It is only recently that such objects have begun to be observed in statistically significant numbers ( Cohen & Kuhi 1979). The relative rapidity of the star formation process means that there are no nearby pre-main-sequence stars, and the fact that they form in denser regions of the interstellar medium favours observations at wavelengths longer than optical. This is partly because stars which are in the process of formation are more difficult to observe. However, our understanding and, without doubt, our predictive power lag well behind. The process by which stars form from their constituent interstellar material is as relevant to all branches of astrophysics, from planets to cosmology, as their subsequent evolution. Here, by examining where theory predicts a particular object ought to lie at a given age, we investigate what properties of a pre-main-sequence star can be determined if these difficulties can be overcome. In addition, any contribution to the light from the accretion disc itself must be subtracted, and obscuration by circumstellar material accounted for (see, e.g., Hillenbrand 1997). The placement of an observed pre-main-sequence star in the theoretical Hertzsprung-Russell (HR) diagram (log L against log T eff) is notoriously difficult because of its sensitivity to distance and reddening (see, e.g., Gullbring et al. Instead, accreting pre-main-sequence stars of around a solar mass are likely to appear older than those of either smaller or larger mass.Īccretion, accretion discs, stars: evolution, stars: formation, Hertzsprung-Russell (HR) diagram, stars: pre-main-sequence 1 Introduction We also show how a set of coeval accreting objects do not appear coeval when compared with non-accreting tracks. Instead, we stress the unreliability of any such comparisons, and use the disparities between various tracks to estimate the likely errors in age and mass estimates. We do not claim that ages obtained by comparison with these particular accreting tracks are likely to be any more reliable than those from comparisons with non-accreting tracks. Knowledge of the current accretion rate, together with an HR-diagram position, gives information about the rate of accretion in the past, but does not necessarily improve any age estimate. Because of the balance established between the Kelvin-Helmholtz, contraction time-scale and the accretion time-scale, a pre-main-sequence star remembers its accretion history. We present a number of accreting protostellar tracks that can be used to determine age if the initial conditions can be determined and the underlying accretion rate has been constant in the past. However, the age determination can be very misleading, because it is significantly (generally different by a factor of 2 to 5) dependent on the accretion rate and, for ages less than about 10 6 yr, the initial state of the star. On the assumption that the stellar models are accurate, we demonstrate that, if the metallicity is known, the mass obtained is a good estimate. Saving Earth Britannica Presents Earth’s To-Do List for the 21st Century.The position of pre-main-sequence or protostars in the Hertzsprung-Russell diagram is often used to determine their mass and age by comparison with pre-main-sequence evolution tracks.Britannica Beyond We’ve created a new place where questions are at the center of learning.100 Women Britannica celebrates the centennial of the Nineteenth Amendment, highlighting suffragists and history-making politicians.COVID-19 Portal While this global health crisis continues to evolve, it can be useful to look to past pandemics to better understand how to respond today.Student Portal Britannica is the ultimate student resource for key school subjects like history, government, literature, and more.This Time in History In these videos, find out what happened this month (or any month!) in history.#WTFact Videos In #WTFact Britannica shares some of the most bizarre facts we can find.Demystified Videos In Demystified, Britannica has all the answers to your burning questions.Britannica Explains In these videos, Britannica explains a variety of topics and answers frequently asked questions.Britannica Classics Check out these retro videos from Encyclopedia Britannica’s archives.
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