft X-ray observations of pre-main-sequence stars in the Chamaeleon dark cloud. Astrophys. J. 338, 262-276. 

POLARIZATION OF LIGHT BY PRE-MAIN-SEQUENCE STARS IN THE VISUAL WAVELENGTHS... 

The science datacube selected for the next simulations corresponds to a proto-planetary disk surrounding a Herbig Ae star 10,000 K). As presented earlier in this chapter, Herbig Ae/Be stars are pre-main-sequence stars. The main difference with T Tauri stars is the mass, this being Af+ > Mq. Spectrally, their SED shows strong infrared radiation excess due to the presence of the drcumstellar accretion disk (Hillenbrand et al. 1992), this is, the thermal emission of circumstellar dust. 

Key words Infrared imaging Embedded clusters — Pre-main sequence stars... 

Key words pre-main sequence stars - line profiles - accretion disks - stellar winds... 

The issue of duplicity among pre-main sequence stars has been receiving new attention (Ghez et al. 1992 Simon et al. 1992 Moneti Zinnecker 1991). In table 1 we list the six weak line objects imaged, in which half were found to be binary. The designation and K magnitude is from Walter et al. 1988. 

DO PRE-MAIN-SEQUENCE STARS DRIVE BIPOLAR MOLECULAR OUTFLOWS ... 

Abstract. In some interpretations, the onset of a high vdodty molecular outflow signals the end of the Class I stage of pre-main-sequence stellar life and marks an evolutionary period after whidi a Class II pre-main-sequence star remains. Since all but a handful of the hundreds of known molecular outflow sources are associated with Class I sources, this interpretation appears reasonable. The handful of more evolved Class II sources that apparently drive outflows are exceptions, hi our infrared, polarimetric imaging program, we have made serendipitous discoveries that indicate the number of Class II sources that drive outflows may now be smaller by at least two than was previously thought. 

These discoveries suggest that some physical attributes previously associated with each of these two pre-main-sequence stars may be incorrect. The attributes have been derived from apparent associations of the stars with nesuly spatiaUy coincident outflows and from models based on their apparent spectral energy distributions (SEDs). The observed SEDs may combine fluxes from at least two objects, one a weakly embedded object, the second a deeply embedded object. If the associations with the outflows are incorrect, the observed SEDs may also be incorrect. 

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