When Do Young Stars Leave Home?

Дата канвертавання24.04.2016
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When Do Young Stars Leave Home?


Methods: Using huge new censes of young stars in concert with new high-dynamic range maps of the distribution molecular gas and dust, we can answer several key long-standing questions related to the timescales of the star and planet formation process: How long does a forming star stay with its natal core? How long does it remain associated with the “lower-density structure” (e.g. a filament in a dark cloud), or even the cloud complex where it originally formed? What kind of environment does a star-disk system need to keep accreting, or to produce an outflow, and when might that reservoir no longer be available to the system?

The fundamental reason these questions have remained unanswered to date is a statistical one. In the past, limited datasets, particularly at mid/far-infrared and millimeter wavelengths have meant that most studies of star formation have had to concentrate on studying a small number of young stars and their immediate environment. However, recent simulations (see Table 1) and observations (see p. 4) suggest that the star formation process is potentially highly dynamical, so that observations of the location and environment of a star “now” may provide little real insight into the conditions of the gas from which it formed. Our proposed program uses data from very large, unbiased, new surveys both of the star-forming interstellar medium, and of the young stellar population itself, to investigate the secular evolution of a young star’s environment in a statistical manner. This work will lay a critical foundation for research on the effects of the star-forming environment on the formation of planetary systems—once those systems can be detected around young stars.

The main analysis technique will be to establish the spatial distribution of gas density, and then to compare the spatial distributions of young stars in coarse age bins (e.g. <0.5 Myr, ~1 Myr, ~10 Myr) with the gas distribution. In sparsely populated regions, it will be possible—for the youngest stars—to associate individual YSOs with individual density peaks. In more complex regions, and for older stars, one-to-one correspondences are unlikely to be found, and our approach will be a statistical one, measuring the spatial frequency of YSOs of various types and comparing that with the spatial frequency of assorted density structures (e.g. where structures are defined as gas above a certain density threshold). Once assembled, our statistics, will suggest how far, how fast, and through what, stars of various ages may have moved since they began to form. Direct proper motion and radial velocity measurements for young stars will be used, in the rare cases where they exist, to evaluate the overall calibration of our more statistical “velocity” measurements.

Data: For two >10-pc scale nearby star-forming regions (Perseus and Ophiuchus), we will use primarily a combination of X-ray (primarily ROSAT) and infrared (primarily 2MASS and Spitzer/c2d1) data to characterize the age and mass distributions of young stars. New molecular line and dust continuum observations from the COMPLETE2 Survey, as well as extinction maps constructed from 2MASS and c2d color excess measurements will be used to understand the distribution and physical properties of high-density gas. Molecular line, sub-mm continuum, and wide-field H images will also be used to mark the presence of disks and outflows around the YSOs in our study areas, and the disk/outflow properties will be used to refine ages based on Spectral Energy distributions constructed from X-ray and near-IR data. The range in spatial scales of the data we will include will extend from ~0.01 to 10 pc, and we expect to be able to sort stars into (at worst) age bins of <0.5 Myr, ~1 Myr, ~10 Myr with less than 10% misclassification.

Motivation and Background (Text begins on p. 3, and Table 1 is discussed on p. 4.)

Table 1: An Heuristic View of “Dynamical” Star Formation

Simulation Snapshots (based on Bate, Bonnell & Bromm 2003)

Time, Box Size

An overall dense molecular cloud, before star formation ensues. Structures are largely transient, and determined by MHD turbulence (e.g. Ostriker, Stone & Gammie 2001).


80,000 yr,

82,500 A.U.

(=0.4 pc)

A self-gravitating region of the cloud starts to emerge. Observationally such structures are associated with “dark clouds” or “cores,” depending on scaling.

Data: COMPLETE (FCRAO & 30-m, SCUBA, 4-m class NICER maps)

160,000 yr

82,500 A.U.

200,000 yr

82,500 A.U.

zoomed in to

202,000 yr

5,156 A.U.

(zoom is x 1/16)

The panels at right, and above right show young stars beginning to form from the fragmenting core.

Data: Gas/dust distribution from COMPLETE (FCRAO & 30-m, SCUBA, 4-m class NICER maps); YSO distribution from ROSAT, 2MASS, Spitzer

210,000 yr

5,156 A.U.

Star symbols () in all panels show the positions of the young stars formed.

A large fraction of stars are ejected from their dense gas homes early on in their lives: many travel at several km s-1.

Data: Gas/dust distribution as above; YSO distribution as above; additional data will include proper motion measurements (Hipparcos, Tycho 2) and radial velocities from new IR spectroscopy

229,000 yr

5,156 A.U.

253,000 yr

5,156 A.U.

zoomed out to

266,000 yr

82,500 A.U.

(zoom is x 16)

The prevailing (analytical) theoretical picture of star formation envisions stars forming inside dense cores, which are in-turn embedded in larger, slightly lower-density structures often called “dark clouds.” A disk surrounds each forming star, and, when it is very young, the star-disk system produces a collimated bipolar flow, in a direction perpendicular to the disk. In its broad outlines, this paradigm is very likely to be right. In detail, though, many questions concerning the timing and physical scaling of this series of events remain.

In reality, star formation is potentially complicated by the fact that the young stars and the reservoir of gas from which they form move with respect to each other. The spatial density-velocity structure of the reservoir is changing with time, as it is a turbulent medium. The stars themselves are affected much more by gravity than by gas pressure (unlike the gaseous structures, which are affected by both), so they can easily move independently of the gas. And, to make matters worse, in dense “enough” regions, dynamical interactions (driven by gravity) amongst stars and dense clumps are statistically likely.3 So, whether because a turbulent flow moves gas away from the star that formed it, or because dynamical interactions eject a forming star from its natal environment, it is likely that a star does not stay sheltered by its natal cocoon for very much of its childhood. The work proposed here will quantify the answer to the question of “When Do Young Stars Leave Home?”, by defining “home” more carefully than has been possible in the past, and by creating a statistical description of a star’s early environment, as a function of time.

The snapshots in Table 1 were selected from the animation of Bate, Bonnell & Bromm’s (2003) simulation of the formation of young star cluster (animation available at http://www.ukaff.ac.uk/starcluster/). The conditions used in these simulations are extreme4, so we do not offer these images (or their temporal and spatial scaling) here as a detailed quantitative illustration of how we think star formation typically proceeds, but instead as a heuristic for thinking about the key phases of a “dynamical” star formation process—even in a case where gravity is less important to determining gas-star motion than it is here. In the table, we point out the key physical attribute of each simulation snapshot shown and associate it with common semantic descriptions. The “Data” notes on which observations are useful in which regime are likely to be more useful to the reader as a reference guide, once the whole proposal is read.

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