The MIT/Chandra Science Star Formation Team
Prof. Claude Canizares
Joel Kastner
Norbert S. Schulz
Dave Huenemoerder
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The MIT/Chandra Science Star Formation Team
Prof. Claude Canizares
Joel Kastner
Norbert S. Schulz
Dave Huenemoerder
Overview
Young stellar associations embedded in extended HII regions are known to be prime locations for early stellar evolution. Prominent examples are cloud complexes in the Orion OB1 association containing the Orion Nebula (NGC 1976) or the Monoceros OB2 association containg the famous Rosette Nebula. The latter one is excited by the compact cluster NGC 2244 which is the youngest subgroup of stars in this association. The observation of very young open stellar clusters has been in the focus of stellar X-ray astronomy since the discovery by the EINSTEIN observatory that a large fraction of T Tauri stars radiate in X-rays at a level 10^3 - 10^4 times that of ordinary main sequence stars. Cluster stars usually share a variety of common properties such as chemical composition, environment, distance and to some extend age which makes them a quite canonical ensemble. Very young clusters (< 30 Myr) are especially attractive because of the relatively large X-ray luminosities of even their lower-mass memebers and because they allow surveys with one single exposure. On the other hand, they mostly appear within molecular clouds and large areas of obscuring dust, which makes optical identification with known T Tauri stars quite difficult, especially in the case of the more distant clusters. The identification is necessary in order to correlate stellar X-ray luminosities and spectral properties with fundamental stellar parameters such as bolometric luminosities and/or spectral types. In so doing, we can better establish, e.g., the "dividing line" between young stars that generate X-rays via radiation driven winds (i.e. early type stars) vs. those that produce X-rays through coronal activity (late type stars).
Although not the closest young cluster - the TW Hya associationi, at a distance of about 50 pc, has that distinction - the Orion OB1 association (located at a distance of about 400 pc) may well be the richest, nearby cluster, and is certainly the nearest young cluster containing relatively massive stars (including a few deeply embedded, highly energetic protostars). A ROSAT PSPC observation of the Orion Nebula region exhibited 171 X-ray sources within a 20 arcmin radius of the Trapezium with X-ray luminosities above 3.5 times 10^{29} erg/s. Most of these sources were identified with PMS stars; 51 of them show spectral types later than F0. Similar to stellar clusters in NGC 2023 and NGC 2024, these sources all show hard ( about 1 keV) spectra and luminosities up to 10^{31} erg/s.
Despite the very high stellar X-ray luminosities (up to 10^{33} erg/s for early type stars), the limited effective area of the HETGS makes observations of distant young stellar clusters extremely time consuming. Therefore we aimed for two associations that are relatively close in distance and hence will allow us to study a variety of key issues in the physics of very young stars and clusters. These issues involve:
High Resolution X-ray Spectroscopy of the nearest and oldest TTauri Star
Classical T Tauri Stars.
Study X-rays from a System that is very similar to the young Sun
Resolve the Orion Trapezium Cluster in X-rays
X-ray Spectroscopy of Colliding or Eclipsing Stellar Winds
Spectroscopy with the HETGS of a Crowded Field
TITLE: TW Hya: one of the nearest, and oldest classical T Tauri stars known.
At a distance of only 57 pc and age of about 20 Myr, the isolated pre-main sequence star TW Hya is the nearest, and one of the oldest, classical T Tauri stars known (Kastner et al. 1997, Science 277, 67). It is surrounded by a molecular, protoplanetary disk, and is an intense source of X-rays (flux ~2.5 times 10^{-12} erg/cm2/s, or L_x ~ 10^{30} ergs/s). Due to its proximity, TW Hya affords us a unique opportunity to study in detail a system that likely bears a close resemblance to the young Sun and solar system. With AXAF/HETG, we will obtain the highest-quality X-ray spectrum yet obtained for any T Tauri star. High resolution X-ray spectroscopy of TW Hya will yield the intensities of many individual, diagnostic emission lines in the 5--20 Å region. These data thereby will provide unprecendented detail concerning the physical conditions in the X-ray emitting region surrounding a young, Sun-like star.
The Orion Trapezium Cluster.
TITLE: The Orion Trapezium Cluster spatially resolved in X-rays.
At the core of the Orion Nebula lies the Trapezium cluster, a small group of young OB stars responsible for the bulk of the excitation of the visible nebulosity. Like most OB stars, the Trapezium stars are known to be luminous X-ray sources, though the X-ray emission mechanism of such hot stars is poorly understood. Spectroscopy of the Trapezium region with Chandra and HETG should help astronomers understand the origin of the X-ray emission in early type (OB) stars. In X-rays the Trapezium has not yet been fully resolved. Three of the main components are separated by less than 3". The HETGS 0th order Image of the Trapezium will be in the best focus AXAF can offer and thus resolve the complex at subarcsec spatial resolution.
TITLE: High Resolution X-ray Spectroscopy of the Orion Trapezium Cluster.
The Trapezium lies at the front surface of a massive, dusty molecular cloud that is teeming with newborn stars. Most young, Sun-like (T Tauri) stars are also X-ray sources; hence the potential for confusion in the identification of a dispersed photon with a unique source makes the Trapezium region a challenging target for HETG. For the observation planned here we will not integrate long enough to encounter much confusion from the many T Tauri stars in the field, but the resulting spectra may be susceptible to confusion due to the many intense X-ray sources associated with the hot OB-star open cluster. Thus we will observe a manifold of spectra from hot early type B-stars. By resolving the closest components (see above) we should see spectral signatures of either colliding or eclipsing stellar winds.
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